Speed of light? [was Re: GPL vs BSD Licence]

[Marc Ramirez]

On Tuesday 02 November 2004 07:20 pm, you wrote:
> On Tue, 2004-11-02 at 14:12, Marc Ramirez wrote:
> > More precisely, gravity is what we call the curvature of spacetime. 
> > Light always heads in the straighest possible line, but in a curved
> > spacetime... Also, if light is emitted in an area of lower potential and
> > is absorbed in an area of higher potential (e.g., from Sun to Earth) it
> > will be redshifted. Other way around, it will be blueshifted. If you are
> > using the frequency of light as a clock (and you basically have no other
> > choice), you will notice this effect as "time moving more slowly around
> > massive bodies."
> >
> > Marc.
>
> Doesn't this equate to defining the speed of light as constant, and then
> adjusting the metre and second appropriately?  I thought the constancy
> of the speed of light was a basic assumption taken by Einstein; the
> slowing of time and reduction of lengths of moving objects, or those in
> a gravitational field are a consequence.

Yes, that's absolutely right.  In fact, one way to unify space and time under 
GR is to define distance in seconds.  (I remeber Grace Hopper coming on the 
Late Night back with Dave Letterman and giving him a piece of wire that was a 
nanosecond long.) 

The invariance of the speed of light under various states of movement, 
rotation, acceleration, and gravitation is pretty well established 
experimentally.  Relativity takes this excellent experimental agreement, and 
moves it up to the status of a postulate (a.k.a. definition). The challenge 
now, of course, is to think up new experiments. Right now, we know of no 
conditions where the speed of light is anything but c.  But without that, 
yes, the speed of light is defined as constant in relativity.  

The postulates of special relativity:

1. The laws of physics are independent of the observer's reference frame.
 "The laws by which the states of physical systems undergo change are not 
affected, whether these changes of state be referred to the one or the other 
of two systems of co-ordinates in uniform translatory motion."

2. The speed of light, c, is the same in all reference frames.
 " Any ray of light moves in the 'stationary' system of co-ordinates with the 
determined velocity c, whether the ray is emitted by a stationary or by a 
moving body. Hence velocity equals [length of] light path divided by time 
interval [of light path], where time interval [and length are] to be taken in 
the sense of the definition in §1."

General Relativity extends this to:

3. Uniform acceleration is equivalent to a gravitational field (the Principle 
of Equivalence). (Can't find the Einstein quote, sorry :)

These are the general constraints (you'll see your comment restated as 
postulate #2).

These are the constraints (along with extra assumptions I won't go into), and 
General Relativity is an explanation of the universe which fits within these 
constraints.  Like I say, so far all of these have been confirmed to a great 
degree of accuracy, so any new theories will have to either accept these 
postulates or explain why they seem to be so pervasive.

Thanks for taking my mind off Lisp! :)

Marc.

-- 
Marc Ramirez
Blue Circle Software Corporation
513-688-1070 (main)
513-382-1270 (direct)
http://www.bluecirclesoft.com
http://www.mrami.com (personal)
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