>The gravity from the positive mass galaxy attracts negative masses from all directions, and as the negative mass fluid comes nearer to the galaxy it in turn exerts a stronger repulsive force onto the galaxy that allows it to spin at higher speeds without flying apart.
Meaning that positive and negative masses will accelerate in opposite directions in response to the same applied force. The effect is different, but it's a bit like how when you are in a car and hit the gas pedal, and your kid is in the back seat holding a helium balloon, the kid feels pushed backward into the seat but the balloon flies forward toward the windshield.
F = GMm/r^2
Meaning that gravitational forces between opposite signed masses is the opposite direction from the force between masses of the same sign.
Thus, the gravity force repels the normal mass away, but this repulsive force actually drags the negative mass along with it, because the negative mass accelerates the wrong way when "repelled".
I'm intrigued. Isn't the reason a balloon moves forward more of buoyancy? If you do the same experiment with the window open, doesn't it do as expected by most people?
I didn't read that positive matter is attracting negative matter. What I gathered is that the presence of a cloud of negative matter in the right place would be held in place by mutual repulsion of the outer arms of the galaxy and the inner galaxy. And that the neg matter supplies enough repulsion to the diffuse outer galaxy that it can spin a bit slower than it would have to otherwise.
Maybe it was modeled as a curvature of space? So positive masses curve space so that everything moves toward them - including light, massless particles, and negative mass. And negative mass repels everything, including other negative mass, by shaping space differently.
[Edit: jbay808 has a better explanation below, and negative mass attracts negative mass.]
Yeah this helped me create a mental picture that kind of makes sense. It would clarify why negative mass is also concentrated in areas of high mass, and could also provide the mechanism for constant universe expansion, as the space between masses is constantly being pushed away by the diffuse negative mass. Heh, armchair physics is fun :)
I think what they're saying is that positive mass galaxies attract the negative mass, even as the negative mass repels the positive mass (and all other negative mass). In a stationary system, eventually the two will reach an equilibrium.
However, the effects of this in a system in which the positive mass galaxy is spinning is that the negative mass counteracts the forces of inertia, allowing the galaxy to spin at higher speeds without shattering like a cd spun too quickly. Now, whether the math of all that makes sense, I'm am not in a position to comment on.
Wouldn't that allow for a "perpetual motion" setup where a blob of normal mass is in front of a blob of negative mass? The negative mass would push the positive mass forward, and then be attracted to it and follow. What am I misunderstanding?
I think you meant to say that the mass accelerates opposite to forces acting upon it.
An idealized -M + M = 0 pair of particles (one normal, one negative mass) could experience runaway acceleration. Author touches on this in the paper as a potential source for the incredibly high-energy extra-galactic cosmic rays.
Granted, the paper describes a model in which something that acts like a particle with negative mass exerts this force upon things, but has no claims as to what a physical manifestation of this would look like.
It would be perpetual motion, but momentum and energy would remain the same. Both depend on mass (so negative mass accelerating creates negative momentum and energy).
You can't ignore the gravitational force of the normal mass. The two would rapidly reach a zero-value equilibrium at some point where the repulsive force of the negative mass is perfectly cancelled out by the attractive gravity of the positive mass, so you have a net force of 0 at some distance.
For an easier to visualize example imagine placing one magnet positive pole up inside of a tube, and then you place another magnet positive pole down in the same tube. It would create a similar effect. There is a repulsive force being perfectly balanced out by an attractive force and the two reach an equilibrium where the magnet on top is 'floating' with the net of all forces in this closed system being 0.
Positive matter creates a gravity well. This much most people are familiar with.
Seems to me what they're saying is that this well will pull in negative matter until a balance is reached, this will, in turn exert force on the internal structure of the galaxy.
The galaxy is in the center, and with a large positive mass it attracts negatively-massed matter towards it. But the negatively-massed matter repulses rather than attracting, so it has the effect of "squeezing" the galaxy in on its self.
Negative mass is like playing opposite day. Negative mass is attracted to other negative mass, but because of the inverse acceleration to force relationship of negative mass the end result is that it is pushed away from other negative mass. Negative mass would also repel positive mass, which is straightforward for the positive mass but for the negative mass (again, inverse acceleration) it would end up with an "attractive" motion toward the positive mass.
Every "dark" physics piece is essentially a science opinion piece, with "darkness" included in the title as the general prevailing clickbait advertising strategy, because the layperson interpretation is that it implies sinister mysteriousness.
It's never news, it's just a refresher listing of unknowns.
The word 'dark' is used in titles because the articles are about the nature of dark matter and dark energy, which are widely accepted terms. You would have to go out of your way not to use the word dark when talking about these subjects and it would make titles less descriptive.
The biggest tell that the Electric Universe theory is bunk is that anybody who ever talks about it can't hardly get two sentences in before dropping in the "but nobody ever talks about magnetic fields or electric currents at this scale"... which is easily observed to be utterly false. Astrophysicists talk about it a lot, and you can find even more discussion if you add "plasma" to the search terms.
If Electric Universe advocates are so wrong about something so easy to check, it doesn't give me much reason to check out anything else they say. (The fact that I have done so nevertheless and I've never managed to find them making an actual prediction is another. It seems to be one of those recursive handwave theories that never quite lands on anything concrete no matter where you search, but it sure does know that the standard theory is wrong.)
Not a physicist, but my summary of the author's point would be:
If you allow for matter with a negative mass and plug it into a n-body simulation. You seem to get what looks like dark matter from first principles. Namely, a halo of non interacting matter around galaxies in just the right "non-cuspy" shape. Their conclusion is that this is worth considering, as almost all mainstream cosmology makes the assumption that mass >= 0, despite the fact mass <=0 doesn't seem to violate any physical invariants we know.
The idea is persuasively simple. Every other force besides gravity is polarised, so why not also gravity? It seems interesting to me, but I'm nowhere near knowledgeable enough to know whether it can be easily refuted.
Is it true that every other forces besides gravity is polarised? There's a repulsive nuclear strong force and an attractive nuclear weak force?
EDIT: Partial answer is that there is a three-"color" polarization for strong force. I feel like it somewhat weakens the argument of "everything else is polarized, why not gravity" if the other forces are polarized in unique ways.
Apologies, you are correct, jumped the gun a bit there.
It's repulsive at short distances, attractive at longer distances. And oddly, the longer the distance, the stronger the force, it doesn't follow the inverse square law!
Matter-antimatter annihilation would produce gamma rays that are detectable. I believe there was a study that went looking for that but didn't find anything.
To clarify (presuming I understand correctly): Antimatter might have negative gravity, but dark matter can't be antimatter, because we'd see the gamma rays from the collisions.
Because it's still in places where there's interstellar gas (if this theory is right). And repelling each other gravitationally doesn't work very well at the molecular level.
I know you put the "/s", but in case anyone isn't aware, as far as we know the photon is its own anti-particle. So light from anti-matter stars would still just be light.
Given the spacetime interval between photon creation and destruction is always zero, I'd find it moderately bizarre if there was an antimatter version of them.
They could repel each other gravitationally too. Seems like that’s how antigravity would would work. I don’t even think we know that about antimatter yet.
"Every other force besides gravity is polarised, so why not also gravity?"
Gravity is mediated by a spin-2 gauge boson, the graviton, whereas the other known fundamental forces are mediated by vector (spin-1) bosons. There is an exercise in Peskin & Schroeder (I believe) working out the lack of a gravitation "repulsion" resulting from this difference.
That being said, I think this kind of phenomenology is excellent and raises lots of interesting questions. There may be reason to rule out this author's new idea, or maybe rethink an established tenet. That's why science is such an exciting field.
Exactly. What I liked about MOND was that it basically amounted to "we don't know." The populist notion that dark matter is conclusively matter, and dark energy is energy seems a bold claim for such a young species.
Crazy theories like non-universal constants are probably crazy, but they do explore unconventional lines of thought. One of those, most likely an unknown unknown, may be answer. Focusing all our energy on one explanation (that has been falsified more than once) seems like a waste.
“What’s most impressive is that the predictions of dark matter were first made in the 1970s and 1980s, and were observationally confirmed later. This is not a case of tweaking the model to fit the data; this is a case of the best kind of science you hope for: where you make predictions, make the observations, and what you see validates and confirms the predictions you had made.
And yet, even 35 years later, there are no modifications of gravity that achieve the galaxy-scale successes of MOND that also explain these other observations. The best tests of dark matter vs. MOND, which are on large, cosmic scales, have a clear winner and a clear loser.”
In short: looking at everything we can observe and can model, we do know that the “dark matter” is matter. Because we first made predictions (“it should behave as matter”), then confirmed them, many times. And MOND can’t even explain that what’s already observed.
"Every genuine test of a theory is an attempt to falsify it, or to refute it. Testability is falsifiability; but there are degrees of testability: some theories are more testable, more exposed to refutation, than others; they take, as it were, greater risks."
That's putting the effect before the cause. The dark matter map can also be a map of "how wrong we are," not only "how much invisible mass is there." An incomplete periodic table of physics, if you will.
We need to produce a WIMP here, on Earth, and watch it decay. That's a "thruthifiable" experiment, but it'd be enough to convince me that we've found the correct line of thinking (which the popular opinion could very well be, don't get me wrong - I simply believe that we're overconfident).
> We need to produce a WIMP here, on Earth, and watch it decay.
No. You can't "need" something if that is simply unreachable. There’s reasonable chance that to produce the particles (which still don’t have to be the WIMPs as we imagined them, and still would be „matter“) the energies needed are simply beyond those that will ever be available to humanity. It wold not mean more than that: that we, humans, just can't repeat with our technology the extreme conditions that the Universe had/has.
The WIMPs in the form discoverable by the energies that are available to us at the moment would have been nice luck, but the existence of dark matter is not anything less certain when the dark matter is simply different from that. It would still be "matter" and not something else.
That doesn’t disprove the “matter-like” observed behavior, it just shows that the particular assumptions under which exactly these non-found WIMPs were imagined were false.
If you have an infinite space of particle parameters and a series of tests keeps eliminating large swaths of this space, at some point it feels like you're playing a god of inbetweens game a la creationists arguing using gaps in the fossil record. There is a categorical difference, of course, but as a practical matter the parallels are useful, at what point do we table (for now) the search for the populist position and break for alternatives?
"Five Reasons We Think Dark Matter Exists: No other idea explains even two of these."
"1.) Galaxy Clusters"
"2.) Galactic Rotation Curves"
"3.) The Cosmic Microwave Background"
"4.) The Bullet Cluster"
"5.) Large-Scale Structure Formation"
From these 5, only dark matter matches all. Whereas MOND theories match only one, the "item number 2" and only as an attempt to after-the-fact-explanation of what's anyway observed, not as any prediction of anything new.
As I've quoted in my another message here:
"the predictions of dark matter were first made in the 1970s and 1980s, and were observationally confirmed later. This is not a case of tweaking the model to fit the data; this is a case of the best kind of science you hope for: where you make predictions, make the observations, and what you see validates and confirms the predictions you had made."
Moreover, there's nothing "large" of the possible space that is now excluded. The possible space includes everything that "behaves like matter". What's (still just "probably") eliminated is just what were for us, humans, in this particular moment of our history very convenient to happen: that the energies needed are exactly such that WIMPs that were conveniently postulated to be such can be generated by the device which design and maintenance costs less than 2% of the total cost of the F-35 fighter planes (the money invested and planned to be invested in F-35 is 75 times bigger than the international investment in LHC). It would have been luck if it had happened, but the possibilities are much bigger and include such which are going to remain physically unreachable to be produced by our technologies.
That we, humans, can't produce some particles doesn't disprove that dark matter exists when we have precise observations that can only be explained by dark matter and effectively by nothing else.
Dark matter can be placed arbitrarily so it can match pretty much anything. I remember recently there was a galaxy with negligible deviation of the GR-predicted rotation curve and that was taken as evidence for dark matter too (even though MOND was still the better fit for that case).
