emil Here is an even clearer one... Please Register or Log in to view the hidden image! GrumpyPlease Register or Log in to view the hidden image!
Why? Can you give something that supports your statement? A mathematical calculation, logical explanation, a link, something ...?
I use this post only as an intro to the subject. It is often misunderstood and sometime misrepresented that the Michelson and Morely experiments disproved the existence of an aether. This is not actually the case. The experiment as with many that followed were set up to detect the Earth's motion relative to "the aether" or light medium.., and failed to do so. They were null results. What they proved is that they were unable to detect any motion of the Earth relative to the ether. That is not the same as proving that there is no ether. They were not even measuring the velocity of light in any medium. They were using an interferometer to try and determine if light traveld at different speeds in different directions. Their equipement was sufficiently sensitive that it should have been possible. All they proved is that they could measure no difference in the speed of light in a wide range of directions relative to the Earth's rotation and orbit. Likewise, it is often said that Einstein tossed out the ether with the introduction of SR. This also is not true. All Einstein did was to prove that experience could be explained without the depending on "an ether". Now, based on the knowledge we have today it would be a fairly secure bet that the ether as define in the late 1800's does not exist. It by the way was completely Newtonian in character and as such unsupported within the context of relativity, special or general... Space is the medium through which EM waves and light propagates. There is still some debate as to exactly what "empty space" is, but one thing it is not, is empty of anything but matter. Part of the discussion about a medium and light and how light propagates from a point of origin, seems to be stuck in a more or less Newtonian or in the least flat spacetime of special relativity. Space is neither, it is a dynamic counterpart to matter, it is curved, warped and possibly even moves, within some limited sense. General relativity gives space an intrinsic substance, that is smooth and has no parts that can be observed over time.., and yet also requires it to be dynamic and in some respect changing. Quantum mechanics endows space with vacuum energy or zero point energy and virtual particles, a medium of varying EM fields or potentials. The reality probably lies somewhere in between, but in either case space is not empty and whatever it is, it is the medium through which EM radiation propagates.
Emil: Let me go back and answer some questions you posted earlier. This says nothing about the motion of the observer. There will still be a Doppler effect if the observer is moving towards or away from the source. As long as the source isn't moving. This is approximately correct provided you're talking about waves moving through a medium (i.e. waves other than light in a vacuum), although I'm not sure what you're saying is "proportional" to what other thing. Recall that light needs no medium to travel, and the speed of light is constant in all reference frames. Therefore, for light in vacuum, the observer can never see a different wave speed. Also, it can be important to know that for light in a vacuum there is no distinction between a moving source and a moving observer. The only thing that matters as far as Doppler shifts are concerned is the speed of the source relative to the observer. The same is not true in a medium. This picture correctly shows the point of view of a reference frame in which the source moves. Whether there is a Doppler effect or not depends on where the observer is located and how the observer is moving. In contrast, the picture above correctly shows the point of view of a reference frame in which the source is not moving. Again, the Doppler effect will depend on what the observer is doing. In other words, both of these pictures correctly show how the light waves are moving in two different reference frames, but they do not specify what an observer is doing or even where the observer is located relative to the source. Recall that there is no medium when you're talking about light in a vacuum. If you're talking about light, I guess you mean the observer is stationary relativity to the source. If you're not talking about light, I assume the picture is showing the waves in the rest frame of the medium. The change isn't proportional to the source speed, but there is a change in frequency. This is ok for a wave other than light, but as I said before, in a vaccum the speed of light is constant so the observer can't see a different wave speed in the case of light in a vacuum. You keep using the word "proportional". That word has a precise mathematical meaning. In most of your examples here, it does not apply. This animation is correct for light in vacuum, in a reference frame in which the source is moving (at less than the speed of light). Whether there is a Doppler effect in this situation will depend on the motion of the receiver of the light waves relative to this reference frame, which the picture does not show. ---- Now to your recent reply to me: I think you're confusng "medium" with "reference frame". By going to a different reference frame, I can make any source stationary or moving forwards or moving backwards or whatever. What can't be changed by a change of reference frame is the speed of the source relative to the medium. Of course, in the case of light in vacuum there is no medium.
Emil In best laboratory vacuum. The aparatus does not measure lightspeed directly, it splits a pure color beam or laser(in modern instruments)into two beams at right angles which travel identical distances to two mirrors which reflect them(sometimes multiple times)back to a recombining optic. If there is any difference in the lightspeed in either leg the screen will show an interference pattern. And if there were any sort of medium the various movements of the Earth would appear as cyclic interference patterns in various cycles. These instruments are very large and the various vibrations in our daily suroundings would have jiggled the mirrors enough to only see noise, so the instruments were mounted on heavy bases floated on Mercury pools, and these pools allowed turning the instrument in any horizontal direction. The point is the instrument and all the more sophisticated modern versions completely failed to provide any evidence that lightspeed varied over all the different frames the Earth goes through. No result outside of the range of error of any instrument has ever been confirmed, lightspeed is the same in any frame it is measured in, no matter which direction you are moving, and no matter what speed you are travelling. And it is the same for every observer in the Universe. You only see differences and the effects of Relativity when observing other frames. And you comment on this diagram? Please Register or Log in to view the hidden image! GrumpyPlease Register or Log in to view the hidden image!
