## Quantum Interpretations

Posted by on Friday, March 20, 2015

Earlier this week I posted an opinion piece on whether or not untrained amateur scientists could ever again make major contributions to the fields of science, and specifically to the area of modern theoretical physics. In that editorial, I commented on a few topics that are as yet unresolved, but which contain many subtle technical details that require many years of specialized training to even understand the problem. In response I have had a number of inquiries about two specific physics questions, and a request to give a simple overview of them.

And so that is the subject of today's two articles. The first is the question of why quantum mechanics works. (I also apologize in advance to more advanced physics students and teachers for quite probably oversimplifying some very complicated theories.)

Quantum mechanics has been studied for over a century now, but it is still not understood. Let me begin with what is known, using a system of one particle as an example. All of the information about this system - the position of the particle, its momentum, etc - is encoded in a wave. This wave then propagates forward in time, changing its properties according to one of several equations (ie Schrodinger equation, Heisenberg matrices, Klein-Gordon equation, Dirac equation, or any of the field theory formulations. These equations are all equivalent, but some are approximations that are easier to use in some situations than others.). Then a measurement is made, and the wave instantly changes to another wave with different properties.

But what exactly is meant by "a measurement is made"? Nobody knows.

The

The

Those are the two primary interpretations, but there are many others. Some include waves propagating backwards in time and interacting with their own past. Some argue that humans must necessarily be part of the wave, and that there is no change to the wave but rather two parts of the wave are interacting. Some argue that the wave contains additional information that we are unaware of, and that when these hidden variables are included the wave will cease to be an issue. Perhaps one day we will discover a system for which quantum mechanics does not work at all.

At present no one knows why quantum mechanics works. And to be honest, the vast majority of physicists, chemists and engineers do not really care so long as the calculations give the right results. But it is worth noting that this cornerstone of modern physics is built on shifting sand, and that no one really knows how it works at all!

And so that is the subject of today's two articles. The first is the question of why quantum mechanics works. (I also apologize in advance to more advanced physics students and teachers for quite probably oversimplifying some very complicated theories.)

Quantum mechanics has been studied for over a century now, but it is still not understood. Let me begin with what is known, using a system of one particle as an example. All of the information about this system - the position of the particle, its momentum, etc - is encoded in a wave. This wave then propagates forward in time, changing its properties according to one of several equations (ie Schrodinger equation, Heisenberg matrices, Klein-Gordon equation, Dirac equation, or any of the field theory formulations. These equations are all equivalent, but some are approximations that are easier to use in some situations than others.). Then a measurement is made, and the wave instantly changes to another wave with different properties.

But what exactly is meant by "a measurement is made"? Nobody knows.

The

**Copenhagen Interpretation**effectively says that a person looks at the system, and the wave instantly changes. There is nothing more to it than that. It works, and it gives the right answer. (which is why it is sometimes also referred to as the*Shut Up And Calculate*method). But what is an observer? Does it have to be a human, or can a cat do it? If a cat, what about a flea? Does it have to be alive, or could a machine collapse the wave? What does it mean to have a wave instantly change? Can it happen if the observer and the system are far apart? Does the wave in one place change simultaneously with the part of the wave lightyears away (because these waves can span the entire Universe in theory)? That would violate the theory of relativity's ban on simultaneity. So basically the Copenhagen Interpretation can give the right answers for most simple systems so long as you don't worry about any details.The

**Manyworlds Interpretation**was developed nearly thirty years later as an alternative. It argues that the Universe divides into multiple copies of itself, and in each the particle will have slightly different properties. Suddenly there are billions of Universe's, each with a copy of the particle and a copy of every person in the world. The the measurement is actually the observer checking which copy of the Universe they have landed in, knowing that somewhere there will be numerous duplicates of themselves measuring a different outcome. As before though, there are still questions. Does nature really require billions of copies of the world for every trivial decision? How do the particles interact across different worlds, since the split is not instantaneous? Does the split occur everywhere at once, or does it propagate out from a central point? The Manyworlds Interpretation is better in some ways, but it still isn't perfect.Those are the two primary interpretations, but there are many others. Some include waves propagating backwards in time and interacting with their own past. Some argue that humans must necessarily be part of the wave, and that there is no change to the wave but rather two parts of the wave are interacting. Some argue that the wave contains additional information that we are unaware of, and that when these hidden variables are included the wave will cease to be an issue. Perhaps one day we will discover a system for which quantum mechanics does not work at all.

At present no one knows why quantum mechanics works. And to be honest, the vast majority of physicists, chemists and engineers do not really care so long as the calculations give the right results. But it is worth noting that this cornerstone of modern physics is built on shifting sand, and that no one really knows how it works at all!