"Dark Matter" started out ok but has become a "god of the gaps" argument, I expect eventually MOND or similar solutions will be developed for all those issues.
Also, I gather that while MOND does seem not predict these observations exactly, it is not fully developed yet and often the presence of some "normal" cold gas (undetectable via light) could make it fit. Also, to get dark matter to fit the observations requires post-hoc assumptions and parameter adjustments. Eg:
"The amplitude of the third peak observed by WMAP merely falsifies the simple ansatz I used to make the prediction, not MOND itself. Indeed, I pointed out in the original papers that the ansatz must fail at some level, so I am hardly surprised that it does. All this means is that there are degrees of freedom (like a scalar field) that can oscillate separately from baryons in whatever the relavistic parent theory of MOND might turn out to be.
The real test was for ΛCDM: the third peak had to be higher than predicted by pure baryonic damping. ΛCDM survives this test. It "wins ugly" in that, in order to obtain a fit, we have had to nearly double the baryon density over what it was so confidently known to be before it wasn't. That, and the no-CDM prediction for the second peak is still bang on. Amazing coincidence, that."http://astroweb.case.edu/ssm/mond/
>"the predictions of dark matter were first made in the 1970s and 1980s, and were observationally confirmed later."
I'm afraid I have never seen this although I have looked into this topic (as a layperson). Can you give an example of this? It would make the dark matter idea much more convincing.
> Dark matter can be placed arbitrarily so it can match pretty much anything.
Quite the opposite, and that statement is obviously uninformed. The science behind dark matter is very precise and exact. That's why the alternative theories can't come even near: once the same rigor which was applied to test dark matter is applied to the alternatives, the alternative theories simply break.
> I'm afraid I have never seen this ... Can you give an example of this?
I've given you the link to the complete articles and also highlighted the major points. Try to read and understand them fully, then search for more. For example, even on the MOND page that you linked you can read:
"2013: ΛCDM provides an excellent fit to the improved CMB data just as MOND provides an excellent fit to improved rotation curve data. As before, one's interpretation depends entriely upon which you find more impressive."
Note: "rotation curve" from that quote is the "item 2" from my posts before. So, no, still the match of the single "item 2" is definitely not "more impressive." The "CMB" mentioned on your page is the "item 3" from all 5 which are explainable by ΛCDM (and specifically, ΛCDM is the model part of which the "dark matter" is: https://lambda.gsfc.nasa.gov/education/graphic_history/univ_... see there 4 and 5).
Edit: response to your later response below:
> The claimed predictions from the 1970s and 1980s are not mentioned on that page.
They are mentioned even on the very page that you linked to: that page, among other points, followed the history of the tests and effectively concludes that, yes, finally, in 2013 ΛCDM wins the "item 3". Read carefully. Learn what ΛCDM is. MOND can still only claim "item 2." Which actual "authority" can show anything else? Also, behind "darkmattercrisis" blog are just a few guys with very fringe positions.
>"Quite the opposite, and that statement is obviously uninformed. The science behind dark matter is very precise and exact."
From what I have seen they can get precise post-dictions but not precise pre-dictions because there are so many free parameters (basically you can put dark matter wherever you need it).
>"I've given you the link to the complete articles and also highlighted the major points. Try to read and understand them fully, then search for more."
The claimed predictions from the 1970s and 1980s are not mentioned on that page.
Also, that "startswithabang" guy is more like a catholic inquisitor than a scientist. I wouldn't consider him a reliable source because this behavior indicates he lacks the required capacity for curiosity, he can only repeat what authority figures say:
"The blogger (@StartsWithABang) contacted @scilogscom on January 24 by replying to a 15-day old tweet that announced our blog’s move to the new domain. He tweeted “Bummed that @scilogscom is in the business of promoting contrarian scientist viewpoints.”, and asks the SciLogs.com community manager (@notscientific) “[Why] are you allowing @scilogscom to promote contrarian voices that undermine public understanding of [science]?”, adding “You have taken on “Dark Matter Crisis” blog, whose mission is to undermine all of physical cosmology & promote MOND.”
The two agreed to discuss the issue via email, with the blogger adding that he was “personally worried that you are promoting clicks & false controversy over quality science content”, and states that he is “very, VERY disappointed about this move that @scilogscom has made”.
By now the SciLogs.com community manager has explained to us what happened after these tweets. He and the publishing director responsible for SciLogs.com unfortunately assumed that the blogger’s criticism was justified. They decided to close our blog without conferring with others or asking us for a statement. After we complained about the discontinuation, they performed an internal investigation, which involved reaching out to astrophysicists and other people, and have realized that discontinuing our blog was a big mistake. We attribute SciLogs.com’s poor judgement to two factors: neither the community manager nor the publishing director has an (astro)physical background, it was the first time that SciLogs.com had experienced an attack against one of its blogs."
https://darkmattercrisis.wordpress.com/2013/03/08/the-dark-m...
Anyway, I can tell you know even less about this topic than me so let's leave it at that.
> The claimed predictions from the 1970s and 1980s are not mentioned on that page.
They are mentioned, they just aren't elaborated: you have to do your own homework if you apparently "understand" the subject enough to claim your own opinion. A begin of that homework, however, as I've already written, you even have on the initial page that you chose to post, specifically:
"2013: ΛCDM provides an excellent fit to the improved CMB data"
As I've said, that's the "item 3" from the 5 item list, and it's ultimately confirmed by ESA's Planck satellite results in 2013. That is a strong confirmation.
Note: more details and the timeline are on the very page you've initially posted. Ad hominems don't change the facts.
No, they were not mentioned. If they were, you would be quoting it.
>'"2013: ΛCDM provides an excellent fit to the improved CMB data"
As I've said, that's the "item 3" from the 5 item list, and it's ultimately confirmed by ESA's Planck satellite results in 2013. That is a strong confirmation.'
You have to read the entire quote. This "fit" was a post-diction, not a pre-diction:
"in order to obtain a fit, we have had to nearly double the baryon density over what it was so confidently known to be before it wasn't."
Post-dictions can be interesting as part of model development but are unable to confirm a model. The new, (possibly adjusted) model then need to be checked against new data.
What is your support for that claim? Can you quote other sources? I can quote my sources: Here are documented two independent approaches in estimating baryonic density: one is calculations based on the Big Bang Nucleosynthesis D/H ratio and another one is the values measured by the satellites observing the SMB. Note the two decades of similar values using both approaches:
> It looks like this doubling happened in ~2000 so it wouldnt show up in your chart.
Since 2000 many new satellite measurements were done and the values improved. As I've quoted your page Planck in 2013 ultimately made the most precise CMB measurements that most conclusively fit the "dark matter" model and provably don't fit the "MOND" model. And for that proof there were no "factor 2" corrections, as you also confirm, at least since 2000. The way you wanted to believe, and how you quoted some other parts of the same document, was that that had to be done for that 2013 proof, and it obviously haven't had. BTW the nature of the measurements, especially those made decades ago, is such that when we started with some value and then later measured something only 2 times smaller or bigger (but then all repeated measurements remain closer to one another), it's still OK. It's not the absolute values that prove this or that, it's the measured shapes that match or not the very complex calculations. And that's what happened in 2013 with Planck: what was measured for more than a decade with other satellites, and finally most precisely with Planck, shows the shape that in practice can't match the MOND predictions but nicely matches what the "dark matter" model predicts.
>"Since 2000 many new satellite measurements were done and the values improved."
The point is that "suddenly" people started measuring a higher baryon density to make it fit what was required for lambda-CDM model to fit the CMB measurements. Ie, the lambda-CDM model did not pre-dict the spectrum beforehand, they had to tune it to match the data.
I have to say, it is very frustrating to discuss this since you seem to misunderstand every simple point being made, and before that has been corrected you have gone on to miss another point that needs to be addressed. We are not discussing this at a very deep level... Perhaps English is not your native language, but I am just letting you know. I hope no offense is taken.
> The point is that "suddenly" people started measuring a higher baryon density to make it fit what was required
W...what? Do you really believe that different scientific organizations in different countries around the world financed and sent different satellites into space but misrepresented the actually measured values only to "fit" some old theoretical predictions?
Really?
Then there's really no point discussing anything about science with you.
There's no point to you discussing science with scientists then. Because that is exactly what scientists are worried about. Why? Because it happens all the time.
>"I do disagree with the assertion that I often see, and remains a widespread misconception among scientists, that MOND does not do a good job of explaining cosmic phenomena. I’ve been through all that, and it is simply incorrect to say dark matter is always better outside of galaxies. Sure, it has shortcomings, but often dark matter only appears to be better because it declines to make a comparably testable prediction. Clusters of galaxies are a great example: MOND is off by a factor of 2 in mass (20% in velocity). Dark matter makes no prediction. Anything over the luminous mass is OK. The rest is dark. It doesn’t matter if that’s a factor of 2 or 5 (the modern value) or 6 (the cosmic value) or ~100 (Zwicky’s value). Being a factor of 2 off only sounds bad because there is a definite prediction! With dark matter we don’t care what that factor is because we have no prediction – at least, not one that can not be fudged."
At the time you do the initial placement. But that's not the only relevant time.
You can scatter any type of DM on a spacelike hypersurface any way you want, but then you have to figure out the successor (and predecessor) spacelike hypersurface, and there you have to bite a bullet: you can constrain the DM or throw away the well-posedness of the initial-value problem.
Your constraints are dominated by the hyperbolic-elliptic PDEs. The hyperbolic portion lets you lay down initial values for your dark matter on a spacelike slice. You then need an equation of motion for your DM, as you do for your visible matter, so that you can evolve the contents of that initial spacelike slice in time. The elliptical portion are the Hamiltonian and momentum constraints.
Without satisfying these constraints you tend to lose uniqueness, which destroys the predictive power of your model.
If you're spraying down a dust of non-interacting DM particles within some spacelike region, as you evolve from your initial configuration you end up with each particle growing into an everywhere-timelike worldtube. (Non-(self-)interaction is just to make this really clear without the distractions of decays or daughter products from scatterings. Each dark matter particle has its own worldtube extending arbitrarily far in to the future or past.)
The simplest Cold Dark Matter models have a conventional nonrelativistic and relativistic dynamics determined by two parameters: particle rest-mass and interaction cross-section. In our simple non-interacting cold DM dust case the interaction is purely gravitational, so we're left with every particle having an identical rest mass.
Now, lay down the visible matter in a galaxy (or cluster!). You can lay down the dark matter dust arbitrarily to form a halo which could support a realistic galaxy rotation curve on your initial surface. You are right that you can put it practically anywhere, and are not restricted to uniform density. However, when you evolve in time, you inevitably find that you are stuck with a narrow range of mass densities at various points in the galaxy (or cluster!), but still have freedom to set down differing particle number density, higher numbers at lower particle mass. If your particles are too light, the gravitational coupling with the visible matter is too slight; worse, the gravitational coupling between the DM particles is too slight too, and your galaxy quickly disassembles. If your particles are too heavy, instead you get dynamic heating of hydrogen gas, and you get far too much star formation and metal enrichment.
You can go the other way too: first put down a test particle DM halo and then add hydrogen gas, or a modern galaxy, or whatever else, and see how the system evolves. It is a feature of the standard galaxy astrophysics that DM and visible matter do not have to be near each other at all times. (This gets important when you consider galaxy clusters rather than just galaxies; laying out CDM is hard work, as I've been describing, but you can't just drop in MOND. It's also testable at solar-system scales; we can look for post-Newton dynamics in the Jupiter system, or in the inner solar system, and test whether it's compatible with General Relativity (with a fine dark matter dust of up to a few Phobos-masses inside Neptune's orbit) or whether it's compatible with a different post-Newtonian dynamics, like MOND, where the galaxy's MOND corrections will generate a quadrupole anomaly stronger around the outer planets and their moons than around the inner planets).
More complicated particle CDM theories are still stuck with the general rules of the simple model above: the stress-energy in the large has to hang around in the right places for long periods of time; the stress-energy can't just pop in and out of existence at a point (it needs to be thermal, so its worldtubes cannot ev...
