Universe not accidental: Is this Steinhardt statement "rather pathetic"? If so, why?

marcus

I think the heyday of string multiverse talk probably came somewhere in 2003-2006. Except for popularizations we hear little about it these days compared with 5 or 6 years ago. Paul Steinhardt (Albert Einstein Professor of Physics at Princeton) deserves substantial credit for this as one who led off in January 2005 with a strong statement in opposition.
http://edge.org/response-detail/805/...annot-prove-it
Interestingly, when I gave this link in another thread, Chalnoth replied in a way that begs for explanation/discussion.
Steinhardt was supported by influential members of the string community such as David Gross and to some extent also by his Princeton colleague Edward Witten. Multiverse papers were excluded from the "Strings 2008" conference at CERN and have made little or no showing at subsequent Strings XXXX. The anthropic bandwagon started by Leonard Susskind with his 2003 "Anthropic String Landscape" paper, which certainly affected planning of Strings 2005, lost a much of its momentum and Steinhardt took the lead in speaking out on this issue.

This statement was published around January 1, 2005 by the Edge online magazine in response to their annual queston, which in 2005 was "WHAT DO YOU BELIEVE IS TRUE EVEN THOUGH YOU CANNOT PROVE IT?"

==quote Edge 2005 annual question==

Paul Steinhardt:

I believe that our universe is not accidental, but I cannot prove it.

Historically, most physicists have shared this point-of-view. For centuries, most of us have believed that the universe is governed by a simple set of physical laws that are the same everywhere and that these laws derive from a simple unified theory.

However, in the last few years, an increasing number of my most respected colleagues have become enamored with the anthropic principle—the idea that there is an enormous multiplicity of universes with widely different physical properties and the properties of our particular observable universe arise from pure accident. The only special feature of our universe is that its properties are compatible with the evolution of intelligent life. The change in attitude is motivated, in part, by the failure to date to find a unified theory that predicts our universe as the unique possibility. According to some recent calculations, the current best hope for a unified theory—superstring theory—allows an exponentially large number of different universes, most of which look nothing like our own. String theorists have turned to the anthropic principle for salvation.

Frankly, I view this as an act of desperation. I don't have much patience for the anthropic principle. I think the concept is, at heart, non-scientific. A proper scientific theory is based on testable assumptions and is judged by its predictive power. The anthropic principle makes an enormous number of assumptions—regarding the existence of multiple universes, a random creation process, probability distributions that determine the likelihood of different features, etc.—none of which are testable because they entail hypothetical regions of spacetime that are forever beyond the reach of observation. As for predictions, there are very few, if any. In the case of string theory, the principle is invoked only to explain known observations, not to predict new ones. (In other versions of the anthropic principle where predictions are made, the predictions have proven to be wrong. Some physicists cite the recent evidence for a cosmological constant as having anticipated by anthropic argument; however, the observed value does not agree with the anthropically predicted value.)

I find the desperation especially unwarranted since I see no evidence that our universe arose by a random process. Quite the contrary, recent observations and experiments suggest that our universe is extremely simple. The distribution of matter and energy is remarkably uniform. The hierarchy of complex structures ranging from galaxy clusters to subnuclear particles can all be described in terms of a few dozen elementary constituents and less than a handful of forces, all related by simple symmetries. A simple universe demands a simple explanation. Why do we need to postulate an infinite number of universes with all sorts of different properties just to explain our one?

Of course, my colleagues and I are anxious for further reductionism. But I view the current failure of string theory to find a unique universe simply as a sign that our understanding of string theory is still immature (or perhaps that string theory is wrong). Decades from now, I hope that physicists will be pursuing once again their dreams of a truly scientific "final theory" and will look back at the current anthropic craze as millennial madness.

==endquote==

Chalnoth

Yes, I am aware that many people object to the multiverse ideas. I still find the objections pathetic. Specifically, I object to this statement:

"The anthropic principle makes an enormous number of assumptions"

Except that in reality, it requires more assumptions to assume a unique universe than it does to assume a prolific universe-generation process: you still need a universe-generation process, except now it can only ever occur once. And forcing that universe-generation process to only occur once requires additional and completely unreasonable assumptions.

If these people were only objecting to the way in which the multiverse ideas were used, I would have no problem. There are surely many very bad ways to make use of multiverse ideas, as there are bad ways to use a great many theoretical ideas. But the objections to the multiverse ideas as a matter of principle are pathetic and irrational. Physicists really should know better, and it is utterly sad that they don't.

But by the way, I don't expect any reduction in overall enthusiasm for string theory, but a reduction in string theory work is natural and expected as the LHC started to turn on. Scientists in general are going to tend to be more interested in real data than in pie-in-the-sky ideas that we don't yet know how to connect to experiment. When the LHC becomes mature, I'm sure that we will see a dramatic uptick in string theory again, barring the emergency of any more compelling alternatives.

