Here's a question I was always curious to hear a good intuitive explanation for --
Why does a neutron star not decay, as it is composed of neutrons, and free neutrons should decay in 15 minutes?
Is it because the neutrons are in an energetic "well" and to decay out would actually require energy? In collapsing under gravity to neutron degeneracy, did the neutrons say, radiate away their ability to decay any more?
Well that's the decay rate for a "bare" neutron. Neutrons in atoms obviously last longer than that. And the neutrons in a neutron star are all smashed up next to each other, kinda like a star-sized atom.
The neutron would have to emit an electron (and an antineutrino). A neutron star does have electrons (and protons) in it, to the extent that the electrons have filled all energy levels into which a neutron decay electron could go. In other words, the decay is prevented by the presence of these electrons and the Pauli exclusion principle.
It seems there are several things going on. Protons absorbing electrons becoming neutrons doing the opposite of decay, a sort of connective cycle that radiates away neutrinos while enabling decay and reverse decay, and plain old Pauli exclusion where there is no room for electrons.
But it all comes down to lots of things being possible in such a crowded place and neutrons not being “free” in a neutron star. Nuclear chemistry is full of crazy stuff happening constantly in stable situations that all kind of balances out.
A free neutron decays spontaneously into a proton, electron and neutrino, because decaying provides energy, because the mass of a neutron is higher than the sum of the masses of the decay products.
A free proton does not decay because none of the possible decay modes can produce particles with a lesser mass.
This is the same reason why your body does not fragment spontaneously in separate parts, but some external energy is required for that, e.g. someone wielding a meat cleaver.
The neutrons forming a neutron star are bound together by the gravitational force. When the neutron star has formed, the energy equal to the binding energy has been lost, so the average mass of a neutron in a neutron star, i.e. the mass of the star divided by the number of neutrons, is less than the mass of a free neutron and it is also less than the mass of a free proton and even less than the average mass of a nucleon inside the nucleus with the highest binding energy (iron 56).
Otherwise the star would have remained composed of ordinary nuclei instead of becoming a neutron star.
To extract a free neutron from a neutron star you must provide an energy at least as large as corresponding to the difference in mass between a free neutron and the mass of a neutron bound in the neutron star.
To make it "decay" (of course, that is not decay, because it is not spontaneous) while remaining in the neutron star, you need to provide some lower energy, which could convert a neutron into a proton, electron and neutrino, creating an excited state of the star, like an excited state of a nucleus or atom. Soon after that, the difference in energy will be radiated, either when the proton and electron would recombine again, or the proton will spontaneously decay into a neutron and a positron (which will later annihilate with the electron).
So a neutron star should behave like any other bound system. The state with the lowest energy is the state when all the nucleons are neutrons, unlike the state with the lowest energy of an ordinary nucleus, where a part of the nucleons must be protons.
This being the state with the lowest energy, no decay processes can exist. External energy can produce excited states, where a few protons, electrons and positrons may exist, but these other particles will decay, combine or annihilate, so the base state will be reached again.
The same happens with atomic nuclei, which are bound by strong nuclear forces instead of gravitational forces. The neutrons in stable nuclei or in nuclei with excess protons do not decay. On the contrary, the protons in nuclei with excess protons over the corresponding stable nucleus decay into neutrons and positrons (or they capture electrons).
So a neutron star behaves in the same way as a nucleus where the state with the lowest energy happens to be the one with no protons.
The question I want answered is not why a neutron takes so long to decay - that seems understandable as it's mitigated by the weak force - but why is that time of ~14.63 minutes the actual time it is?
When the W- boson decays into an electron and antineutrino it happens millions of times faster than the life of the neutron itself. What makes that trigger point happen when it does?
Generally decay rates scale with the energy release (Q) which is relatively small here compared to the W decay. Decay is a tunneling process so less Q generally means a slower decay since there are fewer final states available (but the details are complicated).
Fermi's Golden Rule in principle allows you to calculate the decay rate. In practice we dont know how to calculate all the relevant quantities since QCD is hard.
All things decay and have half life time. I don't get what's so mysterious about it. Neutrons have some not well understood structure and that structure is unstable in the dangerous waters of quantum turbulence.
