Part 7! What is the many-worlds interpretation of quantum mechanics? How does decoherence play a critical role? What are the strengths and weaknesses of this idea? I discuss these questions and more in today’s Ask a Spaceman!

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EPISODE TRANSCRIPTION (AUTO-GENERATED)

Before I dig into today's topic, I wanted to mention that this episode is brought to you by my friends At Chirp. Chirp is an audio book retailer known for awesome deals without any commitments or subscription, and I have an audio book club with them. I know you're always asking me for book recommendations. Well, here it is, folks. At the start of every month, I'll announce the pick here, and Chirp will deeply discount the audio book for a limited time. We'll listen to the audio book together, and at the end of the month, you'll have a chance to share your thoughts and see what other club members thought to my next pick is Chasing the Sun by Linda Geddes. I've often talked about our fundamental connection to this guy. You know, that's like a thing of mine. That connection can take on more than cultural and symbolic meanings. That connection can be very literal, especially when it comes to the sun. My pick this month Chasing the Sun explores our relationship with our nearest star and how the regular rhythms of sunlight impact our lives in so many surprising ways to join go to chirp books dot com slash spaceman and grab my next pick chasing this on on sale for over 75% off for a limited time.

I wasn't kidding about the great deals. And be sure to press follow to join my club, to stay in the loop on future picks and other exclusive content from me. That's chirp. Books dot com slash spaceman. I have a game for you and let me know if you'd like to play it. So here's the setup. I'm gonna concoct some sort of quantum dice roller. You know, maybe it's a radioactive decay of an element. Maybe it's the the spin position of of an electron. When you look at it, you know, whatever. It's some sort of quantum element that is gonna give a 50 50 chance. Perfect odds. 50% chance one direction, 50% chance the other. If this random quantum process goes one way, I will give you a billion dollars. That's right. Not a million, not 100 million. We're going straight for the big B. Carl Sagan would be proud.

Billion dollars and I have it. You can check it. I've got suitcases stuffed with $100 bills I There's a lot of money, and you can see it. So you know I'm not lying. I'm not fitting. You're gonna get a billion dollars if this quantum process goes in one direction. If it goes in the opposite direction, you're gonna die. It's wired up to grenades sitting at your I don't know it. It doesn't matter. Poison gas. Who cares? It doesn't matter. 50 50 chance. Quantum process. If you win, it goes in one direction. You walk away with a billion dollars. If you lose, you die. Would you take that bet? I'm guessing you wouldn't. Unless you're like, weird. If you're not weird, you're not gonna take a bat. Why? Because that possibility seems real to you. The possibility of your death that 50 50 shot that you will be well shot is real to you.

That quantum mechanical process, the indeterminacy, the nondeterministic, the the fact that it's random and probabilistic that 50 50 it seems real. And we've been exploring in the past few episodes. What does quantum mechanics teach us about reality? What is actually going on down there? Einstein argued that God does not play dice. He argued that the universe should follow a deterministic path and that we shouldn't have to take a bet like this, that the appearance of this random possibility this 50 50 chance was just an illusion that we were missing something deeper in quantum mechanics. Now, in his arguments against quantum mechanics, he lost a lot of battles. But he ultimately won the war. He pointed out that quantum mechanics, in order to survive in order to be consistent with itself, had to be nonlocal. Things across the universe could influence other things across the universe. He despised this.

He thought it was a sign that quantum mechanics was incomplete and that if we had a fuller, more complete theory of the subatomic world, then non locality would go away then. Then Nondeterminism would go away that all these probabilities and uncertainties would just vanish and be replaced with a more fundamental theory. You wouldn't have to take the bet. You would know what that little subatomic particle was gonna do. It wasn't gonna be random anymore. In the last episode in the series where we are currently in episode number seven, we haven't been this deep in something since string theory. We saw how Einstein died believing that quantum mechanics was incomplete and that we were missing a deeper understanding of nature, he railed against the Orthodox quantum mechanics interpretation what we call the Copenhagen interpretation. It is the standard interpretation, even though it's not really standard. And nobody at the time really agreed on what makes up the Copenhagen interpretation. And nobody today really 100% agrees on what makes up the Copenhagen interpretation.

