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.

What is a microfiber cloth and how does it work?

Microfibre cloths have become really popular during the last years. I just got one as an advertisement gift from the phone company. They’re handy to clean glasses, all kinds of screens, windows, mirrors and plastic surfaces, quickly and without the use of water. If dirty, put the cloth in the laundry, add detergent, and they’re as good as new.

But what are microfibers and what can they do that a Kleenex can’t?

Cotton cloths or paper wipes are mostly made of cellulose. Cellulose is a polymer, a long molecule that repeats a shorter structure up to some thousand times. Cellulose is hydrophile, meaning it likes to bind to water molecules. What it doesn’t like though is binding to fat molecules, which are themselves hydrophobic and don’t like to bind to water.

Cotton and paper tissues thus work badly for removing fatty stains, such as finger prints from glasses. If you want to get rid of these you have to use water and detergents on the cloth. Detergents are made of molecules that allow mixing water with fatty substances. With the detergent, the wet cotton cloth does a good job with the grease. Except that then it takes a long time to dry because now all the water molecules are attached to the cellulose polymers.

Cross-section of single microfibre,
electron microscope. Image Source.

Microfibers are also polymers, but that’s where the similarities end. Microfibers are synthetic polymers and usually much longer than the cellulose fibers won from organic materials. They are also about an order of magnitude thinner, typically only a few micrometers.

The microfibres used for cleaning cloths are normally a mixture of polyester and polyamide. Polyesters like to bind to fat, which is why the cloths can be used to wipe away grease without adding detergents. Polyesters however don’t like to bind water. Some polyamides do, but the water absorption of the microfibre cloths comes mostly from a clever production technique that increases the surface area of the fibres and allows for capillary action to suck up the water.

This technique works as follows. The long microfibers are not produced separately, but in a mixture of polyesters and polyamides that are arranged as alternating wedges, much like pieces on a cake. These mixed fibres are later split up by high pressure water jets (the image above shows the result). This procedure allows to produce much finer fibres than would be possible to produce directly, and since the microfibers are thin to begin with, it creates very porose materials that have a large surface in a small volume.

Cross-section of microfibre cloth, electron
microscope image. Source: hotrodworks.net

These splitted fibres are then woven or pressed into textiles (see image right). The resulting cloth is lightweight and binds to fat so you can wipe those fingerprints away easily. The material sucks up water, but since most of the water is stored in the pores between the fibres rather than binding directly to them (as is the case with cotton), microfibre cloths dry much faster than cotton.

Microfibres are not a new invention. The production technique goes back to research in the 1950s, but it wasn’t until the early 90s that they were marketed to households, a trend apparently started by the Swedes. During the last decade or so, microfibers have become quite common, especially for cleaning purposes, and, because they dry quickly, for sport and outdoor clothes.

So the next time you wipe the earwax off your display, I hope you appreciate the science behind this not-so-simple cloth.

Interna

Lara, putting on her shoes.

May 1st is a national holiday both in Sweden and in Germany. A good opportunity, I thought, to update you on our attempts at normal family life.

Lara and Gloria are now talking basically non-stop. Half of the time we have no idea what they are trying to say, the other half are refusals. Gloria literally wakes up in the morning yelling "Nein-nein-nein". Saying it's difficult to get her dressed, fed, and to daycare makes quantizing gravity sound like an easy task. Yesterday she insisted on going in her pajamas. Good mother that I am, I thought that was a brilliant idea.

Gloria is proud of her new hat.

Lara isn't quite as difficult as Gloria, but she is very easily distracted. If I ask her to get into the stroller, she'll first spend five minutes inspecting the stones by the road or take off her shoes and put them back on, just because.Time clearly flows very differently when you're two years old than when you're forty. I try to use the occasions to check my email. Time flows through my iPhone, I'm sure it does.

We finally made progress on our daycare issue, which is presently only half a solution. A new daycare place opened in the area, and due to my time spent on the phone last year, asking people to please write down my name and call me back if the situation unexpectedly changes, somebody indeed recalled my name and we made it top of the list for the new place. So there'll be another adaption phase at another place, but this time it's a full-day care that will indeed cover our working hours. It is also, I should add, considerably less expensive than the present solution with a self-employed nanny. This, I hope, will make my commuting easier for Stefan to cope with.

