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The mysteries of dark matter and dark energy may be evidence that we don’t fully understand the force of gravity. But when it comes to a force that has been studied mathematically and probed observationally for hundreds of years, what do we still need to learn? What questions are being asked? What research is pursued at the cutting edge? Would a new theory of gravity lead to a grand revolution in science, or do our present theories just need to be tweaked? This program is part of the Big Ideas Series, made possible with support from the John Templeton Foundation.Learn More

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RICHARD PANEK, AUTHOR : Good evening. Thank you for coming out tonight and thank you to the World Science Festival for hosting this program. And thank you to our other participants who are now going to be joining me on stage and then I will introduce them starting from-at the far left, Rachel Rosen is an assistant professor of theoretical physics at Columbia University. Her research focuses on gravity, Quantum Field Theory, and the intersection of the two. Thank you Rachel. Szabolcs Marka is the leader of the Columbia Experimental Gravity Group in LIGO and a professor of physics at Columbia University. Maria Spiropulu is a physics professor at Caltech. She’s been researching elementary particles and their interactions at Fermilabs Tevatron and CERN’s Large Hadron Collider. David Gross is the chancellor’s chair professor of theoretical physics at the University of California Santa Barbara. He was awarded the 2004 Nobel Prize in Physics for his discovery of asymptotic freedom, which led to the formulation of quantum chromodynamics. And Pedro Ferreira is a professor of astrophysics at the University of Oxford. His most recent book, The Perfect Theory, is published in over-more than 20 countries.

So I’d like to open tonight-what we’re going to do. You know there’s been a lot of talk about gravitational waves this week and a lot of it has been technical and there’s going to be another talk after this in the Skirball Center. And a lot of it is technical and hard science and I thought that maybe tonight, here in this panel, we could have a little a little looser interpretation. Maybe a more philosophical metaphysical maybe not strictly scientific, more speculative perhaps. So I’m going to open up with a question that I actually asked Kip Thorne a few weeks ago on the phone. Kip was one of the founders of LIGO. And one of the recipients of the Kavli Prize earlier in the week at the festival. So I’m going to ask this question to the panel and they can answer it, or not answer it, in any way they prefer. And then after you’re done, I’ll tell you what Kip said. So my question is, what is gravity? Stunned silence.


PANEK: No. Just jump in please.

GROSS: I wish we really knew. I mean we can certainly- it’s easy enough to describe properties of gravity. It’s easy enough to describe theories of gravity. We’ve had two marvelous theories but what it is, for somebody like me, raises question marks and not the answers that we already know. So in a deep sense, we don’t think we know the answer to that. Because of some of the things will be probably discussing later. But in a practical sense, if I want to send a moon- if I want to send a rocket to the moon, we know precisely what is gravity. Or, by the way, if I want to calculate the ripples, the waves that might move the arms of LIGO by a fraction of the size of the proton, I can also calculate that. So we have a pretty good practical understanding of gravity but conceptually, I don’t know.

PANEK: That’s one of the ideas I wanted to introduce to the audience is that’s one thing that we take for granted, that we live with, we don’t even think about except when it’s in the headlines, is something that we don’t really know what it is. And Kip Thorne’s answer was it’s a meaningless question.

MARIA SPIROPULU, EXPERIMENTAL PHYSICIST: But probably the answer to that, you would get the same answer to that question. Like as David’s answer. If you ask what is dark matter. So we know the properties of dark matter and it has to do with gravity, but we don’t know what dark matter is. And the other part of this discussion-t’s even worse actually yes. We ask what is dark energy? And again it has to do with gravity, but you can’t say what it is. So I don’t think it’s a meaningless question, but it’s a question to which the answer we don’t have today. In a way that we have answers to the question, What is the Higgs boson or what is electroweak symmetry or what is the standard model? We don’t have the same kind of answer to what is gravity.

SZABOLCS MARKA, EXPERIMENTAL PHYSICIST: You’re right. And if you if you wish, from an experimental viewpoint, we have-we have models and theories of gravity successfully better and better. And they describe nature very well. It affects so well that we don’t find a way out of it. We just know or suspect it must be wrong, because it’s not a quantum theory. So that’s why Rachel is here.

[00:05:26] PANEK: So let’s take let’s begin with a historical perspective. I’ll ask Pedro to begin the discussion as I said as I as everybody has said, we don’t really know what gravity is and for most of the history of civilization, gravity as a concept wasn’t really thought of. It wasn’t until Newton came along that people started to use the word gravitation and to think about well, what is this thing and what are its properties? And then we asked Pedro to take us briefly from Newton and especially Einstein as the author of the book The Perfect Theory. You’ve dealt a lot with…

PEDRO FERRERIA, ASTROPHYSICIST: Well I can give you a whistle stop tour through it. And I actually can relate it to something that David said. He said two things. He said we can send rockets to the moon and we can calculate ripples in space time. We can send rockets to the moon basically because you know, 300 years ago, a guy called Newton figured out that he could explain why apples fall to the surface of the Earth, why planets go around the sun. In terms of the very simple equation, a very simple law. And it’s the law of universal attraction, which is proportional to the mass of objects and inversely proportional to the square of the distance. In other words, the further away things are the weaker forces and it works beautifully and it’s a fantastic. It’s fantastic. You can calculate, you can calculate how you send rockets to the moon very well. The interesting thing about this is that it has a bizarre property in that it’s-it has an action at a distance. In other words, it happens instantaneously. There’s no there’s no speed in this, there’s no information there’s no information of how far how fast this this force propagates between objects. And by the turn of the beginning of the 20th century, this was a problem. Because Einstein had figured out how to marry the laws of mechanics, the laws of how things move, with the laws of light.

And I’m not going to go into this, but one of the things that came out of this is that there was a cosmic speed limit. It was-things couldn’t propagate faster than a certain speed. So there was this inconsistency between the law of gravity that works remarkably well and understanding of physics at the beginning of the twentieth century. Over seven years, he tried to figure out how to bring things together and he came up with his theory of gravity. It at some point, he was completely stuck. He didn’t know how to do it. He went to his friends and this is driving me nuts. I just don’t know how to do it. His friend gave him a paper and a textbook on Riemannian geometry and Einstein had to learn this stuff and he figured out that his theory, the way to couch his theory, the way to explain his theory. And so an answer to what is gravity is the following. If you put stuff in space time, it’ll still form space time. If you now throw stuff through space time, the stuff that you throw through space time will feel the deformations of space time and will curve and bend as if it was feeling a force coming from the thing that you put in there initially. So that’s the kind of simple summary of Einstein’s theory of gravity of general relativity. So that was you know, that’s a three minute summary of the history of general relativity and gravity.

PANEK: But, within 10 years after Einstein’s creation of general relativity, it ran into a problem with quantum mechanics and I was going to ask David to address that.

