But getting back to the OP, even though we can’t get to the edge, it still exists (right?), so what’s on the other side of it?
Pac-Man’s world, or the world of the Asteroids game, is not curved, nor does either require more than two dimensions. The world of Asteroids is a two-dimensional flat space which just happens to be topologically identified on its edges. Now, you can accomplish a topological identification by bending edges around in some higher-dimensional space, but you can also do it without the higher dimensional space, and even if you do bend the space around, that doesn’t imply that it’s curved. In geometric terms, “flat” just means that the sum of the angles of a triangle is 180 degrees, and that’s true regardless of whether the space is identified.
Quoth CoG888:
The idea is theoretically plausible, but the dodecahedron case (at least, on any observable scale) has been ruled out by observation. The Universe could still be identified in any of a variety of shapes on scales larger than we can observe, however.
I have never seen (or heard of) any evidence (convincing or not) that such an edge exists.
If I had infinite eyesight, and I could see to an infinite distance instantly (without having to wait for the light from that source to actually get to me), if the universe is wrapped around itself, wouldn’t I see an infinite number of “me’s” staring off into the distance?
Yes. Except you forgot to add “and if nothing was blocking my view”.
Actually no because your “infinite eyesight” will not work.
Get far enough away and the Universe is expanding faster than the speed of light (this is not a violation of the speed of light limit). Any light emitted towards us from further away (I know that is not precisely true but good enough for this I think…there is “far enough away” where it is absolutely true) will never reach us therefore you will never see it. The photon never reaches your eye.
In essence anything beyond this horizon is “out” of our universe as perceived from the earth. It can have no effect on us even though it is there (from their perspective we drop off their radar as well and disappear apparently receding faster than the speed of light).
So there is your edge I guess for the OP. Thing is if I magicked you out there everything would look normal and the edge would appear to be around where the earth is.
In short, wherever you are, you are at the center of the universe (as was mentioned above). Hence you cannot reach the edge because you are always at the center. Weird but true.
Can’t we just send out three satellites and have them fly really far away from each other then use lasers to form a triangle and measure the angles to determine the universe’s topology?
At least in theory? (I suppose they would have to go to very deep space to avoid local curvature effects caused by the planets and sun and such)
I just watched a show on TV called Parallel Universes, I think it was on Nova, but I could be mistaken. They talked about having done just this. They used lasers to make some ginourmous triangle and then measured the angles. If the universe is flat the angles should add up to exactly 180 degrees. I thought they said it came out to a bit more then 180 meaning the universe was curved.
ETA the mechanics of doing this weren’t really explained, so don’t question me on how it was accomplished.
I asked Chronos a question on this effect (expansion) in a sister thread:
http://boards.straightdope.com/sdmb/showpost.php?p=10874422&postcount=5
Care to have a crack at it?
:smack:
There are actually different ways of measuring distances on the scale of the Universe and they do not agree. Probably the best way to give you the idea is to have you look at this link: The Distance Scale of the Universe
The “as perceived from the earth” is a very critical weasel phrase here.
According to your explanation, there are an infinite number of universes, each centered at the viewer’s location. If you are here, and I’m a googol light-years away, then even though we can see each other, my universe extends a googol light years further than yours in this direction, and your universe extends a googol light years further than mine in the other direction.
This totally changes the meaning of the word “universe” from my understanding of the OP. We’re both in the same universe, and the OP wants to know how far it goes.
Not necessarily so. My understanding is that it has not yet been proven whether the effects of gravity move at the speed of light, or whether it might be instantaneous (and I’m confident that “instantaneous” has a clear meaning in this context). I think it is also still unproven whether “spooky action at a distance” might occur faster than the speed of light. Thus, it is possible that objects outside this horizon might indeed have an effect on us, and thus be part of our universe.
The speed of gravity hasn’t been measured yet, but everything points to it being the same as the speed of light.
No, this has been proven. Entangled particles definitely affect each other at superluminal speeds.
Thanks for the interesting read.
Heh. Science. For one question answered, four more pop up.
