Tim Maudlin: Quantum Nonlocality Explained FROM SCRATCH

**Tim Maudlin** (0:00)
Most of what you hear about quantum mechanics is wrong. It's subjectivity, the thing about observation, even the indeterminism. None of that is forced on you. All this talk about how in quantum mechanics, the observer can't be left out, you've heard it a million times.
No, this is so outrageous.

**Curt Jaimungal** (0:18)
This is Tim Maudlin, Professor of Philosophy at New York University, founder and director of the John Bell Institute, and one of the world's leading philosophers of physics. Today, I'm thrilled to bring you a lecture explaining Bell's theorem and non-locality in quantum physics with zero background knowledge required.

**Tim Maudlin** (0:35)
The proof of non-locality is the most astonishing proof of any physical fact in the history of mankind. Facts are facts and proofs are proofs.

**Curt Jaimungal** (0:44)
On this channel, I, Curt Jaimungal, interview researchers regarding their theories of reality with rigor and technical depth. Maudlin is an expert elucidator and you're in for a treat.

**Tim Maudlin** (0:54)
Bell uses Einstein's own tools to prove that Einstein was wrong. Locality is dead.

**Curt Jaimungal** (1:01)
All right. This is the continue of the previous lecture, which went obscenely viral for some reason on quantum mechanics. How do you feel about that?

**Tim Maudlin** (1:09)
Well, I'm happy. I mean, I've been trying to explain this stuff for my entire career.
So quite honestly, to be able to get it presented to so many people at once is very exciting for me. It's kind of a capstone to all of this. Hopefully, it'll make a little difference.

**Curt Jaimungal** (1:29)
Many of the comments, and I'll place them on screen, they say this was one of the best explanations of quantum mechanics ever from someone who doesn't know much quantum mechanics, but they've heard quantum mechanics explained 20 times. There were many people who recently came out of graduating from physics. So they have their bachelor's, some have their graduate degree, and they said this was the clearest explanation of EPR.
That was just part one. Now we're on to part two. Thank you so much, Professor, for giving me such a treat and the audience such a treat.

**Tim Maudlin** (1:57)
Well, thanks for giving me the chance to do it. My apologies for it going so long, but that's what it takes. I'm going to begin.
If people didn't watch the first one, they might want to watch it. If you did, here's just a really fast summary of the main points of what was proven in the EPR paper. That was more or less where we got to by the end of last time. All of this is things I've said before, including slides that were before, but just so they're fresh in mind.
The EPR paper asks the question whether quantum mechanical description of reality, meaning the wave function is complete. And so these are now just direct quotes from the paper. It's a very short paper and a very clear paper.
And they tell you what they're after. So I'm just going to read it again. In attempting to judge the success of a physical theory, we may ask ourselves two questions. First, is the theory correct? And second, is the description given by the theory complete? It's only in the case in which positive answers may be given to both of those questions, that the concepts of the theory may be said to be satisfactory. The correctness of the theory is judged by the degree of agreement between the conclusions of the theory and human experience. So again, this is what we would call empirical adequacy. Is this theory making good predictions as far as we can tell? This experience, which alone enables us to make inferences about reality, in physics takes the form of experiment and measurement. It is the second question, the completeness question that we wish to consider here, as applied to quantum mechanics. So they are no where questioning the accuracy of the quantum mechanical predictions. That wasn't the issue.
But then, what do you mean by a complete theory?
Whatever the meaning assigned to the term complete, the following requirement for a complete theory seems to be a necessary one.
Every element of the physical reality must have a counterpart in the physical theory. So think of the physical theory as the mathematical apparatus together with all the commentary and so on that you're given when you're given the mathematical apparatus so that it becomes an attempt to represent physical reality. We should call this the condition of completeness. The second question is thus easily answered as soon as we're able to decide what are the elements of the physical reality. If we know something's real, we can then ask, is that represented in this formalism? What they're going to give us is a criterion for the existence of an element of physical reality. And again, you have to remember, a criterion is not a definition. It's not something that will cover every case. It is just a sufficient condition. If the criterion is met, then they say there must be an element of physical reality. Why? Because of course, how do we get our hands on the elements of physical reality? So the elements of physical reality cannot be determined by a priori philosophical considerations, but must be found by an appeal to the results of experiments and measurements. A comprehensive definition of reality is however, unnecessary for our purpose. We shall be satisfied with the following criterion, which we regard as reasonable, and this is the main point. If, without in any way disturbing a system, we can predict with certainty, that is with probability equal to unity, the value of a physical quantity, by which they really mean the outcome of an experiment, the outcome of a measurement or something like that, then there exists an element of physical reality corresponding to this physical quantity. If I can predict the outcome of a momentum measurement on a system, predict it with certainty completely accurately, then there is some element of physical reality in that system that corresponds to that. It seems to us that this criterion, how far from exhausting all possible ways of recognizing a physical reality, at least provides us with one such way whenever the condition set down and it occur. Notice they're very stringent conditions. You have to be able to predict with certainty the outcome of your measurement without in any way disturbing the system. Regarded not as a necessary, but merely as a sufficient condition of reality, this criterion is in agreement with the classical as well as the quantum mechanical ideas of reality. Now, that criterion you have to pay attention to because part of the criterion is that you're able to make this prediction without in any way disturbing the system that you're predicting the outcome of.