0

Assuming we have 3 objects d,e,f moving on the same line while e is stationary, d is moving to the left at 'almost speed of light' and f is moving to the right at 'almost speed of light'. What is the speed at which f is moving from the perspective of d?

If its 0.9999999% speed of light, what does it say about their course? To my understanding, the perspectives of d,e and f are as followed:

  • from the perspective of d, e and f are both moving away at the sub speed of light
  • from the perspective of e, d and f are both moving away at the sub speed of light
  • from the perspective of f, e and d are both moving away at the sub speed of light

In order to keep all 3 statements true, all 3 objects (d, e and f) are supposed to be moving away from each other at a speed close to speed of light, so they should be virtually located on the vertices of an "invisible" equally-edged expanding triangle.

So my question is - what happened to the original straight line d,e and f were placed on during the acceleration of d and f to 'almost speed of light'?

Qmechanic
  • 220,844
Uri Abramson
  • 143

1 Answers1

5

To avoid a proliferation of nines, let’s take the leftward velocity of D, and the rightward velocity of F, relative to E to be 0.9 $c$. Then, using relativitic addition of velocities as explained here, we find that D sees F move rightward at

$$\frac{0.9+0.9}{1+0.9\times 0.9}c = 0.9945 c.$$

Similarly F sees D move leftward at this same speed.

There is no triangle, just straight-line relative motion with various relative velocities which are all sub-luminal.

G. Smith
  • 52,489