Passing through cosmic walls

Foam. Image source: DoITPoMS. 

Axions are hypothetical particles that are presently being searched for as possible dark matter candidates. The axion is a particle associated with the spontaneous breaking of a symmetry in the early universe. Unlike the case for the Higgs field, there can be a large number of ground states for the axion field. These states all have the same energy, but different values of the field. Since the ground states all have the same energy, they can coexist with each other, filling the universe with patches of different values of the axion fields, separated by boundaries called 'domain walls'.

The best visualization that came to my mind is a foam-like structure that fills the universe, though you shouldn't take this comparison too seriously.

At the domain walls, the axion field has to change values in order to interpolate between the different domains. This position-dependence of the field however creates a contribution to the energy density. Since the energy density of the domain walls decays slower with the expansion of the universe than the energy density of ordinary matter, this can become problematic, in the sense that it's in conflict with observation.

There are various ways to adjust these models or to pick the parameter ranges so that the domain walls do not appear to begin with, decay quickly, or are unlikely to be present in our observable universe. These are the most commonly used strategies for those interested in the axion as a particle. But in the recent years there has also been an increasing interest in using the domain walls themselves as gravitational sources, and so it has been suggested that they might play the role of dark energy or make contributions to dark matter.

In an interesting paper that appeared recently in PRL, Pospelov et al lay out how we could measure if planet Earth passed through such a domain wall

How do you know if you ran through a wall?
M. Pospelov, S. Pustelny, M. P. Ledbetter, D. F. Jackson Kimball, W. Gawlik, D. Budker
Phys. Rev. Lett. 110, 021803 (2013)
arXiv:1205.6260 [hep-ph]

(Apparently the arXiv-title did not survive peer review.)

The idea is to use the coupling of the gradient of the axion field, which is non-zero at the domain walls, to the spin of particles of the standard model. A passing through the domain wall would oh-so-slightly change the orientation of spins and align them into one direction.

This could be measured with devices normally used for very sensitive measurements of magnetic fields, optical magnetometers. Optical magnetometers consist basically of a bunch of atoms in gaseous form, typically alkali metals with one electron in the outer shell. These atoms are pumped with light into a polarized state of higher angular momentum, and then their polarization is measured again with light. This measurement is very sensitive to any change to the atomic spin's orientation, which may be caused by magnetic fields - or domain walls.

In the paper, and in a more recent follow-up paper, they estimate that that presently existing technology can test interesting parameter ranges of the model when other known constraints (mostly astrophysical) on the coupling of the axion have been taken into account. It should be mentioned though that they consider not a pure QCD axion, but a general axion-like field, in which case the relation between the mass of the particle and its coupling is not fixed.

The sensitivity to the event of a domain wall passing can be increased by not only reading out one particular magnetometer, but by monitoring many of them at the same time. Then one can look for correlations between them. This way one is not only able to better pick out a signal from the noise, but from the correlation time one could also determine the velocity of the passing through the domain wall.

I think this is an interesting experiment that nicely complements existing searches for dark matter. I also like it for its generality. Maybe while searching for axion domain walls, we'll find something else that we're moving through and that happens to couple very weakly to spins.

Science should be more like religion

When historians will look back at the 21st century, they’ll refer to it as the century in which religion died and the scientific revolution was completed. I am saying this not despite the battles of ignorance fought by the religious right and fundamentalists’ seeds of violence, but because of them. They fight, with words or with weapons, because they are afraid. And while these fights draw much attention, the number of non-believers is quietly and inevitably rising.

% of Christian, non-Christian religious,
and non-believers, UK, 1983 - 2009.
Source: Wikipedia.

What are they afraid of? They’re afraid of science. And I, I am afraid we have ourselves to blame for that. All we scientists ever do, it seems to me, is to explain how science differs from religion, ignoring the commonalities with science and the benefits that religion brings. Yes, that’s the scientists ignoring the facts.

Scientists aim to develop consistent explanations for the world. They observe and try to understand. Then they try to use what they have learned. They want coherent stories, want to fit the pieces together. They wonder and they seek, they doubt and they improvise. Scientists try and fail, and try again and gradually weave consistent stories to be shared with everybody.

In contrast, many religious stories are not only internally inconsistent, they are also inconsistent with observation and experience. In the best case, they are simply superfluous. Praying does not cure tetanus. The Earth wasn’t created 10,000 years ago. There’s no god sitting on a cloud throwing lightening down on blasphemous bloggers.

Yes, science clashes with religion. We’ve said it often enough.

The human brain excels at finding patterns, solving problems, and developing accurate theories. It abhors inconsistencies so much it will fake facts to remove them. Consciously accepting inconsistency necessitates a constant effort. Religions require the believer to accept these inconsistencies and not to ask. That takes time and energy. Believers must constantly belie themselves. Science doesn’t require you to accept inconsistencies. In fact, it encourages you not to accept them, and thereby frees up all that creative power, the patterns seeking, the story weaving. This clearly speaks for the scientific way. So why are people afraid of science?

They are afraid that science will replace hope with statistical odds, the aurora with equations, and love with oxytocin. They are afraid that science will take the wonder out of life and not give anything back. They are afraid they will have to give up their belief in an immortal soul, in miracle cures, in final answers, and get nothing in return. And we, we’re failing them because we don’t tell them what it is that they get in return.

Almost all scientists I know are atheists. They’re not atheists because they have been rendered unable to believe in God and are now suffering from a meaningless existence. They’re atheists because they don’t need religion. God, as Laplace put it, is an unnecessary hypothesis. And above all, god is a waste of time.

Far from taking the wonder out of human existence, science adds to it. We’re part of nature and science is the only way of understanding our place and our role.

If you’re in love and you read up on what is known on the chemical pathways and neurological activity, far from degrading you to a bundle of synapses, it embeds your love into the course of evolution and the complexity of human culture. If you look at the night sky and know that beyond the Milky Way there are billions of other galaxies, full with solar systems much like our own, your knowledge adds to the wonder. If you are pregnant and you subscribe to one of the dozens of calendars that tells you when your baby’s heart starts beating, how its nervous system develops, and when it is able to hear, then imagine that just 50 years ago you wouldn’t even have gotten an ultrasound image. Now you can have it in 3D. You’re growing a child, in an amazingly intricate and yet virtuously orchestrated process. You’re part of the circle of life. And without science you wouldn’t know much about that circle. You’re part of nature. Enjoy. And don’t forget to take the folic acid.

Religions offer people a community to belong to and a place to go. They offer shared traditions and spiritual guidance. Science doesn’t. Not because we don’t have a community or have nothing to offer. We’re just not letting people know what science has to give, making them believe science will only take away from them. We’re excluding others by not sharing our wonder.

Since the advent of the internet, we’ve gone a long way to making science more human. Scientists speak by themselves and of themselves. But few if any touch ground considered that of religion.

Yalom notes four existential fears: Fear of death, freedom, isolation, and meaninglessness. We all have these fears. They drive many people to religion because it’s the most obvious answer. And while science addresses these fears, scientists shy away from these topics. There are great speakers among the scientists, but most of them preach to the choir of an already scientifically-minded audience. Neil DeGrasse Tyson is one of the few who isn’t afraid crossing this line. He inspires those on both sides. And look how many listen. Carl Sagan did too. I have some more fingers on my hand, tell me who to count.

Most scientists feel awkward if the word “spirituality” is as much as mentioned, and the last thing they want to do is preach. I myself am guilty of course of never writing about living the atheists’ life. I didn’t study physics to be a preacher, and neither did any of my colleagues. So here’s the problem. A communication problem. Who’ll preach the wonders of science to those who most need to hear about them?

Some days ago I buzzed in Jehova’s witnesses thinking it’s DHL. Then I tried to wave them away saying we’re atheists. “Oooh!” said one of the men and raised his arms, pretending to be shocked, “How did this happen?” I asked him what he meant, and he said “Well, one doesn’t get born this way.” He’s wrong of course. We’re all born as atheists. We’re also born being social and in need of each other to talk through our problems. And as long as science doesn’t offer the community that religions provide so effortlessly, as long as scientists stand aloft of spiritual guidance, people will be afraid that their taxes pay scientists to remove the wonder from the world rather than adding to it.

