Of Particular Significance | Conversations About Science with Theoretical Physicist Matt Strassler

Time 2020-12-22 02:03:40

Web Name: Of Particular Significance | Conversations About Science with Theoretical Physicist Matt Strassler

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There have been dramatic articles in the news media suggesting that a Nobel Prize has essentially already been awarded for the amazing discovery of a fifth force. I thought I d better throw some cold water on that fire; it s fine for it to smoulder, but we shouldn t let it overheat.There could certainly be as-yet unknown forces waiting to be discovered dozens of them, perhaps.   So far, there are four well-studied forces: gravity, electricity/magnetism, the strong nuclear force, and the weak nuclear force.  Moreover, scientists are already fully confident there is a fifth force, predicted but not yet measured, that is generated by the Higgs field. So the current story would really be about a sixth force.Roughly speaking, any new force comes with at least one new particle.  That s becauseevery force arises from a type of field (for instance, the electric force comes from the electromagnetic field, and the predicted Higgs force comes from the Higgs field)and ripples in that type of field are a type of particle (for instance, a minimal ripple in the electromagnetic field is a photon a particle of light and a minimal ripple in the Higgs field is the particle known as the Higgs boson.)The current excitement, such as it is, arises because someone claims to have evidence for a new particle, whose properties would imply a previously unknown force exists in nature.  The force itself has not been looked for, much less discovered.The new particle, if it really exists, would have a rest mass about 34 times larger than that of an electron about 1/50th of a proton s rest mass. In technical terms that means its E=mc² energy is about 17 million electron volts (MeV), and that s why physicists are referring to it as the X17.  But the question is whether the two experiments that find evidence for it are correct.In the first experiment, whose results appeared in 2015, an experimental team mainly based in Debrecen, Hungary studied large numbers of nuclei of beryllium-8 atoms, which had been raised to an excited state (that is, with more energy than usual).  An excited nucleus inevitably disintegrates, and the experimenters studied the debris.  On rare occasions they observed electrons and positrons [a.k.a. anti-electrons], and these behaved in a surprising way, as though they were produced in the decay of a previously unknown particle.In the newly reported experiment, whose results just appeared, the same team observed  the disintegration of excited nuclei of helium.  They again found evidence for what they hope is the X17, and therefore claim confirmation of their original experiments on beryllium.When two qualitatively different experiments claim the same thing, they are less likely to be wrong, because it s not likely that any mistakes in the two experiments would create fake evidence of the same type.  On the face of it, it does seem unlikely that both measurements, carried out on two different nuclei, could fake an X17 particle.However, we should remain cautious, because both experiments were carried out by the same scientists. They, of course, are hoping for their Nobel Prize (which, if their experiments are correct, they will surely win) and it s possible they could suffer from unconscious bias. It s very common for individual scientists to see what they want to see; scientists are human, and hidden biases can lead even the best scientists astray.  Only collectively, through the process of checking, reproducing, and using each other s work, do scientists create trustworthy knowledge.So it is prudent to await efforts by other groups of experimenters to search for this proposed X17 particle.  If the X17 is observed by other experiments, then we ll become confident that it s real. But we probably won t know until then.  I don t currently know whether the wait will be months or a few years.Why I am so skeptical? There are two distinct reasons.First, there s a conceptual, mathematical issue. It s not easy to construct reasonable equations that allow the X17 to co-exist with all of the known types of elementary particles. That it has a smaller mass than a proton is not a problem per se.  But the X17 needs to have some unique and odd properties in order to (1)  be seen in these experiments, yet (2) not be seen in certain other previous experiments, some of which were explicitly looking for something similar.   To make equations that are consistent with these properties requires some complicated and not entirely plausible trickery.  Is it impossible? No.  But a number of the methods that scientists suggested were flawed, and the ones that remain are, to my eye, a bit contrived.Of course, physics is an experimental science, and what theorists like me think doesn t, in the end, matter.  If the experiments are confirmed, theorists will accept the facts and try to understand why something that seems so strange might be true.  But we ve learned an enormous amount from mathematical thinking about nature in the last century for instance, it was math that told us that the Higgs particle couldn t be heavier than 1000 protons, and it was on the basis of that `advice that the Large Hadron Collider was built to look for it (and it found it, in 2012.) Similar math led to the discoveries of the W and Z particles roughly where they were expected. So when the math tells you the X17 story doesn t look good, it s not reason enough for giving up, but it is reason for some pessimism.Second, there are many cautionary tales in experimental physics. For instance, back in 2003 there were claims of evidence of a particle called a pentaquark with a rest mass about 1.6 times a proton s mass an exotic particle, made from quarks and gluons, that s both like and unlike a proton.  