From: email@example.com (Henry Spencer)
Subject: Re: Anti Gravity?
Date: Sat, 20 Feb 1999 21:53:10 GMT
In article <firstname.lastname@example.org>,
Joseph Abbott <email@example.com> wrote:
>Prior to 1998 the idea of an anti-gravity force was complete fantasy.
>Einstein's theory on gravity makes absolutely no room for anti-gravity.
Uh, no, actually General Relativity includes a "cosmological constant",
which Einstein introduced as a fudge factor to explain how a non-expanding
universe could be stable. He later, after the discovery of the expansion
of the universe, repudiated this as a mistake.
Quantum theory does in fact predict a non-zero cosmological constant. In
fact, it's difficult to explain why the c.c. is so small that it has very
little impact on physics -- it ought to be quite substantial and have
obvious effects. (Actually calculating it is problematic, because it
depends on the properties of all possible types of particles, including
any we don't know about yet. But it seems improbable that the effects of
all those particle types would beautifully cancel each other out,
perfectly or almost perfectly, and that is what's observed.)
>However, in 1998 astronomers observed that the Universe is expanding
>at an *accelerating* rate.
Uh, no -- astronomers *tentatively reported some evidence* for this.
It is by no means an established fact yet.
>So Einstein's theory of gravity is apparently wrong.
No, if the acceleration is indeed real, it just means that one of several
things -- including a non-zero cosmological constant -- is causing it.
None of which would invalidate General Relativity... or necessarily permit
us to build antigravity machines.
The good old days | Henry Spencer firstname.lastname@example.org
weren't. | (aka email@example.com)