The question addressed here is the following: if a single weak-type neutrino with a definite energy E is a mixture of three mass-type neutrinos (so it does not have a definite mass), why is it that the three mass-type neutrinos travel at different speeds?

The speed v of a particle in Einstein’s relativity can be written in terms of the particle’s mass m and energy E [that's **total** energy, i.e. motion energy *plus* mass-energy E=mc^{2}] and the speed of light c as

- v = c ( 1- [m c
^{2}/E]^{2 })^{1/2}

Recall the raised 1/2 means “take-the-square-root”. If the particle has very high velocity and its total energy E is much, much larger than its mass-energy mc^{2}, then

- v = c (1 – [m c
^{2}/ E]^{2}/2 + …)

where the dots mean that this formula isn’t exact but is an extremely good approximation for E very large. [*In other words, the velocity of a particle traveling near the speed of light differs from the speed of light by a shift equal to half the square of the ratio of the particle's mass-energy to its total energy.]*** So, from this formula you can see if two neutrinos have different masses m _{1} and m_{2} but the same very large energy E, then their velocities will differ by a tiny amount.**

Let’s see what this means. All the measured neutrinos from the 1987 supernova arrived on earth within about 10 seconds of one another. Think about an electron neutrino emitted from the supernova with an energy of 10 MeV (an MeV is a million eV [electron-volts], or 1/1000 of a GeV; read here for the definition of these terms). Well, that electron neutrino was a mixture of neutrino-1, neutrino-2 and neutrino-3, each of which traveled with a slightly different speed! Is this something we would have noticed? We don’t precisely know the masses of the neutrinos, but suppose that neutrino-2 has a mass-energy of 0.01 eV (electron-volts; see this article for the definition) and neutrino-1 has a mass-energy of 0.001 eV. Then their two velocities, remembering that they have the same energy, would differ from the speed of light and from each other by less than a part in a hundred thousand trillion

- v
_{1}– v_{2}= c [ (m_{2}^{2}- m_{1}^{2}) c^{4}/ 2 E^{2}+ ... ] = 0.0000000000000000005 c

(all equations accurate and precise to a one percent or better.) That velocity difference would mean the neutrino-2 part and neutrino-1 part of the original electron-neutrino would both arrive at the earth within a millisecond of each other — a undetectable difference for a variety of technical reasons. *(Keep in mind that OPERA claims a difference of neutrino speeds from the speed of light of one part in 100,000, a much, much larger effect, though the measurement involves neutrinos at an energy a few hundred times larger than those from the supernova.)*

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Matt, isn’t it possible that neutrini have zero mass-energy, travel at exactly c? That they are not ultra-relativistic particles, but pure signals, like photons? No mass-energy, only frequency-energy?

It was once thought possible, but the discovery of neutrino oscillations makes that impossible. http://profmattstrassler.com/articles-and-posts/particle-physics-basics/neutrinos/neutrino-types-and-neutrino-oscillations/ This phenenomenon, which has been observed several times now with extremely high statistical significance, cannot occur if neutrinos travel at c.

Matt: Neutrino oscillation between flavors is a neat trick, but what is lepton flavor? Lepton flavor describes what, muon-ness? The only known difference between electron and muon is the rest-mass. Standard Model would make more sense with one charged lepton (three possible masses) and one neutrino. Neutrinos don’t need flavor to interact with a particle, just the right energy.

lepton flavor is not a conserved quantity — unlike, say, angular momentum or electric charge. So I’m not sure what you’re asking.

” Standard Model would make more sense with one charged lepton (three possible masses) and one neutrino.”It might make more sense to you, but it wouldn’t agree with data. There are three charged leptons; you can check this in many ways, the simplest of which to describe, perhaps, is the measurement of the Z and W particle lifetimes, which would be shorter than predicted and measured if there were only one charged lepton.