A Thought Experiment from “RELATIVITY: THE SPECIAL AND GENERAL THEORY” by Albert Einstein
“..in reality (considered with reference to the railway embankment) [the observer] is hastening towards the beam of light coming B, whilst he is riding on ahead of the beam of light coming from A. Hence the observer will see the beam of light emitted from B earlier than he will see that emitted from A. Observers who take the railway train as their reference-body must therefore come to the conclusion that the lightning flash B took place earlier than the lightning flash A. We thus arrive at the important result: Events which are simultaneous with reference to the embankment are not simultaneous with respect to the train, and vice versa (relativity of simultaneity). Every reference-body (co-ordinate system) has its own particular time; unless we are told the reference-body to which the statement of time refers, there is no meaning in a statement of the time of an event.”
>the [moving] observer will see the beam of light emitted from B earlier than he will see that emitted from A.<
I think this statement is ambiguous.
This “moving observer” will not see the “Actual Event Flash B” occur earlier in time but will merely be receiving his electro-magnetic evidence of “Flash B” before receiving his evidence of “Flash A”. If the speed of light was infinite then both observers would see the Flashes occur simultaneously whatever their velocity and wherever they happened to be -and all events elsewhere as and when they occurred!
>Observers who take the railway train as their reference-body must therefore come to the conclusion that the lightning flash B took place earlier than the lightning flash A.<
I think this conclusion is erroneous.
Because the observer on the train receives the evidence from one flash before the other can indicate nothing unless he knows the two distances separating his present location from the point in Space where they occurred. Einstein says it is the movement of the observer that affects simultaneity but why does he not consider the case where the moving observer starts from a point along the track closer to Flash A so that he arrives mid-way between the Flashes when the two beams meet? In this case he, though still the “mover”, will also see them occur simultaneously. Non-simultaneity is a function of an observer’s position in Space.
What makes the track stationary? The track could itself be the “mover” in the opposite direction, in which case the train decelerates and becomes stationary. Einstein offers no definite meaning for “stationary”.
>Every reference-body (co-ordinate system) has its own particular time; unless we are told the reference-body to which the statement of time refers, there is no meaning in a statement of the time of an event.<
Whilst I would agree that every observer has his own particular relative timescape I maintain that this is fixed by his particular relative position in space AND that there is a meaning to “the time of an event”. Everywhere I look is “ago”, in the Past, and the further away I look the more “ago” it is. The clock I see on the wall across the room I actually see as it was about 20 nanoseconds “ago”. I see from my “Here” the Sun as it was about 8 timeseconds “ago”, shining from an observed position “There” in Space, -whereas it is “Now” about 2 arcminutes further westward from my observation of its position.
Visual and Aural Perspectives: Here-There and their corollaries Now-Then.
The appearance Here (to the eye/ear) of the relative positions There in Space of objects/events.
The appearance Now (to the eye/ear) of the relative positions Then in Time of objects/events.
Suppose there was only sound and no light. How would this affect our knowledge of Then in Time? Sound travels at a fairly constant speed through still air. Suppose two different church clock bells ring the hour together. An observer sat mid-way between them would hear them strike simultaneously but an observer on the move may not. If the moving observer heard them ring separately is it sensible for him to assume they did so at different times because he was “moving”? An effect that would simply depend on his relative separation from each bell at the moment he heard it. Of course, by being able see the (almost) instantaneous light result of clock faces that show the same time (give or take some nanosecs), and recognize that any difference in delay in the aural perception was because of differences in distance from him.
Relativity states that Time dilates and Length contracts for a moving observer. But moving with respect to what? In explanations of the effect (eg. explaining the Twin Paradox) the experience of a stationary observer is compared to that of a fast-moving one. Einstein uses platforms and tracks in his Thought Experiments, but what and where is the “stationary reference” in Space? Could a possible candidate be a point in Space from which light has radiated equally in all directions?
Pursuing this idea I suggest a scenario as follows:
In remote Space an observer “O” creates a brilliant Flash (“O” and “F” in my diagram: see Note 1. below) to emit a pulse of light radiating in all directions. The Flash site becomes a fixed point in space and time, because it is at the centre of an expanding bubble of light-data moving equally at “c” in all directions.
“O” immediately moves away from this central point F at a velocity of 0.5c in a straight course, then somewhen later immediately reverses at 0.5c to return to the site F . My diagram shows four subsequent ‘snapshots’ of ‘events’ with elapse of time shown by the expanding distances the pulse bubble has travelled away from F divided by “c”.
EVENT 1. Shows “O” somewhere on his outward leg. Throughout this outward leg the speed of separation of the pulse straight ahead of “O” is reduced to 0.5c.
EVENT 2. Here “O” stops and instantaneously reverses.
EVENT 3. On his way back to the original position. On this return leg the apparent light speed is gradually being restored to its normal velocity of “c”.
EVENT 4. Back at the site of the original Flash with the pulse at an undistorted “c” in every direction.
My puzzle is that if Relativity’s Time dilation/Length contraction works on the outward journey in maintaining “c” with respect to the separation between “O” and the pulse going ahead of him, how does it also work for the separation of “O” with the light pulse as it goes in other directions?
It seems on his outward journey the speed of separation between “O” and the pulse going in the diametrically opposite direction requires exactly the opposite corrections, viz. Time contraction/Length expansion. And there is always one angular direction, (marked “C”), which requires no corrections at all. Yet there can only be one observed clock rate & length scale for “O”.
In dilating time and contracting length to restore apparent relative light speed to “c”, would not these same “scales” similarly restore the apparent wavelength distortions and remove Doppler red/blue shifts too? But we do see these.
Note 1. My diagram.
Sorry! I just cannot reproduce the visual image of my diagram in your comment box. (I’m an old man of 95 struggling with modern technology on a 10” Tablet).
This is a link to it: https://goo.gl/photos/pY9gPp77MMbXnupV8
I have also posted this whole Comment on my WordPress blog seesomelight.wordpress.com but even here I could only get the diagram right at the top of the text.
Alternatively I think I can email it successfully from email@example.com
Arthur Morris Eastbourne BN23 7PY