Garth

Is not the force of Steinhardt's objection the next part of his sentence:
Of course it depends on what you mean by the Anthropic Principle; Steinhardt seems to identify it with the multiverse concept.

The multiverse is only one of the hypotheses that 'explains' the propitious coincidences.

I always stick with Stephen Hawking's definition: "The universe is as it is because we are."

Garth

Chalnoth

That's not a valid objection either, because a unique universe is also not testable, due to the inability to verify that uniqueness. Furthermore, there is no requirement that the universe accept our demands that it be testable.

Like I said, you can object to the specific ways in which multiverse ideas are applied. I see no problem with that. These ideas can certainly be applied very poorly in certain cases. But it is still, by far, most likely the case that we live in some sort of multiverse.

MathematicalPhysicist

Well this definition of Hawking is blatantly circular.

I mean the universe is the way it is because we are, then it begs the question what came first sentient beings or the stuff that they are made from?

Cause you you can ask why are we the way we are? and you get the answer, because the universe is that way.

MathematicalPhysicist

And I haven't read in depth what the idea of multiverse means.

But if you take the universe as everything that exists, it doesn't matter how do you call it, multiverse, cosmos, etc.

skydivephil

Im curious , is the position of the Earth in the solar system considered to be an accidennt/anthropically selected or is there some underlying theory that allows us to predict where the Earth should be based on the known laws of physics?
If the former is true why cant the laws/constants of nature be the same ? Im not saying they are, but why rule it our a priori?

Chalnoth

Well, there are multiple ideas. Max Tegmark has suggested a three-level hierarchy:

Level I: Inflation predicts that you'll get many Hubble volumes with all possible realizations of the various initial conditions. This can basically be understood as the statement that the universe is much larger than the part of it we can observe.
Level II: Our current knowledge of theoretical physics seems to indicate that the same fundamental laws of physics can potentially lead to very different low-energy laws of physics due to accidents in our past. Combine this with the Level I multiverse and you get that some of these disconnected regions not only have different galaxies, but different low-energy laws of physics altogether.
Level III: Quantum mechanics unambiguously predicts that the part of the wavefunction we observe is not the only part. This really doesn't add anything on top of the previous two multiverse ideas, but it does suggest that the entire multiverse can actually exist as different components of the wavefunction of the universe within our Hubble horizon.

Tegmark has also suggested a fourth level, that of different fundamental laws. I do think his idea here is interesting, but it's really difficult to say anything beyond that.

Fuzzy Logic

Why is it more likely?

jimjohnson

In Brian Greene's 2011 book, Hidden Reality, nine different parallel universe proposals are explained: Quilted, Inflationary, Brane, Cyclic, Landscape,Quantum, Holographic, Simulated, ans Ultimate. In the first chapter he states " I want you to get a sense of how modern scientific investigations.... naturally suggest this astounding possibility" of many universes. The book presents strong arguements for concluding that our universe is not the only one.

Chalnoth

For two main reasons:
1. In general, a unique universe requires more assumptions than a prolific one.
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 (spontaneous symmetry breaking is an essential component of the standard model: no string theory required, though naturally string theory suggests a much greater degree of flexibility).

Garth

Stephen Hawking's definition is succinct, it simply means that if the laws and constants of the universe were not propitious for life then we would not be here. As we are here "the universe is as it is".

garth

bapowell

That's precisely the anthropic argument -- the position and characteristics of the Earth do not arise from some fundamental theory. We live here on Earth simply because we can. Now, in this analogy the Earth is identified with the Universe. Since the Earth exists in a larger space with widely ranging conditions (there are many other, different kinds of planets orbiting many other, different kinds of stars) the analogy necessitates the existence of a multiverse: our Universe among many different kinds. The postulation of the multiverse is what some people find objectionable.

skydivephil

So would you agree that either (a) the above idea on the position of the Earth is unscientific or (b) we cant reject anthropic reasoning a priori on the grounds that it's unscientific?
I cant see a third alternative, but maybe someone else can?

bapowell

No, the main difference is that we have access to the rest of the universe -- we can observe other planets outside of the Earth. This enables us to confirm that there is indeed a rich assortment of possible conditions. We have no such empirical access to other universes. The anthropic principle, as applied to the universe, is therefore non-scientific.

Chalnoth

Except for the simple fact that there are other ways to demonstrate this. As I noted above, spontaneous symmetry breaking events would, unambiguously, lead to different regions of the universe with different low-energy physics. And we can detect the impact of such events through our investigation of high-energy physics.

skydivephil

Before the 1990's we could not observe other planets, so would you have said it was unscinetific then? And what do you say to the many claims that multiverse might be observable , see here:
http://arxiv.org/abs/1109.3473
or:
http://blogs.discovermagazine.com/co...se-guest-post/
or