But it's something like this because, uh, what I'm about to tell you basically just translates the math into words without adding a layer of, you know, understanding. On top of it, it's just English versions of the math equations, one step above the postulates themselves. There's in determinism. You don't know what you're gonna get until you actually measure it. There's the correspondence principle. We live in a classical world. Subatomic particles don't Somehow they meet up, and when we perform a measurement that is a classical operation that is a classical process that gives us a window into the quantum world, we have the born rule, which tells us that the wave function tells us probabilities of where we might see a subatomic particle the next time we go looking for it. When you take a measurement, the wave function collapses onto one of those many possible states, and you get a result. Like I said, Einstein despised this interpretation because it led to the idea of entanglement. Leave aside the question of measurement for now, when two particles out there in the universe interact.

Everything we know about quantum mechanics in the Copenhagen interpretation says that their wave functions overlap and combine, and we can no longer describe the particles as separate local entities anymore, no matter how far away they are. Einstein said this was crazy talk and worked to develop what he called a hidden variables theory, where particles carry around more information than what we have access to in this secret, hidden information tells them how to behave in a very deterministic way. They have a secret code with instructions from headquarters that tells them how to move where and it's all fully deterministic. And if we had access to that knowledge, then all the probabilities, all the stress of this bet would go away, because if you could read the secret envelope inside of that subatomic particle, you would know when which direction it's gonna go. And so you wouldn't be taking the bet you'd be like No, no, no, it's definitely giving me a billion dollars. So I'm gonna go through with this or you say No, no, no. I read the secret envelope. I translated the code.

I know they hidden information that the particle contains and it's definitely gonna kill me, so I'll pass. Einstein spent decades trying to figure out a way out of quantum mechanics, and he couldn't. He was unsuccessful. If only he had patreon patreon dot com slash PM Sutter would have helped Einstein complete his quest for a unified theory of physics that was more fundamental than quantum mechanics. Alas, he did not. Instead, you'll have to go for the next best thing and go to patreon dot com slash PM Sutter to keep this show going as we move into the post World War two era, naturally, development of quantum mechanics took a little pause. Little hiatus here during World War Two, after World War Two, the first generation of pioneers of quantum fury fade from the scene. Einstein, Heisenberg bore, they retire, they they die. And there's a new generation of physicists working with this quantum theory that they have inherited and what they inherit is a quantum theory that seems off.

Einstein's objections still stand. Quantum mechanics appears to be incomplete. Quantum mechanics contain some things that we do not understand, like non locality contains some things that we cannot describe like quantum jumps. But despite that, the majority of physicists continue along the lines lines of Heisenberg and bore in their interpretation. Not all agreeing 100% but continuing in the same tradition, saying, You know what? It doesn't matter. We don't know what happens to subatomic particles. We don't know how quantum jumps actually work. We don't know how non locality entanglement actually works. We can't describe how it works. We just know that it does. That is the essence of the Copenhagen interpretation. But after the dust settles, questions still remain. People get to work, you know, they they they start solving problems. They develop all sorts of cool, uh, quantum mechanics stuff and and they keep advancing the theory. But the voice of Einstein coming from the grave still haunts the minds of the next generation, even as they find success after success in developing quantum field theory, the discovery of strong and weak nuclear forces, antimatter, the standard model, all of it.

They continue advancing quantum theory, even though these questions that the first generation of quantum physicists raised still stand and those questions still linger today, Is quantum mechanics complete? How are we supposed to understand the weirdness of what we're seeing in the subatomic world? How does entanglement work? How does wave function collapse? How does what actually happens? What is a quantum jump? Exactly? Why is quantum physics so different? And how does quantum mechanics how does that weirdness translate into the non weirdness of everyday experience? How do we go from the quantum world to the classical world? Is this it is entanglement real? Are quantum jumps even real? Are we missing something? And now, for a new question, we've seen Schrodinger and Heisenberg square off over the nature of the wave function and the reality of the wave function. Ultimately, the interpretation that it is a measure of probabilities helps way over Schrodinger's interpretation that the wave function meant that objects were physically extended in space.

