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BetsyJ
2014-Mar-06, 01:37 AM
I'm a newbie, so forgive me if this topic has already been covered--or if this isn't the appropriate place to post my question.

Spacetime is sometimes said to be ďcurved.Ē Iíve seen the demonstration of the rubber sheet and the bowling ball. A two dimensional rubber sheet is bent or deformed through a third dimension. And yet Iíve read that curved spacetime does not require, nor does it exhibit, a curving through a fourth dimension. I donít understand this. If itís curving, itís curving through another dimensionĖhow could it be otherwise?

Also, what about the Higgs field? The Higgs field permeates space and gives some (but not all) particles their mass. If thereís no vacuum thatís not permeated by the Higgs field, can we say that space IS the Higgs Field? How would such a field be different from the (supposedly nonexistent) ether?

I personally believe that space is not a backdrop in which objects are embedded, but merely the way observers experience the separation between two objects. However, I get the feeling that physicists are saying something else.

Can anyone enlighten me on any of these topics? It would be much appreciated. (I read lots of nontechnical books on cosmology.)

Cougar
2014-Mar-06, 04:24 AM
A two dimensional rubber sheet is bent or deformed through a third dimension.

The 2-D rubber sheet is supposed to represent the entire universe. Gravity curves the sheet. You're supposed to neglect the third dimension, which isn't part of the (model 2-D) universe.

ShinAce
2014-Mar-06, 04:32 AM
I personally believe that space is not a backdrop in which objects are embedded, but merely the way observers experience the separation between two objects. However, I get the feeling that physicists are saying something else.

Can anyone enlighten me on any of these topics? It would be much appreciated. (I read lots of nontechnical books on cosmology.)

That's a fair belief. To separate space and time is now known to be impossible. To separate spacetime and matter might very well follow in those footsteps and be inseparable.

I don't think it's fair to say the Higgs field is space. There is also the radiation left over from 380,000 years after the big bang which is everywhere. That too might deserve the name of space itself, which still isn't fair. The Higgs field is special because it's scalar(doesn't point in any direction). The Higgs boson is the first ever scalar particle discovered!!!

The curving of space could be said to be more of a mathematical artefact than a real curving. It's not just mass that causes it to curve. If that were the case, Newton's gravity would be all that you need for gravity. These extra terms are all lumped together into a matrix looking thing, called a tensor. At that point, the mathematical abstraction of the process becomes rather large.

Einstein wrote a book called "The meaning of relativity", which does a good job of pointing out what he thought was important and what should be ignored as meaningless details. It's not 'math heavy', but does contain the basic ideas in mathematical form.

Jeff Root
2014-Mar-06, 05:43 AM
My view is that the curvature of spacetime is curvature of
space in the time direction. The presence of matter causes
there to be less space and more time in the vicinity than
there would be without it. That is gravity. Only a very tiny
curvature is required to make Earth's gravity, but the entire
mass of the Earth is required to make that curvature.

Small differences in time can be much easier to measure
than small differences in space. Global positioning system
satellites have precision atomic clocks which are adjusted
to account for the difference in time -- a slowing down of
time due to their orbital speed relative to clocks on Earth,
and a speeding up of time due to their being farther from
Earth's spacetime warpage. At the particular altitude at
which GPS satellites orbit, the difference due to gravity is
greater than the difference due to speed, so there is a net
speeding up of the orbiting clocks. In order to keep them
synchronized with clocks on the ground, they are designed
and built to run more slowly than other clocks when they
are sitting side-by-side.

-- Jeff, in Minneapolis

Shaula
2014-Mar-06, 06:34 AM
Spacetime is sometimes said to be “curved.” I’ve seen the demonstration of the rubber sheet and the bowling ball. A two dimensional rubber sheet is bent or deformed through a third dimension. And yet I’ve read that curved spacetime does not require, nor does it exhibit, a curving through a fourth dimension. I don’t understand this. If it’s curving, it’s curving through another dimension–how could it be otherwise
This is the difference between intrinsic and extrinsic curvature. Extrinsic is as you have describes, the space seen as curved is embedded in a higher dimensional space and it curves into those other dimensions. Intrinsic curvature is a property of metric spaces that is independent of any embedding in higher dimensional spaces. Essentially the metric is described at each point by a set of connections to the next point, how far you have to go in any direction to get to the next point. In the limit of a continuous metric this is a spatially varying function that you integrate over. This describes the metric in terms of paths or intervals.

So what we refer to as curvature is simply a distortion in how far things are from each other. As an analogy it is like graph paper where the length of each side of the squares making it up can vary. No extra dimensions are required, it is a change in the underlying structure of the metric. You can model this as a complex curvature into other dimensions if you like but it is not required to described what is going on.


Also, what about the Higgs field? The Higgs field permeates space and gives some (but not all) particles their mass. If there’s no vacuum that’s not permeated by the Higgs field, can we say that space IS the Higgs Field? How would such a field be different from the (supposedly nonexistent) ether?
The aether was a medium required for light to propagate in its first incarnation. The Higgs field is nothing like that. People have made the Higgs field = aether link before but what it comes down to is how you define aether. The Higgs field in not a physical medium. The Higgs field is not an absolute reference frame. The Higgs field is not required for the propagation of forces. So what are you left with? If you define aether as 'something we find everywhere' then you can call the Higgs field an aether. You could also call any number of other fields an aether since in quantum mechanical terms they are always there, even if their actual strength at any given point is zero.

As for what space 'is' ... sadly the answer is come back later. At the moment our best model to describe spacetime/gravity is general relativity. Our best model to describe pretty much every thing else is quantum mechanics. And we cannot reconcile them. At their most fundamental level they are not compatible. Some new leap is required to bridge that gap, usually expected to be a quantum description of gravity.

Amber Robot
2014-Mar-06, 06:57 AM
If it’s curving, it’s curving through another dimension–how could it be otherwise?

One can mathematically characterize a curved space without the need of a higher dimension. It's not so much that you're describing the curvature of space so much as the curvature of paths through that space.

BetsyJ
2014-Mar-06, 06:49 PM
Thank you. These answers are really helpful. I don’t have the math background, but I think I “get” what you’re saying. From what I know of the history of mathematics, ideas that started as purely mathematical often turn out to have a practical application or are in some sense realized in physical reality. Is it possible that the mathematical curvature of space actually IS a real curvature through another dimension? Or is this simply a manifestation of my ignorance?

Your comments resonate with me, Jeff. Why couldn’t space be “curved” in the time dimension?

ShinAce: I’m intrigued by your statement that: “The Higgs boson is the first ever scalar particle discovered!!!” Could you say more about that?

I’ve just started reading Max Tegmark’s book Our Mathematical Universe My Quest for the Ultimate Nature of Reality. I’m discouraged to find that he asserts that eternal inflation necessarily requires a validation of the many-world’s interpretation of quantum mechanics, something I’ve resisted as-- well, a mess! Infinite space does present some problems, though.

Can't tell you how grateful I am to be in contact with people who are thinking and knowledgeable about these issues. So, thank you again!

Swift
2014-Mar-06, 07:07 PM
By the way BetsyJ, welcome to CQ. Hope you stick around, even after your questions are answered.

