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SR transforms of matter/light interactions

  1. Oct 16, 2006 #1
    Hello

    I'm trying to picture various light based experiments as described from a frame that is in motion with respect to the experiment devices.

    For instance the reflection of a photon in a light clock: the atoms (and their fields) that constitute the surface of the mirror are squashed. How is the reflected photon adapted to match this reality? The same question goes for light refraction.

    The trickiest may be how to deal with light diffraction through a slit: the slit is thinner in the moving frame.

    Do these questions require Quantum Electrodynamics for understanding, or are there simplified answers for the layman?

    Thanks in advance for any hint.
     
  2. jcsd
  3. Oct 19, 2006 #2

    Aether

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    Using SR you can build a first inertial frame that is at rest with respect to the experiment, and a second inertial frame that is in motion with respect to the first. An inertial frame is a coordinate system in which Newton's laws of motion are assumed to hold true. Every "event" (x,y,z,t) in one frame corresponds to an event (x',y',z',t') in the other frame, and the coordinates of these two events are related by a "coordinate transform" (in SR, this is the Lorentz transform).
    Although there is a physical (real) difference between objects in these two frames, a transformation of coordinates from one frame to another doesn't have anything to do with "reality" per se. Upon making our assumption above, we have established a framework in which our experimental results are to be interpreted. If you are primarily interested in "reality", then you must not confuse this artificial framework that we have constructed with what is actually being measured. If however you are primarily interested in applying the framework of SR, then you must not confuse that with "reality" per se.
     
    Last edited: Oct 19, 2006
  4. Oct 19, 2006 #3
    Sorry but there is no proof whatsoever for your claim.
    To assume that there is a physical difference is the position of ether based theories not relativity.
     
  5. Oct 19, 2006 #4
    Thank you Aether and MeJennifer for your replies.

    I'm aware that frame transforms are just different points of view, but I'd understood that through the transforms any phenomenon can be described with equal validity from any frame (the composition of velocities being one of the tools to perform that). For instance length contraction is well in tune with the fact that a moving frame exists with a time slant in another frame. The two borders of the slit in the diffraction experiment exist for another frame in different instants of their own time, hence the reduced width of the slit. I'm beginning to suspect that the light is blue-shifted for the moving observer, which would explain that light diffracts in the thinner slit. Maybe I'm totally off-track...
     
  6. Oct 19, 2006 #5

    Aether

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    There is a physical difference between objects in relative motion to one another. For example, if two identical atomic clocks are synchronized while co-located and at rest in a first inertial frame; then one clock is moved into a second inertial frame for a time; then returned to the starting position; when the elapsed times on the two clocks are compared they are seen to be physically different. This difference is predicted to be the same by both LET and SR. These two theories do not disagree on physical effects, and this certainly is one.
     
  7. Oct 19, 2006 #6
    The above is a very bad example, the "motion" that you describe is certainly not inertial since it involves a turnaraound, so SR does not apply. All you are showing is a manifestation of the Twins Paradox (which isn't a paradox after all).
     
  8. Oct 19, 2006 #7

    Aether

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    I do not want to argue over the semantics of what "between objects in these two frames" means (e.g., the turnaround is a transition between these two frames, etc.). I have clearly stated my point in mathematical terms elsewhere in this forum, provided ample authoritative references (see/search Mansouri-Sexl, and/or Zhang) for the same, and I will refer you to that lengthy discussion for clarification. Suffice it to say that SR is a coordinate-system dependent interpretation of the laws of physics, and as such it should not be confused with "reality" per se.
    All I am showing is the difference between a real coordinate-system independent measurement, and a coordinate-system dependent interpretation(s) of a real measurement.
     
    Last edited: Oct 19, 2006
  9. Oct 19, 2006 #8
    I'm getting confused by the contrary arguments. When a receding frame receives a red shifted EM signal from another frame, is the red shift just interpretation, or real effect? When the 40 m long rocket fits into the 20 m long barn, is it interpretation or real event?
     
  10. Oct 19, 2006 #9

    Aether

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    Measurements of red shifted EM signals (at least over short distances and neglecting gravitational effects) are real because they depend on the dimensionless ratio [tex]\beta=v/c[/tex]. If you assume that c is isotropic when interpreting this measurement, then that is where interpretation comes in.
    Please show the calculation, or provide a link to where you have seen this calculation.
     
    Last edited: Oct 19, 2006
  11. Oct 19, 2006 #10
    If I don't take isotropy for granted, what are the other interpretations?

    It's the (in)famous barn and pole paradox about relativity of simultaneity. The linked page shows the calculations as well as the Minkowski diagram of the case. The rocket fits in the barn only for the barn's frame of reference of course.
     
  12. Oct 23, 2006 #11
    [Mentor's note: I moved 48 postings preceding this point into a new thread, Relativity, LET and Reality.]
    ==================================================

    That's right, I was trying to picture basic Special Relativity transforms applied to electromagnetic interactions between light and matter, in particular reflexion, refraction and diffraction. The common illustrations for SR represent light timelines as simple lines, which is OK for most calculations, but when we want to look at some phenomena in which doppler shifts might play a role, we have to analyse things at a finer level of detail, considering electromagnetic interactions between light and sensors, or light and the mirror's electrons, etc. The simultaneity lines, or planes, or spaces, of the different observers' IFRs cut light paths at different angles, which seems to call for important differences in interpretation. I still have a lot to learn about electromagnetism, so maybe I'm just uttering nonsense.

    Attn Aether: I'm aware that in a stationary ether model simultaneity means a different thing than in SR, so the rocket or pole or ladder will fit into the barn from both points of view (unless the barn has a high opposite ether-relative speed, of course).
     
    Last edited by a moderator: Oct 24, 2006
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