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You have a solid bar that is

  1. Jul 15, 2011 #1
    186000 miles long with zero flex or give. You have an observer at both ends. Observer A has a machine strong enough to turn the bar in either direction. Observer B does not know which way its going to turn. Mr.A turns on the machine and the bar turns to the left. Mr. B sees its direction instantly. (no flex or give in bar) Would there be a frame of reference in which the B end of the bar is seen turning before the A end?? It seems that would be getting the information (bar turning direction) from A to B at faster than light speeds. (It would take a second for the light to get the info there) OK, let me have it, where am I wrong!!
     
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  3. Jul 15, 2011 #2

    xts

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    You just proved that no 'zero flex or give' materials exist in real (relativistic) world.

    Solid steel or even harder diamond are that hard because of electromagnetic forces between atoms of which they are built. Those electromagnetic force propagate with the speed of light. So always the other end of your extremely-hard-even-bigger-Koh-I-Noor would move a little bit later than you push/pull your end.
     
    Last edited: Jul 15, 2011
  4. Jul 15, 2011 #3

    PAllen

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    A bar with those properties contradicts relativity. It is no different than saying suppose I have a bar with negative mass. Nothing can propagate faster than c, including mechanical displacement of a bar. In reality, mechanical displacement of a bar travels at the speed of sound in the material, which is orders of magnitude slower than c.
     
  5. Jul 15, 2011 #4
    Fantastic information! Thanks to both of you!
     
  6. Jul 15, 2011 #5

    DaveC426913

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    To be clear on this, the mechanical pulse travels only at the speed of sound of the material, as PAllen points out. For diamond, the hardest substance known, this is a relatively glacial 15km/s.
     
  7. Jul 15, 2011 #6

    PAllen

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    Only 4 orders of magnitude slower than c :smile:
     
  8. Jul 15, 2011 #7
    And if the bar was made from material at the inside of a massive blackhole? Same answer? Or is a question regarding a density like that part of what makes black holes so interesting?
     
  9. Jul 15, 2011 #8

    xts

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    Then no answer at all, as no observer from inside of a black hole may report you the result of an experiment, so you ask meaningless question.
    I believe neutron matter (of which neutron stars are made) has it speed of sound much higher than diamond, but still lower than c.
    And no material in may propagate any signals (by any means) faster than c. That is just a foundation of SRT...
     
  10. Jul 15, 2011 #9
    You mean no known answer I take it? If matter that dense does exists, then maybe relativity is incorrect? Maybe we can gain access to that matter at some point, or find other objects with equal densities? Not doing anything other than thinking out loud and asking questions that bother me due to the little I know most likely!
     
  11. Jul 15, 2011 #10
    It's actually not accurate to say that relativity forbids anything traveling faster than c, it just forbids anything with mass traveling at c. Tachyons, particles with mass that always travel faster than c, are theoretically possible. But they would be capable of traveling backward in time and they've never been observed, so they are assumed not to exist.

    Density of matter doesn't make a difference here, however. The speed of sound, by definition, must be slower than the speed of light, since sound waves are ultimately caused my electromagnetism, which moves at the speed of light.
     
  12. Jul 15, 2011 #11

    bcrowell

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    Well, it's not just that they haven't been observed and violate causality. There are even more problems than that with tachyons. The WP article goes into gory detail. And "with mass" may be a little misleading; their mass would be an imaginary number.
     
  13. Jul 15, 2011 #12

    DaveC426913

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    It is not incorrect; it is that our current physics stops applying. We call that the singularity.

    The very thing that puts it out of the reach of our understadning of physics is what puts it out of the reach of ever gaining access to it -billions of g's - enough to prevent even light itself from escaping.
     
  14. Jul 15, 2011 #13
    Yes, imaginary mass opens up a whole 'nother can of worms.
     
  15. Jul 15, 2011 #14
    If conditions exist somwhere in our universe where the laws of physics stop applying, and nothing unknown is added to the event, (say an unknown material or force for example), wouldnt you think something is wrong (or at least a bit off) with the laws that we are using? It seems there would have to be an explanation. Is there anywhere we could create the same kind of conditions on a miniature scale and observe what is going on? Can or is the LHC capable of doing something like that? The thought of the known laws breaking down creates an unease in my stomach thats hard to describe, and I am just an old guy trying to learn all this as fast as possible. I wish I would have had interest when I was younger. All of it is so fascinating and mind stretching!
     
