# Neutrinos - what is their mass when they travel so close c?

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1. Sep 20, 2013

### keepit

neutrinos -- what is their mass when they travel so close c?

I understand that neutrinos move at very near the speed of light and that they have a very small amount of mass. This being true, why do they not have a great deal of mass at that speed?

2. Sep 20, 2013

### Staff: Mentor

Neutrinos have a very small rest mass. The "mass" that gets very large when particles move close to the speed of light is "relativistic mass", better known as "energy".

3. Sep 20, 2013

### Staff: Mentor

When we say that neutrinos have a very small mass, we mean that they have a very small rest mass. When physicists talk about mass, they usually mean "rest mass", not "relativistic mass."

4. Sep 20, 2013

### keepit

i'm missing something. Why does this rest mass not increase with the increase in velocity?

5. Sep 20, 2013

### Staff: Mentor

Rest mass is the mass that is measured by an observer who is at rest with respect to the object. Thus, it's not affected by the fact that the object might be moving with respect to other observers - you are moving at .99c relative to some observer somewhere, but that doesn't mean that you think that your mass is any different.

6. Sep 21, 2013

### BruceW

yeah. what Nugatory said is the important thing to keep in mind. But I should also add that it is more complicated when you go into the quantum physics of it all.

7. Sep 21, 2013

### Staff: Mentor

The formula for rest mass is $m^2 = E^2 - p^2$ (in units where the speed of light is 1). As the object increases in velocity, $E$ and $p$ (energy and momentum) both increase, but they do so in a way that keeps $m$ constant.

8. Sep 21, 2013

### Dappy

I always thought that the rest mass was the mass that a particle would have if it could be at rest. That would not change no matter what the speed of the particle.

9. Sep 21, 2013

### Staff: Mentor

That is correct and consistent with the answers in #5 and #7... But note that massless particle cannot be at rest ever.

10. Sep 21, 2013

### Dappy

I think I may of interpreted those answers differently, but thanks for reassuring me. I think the term at rest is merely an ideal which cannot truly be obtained by any particle, but that the term cannot be applied to massless particles. I assume that neutrinos can obtain sufficiently high energy levels since they are detected by the use of Cerenkov detectors.

11. Sep 21, 2013

### Staff: Mentor

Indeed the term "rest mass" is semantically inconsistent with the behavior of massless particles. That's one reason why many physicists prefer the term "invariant mass". However, "rest mass" is so deeply ingrained in the popular and introductory literature about relativity that we'll never be able to eradicate it, so we simply have to live with it.

12. Sep 21, 2013

### dauto

I like rest mass despite being a physicist.

13. Sep 21, 2013

### Dappy

Where as I like invariant mass and I am not a physicist.
(Invariant). The bit that remains when at rest.
It's all a matter of preference.

14. Sep 21, 2013

### BruceW

this is a good trick: when you hear the phrase "rest mass", just think of the word "rest" as being completely unrelated to a lack of motion. That is what I do. (and therefore I do not mind the phrase "rest mass")

15. Sep 21, 2013

### Staff: Mentor

+1

I like invariant because it points out the important part, which is that all reference frames agree. The fact that for massive objects it is also the mass in its rest frame is, to me, not nearly as important.

16. Sep 21, 2013

### Dappy

I agree but there are deeper hidden factors behind that.

17. Sep 21, 2013

### Dappy

I find that funny.I have too much rest.

18. Sep 21, 2013

### strangerep

+2

19. Sep 22, 2013

### Bill_K

-2

I think both physicists and nonphysicists alike are well-advised to use standard terminology, even though they might consider it less than ideal. To do otherwise conveys, rightly or wrongly, a lack of familiarity with the subject, or else a preoccupation with its trivial aspects that others have long since moved past.

In my experience, real physicists reserve the term "invariant mass" for the combined energy-momentum of two or more particles.

20. Sep 22, 2013

### Staff: Mentor

I agree, but that is not my experience. In my experience invariant mass is considered standard terminology. I have never come across such a restriction.