Interact directly with them and be informed at the same time, energized by scientific ideas and at bell house in brooklyn. Back here tonight, the kickoff of the fifth many season of secret science club north and we are happy to kick off symphony spaces this season behind the new republic. And joanna thompson, rebecca wise and mary mead, the most helpful. A very special thanks, they created the libation of the night. The quantum cocktail, exhibit a. And a smoky ginger ale topped with lemon. We get it at the bar. If you are bored by me. A heads up part of the many season, by december 3rd with astronaut kathy sullivan, former nasa astronaut, the First American woman to walk in space and a former director of noaa. You might have read some of her thoughts yesterday in the paper. You can visit our website at we are so high tech we have a blog spot. You can sign up for our mailing list. Our wonderful secret tonight, sean carroll has a new book out. It came out today. And it is a book party. The lovely folks at books on call nyc are booksellers tonight, and we will be signing copies after his talk and the q and a with you. Our wonderful audience. We are excited to have sean carroll with us tonight and a special guest to introduce sean carroll. Brian greene is a professor at Columbia University and director of the center for theoretical physics and also a playwright, sometimes an actor and bestselling author of several Popular Science books including the elegant universe and the fabric of the cosmos. And the pbs nova miniseries based on his book, he is the founder of the good old world science festival. Brian greene. [applause] thank you. Thank you very much. The quantum cocktail. I was shocked Dorian Devins did not follow it up by saying you get it at the h bar. I will leave now. You have been a great audience, thank you very much. Tonights focus with sean carroll is on Quantum Mechanics. As many of you know, we have a wonderful, quite straightforward understanding of the universe. Classical physics. We you tell me how things are today and i use the equations of newton or maxwell or einstein to figure out what they will be like tomorrow and then it all changed. In the early decades of the 20th century beginning with the work of einstein himself the paradigm changed and the new way of thinking about the world is tell me how things are now in the best we can do is probability of what they will be like tomorrow. That was not an easy transition in the era to take on board. It was an emotional upheaval to make this transition but heres the thing. As sean no doubt will describe in more detail, the laws of Quantum Mechanics even though they are probabilistic, make predictions that agree with observations with fantastic accuracy. Decimal point digit digit digit onward to 9 or 10 decimal places and that agreement between theory and experiment is so potent, so powerful that people began to stop thinking about what the theory means, what it tells us about the nature of reality and began to use it as a wonderful structure to make predictions about the world, to control the microworld to yield all the wonderful technological gadgets that we walk around with all the time. And the meaning of Quantum Mechanics, what it was really telling us about how the world works. They were looked on as somebody who lost their way, it wasnt something we were supposed to do. Few people are trying to figure out in particular how we transition from this probabilistic description of the world to the definite reality that we all experience and many ideas we put forward, wild ideas. Sean carroll will talk about the most wild of those ideas, the possibility that we are part of a multiverse, many worlds, parallel universe. A very strange idea but i will tell you this. Ever since the events at least i have found in talking about those ideas, the events of november 8, 2016, people have a much easier time understanding the concept of parallel universes. [laughter] with that, gives me great pleasure to introduce tonights speaker. You all know him from his many bestselling books on the nature of time, the higgs particle, the nature of reality, a physicist we all know and love, one of the great leaders, pioneers in understanding cosmology and quantum physics, tonight you have a great treat to hear from one of the profound thinkers who are willing to push into the meaning of quantum physics to try to figure out what it tells us about the nature of reality. Give a warm welcome to my friend and colleague, sean carroll. [applause] thank you. Who do i need to thank . Thank you secret science club, thank you for figuring out the secrets that you can come here. I dont know how that happens but im happy it does. When you become a physics professor, you teach other people even if it is an abstract physics concept it is best to do demonstrations and bring it back to earth. I will start by doing a demonstration, push a button which will send a signal over the internet to a laboratory in switzerland which will send a photon of particle of light down the tube and a beam splitter, it will go left or right with 50 50 probability and depending whether it goes left or right it will come back here and tell me whether i should hop to the left or the right. Ready . Here we go. The device is ready, photon is emitted, quantum event, the universe just split, hop left. Ready . That is the demonstration. Why is this demonstration worth anything at all . Physical comedy, yes. In my favorite version of Quantum Mechanics, the many worlds version of Quantum Mechanics, it predicts, i believe it is true, there is another reality where i hop over there and him standing right there. There is a different version of me that did something different. Like i said, i believe this is true. What you should be saying is what kind of crazy person would believe this is true . That is what we are here to figure out. Let me see if i can do this, technology is a good thing. The reason we talk about these things is Quantum Mechanics is enormously successful, we