# Simultaneity, and the chronon

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17 hours ago, AbstractDreamer said:

So if two stars supernova, and are recorded as simultaneous by an observer equidistant to each, then let me say (for the remainder of this thread) the supernovae were "objectively" simultaneous.  That is, there is more than one coordinate where, had a recording been made, then a simultaneous reading would have been obtained, and that all such coordinates lie on a plane perpendicular to the line between the two events.

It's not enough to just "see" the events simultaneously to say that they were simultaneous, even if you are halfway between the events when you saw them.

For the observer to claim that those events were simultaneous, he also would have to have be either stationary with respect to these events or the relative motion between himself and those events would have to be on a line perpendicular to the line joining the events.

Another observer moving parallel to that joining line, even if he was at the exact same point as the first observer, and "saw" the events simultaneously, just like the first observer did, would conclude that the events themselves were not simultaneous.

So for example, this shows events according to the observer at rest with respect to the events. The white expanding circles is the light produced by the events

For him, the events occur simultaneously. Both light flashes and the railway car observer arrive at his position at the same time.

However, for the railway observer, the order for the same events is different:

The light from both flashes meet at his position at the same moment as he is adjacent to the track observer, but the events producing those flashes did not occur at the same time.

This is because he must measure light as traveling at c relative to himself.  In other words if he were to measure the speed of either light flash as it passed him, he would measure both as moving a at relative to himself.   Thus for him, the light flashes, once emitted, expand outward from the point of emission at c. (the red dots move away from these points, but we don't care what they do after the flashes are emitted)

Now for him, the time it takes for a flash to reach him from either event is equal to the distance between him and the event at the moment the event took place divided by c

Since the events have to occur prior to his seeing the light flashes, they had to occur before he was at the halfway point between them.  He had to be closer to one event than he was the other.   Different distances means different travel times at c, and therefore when he sees both flashes at the same time, he also knows that one of those flashes had to travel a longer distance and take a longer time to reach him, and ergo, the events producing those flashes could not have occurred simultaneously.

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Thanks for the visuals.

So Simultaneity is just subjective.  But what about concurrency?

Lets presume both you and I exist.    Is the star shining concurrently for you and me, or sequentially?  That is, my detection of photons are registered sequentially by me, and your detection of photons are registered sequentially by you.  But are our combined detection of photons registered collectively sequentially, or could some be registered concurrently?

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11 hours ago, AbstractDreamer said:

Thanks for the visuals.

So Simultaneity is just subjective.  But what about concurrency?

Lets presume both you and I exist.    Is the star shining concurrently for you and me, or sequentially?  That is, my detection of photons are registered sequentially by me, and your detection of photons are registered sequentially by you.  But are our combined detection of photons registered collectively sequentially, or could some be registered concurrently?

It could be either. It depends on where the two observers are and what their motion is, relative to the source.

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