Sound Travels Faster Than Light

Superluminal sound set-up — In this schematic of the acoustical test system, the scientists could create superluminal group velocity of sound waves, as well as negative group velocity. In the latter case, the tiptop of the output pulse traveling through the loop filter exited the filter earlier the peak of the input pulse had reached the first of the filter. Paradigm credit: Neb Robertson, et al.

For the kickoff fourth dimension, scientists have experimentally demonstrated that sound pulses tin travel at velocities faster than the speed of light, c. William Robertson's team from Middle Tennessee State Academy also showed that the group velocity of audio waves can get infinite, and fifty-fifty negative.

Past experiments have demonstrated that the group velocities of other materials' components—such equally optical, microwave, and electrical pulses—tin can exceed the speed of light. But while the private spectral components of these pulses accept velocities very close to c, the components of audio waves are about six orders of magnitude slower than low-cal (compare 340 thousand/southward to 300,000,000 m/s).

"All of the involvement in fast (and slow) wave velocity for all types of waves (optical, electric, and acoustic) was initially to gain a fundamental agreement of the characteristics of wave propagation," Robertson told PhysOrg.com. "Phase manipulation can change the phase relationship between these materials' components. Using sound to create a grouping velocity that exceeds the speed of light is significant here because it dramatically illustrates this point, due to the large difference betwixt the speeds of sound and lite."

The experiment was conducted by two undergrads, an area high school instructor and two high school students, who received funding by an NSF Step (Science, technology, engineering, math Talent Enhancement Program) grant. The grant aims to increment recruitment and retentivity of students to these subjects.

In their experiment, the researchers achieved superluminal audio velocity by rephasing the spectral components of the sound pulses, which subsequently recombine to form an identical-looking function of the pulse much further forth within the pulse. So information technology's non the actual sound waves that exceed c, merely the waves' "group velocity," or the "length of the sample divided by the time taken for the peak of a pulse to traverse the sample."

"The sound-faster-than-light result will non be a surprise to the folks who work closely in this area because they recognize that the grouping velocity (the velocity that the peak of a pulse moves) is not merely connected to the velocity of all of the frequencies that superpose to create that pulse," explained Robertson, "but rather to the manner in which a material or a filter changes the phase relationship between these components. By appropriate stage manipulation (rephasing) the group velocity can be increased or decreased."

To rephase the spectral components, the sound waves were sent through an asymmetric loop filter on a waveguide of PVC pipage, about viii m long. The 0.65-meter loop dissever the sound waves into two unequal path lengths, resulting in destructive interference and standing wave resonances that together created transmission dips at regular frequencies.

Due to dissonant dispersion (which changes the wave speed), sound pulses traveling through the loop filter arrived at the exit sooner than pulses traveling directly through the PVC. With this experiment, the group velocity could actually reach an infinitely modest amount of time, although the individual spectral components still travel at the speed of sound.

"We too accomplished what is known as a 'negative group velocity,' a situation in which the peak of the output pulse exits the filter before the superlative of the input pulse has reached the showtime of the filter," explained Robertson. "Using the definition for speed equally existence equal to altitude divided past time, we measured a negative time and thus realized a negative velocity."

Information technology might non seem that a negative velocity would exceed the speed of light, merely 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 example," Robertson said. "Say the peak of the output pulse exits the filter at exactly the aforementioned time as the input pulse reaches the beginning. In this less dramatic instance, the transit time is null 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 fourth dimension corresponding to a negative group velocity of -52 thousand/s."

Although such results may at start appear to violate special relativity (Einstein's law that no cloth object can exceed the speed of calorie-free), the actual significance of these experiments is a fiddling dissimilar. These types of superluminal phenomena, Robertson et al. explicate, violate neither causality nor special relativity, nor practise they enable data to travel faster than c. In fact, theoretical work had predicted that the superluminal speed of the grouping velocity of sound waves should exist.

"The key to understanding this seeming paradox is that no wave energy exceeded the speed of calorie-free," said Robertson. "Considering we were passing the pulse through a filter, the sped-up pulse was much smaller (by more a factor of 10) than the input pulse. Substantially, 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 free energy of the moving ridge 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 direct pipe instead of through the filter."

Is this miracle simply the upshot of a clever set-upward, or tin it actually occur in the existent world? According to the scientists, the interference that occurs in the loop filter is directly analogous to the "comb filtering" consequence in architectural acoustics, where sound interference occurs between audio straight from a source and that reflected by a difficult surface.

"The superluminal acoustic outcome we have described is probable a ubiquitous only imperceptible miracle in the everyday world," the scientists conclude.

Citation: Robertson, W., Pappafotis, J., Flannigan, P., Cathey, J., Cathey, B., and Klaus, C. "Sound across the speed of calorie-free: Measurement of negative group velocity in an audio-visual loop filter." Applied Physics Messages xc, 014102 (2007).

By Lisa Zyga, Copyright 2006 PhysOrg.com.
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