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How does light slide sideways?

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  1. Mar 19, 2015 #1
    The laser ranging of the moon doesn't make sense. The moon has a special mirror on it that doesn't reflect incoming light along the same angle as the angle of incidence only on the opposite side of the perpendicular, but instead reflects light back exactly on the same path as the light's approach. A telescope is aimed from earth directly to the moon's reflector. A laser is aimed through the telescope, the laser beam hits the reflector and bounces back exactly to the same telescope for detection and time of flight measurement.

    Consider the fact that the moon and earth are in parallel motion in space at 370 kilometers per second. The problem is that in the 2.5 seconds trip of the laser beam to the moon, the moon has moved and the light should miss the reflector completely. Even worse, in the 5 seconds of the round trip of the laser beam, the earth has moved quite a bit through space and the light should miss the telescope which has moved along with it.

    The only way for this to work is if light slides sideways to parallel the motion of the moon and earth which keeps the light beam always at the same angle of approach to the reflector, no matter what the path of the laser beam through space actually is. This sliding simulates the conditions as if the earth and moon were both hanging motionless in space when the measurement is done. Arguing that the telescope is aimed forward to intersect the moon in the same way a quarterback aims for his receiver where he thinks he will be when the football arrives doesn't work because the reflector still returns the light back to where the telescope was when the laser beam was first sent, and not to where the telescope has moved to.

    Does light move sideways? By what mechanism? SR doesn't really explain it. Do its postulates even predict it? How else could laser ranging be made to work?
     
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  3. Mar 19, 2015 #2

    PeterDonis

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    Motion is frame-dependent, and changing frames cannot affect any physics. So you can just analyze the experiment in a frame that is also moving at 370 km/sec in the same direction as the earth and the moon. Then the only motion involved is the residual relative motion of the earth and the moon.

    (Btw, this 370 km/sec figure is arbitrary anyway. There is no absolute motion, so I could just as easily say the earth and moon are moving at 99.99999% of the speed of light relative to a cosmic ray passing by. That doesn't change the physics at all.)

    No, all that means is that the laser should point at where the moon is going to be in 1.25 seconds, instead of where it is right now. (Btw, it's 1.25 seconds one way and 2.5 seconds round trip, not 2.5 seconds one way and 5 seconds round trip.) The light doesn't have to slide sideways; it just has to be aimed right. It's no different than shooting at a moving target: you have to lead the target in order to hit it.

    No, all that means is that the beam has to be wide enough to encompass the relative motion of earth and moon during the time of flight of the light. In fact, as the article linked below states, the beam is 1.8 km wide when it reaches the moon, and the reflected beam expands similarly on its way back to earth.

    Check out this article on the basics of lunar laser ranging:

    http://physics.ucsd.edu/~tmurphy/apollo/basics.html
     
    Last edited: Mar 19, 2015
  4. Mar 19, 2015 #3

    FactChecker

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    In any non-accelerating inertial reference frame, the behavior of light observed in that reference frame is as though the reference frame is stationary. That is the basis of Special Relativity. There is no "ether" in space that would make the light beam you are talking about fall behind that straight line from the Earth to the moon.
     
  5. Mar 19, 2015 #4
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    My bad about the time of flight. I remembered it wrong. As to your point about leading, it can't work. Both the moon and the earth are each others' target and the automated reflector on the moon does not lead the earth when it returns the light beam. I mention that in the paragraph of my original post.
     
  6. Mar 19, 2015 #5

    DaveC426913

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    You're still missing the point. Earth and Moon are stationary. They are not moving at all. (You are always at rest in your own frame of reference). It is the rest of the universe that is moving.

    As Peer says:
    Consider: if you throw a tennis ball at a fellow passenger aboard a plane moving at 500mph, does the tennis ball have to "slide sideways" at 500mph?
     
  7. Mar 19, 2015 #6

    PeterDonis

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    It does if both the earth and the moon are motionless, since no "leading" is required then. They aren't exactly motionless with respect to each other, so there is a small effect due to that relative motion during the time of flight of the light; but as I noted in my post, the spreading of the beam compensates for that--the beam is not an infinitely thin line, so it doesn't all travel in one single direction and is not all reflected in one single direction.

    The real question is, given that my analysis is correct in a frame in which the center of mass of the earth-moon system is motionless (so the only motion is the relative motion of earth and moon in the center of mass frame), how would SR analyze the experiment in a frame in which that center of mass is moving at 370 km/sec? I'll leave you to ponder that one, but here's a hint: Lorentz transformations affect angles as well as lengths and times.
     
  8. Mar 19, 2015 #7

    PeterDonis

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    Not completely; they do have some relative motion even in the center of mass frame of the Earth-Moon system. But it's a lot smaller than 370 km/sec, and the spreading of the beam during flight is more than enough to compensate for the relative motion during flight.
     
  9. Mar 19, 2015 #8
    Ok, so light is sliding sideways. Is there a mechanism for the sideways slide or just supposition? Is the light coming from the laser connected to the laser and therefore sliding sideways with the laser independent of the target? Also, is the reflected light from the target sliding sideways with the reflector independent of the laser?
     
  10. Mar 19, 2015 #9

    DaveC426913

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    Don't want to confuse him with details. :smile:
     
  11. Mar 19, 2015 #10

    DaveC426913

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    Please. No. Throw away your erroneous absolute reference frame.
     
  12. Mar 19, 2015 #11

    PeterDonis

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    That's not what he said.

