"Conceptually, we demonstrate that any warp drive, including the Alcubierre drive, is a shell of regular or exotic material moving inertially with a certain velocity. Therefore, any warp drive requires propulsion. We show that a class of subluminal, spherically symmetric warp drive spacetimes, at least in principle, can be constructed based on the physical principles known to humanity today"
Generally, the math works, and it looks like it should be possible. We've even got some ideas for how to physically build the thing.
The main problem is that it requires exotic matter with negative mass or negative energy. I don't think we have any good evidence to suggest such a state of matter is possible in this universe.
They also say: "We present the first general model for subliminal positive-energy, spherically symmetric warp drives"
Not that I understand the physics well enough to do anything with this, but as you say, there is other work that's definitely (claiming) positive-only energy densities.
> the math works, and it looks like it should be possible
The math can only work (if it even does) in exceptionally constrained circumstances.
The main purely-theoretical issue is that the paper's <https://arxiv.org/abs/2102.06824v2> solution is wholly spherically symmetric, and with vacuum everywhere but in the warping region.
"Fig. 1: Asymptotically-flat vacuum background ... general stationary curved region with a spherical topology ... (the warping region) ... flat inner region ... ('passenger' space). ... As we discuss in Section 5, warp drive spacetimes require some form of propulsion in order to accelerate. For this reason, in physical realisations of such spacetimes, the front and rear parts are likely asymmetric". [emphasis mine]
Do the maths fall apart if we deviate from total spherical symmetry, either by changing the front and rear parts, or by breaking the vaccuum condition on the inside and outside of the shell, especially if our matter is not precisely spherically distributed?
They do a review of this, for limited deviations from spherical symmetry, in §4. However, the vaccum condition is what they study, not straying very far from their earlier "if [the warp region] were replaced by Minkowski spacetime, the whole spacetime would be Minkowski space."
The paper declines to answer the question about the distribution of matter (i.e., not-vacuum): "all warp drive spacetimes are asymptotically-flat ... no metric which describes an accelerating warp drive solution has been presented in the literature ... metrics for ... accelerating objects and more general axisymmetric objects which preserve their shape and mass ... remains a subject for future studies". That last part suggests that the inside part might be kept appropriately symmetrical through dynamic reconfiguration of the things inside (i.e., the ship, cargo, fuel, passengers).
But what about outside the warping region? In our solar system, and in our galaxy, we are not in Minkowski space.
The outer part of the warp region, being asymptotically flat, might be amenable to a "thin shell" (Israel/Darmois junction formalism) with the warp bubble far enough away from everything outside that it feels no tidal forces, which for the purposes of the text above is "acceleration". What determines how far is far enough away? The authors don't attempt an answer.
There are plenty of things (stars, most notably) in our part of the galaxy that will induce tides. The solar component of the lunisolar tide in Earth's oceans is substantial at 1 astronomical unit from our sun. That solar component doesn't go away when a space craft is (for example) 2 a.u. away from Earth&Moon on the opposite side of the sun. Can one zip this warp bubble safely past a 1 solar mass star at a distance of 1 au? Ten au? Can the warping region or passenger region be adjusted to allow for close approaches/operation within a star system? All questions for some possible future paper.
Apart from stars, we inhabit a region of space which is dotted with things you can smack into (from the very solid like rocks and iron to the very wispy, stuff will still tend to slow you down when you hit it; deceleration and acceleration are the same issue). Near luminous enough stars, radiation pressure might also induce non-negligible accelerations. Is any of that relevant? If relevant, is it "fixable"? How? Alter the shape of the warping region, or the passenger region, or both?
Questions for some possible future paper.
So, even if one accepts for the sake of argument that the maths are flawless and further one just assumes the existence of controllable negative energy densities, this paper's maths are designed for a spacetime which overall isn't very much like what we have within a few hundred lightyears of here. Maybe some future...
Two different groups tried this move around the same time, one proved that subliminal drives using only positive mass were possible, the other proved the contrary. Such is the difficulty of GR field equations that it's not yet clear who's correct.
I hope that some day there’s a paper like this to look back on like we do with Lovelace and Babbages stuff (foundational for something that seemed like magic at the time)
Would be cool if it happened during my lifetime though.
Generally speaking, all three of FTL (travel/communication), causality and relativity cannot all be true at the same time. If you can FTL-travel/communicate (they are the same really), then you can come up with a scenario in which an observer can see an event happening, and then see the cause of that event after it happened.
Extending that logic, if the observer can also move at superluminal speeds, he could prevent the cause of the event after seeing the event happen, leading to a paradox.
> Extending that logic, if the observer can also move at superluminal speeds, he could prevent the cause of the event after seeing the event happen, leading to a paradox
This is a guess, i.e. one possible outcome physicists are considering.
People have proposed alternative outcomes of FTL like the (in my opinion much more sensible) Novikov consistency principle, which roughly proposes that spacetime and the entities it contains (e.g. an observer's wordline) should be looked upon as a whole, in the sense that they need to be self-consistent. Spacetime is not time-dependent and does not evolve, so it does not make much sense to say "something something leads to a [spacetime] paradox".
Periodicity in time is unusual to think about in physics, partly because you start to get wacky results. If you could establish a CTC in nature, it would allow for computers that can efficiently (i.e. in polynomial time) solve not just NP problems but actually even all of PSPACE [0]. You can interpret this in two ways. There’s the hopeful way: “we should spend a boatload of money trying to find CTCs in our universe since they’ll let us create super-ultra computers”. And then there’s the pessimistic way, that nature probably isn’t going to give us a free lunch like that. Sadly, I’m in the pessimist camp on this one!
This explanation, like previous explanations for "FTL implies time travel" I've read, presupposes that the signal is actually moving at superluminal speeds, i.e. actually covering a distance greater than 299,792,458 meters in a single second from the signal's perspective. This would not be relevant for Alcubierre drives or wormholes or anything else that warps spacetime, since the whole point of such things is to stretch/contract space such that the thing is traveling a much shorter distance per second.
In short, if I were to instantaneously poof out of existence here on Earth, poof into existence on Mars a second later, grab a rock, poof out of existence again, and poof back into existence back on Earth again another second later and hand you that rock, it doesn't seem reasonable to assert that I traveled backward in time when I clearly traveled forward by two seconds. The whole lecture on that external observer is irrelevant, since there's nothing to observe unless the observer happens to be in my bubble/wormhole/whatever - and even then, one'd only be observing subluminal actions/signals within that region of spacetime.
The problem is that if FTL travel exists then our existing theories about spacetime are wrong in some way (despite making lots of great predictions) or time paradoxes are possible in which case we have no idea what the consequences could be.
Your example assumes there is some underlying rate at which time advances for the universe (or at least Earth and Mars) and that spacetime as we know it (including relativity and time dilation) are just some kind of modifier on top.
But theory and experiment so far point to that not being the case. There is no "pop out of existence here and pop in over there" without time travel (as best as we can tell). The whole light cone / worldline explanations are more formal explanations of that.
Now you can magic this problem away by proposing any number of schemes... like saying the entire universe's worldline is exists within a metaworldline and time travel actually resets the state of the universe as it was in the past then re-runs the universe... but all of that always proceeds forward in the metaworldline. In other words all past histories existed in a causal order, changing the past just adds "new commits" to the universe but history is never really rewritten. Bam! Our magic theory solves all paradox problems without requiring billions of parallel universes and allows time travel! But it's not a theory we can test or make predictions with so it isn't a useful scientific theory. It might as well be literal magic.
Similarly you could propose that GR is wrong... but your new theory is gonna need to match GR's predictions that have proven true while making some new ones we can test, while also avoiding or explaining causality and paradoxes.
