If Two Spaceships close at a speed of 1.4c ?

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Discussion Overview

This discussion explores the implications of two spaceships approaching each other at relativistic speeds, specifically focusing on the concept of closing speeds and the exchange of information between observers in different frames of reference. The conversation touches on aspects of special relativity, the behavior of light signals, and hypothetical scenarios involving particle collisions.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • Some participants propose that if two spaceships are closing in on a target at .7c each, they cannot be moving towards each other at 1.4c, as the speed of light remains the ultimate speed limit for information transfer.
  • Others argue that while the two ships may appear to be moving apart faster than light from a third frame of reference, this does not violate special relativity since no mass is actually moving at the speed of light.
  • A participant references the concept of closing speeds, suggesting that observers may perceive two particles approaching each other at speeds approaching 2c, but this perception does not reflect the actual velocities as measured in their respective frames.
  • Some participants emphasize that light signals will always travel at c, meaning that the ships will see each other before a collision occurs, regardless of their closing speed as perceived by an observer at rest.
  • There are repeated assertions that in the frame of either spaceship, the other will be approaching at less than c, which aligns with the principles of special relativity.
  • One participant questions the implications of protons colliding at speeds approaching 1.99c, suggesting that this would lead to scenarios where an observer would not be aware of a collision until after it occurred, due to the limitations of light speed communication.

Areas of Agreement / Disagreement

Participants express disagreement on the interpretation of closing speeds and the implications of special relativity. While some agree on the fundamental principles, there is no consensus on the specific scenarios presented or the conclusions drawn from them.

Contextual Notes

The discussion involves complex relativistic concepts that depend on the definitions of speed and the frames of reference used. There are unresolved mathematical steps regarding the calculations of velocities and their implications for information exchange.

  • #31
Lightheavyw8t said:
This is terrible news for the Cosmological Red Shifters - but maybe GOOD news for Halton Arp...
Again it seems like you're trolling. If you don't explain your basis for the claim that the closing speed must be c (or find a single example of a reference which says this), or explain why you think this is in conflict with the idea of cosmological redshift, then I suspect this thread will soon be closed, this forum is supposed to be for substantive discussion rather than taunts and one-liners (also see the IMPORTANT! Read before posting thread, which points out that people aren't supposed to use this forum as a soapbox for arguing that mainstream conclusions in relativity are flawed).
 
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  • #32
yuiop said:
The bottom line is the maths. If ships A and B are a distance L apart in the Earth frame when they turn their headlights on then they will see each other after a time t_{see} = L/(v+c) and they collide after a time t_{collide} = L/(2v). For any v<c, (v+c) will always be greater than (2v) so it will always be true that t_{see} &lt; t_{collide} and they will always see the other ship before the collision. Neither ship is "surprised".

Thank you for the math, it's really flattering. But no matter how you slice it, light is NOT going to go from A to B faster than c - unless you, too, wish to refute Einstein...
 
  • #33
yuiop said:
The bottom line is the maths. If ships A and B are a distance L apart in the Earth frame when they turn their headlights on then they will see each other after a time t_{see} = L/(v+c) and they collide after a time t_{collide} = L/(2v). For any v<c, (v+c) will always be greater than (2v) so it will always be true that t_{see} &lt; t_{collide} and they will always see the other ship before the collision. Neither ship is "surprised".

Just for fun I thought I would add that if only one ship turns their headlights on, so that they are relying on the radar reflection to warn them of imminent collision, then the equation for the radar return time is 2Lc/(v+c)^2 and this is always less than the collision time L/(2v) so they will still always have a warning period before the collision even if they have to wait for a radar reflection.
 
  • #34
Lightheavyw8t said:
Sigh - another Einstein refuter, apparently...
You would do well to check your attitude at the door. Janus and JesseM are well qualified and understand relativity very well. The fact is that you do not understand the terminology being used.

Specifically, a "closing speed" is not the velocity of either information or any object, so it is not limited to c. Here is a brief glossary:

velocity - the derivative of the coordinate position wrt time
speed - magnitude of the velocity
displacement - the difference in the coordinate positions of two objects
distance - the magnitude of the displacement
closing speed - the derivative of the displacement wrt time
relative velocity - the velocity of an object in a coordinate system where the reference object is stationary

From these definitions all of what the other posters have been trying to teach you follows.
 
  • #35
Lightheavyw8t said:
Thank you for the math, it's really flattering. But no matter how you slice it, light is NOT going to go from A to B faster than c - unless you, too, wish to refute Einstein...
You've given us a good chuckle, but enough's enough. Thread closed.
 
  • #36
Lightheavyw8t said:
Thank you for the math, it's really flattering. But no matter how you slice it, light is NOT going to go from A to B faster than c - unless you, too, wish to refute Einstein...

If anyone here is refuting Einstein, it is you.

The following quote is taken from his own book on Relativity:

http://bartleby.com/173/9.html

Now in reality (considered with reference to the railway embankment) he is hastening towards the beam of light coming from B, whilst he is riding on ahead of the beam of light coming from A.

In his own words he is saying that the closing speed of light is not always c.

Before you accuse people of refuting Einstein, be sure you know what he actually says first.
 

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