Stephen Hawking’s 75th Birthday Conference: Impressions

I’m back from Cambridge, where I attended the conference “Gravity and Black Holes” in honor of Stephen Hawking’s 75th birthday.

First things first, the image on the conference poster, website, banner, etc is not a psychedelic banana, but gravitational wave emission in a black hole merger. It’s a still from a numerical simulation done by a Cambridge group that you can watch in full on YouTube.

What do gravitational waves have to do with Stephen Hawking? More than you might think.

Stephen Hawking, together with Gary Gibbons, wrote one of the first papers on the analysis of gravitational wave signals. That was in 1971, briefly after gravitational waves were first “discovered” by Joseph Weber. Weber’s detection was never confirmed by other groups. I don’t think anybody knows just what he measured, but whatever it was, it clearly wasn’t gravitational waves. Also Hawking’s – now famous – area theorem stemmed from this interest in gravitational waves, which is why the paper is titled “Gravitational Radiation from Colliding Black Holes.”

Second things second, the conference launched on Sunday with a public symposium, featuring not only Hawking himself but also Brian Cox, Gabriela Gonzalez, and Martin Rees. I didn’t attend because usually nothing of interest happens at these events. I think it was recorded, but haven’t seen the recording online yet – will update if it becomes available.

Gabriela Gonzalez was spokesperson of the LIGO collaboration when the first (real) gravitational wave detection was announced, so you have almost certainly seen her. She also gave a talk at the conference on Tuesday. LIGO’s second run is almost done now, and will finish in August. Then it’s time for the next schedule upgrade. Maximal design sensitivity isn’t expected to be reached until 2020. Above all, in the coming years, we’ll almost certainly see much better statistics and smaller error bars.

The supposed correlations in the LIGO noise were worth a joke by the session’s chairman, and I had the pleasure of talking to another member of the LIGO collaboration who recognized me as the person who wrote that upsetting Forbes piece. I clearly made some new friends there^^. I’d have some more to say about this, but will postpone this to another time.

Back to the conference. Monday began with several talks on inflation, most of which were rather basic overviews, so really not much new to report. Slava Mukhanov delivered a very Russian presentation, complaining about people who complain that inflation isn’t science. Andrei Linde then spoke about attractors in inflation, something I’ve been looking into recently, so this came in handy.

Monday afternoon, we had Jim Hartle speaking about the No-Boundary proposal – he was not at all impressed by Neil Turok et al’s recent criticism – and Raffael Bousso about the ever-tightening links between general relativity and quantum field theory. Raffael’s was the probably most technical talk of the meeting. His strikes me as a research program that will still run in the next century. There’s much to learn and we’ve barely just begun.

On Tuesday, besides the already mentioned LIGO talk, there were a few other talks about numerical general relativity – informative but also somehow unexciting. In the afternoon, Ted Jacobson spoke about fluid analogies for gravity (which I wrote about here), and Jeff Steinhauer reported on his (still somewhat controversial) measurement of entanglement in the Hawking radiation of such a fluid analogy (which I wrote about here.)

Wednesday began with a rather obscure talk about how to shove information through wormholes in AdS/CFT that I am afraid might have been somehow linked to ER=EPR, but I missed the first half so not sure. Gary Gibbons then delivered a spirited account of gravitational memory, though it didn’t become clear to me if it’s of practical relevance.

Next, Andy Strominger spoke about infrared divergences in QED. Hearing him speak, the whole business of using soft gravitons to solve the information loss problem suddenly made a lot of sense! Unfortunately I immediately forgot why it made sense, but I promise to do more reading on that.

Finally, Gary Horowitz spoke about all the things that string theorists know and don’t know about black hole microstates, which I’d sum up with they know less than I thought they do.

Stephen Hawking attended some of the talks, but didn’t say anything, except for a garbled sentence that seems to have played back by accident and stumped Ted Jacobson.

All together, it was a very interesting and fun meeting, and also a good opportunity to have coffee with friends both old and new. Besides food for thought, I also brought back a conference bag, a matching pen, and a sinus infection which I blame on the air conditioning in the lecture hall.

Now I have a short break to assemble my slides for next week’s conference and then I’m off to the airport again.

Stephen Hawking turns 75. Congratulations! Here’s what to celebrate.

If people know anything about physics, it’s the guy in a wheelchair who speaks with a computer. Google “most famous scientist alive” and the answer is “Stephen Hawking.” But if you ask a physicist, what exactly is he famous for?

Hawking became “officially famous” with his 1988 book “A Brief History of Time.” Among physicists, however, he’s more renowned for the singularity theorems. In his 1960s work together with Roger Penrose, Hawking proved that singularities form under quite general conditions in General Relativity, and they developed a mathematical framework to determine when these conditions are met.

Before Hawking and Penrose’s work, physicists had hoped that the singularities which appeared in certain solutions to General Relativity were mathematical curiosities of little relevance for physical reality. But the two showed that this was not so, that, to the very contrary, it’s hard to avoid singularities in General Relativity.

Since this work, the singularities in General Relativity are understood to signal the breakdown of the theory in regions of high energy-densities. In 1973, together with George Ellis, Hawking published the book “The Large Scale Structure of Space-Time” in which this mathematical treatment is laid out in detail. Still today it’s one of the most relevant references in the field.

Only a year later, in 1974, Hawking published a seminal paper in which he demonstrates that black holes give off thermal radiation, now referred to as “Hawking radiation.” This evaporation of black holes results in the black hole information loss paradox which is still unsolved today. Hawking’s work demonstrated clearly that the combination of General Relativity with the quantum field theories of the standard model spells trouble. Like the singularity theorems, it’s a result that doesn’t merely indicate, but prove that we need a theory of quantum gravity in order to consistently describe nature.

While the 1974 paper was predated by Bekenstein’s finding that black holes resemble thermodynamical systems, Hawking’s derivation was the starting point for countless later revelations. Thanks to it, physicists understand today that black holes are a melting pot for many different fields of physics – besides general relativity and quantum field theory, there is thermodynamics and statistical mechanics, and quantum information and quantum gravity. Let’s not forget astrophysics, and also mix in a good dose of philosophy. In 2017, “black hole physics” could be a subdiscipline in its own right – and maybe it should be. We owe much of this to Stephen Hawking.