We saw Einstein's question as to the nature of no no non locality saying, Wait, are you seriously saying quantum mechanics is non local because any rational theory of physics should be 100% local. To which the response was, Yeah, it's non local, and we don't know how it works. Here's a new one. Measurement measurement plays such a fundamental role in quantum mechanics, especially in the Copenhagen interpretation. Measurement is what collapses. The wave function measurement is what goes from this fuzzy cloud of probabilities to an actual result. Measurement is critical. Measurement is what we do. Measurement takes the quantum world and and gives us a classical result. So what exactly is a measurement and how is it different than any other interaction? This show is sponsored by better help. One of the most awesome things about physics is that it's like a user manual for the universe.

You can literally use it to predict the future Now. Now, humans are a little bit more complicated. Believe it or not, people are more complicated than quantum physics. I am not joking, and and life and dealing with people does not come with a user manual, and the next best thing is therapy. I have been using therapy for years It's such a powerful tool for me to to answer life's questions when those questions don't come in the form of of of physics problems. And I think you will benefit a lot from it, too. And that's why I'm proud to have better help as a sponsor. Better help is the world's largest therapy service. Better Help has matched 3 million people with professionally licensed and vetted therapists available 100% online. Plus it's affordable. Just fill out a brief questionnaire to match with the therapist. If things aren't clicking, you can easily switch to a new therapist any time. It couldn't be simpler. No waiting rooms, no traffic, no endless searching for the right therapist to learn more and save 10% off your first month at better.

Help dot com slash spaceman. That's better. Help HE LP dot com slash spaceman. If we take the Copenhagen interpretation at face value, then we're left with a little bit of a puzzle. Quantum objects do two things. They can slowly evolve in time according to the Schroedinger equation, and they can spontaneously jump to a certain state once we measure it. Wait, wait, wait, wait, wait, wait. So if I take a matter wave. OK, fine. It's not a physically extended object. Fine. It's a It's a cloud of probabilities. Fine, whatever. I know how that wave evolves in time. The short equation tells me how that wave of probabilities uh uh, shifts with time, Uh, depending on on the particular situation. Just like if I start a ball rolling down the hill uh, the the the force of gravity and the equations of motions of gravity Tell me how that ball will evolve with time. Schroedinger equation tells me how this wave of probabilities will evolve with time, and it's smooth.

It's continuous. It's just coasting along, doing its thing, adjusting, you know, whatever. And then all of a sudden I do a measurement and it vanishes. It disappears. Quantum objects slowly evolve According to the Schroedinger equation, when we're not looking, then when we start looking, they jump to a certain state. What the heck? We call this, uh, like, just This is the collapse of the wave function. And then we move on and like, you're like, Wait, wait, wait, wait, wait. Step back Here. Collapse of the wave function. What in the world are you talking about? Why does this work? How does it work? Measurement takes this primary importance that seems weird and special. Why should physical systems care or notice that they're being measured and act completely differently? If there's a little electron in a box, it's floating around. No one's looking at it. I'm not looking at it. It's wave function. It's cloud of probabilities is evolving, according to the Schroedinger equation, just fine. Then I open the box and I look at it and then all of a sudden I measure it.

And as one particular state, the wave function went away. Where did it go? Where did the wave function go? Is there like any dust or smoke? Maybe a little sparkles left behind. Where does it go? And like OK, what if, instead of measurement, I just have a proton in space and it bumps into another proton in space all alone? No one's looking. Everyone turn around. Let these protons bump into each other. A purely quantum interaction, right? A purely quantum interaction leads to entanglement. We all agree on this. This is how how we get entanglement. When two quantum particles interact, their weight functions overlap now they're entangled. OK, but what if I take that same Proton or those same two protons, grab them and put them in a accelerator and slam them into each other in my laboratory? Now it's a measurement where entanglement has gone away, and instead I'm just left with these two particles in their positions. Why is that different? Literally Name one other physical system that does that. Go ahead.

