Daniel and Jorge Explain the UniverseWhy can’t we see quantum effects in everyday objects?
Daniel and Jorge talk about quantum decoherence and why big things don't act like quantum particles.
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Hey Daniel, can you pass through walls?
No?
Keep trying and just keep getting more bumps on my head?
How about can you be in two plays at the same time?
Sometimes it feels like I'm supposed to be, but I've never actually managed it.
So I guess you're not a quantum object.
No, actually, I'm quite classical.
But you're a particle physicist. Aren't you made of particles? And aren't those particles quantum mechanical? I study particles and I'm made of particles. But I follow the rules of classical physics. Classic. Do I mean you're classic, Daniel?
I'm not new Coke Daniel.
I am Horehem, a cartoonist and the creator of PhD comics.
I'm Daniel. I'm a particle physics but I'm quite classical in my tastes.
Are you a fan of classical music.
Then it's better than quantum music. Let me tell you.
Oh, that's a new genre to make a new category at the Grammys.
Oh, I wish it was a new genre. But I'm sure if we type it into Google we will find something for quantum music.
I feel like that's all music these days. It's both good and bad. It's kind of incoherence. It's both original and stale. Welcome to our podcast, Daniel and Jorge Explain the Universe, a production of iHeartRadio.
In which we explore everything in the universe and try to make sense of it. We try to understand how things move in our world and how tiny particles move. We try to understand the rules of the universe, whether they govern supernovas and black holes or tiny little electrons, and we try to make sure that you understand them.
Yeah, because we try to make this podcast a superposition of both fun and real.
Science, physics and banana jokes.
People often think those two can't be in the same place at the same time.
What makes say that you haven't met any fun physicists.
You mean the physicists is fun, or they know how to have fun, or.
Just doing physics is fun. Why doesn't everybody see that?
Come on, I think that's the problem, Daniel, that's the problem. But yeah, it's a weird and strange universe out there, and so we like to talk about all of the things that make it weird and strange. It's pretty unintuitive to universe it is, and physics has done a great job of building this edifice to help us understand the way the universe really is, not the way we think the universe should be or might be, or the way that makes sense to us based on our limited experience, but actually revealing to us the true nature of reality. But sometimes what we learn is pretty hard to swallow. Yeah, and specifically quantum mechanics. I feel like that really trips people up. It trips me up, for sure, And it's kind of hard to wrap your head around all of the weird, kind of unintuitive phenomenon that happens at the quantum level.
Yeah, it is pretty weird because quantum particles seem to be following different rules. They see to be able to break rules that are hard and fast for things like baseballs and basketballs and scoops of ice cream. And that's a hard thing to understand because it's weird and it's new. It's also hard to understand, like why are there different rules? And you know, what's the difference between an electron and a baseball? You know, where's the sort of threshold between those two where the quantum rules take over or the classical rules take over.
Yeah, it is kind of weird that, you know, things are so weird at that level, at the microscopic level, but then once you scale up, things feel more gosh normal, more solid, less uncertain, or.
At least more familiar. Right. Science is all about delving into the unknown, and typically we try to explain the unknown in terms of the known. But that fails if the unknown is something really new, something different, something that fundamentally can't be described by what we already know. We've often talked about how physics is like exploring the universe, But it's just been studying the tail of the elephant, and when you look at the rest of the elephant, it's not true that what you learn from the tail can help you understand the rest of the elephant. Sometimes you really do discover something weird and different.
Yeah, but is that elephant quantum mechanical, Daniel, Is it really there or not there?
It's a theoretical thought experiment elephant. So it's just totally not there. That's classically non existent.
All right. Well, that connection between the quantum world and our regular, everyday world is what we'll be exploring today on this episode. Today on the podcast, we'll be asking the question what is quantum decoherence? That's hopefully a coherent question.
Yes, And quantum decoherence is the key concept to understanding the answer to this question. Why do big objects not seem to follow quantum rules? What is the difference between quantum objects and big objects? Where is the threshold? Why do we seem to have two sets of rules or is it just that one set of rules sort of morphs into the other.
Right, it's an important concept, and it seems to be sort of related to this idea of quantum measurements. I think maybe that's something that if you've heard of quantum mechanics or have talked about it or seen any videos about it or read about it, it's something that seems to be important that you know things are quantum, but then when you poke at them or measure them or try to look at them, things collapse for some reason.
