The 2020 Nobel Prize for Physics

I don’t know about you but I was a bit surprised by this year’s announcement of the Physics Nobel Prize but that’s largely because it went to something cosmic last year and not because I disapprove in any way. Roger Penrose’s work in the 1960s on the black hole singularity theorems is rightly famous and the observational discovery of the supermassive black hole in the centre of the Milky Way is also more than worthy of recognition.

Congratulations to Roger Penrose, Reinhard Genzel and Andrea Ghez!

45 Responses to “The 2020 Nobel Prize for Physics”

  1. It should also have gone to Roy Kerr

  2. Anton Garrett Says:

    Penrose’s work on the singularity theorems in GR is excellent, and perhaps this award is also a nod to his collaborator the late Stephen Hawking. Penrose has also done excellent work in pure mathematics with his tiling theorems. I’m less convinced about his twistor program, for reasons I give on this thread:

    https://telescoper.wordpress.com/2010/11/28/doubts-about-the-evidence-for-penroses-cyclic-universe/

    • Phillip Helbig Says:

      Right, but he got it for the singularity-theorem work, not for twistors, or quantum consciousness, or conformal cyclic cosmology.

      If I ever build my own house, then it will have Penrose tiling. 🙂

      • Anton Garrett Says:

        Yes, I have generalised the subject. Quantum consciousness fills a much-needed gap. I don’t claim to understand conformal cyclic cosmology.

      • In his interview with someone from nobel foundation he talked about CCC and also mentioned that he does not believe in inflation

      • Phillip Helbig Says:

        Yes, I heard all the interviews (and several in German with Genzel).

        Penrose has been a critic of inflation since the start, his argument being essentially that the conditions for inflation are more improbable than the improbable stuff inflation is supposed to explain (which is actually not that improbable—at least not based on what was known 40 years ago—but that is another topic).

        The late Wolfgang Rindler also regretted that his excellent work on cosmological horizons in some sense inspired the idea of inflation.

      • It would be interesting to see how most mainstream cosmologists and string theorists react, given Penrose’s strong objections to string theory, inflation. Don’t know his views about dark matter and dark energy, but not sure he buys those arguments too.

      • Phillip Helbig Says:

        “t would be interesting to see how most mainstream cosmologists and string theorists react, given Penrose’s strong objections to string theory, inflation. Don’t know his views about dark matter and dark energy, but not sure he buys those arguments too.”

        Any sensible scientist would agree that he deserves the prize for the contributions for which it is awarded.

        Well, string theorists probably will object (cue right-wing rant from Lubos Motl) or at least not be amused, but the majority of physicists have nothing to do with string theory, not even the majority who work on something related to Penrose’s work. As for inflation, there are others, such as the late, great Wolfgang Rindler, who are sceptical, and most serious scientists would agree that it is on less secure footing than stuff which we suppose happened later in the history of the Universe. Inflation is not my field (pun, as always, intended), but as far as I know no-one has rebutted Penrose’s anti-inflation arguments (whereas his quantum-consciousness stuff has been rebutted and probably refuted).

        Penrose likes a universe with negative spatial curvature. I don’t know precisely what is current thinking is, but I doubt that he would debate the observational fact that the Universe is very close to flat (though we don’t know the sign of the curvature; others prefer positive curvature for other reasons) and other elements of the concordance model, which imply dark matter and the cosmological constant.

  3. Phillip Helbig Says:

    It is a bit unexpected that both last year and this year are essentially astrophysics prizes.

    On the other hand, what have the particle physicists done which hasn’t already been awarded a Prize?

    I’d been thinking for a while that Penrose should get it for this work (and if Hawking had still been around, also for that and/or, together with Bekenstein (also no longer around) for the black-hole thermodynamics stuff.

    • Anton Garrett Says:

      Interestingly, although the gauge theory of gravity pioneered by Tom Kibble and Dennis Sciama has local predictions identical to GR in the absence of fermion spin, its field equations are first order differential rather than second, and this does have some consequences for global properties of solutions.

