When I was young, my main scientific interest was in astronomy, and to prove it there’s a very geeky picture of me with my telescope on display in my apartment, causing much amusement to my guests (no way will I ever allow it to be digitized, I must ensure that it never appears on the web). By the time I got to college, my interests had shifted to physics, and since that time I’ve hardly at all kept up with what is going on in astronomy. Like everyone, I’m still fascinated by the amazing pictures coming out of the field, and like most particle physicists, I’m deeply jealous of astronomers for the fact that they have a wealth of exciting new data to work with, together with promising prospects of lots more to come.
This week there’s a big meeting of the American Astronomical Society going on in Seattle, producing lots of astronomy news. Many bloggers are in attendance, including Rob Knop, Steinn Sigurdsson, Phil Plait, and C.C. Petersen. Rumors that celebrity couple Sean Carroll and Jennifer Ouellette were there turned out to be partially unfounded. Lots of press releases are being generated, including one from the University of Washington full of the usual overhyped claims about cosmic superstrings.
This week’s Science has a special issue on particle astrophysics, with lots of articles worth reading, including a nice summary of the exciting things happening in the field by Adrian Cho. He reports that many experimental particle physicists have moved into the field, partly because of the opportunities there, partly because of the difficult situation of experimental particle physics, especially in the U.S. Michael Turner is quoted explaining that the particle physicists have brought to the field some ambitious ideas, due to their habit of “thinking big”:
These are not people who are afraid to ask for big things, and they’re used to people saying yes.
An example of this is the IceCube neutrino experiment being put together under the ice in Antarctica, employing 400 researchers and costing $271 million.
Turner also has an article summarizing the situation in cosmology, where he notes that many string theorists are now pinning their hopes on making some connection to the real world in this context:
Nowhere in particle physics are the stakes higher than for string theory. If string theory is to live up to its billing as “the theory of everything” rather than, as some say, a theory of nothing, it needs a home run. Because most of its current predictions exceed the reach of terrestrial laboratories, many string theorists are pinning their hopes on a cosmological home run, such as a fundamental understanding of inflation (or a more attractive alternative), a solution to the puzzle of cosmic acceleration, or insight into the nature of the Big Bang itself.
For something truly bizarre, check out the cover story of the February issue of Astronomy magazine, entitled “What if string theory is wrong?” (mentioned earlier here). It confirms me in my opinion that I shouldn’t write about things I don’t know much about, like astronomy, since it’s by an astronomer who clearly knows very little about particle physics, especially about supersymmetry:
Supersymmetry is a mathematical principle that allows force-carrying particles, such as photons and gluons, to transform into one another. It also allows the unification of gravity with other forces because its particle, which some call the graviton, can transform into one of the other force-carriers…. If extra dimensions don’t exist, then supersymmetry doesn’t either… Without supersymmetry, some physicists have proven that the energy of empty space would be so enormous the universe would instantly collapse. Only by understanding physics beyond the standard model can we hope to understand how the vacuum works and the universe’s dark side. And only string theory appears able to serve as a reliable mathematical guide to that larger universe.
Lenny Susskind provides the usual over-the-top outrageous quote:
It is hard to find a serious paper about particle phenomenology that doesn’t in some way use the tools of superstring theory.
The author seems to believe that there’s some sort of experimental evidence of string theory and that it is just like general relativity:
While string theory is sparse on experimental validation, the situation is not so different from general relativity in its early days, when difficult mathematics made calculating a prediction extremely challenging.
and somehow thinks that string theory is the only hope for the future of physics:
Without superstring theory, we’d lose the intriguing prospects for the multiverse, with its infinite and eternal creativity in spawning new universes… More immediately, dark matter and dark energy would remain imponderable enigmas, shorn of any clues about where they come from. Astronomers can live without knowing the quantum properties of gravity. But to learn that 96 percent of the cosmos is unknowable would be a bitter pill to swallow. It would be even worse for physicists. Without a logical framework in which to pose and answer questions, our inquiries into the fundamental aspects of the physical world would devolve into semantic quibbles.
