# Simultaneity Problem

I'm having trouble understanding the simultaneity experiment with Alice on the railroad car, and Bob observing from the ground. If 2 lightning bolts hit the front and back of Alices car at the same time in Bob's frame, it won't be instantaneous in Alices due to simultaneity. My question is simple, if instead of 2 separate lightning bolts, we use 1 laser pointed directly at Bob and Alice at t=0, where both persons have 2 beam spliters to send some of the light to the left and some to the right in both frames to the front and back of Alices car where she has two detectors that will emit flashes once light is detected, how can simultaneity still hold? If for any reason Bob and Alice disagree about the simultaneity of flashes at the end of her railroad car, then in at least one frame, the right or left moving light went slower or faster than the other which is obsurd.

## Answers and Replies

Doc Al
Mentor
Disagreement about simultaneity doesn't imply that the light goes slower in one frame, just that it has a different distance to travel to the end of the car. For train observers, the two beams of light are shot off from the middle at the same time and travel the same distance to reach the ends of the car. So they reach the ends simultaneously.

But for ground observers, one end of the train is moving away from the light while the other is moving towards the light. Of course the light beams arrive at different times.

any more absurd than the idea that both observers calculate the speed of light to be c even though one of them is moving?

it is the assumption that the speed of light is constant for all observers that leads to the conclusion that there is a loss of simultaneity. you have forgotten to take the observers motion into account.

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Disagreement about simultaneity doesn't imply that the light goes slower in one frame, just that it has a different distance to travel to the end of the car. For train observers, the two beams of light are shot off from the middle at the same time and travel the same distance to reach the ends of the car. So they reach the ends simultaneously.

But for ground observers, one end of the train is moving away from the light while the other is moving towards the light. Of course the light beams arrive at different times.

Let me rephrase the experiement. Bob holds the only beam splitters and at t = 0 he has split the light and it has reached the two ends of the railroad car at the same time in his frame. Alice would say in her frame, they were not simultaneous, and I would tell Alice that she's wrong because they came from a point source.

Doc Al
Mentor
Let me rephrase the experiement. Bob holds the only beam splitters and at t = 0 he has split the light and it has reached the two ends of the railroad car at the same time in his frame.
Bob is on the ground. Why would he say the light beams reach the two ends of railroad car at the same time? (The train is moving.)
Alice would say in hereframe, they were not simultaneous, and I would tell Alice that she's wrong because they came from a point source and she's wrong.
I think you have Alice and Bob mixed up.

both observers will see the light move at c. alice would therefore not expect to see the light hit front and back at the same time. why does that seem like a contradiction to you?

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Bob is on the ground. Why would he say the light beams reach the two ends of railroad car at the same time? (The train is moving.)

I think you have Alice and Bob mixed up.

Bob can set up the experiement in a way that ensure both beams arrive the ends of the railroad car at the same time in his frame if he knows her speed and associated contracted length. If her railroad car lenght is 10m at here speed he could set the experiement up such that when Alice is directly in front of him, the split photons are exactly 5m apart.

Let me rephrase the experiement. Bob holds the only beam splitters and at t = 0 he has split the light and it has reached the two ends of the railroad car at the same time in his frame. Alice would say in her frame, they were not simultaneous, and I would tell Alice that she's wrong because they came from a point source.

It doesn't matter that the light came from a point source. Only the events of light hitting the detectors are in question. Those events are simultaneous in Bobs frame and not simultaneous in Alices frame. Alice is just as entitled to say Bob is wrong as Bob is to say Alice is wrong.

In fact, if you forget the detectors and focus on the beam splitter Bob will say the light was emitted forward and backward at the same time. Alice would say the light was emitted forward first and backward second. Alice would still be as free to say Bob is wrong as Bob is to say Alice is wrong.

Bob will say the light was emitted forward and backward at the same time. Alice would say the light was emitted forward first and backward second.

say what? 2 events that occur at the same place and at the same time will do so for all observers regardless of their velocity.