> Your example assumes there is some underlying rate at which time advances for the universe (or at least Earth and Mars) and that spacetime as we know it (including relativity and time dilation) are just some kind of modifier on top.
Not necessarily; only that time is advancing in a forward direction. Whether 1 second on Earth is 1 second or 10 seconds or 0.1 seconds or what have you on Mars doesn't change the underlying premise: something disappeared from one place and appeared some positive amount of time later in another place. The only way I see that implying backward time travel is if time on Earth or Mars is already advancing backward, and if that's the case then the effects of Alcubierre drives on causality are probably the least of our worries.
And on that note...
> There is no "pop out of existence here and pop in over there" without time travel (as best as we can tell). The whole light cone / worldline explanations are more formal explanations of that.
The whole concept of a "light cone" seems to assume that spacetime is uniform (or at least doesn't have bubbles or holes in it). If spacetime is lumpy / Swiss cheesy (as Alcubierre drives or wormholes would cause, respectively), then that would result in similar lumpiness or holeyness in the light cone. In other words: why assume that it's "cone" shaped in situations that would in all likelihood dramatically deform that cone? In other other words: the light cone / worldline explanations don't really address cases where spacetime is outright deformed to shorten the distance something has to travel in order to go from point A to point B.
Further, the "light cone" argument (as presented in the article) seems to hinge on when observers find out about events... but just because an observer observed something to happen in a given order doesn't mean it actually happened in that order. If the light from Mars blowing up reaches us one second before the light from Pluto blowing up reaches us, does that mean that Mars blew up one second before Pluto did? It doesn't seem like observations are absolute truths, and I'm failing to understand why we're treating them as such.
I think you're missing one factor, though: the fast-but-subluminal observer. You're only considering Earth and Mars, and someone poofing between the two.
The issue is that if you have a subluminal (but moving at a significant fraction of the speed of light) observer outside the reference frames of Earth and Mars, there are conditions where they could see you appear on Mars, and then communicate back to Earth -- before you left -- to tell you not to poof to Mars in the first place.
This doesn't have anything to do with the idea of physically moving through space at some rate (that is, "touching" every point between Earth and Mars during your journey there); poofing from one place to another would have the same effect. And I don't think it matters how much time you spend on Mars, whether it's 1 second or 1 day, before poofing back to Earth.
Also, this explanation does not suggest that the poofer has time-traveled to the past (as you argue against); it's the fast-but-subluminal observer who has done so.
At least that's how I understand it; I'm no physicist.
> I think you're missing one factor, though: the fast-but-subluminal observer. You're only considering Earth and Mars, and someone poofing between the two.
There wouldn't be anything meaningful to observe:
- An observer on Earth would see me poof out of existence and poof back into existence with a rock in my hand; a few minutes later, with a really good telescope, that observer might see me poof into existence on Mars, take a rock, and poof back out of existence.
- An observer on Mars would see me poof into existence, take a rock, and poof back out of existence; a few minutes later, with a really good telescope, that observer might see me poof out of existence on Earth and poof back into existence while holding a rock.
- An observer somewhere in between with a really good telescope might be able to see the poofing in and out of existence on Earth and/or Mars, but would only receive that light after I had already returned to Earth, and would lack the necessary information to reliably assert which happened first.
The relevance of that fast-but-subluminal observer is dependent on me actually traversing every last micron of space from Earth to Mars and back in those two seconds, but that ain't what's happening. Rather, I'm taking a shortcut, and in order for the observer to observe anything other than the endpoints said observer would need to be taking that same exact shortcut alongside me - otherwise, at worst, the observer just sees two copies of me (one on Earth, and one on Mars), and by the time the observer thinks to do anything about that I would already have handed you a Mars rock.
----
The more mathy explanation of this involves the Lorentz factor, which in Lisp (because I'm on my computer and Emacs is handy) is (assuming c = 1):
where v is the relative velocity between to reference frames. So, (lorentz-factor 0.1) would correspond to something moving at 0.1c, (lorentz-factor 1) would correspond to something moving at the speed of light, and (lorentz-factor 2) would correspond to something moving at twice the speed of light.
You'll notice that (lorentz-factor 1) produces a division by zero, and that anything past that produces imaginary numbers. That's the basis for the "FTL implies time travel" argument; it assumes that something is actually traveling at a faster-than-light velocity (i.e. actually moving through every last micron of the space from Earth to Mars and back within those two seconds) and thus producing a Lorentz factor which - when plugged into a full Lorentz transformation - would imply backward time travel.
However, that ain't really applicable to the "poofing" above (nor is it applicable to Alcubierre drives or wormholes, of which said "poofing" is an abstraction), because the specific premise here is that I am not actually moving at a velocity significantly above 0; instead, I'm stretching the space behind me / contracting the space in front of me (in the case of an Alcubierre drive) or punching a shortcut between two points in space (in the case of a wormhole) such that I don't have to move at a speed significantly greater than zero. Since my velocity remains basically 0, my Lorentz factor ends up being basically 1, and thereby eliminates the mathematical basis for my "poofing" having any implication of backward time travel.
Space-time can expand at faster-than-light speeds - this is known for sure, since we live in a 93-billion light-years wide universe that's only 13 billion years old.
As far as I understand, the current conjecture is that an Alcubierre drive could move at faster than light speeds (if negative mass/energy to build it existed), but that it it tried to move to its own past, it would either destroy itself because of some conjectured quantum gravity phenomenon - this is called the "chronology protection conjecture" and Alcubierre himself talked about it:
> The conjecture has not been proven (it wouldn't be a conjecture if it had), but there are good arguments in its favor based on quantum field theory. The conjecture does not prohibit faster-than-light travel. It just states that if a method to travel faster than light exists, and one tries to use it to build a time machine, something will go wrong: the energy accumulated will explode, or it will create a black hole.
> Space-time can expand at faster-than-light speeds - this is known for sure
Not exactly. In the standard cosmology, in an equatorial slicing (cf. slicing a cone with each plane perpendicular to the axis, giving circles) of the expanding universe, space expands very slightly at each point in space as the cosmological time ticks by. If we choose a point (p,t) and some coarsening procedure to reduce the count of points immediately around (p,t) to a finite number, then very soon after at (p,t+\epsilon), and using the same coarsening procedure, there will be more points immediately around p. So, for example, for something (p=const,t_0) we might count seven points in which we could find that something at t_1: {(p,t_1),(p x+1,t_1),(p x-1,t_1), (p y+1, t_1), ...}. But there might be, say, twelve points immediately around (p,t_n), twenty-four around (p,t_{n+n}), etc. The same thinking applies at every point in space with the same value of t. As the universe ages, the numbers of points in space is already enormous and at each of those points in space we add more points.[1]
To that last paragraph we add a system of coordinates where there are cosmological observers[2] "stuck" at a particular spatial coordinate like (x,y,z) at all times, observing more space appearing between them, requiring coordinates in between to label that space. Physically these observers, if already distant, see each other's image becoming smaller, dimmer, and redder, as if they were accelerating away from each other. Physically neither observer detects such an acceleration -- they are in perfect free-fall.
The metric expansion, and accelerated expansion, can be (and is usually treated as) purely local. Nothing interacts superluminally, and there is no need from observation to have large nonuniformities in local expansion in the known universe. That is, at large enough distance scales, the local expansion at any point is well-modelled as constant at all points and at all times: the cosmological constant. (At smaller distance scales, in regions dominated by gravitationally collapsing matter (including dark matter), the local expansion is zero. "Manhattan is not expanding", nor is the rest of the solar system or anything in our galactic cluster as far as we can tell).