I'll wait. That's right. You can't. No other physical system has this split personality where when we're not looking, it's obeying the time evolution of of a math equation. And then when we are looking, it's completely different. What? It gets worse. Presumably, everything is a quantum system, right? The atoms inside our experiment that we're studying are quantum systems. If I put an electron in a box and I'm gonna poke at it, obviously that electron is a quantum system. But the box itself is made of atoms, which are quantum systems, the microscope that I'm using to look at everything the laser that I'm using to to flash light on it. Those are quantum objects. The test tube is a quantum object. My hand is a quantum object that the floor I'm standing on is a quantum object. The the cells in my brain are quantum objects. We're all just particles interacting with other particles. So what exactly is a measurement? How is that special?

How is that different? Like if I if I want to probe the nature of an electron I, I bounce some light off of it. You know, I, I shine a photon off of it. Well, if that happens in deep space, all of a sudden it's a It's a quantum entanglement, bro. But then, if I do it in my laboratory, it's a measurement. But the photon itself is is a quantum object. The atoms inside of the laser that shoot the photon are quantum objects. The cable running out the back of the laser is a quantum object. My box that has a little thingy on it, a little dial that wiggles back and forth and I don't know, it's a measurement, I guess, Uh, those were all quantum objects. How does an electron know to collapse its wave function If it's all just quantum objects interacting with quantum objects? If you say no. No, no, no, no, no, no, no. No. Paul. Paul, Calm down. You and me. We're classical objects, not quantum objects. And all this wave function collapse happens when the classical world interacts with the quantum one, right?

Yeah. A photon and electron. They bounce off of each other in deep space. That's purely quantum interaction. But I me, you know me, I'm classical. We're cool, right? I'm a classical system interacting with a quantum object. And so that's what triggers the collapse of the wave function. OK, Neils Bo was a huge fan of this argument that has a name. It's called the Heisenberg Cut. Where is the line between the quantum and classical worlds? Where does the quantum world end in the classical world start What happens at that boundary? What is the proper physical description of that boundary? Where and when does that happen? In the act of measurement, why does a boundary even exist? That boundary must be met every single time we take a measurement. But where I have an electron, I shoot some light at it. Quantum. The light came out of a test tube or reflects back and bounces onto a detector. Quantum. It sends an electrical signal. Quantum. It wiggles like it's all quantum. So where where does the classical world? Start of all I have is a chain of quantum interactions. You're not really solving the problem.

Excuse the expression. But as Schroedinger was exiting the world of quantum mechanics to work on other problems, he gave one last, uh, middle finger to the world of quantum mechanics, the theory that he helped create that he now despised. It's Schrodinger's cat. Put a cat in the box, close the lid, set up some quantum device like a radioactive decay SM element that'll get 50 50 probability. The cat is now entangled with that quantum object because no one's looking. No one's doing a measurement, and then you open the box and there's either going to be a dead cat or a live cat. But if we don't open the box, you're really saying that the cat is both dead and alive. This is what everyone says. Oh yeah, it's a quantum system. The cat is both dead and alive. Come on. How can the cat be both dead and alive? She tell me. Here I am. I've seen dead cats. Didn't expect to say that sentence in this podcast.

I see. I've seen dead cats. I know what they look like. I've seen alive cats. I know what they look like. Show me a cat that is both dead and alive. Describe it to me. Go ahead. I'm all years. Walk me through this, paint a picture. And then when I open the box, how does it know it's being observed? If all all it is is just even more quantum entanglements and quantum interactions, What's so special? How does the process actually unfold? Does this even if you could describe this like all dead and all alive simultaneously? Quantum Cat, How does it morph into a dead cat or a live cat? Again and again and again. What is so special about measurement and what's actually happening? This was Schrodinger's constant attacks on quantum mechanics. Tell me what's happening. Einstein's approach and they were very good friends and exchanged a lot of letters. And Einstein even got Schroedinger started on this whole cat in the box experiment thing. Einstein's attacks were more. Look, you're missing something. Look, you're missing something.

