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As physicists attempt to answer some of science’s biggest questions about the universe, they are testing the limits of experimental and observational science itself. In this program leading physicists, astronomers, and astrophysicists discuss how to push the boundaries of scientific imagination to develop experiments that test the seemingly untestable theories of multiverses, eternal inflation, and exotic particles. Join the conversation about their plans to recreate the Big Bang in particle accelerators here on Earth, as well as their quest to sift through signals from the farthest edges of space for the existence of a multiverse.   This program is part of the Big Ideas Series, made possible with support from the John Templeton Foundation.Learn More

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So we have a wonderful panel here tonight. Our next participant is a professor at the Institute for Advanced Studies at Princeton. He has made many influential and creative contributions to our understanding of the early universe, particle astrophysics and cosmology as probes of fundamental physics. Please welcome Matias Zaldarriaga.

Our next guest is a theorist with a wide ranging interests in fundamental physics from high energy physics and string theory to cosmology and collider physics. He was a professor of physics at Berkeley at Harvard and Harvard before joining the Institute for Advanced Study in 2008. Please welcome Nima Arkani-Hamed

Our final participant is an astrophysicist whose multitude of scientific contributions include observations and experimental astrophysics and space borne instrumentation. She served as NASA’s chief scientist and received the agency’s highest honor the Distinguished Service Medal. Currently she is the head of the seven point five billion dollar independent federal agency charged with advancing the scientific discovery, technological innovation in science education. Please welcome Director of the National Science Foundation France A. Cordova.

So first let me just try to set the stage for our conversation here. The past few decades have been fairly spectacular for fundamental physics. On the grand scale of the cosmos, we have not only detected the cosmic microwave background, which is you know the afterglow of the Big Bang, but also by measuring the properties of this radiation, especially of the fluctuations in this thing, we were able to confirm a broad brush picture of our universe. What is within it? What its properties are? We determined six parameters to a high accuracy that determine our universe.

We know, now a lot about you know what the universe is made of. We know that ordinary matter for example the stuff that we’re made of stars are made of, galaxies are made of is less than five percent of the cosmic energy budget. About twenty five percent or so is dark matter which is matter that while has a gravitational influence it does not admit or absorb any light. The rest seventy percent is in the form of some smooth form that fills all space, which we sometimes call dark energy. It’s consistent with Einstein’s famous cosmological constant. We measured the expansion rate of the universe and we know that rate locally with an error no bigger than 2.4 percent, which means we can determine the age of the universe with that type of accuracy. I actually looked it up and it turns out that with all the medical techniques we have today, we cannot determine the age of a person with that type of accuracy.


But we can determine the age of the universe with that.

So this is on the grand scale of things. On the smaller scale we have a standard model of particle physics Where we know what we think are the basic constituents of matter. These are quarks, leptons. We also know that they are four force carriers, you know carrier of the electromagnetic, of the weak, of the strong interaction. And this culminated in 2012 with the discovery of the Higgs boson or at least a particle that has those properties which is again associated with the field that permeates all space and which gives mass we think to all the particles that we all know and love. So this is also an amazing achievement. We discovered for the first time and managed to detect gravitational waves. These are ripples in spacetime predicted by Einstein’s theory of general relativity and looked for decades and we finally found them. So this is all amazing.

Now. In spite of all of these enormous successes there have been a few surprises. And some might even say disappointments at some level. So for example, in the cosmic microwave background there is a strong prediction from our model which is called inflation, that you know the universe was when one was a tiny fraction of a second all the expended like crazy. That there should be some imprint in the cosmic microwave background in the form of polarization, some some form. Now those have not been detected yet. This does not mean they are not there it’s just that they haven’t been yet detected.

In the particle physics side there have been strong predictions or expectations I should say that in the Large Hadron Collider we will discover supersymmetry that namely that each particle that we all know and love would have a partner that has a spin that is half a unit removed from that. Those haven’t been detected yet. The mass of the Higgs boson was a surprise to some and in particular it was thought that maybe the mass that was found means that there should be other particles in its vicinity. Those were not found.

So basically we understand now that there is still a lot of work to be done. So now I will turn to the participants here and you know let’s see how we can make progress. So I would start by asking each one of you to tell me very briefly, I mean in something like three minutes, what are you working on right now?   So I task with you Matias.

Yes. So right now I’m working on two things so I’m very interested in the things that could be leftover from the very beginning, the beginning of the hot big bang in the form of these small fluctuations that then grow to form this structure that we see in the universe. And the good thing about them is that they have a lot of memory, so when we look at things today we can kind of play back the picture and try to understand how they started. I’m interested in several of those properties and I’m working in trying to improve the ways, the ways that we run this movie backwards. To try to extract more information about how things started because that’s to me one of the big mysteries.


Well I’m thinking about things on the two extremes that you alluded to on the very very short distance frontier, a very high energy frontier. For the past number of years I’ve been thinking a lot about how we could experimentally study, in as much detail as possible, and what we could learn if we got this experimental information about properties of the Higgs particle. The Higgs particle is a very strange particle. We’ve never seen anything like it before and it’s more pointlike than you would naively expect it to be.

