When do stationary observers receive signals from a relativistic spaceship?

[PeroK]

Let me ask you all a question that is at the very heart of my understanding of relativity. If A and B are two events in Spacetime and light emitted at those events meets at point M midway between A and B, were A and B simultaneous? Note, this is without defining any frames of reference and without defining any observer, let us say it is an objective view that could be measured from anywhere.

Sadly, your question takes as a premise that simultaneity of two events is absolute. If you assume this, then you are bound to run into a contradiction at some point, given that SR shows that simultaneity is relative.

 

[Ibix]

I think I misinterpreted Grimble. I am not sure I understand what he wanted to say. Does he unwittingly assigns simultaneity of events to the train and admits that rays of light will meet in the centre of the train, if they were simultaneous in embankment frame?

I'm not sure. From his last post, I suspect he's figured out the block universe model but has not quite worked through the idea that there is no preferred direction in which to view it. I could be wrong...

 

[jbriggs444]

A frame of reference is no more than a map of Spacetime based upon a particular event - a point in space at a point in time and therefore every frame of reference is at rest relative to that initial event.

An event has no definable state of motion. You cannot be "at rest" relative to something for which no state of motion is defined.

 

[PAllen]

Ah! now I do see what you are all saying and why we seem to be using the same words yet speaking different languages!
You say I am giving motion to events when I say their locations are at rest in the rest-frame of an observer - because that 'rest frame' is only 'at rest' measured from itself! From any other frame it is moving - and because 'at rest' has to be relative to something and that something has a state of motion relative to everything else.

Because I have tried to explain how I understand relativity using your framework it doesn't work (for explaining my understanding); because I immediately place myself within the constraints of your views using anthropomorphised frames of reference (well not exactly given human form but at least given physical form - embankments and trains); because that immediately gives rise to 'preferred frames' - usually the embankment - e.g. when we say that the lightning flashes were simultaneous in that frame.
I believe there is a fundamental error in that very phrase for events A and B are not simultaneous in the Embankment frame but are measured to be simultaneous in that frame.

I do not believe that Spacetime has any rest state. That everything moves relative to everything else. That every observer measures Spacetime from their own rest frame. That is not stating that any frame is truly at rest for the very concept does not exist for there is no way to assign a state of rest in Spacetime.
A frame of reference is no more than a map of Spacetime based upon a particular event - a point in space at a point in time and therefore every frame of reference is at rest relative to that initial event.
I am sorry if I do not use the correct phrases for I am not a professional scientist, but I am trying to explain what my understanding is.

Let me ask you all a question that is at the very heart of my understanding of relativity. If A and B are two events in Spacetime and light emitted at those events meets at point M midway between A and B, were A and B simultaneous? Note, this is without defining any frames of reference and without defining any observer, let us say it is an objective view that could be measured from anywhere.

To answer your last question, it again shows a very basic misunderstanding. Midpoint between A and B in spacetime is an event with space like separation between A and B, therefore no signals from A and B could possibly reach it. So, to define a reachable event you have to posit a world line through M, and there are an infinity of such choices, thus your question has no meaning without a frame of reference - which picks which world line through M is considered to be stationary. Given a choice such that signals from A and B arrive at the same event on this world line, you can say that in the frame where this particular world line is stationary, events A and B are simultaneous. In every other frame, which pick different world lines through M as the stationary one, they are not simultaneous.

 

[PeterDonis]

were A and B simultaneous? Note, this is without defining any frames of reference

Then your question makes no sense, because "simultaneous" has no meaning unless you define a frame of reference. This has been said many times in many ways in response to your posts. Enough is enough. Thread closed.