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Q & A: light's sonic boom

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Most recent answer: 07/30/2012
Q:
light sonic boom. im just curious if such a thing were possible and what would be the effects. i've been reading over a few of your questions mostly concerning magnetism, photons, and light. what would such a thing look like? and what would be the effects of something caught in its wake. im just curious what you think. im a big laurence krauss fan and believe hes onto something with his "universe from nothing" theory. he states in his lectures that the universe is expanding faster than the speed of light and that objects in the universe are moving(possibly only away from earth) at faster than the speed of light. these objects are in a vacuum or are part of it and moving faster than the speed of light. i read in a post that things in a vacuum shouldnt be able to move faster than the speed of light but inside materials they can. might that have something to do with it? im not trying to trap you and make you back peddle as ive seen some do. im just generally curious. and also you might disagree with mr krauss. thanks brett. also if you feel this has been covered please just link me i couldnt find one that i thought really answered my questions. though the black hole and light orbiting it ones were very interesting.
- brett (age 31)
calgary alberta canada
A:
I don't think we have covered that. Thanks for checking!

A sonic boom arises when an object travels through a medium (say air) faster than sound speed in the air. This causes sound waves emitted at different times to overlap, building up into the boom. To get something light that for light, you'd need a particle which interacts strongly with light to travel through a medium faster than the speed of light in the medium. Since the speed of light in a medium is lower than the universal speed limit, c, that's possible. An example would be a very energetic proton (charged) traveling through a piece of glass. The boom-like radiation that's emitted is called Cherenkov radiation. () It's used quite a bit in particle detectors.

As for the cosmic questions, no, it's not connected. The expansion of the universe doesn't consist of something moving through some medium. Near some part of the universe, that's all there is. So there's no boom-like effect. The "faster than light" part does describe the growth of the spatial coordinate of something in our conventional coordinate frame, but it doesn't describe the motion of something as seen by its neighbor.

As for Krauss' "something from nothing", that idea has stimulated a great deal of philosophical argument. So far as I can see, the argument is semantic, just concerning what people are willing to call "nothing".

Mike W.

(published on 07/30/2012)

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