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https://www.quantamagazine.org/theory-of-fluids-enters-the-21st-century-20260817/
quantamagazine
fluid dynamics
Theory of Fluids Enters the 21st Century
From the 1800s to the early 2000s, physicists used the same theory to understand the behavior of fluids. Now, using a modern insight, they have redefined fluids from the bottom up.
Charlie Wood August 17, 2026

In the second half of the 20th century, a conceptual tsunami swept through physics. The discovery that our world emerges from a microscopic world of molecules, which emerges from an even more microscopic world of subatomic particles (which in turn emerges from even stranger stuff) triggered the rewriting of our theories of matter.

But the revolution didn’t reach fluids. Their governing equations remained in their simple, vintage form: the Navier-Stokes equations, first developed in the 19th century. The Navier-Stokes equations are enormously successful at predicting how fluids flow and swirl. But they fail to account for the existence of swarms of microscopic bits that make up matter.

Now physicists have a theory that does. It is the fruit of a 20-year effort to rebuild the theory of fluids from the ground up. Along the way, physicists have come up with a whole new way of defining what it means to be a fluid, based on fundamental properties known as symmetries, and have shown that the Navier-Stokes equations are a consequence of symmetries, which explains why the equations take the forms that they do.
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Lucas and his collaborators integrated this symmetry into Liu’s effective field theory in 2024(opens a new tab). “Our main contribution was to justify what Liu was doing,” Lucas said. “To me it’s a closure of a story.”

While the effective field theory framework has catalyzed new calculations, the real prize may be more conceptual: a new definition of what makes a fluid a fluid. A fluid is any material with a particular set of symmetries, no matter what it’s made of.

“There’s something magical about fluid dynamics,” Nicolis said. “There’s oil, water, mercury. These are all very different, microscopically, very different things that, in practice, behave very similarly when you look at how they flow.”