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Thought Experiments

  1. Jul 9, 2008 #1
    Here's a thought experiment that I would like you to comment on.
    Suppose you could construct two spaceships of exact design. I will call them spaceship A and B. Then let us also suppose you could construct a cable, or any tether device, sufficient enough in strength to be attached to the rear of both spaceships.
    Now, in this thought experiment, the two spaceship are attached by the tether and fly in opposite directions, reducing the slack of the tether until it is taunt. The speed of the spaceships will be sufficient to accomplish this within a reasonable amount of time, but not too forcefull as to break the tether. At some point, the spaceships will reach an equiliberium where the force of the thrusters is balanced with the tension in the cable between the ships so that the speed of each ship is 0, but the thrusters are still on.
    The distance between spaceship A and B should be at an optimum distance, say 3 or 5 or 10 light seconds apart for the next part of our experiment. After the equiliberium is reach and both ships are holding steady at a foward speed of 0, space ship A will release the tether and simultaneously send a beam of light to a receptor on the back of spaceship B. Spaceship B will recieve the beam of light in a detector 3 or 5 or 10 seconds later. This will be a way for spaceship B to know the precise moment spaceship A released the tether.
    Will spaceship B immediately begin accellerating foward, having been released of the force holding it back (i.e spaceship A's release of the tether)? Will it remain at a speed of 0 for the 3 or 5 or 10 seconds it takes for that "information" to travel from spaceship A to spaceship B? If it remains under the force of the tether for a few seconds, what is holding it in place since it is now free of the tether?
    If it moves foward immediately, then how is that "information" transfered faster than the speed of light (i.e the light beam sent from spaceship A)?
  2. jcsd
  3. Jul 9, 2008 #2


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    The release of the tether from spaceship A will take a significant length of time to travel the length of the tether. How fast that is depends on the speed of sound through the material that the tether is made of.

    The tether is not a rigid object. There is no such thing as a rigid object in SR.
  4. Jul 9, 2008 #3


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    Apart from the speed of sound issue, we also have the electromagnetic force (i.e. all the lattice bonds that form the tether) being limited to the speed of light. Even if the speed of sound was unbounded in that medium, such a release would still be restricted.

    As said above, SR is not compatible with rigid bodies. That however is a feature, not a bug.
  5. Jul 10, 2008 #4


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    Space ship B won't move before the light gets to it if tthats what you wanted to know. Its a good question though.
  6. Jul 10, 2008 #5
    Why is that? On a far greater scale, which would better demonstrate what happens on the smaller scale in this experiment, such as perhaps 2 AU, when ship A released, and it flashed the beam of light, would Ship B not begin progressing immediately? I'm not very knowledgeable about what would happen in an experiment such as this, but it just seems to defy logic that Ship B would be forced to wait for the light to reach it before it could begin moving, when the opposing force had already ceased.
  7. Jul 10, 2008 #6


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    You're missing the crux of the issue: the tether is not rigid. When Ship A releases it, the "snap-back" will not be instantly felt at the opposite end. The snap-back will travel the length of the tether like a wave until it reaches the other end where Ship A will experience a release in tension.

    This pulse, far from traveling at the speed of light, will indeed travel quite slowly, merely at the speed with which the pulse can be transferred through the medium of the tether. This could take minutes, hours or even days. But there is no physical material, even in principle, that could transmit this pulse as fast as the speed of light.

    So, what Ship B would experience is:
    - a light signal saying "I've released the tether" (ship B thinks this statment is false)
    - nothing for a while
    - a single wave - a pulse - will be seen travelling from the far end the tether, racing torward ship B (while this is happening, ship B is still held rigidly to the tether)
    - the pulse will reach ship B, which will suddenly be free
  8. Jul 10, 2008 #7
    I agree with the sequence of events, but I disagree with the mechanics that seem to be proposed for the cause of the delay. since I'm relatively new and not an in-depth expert in physics, I'll offer it as a question.
    It seems to me that the motion of the tether would travel as a molecular propagation so to speak. it would be similar to the speed of an electromagnetic wave in a solid state medium, that is at a speed that is reduced from c due to the solid vs. vacuum e and u. it could be on the order of 2/3 c. is there evidence that movement of an object propagates along the object's length as a function of acoustics? I think that is absurd, take for instance a ship that is a quarter of a mile long. the bow would start moving a full second after the stern started moving? I can picture an experiment, I might be able to do myself in the rf lab, with motion detectors at the ends of a length of say piano wire and a 2 channel scope in the middle. push and pull the wire back and forth and see either a nanoseconds-scale difference in the ends moving or a milliseconds-scale difference. I know which one I expect to see.
  9. Jul 10, 2008 #8
    The what lab with the motion detectors and nanosecond scale difference anddddd....

