Suppose humanity has settled the stars, but there is a problem. The planets that humans have chosen to settle on are so far from earth that it takes a long time to transpher any information (barring the development of faster than light travel, which for the prurposes of this question we will assume that it hasn’t/cannot be done). Lasers and radio only travel at the speed of light, this is unacceptable for communication when the distances are measured in light years.
So my question is, could the civilizations communicate using a really long rod (for the purposes of this question, assume that the rod is long and strong enough to work, and does not stretch or compress). ie, the communicating parties push or pull the rod to communicate using a sort of morse code. Now since the rod doesn’t stretch, when party A pulls on one end of the rod, the other end of the rod should instantly pull away from Party B. Something like a pool cue being moved. ** Have I broken any laws of Physics yet? [B/] I’m afraid I only have High School Physics.
Now if that works, the people could upgrade their rod comm (rod communication) to having a computer push and pull the rod for them. As technology improves eventually the civilization will have a rod comm that can be pushed or pulled thousands of times per second (kHz).
Sound is defined as vibrations in a medium. The rod comm should be able to be vibrated at a frequency that we would hear (with aplification) as sound. ** however this seems to have broken the light speed restriction [B/] According to my physics, light speed is the fastest possible speed. The sound should not be able to travel along the rod comm faster than light, but if rod cannot stretch or compress, then when the rod is pulled at the source, it must be pulled away from the destination at the exact same time, leading to the vibrations being recreated at the destination at the same time they are created.
This would give the sounds a speed of (X light years)/(0 time) = Undefined, or (some number around positive infinity). ** So, I must have made some physics mistake. Could someone please explain (In terms that I would understand) where I went wrong?[B/]
Thanks!
I’ll reference you all in my Noble Prize acceptance speech if this works
Congratulations! You’ve just shown that relativity proves this assumption is impossible! (Not that anyone thought it was possible as an engineering problem, but it’s actually physically impossible.)
This sort of argument is known as reductio ad absurdum-make an assumption, show something contradictory or impossible follows from it, and conclude that the assumption (in this case, that a completely incompressible object can be created) is false.
Sorry. The vibrations down the rod will not propagate faster than light speed. You pull on one end of a rod one light year long, and the other end won’t move sooner than one year later.
There’s no such thing as a material that’s entirely noncompressable. The force of the pull has to be transferred from atom to atom in the rod. In fact, the force in question is electromagnetic, which, as we all know, obeys the light speed rule.
One of the early topics in relativistic physics was the nonexistance of truly rigid bodies. Even on the level of a pool cue, when you push on one end, those atoms push on the next ones out and so on. The disturbance propagates until the other end is moving. The pool cue is just so short and you’re pushing so weakly that you can’t see it deforming.
Out of curiosity, just what did you plan to push this rod with? I’ve done some back-of-the-envelope calculations and discovered that a rod one inch in diameter and one light year long would be approximately one million trillion cubic inches of material, and if made of tungsten, this would weigh approximately 11 thousand trillion tons, or more than a quarter the weight of the moon.
I’ll leave it up to someone else to check my rough calculations… and to decide how much energy it would take to actually move the damn thing (and how much it would take to make it stop once you’d moved it).
I was going to have the thing made of gold and figure out how much it would cost but I’ll leave that, too, for some other diligent soul.
