What is asymptotically safe gravity and what does it save?

“Infinite Grid” by Georg Koch.

Einstein’s theory of general relativity, which describes gravity as the curvature of space-time, stands apart from the other interactions by its refusal to be quantized. Or so you have almost certainly read somewhere. But it’s not true.

To begin with quantizing gravity isn’t all that difficult. Gravity can and has been quantized much like the other interactions by promoting gravitational waves to quantum waves. That is called perturbative quantization and technically somewhat annoying but perfectly doable. The problem starts only after this because the so quantized theory does not make sense anymore when gravity becomes strong. It delivers infinities as results, no matter what, and that makes it not only useless but also meaningless as a fundamental theory.

You might shrug shoulders on the infinites because you have also probably heard that quantum field theory has a problem with infinities anyways. But that too is not true...

Yes, the occurrence of infinite results was historically a big issue. But quantum field theory development hasn’t stopped in the 1930s and we know today very well how to do these calculations. Whenever you get an infinite result, you need to use a measurement to fix a physical parameter. We call it renormalization and it’s a mathematically clean procedure.

However, if you need an infinite amount of measurements to fix parameters, then you have a real problem because now your theory is no longer predictive: You need infinitely many measurements before you can make a prediction about the next measurement. A theory with that problem is said to be perturbatively non-renormalizable and on that diagnosis most physicists will not resuscitate the patient. Perturbatively quantized gravity has exactly this disease.

As long as gravity is weak, for all practical purposes you need only a finite amount of parameters to get to a desired precision. You can use the theory there. But once gravity gets strong, the theory becomes useless. This means it is not a candidate for a fundamental theory.

The use of quantum field theories and their properties thus depend on the energy scale. Interactions can for example become weaker or stronger depending on the energy of the interaction. Quantum Chromodynamics famously becomes weaker at high energies; it is “asymptotically free” and that insight was worth a Nobel Prize in 2004.

But how theories in general depend on the energy scale has only been really understood within in the last two decades or so. It has been a silent development that almost entirely passed by the popular science press and goes under the name renormalization group flow. The renormalization group flow encodes how a theory depends on the energy scale, and it is at the basis of the idea of effective field theory.

The dependence of a quantum field theory on the energy scale that is used to probe structures is much like Ted Nelson’s idea of the stretch-text, a text in which you can zoom or click into layers of more detail. The closer you look, the more new features, new insights, new information you get to see. It’s the same with quantum field theory. The closer you look, the more new layers you get to see.

Based on this, Weinberg realized in 1976 that perturbative renormalizability is not the only way for a theory to remain meaningful at high energies. It is sufficient if, at high energies (technically: infinitely high), you need to fix only a finite number of parameters. And none of these parameters should become infinite itself in that limit. These two requirements: A finite number of finite parameters that determine the theory at high energies are what make a theory asymptotically safe.

This then raises the question of whether quantum gravity, though perturbatively nonrenormalizable, might be asymptotically safe and meaningful after all. That this might be so is the idea behind “asymptotically safe gravity”. While the general idea has been around for almost four decades, it has only been in the late 1990s, following works by Wetterich and Reuter, that asymptocially safe gravity has caught on.

As of today, it has not been proved that gravity is asymptotically safe, though there are several arguments that support this idea. The problem is that doing calculations in an infinite dimensional theory space is not possible, so this space has to be reduced. But then the result can only deliver a limited level of knowledge. The other problem is that even if the theory is asymptotically safe, it might be physically nonsensical at high energies for other reasons.

Another criticism on asymptotically safe gravity has been that it does not seem to take into account that space-time fundamentally might be described by degrees of freedom different from those used in general relativity. While that arguably is so in existing approaches, the idea of renormalization group flow is in principle perfectly compatible with changing to different – more ‘fundamental’ – degrees of freedom at high energies, as Percacci and Vacca have pointed out.

That is to say, this approach towards quantum gravity has its problems, its friends and its foes, as has every other approach towards quantum gravity. But it is a strong competitor. What makes this approach so appealing is its minimalism: Maybe quantum gravity makes sense as a quantum field theory after all! Depending on your attitude though you might find exactly this minimalism unappealing. It’s like at the end of a crime novel the murder victim comes back from vacation and everybody feels stupid for their conspiracy theories.

Whatever your attitude, asymptotically safe gravity has made some contact to phenomenology, mostly in the area of cosmology, though for all I know these studies haven’t yet resulted in a good observable. Most interestingly, asymptotically safe gravity has been shown to also lead to dimensional reduction and it has recently been argued that it might be related to Causal Dynamical Triangulation. It seems to me that whatever quantum gravity ultimately looks like, asymptotically safe gravity will almost certainly be part of the story.

So the next time somebody tells you that we don’t know how to quantize gravity, keep in mind the many layers of details underneath that statement.

10 Misconceptions about Creativity

Lara, painting. She says
it's a snake and a trash can.

The American psyche is deeply traumatized by the finding that creativity scores of children and adults have been constantly declining since 1990. The consequence is a flood of advice on how to be more creative, books and seminars and websites. There’s no escaping the message: Get creative, now!

Science needs a creative element, and so every once in a while I read these pieces that come by my newsfeed. But they’re like one of these mildly pleasant songs that stop making sense when you listen to the lyrics. Clap your hands if you’re feeling like a room without a ceiling.

It’s not like I know a terrible lot about research on creativity. I’m sure there must be some research on it, right? But most of what I read isn’t even logically coherent.

1. Creativity means solving problems.
   
   The NYT recently wrote in an article titled “Creativity Becomes an Academic Discipline”:

   “Once considered the product of genius or divine inspiration, creativity — the ability to spot problems and devise smart solutions — is being recast as a prized and teachable skill.”

   Yes, creativity is an essential ingredient to solving problems, but equating creativity with problem solving is like saying curiosity is a device to kill cats. It’s one possible use, but it’s not the only use and there are other ways to kill cats.
   
   Creativity is in the first place about creation, the creation of something new and interesting. The human brain has two different thought processes to solve problems. One is to make use of learned knowledge and proceed systematically step by step. This is often referred to as ‘convergent thinking’ and dominantly makes use of the left side of the brain. The other process is a pattern-finding, a free association, often referred to as ‘divergent thinking’ which employs more brain regions on the right side. It normally kicks in only if the straight-forward left-brain attempt failed because it’s energetically more costly. Exactly what constitutes creative thinking is not well known, but most agree it is a combination of both of these thought processes.
   
   Creative thinking is a way to arrive at solutions to problems, yes. Or you might create a solution looking for a problem. Creativity is also an essential ingredient to art and knowledge discovery, which might or might not solve any problem.
   
   
2. Creativity means solving problems better.
   
   It takes my daughter about half an hour to get dressed. First she doesn’t know how to open the buttons, then she doesn’t know how to close them. She’ll try to wear her pants as a cap and pull her socks over the jeans just to then notice the boots won’t fit.
   
   It takes me 3 minutes to dress her – if she lets me – not because I’m not creative but because it’s not a problem which calls for a creative solution. Problems that can be solved with little effort by a known algorithm are in most cases best solved by convergent thinking.
   
   Xkcd nails it:
   
   
   But Newsweek bemoans:

   “Preschool children, on average, ask their parents about 100 questions a day. Why, why, why—sometimes parents just wish it’d stop. Tragically, it does stop. By middle school they’ve pretty much stopped asking.”

   There’s much to be said about schools not teaching children creative thinking – I agree it’s a real problem. But the main reason children stop asking question is that they learn. And somewhat down the line they learn how to find answers themselves. The more we learn, the more problems we can address with known procedures.
   
   There’s a priori nothing wrong with solving problems non-creatively. In most cases creative thinking just wastes time and brain-power. You don’t have to reinvent the wheel every day. It’s only when problems do not give in to standard solutions that a creative approach becomes useful.
   
