Mindscape Ask Me Anything, Sean Carroll | September 2026

Sean Carroll (0:01)

Hello, everyone, welcome to the September 2026 Ask Me Anything edition of the Mindscape Podcast. I'm your host, Sean Carroll. And we should start today's AMA with corrections. I should do like a corrections column, just like the New York Times has to do or something like that, right? Usually I try not to have many mistakes that need correcting, but had a couple of whoppers over the past month. I'm not gonna blame Advancing Age, but perhaps the onset of the new semester. We're now teaching again, it's the new semester starting, has distracted me from getting things correct. So one obvious blunder was in the episode with Vitor Cardoso. I introduced him and put his institute, the Center of Gravity, in Copenhagen, which I then mentioned was in the Netherlands. Which is not true, that is not where Copenhagen is. Copenhagen is in Denmark. Amsterdam is in the Netherlands. I've been to both cities, they're both very nice, but they're also different. They're not the same. And so that was just a little brain fart there. I did manage to catch it, but only after it went live. So I tried to replace it, and what happens is, once you publish the podcast episode, it gets distributed and put into people's caches and memory, and it takes a long time to update it. So I could update the YouTube video right away. I say video, but of course, it's just audio.

But it takes a long time for the actual audio regular podcast to promulgate to all the right places. So, sorry about that. To Vitor, to the people of Copenhagen, and the people of the Netherlands and Denmark. Sorry about that. The other one is more interesting, and a more subtle mistake on my part, but still a pretty egregious blunder. I'm supposed to know about these things. It was based on the AMA question from Michael Bright last month, and people did catch this also. I will mention Ashley Driver and Jeffrey Siegel, both of whom asked AMA questions pointing out my goof. They're too polite to just say you goofed, but I did. So, here is the setup, but I'll just try to explain it and get it right.

You know, you may have heard of the double slit experiment in quantum mechanics. It says if I take a source of electrons, and I fire the electrons at a barrier with two openings, two vertical slits as openings, and I have a detector screen on the other side, what will I see? If I fire the electrons one at a time, according to quantum mechanics, the electrons are described by wave functions, as long as we're not observing them, and therefore, if the electron goes through both slits on the other side, it will emerge from both slits as a wave, as two different waves that can then combine and either reinforce each other or cancel each other out. And therefore, what we see at the detector, for every electron, it's just a dot, it's just where you detected the electron signal, but if you accumulate many, many dots by doing this over and over again, you see an interference pattern. You see bands where there's lots of electron dots, bands where there's almost none, and that's interpreted as saying that there was a wave going through the two slits. This is evidence of the wave-like behavior of quantum matter. And then the spin is, or the twist, I should say, if you try to look for which slit the electron went through, the left one or the right one, that means you've made a detection, you've made a measurement, and the wave function collapses in the usual way of talking about it. And once that happens, the electron only goes through one slit, it goes through the one you saw it go through. And therefore, coming out the other side, it is a wave once again, you're done detecting it, because you didn't stop it, you just sort of noticed which slit it went through. But there's only going through one slit, so there's nothing to interfere with, there's now only one source. And what you see is a pattern on the detector screen, which is just a single slit pattern, a single blob, if you want, no interference bands, no oscillating, yes-no sort of bands of electron dots. And so that's one very famous thought experiment in quantum mechanics, which was the first thought experiment. Now eventually it's been done. The other famous thought experiment that Michael's question combined with it is the EPR thought experiment. Einstein, Podolsky and Rosen, they say construct two particles that are entangled with each other, let them go very, very far away, and measure one, and instantly the state of the other one is collapsed to whatever it needs to be to be compatible with the entanglement. You can't send signals using entangled particles because the person who would detect the second particle doesn't know that the first one has been detected and observed to be in a certain state. And therefore, as far as they're concerned, the probability of getting whatever measurement outcome they might get is just whatever it was before. So no signal has been sent. So Michael says, let's combine both of these. Let's imagine an EPR-like experiment where you have two particles that are entangled and moving in opposite directions. One goes toward Alice, one goes toward Bob. But rather than just detecting them, Alice and Bob have a double slit experiment. Each of them do, right? And they would both go through and maybe make interference patterns. And the question was, if Alice, at her double slit experiment, starts detecting which slit it goes through, then does the wave function for Bob collapse and now you know which slit it's going to go through, and therefore the interference pattern disappears. And my mistake was, well, so what I said was, the interference pattern will disappear, and I gave a perfectly sensible argument for why that's true, because it will, but the problem is, it was never there. That's what I got wrong. There never was an interference pattern in that case. And the reason why is because the particles would have already entangled with the rest of the world by the setup, right? When you do the original version of the double slit experiment, and the particle just goes through two slits, and you quote unquote detect which it goes through, what that means is that you've entangled that particle with the rest of the world, in this case, your detector for the location of the electron. As long as that entanglement is there, the electron is no longer in a pure state all by itself, it's connected to the rest of the world, and that's what destroys the interference pattern on the other side. Here, I was so focused on what happens if you start measuring, what happens when the particle goes through the slit, that it didn't stop to think, well, what if you don't measure? What if you just let it go through? Here, the particle's already entangled with the rest of the world by the setup of the question, and therefore, what it emerges on the other side of the slits is not in a pure state that can interfere, okay? So, you already don't get an interference pattern. You've basically pre-detected, pre-measured, even if you don't know the outcome of the measurement, by putting the electron or the particle moving into an entangled state, you have implicitly measured which one, which slit it goes through, and therefore, you've destroyed the interference pattern. And that's good news. This was pointed out to me by an email from a physicist friend, Guy Franzman, but also the AMA questioners this month, Ashley and Jeffrey. So it's good news because if you could do this, if you could actually have an interference pattern and turn it on and off, just by detecting particles at one of the detectors, and therefore have the interference pattern go on and off, appear and disappear on the other one, you could send signals faster than the speed of light. And that is bad. And there are theorems that say you can't do that. So maybe you should always be careful about these things, right? Like maybe your theorems are not right, or maybe your theorems are assuming something that is not applicable to this situation. In this case, the theorems are fine, and the implication of the theorems are completely correct. You should not be able to send signals faster than the speed of light, and therefore this wouldn't work. So I apologize to Michael for bungling his question. The answer is, you're never gonna see any interference patterns either way, whether or not you personally start looking for which lit the electron goes through in that particular setup. All right, well, here we are, a whole new month of AMA questions. More opportunities for me to make mistakes and bungle, but I do promise that if the mistake is like straightforwardly factual, I will try to let you know about it and try to fix it. If it's a mistake of judgment, or I like a different TV show than you do, then I'm not going to fix that. You just have to live with those kind of more subjective kinds of disagreements. That's okay. Thanks as always to the Patreon supporters of Mindscape. They're the ones who make this AMA possible, and they're the ones who ask the questions. You can be a Patreon supporter of Mindscape. Just go to patreon.com/seanmcarroll for a few bucks a month. I still need to switch the payment schedule. Sorry about that. It's going to happen soon. Big disruption. Sorry about that. It won't really be a disruption for anybody. And then the benefits of being a Patreon supporter are that you get to ask the questions for the AMAs, and also you get ad-free versions of the podcast, as well as just the feeling that you're supporting something that you like and giving some money back. So many, many thanks to everyone who does that and has been doing that for years now. It absolutely keeps me going. So with that, let us all go.