GROSS: Before I do, let me say another point about gravity, which is so you know I often give talks about the big questions in physics and what we understand about the fundamental laws of nature and-and I explain to people like you that gravity is central because in a sense, most of the time, it’s the only force any of you ever feel. Now actually, it’s-in ordinary energy scales, or even within the atom, it’s by far the weakest possible imaginable force and totally negligible and we particle physicists do experiments at LHC, for example like Maria, or theorists who work on the structure of matter, atoms, molecules, ignored gravity completely for years and years. Because it’s so weak. 40 orders of magnitude, that’s a million, million, million, million, million, million, about another million times weaker than the force of electricity inside the atom. But it’s the only force you ever feel. And people find that sort of jarring now. Why. Why do you feel it if it’s the weakest possible force of nature? And the reason is because for two reasons. One is that there’s no anti-gravity. There’s no way of shielding it. Two, it’s universal. Newton called his theory the universal theory of gravity, because it acts on everything that possesses mass or energy.

[00:10:26] GROSS: And so everything feels it. And the reason you feel gravity is that when you get up in the morning, the whole earth is pulling you down. Every atom on the earth is acting on you. And it’s-there’s no nothing just stopping it. There’s no anti-gravity. That’s not true of all the other forces. Electricity, there is positive charge, there is negative charge. They attract, but they also create when they come together, a neutral object that has no charge. Atoms, unless you ionize them, are neutral. That’s why you don’t feel the infinitely stronger force of electricity in atoms. And then there’s the force inside the proton in the nucleus that holds the quarks together, which is much, much stronger than electricity. And yet you’ve never felt it. That’s because protons are neutral. They too have a charge, we call color charge, which is neutralized by these quarks coming together and forming a neutral object. So the only forces that remain, are sort of weak molecular forces between neutral atoms and weak, not that weak, nuclear forces between protons and neutrons. But by and large it’s shielded. Gravity is universal and as was just explained, gravity is universal because, in effect, has to do with the dynamics of space and time.

And space and time are our most universal basic concepts of physical reality. We all construct this picture of space and time. And Einstein tells us that space and time is dynamical. Energy and mass curve it and that’s what gives rise to what we used to call the gravitational force. So it has a certain special role in science, which differentiates it from the other much, much stronger forces. Universality, no anti-gravity, nothing can shield it. But when you extrapolate what we understand about gravity from Newton and or Einstein to very high energies and very small distances. Quantum mechanics comes into play, just as it did with a much, much stronger forces inside the atom and the nucleus. Because it’s so weak, we can’t imagine observing directly those quantum effects. Theorists however, can imagine doing experiments at very high energies and then we run into all sorts of problems and they have to do really with the fact that gravity is a theory of dynamical spacetime. And in quantum mechanics, everything is fluctuating. You try to observe anything, you set it in motion and therefore we would suspect that in gravity as well. If you do observations at very short distances where the gravitational force becomes strong, you would see fluctuating space and time. Space just wouldn’t be something that exists there. It’s dynamical and quantum mechanically fluctuating like mad in ways which we can’t yet control. So that’s a conceptual problem we face. And there are many other conceptual problems that we might get to, which motivates theorists, by and large, to try to marry quantum mechanics and gravity. We are moving along that path with some recent successes. But the answer is far from clear and the speculations which we might get to later are mind boggling.

PANEK: Yes I want to get to the speculations later. But when you say that space is fluctuating, what are you- what do you mean by that?

GROSS: Space, in Einstein’s theory, is a dynamical entity. You know the naive picture that infants create of space, that’s when you learned about space, by the way, that’s how you learn how to get from-crawl across the room. You did it somehow I don’t know.  And it’s a major achievement.

PANEK: You also learned about gravity.

[00:15:27] GROSS: Well you learned about something that was holding you to the floor. But more, I think, much more important, you constructed a model of of events taking place in space and in time. Not trivial task because nobody knows how to teach a computer to construct a model of space and time. But then we learn that you know, it’s really spacetime and then we learn, both from Einstein, that it’s not just there it’s not just a coordinate system. It’s just it’s not just a rigid. And it’s not. It’s dynamical it can be curved. If I put a sun into this room, it would slightly curved space. It would no longer be Euclidean flat. It would be curved like the surface of the Earth is curved. And that you can explain the motion of particles in the vicinity of the sun by saying they’re moving in straight lines except in this curved space. So it looks like a deviate. But anything dynamical in quantum mechanics is fluctuating. You know that’s Heisenberg’s uncertainty principle. You can’t be at rest at a given place because if you try to observe that, you have to interact with that system, and you set it in motion. Well space itself is dynamical. And when you try to observe the structure of space, what is space really? You set it in motion and that’s such an incredibly small effect in this room that we don’t worry about it. We can simply say space is fixed. But if you try to observe space, the structure of space very short distances according to Einstein’s theory and rules of quantum mechanics, the fluctuations of space itself would be so immense and uncontrollable that we suspect that something goes wrong with our model of space and time, which largely dates back to the first few months of our lives.

PANEK: Maria?

SPIROPULU: Yeah I just wanted to say that this alludes to the fact that in the origins of spacetime are being considered now to be quantum mechanical. So quantum mechanics is the place where spacetime emerges. It’s not just classical-Einstein’s theory of relativity is a classical theory, but the underpinning of spacetime as a dynamical system, which is not just a stage where we move on a stage but it actually emerges, is quantum mechanical and that’s where the fluctuation is, tiny fluctuations, can be thought of like…

PANEK: So it’s like the fluctuations were at the beginning and then they smoothed out. From our perspective.

GROSS: They look smooth, because you observe them at large scales. Just like you look smooth. But I know you’re mostly vacuum.

PANEK: So says my wife.

SPIROPULU: But also we can say one thing that kind of a question that emerges from this is that if you are considering the mass in general theory of relativity as the source of the curvature of spacetime and therefore gravity exists, and then we learned that elementary particles gets their mass through a quantum theory quantum field theory for not phenomenon. The phenomenon of the Higgs mechanism. So the question that people ask a lot of times, how does the Higgs, who endows mass to the particles, connected to gravity? And these are questions that we don’t have answers to. But-but obviously, we start seeing that somehow all of this is connected and the conundrum of gravity has to come to some sort of an understanding I think sooner or later. Probably sooner.

PANEK: Now for all that we’re talking about general relativity and we’re all treating it in this kind of common way. There was a period, Which I’ll ask Pedro to expand on a little bit, when general relativity fell into disfavor or just, invisibility? How you would want to characterize it?

FERRERIA: Well so Einstein comes up with this theory and for the next 15 years, it’s a lot of people are doing calculations and coming up with wonderful things. They solve for what happens if you put an object in space time. They figure out what happens to the whole universe. It’s what a graduate student of mine said. It’s you-they were picking the low hanging fruit. When you come up with a theory in the initial-initial few years, there’s all this beautiful stuff you can calculate quite easily. And then it’s younger sister, quantum mechanics, came on the scene and stole the thunder. Quantum mechanics is great because general relativity applies to a very large, quantum mechanics applies to the very small. It applies to stuff you can do in the lab, it’s practical. People can do calculations. People can go and do measurements. It’s so much more interesting for physicists and not only that, it’s useful.