   
3. Happiness makes you creative.
   
   For many people the problem with creative thought is the lack of divergent thinking. If you look at the advice you find online, they’re almost all guides to divergent thinking, not to creativity: “Don’t think. Let your thoughts unconsciously bubble away.” “Sourround yourself with inspiration” “Be open and aware. Play and pretend. List unusual uses for common household objects.” And so on. Happiness then plays a role for creativity because there is some evidence that happiness makes divergent thinking easier:

   “Recent studies have shown […] that everyday creativity is more closely linked with happiness than depression. In 2006, researchers at the University of Toronto found that sadness creates a kind of tunnel vision that closes people off from the world, but happiness makes people more open to information of all kinds.”

   Writes Bambi Turner who has a business degree and writes stuff. Note the vague term “closely linked” and look at the research.
   
   It is a study showing that people who listened to Bach’s (“happy”) Brandenburg Concerto No. 3 were better solving a word puzzle that required divergent thinking. In science speak the result reads “positive affect enhanced access to remote associates, suggesting an increase in the scope of semantic access.” Let us not even ask about the statistical significance of a study with 24 students of the University of Toronto in their lunch break, or its relevance for real life. The happy people participating this study were basically forced to think divergently. In real life happiness might instead divert you from hacking on a problem.
   
   In summary, the alleged “close link” should read: There is tentative evidence that happiness increases your chances of being creative in a laboratory setting, if you are among those who lack divergent thinking and are student at the University of Toronto.
   
   
4. Creativity makes you happy.
   
   There’s very little evidence that creativity for the sake of creativity improves happiness. Typically it’s arguments of plausibility like this that solving a problem might improve your life generally:

   “creativity allows [people] to come up with new ways to solve problems or simply achieve their goals.”

   That is plausible indeed, but it doesn’t take into account that being creative has downsides that counteract the benefits.
   
   This blog is testimony to my divergent thinking. You might find this interesting in your news feed, but ask my husband what fun it is to have a conversation with somebody who changes topic every 30 seconds because it’s all connected! I’m the nightmare of your organizing committee, of your faculty meeting, and of your carefully assembled administration workflow. Because I know just how to do everything better and have ten solutions to every problem, none of which anybody wants to hear. It also has the downside that I can only focus on reading when I’m tired because otherwise I’ll never get though a page. Good thing all my physics lectures were early in the morning.
   
   Thus, I am very skeptic of the plausibility argument that creativity makes you happy. If you look at the literature, there is in fact very little that has shown to lastingly increase people’s happiness at all. Two known procedures that have proved some effect in studies is showing gratitude and getting to know ones’ individual strengths.
   
   For more evidence that speaks against the idea that creativity increases happiness, see 7 and 8. There is some evidence that happiness and creativity are correlated, because both tend to be correlated with other character traits, like openness and cognitive flexibility. However, there is also evidence to the contrary, that creative people have a tendency to depression: “Although little evidence exists to link artistic creativity and happiness, the myth of the depressed artist has some scientific basis.” I’d call this inconclusive. Either way, correlations are only of so much use if you want to actively change something.
   
   
5. Creativity will solve all our problems.
   
   

   “All around us are matters of national and international importance that are crying out for creative solutions, from saving the Gulf of Mexico to bringing peace to Afghanistan to delivering health care. Such solutions emerge from a healthy marketplace of ideas, sustained by a populace constantly contributing original ideas and receptive to the ideas of others.”

   [From Newsweek again.] I don’t buy this at all. It’s not that we lack creative solutions, just look around, look at TED if you must. We’re basically drowning in creativity, my inbox certainly is. But they’re solutions to the wrong problems.
   
   (One of the reasons is that we simply do not know what a “healthy marketplace of ideas” is even supposed to mean, but that’s a different story and shell be told another time.)
   
   
6. You can learn to be creative if you follow these simple rules.
   
   You don’t have to learn creative thinking, it comes with your brain. You can however train it if you want to improve, and that’s what most of the books and seminars want to sell. It’s much like running. You don’t have to learn to run. Everybody who is reasonably healthy can run. How far and how fast you can run depends on your genes and on your training. There is some evidence that creativity has a genetic component and you can’t do much about this. However, you can work on the non-genetic part of it.
   
   
7. “To live creatively is a choice.”
   
   This is a quote from the WSJ essay “Think Inside the Box.” I don’t know if anybody ever looked into this in a scientific way, it seems a thorny question. But anecdotally it’s easier to increase creativity than to decrease it and thus it seems highly questionable that this is correct, especially if you take into account the evidence that it’s partially genetic. Many biographies of great writers and artists speak against this, let me just quote one:

   “We do not write because we want to; we write because we have to.”

   W. Somerset Maugham, English dramatist and novelist (1874 - 1965).
   
   
8. Creativity will make you more popular.
   
   People welcome novelty only in small doses and incremental steps. The wilder your divergent leaps of imagination, the more likely you are to just leave people behind you. Creativity might be a potential source for popularity in that at least you have something interesting to offer, but too much of it won’t do any good. You’ll end up being the misunderstood unappreciated genius whose obituary says “ahead of his times”.
   
   
9. Creativity will make you more successful.
   
   Last week, the Washington post published this opinion piece which informs the reader that:

   “Not for centuries has physics been so open to metaphysics, or more amenable to an ancient attitude: a sense of wonder about things above and within.”

   This comes from a person named Michael Gerson who recently opened Max Tegmark’s book and whose occupation seems to be, well, to write opinion pieces. I’ll refrain from commenting on the amenability of professions I know nothing about, so let me just say that he has clearly never written a grant proposal. I warmly recommend you put the word “metaphysics” into your next proposal to see what I mean. I think you should all do that because I clearly won’t, so then maybe I stand a chance then in the next round.
   
   Most funding agencies have used the 2008 financial crisis as an excuse to focus on conservative and applied research to the disadvantage of high risk and basic research. They really don’t want you to be creative – the “expected impact” is far too remote, the uncertainty too high. They want to hear you’ll use this hammer on that nail and when you’ve been hitting at it for 25 months and two weeks, out will pop 3 papers and two plenary talks. Open to metaphysics? Maybe Gerson should have a chat with Tegmark.
   
   There is indeed evidence showing that people are biased against creativity to the favor of practicality, even if they state they welcome creativity. This study relied on 140 American undergraduate students. (Physics envy, anybody?) The punchline is that creative solutions by their very nature have a higher risk of failure than those relying on known methods and this uncertainty is unappealing. It is particularly unappealing when you are coming up with solutions to problems that nobody wanted you to solve.
   
   So maybe being creative will make you successful. Or maybe your ideas will just make everybody roll their eyes.
   
   
10. The internet kills creativity.
    
    The internet has made life difficult for many artists, writers, and self-employed entrepreneurs, and I see a real risk that this degrades the value of creativity. However, it isn’t true that the mere availability of information kills creativity. It just moves it elsewhere. The internet has made many tasks that previously required creative approaches to step-by-step procedures. Need an idea for a birthday cake? Don’t know how to fold a fitted sheet? Want to know how to be more creative? Google will tell you. This frees your mind to get creative on tasks that Google will not do for your. In my eyes, that’s a good thing. 

So should you be more creative?

My summary of reading all these articles is that if you feel like your life lacks something, you should take score of your strengths and weaknesses and note what most contributes to your well-being. If you think that you are missing creative outlets, by all means, try some of these advice pages and get going. But do it for yourself and not for others, because creativity is not remotely as welcome as they want you to believe.

On that note, here’s the most recent of my awesomely popular musical experiments:

What is analogue gravity and what is it good for?

Image source: Redbubble. 

Gravity is an exceedingly weak force compared to the other known forces. It dominates at long distances just because, in contrast to the strong and electroweak force, it cannot be neutralized. When not neutralized however the other forces easily outplay gravity. The electrostatic repulsion between two electrons for example is about 40 orders of magnitude larger than their gravitational attraction: Just removing some electrons from the atoms making up your hair is sufficient for the repulsion to overcome the gravitational pull of the whole planet Earth.

