Sunday, July 24, 2016

Can we please agree what we mean by “Big Bang”?

Can you answer the following question?

At the Big Bang the observable universe had the size of:
    A) A point (no size).
    B) A grapefruit.
    C) 168 meters.

The right answer would be “all of the above.” And that’s not because I can’t tell a point from a grapefruit, it’s because physicists can’t agree what they mean by Big Bang!

For someone in quantum gravity, the Big Bang is the initial singularity that occurs in General Relativity when the current expansion of the universe is extrapolated back to the beginning of time. At the Big Bang, then, the universe had size zero and an infinite energy density. Nobody believes this to be a physically meaningful event. We interpret it as a mathematical artifact which merely signals the breakdown of General Relativity.

If you ask a particle physicist, they’ll therefore sensibly put the Big Bang at the time where the density of matter was at the Planck scale – about 80 orders of magnitude higher than the density of a neutron star. That’s where General Relativity breaks down; it doesn’t make sense to extrapolate back farther than this. At this Big Bang, space and time were subject to significant quantum fluctuations and it’s questionable that even speaking of size makes sense, since that would require a well-defined notion of distance.

Cosmologists tend to be even more conservative. The currently most widely used model for the evolution of the universe posits that briefly after the Planck epoch an exponential expansion, known as inflation, took place. At the end of inflation, so the assumption, the energy of the field which drives the exponential expansion is dumped into particles of the standard model. Cosmologists like to put the Big Bang at the end of inflation because inflation itself hasn’t been observationally confirmed. But they can’t agree how long inflation lasted, and so the estimates for the size of the universe range between a grapefruit and a football field.

Finally, if you ask someone in science communication, they’ll throw up their hands in despair and then explain that the Big Bang isn’t an event but a theory for the evolution of the universe. Wikipedia engages in the same obfuscation – if you look up “Big Bang” you get instead an explanation for “Big Bang theory,” leaving you to wonder what it’s a theory of.

I admit it’s not a problem that bugs physicists a lot because they don’t normally debate the meaning of words. They’ll write down whatever equations they use, and this prevents further verbal confusion. Of course the rest of the world should also work this way, by first writing down definitions before entering unnecessary arguments.

While I am waiting for mathematical enlightment to catch on, I find this state of affairs terribly annoying. I recently had an argument on twitter about whether or not the LHC “recreates the Big Bang,” as the popular press likes to claim. It doesn’t. But it’s hard to make a point if no two references agree on what the Big Bang is to begin with, not to mention that it was neither big nor did it bang. If biologists adopted physicists standards, they’d refer to infants as blastocysts, and if you complained about it they’d explain both are phases of pregnancy theory.

I find this nomenclature unfortunate because it raises the impression we understand far less about the early universe than we do. If physicists can’t agree whether the universe at the Big Bang had the size of the White House or of a point, would you give them 5 billion dollars to slam things into each other? Maybe they’ll accidentally open a portal to a parallel universe where the US Presidential candidates are Donald Duck and Brigitta MacBridge.

Historically, the term “Big Bang” was coined by Fred Hoyle, a staunch believer in steady state cosmology. He used the phrase to make fun of Lemaitre, who, in 1927, had found a solution to Einstein’s field equations according to which the universe wasn’t eternally constant in time. Lemaitre showed, for the first time, that matter caused space to expand, which implied that the universe must have had an initial moment from which it started expanding. They didn’t then worry about exactly when the Big Bang would have been – back then they worried whether cosmology was science at all.

But we’re not in the 1940s any more, and precise science deserves precise terminology. Maybe we should rename the different stages of the universe that into “Big Bang,” “Big Bing” and “Big Bong.” This idea has much potential by allowing further refinement to “Big Bång,” “Big Bîng” or “Big Böng.” I’m sure Hoyle would approve. Then he would laugh and quote Niels Bohr, “Never express yourself more clearly than you are able to think.”

You can count me to the Planck epoch camp.

Monday, July 18, 2016

Can black holes tunnel to white holes?

Tl;dr: Yes, but it’s unlikely.

If black holes attract your attention, white holes might blow your mind.

A white hole is a time-reversed black hole, an anti-collapse. While a black hole contains a region from which nothing can escape, a white hole contains a region to which nothing can fall in. Since the time-reversal of a solution of General Relativity is another solution, we know that white holes exist mathematically. But are they real?

Black holes were originally believed to merely be of mathematical interest, solutions that exist but cannot come into being in the natural world. As physicists understood more about General Relativity, however, the exact opposite turned out to be the case: It is hard to avoid black holes. They generically form from matter that collapses under its own gravitational pull. Today it is widely accepted that the black hole solutions of General Relativity describe to high accuracy astrophysical objects which we observe in the real universe.

The simplest black hole solutions in General Relativity are the Schwarzschild-solutions, or their generalizations to rotating and electrically charged black holes. These solutions however are not physically realistic because they are entirely time-independent, which means such black holes must have existed forever. Schwarzschild black holes, since they are time-reversal invariant, also necessarily come together with a white hole. Realistic black holes, on the contrary, which are formed from collapsing matter, do not have to be paired with white holes.

(Aside: Karl Schwarzschild was German. Schwarz means black, Schild means shield. Probably a family crest. It’s got nothing to do with children.)

But there are many things we don’t understand about black holes, most prominently how they handle information of the matter that falls in. Solving the black hole information loss problem requires that information finds a way out of the black hole, and this could be done for example by flipping a black hole over to a white hole. In this case the collapse would not complete, and instead the black hole would burst, releasing all that it had previously swallowed.

It’s an intriguing and simple option. This black-to-white-hole transition has been discussed in the literature for some while, recently by Rovelli and Vidotto in the Planck star idea. It’s also subject of a last week’s paper by Barcelo and Carballo-Rubio.

Is this a plausible solution to the black hole information loss problem?

It is certainly possible to join part of the black hole solution with part of the white hole solution. But doing this brings some problems.

The first problem is that at the junction the matter must get a kick that transfers it from one state into the other. This kick cannot be achieved by any known physics – we know this from the singularity theorems. There isn’t anything in the known physics can prevent a black hole from collapsing entirely once the horizon is formed. Whatever makes this kick hence needs to violate one of the energy conditions, it must be new physics.

Something like this could happen in a region with quantum gravitational effects. But this region is normally confined to deep inside the black hole. A transition to a white hole could therefore happen, but only if the black hole is very small, for example because it has evaporated for a long time.

But this isn’t the only problem.

Before we think about the stability of black holes, let us think about a simpler question. Why doesn’t dough unmix into eggs and flour and sugar neatly separated? Because that would require an entropy decrease. The unmixing can happen, but it’s exceedingly unlikely, hence we never see it.

A black hole too has entropy. It has indeed enormous entropy. It saturates the possible entropy that can be contained within a closed surface. If matter collapses to a black hole, that’s a very likely process to happen. Consequently, if you time-reverse this collapse, you get an exceedingly unlikely process. This solution exists, but it’s not going to happen unless the black hole is extremely tiny, close by the Planck scale.

It is possible that the white hole which a black hole supposedly turns into is not the exact time-reverse, but instead another solution that further increases entropy. But in that case I don’t know where this solution comes from. And even so I would suspect that the kick required at the junction must be extremely finetuned. And either way, it’s not a problem I’ve seen addressed in the literature. (If anybody knows a reference, please let me know.)

In a paper written for the 2016 Awards for Essays on Gravitation, Haggard and Rovelli make an argument in favor of their idea, but instead they just highlight the problem with it. They claim that small quantum fluctuations around the semi-classical limit which is General Relativity can add up over time, eventually resulting in large deviations. Yes, this can happen. But the probability that this happens is tiny, otherwise the semi-classical limit wouldn’t be the semi-classical limit.

The most likely thing to happen instead is that quantum fluctuations average out to give back the semi-classical limit. Hence, no white-hole transition. For the black-to-white-hole transition one would need quantum fluctuations to conspire together in just the right way. That’s possible. But it’s exceedingly unlikely.

In the other recent paper the authors find a surprisingly large transition rate for black to white holes. But they use a highly symmetrized configuration with very few degrees of freedom. This must vastly overestimate the probability for transition. It’s an interesting mathematical example, but it has very little to do with real black holes out there.

In summary: That black holes transition to white holes and in this way release information is an idea appealing because of its simplicity. But I remain unconvinced because I am missing a good argument demonstrating that such a process is likely to happen.

