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universes: fictional and/or frictional



At 03:02 PM 8/12/00 -0400, Lisa Gardner wrote:

Is there really only one universe? Why can't there be
many of these universes in a mega-verse?

Any universes beyond our own are fictional. You can say whatever you want
about fictional universes; it doesn't make any difference.

* Suppose there are no other universes beyond our own.
* Suppose there are 13 other universes.
* Suppose there are 17 other universes.

It just doesn't make any difference.

1) Everything that can affect us is in our universe, by definition.
... therefore ...
2) Nothing in any other universe could possibly affect us.
(The second statement follows from the first by a simple application of the
rules of logic.)

You are free to dream up fictional universes in which the laws of physics
apply. You are also free to dream up fictional universes in which the laws
of physics don't apply.

People generally agree on what laws of physics apply in our universe, but
you should not expect the universes you dream up to agree with the
universes anybody else dreams up.

It seems intuitive to me that each universe would be finite, because
at some point there will be a loss of energy so that things would
start getting attracted back into itself.

Your intuition is incorrect. Your intuition is apparently based on inexact
observations of small objects. If you look more closely, you will find
that energy is never really lost, although it might go into
hiding. (Macroscopic kinetic energy can be converted to thermal energy,
which is often less noticeable.) More to the point, momentum is never
really lost, and cannot easily be hidden.

By way of analogy: Consider a satellite launched from earth, and neglect
for a moment all other stuff in the universe. If the satellite has enough
initial velocity, it will never return. The earth will always exert _some_
gravitational pull on the satellite, but the amount of pull decreases
rapidly as the satellite moves away, and even if you add up the effect over
infinite time there won't be enough pull to stop the satellite or bring it
back. For homework, calculate the required initial velocity. This is
called "escape velocity". It's an easy calculation.

We now extend this idea and apply it to the entire universe: If the
universe is expanding fast enough, it will just keep on expanding
(according to standard theories). The effect of gravity will slow down the
expansion, but even if you add up the effect over all time it won't bring
the expansion to zero or reverse it.

Whether or not our universe actually has a large enough velocity (relative
to its mass) remains an open question.