Namely has no structure.

Yeah. It doesn’t seem to have any any structure. Seems to be like like purely pointlike. Doesn’t seem to have any sub substructure at all and a whole bunch of people are are studying the prospects of having another accelerator after the LHC could be 100 kilometers around. And one of the things that would do is put the Higgs under a much more powerful microscope than we could get from the LHC and answer very basic questions that are going to be left open even after we’ve finished running everything about the LHC, about this burning theoretical mystery about its substructure.

In the opposite direction of, I was literally working on the train on the way up here is, has to do with some questions about the cosmology. Cosmology is just the most glorious of the historical sciences but like all historical sciences it has an interesting relationship with the notion of time. None of us were around during the early universe. But we infer this past and the Big Bang and even inflation and all of that stuff because we make measurements today at very late times about sort of correlations in space. And we decide that the best way of making sense of those correlations in space is by inferring the existence of a time and cosmological time and evolution that came before that, a lot like a paleontologist infers the existence of dinosaurs from the existence of big bones lying around. But for many reasons we expect the notion of time has got to ultimately break down. It can’t be fundamental, especially when we get back to the Big Bang, probably to notion of time which is breaking down. So that’s suggests theoretically there should be some way of talking about things without making such heavy use of this concept of time. And so I’ve been working theoretically on trying to find some interesting new mathematical structures that could replace time and our understanding of where these spatial correlations come from.

Right and time within that sense would be an emergent property.

Time would be an emergent property. Yeah

Thanks. France I realize that, you know, you are somewhat different in terms of what you do so in your case I would insist on, I mean what is currently most occupying you know your time.

I run a big agency that funds all of science and engineering. Everything except for the biomedical sciences which the National Institutes of Health does. So we fund everything from social and behavioral sciences to geosciences, biological sciences, computer sciences, clearly physics, astronomy, chemistry, material sciences, mathematics and and so on. I’m sure I left something out. We also run the U.S. Antarctica program and so we have a lot of science there at the South Pole, including an experiment that’s called Ice Cube which is a big neutrino detector at the South Pole, about a square kilometer array of photo multipliers that are that go down a kilometer into the ice and detect neutrinos from the heavens.

But on the practical side of serving science we’re building some very big telescopes for the future. And we’re also trying to make better the telescopes that we currently have. So as one example on the gravitational wave experiment we know there are lots of noise that affects the spectrum of, the frequency spectrum for detecting gravitational wave from seismic noise to thermal noise to shot noise. And so if we can improve the equipment. So those are technological advances that we are working on and we have in the laboratories where these people work and all over the country we have young people working on things like using quantum physics to squeeze slides so we can get a big better focus on the laser light source for LIGO.

So we’re trying to reduce, increase the sensitivity, reduce the noise and thereby be able to detect sources of gravitational waves much farther out. So already this in this run that we’re having now, which is called the second run of this facility, we have improved by a factor of 20 to 30 percent the sensitivity. So we’ll keep going in that direction while also building a lot of new telescopes to observe dark energy, dark matter in the ways you talked about.

And you hope to reach a factor of three right at least in new sensitivity, when everything is said and done?

Yeah yeah.

Good Matias, I know that you have thought quite a bit and presumably still work on this thing called non-Gaussianity in the cosmic microwave background. So you know everything in life we think is Gaussian you know. It depends on this bell shaped curve. But in the cosmic microwave background everything is also Gaussian but different theories predict small, you know, deviations from this Gaussianity. So explain to us a little bit you know what is involved there and what does it mean if it is being detected?

Yeah so I think the first thing to point out is that we are always looking for things that are leftover from the very beginning and that the subsequent evolution of the universe has a difficult time changing. So, for example, natural thing that we would look try to see if the universe has a different composition in different places. But even a universe that started with different compositions, some physical processes in between can make the composition all the same. So you can erase composition differences. However when we look at the statistics of these fluctuations, these properties, the Gaussianity of it, is something that is pretty much impossible to at least on the very large scales, to erase. And so in that sense it’s kind of a very nice thing to look at because if we find it it’s telling us something about really the very beginning of how these fluctuations that lead to structure arose. What it involves doing in terms of the observations is making maps of the universe as big as we possibly can because these are statistical measurements that you want a lot of samples to be able to infer the distribution of matter and difference on different size scales and at different times in the history of the universe so the bigger the map the better and try to learn how to interpret these in the best possible ways.

So just a small follow up on this. I mean can you, do you think we can do this with the current existing data from Planck and W-MAP and so on? Or does this need the next generation of cosmic microwave background detectors?