You know people built the atom bomb. And this meant that a lot of the intellectual activity went into quantum mechanics was sucked away from general relativity. One of the problems with general relativity was there were few early measurements that tested it. And then there was nothing else you know there was really nothing that could be measured. There was there was then there was a meeting in 1957 organized by a guy who worked on quantum gravity, Bryce DeWitt. And he brought the relativists together to say what are we going to do. You know what are we what’s the future of general relativity. And Feynman went to this meeting and I’ve got a quote. He went to this meeting and he was very interested. He’d done quantum electrodynamics. He was looking for one of the next things to do. And he summarized and he said, there exists one serious difficulty and that is the lack of experiments. Furthermore, we’re not going to get any experiments. So we have to take the viewpoint of how to deal with the problems where no experiments are available. He said, well the best viewpoint is to pretend that there are experiments and calculate. In this field, we are not pushed by experiments, but pulled by imagination. And this really characterized general relativity for 30 years until the early 1960s.

[00:20:45] GROSS: It still does. I mean it doesn’t change.


GROSS: No, LIGO used general relativity, uses gravitational-will use gravitational waves to learn about things. To learn about compact objects in the universe and even black holes. But the kinds of things that Bryce DeWitt was talking about was trying to understand quantum gravity and we’re in the same situation. And it’s because as I said, gravity is such a weak force. It’s effects at-so the highest energy microscope we have that probe short distances is the one that Maria works at the LHC. And it can probe down to nano, nano meters. A billionth of a billionth of a meter. Or even farther by now, a hundred times farther and they at the LHC never talk about gravity. There are some wild speculations that they might see extra dimensions and maybe gravity effects, but those are pretty wild. They don’t. They don’t have to worry about it or the other way around. They are not likely enough to be able to explore it. It’s still, experimentally, the problems that DeWitt was talking about, how to formulate a consistent quantum mechanical theory of gravity are unimaginably beyond experimental direct study still. And it’s still the case that the biggest advances we make in the field of theoretical- trying to understand how to reconcile quantum mechanics and general relativity are done by performing thought experiments, much as Einstein did in formulating the theory, Gedankenexperiment.

PANEK: Szabolcs, you were going to say something?

MARKA: So. So in my view it was quite natural that observation lagged behind in GR, general relativity. Because humanity was not ready not ready technologically and for even gravitational waves we had to wait 100 years from the idea of gravitational waves until we could observe. It’s not because we were stupid. We had to wait because we didn’t have the right technology. We had to invented it in 2016 and be ready with the technology. I mean it’s not a new idea. In the 1950s, people already had an idea of, say a laser interferometer. It was not called laser interferometer, because laser was not invented yet. But they actually formulated the fact that you can use light to tell gravitational waves. But you know 1950s and 2016, it’s a long time. You know. A couple of lifespans, because we didn’t have the technology. And I think I think it’s an experiment that has to expressed that there will be a test of general relativity, quantum gravity. But maybe, we have to get ready. Maybe-maybe technology will not be ready in my lifetime. Maybe-maybe it will be the job of the young people sitting in this audience. So. I would like to encourage you to learn about gravity.

PANEK: So what changed in the 60s or after the 1958 meeting? What-was it just technology or was it theory working with technology?

[00:25:47] GROSS: Chapel Hill, this famous meeting that they were talking about. So what happened was there was a transfer of theoretical technology from Quantum Field Theory which had the first quantum electrodynamics into this classical field. So from a field that was dominated by people following Einstein and doing classical calculation, you had people who were-knew about relativistic quantum field theory which was a very difficult subject in its own. But it enormously advanced in the end of the 20th century. That’s the basis of our standard theory of particle physics. Even more, and this is really I think the crux of the matter, is that we so you know I was a particle theorist. I took a great course on general relativity from Steven Weinberg when I was a graduate student. It was wonderful but that was just for cultural reasons. It had no influence on the work I was trying to do to understand the strong force. However, in the course of constructing this incredibly successful standard model of particle physics, we discovered that the underlying conceptual structure of the standard model of particles is, in a deep sense, very similar to general relativity. There is a unity of nature that was discovered and it didn’t take long, therefore for people doing particle physics already in the middle 70s, to extrapolate their theories to the regime where gravity becomes equally important and to discover that they all seem to emerge at the same place. And furthermore, that new theories, like string theory can unify. And to some extent that effort to unify the other forces together with gravity. Does begin to resolve some of the paradoxes that-that we had. But so nature taught us something coming from understanding electromagnetism and the nuclear forces about gravity. And it suggests what has been for the last 30 years the most successful attempt to try to reconcile the two.

FERRERIA: I mean but that is one strand of the development of general relativity. I mean I think the thing that happened post-in the early 60s is there was this field that was created because of all these people who work during the war on radar, which is radio astronomy. They brought these techniques. They looked at the sky and they discovered that the universe was teeming with these objects which were very powerful radio sources, incredibly powerful. And if you worked out how much energy they had and you know early 60s people measured how far away they are, they realized that these objects were immense. And these objects-to understand these objects, you had to go back to general relativity. These were very massive objects where space time would be very curved and you had to go back to general relativity. So that was one of one of the things that kicked off and so I think general relativity really came back. Kip has called it the golden age of general relativity because of what’s been called relativistic astrophysics. Is this merger of astronomers and relativists and trying to understand these objects.

SPIROPULU: And the advent of black hole physics is the-the lab for the people who are studying…

FERRERIA: It’s really remarkable. I mean if you read the history. So black holes become a reality in the 1960s. Yet in the early 1970s they’re established. But if you would talk to astronomers, they still you know it was still-you were pushing it. Black holes weren’t really that real. Nowadays if you talk to my colleagues and my I mean I’m a theorist in an astrophysics department and you talk to them. There’s no doubt that there are black holes. There black holes everywhere. There is a black hole at the center of every galaxy. And it drives a lot of the big research programs in astrophysics for example in the next couple of years, a telescope called the event horizon telescope is going to try and look at the black hole at the center of our galaxy. So I would say that. It’s true what David is saying about the evolution of quantum gravity, but the way that general relativity then contained astronomy is you know it’s a separate thing.

[00:29:23] GROSS: Quantum effects are again totally irrelevant you know but there’s one other thing you didn’t stress which is equally important in that whole era starting with Einstein immediately. Which was physical cosmology. In 1915, a 100 years ago when the theory was formed. People still thought you know the Milky Way was the universe. And there weren’t even other galaxies and actually knew nothing about the universe except there were all these points of light up there. They knew-they didn’t know about the composition of stars. What makes them shine anything. And a hundred years later we have this unbelievable history of the universe in great detail. All of that especially, the large scale cosmology part, required general relativity, which Einstein was the first to construct the model of. And he only did so initially you know it’s very interesting, a marvelous quote recently from Einstein in which he writes to his friend, Erwin Freundlich, describing his first attempt to construct a cosmological model of the universe and that was-and I understood finally what was motivating them. He wrote down these equations, describes gravity. He calculated the perihelion of mercury. He predicted the deflection of light and then he said, OK I have these equations. They describe the dynamics of space and time. What is the universe? It’s spacetime. I should be able to construct a model of the universe. But he clearly was scared that it wouldn’t work because, what an extrapolation. It’s even bigger than Newton to go from describing gravity , motion of planets, to having a theory of the universe. So he constructed a pretty silly model. Which was unstable and introduced a cosmological constant as you saw. And he wrote to Erwin Freundlich, wow I feel so relieved. And I understand why he felt relieved. I think because he was worried that if he took this theory, constructed to explain the motion of planets and so on. Consistent with the relativity principle, and then applied it to the universe, it would be totally inconsistent. You take a theory, you construct for a limited set of phenomena, then you extrapolate it, because you must extrapolate and it fails. But it didn’t totally fail for at least a few months. It was sensible. And he felt so relieved that his equations were logically consistent when extrapolated to the ultimate domain.

