I had been intending to write something here on the blog about this essay by George Ellis, so when I was contacted by someone at Inference about writing a letter in response, I did so for publication there. It has now appeared in their latest issue, with the title Theorists Without a Theory.
The topic is one I’ve addressed here all too often, but the main point I was trying to make is perhaps a new one. When I was writing here about the controversy over inflation one thing that struck me was that the pro-inflation side was responding to arguments that their theory didn’t solve the problem it was supposed to by in effect saying “the real theory is much more complicated” (see the paragraph beginning “Besides our disagreement…” on page 3 here). One way of seeing part of what is going on here is that most of what gets advertised as “theories” of inflation are actually more appropriately described as toy models. They involve a single inflaton field with a simple potential and unknown couplings to matter, intended as a toy model for the real theory (which will have lots of fields, complicated potentials and specified couplings to matter). An aspect of the controversy is one side pointing out that this theory doesn’t solve problems it is supposed to solve, with the other side arguing that it’s just a toy model.
People sometimes note that there’s a terminological problem with “string theory”, in that the public is often told that “theories” are solidly tested parts of science, which is not true in this case. The actual usage among physicists is different though, with “theory” often used to mean a specific mathematical model or set of models, with no implicit claim of a successful experimental test. A lot of the problem with the usage “string theory” is that no one knows what the actual theory is: it’s a conjecture that a theory with certain specific limits exists. The main point I was trying to make in this piece is that to a large degree the arguments over the scientific status of string theory (and of its supposed landscape and multiverse) revolving around its lack of testability are moot, since the underlying problem is something different: that there is no real theory to argue about. String theorists often try and evade this problem by a terminological shift: string theory is not a “theory”, it’s a “framework”. “Framework” is a much more ill-defined term than the already ill-defined “theory”. A theorist who says “I have a framework, not a theory” is actually saying nothing more than that they are a theorist without a theory.
Genuine question, out of pure curiosity: From this perspective, would you consider “the old quantum theory” to have been a theory? (https://en.wikipedia.org/wiki/Old_quantum_theory) Or a collection of “toy models”? Or a “framework”? Clearly it played an important historical role in the eventual development of what became the standard Dirac-von Neumann formulation of quantum theory.
Obviously the analogy with inflation is not perfect. No analogy ever is. But there are some interesting parallels. (And, for the record, I’m asking solely about the analogy with inflation, which is connected with experiments and observations. I’m not attempting to create an analogy with string theory.)
I honestly don’t know what a “framework” is, it’s a term that as far as I can tell only gets used in theoretical physics as part of bogus arguments about string theory vs. QFT.
The old quantum theory was a theory, albeit an incomplete and inaccurate one, although one that was clearly capturing part of the story because it gave very non-trivial predictions that were close to observed results in some cases. I don’t see any relation of that story to string theory, and little if any to usual inflation models.
Granted, it was way easier to do arbitrarily many replicable measurements of small systems than of the whole universe! But we do have a lot of cosmological data. Am I wrong in thinking that inflation can give an explanation for many of these data points, including the scale-invariance of fluctuations in the CMB, and all within basically standard GR plus some scalar fields and small quantum corrections?
I’m also rather curious about whether you have an opinion about where inflation stands. Perhaps you’ve maintained a wise level of ambiguity! But do you feel inflation is on par with string theory in terms of being not even wrong, disconnected from experiment, and a wrong direction that people should turn away from?
So what’s the way forward? We know Prof. Ellis doesn’t think naturalism is the final framework (I’ve read some of his eloquent writings on this) and some multiverse proponents think naturalism *is* the right path. Therefore … ?
I don’t think that problems with the theory of inflation are anywhere near as severe as the situation with the complete failure of string theory as a unified theory. Looking at the arguments discussed in the posting linked to above, the situation is complicated and I’m well aware I’m not a cosmologist.
Sometimes inflationary theory proponents claim evidence for a theory that will produce a multiverse and different physics in each universe. I think it’s important to point out that when they do this, they don’t have an actual theory that does what they claim, certainly not a testable one. If you look into such claims, they rely upon failed ideas from string theory.
I don’t think any of this is relevant to debates over “naturalism” and theology. If you’re looking for answers to such questions in theories of inflaton fields, I think you’re making a category mistake.
As for people who claim that they’re furthering the cause of naturalism by promoting an untestable scientific theory based on a failed research program, justified by appeals to authority, I think they’re making a really huge mistake. If you make this an argument about which untestable idea one should believe, most people are going to quite sensibly chose the church of their ancestors over Lenny Susskind.
Thanks for reply Peter. I was hoping some might take up the baton a bit. I’m really hoping one day for something really meaningful through and through (and I have a physics degree) and see little in the multiverse (though never understood it to be a reaction against theological issues) that impacts on my (irreducibly meaningful) dot of an existence. I smiled at your last point. Best.
The ‘String Hypothesis’ then! Would that be a fair assessment?
Inflation is a real theory, implemented as a series of toy models, but generalizable to more complex models. It’s exactly like gauge theory in the 1960s: a great general idea, implementable in an infinity of working models, waiting for specific implementation consistent with observation.
Inflation is compatible with different “patches” of spacetime having different symmetry breakings. What that has to do with the string “multiverse” has never been clear to me.
Anyway, inflation, unlike strings/M-theory, provides an economical explanation for a series of otherwise “accidental” observations and makes a general prediction for the form of the primordial fluctuation spectrum that so far is in good agreement with observation. It’s a pretty successful idea. It’s doubtful we’ll ever have a fully detailed theory of it, however, because it applies to regime we’ll probably never have access to, like evolutionary biologists trying to figure out how life originated. But who knows?
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