# How fast do neutrinos actually go?

1. May 14, 2004

i wanted to find out what percent of C neutrinos travel at.

so i got on the web and found pages quoting 'light speed' and 'almost light speed'.

my understanding was that they travel slower than light becuase they change flavour as they travel, (they couldn't change if going at light speed).

so how fast do they travel through space?

2. May 14, 2004

### ahrkron

Staff Emeritus
I depends on their energy. If they are actually massive (as seems to be the case), it should be possible to find neutrinos at any speed (up to c).

3. May 14, 2004

### jcsd

Yes, *I think* inorder for there to be uncertainty in the flavour there must be unceratnity in the mass, so the mass must be non-zero. Therefore they can travel at any speed lower than c.

Of course in the standard model neutrinos don't have mass and therefore travel at c (which is why some siters say they travel at c), but as far as I'm aware observational evidence overwhelmingly suggest that neutrinos do have a non-zero mass.

4. May 14, 2004

### krab

Neutrinos from nuclear reactions have energies on the order of 1 MeV. Their mass if nonzero is thought to be around .1 eV. This means
$$\gamma=10^7$$
But
$$\gamma={1\over\sqrt{1-\beta^2}$$
so $\beta$ which is the ratio of speed to speed of light, is veryvery close to 1:
$$1-\beta\sim 10^{-14}$$