Anthropic Principle and Infinitely Minute Variations

[Chalnoth]

My first point is that these links you posted are only arguing about relatively narrow models. They don't even come close to talking about the plausibility of a multiverse where different low-energy physical laws are represented in different regions of space-time. At most, the arguments presented only paint a picture of how one specific mechanism of producing such a multiverse might not be correct.

Good question. See here for a sketch of the general idea:

http://blogs.discovermagazine.com/c...anks-contra-eternal-inflation-2/#.Ut1L1hAo6Uk

This is very hard to follow, as it's so incredibly dense with jargon. But it sounds to me like his primary complaint is two-fold:
1. Eternal inflation assumes an infinite phase space, while in reality the phase space is likely finite and fixed.
2. Eternal inflation uses the approximation of quantum field theory in a curved space-time, but quantum gravity is likely to radically change the picture on these scales.

I don't see how the first point is any argument at all against eternal inflation. The infinite phase space of eternal inflation is due to its use of quantum field theory in a curved space-time, and as long as the available phase space is sufficiently large, many of the qualitative features of eternal inflation could easily be recovered.

The second is a more serious criticism, but it's hard to say if it holds up, because as near as I can tell we only have limited understanding of the full dynamics of the string theory landscape. It's very hard to say that no part of the string theory landscape permits eternal inflation (or something that looks rather like eternal inflation) when you don't know the full behavior of the landscape.

 

[Haelfix]

My first point is that these links you posted are only arguing about relatively narrow models. They don't even come close to talking about the plausibility of a multiverse where different low-energy physical laws are represented in different regions of space-time. At most, the arguments presented only paint a picture of how one specific mechanism of producing such a multiverse might not be correct.

I am arguing only about relatively well known facts in string theory here, and the latter link is a review about string theory cosmology which discusses some of them. The problems listed are quite general within that framework. This may or may not be a good thing depending on your philosophy, since it might imply a rather stringent selection mechanism on the landscape. Anyway, at this time, there is not a single model where we have a full blown solution which proceeds in full mathematical generality from compactification to a 4dimensional effective lagrangian which outputs a realistic inflaton candidate, the standard model and other field content that doesn't spoil some phenomenology in some way. What seems very hard to reconcile here theoretically (Besides simply finding such a vacua), is the following facts.

1) In the standard effective field theory picture of inflation, the flatness of the potential is problematic. Generically, it introduces sensitivity to dimension six and higher operators that drags the mass of the inflaton up towards a higher scale. This is what is known as the Eta problem and prevents enough Efolding from occurring to match observation. String theory makes the problem worse here, b/c there are typically a lot of extra scalar fields that are lurking around after compactification. If you make them too heavy, they directly contribute to the Eta problem, and if you make them too light, they generate unobserved fifth forces and contribute to the Moduli problem of cosmology. There seems to be general inequalities (known as the weak gravity conjecture) inherent within the logic of string theory that prevents the effective field theorist from simply finetuning the eta problem away, therefore they are forced to deal with it head on.

2) In large field models, there are large excursions in field space that requires an unknown symmetry to protect various coefficients in the effective operator expansion otherwise you won't get the predicted tensor modes, which is another example of the problem of having a very flat potential. String theorists are thereforce forced to consider models with certain extra symmetries, like a shift symmetry in order to tame some of the extreme UV sensitivity of the inflationary effective field theory. This proves hard to do in quantum gravity (recall that there should not be any global symmetries in gravity), although there are some toy model tentative proposals in the recent literature (like axion monodromy) which are quite complicated.

Again, none of these things 'prove' that there can't be Eternal inflation, much less a large Type I/II multiverse, but they do emphasize that we don't really have a single tangible model that really accomplishes EI from first principles within string theory, and they generically seem hard to accomplish. I feel that it is simply wrong to insist that string theory predicts Eternal inflation, based on vague conjecture following the KKLT paper, we just don't know that with any real certainty.

As far as Banks and Fischlers ideas, let's just say there is a lot more to it than that. In their logic, there is really only one holographic universe in a Causal diamond and a lot of highly scrambled gauge copies elsewhere. I suppose they believe that going beyond the particle horizon is essentially asking a meaningless question. Anyway there is a lot to this idea, and it is extremely subtle and quite beyond my depth, but is an active school of thought within the black hole and quantum gravity community (see discussions about Blackhole firewalls and particularly about the A ? Rb conjecture).

