Was it the Michelson-Morley experiment?But I read that it only established non existence of aether.

Dont want the deep technical details but only a conceptual description of one such experiment. What bothers me exactly is this:

In most non mathematical explanation of this topic, first a standing in the midpoint observation is given as a definition for checking simultaneity. And then it proceeds that such an observation cannot be made for events in moving frames as we(or they)will be moving towards one of those event, so the midpoint observation fails. Maybe we can modify the midpoint; but maybe that is difficult to accomplish. Can it be demonstrated?

But a more important question for me is why do we maintian this kind of explanation if constancy of light speed we postulate? One event is coming towards me-isnt that the same as one event is coming faster at me? Say I am moving and i want to check 2 events on the ground for simultaneity and then if i was really at the midpoint then light would surely meet me eyes from both events simultaneously if they were actually simultaneous on the ground right? i.e, given the constancy of light speed for me and people on the ground. ???

So how is relativity of simultaneity compatible with constancy of light speed in the sense of above non mathematical explanation? Or am i wrong?


The Michelson-Morley experiment showed that light moves at the same speed regardless of direction. Since the earth is moving through space rather quickly, this is already quite suggestive of the fact that light speed is constant in all frames- if it varied, we would expect it to depend on direction.

From this, Einstein assumed the speed of light was constant in all reference frames while making his theory of special relativity. This constancy of light speed implies a lack of absolute simultaneity, as well as numerous other surprising effects (e.g. time dilation) which have been directly verified.

In your problem, you're assuming that the two people on the ground emit light simultaneously, so of course the events look simultaneous. The resolution is that they are not emitted simultaneously- simultaneity is relative.

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Events don't move: They are points in space-time, for example, the emission of a light pulse.

The constant-speed-of-light postulate (for inertial observers) requires one to give up the idea of universal simultaneity.

So if you're in the middle of a train car and 2 light pulses are sent to you from each end: boom, you will agree their emissions were simultaneous.

The observer on the platform sees them arrive at you at the same time, but working backwards at the speed of light to the (moving) car ends, the distance to the front is clearly shorter. Since the speed of light is constant: it must have been emitted after the rear-pulse.

That's the basics of simultaneity--to get the technical details to work, you have to include time dilation and length contraction.

(Also, you hinted at linear velocity addition: this simply does not work, and relying on it will lead to confusion).

Clarification: Note that nothing is moving here except the train observer with respect to the platform observer. The pulse emissions and detections are fixed events at fixed $(t, x, y, z)$ in space-time. The light could come from lasers in the car, or from signal lights fixed on the tracks-yes, they Doppler shift differently--but that does not matter. All that matters is when and whence the light comes.

The speed of other projectiles (say, a 1 GeV pion beam) is not relevant, and understanding how they would be observed is technical, requiring Lorentz transformation and/or linear additions of rapidity--but you do not need to go there to grasp the fact that a universal constant speed-of-light for all observers blows up a universal clock for all observers.

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  • $\begingroup$ How is the distance to the front closer? Are you not snekily adding to the velocity of light when saying so?Would this make any difference if it was 2 gunshots instead of light pulse? How would the gunshot be different? $\endgroup$ – Anandhu Dileep Nov 15 '17 at 16:29
  • $\begingroup$ Who says light from the front of the train is not blueshifted? After all light from the Andromeda galaxy is blueshifted as we move toward it. The Michaelson and Morley experiment was Considering light going out and coming back and that is different than light being simultaneously emitted from both ends of the train. $\endgroup$ – Bill Alsept Nov 15 '17 at 16:38
  • $\begingroup$ The pulse detection is not moving. It is a stationary event w.r.t the platform. Dial it back 10ns and the light pulses are 10ft from the detection point, but the train's (say, at 0.5c) frontend is now 5ft closer and the back is 5ft farther (and so on until emission). The front pulse reaches it's end 1st--and since we're working backwards, that is after the rear pulse was/is emitted. You simply cannot have universal "c" and universal simultaneity. $\endgroup$ – JEB Nov 15 '17 at 16:48
  • $\begingroup$ Note: The Doppler shift is technical--in S.R it includes a space part (wavelength change) and a time part (time dilation)--they are frame dependent observables and transforming them involves Lorentz transformations; per @BillAlsept, it does not affect the distance between "events". $\endgroup$ – JEB Nov 15 '17 at 16:54
  • $\begingroup$ If your gun shots are from a laser gun, then: no, it would not make a difference. The speed of bullets are not a universal constant, their velocities do not add linearly, being thing into the mix will only cause confusion regarding the speed-of-light and simultaneity. I'm going to add to my answer. $\endgroup$ – JEB Nov 15 '17 at 17:12

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