A parable

Recently, I have read a paper in a prestigious journal in physics, whose logic was a bit stretched. Let me paraphrase it for you.

Italians are known to be good soccer players. Recently, some authors have noticed that Singaporeans may also be pretty decent soccer players. In this paper, we prove that Singaporeans can even be better than Italians.
For the test, the Singaporeans were chosen from one of the many soccer schools in the island; the Italians were chosen among the finalists of the certamen ciceronianum, the most famous competition of Latin prose writing. The age and bodily weight distributions were the same for both samples.
Each player was asked to try and score a penalty kick with the heel. Remarkably, the Singaporeans fared far better than the Italians. This conclusively proves that Singaporeans can be better soccer players than Italians in some tasks.

Reference: Xxxxx et al, Nature Physics Y, yyyy (20zz)


It will not backfire

Yesterday I was talking with a colleague about some sloppy papers published in the (supposedly) best journals — don’t try to find out which papers: you won’t guess, the sample is too big. At some point, my friend said about the authors: “It will backfire on them”. He said it with a point of sadness, because he has sincere concern for those people. I have thought it myself many times. But… will it really?

How many people are prisoners of their lust and commit horrible crimes! Many, probably most, go unpunished, even by their own conscience that they have managed to silence. Why should anything happen to people who are just prisoners of their mathematics, whose only mistake consists in not noticing that their definitions do not describe reality?

How many people commit financial crimes and injustices that ruin lives, and live without worries other than that of being one day stolen of their riches by poor fellows who will be called “criminals”! Why should some scientists, very decent fellows, be the victims of unfailing divine wrath just because they embellish (I am not speaking of faking) their results in order to get additional 5k$ per year of travel money?

No, I don’t think it will backfire: I don’t think we will see those sloppy works denounced and their authors forced to make amends. When the lobbying that keeps them going comes to an end, they will probably just be forgotten… but so will most of the works that one can consider “serious”: time is quite blind in erasing stuff.

If you want to uphold supposedly high standards, you must find other motives than the mere fear of being criticized one day.

Determinism and the speed of light

How do we know that no signal can travel faster than the speed of light? There cannot be a direct evidence of this fact. The indisputable fact is that the speed of light is the same in all reference frames.

In my knowledge, augmented with the browsing capability of my students, the only answer lies in the following deduction:

(1) The speed of light being the same in all reference frames and the principle of relativity lead, as well known, to the Lorentz transformation.

(2) Armed with the Lorentz transformation, one can rather easily show that a signal propagating faster than light could allow me to send a message to myself from the future to the past. Einstein himself was well aware of this (I don’t know if he was even the first to notice it).

Now, why is it a problem, that I can send a message to myself from the future to the past? Normally, the answer lists all kind of crazy things I could do, like winning all my bets and analog stuff. For me, the most dramatic consequence is all that I, and many others, can no longer do. First of all, when I receive a message at time t0 sent at time t1, I know that for sure I’ll have to send the message at time t1: it’s unavoidable. Moreover, all the events happened between t0 and t1 that the message informs me about have also become unavoidable. For instance, the message may inform me that a friend who is walking alongside me at t0 will be killed by a car, and even if this is going to happen one hour later, all I can do is to inform him that his life is about to end. Isn’t this absurd enough?

Well, I definitely find it is… but not necessarily someone who believes in full determinism! For such a person, all the information about what is going to happen in the future is already fully contained in the universe now, and always has been. There is nothing wrong in the physical universe that some existing piece of information gets stored in the neurons that call themselves “I” before the corresponding fact can be observed by all the brains: after all, that is what happens when we predict something with the laws of physics (say, the passing of an asteroid close to the earth).

The deduction made above is absolutely conclusive only if one believes in the possibility of creation of information that was previously not available. It is not such an outlandish belief: people believing in human free will have uphold it for centuries, and within physics, quite a few interpretations of quantum physics also uphold it. But it is funny to find it appearing in a topic usually supposed to derive from special relativity alone.

Two disclaimers. First, I am NOT claiming that I believe something can go faster than light — indeed, since I believe in free will, I do find the deduction above perfectly convincing. Second, I am also aware that, if anything would go faster than light, it should be massless: for massive objects, the Lorentz transformation predicts infinite inertia at a speed approaching that of light, independently of any argument about signaling to the past.

