70 replies to this topic

[bgwowk]

For all those people hung up on the problem of how the universe can be of finite size or age, here is the essence of it:

Imagine that the expansion of the universe just stops, and we hold everything still long enough for you to explore everything as long as you want. What you will find is that if you travel in a straight line in any direction, you will eventually come back to where you started. Even if you change direction, and fly around randomly everywhere, you will find that you just keep exploring the same space over and over again no matter how long you travel. An analogy is a two-dimensional being exploring the surface of a sphere: Its universe is unbounded-- no walls --yet it is neverthless finite. Similar reasoning applies to time. Hawking has compared asking what was before the Big Bang to asking what is south of the south pole.

Our brains are wired to have a conceptual model of space extending in three infinite orthogonal directions, and time as an independent one dimensional coordinate. That is because that model was most useful to our primate ancestors for swinging through trees. That model does not pertain to the real universe at cosmic distances (or even locally when high speeds or strong gravity are involved), for which we must defer to mathematics. The mathematics of space and time (General Relativity), and the experiments that support it, tell us that space and time are not what our brains are wired to think they are.

[halcyondays]

For all those people hung up on the problem of how the universe can be of finite size or age, here is the essence of it:

Imagine that the expansion of the universe just stops, and we hold everything still long enough for you to explore everything as long as you want.  What you will find is that if you travel in a straight line in any direction, you will eventually come back to where you started.  Even if you change direction, and fly around randomly everywhere, you will find that you just keep exploring the same space over and over again no matter how long you travel.  An analogy is a two-dimensional being exploring the surface of a sphere: Its universe is unbounded-- no walls --yet it is neverthless finite.  Similar reasoning applies to time.  Hawking has compared asking what was before the Big Bang to asking what is south of the south pole.

Our brains are wired to have a conceptual model of space extending in three infinite orthogonal directions, and time as an independent one dimensional coordinate.  That is because that model was most useful to our primate ancestors for swinging through trees.  That model does not pertain to the real universe at cosmic distances (or even locally when high speeds or strong gravity are involved), for which we must defer to mathematics.  The mathematics of space and time (General Relativity), and the experiments that support it, tell us that space and time are not what our brains are wired to think they are.

I thought that was just one theory of many and wasn't proven? That if you head off in one direction that space doesn't warp around and you wouldn't end up back where you started. Is there a Universally agreed upon theory that says if you head off in one direction you will end up back where you started? I have read about this theory but I didn't think it was widely accepted as being "fact".

[DukeNukem]

What you will find is that if you travel in a straight line in any direction, you will eventually come back to where you started.

This is not proven, though it's a possibility (and frankly, I think will not prove itself correct). The idea here is that there's enough of a gravitation pull within the universe, due to all of its contents, that space warps back upon itself. The result is like a game of Asteroids, where you leave one side of the screen and wrap around to the opposite side (a weak analogy, but I still like it).

[bgwowk]

I thought that was just one theory of many and wasn't proven? That if you head off in one direction that space doesn't warp around and you wouldn't end up back where you started. Is there a Universally agreed upon theory that says if you head off in one direction you will end up back where you started? I have read about this theory but I didn't think it was widely accepted as being "fact".

IF the universe is finite, and IF expansion were stopped, then that is the manner in which finitude would manifest itself. That is a fact.

My main point is that there is no a priori reason to reject a finite universe. It cannot be rejected just because our brains can't intuitively grasp it, or because it is philosophically objectionable. Reality is what it is.

[Infernity]

. It cannot be rejected just because our brains can't intuitively grasp it

[thumb] hahaha lol, no, it can't ^_^

-Infernity

[bgwowk]

What's so funny? Did I miss something?

[AdamSummerfield]

She meant that our brains are built in the fourth dimension (three spatial and one temporal). And so we cannot imagine higher dimnesions, or infinity.

[AdamSummerfield]

Our brains are wired to have a conceptual model of space extending in three infinite orthogonal directions, and time as an independent one dimensional coordinate.  That is because that model was most useful to our primate ancestors for swinging through trees. 

