[ExI] speed of light at the speed of light

[Anders Sandberg]

On 22/08/2013 20:58, John Clark wrote:
> On Thu, Aug 22, 2013 at 7:29 AM, Gordon <gts_2000 at yahoo.com 
> <mailto:gts_2000 at yahoo.com>> wrote:
>
>     > I am in a spaceship accelerating away from earth and approaching
>     the speed of light. I have a flashlight. My instruments tell me
>     that the light emitted from my flashlight travels at c. This is
>     true no matter whether I shine it forward in the direction of my
>     travel or backward toward the rear of the ship.
>     I am accelerating away from earth such that my ship's velocity
>     relative to earth compared to c is halved in each time period. For
>     example at time t, my ship is travelling at 90% of c. At t2, my
>     ship is travelling at 95% of c. At t3, my ship is travelling at
>     97.5% of c, and so on for an infinite amount of time as I approach c.
>
>
> It's OK for thought experiments to be wildly impractical but they must 
> be physically possible, and the above experiment would not only take 
> an infinite amount of time to perform it would also take an infinite 
> amount of energy.

However, the conclusion about infinite time and energy are a *result* of 
doing the Einstein thought experiment properly. You cannot dismiss the 
lightspeed case straight away.

Einstein had a nifty way of showing that there is something problematic 
going on in this case, which I think was one of his primary reasons for 
developing the full theory: suppose you run past an electromagnetic wave 
while travelling at c. What do you see? It ought to be static in your 
reference frame, but in that case it breaks Maxwell's laws. So either 
electrodynamics is wrong, or the velocity addition formula is wrong (and 
we already have some suspicions since light seem to move at c regardless 
of speed). So let's see what happens if we assume the velocity addition 
formula has to be something else...

-- 
Anders Sandberg,
Future of Humanity Institute
Oxford Martin School
Faculty of Philosophy
Oxford University

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.extropy.org/pipermail/extropy-chat/attachments/20130822/ea5dbcda/attachment.html>