I'm really excited about the workshop for science writers that I'm organizing with George. We now have an (almost) complete schedule, I've ordered food and drinks and sorted out the lab visit, and I'm very much looking forward to the meeting. Directly after this workshop, I'll attend another workshop in Munich, "Quantum Gravity in Perspective", where I'll be speaking about the phenomenology of quantum gravity. I have some more trips upcoming this summer, to Bielefeld and Aachen and, in fall, to Vienna to speak at a conference on "Emergent Quantum Mechanics."

I was invited to take part in this KITP workshop on black hole firewalls but I eventually decided not to go. Partly because I'm trying to keep my travels limited to not burden Stefan too much with the childcare. But primarily because I don't believe that anything insightful will come out of this debate. It seems to me there are more fruitful research topics to explore, and this discussion is a waste of time. I also never liked SoCal in late summer; too dry for my central-European genes.

Lara and Gloria, eating cookies at a visit to the zoo.

We'll be away for the next couple of days because Stefan's brother is getting married. This means a several-hours long road trip with two toddlers who don't want to sit still for a minute; we're all looking forward to it...

Book review: “Time Reborn” by Lee Smolin

Time Reborn: From the Crisis in Physics to the Future of the Universe
By Lee Smolin
Houghton Mifflin Harcourt (April 23, 2013)

This is a difficult review for me to write because I disagree with pretty much everything in Lee’s new book “Time Reborn,” except possibly the page numbers. To begin with there is no “Crisis in Physics” as the subtitle suggests. But then I’ve learned not to blame authors for title and subtitles.

Oddly enough however, I enjoyed reading the book. Not despite, but because I had something to complain about on every page. It made me question my opinions, and though I came out holding on to them, I learned quite something on the way.

In “Time Reborn” Lee takes on the seemingly puzzling fact that mathematical truth is eternal and timeless, while the world that physicists are trying to describe with that mathematics isn’t. The role of time in contemporary physics is an interesting topic, and gives opportunity to explain our present understanding of space and time, from Newton over Special and General Relativity to modern Cosmology, Quantum Mechanics and all the way to existing approaches to Quantum Gravity.

Lee argues that our present procedures must fail when we attempt to apply them to describe the whole universe. They fail because we’re presently treating the passing of time as emergent, but as emergent in a fundamentally timeless universe. Only if we abandon the conviction, held by the vast majority of physicists, that this is the correct procedure, then can we understand the fundamental nature of reality – and with it quantum gravity of course. Lee further summarizes a few recent developments that treat time as real, though the picture he presents remains incoherent, some loosely connected, maybe promising, recent ideas that you can find on the arXiv and I don’t want to promote here.

More interesting for me is that Lee doesn’t stop at quantum gravity, which for most people on the planet arguably does not rank very high among the pressing problems. Thinking about nature as fundamentally timeless, Lee argues, is cause of very worldly problems that we can only overcome if we believe that we ourselves are able to create the future:

“We need to see everything in nature, including ourselves and our technologies, as time-bound and part of a larger, ever evolving system. A world without time is a world with a fixed set of possibilities that cannot be transcended. If, on the other hand, time is real and everything is subject to it, then there is no fixed set of possibilities and no obstacle to the invention of genuinely novel ideas and solutions to it.”

I’ll leave my objections to Lee’s arguments for some other time. For now, let me just say that I explained in this earlier post that a deterministic time evolution doesn’t relieve us from making decisions, and it doesn’t prevent “genuinely novel ideas” in any sensible definition of the phrase.

In summary: Lee’s book is very thought provoking and it takes the reader on a trip through the most fundamental questions about nature. The book is well written and nicely embedded in the long history of mankind’s wonderment about the passing of time and the circle of life. You will almost certainly enjoy this book if you want to know what contemporary physics has to say, and not to say, about the nature of time. You will almost certainly hate this book if you're a string theorist, but then you already knew that.

The Enantiomers’ Swimming Competition

Image Source.

The spatial arrangement of some large molecules can exist in two different versions which are mirror images of each other, yet their chemical composition is entirely identical. These mirror versions of molecules are said to have a different “chirality” and are called “enantiomers.” The image to the right shows the two chiralities of alanine, known as L-alanine and D-alanine.