Update: There's an interesting post from Lubos on the topic. He makes some goods points.

Testing spontaneous localization models with molecular level splitting

Gloria's collapse model.

We in the quantum gravity groups all over the planet search for a unified framework for general relativity and quantum theory. But I have a peripheral interest also in modifications of general relativity and quantum mechanics since altering one of these two ingredients can change the rules of the game. General relativity and quantum mechanics however work just fine as they are, so there is little need to modify them. In fact, modifications typically render them less appealing to the theoretician, for not to say ugly.

Spontaneous localization models for quantum mechanics are, if you ask me, a particularly ugly modification. In these models, one replaces the collapse upon observation in the Copenhagen interpretation by a large number of little localizations that have the purpose of producing eigenstates upon observation. These localizations that essentially focus the spread of the wave-function are built into the dynamics by some stochastic process, and the rate of collapse depends on the mass of the particles (the higher the mass, the higher the localization rate). The purpose of these models is to explain why we measure the effects of superposition, but never a superposition itself, and never experience macroscopic objects in superpositions.

Unfortunately, I have no reason to believe that nature gives a damn what I find ugly or not, and quite possibly you don’t care either. And so, as a phenomenologist, the relevant question that remains is whether spontaneous localization models are a description of nature that agrees with observation.

And, to be fair, on that account spontaneous localization models are actually quite appealing. That is because their effects, or the parameters of the model respectively, can be bounded both from above and below. The reason is that the collapse processes have to be efficient enough to produce eigenstates upon observation, but not so efficient as to wash out the effects of quantum superpositions that we observe.

The former bound on the efficient production of observable eigenstates becomes ambiguous however if you allow for a many worlds interpretation because then you don’t have to be bothered by macroscopic superpositions. Alas, the intersection of the groups of many worlds believers and spontaneous localization believers is an empty set. Therefore, the spontaneous localization approach has a range of parameters with macroscopic superpositions that is “philosophically unsatisfactory,” as Feldman and Tumulka put it in their (very readable) paper (arXiv:1109.6579). In other words, if you allow for a many worlds situation whose main feature is the absence of collapse, then there really is no point to add stochastic localization on top of that. So it’s either-or, and thus requiring absence of macroscopic superpositions bounds possible parameters.

Still, the notion of what constitutes “macroscopic reality” is quite fuzzy. Just to give you an idea of the problem, the estimates by Feldman and Tumulka go along such lines:

“To obtain quantitative estimates for the values [of the model parameters] that define the boundary of the [philosophically unsatisfactory region], we ask under which conditions measurement outcomes can be read off unambiguously... For definiteness, we think of the outcome as a number printed on a sheet of paper; we estimate that a single digit, printed (say) in 11-point font size, consists of 3 x 10¹⁷ carbon atoms or N = 4 x 10¹⁸ nucleons. Footnote 1: Here is how this estimate was obtained: We counted that a typical page (from the Physical Review) without figures or formulas contains 6,000 characters and measured that a toner cartridge for a Hewlett Packard laser printer weighs 2.34 kg when full and 1.54 kg when empty. According to the manufacturer, a cartridge suffices for printing 2 x 10⁴ pages...”

And so on. They also discuss the question whether chairs exist:

“One could argue that the theory actually becomes empirically refuted, as it predicts the nonexistence of chairs while we are sure that chairs exist in our world. However, this empirical refutation can never be conclusively demonstrated because the theory would still make reasonable predictions for the outcomes of all experiments...”

Meanwhile on planet earth, particle physicists calculate next-to-next-to-next-to leading order corrections to the Higgs cross-section.

Sarcasm aside, my main problem with this, and with most interpretations and modifications of quantum mechanics, is that we already know that quantum mechanics is not fundamentally the correct description of nature. That’s why we teach 2nd quantization to students. To make matters worse, most of such modifications of quantum mechanics deal with the non-relativistic limit only. I thus have a hard time getting excited about collapse models. But I’m digressing - we were discussing their phenomenological viability.

In fact, Feldman and Tumulka’s summary of experimental (ie non-philosophic) constraints isn’t quite as mind-enhancing as the nonexistent chair I’m sitting on. (Hard science, my ass.) Some experimental constraints they are discussing: The stochastic process of these models contributes to global warming by injecting energy with each collapse and since there’s some cave in Germany which doesn’t noticeably warm up in July, this gives a constraint. And since we have not heard any “spontaneous bangs” around us that would accompany the collapses in certain parameter ranges, we get another constraint. Then there’s atom interferometry. And then there’s this very interesting recent paper

Are collapse models testable with quantum oscillating systems? The case of neutrinos, kaons, chiral molecules
M. Bahrami, S. Donadi, L. Ferialdi, A. Bassi, C. Curceanu, A. Di Domenico, B. C. Hiesmayr
Nature: Scientific Reports 3, 1952 (2013)
arXiv:1305.6168 [quant-ph]

In this paper the authors calculate how spontaneous localization affects quantum mechanical oscillation between two eigenstates. If you recall, we previously discussed how the observation of such oscillations allows to put bounds on decoherence induced by coupling to space-time foam. For the space-time foam, neutral Kaons make a good system for experimental test. Decoherence from space-time foam should decrease the ability of the Kaons to oscillate into each other. The bounds on parameters are meanwhile getting close to the Planck scale.

For spontaneous localization the effect scales differently with the mass though, and is thus not testable in neutral Kaon oscillation. Since the localization effects get larger with large masses, the authors recommend to instead look for the effects of collapse models in chiral molecules.

Chiral molecules are pairs of molecules with the same atomic composition but with a different spatial arrangement. And some of these molecules can exist in superpositions of such spatial arrangements that can transform into each other. In the small temperature limit, this leads to an observable level splitting in the molecular spectrum. The best known example may be ammonia.

Now if collapse models were correct, then these spatial superpositions of chiral molecules should localize and the level splitting, which is a consequence of superpositions of two eigenstates, become unobservable. The authors estimate that with current measurement precision the bound from molecular level splitting is about comparable to that of atom interferometry (where interference should become unobservable if spontaneous localization is too efficient, thus leading to a bound). Molecular spectroscopy is a presently very active research area and with better resolution and larger molecules, this bound could be improved.

In summary, this nice paper gives me hope that in the soon future we can put the ugly idea of spontaneous localization to rest.

Phenomenological Quantum Gravity

Participants of the 2012 conference on 
Experimental Search for Quantum Gravity.

The search for quantum gravity and a theory of everything captures the public imagination like no other area in theoretical physics. It aims to answer three questions that every two-year old could ask if they would just stop being obsessed with cookies for a minute: What is space? What is time? And what is matter? We know that the answers we presently have to these questions are not fundamentally correct; they are merely approximately correct. And we want to know. We really really want to know. (The cookies. Are out.)

Strictly speaking of course physics will not tell you what reality is but what reality is best described by. Space and time are presently described by Einstein’s theory of general relativity; they are classical entities that do not have quantum properties. Matter and radiation are quantum fields described by the standard model. Yet we know that this cannot be the end of the story because the quantum fields carry energy and thus gravitate. The gravitational field thus must be compatible with the quantum aspects of matter sources. Something has to give, and it is generally expected that a quantization of gravity is necessary. I generally refer to ‘quantum gravity’ as any approach to solve this tension. In a slight abuse of language, this also includes approaches in which the gravitational field remains classical and the coupling to matter is modified.

Quantizing gravity is actually not so difficult. The problem is that the straight-forward, naive, quantization does not give a theory that makes sense as a fundamental theory. The result is said to be non-renormalizable, meaning it is a good theory only in some energy ranges and cannot be taken to describe the very essence of space, time, and matter. There are meanwhile several other, not-so-naïve, approaches to quantum gravity – string theory, loop quantum gravity, asymptotically safe gravity, causal dynamical triangulation, and a handful of others. The problem is that so far none of these approaches has experimental evidence.

This really isn’t so surprising. To begin with, it’s a technically hard problem that has kept some of the brightest minds on the planet occupied for decades. But besides this, returns on investment have diminished with the advent of scientific knowledge. The low hanging fruits have all been picked. Now we have to develop increasingly more complex experiments to find new physics. This takes time, not to mention effort and money. With that, progress slows.

