
How long do neutrons live? Physicists close in on decades-old puzzle - okket
https://www.nature.com/articles/d41586-019-01203-9
======
not2b
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.

------
schemathings
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!)

[https://en.wikipedia.org/wiki/S-process](https://en.wikipedia.org/wiki/S-process)

~~~
kwoff
[http://pdg.lbl.gov/2018/reviews/contents_sports.html](http://pdg.lbl.gov/2018/reviews/contents_sports.html)

~~~
schemathings
Maybe next "semester"! :) I love the misleading URL - those particle
physicists have a sense of humor.

~~~
kwoff
If you haven't seen this series:
[https://archive.org/details/The_Mechanical_Universe_and_Beyo...](https://archive.org/details/The_Mechanical_Universe_and_Beyond_51_Atoms_to_Quarks)
(it's a bit outdated, but the part around 23 minutes is still mind-blowing to
me, especially in the context of the video)

~~~
schemathings
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](https://www.youtube.com/watch?v=U6tFWA9FyGE)

~~~
kwoff
Thanks, I'll watch those. I think you might also like
[https://www.youtube.com/watch?v=0_ldatF3-aI](https://www.youtube.com/watch?v=0_ldatF3-aI)

------
waveforms
This made me curious how long a photon lives.

"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"

[https://futurism.com/science-explained-long-can-photons-
live...](https://futurism.com/science-explained-long-can-photons-live-will-
ever-die)

~~~
pdonis
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.

~~~
tinus_hn
If they are moving with the speed of light no time passes for photons so
‘lifetime’ has no real meaning either.

~~~
pdonis
_> 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.

~~~
omarchowdhury
Wouldn't c have to be infinite for "no time to pass", since light is still
traversing space at a finite speed?

~~~
pdonis
_> 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.

~~~
omarchowdhury
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?

~~~
etaoins
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.

~~~
omarchowdhury
> 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?

~~~
pdkl95
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.

[1]
[https://www.youtube.com/playlist?list=PLsPUh22kYmNCLrXgf8e6n...](https://www.youtube.com/playlist?list=PLsPUh22kYmNCLrXgf8e6nC_xEzxdx4nmY)

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).

~~~
pdonis
_> 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.

~~~
pdkl95
That's interesting... could you provide an example of such an event?

~~~
pdonis
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).

------
phkahler
How does one contain neutrons in a "bottle"?

~~~
SiempreViernes
Magnetic fields and lots of cooling.

[https://www.nature.com/articles/47444](https://www.nature.com/articles/47444)

~~~
ben509
Ah, but how do you capture a neutral particle with a magnetic field?

(As I understand, they're a _little_ bit magnetic because their constituent
quarks are charged.)

~~~
pdonis
_> As I understand, they're a little bit magnetic because their constituent
quarks are charged._

More than a little bit. The neutron's magnetic moment is about 2/3 the
magnitude of the proton's, and of opposite sign.

~~~
hanniabu
Hmmm, if neutron's are of the opposite sign, is this what keeps the electrons
repelled enough stay on the "outside" in orbit?

~~~
fsh
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.

------
craftyguy
Why do neutrons bound with a proton 'live' substantially longer?

~~~
davrosthedalek
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.

------
eruci
“We’re on the way to pinning this down,” ok, but that can be said about
anything we haven't pinned down yet

~~~
foxyv
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."

~~~
stcredzero
_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.

