Light speed barrier / closing speed

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Discussion Overview

The discussion revolves around the implications of relativity on the concept of closing speeds between two spaceships approaching a stationary space station at relativistic speeds. Participants explore the nuances of relative motion, the speed of light, and how these concepts interact in different reference frames.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions how two spaceships approaching each other at 0.75c can have a closing speed of 1.5c, raising concerns about the implications for the speed of light and reference frames.
  • Another participant clarifies that the closing speed of 1.5c is calculated from the perspective of the space station, not the relative speed of the spaceships themselves.
  • It is noted that, according to the relativistic addition of velocities formula, the relative speed between the two spaceships is actually 0.96c.
  • A participant suggests that an observer at the space station might see ship B reach ship A before its own light, prompting further inquiry into the nature of closing speeds and light propagation.
  • Another participant disputes this idea, asserting that the light from ship B will always travel at c and thus will reach ship A first, regardless of the closing speed calculation.
  • Links to additional resources on the topic of relativistic velocity addition are provided for further exploration.

Areas of Agreement / Disagreement

Participants exhibit disagreement regarding the interpretation of closing speeds and the implications for light travel times, with no consensus reached on the nuances of these concepts.

Contextual Notes

The discussion highlights the complexities of relativistic physics, particularly in the context of reference frames and the speed of light, without resolving the underlying assumptions or calculations involved.

Mojo-electro
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This is something that has always bugged me about relativity... I must be missing something...

Lets say you have a space station sitting stationary out in space... a spaceship approaches it from one side at .75c...another ship approaches the station from the other side, also at .75c, still no laws of physics violated yet that I can think of... The closing speed of the 2 ships relative to each other is now 1.5c (unless I am missing something)... the 2nd ship turns on it's headlights, what would the the first ship see?

Given the invariance of the speed of light, it seems that the light from the other ship would approach at c (which, as is my understanding, it would in any reference frame) but the ship itself is approaching at 1.5c...

This leads me to something I have never understood fully... according to relativity, all descriptions of motion only make sense in terms of motion relative to some other frame of reference... yet there is an absolute speed limit, the speed of light... "nothing can move faster than the speed of light"... nothing can movine faster than light relative to what exactly? Can 2 spaceships not approach each other at faster than c? If so, than can c be exceded in some reference frames?

Is there really a state of absolute rest relative the the background universe.. is this the reference frame I am missing?
 
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Mojo-electro said:
Lets say you have a space station sitting stationary out in space... a spaceship approaches it from one side at .75c...another ship approaches the station from the other side, also at .75c, still no laws of physics violated yet that I can think of... The closing speed of the 2 ships relative to each other is now 1.5c (unless I am missing something)...
Right. But that closing speed is according to the space station. It is not the relative speed of the spaceships.
the 2nd ship turns on it's headlights, what would the the first ship see?
Light coming at it at its usual speed.

Given the invariance of the speed of light, it seems that the light from the other ship would approach at c (which, as is my understanding, it would in any reference frame) but the ship itself is approaching at 1.5c...
From the frame of spaceship A, spaceship B is not approaching at 1.5c. The relative speed--given by the relativistic addition of velocities formula--is only 0.96c.
 
Doc Al said:
Right. But that closing speed is according to the space station. It is not the relative speed of the spaceships.

Light coming at it at its usual speed.


From the frame of spaceship A, spaceship B is not approaching at 1.5c. The relative speed--given by the relativistic addition of velocities formula--is only 0.96c.

Interesting.. thanks!.. so would it be correct to say that an observer at the station would see ship B beat it's own light to ship A, but the reverse is true as seen by the pilot of ship A? I'll have to research the addition of velocities a bit more...
 
Mojo-electro said:
so would it be correct to say that an observer at the station would see ship B beat it's own light to ship A,
No, why would you think that? An observer at the station sees ship B traveling at 0.75c. The 'closing speed' is not really the speed of anything, it's just the calculated rate at which the two ships close the distance between themselves according to the station frame. The 'closing speed' of the ships is 1.5c, but the 'closing speed' of ship A and the light from ship B is 0.75c + c = 1.75c. The light wins the race no matter how you look at it.

I'll have to research the addition of velocities a bit more...
You'll find many threads on that topic.
 
A few links to explore:

http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/einvel.html"

http://www.edu-observatory.org/physics-faq/Relativity/SR/velocity.html"
 
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