We all know that (almost) every star seems to be receedind away from us and the farther it is away from us the faster it appears to be receeding. The layman’s explaination/demontration of this is in a 2D form is usually an inflating ballon covered with dots.
And we also know earth is not in a special place in the universe - the effect would be the same no matter which star you were on.
Now if you do a little mind experiment and try to construct a 3D model of this phenom (wires, gears and supports discounted) you will find it quite impossible.
Isn’t this proof positive that we live in (at least) a 4D cosmos? And if so, doesn’t that make the space bending for FTL communication a little more realistic?
For your thought experiment try a loaf of raisin bread. We’re riding on a raisin. As the bread rises the rest of the raisins recede from us. No wires, gears and supports to discount.
Even if this model seems incomplete to you, how does it then follow that “the space bending for FTL communication [would become] a little more realistic”?
The hyper-cube, silly! 
What puzzles me is that there is a center, but it is not measurable in 3D. There is no 3d edge. Anywhere in the cosmos will look similar to other places. What if earth was closer to the center of the milky way. How would our starry nights look? Like 100000 or more stars at night. What is the universe expanding into? Space gets created with expansion. What is space displacing? It is all in a dimension that we cannot measure. The end of “Men in Black” gave the most whimsical, yet possibly accurate measure. We are just huge cosmic marbles being played with by children.
It’s simplest, perhaps, to think of the Universe as being finite in size, like the surface of a balloon, and it may well be so (the 3-d “surface” of a hypersphere), but that’s not necessarily correct. In fact, recent evidence all seems to suggest very strongly that the Universe is, in fact, flat (Euclidian) on the largest scales. There’s still no edge, though, so we don’t need to worry about what it’s expanding into: It would only expand into something else at an edge. As for the space-bending, we already have the capability to bend space-- Any old mass will do that. However, if we want to bend space in just such a way as to allow for FTL communications, we can’t just use any old matter, we need matter with negative mass. Unfortunately, nobody’s ever found any such stuff, and there’s a good chance that it’s impossible for it to exist. But then, if we did know for certain, I’d be out of a job 
Slightly (cough cough) tangential, but still to the point (cough cough): How many dimensions are there and what are they?
you mean galaxy, not star
we do…3 dimensions of space and 1 of time
I know I saw a relevant link for this exact question somewhere, but all I can find at the moment is this…
http://image.gsfc.nasa.gov/poetry/ask/a11331.html
As Chronos said, space is not expanding into anything (nothing, at least, that we can imagine). The universe is either finite, in which space curves back on itself and has no expanding edge, or is infinite, and again has no expanding edge.
I too have wondered about a 4th dimension of space in order to explain the curvature of 3D space…but as far as I can tell, a 4th space dimension is not needed (and has no evidence of its existence) because you can’t really examine the curvature of 3D space without considering the 4th dimension of time. Perhaps Chronos can explain further.
Actually, the latest thought on this is that this universe has 11 dimensions. This comes from the string theory. We cannot see the other dimensions because they are infinitessimally small.
One theory holds that there are other universes, probably an infinite number. A side from this has to do with quantom physics and Einstein’s thought experiment re the cat in the box. (Whether the cat is dead or alive is not known until the door is opened, but the equally possible alternative occurs, but in another universe.)
Physics is only concerned with what can be observed. If you want to hypothesize that our Universe is embedded in higher dimensions that we can’t detect, that’s fine, but it’s not physics. The same is true, to a large degree, of the many-worlds model: So far as we know, there’s no way to test it. When we talk about the curvature of the Universe, we’re sticking to things that we can measure from inside the Universe, such as the angles of triangles. A closed Universe (with positive curvature) could, in fact, be embedded in a Euclidean space of one higher dimensionality, and it’s easier to visualize that way, but we can’t say whether it’s true. A uniform open Universe (negative curvature), however, cannot be embedded in a Euclidean space.
As to the extra dimensions of string theory, recent theories posit that the extra dimensions may be as large as a millimeter, and remain undetected because gravity is the only force that can propogate through those dimensions. (See Scientific American, August 2000, pg. 62 “The Universe’s Unseen Dimensions”, by Nima Arkani-Hamed, Savas Dimopoulos, and Georgi Dvali) Personally, I don’t think there’s anything to it, but science doesn’t care what I think. The important part is that it’s testable: There’s experiments in the works now, and we should have results within a few years. If they’re right, it’ll probably pave the way for the long-sought Theory of Everything.
