# shinoteki comments on Particles break light-speed limit? - Less Wrong

9 23 September 2011 11:00AM

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Comment author: 23 September 2011 10:37:32PM 1 point [-]

It shouldn't. Moving from B to A slower than light is possible*, moving from A to B faster than light isn't, and you can't change whether something is possible by changing reference frames.

*(Under special relativity without tachyons)

Comment author: 24 September 2011 02:46:37PM *  0 points [-]

What I'm trying to get at is, What does a physicist mean when she says she saw X move from A to B faster than light? The measurement is made from a single point; say A. So the physicist is at A, sees X leave at time tX, sends a photon to B at time t0, and gets a photon back from B at time t1, which shows X at B at some time tB. I'm tempted to set tB = (t0+t1)/2, but I don't think relativity lets me do that, except within a particular reference frame.

"X travelled faster than light" only means that tX < t1. The FTL interpretation is t0 < tX < tB < t1: The photon left at t0, then X left at tX, and both met at B at time tB, X travelling faster than light.

Is there a mundane interpretation under which tB < tX < t1? The photon left A at t0, met X at B at tB, causing X to travel back to A and arrive there at tX.

The answer appears to be No, because X would need to travel faster than light on the return trip. And this also explains that Owen's original answer was correct: You can say that X travelled from A to B faster than light, or from B to A faster than light.

Comment author: 24 September 2011 02:33:14PM *  0 points [-]

My question is whether he meant to say

• moving from A to B faster than the speed of light in one reference frame is equivalent to moving from B to A faster than the speed of light in another reference frame

or

• moving from A to B faster than the speed of light in one reference frame is equivalent to moving from B to A slower than the speed of light in another reference frame

both of which involve moving faster than light.

Comment author: 25 September 2011 04:35:04AM 1 point [-]

I meant the first one: faster than light in both directions.

You can think of it this way: if any reference frame perceived travel from B to A slower than light, then so would every reference frame. The only way for two observers to disagree about whether the object is at A or B first, is for both to observe the motion as being faster than light.

Comment author: 24 September 2011 02:53:28PM *  0 points [-]