And quantum gravity is a particularly difficult area for experiment. It’s not just a weak force, it’s weaker than the weak force! This grammatical oxymoron is symptomatic of the problem: Quantum effects of gravity are really, really tiny. Most of the time when I estimate an effect, it turns out to be twenty or more orders of magnitude below experimental precision. I’ve sometimes joked I should write a paper on “50 ways one cannot test quantum gravity”, just to make use of these estimates. It’s clearly not a low hanging fruit, and we shouldn’t be surprised it takes time to climb the tree.

Some people have claimed on occasion that the lack of a breakthrough in the area is due to sociological problems in the organization of knowledge discovery. There are indeed problems in the organization of knowledge discovery today. We use existing resources inefficiently, and I do think this hinders progress. But this is a problem which affects all of academia and is not special to quantum gravity.

I think the main reason why we don’t yet know which theory describes gravity in the quantum regime is that we haven’t paid enough attention to the phenomenology.

One reason phenomenological quantum gravity hasn’t gotten much attention so far is that it has long been believed experimental evidence for quantum gravity is inaccessible to experiment (a belief promoted prominently by Freeman Dyson). The more relevant reason is though that in the field of theoretical physics it’s a very peculiar research topic. In all other areas of physics, researchers share either a common body of experimental evidence and aim to develop a good theory. Or they share a theoretical framework and aim to explore its consequences. Phenomenological quantum gravity has neither a shared theory nor a shared set of data. So what can the scientist do in this situation?

Methodology

The phenomenology of quantum gravity proceeds by the development of models that are specifically designed to test for properties of the yet-to-be-found theory of quantum gravity. These phenomenological models are normally extensions of known theories and are developed with the explicit aim of testing for general features. These models do not aim to be fundamental theories on their own.

Examples of such general properties that the fundamental theory might have are: violations or deformations of Lorentz-invariance, additional space-like dimensions, the existence of a minimal length scale or a generalized uncertainty principle, holography, space-time fluctuations, fundamental discreteness, and so on. I discuss a few examples below. If we develop a model that can be constrained by data, we will learn what properties the fundamental theory can have, and which it cannot have. This in turn can serve as guidance for the development of the theory.

In practice, these phenomenological models quantify deviations from general relativity and/or quantum field theory. One expects that the only additional dimensionful scale in these models is the Planck scale, which gives a ‘natural’ range for the expected size of effects in which all dimensionless constants are of order one. The aim is then to find an experiment that is sensitive to this natural parameter range. Since most of these models do not actually deal with quanta of the gravitational field, I prefer to speak more generally of “Planck scale effects” being what we are looking for.

Example: Lorentz-invariance violation

The best known example that demonstrates that effects are measureable even when they are suppressed by the Planck scale are violations of Lorentz-invariance. You expect violations of Lorentz-invariance in models for space-time that make use of a preferred frame that violates observer-independence, for example some regular lattice or condensate that evolves with some special time-slicing.

Such violations of Lorentz-invariance can be described by extensions of the standard model that couple to a time-like vector field and these couplings change the predictions of the standard model. Even though the effects are tiny, many of them are measureable.

The best example is maybe vacuum Cherenkov-radiation: the spontaneous emission of a photon by an electron. This process is normally entirely forbidden which makes it a very sensitive probe. With Lorentz-invariance violation, an electron above a certain energy will start to lose energy by radiating photons. We thus should not receive electrons above this threshold from distant astrophysical sources. From the highest energies of electrons of astrophysical origin that we have measured we can thus derive a bound on the possible violation of Lorentz invariance. This bound is today already (way) beyond the Planck scale, which means that the natural parameter range is excluded.

This shows that we can constrain Planck scale effects even though they are tiny.

Now this is a negative result in the sense that we have ruled out certain properties. But from this we have learned a lot. Approaches which induce such violations of Lorentz-invariance are no longer viable.

Example: Lorentz-invariance deformation

Deformations of Lorentz-invariance have been suggested as symmetries of the ground state of space-time. In contrast to violations of Lorentz-invariance, they do not single out a preferred frame. They generically lead to modifications of the speed of light, which can become energy-dependent.

I have explained a great many times that I think these models are flawed because they bring more problems than they solve. But leaving aside my criticism of the model, it can be experimentally tested. The energy dependence of the speed of light is tiny – a Planck scale effect – but the measurable time-difference adds up over the distance that photons of different energies travel. This is why highly energetic photons from distant gamma ray bursts are presently receiving a lot of attention as possible probes of quantum gravitational effects.

The current status is that we are just about to reach the natural parameter range expected for a Planck scale effect. It is presently a very active research area.

Example: Decoherence induced by space-time foam

If space-time undergoes quantum fluctuations that couple to all matter fields, this may induce decoherence in quantum mechanical oscillations. We discussed this previously in this post. In oscillations of neutral Kaon systems, we are presently just about to reach Planck scale sensitivity.

Misc other examples

There is no lack of creativity in the community! Some other examples of varying plausibility that we have discussed on this blog are Craig Hogan’s quest for holographic noise, Bekenstein’s table-top experiment that searches for Planck-length discreteness, massive quantum oscillators testing Planck-scale modified commutation relations, and searches for evidence for a generalized uncertainty in tritium decay. There is also a vast body of work on leftover quantum gravitational effects from the early universe, captured in various models for string cosmology and loop quantum cosmology, and of course there are cosmic (super) strings. There are further proposed tests for the idea that gravity is just classical (still a little outside the natural parameter range), and suggestions to look for dimensional reduction.

This is not an exhaustive list but just to give you a sense of the breadth of the topics.

Demarcation issues

What counts and what doesn’t count as phenomenological quantum gravity is inevitably somewhat subjective. I do for example not count the beyond the standard model physics of grand unification, though, if you believe in a theory of everything, this might be relevant for quantum gravity. I also don’t count applications of AdS/CFT because these do not describe gravitational systems in our universe, though arguably they are examples for some quantized version of gravity. I also don’t count general modifications of quantum theory or general relativity, though these might of course be very relevant to the problem. I don’t label these phenomenological quantum gravity mostly for practical reasons, not for ideological ones. One has to draw the line somewhere.

Endnote

I often get asked which approach to quantum gravity I believe in. When it comes to my religious affiliation, I’m not only an atheist, I was never Christianized. I have never belonged to any church and I have no intention to join one. The same can be said about my research in quantum gravity. I don’t belong to any church and have never been Christianized. I have on occasion erroneously been called a string theorist and I have been mistaken for working on loop quantum gravity. Depending on the situation, that can be amusing (on a conference) or annoying (in a job interview). For many people it still seems to be hard to understand that the phenomenology of quantum gravity is a separate research area that does not built on the framework of any particular approach.

The aim of my work is to identify the most promising experiments to find evidence for quantum gravity. For that, we need phenomenological models to quantify the effects, and we need to understand the models that we have (for me that includes criticizing them). I follow with interest the progress in various approaches to quantum gravity (presently I’m quite excited about Causal Sets) and I try to develop testable phenomenological models based on these developments. On the practical side, I organize conferences and workshops to bring together theoreticians with experimentalists who have an interest in the topic to stimulate exchange and the generation of new ideas.

What I do believe in, and what I hope the above examples illustrate, is that it is possible for us to find experimental evidence for quantum gravity if we ask the right questions and look in the right places.

Nordita’s First Workshop for Science Writers, Summary

Patrick Sutton

George and I came up with the idea for this workshop one year ago at a reception of an earlier Nordita workshop. Yes, alcohol was involved. We talked about how science writers often feel like they’re running on a treadmill, having to keep up with the frenetic pace of publishing, only seldom getting a chance to take a few days off to gain some broader perspective. And we talked about how researchers too are running on a treadmill, having to keep up with the pace of their colleagues’ publications, and often feel that science writers miss the broader perspective.

And so we set ourselves the goal to get everybody off the treadmill for a few days.

Our “workshop for science writes”, which took place May 27-29, was devised for both, the writers and the physicists: For the writers to hear what topics in astrophysics and cosmology will soon be on the agenda and what science journalists really need to know about them. And for the physicists to share both their knowledge and their motivation, and to caution against common misunderstandings.

