Protector Domesticus
how do we know space is infinite if we've never been farther than our own region of space?
if space is infinite why do scientists always make up a number of how many stars and galaxies there are in the universe? why not just say there are a infinite number instead of billions of trillions?
that just makes the public think they have no idea what they are talking about.
http://www.youtube.com/watch?v=bENTuGmw0Sg
Biarchus
Re: how do we know space is infinite if we've never been farther than our own region of space?
Well,this way you're saying that space has to have stars and galaxies everywhere in it,so the space can be infinite but the mass in it cannot be infinite too...But even if it is this way,the fact that there are infinite does not stop us identify,put them in categories and such and study them a bit deeper.
About the fact on how we know about the space being infinite,I don't think we have a practical evidence of this fact,we know it,only in theory...I might be wrong though.
Senator
Re: how do we know space is infinite if we've never been farther than our own region of space?
They are almost always referring to the visible universe.
Re: how do we know space is infinite if we've never been farther than our own region of space?
Giving the big bang was supposedly a singularity of infinitely compressed spacetime containing all the energy of the universe the physics are not really easy to understand (I might have fumbled in that one sentence alone).Originally Posted by Jack04
I'd also say that scientists say the number of stars/galaxies not as definitive but as educated guesses of observed densities to show how big the universe is even assuming it is low estimates based on the visible evidence we have. The whole point being that we can only guess about the visible universe since everything passing beyond the observer cone vanishes from our field of view and we cannot know how much stuff already did do so or appeared beyond that horizon.
As a mathematical concept space is infinite but since the roughest idea about the shape of the universe seems to be an expanding bubble based on four dimensions through whatever said bubble expands would still be space, I guess, though the universe itself would be finite...
All in all alot of stuff I have to read more about but can't stop watching when one of those scientists talk about it.
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Senator
Re: how do we know space is infinite if we've never been farther than our own region of space?
I don't think that cosmologists do claim that space is infinite in our universe. If the universe extends infinitely, then it must be infinitely old to have expanded to that state. However, we know that this is absurd since you run into all sorts of paradoxes. The most straightforward I can think of is Olbers' Paradox:if space is infinite why do scientists always make up a number of how many stars and galaxies there are in the universe? why not just say there are a infinite number instead of billions of trillions?
that just makes the public think they have no idea what they are talking about.
http://www.youtube.com/watch?v=bENTuGmw0Sg
If the universe is infinitely old and there are an infinite number of stars, why is night dark? The light of an infinite number of stars should constantly illuminate the Earth. Obviously this is not the case.
So the extent of the universe as we can observe it cannot be infinite and nor can it be infinitely old. Hence to say there are an infinite number of stars would just be plain dishonest and I doubt the number of stars, but are estimates (with a large amount of potential error) from the information that we have. Of course, the general public is not always informed about standard form and how to deal with errors in measurement so this means that qn explanation of how many stars therev are tends to go into the "billions and billions" realm just to emphasise the point that the number of stars is very big, but not infinite.
Praeses
Re: how do we know space is infinite if we've never been farther than our own region of space?
The universe is indefinite, visible stars and galaxies are quantifiable (finite).
Theoretically the universe should be 27.4 billion light years accross at maximum, given the fact that the universe can't be older than 13.7 billion years and nothing can travel faster than the speed of light.
The whole problem with the world is that fools and fanatics are so certain of themselves, but wiser people are full of doubts.
-Betrand Russell
It's a conspiracy!
Re: how do we know space is infinite if we've never been farther than our own region of space?
The observable universe is indeed 27.4 billion light years across, however, we are not the center of the universe.
To an observer on the edge of our observations the universe would appear just as big in every direction as it does to us. The expansion of space itself is not limited by the speed of light.
Praeses
Re: how do we know space is infinite if we've never been farther than our own region of space?
That is no paradox, light will never reach us from beyond the visual universe. Indeed there will come a time when no light will reach us from anything outside the Milky Way. But that won't mean the other galaxies didn't exist.The most straightforward I can think of is Olbers' Paradox:
If the universe is infinitely old and there are an infinite number of stars, why is night dark?
As Jack says, most stellar statistics are only about the visible universe. We don't know what all is beyond our horizon, and we may never know.
Protector Domesticus
Re: how do we know space is infinite if we've never been farther than our own region of space?
aren't we in some weird corner of space where there actually is night?
Get born again.