In the 1980s, Hawking worked with Jim Hartle on the no-boundary proposal according to which our universe started in a time-less state. It’s an appealing idea whose time hasn’t yet come, but I believe this might change within the next decade or so.

After this, Hawking tries several times to solve the riddle of black hole information loss that he posed himself, most recently in early 2016. While his more recent work has been met with interest in the community, it hasn’t been hugely impactful – it attracts significantly more attention by journalists than by physicists.

As a physicist myself, I frequently get questions about Stephen Hawking: “What’s he doing these days?” – I don’t know. “Have you ever met him?” – He slept right through it. “Do you also work on the stuff that he works on?” – I try to avoid it. “Will he win a Nobel Prize?” – Ah. Good question.

Hawking’s shot at the Nobel Prize is the Hawking radiation. The astrophysical black holes which we can presently observe have a temperature way too small to be measured in the foreseeable future. But since the temperature increases for smaller mass, lighter black holes are hotter, and could allow us to measure Hawking radiation.

Black holes of sufficiently small masses could have formed from density fluctuations in the early universe and are therefore referred to as “primordial black holes.” However, none of them have been seen, and we have tight observational constraints on their existence from a variety of data. It isn’t yet entirely excluded that they are around, but I consider it extremely unlikely that we’ll observe one of these within my lifetime.

For what the Nobel is concerned, this leaves the Hawking radiation in gravitational analogues. In this case, one uses a fluid to mimic a curved space-time background. The mathematical formulation of this system is (in certain approximations) identical to that of an actual black hole, and consequently the gravitational analogues should also emit Hawking radiation. Indeed, Jeff Steinhauer claims that he has measured this radiation.

At the time of writing, it’s still somewhat controversial whether Steinhauer has measured what he thinks he has. But I have little doubt that sooner or later this will be settled – the math is clear: The radiation should be there. It might take some more experimental tinkering, but I’m confident sooner or later it’ll be measured.

Sometimes I hear people complain: “But it’s only an analogy.” I don’t understand this objection. Mathematically it’s the same. That in the one case the background is an actually curved space-time and in the other case it’s an effectively curved space-time created by a flowing fluid doesn’t matter for the calculation. In either situation, measuring the radiation would demonstrate the effect is real.

However, I don’t think that measuring Hawking radiation in an analogue gravity system would be sufficient to convince the Nobel committee Hawking deserves the prize. For that, the finding would have to have important implications beyond confirming a 40-years-old computation.

One way this could happen, for example, would be if the properties of such condensed matter systems could be exploited as quantum computers. This isn’t as crazy as it sounds. Thanks to work built on Hawking’s 1974 paper we know that black holes are both extremely good at storing information and extremely efficient at distributing it. If that could be exploited in quantum computing based on gravitational analogues, then I think Hawking would be in line for a Nobel. But that’s a big “if.” So don’t bet on it.

Besides his scientific work, Hawking has been and still is a master of science communication. In 1988, “A Brief History of Time” was a daring book about abstract ideas in a fringe area of theoretical physics. Hawking, to everybody’s surprise, proved that the public has an interest in esoteric problems like what happens if you fall into a black hole, what happed at the Big Bang, or whether god had any choice when he created the laws of nature.

Since 1988, the popular science landscape has changed dramatically. There are more books about theoretical physics than ever before and they are more widely read than ever before. I believe that Stephen Hawking played a big role in encouraging other scientists to write about their own research for the public. It certainly was an inspiration for me.

So, Happy Birthday, Stephen, and thank you.

"Rate your Supervisor" comes to High Energy Physics

A new website called the "HEP Postdoc Project" allows postdocs in high energy physics to rate their supervisors in categories like "friendliness," "expertise," and "accessibility."

I normally ignore emails that more or less explicitly ask me to advertise sites on my blog, but decided to make an exception for this one. It seems a hand-made project run by a small number of anonymous postdocs who want to help their fellows find good supervisors. And it's a community that I care much about.

While I appreciate the initiative, I have to admit being generally unenthusiastic about anonymous ratings on point scales. Having had the pleasure of reading though an estimated several thousand of recommendation letters, I have found that an assessment of skills is only useful if you know the person it comes from.

Much of this is cultural. A letter from a Russian prof that says this student isn't entirely bad at math might mean the student is up next for the Fields Medal. On the other hand, letters from North Americans tend to exclusively contain positive statements, and the way to read them is to search for qualities that were not listed.

But leaving aside the cultural stereotypes, more important are personal differences in the way people express themselves and use point scales, even if they are given a description for each rating (and that is missing on the website). We occasionally used 5 point rating scales in committees. You then notice quickly that some people tend to clump everyone in the middle-range, while others are more comfortable using the high and low scores. Then again others either give a high rating or refuse to have any opinion. To get a meaningful aggregate, you can't just take an average, you need to know roughly how each committee member uses the scale. (Which will require endless hours of butt-flattening meetings. Trust me, I'd be happy being done with clicking on a star scale.)

You could object that any type of online rating suffers from these problems and yet they seem to serve some purpose. That's right of course, so this isn't to say they're entirely useless. Thus I am sharing this link thinking it's better than nothing. And at the very least you can have some fun browsing through the list to see who got the lowest marks ;)

Does the arXiv censor submissions?

The arXiv is the physicsts' marketplace of ideas. In high energy physics and adjacent fields, almost all papers are submitted to the arXiv prior to journal submission. Developed by Paul Ginsparg in the early 1990s, this open-access pre-print repository has served the physics community for more than 20 years, and meanwhile extends also to adjacent fields like mathematics, economics, and biology. It fulfills an extremely important function by helping us to exchange ideas quickly and efficiently.

Over the years the originally free signup became more restricted. If you sign up for the arXiv now, you need to be "endorsed" by several people who are already signed up. It also became necessary to screen submissions to keep the quality level up. In hindsight, this isn't surprising: more people means more trouble. And sometimes, of course, things go wrong.