    • I doubt that Hawking would have got it, even if he had been alive. As with the Higgs prize, the rule nowadays is no prize for theory until confirmed by experiment. Here, the Swedish Academy is saying that despite all the circumstantial evidence it is only Genzel & Ghez’s work on Sgr A* that confirms that black holes really exist. There is no experimental evidence at all for black hole thermodynamics and Hawking radiation. In fact I have a sneaking suspicion that they waited until Hawking was dead before awarding it to Penrose to avoid controversy.

      • Phillip Helbig Says:

        Hawking might have got it with Penrose for the singularity theorems. As for Hawking radiation etc., true, it is not an “invention or discovery” and maybe a bit too shaky for the committee. Similarly with flat rotation curves of galaxies: just an observation, though somewhat unexpected, and there is no real independent confirmation of the cause, nor is it even clear what the cause is. (In other words, various pundits who claim that Vera Rubin would have been awarded a Nobel Prize had she been a man are almost certainly wrong. And had she been awarded one, then surely with Kent Ford, who is rarely mentioned.)

  4. Phillip Helbig Says:

    Official announcement: https://www.nobelprize.org/

    Yesterday’s medicine prize went to the three who had discovered that hepatitic C is caused by a virus.

  5. Phillip Helbig Says:

    For those who are wondering how the prizes can be split up (note that splitting them and giving them for work not done in the previous year are departures from Nobel’s will; it is sometimes claimed that there should be more departures; I think that it is good to be conservative here, but agree with the changes which have been made):

    The possibilities for shared prizes are: two get a half each, one gets a half and two get a quarter each (as here and in physics last year), or three get a third each. While each year has a general theme (e.g. astrophysics, nuclear physics), prizes which are split can be awarded for work on the same specific topic (e.g. supernova cosmology in 2011—also 50/25/25) or for slightly different topics on the same theme (as here and last year), except that awards of a third each are always for the same specific topic (QED is an example of 1/3 each), as are the two quarter awards. That says nothing about whether joint awards are for joint work; in this respect, all combinations are possible and have actually occurred at least once. (Exercise for the reader: for each of the possibilities mention above, find an example with and without collaboration, without using any reference material.)

  6. Anton Garrett Says:

    The citation is about black hole formation definitely being a consequence of GR, but I thought this was established before Penrose, since matter does not have to contract to a singularity for a black hole to form, but merely inside its event horizon. (When was it settled that the coordinate singularities in black hole solutions of the field equations were also physical singularities?) The great physical consequence of the singularity theorems – subject to their axioms, of course – is rather the Big Bang. So is it the singularity theorems that the Nobel committee had in mind? If not, what other work of Penrose relating to black holes? I find this a little confusing.

    • Phillip Helbig Says:

      Mainly:

      Penrose, R., 1965, “Gravitational collapse and space-time singularities”, Physical Review Letters, vol. 14, no. 3. pp. 57–59, 1965. doi: 10.1103/PhysRevLett.14.57.

      More infomration in the PDF linked to from https://www.nobelprize.org/prizes/physics/2020/advanced-information/

      • Phillip Helbig Says:

        Hawking’s paper and the Hawking and Penrose paper on singularity theorems came a bit later.

      • Anton Garrett Says:

        Thank you! I hadn’t known that PainlevĂ© was a politician as well as a mathematician. If I had known that there was a politician of that name then I would probably have assumed that he was a relative of the mathematician, as with the PoincarĂ©s.

      • Phillip Helbig Says:

        “We now know that [the Schwarzschild radius] has an important global significance as the point of no return, which we call the event horizon. Its position precisely coincides with the value proposed long ago by Michell and Laplace, based on Newtonian gravity and the assumption of a particle nature of light. Was it just serendipity that they had found the correct expression as early as the end of the 18th century? Not completely. In special relativity, time dilation in the form of a non-trivial Lorentz factor, gives rise to the relativistic expression for kinetic energy, while gravitational time dilation is associated with potential energy. To obtain the escape velocity, the energy of the test particle moving radially in the Schwarzschild background should be equated with the rest energy of the particle positioned far away from the gravitating mass.”

