Neutrons are electrically neutral but they still interact electromagnetically (they're composed of charged quarks). They have a magnetic moment, for instance [1]. So they wouldn't be dark the way "dark matter" appears (heh) to be.
Dark matter isn't made of protons, neutrons, or anything as pedestrian as that. It's a sort of "placeholder name" for an effect seen in situations like the collision of galaxies, where there seems to be more gravitational attraction between the two of them than there should be given the matter observed in them. It's as if they had a whole bunch more mass to them, but for some reason that mass is interacting gravitationally, and not in any other way or through any other forces. Scientists can model this by assuming that this type of gravitational-only matter is real, calling it "dark matter", because we don't have a better sense of what it really is.
The stuff made in this experiment, on the other hand, has more to do with what you find in something like a neutron star—clumps of neutrons (which are regular, non-dark matter) packed closely together—albeit in an extremely small scale compared to the star.
There aren't any known arrangements of neutrons that are stable without protons. Solo neutrons don't exist for longer than 10-20 minutes before decaying into protons and I doubt these tetraneutrons exist for longer than a few seconds unless they're moving at relativistic speeds (and even then, for not much longer). Unless there is something drastically different going on in the interstellar medium, neutrons as dark matter is extremely unlikely.
Why is this reasoning unsound? The fact that, after centuries of theoretical and experimental physics, we have yet to see stables neutrons in any configuration other than with protons in an atomic nucleus? We know pretty accurately the decay rates of single neutrons and now we know that all the way up to a tetraneutron that the particles are unstable on the order of minutes and seconds. What evidence is there that we are wrong?
Fun fact: the time from the discovery of the neutron to detonating an atomic bomb was a mere 13 years. From the discovery of nuclear fission it was only 7 years.
They do decay into protons and electrons, and then are immediately squished back into neutrons by the immense pressure. (There is a missing neutrino to wonder about though.)
Another way to think about it:
They are unable to decay into protons plus electrons because there isn't enough "room" for the two particles to exist in the space formerly taken up by one particle. See: Electron degeneracy pressure.
Basically all particles decay, unless there is something stopping them - for example there is nothing lighter for them to decay into.
You could ask why don't the neutrons in nucleus's decay into protons? It's because it's not energetically favorable for them to do so.
That's basically all decay is: Is it energetically favorable to be something else. The more energy is involved the "faster" it decays (sort of, it's more like the more energy relative to the barrier).
> They are unable to decay into protons plus electrons because there isn't enough "room" for the two particles to exist in the space formerly taken up by one particle. See: Electron degeneracy pressure.
Just being pedantic here but in a neutron star, it's neutron degeneracy pressure that's at play. Electron degeneracy pressure keeps a white (and later brown) dwarf from collapsing further, but if there's enough mass it is overcome and the next thing that kicks in is neutron degeneracy pressure, which for whatever reason is a lot stronger, and holds up another solar mass or two before it passes the limit where collapsing into a black hole is inevitable.
The principles for both types of degeneracy are similar (it's the Pauli exclusion principle at work basically) but the particles are different.
GP is correct-- the neutron degeneracy pressure prevents the stellar remnant from getting any smaller, but it is the electron degeneracy pressure which makes beta decay energetically unfavorable. In order to be produced by neutron beta decay, an electron must enter a valid state. Pauli exclusion (electrons are fermions) requires the electron to enter a state at least as energetic as the electron Fermi level, which due to the neutron star's gravity is comparable to the rest mass of the electron. Neutron decay is energetically favorable, but not that favorable. The reverse process accelerates and equilibrium is reached.
A neutron decays into a proton by emitting an electron and an electron antineutrino.
A proton decays into a neutron by emitting a positron and an electron neutrino. (These are the antiparticles of the ones emitted in neutron to proton beta decay, above.)
They are. Subatomic decay doesn't happen with particles "contained within" other particles; a neutron does not contain a proton but can decay into a proton, electron, and electron antineutrino via the W boson.
One idea is it is a very thin gas permeating the entire galaxy. Because it does exchange photons it doesnt chemically clump together like regular matter and stays a diffuse gas concentrated only by gravity.
Except neutrons are protons bound to electrons, so that's really just 4 of each, so maybe this is just some exotic Beryllium, or a clump of degenerate neutron gas as might escape from a dead-ish star.
To be more specific, you can turn a proton into a neutron by electron capture or beta+ decay, but that's not what a neutron is.