PANEK: Well partly what defeated him I guess in terms of predicting the expansion of the universe is that he was only-that his universe was one galaxy big. And that galaxy was not…

GROSS: Well also, you know, Einstein was a genius but he, like all of us suffer from prejudices or hidden assumptions. And if you go out at night and look at the sky, it’s beautiful. You go out the next night, hasn’t changed. So obviously you assume that it never changes. It’s eternal. There is motion because we’re going around the sun and so on, but it’s static. So his goal was to construct a universe that never changed. A static universe with no beginning no end just permanent. And-and boy that was a big mistake because he otherwise could have predicted the Hubble expansion of the universe which would have, in my mind, been the greatest prediction ever made. But he didn’t make it.

PANEK: Well to rewind from the expansion of the universe you go back. And you go back to the initial point whatever you want to call it. And that was the cosmic microwave background, the afterglow of that event was discovered in the 60s and that also helped revive the ideas of general relativity, because you would need that to be-to explain the conditions that were at that dense. And the expansion of the universe. So that that opens up. What was the phrase you used?

FERRERIA: Which one?

PANEK: The good one. Yeah, thank you, The Golden Age that. So that was part of the Golden Age.

FERRERIA: Can I just say something? It’s interesting I didn’t mention cosmology. What I do is cosmology. So it’s interesting that I ignored.

PANEK: So yeah.

MARKA: So one note which is which is less about the theoretical development but more about the science sociological development. I mean gravity up until quite recently was kind of a boutique-boutique science that people very few people were interested in. And now know that that gravitational waves are on the front page of every newspaper. Just think about that. You know less than 10 million of humanity actually put serious time into this this science. So in science you need a critical mass of minds thinking about the same thing to have a conversation to have advance in science. And actually that really happened after the 1960s that people really started to congregate that, OK. So this is the next big thing we should think about. Yeah. So. So I think that there is more than you know what we see scientific development. You need-you need human brains actually addressing and getting interested in this topic.

[00:35:20] SPIROPULU: And experiments. I mean now we have, for cosmology we have so many missions, so many experiments that it’s spectacular. It goes to accuracy of the level that is the same as particle physics for example which is astonishing. If you told somebody 30 years ago they would laugh at you when you say I’m doing precision cosmology. They would completely laugh at you.

MARKA: They laughed at me when I changed from particle physics to LIGO. They said you have perfectly good science.

PANEK: So in terms of cosmology, there were a couple of developments in the last the last generation last 20, 30 years that I’d like to discuss and they ultimately bring us back around to the quantum general relativity disparity. But-but before we get to that, let’s-let’s walk through those anomalies. Dark Matter. Maria you’ve been working on that. Can you introduce us to?

SPIROPULU: Right so.

PANEK: Where dark matter was-why it was invoked?

SPIROPULU: Yes. Dark Matter. In fact it’s been more than 70 years when Zwicky, at Caltech, he introduced dark matter and the clusters of the galaxies to explain-explain the total energy conservation essentially in the sky based on what you see from the light of the stars and the galaxies and what you can calculate. So he invoked something which is called the “dunkle materie”, the dark matter. It was a phenomenon that had to do with gravity and there was no characterization of it. And in the past 40 years we have seen that when we look at the rotation curves in galaxies of star, they they seem to be instead of following the Newtonian law and going down. They used to…Yes. That also, but let me go first I will explain the bullet cluster as well. But we have the instead of following Newton’s law as we get out from the galaxy, it looks like they plateau as if there was extra gravity, as if there was extra gravity which if it wasn’t there it wouldn’t hold the stars together so galaxies will- the stars in the galaxies would fly apart. So that’s kind of important because it looks like excess gravity. We can measure it but we don’t know what is the source of this extra gravity. What is it exactly? When we see with the progress in all the astrophysical astronomical observations we are able to look at the sky in radio, in X, in all these different wavelengths of the electromagnetic spectrum. So we can see what we can see the universe at the electromagnetic spectrum and we can see the universe in terms of gravity, as we said with the velocity curves.

When we look at the picture that we have here, we look at the collision, the bullet cluster collision, which takes five hundred thousand years where the-the-the cluster collision leaves dark matter it seems not to be touched at all. And all that and it’s the-the blue stuff and all the red stuff is the is the gas where the particles are actually colliding. So what is the stuff that galaxies come together? They collide. But this stuff doesn’t touch at all and completely different than the matter that we know. Completely different than the interactions of matter than we don’t know. No electromagnetic at all. We call this dark matter and we have developed a different kind of scenario for what it could be. Could it be dark stars? Could it be the black hole? Could it be…? What could it be? We made a lot of observations astronomers. Cosmologists made a lot of observations and they have excluded all sorts of large objects in the sky that could compose dark matter. And then there was this revelation that perhaps we can explain dark matter as a new type of particle, a new type of if you will, something that it is not something that is not included in the standard model, but it’s an exotic particle. And what were the properties of this particle? This particle should be weakly interacting. So it’s like a neutrino. It doesn’t it doesn’t it doesn’t interact with us direct with us. Very-with normal matter super weakly, in fact. In fact super, super, super weakly, much weaker than the Higgs or anything else of the electroweak theory. It’s massive so that it accounts for the evolution of the universe with all the cosmological parameters.

[00:40:15] SPIROPULU: What is the abundance from the beginning of the universe and- and we can produce it, the colliders or we can observe it in the sky when it annihilates with each other. Or in fact we can wait for it in detectors that are massive detectors and wait for a dark matter particle to go and interact with the nuclear interaction with the nucleus of-of-of an atom. If we have a massive detector let’s say with water or with- we can design many detectors or in the detector with silicon and we can figure out from the recoil if actually this is a dark matter particle or something else. So in particle physics, we developed this idea that it could be a particle and we had a very good theory for that. It’s called supersymmetry that was giving us such a candidate. I’m speaking about was and in the in the past tense because this there have been about 20 years where we were-we have been convinced that dark matter is a particle and we are going to find it. But you all read in the newspapers that supersymmetry, we haven’t found any supersymmetry or any hints for supersymmetry. So now we are looking at the different detectors the different detectors that we produce. We think we can produce dark matter. We look at it in the form of a more exotic form, not just in the form of supersymmetry.