Re: how do we know space is infinite if we've never been farther than our own region of space?
huh? no, most of the galaxy is probably pretty dark since stars are generally pretty far from each other. might be different on any planets that happen to exist close to the center of galaxies and other tight clusters, though, but then again, there are three stars (alpha centauri A and B, as well as Proxima Centauri, which isn't even visible from earth-borne telescopes) approximately 4 light years away, which is fairly close by astronomical standards, and they don't really illuminate the night-side of the planets in our solar system...
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Senator
Re: how do we know space is infinite if we've never been farther than our own region of space?
My point (and that of the paradox) was that if there is a universe infinite in extent that has an infinite past, the visible universe is infinite in extent.
Praeses
Re: how do we know space is infinite if we've never been farther than our own region of space?
You must have missed the last 60 years of cosmology. The paradox assumes the distance between Earth and all stellar objects is constant, this is not the case.My point (and that of the paradox) was that if there is a universe infinite in extent that has an infinite past, the visible universe is infinite in extent.
The visible universe is limited by the speed of light vs. the rate of expansion of the universe; far enough away from the Earth the relative rate of expansion of the universe is faster than the speed of light. And since every year our horizon shrinks, if the universe were indeed infinitely old we would see nothing outside the Milky Way.
(Credit for this observation should go to Prof. L. Krauss, it is not an original thought on my part)
Last edited by Sphere; July 06, 2012 at 08:27 AM.
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Re: how do we know space is infinite if we've never been farther than our own region of space?
Because it cannot be finite? How can it be? If there is some kind of border to limit it, there would also be things outside of that border, thus it's always infinite.if space is infinite why do scientists always make up a number of how many stars and galaxies there are in the universe? why not just say there are a infinite number instead of billions of trillions?
Tiro
Re: how do we know space is infinite if we've never been farther than our own region of space?
The diameter of the observable universe is not 27.4 billion light years. The universe has been expanding for 13.7 billion years. The light we now see from the most distant objects was emitted when they were 13.7 billion light years from the Earth. They are much farther away now due to the continuing expansion. IIRC the diameter of the observable universe is over 90 billion light years.
Dark energy causes the universe to expand. The more distance between two points the more pronounced the expansion. If an object is far enough away then the light it just emitted will never reach the Earth. However, the light it already has emitted will reach the Earth. This is because the object was not far enough away when the light was emitted. So light from objects beyond the currently observable universe will still reach us…for a while and then stop.
For the universe to be infinite in size it would have had to expand for an infinitely long time, or expand at an infinite rate or have been infinitely large right from the start. Only the last point cannot be excluded scientifically.
Last edited by Agent Miles; July 06, 2012 at 02:38 PM.
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Primicerius
Re: how do we know space is infinite if we've never been farther than our own region of space?
How can it be 90 billion lightyears? Thats 45 billion lightyears in one direction, which means matter was there 45 billion years ago, long before the universe existed. Unless matter traveled faster than the speed of light after the big bang or all matter didn't originate from a single point.
Ducenarius
Re: how do we know space is infinite if we've never been farther than our own region of space?
Space/time can expand faster tan the speed of light. Thought I think that he was saying in that in the 13.7 billion years it took that light to reach us it traveled another 13.7 billion light years. Thought i assume that is an object traveling at light speed.
Ducenarius
Re: how do we know space is infinite if we've never been farther than our own region of space?
Lag, sorry for the double post.
Biarchus
Re: how do we know space is infinite if we've never been farther than our own region of space?
That's not the case my friend since,during the expansion it's not matter that travels,but space(so time too,according to spacetime theories)expands.This means that two objects remain in their initial points,but the distance between the points is the one that has become larger.Imagine space like a sheet of plastic(random material that can be stretched permanently).Now imagine the two object like two drawn dots in that sheet.Expansion is like pulling the sheet from both sides and it is stretched.Now the two dots are further away than each othe,but this does not mean that ink moved.Sorry for the poor example,but I am not an expert my self.Also I don't think there is a bound that makes space not expand faster than the speed of light.
EDIT:
Sorry tco,we are saying the same thing,I just had left the thread open from yesterday night and your post had not yet appeared.
Last edited by diez; July 07, 2012 at 09:26 AM.
Primicerius
Re: how do we know space is infinite if we've never been farther than our own region of space?
Oh that makes sense. I always pictured space-time bending like putting a bowling ball on a blanket where it deforms but doesn't expand. So instead of it being a blanket that is a definite size and doesn't stretch, its like a bungee cord that expands and stretches.
Biarchus
Re: how do we know space is infinite if we've never been farther than our own region of space?
Actually the blanket example isn't exaclty wrong but it focuses on the interaction of objects with space-time...But yeah,such an example would be more accurate.
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