Dec 23 2014

A New Wrinkle in Quantum Mechanics

The press release of this news items proclaims: “Quantum physics just got less complicated.” I’m not sure I agree. Perhaps in the minds of physicists who actually understand quantum mechanics (as well as it currently can be understood). To the rest of us this new finding is just as strange and incomprehensible as QM itself.

QM describes the universe at the atomic and subatomic levels. At that scale nature behave very differently from what we are used to at the macroscopic level, which is often referred to as the realm of classical physics. The dividing line between the quantum world and the classical world remains a matter of research and debate, but it is somewhere at the level of molecules.

There are several aspects to QM which essentially describe the results of careful experiments. We don’t currently have a proven theoretical framework to explain why the universe behaves this way – that is a breakthrough waiting to happen.

One aspect of QM is known as wave-particle duality. When particles, such as photons, shine through two narrow close slits (the famous double slit experiment) the pattern of light that hits the wall (or film or detector) behind the slits is in a light and dark banded pattern that resembles the interference pattern that results when two waves intersect. The light is clearly traveling as a wave through the two slit and those waves are interfering on the other side.

However, if you place a detector in the setup to determine which slit a photon is traveling through, then the photons no longer form an interference pattern, but pile up in two clumps behind each slit. Detecting the path of each photon makes them behave as particles. They only behave as waves when no one is looking.

Another weird aspect of QM is so-called quantum entanglement. If you create a virtual pair of particles they will have entangled properties. If one is spin up, the other will be spin down. These properties will remain entangled even after the particles have traveled for billions of light years across the universe. This entanglement breaks down, however, as each particle interact with the environment, in a process called decoherence. This is why macroscopic objects are not entangled, their particles are all interacting with each other abolishing any entanglement.

A third weird property of the universe described by QM is Heisenberg’s uncertainty principle. This law states that any two related properties of quantum systems will have a minimum amount of uncertainty. If we take position and vector, for example, the more you know about the position of a particle the less you know about its speed and direction. The sum of the uncertainty of these two properties has a minimum value.

The really strange thing about uncertainty is that it has nothing to do with our technical ability to measure these properties. It is inherent to the properties themselves. There is something about the universe at that scale, the very nature of particles themselves, that is fuzzy. (It’s like the old joke, Bigfoot itself is blurry, which explains all the photographs.)

Originally it was thought that the uncertainty derived from measurement, that there is a minimum amount of disturbance to a system that results from the act of measuring. If you bounce a photon off a particle to measure its location, you change its momentum. However, recent experiments show that this is not the case. Even when the act of measuring produces less uncertainty than the minimum required by Heisenberg, the absolute minimum uncertainty still exists. Measurement does not explain uncertainty. Bigfoot is blurry.

This is where the new study now comes in – it has to do with the relationship between the wave-particle duality and the uncertainty principle. One debate within the world of quantum physics is whether or not these two properties are related or independent. The new study provides evidence for the interpretation that wave-particle duality is simply a manifestation of uncertainty. ┬áThe authors summarize:

Such wave-particle duality relations (WPDRs) are often thought to be conceptually inequivalent to Heisenberg’s uncertainty principle, although this has been debated. Here we show that WPDRs correspond precisely to a modern formulation of the uncertainty principle in terms of entropies, namely the min- and max-entropies. This observation unifies two fundamental concepts in quantum mechanics. Furthermore, it leads to a robust framework for deriving novel WPDRs by applying entropic uncertainty relations to interferometric models. As an illustration, we derive a novel relation that captures the coherence in a quantum beam splitter.

In other words, the more you try to nail down a particle’s position and momentum, the more wavelike it gets. The wave is a probability wave defining the uncertainty in the particle’s position. This concept is not new, but it seems that the authors have now been able to show mathematically that the equations work – using equations that describe a particular formulation of uncertainty called entropic uncertainty corresponds to the wave particle duality in an experimental setup.

If this hold up once it goes through the meat-grinder of peer review, replication, etc., then that would mean that wave-particle duality and quantum uncertainty are simply two manifestations of the same underlying quantum property.

While that is a neat idea, it doesn’t really make QM less mysterious and weird, in my opinion. I still can’t wrap my head around why the universe requires a minimum uncertainty.

Hopefully, however, advances such as this one are leading us closer to an explanatory theory that will tie everything together. Such theories are called interpretations of QM. The experimental results are fairly clear and robust. It is the interpretation of those results that is hotly debated.

Unfortunately, this has opened the door to interpretations that lend themselves to mysticism. We refer to this phenomenon as “quantum woo.” One common (and wrong) quantum woo interpretation is that the wave-particle duality experiments indicate that consciousness affects reality. This is simply not true. Consciousness is not a necessary component of wave-particle experiments. You just need a detector, or any physical interaction. No consciousness is required.

A second common quantum woo interpretation is that quantum entanglement means “everything is connected.” Therefore this explains ESP, astrology, The Secret, or whatever magic you wish to explain. They ignore decoherence and the fact that quantum entanglement cannot be exploited to transmit information.

So far I have not heard a quantum woo exploitation of the uncertainty principle. (If anyone has, please share.)

I would hope that as our theoretical understanding of QM improves, such pseudoscientific exploitation will wane, but I suspect this is not true. In any case, people will latch onto whatever the next cutting edge science is to justify their mystical beliefs.

QM seems ideally suited for such exploitation, however, because it is fundamentally weird, as least to our brains, adapted as they are to the classical world.

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