 

[Chalnoth]

I am arguing only about relatively well known facts in string theory here, and the latter link is a review about string theory cosmology which discusses some of them. The problems listed are quite general within that framework. This may or may not be a good thing depending on your philosophy, since it might imply a rather stringent selection mechanism on the landscape. Anyway, at this time, there is not a single model where we have a full blown solution which proceeds in full mathematical generality from compactification to a 4dimensional effective lagrangian which outputs a realistic inflaton candidate, the standard model and other field content that doesn't spoil some phenomenology in some way. What seems very hard to reconcile here theoretically (Besides simply finding such a vacua), is the following facts.

1) In the standard effective field theory picture of inflation, the flatness of the potential is problematic. Generically, it introduces sensitivity to dimension six and higher operators that drags the mass of the inflaton up towards a higher scale. This is what is known as the Eta problem and prevents enough Efolding from occurring to match observation. String theory makes the problem worse here, b/c there are typically a lot of extra scalar fields that are lurking around after compactification. If you make them too heavy, they directly contribute to the Eta problem, and if you make them too light, they generate unobserved fifth forces and contribute to the Moduli problem of cosmology. There seems to be general inequalities (known as the weak gravity conjecture) inherent within the logic of string theory that prevents the effective field theorist from simply finetuning the eta problem away, therefore they are forced to deal with it head on.

2) In large field models, there are large excursions in field space that requires an unknown symmetry to protect various coefficients in the effective operator expansion otherwise you won't get the predicted tensor modes, which is another example of the problem of having a very flat potential. String theorists are thereforce forced to consider models with certain extra symmetries, like a shift symmetry in order to tame some of the extreme UV sensitivity of the inflationary effective field theory. This proves hard to do in quantum gravity (recall that there should not be any global symmetries in gravity), although there are some toy model tentative proposals in the recent literature (like axion monodromy) which are quite complicated.

Again, none of these things 'prove' that there can't be Eternal inflation, much less a large Type I/II multiverse, but they do emphasize that we don't really have a single tangible model that really accomplishes EI from first principles within string theory, and they generically seem hard to accomplish. I feel that it is simply wrong to insist that string theory predicts Eternal inflation, based on vague conjecture following the KKLT paper, we just don't know that with any real certainty.

As far as Banks and Fischlers ideas, let's just say there is a lot more to it than that. In their logic, there is really only one holographic universe in a Causal diamond and a lot of highly scrambled gauge copies elsewhere. I suppose they believe that going beyond the particle horizon is essentially asking a meaningless question. Anyway there is a lot to this idea, and it is extremely subtle and quite beyond my depth, but is an active school of thought within the black hole and quantum gravity community (see discussions about Blackhole firewalls and particularly about the A ? Rb conjecture).

But again, you started this conversation talking about the multiverse in general, and insisted that such a thing is actually hard in string theory.

I don't think that statement is remotely plausible. At most you've shown that one particular mechanism, that of eternal inflation through the mechanism of inflation that never ends, is difficult to accomplish within string theory.

The main reason why I don't think it's even close to plausible that string theory doesn't support a multiverse view is because the only way that string theory could not support a multiverse view is if there were a unique stable state for the theory (the state we happen to be in). I can see no reason to believe it is even remotely plausible that a theory with some $10^{400}$ possible string compactifications won't have a very large number of local metastable minima.

 

[Chronos]

I fail to see how you can have a multiverse without each 'verse' being causally disconnected. Were this not true there would be boundary conditions that would, to put it mildly, be difficult to comprehend. If all 'verses' are truly causally disconnected you are forever, and irreconcilable stuck in whatever 'verse' you happen to reside with no possibility of detecting the influence of other 'verses'. That leaves the task of a proof that has no phenomenological basis - which appears to orphan it outside the realm of science. I agree with Haelfix that string theory does not 'require' a multiverse, it's merely an interpretation that seems popular.

 

[Chalnoth]

I fail to see how you can have a multiverse without each 'verse' being causally disconnected. Were this not true there would be boundary conditions that would, to put it mildly, be difficult to comprehend. If all 'verses' are truly causally disconnected you are forever, and irreconcilable stuck in whatever 'verse' you happen to reside with no possibility of detecting the influence of other 'verses'. That leaves the task of a proof that has no phenomenological basis - which appears to orphan it outside the realm of science.

