DarkRange55
Enlightened
- Oct 15, 2023
- 1,725
Quantum mechanics is arguably the most successful theory we have in physics. It's the most fundamental theory that we have. It's in some sense more fundamental than General Relativity which is space-time and gravity. GR is very much in the Newtonian tradition, it's in the classical mechanics paradigm that happened before quantum mechanics. QM says there shouldn't be definite things, even space-time itself. There should be wave functions that give you the probability of observing certain things. For a long time physicists have been trying to reconcile QM with GR in a program called quantum gravity. And there's various approaches like super string theory or loop quantum gravity. But there is no consensus on whats right. Quantum mechanics like classical mechanics before it is not a specific theory. It's a family, a framework, it's a paradigm for doing theories.
In quantum mechanics, when describing something like a particle, you don't talk about its position or velocity or anything like that. Those are things you can observe about it. But they're not what it is, they're not the fundamental way that we have of describing it. We describe it as this cloud of probability called the wave function. And the Copenhagen interpretation says there is a wave function and it evolves according to the Schrödinger equation. But when you look at it, when you make an observation the wave function collapses and returns some answer with certain probability for certain outcomes. But it's very vague on exactly what you mean by looking at it. Does it need to be a person, do they need to be conscious, does it happen all the time, does it have to be macroscopic, how quickly does it happen, when does it happen - none of these questions are really addressed. So many people for years have been insisting that we need a better, more precise version of quantum mechanics. And the many worlds hypothesis is one of the examples.
In conventional textbook quantum mechanics something like an electron or any subatomic systems evolves very differently when you are looking at it and when you are not. So the double slit experiment is an example of that. If you send an electron wave function in the vicinity of two slits that it can pass through and then be absorbed on the other side, there's many weird things. One is when the electron is detected on the other side, it's detected as a dot, a particle, a single point on the detection screen. But if you do that many, many times what you see is a pattern of all the dots that get absorbed by the screen. And that pattern has interference paths, as if, the electron was actually a wave when it went through two slits and the wave is interfering with itself. So it's kind of like a particle, kind of like a wave. But if you set up another camera to keep track of which slit the electron is going through, it detects it goes through one or the other and the interference pattern goes away. So it's acting like a particle when you observe it. It's acting like a wave when you don't. And thats a wave function collapse essentially. Thats what Neil Boer would have called complementarity or duality that the particle/wave function thing, whatever you want to call it, is wave like when you're not looking at it and then collapses to be a particle when you do look at it.
Hugh Everett who pioneered the many worlds interpretation referred to what he called the universal wave function. Every version of quantum mechanics believes there is something called the wave function of the universe. Where different versions of quantum mechanics differ is whether or not they think there is also something else in addition to the wave function. Which many worlds says there's not. And how the wave function evolves over time. Some theories say it evolves differently depending on whats happening and many worlds just says no it always obeys the same Schrödinger equation no matter whats going on. And its a straight forward implication of that simple view that when you observe a quantum system, its not that the wave function collapses, its that you, the observer, become entangled with the wave function of whatever you're looking at. And it branches into different possibilities. And in each one of these possible branches there's a whole world where there was some measurement outcome and that's what you saw. Many worlds does not assume any classical structure to begin with. You can derive everything from quantum mechanical underpinnings.
In order to answer what actually happens when a wave function collapses you have to pick a rigorous formulation of quantum mechanics which Copenhagen isn't. It doesn't answer that question. So in other more rigorous and well formulated theories you can answer that. For example, in many worlds the answer is no such thing as a wave function collapse. They don't collapse. So it's simply an effect of something else. You have your own wave function and that plays in. And the wave function of the universe branches into different parallel worlds that don't interact with each other and you can't tell precisely when that happens. When some macroscopic objects becomes entangled with some a microscopic one thats in a superposition with different possibilities. It's just a feature of quantum theory that once that interaction happens and the wave function branches each branch of the wave function evolves forward all by itself independently. They are parallel and simultaneous existing to the universe that we're in and they're being formed all the time. Whenever a radioactive atom decays or doesn't, whenever a spin is observed.
Our ordinary english language just wasn't designed to talk about these concepts. There is one quantum mechanical wave function that encompasses all of the worlds of the many worlds interpretation of quantum mechanics. Branching of the wave function happens forward in time but not backwards. So there were fewer worlds in the past than there will be in the future. So all the worlds that exist now we think came from a common beginning. Only mathematics - equations give you precision.