In the unknown early universe, various approaches to cosmic inflation essentially generatesa lot more new points around old points than expansion does, and the difference in inflation around any point can be large (loosely, the number-of-points-generated-at-point-p gap between "expanding" and "not expanding" is much narrower than the gap between "inflating" and "inflating less" let alone "not inflating").
> 93-billion light-years wide universe that's only 13 billion years old
There are a lot more points between galaxy clusters in an expanding universe than there is in a non-expanding universe.
(Inflation already stopped making new points in space long before the first protons formed, let alone galaxies).
For the most part part galaxy clusters tend to become more compact over time; the matter in them is thick and trending thicker. Expansion doesn't arrest that trend at all. Galaxy clusters shine across the electromagnetic spectrum, and also expel neutrinos and some amount of hot dust and gas. That thin expelled matter does not block expansion very close to it, and so thin matter gets smeared across new points as they appear in its immediate neighbourhood. In the cosmological frame this means stretching their wavelengths or equivalently reducing their kinetic energy or equivalently reducing their temperature adiabatically. However, the expansion is not strong enough to break molecular bonds, so molecules will cool; they won't snap apart because of standard expansion. (Likewise expansion doesn't ionize atoms or fission nuclei; it barely distorts clouds or streams of thick-enough molecular gas that float out of galaxy clusters. The ke...
It's the difference between scaleable quantum computing, and hypercomputation: one appears to be a matter of engineering, which may turn out to be completely impractical/require a million years/whatever. The other is ruled out by our understanding of the universe.
Pre-Wright, flight was the former, not the latter.
Our understanding of gravity isn’t complete. General relativity looks relatively unbreakable but the smartest minds agree non particle gravity just doesn’t make sense in an otherwise QFT universe so we’re missing something deeper.
Whether that will provide a way to travel FTL in space time or outside it, we don’t know. But the science is not as settled as it seems even if mere mortals cannot think up a theory to beat the truly incredible power of GR.
No no you don't understand. Science was wrong and incomplete in the past but now (as everyone learns in school) The Science has been completed. What we know is infallible and what we don't know is unknowable.
The lesson here is an idea, no matter how implausible, can have legs as long as people want it to be true. That particular idea here is FTL travel (or communication).
Nearly all of these ideas spring from a basic inability to understand the domain of a function. The domain is the set of values for which the function is defined. Any other values are undefined f(x) = 1/x for example has a domain of anything but 0.
So what happens is that people plug negative values into quantities like mass without any basis for what negative mass is and then use that as a basis for [insert FTL system here]. Why stop there? Why not use imaginary masses?
There is no negative mass. There is no negative energy. The domain of velocity is [0,c].
Even if you ignore that, no one goes far enough to calculate the energy requirements of, for example, "folding space" or creating a wormhole. If you do the math, it turns out you need to convert a significant portion of a stellar mass into energy.
The future isn't Star Trek nor Star Wars no matter how much you want it to be.
The article here actually presents a warp drive that doesn't require negative mass/energy, but also doesn't move at speeds greater than c. It's interesting because it seems to be permitted by all of the laws of GR and also build-able from known materials, and it could represent a new mode of propulsion.
It's also important to remember that space has expanded far faster than the speed of light in the past - c is only a limit for the speed that matter or energy can move at, it's not a limit for how space-time can change shape. We know this because the observable universe is ~93 billion light-years in size, but only ~13 billion years old - so it's size has grown faster than c.
I'm with you on the idea that none of this means practical controlled faster-than-light travel will ever exist, though. Jury's still out on slower-than-light folding-space drives, though.
Doesn't this warp drive depend on negative energy density (whatever that means) or has the fundamental flaw from out-of-domain values shifted in this version to something else?
> We show that a class of subluminal, spherically symmetric warp drive spacetimes, at least in principle, can be constructed based on the physical principles known to humanity today.
I'm no physicist to be able to evaluate the claim more deeply, but it is a published peer-reviewed article.
The more famous Alcubierre drive, the one that can go faster than light, does require negative energy density.
Notably, energy conservation is a local property of spacetime. Meaning, if you can manipulate the metric, you should be able to create (or vanish) energy.
Creating the warp-bubble clearly takes energy dependent only on its volume, not the mass contained therein.
Here, with a subluminal warp drive, you could hoist matter out of a gravity well and let it fall down afterwards.
> if you can manipulate the metric, you should be able to create (or vanish) energy.
No, you can't. That would violate local energy conservation.
The fact that energy conservation is a local property just means that there is no well-defined global energy in a general spacetime (though there is one in some special classes of spacetimes). It does not mean that stress-energy can be created or destroyed.
> with a subluminal warp drive, you could hoist matter out of a gravity well and let it fall down afterwards.
But the hoisting process would require you to expend at least as much energy as you would be able to collect from the falling down afterwards.
> We show that a class of subluminal, spherically symmetric warp drive spacetimes, at least in principle, can be constructed based on the physical principles known to humanity today
This is awesome. Ever since I read about the Alcubierre paper and it’s followups it seemed obvious to me that while there are huge basic physicality issues with FTL those same issues don’t exist for a near or at light speed warp drive.
Such a drive would not necessarily reduce the energy requirements of travel near light speed, but I wonder if it would eliminate some of the other huge problems. Things like a single dust particle annihilating you or blue shift turning all incident radiation into a gamma ray laser aimed at the front of your ship.
Would it have any impact on time dilation? If you approach the speed of light conventionally you can experience the trip subjectively as near instantaneous due to time dilation. Is that still true if you are in a warp bubble?
Light speed gets you to the centauri system in about 4.5 years Earth time. With time dilation the main cruise phase of the flight would seem instantaneous on board the spacecraft. (It would be years on Earth of course.)
> Light speed gets you to the centauri system in about 4.5 years Earth time. With time dilation the main cruise phase of the flight would seem instantaneous on board the spacecraft. (It would be years on Earth of course.)
Oh? But if there is no ether, and no absolute dead reckoning for motion in the vacuum, then for all we know, the Earth system is moving at 0.8c in one direction and the spacecraft would be moving at 0.2c in the opposite direction.
Subluminal warp drive only needing positive energy seems like cool method to get around conservation of momentum (remember EmDrive?). I wonder if it could be used at very small scales with lower energies for some applications.
Another interesting reactionless drive within possibility:
> "Swimming in spacetime" is a general relativistic effect, where an extended body can change its position by using cyclic deformations in shape to exploit the curvature of space, such as due to a gravitational field.
If either is possible it nearly proves that we must be very early for intelligence in the universe. A universe with intelligent life and FTL turns into a party pretty quick as soon as someone invents it.
If we even proved it was possible at table top scale I’d be about 99% convinced we are first in at a bare minimum our galaxy.
I would also argue that we should scale it up and get a presence out there fast in case someone less nice than us is also inventing one right now. (And we aren’t all that nice.)
My thinking is that if time travel / warp drive is possible, then words like "early" and "first" have no meaning. After all, if humans invented it, I would 100% expect someone to be abusing me with it right now. People would steal from the past to pay for the future.
You can always play with the factors. Maybe FTL is possible, but requires a sizable fraction of the planet's resources. Or you can eventually reach another planet, but terraforming it and building the technology for creating more FTL ships will take millions of years. Basically: FTL is fast but pointless, because getting to FTL takes longer than getting to your destination.
Maybe space is a bit more emptier than you think, and FTL is a bit slower.
Or maybe it is busy, but we haven't waited long enough. I mean think of a native American in the 1400s. If transoceanic travel is possible, why haven't we seen visitors from other continents yet?
