Monday, 20 October 2014

The fastest Mozart you will ever hear

Large prisms used in a tunnelling experiment
In theory, science is very flexible. It is the absolute opposite of a rigid, fundamentalist religion, because there are no absolute truths in science. Theories are just as good as the evidence available - and it's entirely possible that evidence will come out tomorrow that make a widely supported theory untenable.

However, scientists are also human, and have a tendency to cling on to favourite theories beyond their sell-by date. It's not that they go into fundamentalist mode and ignore the evidence - they are more flexible than that. But they will change and patch up a favoured theory so that it matches the latest data. A good example is the big bang theory, which has been patched several times as new data emerged. (And may need patching again if it turns out that inflation wasn't really the way we used to think.) This is not surprising, though it can be arbitrary in the short term. The great British astrophysicist Fred Hoyle, for instance, pointed out to his death bed that the steady state theory he championed, an alternative to big bang, which was ruled out by new evidence, could just as easily have been patched up to match the conflicting data.

Just how flexible scientists are liable to be can depend on solid the theory is considered. Biologists, for instance, are always happy to hang bells and whistles on evolution, but it is hard to see it ever going away. Similarly, physicists are remarkably fond of the second law of thermodynamics. The astrophysicist Arthur Eddington famously said:
If someone points out to you that your pet theory of the universe is in disagreement with Maxwell's equations - then so much the worse for Maxwell's equations. If it is found to be contradicted by observation - well these experimentalists do bungle things sometimes. But if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation.
Amusingly, the comparison Eddington gives, Maxwell's equations is probably now another example of a 'difficult to counter' theory. And so is the implication from Einstein's special relativity that nothing - and particularly no information - can travel faster than light.

This is why some experiments, mostly undertaken towards the end of the twentieth century, are particularly interesting. These 'superluminal' experiments sent quantum particles - typically photons - faster than light. (I will cover these experiments in more detail in another post.)

They did this by making use of an oddity of quantum physics. Left to its own devices, a quantum particle ceases to have a definite location and exists as a three dimensional array of probabilities. It is only when it interacts with something that its location is pinned down, according to those probabilities, which evolve over time as predicted by Schrödinger's equation. One implication of this is that particles can tunnel through a barrier and appear the other side without passing through the space in between. There is good experimental evidence that tunnelling time is zero for quantum tunnelling.

Now think of a quantum particle, specifically a photon of light, travelling from A to B. Along the way it passes through a barrier with zero tunnelling time (such as the gap between the prisms in the illustration above). This means that the photon covers the distance from A to B in less time than it should. It travels faster than light. There are many arguments between different physicists over whether or not this is truly 'superluminal' or whether it is an effect of a change in the shape of a wavefront or other obscure possibilities. But one thing is certain. When one experimenter, Raymond Chiao, said that it didn't matter if it was superluminal because you could never send a signal this way,  only random photons, he was wrong. To demonstrate this graphically, another physicist, G√ľnter Nimtz sent a recording of Mozart's 40th symphony over four times light speed. And for your entertainment you can listen to that superluminal Mozart here. There's a lot of hiss, but it's hard to deny there's a signal.

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