        The two are not completely equivalent, of course. While one can’t get out of a classical black hole ballistically because one can’t reach the escape velocity of light (allowing for the moment classical physics but a maximum speed), one could get out with a rocket, or a ladder, or whatever. In GR that’s not the case. With a big black hole, one wouldn’t even notice anything at the event horizon. You could of course drop a rope in from a safe distances, but not use it to climb back out. To me, that is more bizarre than the concept of an event horizon itself.

      • Phillip Helbig Says:

        “If I had known that there was a politician of that name then I would probably have assumed that he was a relative of the mathematician, as with the PoincarĂ©s.”

        Check out the von Weizsäcker family—there are probably more than you think.

    • The existence of rotating black holes as a consequence of GR was established by Roy Kerr in 1963 before the work of Roger Penrose on the singularity theorems. (Penrose cites it in his 1965 PRL.) Before Kerr’s solution astrophysicists had assumed that adding angular momentum could prevent black hole horizons from forming, and since every collapsed object in the Universe rotates the only known (Schwarzschild) solution was widely regarded as a mathematical curiosity. Kerr’s solution established black hole horizons as physical.

      It is indeed the horizon property that is crucial for black holes, rather than singularities (which may not even exist). The inevitability of singularities, established by Penrose in the 1965 PRL, is a signal that GR breaks down.

      Even after the 1960s astronomers were extremely conservative about accepting black holes, and for many decades referred to observations of “black hole candidates”. Kerr did not receive much recognition for decades, despite being championed by Chandrasekhar. Kerr was awarded the Crafoord Prize for Astronomy by the Royal Swedish Academy in 2016, which had also gone to Genzel and Ghez in 2012 for the same work that they got the Academy’s more famous prize for this year.

      This morning (NZ time) I awoke to emails from colleagues who like Shantanu above were surprised at the Nobel committee’s citation given the defining property of black holes are their horizons, established by the Kerr solution. Well, in the 1960s Penrose certainly did as much or more for the science of black holes than anyone else. So the prize is well deserved!

      The Nobel citation is about the formation of black holes, rather than their existence. The title of the Penrose’s 1965 PRL and the fact that it is known as the first “singularity theorem” paper are unfortunate. Penrose did establish crucial black hole concepts in the 1965 PRL, namely closed trapped surfaces, the outer boundary of which is the “apparent horizon”. Given arbitrary matter obeying reasonable energy conditions he showed compact regions will form from which light cannot escape, even in dynamical nonequilibrium situations. Thus the 1965 PRL was a major step in realizing black holes as actual astrophysical objects. Nowadays we have more general notions of dynamical horizons which are used in modeling the merger of two black holes – just as the Kerr solution and its perturbations (studied in detail by Chandrasekhar) are relevant for gravitational wave production during the “ringdown”.

      • Anton Garrett Says:

        Yes Kerr should have had it too. But he comes from an unfashionable place. Rutherford was from there too, and won the Nobel in chemistry for radioactivity research.

      • Phillip Helbig Says:

        What should it matter where he comes from? As you yourself point out, Rutherford was also from New Zealand and won a prize.

        There will always be debate, some informed and some not, about who should have won, but it seems strange to suggest that Kerr hasn’t (yet) been awarded the prize because he is from an unfashionable place. (But who says that it is unfashionable?)

        Nobel prizes are rarely if ever given for work which is a) theoretical and b) has no direct non-theoretical consequences. But even if Kerr somehow qualified according to those criteria, not everyone who qualifies wins a prize.

      • Anton Garrett Says:

        Of course it shouldn’t matter. But I reckon it does.

      • Phillip Helbig Says:

        Why do you think so?

      • Anton Garrett Says:

        Because Kerr hasn’t got it!

