SR - car with headlights - new twist

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Discussion Overview

The discussion revolves around the relativistic effects of two objects traveling at significant fractions of the speed of light in opposite directions. Participants explore the implications of relativistic velocity addition and how observers in different frames perceive the speeds of these objects relative to each other.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether two ships traveling at 1/2c in opposite directions would perceive each other moving away at the speed of light and whether they could see each other.
  • Another participant emphasizes the need for relativistic velocity composition rather than simple vector addition, referencing an external source for clarification.
  • A participant suggests that since each ship can determine its speed relative to a stationary frame, they might conclude that the distance between them is increasing at rates that exceed the speed of light.
  • It is noted that while the separation rate as measured from a stationary frame can exceed c, this does not represent the speed of any object.
  • One participant argues that the ships would not perceive the other ship's speed as exceeding c, providing specific calculations for the speeds as perceived by the ships themselves.
  • A later reply points out that the concepts being discussed are not new and suggests reviewing the provided links for further understanding.

Areas of Agreement / Disagreement

Participants express differing views on how speeds are perceived in relativistic contexts, with no consensus reached on the interpretation of the scenarios presented.

Contextual Notes

Participants reference relativistic effects and velocity addition without resolving the underlying assumptions or implications of their claims. The discussion remains focused on theoretical interpretations rather than definitive conclusions.

elegysix
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What happens if two objects travel at speed 1/2c in opposite directions?

Like suppose two ships left the ISS in opposite directions, each going speed 1/2c.

Wouldn't either of the ships determine that the other is moving away at the speed of light?
Could they see each other?

What if they both went 3/4c in opposite directions? would they conclude the speed between them was greater than c?
 
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I understand that for two frames, but there's three frames here - one for each ship and one 'stationary' that they move away from.

We can determine that an object is moving at 1/2c or 3/4c, relative to us, right?

which means if a different object is moving at 1/2c or 3/4c in the opposite direction, we would conclude that the distance between them is increasing by c or 3/2c, respectively, right?
 
elegysix said:
which means if a different object is moving at 1/2c or 3/4c in the opposite direction, we would conclude that the distance between them is increasing by c or 3/2c, respectively, right?
Yes, the separation rate as measured by that third frame can be greater than c. But that's not the speed of any object.
 
elegysix said:
which means if a different object is moving at 1/2c or 3/4c in the opposite direction, we would conclude that the distance between them is increasing by c or 3/2c, respectively, right?
Just because we would conclude that does not mean the people on the spaceships would. They would see the other spaceship as moving at 4/5 the speed of light in your first example (we see two spaceships each moving at 1/2 c in the opposite direction), and at 24/25 c in your second example (we see two spaceships each moving at 3/4 c in the opposite direction).

Read the link I provided.
 
By the way, this is not a "new twist". It's over a century old.

It would be good to read the suggested links.
 

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