Grumpy gave a good description of the basic set up and purpose. The only thing I would add is that the equipement did not require that the two paths that the spit beam traveled be exactly the same, since all they were doing was looking for a change in the interference pattern when the two breams were brought back together. Morley I believe was enlisted by Michelson, for his talent in constructing the equipement. Everything had to be custom made... I know that some of the experiments they conducted did involve vacuum. I am not sure if the interferometer itself did. Once again that would not have been an issue anyway as they were not measuring the velocity.
Here is a an excellent primer by rpenner. Here is an explanation that is less math intensive. If these aren't good enough for you just google Special Relativity - it isn't like this information is hidden or something!:shrug:
Grumpy - I meant to tell you earlier that I like your graphic. I think that anyone could understand it - it is very clear..
. Hi Emil, everyone. I had a few minutes so I read through this discussion to look for (as is my wont, my nom de plume) any possible cross-purpose misunderstanding going on. I think I have identified a 'disconnect' between what Emil is querying and what everyone else is answering. I will 'overlay' an imaginary light clock on the animations (ie, a traveling light-clock setup involving parallel-mirrors moving 'to the right' at constant velocity parallel to the surfaces of the parallel-mirror-pair) to help me illustrate the crucial aspect involved..... Let's make the 'red dot' a reflective atom on one mirror surface, and let's make the 'upper boundary of the box' the other mirror surface. OK? Now in the accepted view of what happens in the moving light clock, a light beam emitted from one mirror (in this case the red dot 'source') and propagates across the 'space' between the mirrors (in the frame of the moving mirror-pair assembly) so that the light beam will be reflected from the 'upper box-boundary) mirror surface and back to the red dot surface as if they 'still stationary'. Now look at the following animation and imagine that the UPPER-BOUNDARY DIRECTED 'radial beam' is the ONLY part of the omni-directional pulse that is involved in this 'moving light clock' perspective (and ignore all the other 'radial beam' components of the omni-directional pulse depicted by the expanding blue sphere/circle). Please Register or Log in to view the hidden image! So, for the light clock, whether it is moving as described or not, the beam always moves 'up' and 'back' as if the source/reflecting mirror surfaces had not moved at all, since the light beam moves PERPENDICULARLY to and fro between them as if the source and reflecting points are always at the 'same place' OR as if the light beam radial 'moves along with' the mirror source/reflector points. In other words, this would look like the above case where the 'upper-directed' radial beam (component of the omni-directional pulse) is STAYING ON THE SAME LINE between the red dot and the upper boundary irrespective of whether the red dot moves or not. Now that is the FIRST CASE SCENARIO which Emil is pointing to with that animation of the red dot always at the centre of the radiated light pulse/beams. Now let's compare this to the following scenario..... Please Register or Log in to view the hidden image! We note that the FIRST omni-directional sphere/circle of light has a 'beam radial' component directed to the upper boundary 'mirror surface, BUT IN THIS SCENE the red dot 'mirror' has apparently moved BUT THE UPPER mirror surface reflecting point has NOT, BUT as in every such light clock setup, the reflection mirror point SHOULD 'keep pace' with the red dot 'source' mirror point. So what is going on? In THIS depiction, the 'space' radial seems FIXED in place along that ORIGINAL EMISSION space radial while the source/reflecter points 'move on' with respect to that original emission line radial between them (at original emission point in 'space'). THAT different view is the 'disconnect' between what is being queried and what is being answered here, since the two cases are not comparable IN PRINCIPLE let alone in fact, at least insofar as the behaviour of a light beam between the mirrors of a moving/stationary light clock according to conventional science understandings/experiments makes it quite clear that the first case is the 'real' case and the second case is 'not so'. In the first case, the 'light clock' scenario is the VALID ONE.....and YET the latter case is being also presented by some as the 'answer'....even though this 'answer' clearly contradicts the 'light clock' CONVENTION/EXPERIMENT results that say that there is no 'space frame' for the light beam, and hence there cannot be a situation where the red dot can move out of the 'centre' of the ORIGINAL/FIRST light pulse 'outermost expanding blue ring' (of which the upper-directed radial beam component is a part, and which should act like in the light clock setup). So Emil in effect just wants to know why the first case (is as in the light clock where the red dot WOULD effectively 'always remain in the centre') is being ignored by other 'explanations' which claim that the red dot DOES 'move out of that centre' as if the WAS some 'preferred space frame' point at initial emission? IMPORTANT NOTE: I am at this stage ONLY concerned with the INITIAL single radial 'beam' directed up and reflected back as it would be in a light clock parallel mirror setup. ANY ONGOING pulse emissions and doppler effects involved as the continuing/omni-directional ongoing FURTHER pulses directed FORWARDS are NOT being treated in this observation in order to simplify and identify the crux of the 'disconnect' and its root cause. I leave that part alone as I am not interested in it until the basic 'disconnect' I have pointed to has been addressed and settled. OK guys? Anyhow, that's all I wanted to point out for now. I have to go now, but will read through again tomorrow or next day to see how the discussion progresses. Cheers, Emil, everyone! Please Register or Log in to view the hidden image! .