> If you have an infinite space of particle parameters and a series of tests keeps eliminating large swaths of this space, at some point it feels like you're playing a god of inbetweens game a la creationists arguing using gaps in the fossil record.
Or you're getting closer to finding out what the real parameters are?
If you eliminate all possibilities and have to make up some new parameters, sure, that feels like you're not doing science and might as well be a creationist. But what you describe, narrowing down the space of possible explanations from infinitely many to only a few, is the dream of every theoretical physicist.
Lots, we just patch it and move on. Iron orb should glow blue unless this one variable is this one value? "patch it" by calling that one value plank's constant and see, the models still good!
Our math says the universe comes up short on mass by over 3/4ths? Must be invisible matter we can't see or interact with see, the models still good!
Over simplification in the extreme, but also a far cry from the rock solid foundations it claims to be.
Given there have been no detections of individual gravitons, saying it's a spin-2 gauge boson is speculation at best (though it might be what the model predicts)
If it weren't, general relativity would be wrong as well and we know that (in its domain of applicability) it's a highly accurate model. GR models gravity as fluctuations in the metric tensor of spacetime, which is a symmetric rank-2 tensor. This is all you need to know to conclude that a graviton (a quantize fluctation of that tensor) must be spin-2.
https://en.wikipedia.org/wiki/Graviton mentions "[I]t can be shown that any massless spin-2 field would give rise to a force indistinguishable from gravitation, because a massless spin-2 field would couple to the stress–energy tensor in the same way that gravitational interactions do. This result suggests that, if a massless spin-2 particle is discovered, it must be the graviton." It links to other resources from there.
Well, in classical GR, which is still our best and most accurate theory of gravity, gravity is only an apparent force and thus isn't (or can't be) mediated by anything. Stating as fact that a graviton must exist or otherwise GR "would be wrong" is a bit of stretch IMHO. The graviton is plausible speculation, backed by some theoretical justifications, but nothing more yet.
as far as I understand, the space-time curvature is just an interpretation of the math. Another perfectly acceptable interpretation is as a relativistic classical field that couples with the stress-energy tensor field (thus, indirectly, with any other field).
In fact I think it is possible to come up with a space-time curvature interpretation of the EM field, for a toy universe where everything has a EM charge.
Now, given that the gravitational field couples with other fundamental fields and those are quantized, the gravitational field must necessarily also be quantized. The math straight forward quantization works fine for low energies, thus the spin-2 boson described elsethread. The issue is that at low energy, the gravitational effects are so small that it is impossible to come up with an experiment that would detect the difference between a classical field and a quantized field.
At high energy the straightforward derivation breaks down because of infinities (other quantum fields had similar issues, but the math tricks used to resolve them do not work with the gravitational field). There are multiple theories (string theory, loop gravity, etc) that try to resolve this problems, but the experimental apparatus required to distinguish between them are colossal (as in particle accelerators with radii measured in AU).
>"If it weren't, general relativity would be wrong as well and we know that (in its domain of applicability) it's a highly accurate model."
Somehow I doubt it. I bet as a save someone would propose "dark spin" or some elaborate calculations that shows a few factors happen to cancel out exactly but no one noticed before now.
Well GR is potentially very wrong both at very large scales (seemingly requiring dark matter and dark energy) and very small scales (due to known present incompatibilities with quantum mechanics). It's only been shown to be a highly accurate model everywhere in between.
I think both dark matter and dark energy are perfectly consistent with GR. Hitherto unobserved matter would contribute to the stress energy tensor and dark energy is (I believe - has this changed?) the cosmological constant.
Yes, they are consistent. In fact, they are the result of the following line of thinking: given that we are observing unexpected results once we account for (using the GR paradigm) all the clearly apparent mass and energy, what else could we change so that GR doesn't have to change? In other words, the consistency with GR is already baked in. That's not to say that the approach doesn't have huge merit.
Um, correct my amateur understanding if it's wrong, but... we don't actually have a working quantum theory of gravity, do we? So how certain are we that gravity is mediated by a spin-2 gauge boson?
That does not appear to have any details that relate to your comment whatsoever. All it says is that renormalization doesn't work on gravitons.
However, if I understand your comment correctly, you're saying that the underlying non-quantum theory (GR) uses a symmetric rank-2 tensor, therefore you have to have a spin 2 particle - as opposed to photons, whose underlying non-quantum theory has an antisymmetric rank-2 tensor, and therefore give rise to a spin 1 particle. Do I have that right? If so, could you point me to something that would explain why?
Please see my comment above. You can use the language of gravitons and Feynman diagrams even for classical gravitational physics, but the calculations will be intractable for strong fields. See https://en.wikipedia.org/wiki/Graviton#Gravitons_and_renorma... for example
It's hypothetical, but less so than a lot of the things theoretical physicists consider.
Any quantum theory which reduces to general relativity in the classical limit will look like a theory of gravitons when quantum effects are small. This is just how the math works out in effective quantum field theories involving massless spin-2 particles.
The definitions you brought are tailored to what is our best understanding of the universe. Nobody ever said that these are immutable. Newtonian physics also must have felt rock solid, time and time again explaining new phenomena until it did not and there came about a better explanation.
Sidenote, but I think it is very important to emphasize that Newtonian physics is actually still quite rock-solid under certain conditions. These conditions are actually quite common and useful for humans. Many engineers utilize Newtonian physics everyday.
> In theories of quantum gravity, gravitation is mediated by the graviton – a massless, spin-2, boson. This means that any pair of negative masses would attract, and not repel as suggested in this theory. However, there are also theoretical arguments that gravitons cannot, and will not ever, be detected experimentally (Rothman & Boughn 2006). There appear to be two options: either it is possible that the graviton could be modelled as a bound state of a positive and a negative mass, in a theory of composite gravity or some other mechanism which provides a modification of graviton properties. Alternatively, this could also indicate that the proposed theory cannot be modelled by real, physical, particles, but rather by the presence of effective negative masses within a superseding theory.
Edit to add: the abstract pages also often contain useful extra information, such as (in this case) a link to YouTube videos of the simulations -- https://www.youtube.com/channel/UC8ltFtaETXDphec0l-VxMsg -- or the publication history (in this case, a DOI is given).
I am of the opposite opinion. Don't post links to the abstracts as the pdf's contain them too. But the pdf's also contain figures and other things which make it easier to browse, whereas the arXiv abstract page contains a lot of unnecessary fluff.
If you think about it, "mass" actually means two different things. One one hand, there is mass as the "charge" of gravitational interaction, that is, the gravitational field generated by a body is proportional to its mass. Let's call this concept "gravitational mass". On the other hand, you have mass in the sense of "intertial mass", a body's resistance to change of velocity. Although we kinda take it for granted, it's weird that these two concepts coincide, and not something that would be necessarily true a priori.
Asserting this equivalence as a principle is the starting point for Einstein's theory of General Relativity, which is one of the most beautiful and thoroughly tested theories in modern physics.
On the other hand, you can show that the inertial mass cannot be negative (else the Lagrangian would not be bounded from below and therefore you could not minimise it; the whole of physics then comes crumbling down). Therefore, if that equivalence stands, the gravitational mass also cannot be negative.
I’m not sure this stands true when you think about fields.
Think of charged particles in a magnetic field and how polarity and and strength of the charge impacts their interaction the strength would do the same thing for both particles while the polarity would mirror their interaction.
> On the other hand, you can show that the inertial mass cannot be negative (else the Lagrangian would not be bounded from below and therefore you could not minimise it; the whole of physics then comes crumbling down).
Here's a purely thought-experiment argument for inertial mass not being negative:
If you had an object with negative inertial mass, then pushing on that object would cause it to accelerate into your hand. This is a positive feedback loop that immediately results in the object and your hand exerting infinite amounts of force on each other, because applying force just leads to applying more force.
Specifically the interaction is: objects with positive inertia being able to 'push' objects with negative inertia.
This brings up an interesting question, is gravity about exchanging momentum? My mental model of gravity being spacetime curvature would imply no, but that intuition is likely wrong.
If inertia is resistance to change then negative inertia would be promotion of change? I really dont know, the idea of negative inertia seems wildly counterintuitive.
Inertia isn't "resistance to change". It's defined by Newton's second law, F = ma. Force and acceleration are directional vectors, and mass is scalar.
In the form of the equation favored by my high school physics teacher, a = F/m, we see that when a force is exerted on an object with positive mass, the object will accelerate in the same direction as the force, in inverse proportion to its own mass.
But it's very easy to put a negative number in for the mass, and then you see that such an object would accelerate in response to force just as much as an object of equal positive mass, but in the opposite direction.
Thanks, that's obviously a much better way to think about it. However, you're wrong in at least the typical physics definition of inertia found in dictionaries is in fact, "resistance of change (velocity, acceleration, momentum)."
In that same framework, the maths definition of inertia is also momentum = mv and a google search confirms this. What would negative momentum even look like? Because that's not just movement in the opposite direction.
Velocity, the v in the definition of momentum, is also a vector, so yes negative momentum is just momentum in the opposite direction from positive momentum, whatever direction that may be.
Inertia is generally not defined as being momentum, because an object's momentum in any direction increases along with its velocity in that direction, while we like to think of inertia as being determined solely by mass.
https://en.wikipedia.org/wiki/Inertia begins "Inertia is the resistance, of any physical object, to any change in its velocity". That's specifically resistance to acceleration ("change in velocity"), not resistance to the concept of change in general.
(It does go on: "In common usage, the term "inertia" may refer to an object's "amount of resistance to change in velocity" (which is quantified by its mass), or sometimes to its momentum, depending on the context.")
But that's what I just said: resistance to change in v, a, p are the dictionary definitions I found on Google.
You're right about v. I thought a negative velocity was equally absurd, but it looks like it's defined as a vector which makes sense of course. The last time I took a proper physics course was in high school. How embarassing though, as I've read and watched more popular science/physics material than most people, probably. Hah.
Here's a thought experiment that shows that there can't be electrons:
"If you had an object with negative charge, then putting that object near a positive charge would cause it to accelerate towards the positive charge." Getting closer increases the force. "This is a positive feedback loop that immediately results in the object and the positive charge exerting infinite amounts of force on each other, because" getting closer "just leads to applying more force."
Thus, there are no electrons.
So, I don't think the thought experiment rules out negative mass. In fact, the article explicitly says "if a negative mass was pushed, it would accelerate towards you rather than away from you.", and I assume that if that's an obvious contradiction, the author would have caught it.
So, electrons are attracted to protons by the electromagnetic force, but they don't make it all the way there because they are repelled by some other force. You don't have a paradox because our model of the electromagnetic force doesn't specify that it's the only force there is.
But if a negative mass is moving towards you and encounters a repulsive force, that repulsive force will, by definition, move the negative mass further toward you. It would need to encounter an attractive force to reach equilibrium.
Note that, unlike in the electron example, it doesn't matter what kind of force is being applied; the object's negative mass tells us that the response to any repulsive force is acceleration towards the force.
> they don't make it all the way there because they are repelled by some other force
A repulsive force is not the best way to think about it. The potential of the nucleus is the usual -1/r, and goes to (negative) infinity at zero. A repulsive force would be incorporated into the potential and appear as a bump around the nucleus, and would mess up the electron orbital.
A hand-wavy explanation of why the electron doesn't fall in: if you try to push the electrons into the nucleus, you necessarily localize the electron into a smaller volume; this means its wavefunction must get "spikier" and therefore it has more kinetic energy. This kinetic energy rises faster than the potential energy drops, so the state of lowest energy is actually found at an average radius > 0.
Note that it took quantum mechanics to rescue the atom. A classical electron could fall into the nucleus.
I do not see any problem with a negative-mass particle accelerating towards the force. The analogy with opposite charges seems right to me.
A different hand-wavy explanation. The places that an electron can be found are described by a wave pattern. If the electron is staying in place around a nucleus, that wave pattern has to be a standing wave that reinforces itself. To reinforce itself it has to wrap around the nucleus an integer number of times.