Some days I think that there’s definitely a more realistic view of string theory out there, other days I’m not so sure…
Update: It seems that Edward Witten is attending the AAS meeting, although not speaking there. See the comment from David Cobden, and Steins Sigurdsson’s blog entry from the conference Trendspotting, where he reports:
On a completely unrelated note, Ed Witten was spotted wandering the halls…
Now there is always some cosmic string or quantum cosmo thingy going on here, but what we ask (and, yes, I did actually ask), was he doing in the extrasolar planet session?
Ed likes exoplanets!
Dood.
Update: Science a Gogo has an article about this, String Theory? Knot!, which uses my characterization of Susskind’s quote as “over-the-top”, but then uses the wrong quote, using something from the Astronomy magazine article which wasn’t written by Susskind.
Update: The University of Washington press release on cosmic superstrings, based upon a poster presented at the AAS meeting, has made it to Fox News (via Lubos).
“Without superstring theory, we’d lose the intriguing prospects for the multiverse, with its infinite and eternal creativity in spawning new universes…”
I don’t know why, but I find that quote laugh-out-loud funny. One pictures mournful physicists lamenting the good old days when the multiverse had infinite and eternal creativity.
Google Translate
Translate Text
Original text:
Breathless Popular Science Hype to English
maybe all your quotes are meant to be somewhat overhyped, so I should not add that IceCube was built thanks to the experimentalists who spent many years developing the necessary technology with smaller experiments (from Baikal to Amanda), and because astrophysics suggests that a km^3 detector is needed to probably start seeing neutrino events from astrophysical sources.
Thank you for the quotes from Astronomy. I have always preferred Sky and Telescope to it, but it does not harm to be given additional good reasons now and then…
Cheers,
T.
General Relativity: published 1915.
First exact solution (Schwarzschild)- 1916
Prediction of perihelion shift of Mercury – c. 1916
Bending of spacetime by gravity (Eddington) 1919
Gravitational Red Shift (Pound/Rebka) 1959
String Theory: [this line unintentionally blank]
It is hard to find a serious paper about particle phenomenology that doesn’t in some way use the tools of superstring theory.
What exactly might he mean with ‘the tools of superstring theory’?
Susskind, by making such unfounded statements, must realize he only succeeds in destroying any scientific reputation he may have enjoyed.
As far as Physics is concerned, if he truly believes his statement and is not just engaging in some form of self-promotion, then he is truly delusional.
“Without superstring theory, we’d lose the intriguing prospects for the multiverse, with its infinite and eternal creativity in spawning new universes…”
Actually, you don’t lose anything. I don’t need string theory (or any other theory or even any math) to conclude that we’re in a bubble called the “universe” (or anything you want to call it) and anything (if anything) out there outside this “bubble” could be called another “dimension” (again, whatever you like to call it).
And even with this purely philosophical reasoning I’m not going to be any more right or wrong than any other reasoning of whatever might be out there or not.
It’s kind of like what’s going to happen after death. There’s lots of theories of what might happen with each one as likely to be right or wrong, as no one’s came back to tell what really happened.
So my message to all physicists who happen to read this is: instead of wasting all your life on theories which aren’t even remotely useful for anything practical why not instead do something useful, like figure out how to get fusion reactors working (and when it comes to funding, this is where the money should go).
Lenny Susskind said:
It is hard to find a serious paper about particle phenomenology that doesn’t in some way use the tools of superstring theory.
Obviously Susskind doesn’t know how to do a literature search.
But to learn that 96 percent of the cosmos is unknowable would be a bitter pill to swallow.
Is Susskind referring to the multiverse here? If so, shouldn’t that be “99.9999999…. percent of the cosmos is unknowable”?