It doesn't matter that the light came from a point source. Only the events of light hitting the detectors are in question. Those events are simultaneous in Bobs frame and not simultaneous in Alices frame. Alice is just as entitled to say Bob is wrong as Bob is to say Alice is wrong.

In fact, if you forget the detectors and focus on the beam splitter Bob will say the light was emitted forward and backward at the same time. Alice would say the light was emitted forward first and backward second. Alice would still be as free to say Bob is wrong as Bob is to say Alice is wrong.

I dont think thats true. Relativity is based upon the premise that events actually happen. You can't have some one dead in one frame and alive in another (for all time) like Shrogengers cat. It's only there relative times and locations that vary and are relative. The point in this experiment is taking the different locations out the argument by forcing them to be coincident at one place and one time. If Alice says they arrive at different times, she would be forced to explain to the physics community how a laser can emit S and P light at different times from the same source. That just doesnt happen.

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Doc Al
Mentor
Bob can set up the experiement in a way that ensure both beams arrive the ends of the railroad car at the same time in his frame if he knows her speed and associated contracted length. If her railroad car lenght is 10m at here speed he could set the experiement up such that when Alice is directly in front of him, the split photons are exactly 5m apart.
Sure. But in that case the light beams did not get sent out from the center of the railway car (according to either observer). So why is it a surprise that they reach the ends of the car at different times according to Alice?

(I thought you wanted the light beams to be sent from the center of the car, just as Bob passes Alice. Now you're describing a different set up.)

Sure. But in that case the light beams did not get sent out from the center of the railway car (according to either observer). So why is it a surprise that they reach the ends of the car at different times according to Alice?

(I thought you wanted the light beams to be sent from the center of the car, just as Bob passes Alice. Now you're describing a different set up.)

Yes I changed the experiement because the first one was incorrectly stated and didn't correctly address my issue. This new set up ensures that the two split photons arrive at Alices detectors simultaneously in Bob's frame, and that they came from a point source.

who is moving and who is stationary?

Doc Al
Mentor
I dont think thats true. Relativity is based upon the premise that events actually happen.
The "events"--the emitting and receiving of the light--really do happen.
You can't have some one dead in one frame and alive in another (for all time) like Shrogengers cat. It's only there relative times and locations that vary ro are relative.
Sure.
The point in this experiment is taking the different locations out the argument by forcing them to be coincident at one place and one time.
Which you failed to do.
If Alice says they arrive at different times, she would be forced to explain to the physics community how a laser can emit S and P light at different times from the same source. That just doesnt happen.
Alice surely agrees that the light beams were emitted at the same time. And she observes them reach the ends of the car at different times. No mystery here!

who is moving and who is stationary?

Alice is moving, Bob is stationary.

Doc Al
Mentor
Yes I changed the experiement because the first one was incorrectly stated and didn't correctly address my issue. This new set up ensures that the two split photons arrive at Alices detectors simultaneously in Bob's frame, and that they came from a point source.
Right. So what's the problem? Ask yourself: Where along the train is the emitter when the photons are emitted?

The "events"--the emitting and receiving of the light--really do happen.

Sure.

Which you failed to do.

Alice surely agrees that the light beams were emitted at the same time. And she observes them reach the ends of the car at different times. No mystery here!

Having the two photons emitted at the same time from the same location is one event, not two.

Right. So what's the problem? Ask yourself: Where along the train is the emitter when the photons are emitted?

slightly forward of the train.

I dont think thats true. Relativity is based upon the premise that events actually happen.

That's my point exactly. It's the events of light striking the detectors that counts. Bob claims those events are simultaneous. Alice claims they are not. Both Alice and Bob are free to claim they are right. There is no absolute definition of who is right.

You can't have some one dead in one frame and alive in another (for all time) like Shrogengers cat.

Instead of detectors lets clone Bob and have Bob1 at the front end of the train and Bob2 at the back of the train. I'll (P) take Bobs place at the center of the car. Now let's make the laser lethal and see what happens.

