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Physicists make most precise measurement of neutron’s lifetime
- liquidise 5y ago> [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?
- LatteLazy 5y agoYes, the decay will also release some energy in the form of photons but they didn't mention it.
- sharikous 5y agoThat's inaccurate, please see my reply to the same comment
- a1369209993 5y agoNeutron decay does not produce photons. Rather, the energy difference is carried as kinetic energy by the resulting proton, electron and (anti)neutrino. This can cause light to be emitted (very shortly) later when those particles slam into things, but as sharikous notes that doesn't seem to be they're talking about here.
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- sharikous 5y agoNo, they don't measure decays. 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
- omgJustTest 5y agoAs 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.
- thriftwy 5y agoStupid 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
- perihelions 5y agoFrom 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. https://doi.org/10.3390/atoms6040070 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).
- kzrdude 5y agoHow does the scattering affect the neutrons? When do we start the clock for their lifetime anyway? It sounds like they could absorb + reemit sometimes when being scattered.
- cozzyd 5y agoIt doesn't matter when you start the clock, just that you count at the beginning and end. I doubt scattering plays a big role here at these kinetic energies.
- gus_massa 5y agoIt's one of the nice properties of exponential decay. They are measuring the mean time of life (~15 minutes) but it's easier to explain with half life (~10 minutes). If you have a bunch of neutrons and put them in a box, and look again 10 minutes later, you will see that you have only half of them. If pick all the neutrons that survived for 3 minutes, and put them in a box, and look again 10 minutes later, you will see that you have only half of the neutrons that survived for 3 minutes. If pick all the neutrons that survived for 7 minutes, and put them in a box, and look again 10 minutes later, you will see that you have only half of the neutrons that survived for 7 minutes. If pick all the neutrons that survived for 42 minutes, and put them in a box, and look again 10 minutes later, you will see that you have only half of the neutrons that survived for 42 minutes. When the time is too long, you need to create a really big number of neutrons initially, so enough survive until you start the experiment. So ... the waiting time until the experiment start doesn't matter. It's easier to understand with a discrete model with coins. You have perfectly balanced coins with 50% chance of head and 50% chance of tail. Each minute you flip all the coins at the same time and remove all the "heads". So you can repeat this, and each time you have less coins. You start the experiment, flip the coins 10 times (and remove the heads), and you get 1000 coins that survived. How many additional times should you flip them to remove half of them and have only 500?
- davidhyde 5y ago> Neutrons in beams seem to live longer on average Isn’t this due to relativistic effects? What percentage of the speed of light are these beams?
- fsh 5y agoNeutron lifetime experiments are done with cold beams (tens of K). Time dilation is completely negligible.
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- supernova87a 5y agoHere'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?
- kadoban 5y agoThis is the best answer I've seen: https://physics.stackexchange.com/a/63387 https://physics.stackexchange.com/a/63387 which to my layman eyes basically seems to match your theory.
- savant_penguin 5y agoI'll pretend I understood what the answer says about there not being enough energy and boldly ask: what if the neutron star is spinning _really_ fast?
- jfoutz 5y agoYou are spinning pretty fast on the side of the planet, but you don’t go flying out into space. Same deal, I’d guess as a layman.
- JumpCrisscross 5y agoDoes this mean the neutrons are continuously degrading? Their decay products continuously recombining into new neutrons? (Though that would imply "evaporation" at the surface?)
- kadoban 5y agoI _think_ not, it's just energenically unfavorable so it should ~never happen. https://en.m.wikipedia.org/wiki/Urca_process https://en.m.wikipedia.org/wiki/Urca_process this does seem to be going on though, which tbh I just don't understand so it may contradict what I just said, or not.
- hilbert42 5y agoThe 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?
- wetpaws 5y agoNobody knows
- dnautics 5y agois the annoying answer "if it were really that much shorter we probably wouldn't exist"?
- idontwantthis 5y agohttps://en.wikipedia.org/wiki/Anthropic_principle https://en.wikipedia.org/wiki/Anthropic_principle
- hilbert42 5y agoI actually heard Frank Tipler give a long talk of about 40 minutes on radio about the Anthropic principle several—perhaps even three—decades ago. At the time it was an exciting notion (and he was very animated when discussing the matter which made it all the more interesting). Then I thought his notion likely balmy or eccentric, now I've no opinion as my brain tends to overheat whenever I think about it. ;-)
- kjeetgill 5y agoIt's like how every number n has the factors 1 x n. It's always _an answer_, but never the juicy one.
- hilbert42 5y ago"...much shorter we probably wouldn't exist" Reckon that stands to reason, same goes for the physical constants, c, µ, ε, α, etc. But why remains the question.
- savant_penguin 5y ago"Magnetic fields at the bottom of the bottle prevented the neutrons from touching the surface," So you can control neutrons with magnetic fields??
- ncmncm 5y agoIt 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.
- melbourne_mat 5y agoAs 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.
- ncmncm 5y agoIn a word, slowly. You may reasonably assume that particle physicists are familiar with relativistic time dilation, and how to adjust for it if that were needed. That is not a universally reasonable assumption. Cosmologists apparently never really tried adjusting their expectations for galaxy rotation for general relativity. When a plasma fluid dynamicist facile with the maths looked, the rotation anomaly evaporated. (Cosmologists were assiduously ignoring this, last I checked.) GR maths are considered hard, except by plasma fluid dynamicists, who have to do actually hard maths just to graduate.
- ISL 5y agoThe 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.
- iamtedd 5y agoI read the title as "most precise measurement of Newton's lifetime". I expected an almost comical consideration of relativity and the locations he lived. I'm a little disappointed that wasn't the case.
- adrian_b 5y agoThe research article on Arxiv: https://arxiv.org/abs/2106.10375 https://arxiv.org/abs/2106.10375