Friday, September 23, 2011

Moving Violations

There's an old physics joke: 186,000 mi/s. It's not just a good idea. It's the law.

If this week's announced results prove out, they may just have to issue speeding citations to the staff at CERN, in Switzerland. To summarize, a neutrino experiment appeared to result in speeds 20 parts per million above the speed of light in a vacuum.

That ... is damn weird. It calls into question some fairly fundamental assumptions we've made about how we think the Universe works. By fairly fundamental, I mean some of them go back a couple of hundred years. Allow me to explain.

There's a fairly simple thought experiment we can run through to demonstrate why we think nothing can go faster than light. It relies on just three basic assumptions, that I will list below:

1) There is no universally-preferred frame of reference. A slightly less fancy way to put that is that there's no such thing as absolute motion. Motion only makes sense if you can measure it relative to something else. A corollary to this is that, if you're out in the middle of nowhere and have few or no reference points, you can't tell the difference between sitting still and moving at a uniform speed in a straight line. This is called Galilean invariance.

2) Light moves at the same speed in all inertial frames of reference. Another way of saying that is that light always moves at the same speed in a given medium, no matter where or how you measure it. This is one of the cornerstones of the Special Theory of Relativity.

3) All particles that have zero rest mass, like photons, are constrained to move at the speed of light, and only at the speed of light. They cannot accelerate or decelerate, but they can gain or lose energy in frequency.

Now, bearing those three assumptions in mind, let's imagine two spaceships out in intergalactic space. They're far enough away from any other points of reference that they can't really measure their motion all that well against them, so their only points of reference are each other. Which means, they have no real way of telling if they're at rest, or moving. For all they know, they're sitting still while the other ship zips past them. Now, let's also assume that one of the ship is charged up with several million volts, so that when they pass close enough, a spark jumps between the two ships. We won't say anything about how fast they're moving relative to one another. The speed can be arbitrarily high.

From the first ship, what you see is that you're sitting still, then this other ship zips by, and then FLASH! You're at the center of an expanding shell of photons. The situation looks the same from the other ship -- you're sitting still, zoom, FLASH! You're also at the center of an expanding shell of photons.

Again, the speeds of the ships haven't been specified. It doesn't matter how fast they think they're moving, because no matter which way you slice it, a wave-front of photons is racing out ahead of you. The only logical conclusion is that both ships must be moving slower than the speed of light.

That's all well and good. But now we have, at least potentially, a very sticky problem.

If this recent experiment is correct, if the Swiss scientists haven't made any errors, then one or more of the three basic assumptions above must be wrong.

The implications of this...

On one hand, it staggers the mind. Such fundamental notions about the Universe just don't fall every day. But on the other hand, this only involves such high energies that most of us will never see any kind of difference. And on the other other hand, I'm really only certain about three things anyway[1], so it won't bother me too much if the Swiss scientists are right or wrong. And even if they're right, it'll take quite some time for them to figure out exactly which of the above assumptions are wrong, and in what way. It'll be interesting to watch the commotion no matter how it turns out.

Still. That's going to be one hell of a speeding ticket, and I'm glad I don't have to pay.

[1] Conservation of Mass, Conservation of Momentum, and Conservation of Energy. As far as I'm concerned, everything else is open for speculation.

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