And so do all the direct dark matter experiments that are waiting there for a nuclear interaction to happen with the dark matter particle and the range of the masses that we are exploring are from the very large masses of the G.V. level. At the mass of the problem let’s say but it will be weakly interacting all the way to 500 G.V. to the large hadron collider. So if the particle if the dark matter particle whether it is supersymmetric or something else is there, we have a very good chance to triangulate it among these three ways of experiments. The experiment, the indirect experiments in the sky, where dark matter annihilates with each other and produces different particles, excesses of particles and we can see. Or in the collider experiments where we actually produce dark matter. Or in the direct dark matter underground experiments, a huge project-huge program of underground experiments where we see the nuclear interactions with the active volume of a detector. So the study has-there is a proliferation of dark matter experiments and we have been saying in the past you know 10 years we have been saying that we are right around the corner to discover dark matter. And I think we are. If the dark-dark matter is a particle, we really are right around the corner because we have cornered it from so many experimental sources that it has got to be. It has got to be reachable in the very close foreseeable future with all the experimental program that we have set it apart.

PANEK: I’m not contradicting you but I know from my research that beginning around 1980, people would say, we’ll have the dark matter particle in five years. And then in 1985, it was five years, so on and so forth.

SPIROPULU: Yes that is correct.

GROSS: It’s like cutting the deficit.

SPIROPULU: It has to the same however I will I will remind you it has been the same for the Higgs. It was right around the corner for five years. Every year for a very long time and you know it took a little bit of a corner to go from the Tevatron to the LHC and then we turned that corner but that-I think if it’s a particle, it has to reveal itself in this and the next generation of experiments. W-e cannot we are not going to be going to be shooting in the dark.

SPIROPULU: If if if we go beyond the next generation of experiments and dark matter…

GROSS: I mean it’s not a search for something you don’t know anything about. We know precisely, we don’t know what it is, but whatever matter it is we know how much there is in the universe. And you can see it there. You know, that blue-you might wonder why. Why is that colored blue. That’s because you don’t actually see directly. That’s that what you see are those bright sources. Those white things. Those are quasars behind the cluster. Shining light through it. And then as Einstein once explained in a beautiful paper, he said, you know mass matter is like a lens. Light passes around. Matter gets deflected. So what you see there and the way the observers measure the dark matter even though they can’t see it directly with light. They can measure the bending of light in the gravitational field produced by that matter. So that’s why you can see it. You can measure it. You can count how much there is and it’s we say 90 percent of the matter in the universe is in the form of this dark matter quite precisely, which is amazing. Most of the matter is not made of the stuff that we’re made out of and we don’t know what it is. Could be particles, probably is particles. But since we know how much there is, and we we have strong constraints on what mass it could have.

[00:45:55] SPIROPULU: So this is the plot, the pie chart there. So the 25 percent of the energy of the whole energy density of the universe is a dark matter. If you take mater that’s 90 percent of the mater. And the ordinary matter, the little 5 percent of our magical stuff that we are made of, are the stuff that we can explain with particle physics to 24 orders of magnitude, extremely precisely the phenomenon in energy. So so that dark matter with which all the inference of dark matter is due to its gravitational interactions. But we cannot we cannot really say is it a particle yes or no. We had the-there is something which is called the WIMP miracle and WIMP stands for weakly interacting- weakly interacting massive particle. And people are today they’re talking about the SIMP miracle, which is the strongly interactive massive, massive particle. Because the weakly, weakly interacting one. And this kind of a miracle you would have 100 G.V. of weakly of WIMPs that would fit the-the that would fit the model and would give you the dark matter that we calculate in the universe. Is just not having any result-we don’t have any positive results as a particle. So while we think we have good calculations and we think we have a good we are not going in the dark, as David said we have a theory and we have good calculations of how a particle like that would interact. And we can calculate, we can do a lot of calculations and we have experiments while this is happening. We should keep an open mind and yes, it’s-the particle fits the bill. But until- if you don’t discover it, then it doesn’t. You cannot force it to be. So I think we have to keep an open mind and an open mind on that.

PANEK: And the other part of cosmology in the last generation or two that involves gravity but has left us with a mystery is dark energy and Rachel you’ve been researching that, so can you explain to us what why we needed to invoke dark energy and what kind of research?

RACHEL ROSEN, PHYSICIST: Sure. Yeah maybe I can give a little bit of a history which we touched on before. So as recently as the 1920s as was stated it was widely believed even by Einstein that the universe was static. That it hadn’t evolved in time, that it looked more or less the same, it didn’t have a beginning it didn’t have an end. And in fact when Einstein tried to describe this static universe of course if gravity is the dominant force that acts at very large scales what you would expect is a universe that will collapse over time because you only have this attractive force at large scales. So in order to write down a static universe, Einstein had to introduce this extra factor, an extra parameter in these theories. It didn’t have a good physical interpretation but it was allowed by the theory and you could just pick in this extra factor in order to sort of stabilize the universe, or at least give you a static solution although it wasn’t-wasn’t ultimately stable. But it was at the end of the 1920s, there was a series of observations and in particular those of Edwin Hubble who looked at distant galaxies distant galaxies and noted that they were moving away from us and not only were they moving away from us.

ROSEN: The ones that were farther from us were moving away at a at a faster rate. And so if you believe that our place in the universe is not a special place in the universe, that we’re not at the center of the universe and everything is just moving away from us, what this means is that the universe has to be expanding. That everything in the universe is moving away from everything else so. So this is a somewhat revolutionary idea. What it meant for Einstein’s equations though and for Einstein’s theory is he could get rid of this parameter that he had introduced by hand because you could imagine a universe that started out with some initial kick. That everything was expanding away from everything else. Gravity is still the dominant force at large scales. So over time even though everything is expanding it might slow down and start to contract again. So it was even more revolutionary than when in 1998 people observing distant supernova noticed that not only was the universe expanding, but it was expanding at accelerating rate. So stuff was moving away from us faster and faster. And again, gravity should be the dominant force at large scales. There was nothing there to explain what would be pushing everything away from everything else at a faster and faster rate.

[00:50:36] ROSEN: So there are possible interpretations of what this could be. And I think the most common one is what’s known as vacuum energy. So this is an idea that comes from quantum field theory, which is a theory of particle physics, which tells us that-that empty space is not in fact empty. So this is going back to what we were discussing earlier. As soon as you invoke quantum mechanics, everything becomes dynamical everything is fluctuating. So even if you have a vacuum with no particles in it, what quantum field theory is telling you is that you can produce pairs of particles that then disappear again. But there’s an energy associated with this process. so there’s an energy of empty space. And in fact, it turns out that this energy would have this effect of causing the expansion of the universe to accelerate this vacuum energy. Right so that’s fantastic. So you can go ahead in quantum field theory and you can calculate how big you expect this vacuum energy to be and you can compare it to what we observe. And the answer is off by a 120 orders of magnitude. So that’s a 10 with 120 zeroes after it. So that’s-that’s quite a big difference. So that is the mystery of dark energy. So what dark energy is-is sort of an umbrella term for whatever this thing is that’s causing the universe to expand at an accelerating rate. It could be this vacuum energy, it could be a new force, it could be a new type of matter in the universe. We don’t know what it is, but we call it dark energy and it makes up 70 percent roughly of the of the energy budget of the known universe.

