# Problem with light simultaneity

## Main Question or Discussion Point

Ok so in Einstein's thought experiment with the train, everything makes sense except...

Einstein is stating that an event doesn't happen until someone SEES it. But isn't there a difference between SEEING an event happen and the event happening regardless of whether someone sees it or not?

The answer to his "thought experiment" is that there is more than one answer: the 2 lights strike at once and the front hits train before the back. But the second answer is merely just what the observer in the train SEES. Regardless of what he sees, we all KNOW that the lights hit at the same time.

Ie: let's say physicist wants to set up an experiment.
He is in the train and attaches flashlights in the front and back of the train. They are timed to go on at the EXACT SAME TIME. The physicist knows this and waits. Then he sees the front light before he can see the back light. KNOWING that the flashlights were set to go off at the same time (and testing the equipment an indefinite amount of times), he uses the different times to conclude that ONE light is faster than the other.

How do this be accounted for? Just because someone SEEs a light later doesn't mean it turned on later..

## Answers and Replies

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HallsofIvy
Science Advisor
Homework Helper
Ok so in Einstein's thought experiment with the train, everything makes sense except...

Einstein is stating that an event doesn't happen until someone SEES it.
NO, he doesn't say anything like that! I don't know where you got that idea.

But isn't there a difference between SEEING an event happen and the event happening regardless of whether someone sees it or not?

The answer to his "thought experiment" is that there is more than one answer: the 2 lights strike at once and the front hits train before the back. But the second answer is merely just what the observer in the train SEES. Regardless of what he sees, we all KNOW that the lights hit at the same time.

Ie: let's say physicist wants to set up an experiment.
He is in the train and attaches flashlights in the front and back of the train. They are timed to go on at the EXACT SAME TIME. The physicist knows this and waits. Then he sees the front light before he can see the back light. KNOWING that the flashlights were set to go off at the same time (and testing the equipment an indefinite amount of times), he uses the different times to conclude that ONE light is faster than the other.

How do this be accounted for? Just because someone SEEs a light later doesn't mean it turned on later..
The whole point is that the physicist knows that light does NOT travel faster in one direction than another so knows that the "conclusion" above must be false.

Pardon me. Which of Einstein's thought experiments involving a train is this thread discussing?

Dale
Mentor
Just because someone SEEs a light later doesn't mean it turned on later..
It does if the lights are equidistant.

ghwellsjr
Science Advisor
Gold Member
Ie: let's say physicist wants to set up an experiment.
He is in the train and attaches flashlights in the front and back of the train. They are timed to go on at the EXACT SAME TIME. The physicist knows this and waits. Then he sees the front light before he can see the back light.
This is not correct. This physicist on the moving train will see the two flashes at the same time (assuming he's in the middle of the train). But a physicist on the ground will see the two flashes at different times. Which one is right?

And if the physicist on the ground set up a similar experiment, he would see both flashes at the same time while the physicist on the moving train would see them at different times. Which one is right? Does that make any difference to your understanding of Eisntein's traig experiment.

Ok so in Einstein's thought experiment with the train, everything makes sense except...

Einstein is stating that an event doesn't happen until someone SEES it. But isn't there a difference between SEEING an event happen and the event happening regardless of whether someone sees it or not?
I am also interested in exploring the difference between SEEING an event and the EVENT ITSELF (please see https://www.physicsforums.com/showthread.php?t=460929) - if the dogma police will allow it, that is...

Ok so in Einstein's thought experiment with the train, everything makes sense except...

Einstein is stating that an event doesn't happen until someone SEES it. But isn't there a difference between SEEING an event happen and the event happening regardless of whether someone sees it or not?

The answer to his "thought experiment" is that there is more than one answer: the 2 lights strike at once and the front hits train before the back. But the second answer is merely just what the observer in the train SEES. Regardless of what he sees, we all KNOW that the lights hit at the same time.

Ie: let's say physicist wants to set up an experiment.
He is in the train and attaches flashlights in the front and back of the train. They are timed to go on at the EXACT SAME TIME. The physicist knows this and waits. Then he sees the front light before he can see the back light. KNOWING that the flashlights were set to go off at the same time (and testing the equipment an indefinite amount of times), he uses the different times to conclude that ONE light is faster than the other.

How do this be accounted for? Just because someone SEEs a light later doesn't mean it turned on later..
What do you figure happens after you add another person in the train who sees both lights at the exact same time? Who is right? one person sees both lights at the exact same time and another person sees them go off at different times.

JesseM
Science Advisor
Einstein's thought experiment (discussed in section 8 and section 9 of his book Relativity: The Special and General Theory) is about when the events of the lightning strikes are reconstructed to have happened in each frame, not just when they are seen. For example, if in 2010 I see the light from an explosion 10 light-years away in my frame, and in 2020 I see the light from an explosion 20 light-years away in my frame, then I will retroactively conclude both events happened at the same date (2000) in my frame, even though I saw the light from the events at different times.