Is a universe created each time someone measures an electron's spin? This extremely counter intuitive theory of what happens when we measure properties of particles is actually on solid footing among some of the foremost physicists of the day. Why would anyone seriously entertain such an idea?
Physics attempts to explain the mechanics of the universe, and has made great advances since the Copernican revolution. However, the explanations physics gives us have become more complicated and less related to everyday experience. In 1687 Newton put forth his three laws of motion, the first being that an object will stay at rest or in motion at constant velocity unless acted upon by an external force. With a few examples, and given some thought, we can easily grasp this concept. We might initially think, as Aristotle did, that objects tend to stop moving on their own, but with some additional understanding regarding friction, the validity and elegance of Newton’s law becomes apparent. Once we learn the laws of Newton, they shape our understanding of the world, and our everyday experiences confirms this reality. But the advance of physics brought new laws of motion in 1905, when Einstein put forth the special theory of relativity. Einstein explained that the speed of light is an absolute speed limit for any object and thus the laws of motion defined by Newton had to be modified for objects that travel at speeds comparable to that of light. Here is one of the moments when the explanations of physics begin to become less related to our everyday experience. If you are seated inside a train going smoothly at 100 kilometers per hour and roll a ball toward the front of the train at 10 km/hr with respect to yourself, the ball will be going at the speed of the train plus the speed of the roll, that is, 110 km/hour with respect to the earth. If you are traveling at 100,000 km/second on a train and turn on a flashlight pointed toward the front of the train, the light will travel at 300,000 km/ second with respect to you. How fast will it travel with respect to the earth? Not at 400,000 km/second as you could assume from everyday, Newtonian, experience but at the same 300,000 km/second. Time, however will elapse at a different rate for you than for an observer seated at the train station. Kant considered time, as well as space, to be “a priori knowledge”, that is to say, known to us independent of experience. Special relativity tells us that our a priori concept of time must be radically modified. General relativity says we better start overhauling our a priori concept of space as well. What about Quantum Mechanics?
As we have mentioned in former posts, the Standard Model of quantum mechanics provides us with a tried and true theory for describing the behavior of all the basic particles that we currently know of. The Standard Model uses as a core tool the Schrödinger Equation. Sub atomic particles do not behave in the same way that we are used to when observing everyday objects. Returning to the laws of Newton, we find that Newton’s second law allows us to calculate trajectories as well as energies of objects such as a falling apple or a speeding train. The Schrödinger Equation can be considered as analogous to Newton’s second law for particles. Newton’s Second law tells us what an apple will do when subjected to the earth's gravitational field, in other words when it drops from a tree at a certain height, and it also tells us how much energy it will have at the moment it hits the ground or bumps into a person’s head. Schrödinger’s equation describes the behavior of a particle such as an electron when, for example it is trapped by a proton in a Hydrogen Atom.
However, Schrödinger’s equation doesn’t answer our questions about what the particle is doing in the same way Newton’s second law does. If I want to know how far the apple has fallen, that is to say, what its exact distance from the tree branch is one second after it was released, using the constant for the force of gravity, I find that the apple falls 4.9 meters in one second. In other words, if we ignore the effect of air resistance, we can say with certainty that any apple, or for that matter any object falling to earth can be found without exception at 4.9 meters from where it was dropped after one second. But if we want to know where the electron in a Hydrogen atom is, Schrödinger’s equation can give us a probability of where we will find it, but not its exact position. In fact, the concept of its position, unless it is measured, is no longer meaningful. There are simply probabilities that you will find it at different locations when you actually detect it in some way. If this were the case with the apple, it would mean that as the apple fell from the tree to the ground, its position 1 second after it fell would not necessarily be 4.9 meters from the branch. Of course, this analogy is not quite right because, if an apple acted as an electron, we could not have dropped it from a precise location with a precise amount of potential energy in the first place. Remember Heisenberg and the uncertainty principle. In any case, returning to our electron, the idea is that we do not know exactly where our electron will show up when we observe it, but Schrödinger’s equation gives us more likely and less likely spots. For example, there may be a 50% chance of finding it at a distance A from the center of the atom and only a 10% chance of finding it at a distance B. So if we repeatedly look for the electron in Hydrogen atoms in a similar energy state, we find it more often at point A than at point B, and, eventually we will determine that, in fact, just as our wave equation tells us, it is 5 times more likely to be found at A than at B.