Jeff Root
2014-Mar-06, 08:27 PM
I’ve just started reading Max Tegmark’s book Our Mathematical
Universe My Quest for the Ultimate Nature of Reality.
I’m discouraged to find that he asserts that eternal inflation
necessarily requires a validation of the many-world’s interpretation
of quantum mechanics, something I’ve resisted as-- well, a mess!
That surprises me, and even shocks me. Although I've seen the
terms used together many times, I wouldn't think the two would
have anything to do with each other. Both ideas are highly
speculative. I think that's about all they have in common.



Infinite space does present some problems, though.
Whether the Universe is finite or infinite, either way I think it is
impossible to comprehend. Neither is more incomprehensible
than the other, but one of them is almost certainly reality.
Could be both or neither, somehow...

I forgot to say in my first post: Cougar mentioned that the
2-D rubber sheet is supposed to represent the entire Universe.
That description of it is similar to a photograph in that a photo
compresses everything in front of the camera from 3 dimensions
down into 2 dimensions. I view the rubber sheet or embedding
diagram as representing a slice through the Universe, like a
plan of one floor of a building. That avoids piling up a bunch
of things that are in different places all onto the same part of
the rubber sheet, like objects in a photo that are hidden behind
closer objects. Cougar probably had in mind the rubber balloon
analogy for the entire Universe, rather than a flat rubber sheet
which may be used simply to illustrate spacetime curvature,
and so doesn't need to model even a large area, much less the
entire Universe. But I consider the surface of the balloon to
represent a single flat slice through the entire Universe, too,
not the entire Universe. All that's needed is a representative
slice, and that is what I think the two-dimensional surface of
the balloon most naturally represents. Just as the rubber
actually has a bit of thickness, you can give the slice through
the Universe some thickness, too. It doesn't need to be as
thin as an idealized plane.

The balloon analogy is used to illustrate the cosmic expansion,
how every cluster of galaxies can move away from every other
cluster, how the galaxies and clusters themselves don't have
to expand, and how there can be no center of the expansion.
A flat sheet is usually used to illustrate curvature.

-- Jeff, in Minneapolis
.

Shaula
2014-Mar-06, 09:15 PM
Thank you. These answers are really helpful. I don’t have the math background, but I think I “get” what you’re saying. From what I know of the history of mathematics, ideas that started as purely mathematical often turn out to have a practical application or are in some sense realized in physical reality. Is it possible that the mathematical curvature of space actually IS a real curvature through another dimension? Or is this simply a manifestation of my ignorance?
I don't think it has been ruled out that there are space or time like higher dimensions that spacetime curves into (so called large or universal dimensions to distinguish them from compactified String theory dimensions). The trouble for that idea is that so far there has not been a testable, successful description of them. Some theories have resulted in ways to test or put limits on their properties but these remain fairly theory dependent. So I think Fermilab did some work that constrained the Large dimension theories put forward a few years back, there were some studies based on the weak force and muon magnetic moments too. So we have some results that say "If these dimensions follow these rules then this is what they cannot be like" but no actual tests that give a yes or no answer.

On problem we do come up with is that if there are more dimensions then how many are there? Superstring theory must have more and must have a certain number (which varies by theory) and can have no more or less. GR, by contrast, is absolutely fine with 4 and does not need more (intrinsic curvature works just fine). So adding more - well, if there is one why not two? Why not three? Why not an infinite number? Without a compelling theoretical description of this and tests we can perform it is all rather hard to talk about scientifically!

And of course then you hit the issue that science doesn't measure reality. It compares the output of models to observations to see which fits best. If two descriptions (intrinsic and extrinsic curvature) give identical results for every test we can perform then there is no way to tell between them.

BetsyJ
2014-Mar-07, 01:08 AM
Yes, Jeff, I was surprised too. Tegmark's point is that inflation solves a number of problems in cosmology. He says: “...inflation theory made a testable prediction back in the eighties: our space should be flat...we’ve now performed this test to better than 1% precision and inflation passed with flying colors!”

But inflation also “refuses to stop, forever producing more space.” Inflation ends in some places (our universe) but continues elsewhere. The total volume that keeps inflating doubles forever. But some of that space decays. However, almost all models of inflation lead to eternal inflation.

Later he says: “In summary, in an infinite space created by inflation, everything that can happen according to the laws of physics does happen.” He goes on to say: “Another important thing about physics theories is that if you like one, you have to buy the whole package...In the same way, parallel universes aren’t optional in eternal inflation. They comes as part of the package, and if you don’t like them, then you have to find a different mathematical theory that solves the bang problem, the horizon problem, and the flatness problem, that generates the cosmic seed fluctuations–and doesn’t predict parallel universes.”

I have to go. I'll reply to Shaula later...

BetsyJ
2014-Mar-07, 01:18 AM
I was in a hurry and I misstated this. Tegmark doesn't say inflation automatically favors the many-worlds interpretation of quantum mechanics (although he does subscribe to that). But the argument above amounts to the same thing--you end up with ever-proliferating parallel worlds. But they're all what he calls Level I universes.

Yes, Shaula. Well said. I can't disagree with any of your remarks, (although I'm sure Tegmark would).

Jeff Root
2014-Mar-07, 05:31 AM
Okay: The idea of many (perhaps infinitely many) universes
arising from a single, continuing inflation event is, I'm almost
certain, a completely different idea from the "many worlds"
interpretation of quantum mechanics. They have very similar
names, but otherwise they don't have anything to do with
each other. I can live with that.

I don't like the "many worlds" interpretation of QM, and I'm
not convinced about inflation -- though as you say, it does
rather neatly explain several observations -- but my objection
was to putting both in one package. No way!

-- Jeff, in Minneapolis

BetsyJ
2014-Mar-07, 05:39 AM
I don't know. I haven't gotten to the chapter on quantum mechanics yet, but I'm pretty sure he's going to try to make an ironclad argument for many worlds too! We'll see.

ShinAce
2014-Mar-07, 05:17 PM
Thank you. These answers are really helpful. I don’t have the math background, but I think I “get” what you’re saying. From what I know of the history of mathematics, ideas that started as purely mathematical often turn out to have a practical application or are in some sense realized in physical reality.

You might enjoy Feynman talking about physicists versus mathematicians:
http://www.youtube.com/watch?v=obCjODeoLVw

As for scalar. Here's a general description. Everything spins. Find me a solar system where the planets don't orbit! Find me a galaxy that doesn't spin! You can't! Now, what if even electrons, and nuclei of atoms spin? As a matter of fact, they do. Spin can be generalized to angular momentum. All particles have a certain amount of angular momentum, even if they're not strictly 'spinning'. So now we have this idea that every particle 'spins', which is pretty fundamental to quantum mechanics. You can combine two particle spinning in opposite direction to get a total angular momentum of zero. That's nothing new. What is new about the Higgs boson is that it is not made of two particles, and yet it has no 'spin'. We call that an elementary scalar particle.

Consider this. If you have a particle that doesn't spin, and you suddenly start going around it in circles, won't it look like it's spinning? Not for the Higgs boson. Whether you circle around it or not, it will never appear to be spinning. That's why angular momentum is general, while 'spin' is a term used in common language with an implied meaning which doesn't work in quantum mechanics.

