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Rotwang    323

^ Cool.

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karan129    2

Report on using antimatter orbiting the Earth and Saturn to fuel rockets.

 

http://www.niac.usra.edu/files/studies/final_report/1071Bickford.pdf

 

This has to be one of the coolest ideas I've come across in awhile.

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Rotwang    323

That's pretty weird if it's true. Isn't matter supposed to spin as it contracts, not as it expands?

 

While it's not generally reliable to try applying ordinary intuition about mechanics to situations where GR rules, I believe it's true that things rotate faster as they collapse. But I don't see anything in the article contradicts that; presumably the small effect they observed in the present day would have been much more noticeable when the universe was smaller.

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LuisBSF    26

If only by the law of conservation of angular momentum, then yes. But as you say it's dangerous to apply everyday common sense to such matters as there might be other interactions at work that might not be apparent on a first glance, particularly not when the object being observed is the entire universe.

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Rotwang    323

If only by the law of conservation of angular momentum, then yes.

 

This is a good example to illustrate my point: in GR, unless your spacetime has an exact rotational symmetry (more precisely, a Killing vector) you simply don't have any law of global conservation of angular momentum.

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LuisBSF    26

It couldn't possibly have any. Should the universe be completely symmetric, all matter would've been completely annihilated by antimatter during the first moments of existence, to mention just one direct consequence of it.

 

Plus the law conservation of angular momentum is valid for an object that rotates through a vacuum that exists within space time (any law of physics for all we know is only valid within our universe). The universe might not be such a thing either.

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Rotwang    323

It couldn't possibly have any. Should the universe be completely symmetric, all matter would've been completely annihilated by antimatter during the first moments of existence, to mention just one direct consequence of it.

This assumes that there was an equal quantity of matter and antimatter in the early universe. We don't know whether that was the case or not. Of course, we can tell that the universe doesn't have exact rotational symmetry by simply looking at it.

 

Plus the law conservation of angular momentum is valid for an object that rotates through a vacuum that exists within space time

 

No, in the absence of a Killing vector generating rotations there is no such law. The problem is that angular momentum isn't even defined globally.

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