Tuesday, July 12, 2016

Pulsars could probe black hole horizons

The first antenna of MeerKAT,
a SKA precursor in South Africa.
[Image Source.]

It’s hard to see black holes – after all, their defining feature is that they swallow light. But it’s also hard to discourage scientists from trying to shed light on mysteries. In a recent paper, a group of researchers from Long Island University and Virginia Tech have proposed a new way to probe the near-horizon region of black holes and, potentially, quantum gravitational effects.

    Shining Light on Quantum Gravity with Pulsar-Black Hole Binaries
    John Estes, Michael Kavic, Matthew Lippert, John H. Simonetti
    arXiv:1607.00018 [hep-th]

The idea is simple and yet promising: Search for a binary system in which a pulsar and a black hole orbit around each other, then analyze the pulsar signal for unusual fluctuations.

A pulsar is a rapidly rotating neutron star that emits a focused beam of electromagnetic radiation. This beam goes into the direction of the poles of the magnetic field, and is normally not aligned with the neutron star’s axis of rotation. The beam therefore spins with a regular period like a lighthouse beacon. If Earth is located within the beam’s reach, our telescopes receive a pulse every time the beam points into our direction.

Pulsar timing can be extremely precise. We know some pulsars that have been flashing for decades every couple of milliseconds to a precision of a few microseconds. This high regularity allows astrophysicists to search for signals which might affect the timing. Fluctuations of space-time itself, for example, would increase the pulsar-timing uncertainty, a method that has been used to derive constraints on the stochastic gravitational wave background. And if a pulsar is in a binary system with a black hole, the pulsar’s signal might scrape by the black hole and thus encode information about the horizon which we can catch on Earth.

No such pulsar-black hole binaries are known to date. But upcoming experiments like eLISA and the Square Kilometer Array (SKA) will almost certainly detect new pulsars. In their paper, the authors estimate that SKA might observe up to 100 new pulsar-black hole binaries, and they put the probability that a newly discovered system would have a suitable orientation at roughly one in a hundred. If they are right, the SKA would have a good chance to find a promising binary.

Much of the paper is dedicated to arguing that the timing accuracy of such a binary pulsar could carry information about quantum gravitational effects. This is not impossible but speculative. Quantum gravitational effects are normally expect to be strong towards the black hole singularity, ie well inside the black hole and hidden from observation. Naïve dimensional estimates reveal that quantum gravity should be unobservably small in the horizon area.

However, this argument has recently been questioned in the aftermath of the firewall controversy surrounding black holes, because one solution to the black hole firewall paradox is that quantum gravitational effects can stretch over much longer distances than the dimensional estimates lead one to expect. Steve Giddings has long been a proponent of such long-distance fluctuations, and scenarios like black hole fuzzballs, or Dvali’s Bose-Einstein Computers also lead to horizon-scale deviations from general relativity. It is hence something that one should definitely look for.

Previous proposals to test the near-horizon geometry were based on measurements of gravitational waves from merger events or the black hole shadow, each of which could reveal deviations from general relativity. However, so far these were quite general ideas lacking quantitative estimates. To my knowledge, this paper is the first to demonstrate that it’s technologically feasible.

Michael Kavic, one of the authors of this paper, will attend our September conference on “Experimental Search for Quantum Gravity.” We’re still planning to life-streaming the talks, so stay tuned and you’ll get a chance to listen in.

Monday, July 04, 2016

Why the LHC is such a disappointment: A delusion by name “naturalness”

Naturalness, according to physicists.

Before the LHC turned on, theoretical physicists had high hopes the collisions would reveal new physics besides the Higgs. The chances of that happening get smaller by the day. The possibility still exists, but the absence of new physics so far has already taught us an important lesson: Nature isn’t natural. At least not according to theoretical physicists.

The reason that many in the community expected new physics at the LHC was the criterion of naturalness. Naturalness, in general, is the requirement that a theory should not contain dimensionless numbers that are either very large or very small. If that is so, then theorists will complain the numbers are “finetuned” and regard the theory as contrived and hand-made, not to say ugly.

Technical naturalness (originally proposed by ‘t Hooft) is a formalized version of naturalness which is applied in the context of effective field theories in particular. Since you can convert any number much larger than one into a number much smaller than one by taking its inverse, it’s sufficient to consider small numbers in the following. A theory is technically natural if all suspiciously small numbers are protected by a symmetry. The standard model is technically natural, except for the mass of the Higgs.

The Higgs is the only (fundamental) scalar we know and, unlike all the other particles, its mass receives quantum corrections of the order of the cutoff of the theory. The cutoff is assumed to be close by the Planck energy – that means the estimated mass is 15 orders of magnitude larger than the observed mass. This too-large mass of the Higgs could be remedied simply by subtracting a similarly large term. This term however would have to be delicately chosen so that it almost, but not exactly, cancels the huge Planck-scale contribution. It would hence require finetuning.

In the framework of effective field theories, a theory that is not natural is one that requires a lot of finetuning at high energies to get the theory at low energies to work out correctly. The degree of finetuning can, and has been, quantified in various measures of naturalness. Finetuning is thought of as unacceptable because the theory at high energy is presumed to be more fundamental. The physics we find at low energies, so the argument, should not be highly sensitive to the choice we make for that more fundamental theory.

Until a few years ago, most high energy particle theorists therefore would have told you that the apparent need to finetuning the Higgs mass means that new physics must appear nearby the energy scale where the Higgs will be produced. The new physics, for example supersymmetry, would avoid the finetuning.

There’s a standard tale they have about the use of naturalness arguments, which goes somewhat like this:

1) The electron mass isn’t natural in classical electrodynamics, and if one wants to avoid finetuning this means new physics has to appear at around 70 MeV. Indeed, new physics appears even earlier in form of the positron, rendering the electron mass technically natural.

2) The difference between the masses of the neutral and charged pion is not natural because it’s suspiciously small. To prevent fine-tuning one estimates new physics must appear around 700 MeV, and indeed it shows up in form of the rho meson.

3) The lack of flavor changing neutral currents in the standard model means that a parameter which could a priori have been anything must be very small. To avoid fine-tuning, the existence of the charm quark is required. And indeed, the charm quark shows up in the estimated energy range.

From these three examples only the last one was an actual prediction (Glashow, Iliopoulos, and Maiani, 1970). To my knowledge this is the only prediction that technical naturalness has ever given rise to – the other two examples are post-dictions.

Not exactly a great score card.

But well, given that the standard model – in hindsight – obeys this principle, it seems reasonable enough to extrapolate it to the Higgs mass. Or does it? Seeing that the cosmological constant, the only other known example where the Planck mass comes in, isn’t natural either, I am not very convinced.

A much larger problem with naturalness is that it’s a circular argument and thus a merely aesthetic criterion. Or, if you prefer, a philosophic criterion. You cannot make a statement about the likeliness of an occurrence without a probability distribution. And that distribution already necessitates a choice.

In the currently used naturalness arguments, the probability distribution is assumed to be uniform (or at least approximately uniform) in a range that can be normalized to one by dividing through suitable powers of the cutoff. Any other type of distribution, say, one that is sharply peaked around small values, would require the introduction of such a small value in the distribution already. But such a small value justifies itself by the probability distribution just like a number close to one justifies itself by its probability distribution.

Naturalness, hence, becomes a chicken-and-egg problem: Put in the number one, get out the number one. Put in 0.00004, get out 0.00004. The only way to break that circle is to just postulate that some number is somehow better than all other numbers.

The number one is indeed a special number in that it’s the unit element of the multiplication group. One can try to exploit this to come up with a mechanism that prefers a uniform distribution with an approximate width of one by introducing a probability distribution on the space of probability distributions, leading to a recursion relation. But that just leaves one to explain why that mechanism.

Another way to see that this can’t solve the problem is that any such mechanism will depend on the basis in the space of functions. Eg, you could try to single out a probability distribution by asking that it’s the same as its Fourier-transformation. But the Fourier-transformation is just one of infinitely many basis transformations in the space of functions. So again, why exactly this one?

Or you could try to introduce a probability distribution on the space of transformations among bases of probability distributions, and so on. Indeed I’ve played around with this for some while. But in the end you are always left with an ambiguity, either you have to choose the distribution, or the basis, or the transformation. It’s just pushing around the bump under the carpet.

The basic reason there’s no solution to this conundrum is that you’d need another theory for the probability distribution, and that theory per assumption isn’t part of the theory for which you want the distribution. (It’s similar to the issue with the meta-law for time-varying fundamental constants, in case you’re familiar with this argument.)

