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Have scientists side-stepped the Second Law of Thermodynamics?


Physicists have moved closer to a ‘quantum perpetual motion machine’ – at least on paper. Cathal O’Connell reports.


Physicists have been trying to subvert the Second Law for years with the aim of making a perpetual motion machine.
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Isaac Asimov’s short story The Last Question follows the human race over a trillion-year quest to circumvent the Second Law of Thermodynamics. Now, US and Russian physicists may have found a way to do just that.

In physics terms, this is the equivalent of finding a river that flows uphill, or chucking an ice cube on the fire to fan the flame.

The Second Law of Thermodynamics states that entropy (a measure of disorder) always increases. At root, that’s simply because there generally are a lot more messy states than neat states.

Shuffle a fresh pack of cards, and its initial neat arrangement will get messed up. And if you start out with a neat desk in the morning, it will tend towards messiness as the day wears on.

But physicists, using quantum mechanics to describe particular scenarios of interacting particles where the order of the system increases, appear to have violated this rule, with big ramifications.

The work “could make possible a local quantum perpetual motion machine”, says Valerii Vinokur, a physicist at Argonne National Laboratory and co-author on a study published in Scientific Reports.

But there is nothing to worry about, really. Physics isn’t broken, and the result hinges on the weirdness of quantum mechanics.

The Second Law is actually one of the most profound laws of nature.

It describes why some processes are irreversible (for example, why you can’t unscramble an egg). And it’s possibly what drives forward the arrow of time.

Physicists, as is their wont, have been thinking of ways to break the law for more than 150 years with the practical application of making a perpetual motion machine.

In 1867, Scottish physicist James Clerk Maxwell thought up a somewhat fantastical way to do it. It involved a little demon working as a gatekeeper between two rooms, one hot and one cold. The Second Law (and common sense) says that if you open a door between the two, the temperature will eventually even out between them.

But, by opening and closing the door at just the right moment, “Maxwell’s demon” could allow only the warmest molecules to leave the cold room. This would heat the warm room and cool the cold one.

The new work leads to a quantum version of Maxwell’s demon.

First, Vinokur and his team describe the Second Law of Thermodynamics in quantum terms. They then use it to calculate the entropy changes for a series of thought experiments involving the collision of particles, such as electrons and neutrons.

If the collisions are set up in a particular way, the team found, the entropy change can become locally negative – meaning the particles are in a more ordered state when they come out than when they go in.

As Vinokur explains, this local violation simply means that the entropy decreases in one particular place, while increasing in another.

“What is most important,” he adds, “is that there is not any energy transfer between these two spots.”

That’s a bit like powering a refrigerator on one side of your kitchen using a cooling system on the other, without connecting the two.

This would be impossible in our everyday world, but is made possible by the “non-local” nature of the quantum realm, where distant objects can be connected through entanglement, what Einstein called “spooky action at a distance”.

Now, temporary violations of the Second Law can happen even in everyday experience. Once in a while, a shuffled pack of cards yields a royal flush, five ordered cards (10, J, Q, K, A) all in the same suit.

The importance of the new work is that this emergence of order can be controlled – a bit like the dealer stacking the deck. The bottom line is, it can be used to perform useful work.

Vinokur envisages using the effect to power a nanoscale machine “indefinitely” using the heat of the surrounding environment. This is the so-called “quantum perpetual motion machine”.

Rather than generating energy for free, such a machine would harvest it from its environment in a way that would be impossible in classical mechanics.

The team have yet to validate the basic concept in a real experiment. That’s the next step.

Still, it looks like we’re way ahead of Asimov’s trillion-year schedule.

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Cathal O'Connell is a science writer based in Melbourne.
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