> [isolated neutrons] decay into protons. During the process, each decaying neutron emits an electron and an antineutrino.
> ...detected sparks of light each time a neutron decayed.
Detecting a spark of light would also require photon(s) to be emitted, right? Is this not called out because it is a byproduct of the decay and not part of the decay reaction itself?
As the article says:” The team kept neutrons in the bottle for periods of between 20 seconds and nearly half an hour, and detected sparks of light each time a neutron decayed”
The decay products of neutrons is a proton, and electron and electron antineutrino. Light emissions would most likely be in the from acceleration or impingement of charge particles in the “bottle” magnetic field.
Clearly the article claims they are detecting “sparks”.
The reason the photons are not part of the primary emission is related to the momentum / energy balance in the decay. There are other conserved quantities such as lepton number.
As you say the light is a byproduct of the acceleration/ detections scheme of the charged particles emitted.
It seems like the difference in lifetimes demands new physics. If the magnetically trapped neutron lifetimes match Standard Model predictions, then something involved in getting them into the beam must be changing them, or selecting out longer-lived individuals, both of which seem bonkers.
There is probably a Nobel for whoever solves this.
Capturing some from a beam into a magnetic trap seems like a good start.
The most-likely explanation for the discrepancy between the methods is systematic uncertainty. I think it will be a rather spectacular surprise if the refined beam measurements continue to disagree with the bottle measurements a decade from now.
If the discrepancy persists, then there really will be a problem.
There is plenty of theoretical speculation, especially in the last couple of years, about what such a discrepancy, if true, might mean. Again, it would be a real surprise, but if neutrons have another decay pathway other than the known one, then the bottle method measures the total decay rate, while the beam method measures the electron/beta-decay rate only.
The emerging state of the art, should this discrepancy persist, will become experiments that measure both channels simultaneously.
As a physics layman my first thought when reading that is special relativity: things happen more slowly (ie. decay) the faster something is moving. Not sure how fast the beams are moving though.
Stupid question, do they account for speed dilation on the beam experiment? The difference in lifetime will translate to 50k km/s speed of neutrons or roughly 1/6c
From skimming the review paper from OP (the "source:" in the caption on that error-bar chart), the neutrons in the beam experiments are thermalized, to a mean velocity of ~2,200 m/s. So, slower than 1e-5 c.
(Thermal meaning the neutrons scatter lots of times against atoms in a solid material, until they reach thermal equilibrium. ~km/s is a typical Boltzmann velocity for atom-size things at room temperature).
supernova87a|4 years ago
Why does a neutron star not decay, as it is composed of neutrons, and free neutrons should decay in 15 minutes?
Is it because the neutrons are in an energetic "well" and to decay out would actually require energy? In collapsing under gravity to neutron degeneracy, did the neutrons say, radiate away their ability to decay any more?
kadoban|4 years ago
sp332|4 years ago
pfdietz|4 years ago
colechristensen|4 years ago
But it all comes down to lots of things being possible in such a crowded place and neutrons not being “free” in a neutron star. Nuclear chemistry is full of crazy stuff happening constantly in stable situations that all kind of balances out.
cozzyd|4 years ago
adrian_b|4 years ago
A free neutron decays spontaneously into a proton, electron and neutrino, because decaying provides energy, because the mass of a neutron is higher than the sum of the masses of the decay products.
A free proton does not decay because none of the possible decay modes can produce particles with a lesser mass.
This is the same reason why your body does not fragment spontaneously in separate parts, but some external energy is required for that, e.g. someone wielding a meat cleaver.
The neutrons forming a neutron star are bound together by the gravitational force. When the neutron star has formed, the energy equal to the binding energy has been lost, so the average mass of a neutron in a neutron star, i.e. the mass of the star divided by the number of neutrons, is less than the mass of a free neutron and it is also less than the mass of a free proton and even less than the average mass of a nucleon inside the nucleus with the highest binding energy (iron 56).
Otherwise the star would have remained composed of ordinary nuclei instead of becoming a neutron star.
To extract a free neutron from a neutron star you must provide an energy at least as large as corresponding to the difference in mass between a free neutron and the mass of a neutron bound in the neutron star.