    Look up Lorentz transformations and do the math; start with a frame in which the Earth and Moon are at rest (at least to a good enough approximation), and then transform into a frame in which they are moving at 370 km/sec, and see what you get. It won't be a description of light "sliding sideways".
     
  13. Mar 19, 2015 #12
    Forgive my confusion, but consider a second earth-moon pair. Say pair one is moving east toward an origin at nonrelativistic speed with earth-moon oriented north south. Say pair two is moving west toward an origin with the same orientation. Each pair shoots laser beams between the earth and moon and back again. What will the earth and moon in one pair see when observing the other pair and vice verse? Wouldn't each pair see themselves as stationary and the other as in motion with a laser beam moving sideways at the same rate as the earth and moon that are exchanging them? It tells me that there is some kind of "momentum" the light photons have to carry on the same speed of the sliding motion as the launching platform. Another observer sitting high above the plane with the two pairs will see two pairs moving towards each other and light beams within each pair moving sideways. Clearly I don't fully comprehend the nuances of relativity, because I don't see the mechanism. The lasers are not pointed at an angle, yet the light travels at an angle. The angle of attack is along the line of sight between the earth and moon in each case and not in the direction of propagation. Please explain. Is the scenario flawed?

    BTW, I never mentioned absolute reference frame in this thread. That was another thread to describe a scenario with no second observer. Please don't assume that is what I am assuming here. I asked how is it that light slides sideways and what mechanism. So far in this thread it was said that SR treats the frame as if it is stationary. Ok, In the two pair scenario what is happening? Please disregard relative motion between earth and moon and their individual rotations; those motions are not relevant to my original post.
     
  14. Mar 19, 2015 #13

    russ_watters

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    If the two observers that are stationary with respect to each other see the beam moving straight and the two observers that are moving relative to them see the beam - and the two observers - moving "sideways", is the beam "really" moving "sideways" or not?

    Have you read about the "light clock" thought experiment? You're describing it:
    http://www.emc2-explained.info/The-Light-Clock/#.VQuaQI7F-So
     
    Last edited: Mar 20, 2015
  15. Mar 20, 2015 #14
    I say yes. The motion of the emitter is impressed on the photons. How, I don't know. Consider the observer above the plane looking down.

    No. I just now made it up to focus attention on my real question. I'll look over your thought experiment. Thx for the link.
     
  16. Mar 20, 2015 #15

    A.T.

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    What one has to consider is not the relative motion of the centers, but of the surfaces. But even without the relative motion, with both tidally locked, their common rest frame is rotating, so you have Coriolis effects diverting the beam.
     
    Last edited: Mar 20, 2015
  17. Mar 20, 2015 #16
    How about this: Inside the Laser device sideways sliding light waves are amplified as they reflect back and forth between two moving mirrors.
     
  18. Mar 20, 2015 #17
    The moon is orbiting 1.023 km/s - mean orbital velocity with respect to the earth-moon barycenter
    The moon is 1.25 light seconds away

    In the 1.25 seconds the beam travels from earth to moon:
    the moon advances 1.28km in orbit
    the beam spreads to 1.8km
    the earth's surface at Apache Point Observatory rotates 0.5km

    In the 1.25 seconds the beam returns from moon to earth:
    the moon advances another 1.28km in orbit
    the beam spreads to 15km
    the earth's surface at Apache Point Observatory rotates another 0.5km

    Using line of sight aiming with 1.25 second latency:
    center of beam arrives at moon 1.28km off target reflector
    radius of beam is 0.9km
    target reflector is 0.38km outside beam radius

    I doubt the reflectors are even visible to make a line of sight aim prior to engaging the beam, but with the beam on and the locations of the reflectors known, hunting and sighting the return beam is confirmation that the correct leading is established.
     
  19. Mar 20, 2015 #18

    FactChecker

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    Only real experiments finally convinced scientists. In the late 1800's, scientists thought that light traveled through a stationary "ether" at a certain speed. They tried to experimentally detect the effect on light of the motion of the Earth through the ether. That never worked. They could never detect an effect of non-accelerating motion on the speed of light. The most famous experiment was the Michelson-Morley experiment.
     
  20. Mar 20, 2015 #19

    russ_watters

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    You said before that you weren't looking for absolute motion. It is inadvertent, but I think you are. The only motion that is relevant here is that the source and receiver are stationary with respect to each other. An infinite number of other possible observers might say the light is sliding sideways, but none of them need agree on what direction it is "actually" traveling. So when you say "the motion of the emitter" you are inadvertently suggesting it has an absolute state of motion. Different observers do not have to agree on its direction of motion.

    Hecht, if you add another observer moving in the opposite direction, both might say the beam is moving sideways, but one will say it is moving to the left while the other says it is moving to the right! Which way is it "really" moving?

    It is also worth noting that the "beam" is not a solid object. It isn't required to even be a line, much less travel parallel to itself. If you spin in place while holding a laser pointer (or water hose) the beam will be a spiral. That is in fact the shape seen in the lunar ranging experiment (see above about the coriolis effect).
     
    Last edited: Mar 20, 2015
  21. Mar 20, 2015 #20

    DaveC426913

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    What if you were right? What if remote observer B expects to see light sliding sideways to catch up to a receding Moon?

    OK, now what if there were a third observer C, moving East at twice the speed that E/M is moving? From their reference frame, they actually see E/M moving away from them to the West. So, for them, the light beam obviously must slide West to catch up.

    How can the light beam slide East and West at the same time?
     
    Last edited: Mar 20, 2015
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