That gravity is so weak also means its effects are difficult to measure, and its quantum effects are so difficult to measure that it was believed impossible for many decades. That belief is a troublesome one for scientists because a theory that cannot be tested is not science – in the best case it’s mathematics, in the worst case philosophy. Research on how to experimentally test quantum gravity, by indirect signals not involving the direct production of quanta of the gravitational field, is a recent development. It is interesting to see this area mature, accompanied by the conference series “Experimental Search for Quantum Gravity”.

Alongside the search for observable consequences of quantum gravity – often referred to as the ‘phenomenology’ of quantum gravity – the field of analogue gravity has recently seen a large increase in activity. Analogue gravity deals with the theory and experiment of condensed matter systems that resemble gravitational systems, yet can be realized in the laboratory. These systems are “analogues” for gravity.

If you take away one thing from this post it should be that, despite the name, analogue gravity does not actually mimic Einstein’s General Relativity. What it does mimic is a curved background space-time on which fields can propagate. The background however does not itself obey the equations of General Relativity; it obeys the equation of whatever fluid or material you’ve used. The background is instead set up to be similar to a known solution of Einstein’s field equations (at least that is presently the status).

If the fields propagating in this background are classical fields it’s an analogue to a completely classical gravitational system. If the fields are quantum fields, it’s an analogue to what is known as “semi-classical gravity”, in which gravity remains unquantized. Recall that the Hawking effect falls into the territory of semi-classical gravity and not quantum gravity, and you can see why such analogues are valuable. From the perspective of quantum gravity phenomenology, the latter case of quantized fields is arguably more interesting. It requires that the analogue system can have quantum states propagating on it. It is mostly phonons in Bose-Einstein condensates and in certain materials that have been used in the experiments so far.

The backgrounds that are most interesting are those modelling black hole inflation or the propagation of modes during inflation in the early universe. In both cases, the theory has left physicists with open questions, such as the relevance of very high (transplanckian) modes or the nature of quantum fluctuations in an expanding background. Analogue gravity models allow a different angle of attack to these problems. They are also a testing ground for how some proposed low-energy consequences of a fundamentally quantum space-time might come about and/or affect the quantum fields like deviations from Lorentz-invariance and space-time defects. It should be kept in mind though that global properties of space-time cannot strictly speaking ever be mimicked in the laboratory if space-time in these solutions is infinite. As we discussed recently for example, the event horizon of a black hole is a global property, it is defined as existing forever. This situation can only be approximately reproduced in the laboratory.

Another reason why analogue gravity, though it has been around for decades, is receiving much more attention now is that approaches to quantum gravity have diversified as string theory is slowly falling out of favor. Emergent and induced gravity models are often based on condensed-matter-like approaches  in which space-time is some kind of condensate. The big challenge is to reproduce the required symmetries and dynamics. Studying what is possible with existing materials and fluids in analogue gravity experiments certainly serves as both inspiration and motivation for emergent gravity.

While I am not a fan of emergent gravity approaches, I find the developments in analogue gravity interesting from an entirely different perspective. Consider that mathematics is not in fact a language able to describe all of nature. What would we do if we had reached its limits? We could take out maths as the middle-man and directly study systems that resemble more complicated or less accessible systems. That’s exactly what analogue gravity is all about.

I am sure that this research area will continue to flourish.

(If you really want to know all the details and references, this Living Review is a good starting point.)

8 Years Backreaction!

Thanks to all my readers, the new ones and the regulars, the occasionals and the lurkers, and most of all our commenters: Without you this blog wouldn't be what it is. I have learned a lot from you, laughed about your witty remarks, and I appreciate your feedback. Thanks for being around and enriching my life by sharing your thoughts.

As you have noticed, I am no longer using the blog to share links. To that end you can follow me on twitter or facebook. I'm also on G+, but don't use it very often.

If you have a research result to share that you think may be interesting to readers of this blog, you can send me a note, email is hossi at nordita dot org. I don't always have time to reply, but I do read and consider all submissions.

The eternal tug of war between science journalists and scientists. A graphical story.

I am always disappointed by the media coverage on my research area. It forever seems to misrepresent this and forgets to mention that and raises a wrong impression about something. Ask the science journalist and they'll tell you they have to make concessions in accuracy to match the knowledge level of the average reader. The scientist will argue that if the accuracy is too low there's no knowledge to be transferred at all, and that a little knowledge is worse than no knowledge at all. Then the journalist will talk about the need to sell and point to capitalism executed by their editor. In the end everybody is unhappy: The scientists because they're being misrepresented, the journalist because they feel misunderstood, and the editor because they are being blamed for everything.

We can summarize the problem in this graph:

The black curve is the readership as a function of accuracy. Total knowledge transfer is roughly the amount of readers times the information conveyed. An article with very little information might have a large target group, but not much educational value. An article with very much information will be read by few people. The sweet spot, the maximum of the total knowledge transfer as a function of accuracy, lies somewhere in the middle. Problem is that scientists and journalists tend to disagree about where the sweet spot lies.

Scientists are on the average more pessimistic about the total amount of information that can be conveyed to begin with because they do not only believe but know that you cannot really understand their research without getting into the details, yet the details require background knowledge to appreciate. I sometimes hear that scientists wish for more accuracy because they are afraid of the criticism of their colleagues, but I think this is nonsense. Their colleagues will assume that the journalist is responsible for lack of accuracy, not the scientist. No, I think they want more accuracy because they correctly know it is important and because if one is familiar with a topic one tends to lose perspective on how difficult it once was to understand. They want, in short, an article they themselves would find interesting to read.

So it seems this tug of war is unavoidable, but let us have a look at the underlying assumptions.

To begin with I've assumed that science writers and scientists likewise want to maximize information transfer and not simply readership, which would push the sweet spot towards the end of no information at all. That's a rosy world-view disregarding the power of clicks, but in my impression it's what most science journalists actually wish for.

One big assumption is that most readers have very little knowledge about the topic, which is why the readership curve peaks towards the low accuracy end. This is not the case for other topics. Think for example of the sports section. It usually just assumes that the readers know the basic rules and moves of the games and journalists do not hesitate to comment extensively on these moves. For somebody like me, whose complete knowledge about basketball is that a ball has to go into a basket, the sports pages aren't only uninteresting but impenetrable vocabulary. However, most people seem to bring more knowledge than that and thus the journalists don't hesitate assuming it.

If we break down the readership by knowledge level, for scientific topics it will look somewhat like shown in the figure below. The higher the knowledge, the more details the reader can digest, but the fewer readers there are.

Another assumption is that this background level is basically fixed and readers can't learn. This is my great frustration with science journalism, that the readership is rarely if ever exposed to the real science and thus the background knowledge never increases. Readers don't ever hear the technical terms, don't see the equations, and aren't explained the figures. I think that popular science reporting just shouldn't aim at meeting people in their comfort zone, at the sweet spot, because the long-term impact is nil. But that again hits the wall of must-sell.

The assumption that I want to focus on here is that the accuracy of an article is a variable independent of the reader themself. This is mostly true for print media because the content is essentially static and not customizable. However, for online content it is possible to offer different levels of detail according to the reader's background. If I read popular science articles in fields I do not work in myself, I find it very annoying if they are so dumbed down that I can't make a match to the scientific literature, because technical terms and references are missing.  It's not that I do not appreciate the explanation at a low technical level, because without it I wouldn't have been interested to begin with. But if I am interested in a topic, I'd like to have a guide to find out more.

So then let us look at the readership as a function of knowledge and accuracy. This makes a three-dimensional graph roughly like the one below.

If you have a fixed accuracy, the readership you get is the integral over the knowledge-axis in the direction of the white arrow. This gives you back the black curve in the first graph. However, if accuracy is adjustable to meet the knowledge level, readers can pick their sweet spot themselves, which is along the dotted line in the graph. If this match is made, then the readership is no longer dependent on the accuracy, but just depends on the number of people at any different knowledge background. The total readership you get is the sum of all those.