In principle, we could in this way measure the Universe’s geometry (though in practice, it’d take a much larger triangle than we could send satellites to), but we could not measure the Universe’s topology this way. For a comparison at a more familiar scale: I could draw out a triangle on a flat sheet of paper, and measure the sum of the angles to be 180 degrees. If I then roll the paper up into a tube and tape the edges together, the topology of the paper has changed, but the angles of the triangle would remain exactly the same.
It is not a “weasel phrase”.
We are all in the same universe. However, as a being that perceives the universe the whole of creation is centered on my head as I perceive it. If we say the Universe is 14 billion years old then a light ray stretching from my head to 14 billion light years out is the edge of what I can perceive (note the different measurements I linked in my last post…simplifying here).
Now if you are standing 5 feet away from me your Universe, the one you can perceive, is shifted 5 feet over from mine. Think of it as two bubbles 14 billion light years in radius centered on our heads. Your bubble will not exactly match mine.
That said our bubbles are in an even larger bubble. Due to the inflationary period moments after the big bang the universe is bigger than that 14 billion light years. Just the perception bubble on my head can only go as far as light can travel since the Universe came into existence.
This perception bubble is expanding at light speed so in another billion years my bubble will be 15 billion light years in radius.
This is what is meant that we are all at the center of the universe. Everywhere is the center. We are all in the same universe though.
Gravity propagates at light speed. No information can travel at superluminal speeds. Quantum entanglement seems to be instantaneous but you cannot transmit information using it. There are other things you could get going faster than light speed such as a shadow but a shadow is not a tangible thing and again you cannot transmit info faster than light using one.
Therefore if something can never reach me because it is beyond the horizon where a photon (or gravitational effect) cannot reach me – ever due to the expansion of space – it by definition can have no effect on me. It may well be there but for my purposes it may as well not exist.
Try Light Cones and note the last bulleted item near the top of the page:
“All other events are in the (absolute) elsewhere of E and are those that will never affect and can never be affected by E.”
Anything outside of my light cone cannot effect me nor can I affect it. This goes back to the first part of your question too. Your light cone is slightly shifted from mine.
Okay, technically the cosmic red shift caused by the expansion of the universe is NOT a doppler shift. It’s a different effect.
A doppler shift is the result of the source object moving. The waves are squished when the object is moving toward you, and stretched when it’s moving away, changing the wavelength of the light.
But distant galaxies aren’t really *moving *away from us. Instead the space between us is stretching.
This an important distinction. If distant galaxies were moving then their light would be red-shifted the moment it was emitted. It would travel to us in this red-shifted state and would provide a “fossilized” snapshot of how fast they were moving billions of years ago.
Instead what happens is that the light is emitted normally. But as it travels for billions of years through space that is stretching, it slowly stretches itself. The cosmic red shift isn’t a product of how fast the emitter was moving, it’s a product of how long the light has been travelling.
That’s why more distant objects have a bigger red shift.
I seem to recall statements (but NOT in this thread) to the effect that galaxy’s further away from us appear to be “more redshifted”, “moving faster”, etc, which implied to me that the speaker felt that they were the product of a Universe accelerating in it’s expansion. But I kept thinking “your seeing something as it was “x” years ago…not as it may be now.”
At the end of the universe you will find a brick wall that stretches on all around the universe. All the way up and all the way down. We are in fact encased in this brick wall. If you were to tunnel though you would be bitten by the snapping turtles that hold the wall up, and there are turtles in ranks beyond that extending on infinitely.
Recent observations suggest that the expansion is indeed accelerating. But even if the expansion were constant or decelerating we’d still see a cosmic red-shift.
Imagine you have a rubber band that’s two inches long. You thumbtack one end to the table and make a mark on it halfway along its length.
Now you stretch the rubber band until it’s three inches long. The mark in the middle is 1 1/2 inches from the tack. The end of the band has move 1 inch further away from the tack. But the mark in the middle has only moved 1/2 an inch.
Stretch the band again. Now the end is four inches away from the tack. But the mark in the middle is only 2 inches away.
If you keep stretching the rubber band you’ll notice that the end is “moving” away from the tack twice as fast as the mark is. That’s how expansion works.