Ah bgwowk, you have read 'Hyperspace' By Michio Kaku. I'm on page 84 at the moment.

[DukeNukem]

Wow, now there's an old, probably out-of-date book! I read that 15+ years ago. There are much newer, more up-to-date books that would likely be a more valuable use of your time. ;-) Things like dark energy, quantum gravity, the re-rise of the cosmological constant, and many other significant new findings have come along since that book.

[Centurion]

http://www.activemin...e_equation.html

If only we knew what the hell the values were

[attis]

Actually, LL, technically there is an 'other' side to the spacetime volume expansion if you accept M-Theory, insomuch the volume in which our 4d vacuum [false vacuum according to some theorists] is basically occupying a region of the Bulk in which the membranes exist, notably ours. If you want to know more I believe Paul Steinhardt's website is a great start to look it up.

Personally, I think an infinite domain makes sense insomuch that there would be other domains to consider, thus another unneeded variable to consider, making the proposition more robust. But then again, I'm a metaphysical naturalist too. :-P

[bgwowk]

Even if this universe is finite, many (most?) theories of cosmology and quantum mechanics now predict the existence of an infinite number of alternative universes. Some physicists and philosophers find this pathological. I think it is... wonderful. It's almost religious in its implications.

[caston]

Do gravitons exists? If so does conversation of energy span across all 12 dimensions or just our 3+1?

[xanadu]

I suspect there may be many mini-universes in the totality. They may be separated by a great deal of space and they may be bounded by their own gravity. There is no need to postulate additional dimensions to contain these other universes. Just as a black hole is formed by intense gravity that causes light to bend in on itself, a universe could do the same thing on a larger scale. We might be living inside a black hole. There could be trillions, or whatever number, of these universes inside the real universe. It seems logical.

Now I know someone will archly inform me that scientists have determined that there is not enough mass in the visible universe to make it "closed" as I was speculating about. However, there is not enough visible or detectible mass in any nearby galaxy including our own to explain its rotation. Our galaxy should fly apart at the speed it's rotating but it doesn't. It is now believed by many scientists that over 90% of the mass of the known universe is so called "dark matter". If so, then our universe could easily be finite but unbounded. There is not only nothing on the other side, there is no other side. There could easily be a large number of similar universes a few trillion lights years away from us but we would never be able to escape ours or to visit theirs. Unless perhaps you believe in astral projection but that's another subject altogether.

[bgwowk]

There are many theoretical generating mechanisms for alternate universes that don't require our universe to be closed. Inflation alone may have driven the size of this universe to such a humongous size (10**1000 parsecs) that it might practically be considered a collection of universes, possibly with regions with differing laws of physics. Smolin has written of such things. Even in pre-inflation cosmology, the part of the universe we see out to the 3 K background is only a small fraction of what the Big Bang spawned.

The Oxford interpretation of quantum mechanics (aka MWI) says parallel universes even coexist in the same space we do, but are not detectable (even by gravity) because they are quantum mechanically decoherent with us. QM says that right now you could be sitting in the core of a star in another universe, and not know it; same space, same time, but different QM wavefunction coherence.

[DukeNukem]

I was wondering why it was so hot in my office.

[caston]

QM wavefunction?

http://en.wikipedia....ki/Wavefunction

Edited by caston, 04 November 2006 - 03:00 AM.

[xanadu]

bgwowk wrote:

"Inflation alone may have driven the size of this universe to such a humongous size (10**1000 parsecs) that it might practically be considered a collection of universes, possibly with regions with differing laws of physics."

First of all, what does "(10**1000 parsecs)" mean? I hope you do not mean 10 to the 1000th power because that is obviously nonsensical. Actually, I can find no verification of any of the theories in your post. That doesn't mean it's wrong but it's a lot like saying fairies or pixies control the laws of physics. We can postulate many new things such as "decoherence" in quantum mechanics but is there anything to back it up? Black holes have been seen so there is some experimental basis for the theories I put forward. They do not rely on pixies nor QM decoherence. QM decoherence has not been observed.

