Events vs Effect of Events in Relativity of Simultaneity

[Timbo65]

TL;DR

Thought experiments use the effects of events rather than pure events to explain the relativity of simultaneity. Maybe I am overthinking this, but it does confuse me a bit.

Einstein's thought experiment involves a train carriage, a platform and two observers. The train carriage moves relative to the platform, and one observer is in the middle of the carriage, passing the other observer standing on the platform. When the two observers pass each other, lightning strikes both ends of the train carriage. The observer in the train carriage "sees" the flashes simultaneously, knows that he is equidistance from the ends of the carriage and with the speed of light constant in his frame of reference, concludes the flashes were simultaneous.

My issue is how he can conclude that the flashes were simultaneous without attaching the flash to the end of the carriage (maybe by a burn mark). Take the carriage away and make the event an instantaneous flash, and then he cannot say that he is nearer or further away from the event between the event occurring and him seeing it - events have no frame of reference.

Einstein's thought experiment depends on the flash hitting the end of the carriage to give it a frame of reference. To be sure, the eventual conclusion of the thought experiment (a frame of reference moving towards an event observes that event sooner) is still correct. I might be overthinking it, but the difference between an event and the effect of an event has confused me a bit. Is Einstein using a bit of "poetic licence" to get the explanation across?

 

[Nugatory]

My issue is how he can conclude that the flashes were simultaneous without attaching the flash to the end of the carriage (maybe by a burn mark).

That's pretty much exactly what the thought experiment is doing: The end of the carriage and the point on the track are at the same place when the lightning hits, so both are scorched; the scorch marks are where the lightning hit in both frames. It would have been helpful if Einstein had stated this explicitly instead of assuming that we'd mentally inject tthe scorch marks if we needed clarification of what is meant by "where the lightning hit", but he didn't.

It's easy to accept platform-person's sense that the scorch mark on the track is "where the lightning hit". Somehow we find it harder to accept that train-person's definition of where the lightning hit as being the location of the scorch mark on the train is just as valid. It helps to imagine that the train is at rest while the platform is moving backwards, or even that you are watching the whole thing through a telescope from Mars, so both platform and train are moving at many kilometers a second relative to you).

As an aside, if you find Einstein's presentation confusing, you're in good company. We have lots of older threads about it.

 

[Ibix]

In some ways it's easier to consider a kind of reverse of the setup. When the two observers pass each other they emit forward-going and backward-going light pulses. What do they say about the arrival times of those pulses at the ends of the carriage and platform? They must get simultaneous arrival at the ends of the one with respect to which they are at rest and non-simultaneous for the ends of the other. Hence, simultaneity is frame dependent.

I believe Morin's Relativity for the enthusiastic beginner takes that approach.

 

[Sagittarius A-Star]

The observer in the train carriage "sees" the flashes simultaneously, knows that he is equidistance from the ends of the carriage and with the speed of light constant in his frame of reference, concludes the flashes were simultaneous.

In Einsteins thought experiment, the observer in the train does not see the flashes simultaneously because they do not happen simultaneously with reference to the rest-frame of the train and he is located equidistant to the train-locations, where the events happened.

Source:
https://en.wikisource.org/wiki/Relativity_(1931)/Section_9

 

[ersmith]

My issue is how he can conclude that the flashes were simultaneous without attaching the flash to the end of the carriage (maybe by a burn mark). Take the carriage away and make the event an instantaneous flash, and then he cannot say that he is nearer or further away from the event between the event occurring and him seeing it - events have no frame of reference. Einstein's thought experiment depends on the flash hitting the end of the carriage to give it a frame of reference.

The quoted text concerns me because it seems to express a somewhat muddled view of what a "frame of reference" is. You're correct to say that an event does not have a frame of reference. That's because in special relativity a frame of reference is literally just an assignment of coordinates to events. That is, all events are present in all frames of reference. The frame is not a physical thing, it's just a coordinate system.

There are a number of ways the observer can figure out what coordinates to assign to the events of the two lightning strikes. Scorch marks may make the job easier, but they're not necessary.

 

[Herman Trivilino]

TL;DR Summary: Thought experiments use the effects of events rather than pure events to explain the relativity of simultaneity. Maybe I am overthinking this, but it does confuse me a bit.

I'm also confused by your statement. What do you mean by a "pure event"? And what is the "effect of an event"? Do you mean that a pure event can be a lightning strike, and the effect of that event is arrival of a flash of light? Of course the strike and the arrival of the flash have to be spatially and temporally separated, otherwise there is no passage of time between the two. Without that the thought experiment won't demonstrate the relativity of simultaneity.

Take the carriage away and make the event an instantaneous flash, and then he cannot say that he is nearer or further away from the event between the event occurring and him seeing it - events have no frame of reference.

Events occur in a frame of reference. If the lightning strike and the arrival of the flash are simultaneous, they are the same event. It is simply the lightning strike, and it will be assigned coordinates in all frames of reference.

When the two observers pass each other, lightning strikes both ends of the train carriage.

That statement is true in only one of the frames of reference. It can't possibly be true in both frames, which is Einstein's point.

 

[Timbo65]

Thanks for your replies. I sat down and tried to formulate an example that better articulates my question. As a result, I untangled my mind and realised I was overthinking it - I now get it. Thanks again.