Metaphors and analogies: scientists’ friends or foes?

Metaphors are like men. If you take them
seriously the joke's on you.

The girls were about a year old and I was working from home. As I was reading yet another referee report that came with the preamble “we regret to inform you...,” I watched Gloria trying to push a square block through a round hole. We were really trying the same thing, I thought.

Decoding metaphors and using analogies is a prototypical right-brain task, a pattern finding that helps us get a grip on new situations quickly and that sheds new light on the familiar. Metaphors and analogies are omnipresent in literature and the arts, in humor and also in education. And popular science writing is full of it.

But relying on metaphors is like traveling to a new country and then heading to Starbucks. The very reason to do it is also what limits the experience. It’s familiar and easy to understand, but it prevents us from learning something new. This is why I have a love-hate relationship with Starbucks and other metaphors.

Love: Analogies and metaphors build on existing knowledge and thus help us to understand something quickly and intuitively.

Hate: This intuition is eventually always misleading. If a metaphor were exact, it wouldn’t be a metaphor.

And while in writing, art, and humor most of us are easily able to tell when an analogy ceases to work, in science it isn’t always so obvious.

When it comes to physics I can most often tell when an analogy fails to capture the actual science. But in other areas of science this sometimes is not clear to me. There are for example these artistic images that frequently accompany popular science accounts of new drugs or cancer treatments. You know, the ones with the molecules that fit like keys into locks of other molecules, or that cut through molecular bonds. I am reasonably sure that these explanations suggest a clarity of the underlying mechanism and structure that most often doesn’t quite exist in the actual data. But how much of it is science and how much of it is art is difficult for me to tell.

In a recent Nature comment “Mind the metaphor”, Eleonore Pauwels made a similar point:

“[I]n the late 1990s, computer scientists, physicists and engineers were fuelled by the idea that they might be able to direct cells in the same way that people program computers. In the laboratory, researchers started to use computing and engineering metaphors –switches, oscillators and logic gates, for instance – both to guide the design of synthetic constructs and to understand how natural systems function. Almost immediately, scientists were confronted with the uncertainties and constraints of engineering in the cellular context. Engineering concepts and metaphors could serve only as an inspiration...

Scientists using metaphors among themselves are often aware of, and even careful to point out, the subtleties that could be misconstrued. Problems tend to arise when metaphors are used outside the laboratory...

Faced with explaining the messy complexity and uncertainty of science to the public, it is understandable that scientists reach for metaphors. But [this] sends a message to policy-makers and laypeople that scientists can already make biological systems that are reliable and controllable. It widens rather than closes the gap between scientific realities and the expectations of policy-makers and the public.”

The same problem exists in physics, though at least in the area I work in there aren’t all that many implications for public policy. But I’ve seen it over and over again that people take analogies too seriously and start trying to build arguments on them. Suddenly a rubber sheet isn’t just an analogy for space-time, but it is space-time. The universe is an inflating balloon, the Higgs particle is a rumor, and entangled particles are shoes in parcels.

Except that, well, they’re not. The universe isn’t a clockwork and it’s not a drum either; the brain isn’t a computer, black holes are not cannibals and indeed not even black.

The main reason we use mathematics for scientific theories is that it’s a particularly clean way of thinking, uncluttered from what the right brain wants to associate. An electron isn’t a spinning top, it’s an element of a Hilbert space that transforms under the spinor representation of the Lorentz-group. There is really no metaphor that’ll do equally well. Feynman diagrams seem to be particularly prone to misinterpretation as many people believe they depict physical particles, while they are actually a handy short-notation for lengthy integrals.

But my uneasiness with metaphors and imagery goes beyond the communication issue.

If you spend some time with a set of equations, pushing them back and forth, you’ll come to understand how the mathematical relationships play together. But they’re not like anything. They are what they are and have to be understood on their own terms*.

Thus, as much as I value metaphors for the intuition that can serve as a guide to new ideas, I also mistrust them. We learn much more from the failure of metaphors than from their success.

I admired Gloria for her persistence in trying to push the square block through the round hole. Then Lara took the piece out of Gloria’s hand, opened the lid of the bucket and put the block in. Problem solved. If only it were so easy with my papers…

---

*That is unless you are onto a theory that is truly equivalent (‘dual’) to some other theory.

What is the viscosity of space-time?

The Crab Nebula.
Image Source: Hyperphysics.

One of the most general expectations of quantum gravity is that space-time is not the smooth background of General Relativity, but instead a wildly fluctuating, bubbly, foamy mess. Seeing the quantum properties of space-time directly is not presently possible, but what we can see is whether the quantum gravitational behavior affects the way particles travel through space-time.

One way this could happen is by distorting paths so that photons of different frequency (energy) move at slightly different speeds. Such an effect is referred to as ‘dispersion’. Next to dispersion there is dissipation, which is basically energy loss into the background. While quantum gravitationally induced dispersion has received substantial attention during the last decade, dissipation hasn’t received as much love.

In a nice and straight-forward recent paper dissipation finally got some love from Liberati and Maccione

Astrophysical constraints on Planck scale dissipative phenomena
Stefano Liberati, Luca Maccione
arXiv:1309.7296 [gr-qc]

They start with a general hydrodynamic ansatz that assigns space-time the properties of a fluid, notably a viscosity, which causes dissipation. The microscopic theory that would give rise to such a hydrodynamic behavior they leave unspecified and just ask what observable consequences a non-vanishing space-time viscosity would have. With this ansatz, they make an expansion of the dispersion relation and collect the dissipative (imaginary) contributions.

Then they look at observations of highly energetic photons from a distant source, the Crab nebula. If space-time was viscous, the photons would lose energy during their travel. Already the rather conservative estimate that the photons of the highest observed energies shouldn’t have lost more energy than they have left at arrival leads to very tight constraints. If the photons lose energy faster than that, the spectrum we receive on Earth would be highly distorted and pretty much incompatible with our knowledge of astrophysics.

This constraint from existing data clearly rules out Planck scale effects, ie effects that plausibly have a quantum gravitational origin, at first order. Better constraints can be obtained by drawing upon concrete astrophysical models for the typical energy of photons that are emitted, so it seems likely that in the future we will see even better constraints on this.

Much like with violations of Lorentz-invariance this is a case where nothing has been found. Yeah, Einstein was right, again. But this doesn’t mean that nothing has been learned. We’ve learned that any model for an emergent space-time that does not have a very small, almost vanishing, viscosity is clearly incompatible with observation.

Physics World turns 25

The IOP's member magazine "Physics World" turns 25 and has an anniversary issue out. It's full of interesting articles, some more information about the content is on the Physics World Blog, and you can download the issue for free here. It contains a contribution from me on one of the "five biggest unanswered questions in physics" -- "Can we unify quantum mechanics and gravity?"

For the download you apparently have to agree to end up on a newsletter email list. If you don't want that but would like to read my piece, send a brief note to hossi at nordita dot org.

Shut up and let me think

I recently attended a conference on the foundations of quantum mechanics in Vienna. It was a very interesting and well organized event. The food was good, the staff efficient, and everybody got a conference bag with an umbrella.

I don’t normally have a lot to do with quantum foundations, especially not since I left Perimeter Institute. And so I learned many new things and got feedback on my paper. It was a useful meeting for me – but it was also a little strange.

Most of the feedback I got was people telling me they don’t believe in superdeterminism, wanting to know why I believe in it, not that I’m sure I do. Discussions turned towards final causes and theology. I’m a phenomenologist, I heard myself saying, I couldn’t care less what other people believe, I want to know how it can be tested. Faintly, I heard an echo of a conversation I had with Joao Magueijo at PI some years ago. Boy, I thought back then, does this guy get explosive when asked about his beliefs. Now I think he must have been spending too much time with the quantum foundations folks. Suddenly I’m very sympathetic to Joao’s attitude.

Quantum foundations polarizes like no other area in physics. On the one hand there are those actively participating who think it’s the most important thing ever but no two of them can agree on anything. And then there’s the rest who thinks it’s just a giant waste of time. In contrast, most people tend to agree that quantum gravity is worthwhile, though they may differ in their assessment of how relevant it is. And while there are subgroups in quantum gravity, there’s a lot of coherence in these groups (even among them, though they don’t like to hear that).

