Tuesday, 27 September 2011

Faster than light neutrinos

The first sighting of a neutrino, only 41 years ago
Oh, wow. Physics is in the news. Just for once someone at CERN other than the LHC teams has made the headlines. It seems some neutrinos have been measured moving faster than light. I have seen headlines saying 'Einstein's theory shattered!' or similar. This is baloney. Here's the reality.

Neutrinos are particles produced in nuclear reactions that are almost impossible to detect. Every second about 50 trillion neutrinos pass through your body as they pour out of the Sun. They aren't exactly obvious. Neutrinos can be detected, but only indirectly as a very small percentage of them will interact with matter - what you see is that interaction, not the neutrino itself. It's telling that when a picture was taken of the Sun using neutrinos, the Sun was the other side of the the Earth at the time. Most neutrinos zip through the Earth as if it's not there.

In the CERN experiment neutrinos were sent down a 732 kilometre tunnel, and the timing was out by a matter of 0.00000006 seconds, making it seem that they went very, very slightly faster than light. This, then, is the evidence that is being presented and that has produced statements like this from the BBC:
The speed of light is widely held to be the Universe's ultimate speed limit, and much of modern physics - as laid out in part by Albert Einstein in his theory of special relativity - depends on the idea that nothing can exceed it.
It's really interesting, but I don't think it's earth shattering. The chances are, this is experimental error. Although the experiment has been repeated around 1500 times, it was using the same setup and assumptions. They've only got to get the length of the beam wrong by a tiny amount, for example, for the whole thing to be a mistake. And there is other evidence, comparing neutrinos and light from a cosmic source where there is no such disparity - so there is already some contradictory evidence.

However, even if it is true, the BBC's version is simply wrong. Modern physics doesn't depend on nothing exceeding light speed. We're talking about special relativity here, which is the basis of some modern physics, but light speed being a limit is a consequence of that theory, not a starting point. In fact we already have well established experiments in which particles travel faster than light speed.

This is a consequence of quantum mechanical tunnelling. One of the strange aspects of quantum physics is that particles don't have an absolute location, just a probability of being in various places. This means that particles can jump through an obstacle without passing through the space in between.

This sounds like something obscure and unusual, but it's actually how the Sun (or any other star) works. For nuclear fusion to take place, positively charged protons have to be pushed incredibly close together. So close that even the temperatures and pressures in the Sun aren't enough to get it going. The Sun only works because every second billions of particles tunnel through the barrier of the repulsion and fuse.

That same tunnelling technique has been used to send particles faster than light. All the evidence is that there is zero tunnelling time. A tunnelling particle literally doesn't travel through the space in between. So if you imagine a particle going 1 centimetre at the speed of light, tunnelling 1 centimetre instantly and going a further centimetre at the speed of light, it will have traversed the entire distance at 1.5c - one and half times the speed of light.

I'm not saying this is what is happening in the neutrino experiment, but I do imagine it is going to be something similar. Not a collapse of special relativity, just a way around it. If it's not experimental error, which still seems most likely. Special relativity has been tested so many times and has always delivered. GPS satellites have to be corrected for it, or they'd get more and more inaccurate. Particles demonstrate it in experiments every day. As far as I'm concerned, special relativity is solid as a rock.


  1. I knew it: science is never as exciting as the press make out.

    Still, probably good for the CERN funding that this press explosion happened.

  2. I'm not at all complaining that it made the press, though - it's great to see science making the headlines. But I don't know if they'd get away with such over-dramatisation with, say, a current affairs story.

  3. Good point. I suppose current affairs are commonly everybody's business; physics is often for a more select crowd. People probably need wacky, over-the-top headlines to draw them in.