Many chemical reactions depend not only on the atomic composition of molecules but also on their spatial arrangement, and thus enantiomers can have very different chemical behaviors. Since organisms are not chirally neutral, medical properties of drugs made from enantiomers depend on which chirality of the active ingredient is present. One enantiomer might have a beneficial effect, while the other one is harmful. This is the case for example for Ethambutol (one enantiomer treats tuberculosis, the other causes blindness), or Naproxen (one enantiomer treats arthritis pain, the other causes liver poisoning).

The chemical synthesis of molecules however typically produces molecules of both chiralities in approximately equal amounts, which creates the need to separate them. One way to do this is to use chemical reactions that are sensitive to the molecules’ chirality. Such a procedure has the disadvantage though that it is specific to one particular molecule and cannot be used for any other.

Now three physicists have shown, by experimental and numerical analysis, that there may be a universal way to separate enantiomers

Separation of chiral colloidal particles in a helical flow field
Maria Aristov, Ralf Eichhorn, and Clemens Bechinger
Soft Matter, 2013,9, 2525-2530 

It’s strikingly simple: chiral particles swim differently in a stream of water that has a swirl to it. How fast they travel with the stream depends on whether their chirality is the same or the opposite of the water swirl’s orientation. Wait far enough downstream, and the particles that arrive first will almost exclusively be the ones whose chirality matches that of the water swirl.

They have shown this as follows.

Molecules are typically of the size of some nanometers or so, and the swimming performance for molecules of different chirality is difficult to observe. Instead, the authors used micrometer-sized three-dimensional particles made of a type of polymer (called SU-8) by a process called photolithography. The particles created this way are the simplest example of configurations of different chirality. They labeled the right-handed particles with a blue fluorescent dye, and the left-handed particles with a green fluorescent dye. This allows taking images of them by a fluorescent microscope. Below you see a microscope image of the particles

Next you need a narrow channel through which water flows under some pressure. The swirl is created by gratings in the wall of the channel. The length of this channel is about a meter, but its height and width is only of the order 150 μm. Then you let bunches of the mixed chiral particles flow through the channel and photograph them on a handful of locations. From the amount of blue and green that you see in the image, you can tell how many of each type were present at a given time. Here’s what they see (click to enlarge)

This figure is an overlay of measurements at 5 different locations as a function of time (in seconds). The green shade is for molecules with the chirality that matches the water swirl orientation, the blue shade is for those with the opposite chirality. They start out, at x=32.5mm, in almost identical concentration. Then they begin to run apart. Look at the left tail of the x=942.5 mm measurement. The green distribution is almost 200 seconds ahead of the blue one.

If you aren’t impressed by this experiment, let me show you the numerical results. They modeled the particles as rigidly coupled spheres in a flow field with friction and torque, added some Gaussian white noise, and integrated the equations. Below is the result of the numerical computation for 1000 realizations (click to enlarge)

I am seriously amazed how well the numerical results agree with the experiment! I’d have expected hydrodynamics to be much messier.

The merit of the numerical analysis is that it provides us with understanding of why this separation is happening. Due to the interaction of the fluid with the channel walls, the flow is slower towards the walls than in the middle. The particles are trying to minimize their frictional losses with the fluid, and how to best achieve this depends on their chirality relative to the swirl of the fluid. The particles whose chirality is aligned with the swirl preferably move towards the middle where the flow is faster, while the particles of the opposite chirality move towards the channel walls where the flow is slower. This is what causes them to travel at different average velocities.

This leaves the question whether this study of particles of micrometer size can be scaled down to molecules of nanometer size. To address this question, the authors demonstrate with another numerical simulation that the efficiency of the separation (the amount of delay) depends on the product of the length of the channel and the velocity of the fluid, divided by the particle’s diffusion coefficient in the fluid. This allows one to estimate what is required for smaller particles. If this scaling holds, particles of about 120 nm size could be separated in a channel of about 3cm length and 3.2 μm diameter, at a pressure of about 10⁸ Pa, which is possible with presently existing technology.

Soft matter is not anywhere near by my area of research, so it is hard for me to tell whether there are effects at scales of some hundred nanometers that might become relevant and spoil this simple scaling, or whether more complicated molecule configurations alter the behavior in the fluid. But if not, this seems to me a tremendously useful result with important applications.