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Two Interesting Questions, and Answers

Cosmic Variance
By JoAnne Hewett
Feb 20, 2007 5:00 AMNov 5, 2019 8:12 AM

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A reader sent the following interesting questions:

Question I: Why doesn't light make a sonic boom when it travels. I know its a maseless particles, but the energy in it gives it an effective mass via matter-energy equivialnce. But lets go a step forward. Why don't messenger particles WITH mass like the W and Z boson's make a sonic boom? They do in fact have a true mass. Or even protons in a particles acceleration traviling around Fermilab at near the speed of light make the sonic boom? Does that mean there must be a critical mass to make a sonic boom, and if so, what is it?

A true sonic boom is a shock wave. A sonic shock wave results when an object like a fast plane travels at a velocity greater than that of sound in that medium. The wave travels at an easy-to-calculate angle to the direction of motion of the object, since the object is at the leading edge of the wave creation front, and the wave emanates in a sphere from that point and spreads outward in all directions at the speed of sound. A similar effect results from a boat travelling in water: the V-shaped bow wave is in fact a shock wave. So what about light? Well, almost. When an object like a charged particle travels through a medium (glass, or even air) in which the speed of light is less than c, the speed of light in a vacuum (300,000,000 m/s), it gives off a light shock wave. This sort of shock wave is called Cerenkov radiation, and it is VERY useful to us experimental types because it tells us we have a very fast particle going through our detectors. Now, a Z boson is electrically neutral and will not give Cerenkov radiation. A W boson has charge, and could do so in principle, but in practice its lifetime is so very short it does not travel even a microscopic distance before decaying. As for the protons circulating in the beam pipe at Fermilab, well, that's a vacuum (and a pretty good one) so they don't exceed the speed of light in that medium. Light, or electromagnetic radiation in general, does not cause such a Cerenkov shock wave, but it does exhibit some other odd effects when passing through matter. For photons with wavelength roughly in the visible spectrum and shorter, you get the photoelectric effect (for which Einstein won his first Nobel Prize - it was not relativity), the Compton effect (for which, you got it, Compton won the Nobel), and for really high energy photons (gamma rays) you can get electron-positron pair production, the easiest way to make the antimatter version of electrons, and also very useful for the experimentalists. Then you also have nuclear photoabsorption, and the very odd Mossbauer effect. Happy reading!

Questions II: Why doesn't a duck's quack echo? The only thing I can think of is the fact that the reflecting sound waves quickly colliding negating each other, but that;s just a thought. Truth be told I have no idea why.

Who said a duck's quack doesn't echo? It absolutely must, just like any sound wave, off a reasonably flat surface.

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