I have a long-percolating post that I hope to finish soon (when everything else is finished!) on “Why String Theory Must Be Right.” Not because it actually must be right, of course; it’s an hypothesis that will ultimately have to be tested against data. But there are very good reasons to think that something like string theory is going to be part of the ultimate understanding of quantum gravity, and it would be nice if more people knew what those reasons were.
Of course, it would be even nicer if those reasons were explained (to interested non-physicists as well as other physicists who are not specialists) by string theorists themselves. Unfortunately, they’re not. Most string theorists (not all, obviously; there are laudable exceptions) seem to not deem it worth their time to make much of an effort to explain why this theory with no empirical support whatsoever is nevertheless so promising. (Which it is.) Meanwhile, people who think that string theory has hit a dead end and should admit defeat — who are a tiny minority of those who are well-informed about the subject — are getting their message out with devastating effectiveness.
The latest manifestation of this trend is this video dialogue on Bloggingheads.tv, featuring science writers John Horgan and George Johnson. (Via Not Even Wrong.) Horgan is explicitly anti-string theory, while Johnson is more willing to admit that it might be worthwhile, and he’s not really qualified to pass judgment. But you’ll hear things like “string theory is just not a serious enterprise,” and see it compared to pseudoscience, postmodernism, and theology. (Pick the boogeyman of your choice!)
One of their pieces of evidence for the decline of string theory is a recent public debate between Brian Greene and Lawrence Krauss about the status of string theory. They seemed to take the very existence of such a debate as evidence that string theory isn’t really science any more — as if serious scientific subjects were never to be debated in public. Peter Woit agrees that “things are not looking good for a physical theory when there start being public debates on the subject”; indeed, I’m just about ready to give up on evolution for just that reason.
In their rush to find evidence for the conclusion they want to reach, everyone seems to be ignoring the fact that having public debates is actually a good thing, whatever the state of health of a particular field might be. The existence of a public debate isn’t evidence that a field is in trouble; it’s evidence that there is an unresolved scientific question about which many people are interested, which is wonderful. Science writers, of all people, should understand this. It’s not our job as researchers to hide away from the rest of the world until we’re absolutely sure that we’ve figured it all out, and only then share what we’ve learned; science is a process, and it needn’t be an especially esoteric one. There’s nothing illegitimate or unsavory about allowing the hoi-polloi the occasional glimpse at how the sausage is made.
What is illegitimate is when the view thereby provided is highly distorted. I’ve long supported the rights of stringy skeptics to get their arguments out to a wide audience, even if I don’t agree with them myself. The correct response on the part of those of us who appreciate the promise of string theory is to come back with our (vastly superior, of course) counter-arguments. The free market of ideas, I’m sure you’ve heard it all before.
Come on, string theorists! Make some effort to explain to everyone why this set of lofty speculations is as promising as you know it to be. It won’t hurt too much, really.
Update: Just to clarify the background of the above-mentioned debate. The original idea did not come from Brian or Lawrence; it was organized (they’ve told me) by the Smithsonian to generate interest and excitement for the adventure of particle physics, especially in the DC area, and they agreed to participate to help achieve this laudable purpose. The fact, as mentioned on Bloggingheads, that the participants were joking and enjoying themselves is evidence that they are friends who respect each other and understand that they are ultimately on the same side; not evidence that string theory itself is a joke.
It would be a shame if leading scientists were discouraged from participating in such events out of fear that discussing controversies in public gave people the wrong impression about the health of their field.
Oh good grief, if Paul Davies and John Wheeler are correct, then evolutionary theory is missing a key piece to the purposefully structured puzzle that only creationists and semi-religious foundations, like the templeton organization, support, so don’t pretend that Lynn Marguilis doesn’t call extremists among her own peers, “neodarwinian bullies” because she thinks that the Dawkins mentality isn’t equally over the antifanatical top!
Doncha just love how they only see the other side’s dogma?
Just to bait the readers here — nobody has answered my question: what does one end up with if one promotes dimension to an operator? There are good reasons to think that dimension may be more than just a constant — for example: as energy density increases, the gravitational manifold couples to itself more and more strongly — one could think of this as a “cloud of virtual gravitons” around a point mass, for example — and those virtual gravitons themselves perturb the manifold (no longer a manifold) further (as they are energetic), resulting in more “virtual gravitons”, etc, etc, effectively resulting in a fractal pattern type distortion of the “manifold” in regions of very high energy density. This “fractal” distortion thus changes the Hausdorff dimension in a local region around a point mass.
So one might thing that dimension is in fact an operator related in some way to Hamiltonian density.
What happens if this is done?
To quote James Thurber: Fools rush in where angels fear to tread – but many fools are alive, while all the angels are dead.
