Is Paul Steinhardt's Statement "Rather Pathetic"? Why?

[salvestrom]

Thinking on the topic's title: accidental versus...?

I read the article about non-uniformity in the CMB. Anyone know if they've looked at that spiffy knew pink one we've got now?

I also looked up wave-function collapse. The wikpedia article general says that the Copenhagen Interpretaton is the standard one, but contains no direct mention of what happens to the other probabilities. Many worlds is an extension of that and then, further down it mentions Many Minds. There were nearly a dozen, in all. I've been left with the clear impression that the nature of the wavefunction is quite open to debate and is not unambiguous evidence of a multiverse.

http://www.nature.com/news/quantum-theorem-shakes-foundations-1.9392

The above link (gotta love the over-stated title and leading line) has a further link to a preprint of the paper. They are in agreement with challnoth's view as far as I can tell.

I think I'm sort of with Einstien. He considered the wavefunction statistical, but with an as yet undiscovered underlying reality. My own personal view is the same. I view the wavefunction as a desription of something real, but that what it describes isn't quite what we think it to be.

 

[Chalnoth]

But I can directly detect it's presence, for example, viz. a track in a cloud chamber.

No. What you can do is test the predictions of a model of the electron. Many of the models which include a multiverse are testable in the exact same way.

There is a theory A that makes predictions B, C, and D. I've collected the appropriate data and verified, to some degree of significance, that B and C are true. I have no data to verify D. But, since it is predicted by the same theory that predicts B and C, and since I have adequate data to support predictions B and C, then D is true. Sort of like "true by association." I'm sorry Chalnoth, but I don't buy it. And I don't think Francis Bacon would either. Or really any empiricist for that matter.

This is the normal way that science is done. Every single prediction of a theory cannot possibly be verified. So we test what we can, and the more ways that we can test a theory, the more confident we are in its overall correctness.

With models of the universe that include a multiverse, even if it turns out to be impossible to observe the multiverse component of those models directly, there are most definitely other aspects of those models that can be tested. And even just the standard model plus unitary quantum mechanics makes a multiverse nearly certain.

EDIT: That's not to say that such a situation shouldn't compel one to strongly suspect the validity of D. Your allusion to the transitional fossils made earlier is an example. Yes, given the success of evolutionary theory and its sound logical framework, many pieces of which have been verified scientifically, it is especially likely that transitional fossils should exist (and they do, as I think we all know, but sake of argument here.) However, such a strong suspicion does not abdicate the scientist from his responsibility of finding them. Their absence in no way invalides the theory -- an argument I think you suspected I was making. I was not. But they do not become objective reality simply because they really, really should be there.

And there are many scientists working on creative ways to test for the impact of various multiverse ideas more directly. I don't hold much hope that this will prove to be fruitful. But it does, at least, avoid the useless work of trying to avoid such multiverse hypotheses as a matter of principle.

 

[Chalnoth]

But you apparently have just the right amount of sympathy for those that would argue for a vast and wasteful proliferation of alternate universes.

Why is it wasteful? What does that even mean?

It's exactly this knee-jerk reaction that I have such a big problem with. No matter which way you slice it, multiverse theories require fewer assumptions. It is easier for a theory to be prolific than not. Demanding that the default hypothesis be proven is ludicrous. The default hypothesis needs to be disproven, not proven.

 

[bapowell]

This is the normal way that science is done. Every single prediction of a theory cannot possibly be verified. So we test what we can, and the more ways that we can test a theory, the more confident we are in its overall correctness.

But you still haven't said why you've chosen to accept many-worlds over other interpretations, outside of from what I can tell are purely aesthetic reasons. Unless you have some Bayesian prior on your model space that you aren't telling anyone about.

EDIT: This discussion has actually been helpful because I understand now what we disagree about. It's not so much about the implied correctness of untested predictions or axioms. It's about your staunch acceptance of a version of quantum mechanics based solely on its relative simplicity that I'm not quite in agreement with. Occam's razor is an indispensable guide for selecting the most favored model out of a bunch; but it does not exclude those models it doesn't select.

 

[skydivephil]

But I can directly detect it's presence, for example, viz. a track in a cloud chamber.

It looks like you have your own version of how induction is done, which differs significantly from my view. You seem to be saying the following:

There is a theory A that makes predictions B, C, and D. I've collected the appropriate data and verified, to some degree of significance, that B and C are true. I have no data to verify D. But, since it is predicted by the same theory that predicts B and C, and since I have adequate data to support predictions B and C, then D is true. Sort of like "true by association." I'm sorry Chalnoth, but I don't buy it. And I don't think Francis Bacon would either. Or really any empiricist for that matter.

EDIT: That's not to say that such a situation shouldn't compel one to strongly suspect the validity of D. Your allusion to the transitional fossils made earlier is an example. Yes, given the success of evolutionary theory and its sound logical framework, many pieces of which have been verified scientifically, it is especially likely that transitional fossils should exist (and they do, as I think we all know, but sake of argument here.) However, such a strong suspicion does not abdicate the scientist from his responsibility of finding them. Their absence in no way invalides the theory -- an argument I think you suspected I was making. I was not. But they do not become objective reality simply because they really, really should be there.

I think your example above goes back to what I was saying, that really there is a grey area between what is science and not science.
So let's suppose theory A predicts B, C and D. As you say B and C have veen verified. Should we accept D as true without verififcation? I think I would agree that we should not accept it to the same extent as we accept B and C. However neither should we classify it as the same level of non science as something silly like creationism.

