What would be the speed of the approaching particle?

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When two particles, A and B, approach each other at equal speeds, A observes B moving uniformly at twice their individual speeds, consistent with Galilean relativity. However, if both particles are photons, they would each perceive the other traveling at the speed of light, adhering to Einstein's special relativity, which states that the speed of light remains constant across all reference frames. As particles approach light speed, time dilation occurs, meaning time appears to slow down for them, but photons themselves do not experience time. It is more meaningful to discuss massive particles nearing light speed rather than photons, as photons lack a valid reference frame. The key takeaway is that while massive particles can observe each other approaching light speed, photons do not experience time or relative motion in the same way.
anti quark
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Suppose that 2 particles, A and B, which are far away, are traveling 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 traveling at the speed of light? What would be the speed of the approaching particle then? (here, let's assume A and B are light itself)

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

Thanks in advance guys!
 
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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 traveling 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.
 
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?
 
sk1105 said:
In the second case, if A and B are photons, each would observe the other to be traveling at the speed of light
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.
anti quark said:
According to special relativity, the closer we are to light speed, the slower time passes.
Yes. It is more meaingful to talk about what happens as we approach the limit.

anti quark said:
So for the photon, time is still. Am I right?
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.
 

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