Supersonic and Superluminal Motion: Argument By Analogy

Recently there have been a few interesting investigations where a light or gravity phenomenon was reproduced in sound waves, or vice versa (sometimes only theoretically). While of course there are differences between types of waves, among them that sound requires a medium to propagate, we can still get useful information by observing the behavior of one and extending it by cautious analogy to the other. One excellent example is the study of event horizons by using acoustic black holes (as opposed to the gravitational kind, which are hard to keep in a lab). There have been several papers in the last two decades, among them Matt Visser's, which even provides a sonic equivalent of Hawking radiation.

Conversely, we can also study or at least theorize about the characteristics of a known sonic phenomenon as it might occur with light, and this is what inspired me to write this blog entry. Gamma ray bursts (GRBs) and Double Radiosources Associated with Galactic Nuclei (DRAGNs) are both huge, powerful entities; GRBs are of particular concern to astronomers because they're the most energetic objects in the universe (outshone only by the Big Bang itself) and if one happened anywhere near Earth, we would be cooked. In this case "anywhere near" means within three thousand light years, and they've been invoked as an explanation for the periodic mass extinctions of life on Earth. We still have a poor understanding of why they happen; even if we knew why, we surely wouldn't be able to do anything about them. Gamma ray bursts would also be hard to study in a lab. Is there a sonic equivalent? As it turns out, there is - a sonic boom.

The question was initially investigated in the opposite direction, which makes it all the more intriguing. In his 2007 article in the International Journal of Modern Physics, Manoj Thulasidas at the National University of Singapore built a theoretical model of a hypothetical luminal boom - ignoring for the moment that we don't understand how superluminal motion could be achieved, what would happen if it were achieved?

Guess what? "We calculate the temporal and spatial varation of observed frequencies from a hypothetical luminal boom and show remarkable similarity between our calculations and current observations [of GRBs]." Bonus, the luminal boom model has further explanatory power about current problems in cosmology: "...our model explains why there is significant blue shift at the core regions of radio sources, why radio sources seem to be associated with optical galaxies and why GRBs appear at random points with no advance indication of their impending appearance....our model presents an intriguing option based on how we would perceive hypothetical superluminal motion." Far beyond my math is to begin explaining why provincial violations like Cherenkov radiation don't produce the same devastating boom that exceeding c in a vacuum theoretically would.

Thulasidas clearly points out that there are perceptual problems in relativity that can make something seem as if it's traveling faster than light, but still states the obvious, which is that it's intriguing to speculate that what we're seeing really is something traveling faster than light. He wisely resists the further temptation to speculate that this is not a natural phenomenon driven only by dumb physics - that is to say, that what we're seeing is faster than light motion not by something, but by somebody. In truth, my own speculations to this end motivated me to write this blog entry. But there is every reason to be suspicious of that leap, even if what we're seeing really is superluminal motion; such leaps have proved spurious before, as in the 1960s, when pulsars were initially thought to be little green men signaling to us. And of course, the Big Bang-like energy associated with GRBs would utterly annihilate any nearby matter-based entity, like us, or for that matter, like any conceivable little green men - unless the light-sound analogy holds further. By that I mean, ask a jet pilot onboard a supersonic plane what it's like to break the sound barrier. It's like nothing - onboard, there is no noticeable change in sound at all. It's the people in the boom forest on the ground whose windows break.

For my money, the most interesting take homes from Thulasidas's work is not only how well the model matches observed data, but how it explains other problems in GRBs as well.