We modeled the workshop on “boot camps” organized by the Space Telescope Science Institute, Woods Hole Oceanographic Institute, U.C. Santa Cruz, and other institutions. Our workshop was a very intense and tightly packed meeting, with lectures by experts on selected topics in astrophysics and cosmology, followed by question and answer sessions.

George, wired.

On Tuesday afternoon, we visited the phonetics lab at Stockholm University, which was a fun excursion into a totally different area of science. At the lab, participants could analyze their voice spectra and airflow during speech, and learn the physics behind speech production. They could also take an EEG, which the researchers at the lab use to study which brain areas are involved in language processing and how that changes during infancy.

On Tuesday evening, one of the participants of the workshop, Robert Nemiroff, gave a public lecture at CosmoNova. The fully booked lecture took the audience on a tour through the solar system and beyond, projected on the 17m IMAX screen, while Robert explained the science behind the amazing photos and videos. Besides the stunning images, it was also great to see so many people interested in the laws of physics that shape our universe. (The guy sitting next to me held a copy of Lee Smolin’s new book on his lap which caused me some cognitive dissonance though.)

It was admittedly quite an organizational challenge to find the right level of technical details for an audience that physicists rarely deal with. I think however that the question and answer sessions as well as a large number of breaks were useful for participants to talk to lectures individually. We also had many interesting discussions about the tension between scientific accuracy and popular science writing. As you can guess, I inevitably come down on the side of scientific accuracy.

George turned out to be an excellent organizer, though clearly not used to the physicists compulsive ignorance of deadlines and reminders. I found it quite interesting that when I sent out mass emails to the participants that asked for reply, the first cohort of replies would come almost exclusively from the science writers, frequently within minutes. Among the physicists there were but two who'd answer within 24 hours and meet the deadlines, the rest waited for multiple reminders. The other interesting contrast was that the science writers were considerably more comfortable and engaged with social media.

For me, it was a great pleasure to get to know such an interesting and diverse group of people. I’m neither an astrophysicist nor a cosmologist nor a science writer, and I learned a lot at this workshop - it will probably inspire some more blogposts.

You can find soundbites and links from the meeting on twitter here, and slides of the lectures here.

George Musser, Robert Nemiroff, I, and a bunch of beautiful flowers.

Quantum gravity phenomenology \neq detecting gravitons

First direct evidence for gravitons.

I’ve never met Freeman Dyson, but I’ve argued with him many times.

Almost every time I give a seminar about my research field, the phenomenology of quantum gravity, I find myself in the bizarre situation of first having to convince the audience that it is a research field. And that even though hundreds of people work on it. I have been organizing and co-organizing a series of conferences on Experimental Search for Quantum Gravity, and in each installment we had to turn away applicants due to space limitations. The arXiv is full with papers on the topic, more than I can keep up with on this blog, and it’s in the popular press more often than I’d like*. Why are my fellow physicists so slow to notice? I make Freeman Dyson responsible for this.

Dyson has popularized the idea that quantum gravity is inaccessible to experiment and thereby discouraged studies of phenomenological consequences of quantum gravity. In a 2004 review of Brian Greene’s book “The Fabric of the Cosmos” he wrote:

“According to my hypothesis [...] the two theories [general relativity and quantum theory] are mathematically different and cannot be applied simultaneously. But no inconsistency can arise from using both theories, because any differences between their predictions are physically undetectable.”

And in a 2012 essay for the Edge Annual Question, he still pushed the idea of quantum gravitational effects being unobservable:

“I propose as a hypothesis... that single gravitons may be unobservable by any conceivable apparatus. If this hypothesis were true, it would imply that theories of quantum gravity are untestable and scientifically meaningless. The classical universe and the quantum universe could then live together in peaceful coexistence. No incompatibility between the two pictures could ever be demonstrated. Both pictures of the universe could be true, and the search for a unified theory could turn out to be an illusion.”

The problem with this argument is that he equates the observation of a single graviton with evidence for a quantization of gravity. But the two are not the same. If single gravitons were unobservable, it would not imply that “theories of quantum gravity are untestable and scientifically meaningless.”

It might indeed be that we will never be able to detect gravitons. One can estimate the probability of detecting gravitons and even with extremely futuristic detectors the size of Jupiter put in orbit around a Newton star, chances would be slim. (See this paper for estimates.) Clearly not an experiment you want to write a grant proposal for.

But we don’t need to detect single gravitons to find experimental evidence for quantum gravity.

Look around. The fact that atoms are stable is evidence for the quantization of the electromagnetic interaction. You don’t need to detect single photons for that. You also don’t need to resolve atomic structures to find evidence for the atomic theory. Brownian motion famously provided this evidence, visible by eye. And Planck introduced what is now known as “Planck’s constant” before Einstein’s Nobel-prize winning explanation for the photoelectric effect.

If we pay attention to the history of physics, it is thus plausible that we can find evidence for quantum gravity without directly detecting gravitons. The quantum theory of gravity might have consequences that we can access in regimes where gravity is weak, as long as we ask the right questions.

Some people have a linguistic problem with calling something a “quantum gravitational effect” if it isn’t actually an effect that directly involves quanta of the gravitational field. This is why I instead often use the expression “Planck scale effects” to refer to effects beyond the standard model that might be signatures of quantum gravity.

Interestingly, Christine recently pointed me to a writeup of a 2012 talk by Freeman Dyson, in which he discusses the possibility of detecting gravitons without jumping to the conclusion that an inability to detect gravitons means that quantum gravity is a subject for philosophers. Instead, Dyson is very careful with stating:

“One hypothesis is that gravity is a quantum field and gravitons exist. A second hypothesis is that the gravitational field is a statistical concept like entropy or temperature, only defined for gravitational effects of matter in bulk and not for effects of individual elementary particles… If a graviton detector is in principle impossible, then both hypotheses remain open.”

A hooray for Dyson!

Unfortunately, there are still other people barking up the same tree, for example by pulling the accelerator argument. For example John Horgan writes:

“String theory, loop-space theory and other popular candidates for a unified theory postulate phenomena far too minuscule to be detected by any existing or even conceivable (except in a sci-fi way) experiment. Obtaining the kind of evidence of a string or loop that we have for, say, the top quark would require building an accelerator as big as the Milky Way.”

Horgan is well known for proclaiming The End of Science, and it seems indeed he’s run out of science when he wrote the above. To begin with, string theory doesn’t “postulate... phenomena,” what would be the point of doing this? It postulates, drums please, strings. And I’m not at all sure what “loop-space theory” is supposed to be. But leaving aside this demonstration of Hogan’s somewhat fuzzy understanding of the subject, if we could build a detector the size of the Milky Way, we’d be able to test very high energies, all right. But that doesn’t mean we can conclude this is the only way to find evidence for quantum gravity.

Luckily Horgan has colleagues who think before they write, like George Musser who put it this way:

“[Q]uantum gravity” and “experiment” are… like peanut butter and chocolate. They actually go together quite tastily.

(I had meant to write a summary of which possible experiments for quantum gravity pheno are presently being discussed and how plausible I think they are to deliver results, but I got distracted by Dyson’s above mentioned paper on graviton detection. The summary will follow some other time. Update: The summary is here.)

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*Almost everything I read in the popular press about evidence for quantum gravity is wrong or misleading or both. But then you already knew I would complain about this :p

Why do Science?

I sat down to write a piece explaining why scientific research is essential to our societies and why we should invest in applied and basic science. Then I recalled I don’t believe in free will. This isn’t always easy... So I took out the “should” from the title because it’s not like we have a choice. Evidently, we do science! The question is why? And will we continue?

Natural selection, then and now

Developing accurate theories of nature that allow making predictions about the world are an evolutionary advantage. Understanding our environment and ourselves enables us to construct tools and shape nature to our needs. It makes thus sense that natural selection favors using brains to develop theories of nature.

As it is often the case though, natural selection favored traits that then extend beyond the ones immediately relevant for survival. And so the human brain has become very adept at constructing consistent explanations generally. If we encounter any inconsistency, we mentally chew on it and try to find a solution. This is why we cannot help but write thousands of papers on the black hole information paradox. This is why Dyson’s belief that inconsistencies between quantum mechanics and general relativity will forever remain outside experimental detection does not deter physicists from trying to resolve this inconsistency: It’s nature, not nurture.