I agree there may be other universes and it's not required that this one is closed. A closed universe would be endless but finite. A black hole seems to fit that definition. If our universe is closed because of gravity it could still interact with other universes but on a very long time scale. Those who are not inside a black hole can look at it, measure it's gravity and so on. There may be black hole universes floating around like stars in a galaxy or like galaxies in our universe. They may possibly collide and merge with one another. Would we notice it if our universe merged with another one? Perhaps but it would happen on such a long time scale that to our perspecitve, it would not change at all in a human lifetime.

I suspect our universe pulsates at a regular rate. The big bang is followed by a big crunch which leads to another big bang. Our universe may be a subatomic particle in a larger universe. A trillion years here might be less than a microsecond in that larger universe. But, we know time is relative as is space. Could a black hole be larger on the inside than the outside? Perhaps.

[bgwowk]

First of all, what does "(10**1000 parsecs)" mean? I hope you do not mean 10 to the 1000th power because that is obviously nonsensical.

If 10 to the 1000th power is nonsensical, then what must you think of infinite universe theories?

Actually, I can find no verification of any of the theories in your post.

page 90 of The Bigger Bang by James Lidsey (not Smolin, my mistake)

The part of the universe we occupy may be considerably larger than the region of space that we can observe.  The current distance of 10 billion light years corresponds to roughly 10**26 meters.  In some models of inflation, our region may extend out for a further 10**10000 (10 to the power of 10,000) meters before it meets another region, with a different number of dimensions.  This is a huge number; it is ten followed by ten thousand zeroes!  The inflationary scenario suggests that the region of the universe we observe is just a tiny fraction of the total amount of space that actually exists.  If we thought the observable part of the universe was large, inflation predicts that the entire universe is significantly larger.

That doesn't mean it's wrong but it's a lot like saying fairies or pixies control the laws of physics.

No. Such theories result from adopting a small number of postulates, often subsets of existing laws of physics, and following them to their logical conclusions. They aren't made up like fairy tales. We just have insufficient knowledge at present to know which set of postulates are correct, although inflation is coming under pretty firm footing.

We can postulate many new things such as "decoherence" in quantum mechanics but is there anything to back it up?

Decoherence explanations of the measurement problem of quantum mechanics are not the result adding a new postulate. Decoherence formulations, such as MWI, are the result of deleting the state vector collapse postulate. The collapse postulate is not necessary to generate any of the experimental predictions or observations of quantum mechanics. A survey several years ago showed that 70% of quantum cosmologists and quantum computing experts no longer accept it. Indeed, the entire field of quantum cosmology is nonsensical if the collapse postulate is retained (as if an infinite variety of universes existed in superposition until the first "observer" looked up into the sky and collapsed all but one!).

QM decoherence has not been observed.

On the contrary. Every experimental example of supposed state vector collapse is actually QM decoherence.

See

http://en.wikipedia...._interpretation

[caston]

http://www.physicsfo...ead.php?t=82628

http://en.wikipedia....ation_cosmology

[xanadu]

bgwowk wrote:

"If 10 to the 1000th power is nonsensical, then what must you think of infinite universe theories?"

Don't try to change the subject. You said this universe was 10* 1000 parsecs and you obviously pulled that number out of thin air. There is no observational evidence to support it. Likewise with many of the theories you glibly refer to.

"page 90 of The Bigger Bang by James Lidsey"

Or page 200 of lord of the rings or page 120 of Alice in Wonderland. I'm sorry but the fact that someone dreams of these things does not give them any grounding in reality. I like to see some experimental verification. Einstien's theories would have been in the same catagory if the things he had predicted were not seen in the physical world. Until we observe Hawking radiation, it too falls in that catagory. However, Hawking radiation is relatively solid compared to the fluff you have spoken of.