How much larger this total readership is than the readership in the sweet spot of fixed accuracy depends on many variables. To begin with it depends on the readers' flexibility of accepting accuracy that is either too low or too high for them. It also depends on how much they like the customization and how well that works etc. But I'm a theoretician, so let me not try to be too realistic. Instead, I want to ask how that might be possible to do.

A continuous level of accuracy will most likely remain impossible, but a system with a few layers - call them beginner, advanced, pro - would already make a big difference. One simple way towards this would be to allow the frustrated scientist whose details got scraped to add explanations and references in a way that readers can access them when they wish. This would also have the benefit of not putting more load on the journalist.

So I am cautiously hopeful: Maybe technology will one day end the eternal tug of war between scientist and science writers.

A drop makes waves – just like quantum mechanics?

My prof was fond of saying there are no elementary particles, we should really call them “elementary things” - “Elementardinger”. After all the whole point of quantum theory is that there’s no point - there are no classical particles with a position and a momentum, there is only the wave-function. And there is no particle-wave duality either. This unfortunate phrase suggests that the elementary thing is both a particle and a wave, but it is neither: The elementary thing is something else in its own right.

That quantum mechanics is built on mathematical structures which do not correspond to classical objects we can observe in daily life has bugged people ever since quantum mechanics came, saw, and won over the physics departments. Attempts to reformulate quantum mechanics in terms of classical fields or particles go back to the 1920s, to Madelung and de Broglie, and were later continued by Bohm. This alternative approach to quantum mechanics has never been very popular, primarily because it was unnecessary. Quantum mechanics and quantum field theory as taught in the textbooks proved to work enormously well and there was much to be done. But despite its unpopularity, this line of research never went extinct and carried on until today.

Today we are reaching the limits of what can be done with the theories we have and we are left with unanswered questions. “Shut up and calculate” turned into “Shut up and let me think”. Tired of doing loop expansions, still not knowing how to quantize gravity, the naturalness-issue becoming more pressing by the day, most physicists are convinced we are missing something. Needless to say, no two of them will agree on what that something is. One possible something that has received an increasing amount of attention during the last decade is that we got the foundations of quantum mechanics wrong. And with that the idea that quantum mechanics may be explainable by classical particles and waves is back en vogue.

Enter Yves Couder.

Couder spends his days dropping silicone oil. Due to surface tension and chemical potentials the silicone droplets, if small enough, will not sink into the oil bath of the same substance, but hover above its surface, separated by an air film. Now he starts oscillating the oil up and down and the drops start to bounce. This simple experiment creates a surprisingly complex coupled system of the driven oscillator that is the oil and the bouncing droplets. The droplets create waves every time they hit the surface and the next bounce of the droplets depends on the waves they hit. The waves of the oil are both a result of the bounces as well as a cause of the bounces. The drops and the waves, they belong together.

Does it smell quantum mechanical yet?

The behavior is interesting even if one looks at only one particle. If the particle is given an initial velocity, it will maintain this velocity and drag the wave field with it. The drop will anticipate and make turns at walls or other obstacles because the waves in the oil had previously been reflected. The behavior of the drop is very suggestive of quantum mechanical effects. Faced with a double-slit, the drop will sometimes take one slit, sometimes the other. A classical wave by itself would go through both slits and interfere with itself. A classical particle would go through one of the slits. The bouncing droplet does neither. It is a clever system that converts the horizontal driving force of the oil into vertical motion by the drops bouncing off the rippled surface. It is something else in its own right.

You can watch some of the unintuitive behavior of the coupled drop-oil system in the blow video. The double-slit experiment is at 2:41 mins


Other surprising findings in these experiments have been that the drops exhibit an attractive force on each other, that they can have quantized orbits, they mimic tunneling and Anderson localization. In short, the droplets show behavior that was previously believed to be exclusively quantum mechanical phenomena.

But just exactly why that would be so, nobody really knew. There were many experiments, but no good theory. Until now. In a recent paper, Robert Brady and Ross Anderson from the University of Cambridge delivered the theory:

Why bouncing droplets are a pretty good model of quantum mechanics
Robert Brady, Ross Anderson
arXiv:1401.4356 [quant-ph]

While the full behavior of the drop-oil system is so far not analytically computable, they were able to derive some general relations that shed much light on the physics of the bouncing droplets. This became possible by noting that in the range the experiments are conducted the speed of the oil waves is to good approximation independent of the frequency of the waves, and the equation governing the waves is linear. This means it obeys an approximate Lorentz-symmetry which enabled them to derive relations between the bounce-period and the velocity of the droplet that fit very well with the observations. They also offer an explanation for the attractive force between the droplets due to the periodic displacement of the cause of the waves and the source of the waves and tackle the question how the droplets are bounced off barriers.

These are not technically very difficult calculations, their value lies in making the theoretical connection between the observation and the theory which now opens the possibility of using this theory to explain quantum phenomena as emergent from an underlying classical reality. I can imagine this line of research to become very fruitful also for the area of emergent gravity. And if you turn it around, understanding these coupled systems might give us a tool to scale up at least some quantum behavior to macroscopic systems.

While I think this is interesting fluid dynamics and pretty videos, I remain skeptic of the idea that this classical system can reproduce all achievements of quantum mechanics. To begin with it gives me to think that the Lorentz-symmetry is only approximate, and I don’t see what this approach might have to say about entanglement, which for me is the hallmark of quantum theory.

Ross Anderson, one of the authors of the above paper, is more optimistic: “I think it's potentially one of the most high-impact things I've ever done,” he says, “If we're right, and reality is fluid-mechanical at the deepest level, this changes everything. It consigns string theory and multiple universes to the dustbin.”

Can Planck Stars exist?

Carlo Rovelli and Francesca Vidotto recently proposed a bold solution to the black hole information loss paradox and the firewall problem that they dubbed Planck stars:

Planck stars
Carlo Rovelli, Francesca Vidotto
arXiv:1401.6562 [gr-qc]

In a nutshell they are suggesting that the horizon of the black hole vanishes, due to quantum gravitational effects, at a radius much larger than the Planck length. They call the remaining object a Planck star.

To understand why this is a really radical proposal, let me first give you some context. When matter collapses to a black hole, its radius shrinks and its density increases. Quantum gravitational effects are expected to become strong when the curvature reaches the Planckian regime. The curvature is the inverse of a length square, so that means the curvature is the inverse of the Planck length square or smaller. At which radius the collapsing matter reaches this regime depends on the total mass: The higher the mass, the larger the radius.

The radius at which the collapsing matter reaches the Planckian regime is larger than the Planck length if the mass is larger than the Planck mass. The radius is however always smaller than the horizon radius, so it doesn’t really matter exactly what happens because it’s not in causal contact with the exterior. The curvature at the horizon is weak as long as the total mass of the black hole is larger than the Planck mass. This is somewhat unintuitive, but the curvature at the black hole horizon goes with the inverse of the mass square, ie the higher the mass of the black hole, the smaller the curvature. Thus the often made remark that you wouldn’t notice crossing the black hole horizon - there’s nothing there and space-time can be almost flat if the black hole is large. In particular, you don’t expect any quantum gravitational effects at the horizon.

But the mass of the black hole decreases due to Hawking radiation. Keep in mind that Hawking radiation is not a quantum gravitational effect. It’s quantum fields in a classical gravitational background, a combination often referred to as ‘semi-classical’. If the mass of the black hole has shrunken to the Planck mass, the curvature reaches the Planckian regime and that’s when the semi-classical limit breaks down and quantum gravity becomes important. At that point also Hawking’s calculation breaks down and information can be released. However, the standard argument goes that by this time it’s already too late to get all the information out. Details are subtle but that’s a different story. Suffices to say that Rovelli and Vidotto want information release to be possible earlier, when the radius of the black hole is still much larger than the Planck length and its mass much above the Planck mass.