Yeah, this is a big and still totally unsolved problem in quantum mechanics. Quantum mechanical things can have like multiple possibilities, but we don't observe multiple possibilities. When you poke an electron, as you say, it picks one of them, and we don't really understand how that happens, how one of them gets picked, and actually when that picking happens, you know, does it happen when your finger touches the electron, does it happen when you look at the results, does it happen somewhere in between. That's a really interesting and hard problem, and it's related to quantum decoherence. But they're not quite the same thing. So today we'll try to explain what quantum decoherence is is how it helps us understand the difference between quantum and classical objects. But it's important to understand that it doesn't actually solve this problem of the quantum measurement.
Mmmmm, is that what happens when you pull on the tail of the elephant? Things get real real quick.
You get quantum stumped. It makes quantum music out of you.
All right, well, this is an interesting question what is quantum decoherence? And so as usual we were wondering how many people out there in the real world know what it means. So, as usual, Daniel went o, they're into the wilds of the internet to ask what is quantum decoherence?
So thank you very much to everybody who volunteered your speculation for the podcast. If you would like to volunteer for a future podcast, please write to us two questions at Daniel and Jorge dot com.
Think about it for a second and someone asks you what quantum decoherence is, what would you say it is, or at least guess it is. Here's what people had to say.
I'm not sure if I recognize this right, But I think teakeoherence is about how the quantum and the normal fu colide, so how we can map quantum effects into our reality.
Since coherent means to make sense, I'm thinking quantum decoherence is when quantum particles can't logically follow the rules of the universe, the constant of the universe, or something of the nature.
I'm not sure what quantum decoherence is, but I think it might be wave function collapse, which is how the wave function of a quantity and a quantum system collapses when you measure that quantity.
I don't know what quantum decoherence is, but my guess would be that it is the occurrence of something totally inconsistent that disrupts the quantum realm.
It has got me thinking of something to do with quantum entitlement.
All right, pretty coherent answers for the most part, or at least coherent in there not knowing what it is.
Yeah, nobody actually quite nailed it, but you know, are in the vicinity. They seem to understand that there's a concept of coherence, at least in quantum mechanics.
Somebody said that it is sort of where the real world and the normal world collide. I guess they do, maybe think it has something to do with the connection between the quantum world and our everyday experience.
Yeah, and that one's the closest I think to the right answer, to the right way of thinking about it. Quantum decoherence, in brief, is the idea that explains how quantum objects look like classical objects when they get really big and messy.
All right, well, let's jump into it, Daniel, and I guess let's start with just a quick recap of quantum of a small subject called quantum physics. You know, what is it that we actually call quantum and how can we kind of describe those effects?
So the thing to understand is that when you look at really small objects, things like electrons or photons or individuals, tiny particles, they seem to be following rules that don't apply to bigger objects like baseballs and basketballs. Right, And the key concept to understand when thinking about these tiny particles is that they're not just particles. They're not like tiny versions of a baseball, They're not just like miniaturized little blobs of stuff flying through space. Right. When people were first thinking about the atom, for example, they were thinking about the electron like orbiting the nucleus like a tiny planet around a star. But we pretty quickly figured out that was impossible because if an electron orbits a nucleus, then it's going to give off radiation because it's accelerating, it's giving off radiation. And they did the calculation and discovered, well, that would collapse in like one hundred billionth of a second because they would lose all of its energy. And so instead they had to have a new idea for what controls an electron, what defines what an electron does and how it interacts, and so instead they try to use like wave like properties to describe it. So you've probably heard this phrase called the wave function. The wave function is just a mathematical tool that helps us understand what an electron is likely to do. And the key concept is that the wave function tells you where a particle is likely to be and where it's not likely to be. Sean Carroll says that the wave function is the dopiest name in the world for one of the most profound things in the universe, which really made me laugh.
Well, dope and profound go hand in hand.
I think like physics and fun. Right.
Yeah, well, yeah, I think that is maybe the hardest thing to grasp about quantum mechanics and physics. You know, I think everyone sort of grows up at least in's early in school and definitely in popular culture. You know, the depiction like pictures of an atom always look like little planetary systems, you know, like a bunch of little balls in the middle cluster together, and then other little balls kind of swinging around in large orbits and rings around that. And that's the picture that we have about the atom. But you're saying, at some point we figured out that's not possible, like that doesn't make sense.
Yeah, exactly. It's not like the electron has a trajectory, has a path, and we just don't I don't know it. It doesn't actually have a path. It doesn't have like a position at every moment in time. Instead, what it has is this quantum wave. Now, the quantum wave behaves all sorts of normal wave like rules, but what it does is tell us where the electron is likely to be, and so the electron is likely to be here and it's likely to be there. Now, sometimes people say that means that electron is in two places at once, but that's not actually correct. It means it has the probability to be here or there, and those in both probabilities can exist at the same time. It doesn't mean it's actually in both places at once. If you ask the electron where are you, then it collapses as we talked about earlier, It picks one place or the other. But the wave function describes what's likely to happen.