"Such theories result from adopting a small number of postulates, often subsets of existing laws of physics, and following them to their logical conclusions. They aren't made up like fairy tales. We just have insufficient knowledge at present to know which set of postulates are correct, although inflation is coming under pretty firm footing."

Speculation is all fine and good. However, making up stuff out of thin air like "QM decoherence" is a lot like saying "abracadabra anything is possible". Don't get me wrong, I'm all in favor of imagination and speculation. However, you do need to touch base with reality at some point or you have less credibility than the wizard of oz. Where is the basis or experimental proof for 12 dimensions, QM decoherence or 10* 1000 parsec size of our universe? About the same as the theory that fairies and spirits make the world run. Maybe they do, I don't know but I'm not going to put any energy into that view until someone shows how their theory makes sense in the real world.

There is real world evidence of many of my theories. My theory that our universe pulsates is supported by evidence of the big bang. The big crunch is speculative but the expansion seems to be slowing down and it logically follows that it will eventually end and be followed by a collapse. The detection of large amounts of dark matter in our universe and even in our galaxy lends credence to this theory. Can anyone discuss these things without referring to MWI, decoherence and other unproven fantasies? Particularly if your whole theory rests on those things being a fact.

[jaydfox]

The big crunch is speculative but the expansion seems to be slowing down and it logically follows that it will eventually end and be followed by a collapse.

Logically follows? An object blasted from the surface of the earth at greater than escape velocity will slow as it pulls away from the earth, yet it doesn't logically follow that it will slow and fall back to earth. Is this the type of logic you use when studying physics?

I only ask because you're ridiculing a man with more formal training in physics than you and I put together, yet you speak with an air of authority. If you're going to attack his ideas, use physics and mathematics, not comparisons to pixies. Unless, of course, your intent was not to make a persuasive argument, but merely to make Brian look silly.

[eternaltraveler]

Xanadu go take a physics class before you debate a physicist in physics, though you might want to start by studying logic

[mitkat]

I'm curious as to where "(10**1000 parsecs)" is mentioned/noted.

[xanadu]

Mitkat, look back a few posts. bgwowk mentioned it.

Jay, who am I ridiculing? I am discussing ideas. I ridiculed the concept of 10 to the 1000th power parsecs being the size of the universe. 10^1000 is a number that has no meaning in our world. I doubt there are 10^100 atoms in the known universe. I scoff at saying "decoherence" to cover anything that makes no sense. You could say "magic" to the same effect.

Jay and elrond, can you dispute anything I said? If not, what exactly are you doing beyond what you accuse me of? You are ridiculing my argument with ad hominems saying I should study logic and physics first. I don't claim to know everything, no one does. That's why we are discussing things, to arrive at the truth or closer to it. I simply insist that we start with known facts or things that can be verified to some degree. If I'm wrong about something I said, then point out the error but don't just say so and so has a different opinion so you must be wrong.

Jay, I have no proof that the big crunch will happen. I note that the expansion of the universe has slowed, according to data found by astronomers. The theory of the big bang says the universe was at one time either a point, the size of a proton or some other small size. Hyperinflation supposedly occured and the universe expanded faster than light. None of those things are proven but they seem to fit the known facts. If the universe was at one time very tiny, why would you assume it would not/ could not end it's expansion and contract? If it contracts at all, it will likely collapse into a big crunch. So far it is slowing down the expansion.

bgwowk, I am interested in discussing things with you. Sorry if my argument sounded abrasive. It's just that saying there is a theory that anything can happen does not prove anything to me. Statements like 10^1000 parsecs being the size of the universe sets off my BS detector. If that has been proven or substantial evidence exists, then I will gladly eat crow. Gladly because it means I learned something. You may know a great deal more about the subject at hand than I do. If so, then perhaps you can explain a bit more about decoherence, how it manifests and why you think this is real. Or any of the other things we talked about. Many people might find it interesting. That's what a debate is, discussing facts and theories. If the theories are based on nothing but more theories and no observational data seems to back them up and no predictions can be made and tested based on those theories, then it's like debating how many angels can dance on the head of a pin. That's the way I look at things anyway.

