How To Speed Up 5233?

Given the speed of propagation of light from its source in a vacuum is c in all frames of reference.  If the source is moving relative to us, and is suddenly turned on, how can we measure the velocity of the wavefront as it approaches us, if we cannot see the wavefront until it reaches our receptor?

• I've changed this question to distinguish between the speed of propagation of light from its source 'c', and the velocity of the wavefront as it approaches an observer from a source moving relative to the observer (in any direction). I think our discussion is still useful, though Mark and others may care to add to your comments based on the change. I've also expanded my comment to try to explain the difficulty I am having. I hope this is OK from a 'protocol' viewpoint on the forum. MM  experiment was conducted in a situation where all components of the  experimental apparatus (source, mirrors and receiver) were stationary  relative to each other. Clearly this experiment proved that there  is no aether.... that light travels from its source and is reflected at  the same speed regardless of the motion of the entire experimental  apparatus relative to any other body.  This makes sense to me in terms  of the 'water' analogy. What does not make sense, is the claim  that it experimentally proves that the speed of the ‘wavefront’ cannot  exceed the speed of propagation, where the source is moving toward the  receiver... as that was not tested. To explain my thinking… In my original example, a boat was floating in a current: -       current moving at velocity: v0 (changed from v1) -       the speed of waves in water: v1 (changed from v2) -       frequency of waves emitted from the source: f1 -       wavelength: l1 In  the water analogy, the speed of wave propagation from a source is fixed  by the properties of the medium, regardless of the motion of the source  through the water. In this case, a moving source creates a  'Doppler effect': increasing the frequency (f) and compressing the  wavelength (l) (in the direction of motion) and decreasing (f) and  expanding (l) (in the opposite direction). These effects occur in the wave train itself. However,  with the source moving in a current, relative to fixed points in the  river (front and back), there is no compression or expansion in the  ‘wave train’. Instead, the velocity of the source and the velocity of propagation are additive. It  is this additive effect that creates actual differences in the observed  frequency and wavelength - depending upon the viewpoint and relative  motion of the observer. With water, all observers can see the  wave propagating from the source and see that it is propagating at the  speed of waves in water v1, with a fixed wavelength l1, and frequency f1  - in all directions around the floating boat. They can all see that for the fixed point in front, the ‘measured’ frequency at that point is f2>f1 and wavelength l2<l1. They can all see that for the fixed point in back, the measured frequency at that point is f3<f1 and wavelength l3>l1. All  observers can also see that observed from a point at right angles and  moving parallel to boat at v0, the waves are approaching at v1, with  frequency f1 and wavelength l1 (as if the source and observer are both  stationary – as indeed they are… relative to each other). Of  course, where a source is moving relative to the water, everyone can  also see the actual compression of the ‘wave train’ in front, and the  expansion at the rear. Unfortunately, we cannot do this for light. For light, the speed of propagation from its source is c. It requires no medium and therefore acts like the source moving with the current. And  it makes sense that this is true in all frames in all directions for  light propagating from its source - because there is no aether to limit  its velocity. But what of the velocity of the wavefront (as opposed to the speed of propagation from source)? In  Wikipedia there are two GIF's that shows the MM experiment, with the  second illustrating the 'expected' result for light being propagated in a  'medium' In  the first, the ‘apparatus’ is stationary relative to the page (the  assumed aether).  In the second the whole apparatus is moving to  'through the aether' (to the right of the  page). This second GIF shows the assumed aether creating a phase shift. However, no such shift was observed by MM… clearly invalidating the aether hypothesis. Obviously,  without a medium to limit its speed, as the setup in both cases is  identical, the speed of propagation from the source must be identical -  so there cannot possibly be any difference in arrival of any of the  beams - which is what MM proved. The red and blue shifts fit the  idea that the speed of propagation of  light 'from its source' is fixed  in all frames, but that the speed of the 'wavefront' can be  more or  less than c, depending upon the relative movement of the source and  receiver. Since it is impossible to see the speed of an  approaching wavefront from a beam of light that has just been turned on,  we can only ever measure the apparent frequency or wavelength at the  point of reception, or the time taken between two points. If we  assume that the velocity of the wavefront is always c, then to account  for experimental observation, we must contract length and expand time in  the direction of travel. If we accept c as ‘absolute’ for the  speed of propagation from any source in any frame, but that the speed of  the wavefront is c + v (the relative velocity of the source and  receiver), then we can get the same result, but without the need to play  with length or time. The same with mass.  If a body can move faster than light, it will have more energy and hence greater apparent mass. If  we assume that the frequency and wavelength of light emitted by each  element is the same in all frames (when measured from the source at the  instant of propagation), we can deduce the relative motion of the source  to us. While the mathematics of the gravitational shift may also  'explain' a red and blue shift (in terms of the theory of general  relativity), the 'mathematics' are simply internally consistent.  They  give the result - because speed, time and distance are all  'self-referential'.  If I choose to hold one factor fixed, the maths  will work out and I get the result I expect. I am hoping for an  explanation - not simply a rejection based on the fact that the mental  model does not conform to agreed science. Once again... thank you for hearing me out.

• Answer:

  I'm not sure what you're asking, so let me ramble and see if it helps. Light is naturally a wave phenomenon, so it consists of a train of wavefronts. If you can single out a particular wavefront, e.g., by making it the first of a group in a flash/pulse, then in principle you can time it as it moves between two points and thereby measure its velocity. The trick is that before you can do this, you need to have synchronized clocks or the equivalent thereof at the start and finish points. And in Special Relativity, the Einstein "clock synchronization criterion" basically amounts to "the one-way speed of light must come out to c". So if you want to measure the speed of light from A to B, a distance L apart, first you have to tweak clock B so that when you send light from A to B you get tB=tA+LctB=tA+Lct_B=t_A+Lc. Then you send some light from A to B for real, notice that L/(tB−tA)=cL/(tB−tA)=cL/(t_B-t_A)=c and declare victory for relativity. That reason that different observers travelling at large fractions of the speed of light relative to each other get the same value of c is that they're all applying the Einstein synchronization criterion independently. The reason this isn't a complete cheat is that classically, having carefully contrived this for the direction from A to B, you wouldn't expect it also to work for the direction from B to A - there would be discrepancies proportional to v2/c2v2/c2v^2/c^2. The famous http://en.wikipedia.org/wiki/Michelson%C3%A2%C2%80%C2%93Morley_experiment looked for such discrepancies over a two-way trip and failed to find them. More precisely it compared the discrepancy for a two-way trip in the direction of motion and a two-way trip at right angles to the direction of motion -one should have been twice the other. The reason that the MM experiment doesn't find anything is attributed to length contraction - the apparatus contracts in the direction of motion by just enough to even up the two arm. However there are a bunch of related effects such as time dilation that the MM experiment doesn't directly test but can be confirmed in other experiments. Edit in response to comment thread and updated question. 1. Maybe I'm being too clever and should just answer the actual question posed: if you want to know where/when the signal has been before it gets to you, you make sure the source emits a wide, flat wavefront, put multiple detectors with clocks at different points along its path and also stagger them across the width. Each detector consumes a bit of the width of the wavefront, but leaves some to pass through to the next detector. But then you need a synchronized clock at each detector so that the arrival time can be meaningfully logged, at which point you run into the issue in my original rant. 2. More later.