It's weird, but true.

BetsyJ
2014-Mar-07, 11:26 PM
How do they know this? Just curious...

It seems to me that, if true, the lack of angular momentum implies that space as a three-dimensional frame of reference, in some sense, doesn't exist on the quantum level. So I'm back to my original intuition that space has no separate existence apart from the relationship between objects (and perhaps no existence except in the mind of the observer).

BetsyJ
2014-Mar-07, 11:27 PM
Thanks, Swift. I've read the rules. Is there anything else I need to do?

Shaula
2014-Mar-08, 06:03 AM
How do they know this? Just curious...
Luckily angular momentum is conserved. So with particles like this you add up the spins of everything that pops out and can confirm what the original spin was. The reason that they were looking for a spin zero particle is more complex. The 10 pages of maths sort of complex. What it comes down to is that they had a mechanism (the Higgs mechanism) to give mass to W/Z bosons and from the way that had to behave they extracted the properties of the field it had to have in it and from that they got the boson's properties.

One way to think about it is that forces tend to be associated with vector bosons (they always have a strength and a direction) while non-force fields need not have a direction associated with them. So electric potential is a scalar field, the electric field is a vector field. It does go a bit deeper than that, for example scalar field has certain invariant properties under Lorentz transforms (and some gauge transforms) that vector fields don't.

More accurately the scalar nature of the field derives from Nambu-Goldstone theory (http://en.wikipedia.org/wiki/Goldstone_boson#Theory) (which is a fascinating subject, by the way - underlies a lot of topics in solid state physics too)

kevin1981
2014-Mar-09, 09:59 PM
Confused about space

I am confused about the whole nature of reality, i just can not work it out, nothing makes sense to my mortal human brain ! I think you will fit right in here, welcome :)

Spacedude
2014-Mar-09, 10:17 PM
I am confused about the whole nature of reality, i just can not work it out, nothing makes sense to my mortal human brain !

Ha, me too at times. It's as though for every question we answer we create more questions. Are we just getting dumber? (Ha again) or maybe we're just barely smart enough to realize the true depth of our ignorance. I look forward to the day* when one answer raises just one question, maybe at that time we'll reach an intellectual balance with the universe :) Please continue to confound me.....

*just an expression, not gonna happen

Hypmotoad
2014-Mar-09, 11:07 PM
This is the difference between intrinsic and extrinsic curvature. Extrinsic is as you have describes, the space seen as curved is embedded in a higher dimensional space and it curves into those other dimensions. Intrinsic curvature is a property of metric spaces that is independent of any embedding in higher dimensional spaces. Essentially the metric is described at each point by a set of connections to the next point, how far you have to go in any direction to get to the next point. In the limit of a continuous metric this is a spatially varying function that you integrate over. This describes the metric in terms of paths or intervals.

So what we refer to as curvature is simply a distortion in how far things are from each other. As an analogy it is like graph paper where the length of each side of the squares making it up can vary. No extra dimensions are required, it is a change in the underlying structure of the metric. You can model this as a complex curvature into other dimensions if you like but it is not required to described what is going on.


The aether was a medium required for light to propagate in its first incarnation. The Higgs field is nothing like that. People have made the Higgs field = aether link before but what it comes down to is how you define aether. The Higgs field in not a physical medium. The Higgs field is not an absolute reference frame. The Higgs field is not required for the propagation of forces. So what are you left with? If you define aether as 'something we find everywhere' then you can call the Higgs field an aether. You could also call any number of other fields an aether since in quantum mechanical terms they are always there, even if their actual strength at any given point is zero.

As for what space 'is' ... sadly the answer is come back later. At the moment our best model to describe spacetime/gravity is general relativity. Our best model to describe pretty much every thing else is quantum mechanics. And we cannot reconcile them. At their most fundamental level they are not compatible. Some new leap is required to bridge that gap, usually expected to be a quantum description of gravity.

huh?

Shaula
2014-Mar-10, 06:41 AM
huh?
If you have any questions that are a little more focused I'd be happy to try to answer them. Huh covers a lot of ground.

kevin1981
2014-Mar-10, 07:16 PM
Ha, me too at times. It's as though for every question we answer we create more questions. Are we just getting dumber? (Ha again) or maybe we're just barely smart enough to realize the true depth of our ignorance. I look forward to the day* when one answer raises just one question, maybe at that time we'll reach an intellectual balance with the universe :) Please continue to confound me.....

*just an expression, not gonna happen

I would like to think i am not ignorant, hence the fact that i realize reality makes no sense to me ! Yes, we can use the scientific method and work out how nature works and so fourth. We do that very well and in a sense we know a lot about how nature works and why some things are the way they are ect..

But for me, the fact that we are here having this conversation and the fact there is anything at all just blows my mind. Why is there anything at all ? My answer is, because there can be. However, what came first, nature or the laws of nature... I could go on of course but i have come to the conclusion that what ever is going on is too complex and "out there" for my little human brain to comprehend ! We know a lot, but i am not sure we will ever have all the answers..

kevin1981
2014-Mar-10, 07:17 PM
If you have any questions that are a little more focused I'd be happy to try to answer them. Huh covers a lot of ground.

Haha Very good !

publiusr
2014-Mar-10, 10:03 PM
That's a fair belief. To separate space and time is now known to be impossible.
Well... http://www.scientificamerican.com/article/splitting-time-from-space/

ShinAce
2014-Mar-10, 11:52 PM
Just to count the number of 'might', 'maybe', 'could', in that article makes my head spin.

We're still a long way away from quantum gravity theories. The theprists looking at LHC data are still struggling to reconcile supersymmetry with the lack of observed supersymmetric partners.

BetsyJ
2014-Mar-16, 04:43 AM
Updating my comment of March 6. In chapter 8 Tegmark DOES argue that Level I parallel universes actually "merge with," and are essentially the same as, Everett's many-world universes (which he calls Level III universes). Ugh. The argument is too lengthy to go into here.

WayneFrancis
2014-Mar-17, 03:18 AM
I was in a hurry and I misstated this. Tegmark doesn't say inflation automatically favors the many-worlds interpretation of quantum mechanics (although he does subscribe to that). But the argument above amounts to the same thing--you end up with ever-proliferating parallel worlds. But they're all what he calls Level I universes.

Yes, Shaula. Well said. I can't disagree with any of your remarks, (although I'm sure Tegmark would).

This can be a hard thing to wrap your head around. If the universe is infinite in size, for which we have no evidence for or against (please don't get into the philosophical debate Jeff), and the physical laws are the same for the entire universe that does mean that since there is a finite number of configuration a given finite volume, like the observable universe, can have that means that it would be expected that they would repeat, an infinite amount of times, but at distance scales that scientists would struggle to even explain. If the universe is finite then the chance any region of space would be the same would n where n>0 based on the region in question, the amount of matter/energy in region and the size of the entire universe.