In any case, whether you buy my conclusion or not, it should give you a pause that high energy theorists don’t ever address the question where the probability distribution comes from. Suppose there indeed was a UV-complete theory of everything that predicted all the parameters in the standard model. Why then would you expect the parameters to be stochastically distributed to begin with?

This lacking probability distribution, however, isn’t my main issue with naturalness. Let’s just postulate that the distribution is uniform and admit it’s an aesthetic criterion, alrighty then. My main issue with naturalness is that it’s a fundamentally nonsensical criterion.

Any theory that we can conceive of which describes nature correctly must necessarily contain hand-picked assumptions which we have chosen “just” to fit observations. If that wasn’t so, all we’d have left to pick assumptions would be mathematical consistency, and we’d end up in Tegmark’s mathematical universe. In the mathematical universe then, we’d no longer have to choose a consistent theory, ok. But we’d instead have to figure out where we are, and that’s the same question in green.

All our theories contain lots of assumptions like Hilbert-spaces and Lie-algebras and Haussdorf measures and so on. For none of these is there any explanation other than “it works.” In the space of all possible mathematics, the selection of this particular math is infinitely fine-tuned already – and it has to be, for otherwise we’d be lost again in Tegmark space.

The mere idea that we can justify the choice of assumptions for our theories in any other way than requiring them to reproduce observations is logical mush. The existing naturalness arguments single out a particular type of assumption – parameters that take on numerical values – but what’s worse about this hand-selected assumption than any other hand-selected assumption?

This is not to say that naturalness is always a useless criterion. It can be applied in cases where one knows the probability distribution, for example for the typical distances between stars or the typical quantum fluctuation in the early universe, etc. I also suspect that it is possible to find an argument for the naturalness of the standard model that does not necessitate to postulate a probability distribution, but I am not aware of one.

It’s somewhat of a mystery to me why naturalness has become so popular in theoretical high energy physics. I’m happy to see it go out of the window now. Keep your eyes open in the next couple of years and you’ll witness that turning point in the history of science when theoretical physicists stopped dictating nature what’s supposedly natural.

Friday, June 24, 2016

Where can new physics hide?

Also an acronym for “Not Even Wrong.”

The year is 2016, and physicists are restless. Four years ago, the LHC confirmed the Higgs-boson, the last outstanding prediction of the standard model. The chances were good, so they thought, that the LHC would also discover other new particles – naturalness seem to demand it. But their hopes were disappointed.

The standard model and general relativity do a great job, but physicists know this can’t be it. Or at least they think they know: The theories are incomplete, not only disagreeable and staring each other in the face without talking, but inadmissibly wrong, giving rise to paradoxa with no known cure. There has to be more to find, somewhere. But where?

The hiding places for novel phenomena are getting smaller. But physicists haven’t yet exhausted their options. Here are the most promising areas where they currently search:

1. Weak Coupling

Particle collisions at high energies, like those reached at the LHC, can produce all existing particles up to the energy that the colliding particles had. The amount of new particles however depends on the strength by which they couple to the particles that were brought to collision (for the LHC that’s protons, or their constituents quarks and gluons, respectively). A particle that couples very weakly might be produced so rarely that it could have gone unnoticed so far.

Physicists have proposed many new particles which fall into this category because weakly interacting stuff generally looks a lot like dark matter. Most notably there are the weakly interacting massive particles (WIMPs), sterile neutrinos (that are neutrinos which don’t couple to the known leptons), and axions (proposed to solve the strong CP problem and also a dark matter candidate).

These particles are being looked for both by direct detection measurements – monitoring large tanks in underground mines for rare interactions – and by looking out for unexplained astrophysical processes that could make for an indirect signal.

2. High Energies

If the particles are not of the weakly interacting type, we would have noticed them already, unless their mass is beyond the energy that we have reached so far with particle colliders. In this category we find all the supersymmetric partner particles, which are much heavier than the standard model particles because supersymmetry is broken. Also at high energies could hide excitations of particles that exist in models with compactified extra dimensions. These excitations are similar to higher harmonics of a string and show up at certain discrete energy levels which depend on the size of the extra dimension.

Strictly speaking, it isn’t the mass that is relevant to the question whether a particle can be discovered, but the energy necessary to produce the particles, which includes binding energy. An interaction like the strong nuclear force, for example, displays “confinement” which means that it takes a lot of energy to tear quarks apart even though their masses are not all that large. Hence, quarks could have constituents – often called “preons” – that have an interaction – dubbed “technicolor” – similar to the strong nuclear force. The most obvious models of technicolor however ran into conflict with data decades ago. The idea however isn’t entirely dead, and though the surviving models aren’t presently particularly popular, some variants are still viable.

These phenomena are being looked for at the LHC and also in highly energetic cosmic ray showers.

3. High Precision

High precision tests of standard model processes are complementary to high energy measurements. They can be sensitive to tiniest effects stemming from virtual particles with energies too high to be produced at colliders, but still making a contribution at lower energies due to quantum effects. Examples for this are proton decay, neutron-antineutron oscillation, the muon g-2, the neutron electric dipole moment, or Kaon oscillations. There are existing experiments for all of these, searching for deviations from the standard model, and the precision for these measurements is constantly increasing.

A somewhat different high precision test is the search for neutrinoless double-beta decay which would demonstrate that neutrinos are Majorana-particles, an entirely new type of particle. (When it comes to fundamental particles that is. Majorana particles have recently been produced as emergent excitations in condensed matter systems.)

4. Long ago

In the early universe, matter was much denser and hotter than we can hope to ever achieve in our particle colliders. Hence, signatures left over from this time can deliver a bounty of new insights. The temperature fluctuations in the cosmic microwave background (B-modes and non-Gaussianities) may be able to test scenarios of inflation or its alternatives (like phase transitions from a non-geometric phase), whether our universe had a big bounce instead of a big bang, and – with some optimism – even whether gravity was quantized back them.

5. Far away

Some signatures of new physics appear on long distances rather than of short. An outstanding question is for example what’s the shape of the universe? Is it really infinitely large, or does it close back onto itself? And if it does, then how does it do this? One can study these questions by looking for repeating patterns in the temperature fluctuation of the cosmic microwave background (CMB). If we live in a multiverse, it might occasionally happen that two universes collide, and this too would leave a signal in the CMB.

New insights might also hide in some of the well-known problems with the cosmological concordance model, such as the too pronounced galaxy cusps or the too many dwarf galaxies that don’t fit well with observations. It is widely believed that these problems are numerical issues or due to a lack of understanding of astrophysical processes and not pointers to something fundamentally new. But who knows?

Another novel phenomenon that would become noticeable on long distances is a fifth force, which would lead to subtle deviations from general relativity. This might have all kinds of effects, from violations of the equivalence principle to a time-dependence of dark energy. Hence, there are experiments testing the equivalence principle and the constancy of dark energy to every higher precision.

6. Right here

Not all experiments are huge and expensive. While tabletop discoveries have become increasingly unlikely simply because we’ve pretty much tried all that could be done, there are still areas where small-scale lab experiments reach into unknown territory. This is the case notably in the foundations of quantum mechanics, where nanoscale devices, single photon sources and – detectors, and increasingly sophisticated noise-control technics have enabled previously impossible experiments. Maybe one day we’ll be able to solve the dispute over the “correct” interpretation of quantum mechanics simply by measuring which one is right.

So, physics isn’t over yet. It has become more difficult to test new fundamental theories, but we are pushing the limits in many currently running experiments.

[This post previously appeared on Starts With a Bang.]

Wissenschaft auf Abwegen

Ich war am Montag in Regensburg und habe dort einen öffentlichen Vortrag gegeben zum Thema “Wissenschaft auf Abwegen” für eine Reihe unter dem Titel “Was ist Wirklich?” Das ganze ist jetzt auf YouTube. Das Video besteht aus etwa 30 Minuten Vortrag und danach noch eine Stunde Diskussion. Alles in Deutsch. Nur was für eche Fans ;)

Saturday, June 18, 2016

New study finds no sign of entanglement with other universes

Somewhere in the multiverse
you’re having a good day.
The German Autobahn is famous for its lack of speed limits, and yet the greatest speed limit of all comes from a German: Nothing, Albert Einstein taught us, is allowed to travel faster than light. This doesn’t prevent our ideas from racing, but sometimes it prevents us from ticketing them.