Look, you're missing something. Schrodinger's attacks were Tell me how it works. Tell me how it works. Tell me what's going on face to face. Look at me in the eyes and tell me how all this quantum business happens. What's happening at measurement. This is what we call the measurement problem in quantum mechanics. The Copenhagen interpretation says. Shut up and calculate the Copenhagen interpretation. The interpretation of Heisenberg and Boer. The subatomic world is weird and quit, forcing it to make sense. Here are the rules. Here's the math. Who cares how it all works? Just care that it does work. But, yeah, you can do the math. Yeah, you can solve those problems. But the measurement problem tells us that quantum mechanics is still incomplete. Einstein's objections remain. Quantum mechanics is nonlocal. Quantum mechanics doesn't have a solution for the measurement problem where quantum objects behave one way when no one's looking and then another way, when someone is come on, answer that.

The Copenhagen interpretation has no answer for that option. A dig deep double down on Copenhagen, throw our hands up in the air and say we have no clue what's going on. We just have a set of math rules that work even if they don't make intuitive sense. Option B maybe something else. Maybe this whole quantum jump thing in measurement feels weird and wrong because it is weird and wrong. Maybe the measurement problem really is a problem, and it's a sign that we're misinterpreting quantum mechanics. Like I said, the measurement problem is that quantum objects behave according to the Schroedinger equation. And then all of a sudden, the measurement occurs and the shortener equation just like steps to the side. And there's this new fact that the wave function collapses. What if we took that away? What if we just lived with the Schrodinger equation as it stands without adding anything else in what would that look like? What if we solved the measurement problem by taking away measurement?

You know, if I probe an atom with a laser, it's all quantum this and quantum that. But at some point it stops being quantum and becomes classical, which I call a measurement. But what if I didn't stop? What if I never translated into the classical world? What if every single interaction maintained some sense of the wave function? What if every single interaction led to entanglement? Because that's what happens when you have two quantum particles with their own wave functions. They interact, they entangle. They just have a single wave function that describes both of them simultaneously in the Copenhagen interpretation. At some point you cut that, you stop that and you just look at you're back to local classical physics as you as you know and love. But what if we didn't do that? What if we took Schrodinger's thoughts in his solution to their logical extreme? What if we didn't introduce this artificial thing that we call measurement? What we end up with is entanglement, a lot of it.

The atom that we probe with a photon entangles with the photon. The photon bounces off and then hits a detector. The photon entangles with the atoms in the detector. The atoms in the detector entangle themselves with the electrons carrying the electricity down the wire, which entangles themselves with with the screen readout, which entangles themselves with a new set of photons that hit my eye, which entangles with the the receptors in my eye, which entangles with all the atoms in my optic nerve and then entangles with my brain and then entangles with my feet. And then those atoms entangle with the Earth and the Earth entangles with the photons, uh, from the sun. And then those photons entangle with all the light that is propagated for the past past 4.5 billion years. And then those entangle and you end up with a universal entanglement. This is a new interpretation of quantum mechanics developed in the second generation of quantum physicists, this one by a physicist named Hugh Everett in the 19 fifties.

What if we take entanglement to its logical extreme? What if we get rid of the concept of measurement so that we don't have a measurement problem? What if we're all just one big, huge entangled wave function evolving with time? Then there's no jump from classical to quantum. There's no measurement that collapses the wave function. It's just a never ending series of entanglements that encompasses the entire universe. Biggest advantage to this line of thinking is that solves the measurement problem. There is no such thing as measurement. It's just a series of quantum objects interacting and entangling with other quantum objects without end ending up with a single universal wave function that contains all the entanglements across all of space involving every single particle in the cosmos. It's a good thing that Einstein died a couple of years before ever introduced this idea because if you're not a fan of entanglement in the first place, well, then, But in an imaginary Einstein would have immediately objected to this idea. He would say, Look, if everything is entangled to literally everything else, then how can physics proceed?

How can we trust experiments? It's like his earlier arguments, but on steroids. In response to that, Everett and extensions of this idea since then have a couple of responses. One is called Decoherence may say. In this view, entanglement never gets destroyed. We're all quantum objects entangling with other quantum objects. There's no such thing as measurement, but it just gets too complex to keep track of. Once. I involve too many atoms and too many links in the chain of all the quantum and possibilities become so large that I can't hope to calculate it. And so it just appears as classical physics, it de coheres from entangled quantum systems to just normal, messy classical systems because all the quantum fuzziness just gets lost and scattered and bounced around. So what we call classical physics is really just a bajillion quantum systems that we can't keep tabs on. Sure, when we investigate a single particle, we can easily see its quantum nature. But that gets hard with a lot more particles involved. Once the chain gets too long, we can't sort out all the entanglements, and it just looks like classical physics.