For a professional physicist, solving the Schrödinger equation in this case is quite simple. You can find the details of how the wave function is arrived at and how the probability of finding the electron at a certain distance from the center is calculated on YouTube. But the concept of where the electron is before we measure it is what is really puzzling. Couldn’t it be that we just don’t have enough information to accurately predict that it is in just one single location? Can we even be sure that the electron’s position is not determined until we observe it? This is a great question, and at first glance it seems impossible to answer. But it is not impossible. In 1964 John Bell came up with a theorem that showed that one could test for exactly this question. Ensuing experiments have shown repeatedly that, using Bell’s Theorem, results agree with the presumption that the position of the electron, or any other quantum particle is not simply unknown to us before being observed, it is not defined. How do you explain this? Physicists may not have the definitive answer, but they came up with a cool word for the situation. The electron, they tell us is in a superposition of locations. The question here is what happens when we observe the electron, and find it to be at a certain location. Why does it go from being in a superposition of locations to being in a single location?
Let’s take a step back for a moment and consider what we know and what we don’t. As physics focuses on objects that move faster than we normally encounter, or on things that are much smaller than we perceive, the answers to how things work diverge from what we experience with our unaided senses. Einstein’s theory of relativity posited that time flows at different rates depending on our frame of reference. Should this divergence make us skeptical of what physics tells us? In the example of the train, it is absolutely out of the realm of everyday experience to travel at 100,000 km/second, so for most of the things we do or see, Einstein’s theory has little relevance. But the special theory of relativity has been around now for over a hundred years and has been shown over and over to correctly describe all sorts of natural phenomena. Not only this, but in many cases, the technology we develop and use must take into account special as well as general relativity to work properly. Quantum theory uses the Schrödinger equation to construct a probability wave that describes how particles behave. The idea of a particle’s position not being defined until a measurement is made is, just as the relativistic nature of time, in conflict with our everyday experiences. But, just as in the case of relativity, quantum mechanics and its foundational Schrödinger Equation has been amply tested and corroborated. The behavior of an electron in the example we looked at may seem suspect, but at this point, most physicists accept that this is what it does, and that the Schrödinger Equation is an accurate tool in describing electrons as well as all the other known elementary particles. This we know. What we do not know is exactly what happens when we observe a particle and its location, or any other of its properties, goes from being one of various possibilities to being a definite one.
In his book, Something Deeply Hidden, the renowned physicist Sean Carroll explains this question and takes us on a tour of the different answers that physicists consider. For Carroll, as for a good number of particle physicists, the preferred explanation is the Many Worlds Theory. The Many Worlds Theory, first proposed by Hugh Everett III in the 1950s is seductively simple to many physicists, but outlandish in its apparent implications. The basic concept is that when someone observes an electron and finds it to be at Point A what actually occurs is that this measurement creates a split in the universe, so that all other possible measurements also occur in each different copy. Carroll explains this using the example of the spin property of the electron. I like this example because it uses a situation in which there are only two possible outcomes to our observation. Measuring the spin of an electron can be done using a Stern-Gerlach device, named for the physicist team that performed the original experiment in 1922. Carroll tells us that for his example we take an electron that may have an up or down spin when measured, that is, an electron that is in a superposition of up and down spin. Measuring the spin with our device, we get either an up or down result. If we interpret this according to the Many Worlds theory, what actually happens is that we get both results. How is this possible? According to Everett, at the moment when the electron’s spin is measured, a copy of the observer and all the rest of the universe is created so that in one copy a spin up result was detected and in the other a spin down.
Of course, at every moment many particles are going from a superposition state to one in which spin or location or any other property becomes defined. So the universe is branching at an astounding rate, and copies of you and me abound. Just to make this concept brutally clear, what the Many Worlds interpretation tells us is that, yes, there are many, many universes. Each one is a product of a branching, and at the moment of branching it is the same except for the fact that a particle resolved to be in a different state than in the other universe. This may seem fine for an episode of Rick and Morty, but how can we reconcile something so bizarre and contrary to anything we have ever experienced with our very concept of reality? For that matter, should we even take it seriously?