Its existence was confirmed by multiple experimental groups!  Others, however, didn t see it. It took several years for the community to come to the conclusion that this pentaquark, which looked quite promising initially, did not in fact exist.The point is that mistakes do get made in particle hunts, sometimes in multiple experiments, and it can take some time to track them down. It s far too early to talk about Nobel Prizes.[Note that the Higgs boson s discovery was accepted more quickly than most.  It was discovered simultaneously by two distinct experiments using two methods each, and confirmed by additional methods and in larger data sets soon thereafter.  Furthermore,  there were already straightforward equations that happily accommodated it, so it was much more plausible than the X17.] And just for fun, here s a third reason I m skeptical. It has to do with the number 17. I mean, come on, guys, seriously 17 million electron volts? This just isn t auspicious.  Back when I was a student, in the late 1980s and early 90s, there was a set of experiments, by a well-regarded experimentalist, which showed considerable evidence for an additional neutrino with a E=mc² energy of 17 thousand electron volts. Other experiments tried to find it, but couldn t. Yet no one could find a mistake in the experimenter s apparatus or technique, and he had good arguments that the competing experiments had their own problems. Well, after several years, the original experimenter discovered that there was a piece of his equipment which unexpectedly could absorb about 17 keV of energy, faking a neutrino signal. It was a very subtle problem, and most people didn t fault him since no one else had thought of it either. But that was the end of the 17 keV neutrino, and with it went hundreds of research papers by both experimental and theoretical physicists, along with one scientist s dreams of a place in history.In short, history is cruel to most scientists who claim important discoveries, and teaches us to be skeptical and patient. If there is a fifth sixth force, we ll know within a few years. Don t expect to be sure anytime soon. The knowledge cycle in science runs much, much slower than the twittery news cycle, and that s no accident; if you want to avoid serious errors that could confuse you for a long time to come, don t rush to judgment. The untimely and sudden deaths of Steve Gubser and Ann Nelson, two of the United States greatest talents in the theoretical physics of particles, fields and strings, has cast a pall over my summer and that of many of my colleagues.I have not been finding it easy to write a proper memorial post for Ann, who was by turns my teacher, mentor, co-author, and faculty colleague.  I would hope to convey to those who never met her what an extraordinary scientist and person she was, but my spotty memory banks aren t helping. Eventually I ll get it done, I m sure.(Meanwhile I am afraid I cannot write something similar for Steve, as I really didn t know him all that well. I hope someone who knew him better will write about his astonishing capabilities and his unique personality, and I d be more than happy to link to it from here.)In this context, I m gratified to see that the New York Times has given Ann a substantive obituary, https://www.nytimes.com/2019/08/26/science/ann-nelson-dies.html, and appearing in the August 28th print edition, I m told. It contains a striking (but, to those of us who knew her, not surprising) quotation from Howard Georgi.  Georgi is a professor at Harvard who is justifiably famous as the co-inventor, with Nobel-winner Sheldon Glashow, of Grand Unified Theories (in which the electromagnetic, weak nuclear, and strong nuclear force all emerge from a single force.) He describes Ann, his former student, as being able to best him at his own game.“I have had many fabulous students who are better than I am at many things. Ann was the only student I ever had who was better than I am at what I do best, and I learned more from her than she learned from me.”He s being a little modest, perhaps. But not much. There s no question that Ann was an all-star.And for that reason, I do have to complain about one thing in the Times obituary. It says Dr. Nelson stood out in the world of physics not only because she was a woman, but also because of her brilliance. Really, NYTimes, really?!?Any scientist who knew Ann would have said this instead: that Professor Nelson stood out in the world of physics for exceptional brilliance lightning-fast, sharp, creative and careful, in the same league as humanity s finest thinkers and for remarkable character kind, thoughtful, even-keeled, rigorous, funny, quirky, dogged, supportive, generous. Like most of us, Professor Nelson had a gender, too, which was female. There are dozens of female theoretical physicists in the United States; they are a too-small minority, but they aren t rare. By contrast, a physicist and person like Ann Nelson, of any gender? They are extremely few in number across the entire planet, and they certainly do stand out.But with that off my chest, I have no other complaints. (Well, admittedly the physics in the obit is rather garbled, but we can get that straight another time.) Mainly I am grateful that the Times gave Ann fitting public recognition, something that she did not actively seek in life. Her death is an enormous loss for theoretical physics, for many theoretical physicists, and of course for many other people. I join all my colleagues in extending my condolences to her husband, our friend and colleague David B. Kaplan, and to the rest of her family. It is beyond belief that not only am I again writing a post about the premature death of a colleague whom I have known for decades, but that I am doing it about two of them.Over the past weekend, two of the world s most influential and brilliant theoretical high-energy physicists Steve Gubser of Princeton University and Ann Nelson of the University of Washington fell to their deaths in separate mountain accidents, one in the Alps and one in the Cascades.Theoretical high energy physics is a small community, and within the United States itself the community is tiny.  