Take gravity waves for example, although there has been indirect evidence from binary pulsars there has never been a direct detection despite LIGO being operational for something like 10 years (?). Now let's suppose the pulsar observation had not been made, what should we say about gravity waves? Well I think they would be in this grey area, they are precited by GR and Gr si well verified. Not somehting silly like creastionsim, but neither somehting verified such as time dilation.

 

[Chalnoth]

But you still haven't said why you've chosen to accept many-worlds over other interpretations, outside of from what I can tell are purely aesthetic reasons. Unless you have some Bayesian prior on your model space that you aren't telling anyone about.

EDIT: This discussion has actually been helpful because I understand now what we disagree about. It's not so much about the implied correctness of untested predictions or axioms. It's about your staunch acceptance of a version of quantum mechanics based solely on its relative simplicity that I'm not quite in agreement with. Occam's razor is an indispensable guide for selecting the most favored model out of a bunch; but it does not exclude those models it doesn't select.

1. Many-worlds makes more predictions as to how the universe behaves. Specifically, it makes definite predictions about the nature of collapse. Most other interpretations sweep the nature of collapse under a rug and make no predictions at all about it. Considering that the nature of collapse is becoming more and more important as we try to take advantage of quantum mechanics for computing, this really is an essential feature and can no longer be considered up to personal choice (not that the nature of reality ever was up to personal choice).
2. Many-worlds makes the fewest assumptions. I don't see how there can possibly be any argument about this point.

 

[Chalnoth]

Take gravity waves for example, although there has been indirect evidence from binary pulsars there has never been a direct detection despite LIGO being operational for something like 10 years (?).

This is kind of off-topic, but I just wanted to point out that the upgrade to advanced LIGO is now under construction, and is expected to be up and running somewhere around 2015. The sensitivity is expected to be great enough that it will be guaranteed to detect gravity waves from a number of known sources, barring some unforseen systematic errors.

 

[martinbn]

2. Discoveries in high energy physics point to the existence of spontaneous symmetry breaking, which would lead to different regions of space-time realizing different low-energy laws of physics.

Why? I mean, the spontaneous symmetry breaking does not say that there are universes where each vacuum state is realized. Or am I wrong? I just don't see how spontaneous symmetry breaking is related to many universes!

 

[marcus]

... No matter which way you slice it, multiverse theories require fewer assumptions. It is easier for a theory to be prolific than not. ...

I think this is mistaken. (BTW it's a claim you already were asserting in post#2)
The way I slice it, the appropriate question to be asking at this point is how did the "big bang" come about.
How did the expansion begin and why does it have the observed characteristics?

Bounce theories of how this happened seem to depend on fewer assumptions. They simply have the U extend back further in time, and be in a contracting mode. No different laws from those operating now.

Here's a current survey that briefly describes various approaches to understanding "big bang". It is an invited review for Modern Physics Letters:
http://arxiv.org/abs/1201.4543
Not all the approaches sketched here are "multiverse" and it seems to me some are simpler (as well as more testable.) So I don't think your claim stands.

 

[bapowell]

1. Many-worlds makes more predictions as to how the universe behaves. Specifically, it makes definite predictions about the nature of collapse. Most other interpretations sweep the nature of collapse under a rug and make no predictions at all about it. Considering that the nature of collapse is becoming more and more important as we try to take advantage of quantum mechanics for computing, this really is an essential feature and can no longer be considered up to personal choice (not that the nature of reality ever was up to personal choice).
2. Many-worlds makes the fewest assumptions. I don't see how there can possibly be any argument about this point.

OK. So as I understand it you favor many-worlds due to its parsimony and predictive strength relative to alternatives. This is precisely the way one would go about weighing the relative merits of competing statistical models. But at the end of the day, we are not considering statistical models; we are interpreting the candidate theories as representing objective physical reality. The more complicated, less predictive model may well be correct! Again, these considerations suggest a preference, statistically speaking, for the simpler model. I do not, however, think this is adequate to furnish the kind of certainty and correspondence to objective reality that you are advocating.

 

[marcus]

Brian, hope it's OK to interject. I think the main agenda here is to resolve the cosmological singularity and provide either for inflation or for a substitute mechanism.

Renaldi, in the invited review article I mentioned, mentions string cosmology, loop cosmology, Horava, Jacobson's Einstein-aether, and various others. He gives a brief historical account of the earlier attempts which preceded and led up to these approaches---particularly the first two. In one form or another, most of these involve a bounce.

You might want to glance at the relevant section, which is just 2 pages long. It is section 2 "Lines of Research" and begins on page 2. Here's an excerpt.
==Rinaldi review article http://arxiv.org/abs/1201.4543 page 3==
There are several other models that offer alternatives to the direct quantization of gravity. Recently, Horava has proposed a power-counting renormalizable theory of gravity, based on an anisotropic scaling at high energy ²⁰. Essentially, the fundamental hypothesis is that time and space do not scale in the same way, according to the scheme t → b^zt, xⁱ → bxⁱ, where z is called critical (Lifschitz) exponent and b is an arbitrary constant. By adding higher spatial curvature terms to the standard Einstein-Hilbert action, one can construct a model where, at high energy z ≥ 3, which makes the theory power-counting renormalizable, while at low energy z = 1. Local Lorentz invariance is preserved in the infrared (IR), and it is broken in the UV. The original formulation of this model suffered from un unwanted ghost scalar field, that persisted also in the IR ^21,22. To remove this anomalous degree of freedom one needs to add new terms in the action, that are basically formed by combination of a vector field, orthogonal to constant time surfaces, and its derivatives ²³. In this form, the Hoˇrava-Lifschitz theory becomes very similar to the “Einstein-aether” theory proposed by Jacobson many years before as a vector-tensor theory of gravity 24. Both theories offer non-singular solution to the cosmological equations ^25,26 and the horizon problem is solved without recurring to inflation ²⁷.
==endquote==

 

[Chalnoth]

Why? I mean, the spontaneous symmetry breaking does not say that there are universes where each vacuum state is realized. Or am I wrong? I just don't see how spontaneous symmetry breaking is related to many universes!