Dear Peter & co,
How successful has been string cosmology, including Randall’s & Sundrum braneworld models? How successful has loop cosmology been? I understand that there probably isn’t much in the way of hard quantitative predictions, but what about soft or generic predictions, that would differ and be potentially observable, from current SM+GR? (i.e I am aware string cosmology offers SUSY-partners as DM candidates. Anything else?)
curious
Dan
Dan,
As far as I know, none of “string cosmology”, braneworld models, loop cosmology make any predictions, hard, soft, generic or whatever. The SUSY dark matter candidate is a feature of SUSY, not of strings, branes, loops…
Hi Peter,
Ok thanks. When I google there are a lot of hits, such as arxiv.org/abs/astro-ph/0303499 and arxiv.org/abs/astro-ph/0303499 but I am unsure what real astronomers think
“[T]o learn that 96 percent of the cosmos is unknowable would be a bitter pill to swallow. It would be even worse for physicists. Without a logical framework in which to pose and answer questions, our inquiries into the fundamental aspects of the physical world would devolve into semantic quibbles.”
Perhaps the author is suggesting that if string theory must be abandoned for whatever reason, its cultists will (must!) Drink The Coolaid instead of soldiering on in a purposeless world! The IAS as Jonestown! Well, in that case, maybe it will be sherry, not Coolaid.
Paul Says:
January 9th, 2007 at 6:03 pm
a) If three-dimensional space (let’s omit the “fourth dimension of time”, for a moment, reason given below) is “curved” by gravity, what is it curving into? For example, if you place a bowling ball on a bedspread, it is curving *from* the second dimension *into* the third dimension.
Paul,
The answer to your question is very subtle. To understand curved
space, it isn’t necessary to curve that space into anything. This was discovered by Gauss and Riemann, and is a fact of “intrinsic geometry”.
To see how this works, imagine a curved sheet of rubber. Draw
coordinates (that is a graph) on the rubber. Now mash it flat into the plane. The coordinates will remain on the sheet, but won’t be
flat. The intrinsic curvature is still there, due to the stresses
rubber. On the other hand, if you take a flat piece of rubber
and bend it (without otherwise straining it), it remains intrinsically
flat, though there is now what is called extrinsic curvature.
Einstein’s equations of gravity relate the intrinsic curvature of
spacetime to matter. They say nothing about extrinsic curvature,
i.e. how space-time is embedded in some space of more than
four dimensions.
Implicit in your question is another about curvature of
three-dimensional space, as opposed to four-dimensional
space-time. Many popular books incorrectly explain
gravity as being due to curvature of space. The bowling-
ball/bedspread picture is NOT really the right way to think
about gravity. In the most natural (either comoving or
Schwartzchild) coordinates, the spatial curvature is
extremely small. Newtonian gravity is not at all related to
spacial curvature, in these coordinates, but in space-time
curvature. There is some curvature of three-dimensional space, responsible for some of the shift of the perihelion of mercury,
but it is very small.
The physical way to think about gravity is this… In so-called
comoving, or inertial coordinates, space falls into energy
(which by E=mc^2 includes matter). Particles in this space fall
with it. The bedsheet is slightly curved by the bowling ball, but
Newtonian gravity is due to the bedsheet being sucked into the
bowling ball!
One can choose other coordinates, where spacial curvature
is big, but they are not particularly useful.
Dan, there are plenty of specific models of inflation in string theory. They can involve things like a brane and an anti-brane in the extra dimensions which move toward each other and annihilate. While they move, the universe is expanding (inflation); when they annihilate, lots of particles are produced. Contrary to what Peter says, any specific model of this type does make specific numerical predictions for things like the CMB power spectrum. The trouble is that there are a lot of models of this type, and not enough numbers measured well by experiments to select among them, at least so far.
Please Peter O. and others. This kind of discussion of the basics of GR is off-topic and has nothing at all to do with the posting. This is not intended as a general physics discussion forum, which is something I have no interest in hosting or trying to moderate.
anonymous,
Again, there is no such thing as a “prediction” of string cosmology, or a “prediction” of brane world scenarios about cosmology. These ideas cannot tell you what the next generation of experiments is going to see, or the one after that. There are so many variants of these models and lots of parameters, so much so that these ideas are vacuous in terms of making anything anyone would call a legitimate prediction. If you disagree, tell us what “string cosmology” or brane-world scenarios says the next generation of CMB experiments will see, and be willing to agree that string theory is wrong if they don’t see this.