BetsyJ
2014-Mar-17, 11:49 PM
As of today, we do have evidence. Inflation posits an infinite universe.

profloater
2014-Mar-18, 12:01 AM
As of today, we do have evidence. Inflation posits an infinite universe. No I don't think it does, a) it could be a temporary inflation, and b) a single big bang suggests a finite universe, but while there are plenty of maths models, the evidence is limited by the visual horizon and speed of light. We do not know what is outside that horizon. Could go either way I think. But we will not find out, probably!

BetsyJ
2014-Mar-18, 12:22 AM
From Wiki (article on inflation):

"In many models of inflation, the inflationary phase of the universe's expansion lasts forever in at least some regions of the universe. This occurs because inflating regions expand very rapidly, reproducing themselves. Unless the rate of decay to the non-inflating phase is sufficiently fast, new inflating regions are produced more rapidly than non-inflating regions. In such models most of the volume of the universe at any given time is inflating. All models of eternal inflation produce an infinite multiverse, typically a fractal."

From Our Mathematical Universe, Chapter Five (and others):

"The first shocker is that inflation generally refuses to stop, forever producing more space. This was discovered for specific models by Andrei Linde and Paul Steinhardt."

Yes, there are a few exceptions, but those particular models are in the minority.

Shaula
2014-Mar-18, 06:48 AM
b) a single big bang suggests a finite universe
A single localisable (to us) Bang event could imply a finite universe. However current models have no Bang in them and we don't know if such an event would be localisable in our 3+1 spacetime. So it doesn't really follow from current theory that this statement is true.

profloater
2014-Mar-18, 07:22 AM
A single localisable (to us) Bang event could imply a finite universe. However current models have no Bang in them and we don't know if such an event would be localisable in our 3+1 spacetime. So it doesn't really follow from current theory that this statement is true.Thank ypu fpr the correction but is it the case that from observation we cannot tell? Which is to say we cannot choose between models on the basis of observation?

Shaula
2014-Mar-18, 07:50 AM
Thank ypu fpr the correction but is it the case that from observation we cannot tell? Which is to say we cannot choose between models on the basis of observation?
No, it is the case that we simply don't have a working model that lets us 'go back in time' far enough. Current theory says that the observable universe expanded from a finite sized hot dense state a finite time ago. It does not cover where that state came from, what was going on outside the volume that gave rise to our neighbourhood or what the universe was doing prior to the point where our current models are able to describe it.

Cougar
2014-Mar-18, 11:57 AM
From Wiki (article on inflation):

"In many models of inflation, the inflationary phase of the universe's expansion lasts forever in at least some regions of the universe."

Obviously this is not observed and is speculative.

Vilenkin theorized that our Universe is a bubble of lower vacuum energy that decayed from the very high energy inflation that continues "outside" our Universe. Other bubbles are occurring elsewhere, but since inflation is continually going on between bubbles, no bubble will ever detect another bubble.

Theories that include unobservable parts are not very satisfying.

BetsyJ
2014-Mar-18, 04:49 PM
Obviously this is not observed and is speculative.

Vilenkin theorized that our Universe is a bubble of lower vacuum energy that decayed from the very high energy inflation that continues "outside" our Universe. Other bubbles are occurring elsewhere, but since inflation is continually going on between bubbles, no bubble will ever detect another bubble.

Theories that include unobservable parts are not very satisfying.

The theory we're discussing is inflation--a theory that makes several predictions. Strong preliminary evidence for one of its predictions was announced yesterday. That strengthen's the credibility of its corollary predictions. You can't pick and choose among the predictions. It's all one piece. Max Tegmark on the Randi forum:

"Thanks for raising this important question of whether parallel universes are science or mere speculation! I often hear people assert that it's obviously the latter if parallel universes are unobservable. In my opinion, the question is more subtle.
Most multiverse arguments involve what logicians know as “modus ponens”: that if X implies Y and X is true, then Y must also be true. Specifically, ones argue that if some scientific theory X has enough experimental support for us to take it seriously, then we must take seriously also all its predictions Y, even if these predictions are themselves untestable (involving parallel universes, for example). In other words, ones argues that parallel universes are not a scientific theory, but prediction of certain scientific theories, which are testable if they make additional predictions that we *can* observe. In my book, for example, I argue that there are four implications:

1) Cosmological inflation generically implies Level I multiverse
2) Inflation + string landscape generically implies Level II multiverse
3) Unitary quantum mechanics implies Level III multiverse
4) The Mathematical Universe Hypothesis implies Level IV multiverse

So to argue that some particular type of parallel universes is unscientific, one needs to either make the case that the corresponding implication is incorrect, or that one of the theories (inflation, say) makes no testable predictions and is therefore unscientific."

WayneFrancis
2014-Mar-19, 01:06 AM
As of today, we do have evidence. Inflation posits an infinite universe.

What evidence is there that says the universe is infinite? We can put a lower bounds on the size of the universe but this doesn't equate to an infinite universe. We have no evidence that the universe is finite and unbounded compared to infinite and unbounded.

BetsyJ
2014-Mar-19, 06:57 AM
What evidence is there that says the universe is infinite? We can put a lower bounds on the size of the universe but this doesn't equate to an infinite universe. We have no evidence that the universe is finite and unbounded compared to infinite and unbounded.

Eternal inflation predicts an infinite universe. What more can I say?

Here's an analogy:

The theory of gravity predicts that two bodies attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. Once you've established that, you can predict how planets orbiting a distant star will move. You don't need evidence for the prediction--just for the theory.

Likewise the theory of inflation (which is now supported by evidence) makes certain predictions. You can't admit the theory, but then reject its predictions. It's as simple as that.

I'm sure some people will continue to argue that inflation is bogus and the universe is finite. But it's a much less tenable position if the discoveries announced yesterday are verified by other astronomers.

Shaula
2014-Mar-20, 06:57 AM
Actually in the quote you gave it predicted and infinte multiverse which is quite different.

Infltation theory itself is neutral on the finite or infinite nature of the universe - it is the eternal inflation bit (where each inflationary event can be thought of as creating a new universe and inflationary events happen faster than they decay) that implies an infinite multiverse, and still it does not imply that the individual universes are infnite since individual universes need not expand for ever and inflation is not eternal for them.

BetsyJ
2014-Mar-20, 11:53 PM
I see your point, Shaula. I'll try to be careful of my terminology from now on.

But there are several different versions of inflation, and they predict slightly different things. Some cosmologists refer to Level I and Level II multiverses. (There are also Levels III and IV, but let's not go there.)

@WayneFrancis:

There is evidence.

Speaking just of the dimensions of our unique universe, my understanding is that some measurements of the CBR and BOSS indicate that the space we live in is infinite and that matter is spread randomly throughout it. Here's what one astronomer said about those results:

"One of the reasons we care is that a flat universe has implications for whether the universe is infinite," says Schlegel. "That means – while we can't say with certainty that it will never come to an end – it's likely the universe extends forever in space and will go on forever in time. Our results are consistent with an infinite universe."

http://www.bnl.gov/newsroom/news.php?a=11601

Obviously, not all astronomers agree.

Our universe may be infinite. We can only see those parts from which light has had time to reach us since the beginning of the universe, so I agree with ProFloater. But I DO think there's now evidence for the multiverse, as stated above.

