ABC of relativity

[birdman]

Hello all,

 

Posted this in the 'Relativity' section but no takers..may be a little elementary for that folder, perhaps someone could help here..:

 

'ABC of Relativity by Bertrand Russell - A layman strikes..'

 

Hope someone can help with my understanding of a part of this book. Train is travelling at 3/5 the speed of light and passes a stationary observer at a point which an event in the direction of the train occurs - "An event which happens in the forward direction along the railway, and which the stationary observer judges to be now (or rather will judge to have been now when he comes to know of it) if it occurs at a distance along the line which light could travel in a second, will be judged by the traveller to have occurred 3/4 of a second ago"

 

From what I understand of this, the observer on the train will receive the light from the event before the stationary observer because it is travelling towards it and therefore will see it before the stationary observer (this may be incorrect) but I cannot see why it will be 3/4 of a second ago - can someone run me through this algebraically in order to put my mind at ease?

 

Many thanks for help in advance!

[someguy]

i'm not sure of the exact formulas so i can't show you algebraically but basically time does not hold the same values for objects moving at different speeds. the train would not see the event occur more quickly from its perspective because it is travelling towards the light source. light travels at the same speed no matter how fast you are moving towards it. this is because time does not hold the same value for observers moving at different speeds. what happens is since time is different and yet the train is moving towards the light, the time taken off the expected ETA of the light hitting the train is exactly offset by the difference in time the two observers experience.

 

I would like to better answer your question more specifically for this particular problem but i find it a little confusing. a drawing might help, but also when you speak of seconds you need to define who's seconds they are because the person on the train and the stationary person experience different seconds, though to them they always seem constant.

[birdman]

Thanks for the reply.

 

In that case, if light adjusts it doesn't matter how fast the train is going, the observer on the train and the stationary observer should see the event at the same time if the event happens as they pass?

 

I'm quoting verbatim from the book, so unfortunately I'm not sure whose seconds they relate to..

[someguy]

Thanks for the reply.

 

In that case, if light adjusts it doesn't matter how fast the train is going, the observer on the train and the stationary observer should see the event at the same time if the event happens as they pass?

 

I'm quoting verbatim from the book, so unfortunately I'm not sure whose seconds they relate to..

 

 

ya i think that's right if they both ticked their stop watch when they passed, and that's when the event occurred, one lightsecond away, they would both see it when their stopwatches read one second. though their stopwatches are not ticking at the same rate.

[birdman]

Ok so they'd see it after what they perceived as one second, but how is the "will be judged by the traveller to have occurred 3/4 of a second ago" calculated? And who is it relative to?