Post subject: Re: How can the universe be infinite?
an_outlaw wrote:
If it was finite then what would the end be? If it finite what could be outside?
I actually do this stuff for a living, so hopefully I can help clarify some of these things. Your main question is more of a philosophical one, but the above can be answered more precisely.
When astronomers talk about the "size of the universe," we usually mean "what's the furthest thing we can see?" This is the observable universe and it has a finite size because the universe has a finite age. Light can only have traveled so far in that amount of time.
However, when a non-astronomer asks about the size of the universe, they usually mean something quite different. They want to know about all that "is", even if we can't see it. This question has to do with the topology of the universe.. At the moment, we can't say anything meaningful about the topology -- the universe could be finite or infinite. If it were finite and closed, then there would be no boundary; that is, if you started going in one direction, then eventually you would come back to where you started. Much like what would happen if you started walking along the earth in some random direction, you would eventually come back to where you started.
This sounds like it would be impossible and in normal "Euclidean" space, it is. This is the classical three-dimensional space that everyone's intuition is developed in. Since Einstein, however, the universe has been cast in terms of a four-dimensional space-time in which such things can occur. Don't bother trying to visualize it, we're simply not built to do that.
As for what's "outside" of a finite universe, then the answer is nothing! In such a scenario where the topology is closed, the universe would be all that there is.
I'll give my thoughts on the rest later (my son is screaming).
It's expanding, so it's getting bigger, right? And it can't get bigger if it's already infinite, right? So it must be finite, or am I thinking about this in completely the wrong way? _________________ kill yr idols
And it can't get bigger if it's already infinite, right?
Well, I'm not going to deal with the rest of the topic, since it's late and I'm too tired to give anything resembling an insightful response, but no. There are different magnitudes of infinity, so an infinitely large entity could very plausibly continue to grow.
Post subject: Re: How can the universe be infinite?
sp4cetiger wrote:
If it were finite and closed, then there would be no boundary; that is, if you started going in one direction, then eventually you would come back to where you started. Much like what would happen if you started walking along the earth in some random direction, you would eventually come back to where you started.
So what you're saying is that the universe is shaped exactly like the earth, and if you go straight long enough you end up where you were?
Post subject: Re: How can the universe be infinite?
SilverWalrus wrote:
So what you're saying is that the universe is shaped exactly like the earth, and if you go straight long enough you end up where you were?
Not really, I was just using the earth as an analogy. The similarity is that when you're on the earth, it seems flat, as if it could go on infinitely in any direction. However, if you traveled far enough, sooner or later you'd get back to where you started. This would tell you that the earth is not flat, but really a finite object.
Similarly, the universe *could* be a closed object. The difference is that we're no longer talking about a closed surface, like the earth is, but a closed manifold in three spatial dimensions. A finite universe wouldn't have to be spherical, it could be any shape if you allow for a large enough manifold.
But the universe could be infinite, we just don't know.
Post subject: Re: How can the universe be infinite?
sp4cetiger wrote:
Not really, I was just using the earth as an analogy. The similarity is that when you're on the earth, it seems flat, as if it could go on infinitely in any direction. However, if you traveled far enough, sooner or later you'd get back to where you started. This would tell you that the earth is not flat, but really a finite object.
Similarly, the universe *could* be a closed object. The difference is that we're no longer talking about a closed surface, like the earth is, but a closed manifold in three spatial dimensions. A finite universe wouldn't have to be spherical, it could be any shape if you allow for a large enough manifold.
But the universe could be infinite, we just don't know.
You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum
