## Sunday, November 21, 2004

### Cultures in theoretical physics

Amazon.com has included two popular books in their listings - both of them should appear on April 30th or May 1st, 2005 - about our field.

First of all, it is a very insightful and extremely useful book by Lisa Randall
Warped Passages : Unraveling the Mysteries of the Universe's Hidden Dimensions
by Lisa Randall

Be sure that I will write much more about this book as the date of publishing is getting closer. Yes, I've read this book, and it certainly deserves your attention! On the other hand, I have not seen Lenny's book

An Introduction To Black Holes, Information And The String Theory Revolution: The Holographic Universe
by Leonard Susskind

Well, Lenny Susskind is a character, and it is very natural that he writes a book about all these wonderful ideas - many of them are his ideas. But I can't tell you anything about the book.

The information about these books scared Peter Woit - because every time the public learns more about particle physics and string theory, Peter Woit's struggle to kill string theory becomes more hopeless. And therefore Peter wrote an article "More hype on its way" on his blog "Not even wrong":

http://www.math.columbia.edu/ ... 000109.html

The section with comments was pretty illuminating because we could learn that it is not just string theory and the anthropic principle that Peter Woit hates. He also informed us that he does not like

• The research of the hierarchy problem. In his opinion, it is enough to believe that there is no grand unification, and therefore there is no GUT scale, and therefore we don't have to worry about the hierarchy problem - it does not really exist.
• You might think that I am twisting and trying to humiliate Peter, and he could not write something like that, but you can check yourself! Just to be sure, unless we postulate very new physics, such as large or warped extra dimensions, quantum corrections to gravity only become important at the Planck scale about 10^{19} GeV - which can be calculated without any assumptions of grand unification. If we want to claim that there is any field-theoretical description at energies below the Planck scale, we must explain the parameters, including the obvious gap between the electroweak scale and the Planck scale, a gap that is vulnerable to quantum corrections in the Standard Model.
• Extra dimensions. Obviously, Peter states that the ideas related to extra dimensions are even more ridiculous than string theory itself, and this is not anything new.
• But we also learned some time ago that he is totally unimpressed by gauge coupling unification - and of course by the very idea of supersymmetry.
That's becoming ridiculous, and therefore let me stop. There may be different cultures in particle physics - hep-th and hep-ph cultures - but it is quite clear that the difference between them is not as large as the difference between hep-** and Peter Woit.

That's a good place to say a couple of words about these cultural differences.

In the 1980s, the gap between the theorists and phenomenologists - or, alternatively, between the top-bottom and bottom-up approaches - was huge. String theory and conventional particle physics were two different cultures, and this also affected the separation to two arXivs in the early 1990s.

There has been at least one revolution - the second superstring revolution - in the stringy camp, but virtually no progress in the non-stringy camp. That's one of the reasons that brought these two camps much closer to each other. The other reason is less pragmatic and more important scientifically: namely the specific new discoveries.

In phenomenology, the large and warped extra dimensions were proposed as the new ways to solve the hierarchy problem. These approaches are clearly rooted in string theory; without the influence of string theory, it would seem unlikely that any approach based on extra dimensions would be studied seriously.

The case of extra dimensions in phenomenology was not the first example in which string theory gave bottom-up model builders new ideas and mantinels to probe possible new physics beyond the Standard Model - supersymmetry was another example - and most phenomenologists started to realize that there there is something about string theory if it's able to generate viable paradigms for their own field.

Of course, many phenomenologists don't care about the rest of string theory, besides these stringy-inspired ideas, and it's their legitimate attitude. On the other hand, it is conceivable that the future will show them that they should have taken many more details from string theory than just these general ideas such as extra dimensions and braneworlds. Well, in reality, many phenomenologists are trying to apply other ideas from string theory, too.

The concepts of holography and the AdS/CFT correspondence were very important for convergence of the phenomenological and stringy communities, too. The renormalization group flow looks like an extra dimension - the holographic dimension whose existence seems to be a general feature of quantum gravity. Most string theorists were trained as quantum field theorists anyway, and therefore it is not too surprising that the two communities overlap significantly in their investigation of holography - in the context of Randall-Sundrum models or the AdS/CFT correspondence.