Carroll’s book is fascinating, and includes an imagined question and answer session between a Many-Worlds-Skeptic father and his unflappable physicist daughter. In general, I would say the book makes an extremely compelling case for Many Worlds from the point of view of physics. It also makes a few philosophical observations which I, for one, found unpersuasive. The basic argument is that for many years physicists have used the Schrödinger equation and there is really no question as to its validity. We also know pretty much unequivocally that reality is made up of things that are in a superposition of states. In the words of Carroll, “The enigma at the heart of quantum reality can be summed up in a simple motto: what we see when we look at the world seems to be fundamentally different from what actually is.” The Schrödinger equation doesn’t give us just one outcome when measuring the spin of the electron in a superposition of states. It gives us two. So the purest, simplest way to interpret this, in Carroll’s view is to simply say that both occur but in two different worlds.
In former posts we asked the question of what it means when cosmologists propose that there are universes created beyond the possibility of observation. Many Worlds is like this idea on steroids: worlds being constantly created with duplicates of ourselves which we have no conceivable way of ever detecting.
As I mentioned formerly, Kant was an astronomer as well as a philosopher and you could say he produced his great works on metaphysics while at the same time understanding and contributing works based on science and the laws of physics. Kant also had the great insight of the Noumenon, the thing in itself which is unknowable, in contrast with what we observe which is the phenomenon. It is surprising to read Sean Carroll’s succinct explanation: what we see when we look at the world seems to be fundamentally different from what actually is, echoing this very concept!
Many Worlds is not a certainty, but the fact that it is a preferred explanation of the Schrödinger equation’s mechanic makes it impossible to simply wave it away (no pun intended) as a fringe idea. Should it make any difference to our everyday existence? There is one interesting aside in “Something Deeply Hidden” regarding an app called Universe Splitter which I was thrilled to download on my phone.
According to Many Worlds, the universe splits when a quantum measurement with various possible outcomes is made. What if we base our decision to do something, Carroll gives the example of ordering a pepperoni or sausage pizza, on a quantum event? This would, according to Carroll and the Universe Splitter logic, allow us to eat both types of pizza by splitting the universe in two. The way the app works is that you enter the two options, pepperoni and sausage, and somewhere a quantum measurement is made and you get back one of the options. According to the app and the Many Worlds theory, in a parallel universe a diverging version of yourself gets the other option. I found, unsurprisingly perhaps, that using the app could not be more disappointing. My other self may have enjoyed a different pizza topping, but it made absolutely no difference to me.
If physics finds Many Worlds the best explanation for quantum phenomena, I suppose Carroll is right in arguing that there is really no reason not to embrace it. But what are the real world implications of the Many Worlds interpretation? Well, there is an underwhelming app, Ted Chiang’s fantastic sci-fi story: Anxiety is the Dizziness of Freedom, and some great Rick and Morty episodes. But, aside from some creative thinking, what implications could there possibly be?
A bit of history about the man who proposed Many Worlds. Hugh Everett III was a brilliant but cold and troubled person as described in Peter Byrne’s book, who’s title alone was enough to get me hooked: The Many Worlds of Hugh Everett III: Multiple Universes, Mutual Assured Destruction, and the Meltdown of a Nuclear Family. His now famous theory was originally met with little enthusiasm. If it is correct, it means we must accept a universe that creates endless copies which can never communicate with each other, in other words, whose existence is essentially non-falsifiable. From a philosophical point of view should we take this concept into account at all? Everett drank too much and his family life was not a happy one. He barely knew his two children. Regarding his daughter, Byrne narrates, “Elizabeth made the first of many suicide attempts in June 1982, only a month before Everett died” after Everett’s death, Elizabeth finally succeeded in killing herself. The note she left said she was going to join her father, in another universe.
Nobel laureate Otto Stern, of the Stern-Gerlach experiment initially dismissed Everett’s interpretation as theology. After reading the words Elizabeth Everett left in her note, I would say her father’s interpretation would make for a very sad religion.
Many Worlds is, nevertheless, fascinating, and whether or not it is correct is, indeed, a fundamental question. I have more for you on this subject in the next post. For now, I will leave you with the words of the famed string theorist Juan Maldacena which Byrne quotes, “When I think about the Everett theory quantum mechanically, it is the most reasonable thing to believe. In everyday life, I do not believe it.”
Links
Comments