Quantum physics is based on experimental evidence, much of which was obtained in the first half of the 20th century. The odd behavior has been seen in laboratories around the world, continually agreeing with the theory, despite all common sense. The really strange behavior occurs only on small scales; when you get to the size of cats, the quantum phenomena seems to always take on a definite value. Still, even today, the exact meaning of this strange quantum behavior is up in the air — something that doesn’t trouble most modern physicists who work on these problems.
Some physicists hope that a “theory of everything,” perhaps even string theory, may provide clear explanations for the underlying physical meaning of quantum physics. Among them, Lee Smolin has cited string theory’s failure to explain quantum physics as a reason to look elsewhere for a fundamental theory of the universe — a view that is certainly not maintained by the majority of string theorists. Most string theorists believe that what matters is that quantum physics works (that is, it makes predictions that match experiment) and the philosophical concerns of why it works are less important. All of the interpretations of why quantum physics work yield the same experimental predictions, so they are effectively equivalent.
Einstein spent the last 30 years of his life railing against the scientific and philosophical implications of quantum physics. This was a lively time of debate in physics, as he and Niels Bohr sparred back and forth. “God does not play dice with the universe,” Einstein was quoted as saying. Bohr replied, “Einstein, stop telling God what to do!”
A similar era may be upon us now, as theoretical physicists attempt to uncover the fundamental principles that guide string theory. Unlike quantum theory, there are few (if any) experimental results to base new work on, but there are many Einsteinian critics — again, on both scientific and philosophical grounds. (We get to them in Part V.)
Even with a firm theory that clearly works, physicists continue to question what quantum physics really means. What is the physical reality behind the mathematical equations? What actually happens to Schrodinger’s cat? Some
physicists hope that string theory may provide an answer to this question, though this is far from the dominant view. Still, any successful attempt to extend quantum physics into a new realm could provide unexpected insights that may resolve the questions.
Interactions transform quantum systems: The Copenhagen interpretation
The Copenhagen interpretation represents the orthodox view of quantum physics as it’s taught in most undergraduate level courses, and it’s mostly how I’ve interpreted quantum physics in this topic: An observation or measurement causes the wavefunction to collapse from a general state of probabilities to a specific state.
The name comes from the Copenhagen Institute in (you guessed it) Copenhagen, Denmark, where Niels Bohr and his students helped form quantum physics in the 1920s and early 1930s, before World War II caused many to leave the Netherlands as they picked sides.
In today’s talk, most physicists view the particles in the wavefunction as continually interacting with the world around them. These interactions are enough to cause the wavefunction to go through a process called decoherence, which basically makes the wavefunction collapse into a definite value. In other words, the very act of interacting with other matter causes a quantum system to become a classical system. Only by carefully isolating the quantum system to avoid such interactions will it remain in a coherent state, staying as a wave long enough to exhibit exotic quantum behaviors such as interference.
Under this explanation, you don’t have to open the box for Schrodinger’s cat to take on a definite state. The Geiger counter is probably where the breakdown occurs, and reality makes a “choice” of whether the particle has or has not decayed. Decoherence of the wavefunction takes place well before it ever reaches the cat.
If no one’s there to see it, does the universe exist? The participatory anthropic principle
The participatory anthropic principle (PAP) was proposed by the physicist John Archibald Wheeler when he said that people exist in a “participatory universe.” In Wheeler’s (extremely controversial) view, an actual observer is needed to cause the collapse of the wavefunction, not just bits and pieces bouncing into each other.
This stance goes significantly further than the strict tenets of the Copenhagen interpretation, but it can’t be completely dismissed when you look in depth at the quantum evidence. If you never look at the quantum system, then for all intents and purposes it always stays a quantum system. Schrodinger’s cat really is both alive and dead until a person looks inside the box.
To John Barrow and Frank Tipler (in their popular and widely controversial 1986 topic The Anthropic Cosmological Principle), this means that the universe itself comes into being only if someone is there to observe it. Essentially, the universe requires some form of life present for the wavefunction to collapse in the first place, meaning that the universe itself could not exist without life in it.
