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What is time? >> Was it St. Augustine who said, "Uh, I can tell you what time is if you don't ask, but once you ask me, I can't answer." Um, mathematically, time in both classical physics and quantum physics is a parameter. Things exist at all times. And I can ask what the value of any quantity, observable, whatever, is at any given time. Slight complexity due to spacetime and relativity, but it doesn't change the the fundamental issue there.
Time of course is different from space for us in that we inexorably travel in one direction in time and we don't know why. We might look for a new law of physics that explains why that is. We might look for boundary conditions that say it could have been different, but we happen to live in a universe where time is moving in this direction. Um, we can also look quantum mechanically for something even stranger. A situation where the universe is not evolving in time. Nothing is changing in time. And there are good reasons to think that that would be a good quantum mechanical description of the universe, leaving out for the time being the Big Bang, which we know breaks this symmetry. And there are approaches that can yield the sort of illusion of time in the sense that everything is uncertain, but everything is correlated. That part of my wave function is sitting here now talking to you, and part of my wave function is still back in bed this morning. And that's all right, because if the part of my wave function that's back in bed this morning suddenly woke up and decided to check where you were and called you on your cell phone, it would discover that you were also back in bed. And the part of me now reaches out to you, and any one part of me that's conscious sees things that are consistent with that particular time. There are different formalisms to describe how this might arise, but I'd say it remains one of the deep mysteries of physics.
So you mentioned Bill that asked questions. Great. >> Bill Phillips. Yeah. >> Yes. What is a question someone asked you that you thought you knew the answer, then as you started explaining it, you realized only then that I actually don't know the answer? Well, this happens all the time on all sorts of little technical issues. You know, things that I've taught to graduate courses for decades, and no one asked me a hard enough question to make me stop. The experiment I was telling you about about, you know, how long atoms spend in photons, that would have been one. Atoms spend in photons, photons spend in atoms. I probably would have answered that one incorrectly. And there are many. I mean, I think to be doing interesting science, you should always be at the level of asking yourself questions about things you thought you knew, but questions that go beyond what you've asked before, that lead you to have to think a little bit harder. Um, but I'd say the most exciting one is about what we really learned from Bell's inequalities. And as I said, you know, that's fundamentally the kind of experimentally testable foundations question that drove me to stay in physics. Um, the way we talk about Bell's inequalities has changed over time, and I would usually tell students what we learn from Bell's inequalities is that, or from the violation, the experimental violation of Bell's inequalities is that the world we live in is not local. That you cannot describe what happens to you and what happens to me independently and still have a complete description of reality. And that's pretty shocking. And as I say, you know, it seems to be experimentally borne out. I was reminded recently that a lot of brilliant people disagree with this. And uh, J. Prasar, who was the recent winner of the Turing Medal along with Charlie Bennett, was recently giving us a talk in which he explained that he thinks the rest of us are all wrong, and that in fact, he can prove that the world is local. And while Bell's inequalities have been violated, they do not disprove what he would describe as Einstein's notion of locality. They only disprove this much more stringent notion of locality that Bell introduced. And this goes back to a a brilliant paper by David Deutsch, whom I mentioned before, and Patrick Hayden, whom I don't think I've mentioned before, that that makes this argument that you can understand EPR, Einstein-Podolsky-Rosen correlations and violations of Bell inequalities without needing to resort to thinking about information ever traveling faster than light. And mathematically, it's a very beautiful paper. Interpretationally, every few years I have to go back and try again to see what are, are they really saying about the nature of reality? And I don't know. I don't know if when I teach students about Bell's inequalities, I'm giving them the right picture or not. So there's one thing that I think many of us need to go back and keep thinking about more deeply.
>> Has your work shown that quantum mechanical objects have definite values of position prior to being measured? >> No. I mean, it seems like it because of the weak measurement. And the weak but >> the weak measurement reveals an average. So we've shown that there is a sense in which you can track average positions, look at their correlations, that gives a picture that agrees with the definite positions Bomb would say are real. But Bomb was designed mathematically to agree with quantum mechanics. Our measurement is fundamentally measuring this flux operator that exists in quantum mechanics. The fact that those quantities are equal does not prove that you should think of this one as real or that one as real. And in particular, since we know that we're averaging over a distribution, you could imagine many different theories that have different trajectories that are certain or uncertain, but agree with that average. So, no, I certainly don't think we have an experiment that uh proves that anything is real. I think what we're trying to do is keep reminding people that the jury is out, and there are seemingly conflicting views of reality that are all consistent right now with the formalism that we have. But also that there are measurable quantities that come out of some of these formalisms, or out of weak measurement theory, that seem to be so universal that we're tempted to say they're at least pointing us towards the deeper theory of what's really out there. If there is some deeper theory, maybe it's something that reflects some of these quantities, and that's what we should be looking for. But those are hints. Those aren't rigorous proofs of anything.
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