# What would be the speed of the approaching particle?

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1. Mar 8, 2015

### anti quark

Suppose that 2 particles, A and B, which are far away, are travelling towards each other at a constant equal speed. From A's point of view, will b be having uniform or uniformly accelerated motion? Wouldn't that be same from B's point of view?
And what would happen if A and B are travelling at the speed of light? What would be the speed of the approaching particle then? (here, lets assume A and B are light itself)

Ok. So this was a random thought I had a few days ago..

2. Mar 8, 2015

### sk1105

In the first case, A will observe B to be moving uniformly at twice the velocity of the individual particles; the same would be true for B observing A. This is just Galilean relativity.

In the second case, if A and B are photons, each would observe the other to be travelling at the speed of light, with no doubling of the velocity. This comes from Einstein's postulate of special relativity which says that the observed speed of light is the same in all reference frames. To allow for this, it is the observed passage of time that changes between reference frames.

3. Mar 8, 2015

### anti quark

Ok, so in the second case, the passage of time for particle A would be changed?

According to special relativity, the closer we are to light speed, the slower time passes. So for the photon, time is still. Am I right?

4. Mar 8, 2015

### DaveC426913

It is best to avoid describing what a photon experiences. The speed of light is not a valid reference frame. Photons do not experience the passage of time.

Yes. It is more meaingful to talk about what happens as we approach the limit.

As above.

The best way to express this thought experiment is to discuss massive particles (not photons) moving at near the speed of light. Each particle will observe the other as approaching at near the speed of light.