If we live in an eternally inflating multiverse that contains a vast number of universes, then the other universes recede from us faster than light. We are hence “causally disconnected” from the rest of the multiverse, separated from the other universes by the ongoing exponential expansion of space, unable to ever make a measurement that could confirm their existence. It is this causal disconnect that has lead multiverse critics to complain the idea isn’t within the realm of science.

There are however some situations in which a multiverse can give rise to observable consequences. One is that our universe might in the past have collided with another universe, which would have left a tell-tale signature in the cosmic microwave background. Unfortunately, no evidence for this has been found.

Another proposal for how to test the multiverse is to exploit the subtle non-locality that quantum mechanics gives rise to. If we live in an ensemble of universes, and these universes started out in an entangled quantum state, then we might be able to today detect relics of their past entanglement.

This idea was made concrete by Richard Holman, Laura Mersini-Houghton, and Tomo Takahashi ten years ago. In their model (hep-th/0611223, hep-th/0612142), the original entanglement present among universes in the landscape decays and effectively leaves a correction to the potential that gives rise to inflation in our universe. This corrected potential in return affects observables that we can measure today.

The particular way of Mersini-Houghton and Holman to include entanglement in the landscape isn’t by any means derived from first principles. It is a phenomenological construction that implicitly makes many assumptions about the way quantum effects are realized on the landscape. But, hey, it’s a model that makes predictions, and in theoretical high energy today that’s something to be grateful for.

They predicted back then that such an entanglement-corrected cosmology would in particular affect the physics on very large scales, giving rise to a modulation of the power spectrum that makes the cold spot a more likely appearance, a suppression of the power at large angular scale, and an alignment in the directions in which large structures move – the so-called “dark flow.” The tentative evidence of a dark flow, which was predicted in 2008 had gone by 2013. But this disagreement with the data didn’t do much to the popularity of the model in the press.

In a recent paper, William Kinney from the University at Buffalo put to test the multiverse-entanglement with the most recent cosmological data:
    Limits on Entanglement Effects in the String Landscape from Planck and BICEP/Keck Data
    William H. Kinney
    arXiv:1606.00672 [astro-ph.CO]
The brief summary is that not only hasn’t he found any evidence for the entanglement-modification, he has ruled out the formerly proposed model for two general types of inflationary potentials. The first, a generic exponential inflation, is by itself incompatible with the data, but adding the entanglement correction doesn’t help to make it fit. The second, Starobinski inflation, is by itself a good fit to the data, but the entanglement correction spoils the fit.

Much to my puzzlement, his analysis also shows that some of the predictions of the original model (such as the modulation of the power spectrum) weren’t predictions to begin with, because Kinney in his calculation found that there are choices of parameters in which these effects don’t appear at all.

Leaving aside that this sheds a rather odd light on the original predictions, it’s not even clear exactly what has been ruled out here. What Kinney’s analysis does is to exclude a particular form of the effective potential for inflation (the one with the entanglement modification). This potential is, in the model by Holman and Mersini-Houghton, a function of the original potential (the one without the entanglement correction). Rather than ruling out the entanglement-modification, I can hence interpret this result to mean that the original potential just wasn’t the right one.

Or, in other words, how am I to know that one can’t find some other potential that will fit the data after adding the entanglement correction. The only difficulty I see in this would be to ensure that the uncorrected potential should still lead to eternal inflation.

To add meat to an unfalsifiable idea that made predictions which weren’t, one of the authors who proposed the entanglement model, Laura Mersini-Houghton, is apparently quite unhappy with Kinney’s paper and tries to use an intellectual property claim to get it removed from the arXiv (see comments for details). I will resist the temptation to comment on the matter and simply direct you to the Wikipedia entry on the Streisand Effect. Dear Internet, please do your job.

For better or worse, I have in the last years been dragged into a discussion about what is and isn’t science, which has forced me to think more about the multiverse than I and my infinitely many copies believe is good for their sanity. After this latter episode, the status is that I side with Joe Silk who captured it well: “[O]ne can always find inflationary models to explain whatever phenomenon is represented by the flavour of the month.”

Monday, June 13, 2016

String phenomenology of the somewhat different kind

[Cat’s cradle. Image Source.]
Ten years ago, I didn’t take the “string wars” seriously. To begin with, referring to such an esoteric conflict as “war” seems disrespectful to millions caught in actual wars. In comparison to their suffering it’s hard to take anything seriously.

Leaving aside my discomfort with the nomenclature, the focus on string theory struck me as odd. String theory as a research area stands out in hep-th and gr-qc merely because of the large number of followers, not by the supposedly controversial research practices. For anybody working in the field it is apparent that string theorists don’t differ in their single-minded focus from physicists in other disciplines. Overspecialization is a common disease of academia, but one that necessarily goes along with division of labor, and often it is an efficient route to fast progress.

No, I thought back then, string theory wasn’t the disease, it was merely a symptom. The underlying disease was one that would surely soon be recognized and addressed: Theoreticians – as scientists whose most-used equipment is their own brain – must be careful to avoid systematic bias introduced by their apparatuses. In other words, scientific communities, and especially those which lack timely feedback by data, need guidelines to avoid social and cognitive biases.

This is so obvious it came as a surprise to me that, in 2006, everybody was hitting on Lee Smolin for pointing out what everybody knew anyway, that string theorists, lacking experimental feedback for decades, had drifted off in a math bubble with questionable relevance for the description of nature. It’s somewhat ironic that, from my personal experience, the situation is actually worse in Loop Quantum Gravity, an approach pioneered, among others, by Lee Smolin. At least the math used by string theorists seems to be good for something. The same cannot be said about LQG.

Ten years later, it is clear that I was wrong in thinking that just drawing attention to the problem would seed a solution. Not only has the situation not improved, it has worsened. We now have some theoretical physicists who argue that we should alter the scientific method so that the success of a theory can be assessed by means other than empirical evidence. This idea, which has sprung up in the philosophy community, isn’t all that bad in principle. In practice, however, it will merely serve to exacerbate social streamlining: If theorists can draw on criteria other than the ability of a theory to explain observations, the first criterion they’ll take into account is aesthetic value, and the second is popularity with their colleagues. Nothing good can come out of this.

And nothing good has come out of it, nothing has changed. The string wars clearly were more interesting for sociologists than they were for physicists. In the last couple of months several articles have appeared which comment on various aspects of this episode, which I’ve read and want to briefly summarize for you.

First, there is
    Collective Belief, Kuhn, and the String Theory Community
    Weatherall, James Owen and Gilbert, Margaret
This paper is a very Smolin-centric discussion of whether string theorists are exceptional in their group beliefs. The authors argue that, no, actually string theorists just behave like normal humans and “these features seem unusual to Smolin not because they are actually unusual, but because he occupies an unusual position from which to observe them.” He is unusual, the authors explain, for having worked on string theory, but then deciding to not continue in the field.

It makes sense, the authors write, that people whose well-being to some extent depends on the acceptance by the group will adapt to the group:
“Expressing a contrary view – bucking the consensus – is an offense against the other members of the community… So, irrespective of their personal beliefs, there are pressures on individual scientists to speak in certain ways. Moreover, insofar as individuals are psychologically disposed to avoid cognitive dissonance, the obligation to speak in certain ways can affect one’s personal beliefs so as to bring them into line with the consensus, further suppressing dissent from within the group.”
“As parties to a joint commitment, members of the string theory community are obligated to act as mouthpieces of their collective belief.”
I actually thought we knew this since 1895, when Le Bon’s published his “Study of the Popular Mind.”

The authors of the paper then point out that it’s normal for members of a scientific community to not jump ship at the slightest indication of conflicting evidence because often such evidence turns out to be misleading. It didn’t become clear to me what evidence they might be referring to; supposedly it’s non-empirical.

They further argue that a certain disregard for what is happening outside one’s own research area is also normal: “Science is successful in part because of a distinctive kind of focused, collaborative research,” and due to their commitment to the agenda “participants can be expected to resist change with respect to the framework of collective beliefs.”

This is all reasonable enough. Unfortunately, the authors entirely miss the main point, the very reason for the whole debate. The question isn’t whether string theorists’ behavior is that of normal humans – I don’t think that was ever in doubt – but whether that “normal human behavior” is beneficial for science. Scientific research requires, in a very specific sense, non-human behavior. It’s not normal for individuals to disregard subjective assessments and to not pay attention to social pressure. And yet, that is exactly what good science would require.