I have heard various stories about arXiv moderation gone wrong, mostly these are from students, and mostly it affects those who work in small research areas or those whose name is Garrett Lisi.

A few days ago, a story appeared online which quickly spread. Nicolas Gisin, an established Professor for Physics who works on quantum cryptography (among other things) relates the story of two of his students who ventured in a territory unfamiliar for him, black hole physics. They wrote a paper that appeared to him likely wrong but reasonable. It got rejected by the arxiv. The paper later got published by PLA (a respected journal that however does not focus on general relativity). More worrisome still, the students' next paper also got rejected by the arXiv, making it appear as if they were now blacklisted.

Now the paper that caused the offense is, haha, not on the arXiv, but I tracked it down. So let me just say that I think it's indeed wrong and it shouldn't have gotten published in a journal. They are basically trying to include the backreaction of the outgoing Hawking-radiation on the black hole. It's a thorny problem (the very problem this blog was named after) and the treatment in the paper doesn't make sense.

Hawking radiation is not produced at the black hole horizon. No, it is not. And tracking back the flux from infinity to the horizon is therefore is not correct. Besides this, the equation for the mass-loss that they use is a late-time approximation in a collapse situation. One can't use this approximation for a metric without collapse, and it certainly shouldn't be used down to the Planck mass. If you have a collapse-scenario, to get the backreaction right you would have to calculate the emission rate prior to horizon formation, time-dependently, and integrate over this.

Ok, so the paper is wrong. But should it have been rejected by the arXiv? I don't think so. The arxiv moderation can't and shouldn't replace peer review, it should just be a basic quality check, and the paper looks like a reasonable research project.

I asked a colleague who I know works as an arXiv moderator for comment. (S)he wants to stay anonymous but offers the following explanation:

I had not heard of the complaints/blog article, thanks for passing that information on...  

 The version of the article I saw was extremely naive and was very confused regarding coordinates and horizons in GR... I thought it was not “referee-able quality’’ — at least not in any competently run GR journal... (The hep-th moderator independently raised concerns...)  

 While it is now published at Physics Letters A, it is perhaps worth noting that the editorial board of Physics Letters A does *not* include anyone specializing in GR.

(S)he is correct of course. We haven't seen the paper that was originally submitted. It was very likely in considerably worse shape than the published version. Indeed, Gisin writes in his post that the paper was significantly revised during peer review. Taking this into account, the decision seems understandable to me.

The main problem I have with this episode is not that a paper got rejected which maybe shouldn't have been rejected -- because shit happens. Humans make mistakes, and let us be clear that the arXiv, underfunded as it is, relies on volunteers for the moderation. No, the main problem I have is the lack of transparency.

The arXiv is an essential resource for the physics community. We all put trust in a group of mostly anonymous moderators who do a rather thankless and yet vital job. I don't think the origin of the problem is with these people. I am sure they do the best they can. No, I think the origin of the problem is the lack of financial resources which must affect the possibility to employ administrative staff to oversee the operations. You get what you pay for.

I hope that this episode be a wake-up call to the community to put their financial support behind the arXiv, and to the arXiv to use this support to put into place a more transparent and better organized moderation procedure.

Note added: It was mentioned to me that the problem with the paper might be more elementary in that they're using wrong coordinates to begin with - it hadn't even occurred to me to check this. To tell you the truth, I am not really interested in figuring out exactly why the paper is wrong, it's besides the point. I just hope that whoever reviewed the paper for PLA now goes and sits in the corner for an hour with a paper bag over their head.

Hello from Maui

Greetings from the west-end of my trip, which brought me out to Maui, visiting Garrett at the Pacific Science Institute, PSI. Launched roughly a year ago, Garrett and his girlfriend/partner Crystal have now hosted about 60 traveling scientists, "from all areas except chemistry" I was told.

I got bitten by mosquitoes and picked at by a set of adorable chickens (named after the six quarks), but managed to convince everybody that I really didn't feel like swimming, or diving, or jumping off things at great height. I know I'm dull. I did watch some sea turtles though and I also got a new T-shirt with the PSI-logo, which you can admire in the photo to the right (taken in front of a painting by Crystal).

I'm not an island-person, don't like mountains, and I can't stand humidity, so for me it's somewhat of a mystery what people think is so great about Hawaii. But leaving aside my preference for German forests, it's as pleasant a place as can be.

You won't be surprised to hear that Garrett is still working on his E8 unification and says things are progressing well, if slowly. Aloha.

Book review: "Cracking the Particle Code of the Universe" by John Moffat

Cracking the Particle Code of the Universe: the Hunt for the Higgs Boson
By John W Moffat
Oxford University Press (2014)

John Moffat’s new book covers the history of the Standard Model of particle physics from its beginnings to the recent discovery of the Higgs boson – or, as Moffat cautiously calls it, the new particle most physicists believe is the Standard Model Higgs. But Cracking the Particle Code of the Universe isn’t just any book about the Standard Model: it’s about the model as seen through the eyes of an insider, one who has witnessed many fads and statistical fluctuations come and go. As an emeritus professor at the University of Toronto, Canada and a senior researcher at the nearby Perimeter Institute, Moffat has the credentials to do more than just explain the theory and the experiments that back it up: he also offers his own opinion on the interpretation of the data, the status of the theories and the community’s reaction to the discovery of the Higgs.

The first half of the book is mainly dedicated to introducing the reader to the ingredients of the Standard Model, the particles and their properties, the relevance of gauge symmetries, symmetry breaking, and the workings of particle accelerators. Moffat also explains some proposed extensions and alternatives to the Standard Model, such as technicolor, supersymmetry, preons, additional dimensions and composite Higgs models as well as models based on his own work. In each case he lays out the experimental situation and the technical aspects that speak for and against these models.

In the second half of the book, Moffat recalls how the discovery unfolded at the LHC and comments on the data that the collisions yielded. He reports from several conferences he attended, or papers and lectures that appeared online, and summarizes how the experimental analysis proceeded and how it was interpreted. In this, he includes his own judgment and relates discussions with theorists and experimentalists. We meet many prominent people in particle physics, including Guido Altarelli, Jim Hartle and Stephen Hawking, to mention just a few. Moffat repeatedly calls for a cautious approach to claims that the Standard Model Higgs has indeed been discovered, and points out that not all necessary characteristics have been found. He finds that the experimentalists are careful with their claims, but that the theoreticians jump to conclusions.

