neutrino question

For the discussion of the sciences. Physics problems, chemistry equations, biology weirdness, it all goes here.

Moderators: gmalivuk, Moderators General, Prelates

p1t1o
Posts: 858
Joined: Wed Nov 10, 2010 4:32 pm UTC
Location: London, UK

neutrino question

Postby p1t1o » Fri May 04, 2018 1:16 pm UTC

First, neutrinos are certainly not my field.

I just read that neutrinos come in 3 "flavours" each flavour having a slightly different mass, and that neutrinos can spontaneously convert between flavours in flight.

This must mean that they change mass, which to me raises conservation-of-energy red flags?

Whaddup with this?

I wont understand an answer that involves too many quantum coefficients or has the word "spin" in it too many times, but is this an understood process? Is the mass really changing or is it some virtual thing? If it is, where is the energy coming from to increase mass, and where does it go when it loses mass? Do they speed up and slow down? How could that work, physically? If I knew a little bit more about the specific parts of quantum physics that control this, would I regard these as silly questions?

User avatar
gmalivuk
GNU Terry Pratchett
Posts: 26413
Joined: Wed Feb 28, 2007 6:02 pm UTC
Location: Here and There
Contact:

Re: neutrino question

Postby gmalivuk » Fri May 04, 2018 1:41 pm UTC

There's a lot more to it than I understand, but in general it's good to remember that it's not mass that's conserved, but mass+energy.
Unless stated otherwise, I do not care whether a statement, by itself, constitutes a persuasive political argument. I care whether it's true.
---
If this post has math that doesn't work for you, use TeX the World for Firefox or Chrome

(he/him/his)

User avatar
doogly
Dr. The Juggernaut of Touching Himself
Posts: 5393
Joined: Mon Oct 23, 2006 2:31 am UTC
Location: Somerville, MA
Contact:

Re: neutrino question

Postby doogly » Fri May 04, 2018 1:44 pm UTC

Yeah, you transition to something heavier you slow down, to something lighter you speed up, no problems.
LE4dGOLEM: What's a Doug?
Noc: A larval Doogly. They grow the tail and stinger upon reaching adulthood.

Keep waggling your butt brows Brothers.
Or; Is that your eye butthairs?

Raidri
Posts: 45
Joined: Wed Dec 21, 2016 10:39 am UTC
Location: Germany

Re: neutrino question

Postby Raidri » Fri May 04, 2018 2:11 pm UTC

Unfortunately it is more complicated. There are three flavours and three masses for neutrinos with no one-to-one correlation between flavour and mass. Every neutrino is a superposition of all three flavours and all three masses with the mixing given by the PMNS-Matrix.

And since the three neutrino masses travel at slightly different speed (as doogly mentions) the mix of flavours oscillates. You can find some explanations here and here.
Last edited by Raidri on Tue May 08, 2018 11:27 am UTC, edited 1 time in total.

p1t1o
Posts: 858
Joined: Wed Nov 10, 2010 4:32 pm UTC
Location: London, UK

Re: neutrino question

Postby p1t1o » Fri May 04, 2018 2:37 pm UTC

So mass/energy is conserved due to a change in velocity. That makes a certain sense but Im not happy with something changing speed without a reaction force - or is that way too "classical physics" for this subject?

I saw the word "superposition" there, is it perhaps a quantum weirdness thing where it "appears" to change mass but in fact its all three masses at once? Can velocity be superimposed? You dont know the velocity until you measure it...

User avatar
gmalivuk
GNU Terry Pratchett
Posts: 26413
Joined: Wed Feb 28, 2007 6:02 pm UTC
Location: Here and There
Contact:

Re: neutrino question

Postby gmalivuk » Fri May 04, 2018 3:40 pm UTC

p1t1o wrote:Im not happy with
Honestly, if you won't understand any answer that's got too much quantum in it, you're going to have to settle for remaining not happy with the explanations given.
Unless stated otherwise, I do not care whether a statement, by itself, constitutes a persuasive political argument. I care whether it's true.
---
If this post has math that doesn't work for you, use TeX the World for Firefox or Chrome

(he/him/his)

jewish_scientist
Posts: 895
Joined: Fri Feb 07, 2014 3:15 pm UTC

Re: neutrino question

Postby jewish_scientist » Fri May 04, 2018 3:51 pm UTC

p1t1o wrote:I saw the word "superposition" there, is it perhaps a quantum weirdness thing...

