In Two Place At Once
Unraveling the mystery of quantum superposition
You often hear that quantum objects can be in superposition, meaning they can occupy multiple locations simultaneously. This concept has captured the imagination of many, leading to a plethora of pop culture references and misconceptions. To truly appreciate superposition, though, we must first examine what it really means, why the colloquial interpretation is problematic, and how the concept of information can help us grasp its true essence.
Quantum Physics in 60 Seconds
Before diving into the intricacies of superposition and information, let’s take a moment to revisit the basics of quantum physics. Quantum physics, often used synonymously with quantum mechanics and quantum theory, is the branch of science that humans use to describe the behavior of matter and energy at the smallest scales, such as atoms, electrons, and photons — and quite successfully so, I might add. Though, as we will soon discuss, the theory is now applied well beyond this regime.
At this microscopic level, the familiar and intuitive rules of classical physics that work well to describe our everyday experiences no longer apply. Instead, the particles and their interactions are governed by a set of new rules that were developed in the first half of the 20th century. The rules contain several counterintuitive principles, with names such as wave-particle duality, quantum entanglement, and, of course, superposition. But in repeating these things that are said about quantum physics, I’ve already sinned by suggesting it must be grounded and understood in terms of real physical things, which don’t exist in the way we are compelled to imagine them.
These principles defy our common sense and challenge our preconceived notions of reality, but they have been confirmed time and time again through rigorous experimentation. The field of quantum physics has its fingerprints on every scientific discovery and technological innovation since its inception. Without quantum physics, we would not know what we are made of, let alone what stars and galaxies are made of, nor would we understand the mechanisms of chemistry or biology and that DNA encodes the recipe to make what we are. Hell, without quantum physics, we wouldn’t even be able to accurately explain why the sky is blue. (It’s because air molecules have a quantum energy level structure tuned to higher visible frequencies causing them to absorb and emit more blue light, which is scattered in the sky, by the way.) Quantum physics has revolutionized our modern technology-infused world with innovations such as lasers, transistors, atomic clocks, and every medical scan you’ve ever had or seen on TV.
It’s been said in more ways than one that “nobody understands quantum physics.” Yet, here we are, using it to make the most accurate predictions ever made by humanity. The fact of the matter is many people understand quantum physics. The problem is that, apparently, nobody can provide a genuinely honest and useful explanation of their understanding. Physicists resort to oversimplification and subtle analogies that ought to come with big disclaimers or not be used at all outside of academia. It’s not their fault, though, as we simply don’t have good language to discuss these things.
Personally, the concepts of information theory provide me with a comfortable understanding of quantum physics. I’m going to share some aspects of it with you. Perhaps it’s not objectively “good,” but I think it’s much better than the way we have been doing it for almost a hundred years now. So without further ado, let’s start with the simplest “counterintuitive” concept in quantum physics.
Superposition: Splitting the Quantum and Classical World
The idea of superposition is central to quantum physics. It’s often oversimplified as particles existing in multiple locations at once. But that’s a bit like trying to explain a complex painting with a single brushstroke, without paint on it. In truth, superposition refers to the mathematical representation of a quantum object’s state. You can imagine a particular state as a spreadsheet filled with numbers that encode all the information required to predict the outcomes of experiments performed using that object. We often refer to objects as being “in” states, and that terminology is not limited to quantum physics.
It might be said that you are “in” a state of happiness. But this implies that happiness is a real objective thing or that states, in general, are objective things that objects transition through. But happiness is not objective. Happiness is really a shorthand concept for a bunch of expectations I should have for your actions, like that you will smile, laugh, and engage positively. Another abused two-letter word: is. We might ask, “what color is the apple?” We definitely don’t say, “if white light shines on this apple, I will perceive what I have been told is the sensation of redness.” No, we just say the apple “is” red, even though red isn’t a real physical state. For our everyday experiences, this is unproblematic, though google “pink is not a color,” and you’ll see that it’s not hard to find pedantry in just about anything.
Quantum states are also not objective things that particles pass through — they are mathematical tools that help us understand and predict the behavior of quantum objects in various situations. There are basic states that predict intuitive things like, “if the position of the object is measured, it will definitely be found here” or “if the position of the object is measured, it will definitely be found there.” But, again, instead of writing out sentences that could quickly turn into paragraphs, we use a conceptual short-hand and say the object is “in” the state of being here or, simpler still, the object just “is” here, or there.