This explanation doesn't just explain why it doesn't fall in, it also explains why there are discrete shells that it could be found in, corresponding to how many times it wraps around the nucleus. (It doesn't explain why only a finite number can fit in each shell though. Or why bigger shells can have more electron orbitals. Or...well the actual theory has to be good for something!)
I'm not really interested (here) in the reality of the interaction between the electron and the nucleus. I don't think the existence of an electromagnetic field is a good argument against its own existence as argued by FabHK further up.
I also don't think FabHK's argument works as an analogy to my problem with negative mass. The electromagnetic field is not self-reinforcing in the same way.
Imagine that you're holding a marble of negative glass in your fist. Negative glass is indistinguishable from ordinary glass except that its mass is negative rather than positive.
As we all know, the first step in solving any physics problem is to draw a free-body diagram. ( http://www.smbc-comics.com/comics/20130616.png ) Let's draw one here. First, we'll do one for an ordinary marble:
1. The enormous mass of the earth attracts the marble downward proportionately to the marble's mass.
2. The marble cannot accelerate downward, because it's stuck in your fist. Your fist experiences a downward force equal to the weight of the marble.
3. By Newton's third law, your fist exerts an upward force on the marble equal to the force exerted by the marble on your fist. This is exactly equal to the weight of the marble, but in the opposite direction. The two forces cancel, and the marble is at rest.
Now for the negative marble:
1. The enormous mass of the earth attracts the marble downward proportionately to the marble's mass. Because that mass is negative, the marble attempts to accelerate upward.
2. The marble can't accelerate upward, because it's stuck in your fist. Your fist experiences an upward force equal to the weight of the marble.
3. By Newton's third law, your fist exerts an downward force on the marble equal to the force exerted by the marble on your fist. This is exactly equal to the weight of the marble, and in the same direction, effectively doubling the marble's weight. The marble is now trying twice as hard to accelerate upward into your fist.
2. (Again.) The marble can't accelerate upward, because it's stuck in your fist. Your fist experiences an upward force equal to double the weight of the marble. Nothing has moved; we're still just trying to work out the balance of forces within the system at rest.
3. (Again.) You can see where this is going.
What is the conceptual breakthrough that rescues negative mass from this trap? (Note that saying the marble has negative inertial and gravitational mass, as opposed to negative inertial mass and positive gravitational mass, doesn't help: the marble will be trying to accelerate downward instead of upward, but it will still be doing it with infinite force.)
Maybe that's precisely why objects with negative mass do not exist on the Earth? But what stops them to exist in the void of space, between regular matter? And it doesn't need to be actual "objects", but isolated particles or a particle gas. In relativistic physics mass is dependent on the body's energy, so negative mass implies negative energy.
This is an argument that negative mass cannot be contained, not that it does not exist.
(Though it escapes me why you would assume that I believe in theories about negative mass, which actually seem to me to be very speculative and not very likely.)
> But if a negative mass is moving towards you and encounters a repulsive force, that repulsive force will, by definition, move the negative mass further toward you
I am not sure you understand what "repulse" means..
Joking aside, it's easy enough to imagine negative gravity. Do you mean that such counter intuitive behavior would be the natural consequence of negative inertial mass?
> that repulsive force will, by definition, move the negative mass further toward you.
only if the repulsive force is negative gravity.
If a positive-mass electron attracts a negative-mass electron, the repulsive electromagnetic force will still keep them from making it all the way there.
>> that repulsive force will, by definition, move the negative mass further toward you.
> only if the repulsive force is negative gravity
We're talking about negative inertial mass, not negative gravitational mass. Negative inertial mass means accelerating against the direction of any force applied to you, including but not limited to gravity.
The problem with these papers is there is no compilers to perfectly test the logic except peer review so no one knows if its horse shit or not until someone comes along and comes up with some way to prove it wrong, so I would not be so quick to say “if that’s an obvious contradiction, the author would have caught it”
Physics is highly mathematical and as such, can indeed be "tested" (i.e. proven) to be consistent with mathematical laws. This is fairly straightforward (for suitably mathematically literate people). The real issues are:
1. can the theory make testable predictions that can be verified by observation? e.g. string theory has had issues with this.
2. what does <wierd-mathematical-construct> "mean"? e.g. negative mass, spooky action at a distance, spacetime singularity.
In practice, only 1 really matters. However, 2 tends to bug a lot of people (consider the arguments around what the quantum world really represents). I'm one for whom 2 is important. "But what does it really mean?"
Back to your original point, looking at the paper [0], it doesn't seem too ridiculously hard to validate the math for someone familiar with this space (see what I did there?), unlike me who has forgotten everything I was taught [1].
Why match is better for that? To compute prediction, you will need to use computer and compiler, so it better to talk directly in these terms because of (1) (see above).
Einstein used the best tool he had at hands - math. Now we have mach better tools: computer models, knowledge databases, AI. Math is good because math formulas are very compact, so they are easy to play with, but computer models are much better, because computer can visualize them directly, calculate values correctly, and verify result automatically. Think about Computer Aided Science.
I'm confused - are you trolling us? You do realise that computer science is based on math right? All the calculations that that OpenGL does are mathematical calculations. Even databases are based on relational algebra. AI - linear algebra, probability, multivariate calculus, optimization etc.
There would be no computer science without math.
No, I'm not trolling. English language just have no word for that thing. How would you name something in middle of OpenGL, Kerbal, Wikipedia, Recursive Text, Alpha Go, etc? Something where you can setup your experiment, start it emulation, and then zoom from macro level to quantum level, with description of every effect at each level, with formulas at hand, with links to experiments, papers, reviews, confirmations? Something that can actually answer your questions, teach you actual physics. Something where you can plug your own theory to see is it fits real world better that someone else theory. I have no idea how to name it. Universal Programmable Augmented Disсoverable Science Model?
> If you had an object with negative inertial mass, then pushing on that object would cause it to accelerate into your hand. This is a positive feedback loop that immediately results in the object and your hand exerting infinite amounts of force on each other
No, it doesn't; since this turns the repulsion of normal objects (which is what makes them resist interpenetration) into attraction by switching the orientation of the resulting acceleration, what it results in isn't more force at the boundary (the boundary is a repulsive effect), but interpenetration, which results in force declining to zero as the masses centers are at the same place. (And then switching directions as they cross.)
I'm assuming that you can push in the classical sense but for what negative mass does to the equations, but sure, if you can't then the whole question is moot.
Don't you have two things that can generate gravitational fields though? As distortions in spacetime can be caused by either mass or the momentum of massless particles, such as photons. Or do I have that wrong and photons merely follow the distortions and not cause any of their own? As presumably, you could have a negative gravitational field from a photon traveling backward in time, without having any issues with changes in inertial mass.
> Or do I have that wrong and photons merely follow the distortions and not cause any of their own?
You have that wrong in the momentum of massless particle has mass, independently of its carrier. Massless particles do not actually exist, what exist are particles with zero "mass at rest" and that are never at rest.
Ahh, I was getting a bit confused from photons distorting spacetime off their wavelength and plank's constant, given they also apparently have no mass. Thanks for that. I have to go and re-read a lot of stuff now though. ;)
Besides it being a basic relation for relativity, there's also an entire "parallel" quantum mechanics for particles that have too much energy at the momentum component based on it.
You're talking about relativistic mass, but that isn't a commonly used concept anymore. Today it is indeed the norm to think of "mass" as being the mass at rest. Massless particles are really massless and have momentum without having mass.
In which case, we seem back to having two things distorting spacetime. Could mass just be the momentum of massless particles going in really small circles such that you can pack them together and call them matter?
If you use "mass" to mean the rest mass of something. If you use it meaning the relativistic mass, it's the entire value, not a component.
Yes, using it as the relativistic mass leads to confusing papers and equations, so it's better avoided. Yet, the press does that, what makes everything very confusing for lay people. I should have clarified that the OP usage of the term wasn't optimal (although common on the media he sees), but this following discussion about word definitions seems to be completely missing the point.
Actually the lagrangian is not minimized, it is merely stationary. An example of this is the harmonic oscillator, the lagrangian is clearly not bounded from below.
The action of a trajectory is not minimized either, it is usually at a saddle point, maximal relative to some trajectories and minimal to others. The linked paper has a detailed discussion of this.
I think this might be dumb, but, what if negative mass was just positive mass in some alternative dimension? If mass could be negative with respect to our observable space time, does that actually mean it is a binary kind of thing, or could there be lots of mass in lots of others semi-or-non-interacting dimensions? Maybe even the mass we can observe is only part of anything’s actual substance...
I was thinking, what if body's resistance is actually the norm, even when there is no change of velocity ? But the negative mass would provide the energy to let the body's in movement ?
From our point of view it seems that body's can move by theirselves and it seems also that we have to provide energy to change their velocities.
But if we think the universe as a fluid, body's should be constantly slowed.
It's very strange to have two distinct behavior, except if we have a negative mass in the universe.
Another framing of the issue with negative inertial+gravitational mass is that it would result in perpetual acceleration.
E.g. assume two bodies, one with mass M, the other with mass -M. The standard gravitational model describes a "repulsive" force on each of these objects, in the direction opposite each other. However, since the object of mass -M has negative inertial mass, this force would cause it to accelerate toward the object of mass M.
This results in these objects not accelerating relative to each other, and perpetually accelerating relative to an independent observer. One might attempt to explain this by saying that the negative mass has a growing negative kinetic energy level, but (it seems as a layman) this would violate the second law of thermodynamics.
Edit: Actually just negative inertial mass does this all by itself; it is just in the opposite direction if the gravitational mass is positive.
AFIAIK the "dark matter & energy might be negtive mass" idea has popped up before but has been dismissed because our current observations show that it's gravitational effect isn't lessened by the expansion of the universe.
He addresses this in section 2.3.1 by adding a "creation term" to balance universe expansion with the creation of new negative mass, but that just seems like a dirty hack to me.
Albeit, the cosmological constant was a dirty hack too, so that doesn't really necessarily discount anything.
I'm curious about the idea of "allow[ing] negative masses to not only exist, but to be created continuously." Given the law (in the scientific sense) that matter/energy cannot be created or destroyed, on a scale of 1 to FTL travel, how crazy is this idea?
Yup, that part seemed like a stretch to me too. Besides which, it doesn't seem to address any of the conflicts between QM and GR, so I'm not seeing it as a terribly interesting path. Not saying it's wrong, but it's not where I'd put my money.
He points out that it's a mathematical tool and may not result in particles, e.g. matter or energy, as we currently think of it.
It could be a property of space-time, as space is always expanding everywhere, space it self could have a negative mass.
This could even be some sort of weird energy-neutral balancing mechanism (e.g. some sort of energy - my bet is on potential energy, like how gravity causes things to accelerate through the potential energy of the arrangement - is lost that accounts for the creation of negative mass particles or whatever; conservation is observational, but we don't really know what 95% of the stuff being conserved is in the first place).
Hence it can be consistent with the conservation of mass and energy.
This is very new research without a deep check by other groups and without experimental support, so wait 5-10 years before getting too attached to this result. Anyway, there are weird somewhat similar things that are cannon https://en.wikipedia.org/wiki/Vacuum_expectation_value
The technical details and the experimental support are very very very important to distinguish between weird things that are true and weird things that are false.
Beware the Physical Chem, my son!
The laws that arn’t, the constants that vary.
Beware the phlogiston, my little one
And of the molality be wary.
He took his entropy in hand
Long time the adiabatic foe he sought
So rested he by the delta T
And stood awhile in thought.
And as in Newtonian Thought he stood
The Physical Chem with Beer’s Law Plot,
Came Bohring through the orbital wood
And took quantum leaps when hot!
PV! RT! And with fugacity,
The entropy discharged his wrath.
It reached a degenerate state, and being late,
He returned by the mean free path
If by recognize you mean that it's a variation/remix of "Jabberwocky"[1], then yes.