The only way to do this is to have strong quantum gravitational effects in a region where the curvature of the semi-classical metric is small, much below the Planck scale. In the paper they don’t explicitly say that this is what they do, but of course they have to. You see this most easily when you look at the metric they suggest, equation (14). The third term (containing α) is the correction term that supposedly has a quantum gravitational origin. The validity of the semi-classical limit means essentially that the third term should be smaller than the second as long as the second term is smaller than one. If you convert this into inequalities you find α < m, and that is explicitly the situation they do not consider. Instead α is supposed to start at m and then increase. They do not give any reason given as to why this should be so or what the meaning is of α or what the necessary source terms are for that.

At this point you are probably ready to throw the paper away. There is a reason one of the postulates of black hole complementarity is the validity of the semi-classical approximation near the horizon of a black hole with mass above the Planck mass. That’s because the curvature there is small and no quantum gravitational effects are at your disposal to screw up the semi-classical limit. However, allow me to exercise some good will. I think what Rovelli and Vidotto suggest may be possible if the Planckian-density core behind the horizon displays a very unusual behavior, though that’s a big “if”.

The behavior would have to be such that as the total mass is shrinking, the mass is taken from the center only, leaving behind an increasingly thinner shell of high density at a constant radius (or even an increasing one). This shell would eventually intersect the horizon of the black hole and could do so conceivably at a radius much above the Planck radius. This isn’t a priori in conflict with the semi-classical limit because there is a high density now and also a high curvature region.

However, the metric that is used by Rovelli and Vidotto does not describe such a scenario. (The metric inside a shell has to be flat while their metric is actually singular at the center.) Besides this, there exists no approach to quantum gravity that suggests such a hollow-core behavior. There doesn’t even exist a model that describes such a situation. I also strongly suspect that such a solution, even if it can be created by help of some quantum gravitational pressure (this is almost certainly possible), would be unstable under non-spherical perturbations and just recollapse to form a smaller Planckian-density core. Iterate and end at Planck scale radius as usual.

In summary, this is an ad-hoc proposal. It is not based on anything we know of quantum gravity. Neither is it a complete model. I am reasonably sure that the metric they use cannot describe the situation they want while still maintaining energy-conservation. They do not calculate the curvature that belongs to that metric to check whether their modification is consistent. Neither do they calculate the necessary source that presumably contains a quantum-gravitationally induced stress-energy. It is an interesting suggestion, but I do not think it is very plausible. Planck stars almost certainly do not exist.

Acknowledgements: Carlo Rovelli has been very patient explaining his idea by email, but as you can tell I’ve remained unconvinced...

Got a problem? Good for you...

Natalie Portman

Physicists love good problems, they take them out for dinner and sleep with them. Unsolved problems are
their reason d’etre. And yet it pains me considerably if somebody dismisses a paper or research project with the remark:

“Well, what problem does that solve?”

Indeed, this came up in the discussion of the workshop I attended last week, the criticism that much of current research doesn’t seem to solve any existing problem.

I agree on the underlying sentiment. Yes, most of what gets published in physics these days will almost certainly turn out to be useless to the end of describing nature. But it’s always been this way and will always be this way. It’s in the nature of trial and error that you must try and err.

I disagree though that research is only worthy if it solves, or at least attempts to solve, a known problem.

To begin with good problems don’t grow on trees. Yes, it is often the case that the solution of one problem grows up to be the next problem, ready to pick. But that isn’t always so. Sometimes you have to go and hunt them down. And many problems are found just because researchers – both theorists and experimentalists – followed their curiosity and stumbled upon something interesting. The generation of problems is so important to progress that physicists sometimes are tempted to create problems where there are none, just so they have a target for their methods. Think of superluminal neutrinos, the pioneer anomaly, or the penta-quark.

So research is clearly also important if it draws attention to a problem rather than solving one. A recent example is the black hole firewall. And really, what problem did that solve?

The biggest part of research is dedicated to finding or solving problems, but that still isn’t all of it. Some of research is failed solution attempts. Failing and sharing failure is valuable not only because it can save other people’s time, but also because a failed solution to one problem can turn out to solve another problem. The post-it glue’s failure to stick was also its success. Einstein’s “blunder” eventually turned out to have its use when we discovered the universe’s expansion accelerates. Bubble wrap was conceived as washable wallpaper. Research in string theory was originally pursued to understand the strong nuclear force.

Somebody else’s failure of yesterday might be your solution tomorrow.

And then there is just free-wheeling curiosity that is often a by-product of researchers trying to better understand their gadgets or models. It might or might not turn out to be useful for anything. These are failed attempts to find a problem, or solutions without a problem.

I too used to be cynical about the irrelevance of most papers and their failure to address existing problems. Now I think of them as exercises, as documentations of physicists learning or improving their methods. In fact, often these papers are exactly this: projects give to students or postdocs. Others are reports on somebody’s current interests and thoughts, or their progress in understanding particular relations that will or will not lead anywhere. They might have been out hunting and now want to show off what they found, even if it wasn’t what they were hoping for. Or their idea of a good problem might just not agree with mine.

In the long run, science is much better off with a diversity of interests than with the streamlined attack favored by the dismissive comment “Well, what problem does that solve?”

Interna

I’ll be traveling for the rest of the week, so be warned of a period of silence.

Wednesday I’m giving a seminar in Nottingham, and after that I’m attending a workshop in Oxford. The workshop topic is “The Structure of Gravity and Space-time” and it’s part of the project “Establishing the Philosophy of Cosmology”. Sound more ominous than it is: They’ll have a session on the question whether there exists a “fundamental length”, which is what brought me on their invitation list. There will also be sessions on bi-metric gravity, massive gravity and strings and space-time structure, which sounds very promising to me. We’ll see how much philosophy infiltrates the physics. A preliminary program is here.

The girls are doing well, now attending Kindergarten. Our pediatrician didn’t raise any concerns at the 3-year checkup, except for Lara’s vision problems. She’ll get new glasses next week. The ones she has now always slip down and hang on the very tip of her nose, so we hope that the new ones will stay put better.

Lara and Gloria can open and remove all our children safety locks now and I’ve put away the door keys because I’m afraid they’ll lock themselves in. They also picked up lots of swear words since they attend Kindergarten. They don’t really know how to use them properly, which is often unwillingly funny. We’ve made a little progress with the potty training, but unfortunately the kids declare plainly they’re “too lazy” to go without diaper. It is similarly unfortunate that several older children at the Kindergarten still use binkies. Gloria told me the other day she will learn to use the toilet when she can “reach the ceiling”. She also declared that since Gloria came out of mommy’s belly, Lara must have come out of daddy’s belly. Everything far away is “Stockholm” and that’s a magical place where mommy goes and brings back gifts. They’re getting more entertaining by the day.

I finally replaced my old digital camera because some of the buttons were broken, and now have a Canon DSLR (EOS 1100D) which I am so far very happy with, though the learning curve is steep. I used to have a SLR Camera 15 years ago. You know, one of these things were you had to wind back the film and carry it to some store and wait a week just to see how badly you did. Remember that? The DSLR looks and feels quite different from that, as with all the menus that I keep getting lost in. Maybe reading the manual would help. In any case, I spent some weeks hunting after the kids. Below are some of my favorite photos.

If it quacks like a black hole

Unless you’ve been sitting in a white hole, you probably read somewhere that Stephen Hawking now claims black holes don’t exist. I was about to close my eyes and let this wave of media nonsense pass over me, but even my mother asks what that’s supposed to mean. So here is the brief explanation.

It is often said that a black hole is defined by the presence of an event horizon. The event horizon is the boundary of a region from which no information can escape, ever. The relevant word to pay attention to here is “ever”. The event horizon is a mathematically well-defined property of space-time, but it’s a mathematical construct entirely. You would have to wait literally till the end of time to find out whether an event horizon really is an event horizon in the sense of this definition.

Instead of the event horizon physicists thus often talk about the apparent horizon. The apparent horizon is, roughly, something that looks like an event horizon for a finite amount of time. Since all we can ever measure of anything can be done only in finite times it’s the apparent horizon that we ask for, look for, and observe.