Minor problem with this argument. We don't know how this process of Decoherence actually works. Sounds cool, but we can't suss out the actual physical mechanisms that take us from quantum entanglement to classical normal physics. OK, OK, let's take the face value for now. It's not like Copenhagen can easily explain everything that goes on within it, but so OK, let's turn to the other weird and wonderful but also downright nasty thing about quantum mechanics. The probabilities it's encoded in quantum mechanics. You don't know what you're gonna get until you measure it. But if everything is just a single giant wave function with everything entangled everywhere all the time, how do we reconcile that with the fact that sometimes experiments give one result and sometimes another? How do we go from the fuzziness of possibilities before measurement to a single result. After I do perform experiments, and when I start the experiment, I don't know where the electron is gonna go or what it's gonna do.

And then after the experiment, I know. So how does this concept of never ending entanglements actually give me experimental results that have different probabilities? The answer is what we call the many worlds interpretation of quantum mechanics, where there is one single giant wave function that encompasses the entire universe and within it contains all the entanglements of every single particle interacting with every other single particle. But the possibilities and probabilities of quantum mechanics appear because we can only see one part of the wave function at a time. If I'm presented with two doors, there's a single wave function uniting those doors. But once I open one of the doors, I only get to see what's behind that door. I only get to see what's in that room. I don't get to see the other room. So before I made a choice, before I did an experiment, I was confronted with quantum probabilities.

But then I make a choice, or I perform some experiment and I open one door and then the other result is closed off to me forever. I can't access it anymore because I had to make a choice. I had to go in one particular direction. The whole wave function. The whole entanglement still exists, but we only see what one piece of it in the many worlds interpretation. This is interpreted as the universe splitting every time there's a quantum interaction to me, it's more like an unraveling of separate realities in the many worlds interpretation. All quantum possibilities exist simultaneously, except we only have access to some of them. There is a single universal wave function that encompasses every single cosmos of every single possibility. So if I run an experiment and my electron has two choices, it can be up or down. I run that experiment.

I only get up or I only get down the universe where that provided the opposite result is now closed off to me. I don't get to experience it anymore. Many worlds interpretation is easy to think about at first, just like Copenhagen, interpretation is easy to think about at first, but the more you think about it, the the harder it gets like easy mode, make a choice. I'm I'm here. Two cheeses. Uh, we've got a lovely and bear over here. And then, uh, fontina over there. Make your choice. It's a quantum choice. Remember, all choices are quantum choices because all interactions are quantum interactions in the many worlds interpretation. There's no such thing as classical physics. It's just quantum interactions. Make a choice. You make a choice, you pick fontina. There is now a U experiencing that fontina cheese, But there's also a you who made the other choice the other quantum interaction. Remember, everything is probabilities.

Everything is chance. There's now another U experiencing the came in there the universe has split into, or there's one meta universe or multiverse, and now there is a dividing line between two sections of it to reflect the two different quantum probabilities. OK, that's kind of cool to think about. Like I'm walking around my life, I'm making a bunch of choices. There's a bunch of random things that happen in my life. There's another universe out there, a parallel universe, another section of the universal wave function that I no longer have access to another cosmos that contains me, making all the other different choices in life. Uh, do I move to this city or that city? Well, now there are two universes where in one universe I exist in that city and another universe where I exist in that city. Uh, I make a choice. Uh, should I sit down or stand up? OK, there's now one universe where I stood up in another universe where I sat down.

Do I get the black car or the blue car? Now there's one universe where I got the black car and another universe where I got the blue car. Every interaction is a quantum interaction. Every interaction leads to these branching possibilities. That's kind of cool to think about that. You have these different versions of you out there inaccessible to you, but existing just as much as you do they they don't not exist. They just don't exist here. And it contains all the possibilities of reality that you could have made in your life. But if every interaction is a quantum interaction, then every time there's a quantum interaction, there's a brand new universe. I, uh I don't know. I start up my car, I drive to the grocery store every second that that engine is operating. There are trillions, quadrillions and almost uncountable number of quantum interactions happening in the engine of my car on my way to the grocery store. By the time I've reached my the grocery store, there are now a quadrillion 10 to 100.

