Expert: James Gort - 10/2/2011

Question
QUESTION: Hi James.
I Have some questions related to light. They are stimulated from the recent 'alleged' speed of neutrinos. I thought Einstein stated nothing can be ACCELERATED to faster than light speed? Are photons, and for that matter neutrinos, actually accelerated? Presumably photons and neutrinos are created from, say switching on a light and a nuclear explosion. Could you give me a very simple view of how, and from what, are they created. And crucially, presumably at creation they have no speed. Is there a period of time (however short)when the photon/ neutrino could be said to be accelerating? Or is it viewed to be created and immediately travelling at its final velocity? I tend to doubt these published neutrino speeds, at least for now. But what led me to these questions was the thought that if neutrinos could not be said to have an acceleration period then it would not violate Einsteins theorys. But how does something go from no speed to the speed of light without accelerating!! A short, simple overview of this would be gratefully appreciated. Thanks very much.

ANSWER: Hello Richard,

I'll do my best to answer the question. But the "why" question you ask - why neutrinos have the velocity they do - is not fully known.

First, the photons. Photons have no mass, and must have velocity "c" (taken as a group). Both classical electrodynamics (i.e., Maxwell's Equations) and Relativity predict (actually, require) it. As an aside, individual photons do not necessarily travel at "c". Feynman (and others) developed Quantum Electrodynamics (QED) which predicts something else - that individual photons have a probability of having any velocity, and may take any path. Sounds strange, but please see "QED - The Strange Story of Light and Matter" by Feynman (a little book that says a lot about modern physics ideas).

But let's ignore that little complication, and talk about the average photon travelling at "c". When an event "produces" a photon (say, an electron orbital transition), does the photon suddenly start up from stationary? The answer is that the photon didn't exist as a stationary entity - it wasn't waiting within the atom, waiting to be released. Rather, when the "event" happened, a photon was created as a way to release energy from the atom. The photon always was always travelling at "c" - from its moment of creation. No acceleration was involved.

But what about neutrinos? Here there are more unknowns. If neutrinos have no mass, they must also travel at "c". One can't isolate a massless, stationary particular. But if neutrinos have a small mass and can reside within an atom (trapped within it until released), they why does the neutrino have the velocity it does? No one knows the answer. Various experiments seem to show velocities of <c, =c, and >c. So new theory will have to coincide with better experiments.

But don't worry too much about violation of Relativity. Current Relativity theory is a macro theory. We're still looking for a "quantum theory of Gravity" - which would replace General Relativity at the micro level. QED has already modified Special Relativity at the micro level.

Currently, there's no good theory (the so-called theory of everything) that works at both the micro and macro levels. Perhaps someday...

Cheers,

Prof. James Gort        

---------- FOLLOW-UP ----------

QUESTION: Thankyou for your prompt answer James.
The answer has cleared a few things up for me.
However, I didnt actually ask WHY neutrinos have their speed.
So, I gather from your answer that we dont know much about neutrinos. So, it is possible that in terms of their speed they could be similar to light. That is, they do not undergo an acceleration period. They just travel at their speed, whatever that speed is determined to be. If, a big if, it turns out to be faster than light, would that undermine any theorys we have at present, Einsteins or otherwise?...given that no acceleration has occurred.
Thanks once again.

Answer
You're right - you didn't ask WHY they have the speed they do, but you ask about their "creation" - which then somehow implies speed (and acceleration) after creation. One is sort of tied with the other. When electrons are ejected from an atom after a collision, the speed of the electron depends on the momentum and energy which is required to conserve those those quantities.

So, to answer your last question, if neutrinos are massless, then they're probably like photons in that they cannot exist at rest. When they are "created" (as a mechanism to conserve momentum, etc.), then they do not undergo acceleration.

If they're massless AND travel faster than light, that doesn't violate Relativity, IMHO. One of the consequences of Relativity is that a particle with mass will increase its mass (i.e., some of its kinetic energy will be transformed to mass) as it approaches "c". It will require an infinite amount of energy to reach "c". But massless particles don't have that restriction. One can argue that the faster-than-light neutrinos would "travel backward in time". That's not a problem, either, since that's exactly what anti-particles do! (Again, see that "QED" book)

If they have mass AND travel at or above "c", then that's an interesting area where we don't have the answers. Is there an acceleration mechanism? Is their speed a fundamental constant of a "free" neutrino? If they never pass "c", but are created as faster than light particles (i.e., tachyons), then that doesn't violate Einstein, either. Do we need a new theory? Nobody knows - we're waiting for more definitive experiments to see whether observations fit into the current theoretical framework.

Prof. James Gort