For all practical purposes – and with that I mean actual observations of astrophysical black holes – the distinction between apparent horizons and event horizons is entirely irrelevant. Which is why you and probably many science journalists have never heard of this.

That actual event horizons might not be formed when matter collapses, but only apparent event horizons that eventually vanish, is not a new idea. It’s been discussed in the literature since 20 years or so. In my paper with Lee, we discussed the option of there being no event horizon but only an apparent horizon on very general grounds. See Fig 3, read caption, for further literature check references.

So what then did Hawking mean? The actual quote is:

“The absence of event horizons mean that there are no black holes – in the sense of regimes from which light can’t escape to infinity.”

If you define a black hole as a space-time with an event horizon then that is a correct statement. But then you will still have objects, let me call them “apparent black holes”, that look almost exactly like black holes for times that exceed the lifetime of the universe by several orders of magnitude. You will not, by any observation that is presently possible, be able to tell whether eg the center of the Milky Way harbors a black hole with event horizon or an apparent black hole that looks like a black hole with event horizon.

What Hawking is saying is essentially that he believes that a matter collapse only leads to a temporary apparent horizon but not to an eternal event horizon. That is an opinion which is shared by many of his colleagues (including me) and there is nothing new about this idea whatsoever.

It is very unfortunate that this statement by Hawking has been misinterpreted in this way because there are in fact people who claim that black holes don’t exist. They argue that what we observe are actually just very dark massive objects that never collapse beyond their Schwarzschild radius, but they do have a material surface. This is a fringe opinion to say the least, because it requires substantial changes to Einstein’s theory of gravity, not to mention that it’s in conflict with observation. I am very sure this is not what Hawking was referring to.

Having said that, Hawking’s “paper” is really just a writeup of a talk he gave last year. It’s mostly a summary of his thoughts on the black hole firewall, none of which I found very exciting or remarkable. Had this paper been posted by anybody else, nobody would have paid attention to it.

In summary, nothing has changed in our understanding of black holes due to Hawking’s paper. Move on, there’s nothing to see here.

A moment of silence replaces the big bang

Shhh.

“Big Bang” has become a household expression, but for physicists it’s primarily a Big Headache. Exactly what happened in the first moments of our universe is still not understood. In this early phase, when matter densities were extremely high, quantum fluctuations of space and time were large. We know that much, but we still do not know to describe these fluctuations which would require a quantum theory of gravity. Without that, we cannot reliably tell what banged, if anything.

It is generally expected that quantum gravity will remove the Big Bang singularity that general relativity predicts was the origin of our universe, but we don’t know with what it will be replaced. However, while we do not yet have a full theory of quantum gravity, different models for the early universe have been investigated. These are models based on, but not strictly speaking derived from, theoretical approaches to quantum gravity. The best known of these models are string cosmology and loop quantum cosmology, based on string theory and loop quantum gravity respectively.

Loop quantum cosmology in particular is well known for replacing the Big Bang with a “Big Bounce”: When the density of matter reaches a certain critical density (related to the Planck scale) contraction turns back into expansion. For a recent status update on string cosmology, see here.

A completely different approach to quantum gravity that we discussed recently is Causal Dynamical Triangulation which avoids singularities by discretizing space and time into chunks of finite size. In this approach it was recently found that space-time can exist in different phases, much like water exists in different phases. In the early universe, temperatures were high, and space-time might have been in a different phase, one in which space-time falls apart into causally disconnected pieces.

Phase diagram of space-time in CDT. See earlier post for details

It is thus very interesting that a similar behavior was recently found in loop quantum cosmology, an approach which a priori doesn’t have anything to do with Causal Dynamical Triangulation.

Jakub Mielczarek argues that the modification that arises through a loop-quantization of space-time can be rewritten in a suggestively simply way, as a density-dependence of the speed of light. A brief summary are these conference proceedings:

J. Mielczarek,
Asymptotic silence in loop quantum cosmology
AIP Conf. Proc. 1514 (2012) 81, [arXiv:1212.3527].

The full length paper is here. It’s very technical, but the main conclusion is this: The higher the density, the slower the speed of light. At half the critical density, the speed of light reaches zero – this means points become causally disconnected. But things become even more interesting when the density becomes larger than half the critical density and increases towards the critical density. In this range the speed of light becomes an imaginary number and its square becomes negative. This means that time stops existing and turns into space. Physicists say space-time becomes Euclidean.

This finding realizes the so-called “no-boundary” proposal by Hartle and Hawking, and it also matches well with even earlier, quite general, considerations of what should happen nearby a singularity. In the classical theory, the causal disconnect happens only asymptotically and was dubbed ‘asymptotic silence’. In the quantized case, the causal disconnect happens at a finite time and replaces the singularity, and thus the big bang, by a singularity free region, a “moment of silence.”

I find this an intriguing development because here we have several different routes that point towards the same behavior at high density, much like is the case with dimensional reduction. I will not be surprised if further theoretical support for the moment of silence appears in the soon future. The big question is of course if traces of this silent beginning of the universe are left in observables like structure formation or the cosmic microwave background.

My invisible friend

Here's what I did on the weekend.



As you can see the video quality is still pretty crappy for reasons I can't figure out. Obviously I'm doing something wrong with the mixdown or the compression. To begin with, I probably shouldn't have combined the webcam recording with the rest. I was considerably more successful with the audio quality. Unfortunately, the microphone wouldn't speak to the camera, so I had to record the video and the audio separately and try to match them later, which is why the audio in some places seem out of synch. Sorry about that. I have a totally awesome microphone though and I'm very pleased with the audio. If you're not very careful with the microphone settings you can basically hear the neighbors fart in the recording.

If I find the time I'll do some more recordings and put a beat under this. I only noticed very belatedly that I accidentally mixed up G major with D major. So the piece is more, eh, interesting as it was supposed to be and I've violated the sacred law of amateurs #1: Learn the rules before you break them. Yes, it is clearly therapeutic to every once in a while engage in a project one doesn't know a thing about.

The science of the multiverse.

CMB aniotropies.
Image: NASA/WMAP

A new numerical study of bubble collisions makes a big step towards testing the multiverse hypothesis.

The multiverse might not be the most original or even surprising idea that physicists have proposed recently, but it certainly excelled in capturing the public imagination and in stirring up discussion. On the very top of the discussion list is the question whether the multiverse is a scientific idea at all, or whether it is simply a philosophical retreat for lazy physicists who’ve gotten tired hunting answers that didn’t come knocking on their door.

But there’s no simple answer to that question. Whether the multiverse is a scientific idea depends on what one means with multiverse. As I explained previously, the existence of “multiverses” in many theories is just a consequence of pushing models beyond their phenomenological reach. The only thing these types of multiverses demonstrate is that physicists don’t understand the limits of pure mathematics. In contrast, the best motivated multiverse is bubble universes in eternal inflation. And these can have observational consequences.

Eternal inflation is a variant of inflation, a phase of exponential growth in the early universe. Exactly how this phase proceeds depends on the properties of quantum fields that filled the universe back then. In eternal inflation, it is only parts of the whole space in which the rapid expansion comes to an end and structures like our galaxies form; these parts are referred to as “bubble universes”. In the rest of space, inflation continues and goes on to create new bubbles. This bubble production lasts eternally.

Most of these bubble universes are causally disconnected from ours and we have no chance to ever observe them. However, it is at least theoretically possible that some of these bubbles collide as they expand. This would mean that what is now our observable universe had, when we look back in time, not one seed, but two or more that later came to join. This type of bubble collision can have observable consequences for the formation of structures and leave imprints in the pattern of temperature fluctuations in the cosmic microwave background (CMB).