Well these are things, that were the last generation of CMB experiments made a big progress on that. But basically in the CMB with mapped the entire sky almost to the highest resolution that we can possibly do it. There is we can do better in polarization, so there is a little bit of improvement but not a qualitative improvement. So I think most probably to get a not just a little bit of an improvement, which of course is very nice to have and we are working, people are working on it and it will happen. But to have another order of magnitude we will probably need to look for some other probes, mainly mapping the distribution of matter in the later universe. And there the current constraints on Gaussianity from those from those surveys are significantly weaker than the ones that we have in the CMB. So it means that this field has to catch up quite a bit, learn how to do things. And it’ll be the next few generations of surveys that will do this. And eventually, hopefully, they’ll catch up because indeed there is a big part of the universe we have not yet mapped. So in terms of our data that it’s out there there’s a lot.


Nima, you know we talked about you know some of these things that you know we now know and some of the problems that we have. I know that such people as Savas Dimopoulos and some of his students and ex-students and so on work on tabletop experiments which you know I mean, now Matias now talked about you know bigger experiments and so on. But this is a new type of experiments that perhaps can probe some of the things we’re talking about: dark energy, dark matter and so on. Can you explain a little bit to us how those tabletop experiments work.

Yeah, if I just put it in a little bit of context that we’ve, certainly the generation of people like me and Matias have lived through three decades of amazing experiments probing fundamental physics on every possible frontier, there’s dark matter, the measurements of the universe, colliders, most recently the LHC. And most of these experiments were imagined and conceived by people in the 1980s who had a sort of vision for what the next 30 years was going to look like. And many of these experiments are in their last stages.   And it’s I think a very interesting time to think about what the next 30 years are going to look like because that’s the kind of time scale we talk about in this business. You really have to think sort of three decades on…

At least, I would say.

At least, yeah. If not if not longer. It’s getting longer.

So there are, of course, there’s a whole very important new generation of experiments that Matias was just alluding to to measure everything we possibly can about the distribution of matter in the universe. There is I forget what the factor is but maybe a factor, what is it like ten to the eight times more data potentially out there that we can get as human beings.

A hundred million. It’s a hundred million times.

Then what what what we actually have. There is the experiments I alluded to earlier about taking the sort of next big step after the large hadron collider or a factor of ten, 710 an energy higher than that.

But there are some novel things that people are are talking about which are mostly targeted at looking for things that might be out there that could be related to dark matter that are incredibly weakly interacting with us. Now dark matter is is one of the things that’s supposed to be incredibly weakly interacting with us par excellence. Right. That we’re supposed to think that we’ve only noticed its affects gravitationally and gravity is by far the weakest of all interactions there are. So most of the experiments of the last twenty years that have been looking for dark matter have been assuming, there’s various good theoretical reasons why it’s nice to assume so, have been assuming, that the dark matter particle does participate in some of the interactions that we know about. The weak interactions that are associated with radioactivity are an incredibly weak interaction but still it’s strong enough that people could design all these amazing experiments to look for dark matter particles and in the range that they’re looking for that sort of one of them in every liter in this room you know moving through the room at one one-thousandth the speed of light and they would bang into like very cold big vats of liquid Xenon for example. And you look for the little shakings of the nucleus of these atoms. So those are the kind of experiments people have been doing for a long time. But it’s possible that dark matter doesn’t look like that.

With no results. No results.

We haven’t seen anything. We haven’t seen anything.

Now these things are called, this picture for dark matter is called the picture of WIMPs. The W in WIMPs stands for weakly interacting massive particles and the weak is actually it really is a technical sense of the word the weak interaction. It has that kind of strength interaction. It’s actually possible that the dark matter even is very simple a picture of WIMPS but that the interaction is just too weak. In fact the very very simplest dumbest most straightforward possible picture for what dark matter could be, it just accidentally happens to be so weakly coupled that these experiments are not going to see them. But there have been also long been many other interesting examples of particles that could that could solve many theoretical problems and also be dark matter. Things like axions for example. And these are an interesting kind of particle. There are zillions of them surrounding us all the time. They have very small mass. There are zillions of them surrounding us all the time.

They are more strongly interacting than gravity but way weakly more weakly interacting than then everything else. And so you need a totally different kind of experiment to go looking for them. And what the, this new generation of experiments that you’re referring to use cool methods from atomic physics the our growing ability to quantum mechanically manipulate fairly macroscopic objects in order to look for these things. Just to give you one example, if these particles are out there and they’re dark matter, one of the predictions is that the neutron, the neutron which is a neutral particle would have a tiny so-called electric dipole moment. That would mean that the fact that it would be as if the neutron while it’s neutral has a little bit more charge in one direction in the north pole than in the south pole if it’s spinning with a with a spin going in the north south direction. And so and that tiny tiny electric dipole moment would actually oscillate with time.

And so you can use fancy methods from atomic physics, essentially using the same ideas from nuclear magnetic resonance to pick up and amplify that that oscillating neutron electric

Just to say nuclear magnetic resonance is what is used in all your MRI imaging and so on.

Right. So that’s that’s one example. And there are a number of other examples like this but there is this new frontier of looking for very weakly interacting things that could be if they’re if they’re the dark matter could be filling the universe around us. And that’s I think it’s very exciting.

Right. And maybe I should also add and maybe you can add a little bit too, I mean some of these experiments also do these things with tiny tiny micron sized objects and you know in, acting in gravity between such things which is a force that’s just about the weight of a virus.