Finally, it is still entirely possible that "they" are already there, and there is a cover up. I don't believe it (Occam's razor), but if FTL turns out to be theoretically possible, than I think I will have to update my beliefs :-)
No, but we're destroying the planet that they live on, in the process of harvesting and consuming its resources. We've learned through our own experiences that a being whose intelligence and power transcends our own tends to be bad news for us, even if they don't exist.
* your consciousness is the equivalent of an amoeba and it doesn’t occur to anyone to bother
* you are the first, someone has to be
* theres a whole other way to exist, creatures who live in quantum electrodynamics bound reality are boring
* somebody has been and is negotiating, just not you
* everybody eventually lives a billion years, the thousands of years human civilization has existed barely registers, they’ll get around to it after breakfast
* your species isn’t cool enough and nobody wants to be your friend
* your species keeps turning the salesman into a god
* nobody has noticed you yet, it’s a big universe and there’s plenty of other fun things to do
All good points. A short story by Franz Kafka, "Investigations of a Dog" kind of touches on the concept that the higher beings are invisible to us, and that we fabricate them in our own minds to explain our experience. In it, a dog notices that his fellow dogs engage in strange rituals, after which their food appears. And he wonders where the food is actually coming from. He experiences fleeting glimpses of soaring dog-like beings. If I'm recalling the story correctly, his friends don't want to hear about it, and ostracize him.
I have wondered if some of Kafka's writings belong in the science fiction genre.
This is an abstract of an article behind a paywall so we can't see this details but it is from 2021 and I managed to find an article discussing the concept [1]
> In this case negative energy is not required — rather it’s gravity itself which bends spacetime and gives rise to time dilation. In essence the gravitational field makes the passage of time within the passenger area much slower than the passage of time outside of it. A few minutes for them may be thousands of years worth of space travel, yet to leverage this powerful effect would take enormous amounts of gravity. The paper calls for the compression of an entire planet down to just a thin shell surrounding the passengers. The introduction of a physical warp drive model has changed the question from “Does something like negative energy even exist?” to “How do we compress the mass of a planet down to the size of a spaceship?”
And I'll pull out the most apropos sentence:
> The paper calls for the compression of an entire planet down to just a thin shell surrounding the passengers
Yes, no negative energy or negative energy mass, just an entire planet compressed into a thin shell that somehow doesn't further collapse.
So even if this were possible, how would you accelerate or maneuver such a massive object?
I've seen so many of these proposals over the years. It's all nuclear-grade hopium with a massive flaw or huge, highly theoretical unknown. Always.
It's an article about theoretical physics published in the 21st century, so it would be very surprising if it had not been posted on the arXiv before being published in a journal. And sure enough: https://arxiv.org/abs/2102.06824
I always find these kind of papers interesting. It seems like they found a more general "warp drive" metric. But after skimming it, I don't understand how they want to create it in reality.
The best shot I think we have at metric engineering - unless you want to build something crazy like moving spheres and rings of ultra dense matter - is to look into materials with interesting spin-gravity coupling.
We know that rotating bodies have additional gravitational interactions. The effect is tiny, but measurable in experiments (e.g. Gravity Probe B). We also know that atomic spin is not just an abstract quantum number but actual angular momentum. So there should be some kind of spin-gravity interaction, even though it would be incredibly weak. But even though the absolute force is small, if we manage to make it time dependent we could amplify the effect. Just like a changing magetic field creates an electric field, a changing "gravitomagnetic" field would create a gravitational field. This is basically the insight that took us from compasses and static electricity to the whole of electrical engineering.
I am not sure such materials could even exist. And even though I have a PhD in Physics, I have no idea how one would approach this theoretically. The treatment of particle spin seems to be very rare in GR, and then you would have to marry this to solid state physics and somehow calculate how the situation changes when the spin density changes and so on... The furthest I got was to open a thread on stackexchange about it :-) but who knows, maybe someday somebody looks into this stuff:
I was with you until the analogy to electromagnetism and time dependency. I admittedly don't have a PhD in physics (undergrad astrophysics), so I possibly won't understand your answer, but isn't gravitomagnetism more of a useful fiction than anything people actually believe models reality? I mean the equations look similar, but there's no physical justification: gravity is not a field in the QFT sense.
I guess I'm asking why you'd expect there to be an analog to the interaction between electrical and magnetic fields, when the nature of the thing is so different?
Gravity (more precisely, a dynamical metric) is absolutely a field in the QFT sense. The problem is that its unrenormalizable. That’s a technical term that roughly means the theory doesn’t make predictions to arbitrarily high energies. As a result the QFT describing gravity cannot be used to answer the interesting questions about black holes and Planck-scale physics. Historically this has meant that a new physical mechanism has been ignored that becomes relevant only at large energies, such as stringy physics in string theories or discrete spacetime in LQG. That’s part of why critics argue about the falsifiability of string theory (unfortunately the same critics don’t make the obvious conclusion that unfalsifiability in this sense implies to all models of quantum gravity, not just strings—-and that’s a funding motivated lie by omission. This has to be true since by their own framework, if LQG predicts something and string theory predicts nothing, the experiment distinguishing them could falsify string theory by validating that prediction).
Indeed, if gravity were not described by a QFT at low energies, we would need an extremely convoluted explanation for why quantum effects aren’t always destroyed by the mere existence of gravity. In short, gravitons are the Occam’s razor solution, not having gravitons would be extremely hard to reconcile with the existence of gravity at any scale, and when you dive into the math you see why.
In any case, none of that directly answers your question about gravitoelectromagnetism, which actually is just a way to rewrite the Einstein equations in a particular way that’s valid only sometimes. And that could only be unphysical if Einstein’s equations are unphysical in the same regime. There’s no regime we’ve tested in where the Einstein equations aren’t empirically valid, and they do come naturally as the classical limit of the gravitational QFT (the limit implies there can be small corrections of a particular form, however).
> isn't gravitomagnetism more of a useful fiction than anything people actually believe models reality?
No, it's a real effect and has been measured.
> the equations look similar, but there's no physical justification
Yes, there is; the justification is classical, not quantum, and does not depend on having any quantum theory of gravity. GR is a classical theory, and gravitomagnetism is present in it, similar to the way ordinary magnetism is present in classical electrodynamics. The difference is that electrodynamics is based on a vector field (the 4-potential) while GR is based on a tensor field (the metric); but that difference just changes some particular numerical coefficients in the equations for gravitomagnetism as compared to ordinary magnetism (for example, the gravity analogue to the Lorentz force has a factor of 4 in front of the v x B term). But these are classical fields, not quantum fields.
I really wish I understood this better, but the sense I've gotten is that intrinsic spin needs to couple to gravitation through torsion rather than through the usual curvature we study in GR. Most GR courses and textbooks barely mention torsion at all: IIRC Wald for example specifies "torsion free" as a condition on derivative operators and basically doesn't ever explore the alternative (I think there's a homework problem on it). The torsion free condition is what guarantees that Christoffel symbols are symmetric in their lower indices. Once upon a time while trying to understand all this back in grad school, I wrote up a set of notes extending Wald's calculations of curvature to include the possibility of torsion. I never tried to publish them anywhere, since of course it's nothing new or groundbreaking, but they're on my website here: http://www.slimy.com/~steuard/teaching/tutorials/GRtorsion.p...
Those notes do not discuss the connection to spin, because I was only halfway aware of it at the time and because I didn't have the time to delve into it enough to figure it out. (I also didn't know at the time that this is often called "Einstein-Cartan theory".) One notable thing about torsion is that it's a non-propagating field: as I recall, it's only non-zero inside the material with spin. I'm not entirely sure what the effects of all that might be. This 1976 review article has been lurking at the back of my to-read pile for ages: https://journals.aps.org/rmp/abstract/10.1103/RevModPhys.48....
