Welcome to Quantum Magazine's podcast.

Each episode, we bring you stories about developments in science and mathematics.

I'm Susan Vallett.

The second law of thermodynamics is among the most sacred in all of science, but it's always rested on

19th century arguments about probability. New arguments trace its true source to the flows

of quantum information. That's next.

If you like this podcast, check out The Joy of Why, hosted by me, Steve Stroganz, where curiosity and the pursuit of knowledge take the driver's seat.

We ask questions like, why can prolonged sleep deprivation ultimately be fatal? Where do space, time, and gravity come from? What is life? Learn about all that and more on The Joy of Why, wherever you get your

podcasts. In all of physical law, there's arguably no principle more sacrosynct than the second law of thermodynamics. The notion that entropy,

a measure of disorder, will always stay the same or increase. Nobody's ever seen the second

law violated, and that's not expected anytime soon either. But something about the second law

troubles physicists. Some are not convinced that we

understand it properly or that its foundations are firm. Although it's called a law, it's usually

regarded as merely probabilistic. It stipulates that the outcome of any process will be the most probable one. This effectively

means the outcome is inevitable, given the numbers involved. Yet physicists don't just want

descriptions of what will probably happen. Physicist Hia R. Marletto of the University of Oxford

says they like laws of physics to be exact.

Can the second law be tightened up into more than just a statement of likelihoods?

A number of independent groups appear to have done just that.

They may have woven the second law out of the fundamental principles of quantum mechanics,

which some suspect have directionality and irreversibility built into them at the deepest level.

According to this view, the second law comes about not because of classical probabilities,

...