# How do physicists find the speed of neutrinos?

I have heard that there is evidence for neutrinos traveling close to the speed of light, but how is that done? Since neutrinos barely react with anything, and the only evidence for them is indirect (please correct me if I'm wrong), how could physicists find the speed of neutrinos, or any other particle?

You measure the time it takes to go from one place to another. Just like you would with a car, only you use particle detectors. (For charged particles of known mass and speed less than about 99% of the speed of light you can also measure the relationship between their energy and momentum, but that doesn't apply to neutrinos.)

The speed of light is about $30\,\mathrm{cm/ns}$ (that's one foot per nanosecond in to US physicists) and it is relatively easy to achieve timing accuracy of a few nanoseconds in small detectors so this is a bit of a work-a-day thing for charged particles and is done over distance of a few meters to a few tens of meters.

In the case of neutrinos the low cross-section means that the detectors are relatively large (meters across) and so if you want much precision you have to use a fairly long baseline (meaning a few hundred kilometers).

Not withstanding the hoopla a few years ago at OPERA (which was the result of a faulty cable connection, of all things) no neutrino speed measurement has ever differend significantly from the speed of light.

But that is to be expected as the neutrinos we measure have energies of a few MeV and up, while their masses is none to be rather less than 1 eV. That means the the Lorentz factor is $\gamma > 10^6$.

• There have been neutrinos measured as arriving at about the same time as the light from distant supernovas. I understand that the speed was indistinguishable from light speed. Do you know to what accuracy? And what speed have lab measurements produced? – mmesser314 May 31 '16 at 2:25
• Ummm.... Never mind. I just saw your answer and others at physics.stackexchange.com/q/139/37364 – mmesser314 May 31 '16 at 2:29
• I believe it Mermin suggested introducing the phoot such that the speed of light is 1 phoot/ns. – jim May 31 '16 at 18:28