Quote:
Originally Posted by xelasnave
Steven said....
"A photon can be simultaneously at the start and end of it's journey"
Steven can I trouble you for more help  .
Does this mean a photon that left "the other end of the Universe" to arrive here is at both ends of its journey simultaneously? 
or does it mean that the photo only "thinks" it is?
Or what I should say this seems strange is it possible to explain this to a lay man so it makes sence.
Anyways I have followed this thread with absolute interest and am trying to get my head around it all. 
Alex |
Hello Alex.
The best way to explain this is with a thought experiment.
Suppose I'm driving in my car and I turn my headlights on. You're at a location a few kilometers down the road. The photons from my headlights reach you after a time t= d/c where d is the distance from you when I turned my headlights on and c is the speed of light.
Suppose there are other observers at various locations along the road. They receive the photons after t1=d1/c, t2= d2/c, t3= d3/c etc.
Note that all observers will measure the speed of the photon as c which is independant of the speed of my car as I approach the observers.
Since my car is travelling very much slower than c each observer will note that I turn my headlights on followed by a period of elapsed time when I drive past their locations. The elapsed time is t(1)= d1/u, t(2)=d2/u, t(3)=d3/u etc where u is the speed of my car.
This order of events is consistant with each observer.
Now let's suppose I defy SR and my car is now travelling at u=c. I turn my headlights on. What happens now. Both the car and the photons arrive at the same time for each observer. Each observer will claim I turned my headlights on at the instant I passed their location despite the fact that each observer is in a different location.
At the speed of light all events become
simultaneous.
There is a third scenario. What happens if a photon is able to pass information faster than the speed of light? It can be shown mathematically that causality is violated. Some observers will see event A happen before event B, others will see event B occur before event A.
Hope this is of help.
Regards
Steven