HENRY TYE: Before I start introducing a speaker, I was told to make a few remarks first. First-- anyway, first, I'm Henry Tye. I'm in the physics department.
The first point I have to make is that, if somebody lost an iPod in this room yesterday, please go to physics office to pick it up, OK? The second remark I want to make is that a few of you wonder if these lectures are taped. And the answer is yes. And they should be available next week. If you go to Cornell-- CornellCast. You can either Google CornellCast or go to cornell.edu/video. These five lectures, public lectures, should be available. OK.
This is the fifth lecture, public lecture, by Nima. And for those of you who may not know, most of you don't know, that every morning or before the public lecture, he gave a roughly two-hour-long lecture. So I do a little calculation and I think he has given 15-and-a-1/2-hour lecture this week. Up to now. OK? It's pretty exhausting for us.
The last lecture I think-- I'm going to do two things. First is that I'm not going to put a cutoff since this is fall break started already, you know.
I'll just let him go, OK? I'm sure this record will not be broken by any future messenger lecturers, for sure. That's true.
Another thing is that Nima is going to talk about data, OK? Information from experiments. And as one piece of data which need not have explanation-- not all data can be explained, either by physicists or by anybody. And so he provided me one piece of data, which I want to show you, OK? We're going to have some fun.
So that's where Nima was.
He was in Toronto up to '93. And I only look up baseball and football because otherwise take too much time so--
SPEAKER: Celtics, sir.
HENRY TYE: Oh. Well, yeah. I didn't get to basketball or hockey. So Blue Jays won twice, and then he left town, and Blue Jays never won again.
While he was in-- while he was in Bay Area, 49ers won and San Francisco Giants won. And then he moved to Boston and Red Sox won, OK? That's a big deal if you know what happened to Red Sox before that so-- and of course Patriots won three times.
And then he moved to New Jersey. I allow area of 50 miles area to where he live, OK? Radius. And so I don't know what it means that he spent a week here, so anyway. OK, let's welcome Nima.
NIMA ARKANI-HAMED: Thanks a lot, Henry.
--this one wherever I am, and I'm cursed when I leave. When I arrived at the institute, I told my new assistant, who is a lifelong Phillies fan, that the Phillies were gonna win because I was there. And she thought I was nuts until they did.
OK. So we've had-- we've had a lot of discussion about the theoretical part of fundamental physics. And today is almost completely going to be devoted to the experimental part. And something that you should know is that this decade is actually really an interesting one.
There was a previous golden era of experimental results in fundamental physics that started in the late 1950s, early 1960s. Stretched out about a decade or so and played a very important role in the construction of that standard theory of particle physics that I told you about. And a golden era of cosmology that-- that came after that starting in the very late 1980s, early 1990s that played a very crucial role in the development of that standard cosmological picture.
But for roughly a generation going back, certainly going back to the particle physics side, we have not been in a situation where we've been probing exactly the length scales where we expected something new to happen. Now we've had very high energy accelerators in the interim. Between the late 1970s and now, they've done absolutely spectacular physics. They've taught us a lot about how well our theory is working. But they haven't given us a clue for how things are going to break.
And really, truth be told, there was, of course, always some chance that something would show up. People went to explore shorter distances in the past just for the heck of it, and they found lots of things.
But there wasn't-- there wasn't a completely rock-solid theoretical argument that something should be seen at energies roughly comparable to the energies that are being probed by accelerators now. But there has always been, as I've emphasized a number of times, some rock-solid arguments for something to show up by the energy scales we're about to probe.
So that's why this is a particularly interesting decade from the point of view of the short distance part of our subject. It's really for the first time in 30 years we're going to be getting a window, a peek, into physics at a length scale where we really expect something new and important to happen. And together with that, there's a whole slew of other experiments in astrophysics and cosmology that are all coming together at the same time.
So we really haven't seen a decade with so much promise. We'll see how it turns out. But we haven't seen a decade with so much promise in quite some time. And so I want to tell you about it.
So I'll tell you about three things, three kinds of experiments. The Large Hadron Collider, which is nearest and dearest to my heart. Experiments that are going to look for the dark matter of the universe. This is actually a very exciting subject even right now. Many of us are waiting with bated breath for results from experiments that are looking for dark matter right now and might be releasing their data at the end of October or the end of November. So it's like happening now.
The Large Hadron Collider is also now turned on and is running, but it's going to take probably a little while longer for us to get the interesting data out of it. I'll briefly talk about some of the experiments probing cosmology. And very unfortunately, I won't have time to talk about some of my favorite set of experiments, which are much smaller scale experiments, sometimes tabletop, sometimes much lower energy accelerators, that do what I call-- that go out on what I call fishing expeditions looking for interesting, weird things that might be showing up that could be telling us also something important about fundamental physics.
It's very sad that I won't have time to tell you about that. But some of these things might actually end up turning up extremely exciting evidence for new physics that we'll have to follow up on as well.
All right. So let me start by telling you about the LHC. So that's an aerial view of the region right outside Geneva. There's Lake Geneva. There's the Swiss-French Alps in the background near the border of Switzerland and France.
And, of course, you don't see this red ring on the surface of the Earth. That red ring signifies the place where, if you go deep underground beneath where the red ring is, there is a tunnel, and in that tunnel there is-- there's a beam. And the excitement happens in the beam.
This ring is around 27 kilometers in circumference. That's Mont Blanc. That's what it looks like in the tunnel. So that's deep underground where that red, red ring was. And you see 27 kilometers is a long way, so you can barely see the curvature, OK?
And now inside this-- inside this tube we-- and in this talk, since I'm a theorist, "we" refers to the royal we, so it's the experimentalists. But I'll say "we" over and over again, even though I've never-- if they'd let me near it, I would probably destroy the whole thing, OK?
But we accelerate up to extremely high energies, protons. One set of protons are sent whizzing around the ring in one direction. One set of protons are sent whizzing around the ring in the other direction. They're both inside that beam. They're made to accelerate faster and faster and faster and faster.
Every time they go around, they're given a kick, a bigger kick, a bigger kick, a bigger kick till they're going at roughly 0.9999999 times the speed of light. That's seven nines. That corresponds to the protons having an energy that's roughly 7,000 times their mc squared energy.
And then, at a couple of places around the ring, at a few places around the ring, these protons are made to collide with each other head on. And the debris of those collisions cause us to probe the structure of physics near 10 to the minus 17 centimeters. And we study what comes out.
So around these collision points, we have gigantic detectors that detect the debris of what comes out. Oh, before I tell you that, let me tell you some more about the experiment. Sorry.
So this experiment has a superlative associated in every possible adjective you might use to describe it. It's the largest experiment in human history. There is roughly 9,000 people who work on it from all around the world. America has committed roughly 1,300 people to this effort.
The particular subexperiments that I'm telling you about have 3,000 people working on them each. And it's taken roughly 20, 25 years from conception to the design and actually running of the accelerator. These accelerators are often called the modern-day version of cathedrals, which take a very, very long time to build, often longer than the lifespan of people for cathedrals and the lifespan of careers for people now. So it's a very big-scale, large-scale effort which we undertake because we think it's-- actually, it's really important.
So this is a slightly more schematic version of what happens when we collide particles at the LHC. So I told you we accelerate the protons. Here's the other protons. They're going at 0.9999999 times the speed of light. We make that proton smash into that proton, and something happens.
Now mostly what happens is the following. The proton, if I take my famous magnifying glass and look at it, I see that it's, as I've described before, it's a bag that's made out of quarks and gluons. So inside the proton, we have a bunch of quarks and gluons. These quarks and gluons are point-like, as far as we can tell. The proton is big. The proton is this big, messy bag of quarks and gluons. OK.
So we take one big, messy bag of quarks and gluons, another big, messy bag of quarks and gluons, and you smash them into each other at enormous, enormous velocities. So what do you think happens most of the time?
Most of the time they just go splat, OK? And they just break up. And you get a whole stream and debris of particles going in this direction and a whole stream and debris of particles going in the opposite direction. They're going so fast that the stream and debris more or less goes in the direction that they were coming in. OK?
So they're going really fast this way. They go splat. And they just go splat like that. OK?
That's not what we're interested in. So what happens the vast majority of the time is not what we're interested in, at least for this talk. Some people are interested in that, and there are some interesting physics associated with it. But it's not what we're interested in.
We're interested in doing these things at very high energies because we want to probe very short distances. Again, it's the uncertainty principle over and over again. We've said it a number of times, but if the world was classical, we wouldn't need high energies to probe short distances. We'd build a little Fantastic Voyage spaceship, and we'd make it smaller and smaller and smaller. And we'd zip around slowly, seeing what's going on, taking in the sights, OK?
The problem is that, as we make the spaceship smaller and smaller, the uncertainty principle starts breaking it apart. We have to put more and more energies to probe smaller and smaller distances. So that's just the uncertainty principle.
So what we're actually interested in is the head-on collision of these point-like constituents with each other. That happens much more rarely. That happens much, much more rarely, but it happens sometimes. And we know that it's a head-on collision because the debris of what comes out doesn't just go back to back from the direction of the beam.
It's like two billiard balls hitting each other head on or just a little bit off but really fast. They can come off at big angles relative to the beam. So that's what you look for. You look for stuff coming out at fairly large angles relative to the direction they were coming in at.
Now something new happens in here, OK? But the something new happens-- and the something new happens is typically the formation of new particles. But unfortunately, the new particles don't typically come out with a name tag saying, I'm a new particle. They very rapidly decay and disintegrate in a time scale of order 10 to the minus 27 seconds. That's how long it takes light to travel the distance 10 to the minus 17 centimeters, OK?