The book covers the situation up to March 2013, so of course it is already somewhat outdated; the ATLAS collaboration’s evidence for the spin-0 nature of the Higgs boson was only published in June 2013, for example. But this does not matter all that much because the book will give the dedicated reader the necessary background to follow and understand the relevance of new data.

Moffat’s writing sometimes gets quite technical, albeit without recourse to equations, and I doubt that readers will fully understand his elaborations without at least some knowledge of quantum field theory. He introduces the main concepts he needs for his explanations, but he does so very briefly; for example, his book features the briefest explanation of gauge invariance I have ever come across, and many important concepts, such as cross-sections or the relation between the masses of force-carriers and the range of the force, are only explained in footnotes. The glossary can be used for orientation, but even so, the book will seem very demanding for readers who encounter the technical terms for the first time. However, even if they are not able to follow each argument in detail, they should still understand the main issues and the conclusions that Moffat draws.

Towards the end of the book, Moffat discusses several shortcomings of the Standard Model, including the Higgs mass hierarchy problem, the gauge hierarchy problem, and the unexplained values of particle masses. He also briefly mentions the cosmological constant problem, as it is related to questions about the nature of the vacuum in quantum field theory, but on the whole he stands clear from discussing cosmology. He does, however, comment on the anthropic principle and the multiverse and does not hesitate to express his dismay about the idea.

While Moffat gives some space to discussing his own contributions to the field, he does not promote his point of view as the only reasonable one. Rather, he makes a point of emphasizing the necessity of investigating alternative models. The measured mass of the particle-that-may-be-the-Higgs is, he notes, larger than expected, and this makes it even more pressing to find models better equipped to address the problems with “naturalness” in the Standard Model.

I have met Moffat on various occasions and I have found him to be not only a great physicist and an insightful thinker, but also one who is typically more up-to-date than many of his younger colleagues. As the book also reflects, he closely follows the online presentations and discussions of particle physics and particle physicists, and is conscious of the social problems and cognitive biases that media hype can produce. In his book, Moffat especially criticizes bloggers for spreading premature conclusions.

Moffat’s recollections also document that science is a community enterprise and that we sometimes forget to pay proper attention to the human element in our data interpretation. We all like to be confirmed in our beliefs, but as my physics teacher liked to say “belief belongs into the church.” I find it astonishing that many theoretical physicists these days publicly express their conviction that a popular theory “must be” right even when still unconfirmed by data – and that this has become accepted behavior for scientists. A theoretician who works on alternative models today is seen too easily as an outsider (a non-believer), and it takes much courage, persistence, and stable funding sources to persevere outside mainstream, like Moffat has done for decade and still does. This is an unfortunate trend that many in the community do not seem to be aware of, or do not see why it is of concern, and it is good that Moffat in his book touches on this point.

In summary, Moffat’s new book is a well-done and well-written survey of the history, achievements, and shortcomings of the Standard Model of particle physics. It will equip the reader with all the necessary knowledge to put into context the coming headlines about new discoveries at the LHC and future colliders.

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This review first appeared in Physics World on Dec 4th under the title "A strong model, with flaws".

The hunt for the first exoplanet

The little prince

Today, extrasolar planets, or exoplanets for short, are all over the news. Hundreds are known, and they are cataloged in The Extrasolar Planets Encyclopaedia, accessible for everyone who is interested. Some of these extrasolar planets orbit a star in what is believed to be a habitable zone, fertile ground for the evolution of life. Planetary systems, much like ours, have turned out to be much more common results of stellar formation than had been expected.

But the scientific road to this discovery has been bumpy.

Once one knows that stars on the night sky are suns like our own, it doesn't take a big leap of imagination to think that they might be accompanied by planets. Observational evidence for exoplanets was looked for already in the 19th century, but the field had a bad start.

Beginning in the 1950s, several candidates for exoplanets made it into the popular press, yet they turned out to be data flukes. At that time, the experimental method used relied on detecting minuscule changes in the motion of the star caused by a heavy planet of Jupiter type.

If you recall the two-body problem from 1st semester: It's not that one body orbits the other, but they both orbit around their common center-of-mass, just that, if one body is much heavier than the other, it might almost look like the lighter one is orbiting the heavier one. But if a sufficiently heavy planet orbits a star, one might in principle find out by watching the star very closely because it wobbles around the center-of-mass. In the 50s, watching the star closely meant watching its distance to other stellar objects. The precision which could be achieved this way simply wasn't sufficient to reliably tell the presence of a planet.

In the early 80s, Gordon Walker and his postdoc Bruce Campbell from British Columbia, Canada, pioneered a new technique that improved the possible precision by which the motion of the star could be tracked by two orders of magnitude. Their new technique relied on measuring the star's absorption lines, whose frequency depends on the motion of the star relative to us because of the Doppler effect.

To make that method work, Walker and Campbell had to find a way to precisely compare spectral images taken at different times so they'd know how much the spectrum had shifted. They found an ingenious solution to that: They would used the, very regular and well-known, molecular absorption lines of hydrogen fluoride gas. The comb-like absorption lines of hydrogen fluoride served as a ruler relative to which they could measure the star's spectrum, allowing them to detect even smallest changes. Then, together with astronomer Stephenson Yang, they started looking at candidate stars which might be accompanied by Jupiter-like planets.

To detect the motion of the star due to the planet, they would have to record the system for several completed orbits. Our planet Jupiter needs about 12 years to orbit the sun, so they were in for a long-term project. Unfortunately, they had a hard time finding support for their research.

