- #1
Nantes
- 54
- 5
There is a classical thought experiment on trying to exceed the speed of light: if you push a light-year-long rod, on whose end is a button that will be pressed by the rod, wouldn't you be vastly exceeding the speed of light (which is the speed of cause & effect) by pressing the button instantly? The answer is no, the reason why is summarized in this answer I pulled from a reddit user:
"Mechanical disturbances travel through materials at the speed of sound, which is much slower than the speed of light. Any time you push an object, you are not actually pushing the whole object. You are pushing the end near you, which then gets compressed, bounces back from the compression, causing a part of the object a little ways down to get compressed, and so forth, until the compression wave travels through the whole object to the other side. For small rigid objects, this happens so fast, that it seems like the entire object is moving at once. But for large objects, you simply can't ignore this process."
Ok, but then I started thinking: if such a rod was in space, and you pushed it, you would be pushed back due to Newton's third law. But what would happen to the rod? Would it stay completely immobile until the shockwave traveling at the speed of sound reached the other end (hundreds of thousands of years later), at which point it would finally budge forward?
Or would it somehow start moving forward immediately after the push? But how, if the atoms at a given point in the rod can't go forward because the atoms just ahead of them are blocking the way?
Note: I'm assuming the rod would be made of an impossibly light material, otherwise its total mass would be impossible to budge with human strength.
Thanks
"Mechanical disturbances travel through materials at the speed of sound, which is much slower than the speed of light. Any time you push an object, you are not actually pushing the whole object. You are pushing the end near you, which then gets compressed, bounces back from the compression, causing a part of the object a little ways down to get compressed, and so forth, until the compression wave travels through the whole object to the other side. For small rigid objects, this happens so fast, that it seems like the entire object is moving at once. But for large objects, you simply can't ignore this process."
Ok, but then I started thinking: if such a rod was in space, and you pushed it, you would be pushed back due to Newton's third law. But what would happen to the rod? Would it stay completely immobile until the shockwave traveling at the speed of sound reached the other end (hundreds of thousands of years later), at which point it would finally budge forward?
Or would it somehow start moving forward immediately after the push? But how, if the atoms at a given point in the rod can't go forward because the atoms just ahead of them are blocking the way?
Note: I'm assuming the rod would be made of an impossibly light material, otherwise its total mass would be impossible to budge with human strength.
Thanks