As somebody who primarily works in quantum gravity, I admit that I’m jealous of the quantum foundations people. Because they got data. It is plainly amazing for me to see just how much technological progress during the last decade has contributed to our improved understanding of quantum systems. May that be tests of Bell’s theorem with entangled pairs separated by hundreds of kilometers, massive quantum oscillators, molecule interferometry, tests of the superposition principle, weak measurements, using single atoms as a double slit, quantum error correction, or the tracking of decoherence, to only mention what popped into my head first. When I was a student, none of that was possible. This enables us to test quantum theory now much more precisely and in more circumstances than ever before.

This technological progress may not have ignited the interest in the foundations of quantum mechanics but it has certainly contributed to the field drawing more attention and thus drawing more people. That however doesn’t seem to have decreased the polarization of opinions, but rather increased it. The more attention research on quantum foundations gets, the more criticism it draws.

“Shut up and let me think” is the title of an essay by Pablo Echenique-Robba which you can find on the arxiv at 1308.5619 [quant-ph]. In his personal account Pablo addresses common arguments for why research on quantum foundations is a waste of time. I’ve encountered most of these and I largely agree with his objections. But let me add some points Pablo didn’t mention.

I do have my issues with much of what I’ve seen in quantum foundations. To begin with, most of it seems to be focused on non-relativistic quantum mechanics. That’s like trying to improve the traffic in NYC by breeding better horses. If you can’t make it Lorentz-invariant and second quantized I don’t know why I should think about it. More important, I can’t fathom what most of the interpretation-pokers are aiming at. It’s all well and fine with me to try to find another formulation for the theoretical basis of quantum theory. But in the end I want to see either exactly what the observable differences are or I want to see a proof of equivalence. Alas, there seems to be a lot of talk about, well, interpretations which do neither one nor the other. Again the phenomenologist lacks the motivation to think about it.

Despite these reservations I think that research on the foundations of quantum mechanics is of value, again for a reason that Pablo did not address in his paper, so I want to add.

I’ve been educated in the “shut up and calculate” philosophy with my profs preaching Feynman’s mantra that nobody understands quantum mechanics, so don’t bother trying. Needless to say I, as probably most students, was not so much deterred as encouraged by this, so we dug a little into the literature. If you dig, it gets into philosophy very quickly. That’s not necessarily a bad thing, but most students come around to realize they wanted to study physics, not philosophy, and they move on to calculate. I’m among those who feel comfortable with a mathematical framework that “just” delivers results and that can be used to describe nature. To me science is “just” about making good models.

But those who are criticizing research on the foundations of quantum mechanics on the ground that everything has been understood are dismissing a way to arrive at an improved description of nature, and they are dismissing it based on unjustified arrogance about their superior motives.

Science progresses by evaluating the use of models about nature in the form of specific hypotheses. What we call ‘scientific method’ are procedures that have proved efficient in creating good hypotheses and tests thereof. Not only do these methods change (hopefully improve) over time, what constitutes a ‘good’ hypothesis also depends on beliefs and social dynamics. In the end what matters is not how somebody arrived at a hypothesis, but whether it works. That’s the essence of scientific progress.

The action principle, gauge-symmetry, and unification, for example, have proved dramatically useful in the construction of theories. And that they have been useful in the past is a good reason to employ them in the future search for improved theories. The same goes for naturalness. A theory that isn’t ‘natural’ is typically believed to be incomplete and in need of improvement or at least additional explanation. Yet all that says is that it’s a criterion which researchers draw upon to arrive at better theories. There’s no proof that this will work. It’s a reasonable guess, that’s all. How reasonable depends on your attitude, your beliefs and on whether you think it’ll land you a job.

And so some may guess there is something to be gained by poking around on the foundations of quantum mechanics. You might not believe that the reasons for their interest are good reasons, much like I don’t believe in naturalness and others don’t believe in a theory of everything. But in the end it doesn’t matter. In the end what matters is not what motivated people to study some research question, but only whether it led to something.

My support for quantum foundations thus comes from a live-and-let-live attitude. Maybe studying the foundations of quantum theory will improve our understanding of the fundamental nature of reality. Maybe it won’t. I don’t understand most of their motivations. But then they don’t understand mine either.

Those who are dismissing quantum foundations as a waste of time I want to ask to consider the consequences of this research in fact revealing a different theory underlying quantum mechanics, one that allows us to manipulate quantum processes in novel ways. The potential is enormous. It’s not a stone that should be left unturned.

I’ll shut up now and let you think.

Should the Nobel Prize be given to collaborations and institutions?

It’s a grey and foggy Friday here. The clouds are hanging around like they’ve been out all night and even the leaves are too tired to jump off the trees. A cold is knocking on the door, or at least my brain is mush and I could need an excuse for that. There’s two guys in front of my window tearing off the balcony. If they don’t drink beer and watch me, they make noise and I’m rather unsuccessful in trying to ignore them. In summary, I’m pretty dysfunctional and in a pissy mood. You don’t want me to write a referee report on your paper in this condition.

To cheer me up, I decided I’ll go and disagree with Sean Carroll on something, just for the fun of it. Sean had a recent Op Ed piece in the NYT arguing that “in the future the [Nobel] prize committee should be allowed to consider institutions and collaborations as well as individuals.” It’s well written and worth a read, so have a look. I’ll grab a coffee and wait till you’re back.

There are three ways to approach the question whether the criteria for the Nobel Prize should be changed. One is to look at Alfred Nobel’s original will. He explicitly stated that the prizes be given to “persons”. But then it’s been a while. Second, one can try to guess whether Nobel would have wanted the criteria to be altered if he would be alive today. For me that’s too much psychology and I’ll leave that to somebody else. Third, we can ask whether it would be beneficial for science or for the communication of science and that seems to me the most fruitful approach.

Sean basically argues that science is a community enterprise and if one honors certain discoveries then credit should be given to everybody involved. Scientists take acknowledgement of contributions very, very seriously because it’s essentially what they live from. That’s the reason for long author lists. These lists keep getting longer as the topics we work on become more involved and the experiments become more complex.

However, science has always been a community enterprise. Every single discovery that has been made became possible only through the work of many others before and alongside those who put the pieces of the puzzle together.

Researchers who study the network dynamics of science refer to breakthrough events as ‘pivot points’. They’re combinations of existing knowledge that solve a problem and create a new basis for future research, not seldom founding entirely new fields. You might be interested to have a look at this paper that visualizes pivot points in citation networks with superstring theory as one example.

It has happened frequently in the history of science that major discoveries were made almost simultaneously by several people. That’s not a coincidence but due to the nature of breakthrough discoveries. They typically combine existing knowledge in just the right way. Having the right knowledge at the right time and seeing the potential of this combination is what makes a genius. And that’s what the Nobel Prize honors.

As I argued earlier, while scientists certainly work together, this collaboration is not a case of true collective intelligence. We don’t do distributed information processing in the communities. It’s still single people who contribute ideas and who hand them on to others who work on them and hand on their contributions and so on. It’s just that the interactions have gotten faster and involve more people who are better connected now than they were a century ago. It has become difficult and infinitely cumbersome to track all the small little contributions that people make.

The Nobel Prize, in my opinion, cannot give credits to everybody involved in a discovery because that’s futile. It should then focus on those on whose work was the basis of a new understanding of nature. It is a prize for persons and individual contributions. There are many scientific societies and foundations who give out prizes and awards and nobody ever complains that somebody gets such a prize when there’ve been many other people working on the same thing. That’s because it is understood that these awards are for persons and their dedication and foresight in the first place, and for the specific topic in the second place.

I don’t know anybody who went into science or pursued their research with the aim of winning a Nobel Prize. It is generally recognized that hard work and intelligence is necessary but not sufficient, and that it also takes a good dose of luck which is beyond our influence. So the Nobel Prize doesn’t actually serve as an incentive, or at least not much so. But the mere fact that the Nobel Prize is awarded to (a few) individuals documents the value of personal sacrifice. Giving such an honor to institutions is akin to doing away with private property in communism and believing that everybody cares for the well-being of the group as they do for their own. It doesn’t work because most people want to be recognized as individuals, not as members of collectives. That’s true also for scientists.