The second corresponding verse (so this corresponds to "He took his entropy in hand"), so others can see the resemblance (I chose the second since the first is a bit harder to see):
He took his vorpal sword in hand:
Long time the manxome foe he sought—
So rested he by the Tumtum tree,
And stood awhile in thought.
What's the relationship between negative mass and negative gravity and anti-gravity? Dark fluid is repulsive, so is it theoretically possible to collect a ball of it, put it between you and another positive mass, and fly apart?
Exactly where my mind went. Does the perspective described in the paper make the Alcubierre drive seem more plausible? Or is there something about the nature of the dark fluid hypothesized that would make it useless for something like that?
>Or is there something about the nature of the dark fluid hypothesized that would make it useless for something like that?
Assuming it was, literally, some kind of magical FTL juice... it all seems to be outside the galaxy, which means tens of thousands of years' travel just to get to it. If we're willing and able to do that, why bother with FTL at all?
He is also implying that it spontaneously emerges from space on a continuous basis. If that's the case then it may be possible to setup a harvester of negative mass. The question is how much of it emerges in so many square km of empty space. It could be 1 particle per light year.
Or extremely easy if you believe the UFO reports... the only incorrect assumption is that they would initiate contact. Even Dawkings knew that this would be a stupid assumption:
> It is quite likely that the negative masses in this study are a mathematical tool, rather than real physical matter. This rules out a lot of sci-fi, which is disappointing of course!
To be clear, the author IS proposing that there MAY be negative mass particles, not that this is all mathematical. That statement was simply a tip of the hat to the fact that none have yet been detected and its possible his theory is just a "accurate way to model" dark matter/+dark energy. It's just a nice way saying "My thoery might be the Newtonian Mechanics of Dark Matter/Energy, close but possibly wrong."
Why crap on the fact that this may lead to a warp drive.
There are alternative explanations that theorized that the 2nd law is not linear for extremely weak forces.[1] Thus, two masses that were sufficiently far apart would experience an attraction larger than that predicted by an inverse square law.
I think some particular applications of these theories were proved wrong some years ago, but I can't find those results.
Actually, Star Trek had an episode where a giant space amoeba pulls the Enterprise into a "zone of darkness" where thrust towards the amoeba pushes the enterprise away from the amoeba. Sounds similar. Of course they destroyed the creature with some antimatter.
Would this be only gravitational mass that's negative or would it be both gravitational mass and inertial mass? Because negative inertial mass is zany and wierds me out. Negative gravitational mass behaves in a way that makes sense to me.
Has to be gravitational only. Otherwise positive mass is repelled from negative mass, but the negative mass accelerates towards it. So both masses accelerate in the same direction, until they have infinite kinetic energy!
Conservation of energy & momentum would still hold, because negative mass would have negative energy & momentum, cancelling out that of the positive mass.
But the above surely would be an 'unphysical' situation.
>Negative masses are a hypothetical form of matter that would have a type of negative gravity – repelling all other material around them. Unlike familiar positive mass matter, if a negative mass was pushed, it would accelerate towards you rather than away from you.
Pretty sure the author means both.
A continuously accelerating system seems "unphysical", but we have an example already: the accelerating expansion of the universe.
The paper uses the version where both gravitational and inertial mass is negative. This means that everything wants to accelerate towards positive mass and away from negative mass. So positive mass naturally forms clusters (galaxies) and negative mass naturally spreads out evenly but concentrates around the galaxies (forming the dark-matter halos).
> Negative masses are a hypothetical form of matter that would have a type of negative gravity – repelling all other material around them. Unlike familiar positive mass matter, if a negative mass was pushed, it would accelerate towards you rather than away from you.
That assumes that inertial mass = gravitational mass, which is assumed to be true, not not proven.
> That assumes that inertial mass = gravitational mass, which is assumed to be true, not not proven.
The assertion has been tested experimentally. Inertial and gravitational mass are seen to be equivalent within the bounds of experimental accuracy. The wikipedia article on the "Equivalence Principle" has lists of tests for various versions of the principle:
https://en.wikipedia.org/wiki/Equivalence_principle#Modern_u...
I see I misunderstood that you were specifically questioning the equivalence for this postulated negative mass. I thought your statement was a more general one.
This is explicitly very different from that. There's no claim of "negative mass dark matter particles" in _ever_ location, rather, they have a specific distribution.
No. The negative mass matter clusters around galaxies because it accelerates towards positive mass and away from other negative mass. The area between galaxies becomes largely empty in the simulations.
I invite you to plug in negative mass to Einstein's famous equation. Negative mass is what is required for faster-than-light travel.
What is needed is a craft that has a normal, positive mass shell that contains and holds an equivalent amount of negative mass, thereby giving the entire craft a net mass of zero, allowing unlimited acceleration.
Looks like the rest of the world is about to catch up with Jean-Pierre Petit's theories about negative masses surrounding positive mass objects like galaxies and holding them together... Even the proposal for the modification of Einstein's theories... Times are a'changin.
It took almost ten years (2007-2017) from JPP being shunned by the scientific community (and the highly politicised French Wikipedia) to others "discovering" the same things he had been saying all along. https://arxiv.org/abs/0712.0067
If you put a negative mass and positive mass side by side and release from rest, they will accelerate indefinitely and break conservation of energy. Of course science should be open to any possibility, no matter how strange. But I feel the author understates its strangeness. And requiring negative mass to be continually created makes this theory not all that much "simpler" than the standard dark matter+dark energy.
They would accelerate indefinitely, but it wouldn't break conservation of energy. Gravitational force is inversely proportional to distance^2, so potential energy (integral from 0 to infinity of Force dx) still converges to a finite number.
I think the weirdness is, the distance between masses wouldn't change, they'd just accelerate together, in the same direction, indefinitely.
Both mass would experience negative repulsive force:
F=Gm(-m)/r^2
=-Gm^2/r^2
The positive mass accelerates away from the negative mass, because
F=ma
=> ma=-Gm^2/r^2
=> a=-Gm/r^2
Now here's the kicker. If inertial mass (the m in F=ma), is the same thing as gravitational mass. Then, the negative mass accelerates towards the positive mass. Because
F=(-m)a
=> F=-ma
=> -ma=-Gm^2/r^2
=> a=Gm/r^2
So accelerates in same direction as positive mass.
Technically, conservation of energy & momentum is maintained, because the negative kinetic energy of negative mass would cancel out positive energy of positive mass.
But I think the above can't be natural, because the magnitude of kinetic energy would grow to infinity, leading to even more impossibilities.
So you'd need to 'hack' the theory of gravity and make inertial mass different to gravitational mass. So the particle has negative gravitational mass, but positive inertial mass, to make laws of physics stay sane :)
From what I gather, the negative mass repels the positive mass (which is thus accelerated away), and the positive mass attracts the negative mass, which (because it has negative mass) is thus also accelerated away.
Now, the kinetic energy of the positive mass grows, but the kinetic energy of the negative mass is negative and falls (because its mass is negative...), so energy is conserved.
EDIT to add: I think I gathered wrong. Apparently, there is runaway motion (same direction), energy and momentum is still conserved (since one mass is negative), and since total mass is zero, the two particles reach speed of light. Intriguing. See paper.
The kinetic energy of a negative mass would also be negative, cancelling out the increased speed of both objects. The same would be true of conservation of momentum. There's no "conservation of speed" to be violated.
"Firstly, the theory of positive–negative mass particle pairs
provides clear rules that govern such interactions. The mechanics
of these interactions are governed by the usual physical laws:
the conservation of energy and momentum remain fundamental,
and hence it is unclear why we should object to this potentially
physical law of nature on grounds of aversion alone. Secondly,
and more importantly, observations provide evidence for
significant numbers of ultra-high-energy cosmic rays which are
known to be extragalactic in origin, although the mechanism of
their production remains a mystery (Pierre Auger Collaboration
2017). From this perspective, runaway motion is not a challenge
for negative mass models, but is rather a useful observational
constraint.
The idea that all negative masses in a universe should form
gravitational dipoles and accelerate to high energies is not
supported by the simulations presented here (which have a limited
number of particles), in which no runaway particles can be
identified. While runaway motion is a legitimate physical facet of
negative mass particle interactions, the simulations indicate that
this behaviour is only common for idealised particle pairs and
occurs more rarely as a bulk behaviour within a negative mass
fluid. This is likely as the particles in such a fluid are subject to
numerous counteracting forces from the surrounding medium.
One can assume that some amount of runaway particles must
still exist, although these would likely be highly scattered by
Brownian motion (e.g. Landis 1991)."
Would this also explain galactic voids as regions of large negative mass? It would seem that deviations in the Hubble constant bear a void would confirm this theory.
301 comments
[ 2.3 ms ] story [ 287 ms ] threadI don't get this, seems contradictory.
How can positive matter attract negative matter which at the same time is repelling positive matter?
The author is described as an Astrophysicist at Oxford (a reputable place) so I assume I'm either just not getting it or this is poorly worded.
F=ma
Meaning that positive and negative masses will accelerate in opposite directions in response to the same applied force. The effect is different, but it's a bit like how when you are in a car and hit the gas pedal, and your kid is in the back seat holding a helium balloon, the kid feels pushed backward into the seat but the balloon flies forward toward the windshield.
F = GMm/r^2
Meaning that gravitational forces between opposite signed masses is the opposite direction from the force between masses of the same sign.
Thus, the gravity force repels the normal mass away, but this repulsive force actually drags the negative mass along with it, because the negative mass accelerates the wrong way when "repelled".
[1]: https://www.youtube.com/watch?v=uAJlg8MDAlU
[Edit: jbay808 has a better explanation below, and negative mass attracts negative mass.]
That behavior is described here https://en.wikipedia.org/wiki/Negative_mass#Runaway_motion but it's not what I was expecting either.
However, the effects of this in a system in which the positive mass galaxy is spinning is that the negative mass counteracts the forces of inertia, allowing the galaxy to spin at higher speeds without shattering like a cd spun too quickly. Now, whether the math of all that makes sense, I'm am not in a position to comment on.
It still goes away but the acceleration is in the same direction. It is weird and easy to be confused.
An idealized -M + M = 0 pair of particles (one normal, one negative mass) could experience runaway acceleration. Author touches on this in the paper as a potential source for the incredibly high-energy extra-galactic cosmic rays.
Granted, the paper describes a model in which something that acts like a particle with negative mass exerts this force upon things, but has no claims as to what a physical manifestation of this would look like.
For an easier to visualize example imagine placing one magnet positive pole up inside of a tube, and then you place another magnet positive pole down in the same tube. It would create a similar effect. There is a repulsive force being perfectly balanced out by an attractive force and the two reach an equilibrium where the magnet on top is 'floating' with the net of all forces in this closed system being 0.
When a positive and negative mass particle interact gravitationally the positive mass is repelled and the negative mass is attracted.
[0] https://twitter.com/Astro_Jamie/status/1070303556266283008
[1] https://i.redd.it/qt2tf2tbni221.jpg
Seems to me what they're saying is that this well will pull in negative matter until a balance is reached, this will, in turn exert force on the internal structure of the galaxy.
It's never news, it's just a refresher listing of unknowns.
https://news.ycombinator.com/newsguidelines.html
But so could galactic-scale electric currents and magnetic fields, and is more likely.
No one ever talks about that.
They are not strong enough to produce the necessary effects, they are however interesting phenomena - https://arxiv.org/pdf/1302.5663.pdf.
If Electric Universe advocates are so wrong about something so easy to check, it doesn't give me much reason to check out anything else they say. (The fact that I have done so nevertheless and I've never managed to find them making an actual prediction is another. It seems to be one of those recursive handwave theories that never quite lands on anything concrete no matter where you search, but it sure does know that the standard theory is wrong.)
Gravity is a product of this also.