In his recollection “The First High-Precision Radial Velocity Search for Extra-Solar Planets” (arXiv:0812.3169), Gordon Walker recounts that it was difficult to get time for their project at observatories: “Since extra-solar planets were expected to resemble Jupiter in both mass and orbit, we were awarded only three or four two-night observing runs each year.” And though it is difficult to understand today, back then many of Walker's astronomer colleagues thought the search for exoplanets a waste of time. Walker writes:

“It is quite hard nowadays to realise the atmosphere of skepticism and indifference in the 1980s to proposed searches for extra-solar planets. Some people felt that such an undertaking was not even a legitimate part of astronomy. It was against such a background that we began our precise radial velocity survey of certain bright solar-type stars in 1980 at the Canada France Hawaii 3.6-m Telescope.”

After years of data taking, they had identified several promising candidates, but were too cautious to claim a discovery. At the 1987 meeting of the American Astronomical Society in Vancouver, Campbell announced their preliminary results. The press reported happily yet another discovery of an exoplanet, but the astronomers regarded even Walker and Campbell's cautious interpretation of the data with large skepticism. In his article “Lost world: How Canada missed its moment of glory,” Jacob Berkowitz describes the reaction of Walker and Campbell's colleagues:

“[Campbell]'s professional colleagues weren't as impressed [as the press]. One astronomer told The New York Times he wouldn't call anything a planet until he could walk on it. No one even attempted to confirm the results.”

Walker's gifted postdoc Bruce Campbell suffered most from the slow-going project that lacked appreciation and had difficulties getting continuing funding. In 1991, after more than a decade of data taking, they still had no discovery to show up with. Campbell meanwhile had reached age 42, and was still sitting on a position that was untenured, was not even tenure-track. Campbell's frustration built up to the point where he quit his job. When he left, he erased all the analyzed data in his university account. Luckily, his (both tenured) collaborators Walker and Yang could recover the data. Campbell made a radical career change and became a personal tax consultant.

But in late 1991, Walker and Yang were finally almost certain to have found sufficient evidence of an exoplanet around the star gamma Cephei, whose spectrum showed a consistent 2.5 year wobble. In a fateful coincidence, when Walker just thought they had pinned it down, one of his colleagues, Jaymie Matthews, came by his office, looked at the data and pointed out that the wobble in the data coincided with what appeared to be periods of heightened activity on the star's surface. Walker looked at the data with new eyes and, mistakenly, believed that they had been watching all the time an oscillating star rather than a periodic motion of the star's position.

Briefly after that, in early 1992, Nature reported the first confirmed discovery of an exoplanet by Wolszczan and Frail, based in the USA. Yet, the planet they found orbits a millisecond pulsar (probably a neutron star), so for many the discovery doesn't score highly because the star's collapse would have wiped out all life in that planetary system long ago.

In 1995 then, astronomers Mayor and Queloz of the University of Geneva announced the first definitive observational evidence for an exoplanet orbiting a normal star. The planet has an orbital period of a few days only, no decade long recording was necessary.

It wasn't until 2003 that the planet that Walker, Campbell and Yang had been after was finally confirmed.

There are three messages to take away from this story.

First, Berkowitz in his article points out that Canada failed to have faith in Walker and Campbell's research at the time when just a little more support would have made them first to discover an exoplanet. Funding for long-term projects is difficult to obtain and it's even more difficult if the project doesn't produce results before it's really done. That can be an unfortunate hurdle for discoveries.

Second, it is in hindsight difficult to understand why Walker and Campbell's colleagues were so unsupportive. Nobody ever really doubted that exoplanets exist, and with the precision of measurements in astronomy steadily increasing, sooner or later somebody would be able to find statistically significant evidence. It seems that a few initial false claims had a very unfortunate backlash that did exceed the reasonable.

Third, in the forest of complaints about lacking funding for basic research, especially for long-term projects, every tree is a personal tragedy.

Wolfgang Pauli, 1931, not so dry

In 1931, Wolfgang Pauli went for a long-term stay to Ann Arbor, Michigan. In Ann Arbor, Pauli gave lectures and met, among others, with Otto Laporte, George Uhlenbeck and Arnold Sommerfeld. In the summer 1931, the USA suffered from heat and prohibition. In a letter from July 1st, 1931 to his student Rudolf Peierls, Pauli wrote:

"[T]rotz Gelegenheit zum Schwimmen leide ich sehr unter der großen Hitze hier. Unter der "Trockenheit" leide ich aber gar nicht, da Laporte und Uhlenbeck ausgezeichnet mit Alkohol versorgt sind (man merkt die Nähe der kanadisehen Grenze). Physik (und Physiker) gibt es hier sehr viel, aber ich finde sie zu formal..."

"Despite the opportunity to swim, I suffer from the heat. I do not suffer however from the "dryness," since Laporte and Uhlenbeck have an excellent supply of liquor (one notices the vicinity of the Canadian border). One finds here a lot of physics (and physicists), but most I find too formal..."

Evidently, the supply was ample since, in a letter from later that summer, Pauli reported:

"Dummerweise bin ich neulich (in etwas angeheitertem Zustand) so ungünstig über eine Treppe gefallen, daß ich mir die Schulter gebrochen habe und nun im Bett liegen muß, bis die Knochen wieder ganz sind - sehr langweilig."

"Unfortunately, the other day I fell (somewhat tipsy) on the stairs and broke my shoulder. Now I have to lie in bed till the bones have healed - very boring."

Since drinking was illegal, the official reason for his accident was that he slipped on the tiles at the swimming pool. In the image to the right, you see Pauli with his broken shoulder. Click to enlarge. Image source: CERN archive. Text source: "Wolfgang Pauli: Scientific Correspondence with Bohr, Einstein, Heisenberg a.o." Volume II: 1930-1939, edited by Karl von Meyenn, Springer-Verlag (1985).

Advent calendar #24: Bohr's theory of the Wild West

Today's anecdote about Niels Bohr comes from George Gamow's book "Thirty years that shook physics - The story of quantum theory." This is the same Gamow we have met earlier in correspondence with Wolfgang Pauli. Gamow is the person who famously predicted the cosmic background radiation long before it was discovered. In the late 1920s, he was a student in Copenhagen under Niels Bohr, and tells the following:

The only movies [Bohr] liked were Wild Westerns (Hollywood style), and he always needed a couple of his students to go with him and explain the complicated plots... But his theoretical mind showed even in this movie expeditions. He developed a theory to explain why although the villain always draws first, the hero is faster and manages to kill him. This Bohr theory was based on psychology. Since the hero never shots first, the villain has to decide when to draw, which impedes his action. The hero on the other hand acts according to conditioned reflexes and grabs the gun automatically as soon as he sees the villain's hand move. We disagreed with this theory, and the next day I went to a toy store and bought two guns in Western holders. We shot it out with Bohr, he being the hero, and he "killed" all his students.