The stringy and particle camps are still working independently most of the time, but there is a significant amount of interactions. Harvard is one of the places where the interactions are very strong, and I was explained that the gap between these two cultures is slightly bigger at some places in California, for example.

One may also consider a third culture, namely loop quantum gravity. From the Harvard perspective, the gap between the hep-** cultures on one side, and loop quantum gravity on the other side is virtually infinite. There is a wide consensus between all visible parties at Harvard - string theorists as well as particle physicists - that the precise proposals of the loop quantum gravity to modify the rules of physics (and to ignore the lessons of quantum field theory) is simply rubbish.

That's Harvard, but the people from the Perimeter Institute can tell you stories about their peaceful co-existence with the loop quantum gravity people over there. Of course, it is extremely hard for me to imagine how the string theorists can live so happily with so much anti-physical nonsense around, but it's probably just a matter of mutual brain-washing.

1. To be perfectly fair, its *ok* to not like SuSy, extra dimensions and GUTs from a phenomenological point of view.

In fact there is a small but fairly substantial group of particle physicists who believe in just the standard model and nothing else. Quite a few big names as well. I should note these people still live in the experimentally observed universe, and are safe (at least for the time being).

The arguments against those groups are mostly aesthetic. For instance, you have to be able to stomach vast magnitudes of fine tuning, and naturallness arguments. Moreover, you also have to resign yourself to the level of predictivity of the SM, never expecting that nature will give you anything else.
Personally, I can't live with that, it seems an extraordinary coincidence that we exist in such a state, but ok lets see how far we can take this.

As far as experiment goes the only potentially as yet experimentally probed ironclad departure in the SM comes from Neutrino oscillation experiments. The seesaw mechanism basically waves a flag at you and yells for a GUT, but again its still feasible to just add an extra field.

Lastly, dark matter and the cosmological constant are both experimentally verified now. There is no SM particle that can account for dark matter, and the CC is an embarrasment. It seems to me the vanilla SM people are starting to flow towards obscurity as time progresses, the direction of error is clearly not in their favor.

2. To the previous poster : what precisely is the meaning of "believe in", as you used in "believe in the standard model" ?

For instance, a Muslim "believes in" Allah, and Muhammad as the last Prophet.
A Christian scholar of Islam does not "believe in" Islam in the same way, even though he considers it worth arduous study.

With regard to the standard model, yet another "believe in" could be that one believes that only the SM is unambiguously required by the experimental evidence, all the rest is speculative. "Believe in" has connotations of being sure even in the absence of evidence, believing in things unseen, so to speak.

If I had my way, we wouldn't use the religious term "believe in" in physics at all. I would rather characterize positions as, say, minimalist, as those who don't want to venture beyond the immediately available empirical data, and then a variety of other positions.

3. There has been at least one revolution - the second superstring revolution - in the stringy camp, but virtually no progress in the non-stringy camp.
Lubos, no offense, but you don't know what you're talking about. There has been a lot of progress in "non-stringy" phenomenology. The ninties saw the development of the heavy quark effective theory, for example. Huge progress was made in perturbative QCD, and it's connection with the HERA experiment. Not to mention the massive amount of work that went into understanding the LEP experiments.

Not to tout my own field, but the late nineties saw a lot of progress in lattice field theoy as well.

There's been lots of progress in phenomenology, we're just not so arrogent as to call each bit of progress a "revolution".

4. Matthew, you're comparing uncomparable things.

There may have been massive amount of *work* in the 1990s, but there has been little significant progress and *progress* is what I was talking about. LEP has not seen anything really important in the 1990s. Top quark is just the last quark whose existence was absolutely guaranteed - and in the 1960s and 1970s, they were finding several particles like that every year, and moreover these particles were very new and revolutionary at that time.

Heavy quark effective theory. Well, I hope that you don't claim that it is comparable to the discovery of QCD itself because we would really have to disagree heavily. Heavy quark effective theory is a rather marginal tool, and moreover it was not discovered in the 1990s. Concerning lattice gauge theory, notice that you don't say anything specific. The main progress in that field were more powerful computers, I would say.

