Wieland Schöllkopf: Small particles of matter sometimes act like light

Proving that quantum reflection exists is a little like demonstrating that a ball which has just fallen off a cliff can bounce back up without hitting the ground at all.

A new study in the journal Science describes how tiny bits of matter – stuff – can reflect off of a surface, much like light. “That’s quantum reflection in a nutshell,” said Wieland Schöllkopf, one of the authors of the study, which appeared in Science on February 18, 2011. Dr. Schöllkopf spoke to EarthSky from his office in Berlin:

Quantum reflection is a kind of bizarre variation on the reflection of waves – light waves reflecting off of glass, for example. Sometimes particles of matter are so small, they start to act like light. But, unlike light, quantum particles – miniscule particles – never even have to hit the glass to be reflected.

With his report, Dr. Schöllkopf confirmed that quantum reflection occurs consistently, and with particles larger than a single atom. Which might not sound like a big deal. But, Schöllkopf explained, what his team did is akin to demonstrating that a ball which has just fallen off a cliff can actually bounce back up, long before it hits the ground.

Image Credit: AAAS

It would typically fall down, because that’s where gravity points to, but, in the world of quantum mechanics, there’s a chance … that instead of falling down into the cliff, the quantum particle bounces back from the cliff, even though all the forces are going in the other direction, and that’s the basis of our experiment.

Schöllkopf reiterated that quantum reflection – the bouncing-back stuff – only works when the amounts of matter involved are tiny. His recent experiment, for example, just involved pairs of helium atoms. Why helium? Helium pairs are notoriously fragile – they break apart very easily.

Schöllkopf’s team shot hundreds of pairs of helium atoms against a surface – a wall – at a particular angle. Most of the helium pairs snapped in two. But not all. The intact helium pairs never hit the wall – they had been reflected, a little like light. With one exception …

In our case, the particles bounced back before colliding with the actual wall – about 1-2% of them, maybe.

He said this contradicts the laws of classical physics, which dictate that a surface like a wall should exert an attractive force on small particles – in other words, matter moving toward a wall should just smash into it, and break apart.

Schöllkopf added that the helium particles that managed to dodge the wall had quite a sixth sense, physically speaking – these particles were able to detect and avoid that wall from 40 nanometers away. He explained:

That seems to be a small distance, but, in the world of these tiny atoms or molecules, it’s an enormous distance.

EarthSky asked him why certain helium particles were able to steer clear of the wall, while others drove straight into it, the way classical physics says they should. He responded that it just comes down to probability:

Image Credit: Wieland Schollkopf

Perhaps it’s like in real life, when you are attracted to another person. Usually, you follow this attraction, but in some cases, you might shy back, although the attraction is there.

So, humans and helium molecules can both be a little gunshy. But what’s this knowledge good for? Again, Dr. Schollkopf:

To tell you the truth, I don’t know. But the question reminds me of a great story. When they invented lasers 50 years ago, scientists didn’t know what they were good for, either. And now they’re in everything: DVDs, computers. I like to think that our observation of quantum reflection might prove to be as useful. We just don’t know how yet.

He added that, while his paper has not proven anything completely new, or immediately usable, he said that his team’s findings are a certain demonstration of one thing. He told us:

The laws of nature, the laws of the microcosmos, are really quite bizarre!

That’s suggested by the new paper “Quantum Reflection of He2 Several Nanometers Above a Grating Surface,” which appeared last Friday in the journal Science.

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