      • Phillip Helbig Says:

        There are many people who “deserve” a Nobel Prize but haven’t got one. Without good evidence, what is the point in finding some characteristic (New Zealander, female, more than 2 sigma taller than average, former rock musician, whatever) and claiming that that is the reason, without any evidence?

      • Anton Garrett Says:

        I am all for making people who claim unconscious bias prove their case (eg, judges hand out harsher sentences for the same offence late in the day), but the ignoring of Kerr seems clearer than most if they are have made it a black-hole year.

      • Phillip Helbig Says:

        As Paddy points out above, the committee do interpret the “invention or discovery” rather strictly. It could be argued that Penrose proved that black holes very probably can exist, while Kerr found a solution which has not actually been strictly verified experimentally (though no serious person doubts it). There are actually a huge number of known exact solutions to the Einstein equations:

      • Most of the huge number of exact solutions of Einstein’s equations are not physical. By contrast the Kerr geometry is crucially important astrophysically. It is used for all the phenomena of accretions disks, jets etc of supermassive black holes and active galactic nuclei. It is used in the modeling of gravitational wave production in black hole mergers. If you need to be better informed watch the videos at the 2016 Crafoord Prize Symposium when Roy Kerr went to Stockholm to get that prize along with Roger Blandford.

        Anton does have a point. But it is not where you were born. (Rutherford left NZ and spent most of his career in the UK; he did not win the prize while living in NZ. No NZ-born Nobel laureate ever has.) With the Nobel it is all a question of how many / who are nominating, and all the lobbying. The nomination forms specifically state you are not supposed to discuss it with anyone; but everyone knows that there is widespread lobbying of previous laureates (who have an automatic right to nominate), especially in the country with the greatest number (USA). Then there is another level of lobbying in Sweden itself.

        The Nobel committee are of course aware of all this and take it into account. Given the 3 person rule there are bigger picture issues at stake. This is the first ever Nobel prize for which part has been awarded for theoretical advances in general relativity. The previous two GR prizes (1993, 2017) were purely observational. Kerr did make the first breakthrough that established black holes as physical objects, and the Kerr geometry is the workhorse actually used by astronomers, whereas singularity theorems are not. However, Kerr only made one major contribution, while Penrose did so much more in defining the “golden age of GR” in the 1960s. In deciding for the first time ever to award the prize for (physically relevant) theoretical advances in GR, an honour not even accorded to Einstein, then the choice of Roger Penrose is logical.

      • Phillip Helbig Says:

        “However, Kerr only made one major contribution, while Penrose did so much more in defining the “golden age of GR” in the 1960s. In deciding for the first time ever to award the prize for (physically relevant) theoretical advances in GR, an honour not even accorded to Einstein, then the choice of Roger Penrose is logical.”

        I agree; I just wanted to let someone from Bew Zealand make the point. 🙂

        As for the country of residence playing a role, what matters is certainly the number of scientists at the corresponding level working in the country in question. So it is not completely surprising that no-one living in New Zealand has won a Nobel Prize. There might be higher-order effects, such as more lobbying where there are more people to lobby. However, recent surprises show that the committee does make up its own mind.

        The Golden Age of General Relativity:

        I was there! Here is a video of some of the greats from the golden era of GR reminiscing about the good old days. The whole video is worth watching. Dramatis personae:

        James Anderson (Stevens Institute of Technology)
        Dieter Brill (University of Maryland)
        Cecile DeWitt (University of Texas at Austin)
        Joshua Goldberg (Syracuse University)
        Roy Kerr (University of Canterbury)
        Charles Misner (University of Maryland)
        Ted Newman (University of Pittsburgh)
        Roger Penrose (University of Oxford)
        Wolfgang Rindler (University of Texas at Dallas)
        Louis Witten (University of Cincinnati)

        Rindler died about a year and a half ago. Cecile DeWitt is also no longer with us. Get out the popcorn, sit back, relax, and watch the whole thing. Again, it is all worth watching, but Ted Newman’s contribution is particularly good. (I also have a connection to the protagonist in one of his stories, but the audience rewarded him with more laughter than anyone else.) You can see my balding head from time to time; I’m sitting behind Joe Taylor.

