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Jonathan Oppenheim, a quantum physicist at University College London, in the UK, says we are standing on the brink of our third great revolution in thermodynamics.
The eighteenth century saw the development of the steam engine and the industrial revolution, built on advances in thermodynamics—the science of heat and energy transfer. The twentieth century saw the discovery of new laws that govern the microscopic realm, quantum mechanics. That led to the invention of the transistor and, in turn, today’s digital revolution. But what further advances could we make if were able to combine both of these into one theory of "quantum thermodynamics"?
"The systems we are building are getting smaller and smaller," says Oppenheim. Nanoscale devices, biological motors and quantum computers are just some of the tiny technologies we are trying to perfect. How does thermodynamics work on this quantum level? Oppenheim has recently been awarded an FQXi grant of over $60,000 to investigate.
Oppenheim was inspired to think about quantum mechanics from a thermodynamical perspective during his PhD at the University of British Columbia, in Vancouver, when he read a book by Nobel Laureate physicist Richard Feynman on computation. "For me it was this really beautiful thing and I became really interested in the interplay between information theory and thermodynamics," says Oppenheim.
Quantum Thermodynamics
Jonathan Oppenheim describes his quest to find a meta-theory of physics to Colin Stuart.
The second law of thermodynamics is notorious. It places a restriction on the direction of heat flow in a closed system—namely that heat will not spontaneously flow from a cold object to hot object. It is why, unfortunately, cold cups of coffee never magically reheat themselves. It says the entropy (disorder) of a closed system always increases. Another way of describing the consequences of the second law is that you cannot create a heat engine which extracts heat and converts it all to useful work.
A theory of quantum thermodynamics could similarly uncover limits on the amount of useful work that can be gained from atomic scale devices. The conventional, macroscopic rules of thermodynamics represent a statistical approach looking at the overall behaviour of many atoms or molecules at a time. Oppenheim is investigating whether such macroscopic rules can be applied to a microscopic system too, and, if so, whether the rules of thermodynamics for a single particle is the same as for an ensemble of many.
Free Energy
"It turns out there are rules of thermodynamics for quantum systems," says Oppenheim. Rather than thinking of the second law as the notion that entropy always increases in a closed system, Oppenheim prefers to picture it as stating that the "free energy" always decreases. The free energy is the amount of useful energy in a system that is free to do work. "There are many free energies that all have to go down, but in a macroscopic system they are all equivalent. So you can think of it as a single free energy," he says.
Watt Double-Acting Steam Engine Built by D. Napier and Son (London) in 1859, this now stands in the lobby of the Superior Technical School of Industrial Engineers of the UPM (Madrid). Steam engines propelled the Industrial Revolution. What will be the quantum thermodynamic equivalent? Credit: Nicolás Pérez
Crucially, Oppenheim’s work with others has shown that you cannot play the same trick in a quantum regime. The range of free energies are all different and they all have to decrease in order to satisfy the second law (PNAS 112, 3275 (2015)). His task now is to fully formulate these laws.
Oppenheim’s work on how the other laws of thermodynamics apply in the quantum regime could be just as important. Take the third law, for example. "It says that it takes an infinite amount of time to cool something to absolute zero," says Oppenheim. With many quantum technologies—including quantum computing—you want to be able to cool the system as much as possible, but the third law provides a restriction. "We can use our techniques to get a more quantitative and robust answer to how cool we can get a system of a few molecules if we only have a certain amount of time," he says.
Thermodynamics is not the closed-book, 19th century physics that we thought it was.
- Matt Leifer
Matt Leifer, an FQXi member and physicist at the Perimeter Institute for Theoretical Physics in Ontario, Canada, sees this as important work. "It ties together a number of threads that are becoming increasingly important for our understanding of modern physics," Leifer says. "Thermodynamics is not the closed-book, 19th century physics that we thought it was."
Explaining how thermodynamics works on the quantum scale is surely a key step in developing the technologies of tomorrow. But it is not just about practical applications—this work can provide insights into the very foundation of modern theory. "Understanding the ultimate limits on how we can manipulate small numbers of quantum systems may help us better understand the laws of quantum physics themselves," says Leifer.
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PENTCHO VALEV wrote on March 31, 2017 The End of the Second Law of Thermodynamics
"Davide Castelvecchi recently has an article published in Nature regarding on the same topic. The article highlighted that the laws of thermodynamics are many times "paradoxical," especially the second law of thermodynamics."
This time things are serious. The second law of thermodynamics has long been under attack but a red herring deviating the attention to small, microscopic, quantum etc. systems has been very powerful so far:...
PENTCHO VALEV wrote on April 22, 2016 Almost Obvious Violation of the Second Law of Thermodynamics
When a constant-charge parallel-plate capacitor is immersed in a liquid dielectric, e.g. water, a mysterious pressure emerges between the plates, pushes them apart and so counteracts their electrostatic attraction:
"However, in experiments in which a capacitor is submerged in a dielectric liquid the force per unit area exerted by one plate on another is observed to decrease... (...) This apparent paradox can be explained...
PENTCHO VALEV wrote on April 16, 2016 Daniel Sheehan: "The second law of thermodynamics is considered by most engineers and scientists to be the supreme law of Nature, unbreakable even in principle. Over the last 20 years, however, a silent revolution has begun in the thermodynamics community that has pierced its veil of inviolability. More than two dozen challenges have been advanced into the refereed scientific literature, and experiments now call into question its absolute status. This talk will discuss the history of paradigm...