BetsyJ
2014-Mar-21, 12:58 AM
No, it is the case that we simply don't have a working model that lets us 'go back in time' far enough. Current theory says that the observable universe expanded from a finite sized hot dense state a finite time ago. It does not cover where that state came from, what was going on outside the volume that gave rise to our neighbourhood or what the universe was doing prior to the point where our current models are able to describe it.

Dense, yes. But hot? Heard Andrei Linde say that it's no longer "hot," but few people seem to have noticed! Hard to believe.

WayneFrancis
2014-Mar-21, 03:18 AM
Eternal inflation predicts an infinite universe. What more can I say?

Here's an analogy:

The theory of gravity predicts that two bodies attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. Once you've established that, you can predict how planets orbiting a distant star will move. You don't need evidence for the prediction--just for the theory.

Likewise the theory of inflation (which is now supported by evidence) makes certain predictions. You can't admit the theory, but then reject its predictions. It's as simple as that.

I'm sure some people will continue to argue that inflation is bogus and the universe is finite. But it's a much less tenable position if the discoveries announced yesterday are verified by other astronomers.

Infinite in time and in the infinite future it would be infinite but at every point in time before then, which is infinite, it could be finite in size even with eternal inflation.

IE the size of the universe is a function like f(x). Where x is the time since the big bang. While x is a actual point in time f(x) will be finite. Just as 2x = y where y is finite as long as x is finite. Only when you plug x = ∞ then yes y = ∞ but that isn't a real number.
So right now the universe may or may not be infinite. Just because there appears to be eternal inflation doesn't mean that the universe right now is infinite.

WayneFrancis
2014-Mar-21, 03:22 AM
I see your point, Shaula. I'll try to be careful of my terminology from now on.

But there are several different versions of inflation, and they predict slightly different things. Some cosmologists refer to Level I and Level II multiverses. (There are also Levels III and IV, but let's not go there.)

@WayneFrancis:

There is evidence.

Speaking just of the dimensions of our unique universe, my understanding is that some measurements of the CBR and BOSS indicate that the space we live in is infinite and that matter is spread randomly throughout it. Here's what one astronomer said about those results:

"One of the reasons we care is that a flat universe has implications for whether the universe is infinite," says Schlegel. "That means Ė while we can't say with certainty that it will never come to an end Ė it's likely the universe extends forever in space and will go on forever in time. Our results are consistent with an infinite universe."

http://www.bnl.gov/newsroom/news.php?a=11601

Obviously, not all astronomers agree.

Our universe may be infinite. We can only see those parts from which light has had time to reach us since the beginning of the universe, so I agree with ProFloater. But I DO think there's now evidence for the multiverse, as stated above.

Exactly it may be infinite and it may be finite. We'll probably never know if it is infinite just by the nature of infinity but we do know that if it is finite that it is orders of magnitude larger then the observable universe. For the most part it doesn't matter because most of the universe is not causally connected to us.

WayneFrancis
2014-Mar-21, 03:27 AM
Dense, yes. But hot? Heard Andrei Linde say that it's no longer "hot," but few people seem to have noticed! Hard to believe.

Or perhaps no useful research has come of it. From what I've read he says it was hot, became super cooled, then via a phase transition became hot again. It doesn't change the observations that support the early universe being in a hot dense state followed by a rapid inflation which, via thermodynamics, causes it to cool. This evidence is in the form of the CMBR, element ratios, and other lines of evidence.

Shaula
2014-Mar-21, 06:46 AM
Dense, yes. But hot? Heard Andrei Linde say that it's no longer "hot," but few people seem to have noticed! Hard to believe.
He was specifically talking about the period at the end of inflation. Prior to that point it was required to be at a high enough temperature that the false vacuum was stable.

Edit: And I will apologise if my answers are terse in this thread - multiverse theories are generally highly speculative and based on extrapolation of models we don't even really have yet. So I say little about them because I don't think there is much you can say that is meaningful, in scientific terms.

ShinAce
2014-Mar-21, 12:29 PM
By the way, physicists have a habit of using temperature and energy interchangeably.

We give particle masses in units of energy, and state temperatures in particle accelerators in terms of energy. We really really like energy!!

Cougar
2014-Mar-21, 02:17 PM
So to argue that some particular type of parallel universes is unscientific, one needs to either make the case that the corresponding implication is incorrect, or that one of the theories (inflation, say) makes no testable predictions and is therefore unscientific."

Support for inflation has been growing, slowly, since Guth came up with it. Some corrections were made from Guth's original version. The recent B-mode polarization finding, hinted at since the early WMAP results, provides some significant support. But AFAIK, there is no logically solid "corollary" that inflation implies "other universes," parallel universes, many world universes, fecund universes, or multiverses. Some well-known theorists have speculated that there might be, since this would explain anthropic issues and our so-called fine-tuning problem. But the inflationary era of our universe - the only one we know, or can know - provides no real support that there are other universes beyond our own.

BetsyJ
2014-Mar-21, 02:57 PM
@Shaula: Terse is good.

Though these topics have a way of moving from speculative to testable over time.

@Cougar: That wasn't my quote--it was Tegmark's. Depends on what you mean by "real support." Plenty of physicists and interpreters of physics now find the support for multiverses compelling. These include Carroll, Greene, Krauss, and others. As Krauss says in the link below, if we have a theory that makes 25 testable predictions, all of which have been confirmed and are causally connected--and there's a 26th prediction which hasn't been confirmed--you have to take the 26th prediction seriously.

https://www.youtube.com/watch?v=JF-Lbfksysk

But I get what you mean. (Personally, I have real trouble with multiverses. They make a farce out of human existence.)

George
2014-Mar-21, 03:52 PM
…As Krauss says in the link below, if we have a theory that makes 25 testable predictions, all of which have been confirmed and are causally connected--and there's a 26th prediction which hasn't been confirmed--you have to take the 26th prediction seriously.

His speech comes across as tasteful word salad (ignoring his anti-religious views and anti-Republican snipe). But where’s the beef? This was the original question asked of him (i.e. objective evidence). He chose not to present a single testable prediction of his pet theory but argued just how powerful theories can be, which many are. Duh! The question asked, however, was whether or not the multiuniverse “theory” has any predictions. If he has specific testable predictions, then that was the time he should have stated them. He did not.

He also argued that our universe came from nothing and seem to exclude the idea that the other universes had anything to do with it. How would we know that?

BetsyJ
2014-Mar-21, 04:18 PM
@George -- Uh, no. The exact question was, "Do scientists have evidence that is suggestive of the multiverse?"

To which, Krauss answered, "Yes."

He obviously only had a few minutes to answer, so he did what he could in three minutes.

And he didn't get into the question of whether the other universes had "anything to do with it." He just mentioned that current theories suggest that spacetime, specific to our universe, was created at the universe's inception.

George
2014-Mar-21, 06:16 PM
@George -- Uh, no. The exact question was, "Do scientists have evidence that is suggestive of the multiverse?"

To which, Krauss answered, "Yes."

He obviously only had a few minutes to answer, so he did what he could in three minutes. Ok, so we just need to trust him on it? Yet, this is science not faith, and my concern is that he should have treated it in a manner consistent with scientific argument.