Well, there are two ways to look at this. One is that inflation strongly predicts that this and potentially other symmetry breaking events are only local effects, and that they will occur differently in far-away regions. The second is that whatever physical model you have for our early universe, it is highly unlikely that that physical model is a one-off event.

 

[Chalnoth]

I think this is mistaken. (BTW it's a claim you already were asserting in post#2)
The way I slice it, the appropriate question to be asking at this point is how did the "big bang" come about.
How did the expansion begin and why does it have the observed characteristics?

Bounce theories of how this happened seem to depend on fewer assumptions. They simply have the U extend back further in time, and be in a contracting mode. No different laws from those operating now.

Two points. First, I find these theories highly unlikely, due to the apparent reversal of entropy at the bounce. Second, even if this isn't a problem, there's still no reason whatsoever to believe it's a one-off event. You still have to assume it's a one-off event separately from the physical model.

A tangential point that I'd make is that the way high-energy physics is progressing, it is seeming increasingly unlikely that you could ever achieve the conditions for life with a model that only started one region of space-time with one set of physical laws.

 

[marcus]

Just to be clear, I am addressing the claim of simplicity or fewer assumptions that you made in post #2. It simply is not true.

It is your opinion that the 4 or 5 quantum cosmology approaches discussed briefly in that invited review article are "highly unlikely". Since it happens they are all bounce type. Your opinion could be right or wrong---this is not relevant.

You cannot rightly say that multiverse scenarios require fewer assumptions than other theories being studied that resolve the cosmo singularity.

I don't think you even know what the possible alternative theories are, so it is ridiculous to claim that multiverse theories need fewer assumptions than all the others.

Logically I think what you need to say is that in your opinion the approaches Renaldi covers in his review article (string cosmology, loop, Horava, Einstein-aether...) are "highly unlikely" and if these approaches are excluded then multiverse requires fewer assumptions than whatever theories you know of that resolve the initial singularity.

 

[Fuzzy Logic]

I think there is multiple meanings of the word multiverse being used here.
The problem I have with Steinahrdt's statement is the basis of his argument, in that he beleives the universe was not accidental. Other than that statement. I agree with the rest, in so much as to say that it's equally possible that galaxies are the largest structures or that there is possibly more than one isolated 'universe' existing at the same time.

I agree with Chalnoth in that the universe is not a singular event. More time and a bounce scenario can explain the astronomical probabilities for the conditions of life (as we know it!) just as well as more space.

 

[Chalnoth]

Just to be clear, I am addressing the claim of simplicity or fewer assumptions that you made in post #2. It simply is not true.

Except it is true. There is no possible way to have a unique universe without making that an extra, specific assumption in the theory. This is simply because any physical model of the universe which doesn't explicitly mention other regions of space-time also won't explicitly exclude them. It makes the theory more complex to exclude them. Always.

 

[bapowell]

It makes the theory more complex to exclude them. Always.

But theories are never ruled out on account of their complexity. This is where I disagree with your reasoning. You are essentially performing a Bayesian model selection on your space of competing theories. They all satisfy the data equally well, however, some have additional structure than others that make them either less predictive, more complex, or both. The Bayesian evidence disfavors these models, but it does not exclude them! This is an incorrect interpretation of the statistical method.

 

[marcus]

Except it is true. There is no possible way to have a unique universe without making that an extra, specific assumption in the theory. This is simply because any physical model of the universe which doesn't explicitly mention other regions of space-time also won't explicitly exclude them. It makes the theory more complex to exclude them. Always.

Chalnoth you are not making sense. A bounce cosmology theory does not have any statement in it which says some other bounce in some completely separate realm doesn't exist. It is just a theory whereby the universe that we know and observe, with its physical laws, contracted and rebounded (according to a quantum law of gravity to be tested) resulting in what we now see.

A scientific theory is supposed to explain observations and make testable predictions, this is what we apply the Occam criterion of simplicity to, and fit to data.
A bounce cosmology has no place for some grandiose philosophical speculation about some other completely disconnected realm. Makes no assertion either way.

this is how to get a really simple resolution of the initial singularity.

 

[martinbn]

Well, there are two ways to look at this. One is that inflation strongly predicts that this and potentially other symmetry breaking events are only local effects, and that they will occur differently in far-away regions. The second is that whatever physical model you have for our early universe, it is highly unlikely that that physical model is a one-off event.

But is not an answer to my question, where in the symmetry breaking is the need for many universes!

 

[salvestrom]

But is not an answer to my question, where in the symmetry breaking is the need for many universes!

Is it perhaps derived from not treating any given probability as special, so it is considered that they all playout.

 

[Chalnoth]

But is not an answer to my question, where in the symmetry breaking is the need for many universes!

Spontaneous symmetry breaking is an accidental event which occurs differently in different regions, and which leads to different low-energy laws of physics when it occurs differently.