Light is a circuit
and black holes don't exist
Not a physicist, but my summary of the author's point would be:
If you allow for matter with a negative mass and plug it into a n-body simulation. You seem to get what looks like dark matter from first principles. Namely, a halo of non interacting matter around galaxies in just the right "non-cuspy" shape. Their conclusion is that this is worth considering, as almost all mainstream cosmology makes the assumption that mass >= 0, despite the fact mass <=0 doesn't seem to violate any physical invariants we know.
The idea is persuasively simple. Every other force besides gravity is polarised, so why not also gravity? It seems interesting to me, but I'm nowhere near knowledgeable enough to know whether it can be easily refuted.
EDIT: Partial answer is that there is a three-"color" polarization for strong force. I feel like it somewhat weakens the argument of "everything else is polarized, why not gravity" if the other forces are polarized in unique ways.
https://en.wikipedia.org/wiki/Nuclear_force
It's repulsive at short distances, attractive at longer distances. And oddly, the longer the distance, the stronger the force, it doesn't follow the inverse square law!
https://en.wikipedia.org/wiki/Leidenfrost_effect
Not to mention the occasional universe bleaching bursts of energy that comes from an antimatter galaxy colliding with a regular matter galaxy.
https://cerncourier.com/does-antimatter-fall-up/
Gravity is mediated by a spin-2 gauge boson, the graviton, whereas the other known fundamental forces are mediated by vector (spin-1) bosons. There is an exercise in Peskin & Schroeder (I believe) working out the lack of a gravitation "repulsion" resulting from this difference.
That being said, I think this kind of phenomenology is excellent and raises lots of interesting questions. There may be reason to rule out this author's new idea, or maybe rethink an established tenet. That's why science is such an exciting field.
Exactly. What I liked about MOND was that it basically amounted to "we don't know." The populist notion that dark matter is conclusively matter, and dark energy is energy seems a bold claim for such a young species.
Crazy theories like non-universal constants are probably crazy, but they do explore unconventional lines of thought. One of those, most likely an unknown unknown, may be answer. Focusing all our energy on one explanation (that has been falsified more than once) seems like a waste.
https://medium.com/starts-with-a-bang/theres-a-debate-raging...
In short: looking at everything we can observe and can model, we do know that the “dark matter” is matter. Because we first made predictions (“it should behave as matter”), then confirmed them, many times. And MOND can’t even explain that what’s already observed.
- Karl Popper
We need to produce a WIMP here, on Earth, and watch it decay. That's a "thruthifiable" experiment, but it'd be enough to convince me that we've found the correct line of thinking (which the popular opinion could very well be, don't get me wrong - I simply believe that we're overconfident).
No. You can't "need" something if that is simply unreachable. There’s reasonable chance that to produce the particles (which still don’t have to be the WIMPs as we imagined them, and still would be „matter“) the energies needed are simply beyond those that will ever be available to humanity. It wold not mean more than that: that we, humans, just can't repeat with our technology the extreme conditions that the Universe had/has.
The WIMPs in the form discoverable by the energies that are available to us at the moment would have been nice luck, but the existence of dark matter is not anything less certain when the dark matter is simply different from that. It would still be "matter" and not something else.
What has been falsified more than once?
https://medium.com/starts-with-a-bang/five-reasons-we-think-...
"Five Reasons We Think Dark Matter Exists: No other idea explains even two of these."
"1.) Galaxy Clusters"
"2.) Galactic Rotation Curves"
"3.) The Cosmic Microwave Background"
"4.) The Bullet Cluster"
"5.) Large-Scale Structure Formation"
From these 5, only dark matter matches all. Whereas MOND theories match only one, the "item number 2" and only as an attempt to after-the-fact-explanation of what's anyway observed, not as any prediction of anything new.
As I've quoted in my another message here:
"the predictions of dark matter were first made in the 1970s and 1980s, and were observationally confirmed later. This is not a case of tweaking the model to fit the data; this is a case of the best kind of science you hope for: where you make predictions, make the observations, and what you see validates and confirms the predictions you had made."
Moreover, there's nothing "large" of the possible space that is now excluded. The possible space includes everything that "behaves like matter". What's (still just "probably") eliminated is just what were for us, humans, in this particular moment of our history very convenient to happen: that the energies needed are exactly such that WIMPs that were conveniently postulated to be such can be generated by the device which design and maintenance costs less than 2% of the total cost of the F-35 fighter planes (the money invested and planned to be invested in F-35 is 75 times bigger than the international investment in LHC). It would have been luck if it had happened, but the possibilities are much bigger and include such which are going to remain physically unreachable to be produced by our technologies.
That we, humans, can't produce some particles doesn't disprove that dark matter exists when we have precise observations that can only be explained by dark matter and effectively by nothing else.
Dark matter can be placed arbitrarily so it can match pretty much anything. I remember recently there was a galaxy with negligible deviation of the GR-predicted rotation curve and that was taken as evidence for dark matter too (even though MOND was still the better fit for that case).
"Dark Matter" started out ok but has become a "god of the gaps" argument, I expect eventually MOND or similar solutions will be developed for all those issues.
Also, I gather that while MOND does seem not predict these observations exactly, it is not fully developed yet and often the presence of some "normal" cold gas (undetectable via light) could make it fit. Also, to get dark matter to fit the observations requires post-hoc assumptions and parameter adjustments. Eg:
"The amplitude of the third peak observed by WMAP merely falsifies the simple ansatz I used to make the prediction, not MOND itself. Indeed, I pointed out in the original papers that the ansatz must fail at some level, so I am hardly surprised that it does. All this means is that there are degrees of freedom (like a scalar field) that can oscillate separately from baryons in whatever the relavistic parent theory of MOND might turn out to be. The real test was for ΛCDM: the third peak had to be higher than predicted by pure baryonic damping. ΛCDM survives this test. It "wins ugly" in that, in order to obtain a fit, we have had to nearly double the baryon density over what it was so confidently known to be before it wasn't. That, and the no-CDM prediction for the second peak is still bang on. Amazing coincidence, that." http://astroweb.case.edu/ssm/mond/
>"the predictions of dark matter were first made in the 1970s and 1980s, and were observationally confirmed later."
I'm afraid I have never seen this although I have looked into this topic (as a layperson). Can you give an example of this? It would make the dark matter idea much more convincing.
Quite the opposite, and that statement is obviously uninformed. The science behind dark matter is very precise and exact. That's why the alternative theories can't come even near: once the same rigor which was applied to test dark matter is applied to the alternatives, the alternative theories simply break.
> I'm afraid I have never seen this ... Can you give an example of this?
I've given you the link to the complete articles and also highlighted the major points. Try to read and understand them fully, then search for more. For example, even on the MOND page that you linked you can read:
"2013: ΛCDM provides an excellent fit to the improved CMB data just as MOND provides an excellent fit to improved rotation curve data. As before, one's interpretation depends entriely upon which you find more impressive."
Note: "rotation curve" from that quote is the "item 2" from my posts before. So, no, still the match of the single "item 2" is definitely not "more impressive." The "CMB" mentioned on your page is the "item 3" from all 5 which are explainable by ΛCDM (and specifically, ΛCDM is the model part of which the "dark matter" is: https://lambda.gsfc.nasa.gov/education/graphic_history/univ_... see there 4 and 5).
Edit: response to your later response below:
> The claimed predictions from the 1970s and 1980s are not mentioned on that page.
They are mentioned even on the very page that you linked to: that page, among other points, followed the history of the tests and effectively concludes that, yes, finally, in 2013 ΛCDM wins the "item 3". Read carefully. Learn what ΛCDM is. MOND can still only claim "item 2." Which actual "authority" can show anything else? Also, behind "darkmattercrisis" blog are just a few guys with very fringe positions.
From what I have seen they can get precise post-dictions but not precise pre-dictions because there are so many free parameters (basically you can put dark matter wherever you need it).
>"I've given you the link to the complete articles and also highlighted the major points. Try to read and understand them fully, then search for more."
The claimed predictions from the 1970s and 1980s are not mentioned on that page.
Also, that "startswithabang" guy is more like a catholic inquisitor than a scientist. I wouldn't consider him a reliable source because this behavior indicates he lacks the required capacity for curiosity, he can only repeat what authority figures say:
"The blogger (@StartsWithABang) contacted @scilogscom on January 24 by replying to a 15-day old tweet that announced our blog’s move to the new domain. He tweeted “Bummed that @scilogscom is in the business of promoting contrarian scientist viewpoints.”, and asks the SciLogs.com community manager (@notscientific) “[Why] are you allowing @scilogscom to promote contrarian voices that undermine public understanding of [science]?”, adding “You have taken on “Dark Matter Crisis” blog, whose mission is to undermine all of physical cosmology & promote MOND.”
The two agreed to discuss the issue via email, with the blogger adding that he was “personally worried that you are promoting clicks & false controversy over quality science content”, and states that he is “very, VERY disappointed about this move that @scilogscom has made”.
By now the SciLogs.com community manager has explained to us what happened after these tweets. He and the publishing director responsible for SciLogs.com unfortunately assumed that the blogger’s criticism was justified. They decided to close our blog without conferring with others or asking us for a statement. After we complained about the discontinuation, they performed an internal investigation, which involved reaching out to astrophysicists and other people, and have realized that discontinuing our blog was a big mistake. We attribute SciLogs.com’s poor judgement to two factors: neither the community manager nor the publishing director has an (astro)physical background, it was the first time that SciLogs.com had experienced an attack against one of its blogs." https://darkmattercrisis.wordpress.com/2013/03/08/the-dark-m...
Anyway, I can tell you know even less about this topic than me so let's leave it at that.
They are mentioned, they just aren't elaborated: you have to do your own homework if you apparently "understand" the subject enough to claim your own opinion. A begin of that homework, however, as I've already written, you even have on the initial page that you chose to post, specifically:
"2013: ΛCDM provides an excellent fit to the improved CMB data"
As I've said, that's the "item 3" from the 5 item list, and it's ultimately confirmed by ESA's Planck satellite results in 2013. That is a strong confirmation.
Note: more details and the timeline are on the very page you've initially posted. Ad hominems don't change the facts.
No, they were not mentioned. If they were, you would be quoting it.
>'"2013: ΛCDM provides an excellent fit to the improved CMB data"
As I've said, that's the "item 3" from the 5 item list, and it's ultimately confirmed by ESA's Planck satellite results in 2013. That is a strong confirmation.'
You have to read the entire quote. This "fit" was a post-diction, not a pre-diction: "in order to obtain a fit, we have had to nearly double the baryon density over what it was so confidently known to be before it wasn't."
Post-dictions can be interesting as part of model development but are unable to confirm a model. The new, (possibly adjusted) model then need to be checked against new data.
https://lambda.gsfc.nasa.gov/education/graphic_history/baryo...
"There is general good agreement between the recent CMB and D/H+BBN determinations shown in the figure."
I don't see the "double" anywhere there. And the values could be traced to all the sources mentioned there.
He is currently active at this blog: https://tritonstation.wordpress.com/
It looks like this doubling happened in ~2000 so it wouldnt show up in your chart.
Since 2000 many new satellite measurements were done and the values improved. As I've quoted your page Planck in 2013 ultimately made the most precise CMB measurements that most conclusively fit the "dark matter" model and provably don't fit the "MOND" model. And for that proof there were no "factor 2" corrections, as you also confirm, at least since 2000. The way you wanted to believe, and how you quoted some other parts of the same document, was that that had to be done for that 2013 proof, and it obviously haven't had. BTW the nature of the measurements, especially those made decades ago, is such that when we started with some value and then later measured something only 2 times smaller or bigger (but then all repeated measurements remain closer to one another), it's still OK. It's not the absolute values that prove this or that, it's the measured shapes that match or not the very complex calculations. And that's what happened in 2013 with Planck: what was measured for more than a decade with other satellites, and finally most precisely with Planck, shows the shape that in practice can't match the MOND predictions but nicely matches what the "dark matter" model predicts.
The point is that "suddenly" people started measuring a higher baryon density to make it fit what was required for lambda-CDM model to fit the CMB measurements. Ie, the lambda-CDM model did not pre-dict the spectrum beforehand, they had to tune it to match the data.