On the other hand, the 2nd superstring revolution - M-theory, duality, D-branes, holography - was certainly at least comparable to the 1st revolution in the 1980s. That's a very different situation.

By the way, the statement "there has been no progress whatsoever in phenomenology in the last 10 years (1988-1998), and therefore the argument that string theory is the only game in town is a very powerful argument - certain question will never be answered in quantum field theory which is a clear thing" is a quote from Shelly Glashow, see TEU chapter 9.

5. To the first poster - sure, it's OK not to believe SUSY! Many of us believers are often uncertain, too. It's even OK to believe that there is nothing beyond the nu-Standard Model (nu stands for neutrino masses added), except that you showed at least one example in your own post why it's not likely (e.g. dark matter). I guess that it's the same thing that you said, too.

6. This culture divide is one of the reasons why I work on condensed matter physics. It's rare for me to meet a particle theorist who likes both phenomenology and string theory. I, however, am interested in interacting with theorists and experimentalists of all stripes. By the way, why is it that "theoretical physics" (see your title) always is taken to only include particle/nuclear theory?

7. LEP has not seen anything really important in the 1990s.
To me, confirming the standard model at loop level was important. You are, of course, free to disagree.
Heavy quark effective theory. Well, I hope that you don't claim that it is comparable to the discovery of QCD itself because we would really have to disagree heavily.
Of course it's not comparable to the discovery of QCD. My point was that there has been tremendous progress in our understanding through the development of tools like HQET. Another example is the Soft Colinear effective theory.

You do realize that it's entirely possible we will see violations of the standard model in "low energy" precision experiments?
Concerning lattice gauge theory, notice that you don't say anything specific.
Read my blog. One specific point, out of many, the development and implementation of improved staggered fermions.
The main progress in that field were more powerful computers, I would say.
Then you'd be seriously wrong. Theoretical developments were far more important.
On the other hand, the 2nd superstring revolution - M-theory, duality, D-branes, holography - was certainly at least comparable to the 1st revolution in the 1980s. That's a very different situation.
True, it is very different. All the stuff I'm talking about relates to actual experiments, ongoing or planned.

8. Most string theorists like phenomenology, and a significant fraction of phenomenologists, especially the influential and active ones, like string theory.

On the other hand, there are gaps in different cultures of condensed matter physics, too. Condensed matter physics subfield has become extremely fragmented - see my article with "Marvin Cohen" in the title - and the different fragments have very limited interactions.

Yes, by "theoretical physics" I usually only mean the part connected to high-energy physics and quantum gravity. I realize that it's a bad name because this name should naturally include theoretical condensed matter, theoretical chemical physics, and other fields which I don't quite mean to be included even though they are important and in many sense related.

But I don't have a better label that would include GR, quantum gravity, geometry in physics, as well as high-energy particle physics. If you convince me that there is some better name, I will be happy to use it.

9. Matthew, there are two main reasons why I don't consider "loop tests of the Standard Model" in the 1990s comparable to the discoveries in the 1970s and other decades.

The first reason is that quantum loops in quantum field theory have been measured in the 1930s, and QED is still much much better and more precise. The concept itself is simply not new, and there was no doubt in the 1990s how quantum loops in other gauge theories should be calculated.

The second reason is that the progress is just quantitative - the precision is slightly better than it was before the 1990s. But it's just wrong to say that before the 1990s, our theory was the tree level theory only. Our theory was always the Standard Model with quantum loops, and the difference is only that we have a better precision today that requires us to calculate the loops.

Yes, I don't understand the theories you mention well enough, but yes, I will be skeptical if someone claims that the Standard Model works based on some high precision low-energy measurements. In Brookhaven they almost claimed the discovery of SUSY ;-) a couple of years ago, and it was a sign error in the theoretical 2-loop calculation of the anomalous magnetic moment. These high precision things can only tell you that something is slightly mismatched, but they never quite tell you what causes the mismatch. And even the mismatch itself is a question of your confidence that all subtle errors have been avoided. It's the hard and less illuminating way to do physics.