P.S. Within a few hours from the first post, I noticed another important assumption in the deduction above: the assumption that any physical phenomenon can be harnessed to send a signal — even more specifically, that it can be harnessed to implement the protocol that allows sending messages to the past (which implies sending the superluminal signal to an observer in relative motion and having it reflected back to myself).

Questions without answer

The advantage of holidays: one does not need to be serious all the time… and this is probably the only time when the really serious questions can come to the surface. I have jotted down here three of them, each one followed by an explanation of the context – I have got a few more questions, and more specific ones, but these will becomes research projects for my students and I am not going to reveal them here.

Question 1: is quantum physics imposing on us by its sheer weight?

The inadequacy of classical physics to describe nature is an established fact. Post factum, even the source of the problem has been clearly identified: the “classical prejudice” consists in believing that a definite value (true or false) can be assigned to any physical property at any time. But physics is not in a state of despair: positively, quantum physics has scored an impressive list of successes and is presently unchallenged. But what is quantum physics?

There is no characterization of quantum physics in terms of a small set of physical principles or phenomena. Relativity arises from the constancy of the speed of light, while the only compact definition of quantum physics is the description of its mathematical recipes. The legitimacy of those recipes is justified a posteriori: the number of correct predictions that are obtained is so large, that one must be very careful before even daring challenging a theory with such achievements. This is very reasonable, but should we just be content with it? I would really like to be able to say what quantum physics is in a few sentences, instead of burying the question under the sheer weight of the number of its achievements.

This question has no answer yet, but the answer may come one day through works like http://arxiv.org/abs/1112.1142. The next two questions are more undefined.

Question 2: how necessary is quantum physics?

Here it gets very speculative – but don’t forget, I was on holidays. It is well known that our universe is extremely well tuned, and this suggests prima facie that it has been tuned by an intelligent being. For several reasons, it seems desirable to have at least an alternative to such a conclusion. The currently fashionable alternative goes along the following lines: our universe would be extraordinary if it were unique; however, if all kind of universes are being “tried” in parallel, there is nothing astonishing in us living in the “right” one – by definition, if the universe is the wrong one, we cannot be there.

At first, this solution is meant to convey the idea of universes similar to ours, but in which the values of some physical constant differ. One may also admit that universes with more (or fewer) dimensions co-exist with ours. However, to be rigorous, one must admit the possibility of universes with laws that are absolutely different from ours: not only in the value of constant or in the number of dimensions, but in the very meaning of what “matter” is. Or maybe not? Maybe some features of our universe, duly extrapolated, are a super-universal necessity? Maybe reality is more constrained than speculative logic? As you see, we are not going to find the ultimate answer to this question – but it is good at times to sit at the verge of nonsense and feel its vertiginous call.

Question 3: emergence, seriously?

When Nobel Prize winners feel the need to contribute seriously to humanity, they write books and give talks about their vision of the world in prophetic terms, hoping that future will vindicate part of their vision. In this exercise, particle physicists then tend to adopt a bottom-up approach: everything is made of elementary particles and, in principle, everything could be explained at that level (though admittedly we are happy not to have to, when it comes to putting a satellite in orbit). Condensed matter physicists like to convey a different view, one in which the mess… sorry, the complexity they deal with every day is presented as irreducible: they like then to say that there is new physics emerging at larger scales than that of elementary particles. Their favorite example is the quantum Hall effect: it cannot arise without disorder in the atomic arrangement, and yet most of its features are described only in terms of elementary constants (think it calmly: each atom around itself sees some disorder, but somehow all the atoms together manage to forget the details of the mess and act in a clean, universal way). Sounds nice… but somehow, emergentists have never managed to look consistent in my eyes.