And see how the squares and triangles start moving upwards once the blue circles appear: http://www.astro.umd.edu/~ssm/mond/BBNLCDMMOND.jpg
Remember the oil drop experiment mentioned by Feynman in his cargo cult science talk? That is what this looks like to Mcgaugh: http://calteches.library.caltech.edu/51/2/CargoCult.htm
I have to say, it is very frustrating to discuss this since you seem to misunderstand every simple point being made, and before that has been corrected you have gone on to miss another point that needs to be addressed. We are not discussing this at a very deep level... Perhaps English is not your native language, but I am just letting you know. I hope no offense is taken.
W...what? Do you really believe that different scientific organizations in different countries around the world financed and sent different satellites into space but misrepresented the actually measured values only to "fit" some old theoretical predictions?
Really?
Then there's really no point discussing anything about science with you.
Here in a recent comment he addresses the "galaxy cluster" issue: https://tritonstation.wordpress.com/2018/11/21/hypothesis-te...
Yes.
> so it can match pretty much anything
At the time you do the initial placement. But that's not the only relevant time.
You can scatter any type of DM on a spacelike hypersurface any way you want, but then you have to figure out the successor (and predecessor) spacelike hypersurface, and there you have to bite a bullet: you can constrain the DM or throw away the well-posedness of the initial-value problem.
Your constraints are dominated by the hyperbolic-elliptic PDEs. The hyperbolic portion lets you lay down initial values for your dark matter on a spacelike slice. You then need an equation of motion for your DM, as you do for your visible matter, so that you can evolve the contents of that initial spacelike slice in time. The elliptical portion are the Hamiltonian and momentum constraints.
Without satisfying these constraints you tend to lose uniqueness, which destroys the predictive power of your model.
If you're spraying down a dust of non-interacting DM particles within some spacelike region, as you evolve from your initial configuration you end up with each particle growing into an everywhere-timelike worldtube. (Non-(self-)interaction is just to make this really clear without the distractions of decays or daughter products from scatterings. Each dark matter particle has its own worldtube extending arbitrarily far in to the future or past.)
The simplest Cold Dark Matter models have a conventional nonrelativistic and relativistic dynamics determined by two parameters: particle rest-mass and interaction cross-section. In our simple non-interacting cold DM dust case the interaction is purely gravitational, so we're left with every particle having an identical rest mass.
Now, lay down the visible matter in a galaxy (or cluster!). You can lay down the dark matter dust arbitrarily to form a halo which could support a realistic galaxy rotation curve on your initial surface. You are right that you can put it practically anywhere, and are not restricted to uniform density. However, when you evolve in time, you inevitably find that you are stuck with a narrow range of mass densities at various points in the galaxy (or cluster!), but still have freedom to set down differing particle number density, higher numbers at lower particle mass. If your particles are too light, the gravitational coupling with the visible matter is too slight; worse, the gravitational coupling between the DM particles is too slight too, and your galaxy quickly disassembles. If your particles are too heavy, instead you get dynamic heating of hydrogen gas, and you get far too much star formation and metal enrichment.
You can go the other way too: first put down a test particle DM halo and then add hydrogen gas, or a modern galaxy, or whatever else, and see how the system evolves. It is a feature of the standard galaxy astrophysics that DM and visible matter do not have to be near each other at all times. (This gets important when you consider galaxy clusters rather than just galaxies; laying out CDM is hard work, as I've been describing, but you can't just drop in MOND. It's also testable at solar-system scales; we can look for post-Newton dynamics in the Jupiter system, or in the inner solar system, and test whether it's compatible with General Relativity (with a fine dark matter dust of up to a few Phobos-masses inside Neptune's orbit) or whether it's compatible with a different post-Newtonian dynamics, like MOND, where the galaxy's MOND corrections will generate a quadrupole anomaly stronger around the outer planets and their moons than around the inner planets).
More complicated particle CDM theories are still stuck with the general rules of the simple model above: the stress-energy in the large has to hang around in the right places for long periods of time; the stress-energy can't just pop in and out of existence at a point (it needs to be thermal, so its worldtubes cannot ev...
Or you're getting closer to finding out what the real parameters are?
If you eliminate all possibilities and have to make up some new parameters, sure, that feels like you're not doing science and might as well be a creationist. But what you describe, narrowing down the space of possible explanations from infinitely many to only a few, is the dream of every theoretical physicist.
By that metric, epicycles were rock-solid.
Our math says the universe comes up short on mass by over 3/4ths? Must be invisible matter we can't see or interact with see, the models still good!
Over simplification in the extreme, but also a far cry from the rock solid foundations it claims to be.
https://en.wikipedia.org/wiki/Graviton mentions "[I]t can be shown that any massless spin-2 field would give rise to a force indistinguishable from gravitation, because a massless spin-2 field would couple to the stress–energy tensor in the same way that gravitational interactions do. This result suggests that, if a massless spin-2 particle is discovered, it must be the graviton." It links to other resources from there.
We know what fits GM, but we don't know if it's the only solution.
as far as I understand, the space-time curvature is just an interpretation of the math. Another perfectly acceptable interpretation is as a relativistic classical field that couples with the stress-energy tensor field (thus, indirectly, with any other field).
In fact I think it is possible to come up with a space-time curvature interpretation of the EM field, for a toy universe where everything has a EM charge.
Now, given that the gravitational field couples with other fundamental fields and those are quantized, the gravitational field must necessarily also be quantized. The math straight forward quantization works fine for low energies, thus the spin-2 boson described elsethread. The issue is that at low energy, the gravitational effects are so small that it is impossible to come up with an experiment that would detect the difference between a classical field and a quantized field.
At high energy the straightforward derivation breaks down because of infinities (other quantum fields had similar issues, but the math tricks used to resolve them do not work with the gravitational field). There are multiple theories (string theory, loop gravity, etc) that try to resolve this problems, but the experimental apparatus required to distinguish between them are colossal (as in particle accelerators with radii measured in AU).
IANAP, and don't get the full picture. But isn't that definition itself derived by the fact that gravity has a single polarity?
Somehow I doubt it. I bet as a save someone would propose "dark spin" or some elaborate calculations that shows a few factors happen to cancel out exactly but no one noticed before now.
One model for dark energy is indeed the cosmological constant, but there are other approaches, most notably quintessence (https://en.wikipedia.org/wiki/Quintessence_(physics)).
However, if I understand your comment correctly, you're saying that the underlying non-quantum theory (GR) uses a symmetric rank-2 tensor, therefore you have to have a spin 2 particle - as opposed to photons, whose underlying non-quantum theory has an antisymmetric rank-2 tensor, and therefore give rise to a spin 1 particle. Do I have that right? If so, could you point me to something that would explain why?
Any quantum theory which reduces to general relativity in the classical limit will look like a theory of gravitons when quantum effects are small. This is just how the math works out in effective quantum field theories involving massless spin-2 particles.
> In theories of quantum gravity, gravitation is mediated by the graviton – a massless, spin-2, boson. This means that any pair of negative masses would attract, and not repel as suggested in this theory. However, there are also theoretical arguments that gravitons cannot, and will not ever, be detected experimentally (Rothman & Boughn 2006). There appear to be two options: either it is possible that the graviton could be modelled as a bound state of a positive and a negative mass, in a theory of composite gravity or some other mechanism which provides a modification of graviton properties. Alternatively, this could also indicate that the proposed theory cannot be modelled by real, physical, particles, but rather by the presence of effective negative masses within a superseding theory.
https://arxiv.org/abs/1712.07962
Edit to add: the abstract pages also often contain useful extra information, such as (in this case) a link to YouTube videos of the simulations -- https://www.youtube.com/channel/UC8ltFtaETXDphec0l-VxMsg -- or the publication history (in this case, a DOI is given).
If you think about it, "mass" actually means two different things. One one hand, there is mass as the "charge" of gravitational interaction, that is, the gravitational field generated by a body is proportional to its mass. Let's call this concept "gravitational mass". On the other hand, you have mass in the sense of "intertial mass", a body's resistance to change of velocity. Although we kinda take it for granted, it's weird that these two concepts coincide, and not something that would be necessarily true a priori.
Asserting this equivalence as a principle is the starting point for Einstein's theory of General Relativity, which is one of the most beautiful and thoroughly tested theories in modern physics.
On the other hand, you can show that the inertial mass cannot be negative (else the Lagrangian would not be bounded from below and therefore you could not minimise it; the whole of physics then comes crumbling down). Therefore, if that equivalence stands, the gravitational mass also cannot be negative.
Think of charged particles in a magnetic field and how polarity and and strength of the charge impacts their interaction the strength would do the same thing for both particles while the polarity would mirror their interaction.
I need to read the paper a few more times tho.
Here's a purely thought-experiment argument for inertial mass not being negative:
If you had an object with negative inertial mass, then pushing on that object would cause it to accelerate into your hand. This is a positive feedback loop that immediately results in the object and your hand exerting infinite amounts of force on each other, because applying force just leads to applying more force.
This brings up an interesting question, is gravity about exchanging momentum? My mental model of gravity being spacetime curvature would imply no, but that intuition is likely wrong.
Your scenario could create a black hole if I'm understanding correctly. Enough 'negative inertial mass' trapped in a feedback loop..
In the form of the equation favored by my high school physics teacher, a = F/m, we see that when a force is exerted on an object with positive mass, the object will accelerate in the same direction as the force, in inverse proportion to its own mass.
But it's very easy to put a negative number in for the mass, and then you see that such an object would accelerate in response to force just as much as an object of equal positive mass, but in the opposite direction.
In that same framework, the maths definition of inertia is also momentum = mv and a google search confirms this. What would negative momentum even look like? Because that's not just movement in the opposite direction.
Inertia is generally not defined as being momentum, because an object's momentum in any direction increases along with its velocity in that direction, while we like to think of inertia as being determined solely by mass.
https://en.wikipedia.org/wiki/Inertia begins "Inertia is the resistance, of any physical object, to any change in its velocity". That's specifically resistance to acceleration ("change in velocity"), not resistance to the concept of change in general.
(It does go on: "In common usage, the term "inertia" may refer to an object's "amount of resistance to change in velocity" (which is quantified by its mass), or sometimes to its momentum, depending on the context.")
You're right about v. I thought a negative velocity was equally absurd, but it looks like it's defined as a vector which makes sense of course. The last time I took a proper physics course was in high school. How embarassing though, as I've read and watched more popular science/physics material than most people, probably. Hah.
"If you had an object with negative charge, then putting that object near a positive charge would cause it to accelerate towards the positive charge." Getting closer increases the force. "This is a positive feedback loop that immediately results in the object and the positive charge exerting infinite amounts of force on each other, because" getting closer "just leads to applying more force."
Thus, there are no electrons.
So, I don't think the thought experiment rules out negative mass. In fact, the article explicitly says "if a negative mass was pushed, it would accelerate towards you rather than away from you.", and I assume that if that's an obvious contradiction, the author would have caught it.
But if a negative mass is moving towards you and encounters a repulsive force, that repulsive force will, by definition, move the negative mass further toward you. It would need to encounter an attractive force to reach equilibrium.
Note that, unlike in the electron example, it doesn't matter what kind of force is being applied; the object's negative mass tells us that the response to any repulsive force is acceleration towards the force.
A repulsive force is not the best way to think about it. The potential of the nucleus is the usual -1/r, and goes to (negative) infinity at zero. A repulsive force would be incorporated into the potential and appear as a bump around the nucleus, and would mess up the electron orbital.
A hand-wavy explanation of why the electron doesn't fall in: if you try to push the electrons into the nucleus, you necessarily localize the electron into a smaller volume; this means its wavefunction must get "spikier" and therefore it has more kinetic energy. This kinetic energy rises faster than the potential energy drops, so the state of lowest energy is actually found at an average radius > 0.
Note that it took quantum mechanics to rescue the atom. A classical electron could fall into the nucleus.
I do not see any problem with a negative-mass particle accelerating towards the force. The analogy with opposite charges seems right to me.
