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Dr. Christopher S. Baird

When I bump one end of a long metal bar, the other end instantaneously moves. Can I use this to send messages faster than light?

Category: Physics      Published: April 10, 2014

metal bar
Public Domain Image, source: Christopher S. Baird.

When you bump one end of a long metal bar, the other end does not instantaneously move. The movement would be instantaneous if the bar were perfectly rigid, but perfectly rigid materials are fundamentally impossible in the real world. Although the movement of the bar may seem uniform and instantaneous to our human eyes, it is really not. Our human eyes are simply too slow to notice the quick, but non-instantaneous, sequence of events that happens.

When you bump one end of a bar, you only locally deform that end and the rest of the bar is unaffected at first. But by inwardly deforming the near end of the bar, you have created a region of high pressure in the bar surrounded by regions of lower pressure. Said another way, you have forced the atoms in the near end of the bar to get closer to each other than the equilibrium positions of their chemical bonds. Said more simply, you have knocked the first few layers of atoms against the next few layers of atoms.  What happens next?

The near end of the bar which is at a higher pressure pushes on a lower pressure part of the bar a little farther down the bar, causing it to have a high pressure. This process continues and you end up with a pressure wave going down the length of the bar until it finally reaches the far end. We commonly call such a pressure wave by the name "sound". You therefore literally create a sound wave that ripples down the bar when you bump one end. It is not a continuous sound wave like a flute's tone, but it's a sound wave nonetheless. The far end of the bar does not move until the sound wave has had time to reach it and cause it to move. Because the "message" that the bar was bumped is carried to the entire bar by a sound wave, such a setup could only be used to send messages at the speed of sound. The speed of sound is much slower than the speed of light, and is definitely not instantaneous. What exactly is this pressure wave? It is a domino effect of chemical bonds between atoms being compressed, springing back towards equilibrium position, and yanking their neighbors upon springing back. In the simplest picture, the first layer of atoms knocks into the next layer of atoms and gets it moving, which then knocks into the subsequent layer, and so on.

Nothing can go faster than light. The speed of light in vacuum is the universal speed limit. When you tap on an object, the sound wave that conveys the tap to the whole object is fundamentally carried by the electromagnetic fields that make up the bonds between the atoms. Light itself is an electromagnetic wave. Therefore it should makes sense that the fastest an electromagnetic field (such as the atomic bonds in a bar) can propagate signals is the speed of light in vacuum. Therefore, in a deep sense, sound waves such as bar deformations can never travel faster than the speed of light in vacuum. The speed of light in vacuum is about 300,000,000 m/s. In contrast, the speed of sound in iron is about 5100 m/s and in copper is about 3600 m/s. If you yanked one end of a copper bar that was 3.6 kilometers long, it would take a full second before the other end of the bar would experience the yank (assuming the bar does not permanently deform).

Topics: faster than light, light, pressure, sound, speed of light, speed of sound, wave