A Reading for The Evolution of Physics by Albert Einstein
WHAT IS A WAVE? from The Evolution of Physics by Albert Einstein and Leopold Infeld A bit of gossip starting in London reaches Edinburgh very quickly, even though not a single individual who takes part in spreading it travels between these two cities. There are two quite different motions involved, that of the rumor, London to Edinburgh, and that of the persons who spread the rumor. The wind, passing over a field of grain, sets up a wave which spreads our across the whole field. Here again we must distinguish between the motion of the wave and the motion of the separate plants, which undergo only small oscillations. We have all seen the waves that spread in wider and wider circles when a stone is thrown into a pool of water. The motion of the wave is very different from that of the particles of water. The particles merely go up and down. The observed motion of the wave is that of a state of matter and not of matter itself. A cork floating on the wave shows this clearly, ...