    .... Please, tell me how that goes. I'm interested. I've got, um, a piece of string, some tape, and... It sounds like you kinda have better tools. So tell me how that goes!!
  10. Jul 10, 2008 #9
    RF as in Radio Frequency. I'm an engineer and I design rf and microwave circuitry. I use oscilloscopes that can easily resolve picoseconds. I believe I could create a setup to discern whether movement of an object is "acoustic" or "electromagnetic" based. e-m energy takes about a nanosecond to travel 1 foot, sound takes about a millisecond to travel 1 foot in air. that's six orders of magnitude. speed of sound in metal is faster than in air but nowhere near the speed of e-m propagation. it should be easy to tell the difference. just wondering if this has already been done.
  11. Jul 10, 2008 #10


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    How is that different from a sound wave?
    Not acoustics per se, but the speed at which any vibration can physically propogate through a medium - which, yes, also happens to be the speed of sound in that medium.
    The speed of sound through steel is 5km/s, so it would take 9/100ths of a second. Yes. Ships do suffer from twisting, even under perfect conditions.

    If it's steel piano wire, then 5km/s - fifteen times that of the speed of sound in air.

    I'm a bit confused why you think this is implausible. It is a well known phenom. If you strike the front of the oil tanker with a hammer, it will take about 1/10 of a second for the pulse to travel the length of the tanker. Why do you think that a tether would behave any differently? It's the identical force travelling via the identical mechanism.

    If you did this experiment for real, with a 1/4 mile tether, you would indeed see the pulse racing towards you (and at a quite moderate speed compared to c).
    Last edited: Jul 10, 2008
  12. Jul 10, 2008 #11
    I'm just saying that a mechanical wave moving through a medium is different than the medium itself moving. has this been shown either way with a valid experiment?
  13. Jul 10, 2008 #12


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    What you call "molecular propagation" in your post IS the speed of sound. Whereas an electromagnetic wave can move through a medium at nearly the speed of light (0.7-0.8c) ANY interaction within the medium that requires the ions/atoms to actually move will be restricted to the speed of sound, which is very, very slow compated to c.
  14. Jul 10, 2008 #13


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    You are correct that it is molecular em interactions which determine the probagation speed through a solid, that IS the speed of sound in the materiaL It is much less then the speed of light and is not the same a the "conduction" speed for a electrical signal in the material.
    Opps, I failed to refresh, F95toli beat me to it.
  15. Jul 10, 2008 #14


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    You mean besides acoustics itself? Remember, every bit of acoustics is based on this princple. When you pluck a guitar string, the pitch of the wave is based on the length of the string, tension, and elasticity. The propagation of an earthquake is this principle in action. Sonar. Clapping your hands. The pitch of the sound created when you use a hammer. The examples in vibrations/acoustics is are voluminous.

    Another good example is the water hammer effect. When you open or close a valve, the water in the pipe does not start/stop instantly. It starts/stops essentially the same way cars start/stop at a traffic light. Same issue. http://en.wikipedia.org/wiki/Water_hammer

    This is a very thoroughly understood piece of classical physics.
    The speed of sound in steel is 4500 m/s. That means when an aircraft carrier (300m long) crashes into a wave, people at the stern feel it .067 seconds later. Remember, it is the speed of sound in the material, not the speed of sound in air.

    The reason this sounds absurd is that in your everyday perception, force is transmitted faster than your brain can process the delay. You've taken that and turned it into a belief (a very common one) that it is, in reality, instant. That belief is wrong.
  16. Jul 10, 2008 #15
    I'm going to take one more shot at this concept, then I really have to set up something and prove one way or the other.
    I have two push button switches, side by side, 30 feet away. I want to activate both at the same time so I set up two steel rods such that when I push on my end of both rods, both switches get activated at the same time. fair enough?
    well someone absconded with the steel rods so I scrounge up two more metal rods of the same length. one is lead, one beryllium. now when I push both metal rods at the same time, one switch gets activated more than 4 milliseconds before the other.
    is that what you're saying?
  17. Jul 10, 2008 #16


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    It's interesting that your thought experiment works to obfuscate the very results you're trying to observe intuitively. You are using your finger and you are pushing mechanical switches. Both happen at very human speeds and with human imprecision. As Russ points out, with this human-level of imprecision, you will surely get results that seem instant. Your margins of error alone will be comparatively huge.

    I suggest the following: hit the rods with a hammer - a nice nice sharp and discreet blow - no mushiness about it. And use a seismometer or other such vibration recording device, so there's no question of timing.

    I don't know about the 30ms number, I am confident you will observe the beryllium rod transmit in half the time of the lead rod.
  18. Jul 10, 2008 #17
    I guess I see the errors in the thought experiment. The two ships tethered together would be considered "one ship", just as a very large oil tanker that is, say, 3 light seconds in length.
    If the this oil tanker were traviling foward and suddenly smashed into a large space ice asteroid at an angle sufficient to stop the foward motion (bring it to 0 foward speed), the back of the ship would continue foward, perhaps collapsing towards the front of the ship if it is not ridig enough. The back of the ship would not "know" the front stopped, it would continue (as would the middle, and most of the front) pressing foward at least on a molecular scale and may even, like I said, crunch foward. The information from the front of the ship, that it had stopped, would propagate through the rest of the ship as an energy release, not necessarily in the form of foward "speed" but kinetic release, momentum, heat, etc.

    So my error in the two ships experiment is that ship B would remain without foward motion until the energy (like in the oil rig) wave reached it.

    I guess I'll present my other thought experiment, using planets, to try and make my point (post here shortly).
    Thanks for all of the responses!
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