You know how Schoniger's cat is dead and alive at the same time? The word for that is superposition.

p1t1o
Posts: 858
Joined: Wed Nov 10, 2010 4:32 pm UTC
Location: London, UK

Re: neutrino question

Postby p1t1o » Fri May 04, 2018 4:00 pm UTC

gmalivuk wrote:
p1t1o wrote:Im not happy with
Honestly, if you won't understand any answer that's got too much quantum in it, you're going to have to settle for remaining not happy with the explanations given.


Oh I can accept it, its only one of several mind-bending things that I have heard happen in the quantum world, but I dont have to like it.
I thought perhaps my discomfort was caused by thinking too much along classical lines.
I'd even be happier just knowing that there is an answer, regardless of whether I can understand it intimately.

Are there many other examples of speed changing without an applied force in the quantum world?

***

jewish_scientist wrote:
p1t1o wrote:I saw the word "superposition" there, is it perhaps a quantum weirdness thing...

You know how Schoniger's cat is dead and alive at the same time? The word for that is superposition.


Yes I am familiar with that, what I was asking was that if the masses are superimposed like said cat, does this mean that the mass is not "really" changing? Like in the classical sense of "changing". Or if it were somehow subtly different from Schrodingers cat?

I can take *some* quantum language, my experience goes up to Bachelors-degree level of physical chemistry so I have some knowledge of orbitals, symmetry, wavefunctions and such, I think I remember the word "Hamiltonian". But it was far from my favorite subject and have not studied it since.
Last edited by p1t1o on Fri May 04, 2018 4:07 pm UTC, edited 2 times in total.

User avatar
ucim
Posts: 6361
Joined: Fri Sep 28, 2012 3:23 pm UTC
Location: The One True Thread

Re: neutrino question

Postby ucim » Fri May 04, 2018 4:02 pm UTC

But momentum is conserved also, no? How is momentum conserved when a particle "silently" changes speed? And speed measured with respect to... what frame of reference? If it can change speed, then it's not going at the speed of light, and there is a frame of reference in which the neutrino is motionless. What does it "do" in that frame of reference?

Jose
Order of the Sillies, Honoris Causam - bestowed by charlie_grumbles on NP 859 * OTTscar winner: Wordsmith - bestowed by yappobiscuts and the OTT on NP 1832 * Ecclesiastical Calendar of the Order of the Holy Contradiction * Please help addams if you can. She needs all of us.

User avatar
eSOANEM
:D
Posts: 3650
Joined: Sun Apr 12, 2009 9:39 pm UTC
Location: Grantabrycge

Re: neutrino question

Postby eSOANEM » Fri May 04, 2018 5:09 pm UTC

p1t1o wrote:
So mass/energy is conserved due to a change in velocity. That makes a certain sense but Im not happy with something changing speed without a reaction force - or is that way too "classical physics" for this subject?

I saw the word "superposition" there, is it perhaps a quantum weirdness thing where it "appears" to change mass but in fact its all three masses at once? Can velocity be superimposed? You dont know the velocity until you measure it...


Even classically, you only need a force if the momentum changes not the velocity.

Neutrinos are hella complicated and a pain in the butt for all the reasons Raidri mentions.

So, you know how neutrinos only interact with the weak force right? Well, every time a W boson spits out a neutrino it also spits out a corresponding (anti-) electron, muon, or tau meaning that, if created through charged current interactions*, the neutrino has to be created with a definite flavour. Neutrinos with definite flavour do not have a definite mass (in the same way that an sp3 electron is not definitely in either an s or p orbital, but rather a superposition of both) though.

This is important because the particle's propagator depends on its mass and this leads to different parts of the emitted neutrino's wavefunction propagating at different speeds and so splitting. When the neutrinos are received, the bit of the wavefunction with a definite mass has to collapse into a state with a definite flavour in order to interact with the detector.

