It might not seem that a negative velocity would exceed the speed of light, but in this case, Robertson said, the speed of the pulse is actually much faster than c.
“Consider the pulse speed in a slightly less dramatic case,” Robertson said. “Say the peak of the output pulse exits the filter at exactly the same time as the input pulse reaches the beginning. In this less dramatic case, the transit time is zero and the speed (distance divided by zero) is infinite. So we were beyond infinite! (‘To infinity and beyond,’ to steal a line from Toy Story.) In our experiment, we measured a negative transit time corresponding to a negative group velocity of -52 m/s.”
Although such results may at first appear to violate special relativity (Einstein’s law that no material object can exceed the speed of light), the actual significance of these experiments is a little different. These types of superluminal phenomena, Robertson et al. explain, violate neither causality nor special relativity, nor do they enable information to travel faster than c. In fact, theoretical work had predicted that the superluminal speed of the group velocity of sound waves should exist.
“The key to understanding this seeming paradox is that no wave energy exceeded the speed of light,” said Robertson. “Because we were passing the pulse through a filter, the sped-up pulse was much smaller (by more than a factor of 10) than the input pulse. Essentially, the pulse that made it through the filter was an exact (but smaller) replica of the input pulse. This replica is carved from the leading edge of the input pulse. At all times, the net energy of the wave crossing the filter region was equal to, or less than, the energy that would have arrived if the input pulse had been traveling in a straight pipe instead of through the filter.”
Is this phenomenon simply the result of a clever set-up, or can it actually occur in the real world? According to the scientists, the interference that occurs in the loop filter is directly analogous to the “comb filtering” effect in architectural acoustics, where sound interference occurs between sound directly from a source and that reflected by a hard surface.
“The superluminal acoustic effect we have described is likely a ubiquitous but imperceptible phenomenon in the everyday world,” the scientists conclude.
Citation: Robertson, W., Pappafotis, J., Flannigan, P., Cathey, J., Cathey, B., and Klaus, C. “Sound beyond the speed of light: Measurement of negative group velocity in an acoustic loop filter.” Applied Physics Letters 90, 014102 (2007).
By Lisa Zyga, Copyright 2006 PhysOrg.com.
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