Grimble
- 485
- 11
It is all due to Relativity! All Frames of Reference are equal. None is a preferred Frame.
Also, because no Frame is preferred, Spacetime is at rest as mapped from each and every Frame.
Take an Event: A flash of light. It happens at a particular location in space; at a specific instant in time. (In each frame the Space and time coordinates will be different depending on the origin and orientation of that frame)
Yet the one thing that we know; and to which all observers in whichever frame they observe from; is that after 1 second the light emitted at that event will form a sphere, with a radius of 1 light second, centred on the initial flash-of-light event.
Now the difficulties arise whether we are considering moving trains on embankments, lights in carriages, in Rockets etc.
Let us take the last as an example:
From the rocket's frame of reference, after 1 second the light will have reached either end of the stationary rocket; yet from the perspective of an independent observer relative to whom the rocket is moving, the light will reach the rear of the rocket before the front.
Both these view's are correct; the point is that it is all relative; from the perspective of the observer in the rocket, the light flash was in the middle of the rocket (which in his frame of reference is at rest) and the light took the same time to travel to each end of the rocket.
From the perspective of the independent observer, moving relative to the rocket, the light flash happened at a particular fixed point in space, which at that moment coincided with the centre of the moving rocket. The light traveled out from the point occupied by the centre of the rocket, as a perfect sphere, at rest relative to the independent observer, while the rocket moved away at the relative speed between the rocket and the independent observer.
What we do know is that from an event, i.e. a fixed point in space, at a particular moment in time, the light will spread outwards as a perfect sphere at the speed of light.
In every single frame of reference that will happen. In every frame of reference the origin is at rest, because that is the point from which spacetime is mapped using that frame of reference. For the real or virtual observer, permanently at the origin of any frame spacetime is at rest; it has to be because that is what is being mapped.
Therefore the speed of the light emitted at that event will travel at c relative to each and every observer.
Let me say once again - it is all relevant!
We only have trouble dealing with this when we define one particular view, one particular frame of reference, and make everything relative to that.
Also, because no Frame is preferred, Spacetime is at rest as mapped from each and every Frame.
Take an Event: A flash of light. It happens at a particular location in space; at a specific instant in time. (In each frame the Space and time coordinates will be different depending on the origin and orientation of that frame)
Yet the one thing that we know; and to which all observers in whichever frame they observe from; is that after 1 second the light emitted at that event will form a sphere, with a radius of 1 light second, centred on the initial flash-of-light event.
Now the difficulties arise whether we are considering moving trains on embankments, lights in carriages, in Rockets etc.
Let us take the last as an example:
From the rocket's frame of reference, after 1 second the light will have reached either end of the stationary rocket; yet from the perspective of an independent observer relative to whom the rocket is moving, the light will reach the rear of the rocket before the front.
Both these view's are correct; the point is that it is all relative; from the perspective of the observer in the rocket, the light flash was in the middle of the rocket (which in his frame of reference is at rest) and the light took the same time to travel to each end of the rocket.
From the perspective of the independent observer, moving relative to the rocket, the light flash happened at a particular fixed point in space, which at that moment coincided with the centre of the moving rocket. The light traveled out from the point occupied by the centre of the rocket, as a perfect sphere, at rest relative to the independent observer, while the rocket moved away at the relative speed between the rocket and the independent observer.
What we do know is that from an event, i.e. a fixed point in space, at a particular moment in time, the light will spread outwards as a perfect sphere at the speed of light.
In every single frame of reference that will happen. In every frame of reference the origin is at rest, because that is the point from which spacetime is mapped using that frame of reference. For the real or virtual observer, permanently at the origin of any frame spacetime is at rest; it has to be because that is what is being mapped.
Therefore the speed of the light emitted at that event will travel at c relative to each and every observer.
Let me say once again - it is all relevant!
We only have trouble dealing with this when we define one particular view, one particular frame of reference, and make everything relative to that.