Well, I guess you mentioned before that it has a dopey name. Why do you think it's a bad name, And what would you have called the if not the wave function?
And why is it called the wave function? Well, wave, I suppose because it follows a wave equation shorting equation is very much like equations for other waves that we'd seen before, that electromagnetism, And it's just waves in water. Right, there's a certain differential equation which just looks like a wave equation.
Like a wiggle, like a like a like a rippling wiggle.
Yeah, it's like a rippling wiggle, but you know it's not a rippling wiggle in anything physical, right, Like sound waves are a wiggle in air like air pressure. Right, Yeah, light waves are a wiggle in the electromagnetic field, which even if it's hard to imagine, is a physical thing. The wave function is complex, it's imaginary values. It's like you know, one plus three I and so it's not a wiggle in a physical thing itself. So it's hard to understand because it's like this sort of abstract literally complex things in the sense that it exists in the imaginary plane, but it controls something real. So I wouldn't have called it the wave function, you know, I like the word wave, but function to me is sort of confusing.
Would you have called it the imaginary function? Well, I feel like alluding to the imaginary complex plain that is not helping me here, I guess. And you just brought up maybe a source of confusion, which is that you know, you just said that light or a photon is like a ripple in the electromagnetic field, right, So it's a ripple like kind of like a sound waving that It's like, it's more intense here on this intense here are you saying that there's also like a photon would also have another kind of wavness, which is imaginary.
Absolutely, the photon also has a wave function, right, and that wave function determines where the photon is likely to go, like which parts of the electromagnetic field are likely to ripple?
You know?
For example, you know, the classical situation is, imagine you shoot a photon at two slits in a screen. You know which one is it going to go through. It has a probability to go through one and a probability to go through the other. The wave function controls what those probabilities are, right, And if it collapses and picks the one on the left, then you get an electromagnetic wave through the one on the left. So the probability wave and the electromagnetic wave are sort of two different things to keep in your.
Mind, and they both spread differently. Like why did they call them waves in the first place, Like, do these like probabilities ripple out into space also, or do they just look like a ripple that moves around.
They do ripple, and they do follow wave mechanics, and that's why they call it a wave. And that's why they can do really amazing and fascinating things. Because the location of the electron is controlled by something which is fundamentally a wave, it can do things that waves can do, like it can interfere, right, you can have probabilities interfering with themselves. Probability being here and there can interfere with each other and create probabilities in other places, just like actual waves can. Right, you put two hands in a lake or in a bathtub and you make two sources of waves. Those waves can interfere with each other, meaning just that they can add up or cancel out.
So, for example, if I have like an electron here, it has like a ripple of probability emanating from it, or is it kind of static if the electron is static.
Well, electrons can't actually be totally static, right, because they're quantum objects. So you can't just like say an electron is here and it's not moving. That would violate the Heisenberg and certainly principle would effectively be an electron at absolute zero. But if you have an electron you shoot it out of like an electron gun or something, and you want to describe what's it likely to do, where is it likely to go, then you have a probability for all those various outcomes, right, And the key thing to understand is like there's not a real history that's happening between when you shoot the electron and when it hits the wall and you're just learning about it. It's uncertain, right, It has both possibilities existing simultaneously until you measure it. And that's the key thing that's hard to understand about quantum mechanics. It's like how this measurement changes it from like having two possibilities to actually existing in one place, but doesn't exist between the places you measure it only exists where you measure it. In between. They're just probability, all right.
So you're saying that maybe, like an electron is like a little tiny baseball, but we just don't know where it is, and where it is is determined by this ripple in probability.
Yeah, I'm saying an electron is like a little tiny baseball, but it doesn't have a place where it is. It's not like it is someplace and we don't know its place is not determined. The probability of it being in one place or another is determined by the wave function. It's like forty two percent probability to be there, seventy one percent probability to be here, or whatever, so they add up to one. But the probabilities are determined by the wave function, but doesn't actually have a location. It just has those probabilities again until you measure it, which is the weird bit, and.
Then when you measure it, then then it feels like you were hit by a baseball.
It certainly does, and that's why we see these weird quantum effects because these particles like electrons, can do things that waves can do, like if you have two sources of them, you get these interference effects, or they can like tunnel through walls. These are things that waves can do. Probability waves can do, and you get effects because you have these wave like properties of the electrons wave function.
These probabilities can go through walls like the wall doesn't affect it, like it just goes through them.