I concur with all that. Nonetheless, space is classically a vacuum, it does not represent a "stationary medium" that waves propagate in. That explains why an observer in "uniform relative motion" can't tell their velocity (or their speed) in space without reference to other objects. Therefore speed or velocity can only be relative. The situation is quite different in a medium, as I explained, a solid object represents a medium which is stationary for traveling waves (of sound, pressure, heat, whatever). Light is the only "wave" that propagates in an obvious way in a vacuum. GR predicts that gravity does too, but this is not obvious--we can't "see" gravity except as a one-dimensional force.
arfa: you should investigate the wave nature of matter. Then think on this: if an observer is made of waves, he always measures wave speed to be the same.
Yes, I looked at Wikipedia and is as you said. When I learned about Doppler effect, the phenomenon was revealed as I affirmed it. Just as posted Grumpy: Please Register or Log in to view the hidden image! (@ Grumpy, it is correct.) Then came the practical, how can we measure? Yes, I used wrong "proportional". Maybe I ought to say directly (not inversely) linearly dependent. Or change in frequency is proportional. It Is true also for medium. http://en.wikipedia.org/wiki/Doppler_effect#General "Change in frequency: Please Register or Log in to view the hidden image! " In this case I think I used correctly "proportional" :Please Register or Log in to view the hidden image! In this I'm interested, the pros and cons, if in vacuum (space) there is a "medium"?
Emil: Regarding the proportionality thing: the formulae you have quoted are non-relativistic. Also, they are only approximately true for a moving source, even in the non-relativistic approximation. They are completely correct only for a source stationary relative to the medium and a receiver moving relative to the medium.
Emil There is a difference between Doppler in a medium and Doppler in spacetime. In a medium both the emmitter of the sound and the observer of the sound move in relation to the same medium(air), in spacetime there is no such medium, the two can only be called moving in relation to each other. And since both the moving emitter observer and the outside observer see their own frame as stationary then all movement is Relative to the observer. GrumpyPlease Register or Log in to view the hidden image!
Emil That plus General Relativity, we don't consider them two things, they are both parts of Relativity. Special only dealt with inertial frames, General extended it to accelerated frames. GrumpyPlease Register or Log in to view the hidden image!
Do you not think there is a difference between special principle of relativity and special relativity ?
Emil Both papers deal with the same concept of Relativity. The first dealt with frames in constant motion(inetial frames), the first postulate(long since proven beyond reasonable doubt)was that all such frames measure the speed of light to be the same, no matter what the relative motions of the source to the observer. The second(equally well supported by evidence and testing)is that all observers measure the constants of the Universe to be the same within their own frame. The only way this can happen is if time and/or length change according to the speed of the system relative to the observer's frame of reference. Another insight gained from the math is that energy is equal to the mass times the speed of light squared(as proven by atomic weapons and solar fusion). General Relativity tied spacetime, accelerated frames and gravity into the same system and expanded the ability of the theory to explain a lot about our Universe. Many physicists found many of these things before Einstein, but it was he who tied it all together as a coherent whole. And the bottom line is that all of the predictions we can make using the model have been shown to be true, even gravity waves. GrumpyPlease Register or Log in to view the hidden image!
In free space electromagnetic propagation, the velocity is determined by: \(c\quad =\quad \frac { 1 }{ \sqrt { { \mu }_{ 0 }{ \epsilon }_{ 0 } } } \) These are the intrinsic to free space: the magnetic permeability and electric permittivity. They are duals or analogs of the parameters for a medium that propagates a wave. For sound: \(c_s\quad =\quad \frac { 1 }{ \sqrt { { \varepsilon }_{ 0 }{ \rho }_{ 0 } } } \) in which the medium, such as air, has its own intrinsic properties which are mechanical compliance and density.
