# Problem with light simultaneity

• senorhosh
In summary, the conversation discusses Einstein's thought experiment with the train and the concept of events happening only when they are seen. It also explores the difference between seeing an event and the event itself. The answer to the thought experiment is that there is more than one answer, but regardless of what is seen, it is known that the lights hit at the same time. The conversation also brings up the idea of adding another person in the train and how it affects their perception of the event.

#### senorhosh

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..

senorhosh said:
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?

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

senorhosh said:
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.

senorhosh said:
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...

senorhosh said:
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.

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.