What is the Speed of Neutrinos and How Does it Relate to Special Relativity?

[nburns]

What is the neutrino field? How much of what we know about neutrinos is adequately explained by theory?

Thanks.

 

[phinds]

https://www.physicsforums.com/blog.php?b=3588

 

[nburns]

Sorry for the vague question. These fascinating particles don't seem to generate much literature. Is it because of a lack of experimental results?

It seems that these particles could be the key to a lot of new physics, if they were better understood.

 

[phinds]

Sorry for the vague question. These fascinating particles don't seem to generate much literature. Is it because of a lack of experimental results?

It seems that these particles could be the key to a lot of new physics, if they were better understood.

I think you significantly underestimate the extent to which they ARE understood.

 

[nburns]

Is there reading material you could point me to, please?

 

[phinds]

Is there reading material you could point me to, please?

I don't have any specific references but I would think an internet search would turn up quite a lot. Perhaps one of the other members will have something specific.

There are several neutrino detectors around the world. There's one in Japan in particular, but I can't remember the name. I suggest Googling "neutrino detectors" to start with

 

[Vanadium 50]

These fascinating particles don't seem to generate much literature. Is it because of a lack of experimental results?.

No, it's because it's not true.

There were 1433 papers with the word "neutrino" in the title in 2011.

 

[jtbell]

There are several neutrino detectors around the world. There's one in Japan in particular, but I can't remember the name. I suggest Googling "neutrino detectors" to start with

Before the current crop of neutrino detectors, which are mainly focused on studying neutrino oscillations, both Fermilab and CERN used more traditional detectors (both electronic detectors and bubble chambers) to study neutrino beams produced from their fixed-target accelerators (e.g. Tevatron at Fermilab and SPS at CERN). These were active from the 1970s probably into the 1990s.

 

[nburns]

No, it's because it's not true.

There were 1433 papers with the word "neutrino" in the title in 2011.

I realized after I posted it that I was probably mistaken. I don't read the *real* literature, though, I only read what filters down to magazines like Scientific American -- which is the highlights.

As far as I know, the state of knowledge is still that neutrinos appear to have mass, but also appear to travel at the speed of light. And there is no way to reconcile these two things with existing theory.

 

[nburns]

Moreover; apparently, by far, most of the energy of supernovae goes into neutrinos -- they are what blow the star apart. These strange particles seem to have a major role to play in nature. Even if the LHC doesn't find any new particles, it seems like particle physics still has a lot of work to do explaining the ones we already know about.

 

[jtbell]

As far as I know, the state of knowledge is still that neutrinos appear to have mass, but also appear to travel at the speed of light.

Their mass is so small that their speed is so close to the speed of light that the difference is undetectable.

 

[nburns]

Their mass is so small that their speed is so close to the speed of light that the difference is undetectable.

That explanation sounds a little bit too convenient. I believe that there has to be a more satisfying answer.

 

[nburns]

By the way --

Thank you, jtbell, vanadium 50 and phinds for replying on my thread.

 

[Vanadium 50]

As far as I know, the state of knowledge is still that neutrinos appear to have mass, but also appear to travel at the speed of light. And there is no way to reconcile these two things with existing theory.

That's false.

On PF we discourage the style of learning where one person makes a parade of false statements to be corrected by others. We find it ineffective and many people find it irritating. Asking questions by asking questions just works better than asking questions by making incorrect statements.

That explanation sounds a little bit too convenient. I believe that there has to be a more satisfying answer.

Given a particle's energy and mass, we can calculate its velocity - that's true whether the particle is a neutrino, a brick, or a planet. It just so happens that for neutrinos of detectable energy, this works out to very, very, very close to the speed of light. You may wish that the difference were bigger, of course, but it is what it is.

 

[jtbell]

Their mass is so small that their speed is so close to the speed of light that the difference is undetectable.

That explanation sounds a little bit too convenient. I believe that there has to be a more satisfying answer.

Look up the energies of the neutrinos used in these experiments, and the current estimates of their masses. Find the momentum from

$$E^2 = \sqrt {(pc)^2 + (mc^2)^2}$$

and the speed from

$$\frac{v}{c} = \frac{pc}{E}$$

The last time I tried it, I got a speed which was identical with c out to more than ten decimal places. (I can't seem to turn up that thread at the moment, though. It was sometime during the past year.)

 

[nburns]

Forgive me if I gave the impression that I know something about physics. I am just an observer, not a participant. I don't even know why my statements were incorrect. I was trying to help the discussion along by stating my impression of the current state of knowledge, in the hope that someone would correct me wherever I was wrong.

I'm pretty sure that a real physicist must have had those ideas at some point, though. Because I must have gotten them from somewhere.

That's false.

On PF we discourage the style of learning where one person makes a parade of false statements to be corrected by others. We find it ineffective and many people find it irritating. Asking questions by asking questions just works better than asking questions by making incorrect statements.
Given a particle's energy and mass, we can calculate its velocity - that's true whether the particle is a neutrino, a brick, or a planet. It just so happens that for neutrinos of detectable energy, this works out to very, very, very close to the speed of light. You may wish that the difference were bigger, of course, but it is what it is.

I'm not capable of doing the calculations. But that explanation doesn't ring true for me. I think that would be the first known case of a particle traveling at almost the speed of light because it almost doesn't have mass. All the other ones either do or don't.