So on a time scale of order 10 to the minus 27 seconds, they disintegrate eventually into the particles we know and love that are pretty stable. So, for example, that includes electrons and positrons, which are exactly stable. It also includes muons. The muons are not exactly stable, but they go out so fast that the time dilation effect that we've talked about many times easily allows them to survive a long time. And they're essentially stable as far as any observation around the LHC is concerned.
We can make quarks. And the quarks will come out. We can't see them individually, but they get surrounded with another bag of other quarks and gluons and get spat out of something we call jets. But all of this stuff we can see. And we surround this collision region with gigantic detectors so that we can study everything that comes out.
So we study the visible things that come out in order to get-- and try to use it to back reconstruct what actually happened. And then use that to tell us something about whatever the new thing was that was going on.
So here's a schematic picture of one of the detectors that surrounds one of these interaction points. So the beam comes in this way in one direction, that way in another direction. The collision takes place in there. And surrounding it is this big, huge, onion-like structure of detectors upon detectors upon detectors.
These things are gigantic. This ATLAS detector is 24 meters tall, 45 meters long. And you can ask, why is it so big? There's a number of reasons for why it's so big, but there's one very basic one that's really cool.
It's a little ironic that this machine that's letting us study the shortest distances we've ever studied is the biggest thing that we've ever made. 27 kilometers around, these enormous detectors. It's the uncertainty principle again and again. We had to make these things so big so we can accelerate them so fast so we can smash them into each other at such high energy they come screaming out at high energies and, minimally, you should stop the particles if you want to study what they're doing.
The particles coming out, you just have to stop them. But they're coming out so incredibly fast that you just have to put big blocks of material to stop them. And that has to be this big in order to stop them. So even the gigantic size of the detectors is a direct consequence of the very high energies because we're trying to study short distances.
And this is a picture, again, of what it looks like. So a typical collision would have a particle coming in here, the proton coming in there, something happens, and this spray of particles goes out and is detected all over the place in this onion-like structure.
This is a picture of the actual ATLAS detector. That's somebody, OK? Don't know who. Right? But you see it's no joke. It's really big.
A cool thing about this detector is it's really big. It's got all this steel, et cetera, et cetera. If you stuck it in the middle of the Atlantic Ocean, it would float. It's got a lot of air too.
This is the other detector that we-- this is-- there's two detectors that are studying the sorts of physics that we care about. The second detector is called the CMS detector. CMS stands for Compact Muon Solenoid, the "compact" referring to the fact that it's kind of small compared to the other one.
Now I've emphasized the very, very big feature of these detectors. But really, I mean, I could spend an entire series of lectures telling you about how these detectors work, OK? But it-- but there's something going on in every one of these layers. Not one bit of space is wasted. Everything has a purpose and is designed to measure some feature of the particles coming out.
Is it an electron? Was it a jet? Is it a photon? Is it a proton? What the heck is it that's coming out? And all the way down to really, really tiny distances around the interaction region.
In the really tiny distances around the interaction region, there's lots and lots of strips of silicon. It's exactly like the inside of one of your digital cameras, except a very fancy version of it, OK? So this is what it looks like very, very close in to the beam is the interaction region is covered with all these beautiful layers of silicon.
Just to give you an idea of how far underground it is, this is the cavern in which the CMS detector sits before it was in there. So again, those are people. And this detector was lowered through that hole very, very slowly.
Now amongst other things, again, I should resist the temptation to just tell you trivia. But it's just so, so, so cool. When these detectors are so heavy, when they sit, they would actually sag under their own weight, OK? So when they're engineered, they're engineered to be a little bit oblate. So when they sit, they sag and they become perfectly symmetrical. And that's because you have to arrange it so that you know the position of the beam to millimeter accuracy in order to be able to do the physics that we're interested in.
All right. Back to my pictures. So I want to tell you a little bit about how we will look for some of the physics that I told you about theoretically at the LHC. So remember, some of the puzzles that we mentioned yesterday such as this question of why gravity is so weak compared to all the other forces, these fine-tuning problems that I mentioned really demand that the new physics show up right at the distance that we're about to probe.
I want to stress that this is not because we want to invent something to make our lives exciting, OK? The LHC is not simply exciting because it happens to be the frontier of where we are today. It is the frontier of where we are today. It is the shortest distances that we've seen so far. But that's not the only reason why it's interesting.
It's interesting because it's going to a length scale where we've known for 70 years something is going on. As I've said a number of times, something is happening at 10 to the minus 17 centimeters. Something was happening there 2 billion years ago. Something will be happening there 300 million years from now. What's interesting is that we happen to be living in the epoch where we're about to explore that scale experimentally.
And if there is a resolution along the lines I mentioned yesterday for these puzzles for why gravity is weak, and, as I mentioned, there are a number of proposals. Some have involved the ideas of extra dimensions. They're interesting. They have very specific signatures at the LHC. Those signatures might involve the production of black holes that will most certainly not in any conceivable universe eat up the Earth, OK? So please don't worry about that.
When the-- when I was talking to a New York Times reporter about this, I had an hour-long conversation with him where I told him all the reasons why there was no problem. And at the very, very end he said, yes, but are you really, really sure? And I got frustrated. And I said, look, because of the laws of quantum mechanics, anything can happen. So there is some chance--
--there is some chance--
--there is some chance that the LHC might make dragons that might eat us up. But I lose about as much sleep over black holes as I do about dragons.
One of my smart aleck graduate students sent me an email after seeing this quote in the Times saying, but don't dragons violate energy conservation?
I pointed out that there's plenty of mass in the detectors to make a dragon and to stop being such a smart aleck.
Anyway, so I'm not going to talk about extra dimensions. There's a whole-- I mean, it's an interesting possibility it might happen. But I'm going to focus on supersymmetry because we have both more circumstantial hints that it's correct, and I think it would be a deeper-- it would be a deeper and more exciting discovery if it was there.
Anyway, so how would we do it? We'd collide these protons together. Sometimes there is enough energy-- well, and hopefully these guys have got to be-- these quantum dimensions have to show up around 10 to the minus 17 centimeters. These very high energies at the LHC are designed to probe 10 to the minus 17 centimeters.
So sometimes the two quarks inside the proton will smash into each other and pop off the quarks into the quantum dimensions, making the squarks. We drew this picture yesterday. The squarks then come out and typically decay very, very rapidly into ordinary particles, quarks, other quarks, electrons, positrons, as well as things that escape the detector.
These were the photinos that we talked about, the superpartners of the photon, which are neutral, very weakly interacting, and really just escaped the detector without hardly leaving a trace. These were the things that could well constitute the dark matter of the universe, as we mentioned yesterday.
So-- so you can look for this. You can look to see if you've produced new particles and if there is missing energy. The way you look for missing energy is it looks like there is some missing energy in your event. It can look something like that.
This is actually a cleaned-up picture. In a real picture, there would be a lot more junk going on here corresponding to other normal processes, not just the-- not just this spectacular process. It's slightly cleaned up.
But anyway, it's a picture. The protons came in. You produced a whole bunch of particles that go out, which we see. You identify them in different ways, the things that went out that way, things that make it really, really far out. Could be muons. There's other things that you could see. So there's some way of identifying all these guys.
But you can sort of see by eye here that there's something missing. Like, there's a lot of stuff going out in this direction. There's something seems to not be conserving momentum, right? There's some overall momentum flow in that direction. And that stands to reason that there is something canceling that, moving off in the other direction. So that's how you would see that there is missing energy and have some evidence that the stuff was there and is being missed.
So that's how supersymmetry could look at the LHC. OK? And there is an entire experimental program that's devoted to figuring out exactly how we'll measure all the properties of the superpartners from very, very detailed, more and less detailed measurements, of exactly what comes out, exactly the pattern in which the particles come out. You can try to go back and reconstruct quite precisely what it was that you made, how it decayed, what sequence of decays it underwent, and try to get a more detailed handle on the actual-- the masses of all the particles, how they all talk to each other, and so on.
Something that I said was virtually guaranteed to be seen at the LHC was the Higgs. Remember, the Higgs is this condensate that's supposed to fill the universe. And typical particles move along hitting this condensate every 10 to the minus 17 centimeters or so. And if we're probing physics at 10 to the minus 17 centimeters, we'll put ripples in this condensate. And the ripples in that condensate should look like new particles.
Well, one of the dominant ways that this would happen in the LHC is if a couple of the gluons inside the protons, that they're smashing into each other, produce out of the vacuum or take advantage of the fluctuation of a top quark in the vacuum. And the top quark was one of the particles that has a very big coupling to the Higgs. So that gives a particularly efficient way of making the Higgs also out of the vacuum. So we can have gluons and gluons fusing and producing a Higgs particle.
Now what the Higgs particle-- exactly what it wants to do, exactly what happens to it after it's produced, exactly how it decays depends on its mass in some detail. I'm giving you an example of what it would do in a particularly nice range of its masses from a theoretical point of view. But it also illustrates one of the challenges of doing physics at the LHC.
The Higgs, at least in the range of masses that I'm considering, what it really wants to do, if you make it, what it really wants to do is decay almost all the time to another kind of quark. It's kind of a partner of the top quark called, imaginatively, the bottom quark, OK? So the Higgs really wants to decay to a pair of bottom quarks all the time, almost all the time.