GROSS: Can I  give a slightly different answer? So Einstein formulated a principle of symmetry. All observers are equivalent including the accelerated observers. That’s the basis of the general theory of relativity. If you read his paper, he goes through, not the one he wrote in 1950, the one he wrote in 1960. He goes through with that symmetry argument and says these are the only equations I can write down. Now I have no doubt he knew that he was lying because there is a- so the equation. You know a lot of the principles of physics are you take some quantity and you-the equations arise from demanding that this quantity be extremal, be minimal or maximal be extremal. In other words, small variations don’t change it. So just about all the laws of physics can be formulated that way as he did. And that term he extreme-ized was the curvature of space time. But there’s another term, which he definitely knew about called the volume of space. You know every curvature, mean curvature, average curvature, and total volume.

And I have no doubt that he knew he got at such a turn why didn’t he include that? That’s called the cosmological constant term. Why didn’t he include that in his equations? Well Einstein wanted to calculate everything. In writing down that equation, he had to introduce one parameter. That’s Newton’s constant, strength of gravity. But that in a sense is-defines the scale of massive. If we’re going to introduce this other term, the cosmological term, the volume term, it would involve another parameter and he didn’t like to have anything he couldn’t calculate. He really believed that ultimately the final theory. Everything would be calculable. Then he started doing cosmology and he wanted a static universe and he tried to solve the equations and he couldn’t get a static universe. So he said, Ah well there’s this other term which I didn’t put in. I can use that now because it’s more important for me to show my equations work for the universe as well as for a planet so he puts it back in. He knows he’s going to have to pay a price of having an un-calculable constant. He by the way never did dimensional analysis and said I have to introduce an un-calculatable constant. It’s 10- 120 orders of magnitude smaller than what I definitely know must be the case. But anyway. And then when he when he learned that about the Hubble expansion and other theorists were constructing the models of expansion he said well at least I can get rid of this arbitrary constant.

[00:55:00] GROSS: But you know, so there’s another concept. What I think needs to be emphasized is however, that the discovery of cosmic acceleration. And the measurement of that acceleration and within the cosmological standard model is in my opinion, one of the greatest triumphs of general relativity ever. Because the volume term from a geometrical point of view or the vacuum energy if you move that to the other side of the equation as the source of gravity that is observed, fits Einstein’s theory. To 10 percent accuracy. W= -1. That’s the fundamental prediction of general relativity. The biggest success, in my opinion. Lesson we learned from those supernova measurements of the expansion of the universe is that once again, general relativity works. And if you interpret this turn-you can either interpret it as the dynamics of the volume of the universe or a source that produces that dynamics, that dark energy, vacuum energy, any source consistent with general relativity will give this particular form of stuff that sources this part of gravity. But this stuff is weird and stuff has energy and positive energy, that’s always important positive energy. It’s trouble if you have negative energy. But it has negative pressure. So an ordinary gas pushes out. You all know, you try to put it in- gas in a balloon and it pushes out. This stuff pulls in. It’s like it had negative energy, but it doesn’t. It has positive energy, but negative pressure and that negative pressure that causes the expansion, the anti-gravity aspect. But it has exactly, within 10 percent, the same negative pressure as positive energy and that means that it looks the same to all observers. There’s only one form of stuff with energy and pressure that looks the same to all observers. And that’s this kind of stuff. So the fact that that expansion. Is of a particular form that’s allowed by general relativity that Einstein introduced for the wrong reasons, and then was glad to get rid of, is I think a fantastic success in general relativity.

SPIROPULU: Qualitatively.

GROSS: Qualitatively. It’s not-

ROSEN: It’s the value that we have an issue with.

GROSS: The value, you know is a problem for theorists. We’ve got a lot of numbers we can’t calculate yet. We have a lot of numbers very small numbers that we have calculated already, but it’s not an observation. You know experimenters, they don’t care. They measure it. We need to explain it. And it’s a bit hard to explain such a small number but we’ve done so before in other cases by similar orders of magnitude. So

PANEK: 10 to the 120.

GROSS: Yeah. I mean the way I give the story usually is. QCD, a theory I’m very fond of the theory of the nuclear force explains qualitatively you know in astrophysical terms within a factor of 2 to the 10, the ratio of the mass of the proton to the Planck mass, the basic the scale or gravity. You know mass is big enough so that gravity becomes strong. That’s 10 to the 19 ratio of two scales of mass. If you convert the cosmological constant into a scale of masses and furthermore invoke broken supersymmetry, it helps a lot.

ROSEN: It’s 60 orders of magnitude.  

GROSS: No, but you have to take the square root because you’re now you’re comparing mass squared. You know cosmology doesn’t have dimension of mass. So if I you know I could say the QCD explains the mass squared of the proton over the Planck mass squared which is 40 orders of magnitude. So we’ve done it before. We have tricks to do it. The best trick is you take a large number, you take its logarithms, and that logarithm is a reasonable number you can imagine. But whether you measure things with. Whether you measure ratios, dimensionless numbers, or their logarithm depends on the physics. It’s a question of how does physics vary. We’ve learnt the standard model of physics varies logarithmically. That’s our problem, various…

[01:00:07] SPIROPULU: In other words it’s a big problem. But we will solve it.

GROSS: It’s-it’s an enormous theoretical problem but it’s not a mystery at the qualitative first level. It’s exactly what General Relativity would predict. If you had to predict another term not like the kind you’re playing with, that’s consistent with the principles of general relativity in addition to the curvature. Obviously one’s allowed. In fact the standard philosophy in physics nowadays and even Einstein knew that, is that if something is allowed by your general principles, it’s there. You can’t- anything that’s allowed is mandatory. And it has some value. And now the question is why is it so small in some units? You can turn around and say why is Newton’s constant so small?

ROSEN: Well but-but the issue is how it receives quantum corrections, no?

GROSS: Well that’s an issue of gravity generally.

ROSEN: That’s right, but that’s why this is such a mystery whereas say the electron mass is not.

GROSS: No, it’s-it’s the cosmological constant in terms of the mass, which is Newton’s constant. So the number you’re constructing is GM lambda, right? That is a small number. Whether I attribute it to G or lambda or whatever. That’s a dimensionless constant. And as I said it, clearly is a challenge for theorists. But unless it is not just the cosmological constant. Which is predicted by the general principle of relativity and normal physics and worked off the bat to 10 percent maybe better now. 3 percent. So I regard that as a challenge for theorists but not a mystery.

ROSEN: The value is the mystery.

GROSS: So is the structure constant.

ROSEN: Well but that isn’t sensitive to UV corrections in the same way that the cosmological constant is.

GROSS: Sure it is.

ROSEN: I mean I know that if I introduce high energy physics, it’s going to have a huge change on what this UC is going to be.

GROSS: It’s sensitive. So all-everything in physics is sensitive to what happens at the plank.