  16. Jul 15, 2011 #15

    DaveC426913

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    Our laws of physics. We are not done yet.

    The same thing was said earlier in the century when we saw particles living longer when they approached (what were to become known as) relativistic velocities. We didn't have the theories to describe these events yet. But then Einstein came along and tweaked Newton's classical universe. (Highly apocryphal summation of history. Sue me. :smile:)

    As with the limit of velocities, so with the limit of gravitational collapse. We just don't have the models yet.
     
  17. Jul 16, 2011 #16
    There is nowhere in the universe that the laws of physics stop applying. Yes, if it appeared that some phenomena violated the know laws of physics then we would have question our understanding of those laws and refine then. The step up from Newtonian physics to relativity was a big refinement, but in fact that sort of incremental improvement in our understanding happens all the time. As far as we know the laws of physics have always been the same everywhere and that is what defines a law, i.e. it is something that is universally valid. The only thing that changes is our understanding. In the past there a lot of things we did not understand or misinterpreted and mankind as whole still has a lot to learn.
     
  18. Jul 16, 2011 #17

    DaveC426913

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    Well, that's not true.

    The laws of physics, which are laws we wrote, do not apply in the singularity of a black hole.

    Assumed to be valid. But an assumption is not a given.

    And since we have no understanding of the circumstances at the singularity we have no laws of physics to apply.
     
    Last edited: Jul 16, 2011
  19. Jul 17, 2011 #18
    We already know something is wrong with the laws we are using, because relativity breaks down on the microscopic scale, and quantum mechanics is incapable of describing gravity. None of our current theories predicted the existence of dark energy, either. We're missing something, somewhere.

    Some people hope that the LHC will create microscopic black holes that we can study in the laboratory. That may or may not happen.

    Personally, I have a feeling that we are missing something very basic. Somebody will probably come along and look at existing data, interpreting it in a different way, the way Einstein did with relativity. I have a hunch that these increasingly convoluted theories like string theory and loop quantum gravity aren't going to take us anywhere. Just my opinion.
     
  20. Jul 17, 2011 #19
    OK, it seams to be a question of semantics. You understand the "laws of physics" to be something that is the consensus of opinion of our top experts and the mainstream understanding while I understand the "laws of physics" to be something that nature obeys and may not exactly coincide with our understanding. In my opinion nature "writes" the laws of physics and in your opinion humans do. Before Einstein introduced us to relativity was Mercury disobeying the laws of physics or did we just not understand the laws of physics very well at that time?
    Well we do have some understanding of the singularity. We have never observed one but only predict that they exist by extrapolation of our current understanding of the laws of nature. We cannot say a singularity MUST exist because our our understanding of the laws of physics predict it and then turn around in the next breath and say the laws of physics do not apply here. Its very existence is a consequence of the laws of physics. I will use the term "laws of nature" for my version of the "laws of physics" and restate that my position as if a singularity exists it exists because of the laws of nature and obeys the laws of nature. The laws of nature apply everywhere and nature always obeys those laws. We just have yet to make complete sense of them. The common statement that the laws of physics break down at the singularity means:

    1) The General Theory of Relativity does not have unlimited validity and is just an approximation of a more accurate theory just as Newtonian physics is an approximation of GR.
    2) The General Theory of Relativity is a complete theory with unlimited validity and apparent exceptions such as singularities are just a result of our misinterpretations when we extrapolate the theory beyond what we have currently measured.

    I personally think GR is too elegant to be wrong and take position 2, but that is just my personal belief. Hopefully in the future our collective improved understanding will bring us to a point where GR and quantum physics coexist peacefully alongside each other in some sort of unified theory with no contradictions.
     
  21. Jul 17, 2011 #20

    DaveC426913

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    Since we have no idea what happens to atomic matter at the singularity, how do you know it obeys any laws at all?

    It sounds to me like your definition is wholly tautological.


    Its existence sure. Defining something by its boundary does not define the thing.

    I can define the coastline of my country quite well while knowing virtually nothing about the ocean beyond that boundary - even if it is the ocean that defines that boundary.

    This is an empty definition.

    The purpose of a definition is to separate what something is from what it is not. Your definition defines everything as being a law of nature. Thus is makes no distinction, thus it defines nothing.

    Looked at another way, it is impossible not to be true. If a particle decided to not obey a law of nature, it would still be obeying the laws of nature, since whatever the particle does is defined as the laws of nature.

    Since it can't not be true, it is meaningless.
     
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