The wall does affect it. We have a whole fun podcast on quantum tunneling, and it's possible to go through walls, right because you can have a probability to be on one side of the wall and then a probability to be on the other side of the wall, and you can do that without going through the wall. The probability can leak through the wall, giving you a chance to be on the other side of the wall, even if you're never actually in the wall.
All right, And so that means that the baseball would just appear on the other side, or that it is found a way through the wall.
It just appears on the other side. Remember that quantum particles don't have to have paths between where you've seen them. You see it at A and you see it at B doesn't mean it went from A to B. Doesn't have like a secret history how it got from A to B. It was at A and then it was at B. Remember they follow fundamentally different rules. It was at A, then it had lots of probabilities for maybe how it got to be, not like one of them is true and we just don't know it. There are those probabilities. It's undetermined. It's not unknown, it's undetermined. And then later it's at B. So it doesn't have to go from A to B in order to be at A and then be at B. And this is highlighted by the quantum tunneling experiment because you can't go from A to B. There's a barrier there.
All right, Well, let's get into quantum coherence and how that relates or how that ties this uncertainty and this wave function to real things like baseball. But first let's take a quick break.
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All right, we're talking about quantum decoherence, and hopefully, Daniel will we're not. We're not breaking down into decoherence talking about it. But okay, So particles have a wave function which tells you it's like a ripple in the imaginary space. That tells you the probability where that little tiny baseball where you will find it if you were to poke it. Yes, exactly right. So then what does coherence and decoherence mean?
So quantum coherence is when you have two possible outcomes for what's going to happen to your particle, and those possibilities sort of line up, like the wave functions for those two possibilities line up. Now, any solution to the wave equation, any quantum state, you can take it and add it to another quantum same to get a third quantum state, which is a mixture of the two. So you can mix two possible wave functions, Like if you have one that says the electron is gonna arrive at point A, and you have another one that says the electron is gonna rivee at point B, you can have a mixture that like mixes A and B with fifty to fifty odds. So that's a coherent combination. It just says you have two possibilities and you've added them together, and they're coherent because their wave functions are sort of syncd up. They start in the same place and they sort of wiggle in time together.
Right, but they don't end up in the same place. Isn't there some kind of cancelation or some kind of or are you saying coherence is when they don't cancel.
They can cancel, right. The fact that they can cancel is because they are coherent. You only get interference effects from coherent sources of waves. Like go back to the example of like bathtubs. Right, say you put your hand in the water and you slap it, so you make a rhythm and you get waves in the water. You do the same thing with your other hand if you're doing it in the same way, right, If you're like slapping the water in time, then you get the waves which either add up or cancel out coherently. If your secondhand instead is just like random just like randomly slapping it, then they're not going to add up nicely. It's just going to be a big emotion. It's going to spread out to nothing. So you only get these interference effects when the quantum waves are in sync. That's quantum coherence, and that's what allows quantum things to be sort of quantumy.
Hmmm, meaning that they're in sync in time. So what you're saying, or in time and space.
Yeah, they're in sync in time, and so at every point in space they can interfere. At one point in space, you might get cancelation because they're waving in opposite directions, and at another point in space they might support each other because they're waving in the same direction. But they can only do that consistently if they're in sync in time.
So it sounds like, you know, particles have these wave functions, and they some have to sync up in order for them to interact with each other, because if they don't sync up, then then what happens They just cancel each other out, or they can't or what happens if they don't sink up.
If they don't sync up, then you don't get any of these interference effects. It's like if your noise cancelation headphones, right, if those were sort of just like randomly putting out sounds instead of like syncing up to the sounds that are coming at your ear and producing exactly the opposite sound to cancel them out. Right, your noise canceling headphones only work because they produce a coherent noise which cancels out the ambient noise. Otherwise, if they put out just random, arbitrary noise, they wouldn't cancel each other out, and you just get sort of normal stuff you could have like A and B. You wouldn't have like weird interference effects between the incoming noise and the noise produced by the headphones, and all the weird quantumness comes from those like interference effects. These effects of the probability wave, and if you don't have coherence, then you don't get those effects.
M okay, So you need coherence in order to have interference is a little maybe counter in twitter. But it's not a frequency thing, right, It's not like they have to be the same frequency or fit the same number of wavelengths within the same space. It's something a little bit more than that.
It's more than that they have to be linked up. They have to like sync up in time. Otherwise you could have like coherence just for a moment, right, but have to sink up in time to be like consistently giving out these probability waves that interfere. Then they have to be sort of syncd up, which is linked in time. That's to like start at the same place and go down the same time or up at the same time. Either way, they have to like match each other in phase.
