I am trying to learn STR on my own, from various real text books (not the ones for layman). I would like to check my understanding with the following thought experiment. Can someone please confirm if I am correct, or if not, what am I doing wrong? Thanks…(adsbygoogle = window.adsbygoogle || []).push({});

Consider the following:

An stationery observer shoots a particle to the left at a velocity of 0.9c. Immediately following this, independent of the first particle, he shoots another particle to the right at a velocity of 0.9c. So for the observer, the two particles are separating at a velocity of 1.8c. This, I think, is OK and there is no violation of the max speed postulate of STR. The two particles are simply separating apart at a speed of 1.8c and there is no information exchange between them. There is no information being sent at a velocity greater than c. The left and right particles which are carrying information from the observer, themselves are going at velocities of 0.9c, which is still less than c.

Now consider the left particle. What is the speed of the right particle wrt left? Remember they were shot out independent of each other.

I think from the left particle viewpoint also, the right particle is moving at a velocity of 1.8c. We do not have to use the relativistic addition of velocities because the right particle did not jump off of the left particles – the two particles are independent of each other. Again, since the right particle was shot out independent of the left, there is no information exchange between the left and right particles. So the right particle can indeed move at 1.8c wrt the left particle without violating the max speed postulate of STR.

Am I right or did I totally make a fool of myself?

If I am wrong, and I had to use the relativistic addition of velocities, then the right particle will be moving at a velocity of 0.99c wrt left. But now, there will be a paradox:

The stationery observer will calculate the distance between the left and right particle in 1 sec to be 1.8c. But the left particle, in 1 sec, will calculate that the right particle is at a distance of 0.99c.

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# New to STR, trying to confirm my understanding

**Physics Forums | Science Articles, Homework Help, Discussion**