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Name: Brooke D.
Status: student
Grade: 9-12
Location: MI
Country: N/A
Date: 11/15/2005

What happens to a compressional wave when it goes somewhere that is void of matter?

A compressional wave traveling through a material cannot propagate into a vacuum, because the constituent particles do not have anything to "push against". Hence the statement, "Sound cannot travel in a vacuum." One of two things will happen -- or possibly some of each -- the molecules/atoms/electrons at the surface of the material will be displaced so far from their equilibrium position by the molecules/atoms behind it that the bonds will break and molecules/atoms/electrons will be "sputtered" into the vacuum. The photoelectric effect, where electrons are ripped out of the surface of a metal by an applied electric field, is a similar but not exactly the same.

If the energy of the compressional wave is not sufficient to cause molecules/atoms/electrons to break the bonds holding the surface together, then the restoring force will pull the particles back toward their equilibrium position. Along this path of course will be other particles "in the way". Then, depending upon conditions the wave may reflect in the direction opposite of the initial wave. The wave may scatter, that is the reflected wave may move in different directions than the initial compressional wave, and/or the coherence of the compressional wave may be disrupted. If that happens the coherent wave will dissipate as heat -- the atoms/molecules/electrons moving in random directions. Which process occurs will depend upon the details of the experiment. In the "real world", likely all of the processes will occur to some extent.

Vince Calder


The only waves that exist where there is no matter are electric and magnetic waves. Compressional waves, and sound waves, cannot go beyond the end of matter. If the matter ends abruptly, such as the outer surface of a spaceship, the wave will reflect back in. If the matter does not have a clear surface, like our atmosphere, some of the wave energy will reflect back and some will throw molecules forward. A few molecules may even leave the planet.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College

It must bounce off, with inverse polarity. Compression peaks in the forwards wave become the opposite: tension peaks in the reflected wave. The last matter pushed by a compression peak must swing out under it's momentum, get pulled back by a tensile spring force from the matter behind it, and then bump into the matter behind. it. This re-bumping, and the tension that happened before it, start a wave going backwards. Sort like the lose end of a hanging string would do.

You should probably ask yourself how this matter holds itself together at the boundary of the void. A gas cannot simply end abruptly at a vacuum; something else must be holding it back. A liquid can end abruptly at its top (with respect to gravity) surface, but in normal gravity the other sides would need a solid wall, and that wall might change the reflection by being stiffer than the liquid instead of looser, or by locally having different density than the liquid. The top surface of the atmosphere is not like the top surface of a liquid, because the gas fades to vacuum so gradually that some other phenomenon might happen, like wave-absorption or ejected mass or distorted reflection. With a solid there is usually no problem. It is clear that molecules in solids can pull on their neighbors, not just push.

These are natural opposites:

1) a wave hits an infinitely stiffer medium, and reflects with unchanged polarity,

2) a wave hits an infinitely less stiff medium (a vacuum does not push back), and reflects with inverted polarity. "No reflection" is the center point between the two: a wave hitting a new medium with the same stiffness and mass density.

Jim Swenson

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