There is another reason why giving the Nobel Prize to collaborations or institutions is not a good idea. Nobel Prize winners like no other scientists become spokespeople for their field of research – and beyond. They are being heard. Nobel Prize winners play an important role in representing the interests of the scientific community. Granted, not all of them might live up to expectations, but I think that most of them are aware of the influence they suddenly acquire. Giving the prize to institutions would throw away this voice that scientists have to speak for them, and they don’t have many of these voices.

So I think the Nobel Prize committee is doing the right thing in giving the prize to persons. Because scientists want to be recognized as people, not as members of a collective.

It has started to rain and the balcony guys have packed their tools and left me with a semi-deconstructed balcony and empty beer bottles. Time to finally write these referee reports; keep the gear-wheels of the system turning.

Quantum Gravity in the Cosmic Microwave Background?

Gravity waves. They are pretty but have
nothing to do with gravitational waves.
Image Source: UWO.

Krauss and Wilczek recently posted a brief note on the arXiv. They present a dimensional argument that claims signatures of relic gravitational waves in the cosmic microwave background (CMB) would be evidence for quantum gravity.

Using Cosmology to Establish the Quantization of Gravity
Lawrence M. Krauss, Frank Wilczek
arXiv:1309.5343 [hep-th]

Relic gravitational waves are perturbations of space-time created at the Big Bang. They cannot presently be directly detected, but if they exist they would affect the polarization of CMB photons. The Planck satellite mission is about to deliver data on CMB polarization, so Krauss and Wilczek’s is a very timely contribution.

While their dimensional argument is original and compelling in its simplicity, what they say is not particularly surprising and known to researchers familiar with the subject. The argument means essentially if there are no suitable matter sources that could cause space-time perturbations, then the only way relic gravitational waves can have been created is through quantum effects. That’s because it needs a mass scale to get the dimensions right and Newton’s constant will only give a mass-scale when suitably combined with Planck’s constant, thus indicating a quantum effect.

The argument however only works without matter that brings in anisotropic stress. It would still work if the matter was solely scalar fields because these don’t contribute to the anisotropic stress, but electromagnetic radiation could deliver such a contribution. Be that as it may, this means by a purely dimensional argument alone it is hard if not impossible to reverse the logical arrow, that being the question whether relic gravitational waves could have been created in a non-quantum fashion.

Few few people doubt that relic gravitational waves exist and are quantized. It would certainly be exciting to have evidence that this treatment of the early universe is correct, but it must be said that this is not evidence for what the community commonly refers to as quantum gravity.

“Quantum gravity” is normally meant to be the fundamental theory for the quantum nature of space and time. The quantization that is being used for gravity in the early universe is normally explicitly referred to as “perturbatively quantized gravity”. It is expected by all but a few dissidents that perturbatively quantized gravity is the correct effective limit of any theory of quantum gravity. The mere existence of such quantized perturbations thus tells us little. More telling is the spectrum of the perturbations which depends on what happened in the early universe, for example on whether there was a Big Bang or a Big Bounce, and that does indeed depend on the full theory of quantum gravity.

Evidence for relic gravitational waves would give strong support to the validity of perturbatively quantized gravitational waves (essentially quantum field theory in curved background), but it takes more than a dimensional argument to show that other models cannot produce the same observation. And even if that could be shown, the mere existence of the gravitational wave background does not teach us much about the non-perturbative theory of quantum gravity. Thus, Krauss and Wilczek’s argument makes a good point but its relevance for research in quantum gravity is limited.

Kudos to Jakub Mielczarek for helpful communication.

Bonus: Krauss at a recent discussion following his public lecture in Stockholm. Spot the American among the Swedes :p

Lawrence Krauss in Stockholm. Still from this YouTube Video.

Women in Science. Again.

Thank you, you can stop sending me the link to the NYT article “Why Are There Still So Few Women in Science?” I assure you I saw it. It just didn’t seem to say anything we didn’t know already, so I wasn’t about to mention it. Alas, it seems to have triggered another wave of public commiseration about the alleged lack of women in the sciences, and it seems moreover I’m expected to have an opinion, so here you go.

I have a hard time believing this bemoaning of the current state of affairs is sincere. If Americans would take the issue seriously they’d have paid maternity leave to assure employers don’t think twice hiring women in their fertile years who haven’t yet reproduced. If you want more women in science, that’s where you should start, not with complaints about dress code schizophrenia. Everybody with half a brain knows that a pregnant or nursing woman will not be as productive as her testosterone fueled colleague. That’s not a bias, that’s capitalism.

Please don’t hold it against me that I published several papers during my parental leave – these were written much earlier and just submitted while I was learning how to ten-finger type with a baby or two hanging on my nipples.

Paid maternity leave and paid parental leave might not be sufficient, but necessary, hear me. And it’s not only the women who will benefit from this, but it’ll generally level the playing field for those who want to have children before the age of 40.

Having said that, I’m always uncomfortable to address the question of women in science, physics in particular. I’m not “women in physics”, I’m one woman in physics, and I don’t want to speak for others who have made experiences very different from mine. I don’t doubt that many women feel awkward in male-dominated environments or that they don’t like to stand out by wearing ‘feminine’ clothes or that they think it inappropriate if they get hit on by a colleague. But Eileen Pollack, who wrote the recent NYT piece, is similarly one woman in physics, so let me to add my own experience to the points she brings up just for balance.

I’ve never been a girly girl; quite possibly having three brothers played a role in that. My teachers constantly complained that I was too quiet, not social enough, did not speak up often enough, did not play with the other kids and was generally awkward around people. I spent a lot of time with books. I never had problems at school, unless you count that I was about as unsporty as you can be. As a teenager I was very into science fiction. And since I wanted to tell the science from the fiction, I piled up popular science books alongside this. You can extrapolate from here.

I studied math and physics primarily because I don’t understand people. People are complicated. They don’t make sense to me and I don’t know what to do with them. Which is probably why I don’t spend a lot of time thinking about whether or not my male colleagues behave appropriately. They don’t make sense either way. And the women, they make even less sense. Take in contrast a problem like black hole information loss or the recent firewall controversy. Clean, neat, intriguing. So much easier.

Yes, there’ve been some guys who’ve tried to pick me up on conferences but for what I understand of human mating rituals it’s the natural thing to happen among adults and I just say no thanks (the yes-thanks days are over, sorry). Indeed, there’ve been sexist jokes and I try to stay away from people who make them because such jokes come from brains preoccupied with differences between the male and female anatomy rather than the actual subject matter of the discussion. There have been the elderly guys who called me “little girl” and others who pat my shoulders. And yes, that’s probably the reason why I’m sometimes acting more aggressive than I actually am and why my voice drops by an octave when I’m trying to be heard by my male colleagues.

But by and large the men I work with are decent and nice guys and I get along with them just fine. Most of the time I’m not consciously taking note of them having a crinkly chromosome I don’t have, and my subconsciousness was not consulted for this blogpost. Yes they interrupt me when I speak and it’s annoying, but they interrupt each other as well, and I’ll admit that I too have developed the unfortunate habit of cutting off others, patience has never been my strong side. I still paint my toenails pink and I do have baby pictures in my office.

I’ve seen a bunch of do and don’t-do lists for men in academia when talking to their female colleagues. If you’d give me a set of do and don’t-do’s for how to deal with my male colleagues, I would decide it’s too complicated and just avoid talking to them all together. So I don’t think these lists are very helpful. I understand that everybody has their touchy points and they want others to respect them, but society has never worked by people giving instructions to others for how to treat them, so can we please just deal with each other as individuals?

Sure, I have a do and don’t-do wishlist for my male colleages as well. Here’s my biggest wish: Unless I know you (meaning we’ve met and talked at least a few times), don’t bring me in a situation where I have to be alone with you in a closed room. Because I’ve unfortunately made some bad experiences at an early age and a situation like this sets off a major alarm in my brain. Run, it says, get out of here. I’m really sorry about this because I’m sure you’re a nice guy and play table tennis with your kids every weekend, but my neural circuits insist you’re a potential threat. That’s my biggest Don’t. But I don’t actually expect you to know this, so I’ll forgive you.

I am aware I might be stepping on some toes here, but I’m not even sure that we really need more women in physics. Because it seems to me that most women are in fact not very interested in physics, especially in theoretical physics. Of course I think it’s a shame and there are almost certainly social and cultural reasons next to genetic ones, but this doesn’t make these reasons any less real. If some girl is uncomfortable taking on a job that has a male smell to it, I think this is an important factor for their decision and for them to be happy with their life.