Ann and Steve were both justifiably famous and highly respected as exceptionally bright lights in their areas of research. Even for those who had not met them personally, this is a stunning and irreplaceable loss of talent and of knowledge.But most of us did know them personally.  For me, and for others with a personal connection to them, the news is devastating and tragic. I encountered Steve when he was a student and I was a postdoc in the Princeton area, and later helped bring him into a social group where he met his future wife (a great scientist in her own right, and a friend of mine going back decades).  As for Ann, she was one of my teachers at Stanford in graduate school, then my senior colleague on four long scientific papers, and then my colleague (along with her husband David B. Kaplan) for five years at the University of Washington, where she had the office next to mine. I cannot express what a privilege it always was to work with her, learn from her, and laugh with her.I don t have the heart or energy right now to write more about this, but I will try to do so at a later time. Right now I join their spouses and families, and my colleagues, in mourning. [Note Added: Thanks to some great comments I ve received, I m continuing to add clarifying remarks to this post.  You ll find them in green.]It s been a couple of months since the `photo (a false-color image created to show the intensity of radio waves, not visible light) of the black hole at the center of the galaxy M87, taken by the Event Horizon Telescope (EHT) collaboration, was made public. Before it was shown, I wrote an introductory post explaining what the photo is and isn t. There I cautioned readers that I thought it might be difficult to interpret the image, and controversies about it might erupt.So far, the claim that the image shows the vicinity of M87 s black hole (which I ll call `M87bh for short) has not been challenged, and I m not expecting it to be. But what and where exactly is the material that is emitting the radio waves and thus creating the glow in the image? And what exactly determines the size of the dark region at the center of the image? These have been problematic issues from the beginning, but discussion is starting to heat up. And it s important: it has implications for the measurement of the black hole s mass (which EHT claims is that of 6.5 billion Suns, with an uncertainty of about 15%), and for any attempt to estimate its rotation rate. Continue reading THIS POST CONTAINS ERRORS CONCERNING THE EXISTENCE AND VISIBILITY OF THE SO-CALLED PHOTON-SPHERE AND SHADOW; THESE ERRORS WERE COMMON TO ESSENTIALLY ALL REPORTING ON THE BLACK HOLE PHOTO .  IT HAS BEEN SUPERSEDED BY THIS POST, WHICH CORRECTS THESE ERRORS AND EXPLAINS THE SITUATION.Ok, after yesterday s post, in which I told you what I still didn t understand about the Event Horizon Telescope (EHT) black hole image (see also the pre-photo blog post in which I explained pedagogically what the image was likely to show and why), today I can tell you that quite a few of the gaps in my understanding are filling in (thanks mainly to conversations with Harvard postdoc Alex Lupsasca and science journalist Davide Castelvecchi, and to direct answers from professor Heino Falcke, who leads the Event Horizon Telescope Science Council and co-wrote a founding paper in this subject).  And I can give you an update to yesterday s very tentative figure.First: a very important point, to which I will return in a future post, is that as I suspected, it s not at all clear what the EHT image really shows.   More precisely, assuming Einstein s theory of gravity is correct in this context:The image itself clearly shows a black hole s quasi-silhouette (called a `shadow in expert jargon) and its bright photon-sphere where photons [particles of light of all electromagnetic waves, including radio waves] can be gathered and focused.However, all the light (including the observed radio waves) coming from the photon-sphere was emitted from material well outside the photon-sphere; and the image itself does not tell you where that material is located.  (To quote Falcke: this is `a blessing and a curse insensitivity to the illumination source makes it easy to interpret the black hole s role in the image but hard to learn much about the material near the black hole.) It s a bit analogous to seeing a brightly shining metal ball while not being able to see what it s being lit by except that the photon-sphere isn t an object.  It s just a result of the play of the light [well, radio waves] directed by the bending effects of gravity.  More on that in a future post.When you see a picture of an accretion disk and jets drawn to illustrate where the radio waves may come from, keep in mind that it involves additional assumptions educated assumptions that combine many other measurements of M87 s black hole with simulations of matter, gravity and magnetic fields interacting near a black hole.  