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Measuring the space/time grid

  1. Jun 12, 2014 #1
    Something thats been bugging me, but unable to find an answer for..

    The speed of light (in a vacuum) is absolute, well not really, its relative to the space/time its in. ie. The distant galaxies are moving away from us, but their local light speed is relative to their space/time.

    So since the earth is moving through space (around the Sun, around the galaxy etc), does light on earth appear to travel faster in one direction than the other? Has anyone made a device for measuring the speed of light in different directions, as to show how much faster, and which direction? and how would it read as the device is taken across and out of solar system? Wouldnt it be useful for measuring spacetime.
     
    Last edited: Jun 12, 2014
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  3. Jun 12, 2014 #2

    Bandersnatch

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    You're basically talking about Michelson-Morley eperiment from the early 1900s. It found out that light speed doesn't depend on the choice of reference frame, and is indeed always the same for all observers, which later lead to the development of Relativity.
     
  4. Jun 12, 2014 #3
    Thanks Bandersnatch, Reading up his experiment, yes, but his apparatus would not have been accurate enough as to be able to detect it. Today's atomic clocks shouldn't have too much trouble in getting fast/accurate measurements.
     
  5. Jun 12, 2014 #4

    Bandersnatch

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    It only had to be accurate to 1 part in 5000 to detect the anisotropy due to Earth's orbital motion.

    Here's a recent measurement accurate to 1 part in 10^17:
    http://arxiv.org/abs/1002.1284
     
  6. Jun 12, 2014 #5
    I'm not totally sure I understand that experiment, but if its like the original comparisons of resonance frequencies, I wouldn't expect it ever to show anything abnormal, as the thesis assumes c cant deviate upwards.

    If c is truly absolute, surely you can use that fact to measure speed of the apparatus through spacetime?
     
  7. Jun 12, 2014 #6

    Dale

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    That makes no sense whatsoever. How could you possibly use a frame invariant speed to measure the "speed through spacetime"? Let alone "surely".
     
  8. Jun 12, 2014 #7
    might be hard to explain. here's an example:

    I'm in the centre of a carriage of a moving train, say its travelling at 100kph. I'm stood shining a touch in front and behind me. Lightspeed isnt relative, thus shining it forward doesnt create a lightspeed of c+100kph, its still travels at c. So then it should take longer to reach the front of the carriage then it takes when shining it to the back of the carriage. Thus, you should be able to calculate the speed of the train, by measuring these time differences. (though only a slight difference, approx.1/11000000)
     
  9. Jun 12, 2014 #8

    Nugatory

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    Google for Einstein's thought experiment on relativity of simultaneity; "einstein train simultaneity" are good keywords for this search.

    To perform this experiment you have to have synchronized clocks at both ends of the train. The measurement you're describing won't give us the speed of the train through spacetime, it will give us the speed of the train relative to an observer for whom the two clocks are synchronized.
     
  10. Jun 12, 2014 #9

    ghwellsjr

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    No, no one has made such a device. If some one could, Special Relativity would be proved invalid.

    There are two types of devices that can be made:

    1) A device that measures the total time for light to traverse a distance to a reflector and back again. Note that this device cannot determine how much time it took for the light to get to the reflector compared to how much time it took for the light to get back. This device has only one clock and one ruler. When we divide twice the distance by the time interval, we always get the same answer, no matter which direction we point the device in.

    2) A device that compares the speed of the light from two distant relatively moving sources to see if different light traverses the same distance in the same amount of time. Note that this device is not measuring the speed of light, only determining if one light is faster than the other. This device always determines that the light from two different sources takes the same amount of time to traverse a given distance.

    Since your device cannot be made, then your question doesn't have an answer.

    Same answer.

    The fact that we cannot measure light the way you want us to is the reason that Einstein could postulate that the time it takes for light to traverse to the reflector is the same as the time it takes for the light to traverse back from the reflector. He uses this to define remote time. You should read the first couple sections of his 1905 paper.
     