The social and cultural aspects can be changed, though they change only slowly, and I appreciate all efforts into this direction. Especially when it comes to children’s education and role models I believe this can serve to spark interests that otherwise might have gone unnoticed. So I certainly approve of all means to raise interest in theoretical physics, generally and specifically among young women, but I don’t see the benefit of pushing women into professions they’re not comfortable with. Gender quotas don’t make any sense to me as they seem to make the situation worse rather than better by undermining the credibility of women that benefit from it. On the shortlist for my present job there were 5 people, 3 of them women. This gives me some faith that I wasn’t hired just so there would be at least one woman in the faculty here. That did play a role in my decision to move to Sweden and so did knowing that Sweden has laws regulating a decent maternity and parental leave. (Yes, I did have another offer which was better in some sense and worse in others, so it was not a simple decision.)

I do read the studies and so I know that by all chance I’ve been subject to stereotype bias and from what I read I have to conclude that most likely I sometimes judge other women unfairly myself. This bothers me a lot. I think the best we can do is be aware of these shortcomings and try to address them systematically when we can.

But what bothers me most about the perceived male-ness of theoretical physics is that I’m afraid some women who could find much happiness with the fundamental laws of nature or the evolution of the universe never seriously consider this as a potential profession. Part of the problem is that we, myself included, rarely if ever talk about what drives us into theoretical physics and what keeps us there.

If somebody asks me what I do, I’ll tell them about black holes or gamma ray bursts or the cosmic microwave background. I don’t tell them that even after all these years what amazes me so much about theoretical physics is doing a calculation and getting a result that describes observation, something that explains the world around us. Be that the energy levels of the hydrogen atom or the double-slit experiment, Compton-scattering or gravitational lensing – these little scribbles on a notebook capture a truth about the universe. How awesome is that? And where if not theoretical physics do you find this?

I fail to see how the fascination for this connection between math and the nature of reality is a male domain, and that’s what makes me think the present low fraction of women in theoretical physics is at least partly due to misinformation about what this job is all about. But first, please, the maternity leave.

Testing Conspiracy Theories

I'm about to fly to Vienna where I'll be attending a conference on Emergent Quantum Mechanics. I'm not entirely sure why I was invited to this event, but I suspect it's got something to do with me being one of the three people on the planet who like superdeterministic hidden variables theories, more commonly known as "conspiracy theories".

Leaving aside some loopholes that are about to be closed, tests of Bell's theorem rule out local hidden variables theories. But any theorem is only as good as the assumptions that go into it, and one of these assumptions is that the experimenter can freely chose the detector settings. As you know, I don't believe in free will, so I have an issue with this. You can see though why theories in which this assumption does not hold are known as "conspiracy theories". While they are not strictly speaking ruled out, it seems that the universe must be deliberately mean to prevent the experimentalists from doing what they want, and this option is thus often not taken seriously.

But really, this is a very misleading interpretation of superdeterminism. All that superdeterminism means is that a state cannot be prepared independently of the detector settings. That's non-local of course, but it's non-local in a soft way, in the sense that it's a correlation but doesn't necessarily imply a 'spooky' action at a distance because the backwards lightcones of the detector and state (in a reasonable universe) intersect anyway.

That having been said, you might like or not like superdeterministic hidden variables theories, the real question is if there is some way to test if that's how nature works, because one can't use Bell's theorem here. After some failed attempts, I finally came up with a possible test that is almost model-independent, and it was published in my paper "Testing super-deterministic hidden variables theories".

I actually wrote this paper in the hospital when I was pregnant. The nurses kept asking me if I'm writing a book. They were quite disappointed to be drowned in elaborations on the foundations of quantum mechanics rather than hearing a vampire story. In any case, in the expectation that the readers on this blog are somewhat more sympathetic to the question whether the universe is fundamentally deterministic or not, here a brief summary of the idea.

The central difference between standard quantum mechanics and superdeterministic hidden variables theories is that in the former case two identically prepared states can give two different measurement outcomes, while in the latter case that's not possible. Unfortunately, "identically prepared" includes the hidden variables and it's difficult to identically prepare something that you can't measure. That is after all the reason why it looks indeterministic.

However, rather than trying to prepare identical states we can try to make repeated measurements on the same state. For that, take two non-commuting variables (for example the spin or polarization in two different directions) and measure them alternately. In standard quantum mechanics the measurement outcomes will be non-correlated. In a superdeterministric hidden variables theory, they'll be correlated - provided you can make a case that the hidden variables don't change in between the measurements. The figure below shows an example for an experimental setup.

A particle (electron/photon) is bounced back and forth between
two mirrors (grey bars). The blue and red bars indicate measurements
of two non-commuting variables, only one eigenvalue passes, the
other leaves the system. The quantity to measure is the average time
it takes until the particle leaves. In a superdeterministic theory,
it can be significantly longer than in standard quantum mechanics.

The provision that the hidden variables don't change is the reason why the test is only 'almost' model independent, because I made the assumptions that the hidden variables are due to the environment (the experimental setup) down to the relevant scales of the interactions taking place. This means basically if you make the system small and cool and measure quickly enough you have a chance to see the correlation between subsequent measurements. I made some estimates (see paper) and it seems possible with today's technology to make this test.

Interestingly, after I had finished a draft of the paper, Chris Fuchs sent me a reference to a 1970 article by Eugene Wigner where, in a footnote, Wigner mentions Von Neumann discussing exactly this type of experiment:

“Von Neumann often discussed the measurement of the spin component of a spin-1/2 particle in various directions. Clearly, the possibilities for the two possible outcomes of a single such measurement can be easily accounted for by hidden variables [...] However, Von Neumann felt that this is not the case for many consecutive measurements of the spin component in various different directions. The outcome of the first such measurement restricts the range of values which the hidden parameters must have had before that first measurement was undertaken. The restriction will be present also after the measurement so that the probability distribution of the hidden variables characterizing the spin will be different for particles for which the measurement gave a positive result from that of the particles for which the measurement gave a negative result. The range of the hidden variables will be further restricted in the particles for which a second measurement of the spin component, in a different direction, also gave a positive result...”

Apparently there was a longer discussion with Schrödinger following this proposal, which could be summarized with saying that the experiment cannot test generic superdeterminism, but only certain types as I already said above. If you think about it for a moment, you can never rule out generic superdeterminism anyway, so why even bother.

I'm quite looking forward to this conference, to begin with because Vienna is a beautiful city and I haven't been there for a while, but also because I'm hoping to meet some experimentalists who can tell me if I'm nuts :p

Update: Slides of my talk are here.

Gauge Symmetry Violation (Short film)

Symmetry (Short Film) from Apostolos Vasileiadis on Vimeo.
A physics professor loses control over a false theory of his. A student is there to set things right.

Filmed at Nordita/AlbaNova or in tunnel system between the buildings respectively. Apparently some of the students here have, ehem, dark fantasies.

The multiverse is not a paradigm and it’s not shifting anything.

Google “multiverse paradigm” and you get more than a thousand hits. According to Wikipedia a paradigm “describes distinct concepts or thought patterns”. Unfortunately, the multiverse is pretty much the opposite: There’s no distinct concept, but instead a variety of loosely related properties of existing theories that are being construed to have a common theme which, we are then told, is sign of an impending paradigm shift.

I’m starting to take offense in this forward defense. If the spread of multiversal “thought patterns” is sold as a paradigm shift, everybody opposed to the multiverse is discarded as being stuck in yesterday. It’s only the enlightened who are ahead of their time and understand the significance. I really don’t think there’s any paradigm here and certainly nothing is shifting. To see why, it’s helpful to distinguish two different classes of multiverses that are presently being discussed, usually thrown together.

1. The Multiverse of Disappointed Hopes

Science works by constructing models for real world systems. These models can then be used to understand what happens in the real world, and to make predictions. A theory is a map from a model to the real world. The model should not be confused with the theory itself. The theory is what tells you how to identify properties of the model with the real world. The model is the actual stand-in for the real world system.

Einstein’s General Relativity for example is a theory: it’s a prescription for how to deal with space-time and particles moving in it. A model is the space-time of a star or an approximately homogeneous matter distribution. It’s the theory of General Relativity, but the ΛCDM model. Likewise, there’s quantum field theory, and the standard model. Needless to say, not everybody uses this terminology all the time, but that’s how I want to use it.