Right, But they can also cancel each other out and be coherent, right, Like you can be coherent and destructive at the same time. It's different than decoherent.
Just like noise canceling headphones. Right. The cool thing about the quantum wave function is that we actually have a lot of intuition for how waves work, right, All these effects, interference and cancelation, these are normal things. It's just weird when you apply it to the probability for something to happen. So we're very familiar and happy to talk about waves and that. None of that is weird, Like noise canceling headphones are not quantum magic. It's just weird when you apply it to the probability for an experiment to have a certain outcome.
Right, And lasers are also coherent, right, Like, that's kind of what a laser is.
Yes, exactly, a laser is a coherent source of light. All the photons are like in phase and have the same frequency, so they add up together. Right, So it's a very intense source of coherent light.
Okay, So that's at the microscopic level, like electrons and photons and quarks. They can have this coherence. But then something happens when you go up to the bigger things, like when don't we have to worry about coherence at the baseball level?
Right, Well, we don't have to worry about interference of bass balls because they're not coherent quantum objects, like all of their particles are not wiggling in phase. They're all scrambled and random. It's like a huge choir of children all singing different songs at different volumes and different speeds, and so what we see is like a big average goomush. We don't see any sort of like interference effects when two baseballs bounce off each other because they don't have coherent quantum waves. Their phases are all scrambled.
All right, Well, let's talk about that scrambling, because I think maybe that's at the key of what makes things quantumy or not. Like this, maybe step us through, Like, okay, we start with one atom, and the particles inside the atom do have this wave function, and presumably they're coherent, Like are the protons and neutrons and quarks inside of a an atom coherent together or is there already some kind of smooshing at that level?
They can be coherent. Absolutely, there's no limit to how large a coherent system can be. It just gets harder and harder to do because it has to be isolated. The key is that anytime you interact with something, then it becomes part of your system, and so the system sort of grows and grows and grows, so it's easier to start from like a single particle. Take a single electron or a single photon, right, it has a certain wave function, and that wave function is coherent. It has like two possibilities for what it can do, and those possibilities are coherent, and so you get like interesting interference effects. That's why, for example, a single photon going through the famous double slit experiment can interfere with itself. Right, It's two possibilities are interfering. So single photon with two coherent possibilities can interfere with itself. Now things get messy once that photon starts to interact with other stuff because now the two possibilities for the photon interact differently with the environment. You know this interact with the wall or interact with the tool whatever you're using to measure it, and they change the phases of those two different outcomes, and now it's decoherent. So a particle can be coherent. You can even have two particles coherent, but once you touch it, once you interact with it, then you can break that coherence.
Right. But you know, we're trying to build up from like particles up to a baseball, and so like if I assemble you know, three quarts together into a proton. Are they still coherent together or do they start to kind of get out of sync Once I put them together into a proton.
They can be coherent together, and you can have a single wave function that describes just those particles. Now, if you want to have a wave function that describes just those particles, it can't be interacting with anything else, because then you'd need to include that in the wave function. If you want to have a wave function for just your atom, you have to keep it isolated, right, And that gets harder and harder to do as things get bigger. Like it's possible to imagine a photon in an experiment that doesn't interact with anything you've built a special trap or whatever. It's harder to imagine a baseball that doesn't interact with anything, no air, molecules, no photons, no nothing. There's so many particles in there. It becomes harder and harder to keep it isolated. That's why decoherence appears as soon as things get big, because it's really hard to keep larger, realistic sized things isolated and coherent.
Right.
I guess I'm trying to understand when that comes in. So, Like, if I build a nucleus out of protons that are all in sync inside. Then are they all also together in sync like you know, like carbon has twelve of them in the nucleus and they're all compent there together, held by the strong nuclear force. Are they still coherent or are they starting to kind of fuzz out?
If you keep it isolated, it can stay coherent. Absolutely, Okay.
So then I build an atom, I throw in some electrons, and that also gets synced up. And at what point do things start to kind of go awry?
At the point where it becomes impossible to keep it isolated from the rest of the universe.
But then couldn't I be in sync with the rest of the universe Daniel.
Yes, exactly. Some people think that there is a wave function for the whole universe, right. The problem is that now we're inside the wave function. Now the wave function includes us, and so it's hard to see these quantum effects now because we are part of the experiment.
All right, So I got an atom and it's coherent and it's syncd up, and now I add another atom. Is that a problem or does it just get bigger?