The second paper is
This paper is basically a summary of the string wars that focuses on the question whether or not string theory can be considered science. This “demarcation problem” is a topic that philosophers and sociologists love to discuss, but to me it really isn’t particularly interesting how you classify some research area, to me the question is whether it’s good for something. This is a question which should be decided by the community, but as long as decision making is influenced by social pressures and cognitive biases I can’t trust the community judgement.

The article has a lot of fun quotations from very convinced string theorists, for example by David Gross: “String theory is full of qualitative predictions, such as the production of black holes at the LHC.” I’m not sure what’s the difference between a qualitative prediction and no prediction, but either way it’s certainly not a prediction that was very successful. Also nice is John Schwarz claiming that “supersymmetry is the major prediction of string theory that could appear at accessible energies” and that “some of these superpartners should be observable at the LHC.” Lots of coulds and shoulds that didn’t quite pan out.

While the article gives a good overview on the opinions about string theory that were voiced during the 2006 controversy, the authors themselves clearly don’t know very well the topic they are writing about. A particularly odd statement that highlights their skewed perspective is: “String theory currently enjoys a privileged status by virtue of being the dominant paradigm within theoretical physics.”

I find it quite annoying how frequently I encounter this extrapolation from a particular research area – may that be string theory, supersymmetry, or multiverse cosmology – to all of physics. The vast majority of physicists work in fields like quantum optics, photonics, hadronic and nuclear physics, statistical mechanics, atomic physics, solid state physics, low-temperature physics, plasma physics, astrophysics, condensed matter physics, and so on. They have nothing whatsoever to do with string theory, and certainly would be very surprised to hear that it’s “the dominant paradigm.”

In any case, you might find this paper useful if you didn’t follow the discussion 10 years ago.

Finally, there is this paper

The title of the paper doesn’t explicitly refer to string theory, but most of it is also a discussion of the demarcation problem on the example of arXiv trackbacks. (I suspect this paper is a spin-off of the previous paper.)

ArXiv trackbacks, in case you didn’t know, are links to blogposts that show up on some papers’ arxiv sites, when the blogpost has referred to the paper. To exactly which blogs trackbacks show up and who makes the decision whether they do is one of the arXiv’s best-kept secrets. Peter Woit’s blog, infamously, doesn’t show up in the arXiv trackbacks on the, rather spurious, reason that he supposedly doesn’t count as “active researcher.” The paper tells the full 2006 story with lots of quotes from bloggers you are probably familiar with.

The arXiv recently conducted a user survey, among other things about the trackback feature, which makes me think they might have some updates planned.

On the question who counts as crackpot, the paper (unsurprisingly) doesn’t come to a conclusion other than noting that scientists deal with the issue by stating “we know one when we see one.” I don’t think there can be any other definition than that. To me the notion of “crackpot” is an excellent example of an emergent feature – it’s a demarcation that the community creates during its operation. Any attempt to come up with a definition from first principles is hence doomed to fail.

The rest of the paper is a general discussion of the role of blogs in science communication, but I didn’t find it particularly insightful. The author comes to the (correct) conclusion that blog content turned out not to have such a short life-time as many feared, but otherwise basically just notes that there are as many ways to use blogs as there are bloggers. But then if you are reading this, you already knew that.

One of the main benefits that I see in blogs isn’t mentioned in the paper at all, which is that blogs supports communication between scientific communities that are only loosely connected. In my own research area, I read the papers, hear the seminars, and go to conferences, and I therefore know pretty well what is going on – with or without blogs. But I use blogs to keep up to date in adjacent fields, like cosmology, astrophysics and, to a lesser extent, condensed matter physics and quantum optics. For this purpose I find blogs considerably more useful than popular science news, because the latter often doesn’t provide a useful amount of detail and commentary, not to mention that they all tend to latch onto the same three papers that made big unsubstantiated claims.

Don’t worry, I haven’t suddenly become obsessed with string theory. I’ve read through these sociology papers mainly because I cannot not write a few paragraphs about the topic in my book. But I promise that’s it from me about string theory for some while.

Update: Peter Woit has some comments on the trackback issue.

Monday, June 06, 2016

Dear Dr B: Why not string theory?

[I got this question in reply to my last week’s book review of Why String Theory? by Joseph Conlon.]

Dear Marco:

Because we might be wasting time and money and, ultimately, risk that progress stalls entirely.

In contrast to many of my colleagues I do not think that trying to find a quantum theory of gravity is an endeavor purely for the sake of knowledge. Instead, it seems likely to me that finding out what are the quantum properties of space and time will further our understanding of quantum theory in general. And since that theory underlies all modern technology, this is research which bears relevance for applications. Not in ten years and not in 50 years, but maybe in 100 or 500 years.

So far, string theory has scored in two areas. First, it has proved interesting for mathematicians. But I’m not one to easily get floored by pretty theorems – I care about math only to the extent that it’s useful to explain the world. Second, string theory has shown to be useful to push ahead with the lesser understood aspects of quantum field theories. This seems a fruitful avenue and is certainly something to continue. However, this has nothing to do with string theory as a theory of quantum gravity and a unification of the fundamental interactions.

As far as quantum gravity is concerned, string theorist’s main argument seems to be “Well, can you come up with something better?” Then of course if someone answers this question with “Yes” they would never agree that something else might possibly be better. And why would they – there’s no evidence forcing them one way or the other.

I don’t see what one learns from discussing which theory is “better” based on philosophical or aesthetic criteria. That’s why I decided to stay out of this and instead work on quantum gravity phenomenology. As far as testability is concerned all existing approaches to quantum gravity do equally badly, and so I’m equally unconvinced by all of them. It is somewhat of a mystery to me why string theory has become so dominant.

String theorists are very proud of having a microcanonical explanation for the black hole entropy. But we don’t know whether that’s actually a correct description of nature, since nobody has ever seen a black hole evaporate. In fact one could read the firewall problem as a demonstration that indeed this cannot be a correct description of nature. Therefore, this calculation leaves me utterly unimpressed.

But let me be clear here. Nobody (at least nobody whose opinion matters) says that string theory is a research program that should just be discontinued. The question is instead one of balance – does the promise justify the amount of funding spend on it? And the answer to this question is almost certainly no.

The reason is that academia is currently organized so that it invites communal reinforcement, prevents researchers from leaving fields whose promise is dwindling, and supports a rich-get-richer trend. That institutional assessments use the quantity of papers and citation counts as a proxy for quality creates a bonus for fields in which papers can be cranked out quickly. Hence it isn’t surprising that an area whose mathematics its own practitioners frequently describe as “rich” would flourish. What does mathematical “richness” tell us about the use of a theory in the description of nature? I am not aware of any known relation.

In his book Why String Theory?, Conlon tells the history of the discipline from a string theorist’s perspective. As a counterpoint, let me tell you how a cynical outsider might tell this story:

String theory was originally conceived as a theory of the strong nuclear force, but it was soon discovered that quantum chromodynamics was more up to the task. After noting that string theory contains a particle that could be identified as the graviton, it was reconsidered as a theory of quantum gravity.

It turned out however that string theory only makes sense in a 25-dimensional space. To make that compatible with observations, 22 of the dimensions were moved out of sight by rolling them up (compactifying) them to a radius so small they couldn’t be observationally probed.

Next it was noted that the theory also needs supersymmetry. This brings down the number of space dimensions to 9, but also brings a new problem: The world, unfortunately, doesn’t seem to be supersymmetric. Hence, it was postulated that supersymmetry is broken at an energy scale so high we wouldn’t see the symmetry. Even with that problem fixed, however, it was quickly noticed that moving the superpartners out of direct reach would still induce flavor changing neutral currents that, among other things, would lead to proton decay and so be in conflict with observation. Thus, theorists invented R-parity to fix that problem.

The next problem that appeared was that the cosmological constant turned out to be positive instead of zero or negative. While a negative cosmological constant would have been easy to accommodate, string theorists didn’t know what to do with a positive one. But it only took some years to come up with an idea to make that happen too.

String theory was hoped to be a unique completion of the standard model including general relativity. Instead it slowly became clear that there is a huge number of different ways to get rid of the additional dimensions, each of which leads to a different theory at low energies. String theorists are now trying to deal with that problem by inventing some probability measure according to which the standard model is at least a probable occurrence in string theory.

