Solve Simultaneity Problem with Alice & Bob

[Doc Al]

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 propogate 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!

 

[Flexo]

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 propogate 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.

 

[rq704c]

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.

 

[rq704c]

...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.

 

[Doc Al]

...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.)

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.

 

[rq704c]

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!

 

[Dale]

Wow, 56 posts for one thread in a 3 hour period. That has got to be some kind of record.

 

[rq704c]

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.

 

[neh4pres]

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