There is a need for a fourth dimension, and it isn’t really time. According to Einstein’s theories, when getting near speed of light, very strange things start to happen. And the only way to explain these phenomenons is to make use of a fourth dimension in which objects can “rotate”(their 4th dimension counterpart, that is). A simple example is this: The nearer we go towards light speed, the bigger an energy source we need to continue to accelerate. Why? Think of it this way: put a cart on 2 rails. It is easier to push it from behind to make it go forward, and impossible when pushing from the side, right? Well, the nearer you get to the speed of light, the more the angle of pushing !rotates! towards the side. This is why, theoretically, it is impossible to travel to the speed of light by classic propulsion means. Of course, there’s a whole mathematic base to back this up, but you don’t want to go there. Relativity is fascinating stuff.
But yet, there are a lot of things that we don’t understand about the universe. There is still an enormous amount of work to be done in the realm of Astrophysics and theorical physics…
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A side from this has to do with quantom physics and Einstein’s thought experiment re the cat in the box(Whether the cat is dead or alive is not known until the door is opened, but the equally possible alternative occurs, but in another universe.)
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Err, you mean Shroedinger’s Cat? BTW that “experiment” was thought out by Shroedinger to prove a point: That Quantum Mechanics don’t apply to everyday physics. To say that a cat can be in 2 states at the same time(namely dead and alive)because we haven’t checked on it’s status is totally ridiculous. Shroedinger wanted to show that the bizzare nature of Quantum Mechanics only apply to particle physics, not everyday’s concerns, so you can’t say the cat is dead here and alive in another dimension, but a neutrino can(be in 2 states at once, but in different dimensions).
I thought we could see the effect of higher dimensions. What about light from a distant star getting bent as it passes by our sun (as has been confirmed by experiment)?
If photons have no mass there is nothing for another mass to tug on. If, however, a mass warps the third dimension doesn’t this argue for at least one higher (spacelike) dimension beyond the 3[sup]rd[/sup]? Doesn’t the third dimension need someplace to ‘warp’ or bend into? I thought that when the light ‘bends’ around the star it is actually following the shortest path (a straight line) in 4 dimensional space.
Also, I think it’s misleading to say physics is only concerned with what can be observed. When Einstein formed his theories much of what he proposed couldn’t be observed due to a lack of sensitive tools and complex methods. Still, his theories seemed plausible and some of what the theories predicted could be tested. Over time more and more aspects of his theories get tested and for the most part have stood up wonderfully.
Likewise, Superstring Theory may be able to stand even if we can’t probe for higher dimensions. I believe a successful theory needs to describe what we currently see accurately and make predictions for what we should see once we develop methods to check out the next bit. As long as the thoery continues to get most of its answers right it can continue stand with maybe minor modifications as needed.
Actually, the other dimension that you “rotate” into at high speeds is time, but the time dimension acts differently from the spatial ones, so there’s a few minus signs thrown into the equations where you wouldn’t expect them.
As to the bending of light by massive objects, all of the dimensions we know of (3 space and 1 time) are warped by that mass. It’s easier, as the OP was saying, to picture them as warping into some other dimension, but it’s not actually necessary. All we can measure is the effects in these dimensions, and that’s enough to develop the theory and make predictions. This may be due to other dimensions, but then again, it may not be.
I believe that this example was originally proposed by Einstein: Imagine that you have a flat tabletop, and a bunch of little metal rods, all the same length. Now, you put four on the table in a square. If you put on eight more, you can form a total of four small squares, all of which fit together exactly at the edges, and you can put down many, and form a grid consisting entirely of uniform squares.
Now let’s assume that the tabletop isn’t flat: You can put down one square, but if you try to make a whole array of them, you’ll find that the squares start to not fit together exactly, or they get distorted. Just by observing the pattern of how squares fit together on the tabletop, you can deduce that the geometry of the table is not flat.
However, now consider a third case: Suppose that the surface of the table is flat, but it’s hot in the middle, cooling off towards the edge, so that rods placed in the center of the table expand slightly. Again, you’ll find that the squares don’t fit together right, and you can measure and quantify the amount by which the grid gets distorted, but in this case, the table is still strictly two-dimensional. It’s effectively “curved”, in a Riemannian sense, but it’s not “curved into” anything.
If that doesn’t help explain things, then blame Al
Dang, I knew I was forgetting something: The philosophy of physics. It’s one thing to make predictions which cannot be verified with current technology, which is what Einstein did with some of his theories. It’s another thing altogether to make predictions that cannot be tested at all, even in principle. If I have a theory about a pink Unicorn which has as one of Her properties that She cannot be detected, directly or indirectly, then I have stepped outside the bounds of science. If, however, I say that She cannot be directly detected, but Her hoofprints are visible, then I can go around looking for Unicorn hoofprints, and I’ve got a (potentially) scientific theory again. Even if I do find hoofprints, though, I cannot call this scientific evidence that the Invisible Pink Unicorn exists, unless I can also rule out the possibility of an invisible purple unicorn making the prints, or that prints might exist without unicorns.
