From: Andrew Walrond <andrew@...>
Date: 2005-02-07T07:30:32+09:00
Subject: Re: [OT] Physics of black holes (Was: RubyConf '05)

On Sunday 06 February 2005 22:15, Hal Fulton wrote:
> Tim Bates wrote:
> > Josef 'Jupp' Schugt wrote:
> >> Superluminal speed ist nothing that requires a hyperspace. The
> >> relative speed of galaxies for example is known to exceed the speed of
> >> light.
> >
> > No, it's not. You've forgotten your elementary relativity. If you have
> > two galaxies on opposite sides of the universe (call them A and B) with
> > an observer in between (called O) then from O's frame of reference the
> > sum of the speeds of A and B might exceed c (the speed of light),
> > although neither galaxy's individual speed can exceed c, BUT from A's
> > frame of reference B's speed does not exceed c and vice versa.
> >
> > If this sounds bizarre and impossible to you, you haven't studied
> > physics.
>
> I think he's misunderstanding certain observations that seem to indicate
> superluminal velocities. I forget exactly how it works, but it's
> essentially an optical illusion. Something about making the (invalid)
> assumption that light from both ends of a jet left at the same time.
>

Depends how you define 'relative'. If your understanding of "the relative 
speed of galaxies" is "from O's frame of reference the sum of the speeds of A 
and B" then they are actually in full agreement ;)

Of course "speed" as a scaler is misused here; the quantity needs direction to 
make much sence (velocity), but of course the maths becomes interesting as 
space begins to curve...

I have studied physics, but it still "sounds bizarre and impossible" to me ;)

Andrew Walrond

This is the most interesting thread I've read in ages :)