I've read about this topic before and when I saw the link I hoped that there was news, but there isn't. If you count how many neutrons remain, you get a different answer than if you count how many protons are created, the effect is a good deal larger than the accuracy of the measurements, and it seems the logical answer is that sometimes a neutron decays into ... something else. That would be new physics and would be extremely cool. We still have no idea what dark matter is, it could be the first real lead.
Timely for me - I've been down the rabbit hole lately of understanding how the elements are formed and got as far as stellar nucleosynthesis, s-process which involves neutron capture making a heavier isotope and then apparently the neutron turns into a proton through a process called beta decay. (So I'm all about the lifetime of neutrons right now!)
This was what I was watching most recently .. Nucleosynthesis the origin of elements in our Solar System Jim Connelly University https://www.youtube.com/watch?v=U6tFWA9FyGE
"according to the photons frame of reference, Heeck found that its lifetime would be a rather short three years; however, from our frame of reference, light would live about one billion billion (10^18) years"
Note that this article is assuming, for purposes of argument, that photons actually have mass, equal to the current upper limit for possible photon mass based on experiments. But the experiments are all consistent with the current theoretical belief that photons have zero mass; and if they have zero mass, the concept of "lifetime" for a photon (and indeed the concept of "photons frame of reference") is not even well-defined.
> If they are moving with the speed of light no time passes for photons
No, the concept of "time passing" for an object moving at the speed of light has no meaning. That's why "lifetime" has no meaning for a photon if it has zero mass.
No, because the rules of Lorenz invariant field theories mean that the infinity you’re (presumably) expecting is mapped, via the joy of hyperbolic geometry, onto the propagation speed of that field.
Or, to put it another way, the speed of light is where time dilation and length contraction run into each other and it all goes zero-divided-by-zero.
> Wouldn't c have to be infinite for "no time to pass"
You're using a different notion of "time passing". Yes, we, observing photons, certainly observe them to take a finite time to cover a finite distance. But that is not the same as the concept of "time passing" for the photons themselves, for example according to a clock that the photons carried along with them; that is the concept that has no meaning.
If the photons were a conscious subject, would that mean, from their point of view, they are everywhere (everywhere being defined as the entire path the photon will take from its genesis to its final destruction or absorption), all at once? Because if no time is passing for them, doesn't that mean no space is being traversed either?
The flip side of time dilation is Lorentz contraction. As you approach the speed of light objects in your direction of motion will become shortened from your frame of reference.
For example, there are particles from cosmic rays that should not be able to make it to the surface without decaying. However, they're detected all the time. Two valid ways to think of this are:
1. From the Earth's frame of reference time moves more slowly for the particle. This slows down the process of decaying.
2. From the particle's frame of reference the Earth's atmosphere is considerably shorter so it doesn't need to travel as far.
Things get a bit hairy to talk about once you actually reach the speed of light. One way to think of it might be from the photon's frame of reference its entire path has become infinitely short so it had no distance to travel at all.
> One way to think of it might be from the photon's frame of reference its entire path has become infinitely short so it had no distance to travel at all.
From the photons frame of reference, then, they do not move at all?
And the environment that photon "experiences", being the path in the universe that it traverses from our point of view; is the past, present, and future (from our point of view) all in instant simultaneity for the photon?
Unfortunately when talking about physics, sloppy human languages and our tendency to anthropomorphize when describing very non-human-like things cause a lot of communication/learning problems. Concepts like "experiencing an environment" and "time" don't make sense for the photon, which is sort of equivalent to "moving at c" because experiencing something like "time" requires interactions (events) at different places in spacetime.
Saying "neutrino has a very small mass" is roughly equivalent to saying "neutrinos very rarely experience an oscillation event (changing into a different flavor)". The distance between the rare events is the "time" it experiences. These are so far apart in spacetime for the neutrino it's experience of time (the way it evolves over spacetime) is extremely slow. More massive particles are "more massive" because they frequently interact with the Higgs field. More interaction events means their experience of time happens faster.
The photon (and anything else with 0 mass) only experiences two events: it's creation and destruction. It moves at c because it's never being slowed down by experiencing interactions.
For a very good explanation of this (with helpful animations) this[1] short playlist (6 ep) of PBS Spacetime episodes.
edit: TL;DR - When speed-of-light particles pause to interact with things (thus moving < c)slowing it down), we say that particle "has mass". Mass is a measure of how frequently those interactions occur (aka how much "time" it experiences).
