Physicists lay groundwork to better understand the birth of the universe

To date, theoretical physicists have developed theories that explain how parts of the universe work: classical mechanics for objects at everyday sizes and speeds, quantum mechanics for very tiny objects at everyday speeds, special relativity for things that approach the speed of light. But no theory yet has managed to explain the behavior of very small objects that also have very high energy, and "this final case is very important if you want to understand where the universe comes from," says Deffner.

Current theories assume that systems are at least locally stable, which doesn't hold for some of the most interesting cases, from the Big Bang to black holes. "If we want to understand all these cosmological models," Deffner says, "and we want to understand the thermodynamics of the universe, we actually have no means to do that." That may be about to change.

Deffner and Bartolotta's new paper in Physical Review X builds on an explosion of research in the field of quantum stochastic thermodynamics in the last decade. This field describes the laws of thermodynamics -- one of the fundamental pillars of physics -- at a microscopic level for the first time. Importantly, it takes into account how the immediate surroundings affect small systems in a way that differs from how the environment affects larger systems. Deffner and Bartolotta's work extends this field even further to examine tiny systems at very high energies that are changing quickly.

That extension "is really uncharted territory...a completely new idea," says Deffner. "And the reason no one has done it before is because stochastic thermodynamics is only 20 years old. Quantum stochastic thermodynamics is only 10 years old. As a field, we've just learned how to stand. We don't even know how to walk yet."

The work is pioneering, but it fits into a larger ecosystem of existing physics theories. For example, "Quantum field theories are the most general way of attacking a physics problem," broader than special relativity or quantum mechanics, explains Bartolotta. He and Deffner chose to test, using new equations they developed, whether one specific quantum field theory held true in the rare case of a system of extremely small size and extremely high energy -- and it did.

In addition, they found that in their model system with high energy and at small scale, the system was much more likely to return multiple particles upon sending in just one, than to come out with the initial particle and no more, which was a huge surprise. It is possible at high energies, like those generated at the Large Hadron Collider in Switzerland, to end up with a different number of particles than you started with, because the famous equation e=mc2 allows mass to be created from energy when the energy is extremely large. Even so, finding the multiple particle result so frequently was shocking.

The math to calculate it all was "incredibly hard," Deffner reflects, but the end result, which involved finding a way to compute an infinite number of possibilities, "was actually quite beautiful," says Bartolotta. He shares, "Our hope is that this paper will now open the doors for other people in other fields that previously couldn't use these techniques to now use them."

Deffner's overall research goal is to broaden the uses of the framework of thermodynamics, "until we find neat things that no one has ever seen before," and his latest paper fits that bill.

"I'm still totally blown away by the result," Deffner says, but he's also already contemplating possibilities for future research directions. When it comes to untangling the mysteries of the early universe, he notes, "There's a lot that we have to do next."