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If there were only 2 objects in the Universe . . .

If there were only 2 objects in the Universe . . . - Physics Forum

If there were only 2 objects in the Universe . . . - Physics Forum. Discuss and ask physics questions, kinematics and other physics problems.


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  #1  
Old 09-28-2003, 10:42 AM
Interesting Ian
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Default If there were only 2 objects in the Universe . . .



Suppose that our Universe contained only 2 objects, perhaps 2 spheres.
Suppose they are separated by some distance, say 10 light seconds, and they
are orbiting around each other. I'm just wondering whether it would be
meaningful to claim they are actually orbiting around each other since there
is no third perspective? Also would they experience centrafugal force (or
alternatively would they need centrapetal force)?



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  #2  
Old 09-28-2003, 12:00 PM
Martin Hogbin
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Default If there were only 2 objects in the Universe . . .


"Interesting Ian" <[Only registered users see links. ]> wrote in message
news:cWydb.1490$[Only registered users see links. ].net...



It is not strictly possible to answer your question since
what you are saying is 'what would our universe be like
if it were different from how it is?'

However, I take your question about what is known as
Mach's principle. Newton (amongst others) noticed
that, although linear motion at a constant speed appears
to be entirely relative, rotation (and acceleration in
general) appears to be absolute.

Ernst Mach proposed, unfortunately rather vaguely, that
the absolute nature of acceleration might be due to the
effect of matter in the universe. In Newtonian physics
it is fairly simple to put this general idea into a clear
mathematical form. Linear acceleration could, for
example, be relative to the centre of mass of the
universe at any instant.

The best theory of space and time that we have today
is Einstein's general theory of relativity (GR).
Unfortunately, there are several ways of expressing
Mach's principle in terms of GR. Even after you have
chosen one such way, whether it actually applies
may depend on the 'shape' of the universe.

So, to return to your original question, if your two
objects formed what is known as a Wheeler-Einstein-
Mach (WEM) universe (I am no expert on these
matters but I believe that this would depend on the
detailed descriptions of the two masses) then they
would themselves determine their subsequent
moments dependent on their initial dispositions.

I am sorry if this answer seems rather vague but
there are no easy answers, not even any easy
questions.

Martin Hogbin







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  #3  
Old 09-28-2003, 01:22 PM
Interesting Ian
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Default If there were only 2 objects in the Universe . . .


"Martin Hogbin" <[Only registered users see links. ]> wrote in message
news:bl6ih0$s9l$[Only registered users see links. ]...
they
there
(or
Hi,

Thanks for your response. I don't know much about physics, I'm much more of
a philosopher. I'm very interested in the philosophy of this 18th century
philosopher called George Berkeley. He argued against the existence of an
absolute space (as well as time). It's very interesting you should mention
Mach because apparently Mach's philosophy of science was identical to
Berkeley's, and he reached it apparently without knowing anything about
Berkeley's philosophy.

Anyway, from this link [Only registered users see links. ]

Quote:
"In Einstein's Universe, which has no reference point or

common center of mass, inertial forces such as centrifugal forces for

example, are generated even in the absence of the fixed stars, in

contrast to the earlier relativity theories of Berkeley and Mach. A

spinning Earth would, in a mysterious way, generate centrifugal force

at the equator even if all other matter in the Universe was removed.

This reverts back to Berkeley's argument, "How then can we say that

the Earth is spinning and relative to what?""
I have to say that I absolutely agree with Berkeley. The only way we could
say the earth is spinning would be relative to space itself. I, like
Berkeley, find this nonsensical (incidentally I am also an immaterialist
like Berkeley, so this should not be surprising). And I also find it
interesting that Einsteins *relativity* predicts an absolute space!
{confused}


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  #4  
Old 09-28-2003, 01:28 PM
Jim
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Default If there were only 2 objects in the Universe . . .

Interesting Ian wrote:
You don't need two objects, one will suffice - how could you tell
whether it was rotating or not? I believe this to do with Mach's Principle?
Jim

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  #5  
Old 09-28-2003, 01:38 PM
Interesting Ian
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Default If there were only 2 objects in the Universe . . .


"Jim" <[Only registered users see links. ]> wrote in message
news:jrBdb.1597$[Only registered users see links. ].net...
they
there
(or

Yes considering one object makes it simpler. Well you couldn't could you?
That's what I don't understand. It could only be rotating relative to
space, but philosophically I find that hard to countenance.




I believe this to do with Mach's Principle?

I really don't know anything about Mach. I do know just about everything
about Berkeley's philosophy of science though, and Mach's ideas are supposed
to be identical to it.


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  #6  
Old 09-28-2003, 03:00 PM
tony
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Default If there were only 2 objects in the Universe . . .

>
If the obect were spherelike, and if your one object universe had physical
'laws' like this one, I think an observer on that sphere could determine if it
was rotating by experimenting on its surface. Take it to fast absolute
rotation, and think about differences in gravity at the axis of rotation and at
the object's equater

ajw.
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  #7  
Old 09-28-2003, 04:35 PM
Interesting Ian
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Default If there were only 2 objects in the Universe . . .


"tony" <ajw27703@wmconnect.comremoove> wrote in message
news:[Only registered users see links. ]...
you?
supposed
if it
and at

Yes if physical laws are unchanged. But the spinning would only be known
through it's effect. So we either have an effect with no cause, or
alternatively you might claim that the spinning just simple *IS* the
variation in gravity. But if the earth were the only object in the
Universe, and gravity varied accross its surface in said manner, then we
would simply take this as the way gravity is.


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  #8  
Old 09-28-2003, 05:25 PM
tadchem
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Default If there were only 2 objects in the Universe . . .


"Jim" <[Only registered users see links. ]> wrote in message
news:jrBdb.1597$[Only registered users see links. ].net...
they
there
(or
Principle?

If a tree falls in the forest...


Tom Davidson
Richmond, VA


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  #9  
Old 09-29-2003, 12:41 PM
David Granteer
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Default If there were only 2 objects in the Universe . . .

All of physics depends on the existence of a coordinate system. This is
usually implicit. The example you give has only two measures, the distance
between the objects and the diameter of the objects. If the diameter of the
objects is taken as the unit of distance, then the only measurable value is
the distance between them, in multiples of that diameter. There is no way
to determine if they are orbiting one another. There is no way to determine
a measure of time, unless the distance between them is periodic (which it
probably would be if they were orbiting one another as viewed in an external
cood sys.) Then the unit of time would be the period of the variation of
distance between them.

"Interesting Ian" <[Only registered users see links. ]> wrote in message
news:cWydb.1490$[Only registered users see links. ].net...
they
there


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  #10  
Old 09-30-2003, 12:57 AM
ghytrfvbnmju7654
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Default If there were only 2 objects in the Universe . . .

"Interesting Ian" <[Only registered users see links. ]> wrote in message news:<95Edb.1937$[Only registered users see links. ].net>...

Don't forget the Coriolis effect!
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