So you go through all of this bother, you make the Higgs, you produce the Higgs, it's sitting there, and it just decays the bottom quarks almost all the time. Unfortunately, completely standard other processes, just the collision of a couple of these gluons together, all by itself also makes bottom quarks all the time. And the rate for doing that is vastly, vastly bigger than the rate for producing the Higgs.
So you've worked really hard. You've produced the Higgs. It's decaying. Those bottom quarks are telling you everything you need to know about the Higgs being there, but they're buried on top of a gigantic extra amount of bottom quarks coming from other processes that are just hiding it. So at this point you don't just give up. You notice and you realize that very rarely, like 1 in 1,000 times, roughly 1 in 1,000 times, the Higgs decays in other ways which are more visible.
For example, around 1 in 1,000 times, the Higgs decays to a pair of photons. That is much harder to fake with standard physics. And so you can look for that.
So that's an example of what such a Higgs event would look like at the LHC. So the Higgs is produced somewhere in there. And there's a whole bunch of other particles coming out. But here are the two photons. Here are the two photons that are detected somewhere in this onion.
Now I told you that-- I told you that standard physics has a harder time making photons. It still makes plenty of photons. So here is just a plot of number of events. I'll tell you what that means in a second. But this is what's going to end up coming from the Higgs. And you see, standard physics is giving you plenty-- lots and lots and lots of photons, even more than you get from the Higgs.
So you have to take advantage of yet another fact, which is that, when you have this photon and it decays-- when you have this Higgs and it decays to two photons, each one of the photons is exactly 1/2 the energy of the Higgs, right? Now of course, the Higgs could be coming out really fast, but we can still figure out what the energies of those photons were.
And we can figure out if it was coming from the Higgs or not. If it was coming from the Higgs, if I take the two photons and I figure out the frame in which they would be coming from where they would go back to back carrying equal momentum, then if I add their energies in that frame, it should always be the same value, namely, the mass of the Higgs.
Whereas from standard processes, it would come out all over the place. So the event you get from the Higgs would be all piled up in one spot and in the neighborhood of what the mass of the Higgs actually is. So you make a plot like that. And you find that there is a bump sticking out there.
So standard physics is just sort of randomly giving you photons and fewer and fewer of them as you're going to higher and higher energies. But the Higgs has a particular mass, and it would show up as a feature or a bump in a plot like this. So that's how you begin to infer things about the fact that it's there and some of its properties.
So this is actually a completely ubiquitous way of looking for this physics. We're always interested in looking at-- I mean, this is extremely schematic. Any of my experimental colleagues in the audience will just kill me for making their life look so, so simple, OK? But this is roughly what you do.
You look for some kind of configuration of the particles coming out. So we typically call that the final state. You look at the final-- you look at a final state and you figure out how many of those things you would have gotten if all you had was completely known, standard physics. That's something that we can, in principle, calculate because we understand the theory that we have very well.
So we take the theory we have, the standard model. We understand it very well. We turn the crank. And we figure out how much of those processes we would expect to see.
But just like in the example we saw, if there is some new physics beyond what we've seen, then it would be coming in at some characteristic energy, the characteristic energy that we haven't been to yet that we're about to explore but at some characteristic higher energy scale. And you would expect to see it as a bump on some plot like this, OK? And one of the things that helps to distinguish it from just standard processes is the fact that it does have a characteristic energy scale, whereas the other stuff doesn't.
Again, this is a cartoon. And in actual-- in real life it's harder than this. But it's an OK cartoon.
Now let me give you an idea of-- so it's obviously a needle in a haystack problem. Let me give you an idea of the size of the haystack and the needle, OK?
So when we collide the protons at the LHC, the rate at which we have events taking place is about-- there's about a billion collisions per second, 1 billion collisions per second. The rate is actually so high that you can't record, you can't even record the data that would come out from every one of the collisions. That would be vastly more than we could possibly store.
So you already make a decision at the level of every event that took place whether you're going to keep it or not. The decision is made roughly whether enough things are coming out at high enough energies, at large enough angles relative to the beam that we deem that they have a chance to be interesting enough. So you cut out roughly 1/10 of everything before you even get going. And you store all of that data.
The amount of data is around 15 petabytes a year. If you were to store it on CDs, it would be a stack of CDs 30 miles high. And it's farmed out to computer farms all around the world. And that's only 1/10 of everything that's actually going on, OK? OK.
So that's the kind of size of data sets we're talking about. So the rates are about a billion collisions a second. Now let's talk about some standard physics, like the production of top quarks.
Top quarks were what the previous experiment at the previous highest energy accelerator in the world, which is right outside Chicago, it's called Fermilab. It was colliding protons where each proton had an energy roughly equal to its mc squared energy. So the energy at Fermilab is seven times smaller than the energy at the LHC. The LHC is about a factor of 7 leap relative to Fermilab.
And Fermilab was essentially built and conceived-- it had one main purpose, which was to produce this top quark. It was one of the particles that was predicted in the standard theory that people hadn't seen. So after seven years, eight years of work, they managed to produce some few numbers of top quarks, and it was very exciting. It was a discovery of the top quark. Great.
This is in 1995. People were thrilled with 20 top quarks. At the LHC, we're going to make 10 top quarks every second. This illustrates a very important point. Something that happens when you go to higher energies.
I mean, this factor of 7 might not impress you very much. We're just a factor of 7 above Fermilab. How can this factor of 7 make such a huge difference? It's not a factor of, like, 3,000. It's a factor of 7.
Well, it turns out that the rate for producing new particles happens to scale like a pretty high power of the energy that you collide them with. It happens to scale like the energy to the sixth, roughly. So that factor of 7 is really buying you an enormous amount. And it's buying you these factors, hundreds of thousands, a million, which makes a difference between waiting seven years till you make a handful of particles versus making them all the time.
That's why it's when this machine turns on at maximum energy that you expect the biggest jump in our reach. OK? And that's why we do it. So you expect big things to happen relatively early on. OK. So expect to make 10 top quarks every second.
What about supersymmetry, the other things that we're excited about? Well, we expect to make roughly a squark a minute. OK? So that's a nice human scale. If you make a squark a minute, we can accumulate a lot of squarks. But you see, you've got to beat down all of these things in order to be able to see it. OK? All right.
Now I want to take just a little moment to say something about this known physics, OK? So things that can contribute to known physics are, for example, if I collide two gluons in the proton, it can spit out four gluons coming out. And it can do this at some very, very high rate that can happen hundreds of thousands of times a second. And it can mask what would happen like, let's say, if you produce squarks or if you produce other particles.
So for a long time-- and this is what you would think is the sort of grungy physics we understand already very, very well, but you still need to learn how to calculate it. How to calculate it just so we can make some-- we can understand it accurately enough.
I just want to tell you a very, very short story, which also illustrates the really wonderful unity of our subject. Because it turns out in the course of these supposedly grungy calculations, supposedly grungy calculations about understanding what's going on with good old-fashioned physics might be in and of itself a little hint for a new way of thinking about spacetime. And I don't want to spend too much time on this. But let me just tell you a little story.
It turns out that this problem-- remember I told you to draw these stick figures, Feynman told us how to draw these stick figures? It's all standard, we're done, we've understood it for 60 years.
It turns out that, as you start drawing those stick figures for a process of the sort that I told you about, it starts getting hard very rapidly. So for the process that I told you about, there would be 220 of these stick figures, tens of thousands of terms. You wouldn't even do that to a graduate student. Well, tens of thousands is maybe OK. Hundreds of thousands you would pause.
But people learned how to do these calculations by various tricks. You don't need to understand anything, actually, on this slide in detail. All you need to understand is that those tens of thousands of terms collapsed into this one simple expression. One line.
So there were some crazy cancellations that took place in the course of the calculation, and then it collapsed mysteriously to a single term. That sort of miracle turns out to happen more and more and more when you do more and more and more complicated calculations like this. And it actually strongly suggests that the way Feynman taught us how to do these calculations, which after all, as I stressed many times, makes locality in space and time manifest, makes quantum mechanics and locality in space and time manifest. Is making that manifest at the expense of obscuring some much, much deeper structure?
Even sitting here in physics that's been under our noses for 60 years. Under our noses for 60 years is some marvelous new mathematical structure that's governing good old-fashioned standard physics. You don't need to understand any of these words.
But the ideas that I told you from string theory, other ideas, older ideas that I've had no time to tell you about, going back to Roger Penrose, things known as twister theory, and strange branches of old, classical, beautiful branches of mathematics that never had to do with physics in any deep way before now seem to be sitting at the heart of this new mathematical structure that in the past number of years is starting to be uncovered. Associated with good old-fashioned standard physics, the grungy part.
There's something amazing going on, even in the grungy part. Which seems to say that there's some way of doing the scattering without ever talking about spacetime. So there might be a hint for what gets us to replace spacetime in a new way, even sitting here in the standard physics calculations.
This is something that happens in physics over and over again. There is no-- everything is just-- everything talks to everything else. You can make these artificial separations, say, that's the boring part. This is the new part, that physics doesn't care, right? There is an incredible unity to the subject. And any time you start doing something interesting, you find that it ends up bumping into other things that are actually important. Anyway, that was just an aside.
So now let me finish the discussion of the LHC by discussing what we might know by 2020. So after 10 years of running at the LHC, what might happen? Well, of course, we don't know. So all of this is speculation. But I want to go through a variety of-- the implications of a variety of possibilities for what might happen.
The first possibility that everyone asks about is, what if we see nothing, literally nothing? And it's actually literally impossible for us to see literally nothing. This is something I unfortunately don't have enough time to explain properly but have alluded to before.