So, you asked, why not string theory? Because it’s an approach that has been fixed over and over again to make it compatible with conflicting observations. Every time that’s been done, string theorists became more convinced of their ideas. And every time they did this, I became more convinced they are merely building a mathematical toy universe.

String theorists of course deny that they are influenced by anything but objective assessment. One noteworthy exception is Joe Polchinski who has considered that social effects play a role, but just came to the conclusion that they aren’t relevant. I think it speaks for his intellectual sincerity that he at least considered it.

At the Munich workshop last December, David Gross (in an exchange with Carlo Rovelli) explained that funding decisions have no influence on whether theoretical physicists chose to work in one field or the other. Well, that’s easy to say if you’re a Nobel Prize winner.

Conlon in his book provides “evidence” that social bias plays no role by explaining that there was only one string theorist in a panel that (positively) evaluated one of his grants. To begin with anecdotes can’t replace data and there is ample evidence that social biases are common human traits, so by default scientists should be susceptible. But even considering his anecdote, I’m not sure why Conlon thinks leaving decisions to non-experts limits bias. My expectation would be that it amplifies bias because it requires drawing on simplified criteria, like the number of papers published and how often they’ve been cited. And what does that depend on? Depends on how many people there are in the field and how many peers favorably reviewed papers on the topic of your work.

I am listing these examples to demonstrate that it is quite common of theoretical physicists (not string theorists in particular) to dismiss the mere possibility that social dynamics influences research decisions.

How large a role play social dynamics and cognitive biases, and how much do they slow down progress on the foundations of physics? I can’t tell you. But even though I can’t tell you how much faster progress could be, I am sure it’s slowed down. I can tell that in the same way that I can tell you diesel in Germany is sold under market value even though I don’t know the market value. I know that because it’s subsidized. And in the same way I can tell that string theory is overpopulated and its promise is overestimated because it’s an idea that benefits from biases which humans demonstrably possess. But I can’t tell you what its real value would be.

The reproduction crisis in the life-sciences and psychology has spurred a debate for better measures of statistical significance. Experimentalists go to length to put into place all kinds of standardized procedures to not draw the wrong conclusions from what their apparatuses measures. In theory development, we have our own crisis, but nobody talks about it. The apparatuses that we use are our own brains and biases we should guard against are cognitive and social biases, communal reinforcement, sunk cost fallacy, wishful thinking and status-quo bias, for just to mention the most common ones. These however are presently entirely unaccounted for. Is this the reason why string theory has gathered so many followers?

Some days I side with Polchinski and Gross and don’t think it makes that much of a difference. It really is an interesting topic and it’s promising. On other days I think we’ve wasted 30 years studying bizarre aspects of a theory that doesn’t bring us any closer to understanding quantum gravity, and it’s nothing but an empty bubble of disappointed expectations. Most days I have to admit I just don’t know.

Why not string theory? Because enough is enough.

Thanks for an interesting question.

Monday, May 30, 2016

Book Review: “Why String Theory?” by Joseph Conlon

Why String Theory?
By Joseph Conlon
CRC Press (November 24, 2015)

I was sure I’d hate the book. Let me explain.

I often hear people speak about the “marketplace of ideas” as if science was a trade show where researchers sell their work. But science isn’t about manufacturing and selling products, it’s about understanding nature. And the sine qua non for evaluating the promise of an idea is objectivity.

In my mind, therefore, the absolutely last thing that scientists should engage in is marketing. Marketing, advertising, and product promotion are commercial tactics with the very purpose of affecting their targets’ objectivity. These tactics shouldn’t have any place in science.

Consequently, I have mixed feelings about scientists who attempt to convince the public that their research area is promising, with the implicit or explicit goal of securing funding and attracting students. It’s not that I have a problem with scientists who write for the public in general – I have a problem with scientists who pass off their personal opinion as fact, often supporting their conviction by quoting the number of people who share their beliefs.

In the last two decades this procedure has created an absolutely astonishing amount of so-called “science” books about string theory, supersymmetry, the multiverse and other fantasies (note careful chosen placement of commata), with no other purpose than asking the reader to please continue funding fruitless avenues of research by appealing to lofty ideals like elegance and beauty.

And indeed, Conlon starts with dedicating the book to “the taxpayers of the UK without whom this book could never have been written” and then states explicitly that his goal is to win the favor of taxpayers:
“I want to explain, to my wonderful fellow citizens who support scientific research through their taxes, why string theory is so popular, and why, despite the lack of direct empirical support, it has attained the level of prominence it has.”

That’s on page six. The prospect of reading 250 pages filled with a string theorists’ attempt to lick butts of his “wonderful fellow citizens” made me feel somewhat nauseous. I put the book aside and instead read Sean Carroll’s new book. After that I felt slightly better and made a second attempt at Why String Theory?

Once I got past the first chapter, however, the book got markedly better. Conlon keeps the introduction to basic physics (relativity and quantum theory) to an absolute minimum. After this he lays out the history of string theory, with its many twists and turns, and explains how much string theorists’ understanding of the approach has changed within the decades.

He then gets to the reasons why people work on string theory. The first reason he lists is a chapter titled “Direct Experimental Evidence for String Theory” which consists of the single sentence “There is no direct experimental evidence for string theory.” At first, I thought that he might have wanted to point out that string theorists work on it despite the lack of evidence, but that the previous paragraph accidentally made it look as if he, rather cynically, wanted to say that the absence of evidence is the main reason they work on it.

But actually he returns to this point later in the book (in section 10.5), where he addresses “objections made concerning connection to experiment” and points out very clearly that even though these are prevalent, he thinks these deserve little or no sympathy. This makes me think, maybe he indeed wanted to say that he suspects the main reason so many people work on string theory is because there’s no evidence for it. Especially the objection that it is “too early” to seek experimental support for string theory because the theory is not fully understood he responds to with:
“The problem with this objection is that it is a time-invariant statement. It was made thirty years ago, it was made twenty years ago, it was made a decade ago, and it is made now. It is also, by observation, an objection made by those who are uninterested in observation. Muscles that are never used waste away. It is like never commencing a journey because one is always waiting for better modes of transportation, and in the end produces a community of scientists where the language of measurement and experiment is one that may be read but cannot be spoken.”
Conlon writes that he himself isn’t particularly interested in quantum gravity. His own research is finding evidence for moduli fields in cosmology, and he has a chapter about this. He lists the usual arguments in favor of string theory, that it connects well to both general relativity and the standard model, that it’s been helpful in deriving some math theorems, and that now there is the AdS/CFT duality by help of which one might maybe one day be able to describe some aspect of the real world.

He somehow forgets to mention that the AdS/CFT predictions for heavy ion collisions at the LHC turned out to be dramatically wrong, and by now very few people think that the duality is of much use in this area. I actually suspect he just plainly didn’t know this. It’s not something that string theorists like to talk about. This omission is my major point of criticism. The rest of the book seems a quite balanced account, and he restrains from making cheap arguments of the type that the theory must be right because a thousand people with brains can’t be mistaken. Conlon even has a subsection addressing Witten-cult, which is rather scathing, and a hit on Arkani-Hamed gathering 5000 citations and a $3 million price for proposing large extra dimensions (an idea that was quietly buried after the LHC ruled it out).

At the end of the book Conlon has a chapter addressing explicit criticisms – he manages to remain remarkably neutral and polite – and a “fun” chapter in which he lists different styles of doing research. Maybe there’s something wrong with my sense of humor but I didn’t find it much fun. It’s more like he is converting Kuhn’s phases of “normal science” and “revolution” into personal profiles, trying to reassure students that they don’t need to quantize gravity to get tenure.

Leaving aside Conlon’s fondness of mixing up sometimes rather odd metaphors (“quantum mechanics is a jealous theory... it has spread through the population of scientific theories like a successful mutation” – “The anthropic landscape... represents incontinence of speculation joined to constipation of experiment.” – “quantum field theorists became drunk on the new wine of string theory”) and an overuse of unnecessary loanwords (in pectore, pons asinorum, affaire de coer, lebensraum, mirabile dictum, for just to mention a few), the book is reasonably well written. The reference list isn’t too extensive. This is to say in the couple of cases in which I wanted to look up a reference it wasn’t listed, and the one case I wanted to check a quotation it didn’t have an original source.

Altogether, Why String Theory? gives the reader a mostly fair and balanced account of string theory, and a pretty good impression for just how much the field has changed since Brian Greene’s Elegant Universe. I looked up something in Greene’s book the other day, and found him complaining that the standard model is “too flexible.” Oh, yes, things have changed a lot since. I doubt it’s a complaint any string theorist dare raise today.