> The photon (and anything else with 0 mass) only experiences two events: it's creation and destruction.
This is not correct. The worldline of a photon contains events between its creation and its destruction. The spacetime interval between any such pair of events is zero, but that does not mean the events aren't there.
If you flash a laser at a detector on the Moon, there is a whole continuum of events between the source (the laser) and the destination (the detector on the Moon). The spacetime interval between the source and destination events is zero, but there is still a whole continuum of events between them (all the events the photon passes through between the source and the detector).
> From the photons frame of reference, then, they do not move at all?
There is no such thing as "the photons frame of reference". It is not even a well-defined concept.
> And the environment that photon "experiences", being the path in the universe that it traverses from our point of view; is the past, present, and future (from our point of view) all in instant simultaneity for the photon?
No, none of this is correct. The reason I keep insisting that all these concepts are not well-defined for a photon is to make it clear why you cannot draw all these inferences that you are trying to draw--they are all wrong. The only way to stop drawing them is to recognize what "not well-defined" means. It means the questions you are trying to ask about photons are meaningless; they are like asking how long the color red is or how much time passes for it. Photons are simply not in the category of things for which those questions make sense.
Every single comment you've written on this subject closes all discussion on it. So what is there to discuss? Or shall I remain silent in the ignorant darkness of my status as a (physics) layman?
Even Einstein inserted an anthropomorphized frame of reference into the thought experiments of the celestial objects he contemplated, to explain to others. If we can't ask questions about photons in relation to spacetime as we know it, then what questions can we ask? We certainly don't know spacetime as photons "know" it, since its not "well defined."
Interestingly still, you use the term worldline to describe the totality of the temporal-spatial existence of a photon, so clearly there are concessions to be made and all this is more defined than you assert... and you're lightly reproving us for not using the same dictionary as you...
I'm interested in this subject but not interested enough (or rather, have the time and energy) to become a physicist to understand, if that's what you're going to ultimately suggest to get even a dim, but accurate understanding of the nature of light.
> Every single comment you've written on this subject closes all discussion on it
I have done no such thing. I have only closed discussion based on a fallacious premise. If you drop the fallacious premise there is plenty to discuss.
> We certainly don't know spacetime as photons "know" it, since its not "well defined."
I did not say spacetime is not well-defined. Spacetime is not "as photons know it" or as any observer "knows" it. Spacetime is the underlying geometric entity; it requires no "point of view" to exist, or even to be described; you can describe spacetime without ever using inertial frames, which are what your "points of view" actually are.
> you use the term worldline to describe the totality of the temporal-spatial existence of a photon
That's because "worldline" is the standard physics term for it, as used in physics textbooks and peer-reviewed papers.
> you're lightly reproving us for not using the same dictionary as you.
If you want to discuss physics, it helps a lot to use the standard language of physics.
> I'm interested in this subject but not interested enough (or rather, have the time and energy) to become a physicist to understand
You don't have to become a physicist. But you do have to be willing to drop fallacious premises.
Just note that however you try to define "the photon's frame of reference" you end up with a division by zero, so the definition becomes meaningless.
Eg distance traveled is 0 due to length contraction. Time taken is 0 due to time dilation. So what's its velocity? 0/0 = undefined, the question is meaningless. But the velocity is arguably more fundamendal than the distance travelled or time taken, so those aren't truly 0. They're also undefined quantities.
> One way to think of it might be from the photon's frame of reference its entire path has become infinitely short so it had no distance to travel at all.
No, that is not a correct way to think of it. See my other responses to the poster you responded to.
They can't be. You can't make a conscious subject out of something that has zero mass and moves at the speed of light, and therefore does not have a well-defined concept of "time passing".
> if no time is passing for them, doesn't that mean no space is being traversed either?
It is not the case that no time is passing for a photon. What is the case is that the concept of "time passing for a photon" is not well-defined. That concept not being well-defined means that "space being traversed for a photon" is not well-defined either. That does not mean "no space is being traversed". It means the concept of "space being traversed for a photon" is not well-defined.
No, outside observer will measure clocks of photon run slower by a factor of γ (Lorentz factor), which (in scalar form) is equal to 1/sqrt(1 - v^2 / c^2 ). It diverges at v = c.
> outside observer will measure clocks of photon run slower by a factor of γ
This is not correct since "clocks of photon" is a meaningless concept (at least if photons have zero mass, which they do according to our best current models).
> I did not mean to imply that you can attach a clock on a photon.