On the one hand, as noted above, operational emergentism is a necessity: it is a practical impossibility to use many-body quantum physics to describe the motion of a satellite, no human-built computer will ever be able to do that. But if this is the meaning of emergence, it is trivial. The deep question is whether emergence is real. Let us ask it this way: is nature doing physics from bottom-up, performing the computation that we can’t dream of simulating? Or, on the contrary, does it really have layers of complexity? When hard pressed, it seems to me that even the emergentists are scared of what their idea may ultimately mean, namely that order may appear in some cases from nothing below: a very suspicious conclusion, especially for the evolutionist cosmogony of our time…


You want my opinion? Well, when I was a teenager and all my friends started smoking, by that very fact I started finding smoking silly. In the same vein, since the multiverse is so popular, I believe in one single universe, ours; since everyone believes that everything is quantum, I believe that there may a real boundary where the quantum behavior stops; and I am sympathetic with the emergentists. On holidays, one can afford to be wrong.

Complacency in science

I have finally read Galbraith’s Short history of financial euphoria, which Alain Aspect suggested to me during a random dinner chat a few months ago. It’s nice: it’s the first time I understand something about finance. And it triggered a concern about academia.

In finance as well as in academia, people often fall into euphoria over something that is, by all rational standards, rather worthless. In my field of research, for instance, the latest craze is the following process:

  1. Write down a new version of some criterion that tests that “something is quantum” (a new Bell inequality, a new test of contextuality, a version of Leggett-Garg…); the simpler — the more trivial — the better, because of point 2.
  2. Find a couple of friends to do an experiment for you. Better if they have been running their setup for ages and have exhausted all the serious science that could possibly be done with it, because they will be more than happy to learn that their old machinery can still be used to perform “fundamental tests”. Moreover, since your test is simple and simple quantum physics has been tested to exhaustion, you have no doubt that the experimental results will uphold your theory.
  3. If you can, present it as “the first step towards [a big goal]”. Never mind that it is rather the last use of a setup that has made its time (I refrained to use “swan’s song”, because the last song of the swan is supposed to be the most beautiful; the last concert of an 80 years old pop star would be more appropriate a metaphor). If you can’t invoke the future, present it as “the conclusive proof of [some quantum claim]”. Never mind that the claim is usually always the same, namely, that results of measurements are not pre-established, that there is intrinsic randomness, or however you want to phrase it. Also never mind the fact that there cannot be a “conclusive claim” every month.

The euphoria mechanism is entertained as follows:

  • The big journals (Nature at the forefront) prefer to publish tons of poor science rather than risking and losing a single real breakthrough. So, if someone claims to have solved “the mystery of the quantum” (the general readership of Nature finds quantum physics mysterious), better take them seriously.
  • In turn, people notice that “if you do that, you publish in Nature”. Since “that” is not that difficult after all, it’s worth while going for it.
  • Once you have published in Nature (or Science or…), you are hailed as a hero by the head of your Department, by the communication office of your university, by the agencies that granted you the funds.
  • Put yourself now at the other end, namely in the place of the one who would like to raise a dissenting voice and reveal the triviality of the result. All the legitimate instances (peer reviewed journals, heads of prestigious Departments, grant agencies, even popular magazines and newspapers!) are against you. Isn’t it “obvious” then that you are only venting your jealousy, the jealousy of the loser?

So far, the analogy with financial euphoria is clear. I guess (though I have not studied the statistics) that the speed of the crash is also analogously fast: it happens when some of the editors of the main journals take a conscious decision of having “no more of that”, because they realize that there is really nothing to gain. The rumor spreads that “refereeing has become tough”; the journals are accused of having become irrational since “if they accepted the previous paper, why they refuse this one” (while it’s one of their few moments of rationality).

And the consequences? The same too, but fortunately without criminal pursuits, despair and suicides. The very big fish get out unscathed: either their science is really serious (that is, they have invested only a small amount of their scientific capital in the euphoric topic); or their power is really big (that is, they have invested only a small amount of their political power in backing the euphoria). The opportunists will try to follow the wind as they should, and will be forgotten as it should. Those who face uncertain destiny are the young fellows, who were doing serious science when the euphoria caught them at the right time and the right place. Because of this, they have been raised to prominence. Somehow, all their capital is invested in that topic. Will they be able to find their way out and continue doing serious science? Or will they end up teaming with their buddies, set up a specialized journal for themselves and publishing there until their old age? If one day you find me as the founder of a journal called “Nonlocality”, please wake me up.

Happy Easter!