Specific models make specific predictions. Nothing’s going to say that string theory is wrong, but various scenarios make unambiguous predictions and are falsifiable.
Aaron,
“string cosmology” is not falsifiable
“brane world cosmology” is not falsifiable
I have no idea about loop cosmology.
Sure, you can come up with highly complicated, highly speculative models that don’t really explain anything about observable physics but make “predictions”. I could make up a “scenario” which involves God writing the lyrics to Lucy in the Sky With Diamonds in Morse code and imprinting these in the CMB at a certain amplitude. This would make definite predictions, and be completely falsifiable.
I’m not an astronomer, but I sent your quotes to the astonomers I know (there are not that many – but they do include people working at Harvard-Smithsonian). They weren’t even aware that string theory offered any explanation whatsoever for Dark Energy and Dark Matter. They tell me the most likely candidate for Dark Matter is “axions”, which are hypothetical particles that arise from supersymmetry in the standard model (no strings there). The most likely candidate for Dark Energy is said to be some kind of small positive vacuum energy due to something about the vacuum or fields that we don’t understand (again, no strings needed). These astronomers agree that while positive vacuum energy many constrain string theory, there are no constraints of any sort from string theory to observations.
For the record: The astonomers I asked are actually now directly involved in observation projects regarding Dark Energy and Dark Matter. Just thought I’d mention that.
Aaron Bergman,
“Nothing’s going to say that string theory is wrong.”
Please. Look up the definition of the science method somewhere and then rethink your statement.
There is another quote by Pauli, lesser known, but perhaps with more scientific content and less sarcasm than the title of Peter’s book. Pauli pointed out that there is always a continuum of theories that agree with a given phenomena, but the test of a theory is that it predicts something new.
Note Pauli did not say postdict.
Note that string theory fails this test miserably.
Pauli arrived at this observation, in my opinion, because his genius was so great that he could and did easily discover alternate formulations and approaches.
There is so far no compelling reason to believe string theory in its current form and considerable reasons not to, that is unless you are like Susskind and have invested your professional identity with it…and also consider yourself one of the fathers of the theory.
Dan & other anonymous,
models with parallel branes in extra dimensions have been studied hoping that a) their potential energy naturally gives the flat potential needed for inflation; b) their final collision generates the particles we see.
However a) turned out to be false, after that the size of the extra dimensions is stabilized. Furthermore b) when inhomogeneties in the matter density can be computed, their spectral index comes out very tilted, unlike the observed one. And c) in any case the idea that some God liked to start the Universe putting two perfectly parallel branes looks as naive as the ancient oriental cosmology where the Earth stays above a turtle above a turtle…
correction: probably this cosmology is not naive from the modern point of view of String Naturalness: perfectly parallel branes can be justified by invoking the Anthropic Principle.
I am sorry, but for a moment I forgot that Susskind told us that we must use the tools of superstring theory to do something serious.
Dear Ari,
`So my message to all physicists who happen to read this is: instead of wasting all your life on theories which aren’t even remotely useful for anything practical why not instead do something useful, like figure out how to get fusion reactors working (and when it comes to funding, this is where the money should go). ‘
I agree with you. All philosophizing about mere matters of belief,
such as the multiverse, is even culturally irrelevant if our very culture won’t survive the looming climatological catastrophe we
will have to face in the absence of a viable alternative to the present way of fueling or energy needs. I personally believe that fusion with
magnetically confined plasmas will be viable if we manage to
apply genuine knowledge gained about electroweak symmetry breaking (theoretically and at the LHC) to this problem.
High regards.
Dear Dan,
Regarding loop quantum cosmology, this is a class of models, not yet the full quantum field theory. Nonetheless within them, there is a universal mechanism for eliminating the initial singularity. That mechanism implies modification of the power spectrum, at a level that may be observable in future CMB observations, as argued in astro-ph/0411124. Whether this leads to predictions from the full theory rather than models is the subject of ongoing work, see for example astro-ph/0611685.