At any rate, you should not list it as progress because no new discovery using these high-precision tools has been done. Do you agree? And it won't be done in the 1990s anymore because the decade is gone.

OK, improved staggered fermions. I am not claiming that there have been no papers written. I just say that there has been very little progress, and "improved staggered fermions" sounds a random phrase from a random paper - you could also choose "Black Hole Non-Formation in the Matrix Model" - the first paper on hep-th today - to claim that there has been a lot of progress. I hope that this is not the way how you want us to think.

These things are simply unimportant from the long-term viewpoint.

Sorry, Matthew, but the Standard Model is simply a well established theory for events below 100 GeV, and I won't treat *any* result that just confirms it as significant progress. The real progress in phenomenology is the understanding of various hypothetical models beyond the Standard Model, and the most interesting ones involve supersymmetry, new gauge groups, or extra dimensions.

Don't get discouraged - but I think it is important for the scientists (and others) to say what they consider important because some decisions how the forces (and resources) must be divided are always done, and they should be based on some discussion and rational analysis.

10. Matthew, there are two main reasons why I don't consider "loop tests of the Standard Model" in the 1990s comparable to the discoveries in the 1970s and other decades.
Lubos, do you ever read what people write? I never said they were "comparable" I said they were "important". You seem unable to avoid reading things into what I write that I'm not actually saying.

The concept itself is simply not new, and there was no doubt in the 1990s how quantum loops in other gauge theories should be calculated.
I didn't say there was a doubt. I said that I consider it important that such calculations were done, and that they actually agreed with the experiments.

In Brookhaven they almost claimed the discovery of SUSY
That's not quite what the experimental group claimed. They claimed "we have a 2 sigma deviation from the SM prediction". The SM prediction was wrong. It was the theorists who jumped all over it, trying to claim it was evidence for this thing or that thing.

It's the hard and less illuminating way to do physics.
It's also going to be increasingly the reality in an era where high energy machines cost billions of dollars. Get used to it.

At any rate, you should not list it as progress because no new discovery using these high-precision tools has been done. Do you agree?
No, I don't agree.

OK, improved staggered fermions. I am not claiming that there have been no papers written. I just say that there has been very little progress
And you're wrong. There has been lots of progress, many things that could not be computed before can be now.

and "improved staggered fermions" sounds a random phrase from a random paper
It's not.

you could also choose "Black Hole Non-Formation in the Matrix Model" - the first paper on hep-th today - to claim that there has been a lot of progress. I hope that this is not the way how you want us to think.
No, it's not how I want people to think. It's also not what I'm doing (again you don't read what I write). I wouldn't presume to tell you what's important in string theory without actually doing some research on it first. For all I know "Black Hole Non-Formation in the Matrix Model" could be very important, or it might not be. Unlike you, I'm not so eager to shoot my mouth off about subjects I don't know much about. But feel free to keep commenting on developments in lattice QCD, it's amusing to read.

These things are simply unimportant from the long-term viewpoint.
That's rather arrogent of you.

The real progress in phenomenology is the understanding of various hypothetical models beyond the Standard Model, and the most interesting ones involve supersymmetry, new gauge groups, or extra dimensions.
All those things are interesting, no doubt. They're also wild guesses at this point. Unfortunately, we all have to wait for the LHC :)

Don't get discouraged
Aww shucks, thanks...

Rest assured, I'm not discouraged. Wether or not you consider it important, wiser heads have decided that it is. The experimental program in Hadron physics over the next 5-10 years is very interesting, and I think we'll learn lots of important and interesting things.

11. Thank you for the link on Marvin Cohen. It was very interesting.

I should explain what I meant about the cultural divide in physics. It's not just that there's a dvide. There are people who *hate* string theory, including at least two famous phenomenologists that I know personally. And as you say, I heard rumors about a certain university in California. So far I don't know of any examples like this in condensed matter.

I think it would be probably be safe to say that the entire academy has become fragmented. We just know a lot more than we used to and there are too many people. I would guess that in medieval times, no one called themselves a physicist or a biologist. Just a natural scientist. An unfortunate fact of life that a single person can't know everything, but we should still try -- it makes life richer.