        https://nsm.utdallas.edu/texas2013/events/

  7. Very good points by David Wiltshire. I urge everyone to read the excellent biography of Roy Kerr by Fulvio Melia. Regarding Kerr having only one major contribution, so did Peter Higgs and Russell Hulse. Also Kerr discovered the Kerr solution in UK and was also on the faculty at UT Austin. So somewhat disappointed that he was not nominated. Also I am surprised and disappointed that neither Robert Pound nor Irwin Shapiro have not got it for test of GR. Shapiro delay not only provided a firm test of GR, but is routinely used as an astrophysical tool by pulsar timing timing and also comes into play in determining Hubble constant through strong lensing time delays.

    • Phillip Helbig Says:

      There are good reasons, many very good reasons, for not giving it to more than three people per year. So there will alway be some deserving who have been left out. But in any case, surely the fact that Kerr spent some time out of New Zealand shouldn’t be used as an argument to give him the prize, any more than the fact that he has spent most of his life in New Zealand be one not to.

    • Small correction Shantanu: Kerr discovered the Kerr solution in April/May 1963 while a postdoc at UT Austin. They made him Associate Professor later that year as a result.

      BTW – there is a “Golden Age” historical book coming out, edited by Susan Scott and Daniele Malafarina, many years in the making, in which outstanding “second generation” general relativists (i.e., the post-Einstein 1960s generation) are each interviewed by a member of the “third generation”. (Yes, there is a Stephen Hawking chapter – most of the interviews were back in 2015.) I did the chapter on Roy Kerr. Given the wide cast of characters interviewed, the book will make an interesting read.

      • Phillip Helbig Says:

        How big is the overlap with the people in the video linked to above? Who is publishing the book when?

      • Daniele Malafarina writes: “Our latest goal was to finish by the end of this year but that was before the pandemic happened. The events of the last few months have disrupted everyone’s life quite a bit. My hope at present is to have it out in 2021, but after postponing so many times, I would be rather cautious now.

        The publisher is Springer and the list of interviewees currently is:
        – George Ellis
        – Robert Geroch
        – Josh Goldberg (whom, I was just informed, passed away a couple of days ago)
        – Stephen Hawking
        – Werner Israel
        – Roy Kerr
        – Wolfgang Kundt
        – Charles Misner
        – Jayant Narlikar
        – Ted Newman
        – Roger Penrose
        – Ivor Robinson
        – Ray Sachs
        – Kip Thorne
        – Andrzej Trautman
        I hope I haven’t forgotten anyone (I don’t have the list with me right now).

      • Phillip Helbig Says:

        So half of those from the round-table discussion are in the book:

        Joshua Goldberg (Syracuse University)
        Roy Kerr (University of Canterbury)
        Charles Misner (University of Maryland)
        Ted Newman (University of Pittsburgh)
        Roger Penrose (University of Oxford)

        Obviously, at the round table were only people who were there. Ray Sachs changed fields a long time ago. George Ellis is probably too young (although he was at the meeting); the round-table folks are older. Of the others who were still alive (and obviously those who were alread dead), perhaps none were at the meeting. Some were too old/ill to travel; Engelbert SchĂĽcking sent in a video message (he died in 2015).

    • Shantanu: You are correct in comparing Kerr to Higgs, and if this had been a purely theoretical prize, then Kerr would surely have had a share.

      To understand the tribal loyalties that go into the nomination and decision sociology, a huge loyalty is to one’s discipline (physics subfield), not just one’s country. In fundamental physics, even since the 1960s, the number of theoretical physicists in particle physics has always been very significantly larger than in general relativity. That is true internationally, and no doubt also in Sweden. Nobel prizes must be for observationally verified advances but there have been a number of purely theoretically focused prizes in particle physics (e.g., 1999, 2004, 2008, 2013). That is not at all surprising given the relative size of the particle physics community. But the Nobel committee is not yet ready for a prize solely for theoretical advances in gravitational physics. With the advent of gravitational wave astronomy, the relative number of astrophysicists in this area is going to grow. Perhaps that’s a reason underlying two prizes being awarded in cognate areas in successive years. They have realized the scientific frontier is changing and are playing catch-up.