So, the neutrino-space has two different relevant bases. We have the flavour eigen-basis which governs interactions, and the mass eigen-basis which governs propagation and the PMNS-matrix is the transition matrix between these two bases.

So, let's say you have a nucleus undergoing β+ decay and so producing an electron neutrino νe, in order to propagate this collapses into mass-eigenstate parts ν1, ν2, or ν3. This wavefunction propagates spreading out over the universe and the ratios of ν1, ν2, and ν3 parts varies from place to place (because they propagate at different speeds) until eventually an interaction happens at our detector. This detector, in general, will not be at a point where these ratios are the same as at the emitter and so when we convert back to the flavour eigen-basis and collapse the state onto one particular flavour eigenstate it will not necessarily come back out as a νe like it was before, but could come out as a νμ or ντ instead.

In particular, it turns out that if you consider a constant beam of neutrinos of definite flavour, the probability of them being detected as any particular flavour will vary approximately sinusoidally in the distance between the detector and emitter. In particular, νμ and ντ like to oscillate between each other with νe mostly keeping to itself. At longer ranges though, the mixing gets more complete.

*neutrinos can also be created by neutral current interactions but except for a few low cross-section interactions, these Z bosons would have to be produced by annihilation of leptons which, except at resonant energies, will massively prefer to produce photons (which cannot produce neutrinos).
my pronouns are they

Magnanimous wrote:(fuck the macrons)

User avatar
Soupspoon
You have done something you shouldn't. Or are about to.
Posts: 3384
Joined: Thu Jan 28, 2016 7:00 pm UTC
Location: 53-1

Re: neutrino question

Postby Soupspoon » Fri May 04, 2018 6:16 pm UTC

jewish_scientist wrote:
p1t1o wrote:I saw the word "superposition" there, is it perhaps a quantum weirdness thing...

You know how Schoniger's cat is dead and alive at the same time? The word for that is superposition.

Though as Schrodinger* died in 1961, and cats don't often live much beyond a decade, maybe two, I think we can assume the waveform, if not the cat, has by now collapsed. And/or oxidised.

* This keyboard tried to make me type "school dinner", I wish I could get rid of the "suggestion" buttons above it.

User avatar
doogly
Dr. The Juggernaut of Touching Himself
Posts: 5393
Joined: Mon Oct 23, 2006 2:31 am UTC
Location: Somerville, MA
Contact:

Re: neutrino question

Postby doogly » Fri May 04, 2018 6:50 pm UTC

Also it is not both dead and alive. It is dead tensor product alive.
LE4dGOLEM: What's a Doug?
Noc: A larval Doogly. They grow the tail and stinger upon reaching adulthood.

Keep waggling your butt brows Brothers.
Or; Is that your eye butthairs?

User avatar
Eebster the Great
Posts: 2998
Joined: Mon Nov 10, 2008 12:58 am UTC

Re: neutrino question

Postby Eebster the Great » Fri May 04, 2018 9:43 pm UTC

ucim wrote:But momentum is conserved also, no?

This is a problem, right? A particle whose mass and velocity suddenly change can either conserve kinetic energy or momentum, not both.

User avatar
eSOANEM
:D
Posts: 3650
Joined: Sun Apr 12, 2009 9:39 pm UTC
Location: Grantabrycge

Re: neutrino question

Postby eSOANEM » Sat May 05, 2018 1:32 pm UTC

Remember that the neutrinos can be off-shell (so having the "wrong" mass).
my pronouns are they

Magnanimous wrote:(fuck the macrons)

User avatar
ucim
Posts: 6361
Joined: Fri Sep 28, 2012 3:23 pm UTC
Location: The One True Thread

Re: neutrino question

Postby ucim » Sat May 05, 2018 1:40 pm UTC

eSOANEM wrote:Remember that the neutrinos can be off-shell (so having the "wrong" mass).
What does that mean?

Jose
Order of the Sillies, Honoris Causam - bestowed by charlie_grumbles on NP 859 * OTTscar winner: Wordsmith - bestowed by yappobiscuts and the OTT on NP 1832 * Ecclesiastical Calendar of the Order of the Holy Contradiction * Please help addams if you can. She needs all of us.

