To the average person, most quantum theories sound strange, while others seem downright bizarre. There are many diverse theories that try to explain the intricacies of quantum systems and how our interactions affect them. And, not surprisingly, each approach is supported by its group of well-qualified and well-respected scientists. Here, we’ll take a look at the two most popular quantum interpretations.
Does it seem reasonable that you can alter a quantum system just by looking at it? What about creating multiple universes by merely making a decision? Or what if your mind split because you measured a quantum system?
You might be surprised that all or some of these things might routinely happen millions of times every day without you even realizing it.
But before your brain gets twisted into a knot, let’s cover a little history and a few quantum basics.
The birth of quantum mechanics
Classical physics describes how large objects behave and how they interact with the physical world. On the other hand, quantum theory is all about the extraordinary and inexplicable interaction of small particles on the invisible scale of such things as atoms, electrons, and photons.
Max Planck, a German theoretical physicist, first introduced the quantum theory in 1900. It was an innovation that won him the Nobel Prize in physics in 1918. Between 1925 and 1930, several scientists worked to clarify and understand quantum theory. Among the scientists were Werner Heisenberg and Erwin Schrödinger, both of whom mathematically expanded quantum mechanics to accommodate experimental findings that couldn’t be explained by standard physics.
Heisenberg, along with Max Born and Pascual Jordan, created a formulation of quantum mechanics called matrix mechanics. This concept interpreted the physical properties of particles as matrices that evolved in time. A few months later, Erwin Schrödinger created his famous wave mechanics.
Although Heisenberg and Schrödinger worked independently from each other, and although their theories were very different in presentation, both theories were essentially mathematically the same. Of the two formulations, Schrödinger’s was more popular than Heisenberg’s because it boiled down to familiar differential equations.
While today’s physicists still use these formulations, they still debate their actual meaning.
A good place to start is Schrödinger’s equation.
Erwin Schrödinger’s equation provides a mathematical description of all possible locations and characteristics of a quantum system as it changes over time. This description is called the system’s wave function. According to the most common quantum theory, everything has a wave function. The quantum system could be a particle, such as an electron or a photon, or even something larger.
Schrödinger’s equation won’t tell you the exact location of a particle. It only reveals the probability of finding the particle at a given location. The probability of a particle being in many places or in many states at the same time is called its superposition. Superposition is one of the elements of quantum computing that makes it so powerful.
Almost everyone has heard about Schrödinger’s cat in a box. Simplistically, ignoring the radiation gadgets, while the cat is in the closed box, it is in a superposition of being both dead and alive at the same time. Opening the box causes the cat’s wave function to collapse into one of two states and you’ll find the cat either alive or dead.
There is little dispute among the quantum community that Schrödinger’s equation accurately reflects how a quantum wave function evolves. However, the wave function itself, as well as the cause and consequences of its collapse, are all subjects of debate.
David Deutsch is a brilliant British quantum physicist at the University of Cambridge. In his book, The Fabric of Reality, he said: “Being able to predict things or to describe them, however accurately, is not at all the same thing as understanding them. Facts cannot be understood just by being summarized in a formula, any more than being listed on paper or committed to memory.”
The Copenhagen interpretation
Quantum theories use the term “interpretation” for two reasons. One, it is not always obvious what a particular theory means without some form of translation. And, two, we are not sure we understand what goes on between a wave function’s starting point and where it ends up.
There are many quantum interpretations. The most popular is the Copenhagen interpretation, a namesake of where Werner Heisenberg and Niels Bohr developed their quantum theory.
Bohr believed that the wave function of a quantum system contained all possible quantum states. However, when the system was observed or measured, its wave function collapsed into a single state.
What’s unique about the Copenhagen interpretation is that it makes the outside observer responsible for the wave function’s ultimate fate. Almost magically, a quantum system, with all its possible states and probabilities, has no connection to the physical world until an observer interacts or measures the system. The measurement causes the wave function to collapse into one of its many states.
You might wonder what happens to all the other quantum states present in the wave function as described by the Copenhagen Interpretation before it collapsed? There is no explanation of that mystery in the Copenhagen interpretation. However, there is a quantum interpretation that provides an answer to that question. It’s called the Many-Worlds Interpretation or MWI.
Billions of you?
Because the many-worlds interpretation is one of the strangest quantum theories, it has become central to the plot of many science fiction novels and movies. At one time, MWI was an outlier with the quantum community, but many leading physicists now believe it is the only theory that is consistent with quantum behavior.
The MWI originated in a Princeton doctoral thesis written by a young physicist named Hugh Everett in the late 1950s. Even though Everett derived his theory using sound quantum fundamentals, it was severely criticized and ridiculed by most of the quantum community. Even Everett’s academic adviser at Princeton, John Wheeler, tried to distance himself from his student. Everette became despondent over the harsh criticism. He eventually left quantum research to work for the government as a mathematician.
The theory proposes that the universe has a single, large wave function that follows Schrödinger’s equation. Unlike the Copenhagen Interpretation, the MWI universal wave function doesn’t collapse.
Everything in the universe is quantum, including ourselves. As we interact with parts of the universe, we become entangled with it. As the universal wave function evolves, some of our superposition states decohere. When that happens, our reality becomes separated from the other possible outcomes associated with that event. Just to be clear, the universe doesn’t split and create a new universe. The probability of all realities, or universes, already exists in the universal wave function, all occupying the same space-time.
In the Copenhagen interpretation, by opening the box containing Schrödinger’s cat, you cause the wave function to collapse into one of its possible states, either alive or dead.
In the Many -Worlds interpretation, the wave function doesn’t collapse. Instead, all probabilities are realized. In one universe, you see the cat alive, and in another universe the cat will be dead.
Right or wrong decisions become right and wrong decisions
Decisions are also events that trigger the separation of multiple universes. We make thousands of big and little choices every day. Have you ever wondered what your life would be like had you made different decisions over the years?
According to the Many-Worlds interpretation, you and all those unrealized decisions exist in different universes because all possible outcomes exist in the universal wave function. For every decision you make, at least two of “you” evolve on the other side of that decision. One universe exists for the choice you make, and one universe for the choice you didn’t make.
If the Many-Worlds Interpretation is correct, then right now, a near infinite versions of you are living different and independent lives in their own universes. Moreover, each of the universes overlay each other and occupy the same space and time.
It is also likely that you are currently living in a branch universe spun off from a decision made by a previous version of yourself, perhaps millions or billions of previous iterations ago. You have all the old memories of your pre-decision self, but as you move forward in your own universe, you live independently and create your unique and new memories.
A Reality Check
Which interpretation is correct? Copenhagen or Many-Worlds? Maybe neither. But because quantum mechanics is so strange, perhaps both are correct. It is also possible that a valid interpretation is yet to be expressed. In the end, correct or not, quantum interpretations are just plain fun to think about.
Note: Moor Insights & Strategy writers and editors may have contributed to this article.
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