In my frame I see Bob1 and Bob2 die at the same time. Alice sees Bob1 die first and Bob2 dies second. Both Alice and I agree both Bobs are dead. We simply diagree on who died first. I did not claim that either 'Bob' was alive and dead at the same time.

If Alice says they arrive at different times, she would be forced to explain to the physics community how a laser can emit S and P light at different times from the same source. That just doesnt happen.

You're neglecting the beam splitter. The light started out as one source but was split into two sources. After splitting they are two independant signals and that's what counts.

The only thing Alice sees are flashes once the light hits her two detectors. You don't tell her anything about whether or not they came from a point source. All you tell her is that the flashes were simultaneous in Bob's frame and then ask her the question. She'll tell you they arrived at different times.

Doc Al
Mentor
Having the two photons emitted at the same time from the same location is one event, not two.
Yeah. So?
slightly forward of the train.
So if you realize that the light beams were emitted simultaneously at a point towards the front of the train, why would you think that the light would reach the ends of the train simultaneously according to train observers?

Doc Al
Mentor
The only thing Alice sees are flashes once the light hits her two detectors. You don't tell her anything about whether or not they came from a point source. All you tell her is that the flashes were simultaneous in Bob's frame and then ask her the question. She'll tell you they arrived at different times.
Right! It all makes sense, doesn't it?

That's my point exactly. It's the events of light striking the detectors that counts. Bob claims those events are simultaneous. Alice claims they are not. Both Alice and Bob are free to claim they are right. There is no absolute definition of who is right.

Instead of detectors lets clone Bob and have Bob1 at the front end of the train and Bob2 at the back of the train. I'll (P) take Bobs place at the center of the car. Now let's make the laser lethal and see what happens.

In my frame I see Bob1 and Bob2 die at the same time. Alice sees Bob1 die first and Bob2 dies second. Both Alice and I agree both Bobs are dead. We simply diagree on who died first. I did not claim that either 'Bob' was alive and dead at the same time.

You're neglecting the beam splitter. The light started out as one source but was split into two sources. After splitting they are two independant signals and that's what counts.

Your totally right about Bob1 and Bob2 dying at different times or simultaneously depending on the reference frame. Instead put Bob2 in front of the laser and let everyone know that he is from a planet that has no polarized light. If for whatever reason he gets hit with only S or only p polarized light at any instant he will die. Thats what I'm talking about. Alice will say he died, his brother Bob1 will say he's alive.

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Right! It all makes sense, doesn't it?

Read about Bob and his fear of polarized light above and you'll see a problem with that.

Doc Al
Mentor
Your totally right about Bob1 and Bob2 dying at different times or simultaneously depending on the reference frame. Instead put Bob2 in front of the laser and let everyone know that he is from a planet that has no polarized light. If for whatever reason he gets hit with only S or only p polarized light at any instant he will die. Thats what I'm talking about. Alice will say he died, his brother Bob1 will say he's alive.
I'm not seeing the mystery. When the light hits Bob2, he dies. (Everybody agrees on that!) The only thing up for discussion is whether--after all is said and done and records are compared as to the time of death--they died at the same time or not. Simultaneity is frame dependent, so the two frames will disagree. So?

Try and pose a question where some physically impossible situation is created via your setup.

Your totally right about Bob1 and Bob2 dying at different times or simultaneously depending on the reference frame. Instead put Bob2 in front of the laser and let everyone know that he is from a planet that has no polarized light. If for whatever reason he gets hit with only S or only p polarized light at any instant he will die. Thats what I'm talking about. Alice will say he died, his brother Bob1 will say he's alive.

You are now introducing quantum effects into what was originally a SR problem. And to further muddy the waters you are mis-representing the quantum argument anyway. Stick to the original question and Alice and I agree on the outcome even though we don't agree on the simultaneity. That is the correct answer.

I suggest you take you quantum effects question to the quantum physics forum.

Your totally right about Bob1 and Bob2 dying at different times or simultaneously depending on the reference frame. Instead put Bob2 in front of the laser and let everyone know that he is from a planet that has no polarized light. If for whatever reason he gets hit with only S or only p polarized light at any instant he will die. Thats what I'm talking about. Alice will say he died, his brother Bob1 will say he's alive.