It's not a problem. It gets bigger. It's just harder to keep it coherent because you have to keep it isolated from the system. Remember one time we talked about building a macroscopic like big size stuff that behaves in a quantum mechanical way. This is done in a special way using Bose Einstein condensates special form of matter that can be coherent, that can stay together and can stay isolated from the rest of the system. These things don't last very long. They last like, you know, seconds or minutes because it's hard to keep them isolated. So it's like real experimental bravado if you can put more than a few atoms together in a quantum coherence system and keep them coherent, keep them from interacting with the rest of the universe and getting their wave functions sort of muddled up with the rest of the universe.
Yeah, well, I guess maybe a big part of it seems to be this idea of an experiment, and like who's in on the know and who's outside of the experiment, and what does it mean for something to be kind of pristine or not pristine? So maybe let's get a little bit into that, which is I think basically the idea of decoherence, right, all right, let's get into that, but first let's take another quick break.
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All right, we're talking about quantum decoherence, which is kind of at the heart of this kind of headache that people have quantum mechanics and trying to understand it at an intuitive level. So you know, small particles can have quantum effects and wave functions and weird probability existences, but once you start piling them on together, it gets harder to kind of keep pristine, I guess, right, untouched from to an observer from the outside or just to the universe in general, the same, right, because the universe, I imagine doesn't care, right, like the universe if there is a quantum wave function for the entire universe, Like, it doesn't care, Like it doesn't know the difference between something that you would think is incoherent or not.
Yeah, that's right. And the tricky thing here is that we want to have a wave function just for our experiment because we want to see quantum effects. If we become part of the experiment, then we no longer see the quantum effects because we only are like on one branch of that history.
What do you mean we don't see the effects Like we're also existing in multiple places at the same time to somebody outside of our universe.
Yeah, somebody outside of our universe sees a wave function for the whole universe and they see lots of different possible outcomes, right, and those things can exist simultaneously until like if they observe the universe. But we're on just sort of one branch of that history the way we are existing, and so we don't see those other branches necessarily. And so if you want to see a quantum effect, you have to be like outside of that quantum system and take some measurements of it. Right now, as soon as you take those measurements of it, you've sort of inserted yourself in that quantum system and it becomes much muddier.
I think you just blew my mind here, because, like, if we are all part of the quantum function of the universe, that means that there are like multiples of me out there to some alien observer outside of the universe. M Like, I only think that I exist as one person, because what because I am observing myself, I guess.
Yeah, it's complicated, and this whole question of like who is an observer who can collapse the wave function when does the wave function get collapsed. It's very complicated. It's a whole other philosophical question that it hasn't been resolved. We don't know the answer to it. It's like the biggest problem in the foundations of quantum mechanics. We're not going to figure it out today on the podcast, but it is connected to this question of quantum decoherence and quantum coherence because we're interested in observing quantum effects, like when do things look quantumy, and when do things not look quantumy, and if you are inside the experiment, things don't look quantumy. Like I've never talked to a photon. I don't know what it's like to be a photon in a double slit experiment that has experienced one path is it experienced some weird combination of multiple paths. But I know what it's like to be me, and I don't experience superpositions, right, I don't live two lives at the same time. Sometimes it feels like it.
Yeah, okay, so it kind of depends on who you ask whether something feels quantumy or not. Like, you know, we can have a little experiment here in front of us that looks and feels quantity, but the particles inside it doesn't feel quantomy or to an observer outside of our universe, we feel quantumy to them.
Oh absolutely, Like there's a very simple thought experiment to think about that. Say I set up a quantum experiment that can have you know, two outcomes A or B, and I run the experiment. Now, before I read the outcome of the experiment, you could say there's two possibilities A or B. Cool. What if you're running an experiment and your experiment is me running that experiment. So you put me in a box and I do the experiment, and I know the outcome, but you don't yet know the outcome, right, So I am your experiment. Well you know, am I living in the outcomes of both experiments until you ask me what's the outcome of my experiment? Right? So absolutely? Like the quantumans depends on who's doing the asking and who's doing the observing and who is collapsed the wave function, And that's like that deep question of quantum mechanics that we don't know the answer to is like when do wave functions get collapsed and how did they get collapsed?
Right? Like if the cat ensure the Ainger's box was a physicist, Like, the cat knows whether or not it's dead or alive, or you know, the radioactive particle clicked or not. But to us, the cat is dead and alive, But to the cat it's not.
That's right. And so that's why I say you can observe quantum effects if you're outside the experiment, Like you observe a quantum effect on me, because I'm part of your experiment. I observe a quantum effect on the little experiment that I'm running with the cat or whatever. Right, I don't experience the quantum effects that you observe in me.