Models and theories are not only used in physics and don’t necessarily have to be mathematical. Psychologists have models for human behavior that they apply to patients – the ‘real world’. A drawing is a model, in this case the “theory” that connects it to the real world comes for free with your visual cortex. A story is a model, the “theory” is your knowledge of the language that relates letters to real world objects or actions. And so on. The merit of mathematical models is that they have a very strict quality control, which is self-consistency.

And then there are toy models.

Toy models are models that do not have real world counterparts. It’s drawings of creatures that don’t exit or stories of people that have never lived. They’re playgrounds of creativity that can teach us lessons about the theory, which is why studying toy models is a very common and often fruitful exercise. There’s an infinite amount of such toy models. You could say there’s a whole multiverse of them, all these toy models that don’t map to any part of the universe we know. Asking whether what they describe is real is like asking if Harry Potter really exists because a story has been written about him. The difference between fantasy novels and physicist’s toy models is the size of the interested audience, but in spirit they’re the same exercises in creativity.

So, sure there are models that don’t describe the real world, in physics as well as in painting. That’s because mathematical consistency alone does not imply a model describes what we observe, much like using English does not imply you talk about real people. Additional requirements are needed besides consistency to construct a useful model, and these requirements are always agreement with observations, though this isn’t always explicitly phrased this way. When we assume Lorentz-invariance or renormalizability or absence of ghosts, these are physical requirements ultimately based on our experience.

This means a multiverse that you can get rid of by adding the requirement that the model needs to describe observation is neither new, nor surprising, nor something to worry about. It just means that mathematical consistency of whatever theory it is you’re dealing with is not sufficient to make a particular prediction. The string theory landscape is a multiverse of this type. The only reason people talk about this now is that many of them had been hoping string theory would make some requirements that one needs in the standard model unnecessary. Alas, these hopes were disappointed, though the last word might not be spoken yet.

Does it make sense to instead talk about probability distributions over the models you get when you refuse to use existing ties to observations, here specifically the values of certain parameters? No. Because that’s cherry picking the observations you want to neglect.

In the construction of the model there always enter many other observations that are being neglected if one considers such probability distributions, such as the number of (large) dimensions, Lorentz-invariance, or the existence of space-time to begin with – these are not requirements of mathematical consistency, these are physical requirements based on observations. If you wanted to be serious with asking for the probability of particular models, you should sample over all models, in the end over all that is mathematically consistent. You’d be left with Tegmark’s mathematical multiverse. And in that mathematical universe you’d have replaced the question “Which model describes the real world?” with “Where are we in the mathematical universe?” You don’t gain anything.

Once you have seen the power of mathematical models to describe natural systems, it is natural to ask if there is a mathematical model that describes “everything” we see. I believe there is. But people who search for a “theory of everything” today mean more than that. They want in particular a theory that delivers the parameters in the standard model. But even if that would be achieved, we would still have to use other axioms that are ultimately based on observations. So while it is worthwhile to try to find a simpler model that reduces the number of axioms, including values of parameters, we can never avoid using input from observation. If we do, we’ll end up with a multiverse which just tells us that mathematical consistency isn’t sufficient.

So if you have a multiverse that can be eliminated by the requirement that the model is consistent with observation, this isn’t a paradigm shift, it’s just disappointed hopes.

2. The multiverse package deal

But there’s a different type of multiverse, one that you cannot get rid of by requiring match to observation. It’s the case in which a theory applied to a model that describes a real world system necessarily maps into a space that is larger than what we observe. Eternal inflation and the many worlds interpretation of quantum mechanics are of this type. Or, more mundanely, there is nothing in ΛCDM that predicts the universe just ends beyond the distance that we can (presently) observe, so you have a multiverse beyond our observations.

This opens a can of interpretational worms because we can now endlessly discuss whether the not observable images of the map are real or not. Personally, I find this a rather fruitless debate about the meaning of the world ‘real’. To me a model is a tool to describe the real world and if it does that, and if it’s an improvement over other models, I don’t care if there are mathematical elements in the model that don’t correspond to real world observables. Mathematics is full of structures that for all we know don’t correspond to anything we observe anyway. I don’t see a reason why we must be able to observe them all.

But, no, I don’t think you should just shut up and calculate. Because we might be mistaken in thinking that what the theory predicts beyond our observable universe is indeed unobservable. Maybe we just haven’t asked the right questions and there are ways to observe it after all.

So it’s an interesting feature that theories can display, but it’s certainly not a new concept. There’s been a century of discussion about the presence of mathematical objects in quantum mechanics that for all we presently know are fundamentally non-observable. So if that’s a paradigm shift it’s one that has already happened long ago.

3. Wilzcek’s Multiversality

Frank Wilzcek recently had a paper on the arxiv titled “Multiversality”. The first half of the article is a nicely written general introduction, the second half is about axion cosmology and then the paper ends quite abruptly. The most interesting part of the paper are three positive answers to the question

“Are there aspects of observable reality, i.e. the universe, that can be explained by multiversality, but not otherwise?”

It is fruitful to look at the answers to gauge the depth of the existing arguments in favor of the multiverse:

“Yes – one is the apparent indeterminism of quantum mechanics, despite its deterministic equations.”

Wilczek claims here the apparent indeterminism of quantum mechanics can be explained by the many worlds interpretation but not otherwise. That’s an objectionable claim, in particular because the qualifier didn’t include anything about locality.

“Yes – the outrageously small, but non-zero, value of the dark energy density.”

Here he is claiming that there is no other way to explain the measured value of the dark energy density than anthropic reasoning and that anthropic reasoning necessarily implies a multiverse. There are many people who would object on the former and the latter is manifestly wrong. You don’t need a multiverse to do anthropic reasoning, see my post Misconceptions about the anthropic principle.

“Yes – the opaque and scattered values of many standard model parameters that are not subject to the discipline of selection.”

An interesting answer because it is phrased to suggest that the values of the standard model parameters are scattered to begin with. Even if they were however that wouldn’t force us to believe that any possible distribution of values actually exists in a more meaningful sense than Harry Potter exists.

Taken together, these answers tell you aptly just how weak the case for a multiverse really is.

Summary

We should distinguish between multiverses that you can eliminate by adding axioms to the theory that tie the model to the real world, and those that you can’t eliminate this way. The string theory landscape is of the former type, you “just” have to find the right vacuum, and good luck with finding that. Eternal inflation and the many worlds interpretation are of the latter type. In this case you get more than you asked for. One can interpret this type of multiverse as a calculation device which might have its uses. It might also turn out that these multiverses aren’t unobservable after all, so these ideas certainly merit some investigation. In any case however, there’s no paradigm shifting here.

Book Review: “You are not so Smart” by David McRaney

You Are Not So Smart: Why You Have Too Many Friends on Facebook, Why Your Memory Is Mostly Fiction, and 46 Other Ways You're Deluding Yourself
By David McRaney
Gotham; Reprint edition (November 6, 2012)

I know I said no more brain books, but this one’s been in the pipe. I’ll make this review short. McRaney in his book goes through 48 ‘brain bugs’ that are shortcomings of human cognition where evolutionary advantageous procedures are inappropriate to present-day situations. Having meanwhile read several books on the topic, I knew about 40 of these brain bugs and the rest are very similar to the ones I already knew.

What I was hoping for in McRaney’s book was some kind of structure, maybe a classification or categories, a big picture – some insight as to how it all ties together or where it’s going and what’s next. But the book is really just a selection of little essays, apparently the result of a blog by the same name, and that’s also what it reads like.

The 48 sections of the book do come with selected references and summaries of research studies that have been made, but a discussion of how well-established any particular result is and if there is maybe contradictory evidence is entirely lacking. Also lacking is space to address the question how these studies relate to behavior in the real world, what the evidence is for this, and, most important, if people change their behavior when being educated about shortcomings in their default mode of thinking.

In summary, the book is an easy read, but it’s not terribly insightful and somewhat uninspired. If you follow the popular cognitive science literature you’ll know pretty much everything that is in the book. The book might be useful for you however if you want to get a quick overview on what topics are presently being discussed in this area, without too much skepticsm or scientific background. Also, the essays all probably make good conversation starters.

