What would be the speed of the approaching particle?

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Discussion Overview

The discussion revolves around the motion of two particles, A and B, approaching each other at constant speeds, and how their relative speeds are perceived from each other's reference frames. It explores concepts from classical mechanics and special relativity, particularly focusing on uniform versus accelerated motion and the implications of traveling at the speed of light.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants suggest that from A's perspective, B would be moving uniformly at twice the speed of the individual particles, and this would also hold true for B observing A, based on Galilean relativity.
  • Others argue that if A and B are photons, each would observe the other traveling at the speed of light, without any doubling of velocity, due to Einstein's postulate of special relativity.
  • One participant questions whether the passage of time for particle A would change as they approach light speed, referencing the effects of special relativity on time perception.
  • Another participant emphasizes that it is not meaningful to describe what a photon experiences, as photons do not have a valid reference frame and do not experience the passage of time.
  • There is a suggestion that the thought experiment would be better expressed using massive particles moving at near light speed rather than photons.

Areas of Agreement / Disagreement

Participants express differing views on the implications of special relativity for particles moving at light speed, particularly regarding the experience of time and the validity of reference frames for photons. The discussion remains unresolved with multiple competing perspectives on these concepts.

Contextual Notes

Limitations include the assumption of particles moving at light speed and the implications of time dilation, which are not fully resolved in the discussion. The distinction between massive particles and massless particles like photons is also a point of contention.

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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