I have been thinking about the propagation of EM waves vs. mechanical waves and some of their odd cases. One such case that I haven't been able to puzzle out is what happens when a mechanical wave reaches the end of a medium (such as the outer layer of the atmosphere) and the beginning of a vacuum - outer space itself.

Edit: for clarity, I created a simple image demonstrating what I thought should happen: Last particle in a vacuum is launched into space

Basically, my theory was that the "edge" particles (or particles with a large amount of space between them) would continue into space with perpetual motion as the force applied to them is not counteracted by particles in front of them.

My question is, after the wave has affected the last molecules on the edge of a vacuum, what happens to it, and, do the molecules on the very edge continue to move indefinitely (or do they return to equilibrium)?

  • 2
    $\begingroup$ That wave will reflect at the edge of the medium. $\endgroup$ Jun 3, 2014 at 6:41
  • $\begingroup$ You are imagining a sharp edge, like a perfect lattice abruptly cut along a line, and you are looking at this kind of ``edge of vacuum''. In reality, what happens is that the particle number keeps on reducing gradually and beyond a stage, we just don't have enough atoms in a unit parallelopiped (read, density) to constitute a measurable air pressure. We call that vacuum. Of course, there is no sharp edge then - its all hazy. $\endgroup$
    – 299792458
    Jun 3, 2014 at 6:41
  • $\begingroup$ have you ever knocked on a table and hear the sound waves travel to the edges, what happened then? Of course it is not a vacuum, but related to the denseness of the table can give (at least) a hint.. $\endgroup$
    – Nikos M.
    Jun 3, 2014 at 7:40

1 Answer 1


the materials are elastic, so when it reaches the end it would reflect, because the last layer won't have other layer to collide and the elastic properties of the material would make it bounce back, making the wave reflect back. Unless the wave energy is sooo large that would break the material and in that case the particles would indeed leave the material. if you have a bucket of water and drop a tiny piece of something, it would make a wave, but no water would separate, now if you drop something really big, some part of the water would separate from the other part.


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