# Thread: grappling with curved space time and gravity

1. It's my understanding that everypoint is essentially the centre of the universe.
and if you travel out from any point in a straight line you'll eventually arrive back where you started

if this is so could the universe be rightly viewed, from one perspective, as a sphere with radius of 13.8 billion light years, But the surface of the sphere is equally the same point in space time as the centre of the sphere.

if this is so, from the perspective of every point in the universe there is such a sphere around it. At the extremities of these spheres surfaces would overlap. Here, a repulsive nuclear force would simultaneously be an attractive force at the centre.

could gravitation and the strong nuclear force be one and the same, but skewed into 2 apparent forces because of curved space?

Peter

2.

3. Originally Posted by pedronaut
It's my understanding that everypoint is essentially the centre of the universe.
It would be more appropriate to say every point appears to be the centre - because they would see distant galaxies receding from them. In fact, the universe does not have a centre.

and if you travel out from any point in a straight line you'll eventually arrive back where you started
That is one possibility. But not the only one. We don't know enough about the topology (or size) of the universe to be sure. It may be infinite.

if this is so could the universe be rightly viewed, from one perspective, as a sphere with radius of 13.8 billion light years
The observable universe can be. But the universe goes on beyond that.

, But the surface of the sphere is equally the same point in space time as the centre of the sphere.
And there you lost me ...

could gravitation and the strong nuclear force be one and the same, but skewed into 2 apparent forces because of curved space?
Short answer: no. Their properties are too different. I'll let someone else try and give a better answer to that though...

4. Originally Posted by pedronaut
could gravitation and the strong nuclear force be one and the same, but skewed into 2 apparent forces because of curved space?
No, these two forces are very different in nature. The two main differences in terms of behaviour would be that the strong force has a very short range ( unlike gravity ), and becomes stronger with increasing distance ( unlike gravity ).

5. OK thanks guys

"And there you lost me ... "

following on from the "possibility" that if you travel out from a point you arrive back where you started; i was imagining that if you travelled out in all directions from a point, like a radiating sphere, eventually each radius would return to the start. Therefore there is a spherical surface(area approaching infinity) that is one and the same and equally the starting point (volume approaching zero)

but i guess what i'm thinking of would be better represented by 2 cones back to back like <>

.....

I'm aware that the nuclear force acts over a short range and gravity over a long range, but i was suggesting that curved space reconciles this somehow. Because the longest distance in the universe is simultaneously the shortest distance.

Thanks for your comments, i need to learn some more before they send me to the trash can.

6. Originally Posted by pedronaut
but i was suggesting that curved space reconciles this somehow.
Unfortunately it can't. Like I said, these forces do not behave the same. If you look at the underlying models ( quantum chromodynamics versus general relativity ) you will see that they are nothing alike.

7. i find curved space to be a useful model of gravity

8. Originally Posted by ttown
i find curved space to be a useful model of gravity
Just a detail: that is curved space-time (not just space). But I am glad you find it useful.

9. i thought the universe was flat, (WMAP) within the limits of the measurements? this would imply that you wouldn't return to your starting point if you travelled long enough. spacetime is curved though. but then space isn't the same as spacetime. well that is my understanding.

10. Originally Posted by Chrispen Evan
i thought the universe was flat, (WMAP) within the limits of the measurements? this would imply that you wouldn't return to your starting point if you travelled long enough. spacetime is curved though. but then space isn't the same as spacetime. well that is my understanding.
The universe is indeed flat as far out as we can observe it. The problem here is that this does not allow us to draw any conclusions as to the global geometry of the universe as a whole - it could be that all of the universe is flat, but it could also be that the observable part is so small compared to the total size of the universe that it is simply impossible to detect the global curvature in it.

Space and time can never be separated, there is always only space-time.

11. Thanks Markus. there is always a bit of confusion when talking about the universe. does one mean the just observable or the whole lot. maybe i should have said the topology is flat but spacetime is curved. is that anymore accurate or am i digging a deeper hole?

12. Originally Posted by Chrispen Evan
Thanks Markus. there is always a bit of confusion when talking about the universe. does one mean the just observable or the whole lot. maybe i should have said the topology is flat but spacetime is curved. is that anymore accurate or am i digging a deeper hole?

I think the main issue is that we can only observe a small part of the universe - that small part appears flat, but we do not know how that relates to the universe as a whole.
Locally, in the presence of energy, space-time is of course curved; the easiest way to think of this is that space-time and the gravitational field are really one and the same thing.

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