To make it "decay" (of course, that is not decay, because it is not spontaneous) while remaining in the neutron star, you need to provide some lower energy, which could convert a neutron into a proton, electron and neutrino, creating an excited state of the star, like an excited state of a nucleus or atom. Soon after that, the difference in energy will be radiated, either when the proton and electron would recombine again, or the proton will spontaneously decay into a neutron and a positron (which will later annihilate with the electron).
So a neutron star should behave like any other bound system. The state with the lowest energy is the state when all the nucleons are neutrons, unlike the state with the lowest energy of an ordinary nucleus, where a part of the nucleons must be protons.
This being the state with the lowest energy, no decay processes can exist. External energy can produce excited states, where a few protons, electrons and positrons may exist, but these other particles will decay, combine or annihilate, so the base state will be reached again.
The same happens with atomic nuclei, which are bound by strong nuclear forces instead of gravitational forces. The neutrons in stable nuclei or in nuclei with excess protons do not decay. On the contrary, the protons in nuclei with excess protons over the corresponding stable nucleus decay into neutrons and positrons (or they capture electrons).
So a neutron star behaves in the same way as a nucleus where the state with the lowest energy happens to be the one with no protons.
hilbert42|4 years ago
When the W- boson decays into an electron and antineutrino it happens millions of times faster than the life of the neutron itself. What makes that trigger point happen when it does?
cozzyd|4 years ago
Fermi's Golden Rule in principle allows you to calculate the decay rate. In practice we dont know how to calculate all the relevant quantities since QCD is hard.
wetpaws|4 years ago
akomtu|4 years ago
savant_penguin|4 years ago
So you can control neutrons with magnetic fields??
illini1|4 years ago
pfdietz|4 years ago
liquidise|4 years ago
> ...detected sparks of light each time a neutron decayed.
Detecting a spark of light would also require photon(s) to be emitted, right? Is this not called out because it is a byproduct of the decay and not part of the decay reaction itself?
sharikous|4 years ago
They let the neutrons decay for a while and then measure all the remaining neutrons in the trap by lowering there a detector.
This detector has a scintillator so when a neutron is captured it emits some photons and those are converted to an electric signal.
So no. There is no requirement for the neutron to emit an additional photon during the decay that is measured
unknown|4 years ago
[deleted]
omgJustTest|4 years ago
The decay products of neutrons is a proton, and electron and electron antineutrino. Light emissions would most likely be in the from acceleration or impingement of charge particles in the “bottle” magnetic field.
Clearly the article claims they are detecting “sparks”.
The reason the photons are not part of the primary emission is related to the momentum / energy balance in the decay. There are other conserved quantities such as lepton number.
As you say the light is a byproduct of the acceleration/ detections scheme of the charged particles emitted.
LatteLazy|4 years ago
adrian_b|4 years ago
https://arxiv.org/abs/2106.10375
davidhyde|4 years ago
Isn’t this due to relativistic effects? What percentage of the speed of light are these beams?
fsh|4 years ago
ncmncm|4 years ago
There is probably a Nobel for whoever solves this.
Capturing some from a beam into a magnetic trap seems like a good start.
ISL|4 years ago
If the discrepancy persists, then there really will be a problem.
There is plenty of theoretical speculation, especially in the last couple of years, about what such a discrepancy, if true, might mean. Again, it would be a real surprise, but if neutrons have another decay pathway other than the known one, then the bottle method measures the total decay rate, while the beam method measures the electron/beta-decay rate only.
The emerging state of the art, should this discrepancy persist, will become experiments that measure both channels simultaneously.
melbourne_mat|4 years ago
thriftwy|4 years ago
perihelions|4 years ago
https://doi.org/10.3390/atoms6040070
(Thermal meaning the neutrons scatter lots of times against atoms in a solid material, until they reach thermal equilibrium. ~km/s is a typical Boltzmann velocity for atom-size things at room temperature).
(Not a domain expert).
unknown|4 years ago
[deleted]
iamtedd|4 years ago
I expected an almost comical consideration of relativity and the locations he lived.
I'm a little disappointed that wasn't the case.