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From: weemba@brahms.BERKELEY.EDU (Matthew P. Wiener)
Newsgroups: net.physics
Subject: Re: Space and Space Time
Message-ID: <12504@ucbvax.BERKELEY.EDU>
Date: Thu, 20-Mar-86 09:55:11 EST
Article-I.D.: ucbvax.12504
Posted: Thu Mar 20 09:55:11 1986
Date-Received: Sun, 30-Mar-86 07:20:43 EST
References: <66@zeus.UUCP>
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Reply-To: weemba@brahms.UUCP (Matthew P. Wiener)
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In article <66@zeus.UUCP> dando@zeus.UUCP (Dan Donahue) writes:
>	There has been a disagreement as to whether Einstein felt that there
>	was a difference between space and space time. To quote the article:
>
>>					Consider: The average person has no
>>idea of the difference between space and space time. In fact, according to
>>Einstein, there is no difference, if I remember correctly.
>
>	Would someone out there like to try and explain to this average person
>	(and any others who are interested :v >) whether this difference exists
>	and if so what is it?

There certainly is a difference between space and space-time!  One is three
dimensional and the other is four dimensional.  Pure space is instantaneous.
Perhaps better would be to call it timeless.

The real issue is whether there is a difference between curved space and
curved space-time.  The answer is yes.  Roughly speaking, for ordinary
classical objects the difference is not detectable.  In particular, for the
orbit of Mercury, it makes no difference which you used.  But for objects
that are extraordinarily massive (and so extreme space-time warping is
occurring), the difference is significant.  Not only is the 'where' bent
out of shape, causing an object to follow what looks like a curved path,
but so is the 'when', causing further warping as to the calculation of
orbits.  (To specify the path of an object requires both 'where' and 'when'
information.  If either is curved, deviations from Newtonian predictions
occur.  If both are curved, even more deviations occur.)  Near the sun,
which is considered rather small by general relativity's standards, the
main way to detect the full space-time warping is by the light deflection
experiment.  Light travels so fast that its path is affected by even the
small curvatures in 'when'.

Well, I hope this makes some sort of sense.

ucbvax!brahms!weemba	Matthew P Wiener/UCB Math Dept/Berkeley CA 94720