      • David. I am also not sure how well known Roy Kerr is outside the GR community (in NZ and elsewhere). Last year I was at an particle astrophysics meeting and one grad student working on neutrino physics (and based in NZ) had never heard of Roy Kerr. If this is the case in NZ I doubt others outside NZ and outside GR will know him. Also very few physicists know Joe Taylor and Russell Hulse. at the 2013 Texas meeting (which Philip mentioned and was also there), most young cosmologists I talked didn’t know that Joe Taylor had a nobel prize. During coffee, one participant asked Joe (when he mentioned that he came to Dallas from Chicago via train) whether the train fare was costly!

      • Phillip Helbig Says:

        With regard to Joe Taylor, I’m told that he doesn’t get out much, <I<i.e. rarely goes to conferences. But that of course is not a reason not to know of him and his work. I’m not surprised that a “generic” physicist” hasn’t heard of him. Same with Roy Kerr. But of course everyone with some connection knows who they are.

        I was more surprised at the Dallas meeting to talk to some students working on gravitational lensing who didn’t know who Sjur Refsdal was.

        Let’s turn things around: Off the top of your head, do you know what the GIM mechanism is and what the three letters stand for?

  8. The history of the singularity theorems is quite interesting. The first such was Roger’s trapped surface theorem, proved after he knew of the Kerr solution. Like all math theorems, it makes certain assumptions, such as the null energy condition. Much more important is that it assumes that matter is smooth, NOT particulate. His theorem says that there are problems at the “end” of a light-like curve of finite affine length. In the 55 years since this theorem appeared no one has been able to prove that this implies that the limit of the mass inside a small sphere around that point is non- zero as the radius decreases to zero.
    In the late 60’s Stephen Hawking came for a long weekend and said at the start that he had proved that either singularities or closed time-like loops exist. He said he used the Raychaudhuri equation. I spent 2 days working on this by myself and then told him and George Ellis that I could only prove a weaker result. Later that year he published that version without any “thank you” to me.
    The only interesting application of these theorems is to prove that there is some singularity inside the inner horizon of any Black Hole. This is still an open question.
    By the way, the No-Hair theorems prove that all higher moments vanish outside the event horizon. This also applies to the region outside the inner horizon but it does not apply to the interior where the collapsed body resides.

    • Phillip Helbig Says:

      Speaking as someone a bit younger, my impression from my limited contact with it is that the GR community is somewhat less cutthroat than the astronomical community (whicn, in turn, is less so than many or most other fields), but I have heard similar stories about lack of acknowledgements about other important results found during the “golden age of GR”. Has anyone investigated this from a history-of-science point of view?

  9. Philip. G of GIM was on my thesis committee and so I very well know all 3 in GIM. However GR and cosmology fields are close and yet surprised that cosmologists didn’t know Taylor had a nobel prize. But you are right. I don’t think I have seen any interview of Taylor or Hulse after detection of GWs, despite the fact that the discovery and measurements of Hulse-Taylor binary played a pivotal role in LIGO getting funded.

  10. George Ellis Says:

    The dark horse here is Engelbert Schucking. He wrote very little but was very influential. In particular I believe he emphasized to Roger the importance of conformal structure. Then of course there is Dennis Sciama, who persuaded Roger of the importance and interest of General Relativity, and so brought him into the subject; and John Wheeler, who emphasized the importance of the existence of spacetime singularities and so made that a focus of many people’s studies at that time..

  11. […] will no doubt recall the announcement a few weeks ago of the award of the 2020 Nobel Prize for Physics to Roger Penrose, Andrea Ghez and Reinhard […]

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