Dear Lee Smolin & both anonymous
thanks. It seems string cosmology is also a class of models as is loop cosmology? Incidentally Lee, I’ve been hoping you and Bilson publish a followup preon paper 😉
“string cosmology” is not falsifiable
“brane world cosmology” is not falsifiable
I have no idea about loop cosmology.
Sure, you can come up with highly complicated, highly speculative models that don’t really explain anything about observable physics but make “predictions”.
Actually, some of them aren’t particularly complicated, and they do make honest predictions. I don’t understand what your problem with this is. The models can be ruled out.
How long will it take to rule out 10^500 models? How many per second can be ruled out? To do it within 1000 years (3*10^10 seconds) would mean ruling out ~10^490 models per second. The universe is only 4*10^17 seconds old, so to do it over the age of the universe would require ruling out only ~10^483 models per second.
It reminds me of SDI in 1985. Objectors to SDI dismissed it by working out that all the computers needed to shoot down Soviet missiles would take thousands of years to program. It’s too bad that nobody influential is using this argument against the Landscape.
Peter says the models “don’t really explain anything about observable physics,” but for most of us having a reasonably simple model of how inflation could work is pretty nice. As for what “anon y mus” says, I suggest reading more of the literature — it’s not always about exactly parallel branes, and there are some viable scenarios. It turns out one doesn’t even necessarily need slow-roll, brane inflation has suggested scenarios where the kinetic term matters more than the potential. Anyway, Dan, yes, brane cosmology suggests various models, which as Aaron says are falsifiable. What it doesn’t do is suggest a unique model, but neither did old, field-theoretic inflation. So I don’t really know what Peter’s so worked up about.
anonymous ,
If string theorists have to resort to an extremely speculative idea like inflation as evidence for an even more speculative idea, string theory, than that itself is telling. I do not consider such models meaningful.
Dear Dan,
Loop quantum cosmology must be a set of models for two reasons: 1) they incorporate diffrerent models of the inflaton. 2) They describe reductions to different homogeneous cosmologies, i.e. FRW, Bianchi I, Bianchi IX etc. These are harmless, they do not represent different low energy theories or low energy phenomenologies as do the string cosmologies. Moreover there are no fine tunings in the main results, such as the removal of singularities or corrections to the CMB spectrum.
At the same time, one would like to do better and know if the predictions found are genuine predictions of full quantum gravity rather than a quantization of a reduced set of degrees of freedom. This is the aim of some current work.
There is also a more speculative direction of work in which we postulate that, if geometry emerges from a low energy limit of a purely quantum geometry it may do so in a phase transition which leaves observable signatures. This work is very recent, see hep-th/0604120, and hep-th/0611197, but it appears that one can make distinct predictions for CMB observations on the basis of a few simple assumptions about that conjecttured phase transition, see astro-ph/0611695.
Thanks re preons. Work is presently underway, in collaboration with several people.
Thanks,
Lee
Aaron,
“I don’t understand what your problem with this is. The models can be ruled out.”
You decided to not quote the part of my comment that answers this, here it is again:
“I could make up a “scenario” which involves God writing the lyrics to Lucy in the Sky With Diamonds in Morse code and imprinting these in the CMB at a certain amplitude. This would make definite predictions, and be completely falsifiable.”
There’s more to doing science then coming up with “scenarios” based on pure speculation, you actually need some sort of evidence for them. What string theorists are doing now is saying that string theory can’t tell us anything about low energy physics, but it is science because it can tell us about cosmology. When asked what it tells us about cosmology, the answer is “in general, nothing, but it leads to all sorts of scenarios”. And the only evidence for these “scenarios” is that they are somehow compatible with the idea of string theory unification, determining low energy physics. The whole thing is kind of absurd.
anonymous,
I just don’t see how introducing branes improves the situation at all as far as “having a reasonably simple model” is concerned. It seems to do quite the opposite.