He did not site evidence because, as far as I know (though I could be wrong), there is no solid objective evidence for it. To argue in favor of a theory should mention objective evidence that is the foundation for the theory -- a requirement for all scientific theories-- and the theory needs to present claims that are objectively testable at least in principle. Tegmark is a brilliant mathematician and there is a lot of mathematical elegance, apparently, to much of what he says. He is one of the leaders, I think, in the parallel universe "conjenctures". It's lunchtime, however, and I'd rather have a hamburger than a beautiful painting of a steak. Where's the beef?

A theory requires more than faith. So, his answer should have been "No" to the question assuming that mainstream science has no clear objective evidence substantiating the muliverse "theory". So what is the correct answer he should have given? We don't know since he left out the evidence for it.


And he didn't get into the question of whether the other universes had "anything to do with it." He just mentioned that current theories suggest that spacetime, specific to our universe, was created at the universe's inception. It is implied because of the question asked, namely "...you argue that the Big Bang sprang from nothing, but it can't if there are multi universes". Tegmark responded with "Certainly it can. There was nothing there...". In other words, he believes that Big Bang was from nothing AND independent from the other universes. So the exclusion is more than just implied. This is important to his "theory" since it suggests there is no need for causation, apparently. But how much is hard science and how much of it is limited to math? [I am hopeful that great things will come of the amazing work he and others have done, including String Theory, since it may lead us to great places in the future.]

BetsyJ
2014-Mar-21, 06:39 PM
I see what you're saying, George. I just don't agree about what constitutes evidence. As I mentioned above, I'm in good company.

(Do you mean Krauss? The speaker isn't Tegmark.)

A "pocket universe," which could exist in another dimension but in any case is totally inaccessible to ours, has nothing to do with and can be said to be theoretically independent of our universe.

I'm sorry you're uncomfortable with the idea that something can have no cause. But particles, and apparently universes, CAN pop into existence without one. Also, it isn't "his" theory, it's the consensus theory at this point. And there IS observational data, as well as math, to back it up.

I'm not here to convert you. Have a nice day!

Ken G
2014-Mar-21, 06:57 PM
Tegmark argued:


"1) Cosmological inflation generically implies Level I multiverse
2) Inflation + string landscape generically implies Level II multiverse
3) Unitary quantum mechanics implies Level III multiverse
4) The Mathematical Universe Hypothesis implies Level IV multiverse

So to argue that some particular type of parallel universes is unscientific, one needs to either make the case that the corresponding implication is incorrect, or that one of the theories (inflation, say) makes no testable predictions and is therefore unscientific."
Tegmark's argument is a logical fallacy, but it's one that is quite common among theoretical physicists. They should know better by now, but they don't! The fallacy of Tegmark's logic is that he leaves out an important possibility from his limited two-choice scheme: that a physics theory which makes many testable predictions that do pan out, and is selected as good because of them, can also make other predictions that have never been tested because they are either of a different type than the ones we can actually test, or they exist in regimes (like high energies or small size scales or time scales) that have not been tested, and those predictions might not have any direct connection to anything that has already been tested. When that is true, the reasons we adopt the theory are divorced from the new predictions, and we can have no confidence in those predictions.

When have we ever seen examples of this in the history of physics? Actually, the question to ask is when have we ever not seen examples of this, it is a very obvious aspect of how physics works, this is perfectly clear to any student of its history! Yet Tegmark does not even include that as one of his choices-- he does not include the one thing that has always been true of all physics theories to this very day.

It is easy to show that this possibility is left out of Tegmark's scheme, because he is saying that we should require evidence that an extrapolation of a current successfully tests theory to a regime in which it has never been tested is failing, or else we should always adopt said prediction as a kind of default expectation. But this is an incorrect way to regard a physics theory, and a gedankenexperiment involving any such theory. The purpose of a gedankenexperiment is to understand a theory, not to understand reality. There is always just one way for a physicist to understand reality: put the question to an experiment. That's it, that's what separates physics from being a belief system. So when we say something like, modern inflation theory predicts that inflation will tend to continue happening everywhere except in bubbles that are mutually unobservable, our reaction should be, "that's an interesting element of that theory, I wonder if it's true or not?" And then to answer that latter question, we must find a way to test it. If we have no way to test it, then it never goes beyond the question, "I wonder". Anyone who thinks we should take it as a kind of default expectation really should have a look at the history of physics. (Yes, physics theories can make assertions beyond what has been tested already, and we have discovered many new things because a theory predicted its existence, but the winners write the history on that-- we still have no idea until we actually discover it.)

BetsyJ
2014-Mar-21, 07:20 PM
Maybe. Just because one of the predictions hasn't yet been validated--that doesn't negate the theory.

And, yes, I do believe that as a theory is increasing validated and substantiated by other evidence, further predictions are strengthened as well. That's what happened with GR. (Not that modifications aren't possible.)

Also, it's not just one theory that suggests we live in a multiverse (listen to Krauss in the link above). Personally, I'm not proficient enough to evaluate all the evidence, or lack thereof. And I have to agree, as you point out, that we've heard this tune before. :)

Nobody's saying this is set in cement.

However, we've also heard another tune. And that's the one where the pubic refuses to accept an inconvenient truth (Copernicus anyone?) because it has/had unpalatable implications.

Ken G
2014-Mar-21, 07:54 PM
Maybe. Just because one of the predictions hasn't yet been validated--that doesn't negate the theory.
But the question is not if the theory is "negated" or not. All theories "negate" themselves, they are theories. I really have no idea where comes this common idea "maybe this one is actually true." That's just not what physics theories are, they are just not true, in the sense that every one of their ramifications can be taken to be literally correct. Of all the most unscientific modes of thought I can think of, I can think of none more unscientific than imagining that something that has always been the case will suddenly not be the case this time around.


And, yes, I do believe that as a theory is increasing validated and substantiated by other evidence, further predictions are strengthened as well. That's what happened with GR. (Not that modifications aren't possible.)Of course GR will be modified, there are almost no working physicists who think GR "got it right the first time." Maybe it did, but it would come as a huge surprise to almost everyone. I merely extend that same thinking to all physics theories, including inflation, and quite frankly I'm not sure why everyone doesn't do that, it seems a pretty clear implication of how science works.

What this means is, if inflation is a good theory, then like all good theories, that means two things:
1) it helps us understand what happened early in our universe. It doesn't mean that "really happened," nor does it mean that humans a thousand years from now might not have a completely different way of thinking about the whole issue. This is just a little hard for people to get their heads around, for some reason. I believe they just don't want to, which reminds of other modes of thought that have no business being used in science.
2) it helps us know what to look for to test the theory, it motivates the next layer of observations or experiments, that will reveal the next thing we are ready to know. This is a very important part of science, we don't always know what to look for, and we can't just look for everything, especially as the experiments become very expensive.

So inflation may well do both of those things, which will place it among the great theories of physics. But none of those theories were ever meant to be literally correct, and none of them ever were. That's just not what correctness is in science. There is not one single theory of physics that we have today that a majority of physicists believe is exactly correct or literally true, so there is not one that is believed to be "what is really happening." (And note we cannot count string theory because it's not yet an actual theory, but it is probably the closest to what many physicists might believe is exactly correct, which is a telling fact when you consider that it is the farthest from being an actual predictive theory. What do we call a field where what is regarded as the most exact theory is also the farthest one from being testable? Not science, that's for sure.)


