From: wdb3926@zeus.tamu.edu (BRUTON, WILLIAM DANIEL)
Subject: Re: What is the speed of light measured from?
News-Software: VAX/VMS VNEWS 1.41    
Organization: Texas A&M University, Academic Computing Services
Date: Sun, 30 Aug 1992 18:53:00 GMT

In article <1992Aug30.130555.7676@spuddy.uucp>, spark@spuddy.uucp (Steve) writes...
> 
> 
>OK. Someone can perhaps clear this little nagging thought in my head.
> 
>I haven't done any relativity, or any complicated physics, so please
>keep it simple.
> 
>Even though I'm sitting down, I'm still travelling at about (say) 300m/s,
>as the Earth is rotating. Furthermore, the Earth orbits the Sun, so, I'm
>travelling at, say 20km/s (or whatever, + my 300m/s)
> 
>Now, of course, the Solar System is orbiting around the centre of the Milky,
>Way, so add another 150km/s.
> 
>So, really to answer the question 'how fast am I travelling', I have to
>specify - relative to what.
> 
> 
>But here's the bit that's troubling me.
> 
>The speed of light is said to be the fastest anything can go, but no-one
>ever mentions in which direction. As I am travelling already at anything up
>to 150km/s, if I turn on a torch, pointed in the direction I am travelling,
>will the light from it travel at a different speed than if I pointed it
>backwards?
> 

That's the crazy thing about light.....no matter what reference frame you're
in or what direction you point a light beam, the speed of light measured
in any reference frame is c=2.997925 x 10^8 m/s.
You can check me out on this by looking at a "Modern Physics" text
in the special relativty section.
It seems conterintuitive......but see text and compare Galilean transformations
to Lorentz transformations.

>
>Where is the speed of light measured relative to?
> 

Any frame yields the same result....strange eh?

>If I travel at near the speed of light, and look behind me, at something on the
>other side of the universe, which is also travelling at near the speed of light,
>but in the other direction, it will look as if it is going twice as fast as
>the speed of light.
>

I don't think so.  I think the Lorentz transformations will tell you 
that that something is moving even "nearer" to the speed of light when
compared to it's velocity in the "rest" frame. 
It has to do with the gamma factor = (1-((v/c)^2))^-(1/2)

>Do designers of particle accellerators have to take the rotation of the earth
>etc. into account when they are measuring the speed their particles have 
>reached?
> 

The speed of light is invariant with respect to different reference frames,
but the speed of slower object is governed by the Lorentz transformations
(from special relativity).  I don't think that the designers of particle
accelerators have to consider the rotation of the Earth in the manner in which 
you are speaking.  They use a reference frame either fixed with respect
to the accerator or fixed with respect to one group of particles that
is gonna smash another group of particles......  The results from the two frames
are different but I think one uses the later frame as a standard way of
describing the efficiently of an accelerator.

> 
>Steve
> 
>-- 
> 
>        ---- Steve Parkinson (spark%spuddy.uucp@uknet.ac.uk) ----
>        ----- Call Spuddy for Free UK USENET on 0203 638780 -----


Thanks for the questions, helps me prepare for exams.

Dan

"You can go faster than the speed of light,
 you can't, can't, can't....and it's invariant." -- Patton Priene

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