There’s more to doing science then coming up with “scenarios” based on pure speculation, you actually need some sort of evidence for them. What string theorists are doing now is saying that string theory can’t tell us anything about low energy physics, but it is science because it can tell us about cosmology.
Not everything has to be seen through this prism of string theory, spawn of satan or harmless diversion. If I just called them string inspired models, would that be ok? Take DBI-inflation, for example. Outside of its string theory inspiration, is there anything that makes it worse than other models for inflation?
Aaron,
Working on speculative ideas that really have nothing to do with string theory, and justifying this not because of evidence for the ideas, but because they are “string-inspired” doesn’t seem to me like a healthy way to do science. It’s also kind of intentionally misleading, intended to get people believing that “string theory makes predictions”, and writing completely absurd articles for Astronomy magazine.
Dear anonymous-who-wants-literature-for-problem-a): see pages 22,23 of hep-th/0610102 for a short review. In my opinion the fact that the generic idea does not help is more important than the fact that one can avoid some problem by cooking up specific models.
Working on speculative ideas that really have nothing to do with string theory, and justifying this not because of evidence for the ideas, but because they are “string-inspired” doesn’t seem to me like a healthy way to do science.
People aren’t working on string-inspired models because they’re string-inspired, but because they are new models. There are lots of models for inflation, some string-inspired, some not. I think they’re all worth investigating, don’t you?
If ever there was a gauntlet thrown at the feet of an apartment visitor with a cellphone camera, this is it:
I don’t think it’s entirely fair to say that just because you’re not making any predictions you’re not doing science.
Take early days of aerodynamics, for example. All they were doing was trying to understand what’s happening in their wind tunnels, that is, trying to understand real world phenomena.
Even though they weren’t making any predictions (at the time), everyone in the field still seem to think they were doing science.
Susskind is right.
String theorists occasionally use algebra, hence algebra is one of the “tools of superstring theory”.
Very few serious particle phenomenology papers avoid the use of algebra. QED.
Ari,
Experimentalists don’t need to be making predictions, but theorists are supposed to, and it’s theorists I was talking about.
Aaron B.,
Well, in general I don’t think all possible models (of inflation, or of anything else) are equally worth investigating. One is supposed to be concentrating on those for which one has a good scientific reason. If your only reason for deciding to work on one class of models is that it’s “string inspired”, that seems to me to be a problem.
This discussion of inflation is quite interesting. I can’t think of *any* development that was important in yielding the eventual standard models of cosmology and particle physics, that Peter couldn’t criticise along the same lines.
Gauge theory? Just a model. Can’t make a unique prediction, because it isn’t a unique model. Since you can’t falsify gauge theory, its not science.
Einstein’s theory of gravity? Has an infinity of solutions. If your favorite FRW cosmology doesn’t match data, you could choose a different solution. Thats not science.
In both cases, people chose a framework, built models as best they could, and eventually found that ONE describes nature pretty well. For gauge theory that happened to be 3-2-1. For Einstein gravity, initially, the flat FRW solution (which has since been augmented). Other people chose other frameworks, tried to build good models, and failed. String theorists are working in exactly the same way: they choose a framework, and try to build a model that is itself predictive and falsifiable. Smolin uses his framework. Thats fine. As long as each model is testable, there is no problem with the plethora of models being suggested.
Claiming that these are string theory models of inflation, is no more or less misleading than saying that the various Lambda CDM cosmologies (specified by many parameters) are all simple versions of GR + the standard model. The model is far from unique. So what?
(from Ari) All they were doing was trying to understand what’s happening in their wind tunnels, that is, trying to understand real world phenomena.
And that doesn’t involve making predictions? I don’t mean predictions of the “future,” but just predictions of, say, what velocity field you’ll get in the neighborhood of an specific airfoil positioned in a certain way in the wind tunnel. You may have such observations already in hand, which you try to reproduce with an application of the Navier-Stokes equations or some less general model. If you succeed, then you say, okay, let’s predict what will happen if we reposition the airfoil or change its shape in a certain way, whose presumed effects we know how to calculate. It’s the results of this follow-up testing that are of the greatest interest; we want to know if the initial agreement was merely a fluke.