 

[Chalnoth]

But theories are never ruled out on account of their complexity. This is where I disagree with your reasoning. You are essentially performing a Bayesian model selection on your space of competing theories. They all satisfy the data equally well, however, some have additional structure than others that make them either less predictive, more complex, or both. The Bayesian evidence disfavors these models, but it does not exclude them! This is an incorrect interpretation of the statistical method.

The point, however, is that despite the fact that a unique universe is strongly disfavored by the evidence, many people seem to expend all of their vitriol on the most likely models: multiverse models.

 

[marcus]

The point, however, is that despite the fact that a unique universe is strongly disfavored by the evidence,..

:

You amaze me Chally

First you say the multiverse approach to cosmology requires fewer assumptions than any of the alternatives, which it clearly doesn't, as I've shown by examples.

Now you say a unique universe strongly disfavored by evidence! On the contrary, a unique universe is the DEFAULT case that reflecting the traditional meaning of the word universe: everything that exists.

Obviously it is unique by definition. Other cases could be imagined but this is the one (uni) that exists. So you have to start changing the meaning of words if you want to claim what you say.

And there is no evidence for multiplicity whatsoever. All we have to look at is the universe we've got.
The rest is fantasy and speculation.

 

[Haelfix]

There are three different things that are logically distinct.

1) The case for a multiverse
2) The case for the strong anthropic principle (eg scanning the multiverse and selecting out the universe that has the parameters tuned for life) in the sense of an explanatory principle.
3) The multi worlds interpretation of quantum mechanics

They are all logically distinct, you can believe in anyone of the three without having to agree with the other (with the exception that 2 requires 1 obviously but the converse is not true)

Ok!

The case for 1 is very strong theoretically and experimentally. Essentially WMAP disfavored a number of simple variants of inflation, and strongly supported eternal Inflation (which made a bunch of predictions that were subsequently verified).

Even if eternal inflation is incorrect, it now seems necessary for some type of inflation to have existed at some time. Thats great, except the problem is that it implies length and time scales that are astronomically larger than what we are used too in our visible universe... Indeed even basic assumptions about physics starts to have problems with numbers of that magnitude (that's why people worry about ridiculously improbable events, like vacuum decay). Think numbers like 10^120 (in whatever units you want).

The issue there is that if you believe in statistical mechanics, all sorts of crazy phenomenon start to be logically possible over timescales of that magnitude. In particular, it is ridiculously difficult to contrive a scenario where inflation happens once and only once.

You can spout all sorts of philosophy about falsifiability, but the mathematics doesn't care, and numbers like 10^120 don't care either. In that sense, models where inflation only happens once are incredibly fine tuned and implausible. At the end of the day, the issue is exponential expansion, and not details of the physics!

The case for 2 is also unfortunately relatively strong. And it pains me to say it, b/c it is an annoying argument! The exact details for why exactly anthropic selection seems necessary is non trivial, and come from a number of different lines of reasoning. For instance, it might not be obvious to nonphysicists but the fact that the Higgs particle seems to be centered at 125 MeV is a relative boost to the anthropic argument!

It wouldn't surprise me if there was a mechanism or some new physics that made the case for 2 go away, but at this time no such compelling model exists.

The case for 3 is a subject that I don't pay much attention too. All of the interpretations are presumably mathematically identical in their consequences!

 

[marcus]

The case for 1 is very strong theoretically and experimentally. Essentially WMAP disfavored a number of simple variants of inflation, and strongly supported eternal Inflation (which made a bunch of predictions that were subsequently verified).

Even if eternal inflation is incorrect, it now seems necessary for some type of inflation to have existed at some time. Thats great, except the problem is that it implies length and time scales that are astronomically larger than what we are used too in our visible universe... Indeed even basic assumptions about physics starts to have problems with numbers of that magnitude (that's why people worry about ridiculously improbable events, like vacuum decay). Think numbers like 10^120 (in whatever units you want).
...

That's pretty vague Haelfix and I don't think it holds water. Ashtekar has written a couple of papers and given talks mentioning how well the 7 year WMAP data compats with Loop cosmology and its associated inflation. I'll get a link to one or more of his papers. I doubt the inflation he has in mind was disfavored by WMAP---he says the opposite. And it certainly involves no multiplicity of universes!

So I doubt WMAP disfavored everything besides some multifarious type of inflation (like eternal). You aren't very specific about that claim--maybe better give us a link. If it is just departmental scuttlebut it could well be overlooking something.

Here's a link I said I'd get: http://arxiv.org/abs/1103.2475 This refers directly to WMAP data and inflation. Also there is an even more recent invited review artice by Ashtekar http://arxiv.org/abs/1108.0893 See
pages 98-101 and references therein.

 

[Chalnoth]

:

You amaze me Chally

First you say the multiverse approach to cosmology requires fewer assumptions than any of the alternatives, which it clearly doesn't, as I've shown by examples.

No, you haven't. It's very, very simple. Uniqueness is always an additional assumption. There is no way to get uniqueness "automatically".

Now you say a unique universe strongly disfavored by evidence!

Yes, it is. Very strongly. Because it is looking more and more like many aspects of physical law are due to accidents in our past.

On the contrary, a unique universe is the DEFAULT case that reflecting the traditional meaning of the word universe: everything that exists.

That's just silly word play. What I mean by uniqueness is that our big bang event was the only big bang event, and the low-energy physical laws that we observe are the same everywhere. Both positions are highly unlikely.

 

[marcus]

What I mean by uniqueness is that our big bang event was the only big bang event, and the low-energy physical laws that we observe are the same everywhere...