  11. Jun 12, 2014 #10
    Nugatory,
    If the clocks were previously sync'd to each other while together, then separated to each end of the carriage? How would different observers result in different readings? Sorry, if I sound stupid.

    ghwellsjr, on your second point, so a tube of fixed distance, c would always be constant? No matter if the tube was moving? I can understand it would when measured A-B-A, but not A to B.


    I will do some more reading.. thanks for the replys.
     
  12. Jun 12, 2014 #11

    jbriggs444

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    That's "slow clock transport". The difficulty is that one clock is moving "upstream" (from the point of view of the train station) and that one clock is moving "downstream". Accordingly, they are time-dilated to different extents. When they reach the ends of the train, this will have caused them to go out of synchronization (from the point of view of the train station).

    The clock at the front of the train will read slow. The one at the back of the train will read fast.
     
  13. Jun 12, 2014 #12

    PAllen

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    Imagine someone in a train moves two synchronized clocks from the center to the ends of a train. Imagine someone relative to which the train is moving, moves two synchronized clocks a distance apart equal to the rest length of the train. Each such observer finds that their moved clocks are still in synch (per the Einstein synchronization convention). However, each finds that the other's clocks have become out of synch in the process of being moved.

    Of additional interest, if a third clock remains in the center for each observer, then each observer finds the moved clocks to be NOT in synch with the clock that never moved.

    These affects are all small for ordinary speeds, but all of these effects have now been observed directly with ultra-high precision atomic clocks at ordinary laboratory speeds.
     
  14. Jun 12, 2014 #13
    Thinking about it, the clocks don't even have to be sync'd. Measurements could be done, then put the train in reverse as to take comparison readings.
     
  15. Jun 12, 2014 #14

    Nugatory

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    Then the clocks at the ends have to be synchronized with the clock at the middle; otherwise we have no way of calculating the start and end times of the light flight.

    You're posting quickly enough that I'm not sure that you have googled for, read, and understood that relative of simultaneity thought experiment yet. That is a necessary prerequisite for understanding your scenario.
     
  16. Jun 12, 2014 #15
    (..still reading..!). Though everything I've read so far is what I already understood.
    Yep, you wouldn't be able to know the start and end times, but you could still measure the relative changes in length of time (caused by the training moving forward or backward)
     
  17. Jun 12, 2014 #16

    ghwellsjr

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    On my second point, I didn't say that the speeds of the two lights were being measured. I said they were being compared.
     
  18. Jun 12, 2014 #17
    Hi George, yes, though comparing two A to B, or comparing two A to B to A?


    Im halfway through Einstein 1905 ON THE ELECTRODYNAMICS OF MOVING BODIES. I agree with his assertions about time syncing at different coordinates in different motion states, but I disagree as to his reasons why. c can play tricks with observable time, and in his correct observations of this, hes seemed to made wrong conclusions and now confused c with time itself, then goes on stacking up other examples all based on his flawed conclusion. Think I need to read it from the beginning again!
     
    Last edited: Jun 12, 2014
  19. Jun 12, 2014 #18
    So you think Einsteins conclusions were flawed. You're not alone. Many people did. But he was proved right by experiment. You don't seem to grasp the meaning of the principle f relativity. It basically states that there is no such a thing as an object's absolute speed through spacetime. You're not moving through space time.
     
  20. Jun 12, 2014 #19

    ghwellsjr

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    Comparing two A to B.

    You should also read the wiki article on the one-way speed of light.
     
  21. Jun 12, 2014 #20

    Dale

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    Sure, you can do this. You don't even need light, just anything that propagates with an isotropic speed in a given reference frame. This has nothing whatsoever to do with measuring speed wrt spacetime, only speed wrt the given reference frame in which the train is moving.

    We should clear up some terminology which I am afraid may be getting in the way. The speed of light is called "invariant", which means that its value is the same in all reference frames. When the value of something depends on the reference frame then it is called either "frame variant" or "relative". The word "absolute" is rarely used scientifically in this context, and its usual connotation is that it is the value in the "one true reference frame" which is also commonly known as the "aether frame". Since there is no experimental evidence for the "aether frame", despite a lot of effort to find it, it is a discarded concept except by novices and crackpots.
     
    Last edited: Jun 12, 2014
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