Admittedly, the results of such tests may reflect more on our methods of approximating a solution to the model, or on our choice of relevant free parameters, than on the fundamental assumptions of the model itself, but that’s part of the problem of testing the model—find ways to confront the central assumptions with crucial tests, ie, tests whose evaluation is not overly dependent on incidental aspects of the model’s formulation.
String theory seems to have evolved in such a way that crucial observational tests, as opposed to tests of internal consistency or correspondence to (at least some) important aspects of the Standard Model, are impossible to formulate. The best criticism then becomes a meta-criticism—that string theory seems designed to make such testing impossible or in some sense beside the point. (Of course the apparent freedom to choose a background spacetime has played an essential role in all this.)
In the face of this situation one can continue to imagine that one has understand something about nature that wasn’t understood before, but the same can be said of any number of metaphysical ideas. Susskind seems to be protesting the abandonment of this “opportunity” to understand nature. Perhaps there still remains such an opportunity in string theory, but the current situation demands a much deeper response than a proliferation of convoluted variants on Kaluza-Klein models, an interminable exploration of the Landscape, invocations of the Anthropic Principle, or exploration of the vast array of mathematical ramifications of the theory’s formalism.
wow,
Sorry, but I find these claims that there is no difference between the predictivity problems of string theory, and those of gauge theory or GR to just be absurd, and I can’t believe that anyone makes these claims seriously.
For about the 100th time, the difference is not hard to understand. Gauge theory and GR are quite simple, rigid structures, and at the time they were suggested, they correctly postdicted huge amounts of observed physics, and made huge numbers of predictions, some of which were quickly testable.
String theory unification, whether applied to particle physics or cosmology postdicts virtually nothing and predicts nothing. Unlike the case of gauge theory or GR, there simply is no scientific evidence for the idea, nor any plausible prospects for getting any soon. Unlike the case of gauge theory or GR, attempts to connect string theory unification to real world physics aren’t based upon solid predictions that follow logically from certain simple principles. Instead there’s a whole industry of complicated and ugly Rube Goldberg constructions which don’t explain anything, but are are designed purely to avoid contradicting the rules of logic or what is already known about physics.
Dear Lee Smolin,
I am aware that the symmetry reduced loop quantum cosmology may have good semiclassical description but not LQG.
Incidentally would you mind if I cut and paste your comments to wiki here
http://en.wikipedia.org/wiki/Loop_quantum_cosmology?
A hobby of mine is to update wiki’s entries on LQG
(I.e I added material on preons and Kodama state, for example)
http://en.wikipedia.org/wiki/Loop_quantum_gravity
http://en.wikipedia.org/wiki/Preon
I heard about something quite funny and moderately relevant today at the AAS meeting. One of our newer students was giving a poster on detecting gravitational waves from cosmic strings. He described to me how someone with a high-pitched soft voice came up to talk to him, and how the other students around him suddenly went completely silent. The stranger was quite interested and said something like “you’ve got to look at radiation from cusps”. Student: “Kinks?” Stranger: “No, cusps.” Student: “You seem to know quite a lot about string theory.” Audience: smiles! Yes, the man himself, on the lookout for experimental verification!
It seems strange that every harmless motion and remark of a
prominent physicist seems worth commenting on here.
What happens?
F.
F.,
Well, one role of this blog is to retail gossip and keep people up to date on the activities of the celebrities of our field, and for those of us interested in particle theory and mathematics, Witten is bigger than Britney Spears, Paris Hilton and Lindsay Lohan all rolled into one. He has a very large influence on the direction of the field, so for instance if it turns out he has given up on particle theory and decides to devote himself to the study of exoplanets, that would be explosive news. No, I don’t think it’s happening. More likely, like many of us, Witten just thinks a lot of this astronomy stuff is cool.
Dear F.,
What seems strange to me is that anyone could think that EW’s remark was the point of the comment. Oh well.
Dave
Can Witten go to the toilet while attending a string theory seminar without worrying that it might be interpreted as an explosive news?