In the end, I didn’t hate Conlon’s book. Maybe I’m getting older, or maybe I’m getting wiser, or maybe I’m just not capable of hating books.

[Disclaimer: Free review copy.]

Win a copy of Why String Theory by Joseph Conlon!

I had bought the book before I was sent the review copy, and so I have a second copy of the book, entirely new and untouched. You can win the book if you are the first to answer this question correctly: Who was second author on the first paper to point out that some types of neutrino detectors might also be used to directly detect certain candidate particles for dark matter? Submit answer in the comments, do not send an email. The time-stamp of the comment counts. (Please only submit an answer if you are willing to send me a postal address to which the book can be shipped.)

Update: The book is gone!

Away Note

I have a trip upcoming to Helsinki. After this I'll be tied up in family business, and then my husband goes on a business trip and I have the kids alone. Then Kindergarten will be closed for a day (forgot why, I'm sure they must have some reason), I have to deal with an ant-infection in our apartment, and more family business follows. In summary: busy times.

I have a book review to appear on this blog later today, but after this you won't hear much from me for a week or two. Keep in mind that since I have comment moderation on, it might take some while for your comment to appear when I am traveling. With thanks for your understanding, here's a random cute pic of Gloria :)

Thursday, May 26, 2016

How can we test quantum gravity?

If you have good eyes, the smallest objects you can make out are about a tenth of a millimeter, roughly the width of a human hair. Add technology, and the smallest structures we have measured so far are approximately 10-19m, that’s the wavelength of the protons collided at the LHC. It has taken us about 400 years from the invention of the microscope to the construction of the LHC – 400 years to cross 15 orders of magnitude.

Quantum effects of gravity are estimated to become relevant on distance scales of approximately 10-35m, known as the Planck length. That’s another 16 orders of magnitude to go. It makes you wonder whether it’s possible at all, or whether all the effort to find a quantum theory of gravity is just idle speculation.

I am optimistic. The history of science is full with people who thought things to be impossible that have meanwhile been done: measuring the light deflection on the sun, heavier-than-air flying machines, detecting gravitational waves. Hence, I don’t think it’s impossible to experimentally test quantum gravity. Maybe it will take some decades, or maybe it will take some centuries – but if only we keep pushing, one day we will measure quantum gravitational effects. Not by directly crossing these 15 orders of magnitude, I believe, but instead by indirect detections at lower energies.

From nothing comes nothing though. If we don’t think about how quantum gravitational effects can look like and where they might show up, we’ll certainly never find them. But fueling my optimism is the steadily increasing interest in the phenomenology of quantum gravity, the research area dedicated to studying how to best find evidence for quantum gravitational effects.

Since there isn’t any one agreed-upon theory for quantum gravity, existing efforts to find observable phenomena focus on finding ways to test general features of the theory, properties that have been found in several different approaches to quantum gravity. Quantum fluctuations of space-time, for example, or the presence of a “minimal length” that would impose a fundamental resolution limit. Such effects can be quantified in mathematical models, which can then be used to estimate the strength of the effects and thus to find out which experiments are most promising.

Testing quantum gravity has long thought to be out of reach of experiments, based on estimates that show it would take a collider the size of the Milky Way to accelerate protons enough to produce a measureable amount of gravitons (the quanta of the gravitational field), or that we would need a detector the size of planet Jupiter to measure a graviton produced elsewhere. Not impossible, but clearly not something that will happen in my lifetime.

One testable consequence of quantum gravity might be, for example, the violation of the symmetry of special and general relativity, known as Lorentz-invariance. Interestingly it turns out that violations of Lorentz-invariance are not necessarily small even if they are created at distances too short to be measurable. Instead, these symmetry violations seep into many particle reactions at accessible energies, and these have been tested to extremely high accuracy. No evidence for violations of Lorentz-invariance have been found. This might sound like not much, but knowing that this symmetry has to be respected by quantum gravity is an extremely useful guide in the development of the theory.

Other testable consequences might be in the weak-field limit of quantum gravity. In the early universe, quantum fluctuations of space-time would have led to temperature fluctuation of matter. And these temperature fluctuations are still observable today in the Cosmic Microwave Background (CMB). The imprint of such “primordial gravitational waves” on the CMB has not yet been measured (LIGO is not sensitive to them), but they are not so far off measurement precision.

A lot of experiments are currently searching for this signal, including BICEP and Planck. This raises the question whether it is possible to infer from the primordial gravitational waves that gravity must have been quantized in the early universe. Answering this question is one of the presently most active areas in quantum gravity phenomenology.

Also testing the weak-field limit of quantum gravity are attempts to bring objects into quantum superpositions that are much heavier than elementary particles. This makes the gravitational field stronger and potentially offers the chance to probe its quantum behavior. The heaviest objects that have so far been brought into superpositions weigh about a nano-gram, which is still several orders of magnitude too small to measure the gravitational field. But a group in Vienna recently proposed an experimental scheme that would allow to measure the gravitational field more precisely than ever before. We are slowly closing in on the quantum gravitational range.

Such arguments however merely concern the direct detection of gravitons, and that isn’t the only manifestation of quantum gravitational effects. There are various other observable consequences that quantum gravity could give rise to, some of which have already been looked for, and others that we plan to look for. So far, we have only negative results. But even negative results are valuable because they tell us what properties the sought-for theory cannot have.

[From arXiv:1602.07539, for details, see here]

The weak field limit would prove that gravity really is quantized and finally deliver the much-needed experimental evidence, confirming that we’re not just doing philosophy. However, for most of us in the field the strong gravity limit is more interesting. With strong gravity limit I mean Planckian curvature, which (not counting those galaxy-sized colliders) can only be found close by the center of black holes and towards the big bang.

(Note that in astrophysics, “strong gravity” is sometimes used to mean something different, referring to large deviations from Newtonian gravity which can be found, eg, around the horizon of black holes. In comparison to the Planckian curvature required for strong quantum gravitational effects, this is still exceedingly weak.)

Strong quantum gravitational effects could also have left an imprint in the cosmic microwave background, notably in the type of correlations that can be found in the fluctuations. There are various models of string cosmology and loop quantum cosmology that have explored the observational consequences, and proposed experiments like EUCLID and PRISM might find first hints. Also the upcoming experiments to test the 21-cm hydrogen absorption could harbor information about quantum gravity.

A somewhat more speculative idea is based on a recent finding according to which the gravitational collapse of matter might not always form a black hole, but could escape the formation of a horizon. If that is so, then the remaining object would give us open view on a region with quantum gravitational effects. It isn’t yet clear exactly what signals we would have to look for to find such an object, but this is promising research direction because it could give us direct access to strong space-time curvature.

There are many other ideas out there. A large class of models for example deals with the possibility that quantum gravitational effects endow space-time with the properties of a medium. This can lead to the dispersion of light (colors running apart), birefringence (polarizations running apart), decoherence (preventing interference), or an opacity of otherwise empty space. More speculative ideas include Craig Hogan’s quest for holographic noise, Bekenstein’s table-top experiment that searches for Planck-length discreteness, or searches for evidence of a minimal length in tritium decay. Some general properties that have recently been found and that we yet have to find good experimental tests for are geometric phase transitions in the early universe, or dimensional reduction.

Without doubt, there is much that remains to be done. But we’re on the way.

[This post previously appeared on Starts With A Bang.]

Thursday, May 19, 2016

The Holy Grail of Crackpot Filtering: How the arXiv decides what’s science – and what’s not.

Where do we draw the boundary between science and pseudoscience? It’s is a question philosophers have debated for as long as there’s been science – and last time I looked they hadn’t made much progress. When you ask a sociologist their answer is normally a variant of: Science is what scientists do. So what do scientists do?

You might have heard that scientists use what’s called the scientific method, a virtuous cycle of generating and testing hypotheses which supposedly separates the good ideas from the bad ones. But that’s only part of the story because it doesn’t tell you where the hypotheses come from to begin with.

Science doesn’t operate with randomly generated hypotheses for the same reason natural selection doesn’t work with randomly generated genetic codes: it would be highly inefficient and any attempt to optimize the outcome would be doomed to fail. What we do instead is heavily filtering hypotheses, and then we consider only those which are small mutations of ideas that have previously worked. Scientists like to be surprised, but not too much.

