Courtesy of Slashdot (I think) I came across this interesting article reporting arguments that string theory has been the death of physics, or rather that it has basically taken it down a blind alley. Though many disciplines have suffered from ‘physics envy’, none less than economics, it looks like physics has succumbed to a nasty dose of what other ‘less hard’ sciences like economics have suffered from for ages – what I’ve just decided to call ‘epistemological bubbles’.I’m no physicist (or as I heard someone saying to someone a while back ‘I’m no rocket surgeon’). However, at least if it is to be believed, the story seems familiar. A discipline gets seduced into biting off more than it can chew by grand dreams of a ‘theory of everything’. Of course every now and again some grand theory does come along. Newton and Einstein managed theories of everything. Keynes and Smith in economics. (Well not everything but theories of a great deal).
But when you’re not smart enough, or there’s no theory to be had the result of such striving is ’empty science’. Somehow the necessary dialectic between theory and practice, ideas and facts becomes sterile and a few decades pass with little real progress – sometimes there’s even regress.
Here’s an edited version of the article on string theory.
“When it comes to extending our knowledge of the laws of nature, we have made no real headway” in 30 years, writes physicist Lee Smolin of the Perimeter Institute in Waterloo, Canada, in his book, “The Trouble with Physics,” also due in September. “It’s called hitting the wall.”
He blames string theory for this “crisis in particle physics,” the branch of physics that tries to explain the most fundamental forces and building blocks of the world.
String theory, which took off in 1984, posits that elementary particles such as electrons are not points, as standard physics had it . . . [but] vibrations of one-dimensional strings 1/100 billion billionth the size of an atomic nucleus.
Different vibrations supposedly produce all the subatomic particles from quarks to gluons. Oh, and strings exist in a space of 10, or maybe 11, dimensions. No one knows exactly what or where the extra dimensions are, but assuming their existence makes the math work. . . .
But one thing they haven’t done is coax a single prediction from their theory. . . .
Partly as a result, string theory “makes no new predictions that are testable by current _ or even currently conceivable _ experiments,” writes Prof. Smolin. “The few clean predictions it does make have already been made by other” theories.
Worse, the equations of string theory have myriad solutions, an extreme version of how the algebraic equation X2 4 has two solutions (2 and -2). The solutions arise from the fact that there are so many ways to “compactify” its extra dimensions _ to roll them up so you get the three spatial dimensions of the real world. With more than 10 raised to 500th power (1 followed by 500 zeros) ways to compactify, there are that many possible universes.
“There is no good insight into which is more likely,” concedes physicist Michael Peskin of the Stanford Linear Accelerator Center.
If string theory made a prediction that didn’t accord with physical reality, stringsters could say it’s correct in one of these other universes. As a result, writes Prof. Smolin, “string theory cannot be disproved.” By the usual standards, that would rule it out as science.
String theory isn’t any more wrong than preons, twistor theory, dynamical triangulations, or other physics fads. But in those cases, physicists saw the writing on the wall and moved on. Not so in string theory.
“What is strange is that string theory has survived past the point where it should have been clear that it wouldn’t work,” says Mr. Woit. Not merely survived, but thrived. Virtually every young mathematically inclined particle theorist must sign on to the string agenda to get an academic job. By his count, of 22 recently tenured professors in particle theory at the six top U.S. departments, 20 are string theorists.
One physicist commented on Mr. Woit’s blog that Ph.D. students who choose mathematical theory topics that “are non-string are seriously harming their career prospects.”
To be fair, string theory can claim some success. A 1985 paper showed that if you compactify extra dimensions in a certain way, the number of quarks and leptons you get is exactly the number found in nature. “This is the only idea out there for why the number of quarks and leptons is what it is,” says Prof. Peskin. Still, that is less a prediction of string theory than a consequence.
Like I said, I’ve no idea who’s right in physics, but it’s eerily similar to what one might call ‘epistemological bubbles’ we’ve seen elsewhere. We’re accustomed to quoting old dead sciences like phrenology. But modern sciences have bad problems like this too.
Economics has spend more than it’s fair share of time barking up the wrong trees and on wild goose chases. Often the reason is hubris – the practitioners get hungry for grand syntheses when their knowledge is way too puny to support it. I think of the search for micro-foundations in economics as like this. Keynes’ micro-foundations were all that were necessary for macro-economics. That is one knows broadly what human beings are like – they’re fairly rational but also given to various other types of less rational, more emotional behaviour of various well known kinds.