Most physicists believe that the PAP approach places humans in a crucial role in the universe, a stance which went out of favor when Copernicus realized Earth wasn’t the center of the universe. As such, they (rightly, I believe) dismiss this interpretation in favor of those where humans aren’t necessary components of the universe.
This is an especially strong statement of a concept known as the anthropic principle. Recent discoveries in string theory have caused some theoretical physicists who were once strongly opposed to any form of anthropic principle to begin to adopt weaker versions of the anthropic principle as the only means of making predictions from the vast array of string theory possibilities. I explain more about this concept in topic 11.
All possibilities take place: The many Worlds interpretation
In contrast, the many worlds interpretation (MWI) of Hugh Everett III proposes that the wavefunction never actually collapses, but all possibilities become actualities — just in alternate realities. The universe is continually splitting apart as every quantum question is resolved in every possible way across an immense multiverse of parallel universes.
This is one of the most unusual concepts to come out of quantum physics, but it has its own merit. Like the work of Einstein described in topic 6, Everett arrived at this theory in part by taking the mathematics of quantum theory and assuming it could be taken literally. If the equation shows that there are two possibilities, then why not assume that there are two possibilities?
When you look inside the box, instead of something odd happening to the quantum system, you actually become part of the quantum system. You now exist in two states — one state that has found a dead cat and one state that has found a living cat.
Though these parallel universes sound like the stuff of science fiction, a related concept of parallel universes may arise as a prediction of string theory. In fact, it’s possible that there are a vast number of parallel universes — a vast multiverse.
More on this in topic 15.
What are the odds? Consistent histories
In the consistent histories view, the many worlds aren’t actually realized, but the probability of them can be calculated. It eliminates the need for observers by assuming that the infinite complexity of the universe can’t be fully dealt with, even mathematically, so it averages out over a large number of possible histories to arrive at the probabilities of the ones that are more probable, including the one universe that contains the outcome actually witnessed — our own.
Strictly speaking, the consistent history interpretation doesn’t exclude the multiple worlds interpretation, but it only focuses on the one outcome you’re sure of, rather than the infinite outcomes that you can only conjecture.
From a physical standpoint, this is similar to the idea of decoherence. The wavefunctions continually interact with particles just enough to keep all the possibilities from being realized. After you analyze all the possible paths, many of them cancel out, leaving only a couple of possible histories — the cat is either alive or dead. Making the measurement determines which one is the real history and which one was only a possibility.
Searching for more fundamental data: The hidden variables interpretation
One final interpretation is the hidden variables interpretation, where the equations of quantum theory are hiding another level of physical reality. The strange probabilities of quantum physics (under this explanation) are the result of our ignorance. If you understood this hidden layer, the system would be fully deterministic. (In other words, if you knew all the variables, you’d know exactly what was going to happen, and the quantum probabilities would go away.)
The first hidden variables theory was developed in the 1920s by Louis de Broglie, but a 1932 proof by John von Neumann showed that such theories couldn’t exist in quantum physics. In 1952, physicist David Bohm used a mistake in this proof and reworked de Broglie’s theory into his own variant (which has become the most popular version).
The core of Bohm’s argument was a mathematical counterexample to the uncertainty principle, showing that quantum theory could be consistent with the existence of particles that had definite position and velocity. He assumed that these particles reproduced (on average) the results of the Schrodinger wavefunction. He was then able to construct a quantum potential wave that could guide the particles to behave in this way.
In Bohm’s hidden variables theory, there is another hidden layer of physical law that is more fundamental than quantum mechanics. The quantum randomness would be eliminated if this additional layer could be understood. If such a hidden layer exists, it should, in principle, be possible for physics to someday reveal it in some way — perhaps through a “theory of everything.” (Of course, the existence of either a “hidden layer” or “theory of everything” are ideas that aren’t believed by most physicists today.)
