Jun 08 2015

More Quantum Weirdness

Science news outlets are reporting:

“It proves that measurement is everything. At the quantum level, reality does not exist if you are not looking at it,” lead researcher and physicist Andrew Truscott said in a press release.

What scientific understanding gets distilled down to the public consciousness is important, and that is one of the primary missions of good science news reporting. What is the bottom line, “take-home” message that the average person should walk away with regarding any particular experiment or scientific question. This often requires translating complex technical science into accessible every-day concepts without sacrificing accuracy. Obviously technical detail must be watered down, and doing this without making the result wrong is part of the skill of good science reporting.

One of the pitfalls of science reporting is distilling down a complex scientific concept to something that is cool, engaging, and sensational, but fundamentally wrong or at least highly misleading. The public then latches onto the sensational myth and forever misunderstands the actual science.

That is, I believe, what has happened with quantum mechanics, which is perhaps the most challenging science to explain to a non-technical audience. It’s hard enough for scientists to wrap their minds around, even with a highly technical understanding of what the experiments are actually showing. It seems to me that Truscott is perpetuating the greatest myth about quantum mechanics with a very poor choice of words to characterize what his recent experiment has shown.

The idea that reality at some level “does not exist” until it is “observed” is highly misleading, because both of the concepts I included in quotations are not quite right. A better way to state our current understanding is that very small particles (atomic, subatomic, photons, etc,) exist simultaneously in a wave-particle duality until they interact with other particles.

Truscott and his team recently added another experiment to the list of quantum experiments that demonstrate this weirdness. They essentially carried out a famous thought experiment called Wheeler’s delayed choice experiment. This was first proposed by John Archibald Wheeler in 1978, before we had the technology to carry out the experiment.The experiment was actually carried out recently using photons, and Truscott’s group has now carried out the experiment using a single helium atom.

The experiment (at least in some designs) is a variation of the double-slit experiment in which small particles like photons travel through either one or two very thin parallel slits. If photons, behaving like a particle, travel through one slit then they will form a clump of photons on the other side (a blob of light). If the photon act like waves and travel through both slits simultaneously then they will form an interference pattern on the other side of the slit, as the waves of light crisscross each other. This simplest form of the double-slit experiment reveals that light does indeed travel like a wave, going through both slits and forming an interference pattern. The light must them act like a particle when each photon, for example, strikes an electron in the detector.

Le me dispense with the “observer” myth first as this one is much easier. No conscious observer is required for the photon to become a definitive particle. All that is necessary is for the photon to interact with a particle, like an electron. That is what counts as “observing” in these experiments. The choice of words, however, has created the common misunderstanding that a conscious observer is required. It isn’t.

The second misunderstanding, the “does not exist” part, is a bit more tricky. This is still a gross misunderstanding, but the actual reality is more challenging to convey. Things get interesting with the double slit experiment when you starting messing with the details. For example, if you turn your light source down so dim that only one photon will go through the setup at a time you still get an interference pattern. This means that a single photon is capable of traveling through both slits and interfering with itself. OK – that’s not so hard to understand. The photon is literally traveling like a wave. It is a wave until it strikes another particle, then the wave must collapse down to a definitive particle.

Wheeler’s delayed choice, however, adds another admitted really weird dimension. In this experimental setup there are two paths through which a light beam, or in the recent experiment, a helium atom, can travel. The particle can take one path, the other, or both. If it is traveling like a wave, it will take both. If it is traveling like a particle, it will take only one path. The trick of the experiment is to have a later element in the setup that changes whether the photon or atom will travel like a particle or wave, and to randomly alter this element after the photon or atom passed the first decision point – so after it had to already “decide” to take one or both paths.

In Truscott’s experiment, he dropped a single helium atom through laser beams interfering with each other in order to form a grid pattern that would scatter the path of a wave. The atoms then either did or did not fall through a second grid pattern of laser beams that would recombine the scattered waves. However, the decision to turn the second grid pattern on or off was made randomly after the atom already fell through the first grid and either did or did not travel through both paths.

In other words, the atom traveled like a particle or a wave at the beginning of the experiment depending upon whether or not the grid pattern was present at the end of the experiment. It is as if the atom knew what the total setup of the experiment was going to be even at the very beginning, before the setup was determined.

Certainly this is mind-blowing to a non-quantum creature such as myself with no evolved intuition about the behavior of reality at this tiniest of scales. However, I feel comfortable saying that it is very likely misleading to conclude that the helium atom “does not exist” until its state is determined at the end of the experiment. That phrase, of course, is what led to all the headlines. It is a terrible “bottom line” to hand to the public.

Scientists still argue over what the experiment actually shows. It seems that the majority opinion dispenses with the notion that there is any faster-than-light communication, or time travel, going on. Rather we can simply conclude that the atom is both a particle and a wave at the same time, in an indeterminate state, until it is forced to be one or the other. The atom is behaving like both a wave and a particle as it passes through the first grid, but what gets measured at the end depends upon the entire setup that the atom traversed.

Rather than saying that reality does not exist until it is observed, a better common-language explanation would be that very small particles (atoms, subatomic particles) are indetermined (are simultaneously waves and particles) until they interact with other particles.

That might be a bit confusing and require some explanation, but so be it. I would rather have someone not understand than think they do understand but have it fundamentally wrong. You probably can’t really express quantum indeterminancy in a headline or bumper sticker (feel free to try), at least not without requiring a bit more explanation, and that’s OK.

The simplistic but fundamentally wrong headlines cause a great deal of mischief. “Quantum woo” exists for a reason – largely because of this misunderstanding. The purveyors of all kinds of mystical nonsense have realized that if reality is all a subjective experience of the mind, than anything is possible because quantum mechanics. When physicists say irresponsible phrases like “reality doesn’t exist until it is observed,” they are giving powerful support to dangerous nonsense.

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