But we should be cautious: perhaps not all the assumptions are right.  The image shows no conflicts with those assumptions, but neither does it confirm them on its own.Just to indicate the importance of these assumptions, let me highlight a remark made at the press conference that the black hole is rotating quickly, clockwise from our perspective.  But (as the EHT papers state) if one doesn t make some of the above-mentioned assumptions, one cannot conclude from the image alone that the black hole is actually rotating.  The interplay of these assumptions is something I m still trying to get straight.Second, if you buy all the assumptions, then the picture I drew in yesterday s post is mostly correct except (a) the jets are far too narrow, and shown overly disconnected from the disk, and (b) they are slightly mis-oriented relative to the orientation of the image.  Below is an improved version of this picture, probably still not the final one.  The new features: the jets (now pointing in the right directions relative to the photo) are fatter and not entirely disconnected from the accretion disk.  This is important because the dominant source of illumination of the photon-sphere might come from the region where the disk and jets meet.Updated version of yesterday s figure: main changes are the increased width and more accurate orientation of the jets.  Working backwards: the EHT image (lower right) is interpreted, using mainly Einstein s theory of gravity, as (upper right) a thin photon-sphere of focused light surrounding a dark patch created by the gravity of the black hole, with a little bit of additional illumination from somewhere.  The dark patch is 2.5 5 times larger than the event horizon of the black hole, depending on how fast the black hole is rotating; but the image itself does not tell you how the photon-sphere is illuminated or whether the black hole is rotating.  Using further assumptions, based on previous measurements of various types and computer simulations of material, gravity and magnetic fields, a picture of the black hole s vicinity (upper left) can be inferred by the experts. It consists of a fat but tenuous accretion disk of material, almost face-on, some of which is funneled into jets, one heading almost toward us, the other in the opposite direction.  The material surrounds but is somewhat separated from a rotating black hole s event horizon.  At this radio frequency, the jets and disk are too dim in radio waves to see in the image; only at (and perhaps close to) the photon-sphere, where some of the radio waves are collected and focused, are they bright enough to be easily discerned by the Event Horizon Telescope. The short answer: I m really not sure yet.  [This post is now largely superseded by the next one, in which some of the questions raised below have now been answered.]  EVEN THAT POST WAS WRONG ABOUT THE PHOTON-SPHERE AND SHADOW.  SEE THIS POST FROM JUNE 2019 FOR SOME ESSENTIAL CORRECTIONS THAT WERE LEFT OUT OF ALL REPORTING ON THIS SUBJECT.Neither are some of my colleagues who know more about the black hole geometry than I do. And at this point we still haven t figured out what the Event Horizon Telescope experts do and don t know about this question or whether they agree amongst themselves.[Note added: last week, a number of people pointed me to a very nice video by Veritasium illustrating some of the features of black holes, accretion disks and the warping of their appearance by the gravity of the black hole.  However, Veritasium s video illustrates a non-rotating black hole with a thin accretion disk that is edge-on from our perspective; and this is definitely NOT what we are seeing!]As I emphasized in my pre-photo blog post (in which I described carefully what we were likely to be shown, and the subtleties involved), this is not a simple photograph of what s `actually there. We all agree that what we re looking at is light from some glowing material around the solar-system-sized black hole at the heart of the galaxy M87.  But that light has been wildly bent on its path toward Earth, and so just like a room seen through an old, warped window, and a dirty one at that it s not simple to interpret what we re actually seeing. Where, exactly, is the material `in truth , such that its light appears where it does in the image? Interpretation of the image is potentially ambiguous, and certainly not obvious. Continue reading Wow.Twenty years ago, astronomers Heino Falcke, Fulvio Melia and Eric Agol (a former colleague of mine at the University of Washington) pointed out that the black hole at the center of our galaxy, the Milky Way, was probably big enough to be observed not with a usual camera using visible light, but using radio waves and clever techniques known as interferometry .  Soon it was pointed out that the black hole in M87, further but larger, could also be observed.  [How? I explained this yesterday in this post.]   And today, an image of the latter, looking quite similar to what we expected, was presented to humanity.  Just as with the discovery of the Higgs boson, and with LIGO s first discovery of gravitational waves, nature, captured by the hard work of an international group of many scientists, gives us something definitive, uncontroversial, and spectacularly in line with expectations.An image of the dead center of the huge galaxy M87, showing a glowing ring of radio waves from a disk of rapidly rotating gas, and the dark quasi-silhouette of a solar-system-sized black hole.  Congratulations to the Event Horizon Telescope teamI ll have more to say about this later [have to do non-physics work today

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