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Isn't this a weak basis for time paradoxes ?

  1. May 8, 2004 #1
    Your wide spaceship has inside a light and a receiver, widely separated, sideways to direction of travel. As pilot, you notice the blinking light always takes the same time to hit the receiver, whether you are approaching speed of light or not. But an outside observer notices that the path the light travelled was much, much longer when you are in motion, and therefore perceives it as slower (at near C it creeps along the sidewalls) - so the conclusion is: time is slowing for you as the pilot.

    I agree that light travels a much longer distance, but to insist that because of the constancy of C, time must be slowing down, is a jump. How well is the constancy of C established ?

    This must not be confused with the simultaneity experiment: If you were seeing a synchronized huge digital clock on Mars, it would be lagging. Here there is no time compression, the report of time is merely delayed. The absence of an ability to verify simultaneity over huge distances to me does not threaten it as a concept ? We are just finding out late about what time it is, but can still have full confidence in the clock..
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  3. May 8, 2004 #2


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    Very well. For one, it can be shown that it the speed of light were not consistant for all observers, electrormagnetic radiation emitted by one source would not be detected as a wave by an observer moving with respect to the source. (I.E. a radio in a moving car would not be able to pick up any radio stations)

    For another, the effects predicted by the constancy of light (such as the time dilation you mentioned) have already been detected and measured.
  4. May 9, 2004 #3
    Very well. For one, it can be shown that it the speed of light were not consistant for all observers, electrormagnetic radiation emitted by one source would not be detected as a wave by an observer moving with respect to the source. (I.E. a radio in a moving car would not be able to pick up any radio stations)

    Janus - I don't understand what you are saying - can you elucidate
  5. May 9, 2004 #4
    ndvcx - your apprehension re the light clock experiments is justified. Although this hyperbole is to be found in almost every beginning text on SR, it poses some vexing questions. For example, if the beam is aimed so that it bounces back and forth when the two frames are not moving relative to one another, and then the frame that contains the source takes off in a relativistic way - the beam will no longer strike the opposite mirror at the same point unless the motion affects the source in such a way that it modifies the direction in which the photons are launched (note-there is no ether substance within the spaceship to drag the beam forward in the direction of motion) so the launch angle of the photon source must be altered in order for the photon beam to strike the opposite mirror perpendicularly as viewed within the moving
    spaceship. If the launch angle is not modified when the spaceship takes off, the beam will appear to the outside observer to simply go back and forth retracing the same trajectory - whereas inside the space ship it will appear to propagate backward (opposite to the direction of the spaceship's motion). In this case the observer in the spaceship will be able to make a true measurement of the time taken for the beam to traverse the saw-tooth because he can place on board clocks that are comoving with the spaceship - and the actual time recorded thereby will be t = h/c where h is the hypotenuse of the saw tooth - whereas the outside observer will be able to measure the time in his frame with a single clock as t = L/c where L is the width of the spaceship - the two times will be different - but the difference is not an observational one - it represents two actual (proper) times rather than relative observational conclusions.
  6. May 9, 2004 #5
    More on the above - if (instead of a local source within the spaceship) the photon arrives from an external source (the E frame of the earth) , e.g., traveling at a right angle to the bottom mirror in the spaceship - and it enters through a small aperature - it will sawtooth bounce back and forth (up and down) within the moving space ship and the time between cycles can be determined with two clocks separated by a proper distance as measured in the spaceship (S frame). In the E frame, the photon will be seen to bounce up and down covering the same path and so the proper time for a cycle in the E frame can be measured with one clock - this is configuration is analogous to aberration - so it should not be surprising that there is a difference between the passage of time in the two frames. Now if a photon source is attached to the bottom mirror to launch photons vertically upward when the two frames are not in relative motion, it should not be possible to detect a change in the point where the photon strikes the upper mirror when the spaceship is moving at a constant velocity with respect to the E frame (if this were not the case we could detect absolute uniform motion by simply observing how the spot on the upper mirror moved when the S frame moved). So by what means does the photon adjust its internal trajectory so that the upper spot is unchanged when the source is moving - is the photon somehow given a tangential boost that modifies its direction of departure - if so, does this mean as the author of this thread questions, whether the total velocity might be other than c (a vertical component c and horizon component v)? Or is there a relationship between the source velocity (the old emission theory) that takes into account the velocity of the tangent component without affecting the constancy of c? The photon entering from the exterior frame E is obviously unaffected by the motion of S but the interior launched photon is.
  7. May 9, 2004 #6


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    It may be best to consider a whole family of light beams or, better, a circular/spherical wavefront emitted at the source. In this case, there's nothing to aim. Light will be reflected by each distant mirror that faced the source.
  8. May 10, 2004 #7


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    Typo fixed:
  9. May 10, 2004 #8

    Doc Al

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    launch angle modified? No!