I have no idea whatsoever whether any of that is useful for making warp drives or for metric engineering. But I figured I'd share, since it seemed relevant.
Unfortunately, different quantum spin orientations don't appear to evade the weak equivalence principle. Dropping Bose-Einstein condensates in vacuum drop towers is a popular experiment. An example, "Tests of the Universality of Free Fall with Atoms in Different Spin Orientations", 2016, <http://dx.doi.org/10.1103/PhysRevLett.117.023001> which is the published version of <https://arxiv.org/abs/1503.00433v1>. Likewise, one would not expect a ferromagnet and a ferroic glass to fall differently in a drop tower.
Quantum spin is relevant in post-Newtonian (GR-compatible) gravitation, though.
Rough argument: let's represent perturbations of the metric (gravitational waves) as a massless spin-2 boson field on top of some sourceless background geometry; this is essentially the programme of perturbative quantum gravity. (’t Hooft, <https://dspace.library.uu.nl/bitstream/1874/4708/2/16331.pdf> corresponding to DOI10.1142/9789812796653 (_0007).) This is almost General Relativity. GR's "no prior geometry" (MTW §17.6) axiom puts paid to the idea of obtaining a compatible theory by inserting some force into a static background, and ultimately this idea that spacetime is dynamic is what causes PQG to be a poor theory for strong gravitational interactions ('t Hooft ut supra, 2nd paragraph of §4).
Spin-2 interactions are attractive for the same charge. As far as we know, physicality demands PQG treat everything (including the gravitons) in the universe as possessing the same sign of gravitational charge. We can't (quasi-)neutralize with matter or vast collections of gravitons, so we can't charge screen in the sense of <https://en.wikipedia.org/wiki/Electric-field_screening> as opposed to having things like Lagrange points in Newtonian gravitation.
There is a family of theoretical approaches involves appreciable amounts of matter with opposite gravitational charges from everything else or introducing two "opposite" metrics and coupling most matter to our familiar one and a minority of matter to the other. Informally, we make gravitational and inertial masses unequal for some matter, breaking the weak equivalence principle (unlike in my first paragraph). Evidence takes us away from these approaches.
Additionally, opposite rank-2 tensor charges repel, so it's hard to imagine engineering up a scheme that uses any "anti-gravitating" matter with a single metric tensor, and theoreticians would have to imagine some cutoff to prevent runaway acceleration of an anti-gravitating particle let loose inside the solar system; Bondi, 1957, <https://doi.org/10.1103/RevModPhys.29.423>
One sketch of this sort of idea off the top of my head is Hossenfelder's "Antigravitation", <https://arxiv.org/abs/gr-qc/0508013>, which is per the author a toy idea (each Standard Model particle has a gravitational-anti particle) that makes a simplifying assumption that takes one away from general curved spacetimes (ibid., §4 and the para below eq. 17, which is suggestive of Bondi's runaway).
Instead, we might try to contrive a universe with a complement of fun things like pp-waves (which maybe one can get with your spheres and rings). It's unlikely that this can be done...
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[ 1.6 ms ] story [ 128 ms ] threadhttps://arxiv.org/abs/2102.06824
There's plenty of other reasons it's still just a theory, but that isn't one here, if I read it right.
The main problem is that it requires exotic matter with negative mass or negative energy. I don't think we have any good evidence to suggest such a state of matter is possible in this universe.
However, there were some recent papers describing warp drive geometries with only positive mass.
Ex: https://arxiv.org/abs/2201.00652
Not that I understand the physics well enough to do anything with this, but as you say, there is other work that's definitely (claiming) positive-only energy densities.
The ship drags the bubble along with it at sublight speed using normal thrusters.
The math can only work (if it even does) in exceptionally constrained circumstances. The main purely-theoretical issue is that the paper's <https://arxiv.org/abs/2102.06824v2> solution is wholly spherically symmetric, and with vacuum everywhere but in the warping region.
"Fig. 1: Asymptotically-flat vacuum background ... general stationary curved region with a spherical topology ... (the warping region) ... flat inner region ... ('passenger' space). ... As we discuss in Section 5, warp drive spacetimes require some form of propulsion in order to accelerate. For this reason, in physical realisations of such spacetimes, the front and rear parts are likely asymmetric". [emphasis mine]
Do the maths fall apart if we deviate from total spherical symmetry, either by changing the front and rear parts, or by breaking the vaccuum condition on the inside and outside of the shell, especially if our matter is not precisely spherically distributed?
They do a review of this, for limited deviations from spherical symmetry, in §4. However, the vaccum condition is what they study, not straying very far from their earlier "if [the warp region] were replaced by Minkowski spacetime, the whole spacetime would be Minkowski space."
The paper declines to answer the question about the distribution of matter (i.e., not-vacuum): "all warp drive spacetimes are asymptotically-flat ... no metric which describes an accelerating warp drive solution has been presented in the literature ... metrics for ... accelerating objects and more general axisymmetric objects which preserve their shape and mass ... remains a subject for future studies". That last part suggests that the inside part might be kept appropriately symmetrical through dynamic reconfiguration of the things inside (i.e., the ship, cargo, fuel, passengers).
But what about outside the warping region? In our solar system, and in our galaxy, we are not in Minkowski space.
The outer part of the warp region, being asymptotically flat, might be amenable to a "thin shell" (Israel/Darmois junction formalism) with the warp bubble far enough away from everything outside that it feels no tidal forces, which for the purposes of the text above is "acceleration". What determines how far is far enough away? The authors don't attempt an answer.
There are plenty of things (stars, most notably) in our part of the galaxy that will induce tides. The solar component of the lunisolar tide in Earth's oceans is substantial at 1 astronomical unit from our sun. That solar component doesn't go away when a space craft is (for example) 2 a.u. away from Earth&Moon on the opposite side of the sun. Can one zip this warp bubble safely past a 1 solar mass star at a distance of 1 au? Ten au? Can the warping region or passenger region be adjusted to allow for close approaches/operation within a star system? All questions for some possible future paper.
Apart from stars, we inhabit a region of space which is dotted with things you can smack into (from the very solid like rocks and iron to the very wispy, stuff will still tend to slow you down when you hit it; deceleration and acceleration are the same issue). Near luminous enough stars, radiation pressure might also induce non-negligible accelerations. Is any of that relevant? If relevant, is it "fixable"? How? Alter the shape of the warping region, or the passenger region, or both? Questions for some possible future paper.
So, even if one accepts for the sake of argument that the maths are flawless and further one just assumes the existence of controllable negative energy densities, this paper's maths are designed for a spacetime which overall isn't very much like what we have within a few hundred lightyears of here. Maybe some future...
Would be cool if it happened during my lifetime though.
...like violating causality?
http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-ti...
Generally speaking, all three of FTL (travel/communication), causality and relativity cannot all be true at the same time. If you can FTL-travel/communicate (they are the same really), then you can come up with a scenario in which an observer can see an event happening, and then see the cause of that event after it happened.
Extending that logic, if the observer can also move at superluminal speeds, he could prevent the cause of the event after seeing the event happen, leading to a paradox.
This is a guess, i.e. one possible outcome physicists are considering.
People have proposed alternative outcomes of FTL like the (in my opinion much more sensible) Novikov consistency principle, which roughly proposes that spacetime and the entities it contains (e.g. an observer's wordline) should be looked upon as a whole, in the sense that they need to be self-consistent. Spacetime is not time-dependent and does not evolve, so it does not make much sense to say "something something leads to a [spacetime] paradox".