Yeah. And so

And testing how gravity changes on this type of scale.

Just so you have an idea, we we often say that gravity is the weakest of all forces. And if you take a if you take a pair of electrons their electric repulsion is forty two orders of magnitude stronger than the gravitational attraction between

That’s one and forty zeros.

One and forty two zeros. Yeah.   So now of course gravity looks like the most important force. Here it’s keeping our feet to the ground and so on. And that’s because most objects are in the end electrically neutral right, like atoms are electrically neutral. But you can ask, at what distance, if you take a pair of hydrogen atoms at what distance do you have to put them so that gravity gets weaker than even the residual tiny tiny little piece of the electromagnetic interaction that’s left between them? It’s called the van der Waals force. It turns out that distance is around a millimeter. Already at a millimeter, which is a pretty big distance scale, gravity is just getting swamped by these other interactions. And so indeed there are people who manage to control for example the quantum mechanical coherence between two atoms that are separated by distances of this of this order. So you could actually try to measure and see the effects of gravity in these very small scale things.

France, we heard here of you know of large scale experiments and tabletop type experiments you know and so on. How does the NSF work in terms of, prioritizing, or not, big versus small experiments and so on. Because we’re talking here about different classes of experiments. Things that cost billions of dollars and things that our tabletop experiments which probably still cost hundreds of thousands of dollars maybe but you know it’s still very..

More like tens of millions of dollars.

It’s OK

So it’s it’s an interesting question from many aspects because of course you know even though we have in principle seven and half billion dollars to spend it’s just a drop in the bucket when it comes to funding all of science and engineering, so its a question of how one sets one’s priorities and how you balance little versus big things to invest in because you want to have an investment portfolio. We all want it in our personal lives that that balances for different objectives and goals and has a balanced portfolio. So we really depend on the science and engineering community to be the major source of input for that. But believe me we get a lot of input from other sources too. One major source of input is the National Academy of Sciences. So especially in astronomy and physics we have these reports that come out every decade or five years, for the high energy physics thing every few years, and those are those represent hundreds, even thousands of scientists and engineers coming together and decide, and having these kinds of conversations just like this only really intense about, no if we just build this kind of detector than even though it’ll take ten years and so on. And so they put all that together in a report that Congress really respects because it represents different input than the agency itself. And we we pretty much follow the guidelines of those reports. We just draw the boundary at how much money we have to fund them. And so we have, we fund very big things that cost hundreds of millions of dollars. Like I mentioned in LIGO altogether we’ve put in a billion dollars plus and we fund we have a program called major research instrumentation in universities that fund us up four million dollar projects. That the area that we’re most worried about leaving out right now is the area in the middle that cost anywhere from say ten to one hundred million dollars because we don’t have specific pots of money designed for that.

Matias, I want to turn slightly provocative here in the following sense. You know we’ve we’ve been looking for dark matter now for a long time And we’ve not found anything. Now. It is true that the dark matter particles they have a lot of where to hide. But still we have not found anything. So. There are a few people, there are not very many right now, but there are a few people, one of them happens to be a good friend of mine, who suggest that there are no such dark matter particles that instead we need to change our theory. And there have been historical precedents to this. I mean you know like you know. When Einstein was there and it was ether you know and so on. The idea was not to have something that you don’t see. But to change the theory. So. And similarly with precession of mercury right? We had to change the theory. At what point, if any, do we say – well, maybe we shouldn’t build even a bigger experiment or not let’s think harder of changing the theory.

Well whenever there is if there comes a theory that explains everything and more things then everybody will jump of course. So what a problem is that the lack of such a theory but also it’s important to realize that dark matter was introduced as you know because of discrepancies in galaxies and clusters of galaxies but that’s not where, not at all now from where we get most of our information and all of our information of ours how much dark matter there is or how it is distributed. It’s from the cosmic microwave background.   It’s from gravitational lensing. All things that were not, were not there when dark matter first, the first anomalies that lead to people to think about the dark matter came about. And so now the arena, if you want to have another explanation, which by the way is totally fine.


I think it’s great to try to have lot of people do it and it’s not a problem at all. But where you have to focus your attention in my opinion is on the things that where it’s most constraining, where we have the most information, percent measurements of the you know abundance because of how it was distributed four hundred thousand years after Big Bang and in the middle with the gravitational lensing, and later with Galaxy clustering. So we have so much information and unfortunately even you mentioned some of these frameworks they were built to explain the original anomalies.


But they have nothing to say about all of modern cosmology, I would say. And so it’s they’re so at the moment so lacking of being able to use in any way where the excitement is that that’s the reason most people are not using it for cosmology because it has nothing to say.

I agree with you but I will just a small follow up. I mean partly you know maybe I’m just you know raising this as it was a provocative possibility. Most people didn’t accept this new framework to begin with. And so maybe not enough people have thought about this. You’re right that these frameworks don’t explain the peaks in the cosmic microwave background but maybe because not enough people thought of trying to explain.

