# Neutrino mass

1. Nov 28, 2006

### oferar

Hello, I recently read about neutrinos having mass > 0.
(http://news.bbc.co.uk/1/hi/sci/tech/4862112.stm), but also they travel at the speed of light. How can it be possible? If they have mass, their mass should increase with the velocity, and at the speed of light according to special relativity they should have an infinite mass.
Can someone explain what this means? Maybe they should travel slower than light?

Thanks.
fernando

Last edited by a moderator: Apr 22, 2017
2. Nov 28, 2006

### neutrino

Well, they travel at a speed that is very close to the that of light.

3. Nov 28, 2006

### disregardthat

What decides what velocity a object has since everything is relative?

4. Nov 28, 2006

### Danger

Essentially, the observer decides... relative to himself.

5. Nov 28, 2006

### neutrino

However you measure, it should not be greater than c for a particle with nonzero mass.

Last edited: Nov 28, 2006
6. Nov 28, 2006

### disregardthat

but he cannot reach a veloticity "c". so there has to be something that decides the velocity. if he is just below the c level of velocity, another object can't very much faster than that, even though it stands still in his frame. ( if he has the same speed)

7. Nov 28, 2006

Staff Emeritus

Velocity with respect to what? You are writing as if the neutrinos had an ansolute velocity with respect to some fixed frame of reference. But this is relativity; there is no fixed frame nor any absolute velocity, other than c, either. If I see the moving at v, I can be moving at v in the opposite direction at v and the particle be standing still. Or any combination of its velocity and mine that add up to a relative v between us. As long as $$\frac{v^2}{c^2} < 1$$ it's physically possible.

Now if you say, "regarding my lab frame as being at rest, for the moment, can I work out the physics that determines what velocity I see the neutros travel?" Yes I can, using the kinetic energy and momentum just as for a Newtonian particle, but remembering to make proper use of the gamma multiplier $$\gamma = \frac{1}{\sqrt{1 - \frac{v^2}{c^2}}}$$.

Last edited: Nov 28, 2006
8. Dec 11, 2006

### dupont

Neutrinos are usually emitted at high energy (several keV to MeV). Their mass, if it exists, is <1eV, so their speed is close to c for any experimental situation on earth.
In principle, it is possible to have "slow" neutrinos emitted from particle beams in the backward direction. The beam must be very energetic because it has to match the ratio neutrino energy/neutrino mass. Those beams are not available yet and it can be doubted that we would detect slow particles in the mess of a collision or similar events.