A neutron is 2 down (-1/3 charge) quarks and 1 up (+2/3 charge) quark, whereas a proton is 2 up and 1 down quark. Electron (-1 charge) capture changes an up to a down and emits an electron neutrino (0 charge).
Beta decay is when a neutron changes to a proton and emits an electron and an electron antineutrino (beta-), or when a proton changes to a neutron and emits a positron and electron neutrino (beta+).
Even if you wanted to be generous, the grandparent post left out the electron antineutrino.
A proton is neutron bound by an electron? Where did you get that from?
Beta decay goes both ways[1]. In β+ decay, a proton decays into a neutron and positron, and in β− decay, a neutron decays into proton and electron. (I don't know much physics, but I suppose there's a limit to them decaying from one to another, and I suppose each proton/neutron is unique in terms of its mass/energy content.)
"I suppose each proton/neutron is unique in terms of its mass/energy content"
Oh boy, so there's a can of worms. "Unique" would need to be nailed down to test that statement, and runs into some observational problems in that atoms exist and can be found in things like monatomic gasses in enormous enough quantities that you can wash out any contributions from other matter. These gas clouds can be measured and manipulated, and have been. The results limit the variation from proton to proton (and climbing the periodic table of elements, from nucleus to nucleus) to be entirely negligible.
Theory depends on where you look. Protons are usually taken to be indistinguishable particles (each proton obeys the same particle statistics at low energy). Additionally, there are good reasons to believe that any distinguishability -- however small -- leads to observables (an example is elucidated and generalized in the "Bosonic Birthday Paradox" http://arxiv.org/abs/1106.0849 ). Those observables are not seen.
Proton distinguishability also goes to the question of the mechanisms by which the Pauli Exclusion Principle manifests. It does manifest among protons in atomic nuclei, which is important. Retaining that and introducing actual distinguishability at small scales is a challenge.
If protons are (barely) distinguishable to us in experiments, they'll be (barely) distinguishable in decays and in electron interactions too. But if you think of a proton as "a mess"[1] and look at it at, say, the QCD scale, your question might be better put as,"how do confinement (and asymptotic freedom) and colour exchange work?". Peering into the structure of a proton at that scale is certainly interesting, but any information in there can only escape in ways permitted by confinement.
"So there are reasons to go further and describe things as I have elsewhere on this website: a proton is made from three quarks (two up quarks and a down quark), lots of gluons, and lots of quark-antiquark pairs (mostly up quarks and down quarks, but also even a few strange quarks); they are all flying around at very high speed (approaching or at the speed of light); and the whole collection is held together by the strong nuclear force"
[forgive me for combining a reply to an earlier comment you made on neutron stars]
We don't know the equation of state of a neutron star; most viable models have substantial layering and an impressively busy energy-density flow. Most models predict neutron stars to be very bright in neutrinos, although the brightness is typically greatest at the outer layers rather than from the deeper ones where one would expect a superfluid sea of degenerate neutrons to dominate almost to the exclusion of everything else (e.g. there may be small numbers of superconducting protons floating around in it), and mechanisms which produce the Pauli Exclusion Principle are non-negligible. Possible mechanisms are somewhat constrained. How the density profile goes below the superfluid layer is I think mainly speculation; there are BH-remnant-related reasons to hope for a lattice crystal or some block-like structure of subatomic particles forming an inner core.
> the whole collection is held together by the strong nuclear force
to be pedantic, the "lots of gluons and q-antiq pairs" is the strong nuclear force in action
Also, you're talking about virtual particles here, it's acceptable to (and usually this is done) ignore them when describing the "contents" of a particle since they're everywhere (in less concentration perhaps) anyway.
I'm not sure what you mean by "they're everywhere (in less concentration perhaps)".
There's no antiquark in the usual three-quark simplified description of a proton, yet without antiquarks how does one explain the charged leptons and anti-leptons readily observed following high-energy proton collisions?
Are we dancing around a question like, "what really happens close to the IR fixed point"?
I think he means that the attendant “background noise” of virtual particles occurs within the notional radius of the compound proton just as they occur within any other volume of space.
I'm saying that these quarks are not long-lived, they come and go like other virtual particles.
You have this behavior outside of a hadron too, just that you're more likely to see a quark-antiquark pair inside a hadron than outside. The sea quarks within the hadron are still virtual particles, though.
And often virtual particles aren't considered to be there at all, after all, from that logic one might say that a proton contains all particles (it does, since there always is a possibility of a virtual particle pair of any type springing up). Of course, the possibility of the quarks coming up is more likely since they can be obtained from a gluon, but they're still virtual.