User avatar
doogly
Dr. The Juggernaut of Touching Himself
Posts: 5393
Joined: Mon Oct 23, 2006 2:31 am UTC
Location: Somerville, MA
Contact:

Re: neutrino question

Postby doogly » Sat May 05, 2018 2:23 pm UTC

Through the magic of eigenstates, if you measure p(t_1) and then the next thing that happens to that neutrino is you measure p(t_2), you are not going to observe any nonsense.

Going off shell is like tunneling. You want to calculate a transition amplitude between two states, you integrate over all the possible momenta, not just the "allowed" ones.
LE4dGOLEM: What's a Doug?
Noc: A larval Doogly. They grow the tail and stinger upon reaching adulthood.

Keep waggling your butt brows Brothers.
Or; Is that your eye butthairs?

User avatar
Eebster the Great
Posts: 2998
Joined: Mon Nov 10, 2008 12:58 am UTC

Re: neutrino question

Postby Eebster the Great » Sat May 05, 2018 9:19 pm UTC

Not to hijack the thread, but is there any known lower bound on the individual neutrino rest masses (better than 0)?

jewish_scientist
Posts: 895
Joined: Fri Feb 07, 2014 3:15 pm UTC

Re: neutrino question

Postby jewish_scientist » Mon May 07, 2018 12:27 pm UTC

Soupspoon wrote:
jewish_scientist wrote:
p1t1o wrote:I saw the word "superposition" there, is it perhaps a quantum weirdness thing...

You know how Schoniger's cat is dead and alive at the same time? The word for that is superposition.

Though as Schrodinger* died in 1961, and cats don't often live much beyond a decade, maybe two, I think we can assume the waveform, if not the cat, has by now collapsed. And/or oxidised.

* This keyboard tried to make me type "school dinner", I wish I could get rid of the "suggestion" buttons above it.

Unless of course it is quantumly immortal.

User avatar
Quizatzhaderac
Posts: 1533
Joined: Sun Oct 19, 2008 5:28 pm UTC
Location: Space Florida

Re: neutrino question

Postby Quizatzhaderac » Thu Aug 16, 2018 8:36 pm UTC

@p1t1o: Here is my explanation of neutrinos and neutrino oscillation for lay people.

There are some reactions where electrons disappear (without a positron disappearing). There are some reactions where electrons appear. (without a positron appearing). We don't like the idea of appearing from nowhere or disappearing to nowhere, so physicists theorized that the disappearances were electrons changing (we'll call the things they changed to "neutrinos") and the appearances changing back.

Tests were done, and the disappearances matched up nicely with appearances. But! the electrons come in generations (electron, muon, and tau) the generation of the electrons coming out of the two ends don't add up. The numbers of each generation of electron that comes out, depend on what goes in, and the distance traveled. Everything beyond this is just making sense of those facts.

doogly wrote:Yeah, you transition to something heavier you slow down, to something lighter you speed up, no problems.
Yes in this case, but if someone reading wants to understand neutrino oscillation it helps to understand why this is a special case.

Kinetic energy and momentum don't have the same relationship to velocity. Ek = mvproper2 and p = mvproper. As we approach coordinate velocity of c, changes in momentum and kinetic energy (with respect to changes in coordinate velocity) become more similar, but not exactly the same. Because of quantum, close enough is good enough.

p1t1o wrote:That makes a certain sense but Im not happy with something changing speed without a reaction force
If you like, you can think of it as a continuous series of reactions as energy moves between the neutrino fields.
ucim wrote:If it can change speed, then it's not going at the speed of light, and there is a frame of reference in which the neutrino is motionless. What does it "do" in that frame of reference?
It is impossible for us to detect such a neutrino, as our best methods require a neutrino to have a certain amount of energy, and thus a coordinate velocity of >.999c. My best guess would be "Oscillate less".

Also, because of the uncertainty principle we can't say a neutrino has exactly zero momentum. Since the rest mass is so small, "not quite zero" momentum is still very fast.
The thing about recursion problems is that they tend to contain other recursion problems.


Return to “Science”

Who is online

Users browsing this forum: No registered users and 11 guests