And why are you ignoring the fact that that's not the only way to detect a multiverse?

You could come up with similar arguments for neutrinos by pointing out that you could never detect them directly, and then claiming it's "not science" to even discuss neutrinos because the model is inherently undetectable (directly).

I agree with Haelfix that string theory does not 'require' a multiverse, it's merely an interpretation that seems popular.

I don't see how it's possible for string theory to be remotely plausible without one.

 

[Chronos]

You will have to clarify, are you saying a proof is possible without phenomenological evidence? I don't recall any claim the neutrino was more than a theory prior to its detection. 'Direct' detection is not the issue. Scientists routinely accept theories based on indirect evidence. I've yet to hear of a theory considered proven in the absence of phenomenological evidence. I don't believe the multiverse has yet progresses beyond the status of conjecture on that count. Tom Banks offers an interesting perspective on the multiverse [landscape] in his paper http://arxiv.org/abs/1208.5715

 

[Chalnoth]

You will have to clarify, are you saying a proof is possible without phenomenological evidence? I don't recall any claim the neutrino was more than a theory prior to its detection. 'Direct' detection is not the issue. Scientists routinely accept theories based on indirect evidence.

That's my point. There's more than one way to demonstrate a theory is true. Blindly asserting that it can't be done, without thinking carefully about how one might demonstrate the veracity of a multiverse model, isn't exactly scientific.

 

[Chronos]

As you may recall, I brought up the absence of phenomenological evidence. If it is conceded it is not possible due to causal disconnectivity, does that constitute a blind assertion? I think not. I'm perfectly willing to entertain a feasible experiment that would offer unambiguous evidence, direct or indirect, of the multiverse. I haven't seen that yet, and do not feel obligated to take it seriously until then. Does that mean I believe we should stop trying? Of course not. I do believe it can't be bootstrapped to the level of scientific proof solely based on logic.

 

[Chalnoth]

As you may recall, I brought up the absence of phenomenological evidence. If it is conceded it is not possible due to causal disconnectivity, does that constitute a blind assertion? I think not. I'm perfectly willing to entertain a feasible experiment that would offer unambiguous evidence, direct or indirect, of the multiverse. I haven't seen that yet, and do not feel obligated to take it seriously until then. Does that mean I believe we should stop trying? Of course not. I do believe it can't be bootstrapped to the level of scientific proof solely based on logic.

Here's another question, then: why do you think that our ability to experimentally observe a theory should have an impact on what we think is likely to be true? Is a theory which could, in principle, be experimentally observed (though no observations yet exist) more likely to be correct than one that cannot, even in principle, be observed?

 

[julcab12]

"The probabilities in microscopic quantum processes and in the multiverse are both understood as “branching in the entire multiverse state |(or equivalently, as a “misalignment” in Hilbertspace between and the basis of local observables)."​

http://arxiv.org/PS_cache/arxiv/pdf/1105/1105.3796v1.pdf
http://dwave.wordpress.com/2011/06/10/decoherence-myths-and-realities/

Here is my question;

What specific criteria that enable us to conclude a "branching" did/should occur somewhere. Can time dilation and such postulate in QED's time reversal had an effect on the composition of decohered quantum superposition that may look as if they are overlapping or appear absent to interference between the elements of the superposition? Or.. Am i missing something. Help?

http://arxiv.org/abs/arXiv:1311.1095
http://arxiv.org/abs/1304.3582
http://inspirehep.net/record/1263356/plots

 

[Chalnoth]

"The probabilities in microscopic quantum processes and in the multiverse are both understood as “branching in the entire multiverse state |(or equivalently, as a “misalignment” in Hilbertspace between and the basis of local observables)."​

http://arxiv.org/PS_cache/arxiv/pdf/1105/1105.3796v1.pdf
http://dwave.wordpress.com/2011/06/10/decoherence-myths-and-realities/

Here is my question;

What specific criteria that enable us to conclude a "branching" did/should occur somewhere. Can time dilation and such postulate in QED's time reversal had an effect on the composition of decohered quantum superposition that may look as if they are overlapping or appear absent to interference between the elements of the superposition? Or.. Am i missing something. Help?

http://arxiv.org/abs/arXiv:1311.1095
http://arxiv.org/abs/1304.3582
http://inspirehep.net/record/1263356/plots

The cause of branching is an interaction with an external field.