We're just dancing around the question "should we count virtual particles when talking about the contents?". The answer usually depends on context. So it's completely ok to say that a proton is just three quarks, since those are the only real particles in it.
The Strassler links are VERY good, thank you for posting. I'm especially impressed that he answers comments on the posts in detail, even "stupid" questions.
It's a tragedy that he has left full-time theoretical physics for greener pastures as a result of the huge (as in double-digit percentages, and in some cases more than a third) cuts in research budgets in the USA, coupled with his desire to remain working in the States.
Unfortunately there are quite a few good working-scientist bloggers who have been producing much reduced output as a result of lower funding in general. Some have moved out of the USA and continue to blog. Some have had to focus themselves on earning survival income by pushing out many grant proposals in hopes of even partial funding, and simply cannot afford to take the time to explain other scientsists' work to non-scientists. Some have, alas, mostly stopped blogging altogether.
> An up quark is not a down quark plus an electron.
They're all perturbations in fields, and I think you could consider all the various ways particles can combine and split as being in some sense evidence of an underlying sameness to them. That's what the whole idea of grand unification was.
Interesting about the people here claiming that this isn't the case what blahDeeBlahBlah says. I don't know anything about chemistry other than the high school level, but essentially what blahDeeBlahBlah says is what I also learned in high school. Yet another lie they tell young students. They never told anything about quarks as well.
Wikipedia's take on it is here. The last two or three paragraphs before "See also" are provocative.
Nuclear physics are enormously complicated and the simplified picture usually presented of the Standard Model (in charts showing 3-quark particles) is a compromise between getting across the idea of a composite particle like a proton and digging into why a proton is an extremely messy thing filled with pions and gluons. The latter is where the symmetry groups of the Standard Model lead you, and down that path lies enormous many-body problems. Those are almost always irrelevant at the scale of chemistry, nuclear engineering, or even most of astrophysics, where the simplification is good enough. Indeed, most of the time quarks and gluons don't even arise in the solutions to problems in those fields.
Additionally, at that scale one starts running into some problems in the Standard Model which are essentially irrelevant in useful effective field theories, and which are simply not well enough understood by working scientists in the field to boil down to something useful for teaching except at the graduate level. (Well, as long as you believe in the principle that to explain something to a child, or even an adult non-specialist, you have to understand it extremely well or sacrifice accuracy.)
Hello, why writing "particles" with the quotes? Do you consider the reality of a particle to be proportional to its lifetime? By the way, a nanosecond is a very long time when you compare it to the lifetimes of some particles (with no quotes)!
Not the person to whom you're replying, but my lay understanding of it was that it's much more difficult to model mathematically those particles which exist for incredibly small spans of time.
I know nothing about the mathematics. But I think it's fair to say that in general people tend think of particles as being a _stable_ unit of some sort. So the term "particle" becomes less appropriate the less stable something is, and time is often a factor when thinking about stability.
If someone were simply to slam 4 neutrons together, for how long would they need to stay together for this to be deemed a real particle? I think it's for this reason the original commenter quoted the use of the term "particle".
Not the person you replied to either, but roughly speaking a resonance is mathematically the same as a particle. However the two concepts differ of course philosophically. So the shorter the lifetime of a particle is, the more one is inclined to call it a resonance rather than a particle.
Not the person you replied to either, but roughly speaking a resonance is mathematically the same as a particle. However the two concepts differ of course philosophically. So the shorter the lifetime of a particle is, the more one is inclined to call it a resonance rather than a particle.
Maybe if we lay low for a while we can skip this extinction event and mop up after the Reapers are through, which I assume is roughly how the Protheans got started.
It's short enough that it doesn't even touch the detector. The evidence is that the energy from the four neutrons all shows up at once, instead of being split among four individual neutrons. http://arstechnica.com/science/2016/02/no-protons-needed-pos...
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[ 5.7 ms ] story [ 143 ms ] thread[1] - https://en.wikipedia.org/wiki/Neutron_magnetic_moment
The stuff made in this experiment, on the other hand, has more to do with what you find in something like a neutron star—clumps of neutrons (which are regular, non-dark matter) packed closely together—albeit in an extremely small scale compared to the star.
Don't equate the bound states with the unbound states.
They do decay into protons and electrons, and then are immediately squished back into neutrons by the immense pressure. (There is a missing neutrino to wonder about though.)
Another way to think about it:
They are unable to decay into protons plus electrons because there isn't enough "room" for the two particles to exist in the space formerly taken up by one particle. See: Electron degeneracy pressure.