This explanation doesn't just explain why it doesn't fall in, it also explains why there are discrete shells that it could be found in, corresponding to how many times it wraps around the nucleus. (It doesn't explain why only a finite number can fit in each shell though. Or why bigger shells can have more electron orbitals. Or...well the actual theory has to be good for something!)
I also don't think FabHK's argument works as an analogy to my problem with negative mass. The electromagnetic field is not self-reinforcing in the same way.
Imagine that you're holding a marble of negative glass in your fist. Negative glass is indistinguishable from ordinary glass except that its mass is negative rather than positive.
As we all know, the first step in solving any physics problem is to draw a free-body diagram. ( http://www.smbc-comics.com/comics/20130616.png ) Let's draw one here. First, we'll do one for an ordinary marble:
1. The enormous mass of the earth attracts the marble downward proportionately to the marble's mass.
2. The marble cannot accelerate downward, because it's stuck in your fist. Your fist experiences a downward force equal to the weight of the marble.
3. By Newton's third law, your fist exerts an upward force on the marble equal to the force exerted by the marble on your fist. This is exactly equal to the weight of the marble, but in the opposite direction. The two forces cancel, and the marble is at rest.
Now for the negative marble:
1. The enormous mass of the earth attracts the marble downward proportionately to the marble's mass. Because that mass is negative, the marble attempts to accelerate upward.
2. The marble can't accelerate upward, because it's stuck in your fist. Your fist experiences an upward force equal to the weight of the marble.
3. By Newton's third law, your fist exerts an downward force on the marble equal to the force exerted by the marble on your fist. This is exactly equal to the weight of the marble, and in the same direction, effectively doubling the marble's weight. The marble is now trying twice as hard to accelerate upward into your fist.
2. (Again.) The marble can't accelerate upward, because it's stuck in your fist. Your fist experiences an upward force equal to double the weight of the marble. Nothing has moved; we're still just trying to work out the balance of forces within the system at rest.
3. (Again.) You can see where this is going.
What is the conceptual breakthrough that rescues negative mass from this trap? (Note that saying the marble has negative inertial and gravitational mass, as opposed to negative inertial mass and positive gravitational mass, doesn't help: the marble will be trying to accelerate downward instead of upward, but it will still be doing it with infinite force.)
(Though it escapes me why you would assume that I believe in theories about negative mass, which actually seem to me to be very speculative and not very likely.)
I am not sure you understand what "repulse" means..
Joking aside, it's easy enough to imagine negative gravity. Do you mean that such counter intuitive behavior would be the natural consequence of negative inertial mass?
only if the repulsive force is negative gravity.
If a positive-mass electron attracts a negative-mass electron, the repulsive electromagnetic force will still keep them from making it all the way there.
> only if the repulsive force is negative gravity
We're talking about negative inertial mass, not negative gravitational mass. Negative inertial mass means accelerating against the direction of any force applied to you, including but not limited to gravity.
1. can the theory make testable predictions that can be verified by observation? e.g. string theory has had issues with this.
2. what does <wierd-mathematical-construct> "mean"? e.g. negative mass, spooky action at a distance, spacetime singularity.
In practice, only 1 really matters. However, 2 tends to bug a lot of people (consider the arguments around what the quantum world really represents). I'm one for whom 2 is important. "But what does it really mean?"
Back to your original point, looking at the paper [0], it doesn't seem too ridiculously hard to validate the math for someone familiar with this space (see what I did there?), unlike me who has forgotten everything I was taught [1].
[0] https://arxiv.org/pdf/1712.07962.pdf
[1] I have a physics degree but that was 33 years ago and I've been doing nothing but IT since.
No, it doesn't; since this turns the repulsion of normal objects (which is what makes them resist interpenetration) into attraction by switching the orientation of the resulting acceleration, what it results in isn't more force at the boundary (the boundary is a repulsive effect), but interpenetration, which results in force declining to zero as the masses centers are at the same place. (And then switching directions as they cross.)
You have that wrong in the momentum of massless particle has mass, independently of its carrier. Massless particles do not actually exist, what exist are particles with zero "mass at rest" and that are never at rest.
https://en.wikipedia.org/wiki/Energy%E2%80%93momentum_relati...
Besides it being a basic relation for relativity, there's also an entire "parallel" quantum mechanics for particles that have too much energy at the momentum component based on it.
Yes, using it as the relativistic mass leads to confusing papers and equations, so it's better avoided. Yet, the press does that, what makes everything very confusing for lay people. I should have clarified that the OP usage of the term wasn't optimal (although common on the media he sees), but this following discussion about word definitions seems to be completely missing the point.
https://en.wikipedia.org/wiki/Geon_(physics)
http://www.eftaylor.com/pub/Gray&TaylorAJP.pdf
E.g. assume two bodies, one with mass M, the other with mass -M. The standard gravitational model describes a "repulsive" force on each of these objects, in the direction opposite each other. However, since the object of mass -M has negative inertial mass, this force would cause it to accelerate toward the object of mass M.
This results in these objects not accelerating relative to each other, and perpetually accelerating relative to an independent observer. One might attempt to explain this by saying that the negative mass has a growing negative kinetic energy level, but (it seems as a layman) this would violate the second law of thermodynamics.
Edit: Actually just negative inertial mass does this all by itself; it is just in the opposite direction if the gravitational mass is positive.
AFIAIK the "dark matter & energy might be negtive mass" idea has popped up before but has been dismissed because our current observations show that it's gravitational effect isn't lessened by the expansion of the universe.
He addresses this in section 2.3.1 by adding a "creation term" to balance universe expansion with the creation of new negative mass, but that just seems like a dirty hack to me.
Albeit, the cosmological constant was a dirty hack too, so that doesn't really necessarily discount anything.
It could be a property of space-time, as space is always expanding everywhere, space it self could have a negative mass.
This could even be some sort of weird energy-neutral balancing mechanism (e.g. some sort of energy - my bet is on potential energy, like how gravity causes things to accelerate through the potential energy of the arrangement - is lost that accounts for the creation of negative mass particles or whatever; conservation is observational, but we don't really know what 95% of the stuff being conserved is in the first place).
Hence it can be consistent with the conservation of mass and energy.
This is very new research without a deep check by other groups and without experimental support, so wait 5-10 years before getting too attached to this result. Anyway, there are weird somewhat similar things that are cannon https://en.wikipedia.org/wiki/Vacuum_expectation_value
The technical details and the experimental support are very very very important to distinguish between weird things that are true and weird things that are false.
The second corresponding verse (so this corresponds to "He took his entropy in hand"), so others can see the resemblance (I chose the second since the first is a bit harder to see):
[1]: https://en.wikipedia.org/wiki/JabberwockyAssuming it was, literally, some kind of magical FTL juice... it all seems to be outside the galaxy, which means tens of thousands of years' travel just to get to it. If we're willing and able to do that, why bother with FTL at all?
https://www.richarddawkins.net/2017/11/we-just-sent-a-messag...
If they show up every couple thousand years and don't say anything, would we even notice?
> It is quite likely that the negative masses in this study are a mathematical tool, rather than real physical matter. This rules out a lot of sci-fi, which is disappointing of course!
Why crap on the fact that this may lead to a warp drive.
There are alternative explanations that theorized that the 2nd law is not linear for extremely weak forces.[1] Thus, two masses that were sufficiently far apart would experience an attraction larger than that predicted by an inverse square law.
I think some particular applications of these theories were proved wrong some years ago, but I can't find those results.
[1] https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics
https://en.wikipedia.org/wiki/The_Immunity_Syndrome_(Star_Tr...
Conservation of energy & momentum would still hold, because negative mass would have negative energy & momentum, cancelling out that of the positive mass.
But the above surely would be an 'unphysical' situation.
Pretty sure the author means both.
A continuously accelerating system seems "unphysical", but we have an example already: the accelerating expansion of the universe.
That assumes that inertial mass = gravitational mass, which is assumed to be true, not not proven.
The assertion has been tested experimentally. Inertial and gravitational mass are seen to be equivalent within the bounds of experimental accuracy. The wikipedia article on the "Equivalence Principle" has lists of tests for various versions of the principle: https://en.wikipedia.org/wiki/Equivalence_principle#Modern_u...
It would be a lot simpler if inertial mass was negative gravitational mass in this theory.
Then you have repulsive gravity with normal behavior under force.
https://en.wikipedia.org/wiki/Dirac_sea
Rather than have dark matter particles, have negative mass dark matter particles in every other location instead.
From the article:
"This is explained by the Hubble-Lemaître Law, the observation that more distant galaxies are moving away at a faster rate."
If we observe an accelerating universe how can be that galaxies in the past move away at a faster rate?
Written by this guy http://web.mit.edu/physics/people/faculty/tegmark_max.html is does a fantastic job of going from really simple concepts/experiments to blowing your mind.
What is needed is a craft that has a normal, positive mass shell that contains and holds an equivalent amount of negative mass, thereby giving the entire craft a net mass of zero, allowing unlimited acceleration.
https://www.realclearscience.com/articles/2018/08/11/could_p...
Maybe these phenomena are linked, and it's more immediate than cosmology.
via: https://twitter.com/robinhanson
It took almost ten years (2007-2017) from JPP being shunned by the scientific community (and the highly politicised French Wikipedia) to others "discovering" the same things he had been saying all along. https://arxiv.org/abs/0712.0067
http://jp-petit.org/papers/cosmo/2018-AstrophysSpaceSci.pdf
You may give yourselves the smallest pat on the back in the universes.
thanks for pointing out this earlier work
Same with normal gravity, but backwards.
Both mass would experience negative repulsive force:
F=Gm(-m)/r^2
=-Gm^2/r^2
The positive mass accelerates away from the negative mass, because F=ma
=> ma=-Gm^2/r^2
=> a=-Gm/r^2
Now here's the kicker. If inertial mass (the m in F=ma), is the same thing as gravitational mass. Then, the negative mass accelerates towards the positive mass. Because
F=(-m)a
=> F=-ma
=> -ma=-Gm^2/r^2
=> a=Gm/r^2
So accelerates in same direction as positive mass.
Technically, conservation of energy & momentum is maintained, because the negative kinetic energy of negative mass would cancel out positive energy of positive mass.
But I think the above can't be natural, because the magnitude of kinetic energy would grow to infinity, leading to even more impossibilities.
So you'd need to 'hack' the theory of gravity and make inertial mass different to gravitational mass. So the particle has negative gravitational mass, but positive inertial mass, to make laws of physics stay sane :)
Now, the kinetic energy of the positive mass grows, but the kinetic energy of the negative mass is negative and falls (because its mass is negative...), so energy is conserved.
EDIT to add: I think I gathered wrong. Apparently, there is runaway motion (same direction), energy and momentum is still conserved (since one mass is negative), and since total mass is zero, the two particles reach speed of light. Intriguing. See paper.
"Firstly, the theory of positive–negative mass particle pairs provides clear rules that govern such interactions. The mechanics of these interactions are governed by the usual physical laws: the conservation of energy and momentum remain fundamental, and hence it is unclear why we should object to this potentially physical law of nature on grounds of aversion alone. Secondly, and more importantly, observations provide evidence for significant numbers of ultra-high-energy cosmic rays which are known to be extragalactic in origin, although the mechanism of their production remains a mystery (Pierre Auger Collaboration 2017). From this perspective, runaway motion is not a challenge for negative mass models, but is rather a useful observational constraint. The idea that all negative masses in a universe should form gravitational dipoles and accelerate to high energies is not supported by the simulations presented here (which have a limited number of particles), in which no runaway particles can be identified. While runaway motion is a legitimate physical facet of negative mass particle interactions, the simulations indicate that this behaviour is only common for idealised particle pairs and occurs more rarely as a bulk behaviour within a negative mass fluid. This is likely as the particles in such a fluid are subject to numerous counteracting forces from the surrounding medium. One can assume that some amount of runaway particles must still exist, although these would likely be highly scattered by Brownian motion (e.g. Landis 1991)."