It's the fact that if we just take the particles that we've seen so far, even no Higgs, just take the particles we've seen so far and do quantum mechanical calculations and the theory with the particles that we've seen so far, that little subtheory predicts its own demise by 10 to the minus 17 centimeters. Full stop. If you take that theory and you calculate things with it, if you calculate the probability for two-- for some specific processes, you find the total probability for some process starts becoming bigger than 1.
It makes no sense. There is no such thing as probabilities getting bigger than 1. And that's the way the theory-- the subtheory has to tell you, I am sick. Please repair me. I need help.
The Higgs or something like it solves that problem, amongst other things. So we simply must see something. Now it's possible there is something going on and we're missing it experimentally. I'm not saying that it will necessarily be easy. But as an actual physical fact, it's almost impossible to believe that there's nothing there. If there is nothing there, then quantum mechanics is wrong, and there's no way that quantum mechanics is going to be wrong yet.
OK. Another possibility is that by 2020, and actually, again, if this is what's going on, it's really likely that this will start happening earlier, significantly earlier than 2020. I should mention that right now the LHC is running at 1/2 of its ultimate design energy. That 1/2, remember I told you that these rates scale like the energy to the sixth. So that 1/2 to the sixth is a bit of a pain. So it's running right now.
It's possible that we'll see some hints for something new going on already in this current round of data, but it's not really guaranteed. So it's going to run till the end of 2011, and then it's going to shut down for a while while they take it up to its final energy.
Once it gets to its final energy, then, really, if there are things going on there, they should be produced like gangbusters. Fermilab right now has been looking for supersymmetry, has been looking for all these things. And after the result of many, many, many years of effort, it tells us that, if there are gluinos there in nature-- those superpartners of the gluon-- they have to be heavier than, let's say, 300 times the mc squared of the proton.
Let's say they're really there at 350 times the mc squared of the gluon-- of the proton. If they're there at 350 times the mc squared of the proton, then the LHC will produce them once every second. That's again this big jump that you get when you go to high energies. So you expect exciting things to happen relatively soon.
So, but anyway, if we see something like that, either supersymmetry or some of these other ideas that involve the hand that holds up the pencil, the given nice mechanistic explanation for the absence of fine tunings in nature, I think that would be euphoria for our field because we've been waiting for lots of new data like that for about a generation. And if it's particularly supersymmetry, it would be the first extension of our notion of spacetime since Einstein.
It would tell us very vividly that there was no fine tuning required after all for answering the very basic question, why is gravity weak? It would leave us still scratching our heads about the size of the cosmological constant. We still wouldn't have a good explanation for the smallness of the cosmological constant. But we don't always solve all problems in physics at the same time.
Something else that might easily happen by 2020 is that we'll produce zillions of dark matter particles. And we'll see that they're there. There will be all this missing energy. And the LHC could be a factory for making dark matter. We wouldn't really be able to collect it too efficiently, but it's quite conceivable there would be millions of dark matter particles made, just made, in the laboratory, excited out of the vacuum at the LHC. We'll talk about dark matter more in a second.
Now here is what many theorists consider a nightmare scenario. I don't think it's so much of a nightmare scenario, but it's widely considered to be the nightmare scenario for our subject. What if we do eventually find the Higgs? I'll mention a small ironic point. If there is supersymmetry or any of this other exciting physics going on, that could be discovered early. It could be discovered well before the Higgs is. Because for the reasons I explained to you, the Higgs is actually a little harder to see.
So the Higgs, if it's there, we're talking maybe 2015, OK? But anyway, when you're talking about 2020, so what if by 2020 we have evidence for the Higgs and nothing else? That's very, very deeply confusing because these arguments we had that we had to see some new physics at 10 to the minus 17 centimeters to hold up the pencil, to explain why gravity is weak without invoking any fine tuning, that there is something wrong with that argument.
I should say this has never happened to us in such a sharp way before. Actually, it kind of happened to us for the vacuum energy problem. I mentioned to you that if you believe that new physics has got to come in at the relevant length scale to stabilize the pencil, then for the vacuum energy the new physics would have had to show up at a millimeter and it didn't. So if we go to the 10 to the minus 17 centimeters and we don't see anything that stabilizes the Higgs, then you start thinking that perhaps the Higgs is fine tuned. Is it possible? What does it mean?
In any case, it would be mysterious. It would make the fine tuning for the weakness of gravity start looking a lot more like the fine tuning for the smallness of the cosmological constant. And I can't emphasize what an incredibly bifurcatory moment in the history of our subject this represents. The settling of this question one way or the other has incredibly profound implications for the direction of physics in the future.
It's the bifurcation between order and chaos. Do we see something like-- do we see that there really is a mechanism that stabilizes-- the mechanism that explains why gravity is weak? There isn't any fine tuning. There's supersymmetry that's accompanied with these beautiful ideas that the forces are all unified at very high energies. These new symmetries, new sorts of dynamics, extensions of spacetime on the one hand. Or this other picture, the picture that we're a tiny little nothing speck in a vast lethal multiverse. And most of the multiverse is deadly in the few places where the parameters are adjusted so that, for example, the energy of the vacuum is small enough we can be there.
Could it be that there is an explanation like this for the weakness of gravity? Well, remember I told you, if gravity was stronger, we'd be black holes. It's actually conceivable that the weakness of gravity-- it's conceivable, it's conceivable-- that the weakness of gravity has a similar explanation. In fact, if you made gravity even a few times stronger, you would drastically change-- when you look at it in detail, you would really drastically change the nature of chemistry.
And the fact that we have interesting stable atoms turns out to very, very sensibly depend on the precise value of this weak length scale. And if you change it a little bit, you'd completely change what chemistry looks like.
So it does seem like even the value of the weak length is at an interesting, somewhat special value. It's conceivable. There is much, much less evidence for this. I think the case for this is a lot less strong than the case for the vacuum energy. But if we see the Higgs and nothing else, then we might start taking this possibility a little more seriously.
And I don't have time to talk about it, but there are other theories more closely related to supersymmetry and more closely preserving this picture of unification, which would be lost if we just saw the Higgs and nothing else, for which this conclusion that there is some fine tuning could be made even sharper. I don't have time to describe them, but it is possible that by 2020 we will be contemplating this possibility more seriously if we don't see this whole panoply superstructure of mechanisms coming in, extending spacetime, and explaining the question, why is gravity weak?
So it's really a dramatic moment. One way or the other the field will go and physics will go, fundamental physics will go in radically different directions. All right.
So now I want to end by telling you about the other class of experiments that will be happening. So let's talk about dark matter. Now remember, one of the reasons dark matter was interesting is that we had this argument that there was a very good reason to expect dark matter to show up at around the scales we're about to go. It doesn't have to. There could be other kinds of dark matter particle.
But if you ask the sort of simplest possible way that cosmology could synthesize and make in the early universe particles that had the right density to constitute the dark matter of the universe, you would find the answer particles whose mass is hundreds of times the mass of the proton. The range of the interactions are around 10 to the minus 17 centimeters. That's just remarkable. It didn't have to happen that this argument from the opposite end of physics supported this picture that we had to see new things at 10 to the minus 17 centimeters. OK?
I should say that if this is what-- if this is what the dark matter is, then there's roughly one of it per every liter of space here. So right there there's one dark matter particle. There there's a dark matter particle. There there's a dark matter particle. Actually, they're whizzing through here at around 1/1,000 the speed of light. But there's very few of them. They're pretty dilute, but not that dilute. There's lots of them in this room.
So how do you look for dark matter? Well, every way of looking for dark matter really involves the observation that it does interact with ordinary matter in some way. I mean, the range of this interaction is very short, it's 10 to the minus 17 centimeters. It's incredibly weak. But it does interact with ordinary matter in some way.
So we can make it at the LHC, as we just discussed. So the LHC might be a dark matter factory. You can do something else. The dark matter should be out there, should be out there in the neighborhood-- in the galaxy somewhere. In fact, under its gravitational influence, it would want to clump a little bit more towards the center of our galaxy.
There is this big dark matter cloud 100 times bigger than our galaxy. We're sitting in the middle of it. But the dark matter would even be clumping a little bit more towards the center. And every now and then a pair of those dark matter particles could annihilate with each other. Exactly the same sorts of reactions here that make dark matter talk to ordinary matter could make the dark matter particles annihilate. And they have these big masses, hundreds of times the mass of the proton. So the particles that come out will be whizzing out at incredibly high speeds.
And so you can look for anomalous high-energy particles coming from our vicinity in the galaxy. Particularly interesting is looking for antimatter coming at very high energies from the center of the galaxy, because you'd be making particles and antiparticles. And it's not so easy to make high-energy antimatter from ordinary processes. So that's called indirect detection.
And finally, you can look for it directly. So there's one of them per every liter of space. So how would you look for them? Well, you would take a very, very cold vat of stuff, many different kinds of stuff. But you take cold that's of preferably quite heavy nuclei. You put them very, very deep underground to shield them from all sorts of cosmic rays and other things that could be hitting the apparatus and you just wait.
You just wait for one of these dark matter particles to come along, hit one of these nuclei. When they hit them, the nucleus will recoil. And as it recoils, it'll give off some energy. And so what you have is this incredibly cold, dark place where every now and then the nuclei for no reason start moving. And you use that to infer that a dark matter particle came along and hit it. So that's called direct detection.