In fact, our brain is so eager to create consistent theories that it sometimes does so by denying facts which won’t fit. This is why we are prone to confirmation bias, and in extreme cases paralyzed people deny they are not able to tie their shoes or lift an arm (examples from Ramachandran’s book “Phantoms in the Brain.”)

But leaving aside the inevitable overshooting, evolution has endowed us with a brain that is able and eager to develop consistent explanations. This is why we do science.

The question whether we will continue to do science, and what type of science, is more involved than asking whether scientific thinking has benefitted the reproduction of certain genes. The reason is that we have become so good at using nature to our needs that evolution no longer acts by just selecting the phenotypes best adapted to a given environment. Instead, we can make the environment fit to us.

Today, the major effort of societies is eradicating risks and diseases, optimizing crops and agricultural yields, and developing all kinds of technologies to minimize exposure to natural events. Natural selection of course still proceeds. It’s a process that acts on adaptive systems so generally and unavoidably that Lee Smolin famously uses it to explain the evolution of universes. But what does change is the mechanism that creates the mutations among which the “fittest” has an evolutionary advantage. Since we humans now create large changes on the environment in which we have to survive, the technologies that enable us to make these changes have become part of the random mutations among which selection acts. Backreaction can no longer be neglected.

In other words, natural selection can only act on expressions of genes and ideas together. The innovation provided by scientific progress is now part of the mutations that create species better adapted to the environment.

Applied and basic research

The purpose of scientific research is thus to act as an innovation machine. It enables humans to “fit” better to their environment. This is the case at least for applied research. So what then is the rationale to engage in basic research?

First note that what is usually referred to as “basic research” is rarely “non-applied,” but rather it’s “not immediately applied”. Basic research is commonly pursued on the rationale that it is the precursor of applications in the far future, a future so far that it isn’t yet possible to tell what the application might be. This basic research is necessary to sustain innovation in the long run.

Also note that what is commonly referred to as an “application” doesn’t cover the full scope of innovation that scientific research brings. Scientific insight, especially paradigm shifts, have the potential to entirely reshape the way we perceive of ourselves and our place in the world. This can have major cultural and social impacts that have nothing to do with the development of technologies.

Marxist thought for example has thrived on the belief that we differ only in the chances and opportunities given to us and not by heritable talents that lead to different performances, a fact now known to be scientifically fallacious. Planned economy seems like a good idea if you believe in a clockwork universe in which you can make accurate predictions, an idea that doesn’t seem so good if you know something about chaos theory. Adam Smith’s “invisible hand” is based on the belief that self-organization is natural and leads to desirable outcomes, and we’re only slowly learning the problems in managing risk in complex and highly connected networks. The ongoing change in attitude towards religion is driven by science shining light on inconsistencies in religious storytelling. And many scientists seem to be afraid what it could do to society if people realized that they have no free will. All these are non-technological examples of innovation created by scientific knowledge.

Having said that, we are left to wonder about the scientific research that is neither applied (immediately or in the far future) nor has any other impact on our societies. There very possibly is such research. But we don’t know in advance whether or not a piece of research will become relevant in the future. I previously referred to this research as “knowledge for the sake of knowledge.” Now I am thinking that a better description would have been You-never-know-ledge.

Bottomline

Since we have to manage finite resources on this planet, there is always the question how much energy, time, money, and people to invest into any one human activity for the most beneficial outcome. This is a question which has to be addressed on a case-by-case basis and greatly depends on what is meant with “beneficial”, a word that would bring us back to opinions and “should”s. So the above considerations don’t tell us how much investment into science is enough. But they do tell us that we need continuous investment into scientific research, both applied and basic, to allow mankind to sustain and improve the Darwinian “fit” to the environment that we are changing and creating ourselves.

Have your multiverse and eat it

The recent results from the Planck mission have caused a flurry of activity among theoretical physicists, documented on the arXiv in an increasing amount of papers with updates on constraints on various cosmological models. Of particular interest is the question which models of inflation are favored by the data. Interestingly, the simplest potentials for the scalar field that causes inflation are ruled out or disfavored already. For a summary, see Jester’s post Planck about inflation.

Paul Steinhardt and collaborators have taken this as a reason to argue that the data actually hints at cyclic models.

Inflationary paradigm in trouble after Planck2013
Anna Ijjas, Paul J. Steinhardt, Abraham Loeb
1304.2785

Planck 2013 results support the simplest cyclic models
Jean-Luc Lehners, Paul J. Steinhardt
1304.3122

The argument in these papers goes as follows.

The potentials for the inflaton field that are necessary to fit the Planck data are not simple in that they require finetuning, ie delicately adjusted parameters. The finetuning has to produce a suitably flat plateau in the potential, and a power law with coefficients of order one isn’t going to do this. If you’d random pick the potential, it would be very unlikely you’d get a suitably finetuned one.

This, Steinhardt et al argue, is a serious problem because the “inflationary paradigm” draws its justification from our universe being a “likely” outcome of quantum fluctuations that are blown up to produce the structures we see. If the potential, or the initial value of the scalar field, is unlikely, this erodes the basis of believing in the inflationary paradigm to begin with. In the paper this unlikeliness is quantified, and it is noted that the unlikeliness of the initial value of the scalar field can be recast as an unlikeliness of the potential. Then they go on to argue that cyclic models are preferable because in these cases natural parameter ranges for coefficients in the potential are still compatible with the data (they do not comment on how natural these models are in other respects). They then identify observables that could further solidify the case.

There are two gaps in this argument. The first gap is between “inflation” and “inflationary paradigm.”

Inflation is a model that describes very well the observations in our universe by using a familiar framework that makes use of quantum field theory and general relativity. The inflationary paradigm that they refer to (not an expression that is common in the scientific literature) adds requirements beyond the explanation of observation, that being the likeliness of the model.

To begin with, speaking about probabilities makes only sense if one has an ensemble. So to even refer to unlikeliness you have to believe in a distribution over set of possibilities, a multiverse. And for that you must have faith in your model, faith that extends beyond and before and beneath our universe, faith that the model holds outside everything we have ever observed, and that you can actually use it to make a statement about likeliness.

Besides this, the inflaton potential is normally not expected to be fundamental, but some effective limit a few orders of magnitude below the Planck scale. If you want to say anything about the probability of finding a particular potential, you would first have to know the fundamental degrees of freedom and the UV-completion of the theory. Just taking potentials and attempting to assign them a probability doesn’t make a lot of sense.

So talking about probabilities is already a bad starting position. From this starting position then Steinhardt et al argue that the inflationary paradigm says that we should find our universe to be likely. But by going from inflation to the inflationary paradigm, one is no longer talking about testing a model that explains observations. In their own words

“The usual test for a theory is whether experiment agrees with model predictions. Obviously, inflationary plateau-like models pass this test.”

That should be the last sentence of a scientific paper. Alas, there’s a next sentence, and it starts with “However…”

“However, this cannot be described as a success for the inflationary paradigm, since, according to inflationary reasoning, this particular class of models is highly unlikely to describe reality.”

Note the leap from “theory” to “paradigm”. (Let me not ask what “reality” means, I know it’s an unfair question.)

The second gap in the argument is that you could use it to rule out pretty much any model anybody has ever proposed.

In this earlier post I explained that all presently existing theories inevitably lead to a multiverse, a large space of possibilities. It’s just that this multiverse is more apparent in some approaches than in others.

The reason a multiverse is inevitable is that we always need something to specify a theory to begin with. Call it basic axioms or postulates. We need something to start with. And in the context of the theory you’re working with, that postulated basis is an uncaused cause: It was written down with the explicit purpose to explain observations. If you take away that purpose because you’ve misunderstood what science is all about, you are left with only mathematical consistency. And then, layer by layer, you are forced to include everything into your theory that is mathematically consistent. That’s what Tegmark called the “Mathematical Universe.”