I don’t think that’s totally correct. People have tried. People came up with ways and tried to implement them and they didn’t work out. So there are people that are continuing to try just to mention somebody Justin Khoury…

Or Linde.

Or Linde. There are many people that are trying to do it so it’s not the case that and you know I think people should not take the, should not think that in any way shape or form we are not trying to find the answer right. So if there is an idea and people, you know, especially theorists, they like to speculate. And whenever there is something that rings a little bit true they follow up. The problem is that they follow up and you know the typical complaint about the theorists is that there is one event and then there’s, you know at the LHC or whatever, a hundred papers. So every you know theories we follow up everything. And the reason these things have not picked up in my opinion is because the grains of truth that you know ring then when you follow them up so far they lead to nowhere right. While on the other hand dark matter as a particle we have to remember the universe was much hotter than anything, collisions were much higher energy than anything we are probing in the laboratory…

Can I say a little thing about this? I mean because there’s a more general point there’s a more general point here which is often when we talk about dark matter and dark energy especially in the context like this there is there is an obvious question as always some twelve year old kid in the audience who isn’t it just like the ether. You guys are idiots. You haven’t learned anything you know.   Like every time you have a problem you like invent some new crazy thing was supposed to build the universe. And we know that. Of course of course we we all know that. The problem with history, and if you know anything about the history of science you know that you can use, you can take an example from history to illustrate any polemic point you want to make. And in the case of dark matter you alluded to it already but there are there were there were two.

Both ways

There are both ways and there was actually one astronomer, Le Verrier, who was involved in both of them. Mr. Le Verrier predicted the location of Neptune because there were some little anomalies in the orbits of the distant planets and so that was dark matter back then, dark planet was. You could all have said oh let’s mess up Newton’s laws. But no, the right thing was conservative. Almost always the right thing is conservative. That’s right. Then he predicted Vulcan because there was something wrong with the orbit of Mercury and he was wrong in that case. Right. So the same guy was right once and wrong once. So we never know ahead of time.

And that’s why we always keep an open mind especially I’m just echoing what Matias said, especially theoretical physicists, we keep a very open mind. As Robert Oppenheimer said it’s important to keep an open mind not so open that your brains fall out. But you’ve got to keep your mind as open as possible. And that’s really difficult. I’ve spent time thinking about modifying gravity and I’ve written papers about it but exactly what Matias said is right. That there is a certain smell of truth, of logical consistency, of inevitability, of something that works which is nowhere near any of the attempts so far. It doesn’t mean there might not be one someday. But it’s not like we’re dogmatically beating these people over the head who would dare challenge Einstein. Believe me if we could challenge Einstein and be right we would have the greatest thing we could possibly do. So the the difficulty in this subject is how to be radical and conservative at the same time. And especially given that so much works so well already. You can’t just go crashing everything. Because we have this incredible edifice that we’ve built up over 400 years. That’s why we don’t know ahead of time how radical and how conservative to be and we try everything we can.

There’s another point here, so we talked a little bit about the gravitational wave observatory and this is the first time in the history of the planet that we now have an observatory that can detect sources of gravitational waves. So what did the first three sources turn out to be? Something that we didn’t even know existed. Not something that that can’t exist. But nobody was running round writing papers about binary black holes especially if those kinds of masses of twenty, thirty solar masses, and now we know that there is probably a very large population. So I asked one of your contemporaries theorists, Ed Whitton at MIT, I said is is it possible that they could form a constituent of dark matter. And he said actually it’s an interesting question because in parameter space there is a place for them to form, you know not all of it, but how do we know? You know we’re just, you set up a, you invent a new kind of technology that can detect the universe in a whole new way. You discover a class of sources. Maybe there are many classes of sources and they don’t all have to be like that but that’s just the beauty of what we’re involved in right now is that we’re always discovering something new that’s going to shed some light and who knows if those will be connected to the dark matter question. They’re certainly connected to the evolution of the universe in some way.

I want to say something very quickly about that since you since there was something at the very end of your question to Matias. You said shouldn’t we just might, might we’d better not be served by thinking about something new rather than doing more and more complicated experiments. I know I know you’re being provocative but but I think the answer is theorists are theorists and do what theorists do, we’re cheap. But we should do every conceivable experiment we can that we can do using the technology, the greatest technology we have at any time to learn more about the universe experimentally because surprises happen every single time we do.

That’s right

Nima, I want to ask you in this particular concept that I’m going to bring up now actually raises the blood pressure of many of my friends, but I know it doesn’t raise your blood pressure and neither of Matias. I want to bring about the multiverse. So basically in recent years, I think it’s fair to say that it’s fairly recent. I mean it’s been around for a while but not that long. Theorists have come up with the idea, and there are all kinds of reasons for this, that maybe our entire universe is not all there is but rather this is one member of a huge ensemble which could be ten to the five hundred one followed by five hundred zeros or it could be perhaps infinite.

Call it infinite.