And what's been going on in the past couple of years is lots and lots of activity in these two subjects, in these two areas, as the LHC has been ramping up. And we really expect certainly this-- certainly direct detection is really going places, especially in the next two, three, five years. Let me tell you a little bit about that.
So I'm going to describe just in a little bit of detail one of the experiments that's looking for direct-- that's doing direct detection of dark matter. This experiment is known as XENON100. And it's because the vat of stuff in this case is xenon. OK?
So you have-- I won't describe it in too much detail, but you'd have this big vat of liquid xenon. There's some gas up here, some gaseous xenon up here. But you have this vat of liquid xenon. It's deep underground in Gran Sasso in Italy. And you wait for a dark matter particle to come in, hit one of these xenon nuclei.
When it hits one of these xenon nuclei, one of the things it does is it shakes up the atom, which gives off a lot of light. And it also strips off some of the electrons. And those electrons, after you apply some electric field through this xenon, drifts to the top. And when they get to the top, you measure them another way.
So you get, actually, two signals from one of these dark matter particles coming in. First, a flash of light and then after some time another flash of light associated with the electrons. So using these two pieces of information, you can figure out that a collision happened and where it happened, OK?
And this is actually important because this experiment is designed to be big enough that other possible sources of things that could cause these nuclei to recoil become small. The major source of this so-called background, the thing that would fake the signal that you're looking for, is, believe it or not, just the radioactivity in the detectors that surround it. Just trace amounts of radioactivity there will occasionally spit a particle into the vat of xenon, and that particle will hit the nucleus and make it shake and give off some light and so on.
So one thing you do is you make it big enough so that there is a region in the center, which is likely to be shielded from these accidental external radioactivity. There's other detailed things that you can do. It doesn't matter in detail. But it turns out that the way that the dark matter excites the light and excites the electrons is exactly the other way around as the way the radioactivity would. The radioactivity would turn out to give you less light and many more electrons, whereas the dark matter would give you much more light and many less electrons.
So that gives you a way of discriminating what was radioactivity from what was dark matter. It matters a lot for the success of the experiment. So you have these two kinds of signal, the light signal and the electron signal. And when one is much bigger than the other or vice versa, you can deduce if it was dark matter or ordinary stuff.
And again, the details don't matter here, but you see-- so this is a measure for the ratio of the size of those two signals, the electron signal and the photon signal. And you do see that there is this big-- this is the actual data that they released just a little earlier this year. And you see, there is this big spread of things up here which correspond precisely to that radioactivity you're trying to get rid of.
But indeed, it starts going down precipitously as you go in this direction, where you would start expecting it to look more like dark matter. And the signals for dark matter would be expected to be seen in this band, inside this blue band. And you see, there is a little bit of leakage into this blue band. But you then look at these events in detail and actually figure out that so far they're not really seeing evidence, any evidence for dark matter.
But this picture was based on a very small amount of data that they studied. They've been collecting a lot more data than that since then, just this year, and they plan to analyze it, open their box, look at their data. They haven't decided exactly when, but maybe at the end of October, maybe the end of November, maybe January, but sometime in that neighborhood.
And they've collected enough data that, if there is dark matter out there with roughly the properties we would expect, we're talking about 50 events in there. So it's something that would really hit you over the head. Again, when you do experiments that go into uncharted territory for the first time, you expect big bang for your buck early. It makes sense. That's why you do it. All right.
Something else that-- so that's direct detection. Something else that you can look for, as I mentioned, is indirect detection. And here I'll just tell a-- I'll just take a short amusing story because it triggered a fair amount of excitement in the field two years ago. You remember I told you that, if there is dark matter, if there is dark matter out there in the neighborhood of the center of the galaxy, you would expect that every now and then it annihilates, and it could annihilate into very high-energy particles and antiparticles.
And so you would expect to see-- you would expect to see very high-energy antimatter in space. Well, there was a satellite, an Italian satellite, known as the PAMELA satellite, which was up there looking for high-energy antimatter, high-energy positrons and antiprotons, but positrons in particular. And they reported at a conference in August 2008 the following picture, where they were drawing-- you can't even see this here-- but it was the ratio of the number of positrons and the number of electrons, which went down, down, down just as predicted from standard astrophysics theory, and then started dramatically going back up again.
Now the reason I'm showing this picture is because they announced this result at a conference. People got very, very excited because this is exactly what you would have expected if there's dark matter out there. You expect to all of a sudden start seeing an extra new source of antimatter. And this was at around the place where you might have expected to see it.
So people started getting very excited. I got an email from people who were at the conference. I immediately went to the conference website to try to see if I could pull this talk and get the slide. They had pulled the slide. The one slide that had the data in it they had just pulled from the presentation. They weren't making it public.
So people were very frustrated, and they knew that this experiment was going to release their data again, give the same talk two weeks later at another conference. Well, guess what? At the other conference, everyone was ready with their cameras. And when the slide came up, people took a picture. And this is a picture taken by my buddy on his cell phone and emailed to me.
Anyway, later, indeed, it came out. This is the official plot. And indeed, the ratio went down, then it went back up again. So this actually triggered a fair amount of excitement in the field a couple of years ago. And right now the status is really unclear. Is it dark matter? Is it not dark matter?
Our astrophysics-- us particle physicists got very excited, started building theories of dark matter that might explain this and might explain other things. Our astrophysics colleagues got excited and invented theories of pulsars that would explain these things. So we don't know if it's dark matter. We don't know if it's astrophysics. It definitely was unexpected.
And it tells us that-- but there is a bit of a cautionary tale here that anything from indirect detection of dark matter is likely going to take many, many pieces of evidence till you start taking it seriously. It's not going to be as valuable as just the direct detection, something bangs into your vat of xenon in the lab. It's ultimately better to do experiments in the lab here than rely on making observations on what's going on many, many light years away in the galaxy.
There are other things that could happen, by the way, in the next few years that would be more incontrovertible. It turns out that, when you look at the way dark matter is distributed around the galaxy, it's not distributed completely uniformly. And there are places where it's a little bit more clumpy. There are places, in fact, off the plane of the galaxy where it's a little bit more clumpy.
Now if it's a little bit more clumpy over there, you would expect there to be more chance that they could annihilate over there. And so you'd expect to see these signals that we're talking about-- very, very high-energy photons, very, very high-energy, well, particularly very, very high-energy photons-- to be coming now not from the center of the galaxy but from well off the plane of the center of the galaxy.
There's nothing out there. There's no stars out there. There's no pulsars out there. There's no funny stuff out there.
So if you start seeing strange light coming in a clumpy way from off the plane of the galaxy, then that's another very good signal that maybe you're seeing annihilating dark matter. And these are a few pictures for what it might look like in a variety of scenarios. And you really could see that things off the plane of the galaxy would start lighting up and would not only give you evidence for dark matter, which would be exciting enough in itself, but would start actually probing exactly what it looks like, how much of it there is where. All right.
So what might we know by 2020? Well, as I said, this subject is really on the experimental front evolving very rapidly now on all these different fronts. The most exciting one continues to be the direct detection of dark matter. And here is a plot. Again, you don't have to pay attention to the details. Very roughly speaking-- well, roughly speaking-- the vertical axis is a measure of the strength of the interaction of the dark matter with ordinary particles, something we call a cross section, and the horizontal axis is the mass of the dark matter particle.
So basically, this entire range from the mass of something that's theoretically reasonable. These various curves show what a variety of experiments have done putting limits on what these cross sections can be. I won't go through a story that some of these experiments actually claim to have seen something. You know, it's as exciting experimental science should be. It's in a chaotic and quickly developing state right now.
But just so you get an idea, people have sensitivity. For example, the XENON100 experiment. We have sensitivity down to these cross sections of order 10 to the minus-- at the moment, let's call it 10 to the minus 44 centimeters squared. That means that the range of the interaction that's being probed is around 10 to the minus 22 centimeters.
Now it turns out that just from a theoretical point of view, if you ask the theorists what are the sort of reasonable places that you would expect to have-- what are the reasonable values for the cross sections you'd expect. Well, one set of values would live up here. They have been long excluded. But another set of values really live right in the neighborhood of where we're about to probe.
So this is something that's going to improve very quickly. And the experiments are exactly in a region where we expect to start seeing something.
I should make one final comment. If there are WIMPs out there, they will be seen by 2020, almost period. If they're there in this mass range, they'll be seen by 2020 or they won't be seen for a heck of a long time after that. The current round of experiments is going to have sensitivity to cross sections. Not the current round but the planned next generation, certainly by 2020, will have sensitivity to cross sections that will go down another two orders of magnitude from that. OK? So it'll go down another factor of 100 from there.
In that neighborhood, around 100 times lower, around 10 to the minus 47 centimeters squared, you start running into a new background, something new that can fake the signal that you're looking for. And those are neutrinos. I haven't mentioned much about neutrinos in these lectures. They're some of the particles we expect in the standard model. And they're copiously produced in supernova explosions.
So supernova explosions far away give us some stream of neutrinos that are running through the Earth right now. OK? These neutrinos are incredibly, incredibly feebly interacting. That's why I've hardly mentioned them in these lectures. But they would interact with these nuclei strong enough to kick the nuclei by an amount that would be exactly what these events would look like down here at 10 to the minus 47 centimeters squared.
If we don't see the WIMPs by then, then the WIMPs will start being swamped by neutrinos and the experiment will start getting very, very hard, very, very hard. Not impossible, but very, very hard.
