How Classical Kinetic Energy Is Actually A Subset Of Relativistic Kinetic Energy

Many people think that Classical Physics and Relativistic Physics are two different things. Of course, anyone who has studied both can tell you that one can derive many of the classical physics equations from relativistic equations, proving that classical equations are actually special cases of the more general relativistic equations.

In this Don Lincoln's video, he shows how classical kinetic energy that many students learn in General Physics courses can actually be derived from the more general relativistic energy equation, and why we still use the classical physics equation in most cases.

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Electrons Behave Like A Fluid - Exhibit Vortices

This is a rather cool experiment.

They have a direct observation, for the first time, of electrons behaving like an ordinary fluid and exhibiting vortices  when going thorough a channel.[1]

In contrast, electrons flowing through tungsten ditelluride flowed through the channel and swirled into each side chamber, much as water would do when emptying into a bowl.

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“That is a very striking thing, and it is the same physics as that in ordinary fluids, but happening with electrons on the nanoscale. That’s a clear signature of electrons being in a fluid-like regime.”

So far, "ordinary" electron flow behaves like a "Fermi liquid", which is not like ordinary fluid flow. To get electrons to behave this way, they had to make sure that the electrons do not bump into the crystal lattice (the bulk material), so this is not easy since normal-state electrons usually have such interaction (non-zero resistivity).

Just to be clear, this is not the first observation of electrons exhibiting vortex flow. This is a common observation when they are in a superconducting state, where vortices form around magnetic flux lines that penetrates Type II superconductors. But in that case, these electrons are in a superfluid, and what is flowing is the paired electrons (Cooper pairs).

In this experiment, these are individual electrons not in a superconducting state, so this truly is a river of electrons.

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[1] A Aharon-Steinberg et al., Nature 607, 74 (2022).