Steinhardt et al’s elaboration about the possible shape of potentials is an example of this mathematical multiverse beneath the basis. They take away one postulate and replace it by a larger space of mathematical possibilities. Instead of postulating a specific (purpose bound) real-valued, differentiable, scalar function, they replace them with the space of all continuous functions (though they’re not too explicit on the requirements). But why stop there? Why not take the space of all functions and random pick one of these? Almost all functions on the real axis are discontinuous in infinitely many places, which is a fancy way of saying that the probability to get a continuous one upon random picking is zero. Look, I just ruled out both the “inflationary paradigm” and Steinhardt’s cyclic models without referring to any data at all.

To be fair however, Steinhardt et al are just fighting inflation with its own weapons. It is arguably true that the literature is full of arguments about naturalness and how inflation solves this or that philosophical conundrum. If you believe in the multiverse, or eternal inflation specifically, I think you should take the argument put forward in these papers seriously. For the rest of us, those who see inflation as a model with the purpose to describe observations in our universe, there’s no reason to make these leaps of faith. And that’s what they are - at least for now. One never knows what the data will bring.

Who said it first? The historical comeback of the cosmological constant

I finished high school in 1995, and the 1998 evidence for the cosmological constant from supernova redshift data was my first opportunity to see physicists readjusting their worldview to accommodate new facts. Initially met by skepticism - as all unexpected experimental results - the nonzero value of the cosmological constant was quickly accepted though. (Unlike eg neutrino oscillations, where the situation remained murky, and people remained skeptic, for more than a decade.)

But how unexpected was that experimental result really?

I learned only recently that by 1998 it might not have been so much of a surprise. Already in 1990, Efstathiou, Sutherland and Maddox, argued in a Nature paper that a cosmological constant is necessary to explain large scale structures. The abstract reads:

"We argue here that the successes of the [Cold Dark Matter (CDM)] theory can be retained and the new observations accommodated in a spatially flat cosmology in which as much as 80% of the critical density is provided by a positive cosmological constant, which is dynamically equivalent to endowing the vacuum with a non-zero energy density. In such a universe, expansion was dominated by CDM until a recent epoch, but is now governed by the cosmological constant. As well as explaining large-scale structure, a cosmological constant can account for the lack of fluctuations in the microwave background and the large number of certain kinds of object found at high redshift."

By 1995 a bunch of tentative and suggestive evidence had piled up that lead Krauss and Turner to publish a paper titled "The Cosmological Constant is Back".

I find this interesting for two reasons. First, it doesn't seem to be very widely known, it's also not mentioned in the Wikipedia entry. Second, taking into account that there must have been preliminary data and rumors even before the 1990 Nature paper was published, this means that by the late 1980s, the cosmological constant likely started to seep back into physicists brains.

Weinberg's anthropic prediction dates to 1987, which likely indeed predated observational evidence. Vilenkin's 1995 refinement of Weinberg's prediction was timely but one is lead to suspect he anticipated the 1998 results from the then already available data. Sorkin's prediction for a small positive cosmological constant in the context of Causal Sets seems to date back into the late 80s, but the exact timing is somewhat murky. There is a paper here which dates to 1990 with the prediction (scroll to the last paragraph), which leads me to think at the time of writing he likely didn't know about the recent developments in astrophysics that would later render this paper a historically interesting prediction.

Guestpost: Howard Burton - "Justified Optimism"

[Howard Burton, founding director of Perimeter Institute, has a new project, Ideas Roadshow, a weekly magazine dedicated to ideas of all types and shapes. Rather than having the declared aim of spreading fractured pieces with little content, the Ideas Roadshow is for those who are looking for content, and who want to know more than the catchy phrases. The magazine will be published in text and as video (streaming and downloadable).]

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A fairly common reaction when I tell people what I’m doing now with Ideas Roadshow is a quizzical raising of the eyebrows followed by a wry little smile.

“Well, good luck,” they say sceptically. “I certainly think that’s needed right now. But you know Howard, the internet is not exactly about substance. We live in a sound bite world. How do you think you’re going to make money from this? Who is going to watch it?”

So one of the few benefits about careening into advanced middle age is that I’ve witnessed enough by now to recognize that any references to recent golden ages are wildly exaggerated. I don’t remember being brought up in a world awash in substantive, measured discussions of the latest issues in neuroscience or public policy. My high school experience didn’t consist of teachers having to forcibly detach kids from their iPhones, but there was no shortage of ways for us to waste our hours and avoid doing what we were supposed to: Donkey Kong manages to kill time just as well as Angry Birds.

That’s not to say that, by some objective measure, things aren’t getting worse. In some ways they certainly are.

It’s true that newspapers almost everywhere are in deep financial trouble and those that have managed to stay afloat are devoting increasingly less of their time and resources towards long-form analysis and more for mindless knee-jerk responses to the ever-increasing amount of “breaking news”.

But it’s also true that there are now far more effective and salubrious ways for a young ambitious musician to gain a popular audience than by being forced to cavort with sleazy record executives.

Technology, of course, is but a tool. That is so obvious as to border on the cliché. But that doesn’t mean that the message doesn’t sometimes get overlooked.

The notion that, somehow as a result of our developing technology, virtually nobody on planet Earth actually cares anymore about engaging in the world of ideas, is, of course, simply ludicrous. It can’t be true. And it bloody well isn’t.

What technology has done, however, is change the way that those who are interested interact with the world of ideas. In particular, one decidedly ironic effect of the internet has been to intellectually ghettoize people. So while it’s now trivial to meaningfully interact with like-minded people living on the other side of the world, it’s also the case that one is much less likely to be confronted with interesting and stimulating ideas outside of one’s own self-selected area of interest.

Often the most illuminating and stimulating experiences happen when we are forced to encounter people who hold radically different approaches or interests to our own. But the more we spend time with our like-minded friends, the less likelier such encounters are going to be.

This is the core issue. It has, of course, been commented on before. But somehow I don’t think it’s as appreciated as much as it should be.

Conventional newspapers are not collapsing because nobody cares about general ideas. Conventional newspapers are collapsing because their principal revenue stream – print advertising revenue – has dried up. Advertisers are naturally much keener to ensure that their message is being delivered to their particular target audience, which naturally argues for a segmented, specialized approach to sponsorship. Now that technology allows for detailed methods to precisely deliver content and measure its impact, advertisers are increasingly unwilling to participate in scattershot approaches that will clearly be hugely less efficient and effective.

All quite reasonable. But the solution for those who seek a general level of stimulation, for those who are keen to be at play in the world of ideas, is not to bemoan the logic of the marketplace or fall back on dreamy reminiscences of some mythical golden age, but to simply capitalize on the opportunities afforded.

Twenty years ago, or even ten, it would have been completely inconceivable to imagine creating a program where one travels the world and records substantial conversations with a diverse range of fascinating people. Camera technology would have made it prohibitively expensive to develop a professional-quality product; and even had that been somehow circumvented, it would have been virtually impossible to disseminate the results with anywhere near the range necessary to make it profitable.

People interested in ideas have always been a small minority, so to make it work one has to scale globally, or at least nationally. How could a private start-up even attempt such a thing? We’d have had to effectively take over a TV station. Inconceivable.

Recent technology has allowed both of these fundamental obstacles to be overcome. We can not only film for a fraction of the cost of ten years ago, we can also fit all of our cameras, lights and gear into two travelling cases that we can easily travel with anywhere. And once we’ve made our videos and eBooks, we can easily market them to ideas-oriented consumers worldwide.

Of course, just because structural impediments are eliminated, success is hardly guaranteed. One still has to make a product that people actually like. And then one has to establish a new brand and market it successfully.

But let’s be very clear: those are the issues. Not that we are all too superficial now. Or that nobody cares about ideas. That’s just silly.

Starting something new is always a challenge. But there are challenges and then there are challenges.

Being at the front end of a new wave of global niche market digital media products is one thing. But it’s not like some unknown guy trying to build a theoretical physics institute in the middle of nowhere from scratch.

Now that, surely, is impossible.

Dimensional Reduction

Dimensionally reduced scientist.

“Science is the only news,” Steward Brand wrote. My reading of this sentence is that science, the exploration of nature and natural law, is the ultimate source of inspiration. Developing a model and studying its properties can be like discovering a new world, and the discoveries that are the most fascinating are the ones that are surprising and unintuitive.