[mitkat]

Thanks Xanadu, I saw it before, I'm not sure how that was measured empirically...starting to get out of my reach over here [glasses]

[Lazarus Long]

Jay, I have no proof that the big crunch will happen. I note that the expansion of the universe has slowed, according to data found by astronomers. The theory of the big bang says the universe was at one time either a point, the size of a proton or some other small size.

Actually expansion is not slowing, it's accelerating according to most of the recent data and the reason is attributed to dark matter. The current Nobel for physics was awarded over this discovery.

True, there are those that still find it difficult to accept the evidence but the data is pretty conclusive and inescapeable. We are not headed for the big crunch or even the heat death that for so long many predicited but in fact the current model is the big rip and a cold death of never ending expansion.

Is any theory at this time certain?

I wouldn't bet the farm but the stuff coming from WMAP and Chandra, not to mention Hubble have been pretty upsetting for many classical astrophysicists, as it overturned the apple cart pretty well.

I can even hear Copernicus and Galileo laughing all the way from their graves. )

[jaydfox]

The Oxford interpretation of quantum mechanics (aka MWI) says parallel universes even coexist in the same space we do, but are not detectable (even by gravity) because they are quantum mechanically decoherent with us. QM says that right now you could be sitting in the core of a star in another universe, and not know it; same space, same time, but different QM wavefunction coherence.

Actually, just to split hairs for a second, but can we really speak of "same space, same time" in cases as divergent as sitting in the core of a star in another branch? Wouldn't the bending of spacetime alone make a 1-to-1 mapping of spacetime coordinates impossible, or non-sensical at the least?

[bgwowk]

Actually, just to split hairs for a second, but can we really speak of "same space, same time" in cases as divergent as sitting in the core of a star in another branch? Wouldn't the bending of spacetime alone make a 1-to-1 mapping of spacetime coordinates impossible, or non-sensical at the least?

Excellent question. How could we, for example, share spacetime with a black hole? The answer is that for MWI to be true, gravity must be quantized (a testable prediction of MWI), and therefore subject to the same QM decoherence effects as other forces of nature. This would mean that Einstein's model of bent space time in the equations of General Relativity is a model of limited applicability, similar to the way Maxwell's Equations just approximate what is going on with the more generally-applicable quantum electrodynamics.

Xanadu, I understand your reaction. I'd have the same reaction if some guy sidetracked a discussion of mine by suddenly throwing in a bunch of weird ideas seemingly pulled out of a hat. I assure that is not what I'm doing. According to Guth, inflation *at a minimum* predicts that the whole universe is 10**23 times larger (about a TRILLION TRILLION times larger) than the visible universe. The even wilder large numbers apparently come from calculations by Andrei Linde based on models of chaotic inflation. Remember that the main idea of inflation is that the universe we see all looks so similar because we are seeing only a teensy tiny part of a more heterogeneous whole. In modern cosmology, there is no getting away from the inference that much, much more exists than what we can see in our tiny light cone reaching back to Big Bang afterglow.

Decoherence (but not necessarily MWI) is a fact of quantum mechanics. See

http://en.wikipedia....tum_decoherence

Decoherence is what happens when observers are considered part of quantum systems, as they really should be. The tricky part of decoherence is how you philosophically handle observers that decohere from each other. For example, if QM predicts that a double slit experiment will give rise to two observers each seeing a different result, but never able to communicate with each other because of decoherence, and you can subjectively be only one of them, how do you handle the other observer that QM predicts? You can ignore the result as irrelevant for any practical purpose. Or you can accept the result as philosophically real, which is MWI.

[jaydfox]

how do you handle the other observer that QM predicts? You can ignore the result as irrelevant for any practical purpose. Or you can accept the result as philosophically real, which is MWI.

Is the part I bolded the Copenhagen interpretation, or is that something else further?