needed to understand lasers and transistors, particle physics like sunshine and even why chairs and tables dont collapse, why matter is stable itself but as my illustrious predecessor said i think i can safely say no one understands Quantum Mechanics. I usually dont like to appeal to authority by bringing up other physicists but the claim i am making is that nobody understands something, the person who understood things the best is a good authority, that nobody else understands it anyway. How in the world can it be possible that even though we use Quantum Mechanics to make these protections to enormous accuracy Richard Friedman can say we dont understand it . It is because we can make predictions but if us professional physicists what is going on, what is the reality that meets that prediction . They will say i dont know. They will say i dont want to know. Who cares about reality . Im a physicist, i want to make predictions. Isnt that a terrible attitude . My fellow physicists should be embarrassed with themselves. It wasnt always like this. No one would have said this 100 years ago. Only since Quantum Mechanics came on the scene that people attended to think the only reason we do physics is to make predictions, not to understand how nature works. Given that we dont understand Quantum Mechanics, you would imagine Quantum Mechanics is so important, the path to understanding Quantum Mechanics would be the most important thing in all of physics. The people who specialize this are superstars of physics. In the room tonight will, we have several of the worlds leaders in understanding the foundations of Quantum Mechanics, these people should be millionaires living in penthouse apartments. They are not. They are great people and they are underpaid and undervalued. It is not only that we dont understand Quantum Mechanics, it is that we havent even tried. It is a complicated set of reasons why but that is a holder can talk. Maybe you will get that some other time but i want to highlight the degree of embarrassment physicists should have. It reminds me of the fable of the fox and the grapes, fox sees some grapes and hops over and over trying to get the grapes that they are too high and the fox cant get them so the foxes i didnt want those grapes anyway, they are probably sour. In case the parable is not perfectly clear the fox represents physicists, the grapes represent understanding Quantum Mechanics. We use to try, we couldnt do it so we said we never really wanted to understand Quantum Mechanics anyway. Until tonight. Starting tonight everyone is going to understand Quantum Mechanics. Im going to do oral wind tour of why we came up with Quantum Mechanics in the first place and why it led us to these crazy suppositions i will try to entertain, not a historically accurate tour but history is messy and we dont have time for that so i will tell you a story that gets us there in a relatively brief period of time. 100 years ago or a little more scientists put together a picture of the atom. Everyone has seen those pictures was the cartoon picture of an atom. There is a nucleus at the center. Today we know the nucleus is made of protons and neutrons, they didnt know that. Orbiting around sorry. Orbiting around the nucleus like planets orbit the sun are these little particles called electrons. This is very successful for various reasons. It also cannot possibly be right. The reason why, before Quantum Mechanics came along we had extremely successful theory called classical mechanics, the theory given to us by isaac newton and his followers. Classical mechanics makes predictions. When you take an electron, a charged particle and send it zooming in circles or jiggling up and down it emits light. Radio or infrared or visible electromagnetic waves. All the light you are seeing in this room you are seeing comes from an electron jiggling up and down so here you have electrons zooming around being accelerated. They should radiate, they should lose energy and what they should do according to classical mechanics is spiral into the center and collapse into the nucleus. The timescale for this to happen, maybe it takes a long time, less then a picosecond, a short period of time. You, the chair you are sitting on, everything you see in the world should just collapsed. Matter should have 0 extent. It should collapse into a black hole but it apparently doesnt. Scientists had a good theory, it was conceptually completely inadequate meaning we can do better. It was hard to do better. There is a long set of steps i am missing but eventually they came up with the following radical idea. Maybe the electron is not a particle. Maybe the way to think about electrons is as waves, not particles. There was previously a long debate about whether weight is life particles or waves, maybe it is wavelike in certain ways. Calls for ways to represent the electron, the wave function, one of the least romantic names for one of the most import concept in physics but that is what we call it. There is a wave function that spreads out around the nucleus, it fades away and you can do the math problems saying what are the different ways that an electron wave could settle down into an atom . Even though the wave is all energetic and so forth, waving back and forth, there are certain discrete specific shapes the wave can have where it has specific energy, it can sit there and not change. The electron settle into this. This is data, the only time in the talk im showing you any data. This is a picture of science involved in getting there. Pictures of the electron around the helium atom, it is not in some elliptical orbit. It is spread out along the wave function so thats why electrons dont dk. They are not moving. It is not a little point going in a circle. There is this cloud, this wave and it is unchanging. That is why it doesnt radiate. That is pretty good, that is progress. More progress a couple years later from erwin schrodinger who gave us any equation. I need to show you details of the equation because of a quiz we are