Advent calendar #23: Moonshine in Rutherford's brain

Ernest Rutherford is known for his achievements in atomic and nuclear physics, most essentially the insight that the mass of the atom is concentrated in a small nucleus. This is known today as the Rutherford model of the atom, and was experimentally shown by scattering alpha particles on gold. Rutherford won the Nobel prize for Chemistry in 1908 for his investigations into the disintegration of the elements, and the chemistry of radioactive substances.

In 1933, he gave a talk at a meeting of the British Association for the Advancement of Science, from which he was quoted in The London Times of September 12, 1933 about the possibility of energy-efficient nuclear fission as follows:

We might in these processes obtain very much more energy than the proton supplied, but on the average we could not expect to obtain energy in this way. It was a very poor and inefficient way of producing energy, and anyone who looked for a source of power in the transformation of the atoms was talking moonshine. But the subject was scientifically interesting because it gave insight into the atoms.

The 16 September 1933 issue of Nature tells its readers about the talk:

One timely word of warning was issued to those who look for sources of power in atomic transmutations ‒ such expecations are the merest moonshine.

It is easy to invoke this statement as a further example for a severe scientific misjudgement by a senior scientist. But in context, it was perfectly reasonable: Rutherford was discussing nuclear reactions tiggered by the proton beam from the then brand-new accelerator of Cockroft and Walton. Trying to gain nuclear energy that way is about as efficient as producing antimatter at CERN to fuel a matter-antimatter-annihilation engine.

Moreover, in his paper Atomic Energy is "Moonshine": What did Rutherford Really Mean?, historian of science John G. Jenkin argues that Rutherford was well aware that there might be ways to harness nuclear energy, especially using neutrons as tools to induce reactions. He suggests that Rutherford "in all of his later negative pronouncements regarding the possibility of atomic energy, was adopting a quite deliberate policy to disguise and postpone, for as long as possible, the awful prospect that he saw looming over the horizon: a new and dreadful war, a new and devastating weapon, and unprecedented destruction."

On a lighter side, Rutherford also allegedly warned (quoted for example in "The Strangest Man: The hidden life of Paul Dirac" by Graham Farmelo):

"Don't let me catch anyone talking about the universe in my department!"

and said about special relativity (quoted for example in "The Nobel Prize: A History of Genius, Controversy, and Prestige" by Burton Feldman):

"Oh, that stuff. We never bother with that in our work."

Though I am not sure about the origin of this latter quotation.

Rutherford was reportedly skeptic about special relativity in its early days, and for most of atomic physics it can be safely neglected and one does indeed not have to bother. But when in 1930 he prepared a new and updated edition of his book "Radiation from Radioactive Substances", he did add a discussion of the mass defect based on E = mc².

Advent calendar #22: Space, Time and Birds

Today's anecdote about Werner Heisenberg has been preserved by Felix Bloch in his article "Heisenberg and the early days of quantum mechanics" Physics Today, December 1976:

We [Werner Heisenberg and Felix Bloch] were on a walk and somehow began to talk about space. I had just read Weyl's book Space, Time and Matter, and under its influence was proud to declare that space was simply the field of linear operations.

"Nonsense," said Heisenberg, "space is blue and birds fly through it."

This may sound naive, but I knew him well enough by that time to fully understand the rebuke. What he meant was that it was dangerous for a physicist to describe Nature in terms of idealized abstractions too far removed from the evidence of actual observation. In fact, it was just by avoiding this danger in the previous description of atomic phenomena that he was able to arrive at his great creation of quantum mechanics. In celebrating the fiftieth anniversary of this achievement, we are vastly indebted to the men who brought it about: not only for having provided us with a most powerful tool but also, and even more significant, for a deeper insight into our conception of reality.

Advent calendar #21: Bohr and the horseshoe

The web is full with anecdotes and quotations about physicists and mathematicians. It would not be difficult to fill a whole year with stories Google put at my fingertips, but then I could as well make them up myself. The time-intensive part of this advent calendar has not been to find the stories but to find out if they have a reliable source. Inevitably, some widely spread stories, if they had any source at all, turned out to have been altered several times, much like a digital game of Chinese whispers.

One such story is for example that of Niels Bohr and the horseshoe. The version on this website goes like this:

"An American scientist once visited the offices of the great Nobel prize winning physicist, Niels Bohr, in Copenhagen. He was amazed to find that over Bohr's desk was a horseshoe, securely nailed to the wall, with the open end up in the approved manner (so it would catch the good luck and not let it spill out). The American said with a nervous laugh,

"Surely you don't believe the horseshoe will bring you good luck, do you, Professor Bohr? After all, as a scientist --" Bohr chuckled.

"I believe no such thing, my good friend. Not at all. I am scarcely likely to believe in such foolish nonsense. However, I am told that a horseshoe will bring you good luck whether you believe in it or not."

In some other versions that you can find online it's a student who asks the question, in yet some other versions the horseshoe is not above the desk but above the door to Bohr's cottage. The above version is particularly interesting for the amount of irrelevant details that somebody or maybe several people have added. Wikipedia lists the quote as disputed.

