# Relativity of Simultaneity and classic thought experiment

For the following thought experiment, How to do calculation in relativity of simultaneity

Why is simultaneity considered as an act of seeing the bolts? Even for the passenger in the train, can we not argure that the event may actually be simultaneous for him as well, just that he perceived (with eyes) at a later point of time?

Instead of person in the middle of train doing the observation, I have an observer at each end of the train who just note the time of nearest bolt and tally them later. How do you think this changes things for the given experiment?

How is this thought experiment justified for explaining relativity of simultaneity as it's based on human senses ? Even with Galilean transformation wouldn't this experiment be still valid? Can we reformulate this thought experiment to not have human perception for measurement and still validate the relativity of simultaneity?

• How could the person "observe" something they so not have data for? If you mean that it is a delayed realization then perhaps, but it has to be the same for all observers to be a law or result of physics.
– user196418
Dec 27, 2018 at 11:53
• Can't the observation be based on something like a light beam reacting on some.light sensitive plates with a timestamp, which later be taillied by both observers? I am not sure how this experiment establishes perception as basis of non simultaneity.
– FUD
Dec 27, 2018 at 11:56
• That is exactly correct but does not undo relative simultaneity, as you have suggested..
– user196418
Dec 27, 2018 at 12:02
• IMHO a reading of first chapter of Bernard Shutz's intro to GR will best explain this to you. I can reproduce his core argument here but it will take too much space. The key is to take the human element and the perception stuff away from the experiment. Just think of the observer as a coordinate system and recordings as always made at the point where the event happens. He uses this construction to simply illustrate the failure of simulatenity and then goes on to derive the Lorentz transformation from it. Dec 27, 2018 at 12:30
• Sorry got too excited I understand this is an answer to my last question. I will take a look at it . Thanks
– FUD
Dec 27, 2018 at 12:37

Your last statements sort of undo all of modern physics.

1. A person cannot perceive something that is not there. That is not objective reality. And that is one of the corner stones of all modern physics that we cannot remove ourselves from the basic theory. Observation is required for objective results to be discussed.

2. Einstein actually considered the fact (as was known at the time) that our senses are electromagnetic. That is that all we are able to observe using our senses is due solely to the electromagnetic force. If there is a symmetry or constraint in EM theory then there is a symmetry and constraint in all observation and knowledge. He writes about this in Meaning of Relativity.

3. Even if there are deviations in observed simultaneity due to variations in nerve conductivity or delayed perception the observer can be replaced with detectors and what we know from relativity holds for the response of the detectors. So that is on sound and solid ground.

4. Even is you could devise a theory that had things occurring simultaneously in all frames but perceived differently by different observers all you would have done is move a problem to a different spot under the carpet. Science is based on empiricism. They only thing we seek to describe is when we experience. So building layers of reality that are manifestly non-existent does not serve any purpose.

• Instead of person in the middle of train doing the observation, I have an observer at each end of the train who just note the time of nearest bolt and tally them later. How do you think this changes things for the given experiment?
– FUD
Dec 27, 2018 at 12:05
• I have edited my question to make more clear what I meant by human senses. For a person in the middle of the train and person at the ends the perception would be different.
– FUD
Dec 27, 2018 at 12:14
• It doesn't because they know nothing of each others observation until they meet and compare notes. If they are in the same frame of reference, and synchronized then when they meet in the middle they will see different times recorded and measure they same time delay between observations. They could be two synchronized detectors.
– user196418
Dec 27, 2018 at 12:14
• Could you explain why there would be a time difference as there is no distance for the light bolts to travel at each end. Both should see and record bolts at the same time t in the trains frame of reference.
– FUD
Dec 27, 2018 at 12:17
• It's the movement of the train that causes the time delay. The train is moving, right? If not then relativity states that the events are simultaneous.
– user196418
Dec 27, 2018 at 12:22