Basically all particles decay, unless there is something stopping them - for example there is nothing lighter for them to decay into.
You could ask why don't the neutrons in nucleus's decay into protons? It's because it's not energetically favorable for them to do so.
That's basically all decay is: Is it energetically favorable to be something else. The more energy is involved the "faster" it decays (sort of, it's more like the more energy relative to the barrier).
Just being pedantic here but in a neutron star, it's neutron degeneracy pressure that's at play. Electron degeneracy pressure keeps a white (and later brown) dwarf from collapsing further, but if there's enough mass it is overcome and the next thing that kicks in is neutron degeneracy pressure, which for whatever reason is a lot stronger, and holds up another solar mass or two before it passes the limit where collapsing into a black hole is inevitable.
The principles for both types of degeneracy are similar (it's the Pauli exclusion principle at work basically) but the particles are different.
(In their core, they could potentially even contain quarks or hyperons, although recent neutron star measurements make this a bit more unlikely)
Or maybe an ELI-only-took-undergrad-physics-101? :)
A proton decays into a neutron by emitting a positron and an electron neutrino. (These are the antiparticles of the ones emitted in neutron to proton beta decay, above.)
https://en.m.wikipedia.org/wiki/Beta_decay
They are. Subatomic decay doesn't happen with particles "contained within" other particles; a neutron does not contain a proton but can decay into a proton, electron, and electron antineutrino via the W boson.
There is a paper about it, and some hints it exists, but it has not risen to the level of accepted particle.
No, they are not.
A neutron is 2 down (-1/3 charge) quarks and 1 up (+2/3 charge) quark, whereas a proton is 2 up and 1 down quark. Electron (-1 charge) capture changes an up to a down and emits an electron neutrino (0 charge).
Beta decay is when a neutron changes to a proton and emits an electron and an electron antineutrino (beta-), or when a proton changes to a neutron and emits a positron and electron neutrino (beta+).
Even if you wanted to be generous, the grandparent post left out the electron antineutrino.
Beta decay goes both ways[1]. In β+ decay, a proton decays into a neutron and positron, and in β− decay, a neutron decays into proton and electron. (I don't know much physics, but I suppose there's a limit to them decaying from one to another, and I suppose each proton/neutron is unique in terms of its mass/energy content.)
[1] See https://en.wikipedia.org/wiki/Beta_decay
"I suppose each proton/neutron is unique in terms of its mass/energy content"
Oh boy, so there's a can of worms. "Unique" would need to be nailed down to test that statement, and runs into some observational problems in that atoms exist and can be found in things like monatomic gasses in enormous enough quantities that you can wash out any contributions from other matter. These gas clouds can be measured and manipulated, and have been. The results limit the variation from proton to proton (and climbing the periodic table of elements, from nucleus to nucleus) to be entirely negligible.
Theory depends on where you look. Protons are usually taken to be indistinguishable particles (each proton obeys the same particle statistics at low energy). Additionally, there are good reasons to believe that any distinguishability -- however small -- leads to observables (an example is elucidated and generalized in the "Bosonic Birthday Paradox" http://arxiv.org/abs/1106.0849 ). Those observables are not seen.
Proton distinguishability also goes to the question of the mechanisms by which the Pauli Exclusion Principle manifests. It does manifest among protons in atomic nuclei, which is important. Retaining that and introducing actual distinguishability at small scales is a challenge.
If protons are (barely) distinguishable to us in experiments, they'll be (barely) distinguishable in decays and in electron interactions too. But if you think of a proton as "a mess"[1] and look at it at, say, the QCD scale, your question might be better put as,"how do confinement (and asymptotic freedom) and colour exchange work?". Peering into the structure of a proton at that scale is certainly interesting, but any information in there can only escape in ways permitted by confinement.
[1] http://profmattstrassler.com/articles-and-posts/particle-phy...
A neutron has 3 quarks in it, not 3 quarks plus an electron.
That down quark that became an up quark: An up quark is not a down quark plus an electron.
Matt Strassler has a couple relevant articles on his blog site.
The first goes deeper than the 3-quark simplification.
"What's a proton? First and foremost, it’s a mess. A total mess"
http://profmattstrassler.com/articles-and-posts/largehadronc...
The second digs into both protons and neutrons
"So there are reasons to go further and describe things as I have elsewhere on this website: a proton is made from three quarks (two up quarks and a down quark), lots of gluons, and lots of quark-antiquark pairs (mostly up quarks and down quarks, but also even a few strange quarks); they are all flying around at very high speed (approaching or at the speed of light); and the whole collection is held together by the strong nuclear force"
http://profmattstrassler.com/articles-and-posts/particle-phy...
