Re: Talking about distance

On Wed, 21 Jan 2009 08:34:20 -0500, "Dennis Woos" <dpwoos@xxxxxxxxx>
wrote:

In what sense is it true that M31 might no longer exist, or that one's hand
might no longer exist, or that anything and everything might no longer exist
no matter how arbitrarly close we are to it? I have never been comfortable
with this "might not exist" stuff, and maybe it is because of some vague
ideas of Relativity and a "light cone". Anybody have any
thoughts/assistance? How should amateur astronomers be talking about
distance?

I approach this in two different ways, depending on the sophistication
of my audience. The simple answer is more or less along the lines you
give: the light left a long time ago, so we are seeing something as it
was, not as it is. Most people seem to get this; if not, the example of
thunder and lighting works pretty well. Thunder and lightning are
created at the same instant; the lightning tells us when that is, but
the thunder is subject to time-of-flight.

For those advanced enough to grasp some General Relativity (about 12
years old in my experience), I try for a more accurate answer. Under GR,
time does not pass for something traveling at c. So it is perfectly
reasonable to say that we are truly seeing M31 as it is now, because
time and space are inextricably connected. Wheeler had a great graphic
of a universe filled with a latticework of meter sticks with clocks at
each intersection. There are different ways of synchronizing these
clocks, but essentially it amounts to sending a light wave out from one
of them, and as each clock sees that wave pass it gets reset. Each of
those times is the same "now". We think the clocks have different times
only because we (in our mind experiment) are watching the system from
outside the Universe. But that can't be done in reality.

An event that is a million ly distant cannot have any effect on us for a
million years, under GR, that event happens when it is observed, not at
some time propagated backwards based on c and the event distance. This
is the light cone you are referring to (although it is a 4D cone, which
isn't so easy to visualize, so this is usually shown by analogy in a
2D+T universe).

The interesting thing is, some people get this very easily, and some
people don't get it at all. There doesn't seem to be much middle ground.
_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
.

Relevant Pages

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  • Re: Talking about distance
    ... ideas of Relativity and a "light cone". ... For those advanced enough to grasp some General Relativity (about 12 ... clocks, but essentially it amounts to sending a light wave out from one ... some time propagated backwards based on c and the event distance. ...
    (sci.astro.amateur)
  • Re: Is special relativity falsifiable?
    ... As no inertial frame can be found in the Universe, ... All three clocks are set to 0. ... As shown above, at some distance d from P, both clocks on A and B read ... observer A situated at a distance d from observer B ...
    (sci.physics.relativity)
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