# sup

Discussion in 'Physics & Math' started by stef 730, Mar 12, 2002.

1. ### stef 730Registered Member

Messages:
19
thinking big

Hasn't virtually every generation before us thought too small about space in general? What if we too are thinking too small about right now. When I started this thread I was just trying to throw out the idea that maybe the universe is part of something much bigger and that something is also part of something else bigger and so on. I really am intrigued by some of your posts. That is what this site is for, isn't it? Just to throw around ideas.

Last edited: Mar 13, 2002

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3. ### James RJust this guy, you know?Staff Member

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35,782
I think we've got about as big as we can get when thinking about the universe.

In relativity, some theories suggest that our universe is spatially infinite. In quantum mechanics, the <i>many worlds</i> theory says our universe is just one of an infinite number of universes, all differing from each other by the single collapse of a wavefunction.

There are also theories that our observable universe is but one of a potentially infinite number of "bubble universes" floating around out there in the nothing.

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5. ### Joeblow93132Guest

James R,

You sure love the term "infinite", don't you?

Unlike you, I hate the word "infinite". It screws up all the formulas it's in.

If you accept Einsteins explanation of gravity, which I know you do, then his theory of curved space would indicate that space was made out of something. After all, how can you curve "nothing". Now if you accept that space is made out of something then you have to decide what. The only answer is that space must contain a small amount of energy or matter for it to exist.

If you accept that all space contains a small amount of matter or energy, then an infinitly sized universe would have infinite energy and/or an infinite mass. As you can see this would be absurd. After all how would the conservation laws apply. The sum of what two numbers would give infinity??

I believe that the universe is finite in size and energy, and that it will continue to expand until a)gravity stops it from expanding or b) all the matter and energy is converted into space.

When I use the term "universe" I mean everything. Multiple universes would simply be seperate segments of one and only one universe.

Tom

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7. ### James RJust this guy, you know?Staff Member

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Tom,

<i>If you accept Einsteins explanation of gravity, which I know you do, then his theory of curved space would indicate that space was made out of something. After all, how can you curve "nothing".</i>

General reltivity is a theory of geometry. What is curved is the geometry of spacetime. That is not a thing which is made of anything. What Einstein was saying is that measurements of time and space change in the presence of mass and/or energy.

<i>If you accept that all space contains a small amount of matter or energy, then an infinitly sized universe would have infinite energy and/or an infinite mass.</i>

True.

<i>As you can see this would be absurd.</i>

I have trouble seeing that.

<i>After all how would the conservation laws apply.</i>

Which ones do you think would have problems?

<i>I believe that the universe is finite in size and energy, and that it will continue to expand until a)gravity stops it from expanding or b) all the matter and energy is converted into space.</i>

(a) may be right. (b), as far as I know, cannot happen.

8. ### thedIT GopherRegistered Senior Member

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1,105
Aleph_0, Aleph_1 and so on. But that's the point, is it not. An infinite universe requires infinite everything.

James; Think about this. If the Universe where infinite in extent then you start running into problems. It means it has existed for infinity. But we know form the second law of thermodynamics that entropy always increases. So there should be no structure visible as given an infinite time it all falls to the lowest energy state. We see structure so we must either,

1. Live in a finite Universe or,
2. Live at a unique time close to the beginning of the Universe and before entropy takes hold.

but as the Universe is infinite this time never exits.

As per Olbers Paradox there is an infinite mass that would produce an infinite flux of inbound EM radiation. This would be so high, infinite in fact, that life would be toasted.

The non-existence of this inbound infinite flux (the dark night sky that is) requires,

1. The Universe is finite
2. It is expanding
3. Both of the above.

All current indications are that the Universe is finite in scope within and boundless (topologically speaking).

Last edited: Mar 13, 2002
9. ### Joeblow93132Guest

James R,

Thed basically explained why the universe has to be finite in size.
Another reason is that if the universe was infinite in size, the gravitational constant would be infinite.

Now to explain what I meant by saying laws of conservation wouldn't apply if the universe was infinite:

If mass and energy was infinite in our universe, and if you converted one kilogram of mass into energy, how much mass and energy would you have then? The same because if you add or subract a number from infinity you still have infinity.

What if you took half of the infinite energy in the universe and you converted it to mass? How much energy would you have then?

As you can see from the examples above, infinity should NEVER be used in physical formulas. If you do, then you'll notice that math and logic collapse.

Tom

10. ### James RJust this guy, you know?Staff Member

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35,782
thed:

A universe infinite in extent need not have existed for an infinite amount of time. Remember that the big bang happened everywhere at once, not at one point which then expanded.

Olber's paradox is solved by having a finite lifetime for the universe, and also by the Hubble expansion. Both explanations work even for a universe infinite in size.

Tom:

That's not a violation of the physical conservation laws. It's just that different <i>mathematical</i> rules apply when we deal with infinity. If we convert half of an infinite mass to energy, that leaves an infinite mass and an infinite energy. The mass is not the same as before. It just has the same cardinality.

11. ### stef 730Registered Member

Messages:
19
So let me get this clear. The universe, according to some of you guys is in fact, finite. Because if you subtract anything from infinity, you still get infinity. I was also wondering if any of this had to do with the curvature of space-time.