So some huge number of copies of me all at the grocery store all the same, except at some point in the drive. One little oxygen atom went left instead of right. And then another moment in the drive. Uh uh. A little molecule of of gasoline went up instead of down. And then there's a whole other parallel universe where it went down instead of up. Everything else is exactly the same. Except now there are two universes for every single interaction. And then once one of those choices are made, there's a whole new branching set of possibilities. So every time I drive to the grocery store, I create trillions upon trillions upon trillions upon trillions of universes, they all split off from each other. Every time you go to the grocery store, you're making a copy of me trillions of times over. Every time I go to the grocery store. I'm making trillions upon trillions of copies of you every single time.

Every single quantum interaction happening in every single star in the universe right now is creating trillions upon trillions. I can't even think of how big this number could be of copies of us of copies of the entire universe, where the only thing is that is different is one little tiny quantum choice that is ultimately probably insignificant. But these quantum interactions they don't know which one is insignificant. Significant according to human measures, it's all just quantum interactions. All these different branches of universes all simultaneously exist. All simultaneously evolve and all continue to make further branches of themselves. On one hand, I have to give the many worlds interpretation for points for being elegant. It's pretty straightforward, and on the face of it, it does solve the measurement problem. There's no such thing as measurement. There's just quantum interactions, quantum things happening. But maybe it doesn't taste very good. I mean, hey, distaste of this constantly splitting bifurcating, branching off multiverse with every single petty interaction doesn't taste good.

That's not really a valid criticism because there's a lot of parts of Copenhagen interpretation that don't taste very good. The universe do what the universe do, man. We shouldn't be afraid of these infinitely multiplying universes, although I do admit it takes a certain amount of, um, commitment to the idea to realize that it's not just my choices that are made making multiple copies of myself. There's a star in the Andromeda galaxy that is currently fusing hydrogen into helium, and every second that goes by, it's creating a trillion copies of me and you in the entire universe. But, hey, physics is weird, all right, that could be true. But that doesn't mean that there aren't major issues and questions surrounding the many worlds approach. It is intriguing because it does solve the measurement problem, but it has issues of its own. For example, what exactly is the precise mechanism for these universes splitting off what's actually happening? What does it look like or entail? How long does it take?

Where does the universe go? For example, the the cat in the box. OK, we're getting rid of the measurement problem. There's no wave function that's all the way dead and all the alive cat, uh, they collapse into a certain state No, no, no, no. There's there's still in all the way alive and all the way a dead cat before I open the box. Then I open the box and then there's splitting where one universe, it appears dead and the other universe, it appears alive. But why don't we feel or notice or detect that splitting? The answer here depends on decoherence like OK, at some point, classical physics takes over, and I just don't get to sense these quantum splittings anymore of the universe of the many worlds. But this process of decoherence, like I said is, is just as elusive as the original collapse of the wave function. We don't know how decoherence actually works. We can't see the process of measurement unfold to collapse the wave function. We also can't see the process of the universe splitting into its different quantum possibilities. We haven't really advanced the question of how does it actually work?

And yeah, related to that concept that I'm constantly getting new copies of myself, not just due to my own choices, but choices do random stuff happening throughout the universe is constantly making copies of myself. How do I maintain a A sense of self or consciousness through that. If I'm constantly splitting and branching every single moment of every single day trillions upon trillions of times every single second, even due to quantum interactions that I'm not even a part of, Uh uh, but consciousness. My experience of reality takes time to develop and unfold. How can I even maintain continuity of thought if there's a new me Trillions of new me and new use happening all the time? How do you have a single line of threat of experience? How does that work? And yeah, entanglement, Um, if everything is entangled and everything is constantly making new branches of the universe all throughout space all the time, Um, doesn't that seem kind of weird in physics that a star billions of light years away can make a new copy of me here on the other side of the universe are my choice to go to the grocery store.