The term alternate universes can be used but in quantum mechanics we tend to say worlds separate worlds or we talk about branches of the wave function. It would be okay to say universes or parallel universes but then you're not sure whether you're talking about cosmology (cosmological multiverse) or quantum mechanics.
Next we will go into detail about the different multiverse theories in quantum mechanics, including many worlds.
In quantum mechanics, when describing something like a particle, you don't talk about its position or velocity or anything like that. Those are things you can observe about it. But they're not what it is, they're not the fundamental way that we have of describing it. We describe it as this cloud of probability called the wave function. And the Copenhagen interpretation says there is a wave function and it evolves according to the Schrödinger equation. But when you look at it, when you make an observation the wave function collapses and returns some answer with certain probability for certain outcomes. But it's very vague on exactly what you mean by looking at it. Does it need to be a person, do they need to be conscious, does it happen all the time, does it have to be macroscopic, how quickly does it happen, when does it happen - none of these questions are really addressed. So many people for years have been insisting that we need a better, more precise version of quantum mechanics. And the many worlds hypothesis is one of the examples.
In conventional textbook quantum mechanics something like an electron or any subatomic systems evolves very differently when you are looking at it and when you are not. So the double slit experiment is an example of that. If you send an electron wave function in the vicinity of two slits that it can pass through and then be absorbed on the other side, there's many weird things. One is when the electron is detected on the other side, it's detected as a dot, a particle, a single point on the detection screen. But if you do that many, many times what you see is a pattern of all the dots that get absorbed by the screen. And that pattern has interference paths, as if, the electron was actually a wave when it went through two slits and the wave is interfering with itself. So it's kind of like a particle, kind of like a wave. But if you set up another camera to keep track of which slit the electron is going through, it detects it goes through one or the other and the interference pattern goes away. So it's acting like a particle when you observe it. It's acting like a wave when you don't. And thats a wave function collapse essentially. Thats what Neil Boer would have called complementarity or duality that the particle/wave function thing, whatever you want to call it, is wave like when you're not looking at it and then collapses to be a particle when you do look at it.
Hugh Everett who pioneered the many worlds interpretation referred to what he called the universal wave function. Every version of quantum mechanics believes there is something called the wave function of the universe. Where different versions of quantum mechanics differ is whether or not they think there is also something else in addition to the wave function. Which many worlds says there's not. And how the wave function evolves over time. Some theories say it evolves differently depending on whats happening and many worlds just says no it always obeys the same Schrödinger equation no matter whats going on. And its a straight forward implication of that simple view that when you observe a quantum system, its not that the wave function collapses, its that you, the observer, become entangled with the wave function of whatever you're looking at. And it branches into different possibilities. And in each one of these possible branches there's a whole world where there was some measurement outcome and that's what you saw. Many worlds does not assume any classical structure to begin with. You can derive everything from quantum mechanical underpinnings.
In order to answer what actually happens when a wave function collapses you have to pick a rigorous formulation of quantum mechanics which Copenhagen isn't. It doesn't answer that question. So in other more rigorous and well formulated theories you can answer that. For example, in many worlds the answer is no such thing as a wave function collapse. They don't collapse. So it's simply an effect of something else. You have your own wave function and that plays in. And the wave function of the universe branches into different parallel worlds that don't interact with each other and you can't tell precisely when that happens. When some macroscopic objects becomes entangled with some a microscopic one thats in a superposition with different possibilities. It's just a feature of quantum theory that once that interaction happens and the wave function branches each branch of the wave function evolves forward all by itself independently. They are parallel and simultaneous existing to the universe that we're in and they're being formed all the time. Whenever a radioactive atom decays or doesn't, whenever a spin is observed.
Our ordinary english language just wasn't designed to talk about these concepts. There is one quantum mechanical wave function that encompasses all of the worlds of the many worlds interpretation of quantum mechanics. Branching of the wave function happens forward in time but not backwards. So there were fewer worlds in the past than there will be in the future. So all the worlds that exist now we think came from a common beginning. Only mathematics - equations give you precision.
The term alternate universes can be used but in quantum mechanics we tend to say worlds separate worlds or we talk about branches of the wave function. It would be okay to say universes or parallel universes but then you're not sure whether you're talking about cosmology (cosmological multiverse) or quantum mechanics.
Next we will go into detail about the different multiverse theories in quantum mechanics, including many worlds.
