# SR - car with headlights - new twist

1. Jan 20, 2012

### elegysix

What happens if two objects travel at speed 1/2c in opposite directions?

Like suppose two ships left the ISS in opposite directions, each going speed 1/2c.

Wouldn't either of the ships determine that the other is moving away at the speed of light?
Could they see each other?

What if they both went 3/4c in opposite directions? would they conclude the speed between them was greater than c?

2. Jan 20, 2012

### D H

Staff Emeritus
3. Jan 20, 2012

### elegysix

I understand that for two frames, but there's three frames here - one for each ship and one 'stationary' that they move away from.

We can determine that an object is moving at 1/2c or 3/4c, relative to us, right?

which means if a different object is moving at 1/2c or 3/4c in the opposite direction, we would conclude that the distance between them is increasing by c or 3/2c, respectively, right?

4. Jan 20, 2012

### Staff: Mentor

Yes, the separation rate as measured by that third frame can be greater than c. But that's not the speed of any object.

5. Jan 20, 2012

### D H

Staff Emeritus
Just because we would conclude that does not mean the people on the spaceships would. They would see the other spaceship as moving at 4/5 the speed of light in your first example (we see two spaceships each moving at 1/2 c in the opposite direction), and at 24/25 c in your second example (we see two spaceships each moving at 3/4 c in the opposite direction).