Solve Simultaneity Problem with Alice & Bob

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    Simultaneity
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SUMMARY

The discussion centers on the simultaneity problem in special relativity, specifically involving Alice on a moving railroad car and Bob observing from the ground. The participants explore how the speed of light remains constant for all observers, leading to differing perceptions of simultaneity for events occurring in different frames of reference. They conclude that while Bob can set up an experiment to ensure light reaches both ends of the car simultaneously in his frame, Alice will perceive the events as non-simultaneous due to her motion. This disagreement does not imply a change in the speed of light but rather highlights the relativity of simultaneity.

PREREQUISITES
  • Understanding of special relativity principles
  • Familiarity with the concept of simultaneity in different frames of reference
  • Knowledge of light behavior and beam splitters
  • Basic grasp of Lorentz transformations
NEXT STEPS
  • Study the implications of Lorentz transformations on simultaneity
  • Explore the concept of light cones in special relativity
  • Investigate the role of reference frames in relativistic physics
  • Learn about experimental setups demonstrating the relativity of simultaneity
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Physics students, educators, and anyone interested in understanding the complexities of special relativity and the nature of simultaneity in different inertial frames.

  • #31
rq704c said:
The point is that they both came from a SINGLE event regardless of what Alice wants to believe. Light doesn't 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?
 
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  • #32
rq704c said:
The point wasn't to discuss QM, it's to address simultaneity and whether two separate events are truly 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.

rq704c said:
The point is that they both came from a SINGLE event regardless of what Alice wants to believe. Light doesn't 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 independent 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.
 
  • #33
rq704c said:
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.
 
  • #34
rq704c said:
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.
 
  • #35
Doc Al said:
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.
 
  • #36
rq704c said:
I'm accounting for the time it takes light to reach the ends ot the train sultaneously in Bobs frame from the beam splitter.
Huh? What do you mean "accounting for"? Forget Bob, we're talking about the train frame. Do you agree that the light beams were sent out from a single event? Do you agree that this event took place in the forward end of the train? Then of course the light will take different times to reach the ends of the train according to train observers. It's as simple as that.
 
  • #37
paw said:
Ok, I accept that. But it's simpler and easier to understand if you don't introduce more and more variables.



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 independent 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.

Two photon emission from a single source is common via down conversion. Two photons = one event.
 
  • #38
rq704c said:
Two photon emission from a single source is common via down conversion. Two photons = one event.
It's perfectly OK to consider the emission of both photons as a single event. The main thing is that everyone agrees that the two photons were emitted at the same place and time. (That's not the problem.)
 
  • #39
Doc Al said:
It's perfectly OK to consider the emission of both photons as a single event. The main thing is that everyone agrees that the two photons were emitted at the same place and time. (That's not the problem.)

No we agreed that Alice did not see the two flashes simultaneously. Bob set the experiment up in such a way that the two photons hit both of her detectors simultaneosuly in his frame. I'm making a picture.
 
  • #40
rq704c said:
No we agreed that Alice did not see the two flashes simultaneously. Bob set the experiment up in such a way that the two photons hit both of her detectors simultaneosuly in his frame. I'm making a picture.

You are making this much more complex than it needs to be. Please state whether or not you believe simultaneity is not agreed upon. If you say that is it not, what problem do you have with it?
 
  • #41
rq704c said:
No we agreed that Alice did not see the two flashes simultaneously. Bob set the experiment up in such a way that the two photons hit both of her detectors simultaneosuly in his frame. I'm making a picture.
Who needs a picture? If you agree that according to Alice the flashes reach the ends of the car at different times, then what's the problem? (The statement that "Alice did not see the two flashes simultaneously" is ambiguous. It does not mean that she has any reason to think they weren't emitted simultaneously.)

Seems like you are confusing the emission of the photons (which everyone, including Alice, agrees happened at the same time) with their arrival at the ends of the car.
 
  • #42
rq704c said:
Two photon emission from a single source is common via down conversion. Two photons = one event.

Sure, but it's not the creation event we are discussing, it's the two detection events that are of concern.

You can analyse the problem at the level of the creation event but that wasn't the original question. I did try to address the creation event in an earlier post. In that case Bob agreed the photons were created at the same time, Alice did not agree they were created at the same time. A more rigorous analysis would require a quantum explanation which I'm not qualified to give and is really outside the scope of this forum.
 
  • #43
paw said:
Sure, but it's not the creation event we are discussing, it's the two detection events that are of concern.

You can analyse the problem at the level of the creation event but that wasn't the original question. I did try to address the creation event in an earlier post. In that case Bob agreed the photons were created at the same time, Alice did not agree they were created at the same time. A more rigorous analysis would require a quantum explanation which I'm not qualified to give and is really outside the scope of this forum.
Seems like we are off on a side road, but if one frame sees two photons emitted at the same time and place, then all frames will. Including Alice's frame.
 
  • #44
Doc Al said:
Huh? What do you mean "accounting for"? Forget Bob, we're talking about the train frame. Do you agree that the light beams were sent out from a single event? Do you agree that this event took place in the forward end of the train? Then of course the light will take different times to reach the ends of the train according to train observers. It's as simple as that.

Do you agree that the light beams were sent out from a single event?
Yes

Do you agree that this event took place in the forward end of the train?
Yes

Then of course the light will take different times to reach the ends of the train according to train observers. It's as simple as that.

The photons are set up by Bob, to force the photons to reach the two detectors simultaneously in his frame, not at different times.
 
  • #45
rq704c said:
The photons are set up by Bob, to force the photons to reach the two detectors simultaneously in his frame, not at different times.
Right! So what are we discussing here? They arrive at the detectors simultaneously in Bob's frame, but at different times in Alice's frame. Done!