Yeah…of universes. And the reason our universe has the properties it has is because those have to be consistent more or less with the fact that we are here. Namely the values of the constants of nature and the laws of physics are such that they have allowed our being. But there are many other universes, which in which the laws may be different, the constants of nature may be different and so on. There are colleagues, I’m sure you are very aware of, that regard this concept as the end of physics. I want to because, why do they say the end of physics because they say Oh well since these other universes are not observable then this becomes more like metaphysics and not like physics because you cannot test it and so on.

Now, I happen to know that you believe that this is not the case and I want you now to explain this.

So yeah the discussion of multiverse used to, I mean they don’t raise my blood pressure because they think there’s something intellectually wrong with talking about it although, my blood pressure does increase because an enormous amount of nonsense is said about it in both directions, both in the in pro and con directions.   So let me just say one thing just before, just to set the context for the discussion. Even the theory the idea that there is a multiverse is not a theory yet. It’s not even a theory at the level that we’re used to in a theoretical physics. There’s all kinds of things that we talk about that we have not yet verified are there in nature.  For example, things like supersymmetry, ideas like that. These deserve to be called theories because we understand the theoretical structure well enough to know what we’re talking about. To make, to say if this and this and that is true then we can make a lot of other predictions that may or may not be realized in nature but it still deserves to be called a theory. We understand the ingredients. The multiverse is not like that. The ideas and concepts involved with the multiverse or at the very edge of the things that we even conceptually know how to talk about. I think of it as a caricature of something that might be true. It doesn’t even rise to the level of a theory yet.

How do of how to verify?

Oh good, let me say…I’ll just take them, take some of the problems one step at a time. One of the ingredients that you need for something, for a picture like the multiverse to be right is many, many approximate vacuums that one underlying set of laws could have.

Just I’ll stop you for one second. I mean. A vacuum is this thing, which we would call a universe at some level or a pocket universe. So you can have many vaccua.

It’s a lower sort of the lowest possible lowest energy state that we can have where you just empty everything out right and helpfully in an expanding universe that’s what happens.   As the universe expands everything gets more and more dilute and you approach more and more the lowest energy configuration. Now it’s not a crazy possibility, in fact and happens all the time in our simplest theories of a particle physics that you could have theories where you could have a lowest possible energy state and another energy state that could have a different energy and you could get stuck in the sort of a local places where locally you have the smallest amount of energy. But in order to go to find the place where you have the lowest possible energy you’ve got to go far away and somewhere else.

The second that becomes possible we can entertain the idea that these different possible, approximately stable places could exist. That piece of the multiverse could in principle be verified by experiments in our universe. That could be in principle verified by doing,

So give an example

I’ll give you an example. Now, we don’t, since we don’t know the we don’t know the energies involved but for the barriers between one local minimum here another local minimum there, those energies could be gargantuan they could be much much higher than energy, than any energy that we could make ..

Think of a landscape which has valleys but within them there can be huge mountains.

Exactly. There are just enormous mountains but if you have enough energy to climb over the enormous mountains then you can make little bubbles of the other regions, if you can really make them. You could make them in a laboratory. You could send little elementary particles in and say oh, gosh I’m the Higgs particle. Out here I have this mass. In there I have a different mass Wow! And it comes back out and you can actually see all of that. You could in principle see that there are a ten to the five hundred different possibilities. All of that you could actually in principle definitely not in practice as far as we can tell. But in principle it’s not a question of philosophy it is a question of physics.

What we don’t know how to do, and this is the deepest conceptual problem associated with the multiverse, and if someone were to make a theoretical breakthrough on this question it could settle, certainly in my thinking as I’m sure in Matias’ and almost all of our thinking, whether this idea is a deep one or a crap one. We still don’t know. The deepest conceptual problem is how are these different regions realized out there in the universe and what you alluded to, the fact that in our accelerating universe we only have access to what we see now is what we’re ever going to see. So if there are these other regions out there light from then will never make it to us even in principle. That’s a good reason to be suspicious about whether it actually makes sense to invoke them and talk about them. We don’t know if it makes sense to invoke them or talking about them. Invoking and talking about them is a little bit like invoking and talking about what’s on the other side of a black hole of the horizon of a black hole.

And you know in the last twenty years we’ve understood that there is some subtle way that the quantum mechanics lets you see into the inside of a black hole and get the information about what’s in there, out. So it’s possible that similarly there are some very subtle way in which quantum effects now apply to the entire universe will help us make sense of what what’s going on behind the horizon out there in the multiverse. But this is a part which is totally speculation at this point. But imagine we did the experiments that show the these ten to the five hundred different environments existed. It would be very hard not to believe that there should be that they play some role in, in controlling the properties of our world and that part is not philosophy. That part is actually physics.

Right. I just want to add two things one very simple and one a little bit more subtle. One is that you see at that time there was this great astronomer Johannes Kepler and he was really very smart. And he was the great astronomer and he thought that he can explain why in the solar system there are exactly six planets, six were known at this time, because he thought that that is a fundamental property of the universe which can be explained from first principles. Today we know that’s an accident really. In this way it could in principle be that some properties of our universe, which we now regard as fundamental, are in fact accidental. And you know they get different values in these different members of the ensemble.