And as I drive to the grocery store, I'm making new copies of that star way over there. How how do I have this universal influence that seems to fly in the face of physics? Like if you don't like non non locality. You hate many worlds because it's saying I am literally creating new universes through the simple act of going to the grocery store and not just a few trillions of them. Every second. I am God and so are you. And so is a mouse. And so is a a proton floating through some random gas cloud. Each one of us is imbued with the power of creating whole new universes. That's a lot to swallow, especially if you can't say how that splitting actually unfolds or takes place or what it looks like. Lastly, let's talk about probabilities. You know, one of the cornerstones of quantum mechanics and one of the things that makes us so different than classical physics. I don't know what I'm gonna get in any quantum interaction or random process, I don't know.

But in the many worlds interpretation in each universe, I get one result. I'm guaranteed to get that result in that one universe and the opposite result in the other. But how do I go from guaranteed results to the probabilities and chances that we see in quantum mechanics? Take the cat in the box. You know what, scratch that. Let's go back to our bet. Let's make this personal. Let's put you in the box. Set up the radioactive isotope. Whatever. 50 50 you live and die 50 50. Let's make this pleasant. If you die, it's not gonna be violent. It's not gonna be gross. You're gonna peacefully fall asleep or you'll fall asleep naturally. And then the experiment will take place and we'll decide if you're alive or dead so you won't even know that you died. And if you win, if you wake up, you'll have a billion dollars. Now. You would naturally recoil from that, just like at the beginning of the episode, even though I made it as pleasant as possible because you're worried that you die and that there would be no more you. But in the many worlds interpretation, you live and you die.

But you only experience the universe where you live the universe where you die, that branch of the many worlds you don't you're not around to experience it. You won't know when you wake up, you will be alive and you'll have a billion dollars. You are guaranteed to wake up because that is one of the quantum possibilities That's right. If I put you in that box with a 50 50 chance that you'll live or die, you will be guaranteed to be a billionaire overnight when you wake up, because the version of you that doesn't wake up won't be around to talk about it. Let that sink. In the full implications of the many worlds interpretation, you are guaranteed to live because the only version where you wake up is the one where you are literally a billionaire. So why don't you take the bed? You won't be in the branches of the universe where you die. You won't be around to experience it. You only get to experience the universe where you're a billionaire.

That's the only one that you will wake up to. You are guaranteed to be a billionaire. But wait, why wouldn't you still take the bet? If you're guaranteed to be a billionaire, why isn't anyone taking the bet? Why isn't anyone a billionaire through this method? Why hasn't anyone taken the bet? Because it's hard to square up the guaranteed outcomes that you will experience with the probabilities that we know from quantum mechanics. Quantum mechanics says it's a 50 50 shot. But in many worlds interpretation, you're guaranteed a particular result. How do you square that up in the Copenhagen interpretation? The probabilities are taken as a postulate. We're just gonna assume that probabilities exist. But the many worlds interpretation can't take it as a postulate because it's taking a different approach that doesn't automatically fold in probabilities. It doesn't take the born rule. The wave function collapse as a postulate, and so it's not clear how to get the probabilities back. Nobody is exactly sure how to recover the fundamental probabilities at the heart of quantum mechanics. In the many worlds interpretation, I can see why many worlds is alluring.

It's cool to think of parallel universes where we have an infinity of other cells, all living alternate branches of quantum possibilities. But it runs into just as many conceptual headaches as the Copenhagen interpretation. Just different ones. Many people are strongly aligned with one or the other and are willing to fight to the death to defend their preferred interpretation. You'll find leading scientists and major science communicators falling on one side of the spectrum or the other, saying that their favorite interpretation is better or more elegant or simpler or cleaner or whatever. Or maybe we're missing something, and maybe we're not properly understanding what reality really is. But that story will have to wait until next time, thanks to everyone who have asked questions about quantum mechanics. Mihail E on email at Sharman on Twitter. Massimiliano S on Facebook. Isaac P on email at On Twitter. Chris F on Facebook, aka B on email at SMTR on Twitter Albert R on email. Julius M on email. Martin EON email John on Facebook RC on email.

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