Probability amplitudes and wavefunctions are examples of such surprising and unintuitive properties, examples that are now a century old and that have changed the way we think about the world. Holography is a more recent example. And, gathering momentum in the quantum gravity community right now, is dimensional reduction.

Dimensional reduction means that on short distances the dimension of space-time decreases. To quantify what this means one has to be very careful with defining “dimension.”

The way we normally think about the dimension of space is to picture how lines spread out from a point. How quickly the lines dilute into their environment tells us something about the spheres we can draw around the point. The dimension of these spheres can be used to define the “Hausdorff dimension” of a space. The faster the lines dilute with distance, the larger the Hausdorff dimension.

The notion of dimension that is relevant for the effect of dimensional reduction is not the Hausdorff dimension, but instead the “spectral dimension.” The spectral dimension can be found by first getting rid of the Lorentzian signature and going to Euclidean space. And then to watch a random walker who starts at one point, and measure the probability for him to return to that point. The smaller the average return probability, the higher the probability he’ll get lost, and the higher the number of dimensions. One can define the spectral dimension from the average return probability.

Normally, for a flat, classical space, both notions of dimension are identical. However, there have been several approaches toward quantum geometry that found that the spectral dimension at short distances goes down from four to two. The return probability for short walks is larger than expected. One says that the spectral dimension “runs”, meaning it depends on the distance at which space-time is probed.

Surprising. Unintuitive.

This strange behavior was first found in Causal Dynamical Triangulations (hep-th/0505113), where one does a numerical simulation of an actual random walk in Euclidean space. But in other approaches one does not need a numerical simulation; it is possible to study the spectral dimension analytically as follows.

The behavior of the random walk is governed by a differential equation, the diffusion equation, in which there enters the metric of the background space-time. In approaches to quantum gravity in which the metric is quantized, it is then the expectation value of the operator that the metric has become which enters the diffusion equation. From the diffusion equation one calculates the return probability for the random walk.

This way, one can then infer the spectral dimension also in Asymptotically Safe Gravity (hep-th/0508202). Interestingly, one finds the same drop from four to two spectral dimensions. Yet another indication comes from Loop Quantum Gravity, where the scaling of the area operator with length changes at short distances. It is somewhat questionable whether the notion of a metric makes sense at all in this regime, but if one nevertheless constructs the diffusion equation from this scaling, one again finds that the spectral dimension drops from four to two (0812.2214). And Horava-Lifshitz gravity is maybe the best studied case where one finds dimensional reduction (0902.3657).

Surprising. Unintuitive. It is difficult to interpret this behavior. Maybe a good way to picture it, as Calcagni, Eichhorn and Saueressig suggested, is to think of the quantum fluctuations of space-time hindering a particle’s random walk and slowing it down. It wouldn’t have to be that way. Quantum fluctuations could also be kicking the particle around wildly, thus increasing the spectral dimension rather than decreasing it. But that’s not what the theory tells us. One shouldn’t take this picture too seriously though, because we’re talking about a random walk in Euclidean space, so it’s not an actual physical process.

It seems strange that such entirely different approaches to quantum gravity would share a behavior like this. Maybe our theories are trying to teach us a lesson about a very general property of quantum space-time. But then again, the spectral dimension does not say all that much about the theory. There are many different types of random walks that give rise to the same spectral dimension. And while these different approaches to quantum gravity share the same scaling behavior for the spectral dimension, they differ in the type of random walk that produces this scaling (1304.7247).

So far, this is an entirely theoretical observation. It is interesting to speculate whether one can find experimental evidence for this scaling behavior. In fact, this recent paper by Amelino-Camelia et al aims to “explore the cosmological implications” of running spectral dimensions. At least that is what the first sentence of the abstract says. If you read the second sentence though you’ll notice that what they actually explore are modified dispersion relations. And while modified dispersion relations lead to a running spectral dimension, the opposite is not necessarily the case. But is there any better indication for a topic being hot than that people use it in the first sentence of an abstract to draw the readers interest?

A star-rating for scientific news?

Garry Gutting's recent post What Do Scientific Studies Show? at the NYT blogs is utterly unremarkable. Or so I thought, being clearly biased because the guy is a professor of philosophy, and I - I'm at the other end of the circle. But then he puts forward a proposal I think is brilliant: A labeling system for scientific news "that made clear a given study’s place in the scientific process", ranging from the speculative idea and preliminary results all the way to established scientific theory.

I like the idea because it would be an easy way to solve a tension in science news, which is that what's new and exciting, and therefore likely to make headlines, is also often controversial and likely to be refuted later. The solution can't be to not report what's new and exciting, but to find a good way to make clear that, while interesting and promising, this isn't (yet) established scientific consensus.

23andMe has a star rating to indicate how reliable a correlation between a genetic sequence and certain traits/diseases is, based on what has been reported in the scientific literature. (See my earlier blogpost for screenshots showing how that looks like.) They have a white paper laying out the criteria for assessing the scientific status of these correlations. The 23andMe rating serves a similar purpose as the proposed rating for science news. It is handy as a quick orientation, and it is a guide for those who can't or don't want to dig into the scientific literature themselves. It doesn't tell you to disregard results with few stars, just to keep in mind that this might turn out to be a data glitch, and to enjoy or worry with caution.

I think that such a label indicating how established a scientific result or idea is would be easy to use. Writers could just assign it themselves with help from the researchers they have been in contact with while working on a piece. That might not always be very accurate, but undoubtedly bloggers would add their voice. There would most likely be a service popping up to aggregate all ratings on a given topic/press release (probably weighted by the source). I am guessing it would be pretty much self-organized because we're all so very used to these ratings for other purposes.

Do you think such a labeling would be helpful? If so, what criteria would you require for zero to five stars?

Basic research is vital

Last month I had the flu. I was down with a fever of more than 40°C, four days in a row. Needless to say, it was a public holiday.

While the body is struggling to recover from illness, priorities shift. Survive first. Drink. Eat. Stand upright without fainting. Feed the kids because they can’t do it themselves. Two days earlier, I was thinking of running a half-marathon, now happy to make it to the bathroom. Forgotten the parking ticket and the tax return.

We see the same shift of priorities on other levels of our societies. If a system, may that be an organism or a group of people, experiences a potential threat to existence, energy is redirected to the essential needs, to survival first. An unexpected death in the family requires time for recovery and reorganization. A nation that is being attacked redirects resources to the military.

The human body’s defense against viruses does not require conscious control. It executes a program that millions of years of evolution have optimized, a program we can support with medication and nutrition. But when it comes to priorities of our nations, we have no program to follow. We first have to decide what is necessary for survival, and what can be put on hold while we recover.

The last years have not been good years economically, neither in the European Union, nor in North America. We all feel the pressure. We’re forced to focus our priorities. And every week I read a new article about cuts in some research budget.

“Europe's leaders slash proposed research budget,” I read. “Big cuts to R&D budgets [in the UK],” I read. “More than 50 Nobel laureates are urging [the US] Congress to spare the federal science establishment from the looming budget cut,” I read.

An organism befallen by illness manages a shortage of energy. A nation under economic pressure manages a shortage of money. But money is only the tool for the management. And it is a complicated tool, its value influenced by many factors including psychological, and it is not just under national management. In the end, its purpose is to direct labor. And here is the real energy of our nations: Humans, working. It is the amounts of working hours in different professions that budget cuts manage.

In reaction to a perceived threat, nations shift priorities and redirect human labor. They might aim at sustainability. At independence from oil imports. They invest in public health. Or they cut back on these investments. When the pressure raises, what is left will be the essentials. Energy and food, housing and safety. Decisions have to be made. The people who assemble weapons are not available to water the fields.

How vital is science?

We all know that progress depends on scientific research. Somebody has to develop new technologies. Somebody has to test whether they are safe to use. Everybody understands what applied science does: In goes brain, out comes what you’ll smear into your face or wear on your nose tomorrow.