PANEK: But Alpha is logarithmically sensitive.

GROSS: Not if I…

SPIROPULU: Right okay, guys. This is what’s happening when you got theorists arguing all the time. In my opinion as an experimentalists, they’re both correct. But I want to say that the challenge here is actually going back to gravity is actually your initial question. What is gravity? We said what is dark matter? What is dark energy? What is black holes? And to all these questions, we pause. We-we. And that’s a big conundrum where we probably are on the right track. We are not like completely lost in in the sky but. But it’s going to take a lot of work and thinking before we actually say and now we’ve got it. And by the way it’s not so big. Yes it is very big. And we’re going to have to figure that out.

PANEK: Yeah, I think from a layman’s point of point of view I think that one of the exciting things about science. You know there’s a common perception that it’s about coming up with answers and it is, but it’s also about coming up with great questions and that’s what’s happened here with Dark Matter and Dark Energy. I was wondering if you could take us through some of the experimental side of gravity coming up. Now you you’ve worked on LIGO and still are working on LIGO and what-that was initially the detection of the gravitational waves from colliding black holes. But there are other phenomena that could produce gravitational waves that we know at such a scale that we can measure.

MARKA: So it is very important to approach I think gravity from the experimental viewpoint because they never see or have questions. Maybe experiments can even pose more questions or answer some of them. Or uncover information given to them. So you know in LIGO, we had to put on a plethora of sources you know anything, anything which does boom or does this, that generate gravitational waves. I just generated some gravitational wave for you. Yeah. Very small. You can’t see it but. It deformed you I assure you.

SPIROPULU: And the shape of the room.

MARKA: Yes. Yes. Everything. So-so we need large masses. And it was kind of a- kind of a surprise at first, we had seen fairly big black holes merging. But that was very interesting because it was not just the discovery of a black hole binary but it was also a chance to test Einstein’s gravity in a really strong field regime. Around these black holes, you know gravity is fairly violent you know. So-so then they merged. There are kind of three phases. You know, one is, they go around each other, you know, they get closer and closer to each other and then they kind of merge and then the final individual black hole which was just born. And then that would be one black hole.

[01:05:26] MARKA: And this means there are three signals you know, three signals getting information about the same-same phenomenon and comparing those three signals, you can actually derive a lot of information both Einstein’s gravity. We were hoping to find something, you always want to find Einstein wrong and you know, the result is we always find him right. You know. What a tragedy. So-so we did this study and actually it was consistent with Einstein’s theory but-but looking at more black holes, maybe we will find a deviation. Maybe you find the effects we didn’t count on you know. And that had other sources like. Like you know a black hole which was born is roughly as big as Long Island. It’s a fairly big one yeah. So a smaller black hole which is as big as Manhattan or a neutron star, which is very heavy star 1.5 times heavier than our sun. Roughly also the size of Manhattan can be eaten by a black hole or two neutron stars can actually kind of merge and rip each other apart and form a black hole.

Can happen and all of these processes actually-actually has gravity on-on very different scales and rates, you know so. So in a sense, the beauty in gravitational illustration is that that we just converted Black Hole from some mysterious beautiful object in the sky or a specified laboratory. From now on Black Holes and neutron stars can be considered as our backyard laboratories that we can study the properties of gravity in a way we can never studied before. You know. So. So the discovery of the discovery we were so happy to have it. I built like more than 16 years on this. I really wanted to discover something. Why did I leave if I don’t discover something? But-but-but we’re really kind of open up new ways for everybody, theorist, other experimentalists, you know there are people who are actually considering what happens then black holes eat dark matter you know kind of kind of putting black holes and dark matter together. Maybe. Yeah. So. So there are there are actually new ways to have experimental evidence on-on-on-on. Regimes of gravity which was not observed before and hopefully they will give us a clue on-on-on-on. On GR or on the flaws of GR. Of course you know quantum gravity expert will tell us, yeah gravitational waves need big masses and it will be very hard to observe, but not always.

SPIROPULU: And we can say one thing about this experiment which I think astonished me over the course of the years that I was following LIGO. that in order to get the massive- the effects of the collision of these massive, massive objects a billion and a half years ago to the lab, the lab invoked quantum engineering type of instrumentation. So gravity and quantum came together in the actual instrumentation of how you detect these collisions of these hugely massive objects- objects that produce quantum effects on the mirrors of the LIGO. I think this is spectacular in terms of in terms of technology and as you said earlier, this we didn’t have 20 years ago when LIGO was conceived, there was you know talking about interferometry but they were asking quantum optics people in order to go ahead and develop this engineering. So I think this is a feat. And they-they deserve freely all the awards they’re getting all of them although I think you’ve got only $2,000 dollars compared to the other.

MARKA: I was born too late, so bad. But I will make up. But there is also one more important aspect. You know whenever-whenever you consider these systems they are really alien. Just imagine that two objects which are like 20 times, 30 times heavier than our sun going around each other. You know your kitchen blender is slow. They go in on each other they like-like-like-like 60 percent of the speed of light. Yeah?

[10:10:05] MARKA: It’s like it’s like it’s like amazing. During the lifetime this object emanated three times the mass of our sun in gravitational energy. Wow. Nature is having some problem. OK. So. You know it’s like we had a science run and you do the math. We discovered at least one gravitational wave because we published it. And if you if you imagine that LIGO will be three times more sensitive. The thing is that volume, three times three times three like-like 37, 30. So.

MARKA: That’s you know, one a week, one a day.

SPIROPULU: They’re going to be having discovery papers one after the other.

GROSS: Well there’s another aspect that black holes are great things. Amazing. And it’s fantastic to observe them directly. They’re also very weird. They’re called black holes because classically there’s so much mass in a compact region that light can’t escape. Pulled back into the black hole and since light is-travels faster than is the limiting velocity, means nothing can escape from that region of space. And that raises the kind of conceptual problem that general activity in the hands of theorists is full of not experimentalist, unfortunately. So you’ll you’re going to hopefully be able to see within a few times, the horizon of a black hole, the region where light can no longer escape from. But according to classical relativity, there is no way of seeing into a black hole. So what’s in there? And-and this has been an outstanding problem from the moment that black- people accepted that black holes actually probably exist. Because it raises very deep conceptual issues such as the conservation of information.

So one of the quantum mechanics and all of our successful theories and traditional classical physics, you don’t lose information if you know everything about a system where you prepare a system, you follow its evolution, you know you can then do a measurement and you know you know enough so that you can run the experiment backwards in time. You haven’t lost information. In principle, if you’re careful enough. But in a black hole it seemed that you did, because stuff went into the black hole and nothing came out. So you’re either left with this black hole which according to the equations, becomes singular, everything becomes nonsensical. Curvature becomes infinite. Everything gets crossed. Nobody knows what would happen. And then Hawking is famous because he discovered that in a quantum treatment, the black hole does emit particles. It it actually emits thermal radiation. It looks like a hot object. Thermal radiation tells you know nothing about what’s going on inside an object except it’s temperature. Otherwise it looks the same. So no matter what you put into a black hole, what comes out will look the same. Eventually the black hole will evaporate. Nothing will be left. You’ve lost information. Violates principles of quantum mechanics. And Hawking became famous because he said general relativity, Einstein’s theory, and quantum mechanics are mutually inconsistent. And it’s interesting the history of that which happened also in the 60s and 70s, 80s. The relativists, who were general relativists said, OK we’ll give up quantum mechanics. people like me, the particle physicists, said, No, we’ll change general relativity. And that quarrel and study has been going on for the last 40 years. By and large, we won.