Right, So then how does that apply to our baseball? Like is it a thing kind of like as you pile on more particles that are interacting with more things, you're sort of opening up those throlled in group boxes.
Yeah, exactly. And there's two ways to think about it, sort of. One is intuitive and the other's mathematical. The intuitive way is that as the baseball starts to interact with more stuff, that stuff becomes part of the baseball's quantum wave function, right, Like, now you have a wave function for the bat and the baseball together, and so the bat can no longer do like quantum experiments on the baseball, because it's like entangled with the baseball the same way they're like, I'm inside your experiment. And so the intuitive way is like, as a particle starts to interact with the system around it, it gets sort of enmeshed quantum mechanically with the wave function of the larger system. So that system is now part of that you know, particle or baseball's quantum wave function, and it can no longer see the quantum effects of those things. So that's decoherence. And so the mathematical way to think about it is that when a particle interacts with something, what happens is that its phase gets shifted a little bit, and the phases of the different possibilities get shifted differently based on how you're interacting. And so what happens is that all the phases of the baseball when it hits the bat gets shifted a tiny little bit and they all get scrambled. And so now the phases are like out of sync and they can't do quantum stuff together because they have decoher because they're all like random phases.
Wait, so you know I was building this baseball from atoms, and so it isn't it possible to build a baseball that is still coherent, like all of the wave functions inside are in sync and happy and quantumy, in which case the whole baseball is quantumy.
Theoretically possible, practically very very difficult because you'd have to isolate it from the entire universe. Nobody could interact with or observe that baseball.
Right, Let's say I put it inside of a short finger's box. It's there, it's not interacting. Yeah, that baseball is quantumy.
That baseball is quantum me. Now, like practically shortingers box is impossible because you know, no box is impervious to heat and all sorts of other interactions. But let's say theoretically you've built some way to isolate the baseball completely, then yes, it is still quantumy. It has not interacted with anything else.
Right, But I think maybe a key limitation here is that it's not that the baseball can be here or in Mars. The probability of where it is isn't that big, because you know, you're just adding tiny little probabilities, right, Like it can't be here or a meter away that quantum meanness of the isolated baseila is like it's here, or it's a few angs from to the right.
It could actually have quite different possible locations. It depends on how you set it up inside the box. It could be sensitive to one quantum fluctuation which sends it in one direction or the other direction. I think what you're referring to, though, is more like the classical sense of the decohered baseball, a baseball that's like that you're familiar with, that's flying through the air in a normal baseball game. We don't see those quantum effects because they all average out. Because all the quantum effects of all those particles in the baseball are not pulling like in the same directions. You never see like weird interference effects or weird probability distributions because they've all averaged out. They're all decoherent. If they were coherent, then yes, the baseball could do quantum things the way like Schruenninger's cat can do quantum things like be dead or alive. Have those possibilities at the same time.
Okay, so now you're saying that, like if I has this baseball in the box and I open it. That's the same thing as hitting it with a bat.
Yes, because now photons are hitting it and you are seeing those photons.
And like, the bat is connected to the batter, and the batter is connected to the ground, and the ground is connected to me, and there's air in between, and there are photons flying back and forth, meaning that like, I am kind of inextricably tied to this baseball, which means that I am now inside of a larger box with the baseball exactly. Then that's why you don't see quantum effects on big things, because big things are always interacting, you know. Einstein famously asked somebody like, do you believe the moon isn't there when you're not looking, because he was thinking, like, it's silly to imagine that the universe like is uncertain when you're not existing. And the answer is like, of course the moon is there because photons are hitting it and bouncing off of it, and so the universe is always looking because the universe is filled with particles and they're always sort of bouncing off of things in gravity too, right, it's interacting through gravity with us.
Ooh, that's tricky because we don't know if gravity is quantum mechanical and if they are gravitons bouncing around through space. But in principle yes, and so quantum decoherence is just like when an object no longer becomes isolated and its wave function is now like complicatedly mixed up with the rest of the environment so that they don't like add up coherently anymore. Like this little bit of the wave function is mixed up with that part of the wave function from the bat, and that part of the wave function from the ball is mixed up with this other bit of the wave function from the bat. And if you were outside the baseball game, you could view the whole baseball games wave function. Then you could say, oh, I still see quantum effects, right, because I'm looking at the wave function of the whole baseball game. But if you're inside, if you are the batter, right, then now you are only seeing one slice of it.
All right. Well it's weird because you know, I feel like the word decoherence means that things get out of sync, but really it means I got sucked into the box.
Yeah, you are entangled, right, and decoherence right, yeah, exactly, I see how that's confusing.
