Elegant Universe

editor · 2005 January 21st, 14:57

AS you can tell, I\'ve been reading Brian Greene\'s book.
In Chapt 2 he has George and Gracie passing each other in space and noting that each sees the others watch as slow. That\'s OK, but what happens when they have floated completely around the universe and meet up again.( You know, if you go far enough in a universal geodesic you arrive back at the same place. ) What would the watches show ??

joelp · 2005 January 21st, 18:13

whats all this stuff people come out with about cylindrical galaxies and similar stuff? is this real physics? as far as im concerned, its a simple sphere. i cant see any reason why people would speculate about going so far that you end up where you started. Is it a throw off from a current theory?

editor · 2005 January 22nd, 15:28

No. and I made no mention of cylinders.
The theory goes roughly like this:
The universe contains mass which has serious gravitational effects. Gravitation is the curvature of spacetime.
Enough mass and the universe is closed. Space is bent sufficiently that if you could see far enough you would see the back of your head.
The distance is arguable but it\'s in excess of 14 billion light years.
So, no , you can\'t really see the back of your head because you would have to stand there for 14 billion years, but that\'s the theory.

**Fernanda** · 2005 January 22nd, 15:47

who\'s moving where...like in opposite directions? one is standing still?

joelp · 2005 January 22nd, 17:29

sorry editor, i wasnt being funny, or imply that you were talking about cylinders - it was just a bit of a wander off topic. I appreciate what you say about the amount of mass and the curving effect, so i would like to throw up a few points:

- assuming that you are heading away from the average direction of mass (could be called the universes centre of mass, but that harks back to the other thread on the centre), there is even mass behind you on either side, so there would be no net curvature.

- a body in orbit could be described as following a gravitational geodesic (could it?), there seems to be some similarity to what you say about following a geodesic and ending up back where you started. surely, the object in your example could be described as orbiting a mass in the universe, which would not in itself imply that the universe was a closed loop of sorts. does that make any sense, cos it only vaguely does to me!

i know you dont actually mention an object, or indeed an example, its merely an extrapolation of what you say about seeing the back of ones head.

editor · 2005 January 23rd, 15:08

Yes, a body in orbit is following a geodesic. Which is the path equivalent of a straight line but in curved spacetime. That is, Newtons law, somewhat modified by gravity. If no other force is applied an object will continue to follow the geodesic at a constant velocity.
I\'m talking about photons, i.e. light, following the geodesic. So that, if the mass of the universe is sufficient, the photons will arrive back where they started from.
There is no centre of mass, but the total mass in the universe creates a total gravitational effect.

joelp · 2005 January 23rd, 17:00

ok, cheers for that, i get what you say.

the 14 billion light years, where does that come from?

sorry for all the questions... :-)

Xerxes314 · 2005 January 23rd, 18:35

Quote:

Originally posted by joelp
the 14 billion light years, where does that come from?

That\'s just the age of the universe times the speed of light.

Xerxes

joelp · 2005 January 24th, 16:24

i see. So it really isnt guaranteed if there is enough mass to make this a closed universe? plus if there was, it would be nothing nore miraculous than being in orbit of a large mass.

what if George and Gracie were travelling at relatavistic speeds in opposite directions whilst in orbit around a large mass? would this be a similar question to editors first one?

is this question inspired around avoiding the turnaround phase of the twin paradox? would it have any effect? i think its time i reread the FAQs!