Measuring uncertainty relations

In the space of two weeks, two works appear in Nature Physics about measuring uncertainty relations. In the first, an experiment is actually performed to test (and, needless to say, verify) the validity of an uncertainty relation which applies to more situations than the one originally devised by Heisenberg. In the second, it is proposed that the techniques of quantum optics may be used to probe modifications of the usual uncertainty relation due to gravity. Now, to have finally a tiny bit of evidence for quantum gravity, this would really be a breakthrough!

Faithful to my principle of not doing “refereeing on demand”, this is not an unrequested referee report: in fact, I have only browsed those papers, certainly not in enough depth to make judgments. The authors are serious so, by default, I trust them on all the technicalities. The question that I want to raise is: what claims can be made from an uncertainty relation?

An uncertainty relation looks like this:

[something related to the statistics of measurements, typically variances or errors] >= [a number that can be computed from the theory]

which has to be read as: if the left hand side is larger than 0, then there MUST be some error, or some variance, or some other form of “uncertainty” or “indeterminacy”. Let me write the equation above D>=C for shorthand.

Now, let’s see what a bad measurement can do for you. A bad measurement may introduce more uncertainties than are due to quantum physics. In other words, one may find D(measured)=C+B, where B is the additional contribution of the bad measurement. It may be the case that your devices cannot be improved, and so you can’t remove B. Now, the second paper proposes an experiment whose goal is precisely to show that D(measured)=C+G, where G is a correction due to gravity. Obviously, much more than the mere observation of the uncertainty relation will be needed, if someone has to believe their claim: they will really have to argue that there is no way to remove G and not because their devices are performing poorly. The problem is that there is always a way of removing G: a bad measurement can do it for you!

Indeed, a bad measurement may also violate the uncertainty relation. Let me give an extreme example: suppose that you forget to turn on the powermeter that makes the measurement. The result of position measurement will be systematically x=0, no error, no variance. Similarly, the result of momentum measurement will be systematically p=0, no error, no variance. In this situation, D(measured)=0. Of course, nobody would call that a “measurement”, but hey, that may well be “what you observe in the lab”. To be less trivial, suppose that the needle of your powermeter has become a bit stiff, rusty or whatever: the scale may be uncalibrated and you may easily observe D(measured)<C.

So, a bad measurement can influence the uncertainty relation both ways, either increasing or decreasing C.

Now, there are reasonable ways of getting around these arguments. For instance, by checking functional relations: don’t measure only one value, but several values, in different configurations. If the results match what you expect from quantum theory, a conspiracy becomes highly improbable; and indirectly it hints that your measurement was not bad after all. For instance, this is the case of Fig. 5a of the first paper mentioned above.

Still, I am left wondering if the tool of the uncertainty relation is at all needed, since by itself it constitutes very little evidence. Let me ask it this way: why, having collected enough statistics for a claim, should one process the information into an “uncertainty relation”? The information was already there, and probably much more of it than gets finally squeezed into those variances or errors. OK, maybe it’s just the right buzzword to get your serious science into Nature Physics: after all, “generalized uncertainty relation” will appeal to journalists much more than “a rigorous study of the observed data”.

Map your project

“I live in a house with garden in North America”. What is preposterous about this sentence? If you speak about your garden, you would not expect the whole of North America as a location. You’d expect something like “I live in a house with garden in the District D, Town T, Canada”.

Well, that is the level of introduction that you get in many papers and scientific presentations: let me make up one. “Quantum computers will have capacities beyond those of any classical computer. Here, we study how a bi-exciton decays in the quantum dots that our group has been trying to fabricate for 10 years with moderate success” (the last half is usually phrased differently, but everyone knows what that means). The point I am making here is: there is a world between the grand dreams of the field and the specific research topic one is dealing with. Following up on a point of a previous post: it is very useful to take some minutes to locate your project on the map of science, with a gradual zoom:

Continent – Nation – Town – Street – House

or, if you prefer:

Grand field – Big challenge within the field – Approach to the challenge – Specific technique [here is “the specifics of my boss”] – My project


Quantum computing – Experimental quantum coherence – Artificial atoms, Quantum dots – Self-assembled GaAs quantum dots – decay of the bi-exciton

