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Sound Faster Than Light. Come Again?

Sound Faster Than Light. Come Again?

By Steve Robinson

Hold on to your hats, there's some serious science upon us. A team of young scientists in America have managed to construct an experiment where sound waves travel faster than the speed of light.

This is almost unbelievable for two reasons. Firstly, the speed of light is generally seen as an impenetrable barrier faster than which nothing can go. Secondly, sound waves themselves tend to travel at a mere 340 metres per second, six orders of magnitude slower than light usual speed of 300,000,000 metres/sec.

Nevertheless, the experimental setup worked, and what's even more startling is that the study was performed by two high school students, two undergraduates and a teacher.

Quite how they managed this feat is down to the specifics of the experiment. The key feature is that the fact that the velocity of the sound waves is measured as a 'group velocity'.

Sound is emitted from a speaker and 'rephased' via a filter. This means that it's broken down into its component parts. This information is then put back together and emitted to form a replicate of the original signal some distance away from the source. So it is not the sound waves themselves that travel faster than the speed of light, but rather the impression of the sound pulse that reaches the receiver quicker than the speed of light after it is emitted. It is the collective action of these elements, such as the filter, that allow the pulse to be replicated much faster than normal.

The team even found that some of the rephased pulses were exiting the filter at the same time they were entering it. This effectively means the pulse was travelling instantaneously from one end to the other. Hence, since distance over zero time = velocity, the speed of the pulse was infinite! They even managed negative velocities, meaning the pulse left the filter before it arrived! Thankfully, the laws of the universe remain intact, because the pulse carried no information and so doesn't violate causality. Phew.

Apparently this phenomena occurs in every day life with the interaction of sound waves, but is so slight to be unnoticeable. Which is probably a good thing, because just thinking about it theoretically is doing my head in!

We have to hold our hands up here and say that we don't fully understand the ins and outs (or outs and ins if we're talking negative velocities) of all these physics shenanigans. If you can help us out to understand quite what's going on then we'd be very grateful. Thanks.

Steve has tackled other subjects with similar aplomb.  Read stuff here.

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08 Aug 2010
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