MARKA: Correct? I think everyone would agree. In the sense that even though-because we now know enough, understand enough we think about that- even Hawking admitted that black holes do not violate quantum mechanics. Information is not lost.

GROSS: In-formation of a black hole, for which classically nothing can escape thermal radiation, eventually black hole disappears. Information is preserved. We have models of black holes which we can understand by ordinary quantum mechanical means. But how exactly this happens and what happens at these horizons. If you could probe close enough is still a subject of you know it’s the theoretical laboratory in which a lot of attempts to reconcile quantum mechanics and general relativity occur are fantastic laboratories.

[10:15:40] PANEK: We’re going to try to get to one or two questions from the audience. But we did promise early on in the program that we would talk about some of the-the-the-the wilder prospects of how to resolve quantum gravity. Does anybody want to go there?

ROSEN: Well I just want to say so I think it’s a little bit of an overstatement to say that quantum mechanics is incompatible with general relativity. So I would say that in-so we talked earlier about how we can describe general relativity as curvature of space time and this was the original way that Einstein conceived it. But it turns out there’s an entirely equivalent way of describing gravity and that is a force mediated by a certain kind of particle and this is a particle known as a spin-2 particle. So we mentioned the other forces earlier, the electromagnetic force, the strong and weak force. So these are our perfectly consistent quantum mechanical theories of forces that are mediated by particles and you can describe gravity in the exact same language. You can describe it as a force mediated by this spin-2 particle. And when you do this, in fact you can ask what is the theory that I write down if I only want a force that to mediated by a spin-2 particle. And what you find is the unique answer to this question is general relativity. So you come back to the exact same equations that Einstein wrote down originally without ever mentioning you know, curvature of spacetime or equivalence principle or all these other things that we’re used to. And so there is a perfectly valid theory at large distances or low energies of gravity as a particle, as a force mediated by this particle that’s known as the graviton. And I would say that this is a perfectly healthy quantum mechanical theory. We just don’t know what it what it continues to look like at the very, very short scales. But I but I do think it’s unfair to say that gravity is fundamentally inconsistent with quantum mechanics.

GROSS: No, I didn’t say that. Hawkins said that. I never believed it. No he said it for a different reason. Absolutely. It is. Perfectly…

ROSEN: I think it’s correct when it’s a…

GROSS: But you know from the point of view of a low energy theory there’s never been a problem. The problem is with horizons, with and it. But in fact, I don’t think there are many people nowadays who think there is any compatibility of general relativity and quantum mechanics.

GROSS: The final story hasn’t yet to be given.

SPIROPULU: I think we have to keep saying this. Because the usual thing is that all you know generally and quantum mechanics. But it’s not like that.

ROSEN: That’s right, yeah.

PANEK: That’s important. So should we take some questions? Waiting for the lights to go up so we can see. Yes sir. Could you wait for the microphone coming over? Thanks.

AUDIENCE: Do all quantum mechanical processes take place in space time?

GROSS: That’s our model. When you walk, you have a model that allows you to walk in spacetime. By the way, that model that infants construct of space time has been modified by special relativity and general relativity substantially so but it’s still only a model. You don’t directly perceive space and time. You perceive events which we model is occurring and a four dimensional dynamical manifold called spacetime. Some of us now believe strongly and this is just you know it’s just a guide a clue that that it’s probably at- in certain regimes, when extrapolated a very short distances, or a very strong field or very high energy collision where quantum gravity effects become important, that that’s a poor model. It’s only an approximation to a better model of physical reality. It’s in that sense that many people working in the spec- doing these speculations trying to understand properties of black holes or other quantum gravity phenomena say that perhaps we should regard spacetime as much as emergent phenomena. As not being the ultimate way we describe physical reality. But just a sort of course grained or a model of reality that’s good for the kind of physics that we’re-scale’s where it doesn’t break down. There are strong indications of that from many sources of our current speculation and there are even more and more precisely define toy examples of how you can regard space-Time. Space, especially as an emergent phenomena, as an emergent object. Gravity is an emergent object from other ways of looking at the same phenomena which don’t-don’t-don’t have their very formulation either space or gravity.

[01:21:11] SPIROPULU: So not only the quantum mechanical phenomena are happening in the x and t spacetime but one would draw diagrams and we do calculations we have. We have x for x y z and we have T and we calculate this x and t now is emerging and possibly from from the quantum. So the quantum wins. Not geometry anymore the quantum win. I think it’s spectacular thinking on how spacetime emerges. And when we say spacetime, we say gravity and so on so forth. So it’s a revolution in thinking on how these things will evolve.

PANEK: The questions yes.

AUDIENCE: I was just wondering what are your thoughts on the idea of the Alcubierre warp drive?

SPIROPULU: Say what’s that? This is a Star Trek thing. This is the…

AUDIENCE: It’s the idea like there is there’s this Casimir effect. People are able to produce this negative or exotic matter which acts opposite of regular matter, which instead of pushing like down in space. Space time is pushing like on two dimensional view of space time so instead of pushing down, it pushes up so like you could use a spacecraft to…

SPIROPULU: Yeah, I heard about this and somebody-somebody told me recently that NASA’s pursuing some-some experiments on that. I don’t know much about this. I mean I can’t speak to that.

GROSS: My understanding of the Casimir effect are the basic laws of quantum field theory, that it’s simply wrong. Yeah there is no anti-gravity, period. Whether you consider vacuum fluctuations or any other.

PANEK: One more question. Yes.

AUDIENCE: This one’s for Rachel. You did such a fabulous job of explaining dark energy. Does it relate in any way to dark matter? Or is it just that you don’t know what either of them are?

ROSEN: Yeah I think the correct answer is we still don’t know what either of them are. So we gave them both the named dark. There are theories that try and relate the two of them but I think that the current and most popular opinion is that dark matter is probably some particle and dark energy is probably just vacuum energy and that the two things aren’t necessarily related to each other.

SPIROPULU: But how beautiful it would if all the dark and the black things that relate to gravity can kind of like sort of be explained in this in a way that is together.

PANEK: OK. Thank you very much for your attention. And let’s think of panelists.

Big Ideas
Coming to Grips With Gravity

The mysteries of dark matter and dark energy may be evidence that we don’t fully understand the force of gravity. But when it comes to a force that has been studied mathematically and probed observationally for hundreds of years, what do we still need to learn? What questions are being asked? What research is pursued at the cutting edge? Would a new theory of gravity lead to a grand revolution in science, or do our present theories just need to be tweaked? This program is part of the Big Ideas Series, made possible with support from the John Templeton Foundation.Learn More

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