12. Yes, Matthew, I read your texts carefully. I was using the word "comparable to something else" because it is more well-defined than "important". Yes, if you wish, I can state that these things are not important, but you might be insulted. This is why it's better to compare things. Note that the funding is not only comparable, it's even higher, but the results are not.

It's very important to compare things if we judge whether something is important and how important it is!

Another comment. In Brookhaven, they should not have said "we have a deviation from the SM prediction" because it was not true. They only had a deviation from someone's incorrect calculation of the SM prediction. ;-)

Concerning low-energy high-precision physics to do far-reaching predictions. No, I will not get used to "it". High-precision low-energy measurements will never give us the full information about physics we want to know. That's just a fact, get used to it. We either pay for bigger machines, or we will have no new reliable data - which is what is happening today.

Black hole non-formation in the matrix model is one particular interesting exercise in two-dimensional string theory with some canceled leading term, and I think it is obvious that it is not - and it will not be - as important as some other papers by these authors. In fact, the *whole* two-dimensional string theory combined just can't be as important for physics as some other major discoveries, whatever is done in this subfield. Don't be silly. I just think it is very unreasonable and unfair if someone says that something is important just because he or she is working on it (because he or she did not have another choice). Some things are simply NOT that important, and some people are nevertheless fooling themselves that what they do must be important because it's *them* who is doing it.

There is a lot of unimportant stuff going on in particle physics today. Of course, we must scale our ambitions down in the less-than-revolutionary periods, but we should still distinguish.

Sorry, but claiming that something is important just because *you* are working on it is what is arrogance in *my* point of view (as well as the point of view of Nima Arkani-Hamed who would probably tell you the very same things as me).

13. An answer to my condensed matter friend.

Yes, some people HATE string theory - and be sure that I've met many of them, and some of them may be the same people you described. ;-) There are people who HATE the theory of evolution, and there are people who HATE America. There are people who HATE Microsoft, and there are people who HATE capitalism as such. I could go on, but the principle is clear. Any thing that is has big ambitions and that can show that it works - BIG time - has some BIG enemies.

For me, that does not mean anything as the sum. The fact that there are people who HATE it just indirectly shows - it's a circumstantial evidence - that it is an important thing to study. I think that all the people listed in the previous paragraph are wrong, and their mere *hatred*, of course, has very little impact on my thinking (and even the sign is uncertain). It's only arguments that matter, and of course they don't have too many. They don't offer anything big that would scare me, and therefore I have no reason to hate them; I just think that they're pretty irrelevant.

You can take it more specifically as a comment about condensed matter physics. It's an interesting field with millions of interesting questions, but it has no truly big and universal goals, which is why people don't care about it so much and don't hate it so much. ;-)

I feel that two of us agree that the continuing fragmentation of the whole natural science is sort of wrong, but it also seems necessary. We just have too many things to cover. Still, the breadth of someone's knowledge is an important aspect.

14. Sorry, but claiming that something is important just because *you* are working on it is what is arrogance in *my* point of view
I'm not claiming my particularly little corner of the world is important. I am claiming that hadron physics is. There is a difference. Quantitativly understanding hadron physics is an important task. At least to me... to you it would appear that the only "important" field in physics is string theory. I find that a narrow, and sad, view.

(as well as the point of view of Nima Arkani-Hamed who would probably tell you the very same things as me).
Wow, you can drop names of famous people, I'm so impressed.

15. Sorry, Matthew, but hadron physics itself *is* a small corner of physics. It seems to be a general consensus that the main physics questions about the hadrons have been answered, and moreover it's not a critical field for applied science.

No way, I don't claim that string theory is the only important thing in this corner of physics - but this does not mean that everything is equally important.

16. Matthew,

Importance is in the eye of the beholder. I think Lubos is taking the point of view, e.g., that once you know the Schrodinger equation, atomic physics has nothing of importance left. This is a very Einsteinian point of view ""I want to know how God created this world.  I am not interested in this or that phenomenon, in the spectrum of this or that element.  I want to know His thoughts; the rest are details." Let us respect it, but note that that is but one approach to physics. An Einstein would be in no position to know anyone's thoughts but for all the folks who filled in the details.

