Quantum Weirdness: Not so Weird After All?

February 24, 2016 | Joanne Kennell

Artistic representation of an atom

At the subatomic level, reality begins to unravel.

A 64-year-old theory of quantum mechanics is making a comeback.  The theory, developed by physicist David Bohm in 1952, was resurrected after experiments on photons seemed to support it.

Reality at the quantum level gets a little confusing.  For example, in the everyday macroscopic (large-scale) world we live in, the location of objects in space can be estimated with remarkable accuracy, but at the subatomic level, reality begins to unravel.  This unraveling is commonly called quantum weirdness, and it perplexes scientists.

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“We have had geniuses working on it and we still have a problem,” Basil Hiley, a quantum physicist at Birbeck College at the University of London, who worked with Bohm until Bohm’s death in 1992, said to New Scientist.

The famous double-slit experiment, which has been described as the subatomic equivalent of Schrödinger's cat, involved firing photons at two slits. Instead of each photon passing through one slit, hitting a screen on the other side, and making a single mark — as you think would happen if photons were particles — the photons created a series of light and dark bands.  This indicates that a photon behaves like a wave, not a particle, and passes through both slits simultaneously.

Double Slit experiment
Bottom: a series of light and dark “interference” bands results from shining a light through two slits. Photo credit: Jordgette/Wikipedia (CC BY-SA 3.0)

An explanation for this behavior proposed by physicists is called the Copenhagen Interpretation, which states that a photon is neither a wave nor a particle until you make a measurement — and then it becomes one or the other depending on which property you measure.

Another popular explanation is called the Many-Worlds Interpretation, and this states that each possible state of the photon will manifest in an alternate universe, thus creating an infinite number of universes.

In 2011, physicist Aephraim Steinberg, from the University of Toronto, managed to track the trajectories of photons using a series of measurements and then averaging the information.  This method showed that the trajectories looked very similar to the classical intuition of the double-slit experiment — like balls flying through the air.  

However, many physicists were not convinced of his result because the experiment did not take quantum entanglement into account. Quantum entanglement says that two particles can be intimately connected so measuring one instantly affects the other, regardless of the distance between them. Opponents of Steinberg’s findings argued that measuring one particle would lead to an incorrect prediction of the trajectory of the entangled particle, resulting in surreal trajectories.

It was this idea of surreal trajectories that originally resulted in Bohm’s theory of quantum weirdness being dismissed in the 1990s.

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Now, Steinberg has conducted a new experiment that accounts for the effect of entanglement.  What he discovered was that, at the quantum level, particles behave like billiard balls rolling along a table, and that the surreal behavior observed in his previous experiment was caused by the “spooky” influence of the other particle or quantum weirdness.

As was explained by the Copenhagen Interpretation, there is no real trajectory of photons, and scientists can only calculate the odds of a photon being in one place at any given time.

But Steinberg’s experiment tested another interpretation, known as the De Broglie-Bohm theory, which states that photons do have real trajectories and that their paths are governed by a pilot wave that the particles hitch a ride on. According to Steinberg’s results, this theory holds true.

“I’m happy to see this resolution. It restores my taste for Bohmian mechanics,” said Steinberg.  “We want to bring it back to its rightful place among all other interpretations.”

Although the experiments do not prove that Bohm’s theory was correct, it does show that it cannot be excluded as a possibility — and what more can you ask for when trying to solve the mysteries of the quantum world.

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