Indeed, if you look at the scientific enterprise today, almost all of its institutionalized procedures are methods not for testing hypotheses, but for filtering hypotheses: Degrees, peer reviews, scientific guidelines, reproduction studies, measures for statistical significance, and community quality standards. Even the use of personal recommendations works to that end. In theoretical physics in particular the prevailing quality standard is that theories need to be formulated in mathematical terms. All these are requirements which have evolved over the last two centuries – and they have proved to work very well. It’s only smart to use them.

But the business of hypotheses filtering is a tricky one and it doesn’t proceed by written rules. It is a method that has developed through social demarcation, and as such it has its pitfalls. Humans are prone to social biases and every once in a while an idea get dismissed not because it’s bad, but because it lacks community support. And there is no telling how often this happens because these are the stories we never get to hear.

It isn’t news that scientists lock shoulders to defend their territory and use technical terms like fraternities use secret handshakes. It thus shouldn’t come as a surprise that an electronic archive which caters to the scientific community would develop software to emulate the community’s filters. And that is, in a nutshell, basically what the arXiv is doing.

In an interesting recent paper, Luis Reyes-Galindo had a look at the arXiv moderators and their reliance on automated filters:

In the attempt to develop an algorithm that would sort papers into arXiv categories automatically, thereby supporting arXiv moderators to decide when a submission needs to be reclassified, it turned out that papers which scientists would mark down as “crackpottery” showed up as not classifiable or stood out by language significantly different from that in the published literature. According to Paul Ginsparg, who developed the arXiv more than 20 years ago:
“The first thing I noticed was that every once in a while the classifier would spit something out as ‘I don't know what category this is’ and you’d look at it and it would be what we’re calling this fringe stuff. That quite surprised me. How can this classifier that was tuned to figure out category be seemingly detecting quality?

“[Outliers] also show up in the stop word distribution, even if the stop words are just catching the style and not the content! They’re writing in a style which is deviating, in a way. [...]

“What it’s saying is that people who go through a certain training and who read these articles and who write these articles learn to write in a very specific language. This language, this mode of writing and the frequency with which they use terms and in conjunctions and all of the rest is very characteristic to people who have a certain training. The people from outside that community are just not emulating that. They don’t come from the same training and so this thing shows up in ways you wouldn’t necessarily guess. They’re combining two willy-nilly subjects from different fields and so that gets spit out.”
It doesn’t surprise me much – you can see this happening in comment sections all over the place: The “insiders” can immediately tell who is an “outsider.” Often it doesn’t take more than a sentence or two, an odd expression, a term used in the wrong context, a phrase that nobody in the field would ever use. It is only consequential that with smart software you can tell insiders from outsiders even more efficiently than humans. According to Ginsparg:
“We've actually had submissions to arXiv that are not spotted by the moderators but are spotted by the automated programme [...] All I was trying to do is build a simple text classifier and inadvertently I built what I call The Holy Grail of Crackpot Filtering.”
Trying to speak in the code of a group you haven’t been part of at least for some time is pretty much impossible, much like it’s impossible to fake the accent of a city you haven’t lived in for some while. Such in-group and out-group demarcation is subject of much study in sociology, not specifically the sociology of science, but generally. Scientists are human and of course in-group and out-group behavior also shapes their profession, even though they like to deny it as if they were superhuman think-machines.

What is interesting about this paper is that, for the first time, it openly discusses how the process of filtering happens. It’s software that literally encodes the hidden rules that physicists use to sort out cranks. For what I can tell, the arXiv filters work reasonably well, otherwise there would be much complaint in the community. But the vast majority of researchers in the field are quite satisfied with what the arXiv is doing, meaning the arXiv filters match their own judgement.

There are exceptions of course. I have heard some stories of people who were working on new approaches that fell between the stools and were flagged as potential crackpottery. The cases that I know of could eventually be resolved, but that might tell you more about the people I know than about the way such issues typically end.

Personally, I have never had a problem with the arXiv moderation. I had a paper reclassified from gen-ph to gr-qc once by a well-meaning moderator, which is how I learned that gen-ph is the dump for borderline crackpottery. (How would I have known? I don’t read gen-ph. I was just assuming someone reads it.)

I don’t so much have an issue with what gets filtered on the arXiv, what bothers me much more is what does not get filtered and hence, implicitly, gets approval by the community. I am very sympathetic to the concerns of John The-End-Of-Science Horgan that scientists don’t clean enough on their own doorsteps. There is no “invisible hand” that corrects scientists if they go astray. We have to do this ourselves. In-group behavior can greatly misdirect science because, given sufficiently many people, even fruitless research can become self-supportive. No filter that is derived from the community’s own judgement will do anything about this.

It’s about time that scientists start paying attention to social behavior in their community. It can, and sometimes does, affect objective judgement. Ignoring or flagging what doesn’t fit into pre-existing categories is one such social problem that can stand in the way of progress.

In a 2013 paper published in Science, a group of researchers quantified the likeliness of combinations of topics in citation lists and studied the cross-correlation with the probability of the paper becoming a “hit” (meaning in the upper 5th percentile of citation scores). They found that having previously unlikely combinations in the quoted literature is positively correlated with the later impact of a paper. They also note that the fraction of papers with such ‘unconventional’ combinations has decreased from 3.54% in the 1980s to 2.67% in the 1990, “indicating a persistent and prominent tendency for high conventionality.”

Conventional science isn’t bad science. But we also need unconventional science, and we should be careful to not assign the label “crackpottery” too quickly. If science is what scientists do, scientists should pay some attention to the science of what they do.

Sunday, May 15, 2016

Dear Dr B: If photons have a mass, would this mean special relativity is no longer valid?

Einstein and Lorentz.
[Image: Wikipedia]
“[If photons have a restmass] would that mean the whole business of the special theory of relativity being derived from the idea that light has to go at a particular velocity in order for it to exist/Maxwell’s identification of e/m waves as light because they would have to go at the appropriate velocity is no longer valid?”

(This question came up in the discussion of a recent proposal according to which photons with a tiny restmass might cause an effect similar to the cosmological constant.)

Dear Brian,

The short answer to your question is “No.” If photons had a restmass, special relativity would still be as valid as it’s always been.

The longer answer is that the invariance of the speed of light features prominently in the popular explanations of special relativity for historic reasons, not for technical reasons. Einstein was lead to special relativity contemplating what it would be like to travel with light, and then tried to find a way to accommodate an observer’s motion with the invariance of the speed of light. But the derivation of special relativity is much more general than that, and it is unnecessary to postulate that the speed of light is invariant.

Special relativity is really just physics in Minkowski space, that is the 4-dimensional space-time you obtain after promoting time from a parameter to a coordinate. Einstein wanted the laws of physics to be the same for all inertial observers in Minkowski-space, ie observers moving at constant velocity. If you translate this requirement into mathematics, you are lead to ask for the symmetry transformations in Minkowski-space. These transformations form a group – the Poincaré-group – from which you can read off all the odd things you have heard of: time-dilatation, length-contraction, relativistic mass, and so on.

The Poincaré-group itself has two subgroups. One contains just translations in space and time. This tells you that if you have an infinitely extended and unchanging space then it doesn’t matter where or when you do your experiment, the outcome will be the same. The remaining part of the Poincaré-group is the Lorentz-group. The Lorentz-group contains rotations – this tells you it doesn’t matter in which direction you turn, the laws of nature will still be the same. Besides the rotations, the Lorentz-group contains boosts, that are basically rotations between space and time. Invariance under boosts tells you that it doesn’t matter at which velocity you move, the laws of nature will remain the same. It’s the boosts where all the special relativistic fun goes on.

Deriving the Lorentz-group, if you know how to do it, is a three-liner, and I assure you it has absolutely nothing to do with rocket ships and lasers and so on. It is merely based on the requirement that the metric of Minkowski-space has to remain invariant. Carry through with the math and you’ll find that the boosts depend on a free constant with the dimension of a speed. You can further show that this constant is the speed of massless particles.

Hence, if photons are massless, then the constant in the Lorentz-transformation is the speed of light. If photons are not massless, then the constant in the Lorentz-transformation is still there, but not identical to the speed of light. We already know however that these constants must be identical to very good precision, which is the same as saying the mass of photons must be very small.

Giving a mass to photons is unappealing not because it violates special relativity – it doesn’t – but because it violates gauge-invariance, the most cherished principle underlying the standard model. But that’s a different story and shall be told another time.

Thanks for an interesting question!