The accurate mathematical description of these bubble collisions is however difficult because Einstein’s field equations are non-linear. Solutions can normally only analytically be found in cases with many symmetries, and in the case of bubble collisions the geometry prevents one from using such a highly symmetric ansatz. Approximately valid analytical solutions have previously been put forward, but when one wants to make quantitative prediction one needs a sufficiently precise numerical simulation. Such a numerical simulation has to evolve forward in time the metric components, whose fluctuations eventually go to seed the perturbations in the CMB, which we then later measure.

Such a numerical simulation has recently been published in this paper

Simulating the universe(s): from cosmic bubble collisions to cosmological observables with numerical relativity
Carroll L. Wainwright, Matthew C. Johnson, Hiranya V. Peiris, Anthony Aguirre, Luis Lehner, Steven L. Liebling
arXiv:1312.1357 [hep-th]

The authors only study the simplest case, that is the collision of only two bubbles and in addition these bubbles are identical (they have the same vacuum state). This is clearly not the most general case, but even so their simulation allows them to calculate the effects on the CMB better than previously possible.

Roughly speaking, the bubble collision leaves a localized temperature fluctuation - a hot spot or a cold spot - in the CMB that fades off away from the center of the collision. Exactly how it fades is very important for the extraction of such a signal from the data, and yet this could only be estimated before this numerical simulation was completed. Notably, the authors found that the fall-off is faster than was expected from the analytic estimates.

The figure below shows the time-evolution for the scalar field (Φ) that drives the inflation and two functions (a and α) that quantify the behavior of the metric. The horizontal axis is one of the spatial coordinates the vertical axes a measure for the time.

Figure 7 from arXiv:1312.1357 [hep-th]

What you can see in the image is how the configuration starts out being initially split in two halves and then evolves towards a situation where the initial split slowly fades away. That the bubbles both originate at “the same” time can always be achieved by a suitable choice of time-coordinate. The time, or the initial distances between the bubbles, can later be changed to a free parameter by a coordinate transformation.

This numerical study demonstrates nicely that it is possible to connect the underlying model for eternal inflation to observable signatures. It is also valuable in suggesting a useful parameterization for the effects. I find this a very interesting paper which will provide a basis for further studies that are necessary to analyze cosmological data for signals that might show we live in a multiverse.

Trouble in the Ivory Tower: Not an academic problem.

The Ivory Tower.
Image from The Neverending Story.

“Science is the only news,” Stuart Brand told us. And the news is that research misconduct is on the rise while reproducible results are in decline. Peer review, the process in which scientific publications are evaluated by anonymous peers, has become a farce as scientists’ existential worries make it an exercise in forward defense with the occasional backhand offense. Scientists produce more papers now than ever, and then hide them behind journal subscriptions so costly nobody can read them – a good idea because most published research findings are probably false, though that too is probably false. Measures for scientific success have been criticized ever since they began being used, and the academic system chokes on social effects like herding, pluralistic ignorance and groupthink.

Yes, science works, no need to call me names. But science doesn’t work as good as it could, not as good as it should, not as good as we need it to work.

Scientific institutions and scientific management are stuck in the last century. The academic system today is in no shape to cope with the demands of high connectivity in a global and increasing workforce, is unable to deal with complex trans-national and interdisciplinary problems, and can’t handle the amplification of social feedback that information technology has brought.

The academic system, in brief, has the same problem as our political, social and economic systems.

The biggest challenge mankind faces today is not the development of some breakthrough technology. The biggest challenge is to create a society whose institutions integrate the knowledge that must precede any such technology, including knowledge about these institutions themselves. All of our big problems today speak of our failure, not to envision solutions, but to turn our ideas and knowledge into reality.

It’s not that we lack creativity. It’s that the kind of creativity that comes to us naturally does not latch upon problems evolution didn’t endow us to register to begin with. We do not comprehend the interplay of large crowds of people and are unable to individually beat our own psychology, rooted in groups of tens to hundreds, not billions. To arrange our living together in groups larger than we can intuit, we agree on rules of conduct and incentives that align our individual actions with collective trends so that both are to our benefit. This requires systems design. It requires science. And before that it requires we acknowledge the problem.

But we watch. We watch with bewilderment as a video of sunrise is broadcast on Tiananmen square where thick smog forces onlookers to wear breathing masks. We watch with horrified fascination video footages of the big garbage swirl and of birds dying from indigestible plastic pieces. We watch, hypnotized, replays of negotiation failures that make our adaptation to climate change more costly by the day. The way we have arranged, organized, policed and institutionalized our living together leaves us to watch ourselves watching, stunned at our own inability to change anything about it.

And scientists, the ones who should be able to analyze the situation and to devise a solution aren’t any better.

Scientists, of course, know exactly what is wrong with academia. Leaving aside that no two of them can agree on how to do it, they know how to solve the problem. There’s no shortage of proposals for how to fix peer review and scientific publishing and for how to better distribute resources. Futures markets, auction markets, lottery systems, open peer review, and dozens of alternative metrics have been suggested, we’ve seen it all. They write papers about it and send them for peer review. The rest is the same old he-said-she-said.

So far, scientists miserably failed to adapt the academic system to the changing demands of the 21st century. They belabor the problem and devise solutions, but are unable to implement them. And in the ocean of conference proceedings they watch the giant abstract swirl.

Academia mirrors the problem of our societies in a nutshell. The members of the academe, they’re all talk but no walk. We are being told that scientists are studying now the interconnectivity of the multi-layered networks that govern our societies, and we ask for answers and advice, we ask to be informed about how to solve our problems. There’s nobody else to solve these problems.

Social systems adapt to changing demands much like organisms do, by gradual modification and selection. But this process takes time – a lot of time – and it’s time we cannot afford. The only way to accelerate this adaption is the scientific method: a targeted, controlled, and recorded series of modifications. Many existing projects today aim to track and analyze the complex interactions of our highly interwoven networked world. But not a single one of these projects addresses the real problem, which is how to use this knowledge in the very systems that are being studied. It is this feedback of knowledge about the system back into the system that is necessary for our institutions to adapt. It requires a self-consistent scientific approach to institutional design, an approach that doesn’t exist and is nowhere near existence.

We need scientists to help us create social systems that organize our living together in groups so large that our evolutionary brains, trained to deal with small groups, cannot cope with. Trial and error will take too long and the errors are too costly now. But scientists are like the overweight doctor preaching the benefits of blood-pressure regulation, evidently unable to solve their own problems first. They presently can’t help us solve any problems, and we shouldn’t listen to their advice until they’ve solved their own problems.

Science is the only news, but it’s not only news. It’s the canary in the coal mine. Better watch it closely.

Loops vs Strings. Almost discovered for much longer.

I didn't attend this year's FQXi conference, but most of the talks will be uploaded sooner or later to the FQXi YouTube channel. The FQXi conferences always feature a mock debate with a role switch. I've never seen one with an actual exchange of arguments, but the debates have some amusement value. This year we have Carlo Rovelli defending String Theory ("You Loop people... insist that you cannot get any prediction from your theory. But we can't either.") and Raffael Bousso defending Loop Quantum Gravity ("What happened next was the string landscape. You've all heard of the string landscape. It's also known as 'The End of Science.'"). Enjoy.

Shooting strings

Shooting strings.
Source: Meez Forums.

String theory, once hailed as theory of everything, now struggles to demonstrate its use for anything at all.

Most string theorists today, if not working for banks, study the gauge-gravity correspondence. This celebrated idea, arguably one of the most interesting findings in string theory, relates a strongly coupled field theory in flat space to weakly coupled gravity in a higher dimensional space. These higher-dimensional spaces do not resemble our universe, so the interesting applications of the gauge-gravity correspondence are analytical calculations in strongly coupled field theory. Notoriously difficult problems of the field theory can become manageable by reformulating them in the language of gravity.

The most widely promoted use of this gauge-gravity correspondence has been the quark gluon plasma, which is produced in highly energetic collisions of heavy ions, previously at RHIC and now at the LHC. There has been a lot of hype about the low viscosity that was analytically found using the gauge-gravity correspondence and that fit well with observations. But heavy ion physics isn’t just viscosity. There are many other observables that a good model must be able to explain.