Although I am not too familiar with String Theory, it seems to be just a mathematical model with no physical evidence. (no evidence of strings, no evidence of the other dimensions.) It will be interesting to see what research happens in this area.
I am not a scientist and only know what I read in various books and periodicals. I stand corrected as to the ownership of the cat, which as Ranman points out, was Shroedinger’s. However, Ranma says that it proves Shroedinger’s point that qm applies only to particle physics. But does it? It does not rule out parallel universes.
Phobos states that String Theory (ST) is purely mathematical. However, it is my understanding that a cat named Guth, or Goth, or something like that, postulated ST to explain certain phenomena of this Universe, such as the overwhelming preponderance of matter over anti-matter.
Chronos replies that parallel universes and the like are not physics, not even theoretical physics, as they can not be tested and are not subject to being tested: they cannot be tested at all. Perhaps with present technology, a theory cannot be tested, but does that mean it can never be tested? To propose the hypothetical he does, about invisible things which are not amenable to being seen is not to preclude that future technology or breakthroughs will make it possible to test for parallel universes or 11 dimensions created by ST. One cannot state dogmatically that those ideas are impossible to test.
Phobos:
While we’ve never seen a string, there is actually quite a lot of evidence to support the theory, though the evidence is not very ‘hard’. String theory (actually M-theory) looks to be our best bet for a grand unification theory. String theory has been tested against a number of well established (and a few not-so-well established) results and has been in 100% agreement with everything we know today. Some of the ‘fresh’ predictions made by string theory are due to be tested in the Large Hadron Supercollider in Geneva sometime in 2005.
Before you prejudge string theory, I suggest you read “The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory” by Brian R. Greene. He does a really good job of showing the evidence FOR… and the open questions OF… string theory.
First off let me state I’m uncomfortable questioning what Chronos wrote as I’m going up against Chronos (one of the most knowledgeable posters on SDMB as regards physics) and Albert Einstein (one of the most knowledgeable people as regards physics period). Be gentle if I’m way off base…
While the quote above is a good thought experiment to show there are different ways to interpret effects (i.e. what’s caused what you are observing) it doesn’t explain away curved space into higher dimensions.
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There is no center to our universe (i.e. no hot middle with cool edges). At least, I don’t think there is.
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The ‘bending’ light happens near massive bodies (ok…technically any mass).
I don’t think Einstein’s example works in debunking higher dimensions in this case. I understand that it is perfectly acceptable from a physics (read mathematical) sense to use higher dimensions to get consistent answers for their problems and that this does not necessarily imply that such higher dimensions actually exist as a physical ‘place’ (for lack of a better word).
However, doesn’t Occam’s Razor argue for the existence of higher dimensions in this case? The simplest and most useful explanation when looking at these problems is to use dimensions past 3. Without working in those dimensions scientists have to start creating convoluted frameworks to fit what we observe into. Higher dimensions may seem wacky but wouldn’t it be harder for Einstein to point to a hot middle of our universe? Indeed, if there is a middle to our universe doesn’t it have to be in a higher dimension?
For what it is worth I believe String theory does not ask you to believe in a Pink Unicorn that you can never see and just have to assume the hoofprints we see are indeed made by that mythical animal. The higher dimensions suggested by String theory is testable albeit very difficult. IIRC to probe the higher dimensions would take a particle accelerator as large as the earth’s orbit around the sun and/or the equivalent output of a star’s energy to achieve. I may have those examples totally wrong but suffice it to say it would take some ludicrous amount of energy to achieve. Still…it is technically verifiable.
As for parallel universes I agree with Chronos that they probably can never be tested for. Quantum Mechanics may suggest that this is a possibility but for now I think it needs to be relegated to literary tool for Sci-Fi authors.
IIRC to probe the higher dimensions would take a particle accelerator as large as the earth’s orbit around the sun and/or the equivalent output of a star’s energy to achieve.
But Chronos pointed out that a recent article in Scientific America stated that the higher dimensions may be as large as a mm. Can’t that be tested with present technology?
[slight hijack]
This is a thread I started in Great Debates, but it probably belonged here. I was hoping Chronos and more of the physics-gentsia would respond. So this is a transparent attempt to try and get some input in this one. It is basically asking if the it makes sense to discuss fractions of a second before the Big Bang.
http://boards.straightdope.com/sdmb/showthread.php?threadid=34581
[/slight hijack]
cool! let’s accelerate the whole Earth to near-light speed!
JoeyBlades - thanks, I do mean to read that book someday. My first impression of Brian Greene is that he’s a bit odd (part physicist, part cult leader?), but there seems to be enough good things said about that book to let me give him the benefit of the doubt.