So, let me risk being a fool and pose the following question: What’s the physics motivation for String Theory?
I’m serious – what is the physics that is driving this train?
At the start of the 20th century, physicists made theoretical proposals that were at least as unconventional and counter-intuitive as anything in String Theory. A key difference is that physicists were trying to explain experimental observations for which classical physics could not account, which drove the development of quantum mechanics.
What is the experimental observation that is motivating String Theory, that can not be explained with conventional physics? By the way, I personally am in the Feynmann camp. That is – while I think it would be cool if quantum mechanics and gravity could be reconciled into a single theory – until such a theory is developed, that accounts for the observed world, I’m agnostic on whether or not such a theory even exists.
My reference to Feynmann refers to an interview he gave, wherein he was asked about the latest trends in HEP Theory, that seemed to use mathematical elegance as a guide. He allowed that while in the past, physical theories that turned out to be correct were indeed mathematically beautiful, there is no reason to suppose that this would continue, and that it was perfectly possible (in his opinion) that peeling matter down to the next layer might reveal something mathematically complex, lacking all elegance. For what does Nature care whether we find the equations pretty or not?
Now – here’s where my ignorance may have led me int a trap. Is it that, from our understanding of the Big Bang and the evolution of the universe, that at some point in the distant past – for the merest fraction of a second, gravity and QM MUST have been the same? Is that the motivation? (I’m not trying to be snarky – I don’t know). [My own research field is in Experimental Condensed Matter – developing materials for solar cells and doing some collaborating with a Neuroscientist – where we may have found something relevant for understanding of Parkinson’s].
If that is the only physical motivation (that at some point in the early universe gravity and QM must have been joined), then this seems to be thin soup.
Sean, you mention String Theory as being promising. And I agree, it is. But when we hire a promising assistant professor, we give him or her seven years to pay off on their potential. At some point the plug gets pulled. What would you agree to be a fair timeline for String Theory? Eventually I imagine the field will vote with their feet, and researchers will move on.
Some of my colleagues are concerned about blowback. There was a great deal of attention when high temperature superconductivity was discovered, but its now 20 years and the trains still do not levitate. Some early promises have had to be abandoned. String Theory has been presented to a considerable subset of the non-physics professor community. I think many do not recognize the provisional nature of the theory. If it all turns out to be a false approach (of which there have been many in physics, and it is after all how we get to correct theories, crawling over the corpses of incorrect ones) will there be a backlash with the public?
Well, I have to go pick up my son. I think I’ve embarassed myself enough with this post.
And I see that while I was writing, Tim G two posts above gets at my point in 1/4 the space, and better.
There are many very good reasons to believe that string theory has something to do with quantum gravity in the real world. That’s why so many smart people are devoting their professional lives to working on it. It’s much better behaved at short distances than conventional field theories; it has proven fruitful in shedding insight on longstanding problems (such as a microscopic understanding of black-hole entropy); and by dualities it is part and parcel of our understanding of field theory itself. More details will have to wait for the post foreshadowed above — or until more string theorists decide to try harder to explain themselves.
Jim:
That’s a good question. One experimental observable that I think any quantum gravity theory should probably explain would be the value of the cosmological constant (or the properties of dark energy).
None have done so yet. (Model constructions .. not predictions! … aside).
Another example is so ubiquitous people often just forget that it is an experimental observable. That is the fact that, at least macroscopically, our universe has 3+1 dimensions. I think any quantum gravity theory should explain _why_, at macroscopic scales, Minkowski space is the appropriate metric (again, not as a possible construction/”compactification”, but an actual prediction).
Hi Jim.
The problem with quantum mechanics and gravity is not that they’re separate; it’s that they are incompatible. You need to turn quantum mechanical matter into classical gravitation and nobody knows how to do that. Now, maybe the problem is just too hard to solve without experiments to guide us, but the problem is there, and it has to have a solution.
Ellipses:
You raise two excellent observables. Now, I’m not, by any stretch, an expert on String Theory. Is this what the theorists are trying to explain? I thought that rather than account for our 3 + 1 dimensions, we are to take seriously the existence of higher dimensions, which are hiding in very small or very large length scales.
[The geek in me loves the notion that the exctra dimensions may be vary large. I call that the Good Omens Hypothesis – in Neil Gaimen and Terry Pratchets book of the same name, they explain why psychics were unable to detect the “aura” around the son of the Devil – they couldn’t see it for the same reason that you can’t see England when you’re standing in Hyde Park]
Sean:
You said: “There are many very good reasons to believe that string theory has something to do with quantum gravity in the real world.”
I don’t doubt it. but what are the real world arguments for quantum gravity? What physical phenomena have we observed that must include both? No one has observed Hawking Radiation, or anything like it, have they?