Dear Mr. President

Two weeks ago, Barak Obama visited Stockholm and spent half an hour or so on the KTH campus. Since Nordita is officially part of KTH, safety regulations went through the employee email list weeks in advance. Luckily I was away the great day. I was told later the Swedes were so successful scaring people off the impending traffic disaster that Stockholm was basically deserted during the President’s visit and elks were seen chewing licorice in front of the royal palace.

I flew back to Stockholm the following day. Lufthansa online check-in suffered an interesting technical glitch and produced a boarding pass for seat 1A business class. Yeah to software bugs. As I was sitting in the business class with leg space I don’t actually need (I’m not socialist, just short), I couldn’t help but wonder what, if I had 15 minutes, would I tell the President. Hell, what would you tell the man?

From the German perspective, the American political system looks strange, which is ironic given the history of Germany’s representative democracy. The strong role of the US President in particular and the focus on individuals rather than programs in general is the most obvious difference. Stranger even is that the political landscape in America is in practice a two party system. This has created a situation where, instead of different parties offering a spectrum of alternatives, the two parties morph to fit their potential clientele, or make it fit. And, needless to say, the wealthy part of the clientele lobbies for their interests, an influence that’s amplified by the almost complete lack of labor unions.

Yes, from a German perspective it seems strange that a country which values democracy so dearly practices it so badly. But then I’m not a political scientist, I just hope I know enough to put my two X in the right places on Sunday.

During the years I spent overseas, Academic America seemed to be overwhelmingly on the side of Obama’s Democratic Party. I recall many seminars in which an US American speaker would make jokes or political statements that clearly showed they were confident the majority of the audience would sympathize with their political views. And they were right of course. (Provided the audience was mostly American. These jokes don’t fly in Europe.) But during the last year I sense this support base faltering as the conditions for scientific research gradually worsen under Obama’s watch.

There are many things the man must shoulder and I’m sure they weigh heavily. Among all these weighty boulders, there’s a tiny little pebble that made me lose my faith the USA will overcome its anti-scientific congestion. It came with this headline:

Congress Cuts Political Science Research Grants

“Last month [March 2013], President Obama signed 600 pages of legislation to keep the government from shutting down, while shutting down much of the nation’s [political science] studies. Senator Tom Coburn (R-Okla.) secured Democrats’ approval for an amendment to the bill that eliminates the National Science Foundation’s political science studies, except those the NSF director deems relevant to national security or U.S. economic interests.”

By now, the NSF has cancelled the political-science grant cycle.

Dear Mr. President, how could you have let that happen?

Every major problem that this planet presently faces is primarily about organizing human life and negotiating complex problems with uncertain solutions. The existing political, social, and economic systems are insufficient to deal with these problems, and scientific knowledge is insufficiently integrated into decision making procedures. As societies and economies have become more interconnected, political institutions have not kept pace. The technology is there, the knowledge how to use it isn’t. This realization lies behind initiatives like the FuturITC and attempts to predict political unrest. Yes, that’s political science for you.

Today riots are organized on twitter, wars are led on YouTube, and election results predicted on online futures markets. Nobody knows what this means for the future of democracy. Do Facebook and Twitter help spread Democracy and Human Rights? Are the White House Petitions are good idea or do they just create noise? Yes, that’s political science for you. We all have too much information and not the faintest idea how to intelligently aggregate it and use it within our political systems. We need a scientific approach to institutional design. Trial and error is an archaic procedure that takes time that we don’t have, and errors have become too costly.

Just the situation to scrape funding for the political sciences, I see.

I am trying to imagine Angela Merkel suspends all governmental funding for political science. Germany is the land of the poets and thinker, the land of Kant, Hegel, Marx, Engels and Weber. Besides inventing compound nouns, Germans are also good with solidarity, strikes, and nudity. The Americans made very sure each German receives a solid education about the merits of democracy. I can see the outrage. I see the ‘68 students, now at retirement age, clogging the streets, “academic freedom” scrawled over their flopping breasts. “Censorship!” they shout. “Thoughts are free” they sing. Then the President of the United States calls. “Angie,” he says “Wtf?”

The great advantage of the American political system over the German one is however that the US President can only serve two terms, while the German chancellor can run till he or she drops dead.

Dear Mr President: I hope you tried a handful of the salty licorice that the Swedes chew down by the pound. Because that’d make you as sick as I feel when I read what American scientists must endure these days.

Quantum Gravity in Gamma Ray Bursts: Still Nothing

Small wavelength photons (blue) travel faster
than their long wavelength companions (red).
[Image Source]

Gamma ray bursts emit highly energetic photons that, by the time they reach us, have a long journey behind them. That makes these photons excellent candidates to test new physics: Because both the energies and the distance are extreme they potentially give us access to so-far undiscovered effects.

However, in contrast to supernova of type Ia, gamma ray bursts are one of a type – they’re not so much standard candles but surprise fireworks. That makes these photons not quite so excellent candidates to test new physics.

A story that dates back now more than a decade and that has been hailed as a ‘test of quantum gravity’ is that certain quantum gravitational effects could lead to an energy-dependence of the speed of light. In this case, photons of high energy would travel either faster or slower than the low energetic photons (depending on the sign of a parameter). Such an effect is not allowed by the presently established theories, and looking for a signal of an energy-dependent speed of light therefore tests deviations from Einstein’s theory.

Theoretically, there are two different ways this could happen, either by a breaking of Lorentz-invariance or by a deformation of Lorentz-invariance, and these cases have to be carefully distinguished. Both cases lead to an energy-dependent speed of light, but if Lorentz-invariance is broken, meaning there is a preferred restframe, then this would lead also to other effects that we should have seen already. This means if we do see such an unexpected effect in the emissions of gamma ray bursts, we’d know it’s not a breaking of Lorentz-invariance but a deformation. This would be considerably more exciting, but is also much more speculative.

My position on this has been, and still is, that a deformation of Lorentz-invariance is not well motivated and theoretically highly problematic, thus I don’t think an energy-dependent speed of light is plausible. But in the end the question is what the data says.

Data however is a reserved companion who just politely asks to be analyzed, and given that no two gamma ray bursts are alike it’s not at all clear how to do the analysis. It seems to me experimentalists are still poking around and trying out new methods. Occasionally a constraint comes out of this. The most recent constraint came out in two papers by Vlasios Vasileiou and a whole list of other people in no particular alphabetic order (if somebody can fill me in on the authorship order in that part of the community, please enlighten me).

To make a long story short, they propose three new ways to arrive at new bounds, all with advantages and disadvantages, and arrive at a bound that constrains new quantum gravitational effects to be beyond 7.6 times the Planck scale, at 95% confidence level. This means the new bound is both weaker and at a lower confidence level than the bound by Nemiroff et al that we previously discussed, so it’s non-news really. And that doesn’t even take into account that the more ways you try to extract a signal from the data, the less likely it will eventually be a real effect.

In a footnote in the discussion the authors of the new paper criticize the Nemiroff et al result basically for the same reasons that I put forward in my earlier blogpost: The constraint hinges very strongly on a few pairs of photons. But the advantage of the Nemiroff analysis is that it’s a clear and clean method that can rapidly increase to higher statistical relevance with more observations, provided we see just a couple more of such pairs. It merely relies on the statement that it’s quantifiably unlikely that a few photons arrive almost simultaneously if they weren't emitted simultaneously and traveled together – at the same speed. Unfortunately, the significance of that result could also decrease in relevance, and that for reasons that have nothing to do with the energy-dependence of the speed of light, just with the physics at the source.

The new approach in the Vasileiou et al paper is valuable however for trying to take into account an intrinsic dispersion of the source. But I think the great weakness of this bound is the same as the previous bounds: low statistics with results that strongly depend on one or a few gamma ray bursts. I doubt we’ll ever get rid of the possibility that source effects play a role unless red-shift is taken into account and different distances are sampled over. That’s because an energy-dependent speed of light should yield a stronger effect the farther away the source, while a source-dependent effect does not get stronger.

Either way, for me it’s a win-win situation :o) There’s either quantum gravity in the gamma ray burst measurements or there isn’t. If there is, it’s a huge boost for the field I work in. If not, I was right all along saying that there is no effect. At the moment however the situation isn’t entirely settled, so stay tuned.

