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If you have a metal pole sitting on the counter and you push on one end, the whole pole moves at the same time, or so it seems.
What happens if you have a bigger pole? Let’s say a pole that spans the English channel from the shores of England to the Shores of France.
Ignoring the fact that it would weigh too much to push, what would happen if you were able to push it in a straight line without it bending at all? If you were on the English side, would the side in France instantly move?
It may seem like it would, but if it did, that would mean you have broken one of the fundamental rules of the universe.
Nothing can travel faster than the speed of light. So, how would the French side ‘know’ to move at the same instant that you pushed the English side. To put it simply, it wouldn’t.
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The answer can be determined by looking at the metal pole on the microscopic level rather than from our normal point of view. While the pole looks like one solid piece of metal to us, on the microscopic level it is actually billions of cells configured in a crystalline structure.
Everything is held together by the electromagnetic forces that exist between the various cells.
So, when you push on one end of the pole, you are moving those cells at that end, which then move the ones next to it, and so on, all the way to the end of the pole on the French side.
But how fast does that compression on one end work its way through the entire pole to reach the other end?
Well, it depends on what it is made of, the temperature at the time, and several other factors. In general, it can be estimated that the movement will travel down the pole at roughly the speed of sound in a normal scenario.
It is explained on the University of Illinois Urbana-Champaign’s site, saying:
“A little ripple in the density of a piece of steel creates much bigger forces than a similar percentage ripple in the density of some air. So those bigger forces speed up the sound. Another way of saying that is that the sound speed goes up for bigger bulk modulus. (That’s the compressive longitudinal sound. Transverse sound depends on the shear modulus.) For ideal gases, the two factors just cancel and the speed of sound doesn’t depend on the density. In typical solids the bulk modulus goes up by a bigger factor than the density does, compared to gases, so solids typically have faster sound waves.”
That is all science talk to say that in a pole made of iron, the movement would travel at about 5130 meters per second. Given that the shortest distance between England and France is about 33 kilometers (20 miles), you can calculate it out to determine that it would take just under 6.5 seconds.
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That’s right, if you started pushing on the pole, your friend on the other side wouldn’t see it move for 6.5 seconds.
This is still pretty fast, but if you decided to shine a light down the pole at the same time, your friend would see it just .00011 seconds after you turned the light on, illustrating just how much faster light is than sound.
If you think that’s impressive, check out this story about a “goldmine” of lithium that was found in the U.S. that could completely change the EV battery game.