Approaching the speed of light

[ACG52]

Perhaps I misunderstood your explanation. The observer on the ground would see the moving clock as running slower. The observer on the ship would see the earth clock as running slower. They would both see their own clock as running normally.. Given that the earth clock remains 'stationary' (i.e. has no accelerations applied to it), then when the earth clock and the moving clock are brought into the same reference frame and compared, less time will have elapsed for the moving clock.

Edited December 26, 2012 by ACG52

[altergnostic]

The discussion has already gone into the realm of GR, but it may not be too late to be reminded how c is determined in the first place and to remember that c as a constant in every frame is a convention, not an experimental fact.

The speed of light can only be measured with a roundtrip setup, where you send a beam towards a mirror some distance away and wait for it to be detected back at the point of emission. No other setup can consistently measure the speed of light, because we only know where light is when it is detected directly. Einstein knew this and stated that we have no evidence that light travels at c in each trip, the evidence is only that c is constant for the roundtrip, or avarage.

Length contraction and time dilation were developed when it was thought that light moves at c relative to the aether. Since experiments showed that motion relative to the aether didn't affect our measurements of the speed of light, it was postulated that lengths and times changed as we moved with respect to the aether. Einstein built SR using the Lorentz Transfromations, which were formulated upon the concept of the aether, but he realised that we could easily apply those transforms without any absolute aether if everything was relative period, not relative to an aether. So it is not the bodies that are compressed, but "space", which was an abstract concept until then (space could not acquire physical characteristics, like compression). Einstein never showed us why or how length contracts and time dilates, but it is an accepted convention because it is the only way we have married electromagnetism with kinematics consistently: the relative velocities of objects in motion and the constant velocities of electromagnetic waves.

I like to remember that these speeds are not measured in the same way, though. We can measure the speed of a train from any point A to any point B, if we receive light from it constantly (what we call "observation") but we can do the same with light (we can't bounce off photons from a photon to observe it), so we only know the speed of a single beam by emitting it ourselves, controling it's path with mirrors so that it returns to us and recording the time of return.

[DarkStar8]

I know that in non inertial frames space does not behave as a ether like medium, but it seems to behave like a fluid in accelerated frames.

[LaurieAG]

If the ship were trailing a cable with periodic markings, the cable, and those increments, would be shorter. The distance to the ship as measured by that cable would be shorter, but still increasing.

 

EquisDeXD, it is clear from Swansont's post that what is being described is not a real world cable.

 

Take for example a space based observation point at a comoving distance stationary with regard to the hubble flow. The observer could be consistent with the flow and be connected to earth via an inline chain of communication satellites. To keep our communications consistent you would have to send out satellites as the comoving point moved away from our soloar system.

 

Would you really expect to vary the distance between the satellites as the observer moved away?

[swansont]

!

Moderator Note

Black hole discussion split into a separate thread http://www.scienceforums.net/topic/72059-does-moving-close-to-c-turn-something-into-a-black-hole/ .