One of these observables is the energy loss that elementary particles experience when they travel through the plasma. Just by chance it can happen that a particle pair is created but only one of the two particles travels through the plasma and loses energy. The primary particles are unstable and eventually decay to form stable hadrons. By measuring and summing up the momentum of the decay products one can infer the energy loss that happened in the plasma.

We previously saw that the gauge-gravity correspondence seems to work well for the RHIC data, but misses the mark when the more recent LHC is also taken into account. The prediction is far outside the error margin of the data, both in terms of magnitude and in terms of slope. The gauge-gravity correspondence predicts too much energy loss. I call that a bad fit to the data. String theorists call it “qualitatively correct” which seems to mean their prediction has an upward slope.

But heavy ion physics is a messy business where many different processes come together and that makes it difficult to draw unambiguous conclusions. Clearly that situation doesn’t look good. However, as I mentioned earlier, last year I heard a talk by Steven Gubser about an upcoming paper of his addressing the energy loss in the gauge-gravity correspondence. Ficnar, Gubser and Gyulassy now recently posted their new paper on the arxiv:

Shooting String Holography of Jet Quenching at RHIC and LHC
Andrej Ficnar, Steven S. Gubser, Miklos Gyulassy
arXiv:1311.6160 [hep-ph]

In this paper, the authors propose a new description on the gravity side for the particle which on the field theory side loses energy while traveling through the plasma. Previously, this particle was modeled by a string parallel to the boundary that fell towards the black hole horizon. Ficnar et al instead model the particle by a string that ‘shoots up’ away from the horizon. They calculate the energy loss of the endpoint and find that the energy loss is reduced relative to the previous scenario.

They do not motivate the gravitational description and I am left wondering if not there should be an unambiguous procedure to find the gravitational analogue. If one can just choose a different setup and get a different energy loss that does not exactly increase my faith in the predictive value of the model.

Be that as it may, with their new model a sufficient reduction of the energy loss can only be achieved by pushing the crucial parameter (λ, the ‘t Hooft coupling) into a limit where the approximation actually breaks down. This is no good because then the results cannot be trusted.

So then they add higher curvature terms on the gravity side. This introduces an additional parameter, and a suitable choice for this second parameter allows the coupling to remain just about in the okay range. One would expect these higher-order terms to be present, but in principle I’d think the coupling shouldn’t be an independent parameter. In any case, this still doesn’t fit both the RHIC and the LHC data.

Since the interpretation of the data depends on the reconstruction of the effective temperature at the collision, they then speculate that maybe the temperature values are off by 10% or so, in which case their calculation would fit the data just fine.

This model is clearly an improvement though I can’t say I am terribly convinced. What seems to become increasingly clear though is that any successful model for highly energetic heavy ion collisions must use a suitable combination of both weakly and strongly coupled physics. The gauge-gravity correspondence still has a good chance to prove its use for the strongly coupled physics, but that will necessitate getting into all the messy details.

Why quantize gravity?

When I first read DNLee’s story, I mistakenly thought “ofek” is a four letter word, an internet slang for oh-what-the-fucken-heck. Ofek, then, is all I have to say about my recent grant proposals being declined, one after the other. Ah, scrape the word “recent” – the Swedish Research Council hasn’t funded any one of my project proposals since I moved to Sweden in 2009. So all I can do is to continue to write papers as what feels like the only person working on quantum gravity in Northern Europe. Most painfully, I have had to turn away many a skilled and enthusiastic student wanting to work on quantum gravity phenomenology.

To add insult to injury, the Swedish Research Council publicly lists the titles of the winning proposals. You win if your research contains either “nano” or “neuro”, promises a cure for cancer, green energy, or a combination of the above. The strategy is designed for a bad return on investment. Money goes where lots of people poke the always same questions. If many flies circle the same spot there must be shit to find, the thinking is, let’s throw money at it. Our condensed matter people have no funding issues.

As nations face economic distress and support for basic research dwindles, why would anybody want to work on quantum gravity. Srsly. This question keeps coming back to me; its recurrence time conspicuously coincides with the funding agencies’ call cycles. It factors into my reflection index that women, I read, are drawn to occupations that help others, also occupations where they can use their allegedly superior social and language skills. What’ wrong with me? Why quantize gravity if I could cure cancer instead? Or at least write proposals promising I will, superior languages skills and all.

Modern medicine wouldn’t exist without the technologies that have become possible by breakthroughs in physics. There wouldn’t be any nano or neuro without imaging and manipulating quantum things and without understanding atoms and nucleons. Without basic research in physics, there wouldn’t be CT scans, there wouldn’t be NMR, nuclear power, digital cameras, and there wouldn’t be optical fibers for endovenous laser treatment.

At this point in history we still build on the new ground discovered by physicists a century ago. But the only way we can continue improving our circumstances of living is to increase our understanding of the fundamental laws of nature. And at the very top of the list there’s the question what is space and time, and how can we manipulate quantum objects. In my mind, these questions are intimately related. In my mind, that’s the ground the technologies of the next centuries will be built upon. In my mind, that’s how my occupation contributes to society – not to this generation maybe, but to the coming ones. Quantum gravity, quantum information, and the foundations of quantum mechanics are what will keep medicine advancing when nano and neuro has peaked and busted. Which will happen, inevitably, sooner or later.

So why quantum gravity? Because we know our knowledge of nature is incomplete. There must be more to find than we have found so far.

The search for quantum gravity is often portrayed as a search for unification. All other interactions besides gravity are quantized, there’s no unifying framework and that’s what physicists are looking for. It’s an argument from aesthetics, and it’s an argument I don’t like. Yes, it is unaesthetic to have gravity stand apart, but the reason we look for a quantum theory of gravity is much stronger than that: We know that unquantized gravity is incomplete and it is inconsistent with quantum theory. It isn’t only that we don’t know how to quantize gravity and that bugs us, we actually know that the combination of theories we presently have does not describe space and time at the fundamental level.

The strongest evidence for this inconsistency are the occurrence of singularities in unquantized gravity and the black hole information problem. The singularities are a sign that the unquantized theory breaks down and is incomplete. The black hole information problem shows that combining unquantized gravity with quantized matter is inconsistent – the result of combining them is incompatible with quantum theory.

Most importantly, we know that quantum particles can exist in superposition states, they can be neither here nor there. We also know that all particles carry energy and all energy creates a gravitational field. We thus know that the gravitational field of a superposition must exist, but we don’t know what it is. If the electron goes through both the left and the right slit, what happens to its gravitational field? Infuriatingly, nobody knows.

Nobody knows isn’t to say that nobody has an answer. Everybody seems to have an answer, the flies are circling happily. So I’ve made it my job to find out how we can ever know, which leads me to the question how to experimentally test quantum gravity. Without finding observational evidence, quantum gravity should be taught in the math or philosophy departments, not in the physics departments.

The irony is that quantum gravity phenomenology is as safe an investment as it gets in science. We know the theory must exist. We know that the only way it can be scientific is to make contact to observation. Quantum gravity phenomenology will become reality as surely as volcanic ash will drift over Central Europe again.

Every time I go down this road of self-doubts, I come out at the same place, which is right here in my office with my notepad and the books and the piles of papers. Quantum gravity is the next level of fundamental laws. The theory has to be connected to experiment. Quantum gravity is my contribution to the future of our societies and to help advance life on planet Earth. And, so I hope, space exploration, eventually. Because I really want ask those aliens a few things.

Today I talked to a professional photographer. Between the apertures and external flash settings and my attempt to produce a smile, I learned that he too has to write proposals for project funding. In his case, that’s portraits taken by a method which, I gather, isn’t presently widely used and not very popular with the Swedes. It’s neither nano nor neuro and it wasn’t funded.

Money is time, and time flies, and so in the end the most annoying part is all the waste of time that I could have used better than searching for pretty adjectives to decorate my proposals. Your tax money at work. Neuro-gravity anybody? Nano is also a four-letter word.