Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Length and time inside matter

  1. Aug 13, 2006 #1
    Special and General relativity rely on the possibility of having clocks and propagating light rays for defining (locally) time and length. My question is about how to define such things inside matter. Inside a metal block or inside a neutron star, light will not propagate that far, or not at all. Maybe the electromagnetic wave can be made to propagate when going to higher frequencies, but again, one could imagine very dense materials in which this would not be possible. The same question about clocks : How should one imagine a clock sitting inside a dense material ? And who could be there to observe it ? So, it seems that all the nice thought experiments used in special relativity would not apply inside matter. Is this an indication that relativity in general is not strictly valid inside matter (since time and length can not adequately defined there) ?
     
  2. jcsd
  3. Aug 13, 2006 #2

    George Jones

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    No.

    Ideal clocks function perfectly well inside matter. :biggrin:

    To probe the spacetime inside a material body, find what the metric says about worldlines of imaginary particles that don't interact with the matter that makes up the body.

    Interactions and other physics can also be taken into account by, e.g., replacing partial derivatives by covariant derivatives.
     
  4. Aug 13, 2006 #3

    russ_watters

    User Avatar

    Staff: Mentor

    It's a thought experiment, notknowing. Does putting up a roadblock imply that a road ceases to be a road?
     
  5. Aug 13, 2006 #4

    Hm, Hm, thank you for your help but I'm not completely satisfied. To me, a clock is something that can "tick", in the sence that one has some periodic system (for instance an oscillator) and some counting system. To describe reality, one needs real clocks, which are made of real materials and which really function : so idealised or imaginary clocks are to me not part of reality and therefore can not be part of a realistic description of nature. Likewise, how will one find the worldline of an imaginary particle ?
    Special relativity also used (in the thought experiments) real light (or photons). Despite of being thought experiments, such a situation can indeed actually be realised since we know how to generate light and we also know that light propagates in the vacuum between two observers. Such a situation seems not to apply inside matter.
     
  6. Aug 13, 2006 #5
    Strictly speaking this is true.
    But the only location where light rays do not propagate or where there are no "clocks" is at a singularity.

    It seems you have a too narrow definition of a clock.
    The idea of a clock is simply anything that has a period.
    QM teaches us that practically everything can be represented as a wave and waves have periods right?

    In SR and GR when we speak of clocks slowing down, we are not just talking about some human fabricated device. No instead we are talking about all processes slowing down.

    Note that length contraction and time dilation are not symmetrical with respect to spatial dimensions. While length contraction only occurs in the direction of relative motion, time dilation applies to all particles and waves in the obect in relative motion.
     
    Last edited: Aug 13, 2006
  7. Aug 14, 2006 #6


    Your argument about clocks seems OK, but what about the definition of length ? Here one needs some propagating signal (light) to be able to compare different points/events. Inside a dense solid, one can maybe generate an electromagnetic wave which moves a few atom layers from the point it is generated but not further.
     
  8. Aug 14, 2006 #7

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    There's something rather odd with your original question, and your subsequent responses.

    If I put a "clock" in an opaque box where there's no "propagation" of light whatsoever, are you implying that one has problems in defining time and length?

    Note that "light" in SR can almost be considered as nothing more than a euphimism for any electromagnetic interaction. There are EM interaction inside matter - that's who they stay together most of the time, especially in metals where the EM forces are responsible for holding them together in a regular lattice. So the absence of any visible light propagation is irrelevant in the presence of time and length in such a situation.

    Zz.
     
  9. Aug 14, 2006 #8
    First part on the clock : Yes, indeed. A clock alone is not sufficient. To be able to compare things like length (or talk about length contraction for instance) at two different locations, one needs that one can transmit signals (EM wave in general) between these two locations.

    Second part on "light" : I didn't want to imply that one needs visible light. Indeed any wave propagation (at the constant speed of light) will do. My question is whether one really has the possibility to propagate such waves over significant distances in arbitrary matter at arbitrary high densities.
     
  10. Aug 14, 2006 #9

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    But you don't need a "light" clock for time to exist. The light clock in Einstein's SR is simply a thought experiment to demonstrate the principle. The EM interaction within an atom can also be used, but not many people can understand that. I mean, how do you think a cesium atom knows that it makes that many oscillation per second for us to use it as our standard definition of a second? Did it need a "light clock" to be able to do that?

    I'm not sure what that has anything to do with "time" and "length", really. You seem to imply that if I absorb a photon and nothing can get transmitted, that time and length are then no longer defined? That would be utterly strange, and condensed matter physics/solid state physics are using bogus dynamical systems right now to make all these amazing predictions that you are using in your electronics.

    Zz.
     
  11. Aug 14, 2006 #10
    Part I : I've never talked about a "light clock".

    Part II : Length : Please give me then your definition of length inside a material where no photon can be transmitted.
     
  12. Aug 14, 2006 #11
    You do not need to border on the absurd. To measure length you need two things:

    1. two SYNCHRONISED clocks at the two ends of the material you are tryng to measure (not necessarily INSIDE the material, just at its ENDS)

    2. a signal of known speed (em , accoustic, etc) that is timed by each clock (start, finnish)

    Now, you can have the whole setup OUTSIDE the material that you are trying to measure and still get valid measurements, wouldn't it? This would work well for the pathological cases that you seem so preocupied with, like materials that do not allow ANY type of signal to propagate thru them, right?
     
  13. Aug 14, 2006 #12

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    What's wrong with the conventional definition? You do know what a "crystal lattice" is, don't you? Please refer to a solid state text if you don't. Now how do you think we can talk about such a thing inside ALL crystalline material, even in those that have no photon transmission? The metals and semiconductors that you are using depends on that. So maybe it is you who need to reconcile the fact that we DO have "time and length" inside these materials.

    For some odd reason, you seem to have totally ignored solid state physics and the successes from such a field in arguing your point. Why is that?

    Zz.
     
  14. Aug 14, 2006 #13
    I though this was a discussion forum where one could, IN A FRIENDLY WAY, discuss things in an open spirit. Some people here seem to like the agressive way however. It is typical that very fundamental questions may look absurd or trivial to people who only are used to scratch at the surface of things. Therefore I say goodbye to this discussion forum (and further reactions will not be read).
     
  15. Aug 14, 2006 #14

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    There is no "discussion" here, because when I keep bringing examples from solid state physics, you continue to ignore them as if they do not exist. This is not only "unfriendly", but it is also annoying. So your decision to dump this forum when I brought up another solid state example is consistent with that.

    Zz.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Length and time inside matter
  1. Length of time (Replies: 13)

Loading...