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. Wow 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. Nima? 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. Right. 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. Right. 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. OK. 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.