It’s important to keep in mind that states are mathematical objects. The state summarized as “object is here” is actually a spreadsheet of numbers, as is the state summarized as “object is there.” Being made of numbers, we can easily imagine adding them together, giving us a new spreadsheet — a new state. This is a valid description of the object — it can be “in” this state. But what does this mean? Well, technically, nothing beyond what I’ve already said. This state is nothing but information that provides predictions for any experiment performed using the object.
Of course, this is a deeply unsatisfying answer to you, as it has been to generations of physicists. It’s tempting to say that a superposition of the “here” and “there” states is a “here and there” state. Just as we simplified an object being described as “if the position of the object is measured, it will definitely be found here” to “the object is here,” most people simplify the superposition state to “the object is both here and there” — in two places at once! I hope you can see now that this is a ridiculous statement that could have been easily avoided had we not relied on oversimplified short-hand.
Now is a good place to pause and reiterate that states are mathematical objects we assign to phenomena that we use in our theories to make predictions. Sometimes the states can faithfully be interpreted through analogies to everyday experiences. We make copious use of these analogies as they provide enormous conceptual economy. However, many states have no direct connection to our everyday experience. Using the typical analogies then quickly results in nonsensical statements and confusion.
To be clear: an electron, say, is never “in two places at the same.” In fact, in such a superposition state, it is somewhat problematic to even be calling “it” an electron. Electrons are most often considered to be particles, which, if nothing else, have a definite location. The fact that electrons can be in superpositions of the “here” and “there” states forces us to revisit the idea that particles exist at all. There are no particles. There are only phenomena that occasionally behave “as if” they were particles. What is an electron in superposition, then? Information. Such a state provides information, heavily couched in the context of the rest of physics, that allows us to make accurate predictions about what will happen next.
Now, if you have reductionist sympathies, you are likely open to the idea that everything, including the electron, the apple, and the state of happiness, is built up from billions upon billions of fundamental particles interacting according to the laws of physics. The universe is just a collection of particles arranged in various patterns, some of which happen to be very complex. But if I am just a collection of electrons and other particles, which can be in superposition states, why is it that I cannot? Why isn’t superposition a part of our day-to-day lives?
Decoherence: Bridging Two Worlds
When a quantum object is isolated, it can maintain its superposition state and exhibit all that strange and counterintuitive goodness we like to ascribe to the quantum world. However, as soon as it interacts with its surroundings, such as air molecules, dust particles, or even photons of light, it becomes entangled with the environment, and the superposition quickly deteriorates. The more complex the object, the more interactions it has with its environment and the faster the decoherence process unfolds. For example, a dust grain, floating in the dead of space, as far as possible from anything else, including starlight, would cease to be in a superposition of two locations separated by the width of the grain in less than a second. This is the standard story of how the quantum world gives way to the classical world. The problem with this story is that it just waves more quantum jargon in your face. Entanglement? Who ordered that?
Imagine that dust grain again. Initially, suppose it is in one of the “object is here” states. Remember, this simple statement comes with a lot of implicit context. What it means is that if we were to perform an experiment that reveals the location of the dust grain, it would be found in one particular spot. Experiments and measurements are something we take for granted at human scales. One of the most confronting lessons quantum physics teaches us is that measurement requires some interaction with the object of investigation. It’s the context of that measurement that defines the possible outcomes that provide us with information.
For example, superpositions of “here” and “there” are not among the possible states that can be observed if we arrange to measure the position of the dust grain. There are measurements, called interference experiments, that do reveal superposition states, but they are not “position” measurements, and they would be extremely difficult to perform. As noted, such an experiment would at least require an environment more extreme than deep space. Perhaps we will one day perform one of these interference experiments with an object that can be seen with the naked eye. However, there would be absolutely no sense in asking what the superposition states “looks” like because “looking” would necessitate a position measurement, which, again, does not have superposition states as its possible outcomes. The only thing such an experiment would look like is data shown to you on a screen — just more information.
If the interactions between things and a dust grain did not provide information about its location, the superposition state would be maintained. However, the entire concept of location would also be irrelevant in such a situation. It’s the proliferation of location information that gives a “here and there” superposition state meaning and why it features in the theory at all. In effect, our everyday world is a collection of particles with locations because they all mutually encode position information of each other. If it were otherwise, we would not exist. Superposition is not miraculous. It’s the simple, classical “here” and “there” states that provide all the beauty and complexity of our experience. Quantum physics is really a theory about isolation, and it doesn’t apply to you — nor would you want it to.
