-------- Original Message --------
--- On Mon, 7/28/08, joseph Aisenberg <vanveen13@SBCGLOBAL.NET>
wrote:
> S. K.-B.: It's a real challenge to separate the popular
> expositions of Relativity (and their impact on what you
> might call "lay [laid-back?] culture") from the
> hard (truly hard) applied mathematics involved in
> Einstein's "slick" formulae.
...
> Jansy mentioned in a recent post that she ponders setting
> off in an "imaginary" space-ship in order to test
> the Einstein "rejuvenation" formula. In fact,
> Jansy, it definitely works for REAL space-ships. REAL
> astronauts and atomic-clocks have made the journey and they
> do return YOUNGER.
>
> J.A.: This is quite fascinating! How did they know the
> astraunauts were younger? Since they would have been going
> at far far less than the speed of light it doesn't seem
> exactly measurable, unlike say, the loss of bone mass that
> occurs in zero gravity existence. I suppose their clocks
> were a little behind those they had originally been synced
> with? Couldn't have been by much I suspect.
Jerry Friedman: You've got the idea. According to
<http://en.wikipedia.org/wiki/Hafele-Keating_experiment>,
the time differences are in the hundreds of nanoseconds
(billionths of a second). Obviously this had no
measurable effect on the astronauts; it was measured only
by the cesium clocks. Also, the clocks run /faster/ in
lower gravity, and this effect is stronger than the slowing
due to their speed, so the net result was that the
astronauts were a tiny fraction of a second /older/ when
they returned than they would have been if they'd stayed
on Terra firma.
GPS satellites have to adjust their clocks for these
effects. However, as Stan pointed out, relativity is
far easier to measure at particle accelerators. Certain
of Einstein's "slick formulae"--not only time dilation
and E = mc^2, but also the latter's generalization for
energy as a function of momentum, and the relationship
of momentum to speed--simply can't be rejected, and I
don't think they could have been even in 1968 by anyone
who looked at what was going on at accelerators.
> J. A. I must admit as every inch a non physicist
> I've never understood notions of space-time continuums
J.F.: I'd say "space-time continuum" is a way of
understanding certain mathematical facts about the positions
and times of events as measured by observers moving at high
speeds. It's not a way of talking aobut how we perceive
space and time. If you're interested in some simple math,
feel free to e-mail me, or look around on the Web.
> or that time travel could exist in any real or perceivable
> way, since it doesn't seem like time exists. It is
> merely a reification of the clock device
> which we culturally use to measure changes in space: in
> other words the time is always now, with rearrangements of
> matter. How could one move around in "time" since
> this "stuff" isn't really anything at all?
J.F. In principle, we can alter the speed at which we go
toward the future. So if we knew a way to make
spaceships go at almost the speed of light (or to
suspend our lives and then revive ourselves), we could
see the Earth 1000 years from now after experiencing
a much shorter time ourselves. You may or may not
want to call that time travel. Most physicists would
say we can't revisit our pasts.
Jerry Friedman