To use an analogy I found simple, think of a very long loaf of bread that represents space-time. Consider every moment in time to be a "slice" of the loaf. For the observer on the train, he will say that both Bobs died on the same "slice." The observer not on the train, however, has slices that are slanted.(In reality, this is backwards. However, it is easier to visualize.)The non-train observer will say that Bobs died on different "slices."

If we were to examine there entire loaves, the exact same event would happen. The only difference would be which "slice" they occur on.

You are now introducing quantum effects into what was originally a SR problem. And to further muddy the waters you are mis-representing the quantum argument anyway. Stick to the original question and Alice and I agree on the outcome even though we don't agree on the simultaneity. That is the correct answer.

I suggest you take you quantum effects question to the quantum physics forum.

The point wasn't to discuss QM, it's to address simultaneity and whether two separate events are truely separate if you force them to come from the same initial event. You and I agree that Alice will see two distinct flashes if the events are simultaneous in Bob's frame. The point is that they both came from a SINGLE event regardless of what Alice wants to believe. Light doesnt travel different speeds from a single event.

train space-time plot

There is talk of 'someone sees the light reach...' but the only way to know this has happened is for a light signal to reach the observer from that point. The space-time plots show that any observer not in the middle of the rain sees two flashes at different times on his clock.

The first picture shows the trains point of view. The light strikes the back and front at the same time. The second picture shows the platforms viewpoint. The light reaches the ends of the trains at different times.

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To use an analogy I found simple, think of a very long loaf of bread that represents space-time. Consider every moment in time to be a "slice" of the loaf. For the observer on the train, he will say that both Bobs died on the same "slice." The observer not on the train, however, has slices that are slanted.(In reality, this is backwards. However, it is easier to visualize.)The non-train observer will say that Bobs died on different "slices."

If we were to examine there entire loaves, the exact same event would happen. The only difference would be which "slice" they occur on.

In the experiement above as stated, Bob2 dies in Alice's frame, but does not die (at all) in Bob1's frame. I understand your analogy and again I don't mean to talk about QM here.

Doc Al
Mentor
The point is that they both came from a SINGLE event regardless of what Alice wants to believe. Light doesnt travel different speeds from a single event.
What are you talking about? Everybody--even Alice, of course!--agrees that both light beams came from a single event. SO? There is no reason whatsoever for her to conclude that the light beams travel at different speeds. Where are you getting this from?

The point wasn't to discuss QM, it's to address simultaneity and whether two separate events are truely separate if you force them to come from the same initial event.

Ok, I accept that. But it's simpler and easier to understand if you don't introduce more and more variables.

The point is that they both came from a SINGLE event regardless of what Alice wants to believe. Light doesnt travel different speeds from a single event.

This is, I think, the root of the problem. You are seeing the emmision of two photons as a single event. I disagree. The two photons are independant and free to travel different paths. It's not their creation that is of concern to this problem it's the two separate events of their detection that we are discussing. The events of detection are simultaneous in one frame and not simultaneous in the other.

Doc Al
Mentor
In the experiement above as stated, Bob2 dies in Alice's frame, but does not die (at all) in Bob1's frame.
Aren't Bob1 and Bob2 at the ends of the train and thus in Alice's frame?

In any case: A frame is just a framework for capturing measurements of the time and position of various events. Events happen--or don't happen--independent of any particular frame you might use to describe them.

In the experiement above as stated, Bob2 dies in Alice's frame, but does not die (at all) in Bob1's frame. I understand your analogy and again I don't mean to talk about QM here.

This implies that there are two different space-time realities. Think of this in terms of the above analogy, and I think you'll figure it out.

Yeah. So?

So if you realize that the light beams were emitted simultaneously at a point towards the front of the train, why would you think that the light would reach the ends of the train simultaneously according to train observers?

I'm accounting for the time it takes light to reach the ends ot the train sultaneously in Bobs frame from the beam splitter.