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New evidence could break the standard view of quantum mechanics

Just as we were beginning to understand it...

FIONA MACDONALD
24 MAY 2016
 

Quantum mechanics is difficult to understand at the best of times, but new evidence suggests that the current standard view of how particles behave on the quantum scale could be very, very wrong.

In fact, the experiment hints that an alternative view predicted almost a century ago might have been right this whole time. And before you get too bummed about that, the good news is that, if confirmed, it would actually make quantum mechanics a whole lot simpler to understand.

 

So let's step back for a second here and break this down. First thing's first, this is just one study, and A LOT more replication and verification would be needed before the standard view comes crumbling down. So don't go burning any text books just yet, okay? Good. 

Now that we've got that straight, here's what's going on. Right now, one of the most confusing (but important) aspects of quantum mechanics is the idea that particles don't have a location until they're observed.

We've talked about this a few times, but what that basically means is when quantum physicists talk about a particle, there's a cloud of possibilities for its location, and that's described by a mathematical structure known as a wave function.

As soon as a particle is observed, its wave function collapses. And only then does it have a specific position.

The mathematics behind all of that is clear enough, and scientists can use it to work with particles on the quantum scale. But for the rest of us, it's all a little odd.

Even Albert Einstein had issue with this part of the standard view, which we often called the Copenhagen interpretation. Einstein's biographer Abraham Pais remembers this conversation, as Dan Falk reports for Quanta Magazine:

"We often discussed his notions on objective reality. I recall that during one walk Einstein suddenly stopped, turned to me and asked whether I really believed that the Moon exists only when I look at it."

So why did the Copenhagen interpretation become our standard view then? Well, we did have an alternative, known as the pilot-wave theory, or Bohmian mechanics, which states that particles really do have precise positions, whether or not we're observing them.

But it never really took off, in part, because it would mean that the world must be "strange in other ways," as Falk explains.

To simplify it greatly, the weirdest part about Bohmian view is that it insists upon nonlocality, which basically means that anything in the Universe can affect anything else, no matter how far apart these objects are.

That's the same idea behind quantum entanglement - or "spooky action at a distance" - but Bohmian mechanics takes it a step further and suggests that the entire Universe is dependent on actions happening to individual particles.

One of the final blows for this Bohmian view was delivered in 1992 when a study claimed that a particle following these laws would end up taking a trajectory that was so bizarre that they described it as "surreal" - which is saying a lot coming from quantum physicists.

 

But now, almost 25 years later, researchers in Canada have conducted an experiment that they say invalidates that 1992 paper, and suggests that Bohmian mechanics might still have some potential.

The experiment in question is the double-slit experiment. It works like this: you fire a beam of photons at two parallel slits in front of a detector screen, and instead of seeing just two bands of light, or photons, on the other side, you see a stripy 'interference pattern'.

You can see that pattern in the diagram below:

Double-slitNekoJaNekoJa/Wikimedia
The experiment is often used as evidence of the Copenhagen interpretation, because particles don't have a defined position until they're measured, so of course they're showing up all over the place.

Back in 1992, it also disproved Bohmian mechanics. At the time, scientists claimed that if a photon really does have a position, like the Bohmian view states, then it would pass through just one slit. But it somehow always ends up being recorded as having passed through both slits, so the photon would have a "surreal" trajectory. Hence no more Bohmian view.

But now a group of researchers led by Aephraim Steinberg from the University of Toronto in Canada have conducted this experiment IRL, showing that it does make sense with Bohmian mechanics, as long as people remember to consider nonlocality - that idea that particles can affect other particles anywhere in the Universe.

In their experiment, the team used pairs of entangled photons - which are inextricably linked, so that whatever happens to one will automatically affect the other, no matter how far apart they are (there's that spooky action again). This allowed the researchers to 'interrogate' one of the photons to gain information about the path taken by the other.

That interrogation returned "surreal" results, just as the 1992 study had predicted. But Steinberg and his team say that's only a problem without nonlocality. As Falk explains for Quanta:

"The farther the first photon travels, the less reliable the second photon’s report becomes. The reason is nonlocality. Because the two photons are entangled, the path that the first photon takes will affect the polarisation of the second photon. ... 

The problem isn’t that Bohm trajectories are surreal, said Steinberg. The problem is that the second photon says that Bohm trajectories are surreal - and, thanks to nonlocality, its report is not to be trusted."

The results have been published in Science Advances, and if they're verified, it could very well shake up our view of quantum mechanics - potentially making it easier to understand.

"All you have to do to make sense of quantum mechanics is to say to yourself: When we talk about particles, we really mean particles. Then all the problems go away," Goldstein told Falk. "Things have positions. They are somewhere."

"It’s a far simpler version of quantum mechanics than what you find in the textbooks," he added.

For the record, Einstein didn't think much of Bohmian mechanics, and found the whole thing too simplistic. Only time will tell if he was right.

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