Also, it's not just one theory that suggests we live in a multiverse (listen to Krauss in the link above). Personally, I'm not proficient enough to evaluate all the evidence, or lack thereof. And I have to agree, as you point out, that we've heard this tune before. The real problem is, there isn't evidence to evaluate in the first place. There is only evidence that inflation is a good theory in the two ways I listed above, that's the whole story for any physics theory-- there just isn't any other evidence, that's what science produces in the way of evidence of a good theory.


However, we've also heard another tune. And that's the one where the pubic refuses to accept an inconvenient truth (Copernicus anyone?) because it has/had unpalatable implications.Yet we must be careful this sword is not turned back on ourselves. For centuries, science has been trying to get other modes of thought to release their grip on a kind of dogma of convenience, what is held to be right because it seems to make sense and is palatable, but science has had the authority to make that request for one reason and one reason only: it had produced evidence for doing so. Not evidence of some theory, but evidence of a fact that was inconvenient to the other mode of thought. That's it, that's what made it science. Not that it was something a theory suggested might be true, or even should be true (that's just another convenience of thought!) but rather that there was observed evidence for it. Copernicus' speculation about the solar system was not right because it made more sense, because it didn't make more sense to some, and even if it did that's exactly what you mean by conveniences of thought. Copernicus was right for only one reason: Galileo's observations showed that he was right. (And as should come as no surprise, he was wrong about a number of things, but the core idea had great value: that the Earth was like the other planets, and the Sun like the other stars.)

George
2014-Mar-21, 08:27 PM
I see what you're saying, George. I just don't agree about what constitutes evidence. As I mentioned above, I'm in good company. There is subjective evidence and objective evidence. Religion and philosophy are fine with subjective-only evidence, but the more objective claims to any philosophical argument the stronger, or weaker, it becomes as those claims are tested. The differences between the two are critical to understanding what science is. Hypotheses and theories do include subjective elements -- they are ideas after all -- but it is the objectivity of those claims that allows them to be useful in validating any scientific hypothesis or, if broad, a theory.


(Do you mean Krauss? The speaker isn't Tegmark.) Eek!. Thanks, though their views are not that different, I think.


A "pocket universe," which could exist in another dimension but in any case is totally inaccessible to ours, has nothing to do with and can be said to be theoretically independent of our universe. That is a fine philosophical idea. I have not seen, directly or indirectly, any pocket universes, however, so I remain scientifically skeptical. Back to the original video question asking for evidence, where is it if not in faith only? Whenever you, or I, state that something is "totally inaccessible", it suggests that it may be unobservable. If so, we are talking metaphysics or pseudo science. Nevertheless, it is possible that the mathematics that generates these ideas will prove useful in dealing with the world we can observe. For about 1,500 years, the Aristotle/Ptolemy/Thomist Geocentric model was a mathematical construct that proved useful. The heliocentric model wasn't much better, if any, though it was more elegant by far.


I'm sorry you're uncomfortable with the idea that something can have no cause. Is it not a better reason than most to exercise normal scientific scrutiny?


But particles, and apparently universes, CAN pop into existence without one. If this is so, then why does quantum mechanics work so well, though QM is not based on any extra-universe "theory", or am I wrong?


I'm not here to convert you. Have a nice day! Your questions and arguments are more constructive than you likely realize. Keeping science clean and viable as true science is being misrepresented and it is important that we all learn what science is and, more importantly, what it is not. Please consider me appreciative of all your posts, because I am. [I'm certainly no arbiter, so these dialogues are helpful to me.] Nothing I throw should impact you more than an un-rotten tomato. :)

George
2014-Mar-21, 08:44 PM
Of course GR will be modified, there are almost no working physicists who think GR "got it right the first time." Maybe it did, but it would come as a huge surprise to almost everyone. Indeed, I wonder how many times Einstein had to correct it? The race to correct his own math is an interesting story.

BetsyJ
2014-Mar-21, 09:42 PM
Let’s review the indirect evidence for multiple universes.

(For those of you who already know all this, I’m sorry to belabor the obvious. I guess I’m just tired of hearing that there’s “no evidence.”) Yep, Ken, nothing here is a smoking gun. But if you want to kick the crutch out from our understanding of the universe, you have to have some idea of what you could replace it with.

1. OBSERVATIONS (see the experiment I cited above.) Observations indicate that space is flat rather than donut-shaped or spherical. If it’s flat it most likely stretches out to infinity. Why? Because what’s to stop it? A wall?

But if space-time goes on forever, then it must start repeating at some point, because there are a finite number of ways particles can be arranged in space and time.

And if you look far enough, you would encounter another version of you — in fact, infinite versions of you.

Here’s where it gets downright silly.

That means somewhere out there, Ken G. has just stuffed a banana in his ear and is running down the street screaming, “Booga, booga, booga!”

And in another universe, George has done the same thing, only he’s screaming, “Wooga, wooga, wooga!”

(I wish I were kidding, but this kind of thing is one reason some of us find multiples so unnerving.)

2. MATH: The mathematics of quantum mechanics describe the world in terms of probabilities, rather than definite outcomes. Physicists are divided about what this mean. Some believe that all possible outcomes of each situation occur — in their own separate universes. For example, if you reach a crossroads where you can go right or left, the present universe gives rise to two daughter universes: one in which you go right, and one in which you go left.

I won’t go into the Copenhagen interpretation, which is the other way of looking at the math. Just to say, in many ways--to a lot of people, it’s even more unpalatable.

3. FINE TUNING: There’s evidence that the universe is fine-tuned. But what does this mean? Imagine that I choose a particle, somewhere in the universe, and tell you that you have to find it. This could be on Mars–it could be in the middle of the sun, it could be in somewhere in the Andromeda galaxy or in the middle of a another galaxy too far away and too dim for us to see with out telescopes.

Just think of all those particles out there. And you have to find that one particle.

Your odds of finding that one particle are about the same as the odds of living in a universe with the fundamental constants tuned to the extent that they appear to be tuned in ours.

Well, you can believe that God did it. (George are you paying attention?)

Or you can believe that our universe isn’t special. Instead there are universes out there in which every permutation of those constants is realized. We couldn’t have been born in any of those other universes–because they aren’t hospitable to life. So here we are.

It defies credulity to think that a single universe would come into being and that universe would just happen to have the very precise properties needed to develop life ... or, for that matter, to develop anything.

Hey, don’t believe me. Martin Reese wrote extensively about this idea in his classic book Just Six Numbers: The Deep Forces That Shape the Universe.

4. STRING THEORY. String theory, which explains many features of our universe which otherwise puzzle cosmologists, includes a structure to think about where these other universes could be located. The reason physicists like it is that string theory smooths out the mathematical inconsistencies that currently exist between quantum mechanics and the theory of relativity.

5. GRAVITATIONAL WAVES -- And now we have the icing on the cake. The evidence for inflation, which implies multiple (or pocket) universes. I dunno. Maybe it's not conclusive, but it sure seems more than coincidental.