But not everybody understands that this isn’t all of science. Besides the output-oriented research, there is the research that is not conducted with the aim of developing new technologies. It is curiosity-driven. It follows the loose ends of today's theories, it aims to understand the puzzle that is the data. Most scientists call it basic or fundamental research. The NSF calls it transformative research, the ERC frontier research. Sometimes I’ve heard the expression blue-skies research. Whatever the name, its defining property is that you don’t know the result before you’ve done the research.

Since many people do not understand what fundamental research is or why it is necessary, if science funding is cut, basic research suffers most. Politicians lack the proper words to justify investment into something that doesn’t seem to have any tangible outcome. Something that, it seems, just pleases the curiosity of academics. “The question is academic,” has to come to mean “The world doesn’t care about its answer.”

A truly shocking recent example comes from Canada:

“Scientific discovery is not valuable unless it has commercial value," John McDougall, president of the [Canadian National Research Council], said in announcing the shift in the NRC's research focus away from discovery science solely to research the government deems "commercially viable". [Source: Toronto Sun] [Update: He didn't literally say this as the Sun quoted it, see here for the correct quote.]

Oh, Canada. (Also: Could somebody boot the guy, he’s in the wrong profession.)

Do they not understand how vital basic research is for their nation? Or do they decide not to raise the point? I suspect that at least some of those involved in such a decision approve cutting back on basic research not because they don’t understand what it’s good for, but because they believe their people don’t understand what it’s good for. (And they would be wrong, if you scroll down and look at the poll results...)

I suspect that scientists are an easy target, they usually don’t offer much resistance. They're not organized, for not to say disorganized. Scientists will try to cope until it becomes impossible and then pack their bags and their families and move to elsewhere. And once they’re gone, Canada, you’ll have to invest much more money than you save now to get them back.

Do they really not know that basic research, in one sentence, is the applied research in 100 years?

It isn’t possible, in basic research, to formulate a commercial application as goal because nobody can make predictions or formulate research plans over 100 years. There are too many unknown unknowns, the system is too complex, there are too many independent knowledge seekers in the game. Nobody can tell reliably what is going to happen.

They say “commercially viable”, but what they actually mean is “commercially viable within 5 years”.

The scientific theories that modern technology and medicine are based on – from LCD displays over DVD-players to spectroscopy and magnetic resonance imaging, from laser surgery to quantum computers – none of them would exist had scientists pursued “commercial viability”. Without curiosity-driven research, we deliberately ignore paths to new areas of knowledge. Applied research will inevitably run dry sooner or later. Scientific progress is not sustainable without basic research.

As your mother told you, if you have a fever, watch your fluid intake. Even if you are tired and don’t feel like moving a finger, drink that glass of water. The woman with the flu who didn’t drink enough today is the woman in the hospital on an IV-drip tomorrow. And the nation under economic pressure who didn’t invest in basic research today is the nation that will wish there was a global IV-drop for their artery tomorrow.

And here’s some other people saying the same thing in less words [via Steve Hsu]:



I know that on this blog a post like this preaches to the choir. So today I have homework for you. Tell your friends and your neighbors and the other parents at the daycare place. Tell them what basic research is and why it’s vital. And if you don’t feel like talking, send them a link or show them a video.

What do "most physicists" work on?

It always amazes me how skewed the image of physics research in the popular press is. To begin with, the amount of coverage is totally unrepresentative for the actual amount of research on a given topic. Controversial and outright fantastic topics are typically hotly discussed, so is everything that captures the public imagination. On the other hand, down-to-earth research like soft condensed matter or statistical mechanics rarely makes headlines.

The field I work in myself, quantum gravity, is among the over-represented fields. If you believe what you read, the quest for quantum gravity has become the "holy grail" of theoretical physicists all over the planet, and we're all working on it because the end of science is near and there's nothing else left to do.

Since coverage by the media is driven by popularity and not by relevance, one can expect such a skewed representation. It probably isn't much different in other areas of our lives. (Who actually wears those wacky clothes that fashion designers celebrate?) What bothers me much more than the skewed selection of topics is how their relevance is misrepresented even in these articles. I must have read hundreds of times that "many physicists" believe this or that, while in reality most physicists couldn't care less and probably have no opinion whatsoever.

Here are some examples:

"According to the current thinking of many physicists, we are living in one of a vast number of universes. We are living in an accidental universe. We are living in a universe uncalculable by science."

Alan Lightman, The Accidental Universe.

"The team’s verdict, published in July 2012, shocked the physics community."

Zeeya Merali, in a recent nature issue, Astrophysics: Fire in the hole!. We note in the passing that the article doesn't have much, if anything, to do with astrophysics.

"Most physicists believe that space is not smooth, but it is rather composed of incredibly small subunits, much like a painting made of dots. This micro-landscape is believed to host numerous black holes..."

Mihai Andrei, in an article titled Finding black holes at a quantum scale about a deeply flawed paper by Jacob Bekenstein. (Which, depressingly, got published in PRD.)

But why limit ourselves to physicists, let's be bold:

"Many scientists claim that mega-millions of other universes, each with its own laws of physics, lie out there, beyond our visual horizon. They are collectively known as the multiverse."

George F. R. Ellis, Scientific American, Does the Multiverse Really Exist? "They" presumably refers to the "other universes," and not to the "many scientists".

So then let's try to quantify "most physicists" by estimating an upper bound on the fraction of physicists who are working on these topics, a sub-area of quantum gravity. The topics under question here tend to appear on the arXiv under hep-th cross-linked to gr-qc or the other way round. That there is no subject category for "quantum gravity" should already tell you that there aren't all that "many" people working on it. First let us have a look at the arXiv submission rates

The left graph shows the total number of submissions, the right shows the percentage. Blue, which presently accounts for about 10%, is high energy physics and collects hep-th+hep-ph+hep-lat+hep-ex. Note that for historical reasons hep is likely to be over-represented in the arXiv statistics relative to the actual distribution of researchers. In hep, pretty much every paper goes on the arxiv, but the same is not true in other areas (at least not yet). Also, hep tends to be a very productive and communicative field, so looking at the number of arXiv submissions rather than researchers is probably an over-estimate. Be that as it may, the topics we are looking for almost certainly occupy less than 10% of researchers.

More data that tells you that the vast number of physicists aren't working on anything related to quantum gravity can be obtained from the number of members in sections of the German Physical Society. The section on Particle Physics (which includes beyond the standard model physics and quantum gravity) has about 2,500 members. The section on Quantum Optics and Photonics has more than 3,000 members, Physics of Semi-conductors 3,800, Low Temperature Physics 1,450, Atomic Physics together with Hadronic and Nuclear Physics come to about 3,000, Material Physics together with Chemical and Polymer Physics and Thin Films another 3,500. Not all sections have membership numbers online, so this doesn't cover the full spectrum. But this already tells you that "most physicists" don't even do high energy physics, certainly not quantum gravity, and have no business with multiverses, firewalls, or "micro-landscapes of black holes".

But we can try to get a better estimate by seeing how many papers are cross-linked from hep-th to gr-qc, assuming that the opposite cross-linking is similarly frequent. For this, we look at the submission statistics of gr-qc for the first four months of the year 2013. It lists the submissions as well as the cross-lists. Click on any of the months, select "show all" and count the number of times "cross-list from hep-th" appears on the page. The numbers I get for January to April are: 70,71,52 and 67. If you look at the titles, you'll note that the papers you find this way fit well to the topics we're looking for.

Comparing these numbers with the total arxiv submissions per month (about 7500), we can estimate that it's about 1%. Multiply by two to account for gr-qc cross-linked to hep-th.

Now this is a rather crude estimate and I have mentioned several reasons why it's inaccurate: 1) Some fields of research are not as well represented on the arXiv as is hep-th. This means 2% is still an over-estimate. 2) Some fields might be more productive in paper output than others. If hep-th is on the more productive side, this means the 2% is even more of an over-estimate. 3) Not every paper in the area we're looking for might be hep-th cross-linked to gr-qc or vice-versa. This leads to an under-estimate. 4) On the other hand, not every paper cross-listed as such is about quantum gravity or related topics. 5) There are probably more people following the literature than actively working on it, which also leads to an under-estimate.

However, even if you'd add up all these errors, you would still be left to conclude that the above quoted uses of "most physicists" or "physics community" are extremely inaccurate and misleading.