Well it's nice that you say what you mean. About the first. Cosmology is a mathematical science, not philosophy. A mathematical model of the universe does not need to include a statement THIS IS THE ONLY UNIVERSE! It is simply a math model that one fits to the data. A simple model with superior fit wins and is used to make predictions.

So if I have a model which accounts for the big bang, inflation and the present structure and that model does not happen to predict other big bangs, then FINE. That is neither an additional assumption nor an additional conclusion. The issue doesn't even come up among reasonable people.

Now when you talk about regions with different low-energy physics, you may be WEASELING by calling any such instance a "different universe". I've heard that we could have regions with a different QCD θ angle and perhaps different dark energy density within the same universe, all stemming from the same big bang. I think a bounce cosmology should be able to accommodate some regional variation of low-energy physics depending on when and how the spontaneous symmetry cookie crumbles. You don't need a whole other big bounce just to get some measly variation in some measley parameter. You can have different regions in the same universe which we know is the same universe because it stems from the same bounce, or if you prefer, "bang".

 

[Haelfix]

Marcus, the reason WMAP was viewed as a victory for EI and more particularly certain breeds of chaotic inflation, was b/c they made very specific and technical predictions twenty years a priori (regarding such things as the tensor to scalar ratios and spectral index etc). When WMAP came out, huge swaths of inflationary models were ruled out, and yet the ones that fit the details to a T happened to be certain types of chaotic inflation models (yet others in the subclass were ruled out).

Anyway, there are currently hundreds of models that fit WMAP, and I'm sure the LGC groups are well aware of the constraints. Of course, many of these people are postdicting results, which is of course much easier!

As far as the papers you linked. They say more or less exactly the opposite of what you think they do.

They argue for a specific measure regarding the probability for inflation to occur. Ashtekar thinks that this probability is nearly unity! Well, that's great, except that it makes the case for a multiverse much more likely! If the conditions necessary for inflation is generic, then a Hamiltonian system will graciously proceed to recur those identical conditions many times over. Once you have that, the power of exponential expansion takes over and the plurality of the volume of spacetime will be dominated by inflating regions.

So to reiterate to argue against a Multiverse, you have to contrive it so that inflation is astronomically unlikely! But then that defeats the purpose of inflation in the first place (which was to save the big bang model from the mother of all finetuning problems).

 

[marcus]

I don't think you get what Ashtekar is saying. The loop bounce has something unusual a brief period of superinflation--faster than exponential expansion. (Inflation is just ordinary exponential or nearly but not quite exponential). Ashtekar says the probability of adequate inflation ensuing is ~ 1 at the loop bounce.

Not at other times. In a typical model there is one bounce which the entire universe universe undergoes at the same moment. (Other versions are cyclic but for simplicity just think of the one-time bounce.)

Its a model that fits the data including WMAP and has only one inflation opportunity, one which the whole universe participates in at once, and which makes an adequate inflation episode highly probable without fine tuning.

I'm not saying it's right. but I note that it has an increasing number of people interested.

Here is a current invited review article (string cosmology, loop, Horava, Einstein-aether...) by a guy at Belgian institution that discusses the trends. In case you are interested and don't have other sources this may give you an idea what other people outside the department coffee-room are thinking.

http://arxiv.org/abs/1201.4543
Aspects of Quantum Gravity in Cosmology

the author is not a Loop theorist, indeed before 2007 most of his papers were string/M or stringy cosmology. From 2007 he seems to be doing straight cosmology without the branes. In any case I wouldn't expect him to have any pro-Loop bias. So you probably get a clear balanced view covering a halfdozen approaches to resolving the singularity and giving historical perspective on the field.

 

[skydivephil]

I don't think you get what Ashtekar is saying. The loop bounce has something unusual a brief period of superinflation--faster than exponential expansion. (Inflation is just ordinary exponential or nearly but not quite exponential). Ashtekar says the probability of adequate inflation ensuing is ~ 1 at the loop bounce.

Not at other times. In a typical model there is one bounce which the entire universe universe undergoes at the same moment. (Other versions are cyclic but for simplicity just think of the one-time bounce.)

Its a model that fits the data including WMAP and has only one inflation opportunity, one which the whole universe participates in at once, and which makes an adequate inflation episode highly probable without fine tuning.

I'm not saying it's right. but I note that it has an increasing number of people interested.

Here is a current invited review article (string cosmology, loop, Horava, Einstein-aether...) by a guy at Belgian institution that discusses the trends. In case you are interested and don't have other sources this may give you an idea what other people outside the department coffee-room are thinking.

http://arxiv.org/abs/1201.4543
Aspects of Quantum Gravity in Cosmology

the author is not a Loop theorist, indeed before 2007 most of his papers were string/M or stringy cosmology. From 2007 he seems to be doing straight cosmology without the branes. In any case I wouldn't expect him to have any pro-Loop bias. So you probably get a clear balanced view covering a halfdozen approaches to resolving the singularity and giving historical perspective on the field.