All of this suggests that there are multiple universes. Concrete evidence? No. But all the arrows are pointing in the same direction.

antoniseb
2014-Mar-22, 01:36 AM
...But if space-time goes on forever, then ...
Do you see the place where you are trying to apply some gut-feeling common sense to a situation that might be outside the scope of this kind of reasoning?
As a simple example of a gap in your post above: We have a lot of improving on our measurements to do before we can say that the universe is really flat, and not just kinda flat where we can see it (among other things).
This doesn't mean there aren't multiple universes. This doesn't mean our universe is finite. This doesn't mean our universe is infinite. So far we don't know. We have new clues. That's good news.

George
2014-Mar-22, 04:18 AM
1. OBSERVATIONS (see the experiment I cited above.) Observations indicate that space is flat rather than donut-shaped or spherical. If it’s flat it most likely stretches out to infinity. Why? Because what’s to stop it? A wall? You seem to respect Martin Rees, so perhaps his words from the book you cite can assist us...

"...we may be mistaken in thinking that our universe extends uniformly without limit."

Only philosophy and scientific supposition takes us beyond the observable universe.


And in another universe, George has done the same thing, only he’s screaming, “Wooga, wooga, wooga!” That's just silly; I do that now.


2. MATH: The mathematics of quantum mechanics describe the world in terms of probabilities, rather than definite outcomes. Physicists are divided about what this mean. Some believe that all possible outcomes of each situation occur — in their own separate universes. For example, if you reach a crossroads where you can go right or left, the present universe gives rise to two daughter universes: one in which you go right, and one in which you go left. Vegas thrives on their favorable probability at their tables. But it is impossible to win at a table no one can find.


I won’t go into the Copenhagen interpretation, which is the other way of looking at the math. Just to say, in many ways--to a lot of people, it’s even more unpalatable. Ok, besides I heard someone say it is bohring.


3. FINE TUNING: ...

Well, you can believe that God did it. (George are you paying attention?) Yep, I like to catch tomatoes, too. This would be another non-scientific approach to the question. The "why" behind the remarkable fine tuning is not science. The "hows" are worth pursuit.


It defies credulity to think that a single universe would come into being and that universe would just happen to have the very precise properties needed to develop life ... or, for that matter, to develop anything. That is a fair philosophical argument.


Hey, don’t believe me. Martin Reese wrote extensively about this idea in his classic book Just Six Numbers: The Deep Forces That Shape the Universe. Here's another wise statement he makes: "The fundamental question of 'Why is there something from nothing?' remain the province of philosophers."


All of this suggests that there are multiple universes. Concrete evidence? No. But all the arrows are pointing in the same direction.We are only shooting ourselves if the universe is just barely closed with our without other universes, apparently.

Ken G
2014-Mar-22, 05:04 AM
This doesn't mean our universe is infinite. So far we don't know. We have new clues. That's good news.And to follow onto that, let's say we do improve our instruments, and the universe gets even flatter. We now have even better evidence that inflation is a good theory of physics. But what does that mean, what is a good theory? Just look at this list of good theories:
1) Ptolemy's geocentric model
2) Newton's laws of motion
3) Bohr's model of the atom
4) Einstein's model of gravity
What do these all have in common? They all answer questions, they all make predictions that are borne out, they all motivate the next generation of observations-- and they are all wrong (most likely for GR anyway). It is a mistake to even think their purpose was to be right, their purpose was just to be good theories. I don't understand why we still haven't learned that lesson.

Let's stick to inflation, because that's what this thread is about. What do we know? We know that flatness is a prediction of inflation, so the theory motivates us to look for any deviation from flatness. That's good, that's what theories are supposed to do, to tell us what to look for. If we never see any deviations, what can we conclude? We can conclude that we can understand the flatness using inflation. Does that mean we can take literally every aspect of the inflation theory?

Let's look at Newton's laws of motion to answer that. When Newton devised what was obviously a good theory of motion, powerful and predictive, people noticed that it was deterministic. So they made the usual mistake. What they did was say "the universe is deterministic, what does that mean about our place in the grand scheme" and so on. What they should have done instead was remember what science is, and say "that's interesting, this theory is deterministic, I wonder what that tells us about reality that we should be looking for," and used that question to motivate more and more experiments to test determinism. Had they done that, we might have had quantum mechanics sooner than we did-- but we didn't kick that "crutch" (as BetsyJ correctly labeled it, though for different effect) for over three more centuries, because we forgot what science is.

So what does that mean about inflation? It means that even if our observable corner of the universe is very flat, it doesn't require that the whole universe be that. It just requires that we need a theory to explain why our corner is flat, and inflation can do that. But if we forget what science is, we make the same mistake all over again, and think that our theory is telling us something about reality, instead of letting observations tell us about reality and let theories help us understand that.

The list of this kind of thing just goes on and on in physics, I'll just give one more example because I think it is particularly telling. In Newton's time, there was a lot of understanding of particles and waves (largely because of Newton!). So it was natural to wonder if light was a particle or a wave. It refracted, which sounded pretty wavelike to a lot of people, but Newton was convinced it was a particle (I presume because he expected everything to be a particle, given the success of his particle theory-- sound familiar?). So he developed a theory whereby a particle could refract, and it worked so well that his idea won the day-- light was a particle. The problem was, his theory required light particles to speed up in a medium, so they could experience a force that would make them refract. The theory worked so well that no doubt Newton thought it was true (and most everyone else too), but when the technology got good enough to do the observations, it was found that light slows down in a medium, it doesn't speed up! So the light particle was trashed, and light was now a wave. That continued until Einstein's photoelectric effect showed that light was actually a particle after all, but obeyed the laws of motion of a wave, not Newton's theory.

So, was Newton right after all, since light did turn out to be a particle? Who cares, it's not the point of science to happen to be right, and that's a moving target anyway. We should never have thought that Newton was trying to say what light really is or what light really does, because that's just not the role of a physics theory. But we just keep forgetting that, over and over, so we make the same mistake that believers in Newton's theory of light made. Why do we want physics to be a means of arriving at a dogmatic approach to truth, when science is all about not being dogmatic about truth?

Ken G
2014-Mar-22, 05:11 AM
All of this suggests that there are multiple universes. Concrete evidence? No. But all the arrows are pointing in the same direction.But so what? What the arrows point at is where to look next, that's all arrows ever do in science. When they point at "truth", you just aren't doing science any more, you are doing dogma.

Cougar
2014-Mar-25, 01:22 PM
And now we have the icing on the cake. The evidence for inflation, which implies multiple (or pocket) universes....

To me, this appears to be a non sequitur. How do you get from the premise to the conclusion? BTW, I don't argue against multiple universes because I don't like the idea or I find it "inconvenient." What do I care? I'm just skeptical of any theory before I see or hear of some evidence or logical implication supporting it. And having taken a rather rigorous course in logical implication, I'm also fairly skeptical of claims that one thing logically implies another - until the steps are spelled out and the conclusion is seen to be a valid consequence.

Ken G
2014-Mar-25, 08:38 PM
In my opinion, the success of theory A to explain observation X never leads to the logical conclusion that theory A is what is actually happening. I think if there is any one thing about the entire history of scientific theory that is perfectly clear, it is that.