Constant acceleration approach? =/

[Smiles302]

Link to guy's blog post: http://ftlfactor.com/2011/04/23/how-does-special-relativity-allow-effectively-faster-than-light-travel/#comment-117

He says he has a PhD in space physics.

My understanding of special relativity was that the speed of light is always constant no matter what reference frame you are observing from. That's correct yeah?

he says:

According to Special Relativity theory, neither an Earth-based observer nor a traveling observer can ever measure that the traveler travels faster than light speed. This is the headline that is generally known. However, if you look at the whole trip to a another star something interesting happens. Assuming that trip distance is measured at both ends of the trip, the distance will be measured in approximately the Sun’s reference frame so that the Lorentz contraction factor is approximately 1 and length contraction is insignificant. If the traveler considers his or her own experience, then the traveler’s time measurement is the important one and assuming relativistic speeds are reached the Lorentz contraction factor varies from 1 and time dilation is significant. It is in this mixed-reference frame perspective that Special Relativity theory allows travel at effectively faster than light speed.

Does he have special relativity wrong? Or is this any way possible?

I haven't studied general relativity but I thought you had to use general relativity if you were dealing with acceleration?

 

[ghwellsjr]

He has SR wrong, but he apparently knows that because he calls it a "mixed-reference frame perspective". I'm sure he is aware that for the traveler, length contraction is just as significant as time dilation and so the traveler measures his speed exactly the same as an Earth or star based observer would, something less than the speed of light.

SR handles acceleration perfectly well. GR is for situations involving gravity.

 

[Smiles302]

So from what perspective would an observer see light traveling faster than the speed of light?

 

[Dale]

So from what perspective would an observer see light traveling faster than the speed of light?

From no perspective. As he clearly says his comments refer to a "mixed-reference frame perspective", specifically taking the distance from the Earth's perspective and the time from the traveller's perspective.

 

[DrGreg]

When you divide the external observer's distance by the traveller's time, you get something called "celerity" (also known as "proper velocity", a term I don't like). It turns out that the celerity of light is infinite, so there's no "faster than light" travel involved, even though the celerity of an object can exceed the velocity of light. You have to compare like with like, velocity with velocity or celerity with celerity.

So from what perspective would an observer see light traveling faster than the speed of light?

All observers always measure the local speed of light (i.e. light passing very close to the observer) as c, but non-inertial observers may measure the "remote" speed of light to be some other value, larger or smaller.