Why do string theorists seem so certain that this is the only possible answer? (Or am I overestimating their sense of certainty based on message board rhetoric?)
I know next to no one in the field who believes that string theory is the only possible answer. I do know lots of people who believe that it must be some element of the right answer, however.
“… it [string] has proven fruitful in shedding insight on longstanding problems (such as a microscopic understanding of black-hole entropy); and by dualities …” – Sean Carroll
If I can quote a contrary argument:
“The claim to explain black-hole entropy is (as noted in chapter 9) exaggerated, because the string theory results work only for special and aytpical black holes.”
– Lee Smolin, The Trouble with Physics, US ed., p. 277. (In chapter 9, Smolin explains that Andrew Strominger and Cumrun Vafa showed that extremal black holes, i.e., black holes with about maximum possible electric/magnetic charge while remaining stable, have identical thermodynamics to extremal stringy branes.)
Regarding Maldacena’s duality between string theory and gauge theory:
“Even if it is true, the duality conjecture can be useful only if one side of the duality can be defined precisely. So far, it has been possible to define the relevant version of string theory only in special cases.”
– Lee Smolin, The Trouble with Physics, US ed., p. 143.
Aaron:
Thank you. I see. Sorry for being dense.
This reminds me of a conversation I had over ten years ago, with Sir Sam Edwards. He argued that since we were unable to reconcile Quantum Mechanics with General Relativity – that we should toss out General Relativity. After all – the list of experimental confirmations for GR is MUCH shorter than for QM. My teenagers could live without GR, but not without their cell phones, laptops, DVD’s iPods, CD players – none of which are possible without the transistor and/or the laser, neither of which could be invented without QM.
So perhaps there’s a theory of Gravity that can do what GR can, plus be quantized – just as GR could do what Newton could, plus some extra bits. Is this the physical motivation for String Theory? That it intends to replace GR with a new, quantizable theory of gravity? But there are many other alternative models for quantum gravity. I know that some are concerned that these other appraoches are being starved for oxygen , due to all the love garnered by String Theory. Which again raises the timeline question.
Thanks again.
The paper that I referred to is indeed hep-th/0703280 (Chen, Li, Mayes, Nanopoulos) which will be available on the archive tonight. We describe a three-generation model in which the gauge couplings are unified and for which we can calculate all of the Yukawa mass matrices and mixings, as well as the low-energy phenomenology. This is the first model that we are aware of that has all of these properties. It is true that the muon and electron masses are a little off. However, it should be kept in mind that these states are very light and subject to quantum corrections.
Eric
(sorry for the typo: aytpical should be atypical)
You raise two excellent observables. Now, I’m not, by any stretch, an expert on String Theory. Is this what the theorists are trying to explain? I thought that rather than account for our 3 + 1 dimensions, we are to take seriously the existence of higher dimensions, which are hiding in very small or very large length scales.
Yes — in trying to reconcile GR with QM, string theorists (not me, by any means — I’m an experimental particle & astroparticle physicist — if you must know, junior faculty at a major research university) posit an 11-D space, which then is “compactified” at macroscopic length scales. Why one precisely ends up with Minkowski space, though, has _never_ been given a good explanation, to my knowledge.
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As an aside, no one has ever answered my question (comment #52). Anyone want to bite?
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Anxiously awaiting Mayes’ hep-th/0703280 to see if it explains the universe. I’m sitting down.
“Seventh, evolution predicts a common ancestor for all current forms of life. This prediction has been tested with DNA sequencing, most spectacularly with mitochondrial RNA sequencing, and has been proven true.”
Always knew ‘everything’ was related,
Everything is related, and everyone too.
So which came first the chicken or the egg?
“Like all quanta, the photon has both wave and particle properties, exhibiting wave—particle duality.”
So which came first the ‘particle’ or the ‘wave’?
> Anxiously awaiting Mayes’ hep-th/0703280 to see if it explains the universe.
You can read it here.
Arxiv bug making papers available early? Hmm.
So, Eric Mayes, since this apparently is not an April Fool’s joke: does your model have a cosmological moduli problem?
If by cosmological moduli problem you mean the problem of moduli stablization, it has already been shown that all moduli can be stabilized in the model by turning on fluxes (Chen, Li, Nanopoulos ’06). If you’re referring to the the problem of obtaining a cosmological constant, then that is a story we’ll have more on later which will likely involve noncritical string theory.
Eric,
I don’t want to be too harsh on your paper, because I think models like these have some value, but I’d like you to explain:
1) Why did you choose: epsilon^{D1} = 0.061 and kappa^{(1)} = 39.6i. To match the data, right? _That’s not a prediction, my friend_. That is a post-diction to match what experimentalists have already told you.