Book Review: The Universe in the Rearview Mirror

The Universe in the Rearview Mirror: How Hidden Symmetries Shape Reality
By Dave Goldberg
Dutton Adult (July 11, 2013)

In his new book “The Universe in the Rearview Mirror,” Dave Goldberg expounds the important role of symmetries in the fundamental laws of physics. He starts with the discrete operations of charge-conjugation, parity, and time inversion, and their combinations. After introducing the reader to Emmy Noether and her work, he discusses continuous symmetries, homogeneity and isotropy, as well as Lorentz-invariance before continuing with gravity. The later chapters deal with gauge symmetries and symmetry breaking. The book finishes with existing proposals for physics beyond the standard model, grand unification, supersymmetry, and the missing theory of quantum gravity.

Goldberg does a remarkably good job conveying a very technical topic in non-technical terms and with only a handful of equations (yes, E=mc² among them). He works mostly with analogies and writes in an engagingly colloquial way with a large dose of humor, though some readers might find the high density of jokes more distracting than helpful*. The bibliography and the guide to further reading provide helpful references for the readers who wish more details, and the book also has a brief glossary.

Symmetries that “shape reality,” as the subtitle of the book says, are a vast topic of course. Goldberg has focused on these symmetries that (for all we know) shape reality on the most basic level. He does not (except for the purpose of a brief analogy) touch upon the much broader topic of emergent symmetry and order in condensed matter systems, or in other areas of physics and science more generally. This focus has the benefit that the book is relatively lean (291 pages, hardcover) and maintains its momentum, but the blurb could have been more descriptive.

On the downside, the book is confusingly structured and the reader who doesn’t bring prior knowledge might become frustrated in several places. For example the WMAP mission is mentioned in the first chapter, without explanation for what exactly it measures and without an image. The radiation of the cosmic microwave background is again introduced in the third chapter, without referral to the earlier mentioning of WMAP, and temperature anisotropies are briefly mentioned here. Temperature anisotropies are then again introduced at the end of this chapter and here the WMAP image finally appears (low-resolution black-white), alas without the image being mentioned in the text and without explanation for what it shows.

In fact, while the graphics that have specifically been produced to accompany the text are well done and helpful, the book also contains a number of images that are useless and only loosely connected to the text. An image on page 41 I guess shows the Venus transit which is mentioned in the text on this page, or maybe it shows an exercise to find one’s blind spot. On page 109, the reader encounters a Klein bottle and the only reason I can infer is that the next page mentions Emmy Noether “took classes with Hilbert and Klein.” An image on page 118 (no caption) shows Einstein arcs and the explanation in the text amounts to “massive bodies bend light”. The image on page 250 remained a mystery to me until I found it in the Wikipedia entry to “Sisyphus” (mentioned on that page).

The book is also confusing and unstructured in other ways. Goldberg begins to talks about “the elusive dark matter particle” (in itself a questionable phrase) in Chapter 9 without so much as mentioning what dark matter is or what evidence we have for it. He uses the Planck length in chapter 6, but only explains it in Chapter 10. The cosmological constant problem is introduced twice. The elaboration on the twin paradox somehow misses to spell out what the resolution of the paradox is. It is mentioned that inflation was proposed “to get around the horizon problem” but the reader is not actually told how inflation solves the problem. Evidence for inflation amounts to “we’re reasonably certain that it is [correct]”. Goldberg elaborates on the multiverse and later on the compactified dimensions of M-theory, but does not connect the two topics. He speaks about the entropy of matter in the early universe before explaining what happened in the early universe. On page 167/168, I came across the possibly most opaque motivation for quantum gravity that I’ve ever encountered. Luckily there is a considerably better one on page 269. A quotation from Stephen Hawking expressing the opinion that information is not lost in black holes is dumped onto the reader in a description of black holes as “entropy-producing machines” without so much as mentioning the black hole information problem.

I’ll not go down the full list of similar notes that I took while reading; you get the picture.

Goldberg has to be credited for making his text timely by referring to very recent works, for example he mentions Verlinde’s contribution on entropic gravity. This reference (the only reference on the topic) appears in a section on the arrow of time and at least I could not infer the direct connection, besides both having something to do with entropy. Goldberg uses Max Tegmark’s proposed level structure of the multiverse and in the last chapter on physics beyond the standard model we meet Garrett Lisi the surfer without university affiliation who allegedly stunned everybody with proposing his theory of everything. It somehow goes unmentioned that Lisi has PhD in physics. I’m picking at this point not because I don’t think the E8 root diagram is pretty, but because the reader is left with the unfortunate impression that surfing is all you need to understand modern physics. Towards the end of the book the reader can find a very good summary of the recent discovery of the Higgs particle and its relevance.

In summary, the book is valuable for the selection of topics and for conveying the relevance of symmetries in the laws of nature, but the execution leaves wanting. Sean Carroll’s two books for example cover a substantial part of the physics built upon Goldberg’s hidden symmetries, but the reader who does not bring prior knowledge about modern physics will learn a great deal more from Carroll’s more didactic approach. Goldberg however succeeds in inspiring a sense of awe for the power of symmetries, not at least because awesome seems to be one of his favorite words.

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*Humor, of course, is always a matter of taste. So let me just say that messages like “science nerds… spend … many nights alone” or physicists don’t know how to dress elegantly and don’t get invited to dinner parties, strike me more as funny-peculiar than funny-ha-ha.

Whatever happened to AdS/CFT and the Quark Gluon Plasma?

A decade ago, the AdS/CFT correspondence was celebrated as a possible description of the quark gluon plasma. RHIC measurements of heavy ion collisions at that time showed a surprisingly small viscosity that lead to a revision of the previous models. Excitingly, a small viscosity appears naturally in the gauge-theory dual of the AdS/CFT correspondence, nevermind that QCD is neither conformal nor supersymmetric. This development was all the more welcome as it served to demonstrate that string theory is not useless, as critics claimed, but that it can provide insights which improve our understanding of physical processes in the real world.

The gauge-gravity correspondence rapidly became a boom area in high energy physics. After the viscosity, people looked at other observables, notably the energy loss of particles going through the plasma. In highly energetic particle collisions, quarks are produced in pairs, but due to confinement individual quarks are never measured. What is measured instead are color-neutral hadrons that the quarks decay into and that are bundled into the direction of the original quarks. These bundles of hadrons are called jets and in the simplest case there are two of them with total momenta that are back-to-back correlated owing to their common origin from the quark pair.

In a heavy ion collision, one of the quarks may have to pass through the quark gluon plasma and thereby loses energy. This leads to what is known as ‘jet quenching’, a pair of back-to-back correlated jets where the total energy on one side is reduced. The energy loss in the plasma can and has been calculated in different models for heavy ion collisions. There are about a handful of such models, and in the days before the LHC all tried to get in their predictions for the jet quenching at LHC energies, the central question being how the energy loss scales with the increase in collision energy.

After the LHC heavy ion runs, it turned out the data do not agree very well with the scaling expected for energy loss from the AdS/CFT correspondence – in fact from all the models it was the worst prediction. As we discussed in an earlier post, AdS/CFT predicts too much energy loss, the plasma is too strongly coupled.

AdS/CFT confronts data. Image Credits: Thorsten Renk.
For details and references, please refer to this earlier post.

That the scaling doesn’t fit well with the data need not be too much of a worry because these scaling arguments were quite general and in reality the process of propagation through the quark gluon plasma isn’t quite as simple. But clearly the new data called on theoretical physicists working on AdS/CFT to study the observables and improve their model or to call it a failure and move on. Alas, nothing like that happened.

Since the LHC data came in, for two years or so, I’ve now been sitting through AdS/CFT talks that would inevitably be motivated by the low viscosity of the quark gluon plasma and the RHIC data, frog spawn picture and all. And every time I’d raise my hand at the end of the seminar and ask for the speaker’s opinion on the recent LHC data, expecting an update on the work on that matter and that there is no need to worry because the models can be improved to accommodate the data. Instead, it was like the LHC never happened. I don’t work in this field and don’t even follow the literature closely, but it seemed that I knew more about the problems with the LHC results than the people who got paid for talks motivated by yesterday’s data.

What they’d typically say is that nobody really expected AdS/CFT to make quantitative predictions. Alas, even the qualitative prediction, the mere slope of the curve, is wrong. The only prediction that is “qualitatively” correct is that there is some energy loss. Besides this, it’s all well and fine that a new model doesn’t make quantitative predictions, but that’s not a status that should become permanent.

