Opposite beams of photons or electrons

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Discussion Overview

The discussion revolves around the implications of speed and information transfer in the context of particles, specifically electrons and photons, emitted in opposite directions. Participants explore concepts from relativity, including the addition of velocities and the definition of information transfer between receivers.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant claims that two electrons emitted in opposite directions can create a scenario where information appears to travel faster than light between two receivers.
  • Another participant challenges this assertion, stating that the claim is incorrect.
  • A third participant introduces the relativistic formula for adding speeds, emphasizing that speeds cannot simply be added linearly in relativistic contexts.
  • It is noted that while information travels from a central station to two receivers, no information travels directly between the receivers themselves.
  • One participant discusses the subtleties of defining speeds between two objects, suggesting that the relative speed should be calculated using the appropriate relativistic formula rather than a naive addition of speeds.

Areas of Agreement / Disagreement

Participants express disagreement regarding the interpretation of speed and information transfer, with no consensus reached on the implications of the scenarios presented.

Contextual Notes

Participants highlight the importance of relativistic effects on speed calculations and the definition of information transfer, indicating that assumptions about speed may lead to misunderstandings.

motoroller
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With two electrons emitted from a decaying atom in opposite directions, the speed can easily be greater than the speed of light between the two electrons (e.g. 0.7c in each direction). If two receivers then pick up the electrons, hasn't the information traveled between the two receivers at a speed greater than the speed of light? Similarly for two fibre optic beams, the central station can transmit to both receivers so that they receive the light at the same time, so that the information has traveled greater than the speed of light between the receiving stations?
 
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motoroller said:
With two electrons emitted from a decaying atom in opposite directions, the speed can easily be greater than the speed of light between the two electrons (e.g. 0.7c in each direction).
You are wrong about that.
 
In relativity you can't just "add speeds", you use the formula

\frac{u+v}{1+uv/c^2}​

In your example, put u=v=0.7c.
 
When a central station O sends a particle beam or a light beam to receiving stations A and B, then information has traveled from O to A and from O to B, but no information has traveled from A to B.
 
It can be a little subtle how to define "speeds". Sure, I can emmit a particle with v=0.9c in the x direction, and a particle with v=0.9c in the -x direction. Naively, you would say that the relative speed is now 1.8*c, because every second the distance between the two photons increases with 1.8*c meters. But that's from my point of view. Speeds are defined between two objects. So in this case you should define the speed between the 2 particles, and the formula is given by Dr.Greg.
 

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