This thread seems to be going in circles. I suggest you make your point quick.
 
  • #46
Doc Al said:
Seems like we are off on a side road, but if one frame sees two photons emitted at the same time and place, then all frames will. Including Alice's frame.


Here's a diagram
 

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  • #47
rq704c said:
Here's a diagram
OK. So?
 
  • #48
Doc Al said:
Seems like we are off on a side road, but if one frame sees two photons emitted at the same time and place, then all frames will. Including Alice's frame.

Yeah, I think I could have said that better. I'm thinking of the beam splitter as a black box simply emitting two independent photons with the mechanism hidden. This was in an attempt to show that it's not the creation event we are discussing. It seems like every time I try to give an answer the question changes.
 
  • #49
paw said:
This was in an attempt to show that it's not the creation event we are discussing. It seems like every time I try to give an answer the question changes.
I know the feeling. :wink:
 
  • #50
Doc Al said:
OK. So?

If you look at the right diagram, the two photons hit the two detectors simultaneously in Bob's frame at t = 0. The flashes from the detections then propagate back toward Alice and Bob. If Alice says the signals arrive at her head at different times in her frame, we have to conclude that the two photons were created or emitted at different times because the light time distance is equal from the two detectors back to the beam splitter at t = 0. Thus Alice and Bob diagree that the two photon creation event was a single event.
 
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  • #51
rq704c said:
If you look at the right diagram, the two photons hit the two detectors simultaneously in Bob's frame at t = 0. The flashes from the detections then propagate back toward Alice and Bob.
Ah... I finally see what you're talking about. You are concerned with the time that specific observer Alice (in the middle of the train) receives signals from the detectors.
If Alice says the signals arrive at her head at different times in her frame, we have to conclude that the two photons were created or emitted at different times because the light time distance is equal from the two detectors back to the beam splitter at t = 0.
The light travel time from the detectors happens to be equal, but the photons do not arrive at the detectors simultaneously. Alice will deduce that the signals from the detectors were not sent out at the same time. Which makes perfect sense since the original photons had to travel different distances to get to the detectors.

Again: No mystery!
 
  • #52
rq704c said:
If you look at the right diagram, the two photons hit the two detectors simultaneously in Bob's frame at t = 0. The flashes from the detections then propagate back toward Alice and Bob. If Alice says the signals arrive at her head at different times in her frame, we have to conclude that the two photons were created or emitted at different times because the light time distance is equal from the two detectors back to the beam splitter at t = 0. Thus Alice and Bob diagree that the two photon creation event was a single event.

In your scenario, if I understand it right, Alice will also agree that the light reaches each detector simultaneously. I know this is not what you are trying to express, though. You need to understand that to a stationary observer, the light needs to travel different distances to reach each part of the train. This is what results in the disagreement.
 
  • #53
Doc Al said:
Ah... I finally see what you're talking about. You are concerned with the time that specific observer Alice (in the middle of the train) receives signals from the detectors.

The light travel time from the detectors happens to be equal, but the photons do not arrive at the detectors simultaneously. Alice will deduce that the signals from the detectors were not sent out at the same time. Which makes perfect sense since the original photons had to travel different distances to get to the detectors.

Again: No mystery!


...The photons travel the EXACT SAME DISTANCE to the detectors. Look at the picture again.
 
  • #54
rq704c said:
...The photons travel the EXACT SAME DISTANCE to the detectors. Look at the picture again.

However in Bob's frame, they travel different distances back to Alices head beacuse she's moving to the right.
 
  • #55
rq704c said:
...The photons travel the EXACT SAME DISTANCE to the detectors. Look at the picture again.
Look at your own diagram again! In the train frame, the photons travel different distances to the detectors. (The distance would be the same if the photons were emitted at the center of the train, but they are not.)
rq704c said:
However in Bob's frame, they travel different distances back to Alices head beacuse she's moving to the right.
Which, again, makes perfect sense! Everyone agrees that the signals from the detectors arrive at Alice's head at different times. And since the signals leave the detectors at the same time in Bob's frame, they'd better travel different distances in his frame.
 
  • #56
Doc Al said:
Look at your own diagram again! In the train frame, the photons travel different distances to the detectors. (The distance would be the same if the photons were emitted at the center of the train, but they are not.)

Which, again, makes perfect sense! Everyone agrees that the signals from the detectors arrive at Alice's head at different times. And since the signals leave the detectors at the same time in Bob's frame, they'd better travel different distances in his frame.

Agreed!
 
  • #57
Wow, 56 posts for one thread in a 3 hour period. That has got to be some kind of record.
 
  • #58
DaleSpam said:
Wow, 56 posts for one thread in a 3 hour period. That has got to be some kind of record.

Yeah that's a lot.
 
  • #59
ok I am sorry but all your doing is rewording the original problem "if a flash is emitted in the center of the train , one observer will see it hit both ends simultaneously and the other will not" don't try to reword it or change the that scenario, you will only delay your understanding of it. What the scenario is trying to help you understand is that the speed of light is always c to every frame. If your traveling at .5c and you measure the speed of light coming at you from the front it will measure c. if you measure light coming from behind then it will also measure c.. even though you are traveling at .5c already. Also if you emit a beam of light while going .5c you will see it speed off at c and to the stationary observer it will also be going c NOT 1.5c there for the two will argue at the distance the beam traveled from the rocket over a given time... and if you still don't understand... then if you emit a pulse of light while traveling at .5c then u both measure the distace from the rocket the light traveled after one second.. The man in the rocket would say it was 186000 miles from the rocket, and the stationary observer would say it was 93000 miles from the rocket after one second the pulse was emitted ...two people have seen 2 different things and both are true
 

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