 

[Chronos]

Here's another question, then: why do you think that our ability to experimentally observe a theory should have an impact on what we think is likely to be true?
Is a theory which could, in principle, be experimentally observed (though no observations yet exist) more likely to be correct than one that cannot, even in principle, be observed?

We generally accept there is no 'truth' in science, rather that is an evolving body of knowledge based on observation and theory. In the Popperian sense, this requires a theory to be falsifiable and, more generally, recognizes empirical science is the only reliable source of information we have about the world in which we live. In the words of Bertrand Russel "...science is always tentative, expecting that modification in its present theories will sooner or later be found necessary, and aware that its method is one which is logically incapable of arriving at a complete and final demonstration." Richard Feynman put it somewhat less eloquently stating "If you thought that science was certain — well, that is just an error on your part." And Albert Einstein is quoted as saying "It is the aim of science to establish general rules which determine the reciprocal connection of objects and events in time and space. For these rules, or laws of nature, absolutely general validity is required — not proven.".

In a nutshell, the scientific method is traditionally held as: make observations; form a testable hypothesis to explain observations; make predictions based on the hypothesis; search for evidence affirming or denying predictions. This is necessarily an empirical [phenomenological] exercise, not logical process, and entails more than just vanilla empiricism. A defining principle in science is a hypothesis must be falsifiable and independently corroborated. A hypothesis that can explain all observations is neither testable or scientific. It is little more than a solipsism, which merely predicts things are what they are and no observation is possible which cannot be explained away within its own context. That sounds eerily similar to the multiverse hypothesis - the physical constants are what they are because every possible value of the physical constants is realized somewhere in the multiverse. IMO, to achieve scientific validity, the multiverse hypothesis must make specific, unambiguous, phenomenological predictions that can be independently corroborated. If this is impossible, the hypothesis is not testable and fails the fundamental definition of science.

 

[Truenorthnatur]

A bit of quibbling about semantics first:

That's not really the anthropic principle. The anthropic principle generally takes one of two forms, the strong and weak.

The strong anthropic principle states that the universe must be such that it can harbor life. I don't think we need to discuss this.

The weak anthropic principle states that intelligent observers will only be able to observe conditions that allow them to exist. This is definitely a tautology (like much of mathematics), and it's a tautology that might help us to make sense of our own universe. It is, in short, a selection effect.

The statement that the physical constants of our low-energy physics need to be very specific for intelligent observers to exist is the statement that our universe is finely-tuned. This usually takes the form where we might expect some parameter that describes our low-energy physical laws lies between 0 and 1. If so, then we might reasonably expect a parameter value of 0.2353 or 0.7236. But we would think it very strange indeed if the value turned out to be 0.00000000000012, unless there were some physical process that would set it to that value. A universe where certain parameters take on values like that is a finely-tuned universe.

The anthropic principle might help us to make sense of why certain aspects of our universe appear to be finely-tuned, or it might turn out that there is some as-yet-unknown physical process which dramatically reduces the available parameter space (in the example above, of an imaginary constant of 1.2 \times 10^{-13}, what if there were some physical process that limited the possible range of this value to between 1.1 \times 10^{-13} and 1.3 \times 10^{-13}. If there were some physical process, then it would turn out that this parameter value wasn't finely tuned after all.

Re the weak principle. The irony is that humans fail miserably at this. We don't observe the reality of matter and energy...the essence of existence. Our senses and evolved physical brain are mired in a fraction if a fraction of being able to perceive and conceive of reality both in the subatomic quantum level and the cosmological level. We are 'stuck' with puny brains and extremely limited physical senses.

 

[julcab12]

The cause of branching is an interaction with an external field.

Ok. What exactly do you mean with external field. Are they introducing some exotic field(s) to accommodate each state?

Here is a argument for single scalar field

http://arxiv.org/pdf/1301.1174v2.pdf

"Observations suggest, that there may be periods in the history of the universe, including the present
one, in which its evolution is driven by scalar fields. This paper is concerned with the solution of the
evolution equations for a spatially
at universe driven by a single scalar field."​

http://arxiv.org/abs/1304.2785

"Recent results from the Planck satellite combined with earlier observations from WMAP, ACT, SPT and other experiments eliminate a wide spectrum of more complex inflationary models and favor models with a single scalar field, as reported by the Planck Collaboration..."