The issue isn't that you can't attach a clock to a photon in a practical sense. The issue is that even in principle, the concept of "the clock of a photon" is not well-defined. For it to be well-defined, there would need to be an inertial frame in which a photon was at rest. But that is impossible.
> Speed of light is for-all-practical-purposes infinite in the reference frame of the moving object (i.e. photon), thanks to special relativity.
This is not correct, because there is no "reference frame" for a photon moving at the speed of light. The concept of "reference frame" is not well-defined for such objects.
This conversation sounds like it's running into the linguistic barrier that we tend to think of "speed of light" in terms of, well, light, so becomes recursive when talking about photons with mass, which naturally travel slower than what should be thought as "the speed of massless particles".
This according to measured upper limits on photon mass. Strong theory postulates absolute zero mass for the fundamental particle and in this case it will have an infinite lifetime.
Neutrons with low enough kinetic energy (< 300 neV) are reflected by some materials due to the so-called Fermi pseudo potential. These ultracold neutrons can therefore be stored in an actual bottle made from an appropriate material (such as Nickel or Aluminum).
Another approach is to use magnetic fields to create a potential well (neutrons have a magnetic moment) and to rely on gravity to keep them in that well. Ultracold neutrons behave very much like ping-pong balls and cannot jump very high (less than a few meters) due to their low velocities.
are reflected by some materials due to the so-called Fermi pseudo potential
Cool! I learned something new today!
Ultracold neutrons behave very much like ping-pong balls and cannot jump very high
Most of the time. My understanding is that they can tunnel potential barriers, but this isn't that likely to happen in these traps, so it shouldn't affect the results.
Anything can tunnel through a potential barrier, the question is how big the barrier is, and what probabilities you expect from this event. When the barrier is big enough, it basically doesn't happen.
No. The gross structure of an atom is only determined by the electrostatic interaction between the charge of the electrons and of the protons in the nucleus. The interaction between the magnetic moments of the electrons and of the nucleus (which is due to both protons and neutrons) leads to hyperfine structure on a much smaller energy scale.
> if neutron's are of the opposite sign, is this what keeps the electrons repelled enough stay on the "outside" in orbit?
No, but it does affect which spin states of nuclei have the lowest energy. For example, the ground state (lowest energy state) of a deuterium nucleus has spin 1--spin of the proton and neutron are parallel--because the opposite signs of the proton and neutron magnetic moments make that state have a lower energy (more tightly bound).
The sign refers to the direction that the vector points relative to the particle's spin vector. The proton's magnetic moment is positive, meaning it points in the same direction as the proton's spin. The neutron's magnetic moment is negative, meaning it points in the opposite direction to the neutron's spin.
The magnet inside your compass isn't charged either. Magnetic fields act on magnetic moments, not on charge (unless the charge is moving which gives you a Lorentz force).
> (unless the charge is moving which gives you a Lorentz force)
Isn't this always the case, though? I mean, two objects not moving relative to each other is a very specific state and not something that can be held in practice without a negative feedback loop on their relative velocities.
Because that's a lower energy state. The deuteron binding energy is 2.2 MeV, i.e. the mass of a deuteron is 2.2 MeV below the sum of the masses of a proton + neutron. If the neutron inside would decay into a proton, you would have a diproton (^2He). Very unstable, essentially no binding energy, so it's mass is very close to 2 protons, which is still above the deuteron mass. So the neutron can not decay spontaneously. There is not enough energy available.
>Spontaneous processes such as neutron decay require that the final state is lower in energy than the initial state. In (stable) nuclei, this is not the case, because the energy you gain from the neutron decay is lower than the energy it costs you to have an additional proton in the core. https://physics.stackexchange.com/questions/9098/how-come-ne...
An extremely handwavy explanation would be that they do not: the neutron decays into a proton and other stuff. That other stuff is captured by a proton that becomes a neutron. This is borderline metaphoric, but it gives a gist of the physics.
I think it's kinda like an intermittent bug in some application. You don't even know if it's an actual problem or just a user with a bad internet connection. That is, until you find out that it only happens after a period of inactivity.
Suddenly you are "on the way to pinning this down." Instead of "Flailing madly in the dark with wild theories."
I think it's kinda like an intermittent bug in some application.
Like a "stable" civilization?
Sometimes I wonder if clouds of autonomous self replicators, such as our planet and our civilization, aren't viewed as atavistic exponential pests by godlike alien beings who control the entire resources of a civilization in one entity.