It’s not that the data went entirely unnoticed. A few brave souls took on the issue. In this paper Ficnar, Norona and Gyulassy looked at the effects of higher derivative corrections to the gravity sector. It's somewhat ad-hoc, but apparently does reduce the energy loss. There is however no fit to the data and I’m not sure what this does to other observables. In another work, Ficnar also took into account a time-dependence of the configuration, but the conclusions with respect to the jet quenching and LHC data remain vague and amount to “a more thorough numerical analysis is needed.” In a recent paper, William Horowitz summarized the situation as follows:

“Despite significant efforts, AdS/CFT estimates for light quark and gluon energy loss are qualitative at best… it is difficult to imagine that a relatively sophisticated estimate of the suppression would be consistent with data.”

I was thus thrilled when I heard a talk by Stephen Gubser (about recent work with Ficnar) at a conference in Frankfurt this July, because he spoke about a possibility to improve the AdS/CFT model to accommodate the LHC data. Unfortunately, Gubser and collaborators don’t have a paper about this on the arXiv yet, so all I can do is refer you to the slides. My vague recollection is that he said one needs to take into account the momentum on the endpoints of the strings and that this does improve the scaling of the energy loss and fits considerably better with the LHC measurements. Though, if I recall correctly, getting the slope to match the data requires pushing the parameter into a range where one actually shouldn’t trust the model anymore. So in the end this might not solve the problem either.

If that explanation sounds like I don’t really understand the details it’s because I don’t really understand the details. I didn’t take notes, and two months later that’s as much as I can recall when looking at the slides and the Princeton professor has not been very communicative upon my inquiry. I thus just want to draw your attention to this development – if you’re interested in the topic, I recommend you have an eye on Ficnar and Gubser’s next arXiv uploads. For all I can tell, these guys are the only ones who take the issue seriously and so far it doesn’t sound too promising to me. If I’m missing some references, please let me know.

I don’t know enough about the topic to tell how likely it is that the AdS/CFT model can be improved to fit the data, and personally I find the applications to condensed matter systems better motivated. What annoys me about this situation is that people working in the field continue to decorate themselves with false achievements when they use the viscosity of the quark gluon plasma to justify the relevance of their own work and that of string theory by large.

It’s time the community comes clean and draws a conclusion. Either AdS/CFT cannot describe the quark gluon plasma, then please bury this episode in the history books and move on. Or it can, and then I expect to see a curve that fits on the LHC data. At the very least I want to hear it’s on the to-do list. Yes, the LHC really happened.

The Limits of Science

There’s been some buzz going through the blogosphere, following an essay by Steven Pinker on “Scientism”. On the one side of the debate are those who believe scientism is a higher state of consciousness, and on the other side those who think it’s a scientifically transmitted disease with a symptomatic itch that shouldn’t be scratched publicly. And I think they all failed to address the main point: Where are the limits of science? And how do we find them?

If I read essays by philosophers and social scientists and other academics in what is vaguely considered “soft science” I often can’t but sense a certain ring of panic. The physicists are coming, is what I read, they’re planning to take over with mathematics. Then they rush to ensure themselves and everybody else that no, no, some things can’t be described by mathematics. This appeals to the public because nobody likes to be predictable and many people are afraid of math. The softies line with the masses and end up being the good guys for perpetuating cognitive illusions, while the physicists are marked delusional reductionists. Welcome to the 21st century.

Oh yes, the softies will admit, there are meanwhile many mathematical models in the social sciences, and neuroscience has already made some discussions about consciousness entirely redundant. But look, they’ll say, these can only tell you something about statistics (scary math word). Human behavior can’t be modeled mathematically. After all we’ve got free will (unproved). We understand the models about us (irrelevant). Humans are special (said the human), the brain is complex (whatever that means), and it’s got qualia (defined by being unmeasurable). And, most importantly, humans are not elementary particles. (Always good to finish an argument with a completely irrelevant statement that everybody must agree on.)

The problem with these elaborations, besides making me wonder what these people get paid for, is that we presently know of no reason why some observations, like human behavior, cannot be modeled mathematically. But neither does anybody know for sure that it is possible. What we do know however is that it certainly is not presently possible. And that’s what determines the limits of science: our present possibilities, what we can do in practice, and not unknown and quite possible unknowable principles.

It shouldn’t be relevant to my argument, because I’m telling you what matters is what we can do in practice and not what we can do in principle. But just so you don’t misread me: I don’t believe that everything can be described by mathematics (for reasons I’ve laid out here). I’m just saying that we presently don’t know of any reason why it should not possible to describe human behavior mathematically.

Personally, I think it is possible but useless in that such a model would in the best case be a copy of the real system and would not deliver predictions. It would be like trying to understand the sun by simulating it in true resolution and real time on a computer cluster. Then you can either watch the sun or your computer, but besides this you haven’t really gained anything. If you believe that we live in a matrix as study-objects, then we live here because it was not possible to find a simpler way to analyze behavior of human societies than just creating and watching them.

So much about my beliefs. But these are beliefs because we don’t know whether they’re true, and in any case these limits that might exist in principle are far beyond the limits that presently exist in practice.

And of course science has limits. It has limits because our understanding is incomplete. These limits of science aren’t fixed and they are constantly shifting as we learn more about the world that we live in. In that process, topics that were previously inaccessible to the scientific method become accessible, and that creates friction in the communities.

Imagine the world of knowledge as having a core of hard science, surrounded by a belt of soft science, that goes over into interpretations, narrative, opinions, speculations, and eventually fantasy. The hard core expands as we learn: What once was a matter of interpretation becomes measurable. What once seemed beyond computational possibility becomes computable. What once was merely a story becomes supported by evidence. Problems arise if researchers refuse to use the best scientific methods of the day in their field. Then they are simply acting unprofessionally. And when they notice they’ve missed the boat they panic.

Where are the limits of science right now? That’s the discussion that we should have. And it’s not an easy one.

Let me give you three examples of what’s presently off-limits for mathematical modeling.

One is history. You could in principle imagine that it was possible to create a model about human behavior, say, in war-times, that produces outcomes that we can observe today, for example how people expressed themselves in the literature. Then you could analyze the literature to draw conclusions about the circumstances back then. Needless to say, converting experience into writing is so difficult to model mathematically that nobody can do this, and nobody even knows if it is possible. So instead historians go and read the literature and study the paintings, and try to interpret them by taking into account as much as they know, most notably about being human, something that they can do better than any software or equation. At least for now.

The second one is personal identity. Nobody really knows what it takes for a human brain to create a sense of self-awareness and the experience of being an individual actor in possession of a body. Neuroscience has collected a lot of information on that matter, but we’re far off from being able to mathematically model these processes. Much of the literature on the subject is interpretation of data or case-studies. But wait some decades and I’m sure we’ll know much more about what enables “you” to think of “yourself”.

The third example is politics. One often hears that science can only deliver the facts, but humans still have to make the decisions because they have to take into account “morals” and “values” that are off-limits for science. This is however empty vocabulary. Values and morals are just simplifying concepts that arise in our cultures. They are in the first line words that primarily serve the purpose of communicating opinions. Morals and values change over time, people tend to interpret them individually differently, and they might regard them more or less helpful for their self-expression. But there is nothing – in principle! – that prohibits science from predicting the emergence of certain morals and values. Again though, in practice, nobody can do this.

And that’s why science cannot replace politics. Because when you express your opinion about a possible change, you are projecting yourself into the future and try to find out whether or not it would be an improvement. Or, in the Darwinian mindset, whether you’ll be more or less well adapted to your environment. For this projection you need to know some facts, and these facts science can provide – with errorbars. But what science cannot do is to project you and your experience into the future. The best way that we presently know to do this projection is to ask people to do it themselves. There are pitfalls to this, because we are not actually good at predicting what we will think in ten years from now. But presently it’s the best we can do.

The last example also tells you why it is important to know the present limits of science. Because it raises the question what we know about human decision making and whether we can use that knowledge to make better decisions.

In summary. Science has limits, but they change over time. Knowing where the present limits of science are is important because that’s where opinions and interpretations become relevant to decision making. Excuse me for publicly scratching my itches.