[0] https://www.scottaaronson.com/papers/ctc.pdf
In short, if I were to instantaneously poof out of existence here on Earth, poof into existence on Mars a second later, grab a rock, poof out of existence again, and poof back into existence back on Earth again another second later and hand you that rock, it doesn't seem reasonable to assert that I traveled backward in time when I clearly traveled forward by two seconds. The whole lecture on that external observer is irrelevant, since there's nothing to observe unless the observer happens to be in my bubble/wormhole/whatever - and even then, one'd only be observing subluminal actions/signals within that region of spacetime.
Your example assumes there is some underlying rate at which time advances for the universe (or at least Earth and Mars) and that spacetime as we know it (including relativity and time dilation) are just some kind of modifier on top.
But theory and experiment so far point to that not being the case. There is no "pop out of existence here and pop in over there" without time travel (as best as we can tell). The whole light cone / worldline explanations are more formal explanations of that.
Now you can magic this problem away by proposing any number of schemes... like saying the entire universe's worldline is exists within a metaworldline and time travel actually resets the state of the universe as it was in the past then re-runs the universe... but all of that always proceeds forward in the metaworldline. In other words all past histories existed in a causal order, changing the past just adds "new commits" to the universe but history is never really rewritten. Bam! Our magic theory solves all paradox problems without requiring billions of parallel universes and allows time travel! But it's not a theory we can test or make predictions with so it isn't a useful scientific theory. It might as well be literal magic.
Similarly you could propose that GR is wrong... but your new theory is gonna need to match GR's predictions that have proven true while making some new ones we can test, while also avoiding or explaining causality and paradoxes.
Not necessarily; only that time is advancing in a forward direction. Whether 1 second on Earth is 1 second or 10 seconds or 0.1 seconds or what have you on Mars doesn't change the underlying premise: something disappeared from one place and appeared some positive amount of time later in another place. The only way I see that implying backward time travel is if time on Earth or Mars is already advancing backward, and if that's the case then the effects of Alcubierre drives on causality are probably the least of our worries.
And on that note...
> There is no "pop out of existence here and pop in over there" without time travel (as best as we can tell). The whole light cone / worldline explanations are more formal explanations of that.
The whole concept of a "light cone" seems to assume that spacetime is uniform (or at least doesn't have bubbles or holes in it). If spacetime is lumpy / Swiss cheesy (as Alcubierre drives or wormholes would cause, respectively), then that would result in similar lumpiness or holeyness in the light cone. In other words: why assume that it's "cone" shaped in situations that would in all likelihood dramatically deform that cone? In other other words: the light cone / worldline explanations don't really address cases where spacetime is outright deformed to shorten the distance something has to travel in order to go from point A to point B.
Further, the "light cone" argument (as presented in the article) seems to hinge on when observers find out about events... but just because an observer observed something to happen in a given order doesn't mean it actually happened in that order. If the light from Mars blowing up reaches us one second before the light from Pluto blowing up reaches us, does that mean that Mars blew up one second before Pluto did? It doesn't seem like observations are absolute truths, and I'm failing to understand why we're treating them as such.
The issue is that if you have a subluminal (but moving at a significant fraction of the speed of light) observer outside the reference frames of Earth and Mars, there are conditions where they could see you appear on Mars, and then communicate back to Earth -- before you left -- to tell you not to poof to Mars in the first place.
This doesn't have anything to do with the idea of physically moving through space at some rate (that is, "touching" every point between Earth and Mars during your journey there); poofing from one place to another would have the same effect. And I don't think it matters how much time you spend on Mars, whether it's 1 second or 1 day, before poofing back to Earth.
Also, this explanation does not suggest that the poofer has time-traveled to the past (as you argue against); it's the fast-but-subluminal observer who has done so.
At least that's how I understand it; I'm no physicist.
There wouldn't be anything meaningful to observe:
- An observer on Earth would see me poof out of existence and poof back into existence with a rock in my hand; a few minutes later, with a really good telescope, that observer might see me poof into existence on Mars, take a rock, and poof back out of existence.
- An observer on Mars would see me poof into existence, take a rock, and poof back out of existence; a few minutes later, with a really good telescope, that observer might see me poof out of existence on Earth and poof back into existence while holding a rock.
- An observer somewhere in between with a really good telescope might be able to see the poofing in and out of existence on Earth and/or Mars, but would only receive that light after I had already returned to Earth, and would lack the necessary information to reliably assert which happened first.
The relevance of that fast-but-subluminal observer is dependent on me actually traversing every last micron of space from Earth to Mars and back in those two seconds, but that ain't what's happening. Rather, I'm taking a shortcut, and in order for the observer to observe anything other than the endpoints said observer would need to be taking that same exact shortcut alongside me - otherwise, at worst, the observer just sees two copies of me (one on Earth, and one on Mars), and by the time the observer thinks to do anything about that I would already have handed you a Mars rock.
----
The more mathy explanation of this involves the Lorentz factor, which in Lisp (because I'm on my computer and Emacs is handy) is (assuming c = 1):
where v is the relative velocity between to reference frames. So, (lorentz-factor 0.1) would correspond to something moving at 0.1c, (lorentz-factor 1) would correspond to something moving at the speed of light, and (lorentz-factor 2) would correspond to something moving at twice the speed of light.You'll notice that (lorentz-factor 1) produces a division by zero, and that anything past that produces imaginary numbers. That's the basis for the "FTL implies time travel" argument; it assumes that something is actually traveling at a faster-than-light velocity (i.e. actually moving through every last micron of the space from Earth to Mars and back within those two seconds) and thus producing a Lorentz factor which - when plugged into a full Lorentz transformation - would imply backward time travel.
However, that ain't really applicable to the "poofing" above (nor is it applicable to Alcubierre drives or wormholes, of which said "poofing" is an abstraction), because the specific premise here is that I am not actually moving at a velocity significantly above 0; instead, I'm stretching the space behind me / contracting the space in front of me (in the case of an Alcubierre drive) or punching a shortcut between two points in space (in the case of a wormhole) such that I don't have to move at a speed significantly greater than zero. Since my velocity remains basically 0, my Lorentz factor ends up being basically 1, and thereby eliminates the mathematical basis for my "poofing" having any implication of backward time travel.
As far as I understand, the current conjecture is that an Alcubierre drive could move at faster than light speeds (if negative mass/energy to build it existed), but that it it tried to move to its own past, it would either destroy itself because of some conjectured quantum gravity phenomenon - this is called the "chronology protection conjecture" and Alcubierre himself talked about it:
> The conjecture has not been proven (it wouldn't be a conjecture if it had), but there are good arguments in its favor based on quantum field theory. The conjecture does not prohibit faster-than-light travel. It just states that if a method to travel faster than light exists, and one tries to use it to build a time machine, something will go wrong: the energy accumulated will explode, or it will create a black hole.
[0] https://web.archive.org/web/20160318223348/http://ccrg.rit.e... (last three slides touch on this area)
Not exactly. In the standard cosmology, in an equatorial slicing (cf. slicing a cone with each plane perpendicular to the axis, giving circles) of the expanding universe, space expands very slightly at each point in space as the cosmological time ticks by. If we choose a point (p,t) and some coarsening procedure to reduce the count of points immediately around (p,t) to a finite number, then very soon after at (p,t+\epsilon), and using the same coarsening procedure, there will be more points immediately around p. So, for example, for something (p=const,t_0) we might count seven points in which we could find that something at t_1: {(p,t_1),(p x+1,t_1),(p x-1,t_1), (p y+1, t_1), ...}. But there might be, say, twelve points immediately around (p,t_n), twenty-four around (p,t_{n+n}), etc. The same thinking applies at every point in space with the same value of t. As the universe ages, the numbers of points in space is already enormous and at each of those points in space we add more points.[1]
To that last paragraph we add a system of coordinates where there are cosmological observers[2] "stuck" at a particular spatial coordinate like (x,y,z) at all times, observing more space appearing between them, requiring coordinates in between to label that space. Physically these observers, if already distant, see each other's image becoming smaller, dimmer, and redder, as if they were accelerating away from each other. Physically neither observer detects such an acceleration -- they are in perfect free-fall.