    Perhaps I'm misunderstanding your point, but you seem to be saying that the moving ship must somehow change the launch angle of the light beam else things won't work out right from the point of view of an outside observer. Not so. The properties of a light beam in that moving ship are independent of its speed with respect to another frame. The moving ship doesn't know or care about its speed with respect to an outside observer. It may be zero for all they know.

    Of course, the outside observer will observe the light to move at an angle. But this is simply due to the relative motion, not any action on the part of people in the moving ship. Imagine a person in a train car bouncing a ball against the floor of the car. In the car, the ball just goes up and down. But from the point of view of someone watching on the platform, the ball moves at an angle. But obviously the ball-bouncer doesn't change the direction of his bounce to suit the observer on the platform. Light beams are no different.
  10. May 10, 2004 #9

    That’s nonsense. By that same false reasoning, a moving sound observer would not be able to hear any sound.

    Moving cars receive redshifted and blueshifted radio signals all the time, but the effect is not noticed because it is extremely slight at low car speeds speeds, so nobody in the car notices it.

    NASA receives redshifted and blueshifted signals from space craft all the time. They also have to de-tune or re-tune their receivers to receive these redshifted and blueshifted signals.

    There are two causes of the Doppler effects. One is the cause of “stretched out” or “compressed” waves causing a redshift or a blueshift, and the other cause is due to the c - v and c + v effect of the relative speed of the signal and the observer, which also causes a redshift and a blueshift. Droppler explained this in the 1840s and he even predicted the redshifts and blueshifts of starlight, decades before they were ever observed.
  11. May 10, 2004 #10
    Radio, TV, light, and other EM signals operate as a continuous spectrum. Having a car radio move down a highway while receiving a radio signal is the same as having a planet move through space while receiving light from a star or galaxy. Of course we see redshifted and blueshifted star and galaxy light all the time.

    The observed frequency shifts because of those two Doppler causes I just mentioned. Everybody knows about the “stretched out” and “compressed” signals, but not many people today know about the c – v and c + v phenomenon.

    It’s the same way with sound waves. This is why an observer riding on an open car at the end of a train observes the NORMAL tone of the train whistle, even though the sound waves are “stretched out” in the air behind the whistle. The second Doppler cause cancels out the first Doppler cause, so no “Doppler effect” is “observed” by the rear train observer, even though there are two separate and distinct Doppler effects at work in this example.

    If what you are saying were true, no one would be able to observe light waves coming from any star that the earth is moving relative to, which means that because the earth is moving around the sun, earth people, by your “hypothesis”, would not be able to observe any starlight at all, since the earth is moving relative to all stars, and it is moving relative to the light photons emitted years ago by those stars. This produces a c - v and c + v effect relative to the moving earth, at some distance from the surface of the earth, probably several thousand miles.
  12. May 10, 2004 #11
    Doc al - yes - I agree with what you have said - with the ball analogy we have no issue - and if we were emitting electrons rather than waves - the notion of comoving particles of any sort is without question - no coorections are required inside the moving spaceship --- and if photons act like electrons in the their emission(s) then the changed velocity of the S frame after acceleration is accounted for because all elements of the capsule are comoving - but can particle physics be applied to the direction of photon trajectory - thre photon travels with respect to space via wave phenomena - the S frame has gone through an acceleration - how does the photon source get updated as to the fact that the emission source has undergone acceleration - unless there is a component of the velocity that is emission velocity dependent - for example, emission by an equivalent source outside the S frame of the moving spaceship that enters through an aperature will not strike the upper mirror directly above the aperature (because of aberration) but the photon emitted from the same perpendicular laser source (for example) inside the moving frame S will -- so my iintrigue here boils down to the issue of how the two emissions are mechanically correlated. SR of course solves the issue be definition.
  13. May 11, 2004 #12

    Doc Al

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    Particle or wave model, take your pick. Either way you need to specify the direction of the light with respect to some frame. Then you can use the Lorentz transformations to find the angle that the light will make in any other frame.
    The S frame is moving along at a uniform (high) velocity. No need to "update" the photon source--since nothing is happening to it. But the velocity of the light (the direction, at least) is frame dependent. So from the "stationary" frame's viewpoint the light in the ship does move at an angle.
    That outside light is emitted perpendicular to the direction of the ship's velocity as seen in the stationary frame. But observers in the ship will see it as moving at an angle: Lorentz transformation (aberration) again.
    Well, I wouldn't say that SR solves the problem by definition. (I'm not really seeing a problem. :smile: ) But SR certainly explains how the angle of a light "ray" transforms between frames, and how what is perpendicular in one frame moves at an angle in another.
  14. May 11, 2004 #13

    But it doesn’t take any longer for the ball to go up and down for the guy on the train and the guy on the ground, so there is no “time dilation” involved in this phenomenon. The ball is still traveling up and down about 4 feet. The added motion of the train gives the ball an additional sideways motion. But it still covers the 4 feet in the same amount of time. No time dilation.