Yeah, it's more like I got sucked into the box. But the word decoherence kind of, you know, implies like some kind of like noise or some kind of like breakdown of things.
And I think the key there is that, you know, just the ball itself not become decoherent, right. You no longer have just a wave function that describes the ball. You have to describe the ball and the bat, or Jorge and the cat. There's no wave function by itself that now describes the ball because the ball is entangled with the bat, and so the ball's isolated individual wave function is no longer coherent. It's like a part of a larger wave function. It can't be isolated. And so that's why you can't get quantum effects on the ball anymore, because it's complicatedly tied up with the things you want to use to measure those quantum effects.
Because we are the ball now, Daniels, The ball and us are one. That's what decoherence means, right, kind of like it gets so complicated. I guess that's why we use the word decoherence, just because it gets complicated beyond our ability to be outside the box. It's like we're in the box now, and we can't make out what these quantumness effects are.
Yeah, exactly, it's too much for us to calculate. It's too much for us to understand. And so what happens is that quantum mechanics doesn't fail, doesn't go. This is just what quantum mechanics looks like at a big scale. Quantum mechanics over zillions and zillions of objects looks different because you don't see those coherence effects anymore. They only exist when you have like one or two or three little things that you can keep separated, so you can have a wave function just for that. When you're part of the wave function, quantum mechanics says that things look different. They look more smeared and averaged out. So it's not like classical physics is in disagreement with quantum mechanics. It's what quantum mechanics looks like, sort of from a high altitude right.
Like to an alien observer outside of our universe, we are still all coherent. We are you and I in this podcast, and that baseball it still looks like a Priestine quantum universe.
Yeah, and I hope that alien that has that deep understanding of quantum mechanics has a coherent understanding of what we've been talking about today, because it's gotten pretty tricky.
All right, Well, I think hopefully that gives people a sense of kind of the issues involved. You know, it's kind of about what you consider the box to be, what's interacting with what, and how these kind of probabilities add up or don't add up.
And it's not just like an academic question or a philosophical question. It's actually really important for quantum computing. If you want to build a quantum computer, you need cubits. You need weird particles that follow quantum rules so you can have them do quantum computations. And to do that, you need to keep them isolated. And that's okay to do for one cubit, two cubits, three cubits, But imagine having a really big quantum computer with thousands and thousands of cubits or millions or trillions, right, you got to keep them all isolated and all individually coherent. It becomes really difficult. So this is something people are literally working on, is building larger coherent quantum systems.
M yeah, I can't wait for that quantum phone, so I can take quantum pictures of my kid playing quantum baseball.
So you can ignore that email and answer it at the same time.
Yeah, that's right, So I can do everything at the same time exactly.
And this is really closely connected to deep issues in the philosophy of quantum mechanics. You know, who is doing the observing, why does it matter? When does the wave function collapse? And I want to have another episode where we talk about wave function collapse and the measurement problem. But this is sort of like a warm up to that because it helps you understand, you know why sometimes the probabilities are more classical instead of quantum mechanical. Quantum coherence tells you, like, you know what's likely to happen. It doesn't explain why it collapses from two possibilities down to one actual thing.
Mmmm. That's the tricky part.
That's one of the tricky parts. That's the trickiest part.
It's all tricky, but you have to get inside the box and then it's not tricky.
Yeah, exactly.
Then you understand or you don't understand, just like the cat.
Yeah exactly. If you want quant mechanics to go away, just you know, only work on big complicated systems. Where those effects don't appear because they're all decoherent.
All right, Well, we hope you enjoyed dad and got a better sense of quantum mechanics. Thanks for joining us, See you next time.
Thanks for listening, and remember that Daniel and Jorge Explain the Universe is a production of iHeartRadio. For more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows. When you pop a piece of cheese into your mouth, you're probably not thinking about the environmental impact. But the people in the dairy industry are. That's why they're working hard every day to find new ways to reduce waste, conserve natural resources, and drive down greenhouse gas emissions. House US dairy tackling greenhouse gases. Many farms use anaerobic digestors to turn the methane from manure into renewable energy that can power farms, towns, and electric cars. Visit you as dairy dot COM's Last Sustainability to learn more.
As a United Explorer card member, you can earn fifty thousand bonus miles plus look forward to extraordinary travel rewards, including a free checked bag two times the miles on United purchases and two times the miles on dining and at hotels. Become an explore and seek out unforgettable places while enjoying rewards everywhere you travel. Cards issued by JP Morgan Chase Bank NA member FDIC subject to credit approval offer subject to change. Terms apply.
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Get home safely.
Protect our cyclists and pedestrians because they're people too.
Go safely.
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