You don’t need much more than that to investigate various macro-properties of aggregations of such agents. One doesn’t need optimising micro-foundations to tell the stories of macro-economics – bubbles, busts, liquidity traps, steady growth and so on.
But this kind of informality and improvisation was ruled out on grounds which related more to the certain aesthetics of the discipline as it was then coming to be practiced rather than on stricter scientific grounds of realism or usefulness. (Indeed there’s a truly strange little debate begun by Milton Friedman about whether or not you want your assumptions to be ‘realistic’ – but I’ll leave that to one side here.)
Thus it was simply presumed that micro-foundations could only be of an optimising form (even though as Herbert Simon demonstrated, theoretically and from observation, that human beings didn’t do much optimising – they followed rules of thumb).
In addition to this grand hubris, there’s an analogous kind of micro-hubris. It goes like this. Although practitioners are not after some grand theory, it becomes clear that important parts of the story are being left out of the discipline’s formal constructs. The old way of handling this – say Alfred Marshall’s or J.R. Hicks’ way of thinking of economies of scale in trade – is to say ‘these phenomena are both real and important. So they should not be forgotten, but they’re very difficult to model, so we’ll have to try to take them into account as we go – informally.’
But we’ve seen all sorts of strange concoctions in the last few decades that take economics in directions which are useless or postivitely strange. The example I know best is Strategic or New Trade theory. Here economists began modelling scale economies in trade beginning in the 1980s – indeed around about the time string theory got going!
Now no-one can blame anyone for having a crack at solving a formal problem (acommodating scale economies into trade theory) that had eluded economists previously – especially when there are some new formal modelling techniques around. But you’d think that prominent in their work would be some consideration of why it failed in the past and why it could be expected to be successful this time. If anyone can find any such serious consideration (say lasting substantially more than a couple of sentences) in any major milestone article on strategic trade theory in its first decade I’ll donate $100 to their nominated charity.
As it turned out, a few decades passed and nothing much emerged except stuff that was intuitively clear at the outset. Thus strategic trade theory offered itself as a comprehensive rebuilding of trade theory and ended up with a few commonsenscal conclusions – namely:
- that scale economies make the case free trade trickier to prove formally – by creating circumstances where policy makers can succeed in acting strategically; but
- scale economies also make trade itself (and so free trade) more important because with scale economies there are more gains to trade, more gains from specialisation.
In macro, here’s Krugman observing that, despite all the theoretical controversy and formal development of macroeconomics in the last few decades, the basic apparatus developed during Keynesian revolution from the late 30s through to the 1950s remains the most useful apparatus we have for thinking about macro-economic policy problems.
Another example is ‘rational expectations’. [Postscript, if Krugman is regarded as ‘left’ – something that might have surprised him a decade or so ago, then here’s (pdf) Greg Mankiw a Bush economist arguing pretty much the same thing.]
Nothing wrong with trying to accommodate expectations into theory – that was obviously important. It’s that word ‘rational’ that is the problem. The way ‘rational expectations’ works is to insert the formal predictions of some theory into the expectations of the agents we are modelling. The problem is that the world is not like that. Agents have different theories and take different positions, so taking account of changing expectations in macro-markets is a bit like taking account of scale economies in trade you can do it, but you do such violence to what’s going on by doing so that the deprecated alternative of a more informal understanding of the phenomenon might be better.
Then there’s real business cycles, which are truly strange. Real business cycles generate macro-economic phenomena like booms and busts entirely from optimising rational micro-foundations. This usually requires certain parameters (like the income elasticity of labour supply) to be around ten times higher than they appear to those dull and worthy souls who go out into the empirical world to try to measure them. Real business cycles typically model recessions as driven by micro-economic changes (often changes in technology). When some technology shock comes along (say computer technology improves productivity and wages in some sectors and not others, forward looking people redo those labour/leisure optimisation calculations they’re always re-running to make sure things are still optimal (for instance your local bus driver). They then decide to take an unpaid holiday for a few years. Voila recessions explained. (I am not joking).
In a remarkably prescient and powerful piece, Adam Smith argued around two centuries before Thomas Kuhn made ‘paradigm shifts’ all the rage, that aesthetics drives science. He was right – but ‘epistemological bubbles’ illustrate the downside when the aesthetics go awry when they become a self contained echo-chamber rather than existing in a healthy dialectic with the world, and with practical usefulness.