    What you need to do is bounce the ball on an open flatcar while the train is moving and see what happens and you will see how this bouncing ball analogy is not a good one. The guy throws the ball toward the floor of the flat car and the wind blows it toward the back of the flat car. The reason the ball goes up and down with him inside the sealed train car is because the air is traveling with the guy and the ball. But the guy is moving through the air when he is on the open flatcar.

    So there are two things to consider with light: 1) the inertia frame, and 2) the propagating medium. This is just like the guy with the ball inside the car and outside on the flatcar.

    If a guy is bouncing a light beam off a mirror, and we say he is “stationary” with another guy in space (floating in space), when the guy and the laser and mirror start to accelerate and move, there is no reason to believe that the light beam will start accelerating sideways and move along with the guy and the mirror. Once the light beam has left his laser, the guy and his laser have no more influence over it. It is not going to start moving sideways with the guy and his laser and his mirror.

    Yogi is right, when the guy starts to move, the beam is going to miss the mirror.
  15. May 12, 2004 #14
    Well - yes - during acceleration there is a curvature of the beam as seen in the moving S frame because the photon's path is unaffected by the motion of the S frame once it has left the source (this is just a version of Einstein's elevator thought experiment). But the same does not hold for uniform velocity, this is where ether theories can be deceptive - the photon's trajectory is not modified by any ether wind if such exists just as a particle in an ideal fluid is not affected by motion relative thereto (D Alambert's proof). What is curious is that a photon source such as a laser aimed for example, at a distant planet, will not emit photons in the direction of the planet when it is moving perpendicular to the direction of its aim - in other words, the direction of the photon is determined by the sideways motion of the source but not its velocity (if the conventions of SR are correct) --- as opposed to a bullet fired from a rifle that is mounted to a platform moving perpendicular to the direction of aim (e.g., shooting buffalo from a moving train) where the velocities add pythagorean wise
  16. May 12, 2004 #15
    You can’t have a Doppler effect caused by “stretched out” or “compressed” light waves without a medium. It is the medium that stretches out or compresses the light waves of the emitter that is moving through the medium.

    If your moving source always acts like the center of expanding light “spheres”, and the center of the spheres move through space with the source, there is no “stretched out” light waves toward the rear of the source and no compressed waves to the front of it. This is an “inertial” situation and a “no medium” situation, and thus the waves can never be “stretched out” or “compressed”. And the light will be moving sideways with the emitter. A diagram of this will show light being emitted as concentric circles around the emitter, no matter how fast the emitter’s speed is.

    However, if you use this model, that means an observer to the rear of the moving emitter will receive the light at the speed of c – v. And that is what would cause the redshift, since the source is not emitting stretched out waves.

    You can’t have it both ways with “no medium” AND “stretched out” light waves in space trailing behind a moving emitter.
  17. May 12, 2004 #16

    Here is an example of stretched out and compressed light waves in a University Doppler diagram, but you would need a medium for this to occur. The light is traveling at “c” relative to the medium but not “c” relative to the emitter.

    http://zebu.uoregon.edu/~soper/Light/doppler.html [Broken]

    Here is another example and you can see what would happen with your light clock. Click on the gray area (which represents the medium), then drag the mouse to the right. A yellow vector arrow represents v < c.


    Notice that each light beam moving "up" will miss a mirror that is moving with the emitter, and the waves behind the emitter are stretched out. It is the motion of the emitter through the medium that is doing this.

    If there were no medium, then the circles of light around the moving emitter would be concentric, not offset.
    Last edited by a moderator: May 1, 2017
  18. May 12, 2004 #17

    Doc Al

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    Sure there is time dilation. It's just not significant at "normal" speeds. Get that train moving at some good fraction of the speed of light and time dilation will be evident.
    Don't confuse things by thinking of acceleration. Keep it simple. The two frames are in uniform motion with respect to each other. Think of them as being in two different space ships. Do you really think that the person in ship A must do something to aim his light differently just because ship B is moving past him?
  19. May 12, 2004 #18

    Doc Al

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    Actually the velocities add in exactly the same manner for bullets and light beams: relativistically. Of course for low, bullet-like speeds, this reduces to the classical galilean addition of velocities formula.

    As I mentioned in a previous post, the angle that the laser beam moves depends on the frame in which it is observed. So a laser beam aimed perpendicular to the motion of the planet, will be detected as hitting the planet at an angle (per the Lorentz transformations for velocity). This is our old friend aberration again.
  20. May 12, 2004 #19
    Do you have some documentation that proves a bounced ball in a train will bounce at a slower rate if the train is moving? I’ve never heard of such a thing in physics. I think you are getting your imagined light signals mixed up in your thought experiments.
  21. May 12, 2004 #20
    In real life, something can’t start to move or change position without accelerating. The light from the accelerating laser will miss the mirror.

    If we have two guys next to each other in space, and if they have two “light clocks” with the laser light bouncing up and down, then if one of them starts to move, the light from the laser that starts to move isn’t going to start moving sideways for no reason. You are thinking of projectiles, such as machine gun bullets that have mass. A machine gun bullet will move sideways, but light has been proven to not act like projectiles.
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