And the third deals with neutron stability in nuclei, and specifically focuses on the stability of the neutron inside a deuteron.
http://profmattstrassler.com/articles-and-posts/particle-phy...
[forgive me for combining a reply to an earlier comment you made on neutron stars]
We don't know the equation of state of a neutron star; most viable models have substantial layering and an impressively busy energy-density flow. Most models predict neutron stars to be very bright in neutrinos, although the brightness is typically greatest at the outer layers rather than from the deeper ones where one would expect a superfluid sea of degenerate neutrons to dominate almost to the exclusion of everything else (e.g. there may be small numbers of superconducting protons floating around in it), and mechanisms which produce the Pauli Exclusion Principle are non-negligible. Possible mechanisms are somewhat constrained. How the density profile goes below the superfluid layer is I think mainly speculation; there are BH-remnant-related reasons to hope for a lattice crystal or some block-like structure of subatomic particles forming an inner core.
JM Lattimer has a recent (2013) paper: http://arxiv.org/abs/1305.3510
to be pedantic, the "lots of gluons and q-antiq pairs" is the strong nuclear force in action
Also, you're talking about virtual particles here, it's acceptable to (and usually this is done) ignore them when describing the "contents" of a particle since they're everywhere (in less concentration perhaps) anyway.
But yeah, those particles are there.
There's no antiquark in the usual three-quark simplified description of a proton, yet without antiquarks how does one explain the charged leptons and anti-leptons readily observed following high-energy proton collisions?
Are we dancing around a question like, "what really happens close to the IR fixed point"?
You have this behavior outside of a hadron too, just that you're more likely to see a quark-antiquark pair inside a hadron than outside. The sea quarks within the hadron are still virtual particles, though.
And often virtual particles aren't considered to be there at all, after all, from that logic one might say that a proton contains all particles (it does, since there always is a possibility of a virtual particle pair of any type springing up). Of course, the possibility of the quarks coming up is more likely since they can be obtained from a gluon, but they're still virtual.
We're just dancing around the question "should we count virtual particles when talking about the contents?". The answer usually depends on context. So it's completely ok to say that a proton is just three quarks, since those are the only real particles in it.
It's a tragedy that he has left full-time theoretical physics for greener pastures as a result of the huge (as in double-digit percentages, and in some cases more than a third) cuts in research budgets in the USA, coupled with his desire to remain working in the States.
Unfortunately there are quite a few good working-scientist bloggers who have been producing much reduced output as a result of lower funding in general. Some have moved out of the USA and continue to blog. Some have had to focus themselves on earning survival income by pushing out many grant proposals in hopes of even partial funding, and simply cannot afford to take the time to explain other scientsists' work to non-scientists. Some have, alas, mostly stopped blogging altogether.
They're all perturbations in fields, and I think you could consider all the various ways particles can combine and split as being in some sense evidence of an underlying sameness to them. That's what the whole idea of grand unification was.
Wikipedia's take on it is here. The last two or three paragraphs before "See also" are provocative.
Nuclear physics are enormously complicated and the simplified picture usually presented of the Standard Model (in charts showing 3-quark particles) is a compromise between getting across the idea of a composite particle like a proton and digging into why a proton is an extremely messy thing filled with pions and gluons. The latter is where the symmetry groups of the Standard Model lead you, and down that path lies enormous many-body problems. Those are almost always irrelevant at the scale of chemistry, nuclear engineering, or even most of astrophysics, where the simplification is good enough. Indeed, most of the time quarks and gluons don't even arise in the solutions to problems in those fields.
Additionally, at that scale one starts running into some problems in the Standard Model which are essentially irrelevant in useful effective field theories, and which are simply not well enough understood by working scientists in the field to boil down to something useful for teaching except at the graduate level. (Well, as long as you believe in the principle that to explain something to a child, or even an adult non-specialist, you have to understand it extremely well or sacrifice accuracy.)
https://en.wikipedia.org/wiki/Lie-to-children
Is this stable for some reasonable length of time, or is it one of those "particles" with a lifetime in nanoseconds?
This sentence doesn't make any sense to me. Could you please elaborate?
If someone were simply to slam 4 neutrons together, for how long would they need to stay together for this to be deemed a real particle? I think it's for this reason the original commenter quoted the use of the term "particle".
(Positronium is often called the zeroeth element in pop sci)