I have a question for you Marcus of how one gets a singualr universe in LQC inflation , I hope you can help me clear up some issues.
If I've understood Guth correctly inflation is eterrnal becuase of the way that it ends, not becuase of the way that it starts.
http://arxiv.org/pdf/hep-th/0702178v1.pdf
"In fact, in any successful
inflationary model the rate of exponential expansion is always much faster than the
rate of exponential decay. Therefore, even though the false vacuum is decaying, it
never disappears, and in fact the total volume of the false vacuum, once inflation starts,
continues to grow exponentially with time, ad infinitum"

So why is this process different in LQC? is it different?
Also my reading of the field is the main promoters of inflation: Guth, Linde, Vilenkin, Aguirre etc and the main detractors Steinhardt, Turok etc all agree inflation is eternal. I've not heard any comments from the Loop guys, Ashketar, Bojowald, Singh etc argue that inflation is not eternal. Given the attention eternal inflation receives and that they all agree inflation occurs I would expect them to say something. Hence it seems to me that there is somehting of a consensus (maybe that's too strong a word but somehting aproaching that) that inflaiton is eternal.
Have I read this wrong? If so on what basis would you say that?

 

[Chalnoth]

Now when you talk about regions with different low-energy physics, you may be WEASELING by calling any such instance a "different universe".

Weaseling? This is the primary point of interest where "other universes" are concerned. How the hell is cutting straight to the heart of the matter weaseling?

 

[martinbn]

Spontaneous symmetry breaking is an accidental event which occurs differently in different regions, and which leads to different low-energy laws of physics when it occurs differently.

You more or less repeat what you've already said, and I think I understand you say, but I don't understand why you say it. Can you give me a reference for the spontaneous symmetry breaking mechanism where many universes are needed for it?

 

[bapowell]

The point, however, is that despite the fact that a unique universe is strongly disfavored by the evidence, many people seem to expend all of their vitriol on the most likely models: multiverse models.

Yeah, I get that, but my intent is not to debate most likely models. I'm aware that there's plenty of cross fire going on in this thread so it's hard to keep things straight. I am referring back to our discussion regarding the multiverse as being part of objective reality because the theory from which it emerges is well established and accepted -- this is your stance. My point is while many-worlds is perhaps the most likely theory from a Bayesian perspective, it is not definitively so because competing theories are not ruled out using these kinds of simplicity arguments. So, I reject your claim that the multiverse is necessarily part of objective reality because many-worlds has not been singularly identified as the correct theory of quantum mechanics.

 

[bapowell]

You more or less repeat what you've already said, and I think I understand you say, but I don't understand why you say it. Can you give me a reference for the spontaneous symmetry breaking mechanism where many universes are needed for it?

You're missing his point. He's not saying that multiple universes are required for SSB. What he's saying is that given a large enough universe in which SSB occurs, it is inevitable that different causal regions will be characterized by different order parameters (the SSB field will evolve to different vacua.) This conclusion requires only a universe larger than the one we observe and a continuously distributed field facilitating the SSB.

Alternatively, according to many-worlds QM, the SSB that occurs right in our Hubble patch resulted in only one possible low energy vacuum among many. Other vacua in our Hubble patch are selected and evolve in other branches of the wavefunction.

 

[martinbn]

You're missing his point. He's not saying that multiple universes are required for SSB. What he's saying is that given a large enough universe in which SSB occurs, it is inevitable that different causal regions will be characterized by different order parameters (the SSB field will evolve to different vacua.) This conclusion requires only a universe larger than the one we observe and a continuously distributed field facilitating the SSB.

Hm, that is very different than what I think he said. He said that SSB leads to that, and I don't understand why.

Alternatively, according to many-worlds QM, the SSB that occurs right in our Hubble patch resulted in only one possible low energy vacuum among many. Other vacua in our Hubble patch are selected and evolve in other branches of the wavefunction.

This also seems different to me.


Anyway, I am probably lost and need to reread the whole thread.

 

[alt]

Interesting thread. Aside from the multiverse (or not) issue, I'm trying to get my mind around the OP - universe accidental or not.

What do we really mean by use of the word 'accidental' ?

http://dictionary.reference.com/

ac·ci·dent
noun
1. an undesirable or unfortunate happening that occurs unintentionally and usually results in harm, injury, damage, or loss; casualty; mishap: automobile accidents.
2. Law. such a happening resulting in injury that is in no way the fault of the injured person for which compensation or indemnity is legally sought.
3. any event that happens unexpectedly, without a deliberate plan or cause.
4. chance; fortune; luck: I was there by accident.
5. a fortuitous circumstance, quality, or characteristic: an accident of birth.

Also, the etymology of the word ..

http://www.etymonline.com/
accident late 14c., "an occurrence, incident, event," from O.Fr. accident (12c.), from L. accidentem (nom. accidens), prp. of accidere "happen, fall out, fall upon," from ad- "to" (see ad-) + cadere "fall" (see case (1)). Meaning grew from "something that happens, an event," to "something that happens by chance," then "mishap." Meaning "unplanned child" is attested by 1932.

A more consise definition from the dictionary on my computer (Wordweb, downloadable) is ..

1)Anything that happens suddenly or by chance without an apparent cause
2) An unfortunate mishap; especially one causing damage or injury

So I don't feel that any of the definitions above, or even the etymology of the word 'accident', reflects the context of what is meant here when we discuss whether the universe is accidental or not.

Out of curiosity, I also translated the word to old Greek and Latin, then sought the meaning thereof in each case. I mostly came up with 'event, incident, occurrence', etc, be it good, bad or indifferent

All roads seem to basically end to 'good or bad event'. Surely that’s not what is been considered here.

So the question might be whether the universe is with cause, or without cause ? If so, how can anything be without cause ?

Or is the question merely restating the fact that science stops at the first cause (whatever that might be) ?

 

[Chalnoth]

You more or less repeat what you've already said, and I think I understand you say, but I don't understand why you say it. Can you give me a reference for the spontaneous symmetry breaking mechanism where many universes are needed for it?