going to have at the end, youre going to have to solve these boundary conditions. I hope everyone is happy with that but conceptually it is not that hard. The greek letters i represents the wave function of the electron and on the left you have the question that is asked of the wave function, how much energy is in you. On the right you have an answer which says how fast are you moving . Typical wave function will have various different parts that have different energies and schrodingers equation says highEnergy Particles change reveille, low Energy Particles change slowly and that is the whole story. What matters is there is any equation. Scientist love equations, physicists love equations. Any equation is cool employment for scientists and students solving the equation. The equation says now that i know about a particular circumstance i can figure out what the wave function will do in any circumstance so schrodingers equation, gave scientists the ability to say how wave functions evolve with time. Everything was looking good except one obvious dramatic flaw, when you look at electrons, if youre not hightech, this is a recent picture, a reconstruction based on delicate stuff. Usually when you look at electrons they dont look like waves. They look like particles. This is actually a little chunk of uranium that is radioactive so it is emitting particles and this is in a Cloud Chamber which means when a charged particle dips through it, it leaves a string of bubbles behind and every one of those lines you see being created as a result of a charged particle being emitted by the radioactive uranium. I showed you most of it is cool. What i want you to learn from this picture is the prediction of the schrodinger equation is when the uranium atom emits some radioactivity it is emitted in a spherical shell. It goes in all directions equally. That is not what you see when you look at it. What you see is a particle like thing moving. That is why we call them particles in the first place. Schrodinger had the hope that when he started equation what he would find was the wave bunches up and looks like a particle. Then people did actually solve the equation, thats not what happens. It spreads all over the place. There is a dramatic mismatch between the formalism that says the wave function spreads out and the observation which says it localizes and looks particle like. So the following terrible no good idea is wave functions can be spread out but when you look at them they collapse. They are shy. I am you think im kidding you. This is what we teach our students. This is what i got taught. There is the wave function all by itself which has a particularly nice form and on the right it says when you measure, where is the electron, want to take a picture of it with my electron camera, you dont see zack cloud. You see it located in some particular location. Why is that . The word went out that there is something called the measurement process. The measurement process involves collapse of wave function. This was figured out, what people told themselves in 2628. This came to be codified in the copenhagen interpretation of Quantum Mechanics and this is what we teach our students today. If you open a textbook in Quantum Mechanics you will find that if you ask what do wave functions do . There are two sets of rules. Wave functions do one kind of thing when you are not looking at them, they obey the schrodinger equation. They do another kind of thing when someone looks at them. They collapse into a particular value and the probability of getting any particular outcome is what the wave function was squared. When you have something spread out like that on the left it means the probability of measuring the electron is relatively large and the probability out there is relatively small. This is why people tells because no one in the right mind thinks that this is the ultimate way of reality, okay . Highlight two of the problems with this cookbook we are given. One is what we might call the oncology problem. Philosophers use that were to talk about what is real, the study of being. What is the world, not just what you observe but what does it mean when youre not observing it. We noticed we split around on the question when we are being introduced to the copenhagen interpretation. There is a wave function but is it what reality is . Is the one to one representation, the complete story of reality . Or only part of reality, maybe a wave function of the things, particles as well as ways. Maybe it has nothing to do with reality, maybe its just a black box that we can use to make predictions about experiment outcomes. There are people believe all these things. We do not agree about what reality is. Slightly more publicized is what we call the measurement problem which is more obvious. What do you mean, look at . Looking at things changes in, like what does it mean . Do i have to look at it . Could a cat look at it . What about something nonliving like a photograph or video camera . What if i just glanced at it . Does it imply somehow my agency and consciousness, my disembodied mind has something to do with the fundamental recipe of reality . Again there are people who believe all of these things. I dont want to discount them but at least we dont agree on the answer so that is a problem. What are we going to do . I have ideas. The talk is not over yet. There is a very wellestablished tradition and Quantum Mechanics popularization of highlighting how mysterious and weird and spooky thing is and never telling you how to get right. Im going to take at least my papers to get it right. Hugh everett who is a graduate student in 1957 basically acted as a quantum therapist. What he said was look, you have been working too hard. Chill out a little bit, okay . You had some goodlooking rules of what is going on. There are wave functions clearly, they obey the equation a lot of the time, clearly. The simplest dumbest most reporting is to say the wave function is everything. What it does is obey the equation all the tim