ok but now I’m curious about what’s on the whiteboard (unless it’s just random scribbles)
[Referring to the Yan!Professor doodle]
It is indeed a genuine snippet about the Dirac Sea!
Short answer: Dirac proposed the existence of an Anti-world (Dirac Sea) containing antiparticles, a model which was later disproved. It still remains a popular Sci-Fi trope.
Long answer: Let us briefly return to the time when Schrödinger had published his famous equation which described the wave function of a particle. Many physicists asked themselves if this new mathematical formula could be formatted in a relativistic way, according to Einstein's laws.
Now, I won't go into the mathematical details, but you essentially take the time-dependent Schrödinger equation and flip it around using operators and the mass-energy equivalence. What you get, if you're curious, is the Klein-Gordon Equation. If you try to solve it, you will get a -E solution.
Basically, they hit an impasse. Combining Schrödinger's equation with Einstein's relativism resulted in a mathematical solution which suggested there's negative (kinetic) energy. Is negative energy possible? This is where Dirac comes in. He sees the numbers and proposes the following idea: that our world is a sea of particles, and there's another anti-world which contains anti-particles.
The theory was quickly discarded, as it came with too many additional issues. If such anti-particles existed, for example, their mass and gravitational force would be so massive, they should be impossible to ignore. Where were these forces?
Later, Feynman would take over and clear up the confusion. These anti-particles of negative energy merely appear to travel backwards in time, but it's a mathematical trick which can be quickly "fixed" using CPT symmetry. An electron with negative energy traveling from A to B is really just a positron with positive energy that goes from B to A.
While Dirac's Sea turned out to be false, it was a massive source of inspiration for science fiction and fantasy. Surely you must've seen the trope of evil/opposite self from an alternate world. This is where it stems from.