Matias, I’ll ask you one more question and then I want to leave enough time for the audience to ask questions as well. You alluded to this but I just want to sharpen it a little bit. The very early universe is relatively simple. You know only fundamental things happened there. But it is also far away and not that easy to observe. The nearby universe we can observe more easily but it’s a mess because it involves all kinds of processes, star formation and planet formation and galaxy formation and whatnot and so on. Is there a sweet spot somewhere where you know you get the benefit of both worlds?

I think the clear sweet spot is the cosmic microwave background, the history of the universe is more or less much more in between. And that, you perhaps they’re calling it the early universe, but it is four hundred thousand years after the Big Bang. So that’s pretty late for a lot of it depends. But so that’s, then it’s true. It gets more and more complicated. And that’s, for example, when we were talking about the discussions about dark matter, when we start using our theories for dark matter try to understand galaxies or small enough things, but let’s take galaxies for example, there we are getting into trouble. Sometimes they don’t seem to work. And that is the reason why people also are trying to find other. But it’s also the case that all of the complications you alluded to star formation, the black holes in the centers of each galaxy, they play a big role. We see it with our own eyes meaning the telescope. So the more complicated things get sometimes it’s difficult to disentangle. And that’s why in cosmology we try to get out you know just use galaxies for example as points not too much try to understand how they’re made of but, just tracing where the matter is distributed.

We never know where we will be able to make some breakthrough and there there are certain things that you just have to leave to the side even if they’re a very interesting problem. But you have to leave it to the side because it doesn’t look like there’s any, no progress is being made. It’s kind of the business of the game is like this you speculate you try to look for things and you go on from there, right.

I would like to open this for questions from the audience

Ok, it’s a wonderful panel thank you. Obviously mankind when we discovered electrons it changed our world. We now have electricity and all these wonderful things. Now all these new particles that you’re discovering fermions and quarks and all these items. Are we on the verge of taking these particles and revolutionizing our existence as humans?

We don’t know what the, what fundamental breakthroughs in science will eventually give. Michael Faraday famously when he was doing his experiments on magnetism in the basement of the Royal Institute in London some British MP visited him and said what is this good for? He said I don’t know sir but one day you will tax it. And that happened 50 years later. Can I think at the moment of any practical technological application of the discovery of the Higgs particle no. However when you get large groups of people to do very very hard things they, they inevitably have to come up with innovations that have lots of other impacts. A classic example from the field that you were talking about is the invention of the World Wide Web which was invented at CERN to help experimentalists share this enormous amount of data with each other. So even though I had nothing to do with the particles that they actually discovered when you have people pursuing pure ends, very difficult problems that are right on the frontier of what we know how to do good things always come out of it, or have historically.

But you also never do it for, we don’t do it for these reasons.

That’s right.

We do it because we’re curious.

Let me say it another way. If you want to think about what’s going to be exciting in technology ten years from now talk to people in Silicon Valley. If you want to wonder what might be exciting fifty or one hundred years from now it’s gonna come out of fundamental science.

Yes please.

So if new technology reveals new theories can we ever reach a final theory? And since all our theories currently evolve from the Big Bang does anyone question the Big Bang now?

I’m happy to take one shot at this and say that what I think we’re all, we’ve all been doing is pushing the frontier farther and backwards in time all the time. And when people talk about you know a final theory I mean Lord Kelvin famously thought at the end of the 19th century that all problems in physics were solved except for two small problems. And those two small problems actually turned out to lead to general relativity and quantum mechanics. So two big revolutions in physics. So I think that we have now discovered that the more we push, we find new questions. So it’s not I don’t see any danger that we will run out of questions to answer at any point in time. So you know in those terms no theory is truly final. Theories in physics are really, you know, only theories that are good for the data available at a given time. But as new data become available I mean you sometimes have to modify a theory. Sometimes it becomes incorporated in a bigger framework like in Newton’s theory being incorporated in general relativity let’s say. Sometimes it has to be rejected altogether and so on And this process I believe will continue forever so that’s the process we’re going through.

The Big Bang itself is not a theory, right? It’s an observation. It’s a collection of pictures from the past right and pictures and things that we’ve got from the past. So that will never go away. It’s not, it’s just.

Well so I think that maybe I’ll echo the same thing and make a slightly more general point as well. Something I think many people don’t appreciate is that there are various really grand questions about the universe that all of us get excited about. Some fraction of us decide this is what we want to do with our lives and we and we attack them. But there’s something really fascinating as your as your understanding of the world becomes precise enough so that things really work the character of the questions change, changes. The language with which you describe the questions changes and very often the actual questions evolve. We don’t even know what the right questions are until we happen to be in the vicinity of the correct answers. And as we happened to learn more. So you should not have this idea that we have this sort of fixed set of questions we’ve been working on for two thousand years and we’re getting closer and closer and we might hit the end. Something much more interesting is happening that we’re learning more and deeper and more profound things about the world that’s allowing us to ask entirely new kinds of questions things that we weren’t even questions before have become questions and so on.

