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Author Topic: Are there exceptions to the constancy of c?  (Read 1173 times)

Offline Bill S

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Are there exceptions to the constancy of c?
« on: 30/10/2015 20:52:10 »
J Richard Gott uses a thought experiment involving an astronaut who uses a flashlight on a spacecraft travelling faster than c.

He says:  “If an astronaut’s rocket were to travel by us at faster than the speed of light, a light beam he sent forward could never catch up with the front of his rocket.  The light beam could never catch up because the front of the rocket would be moving faster and have a head start.  Any athlete should know that catching another runner who is running faster and has a head start is impossible.  The astronaut’s observations would be most peculiar: he would take out a flashlight and shine it towards the front of his rocket, but he would never see the beam of light arrive.”

The final sentence indicates that he is talking about the observation of the astronaut on board the craft; not an outside observer.

Surely, if the astronaut is on the rocket, the front of the rocket would be stationary in his frame of reference.  At subluminal speed he would see the light beam move towards the front at c.  Why would this be different at superluminal speed?   


 

Offline jeffreyH

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Re: Are there exceptions to the constancy of c?
« Reply #1 on: 30/10/2015 21:05:29 »
All the kinetic energy of the photons would be negative in the forward direction. They would all be traveling backwards. I have no idea what would happen to the photons coming at the craft from in front of it.
 

Offline evan_au

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Re: Are there exceptions to the constancy of c?
« Reply #2 on: 30/10/2015 21:48:37 »
Quote from: J Richard Gott
a thought experiment involving ... a spacecraft travelling faster than c.
The premise is already a problem - we know of no way to accelerate a spacecraft faster than c.

So it's not surprising that describing what would happen is also a problem.
 

Offline alancalverd

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Re: Are there exceptions to the constancy of c?
« Reply #3 on: 30/10/2015 23:13:43 »
“If an astronaut’s rocket were to travel by us at faster than the speed of light

That makes "us" special, which is not permissible.

Suppose there are two astronauts. A is travelling past "us" at 0.7c and B at 1.4c. Then A observes entirely ordinary physics, but according to Gott's hypothesis, B observes extraordinary physics.

But if "we" were on A's rocket, then accordig to Gott, B would observe ordinary physics because he was only travelling past "us" at 0.7c.

So what B observes depends on who we think we are, which is nonsense. 

Hence relativity.
 

Offline Bill S

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Re: Are there exceptions to the constancy of c?
« Reply #4 on: 31/10/2015 18:56:51 »
 
Quote from: Alan
The premise is already a problem - we know of no way to accelerate a spacecraft faster than c.

This is why I mentioned that it was Gott’s thought experiment, rather than some crackpot idea I had had.  To be fair to him, he was using it as an argument against superluminal travel.  It was just the reasoning that gave me pause for thought.
 
Let’s bear with JRG for a moment and assume that his imaginary craft is travelling at exactly c. The astronaut turns on a flashlight, pointing towards the front of the craft.  An outside observer measures the speed of the craft as c, so she would not see the light leave the flashlight.  Now suppose her observation to have been made a minute fraction of a second later; would she then see the beam of light frozen part way to the front of the craft?
 
 

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Re: Are there exceptions to the constancy of c?
« Reply #4 on: 31/10/2015 18:56:51 »

 

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