Once you have established this map, here is my advice on how to structure a 20 minutes (10 slides) presentation, according to the circumstances. Take it with flexibility of course 🙂

  • Undergrad project: 1-1-3-2-3 [your project is probably an incremental step in your supervisor’s field, so it’s better to take a close focus; but two or three slides on the bigger picture are necessary]
  • PhD Thesis defense: 1-2-2-1-4 [your project is more relevant, so it should contribute at least a bit to the “big challenge”, if not to the grand field]
  • Generic conference: 1-3-3-2-1 [people should remember that “you work in that challenge”, they will forget the details]
  • Conference of your grand field: 0-2-3-3-2
  • Conference of your big challenge: 0-1-2-3-4
  • Specialized workshop: 0-0-1-3-6 [here is where people really care about your technicalities]
  • Grant defense: 5-3-0-0-2 [fine print will be lost, but you have to show that you are doing everyday progress]

A last word: contrary to geography, starting from your project you may zoom out in different ways. For instance, a study on quantum dots may also be seen as belonging to material science, or to quantum optics, rather than to quantum computing… Some people like to bring up all those maps at once: it’s a dangerous option, because it may confuse the audience on your motivation and also give the impression that you are trying to “play up”. The safe option consists in choosing one of the possible maps (the one that your public may like most) and stick to it.

Theorists collaborating with experimentalists: a tentative guide

This post is triggered by some of my students. I could give them my own advice directly, but I prefer to write it here because they may benefit from other valuable feedback. The issue at stake is pretty specific: the role of theorists collaborating with experimentalists. The background is my own, limited experience, the classification is as crude as it gets, the analysis proposed here is not the only possible one and is not even always suitable — so, let’s start!

It seems to me that theorist can contribute in four ways to experiments:

  1. Vision: come up with a new concept. It can be very general (say, quantum optics; quantum computing; quantum cryptography) or more specific (say, squeezed states; measurement-based quantum computing; quantum repeaters), but in any case it opens paths that were unexpected before. You won’t find these ideas planned in research proposals: they “happen”. I don’t suggest you base your career on the hope of having such a flash of genius (though you should be ready to recognize it, if it comes).
  2. Tools: develop the necessary theoretical understanding. To continue with the examples above: develop the formalism of quantum optics; invent fault-tolerant architectures; provide the formalism to do security proofs.
  3. Proposals: suggest that a given experimental setup can be used to observe some interesting effect, and do a rough first feasibility study.
  4. Specific description: take a real experiment and describe the physics, to the point where your theory matches the data.

As you can understand, these categories are not tight: for instance, some proposals make it directly into an experiment (nowadays, this is the route of choice to publish your proposal in Nature).

OK, now: here you come and have to decide in which direction to go. You may want absolutely to study a type of experiment: then you need to know what people in that field would appreciate. Or you may want to spend your life (say) inventing proposals: then you need to assess which experimental field is ripe for those. In both cases, the crucial insight for me is: there is a strong correlation between the role of a theorist and the stage of development of the experimental field:

  • At the beginning of a field, proposals are really important; they are also the fast way to celebrity (example: Cirac-Zoller first proposal of a logic gate in quantum computing). If you are up for a longer-term investment, go for tools even at this stage: your effort will be appreciated with a lag, but when the moment comes, you are seen as a pioneer and you are among the few of can really make a difference (example: the work of Norbert Lutkenhaus on unconditional security of quantum cryptography).
  • Once the field is mature, proposals must become really relevant (some physics journals can be seen as a cemetery of irrelevant proposals). Rule of thumb: if no experimentalist cares about your proposals, with overwhelming probability you are not the misunderstood genius, but just the delayed fellow (though, of course, exceptions are possible). At this stage, tools are the most appreciated contribution of theorists: if you develop them, you can choose to keep contact with the labs, or you can take the way of mathematical physics and develop them for their own sake. Both ways are serious; my advice is, but whichever you take, keep an eye on the other.
  • When a field is so mature that even the tools are fully developed, there is little left to do other than specific descriptions (this is the case, for instance, of quantum optics per se, i.e. aside from possible applications in quantum information). Specific descriptions of experiments are tricky. First, you need to check if the experimental group “needs your service”: some groups, for instance, have developed their tools so well, that their experiments are “textbook experiments”, which means that anyone who can follow a textbook can do the theory. Now, if your help is welcome, it is normally very welcome. Since there few people able to do that (compared to the mass of people contributing to the cemetery of proposals), you will be noticed among the experimentalists and may be asked for various collaborations. This is great for a PhD and, if you have such competence, you would do well in keeping it alive for the rest of your career. But you have to do something else as well, if you want to get a position: you need to show that you are also capable of original work (see why in a previous post).