On the other hand, as I mentioned, there's no very good theoretical reason to expect that so low. There's very good theoretical reasons to expect it right around here. And so if they're WIMPs, they're of the sort that theorists have known and loved and talked about for 30 years. They will show up by 2020. And they'll show up in someone's detector.
And once they show up in someone's detector, you can be more confident that maybe you are seeing some signal from far away as well, because you can correlate what you're seeing far away from the properties that you have inferred from the detector. All right. So that's the story of dark matter.
Let me end with just a few brief remarks about cosmology. So the story of this decade in-- coming decade in cosmology is developments on two fronts. One of the interesting things about cosmology is what happened very, very early in the history of the universe.
And particularly, as we've discussed, this wonderful theory of inflation-- that we have lots and lots of not just circumstantial but pretty reasonably hard evidence for, and that evidence will get more and more solid as this decade progresses-- this theory of inflation gave us an understanding for where the tiny, tiny fluctuations in energy density early on in the history of the universe came from.
And these tiny, tiny fluctuations, which in the sky today look like tiny, tiny differences in temperature of the cosmic microwave background in different directions of space, came from these underlying quantum fluctuations during inflation that were stretched out by this rapid accelerated expansion of the universe and turn into ultimately the structure that gave us us today, right?
So that's an amazing thing. And the experiment that throughout the last decade did more than anything to solidify this picture and help us believe it was this wonderful WMAP satellite, the Wilkinson Microwave Anisotropy Probe, and it just takes a picture of the cosmic microwave background and a number of frequency bands. This is what it actually looks like before you process it too much.
So you see there are these regions, different colors here, are just slight temperature variations. You're just looking out of the sky. They're just slight temperature variations. The overall size is around-- the overall size is around 10 to the minus 5, a part in 100,000. The blue regions, as you see from here, are colder. The red regions are hotter. What do you think this is in the middle?
That's our galaxy. So you don't often see the picture look like that. You often see the picture look like that, which is what happens after you subtract the galaxy, which you think you understand.
But anyway, that's what it looks like. This is really a snapshot of what the universe looked like around 100,000 years after the Big Bang. And there are regions which were within a part in 100,000, a little bit hotter, a little bit colder. And as I mentioned before, this little bit hotter and little bit colder got magnified by gravity. So ultimately, we got big galaxies, small galaxies, solar systems, planets, you and me. So the ultimate origin of all the structure in the universe is sitting here in this picture.
And studying this picture in detail, in great detail, tells us a lot about the early inflationary epoch, which could have given rise to it. So WMAP did a huge amount. And now there's a next generation of experiment that's going to improve our measurement of exactly this picture by another order of magnitude or more. And that's known as the Planck satellite, which has been launched by the European Space Agency.
It was launched a little while ago and has just started this year releasing its first pictures. So where it is is-- this is fun. The Planck satellite is actually nowhere near the Earth. It's 1 and 1/2 million kilometers from the Earth. And it's sitting at an interesting point determined by the Earth, an interesting point of the-- determined by the gravitational pull of the Earth and the Sun, which happens to be relatively stable. So it can sit there for a long time.
These special points in the neighborhood of the orbit of the Earth and the Sun are known as Lagrange points. They were discovered by Mr. Lagrange in the 1700s, I guess. And there's five of them. Three of them are-- well, there's two of them that are completely stable. In other words, if you're in that neighborhood and you give a little kick, you just always stick around that region.
And the other ones aren't stable. They're sort of semi stable. In some directions they're stable. In the other directions they are not stable. This thing is in the neighborhood of one of the semi stable ones.
You might wonder why you don't put things in the neighborhood of the very stable ones. It's because things in the neighborhood of the very stable ones also accumulate rocks, other crap floating around, which also get trapped around there. It's a very bad place for a multi extremely expensive experiment doing very fine measurements to sit.
So we actually put it in the neighborhood of one of the semi stable ones where, OK, every now and then it goes off a little, but you have computers on Earth that tell it to go back. OK?
Anyway, and it's just started releasing its first images. That's the first image of the Planck map of the sky. But anyway, so this has not subtracted the galaxy or anything like that. But ultimately, we're going to get much, much more detailed measurements of the microwave background with Planck.
Those detailed measurements could tell us much more and give us a lot more confidence in the underlying inflationary structure. In particular, one of the predictions of inflation is that not only should we have these fluctuations in temperature, but we should also have little fluctuations in what you might call little fluctuations in the spacetime as well. What we sometimes call gravity waves.
So we should see fluctuations that we can interpret as gravity waves as well. If the energy during inflation was as high as makes sense in many of the sensible theories-- certainly not all, but many of the sensible theories-- you would expect to see that, too. And if you see that too, that's a real smoking gun of the underlying inflationary picture, OK?
It's not guaranteed to be seen, but it could be seen. And it's one of those things that, if you do see it, you get extreme confidence that the theory is right. If you don't see it, you still have to look for other things that might give us extra confidence.
So now, the other extreme of cosmology is understanding more the behavior of our current accelerating phase. So that's the other interesting thing that's been going on in the past few billion years is our universe is accelerating at a uniform rate.
Now I've been just telling you the whole time that what causes this is the vacuum energy. But I said it's the simplest explanation for what's causing it. That simple explanation-- however, in that simple explanation, we have to swallow that there is this enormous fine tuning. We don't quite understand why this vacuum energy has the size it does and so on, OK?
Some people think that maybe there's something else that's causing the acceleration of the universe. Maybe it's not a vacuum energy. Now whatever else might be causing it, you still have to explain why the vacuum energy wasn't that the Planck's down, OK? So none of these other explanations actually so far give us any way out of that. So they're no better on those grounds.
But you still might think we understand this so poorly, there's clearly something so wrong with our understanding, that it would be a good idea to try to see if it's true that it looks like a vacuum energy. If it looks like a vacuum energy, then the rate of doubling of the size of the universe should be dead constant. It should show no signs of having varied earlier, of having varied as we go further and further back in time.
So what you can do is design experiments that in one way or another as finely as possible give you measures for the expansion history of the universe between now, going back to the hundred thousands of years after the Big Bang epoch, OK? And by doing those measurements very finely, you can reconstruct what the actual expansion history of the universe looks like and see if there's evidence, see if there's evidence that this vacuum energy, or that the doubling rate, the expansion rate, was actually changing. If it's not changing, it's a dead vacuum energy. If it is changing, it's something else.
So again, that's very dramatic. The difference has dramatic impact on how we think about physics.
A whole variety of experiments will weigh in on this. The Planck satellite will actually weigh in on this. But there are also experiments on the ground. And there's this wonderful telescope that hasn't been built. This is an artist's conception for what it would look like. It's been designed. Called the Large Synoptic Survey Telescope, which would be placed down in Chile somewhere. The astronomy community in the US has decided that this is the most important project to support over the next 10 years because it can do a huge number of different kinds of experiments.
But one of the things that could really help is to nail as accurately as we can whether the thing that's driving the acceleration of the universe corresponds to a constant vacuum energy or not. And once again, don't pay attention to the details of this plot. But this is a two-dimensional plot of various ways that the energy-- that the expansion rate could actually be changing with time. The kind of origin here where this is minus 1 and 0 would correspond to a dead cosmological constant, perfect cosmological constant.
And you can see that a combination of a whole variety of experiments using this LSST and other data could start putting pretty strong constraints on how much you can deviate from an exact cosmological constant. So that's another set of things that we can be looking forward to in the next 10 years.
So in this subject, what might we know by 2020, we may-- it's not guaranteed-- but we may gather very significant additional support for the idea of early inflation. So that's one of the things I'm most looking forward to. I think as far as probing the acceleration of our universe, I think it is overwhelmingly the most likely thing is that we'll just find that to much higher accuracy the cosmological constant is the very best fit to explain the current accelerating universe, OK?
I'm quite sure that that's what's going to happen. But it would be an enormous shock if not. So it would really have to go back to the drawing board again. We're actually still pretty much at it as far as the cosmological constant is concerned. But this would be just really massively confusing to theorists. So this is what I think what the sort of situations we might be in 2020.
OK, so that's it. That's the survey of the experimental situation for the coming decade. And so I've had an enormous amount of fun in this set of lectures telling you about the past, the present, and the future of this wonderful subject. I think one of the things I hope you take away from these lectures, especially from the first couple of lectures, is that-- oh, sorry. Oh, sorry.
One of the things I hope you take away is that we've really come an enormously long way in our understanding of fundamental physics. The understanding we have today encoded in the standard model of particle physics and our understanding of gravity and our understanding of cosmology is, I think, beyond the wildest imaginations of any physicist in the 1950s.
The fact that we understand the history of the universe, the fact that we understand the basic constituents of matter, we understand the basic rules for how things interact. We know roughly all possible worlds that could make sense.
We have reduced our understanding. We've reduced the uncertainties. We've reduced the things that we're confused about and which require deeper explanation to explaining 19 numbers. This is an incredible, incredible amount of progress.
I want to emphasize again. There are not basic questions in the world around us that we don't have a good answer to anymore. It's an amazing thing. It was eons from being true 100 years ago. Even 50 years ago there were lots of basic questions about the world. Just how does radioactivity work? No one knew exactly.
Now we understand the question. Any question you have about the world around us, we have a very good answer for that. Now on the one hand, you can think that that's sort of disappointing. The questions that we're left with are further removed from things that we see in the world around us. But on the other hand, it means that we understand the world so well now that we can ask incredibly deep questions about it.