As I said, I'm definitely not an expert, so I can't offer anything more than my non-expert intuition. But I get the sense that there's not nearly enough data to settle the matter once and for all, so even non-expert intuition has a chance.

 

[Vanadium 50]

The last time I tried it, I got a speed which was identical with c out to more than ten decimal places. (I can't seem to turn up that thread at the moment, though. It was sometime during the past year.)

For a 0.1 eV neutrino at 10 GeV, it's the same to twenty-two decimal places.

 

[Vanadium 50]

I'm not capable of doing the calculations. But that explanation doesn't ring true for me.

The thing about science is that if you can't do the calculation, you don't get to have an opinion. This isn't "I like broccoli" where everyone's opinion is equal. In science, everyone's opinion is not equal - it has to be based on facts.

 

[nburns]

Look up the energies of the neutrinos used in these experiments, and the current estimates of their masses. Find the momentum from

$$E^2 = \sqrt {(pc)^2 + (mc^2)^2}$$

and the speed from

$$\frac{v}{c} = \frac{pc}{E}$$

The last time I tried it, I got a speed which was identical with c out to more than ten decimal places. (I can't seem to turn up that thread at the moment, though. It was sometime during the past year.)

Thanks. I'll have to see if I can make anything out of those equations with what knowledge I have.

The explanation doesn't feel very satisfying, though, does it?

 

[nburns]

The thing about science is that if you can't do the calculation, you don't get to have an opinion. This isn't "I like broccoli" where everyone's opinion is equal. In science, everyone's opinion is not equal - it has to be based on facts.

Agreed. But the calculations are based on assumptions, and those assumptions could prove false.

I'm going to stop pretending to be more of an expert than I am and leave it at that. I'll remain skeptical until the data are all in and then I'll accept the outcome, whatever that is. It doesn't look to me like it will be resolved for at least a few more years.

 

[kloptok]

Agreed. But the calculations are based on assumptions, and those assumptions could prove false.

I'm going to stop pretending to be more of an expert than I am and leave it at that. I'll remain skeptical until the data are all in and then I'll accept the outcome, whatever that is. It doesn't look to me like it will be resolved for at least a few more years.

And what assumptions are you talking about in this case? It has been experimentally established that neutrinos have a very, very tiny mass (tiny in comparison to all other known elementary particles). Given this we can calculate the speed of the neutrino as a function of its energy via well established relativistic formulae. It so happens that when we do this the speed is very, very close to the speed of light.

From the same formulae also follow that a massless particle will travel at the speed of light, so it's not very strange in my opinion that a particle with a very small mass travels very close to the speed of light.

As a scientist you have to be skeptical of experimentally unfounded assumptions and conjectures, that much is true. But in the case of the formulae in special relativity giving the speed of a particle we are talking about experimentally well established results that no physicist would argue about to the current experimental accuracy. The thing is that you haven't "accepted the outcome, whatever it was", you have instead disregarded it as based on possibly false assumptions.

I'm not capable of doing the calculations. But that explanation doesn't ring true for me. I think that would be the first known case of a particle traveling at almost the speed of light because it almost doesn't have mass. All the other ones either do or don't.

Well, neutrinos don't travel at the speed of light. No particle with mass can do that.

The explanation doesn't feel very satisfying, though, does it?

And what would a "satisfying explanation" be, in your opinion? Are you trying to disprove special relativity?

---

On another note, there are definitely unsolved questions regarding neutrinos. But these do not concern the fact that a particle with a small mass would travel very close to the speed of light. Unsolved problems in neutrino physics are for example whether there is cp-violation in the lepton sector, if there are right-handed, sterile, neutrinos and whether the neutrino is a majorana particle or not.

 

[jtbell]

On another note, there are definitely unsolved questions regarding neutrinos. But these do not concern the fact that a particle with a small mass would travel very close to the speed of light. Unsolved problems in neutrino physics are for example whether there is cp-violation in the lepton sector, if there are right-handed, sterile, neutrinos and whether the neutrino is a majorana particle or not.

And why they have such small masses compared to the other fundamental particles. But this is part of a larger question: why do the fundamental particles (quarks and leptons) have the masses that they do? We don't have a generally accepted answer to this yet.

 

[nburns]

And what assumptions are you talking about in this case? It has been experimentally established that neutrinos have a very, very tiny mass (tiny in comparison to all other known elementary particles). Given this we can calculate the speed of the neutrino as a function of its energy via well established relativistic formulae. It so happens that when we do this the speed is very, very close to the speed of light.

From the same formulae also follow that a massless particle will travel at the speed of light, so it's not very strange in my opinion that a particle with a very small mass travels very close to the speed of light.

As a scientist you have to be skeptical of experimentally unfounded assumptions and conjectures, that much is true. But in the case of the formulae in special relativity giving the speed of a particle we are talking about experimentally well established results that no physicist would argue about to the current experimental accuracy. The thing is that you haven't "accepted the outcome, whatever it was", you have instead disregarded it as based on possibly false assumptions.

Well, neutrinos don't travel at the speed of light. No particle with mass can do that.

And what would a "satisfying explanation" be, in your opinion? Are you trying to disprove special relativity?

Is it possible that the masses of the various neutrino subtypes sum to zero, giving the neutrino a net mass of zero? That would evidently require that at least one of the masses is negative. I'm thinking of how a photon can be measured to have various charges at different points in space, but the net charge is zero.

I would not go up against Einstein. I don't have that much hubris...