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!


26 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:

    • 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

    • 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.

  3. 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.

  4. 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.

  5. 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.

    • Anton Garrett Says:

      Thank you! I hadn’t known that Painlevé 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 Poincarés.

    • 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.

      • Anton Garrett Says:

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

      • Anton Garrett Says:

        Because Kerr hasn’t got it!

      • 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.

      • 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.

  6. 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.

    • 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.

      • 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).

      • Barry Straight Says:

        I was at Canterbury in the late ’70s and have a somewhat apocraphal story from Rob Kerr’s childhood, told me by an elderly acquaintance of his parents, illustrating his grasp of quantity.
        His parents ran a mail order business (Agee preserving jars) from Timaru (I think). Rob’s job as a child was to put a number of rubber bands in with each package lot, say 100. Instead of counting them out he would pick a handful out and drop them in. Being frugal, one of his parents (or staff) saw this and counted the rubber bands, which never varied from the prescribed quantity so he was left to do it his way.

    • 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!

  7. 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.

  8. 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.

  9. 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..

  10. […] 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 […]

  11. Kevin Barker Says:

    Roy Kerr was the HOD Professor of mathematics at Canterbury University and taught me Calculus in the mid 1980’s. I was studying physics at CU in the 1980’s. I wonder if there a bias against him being a mathematician? I have just listened to an interview of Professor Kerr on Radio NZ after this weeks photograph of the accretion disc around Sagittarius A. This weekend he turns 88. I recall in the 1980’s university staff spoke about Professor Kerr as a potential Nobel laureate.

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