The metric expansion, and accelerated expansion, can be (and is usually treated as) purely local. Nothing interacts superluminally, and there is no need from observation to have large nonuniformities in local expansion in the known universe. That is, at large enough distance scales, the local expansion at any point is well-modelled as constant at all points and at all times: the cosmological constant. (At smaller distance scales, in regions dominated by gravitationally collapsing matter (including dark matter), the local expansion is zero. "Manhattan is not expanding", nor is the rest of the solar system or anything in our galactic cluster as far as we can tell).
In the unknown early universe, various approaches to cosmic inflation essentially generatesa lot more new points around old points than expansion does, and the difference in inflation around any point can be large (loosely, the number-of-points-generated-at-point-p gap between "expanding" and "not expanding" is much narrower than the gap between "inflating" and "inflating less" let alone "not inflating").
> 93-billion light-years wide universe that's only 13 billion years old
There are a lot more points between galaxy clusters in an expanding universe than there is in a non-expanding universe. (Inflation already stopped making new points in space long before the first protons formed, let alone galaxies).
For the most part part galaxy clusters tend to become more compact over time; the matter in them is thick and trending thicker. Expansion doesn't arrest that trend at all. Galaxy clusters shine across the electromagnetic spectrum, and also expel neutrinos and some amount of hot dust and gas. That thin expelled matter does not block expansion very close to it, and so thin matter gets smeared across new points as they appear in its immediate neighbourhood. In the cosmological frame this means stretching their wavelengths or equivalently reducing their kinetic energy or equivalently reducing their temperature adiabatically. However, the expansion is not strong enough to break molecular bonds, so molecules will cool; they won't snap apart because of standard expansion. (Likewise expansion doesn't ionize atoms or fission nuclei; it barely distorts clouds or streams of thick-enough molecular gas that float out of galaxy clusters. The ke...
Pre-Wright, flight was the former, not the latter.
Whether that will provide a way to travel FTL in space time or outside it, we don’t know. But the science is not as settled as it seems even if mere mortals cannot think up a theory to beat the truly incredible power of GR.
Nearly all of these ideas spring from a basic inability to understand the domain of a function. The domain is the set of values for which the function is defined. Any other values are undefined f(x) = 1/x for example has a domain of anything but 0.
So what happens is that people plug negative values into quantities like mass without any basis for what negative mass is and then use that as a basis for [insert FTL system here]. Why stop there? Why not use imaginary masses?
There is no negative mass. There is no negative energy. The domain of velocity is [0,c].
Even if you ignore that, no one goes far enough to calculate the energy requirements of, for example, "folding space" or creating a wormhole. If you do the math, it turns out you need to convert a significant portion of a stellar mass into energy.
The future isn't Star Trek nor Star Wars no matter how much you want it to be.
It's also important to remember that space has expanded far faster than the speed of light in the past - c is only a limit for the speed that matter or energy can move at, it's not a limit for how space-time can change shape. We know this because the observable universe is ~93 billion light-years in size, but only ~13 billion years old - so it's size has grown faster than c.
I'm with you on the idea that none of this means practical controlled faster-than-light travel will ever exist, though. Jury's still out on slower-than-light folding-space drives, though.
> We show that a class of subluminal, spherically symmetric warp drive spacetimes, at least in principle, can be constructed based on the physical principles known to humanity today.
I'm no physicist to be able to evaluate the claim more deeply, but it is a published peer-reviewed article.
The more famous Alcubierre drive, the one that can go faster than light, does require negative energy density.
Creating the warp-bubble clearly takes energy dependent only on its volume, not the mass contained therein.
Here, with a subluminal warp drive, you could hoist matter out of a gravity well and let it fall down afterwards.
No, you can't. That would violate local energy conservation.
The fact that energy conservation is a local property just means that there is no well-defined global energy in a general spacetime (though there is one in some special classes of spacetimes). It does not mean that stress-energy can be created or destroyed.
> with a subluminal warp drive, you could hoist matter out of a gravity well and let it fall down afterwards.
But the hoisting process would require you to expend at least as much energy as you would be able to collect from the falling down afterwards.
This is awesome. Ever since I read about the Alcubierre paper and it’s followups it seemed obvious to me that while there are huge basic physicality issues with FTL those same issues don’t exist for a near or at light speed warp drive.
Such a drive would not necessarily reduce the energy requirements of travel near light speed, but I wonder if it would eliminate some of the other huge problems. Things like a single dust particle annihilating you or blue shift turning all incident radiation into a gamma ray laser aimed at the front of your ship.
Would it have any impact on time dilation? If you approach the speed of light conventionally you can experience the trip subjectively as near instantaneous due to time dilation. Is that still true if you are in a warp bubble?
Light speed gets you to the centauri system in about 4.5 years Earth time. With time dilation the main cruise phase of the flight would seem instantaneous on board the spacecraft. (It would be years on Earth of course.)
Oh? But if there is no ether, and no absolute dead reckoning for motion in the vacuum, then for all we know, the Earth system is moving at 0.8c in one direction and the spacecraft would be moving at 0.2c in the opposite direction.
Or something like that.
Another interesting reactionless drive within possibility:
> "Swimming in spacetime" is a general relativistic effect, where an extended body can change its position by using cyclic deformations in shape to exploit the curvature of space, such as due to a gravitational field.
https://en.wikipedia.org/wiki/Reactionless_drive#Movement_wi...
No, it isn't. You can't "get around" conservation of momentum.
> Swimming in spacetime
Still doesn't allow you to violate conservation of momentum. The article you link to notes this.
If we even proved it was possible at table top scale I’d be about 99% convinced we are first in at a bare minimum our galaxy.
I would also argue that we should scale it up and get a presence out there fast in case someone less nice than us is also inventing one right now. (And we aren’t all that nice.)
Not under the Zoo hypothesis: https://en.wikipedia.org/wiki/Zoo_hypothesis
Makes me wonder if UFO sightings are from misguided extraterrestrial missionaries trying to teach us about their Lord and Savior Behemecoatyl.
Maybe space is a bit more emptier than you think, and FTL is a bit slower.
Or maybe it is busy, but we haven't waited long enough. I mean think of a native American in the 1400s. If transoceanic travel is possible, why haven't we seen visitors from other continents yet?
Finally, it is still entirely possible that "they" are already there, and there is a cover up. I don't believe it (Occam's razor), but if FTL turns out to be theoretically possible, than I think I will have to update my beliefs :-)
laughs in some 700CE-vintage Polynesian language while munching on a sweet potato
* you don’t have anything to pay for it with
* your consciousness is the equivalent of an amoeba and it doesn’t occur to anyone to bother
* you are the first, someone has to be
* theres a whole other way to exist, creatures who live in quantum electrodynamics bound reality are boring
* somebody has been and is negotiating, just not you
* everybody eventually lives a billion years, the thousands of years human civilization has existed barely registers, they’ll get around to it after breakfast
* your species isn’t cool enough and nobody wants to be your friend
* your species keeps turning the salesman into a god
* nobody has noticed you yet, it’s a big universe and there’s plenty of other fun things to do
I have wondered if some of Kafka's writings belong in the science fiction genre.