Peer review

Recently, I have submitted four papers to a conference with different co-authors. After the peer review process, one was accepted for a talk, the three others for a poster. I do not copy all the reports here because it would be boring, but just the marks we received to the question “is this worth a talk”, ranging from +3 to -3. You will see a pattern emerge.

Paper 1 (the one that was accepted) had three reviewers: marks 3, 2 and 1

Paper 2 had three reviewers: 2, 0, -2

Paper 3 had two reviewers: 1, -2

Paper 4 had two reviewers: -3, 2 (the first reviewer, having noticed a few typos, mentioned “poor right up” [sic] as one of the reasons not to consider our submission).

Do you see the pattern? No? Look more closely… YES, you have got it: peer reviewing is random number generation 😉

Technical corollaries:

(1) With little post-processing, any correlation with the content of the paper can be removed for papers 2-4.

(2) Paper 1 is special, not because there is no spread, but because the average is not centered around 0. This bias is robust and can be eliminated only by suppressing buzzwords.

How to convey the wrong message

Last week, two undergraduates I know managed to convey the wrong impression about themselves and their work in a remarkably instructive way. They have learned from their (ultimately harmless) mistakes. Maybe someone else can learn too.

Case 1: a student presents the progress in his project. He starts by stressing that the title has changed because the initial project proved too ambitious. The rest of the talk is a review of some schemes for atom cooling: nice and clear, but covering pretty well known material and leaving important schemes aside. You are listening to him. What do you conclude?

Here is a pretty reasonable analysis: the student did not match the expectations of the supervisor, so the initial ambitious project was tuned down to something hardly more than a review of literature.

Here is the truth: the incompetent fellow, if anyone, is the supervisor (myself), who did not evaluate correctly the difficulty of the initial project. The student is doing very well, he has done much more than a simple review in terms of calculations and simulations. The review was simple because I asked him to use graphs and pictures instead of equations; it was incomplete because the goal is to describe a real ongoing experiment, not to review the whole field of atom cooling!

Where did the student fail? Here are some hints:

  1. Opening the talk by stressing that the focus of the project has changed was the wrong thing to do: it conveys the message that something has gone wrong. Incidentally, the initial topic was pretty similar and nobody would have noticed the change.
  2. The student embarked in the usual idiotic “atom cooling is a fundamental whatever-not in modern physics, interesting both for our understanding of nature and for applications” and failed to mention the REAL motivation, which is the description of an experiment which is really happening two floors below.
  3. The talk was too simple. One must be clear of course, but if it is a research project, one must devote one or two slides to show off — I mean, to convey what has been done.

Case 2: after a few months of research under the direction of a post-doc, a student is asked in the office of the professor (a Singaporean Chinese, which matters for what follows). The professor starts by asking “What have you learned of this field so far?” and the student replies “Nothing much really”.

Here, you don’t have to be a professor to guess the rest. What is more astonishing is the background story.

The truth is that the student had learned a lot and was aware of it. But she feigned ignorance in order to give the professor the chance to explain things from the beginning and learn from his insight! She thought that, by acting this way, she would show how eager to learn she is and how much she appreciates the wisdom of the professor.

This attitude, to this extreme, can only be found in students who have been formed in the Confucian style. But a milder form happens everywhere: for instance, when a professor asks in a lecture “Do you know this or do you need a reminder?”, almost always the students ask for the reminder (maybe there it’s also a trick to slow down the pace and trick the professor into not covering too much new material). In the context of master-to-disciple relationships, it may have its value (though I personally hate it). But if you are going into research, you have to show what you know and admit what you really don’t know: both false humility and false confidence will be detected and signal the end of your application.