These questions about where does spacetime come from. These questions about why is the world big. These are questions that at some level you might have thought you could have asked 100 years ago, 300 years ago, 2,000 years ago. But in any of those previous periods, it was simply completely the wrong time to ask that question. There'd be no way to even start answering it.
That's one of the wonderful things about progress in physics. The more and more we learn, the more and more deeper questions we generate. And we're allowed to ask them. There become sensible questions that we can ask and work on and start to make progress on.
So the fact that we understand the world around us so amazingly well means that finally this is the epoch, this is a time in which these questions about the ultimate nature of space and time become accessible and become things that you can go to graduate school, learn about, start working on, work on in your office, right?
It's not something Aristotle could have done. It's not something Einstein could have done. It's not something Feynman could have done. But it's something that we can do now. And it's a testament to the power of this scientific process that it does this.
So we've come a very long way in our understanding of fundamental physics. But I also hope that you've seen from the last number of lectures and from the exciting prospects for new data in this coming decade that you should stay tuned because the best is yet to come. Thanks a lot.
HENRY TYE: I will give people the 10, 15 seconds if they want to leave, and then Nima can take some questions.
NIMA ARKANI-HAMED: Oh, thanks.
HENRY TYE: 17 hours.
PAUL: One tiny typo. You did, Plato, you did invent a new field of mathematics.
NIMA ARKANI-HAMED: What? Really?
NIMA ARKANI-HAMED: No. No, I didn't. No, I didn't. I don't think I did.
PAUL: Allegoric technology.
NIMA ARKANI-HAMED: [LAUGHING] Allegoric geometry.
PAUL: Actually, elegiac.
NIMA ARKANI-HAMED: Oh, you're right. "Elegraic." Oh, geez, hold on. Thanks, thanks, Paul. Yeah, that's funny. Sorry. Just one second. I'm sure this was bothering everyone as much as it was bothering Paul.
PAUL: I'm a happy man.
NIMA ARKANI-HAMED: You're a happy man. All right, great.
PAUL: It's on 190.
NIMA ARKANI-HAMED: Oh, sorry. There we go. Oh, let me leave this up. Sorry, you don't need to see my desktop. We've come a long way. Stay tuned. There we go. Great. Yes, any questions?
AUDIENCE: You mentioned [INAUDIBLE] said, you know what we have to do to understand quantum mechanics and understand what you [INAUDIBLE]. Do you understand what I mean?
NIMA ARKANI-HAMED: Sure, sure, sure. Yes. Well, I mean, so everything about the measurement problem is explained by decoherence. Every puzzle, every confusion, every everything. If you just follow-- if you just think about the whole big system and you always include the system and the apparatus, everything together, there's never a problem. Now there is one thing that the people sometimes talk about, which is, I think that the nub of the matter for a lot of people with the interpretation of quantum mechanics is the following situation.
That you do an experiment. Let's say you see whether a spin is up or down, whatever. So the state is the spin is up or down. And then after you do the experiment, you have a state where the spin is up. The apparatus says it's up. Your brain thinks it's up. Plus, you know, it's down. The apparatus says it's down. Your brain thinks it's down.
So it looks like the whole state, including you, is in some strange super position between the two possibilities. And so what people wonder is, why, therefore, after any one experiment do I feel certain that it was either up or down? And I don't feel like I'm in this strange super position.
As far as I can tell, most of these issues come down to that feeling question, OK? Why don't I feel like I'm in this strange super position? Well, if you ask the question correctly, if you ask the question in the language of quantum mechanics, I mean, that's what we have to do when you test a theory. You have to take the theory on its terms. Ask a question in the language the theory likes.
Then you find that you can invent an answer to the question, am I certain of the outcome of the experiment? And the answer is yes, despite the fact that you're in this strange, entangled state.
So I'm stressing this because I really believe that people say lots of nonsense about the interpretation of quantum mechanics. And they make it seem like there's something deep and crazy going on when you do something as dumb as look at this cup of coffee, right? It just sounds implausible, right? You know, we look at coffee all the time.
It would be very strange if the future of everything about physics depends on understanding that interaction. Which is not to say there isn't something deep and mysterious about quantum mechanics. There is. It's very strange that the sharp observables that you talk about necessarily invoke infinities, necessarily invoke infinitely large apparatuses with an experiment done infinitely many times.
But in order to come to grips with how strange that is, it makes sense to go to a situation where that strangeness becomes important. And that strangeness becomes important when the words "gravity" and "cosmology" are involved. So that's why I say, unless the words "gravity" and "cosmology" come up in the discussion of quantum mechanics, there just cannot be a mystery. Or all the mystery is in our head. Which is not to say that there aren't mysteries, but the mysteries start happening there, as I described.
AUDIENCE: All right. Are there any experiments involving gravitational lensing to find dark matter?
NIMA ARKANI-HAMED: Sure. Yes, yes. In fact, there's a whole slew of indirect experiments like that. Those experiments-- I mean, there's already indirect evidence for dark matter around the galaxy from gravitational lensing. Right now there are kind of at the factor of 2 level. So it's not sort of sharp enough that-- and also, you know, there are people who talk about really radical alternatives that, again, gravity is modified in some way. And since the size of these effects can almost be determined by dimensional analysis, it doesn't necessarily, necessarily.
It's consistent, perfectly consistent, with dark matter. But you might imagine someone else saying, oh, it's consistent with my theory too. Those theories don't make very much sense. But we're very much as-- means really not very much. But anyway. But it's really not as good as having something hit your detector deep underground, right?
I think if we don't see WIMPs, it doesn't disprove the idea of dark matter by any means. There could be other things that could be dark matter. But it would be very nice. I mean, if we do see it, we do have direct evidence for dark matter, then we can just close that little-- there is this possibility that gravity is modified at very large distances in some funny way. And you know, one night out of 100 I wonder whether that might not be the right answer to this problem. If we actually discover dark matter, then I just won't worry about it anymore.
HENRY TYE: Maybe you should explain what WIMPs is.
NIMA ARKANI-HAMED: Oh, yes. I think I said last time, but WIMP stands for Weakly Interacting Massive Particles. OK? I guess the astronomers took 15 years to catch up with particle physicists in giving things dumb names. But there are WIMPs and there are MACHOs, which were Massive Compact Halo Objects. And so anyway, I really hope we return to an era of sanity in naming. Yes. Yes.
AUDIENCE: An abiding question about what we mean when we say space times 2. I was thinking that [INAUDIBLE] you might have already said it. You might be saying that spacetime is not fundamental, that it's--
NIMA ARKANI-HAMED: That's exactly what I'm saying.
AUDIENCE: --some more basic thing.
NIMA ARKANI-HAMED: Yeah. That's exactly what I meant.
AUDIENCE: Even if it means just spacetime doesn't stick, it doesn't exist.
NIMA ARKANI-HAMED: No, no. It doesn't mean it doesn't exist. It means it doesn't exist fundamentally. I'll give you the perfect analogy for it is the word "position" and "momentum." Position and momentum don't exist. They don't fundamentally exist. Because of quantum mechanics, position or momentum exists. An "and" turned into an "or" as we went from classical to quantum physics.
So at a fundamental level, position and momentum don't exist. Of course, no one disputes that it's an incredibly good approximation to a huge class of questions. And of course, spacetime is an incredibly good approximation to a huge class of questions for us. But for these questions of principle, knowing that an "and" turns into an "or" makes a very radical difference in how you think the next layer of physics should be described.
AUDIENCE: So you don't mean to suggest that spacetime is like the [INAUDIBLE] equivalent?
NIMA ARKANI-HAMED: Definitely not. You see, that's, as I said, there was a small group of theoretical physicists who take this attitude. It's something in my previous lecture I referred to as atoms of spacetime. That saying spacetime is doomed is like saying this piece of wood is doomed. If I go to very short distances, it doesn't look like wood anymore. I see all the atoms and molecules that make up the wood, right?
So you go-- or there's some kind of granular structure. You go to very short scales and you see that it's like a discrete lattice or something like that, OK? That's a sort of very naive idea of what would happen if spacetime broke down.
And just from this thought experiment with a magnifying glass looking at short distances, you might say, oh, that's all it means. It means that I can't actually talk about much shorter distances in time. So it's like a little lattice, just a little lattice sitting there. It's like atoms.
There's a very fundamental difficulty with that idea, which has to do with relativity, which is that the size of that lattice would not look the same to every observer. And that's a real hint that what's going on is much more interesting than that. And in the cases where we understand it very well, you saw exactly how much more interesting it is.
Spacetime really was doomed. The inside really was gone. And there was some description instead involving this holographic idea, with things living on boundaries and infinity.
That's, I think, one of the deepest sort of qualitative lessons we've learned about quantum gravity in the last 50 years. And people said it and knew it 40, 50 years ago. But not till 13 years ago did we see very sharply how it could actually work.
It's something I haven't had a chance to-- it's really-- it's an unfortunate thing about lectures like this that you can't see the sort of magic of it really working. And when you see these things work precisely, sharply, mathematically, the degree of confidence you get in them is exponentially larger than when you hear about it. But it's unfortunately the sort of thing that's harder to get across in this sort of lecture. Yes.
AUDIENCE: When you spoke about the multiverse in your 2 to the 1,000 energy levels and so on--
NIMA ARKANI-HAMED: Yes, yes. Right.
AUDIENCE: --do we live in the case where that--
NIMA ARKANI-HAMED: Yeah, we live in the one that's damn close to 0. That's right.
AUDIENCE: Yeah. What are the chances there's gonna be a fluctuation in the next 10 to the minus 40 seconds and those numbers are gonna change and then suddenly we're expanding again?