I don't get what your hangup is here. This is just the way spontaneous symmetry breaking works. There tends to be some finite distance across which the spontaneous symmetry breaking takes on the same value, beyond which there are different values. Since these different spontaneous symmetry breaking results lead to different low-energy laws of physics, these other regions can be thought of as separate universes (and are one of the main multiverse ideas).

 

[Chalnoth]

Yeah, I get that, but my intent is not to debate most likely models. I'm aware that there's plenty of cross fire going on in this thread so it's hard to keep things straight. I am referring back to our discussion regarding the multiverse as being part of objective reality because the theory from which it emerges is well established and accepted -- this is your stance. My point is while many-worlds is perhaps the most likely theory from a Bayesian perspective, it is not definitively so because competing theories are not ruled out using these kinds of simplicity arguments. So, I reject your claim that the multiverse is necessarily part of objective reality because many-worlds has not been singularly identified as the correct theory of quantum mechanics.

And at some point more physicists will realize that making up additional dynamics to explain something that is already explained by the known and tested wavefunction dynamics is just plain engaging in a worthless endeavor.

 

[bapowell]

And at some point more physicists will realize that making up additional dynamics to explain something that is already explained by the known and tested wavefunction dynamics is just plain engaging in a worthless endeavor.

Totally agree. But simplicity is a statistical nicety; not necessarily a physical one. The universe simply might not work that way. Multiverses simply might not exist.

 

[marcus]

Totally agree. But simplicity is a statistical nicety; not necessarily a physical one. The universe simply might not work that way. Multiverses simply might not exist.

Exactly. The primary issue in the thread is whether it's appropriate to call Steinhardt's position (stated in post #1) is "rather pathetic".

I think it has been amply shown that it is not.

As you say "multiverses simply might not exist". Here's another way of saying it (let me know if it is not close enough to suit you.) I want to reflect the fact that cosmology is based on the construction of mathematical models rather than verbal/philosophical description:

"To get a good model of our big bang we don't need to include other big bangs having occurred."

 

[marcus]

It might be worthwhile for anyone who can't remember how the discussion started to review post #1 to see what we are talking about. Do we have to give up on the traditional reductionist program of explanation and resort to anthropics, or not?

The issue arose concisely in this post:
https://www.physicsforums.com/showthread.php?p=3725098#post3725098

I linked to Steinhardt's statement and Chalnoth replied that it was pathetic.

Here's the post we began the thread with:
===Originally Posted by Chalnoth===

==Originally Posted by marcus==
http://edge.org/response-detail/805/what-do-you-believe-is-true-even-though-you-cannot-prove-it

or google "Steinhardt annual question 2005"
==endquote==​

I've always found that response to anthropic arguments to be rather pathetic.
===endquote===

In fact Steinhardt's position has largely prevailed, physics has moved on and Chalnoth's complaint is out of date.
In scientific discussion (i.e. outside popular media) one does not hear much anthropics talk these days.
I think that's great and I'm grateful to Steinhardt for leading the attack on it.

There is a lot of obfuscation about this issue and natural confusion as well so I urge anybody who is not familiar with it to take a close look at post #1.

 

[Chalnoth]

Totally agree. But simplicity is a statistical nicety; not necessarily a physical one. The universe simply might not work that way. Multiverses simply might not exist.

That's extremely unlikely.

With a bit of hindsight, now that we know that the appearance of collapse was built into the wavefunction dynamics from the start, it is patently obvious that the entire process of attempting to explain wavefunction collapse was always an unnecessary enterprise. We were trying to explain something that was already explained by the wavefunction dynamics.

The whole enterprise of searching for another interpretation of quantum mechanics is akin to having Newtonian gravity before you and attempting to come up with some rule by which planets can have elliptical orbits. The very idea is nonsensical: Newtonian gravity explains it just fine. Why look for anything extra?

 

[Fuzzy Logic]

As I said in an earlier post, the only problem I have with Steinhardt's statement is his reason in that the universe was not accidental. Of course it was. Saying otherwise infers design. That answers nothing and complicates things immensely.

 

[Chalnoth]

In fact Steinhardt's position has largely prevailed, physics has moved on and Chalnoth's complaint is out of date.

Um, I guarantee you that nearly all theoretical physicists would find that a complete and utter surprise.

In scientific discussion (i.e. outside popular media) one does not hear much anthropics talk these days.

Only because these ideas are very difficult to nail down, and there are other areas where we have lots of good new experimental data to examine. As I said before, we can expect them to resurface once the LHC becomes mature and precision cosmology enters a lull in new data.

I think that's great and I'm grateful to Steinhardt for leading the attack on it.

I guarantee you he had nothing to do with it. Bad, irrational arguments like the one you quoted aren't likely to have had any impact on the scientific community.

 

[marcus]

As I said in an earlier post, the only problem I have with Steinhardt's statement is his reason in that the universe was not accidental. Of course it was. Saying otherwise infers design. That answers nothing and complicates things immensely.

That's great. Then you agree with the main body of the statement (we don't have to give up on the effort to find natural explanations) and just disagree verbally with how he chose to phrase the headline.

I'm sure you agree that Steinhardt was not arguing for "design"!
His basic message is there is no indication that we are through explaining yet. There are further layers of the onion.
We have not yet reached the end of physics where we have found the deepest explanation and where beyond that it "just is the way it is."

