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Simultaneity

Simultaneity
From different reference frames, there can be disagreement on the simultaneity of events. This is a fundamental result of special relativity.


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Rating: 2.9 / 5 (1010 votes)
Posted by nova on June 30, 2007
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Comments

Posted by John Q. Public on June 30, 2007 at 1:02 pm
This is a confused and in fact INCORRECT description of the phenomenon. Even the short written description above contains a minor error.

At the time of the lightning strikes ("events"), each observer is shown equidistant from both events. Since light travels at the same velocity regardless of frame of reference, BOTH observers will perceive the events to be simultaneous. The motion of the train is completely irrelevant in this situation.

That would not be so if one or both observers were closer to one event than the other. But that is not how this flawed "example" was set up.

Different frames of references can indeed result in different observations of simultaneity. That is real. But this is not a valid example.

And I am not trying to nitpick, but the statement above that from two different frames of reference, there can "never" be agreement on simultaneity, is simply incorrect. The example shown is actually one of the rare cases where there WOULD, in fact, be agreement. The statement should have been, "From different frames of reference, there CAN BE valid disagreement about the simultaneity of events." There is nothing "never" about it.
Posted by John Q. Public on June 30, 2007 at 1:09 pm
Except in the sense that the ratio of the distances from each observer to each event will "never" be EXACTLY the same, of course... however, it can be close enough that the difference is not measurable.
Posted by nova on June 30, 2007 at 2:41 pm
The description has been updated, thanks.
Posted by John Q. Public on July 3, 2007 at 3:16 am
Pardon if that sounded like criticism. That was not intentional. I just wanted to set the record straight.
Posted by nova on July 3, 2007 at 5:05 am
I know and I really appreciate it :)

My health is too poor for me to go to university so I made this site to learn, whether it be from the videos or the community ;)
Posted by John Q. Public on July 7, 2007 at 2:07 am
I examined this in more detail, and if we want to be technical (we should), I was wrong about something, too.

Since at the instant of the flashes (we assume they are instantaneous), observer Y on the platform is equidistant from them both. So the spheres of light will converge in the middle, directly in front of him, exactly where the other observer (X) is at the time the flashes occur. But by the time the light reaches the center, X will have moved to the right. So Y will indeed see X reach the right-hand light first.

However, since observer X is equidistant from both light flashes, and the speed of light is independent of frame of reference (speed or direction), then X MUST also see both flashes of light at the same time.

So they do indeed see things differently. However, the narrator is still incorrect in stating that X will see one flash first. She could not.
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