For the other set of physics values, e.g., read S. Chandrasekhar's lecture (scroll down) at http://www.sawf.org/Newedit/edit02192001/musicarts.asp, and specifically, about Lord Rayleigh.

-Arun

17. Hi Arun, thanks for your clarifications for Matthew.

The page about Chandra is very poetic, but I am not sure whether it will exactly describe Matthew's other values about physics. ;-)

While I share the basic Einsteinian values, you are exaggerating. Of course that our understanding of more complex systems - even those whose equations we already know - is also important. It just seems to me that the progress in hadronic physics related to these questions has not been too big either.

You know, in 1975, all the fathers of QCD knew it was a correct theory that describes all properties of strongly interacting systems correctly - and they've been waiting for their QCD Nobel prize since then (for 29 years). My guess is that if you asked them whether the properties of protons etc. would be calculated in 30 years, and you would tell them about the progress of the computers that was waiting for us, they would probably say "yes".

Even the full understanding, allowing us to calculate the properties of protons and neutrons and nuclei and their low-energy interactions with 0.01% precision, would not be as important as the discovery of QCD. But we have not moved too much towards this goal either (in the 1990s). It's a set of interesting attempts and quantitative progress, but no complete breakthroughs and full solutions.

I *am* interested in complex systems, especially in the way how they're contained in the elementary laws, and in all possible quantitatively sharp and mathematically studiable emergent phenomena. But even with this statement, it seems absolutely clear that the progress in the 1990s was probably smaller than in all previous decades of the 20th century. If you disagree, try to list your important discoveries in the 1990s, and I will try to give you bigger discoveries of a similar kind in all previous decades of the 20th century.

18. Lubos,

You may well be right that the 1990s were relatively barren compared to any other decade in the 20th century. I would hazard a guess that the WWII decade of the 1940s might turn out to be relatively barren as well. Or if any 10 year period is to be considered, maybe 1937-47. I wonder how this decade will turn out to be.

-Arun

19. Hi Arun!

1937-1947 might have been a relatively dumb period in theoretical physics (and the interaction between theoretical physicists decreased a lot during the war), but it was an incredible period for technology and applied physics.

You know, it's not just the nuclear bomb that was constructed and tested. It was a lot of things associated with airplanes, radars, and so forth.

I have no idea about 2000-2010. So far it seems that it will be more groundbreaking for biology than physics - but we have some expected events coming in this decade, so it's certainly too early to make conclusions! ;-)

All the best
Lubos

20. The real rebels are very artistic I think. Autistic asks that we replace the u with an r and I will have changed the nature of Peter comments, so moving on.

By nature, I find the artistic implications and mathematical skills in observance, closely tied together. Why not, as many examples of what could be offered for inspection in regards to quantum gravity? How many models are there?

21. I forgot to include current post I was working on.

22. I would say the defining moment of the 2000's was the WMAP experiment. Many orders of magnitude in nature were suddenly made precise and in agreement with the concordance model, and of course all the nastiness that it implies for our fundamental understanding of the universe. Many people were skeptical of the concordance model, so it obviously is a theoretical win as well.

In my opinion, the 2000s are already more important for science than the 1990s. Perhaps theoretically, nothing that new in terms of ideas has emerged yet (well large extra dimensions is probably a good attempt), but then again as a scientist im less interested in ideas, and more interested in nature herself.

23. This comment has been removed by a blog administrator.

24. The orthodox approach has really become a part of math with very little consequences for physics. The developments in physics during the last 40 years just seem to have very little to do with these abstract comments about operator, C* algebras and their representations - with two-dimensional conformal field theory being the exception (but even there, the old insights are just a very small part of the important knowledge about CFT that we have).

This formalism is just not fit to deal with the renormalization group, the language of effective field theory, with gauge theories, confinement, S-duality, quantum gravity, and so forth.

25. Uuups! I accidentally deleted my previous post, so lumo answer is not very understable. In case you didn't read it, the previous post was trying to elucidate the role of math-ph; I was calling it the "orthodox approach" :-)