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Can you determine absolute motion?

  1. May 9, 2011 #1
    I have heard it said that an object not feeling acceleration cannot determine if he is in motion unless he sees another object to compare himself too.

    But what if you had an apparatus that was made of two spheres. One inside the other. Perfectly centered on each other. The inner sphere emits light pulses at a given frequency. The outer sphere is lined with photo receptors and are ALL equi-distant from the surface of the inner sphere. All Photo receptors have a clocked that is syncronized with only the photo receptors immediatly next to it (minimal distance between them).

    We know that the propogation of light is emitted from its 3d point in space and is subject to the doppler shift.

    With that said if this entire apparatus were "absolutely still" thus having "zero motion" a pulse of light from the center would reach ALL surfaces of the outer sphere at the same time and all clocks would register as recieving the light at the same time.

    However, if that apparatus were in motion in a given direction that same emition of light would hit the receptor that is in the opposite direction of travel earlier than the others ending with the last receptor recieving the light being the direction of travel. Since all clocks are synced to each other there is a time difference between the first and last and thus can determine the speed at which it is going.

    Now that you have speed and direction you have Velocity.

    If this is the case then can we say that something is at "absolute rest"?
  2. jcsd
  3. May 9, 2011 #2
    The short answer is "no".
    Absolute velocity has no physical sense. This is actually the foundation of the theory of relativity.
  4. May 9, 2011 #3
    True it is the foundation of the theory of relativity. Not arguing that. Just saying what would this imply and would it the measurements and predictions be accurate according to the setup I made?
  5. May 9, 2011 #4


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    This is not correct. Light always travels at c, you can't "add" a speed to it. So light would hit all the receptors at the same time.
    Remember that EVERYONE will always measure the same speed of light(c).
  6. May 9, 2011 #5
    In addition to what f95toli said, it wouldn't work even if you used something other than light. This spherical object is never in motion relative to itself, you could use light, sound, water, gas, coca-cola, or anything else you wanted and it still wouldn't work.

    The relative motion of the device wouldn't change anything, the signals would always hit the receptors at equal times regardless of the type of signal unless the object is accelerating, which can already be determined with a simple pendulum.
  7. May 9, 2011 #6
    Slightly off. Light does travel at the speed of light correct. It does this from the point of origin. Hence why we see light of a moving object having a doppler shift. Classic proof is the Red/blue shift of the light of stars we see.

    It is true that there is a relativistic idea that light approaches all objects and the speed of light but that is not supported by what we see with the red and blue shifts of light.

    a good quick link for this would be http://www.astro.ucla.edu/~wright/doppler.htm
    Last edited: May 9, 2011
  8. May 9, 2011 #7


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    Wait, what? Are you seriously suggesting that red/blueshifted light is actually traveling at a velocity different from c?
  9. May 9, 2011 #8
    Red/blue shift is from a lengthening/contraction of wavelengths and has nothing to do with the speed of the photons.

    And, even so, this is irrelevant because in order for red/blue shift to occur, the source has to be accelerating to/away from the observer, which could never happen in your device since it's two spheres mounted to one another as one piece, with the outer one being the "observer" and the inner one being the "source."
  10. May 9, 2011 #9


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    It's not just an idea: the whole of relativity is based on that assumption and there is overwhelming experimental evidence to support relativity.

    The Doppler effect is 100% compatible with relativity: see Relativistic Doppler effect. The shifts are a change of frequency, not a change of velocity.

    And the answer to the question "Can you determine absolute motion?" No. The whole of relativity is based on that assumption and there is overwhelming experimental evidence to support relativity.
  11. May 9, 2011 #10
    No. The photons are traveling at the speed of light. From the point in which they were emitted. The closing speed to a reference point is not necessarily C if the reference point is in motion.
  12. May 9, 2011 #11
    Yes it is..
  13. May 9, 2011 #12
    One thing should be certain:
    Light travels at the "speed of light" or c in which ever reference frame you are.
    Always travels at v=c. If you shine light at someone who is going at you at 200,000,000 m/s and they see the light traveling towards them at c. The same speed as any light they see.
    All this is of course in vacuum.
  14. May 9, 2011 #13
    There needs no acceleration. Just a closing or opening speed of the objects in question. Also, you are correct that we see these shifts due to the wave length change. This is also has an effect on the spacings of the photons (which are the same thing).

    Photons emitted at a set interval expand out in a perfectly spherical pattern and if the light source is in motion the spacing between the surface of these spheres are not the same distance all they way around. (hence doppler shift).
  15. May 9, 2011 #14
    If we can't determine absolute motion, what happens when a spacecraft compares its clock to a clock on earth? Why would we be unable to discover which direction of motion causes the spacecraft's clock to run faster than the clock on earth?
  16. May 9, 2011 #15
    When you say opening/closing speed what do you mean?
  17. May 9, 2011 #16
    Well, in the spacecraft, clocks on earth run slow, but on earth, the clocks in the rocket run slow. It's, you know, relative.
  18. May 9, 2011 #17
    If objects are overall seperating this is an opening speed but if they are overall getting closer together this is closing speed. You can use the same term for both and just have it negative in the opposite direction. IE i run away from you i have a negative closing speed.
  19. May 9, 2011 #18
    and frequency is the the number of wavelengths in a given interval of time. The shorter the wavelength the higher the frequency. Thus if the wave length is getting shorter so would the space between the photons. (if it is in the direction of travel... opposite effect in the if you are standing behind the objects path.
  20. May 9, 2011 #19
    I misspoke, but what I said applies. A closing or opening speed between your source and reciever can't happen either if the spheres are mounted to each other.
  21. May 9, 2011 #20
    I have to object to your idea of "space between photons".
    A single photon has some wavelength. Even if you have just one photon you could - in principle - assign it with some color, ie. wavelength/frequency, whatever you want.
    The space between the photons is -here- irrelevant.
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