2) Same for v^1_u, v^2_u, v^3_u, etc. etc. etc.
3) Where are the CP phases in either the CKM or PMNS matrices? The latter would be especially nice to know because it would be a _prediction_ (you haven’t forgotten what that means, have you?)
I have my physics diploma from 10 years ago. Now I earn my money with software and physics is kind of a hobby. So at least I have no personal stake in the subject.
My 50 cent are, that string theory is loosing the public debate, because Woit and especially Smolin have the better arguments. To me it looks just that simple. You can say I am too dumb because I am not able to follow the complicated mathematics of string theory (still struggeling with ordinary field theory) (@Sean: I was however able to follow your three hours introduction to general relativity, which is available online, very nice). But so far I have not read a reply to Smolin’s book from a string theorist that really convinced me. The ideas presented by Lee (Loop quantum gravity, deformed special relativity) just seem more exciting and really falsifiable.
“Ockham’s razor tells you that LQG is closer to reality.”
Ockham’s razor is a load of crap. The “aether” was seemingly simple and straightforward, yet it was wrong. a^4 + b^4 + c^4 = d^4 was long thought to have no integer solutions. That was wrong too. Goethe (or somebody) said that once you scratch the surface of something “simple”, you find it’s complex and deep.
Eric, sorry — what I actually meant to ask was, do you have a moduli-induced gravitino problem? (Moduli decaying to the gravitino, which is long-lived, and can screw up BBN etc…)
Sean,
The proposed title “Why String Theory Must Be Right” falls into the rhetorical trap-hole that both string theorists and their detractors have collaborated in digging. I know you don’t actually believe that it is necessarily right and therefore experimental verification is merely a filigree on its mathematical edifice, but that is the impression such a title gives.
In my limited understanding a more accurate, but less catchy title would be “Why String Theory Must Be Useful [in constructing a theory of quantum gravity, whatever that theory turns out to be in the long run].” Sometimes, a theory that is useful is even more worthwhile than one that might in the long run be right. I didn’t watch any of the Horgan/Johnson exchange because I’m not interested in the rhetoric surrounding this subject. One thing that string theorists have not always succeeded in getting across is that (so far as I know) constructing a mathematically consistent and elegant theory that reduces to our current low-energy understanding of the universe is difficult. So working out such a theory would be a major mathematical-physics success even if it made no new experimental predictions.
Part of the problem is that string theory is legitimately difficult to explain, and the culture of superiority that has existed among high-energy theorists (We are the most fundamental, etc) has meant that they haven’t always bothered to try. This has alienated even many fellow scientists, let alone the public. (I am not a theorist, but I did go to physics grad school at Rutgers, so I am quite familiar with this phenomenon.) I’ve seen Ed Witten give a brilliant talk in which he made us feel for half an hour like we sort of understood string theory. An hour later, I realized I still didn’t understand it at all. But, he was talking to the audience, rather than engaging in triumphalist rhetoric. I do think one reason string theory is getting this flak is that people, including PhD physicists, are threatened by things they don’t understand, especially when the proponents are overbearing.
Sean,
Don’t you think there’s something funny going on here when you and others assume that a string phenomenology paper is an April Fool’s joke, but its authors don’t seem to think so?
“[string theory is] much better behaved at short distances than conventional field theories”
This is really a misleading statement. Yang-Mills theories, the ones that are the main component of the Standard Model, are precisely formulated even outside of perturbation theory, and asymptotically free. Their short distance behavior is arguably better than that of string theory, where one doesn’t have a non-perturbative definition. Perhaps you were just referring to “conventional” qfts of quantum gravity, but if so you should make that clear. And even there, the recent evidence about finiteness of N=8 supergravity may make this statement incorrect.
Rob,
I’ve never claimed there is anything wrong with working on speculative ideas that can’t be immediately tested. That’s a misinterpretation of things I have written. I actually spend as much of my time as I can manage working on such speculative ideas.
You’re right that the crucial question is how such work gets evaluated. It’s not true that working on something speculative that isn’t successful will destroy one’s career. Lots of people doing this have tenure. The people participating in these public debates all have tenure, and aren’t going to lose their jobs if they lose the debate (my situation is somewhat special: I don’t have tenure, but do have a pretty secure permanent position). The problem is mainly for junior people, where the reward structure is such that getting another job and getting tenure requires that they either work on certain widely accepted speculative ideas, or, if they decide to do something else, are both lucky and brilliant enough to make a big breakthrough. This reward structure would make sense if these widely accepted speculative ideas seemed to be working out, it’s counter-productive when there’s strong evidence that they are not working out as hoped.