Going back to what Matias said, we have as much evidence as we’ll, and will only get more, than that that the earth, that the universe is expanding as you run the picture backward in time it got denser and hotter and that hot dense early period of the universe is what we colloquially call the Big Bang. What you were referring to as the Big Bang and many people refer to the Big Bang as a sort of mathematical singularity of a point where everything starts back in time. And that’s a thing, which we don’t understand. We don’t understand and what’s very likely going on is that the whole notion of time is breaking down there. It’s not a question of figuring out what came before. It’s the whole notion of time was probably born there or is certainly breaking down there. And if that sounds like a very tall order to figure out what it means it is a tall order to figure out what it means. OK and that’s what people are trying their best to you know take little bits and pieces off that problem and make some progress on it.

This gentlemen over here.

Is it felt that the universe is infinite? And if it is felt to be infinite in what sense is it infinite?   And how did something that was finite become infinite in finite time?

The easy answer would be to say that we only see some region of the universe and what’s outside I have no idea. That would be the standard answer that I would give. I don’t know if it’s infinite or it’s curved and it comes back and is like the surface of a ball and it’s really infinite. We don’t know. For all we see we don’t see any curvature of this ball. We, when we see further away things look more or less the same, so it looks pretty much homogeneous. We clearly see that if it you know finishes or it has a curvature it’s much bigger. The region that looks like the part of the universe that we can see is probably goes on for quite for a while. Other than that we don’t really know. And also how this started started is also some of the you know these are questions whose answer we don’t know and whether or not our universe is connected to places which are completely different and is so much bigger than what we see and the laws of physics are completely different.

But just say one thing about the question of a finiteness, something very important happened in the late 1990s when we discovered the universe was accelerating which is what we see in the universe. Whatever you might imagine in your mind’s eye the universe going on and on..that’s what we’ve been referring to. Because the universe is accelerating what we see now in the universe is what we’re ever, ever going to see. And that’s kind of an amazing thing. If the universe was not accelerating, if there was a picture from what people talked about in the books in the 1980s and the universe just kept expanding forever then it would be an experimental question if it was infinite or not. If you waited long enough your great great great great great great great grandchildren would see more and more and more and more of the universe. We can’t now. I mean what we see now is what we’re ever going going to see.


Of course. Believing that the acceleration of the universe.

Exactly.True, true.

I’ll make this even sharper. If the universe continues to accelerate as it does then maybe a trillion years from now, actually if astronomers still live here they will not be able to see anything, right?

One galaxy.

Just our galaxy, that will be it. And then all these stories about the universe you know these would be like mythology.

  1. Can we have this young woman here?

If there is a multiverse how would our universe interact with the other universes? How would that work? Do we know?

Do you want to say something?

Yeah, well, most obviously it wouldn’t. That’s one of the problems. That’s one of the conceptual problems is that all of this stuff is out there and it’s beyond our cosmological horizon which because we’re accelerating we won’t see we won’t see any of that stuff. Now that might make you think, that and we’ve gotten very wary in physics for many good reasons, we’ve gotten very wary of concepts and ideas that we can’t even in principle see. And what I’m just talking about is not a question of practicality right. Our acceleration makes it impossible for light from those regions to years to reach us. Now it’s conceivable sometimes people talk about if we came from some parent, underlying vacuum that that gave rise to us that there could be other little bubbles and those bubbles could collide with each other. This is something that people talk about. It’s an ‘in principle’ possibility. I have to tell you it’s so vanishingly unlikely to happen that if you hear about it in the press you should be very skeptical it’s not, I mean even theoretically it’s incredibly unlikely for it for it to happen but it is in principle possible.

Now, but having said all of this, and this is part of the reason it’s theoretically difficult, if it was clear that nothing about these other vaccua, these other regions could have any effect on us at all then we would be almost certain that it’s garbage and we shouldn’t think about it. But it’s not a one hundred percent obvious. And the reason is that you can imagine other parts, you can imagine futures that we could have where if we exited our vacuum but we went into another kind of vacuum in the future those observers could look back and look at the night sky and see eventually if they waited long enough all these collisions happening with all of these other bubbles and they could see if you waited long enough there was somebody that could see the entire multiverse. It’s not us but in principle there are some people if you waited long enough you could. But anyway. But the short answer to your question is no conceivable way we can imagine now other than these vanishingly unlikely things involving collisions of bubbles.

Thank you very much Nima. Thank the panel and thank all of you for attending.

Big Ideas
Testing the Limits of Cosmology

As physicists attempt to answer some of science’s biggest questions about the universe, they are testing the limits of experimental and observational science itself. In this program leading physicists, astronomers, and astrophysicists discuss how to push the boundaries of scientific imagination to develop experiments that test the seemingly untestable theories of multiverses, eternal inflation, and exotic particles. Join the conversation about their plans to recreate the Big Bang in particle accelerators here on Earth, as well as their quest to sift through signals from the farthest edges of space for the existence of a multiverse.   This program is part of the Big Ideas Series, made possible with support from the John Templeton Foundation.Learn More

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