To find the origin of that story it is useful if you speak German, since it goes back to Werner Heisenberg's book "Der Teil und das Ganze" (The part and the whole). Most of the book is a recollection of conversations Heisenberg had with Niels Bohr and Wolfgang Pauli, among others. Heisenberg wrote down these conversations long after they had taken place, so one should not expect the exchange to have been word by word exactly as he reported. Heisenberg finishes Chapter 8 on "Atomphysik und pragmatische Denkweise" (atomic physics and pragmatism) with an anecdote that Niels Bohr told:

Niels schloß das Gespräch ab mit einer jener Geschichten, die er bei solchen Gelegenheiten gern erzählte: "In der Nähe unseres Ferienhauses in Tisvilde wohnt ein Mann, der hat über der Eingangstür seines Hauses ein Hufeisen angebracht, das nach einem alten Volksglauben Glück bringen soll. Als ein Bekannter ihn fragte: "Aber bist du denn so abergläubisch? Glaubst du wirklich, dass das Hufeisen dir Glück bringt?", antwortete er: "Natürlich nicht; aber man sagt doch, daß es auch dann hilft, wenn man nicht daran glaubt.""

Niels finished with one of these stories he liked to tell on such occasions: "Near by our vacation house in Tisvilde lives a man who has a horseshoe above his door, after the old superstition that it brings luck. When a friend asked him "Are you superstitious? Do you really believe the horseshoe brings luck?" He replied "Of course not; but they say it also helps if you don't believe it."

~Werner Heisenberg, Der Teil und das Ganze, 1973, p. 112/13

So if you plan on winning a Nobel prize, be careful with the anecdotes you tell. Later generations might unashamedly turn the narrator into a subject and the details may be buried in translation.

Advent Calendar #20: Fermi's driver

Enrico Fermi had quite a dry sense of humor. George Gamow, in his book Thirty Years that Shook Physics: The Story of Quantum Theory, relates the following story from the time when Fermi was a professor in Rome in the 1930s:

Once Fermi had to attend a meeting of the Academy of Sciences at the Palazzo di Venezia, which was strongly guarded because Mussolini himself was to address it. All other members arrived in large foreign-made limousines driven by uniformed chauffers, while Fermi drew up in his little Fiat. At the gate of the Palazzo he was stopped by two carabinieri who crossed their weapons in front of his little car and asked his business there. According to the story he told to the author of this book, he hesitated to say to the guards: "I am His Excellency Enrico Fermi," for fear that they would not believe him. Thus, to avoid embarrassment, he said: "I am the driver of His Excellency, Signore Enrico Fermi." "Ebbene," said the guards, "drive in, park, and wait for your master."

Advent Calendar #19: Confident Einstein

In September 1919, Ilse Schneider was working on her Ph.D. thesis in philosophy at the university of Berlin on the "space-time problem in Kant and Einstein". She did profit from the fact that the creator of the theory of relativity was a professor in the physics department: She attended Einstein's lectures, and met regulary with him to discuss the meaning and implications of his theory.

At that time, Einstein was eagerly waiting for news about the results of the British eclipse expedition by Eddington, who had tried to measure the deflection of light by the sun as predicted by the general theory of relativity. Einstein's theory of general relativity is a remarkable achievement of a brilliant mind that knew how to make use of mathematics. Einstein had to try around somewhat before he found the correct equations, but once he had arrived there, he had little doubt they did describe nature correctly.

In her memoir "Reality and Scientific Truth: Discussions with Einstein, von Laue, and Planck", Ilse Rosenthal-Schneider remembers on of her meetings with Einstein from that time:

Suddenly Einstein interrupted the reading and handed me a cable that he took from the window-sill with the words, "This may interest you." It was Eddington's cable with the results of the famous eclipse expedition. Full of enthusiasm, I exclaimed, "How wonderful! This is almost the value you calculated!" Quite unperturbed, he remarked, "I knew that the theory is correct. Did you doubt it?" I answered, "No, of course not. But what would you have said if there had been no confirmation like this?" He replied, "Da könnt' mir halt der liebe Gott leid tun. Die Theorie stimmt doch." ("I would have had to pity our dear God. The theory is correct anyway.")

We thank Toby Bryant for reminding us of that story! According to the Einstein biography by Albrecht Fölsing, Einstein did receive a telegram from Lorentz in Leiden on September 22, 1919, reporting preliminary results on the light deflection as compatible with the prediction of general relativity.

Advent calendar #18: Heisenberg and the microscope

Werner Heisenberg is well known for his analysis of the inevitable uncertainty in observations with a microscope that eventually lead him to formulate the uncertainty principle. Less known is the origin of his obsession with microscopes. In 1923, Heisenberg was heading towards the final oral examination for his doctorate. He passed mathematics, theoretical physics and astronomy just fine, but he run into troubles with experimental physics where he was to be examined by Wilhelm Wien.

Wien had required that Heisenberg did a "Praktikum" (basically a practice in physics experiments), but there was some equipment lacking and Heisenberg wasn't interested enough to find out where to get it. He thus turned towards other things without looking much into the experiments he was supposed to do, for example measuring the splitting of spectral lines by help of an interferometer. Then came the day of the oral exam:

"Wien was annoyed when he learned in the examination that Heisenberg had done so little in the experimental exercise given to him. He than began to ask him questions to gauge his familiarity with the experimental setup; for instance, he wanted to know what the resolving power of the Fabry-Perot interferometer was... Wien had expained all this in one of his lectures on optics; besides, Heisenberg was supposed to study it anyway... But he had not done so and now tried to figure it out unsuccessfully in the short time available during the examination. Wien... asked about the resolving power of a microscope; Heisenberg did not know that either. Wien questioned him about the resolving power of telecopes, which [Heisenberg] also did not know."

(From Jagdish Mehra, Helmut Rechenberg: "The Historical Development of Quantum Theory Vol. 2 - The Discovery of Quantum Mechanics 1925" p. 67)

Wien wanted to fail Heisenberg, but Sommerfeld, in whose exam on theoretical physics Heisenberg had excelled, put in a strong word for Heisenberg. Heisenberg passed the doctoral examination with the lowest possible grade. Many years later Heisenberg would recall

"So one might even assume, that in the work on the gamma-ray microscope and the uncertainty relation I used the knowledge which I had acquired by this poor examination."