Here is a interesting counter proposal

http://arxiv.org/abs/1312.7619

"Models of cosmic inflation posit an early phase of accelerated expansion of the universe, driven by the dynamics of one or more scalar fields in curved spacetime. Though detailed assumptions about fields and couplings vary across models, inflation makes specific, quantitative predictions for several observable quantities, such as the flatness parameter (Ωk=1−Ω) and the spectral tilt of primordial curvature perturbations , among others---predictions that match the latest observations from the {\it Planck} satellite to very good precision. In the light of data from {\it Planck} as well as recent theoretical developments in the study of eternal inflation and the multiverse, we address recent criticisms of inflation by Ijjas, Steinhardt, and Loeb. We argue that their conclusions rest on several problematic assumptions, and we conclude that cosmic inflation is on a stronger footing than ever before."The direction toward multiverse remains consequential and somewhat hard to swallow to be honest (at least for me). I understand the convenience of jumping to speculative ideas coming from a semi(lack of better word) ad-hoc model that works pretty well in solving most of the problem in cosmology. I had a lot of fun thinking about it. Besides, who wouldn't love a dose of infinite possibilities with less fairies and closer to everyday experience?!..^^

I don't want to dwell in philosophy but somewhere down the line of all the article I've read and based upon my current understanding on the subject. I'll remain skeptic with a pinch of awe and a bit of weird lingering after taste. ^^

 

[Chalnoth]

Ok. What exactly do you mean with external field. Are they introducing some exotic field(s) to accommodate each state?

No, no. Nothing exotic. Just a field that is different from the system that we're paying attention to. Frequently for normal matter it's an electromagnetic field (i.e., photons) that will cause the decoherence, but it could just as easily be other matter (electrons, protons), the gravitational field, or anything else that interacts with the system in question.

Here is a argument for single scalar field

http://arxiv.org/pdf/1301.1174v2.pdf

"Observations suggest, that there may be periods in the history of the universe, including the present
one, in which its evolution is driven by scalar fields. This paper is concerned with the solution of the
evolution equations for a spatially
at universe driven by a single scalar field."​

http://arxiv.org/abs/1304.2785

"Recent results from the Planck satellite combined with earlier observations from WMAP, ACT, SPT and other experiments eliminate a wide spectrum of more complex inflationary models and favor models with a single scalar field, as reported by the Planck Collaboration..."

Here is a interesting counter proposal

http://arxiv.org/abs/1312.7619

"Models of cosmic inflation posit an early phase of accelerated expansion of the universe, driven by the dynamics of one or more scalar fields in curved spacetime. Though detailed assumptions about fields and couplings vary across models, inflation makes specific, quantitative predictions for several observable quantities, such as the flatness parameter (Ωk=1−Ω) and the spectral tilt of primordial curvature perturbations , among others---predictions that match the latest observations from the {\it Planck} satellite to very good precision. In the light of data from {\it Planck} as well as recent theoretical developments in the study of eternal inflation and the multiverse, we address recent criticisms of inflation by Ijjas, Steinhardt, and Loeb. We argue that their conclusions rest on several problematic assumptions, and we conclude that cosmic inflation is on a stronger footing than ever before."

These are all talking about cosmology, and are quite independent of the concept of a quantum multiverse.

The direction toward multiverse remains consequential and somewhat hard to swallow to be honest (at least for me). I understand the convenience of jumping to speculative ideas coming from a semi(lack of better word) ad-hoc model that works pretty well in solving most of the problem in cosmology. I had a lot of fun thinking about it. Besides, who wouldn't love a dose of infinite possibilities with less fairies and closer to everyday experience?!..^^

I don't want to dwell in philosophy but somewhere down the line of all the article I've read and based upon my current understanding on the subject. I'll remain skeptic with a pinch of awe and a bit of weird lingering after taste. ^^

The #1 thing that I'll keep coming back to is that a multiverse should be our default assumption and we should require evidence to be convinced that there is no multiverse, for the reason that multiverse proposals inherently require fewer assumptions than non-multiverse proposals.