Skulls in the Stars

This is part 3 in a lengthy series of posts attempting to explain the idea of quantum entanglement to a non-physics audience.  Part 1 can be read here, and Part 2 can be read here.

Here, in part 3, we will at long last introduce entanglement! But, before we do, we need to be sure we really understand what the wave properties of a quantum particle imply about its behavior.

So, by the late 1920s, physicists knew that discrete bits of matter — electrons, for example — sometimes act like a wave and sometimes act like a particle.  This seemingly contradictory nature is often referred to as wave-particle duality.  It was not immediately obvious how to interpret the wave properties of matter, but physicists finally settled on what is referred to as the “Copenhagen interpretation,” after the city in which it was more or less developed.  If we consider the motion of a single electron, the Copenhagen description of quantum behavior could be summarized as follows:

• While freely propagating through space, the electron and all of its properties evolve as a wave.
• The wave may be described as a “wave of possibilities”: the amplitude of the wave only describes the probability of the electron to be found in a certain position or configuration.
• While evolving as a wave, the electron in general has no definite position or configuration — it is, roughly speaking, existing in all possible configurations simultaneously.  In Young’s double slit experiment, for instance, it is often said that the electron goes through both slits.
• When a property of the electron is measured in an experiment, where the property must take on a definite value, the electron “chooses” an outcome, based on the wave probabilities mentioned above.  The wave “collapses” into that single outcome.  For instance, if we have a detector to measure the position of the electron, we will see that electron at a single definite location on the detector.
• If the particle still is free to continue moving, the process repeats itself, the collapsed wave evolving again, usually resulting in a wave of many possible outcomes again.

This is part 2 in a lengthy series of posts attempting to explain the idea of quantum entanglement to a non-physics audience.  Part 1 can be read here.

So, by the mid 1920s, physicists had made significant progress in developing the new quantum theory.  It had been shown that light and matter each possess a dual nature as waves and particles, and Schrödinger had derived a mathematical equation that accurately described how the wave part of matter evolves in space and time.

But it was not clear what, exactly, was doing the “waving” in a matter wave.   Water waves arise from the oscillation (waving) of water, sound waves arise from the oscillation of molecules in the air, but what is oscillating in a matter wave?  Or, to put it another way, what does such a wave represent?

We will try and answer this question by looking at how a matter wave manifests in an actual experiment.  It turns out that the best example for demonstrating the wave properties of matter is also the best example for demonstrating the wave properties of light: Young’s classic double slit experiment!  However, the double slit experiment was not done with electrons until decades after the foundation of quantum mechanics, so we must briefly step away from our historical discussion to investigate it.

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If you follow science, or science fiction, to any degree, great or small, you’ve probably heard the term “quantum entanglement” before.  You may also have heard it referred to as “spooky action at a distance,” and understand that it somehow involves a weird connection between separated quantum particles that can “communicate,” in a sense, over long distances instantaneously.  You may have read that quantum entanglement is a key aspect in proposed technologies that could transform society, namely quantum cryptography and quantum computing.

But it is difficult for a non-physicist to learn more about quantum entanglement than this, because even understanding it in a non-technical sense requires a reasonably thorough knowledge of how quantum mechanics works.

In writing my recently-published textbook on Singular Optics, however, I had to write a summary of the relevant physics for a chapter on the quantum aspects of optical vortices. I realized that, with some modification, this summary could serve as an outline for a series of non-technical blog posts on the subject; so here we are!

It will take a bit of work to really get at the heart of the problem; in this first post, I attempt to outline the early history of quantum physics, which will be necessary to understand what quantum entanglement is, why it is important, and why it has caused so much mischief for nearly 100 years!

Small disclaimer: though I am a physicist, I am not an expert on the weirder aspects of quantum physics, which have many pitfalls in understanding for the unwary! There is the possibility that I may flub some of the subtle parts of the explanation. This post is, in fact, an exercise for me to test my understanding and ability to explain things. I will revise anything that I find is horribly wrong.

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