> In this case negative energy is not required — rather it’s gravity itself which bends spacetime and gives rise to time dilation. In essence the gravitational field makes the passage of time within the passenger area much slower than the passage of time outside of it. A few minutes for them may be thousands of years worth of space travel, yet to leverage this powerful effect would take enormous amounts of gravity. The paper calls for the compression of an entire planet down to just a thin shell surrounding the passengers. The introduction of a physical warp drive model has changed the question from “Does something like negative energy even exist?” to “How do we compress the mass of a planet down to the size of a spaceship?”
And I'll pull out the most apropos sentence:
> The paper calls for the compression of an entire planet down to just a thin shell surrounding the passengers
Yes, no negative energy or negative energy mass, just an entire planet compressed into a thin shell that somehow doesn't further collapse.
So even if this were possible, how would you accelerate or maneuver such a massive object?
I've seen so many of these proposals over the years. It's all nuclear-grade hopium with a massive flaw or huge, highly theoretical unknown. Always.
[1]: https://medium.com/predict/the-first-physical-warp-drive-mod...
True, the planet shell thing is nuclear grade hopium. But negative energy is big bang level hopium. This is a big downgrade.
The best shot I think we have at metric engineering - unless you want to build something crazy like moving spheres and rings of ultra dense matter - is to look into materials with interesting spin-gravity coupling.
We know that rotating bodies have additional gravitational interactions. The effect is tiny, but measurable in experiments (e.g. Gravity Probe B). We also know that atomic spin is not just an abstract quantum number but actual angular momentum. So there should be some kind of spin-gravity interaction, even though it would be incredibly weak. But even though the absolute force is small, if we manage to make it time dependent we could amplify the effect. Just like a changing magetic field creates an electric field, a changing "gravitomagnetic" field would create a gravitational field. This is basically the insight that took us from compasses and static electricity to the whole of electrical engineering.
I am not sure such materials could even exist. And even though I have a PhD in Physics, I have no idea how one would approach this theoretically. The treatment of particle spin seems to be very rare in GR, and then you would have to marry this to solid state physics and somehow calculate how the situation changes when the spin density changes and so on... The furthest I got was to open a thread on stackexchange about it :-) but who knows, maybe someday somebody looks into this stuff:
https://physics.stackexchange.com/questions/46099/materials-...
I guess I'm asking why you'd expect there to be an analog to the interaction between electrical and magnetic fields, when the nature of the thing is so different?
Indeed, if gravity were not described by a QFT at low energies, we would need an extremely convoluted explanation for why quantum effects aren’t always destroyed by the mere existence of gravity. In short, gravitons are the Occam’s razor solution, not having gravitons would be extremely hard to reconcile with the existence of gravity at any scale, and when you dive into the math you see why.
In any case, none of that directly answers your question about gravitoelectromagnetism, which actually is just a way to rewrite the Einstein equations in a particular way that’s valid only sometimes. And that could only be unphysical if Einstein’s equations are unphysical in the same regime. There’s no regime we’ve tested in where the Einstein equations aren’t empirically valid, and they do come naturally as the classical limit of the gravitational QFT (the limit implies there can be small corrections of a particular form, however).
No, it's a real effect and has been measured.
> the equations look similar, but there's no physical justification
Yes, there is; the justification is classical, not quantum, and does not depend on having any quantum theory of gravity. GR is a classical theory, and gravitomagnetism is present in it, similar to the way ordinary magnetism is present in classical electrodynamics. The difference is that electrodynamics is based on a vector field (the 4-potential) while GR is based on a tensor field (the metric); but that difference just changes some particular numerical coefficients in the equations for gravitomagnetism as compared to ordinary magnetism (for example, the gravity analogue to the Lorentz force has a factor of 4 in front of the v x B term). But these are classical fields, not quantum fields.
I really wish I understood this better, but the sense I've gotten is that intrinsic spin needs to couple to gravitation through torsion rather than through the usual curvature we study in GR. Most GR courses and textbooks barely mention torsion at all: IIRC Wald for example specifies "torsion free" as a condition on derivative operators and basically doesn't ever explore the alternative (I think there's a homework problem on it). The torsion free condition is what guarantees that Christoffel symbols are symmetric in their lower indices. Once upon a time while trying to understand all this back in grad school, I wrote up a set of notes extending Wald's calculations of curvature to include the possibility of torsion. I never tried to publish them anywhere, since of course it's nothing new or groundbreaking, but they're on my website here: http://www.slimy.com/~steuard/teaching/tutorials/GRtorsion.p...
Those notes do not discuss the connection to spin, because I was only halfway aware of it at the time and because I didn't have the time to delve into it enough to figure it out. (I also didn't know at the time that this is often called "Einstein-Cartan theory".) One notable thing about torsion is that it's a non-propagating field: as I recall, it's only non-zero inside the material with spin. I'm not entirely sure what the effects of all that might be. This 1976 review article has been lurking at the back of my to-read pile for ages: https://journals.aps.org/rmp/abstract/10.1103/RevModPhys.48....
I have no idea whatsoever whether any of that is useful for making warp drives or for metric engineering. But I figured I'd share, since it seemed relevant.
That's because nobody knows how to do that, and statements in papers like these that claim it can be done are basically hand-waving.
Quantum spin is relevant in post-Newtonian (GR-compatible) gravitation, though.
Rough argument: let's represent perturbations of the metric (gravitational waves) as a massless spin-2 boson field on top of some sourceless background geometry; this is essentially the programme of perturbative quantum gravity. (’t Hooft, <https://dspace.library.uu.nl/bitstream/1874/4708/2/16331.pdf> corresponding to DOI10.1142/9789812796653 (_0007).) This is almost General Relativity. GR's "no prior geometry" (MTW §17.6) axiom puts paid to the idea of obtaining a compatible theory by inserting some force into a static background, and ultimately this idea that spacetime is dynamic is what causes PQG to be a poor theory for strong gravitational interactions ('t Hooft ut supra, 2nd paragraph of §4).
Spin-2 interactions are attractive for the same charge. As far as we know, physicality demands PQG treat everything (including the gravitons) in the universe as possessing the same sign of gravitational charge. We can't (quasi-)neutralize with matter or vast collections of gravitons, so we can't charge screen in the sense of <https://en.wikipedia.org/wiki/Electric-field_screening> as opposed to having things like Lagrange points in Newtonian gravitation.
There is a family of theoretical approaches involves appreciable amounts of matter with opposite gravitational charges from everything else or introducing two "opposite" metrics and coupling most matter to our familiar one and a minority of matter to the other. Informally, we make gravitational and inertial masses unequal for some matter, breaking the weak equivalence principle (unlike in my first paragraph). Evidence takes us away from these approaches.
Additionally, opposite rank-2 tensor charges repel, so it's hard to imagine engineering up a scheme that uses any "anti-gravitating" matter with a single metric tensor, and theoreticians would have to imagine some cutoff to prevent runaway acceleration of an anti-gravitating particle let loose inside the solar system; Bondi, 1957, <https://doi.org/10.1103/RevModPhys.29.423>
One sketch of this sort of idea off the top of my head is Hossenfelder's "Antigravitation", <https://arxiv.org/abs/gr-qc/0508013>, which is per the author a toy idea (each Standard Model particle has a gravitational-anti particle) that makes a simplifying assumption that takes one away from general curved spacetimes (ibid., §4 and the para below eq. 17, which is suggestive of Bondi's runaway).
Instead, we might try to contrive a universe with a complement of fun things like pp-waves (which maybe one can get with your spheres and rings). It's unlikely that this can be done...
1 https://en.wikipedia.org/wiki/Flood_(Baxter_novel)
2 https://en.wikipedia.org/wiki/Ark_(novel)
3 https://en.wikipedia.org/wiki/Stephen_Baxter_(author)