NIMA ARKANI-HAMED: Right. Yeah. So yeah. This is very important. So in that picture, we had these-- in that picture I drew, actually, a little well with a barrier sitting there, right?
And if there is any reasonable size barrier, also the tunneling rates start becoming very, very low. So just like the rate from my tunneling across the wall is 10 to the minus 10 to the 30, the typical tunneling rates from one side to the other can be very small. That means that you sit in any one of them for quite a long time, OK?
Now the important thing, all that matters, in fact, is if you tunnel out slowly enough that this picture of eternal inflation takes over. So let's say the doubling time of the universe when you're sitting in your first configuration is 10 billion years, OK? Or let's say it's 100 years. Then if you tunnel out in 1,000 years, then that's fine. You still have an eternally inflating universe because you tunnel out here.
You start expanding at the speed of light. The universe is doubling in size every 100 years. 1,000 years later you tunnel out again, forget it. Those bubbles are never going to hit each other. And so the process just keeps going on and on and on and generates this infinite universe.
Now I should say, again, that we certainly don't know experimentally that that's correct. We certainly don't know experimentally that's correct. That's what happens if we just take the laws as we know them. And we take the radical conservative attitude of pushing it and seeing what it predicts. And if I haven't said it before, I will say it now that theorists have much, much more often made mistakes not taking their theories seriously enough than taking them too seriously.
So really-- and the mistakes are that you refuse to believe predictions that your theory wants to make, which has happened over and over again. But so if you do that, you're led to this picture. The picture is incredibly confusing. And properly interpreting it is a major conceptual challenge.
But it's not a philosophical challenge. It's a physics challenge. And we may not quite have the mathematical tools to attack it head on yet. But we really may in the next 10 or 20 years develop those tools.
HENRY TYE: A question.
NIMA ARKANI-HAMED: Yes.
NIMA ARKANI-HAMED: Yes.
AUDIENCE: I need some information about the [INAUDIBLE].
NIMA ARKANI-HAMED: Yes.
NIMA ARKANI-HAMED: That's right.
NIMA ARKANI-HAMED: That's right, that's right, that's right.
NIMA ARKANI-HAMED: We don't know. I wish I knew. That's really a problem. So we suspect that we have to figure out what emergent time means. We suspect that because we've already seen that space is emergent. We know there is relativity that mixes up space and time. And we also see that all of our real confusions now with quantum mechanics and gravity and these very, very deep conceptual puzzles all involve cosmology. They all involve the time dependence of the universe that we have.
So it's just a suspicion. I can't prove that time is emergent. But it's a suspicion. The trend that things are going seems to suggest that we need to make sense of the idea of emergent time somehow. But it's a massive challenge to figure out what it means.
Because if you don't have space, that's one thing. That's maybe kind of-- that's OK. Things can still evolve. There could still be time. I mean, physics is about ultimately, we think for 2,000 years, describing how things change in time from one time to another.
We don't know how to get rid of time. But 15 years ago we didn't know how to get rid of space and gravity, and we know how to do that now. So progress happens by people chipping away, doing the kind of-- you can't make progress asking an incredibly hard question that you have no idea how to get started. And you don't want to spend all your time playing around with things that you can do so well you're doing them with your left hand.
So the bleeding edge of progress in theoretical physics is to attack problems that are just hard enough that are hard but are just easy enough that you can articulate them. And you can start attempting to lasso your way around them and do some calculation that might shed some light on something and just start hoping a surprise happens. It's a chaotic process, but it's worked for a long, long time. And we suspect and we hope that emergent time, it's now the right time to start taking it seriously.
It might not be. Maybe we'll have to wait another 50 years before we have the right language. Maybe we have to wait another 200 years before we have the right language. But I think we suspect that it's the next thing. It's the next set of things that we're going to have to come to grips with and understand.
HENRY TYE: Maybe last question.
AUDIENCE: You just mentioned the name of those last class of experiments. Some of the names.
NIMA ARKANI-HAMED: Oh, yeah. Well, so there's many wonderful experiments. So there are experiments, for example, that look for things like some of the elementary particles like the electron. I told you the electron has some magnetic property. It spins, OK? It could also have a funny electric property.
So we think the electron is like point-like. And it has negative charge, but there is no sort of separation of that negative charge into a little more negative charge on the north pole of the electron, a little less on the south pole, OK? Of course, you can't really think of it as a little sphere, but if you did.
Now if that little separation existed, it would be called something. It would be called an electric dipole moment of the electron. And the electric dipole moment of the electron is something which we suspect should be there in many, many of the extensions of the standard model that we've been talking about. But which fortunately, the standard model itself predicts a completely minuscule size for it.
So it's the sort of experiment that if you see anything positive, it immediately tells you there's something beyond the standard model. These are wonderful atomic physics tabletop experiments, beautiful experiments. They're very cheap, I mean, on the scale of things. And they're ongoing. So that's one of my favorite classes of experiments, experiments looking for the electric dipole moment of the electron.
There's classes of experiments that are looking for deviations from or measuring Newtonian gravity at very, very short distances, about a tenth of a millimeter by now. So some of these experiments were inspired by these ideas involving large extra dimensions of space that people talked about 10, 12 years ago now. And gravity at that time had been measured down to around a millimeter. Actually, a little more. Actually, more like a centimeter.
And if you ask why hadn't people measured gravity to much shorter distances, you remember, I said it in these lectures and many of you have heard, we say the statement that gravity is incredibly weak, except at large distances it dominates because everything else neutralizes, right? Now even when you neutralize, even if you take two neutral atoms, a hydrogen atom and another hydrogen atom, there is a tiny residual, residual electric force between them. It's called the Van der Waals force, OK?
That Van der Waals force becomes comparable to gravity around a centimeter and then gets much stronger than gravity as you go to shorter distances. So if you're going to measure short-range gravity, you have to screen out incredibly accurately all of these other stray effects. And people didn't have particular motivation to do it so they didn't.
But after these proposals came out, this absolutely wonderful group at the University of Washington led by Eric Adelberger and Blayne Heckel did these experiments and pushed and measured Newtonian gravity down to distances now about 100 microns or so, shorter than a millimeter. 200 microns, 100 microns. And they're continuing. They're continuing. They're going to go to shorter distances.
Now this 100-micron scale is interesting because this is around the neighborhood where, if you believed there was some modification of gravity that would solve the cosmological constant problem, it would show up around there. I told you it should show up around a millimeter. And I glibly did this experiment with my fingers and I said we didn't see anything, OK, at a millimeter.
But it's just conceivable. I think very unlikely, but just conceivable that there are some purely gravitational thing which is going on at those distances and is somehow shutting off gravity at very short distances, shorter than 100 microns. And anyway, these are absolutely beautiful, ingenious experiments, absolutely pushing the frontier of what we can do sitting on a tabletop. And they're going to keep going for a while.
There's another class of experiments that's motivated by-- partially motivated by some of these anomalies involving dark matter. But really, they're quite, quite generally, which has to do with the possibility that, instead of going to much much, much higher energies to see all these extensions of spacetime that we're talking about, that actually some of this physics, like supersymmetry, could be sitting there at much lower energies, much, much lower energies but just not involving our particles and forces but some other particles and forces. And that those other particles and forces could be relatively weakly interacting with us.
Not that weakly interacting, like 1,000 times less strongly interacting than the ordinary things. Not a million times, not a gajillion times. 1,000 times more weakly interacting.
That ends up being a perfectly viable possibility. It could be that you have these extremely light particles lying around that we've missed for 50 years. They are just 1,000 times less strongly interacting than the ordinary stuff, OK?
And they could have supersymmetry. They could have all of this wonderful stuff sitting there not at incredibly high energies but sort of next door, weakly coupled to us through some little portals that we can try to go through, through these weak couplings.
So there's another class of experiments that involve not very high energy accelerators but low-energy accelerators with very high intensities. And they're starting to happen now. Actually, just earlier this summer, there's a new class of experiments of that sort that are starting. They might see something.
You know, that's what I said. These are fishing expeditions. If they see something, it could radically change the future of the field. Like, let's say one of these low-energy experiments sees something. Then all of a sudden you could start looking for supersymmetry. You could study all these properties about the extension of spacetime at low-energy machines using incredibly high intensities. Not with 27-kilometer-around machines, OK?
But as I said, unlike what you see here, there is a basic guarantee, right? We are probing a length scale where definitely something is going to happen. Things talk to us. We know something is going on. These things are fishing expeditions because there are interesting theories that would say that something's happening there, but there's no a priori guarantee.
I mean, there's never a complete guarantee of anything. But these are just things where it could be. It would be nice to go look. But if you don't see anything, it doesn't rule anything out. It doesn't.
Those are three examples. There are even more. But I'm a very big fan of these relatively low-cost cool experiments looking for a whole array of new phenomena. And there's more of them. Yeah.
HENRY TYE: OK, that's enough.
NIMA ARKANI-HAMED: OK.
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Renowned theoretical physicist Nima Arkani-Hamed delivered the last in his series of five Messenger lectures on "The Future of Fundamental Physics" Oct. 8.
Formerly a professor at Harvard, Arkani-Hamed currently sits on the faculty at the prestigious Institute for Advanced Study in Princeton, New Jersey, where Einstein served from 1933 until his death in 1955.
The Messenger lectures are sponsored by the University Lectures Committee. The lectures were established in 1924 by a gift from Hiram Messenger, who graduated from Cornell in 1880.