Notice how Steinhardt uses the word "desperation" in analyzing the motives of those who resort to anthropics. The pernicious thing is the cop-out: there are lots of different big bangs and this one just is the way it is, so don't ask why. He attributes it to an desperate attempt to excuse some theorist's failure to come up with a unique fundamental theory (by fundamental I mean good up to Planck scale.) A multiplicity of big bangs let's them off the hook.

Since 2006 it seems to me as an onlooker that the community has pretty much decided that theorists are NOT going to be let off the hook. The anthropic excuse for failure has been discarded or has lapsed into disuse.

 

[Fuzzy Logic]

That's great. Then you agree with the main body of the statement (we don't have to give up on the effort to find natural explanations) and just disagree verbally with how he chose to phrase the headline.

You miss the point. While our current model may be sufficient to explain what we already know, it doesn't explain everything. His statement is fine by scientific standards and I agree that the anthropic principle is not enough reason to justify a multiverse theory. I agree that the anthropic principle has no merit.
Would I say it's rather pathetic? No, but it is not a valid argument against multiverse theories, only the anthropic principle.

There are many compelling reasons to consider multiverse theories.
Most importantly, what happened at T=0.

 

[Chalnoth]

I agree that the anthropic principle has no merit.

That is taking things way way too far. The anthropic principle is, first of all, necessarily true. Because of its necessary truth, it manifests itself as a selection effect that must be applied whenever considering any theory for the birth of a new region of space-time. Failure to apply the selection effect that is the anthropic principle in such a situation will always lead you to incorrect conclusions.

Now, the anthropic principle can be used badly, as can most anything, but that's no reason to disregard it.

 

[Fuzzy Logic]

That is taking things way way too far. The anthropic principle is, first of all, necessarily true. Because of its necessary truth, it manifests itself as a selection effect that must be applied whenever considering any theory for the birth of a new region of space-time. Failure to apply the selection effect that is the anthropic principle in such a situation will always lead you to incorrect conclusions.

Now, the anthropic principle can be used badly, as can most anything, but that's no reason to disregard it.

No, I say it has no merit, because we don't know what the conditions for life really are. Life could just as easily prosper in a universe with completely different physics from our own. The fact that life is so prolific on Earth contradicts the idea of a finely tuned universe. The fact that we can't find life outside of our own planet only proves that the universe is vast, not that life is unique.

If you want to say that it's a necessary truth, then it is no different than saying "I think, therefore I am".

 

[marcus]

His statement is fine by scientific standards and I agree that the anthropic principle is not enough reason to justify a multiverse theory. I agree that the anthropic principle has no merit.

I agree. I think Steinhardt in 2005 led the attack that established that the anthropic principle has no place in fundamental physics. His statement was effective and influential. It was echoed and anthropics lost out e.g. as reflected in subsequent StringsXXXX conferences. Ridiculous to call it pathetic.

Would I say it's rather pathetic? No, but it is not a valid argument against multiverse theories, only the anthropic principle.

That was my point at the outset, specifically regarding the anthropic cop-out.
===Originally Posted by Chalnoth===
==Originally Posted by marcus==
http://edge.org/response-detail/805/...annot-prove-it

or google "Steinhardt annual question 2005"
==endquote==
I've always found that response to anthropic arguments to be rather pathetic.
===endquote===

BTW don't mind predictive testable theories that explain stuff involving subsequent big bangs as a byproduct. If a particular model of our big bang (as long as it is testable) just has to produce baby big bangs/reheats somewhere down the road, that is OK with me.
I just get disgusted when I see arguments like "it's the way it is because otherwise we wouldn't be here and we can't explain any more".

There are many compelling reasons to consider multiverse theories.
Most importantly, what happened at T=0.

Please explain, what do you think happened at T=0 that makes this or that compelling. Are you thinking of socalled rogue regions where inflation did not stop and is still going on---or where reheating occurred later? That was discussed in an interesting thread last summer. You might be intrigued by some of what Ben Crowell had to say regarding a conference at Perimeter about problems with the prevailing early universe pictures and related stuff
https://www.physicsforums.com/showthread.php?p=3403876#post3403876

 

[Fuzzy Logic]

Please explain, what do you think happened at T=0 that makes this or that compelling. Are you thinking of socalled rogue regions where inflation did not stop and is still going on---or where reheating occurred later? That was discussed in an interesting thread last summer. You might be intrigued by some of what Ben Crowell had to say regarding a conference at Perimeter about problems with the prevailing early universe pictures and related stuff
https://www.physicsforums.com/showthread.php?p=3403876#post3403876

I have no opinion on simultaneous multiverse theories, though because of intuition and the singularity problems I am inclined to a big-bounce theory. I don't think that we should be extrapolating back to exactly 0. I expect that the universe bounced slightly before. What we say is T=0 in the big-bang model is more like Planck time. What happened before that was the end of the collapse of the previous universe when it reached critical density and entropy surpassed gravity. If I say too much I'll surely paint myself into a corner. That is just my layman interpretation of what I think happens. I don't pretend to know enough details to argue for it.

I'll check out that thread, thanks.

Edit: sorry I didn't actually address your question. Why is it compelling.
I think it's a compelling reason because we can't explain the singularity event. Not only can we not explain what happened in the first moments, we can't explain how the universe came into existence at all. If I have to choose between saying the the universe just popped into existence and time began or saying that the universe has existed forever, without any direct evidence in favour of either argument the circumstantial evidence would suggest that the life cycle of the universe is closed system, just like the rest of nature. Nowhere do we have any evidence of spontaneous existence, but everywhere we can see examples of cyclic evolution.