Advent Calendar #17: Fermi's paper snippets

Enrico Fermi is famous for his ingenious ways to arrive at quantitive estimates for the solution of complicated physical problems. One of the most legendary examples is his estimate of the energy released by the first atomic bomb. As Fermi himself recalls in My Observations During the Explosion at Trinity on July 16, 194,

About 40 seconds after the explosion the air blast reached me. I tried to estimate its strength by dropping from about six feet small pieces of paper before, during, and after the passage of the blast wave. Since, at the time, there was no wind I could observe very distinctly and actually measure the displacement of the pieces of paper that were in the process of falling while the blast was passing. The shift was about 2 1/2 meters, which, at the time, I estimated to correspond to the blast that would be produced by ten thousand tons of T.N.T.

Emilio Segrè, who witnessed the event together with Fermi, gives a few more details. In his biography Enrico Fermi, Physicist, he writes that Fermi had done the necessary calculations in advance, "having prepared himself a table of numbers, so that he could tell immediately the energy liberated from this crude but simple measurement."

At Los Alamos, Enrico Fermi had the role of an "oracle": Because of his enormous knowledge and competence in all areas of physics, he was consulted for all kinds of physical problems. However, his mastery of physics could be intimidating to other physicists.

As a bonus, here is a story remembered by Subrahmanyan Chandrasekhar, who was a colleague of Fermi at the University of Chicago in the early 1950s (Bull. Amer. Math. Soc. Vol. 84, No. 3 (1978), p. 431):

Some twenty-five years ago, I met a colleague of mine emerging from the office of Enrico Fermi. He told me that he had been discussing physics with Fermi; and after a moment's pause asked, "Why am I doing physics? I should probably be a grocer".

Advent calendar #16: Stern's cigar

This is a story one cannot escape if one studies physics in Frankfurt am Main.

In 1922 Otto Stern and Walther Gerlach demonstrated the directional quantization of angular momentum by sending silver atoms through an inhomogeneous magnetic field. Silver has only one electron in the valence shell, so the orbital angular momentum vanishes and only the electron spin contributes to the total angular momentum of the atom. Depending on the orientation of the spin relative to the magnetic field, the atom takes one out of two trajectories, leading to a discrete splitting of the beam after it passed the magnetic field. Classically, one would expect a smooth distribution. This experiment, conducted in Frankfurt am Main, is known today as the Stern-Gerlach experiment, and was one of the milestones on the way to quantum mechanics.

But it was not just the ingenuity of the experimenters that lead to success since originally Stern and Gerlach couldn't see anything on the screen that should be showing two discrete lines. Dudley Herschbach, who won the Nobel prize for Chemistry in 1986, retold Stern's description of the discovery as follows:

"After venting to release the vacuum, Gerlach removed the detector flange. But he could see no trace of the silver atom beam and handed the flange to me [Stern]. With Gerlach looking over my shoulder as I peered closely at the plate, we were surprised to see gradually emerge the trace of the beam... Finally we realized what [had happened]. I was then the equivalent of an assistant professor. My salary was too low to afford good cigars, so I smoked bad cigars. These had a lot of sulfur in them, so my breath on the plate turned the silver into silver sulfide, which is jet black, so easily visible. It was like developing a photographic film."

The complete story of Stern and Gerlach's experiment can be found in Physics Today 56 (December 2003) Stern and Gerlach: How a Bad Cigar Helped Reorient Atomic Physics (pdf), by Bretislav Friedrich and Dudley Herschbach. They also went on to test the plausibility of this story and repeated the original experiment at its 80st anniversary. The found that bad breath alone wouldn't do the trick, but that more likely Stern was actually puffing on a cigar when Gerlach handed him the invisible result.

Advent calendar #15: The end is nigh

In 1903, briefly before the dawn of Special Relativity and Quantum Mechanics, Albert Abraham Michelson offered his view on physics:

“The more important fundamental laws and facts of physical science have all been discovered, and these are so firmly established that the possibility of their ever being supplanted in consequence of new discoveries is exceedingly remote.”

~A.A. Michelson, Light waves and their uses, University of Chicago Press (1903)

Advent calendar #14: From Hilbert with Sympathy

Hilbert had a student who one day presented him with a paper purporting to prove the Riemann hypothesis. Hilbert studied the paper carefully and was really impressed by the depth of the argument; but unfortunately he found an error in it which even he could not eliminate. The following year the student died. Hilbert asked the grieving parents if he might be permitted to make a funeral oration. While the student's relatives and friends were weeping beside the grave in the rain, Hilbert came forward. He began by saying what a tragedy it was that such a gifted young man had died before he had an opportunity to show the world what he could accomplish. But, he continued, in spite of the fact that this young man's proof of the Riemann hypothesis contained an error, it was still possible that some day a proof of the famous problem would be obtained along the lines which the deceased had indicated. "In fact," he continued with enthusiasm, standing there in the rain by the dead student's grave, "let us consider a function of one complex variable..."

Quoted from "Hilbert" by Constance Reid, where it is noted that this story is "perhaps apocryphal."

Advent Calendar #13: A Postdoc's Nightmare

Pascual Jordan was, along with Werner Heisenberg, Paul Dirac, and Wolfgang Pauli, one of the Wunderkinder contributing to the development of quantum mechanics. He had obtained his Ph.D. in 1924, at the age of 22. In the following year, together with his Ph.D. advisor Max Born and with Heisenberg, he created the matrix formulation of quantum mechanics, formulating the canonical commutation relations between position and momentum. Jordan kicked off quantum field theory, and found the anti-commutation relation for creation and annihilation operators of particles with spin 1/2. These particles, now known as fermions, actually could be linked directly to Jordan, were it not for a case of extremely bad luck. As Max Born remembers:

In December of 1925 I went to America to give lectures at MIT. I was editor of the Zeitschrift für Physik, and Jordan gave me a paper to be published in the journal. I didn't find time to read it and put it in my suitcase. I forgot about it, and when I returned half a year later and unpacked, I found the paper at the bottom of the suitcase. It contained the Fermi-Dirac statistics. Meanwhile both Fermi and Dirac had discovered it. But Jordan was the first.

The Max Born quote, and more about Jordan, can be found in Engelbert Schuckings reminescences "Jordan, Pauli, Politics, Brecht... and a Variable Gravitational Constant" (in On Einstein's path: Essays in Honor of Engelbert Schucking).