Re: Speed of light? [was Re: GPL vs BSD Licence]
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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.=20
> > 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 und=
er=20
GR is to define distance in seconds. (I remeber Grace Hopper coming on the=
=20
Late Night back with Dave Letterman and giving him a piece of wire that was=
a=20
nanosecond long.)=20
The invariance of the speed of light under various states of movement,=20
rotation, acceleration, and gravitation is pretty well established=20
experimentally. Relativity takes this excellent experimental agreement, an=
d=20
moves it up to the status of a postulate (a.k.a. definition). The challenge=
=20
now, of course, is to think up new experiments. Right now, we know of no=20
conditions where the speed of light is anything but c. But without that,=20
yes, the speed of light is defined as constant in relativity. =20
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=20
affected, whether these changes of state be referred to the one or the othe=
r=20
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 t=
he=20
determined velocity c, whether the ray is emitted by a stationary or by a=20
moving body. Hence velocity equals [length of] light path divided by time=20
interval [of light path], where time interval [and length are] to be taken =
in=20
the sense of the definition in =A71."
General Relativity extends this to:
3. Uniform acceleration is equivalent to a gravitational field (the Princip=
le=20
of Equivalence). (Can't find the Einstein quote, sorry :)
These are the general constraints (you'll see your comment restated as=20
postulate #2).
These are the constraints (along with extra assumptions I won't go into), a=
nd=20
General Relativity is an explanation of the universe which fits within thes=
e=20
constraints. Like I say, so far all of these have been confirmed to a grea=
t=20
degree of accuracy, so any new theories will have to either accept these=20
postulates or explain why they seem to be so pervasive.
Thanks for taking my mind off Lisp! :)
Marc.
=2D-=20
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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