⚛️ Quantum Physics

Classical physics predicted that a hot object should radiate infinite energy at high frequencies — the "ultraviolet catastrophe." Max Planck solved it in 1900 by assuming energy is emitted in discrete chunks, or quanta, proportional to frequency: E = hf.

This wasn't supposed to be a real physical claim — Planck called it a "mathematical trick." But it worked perfectly, and five years later Einstein showed it was actually true. Energy doesn't flow continuously. It comes in packets.

Shine light on metal and electrons pop off. Classical physics said brighter light should eject faster electrons. Wrong — frequency matters, not brightness. Below a threshold frequency, no electrons are ejected no matter how bright the light.

Einstein explained this in 1905 by treating light as particles (photons) with energy proportional to frequency. Each electron needs a minimum energy to escape — one photon per electron. This was the definitive proof that light is quantized. It's also what Einstein actually won the Nobel Prize for, not relativity.

De Broglie proposed in 1924 that if light can be a particle, matter can be a wave. Schrödinger gave that wave a mathematical description: the wave function ψ, which encodes the probability of finding a particle in any given location.

The wave function doesn't say where the particle is — it says where it might be, and with what probability. Before measurement, a particle exists in a superposition of all possible states. Measurement collapses the wave function to a single outcome. This is not ignorance of the position — the position genuinely doesn't exist until measured.

Classical physics said electrons orbiting a nucleus should continuously radiate energy and spiral inward — atoms should collapse in nanoseconds. They don't. Bohr proposed in 1913 that electrons can only occupy specific energy levels, and only emit photons when jumping between them.

The modern picture is more nuanced: electrons don't orbit like planets. They exist in probability clouds (orbitals) defined by quantum numbers. The Heisenberg uncertainty principle means you can't know both position and momentum precisely — the electron's "orbit" is fundamentally smeared across space.

The Standard Model is the most precisely tested theory in science. It describes 17 fundamental particles: 6 quarks, 6 leptons, 4 force-carrying bosons, and the Higgs boson. Every atom, every interaction you've ever experienced, is built from these.

The Higgs field, confirmed in 2012 at CERN, permeates all of space. Particles gain mass by interacting with it — the more strongly they interact, the more massive they are. Photons don't interact with it at all, which is why they have no mass and travel at c.

Every fundamental particle has an antiparticle with opposite charge. When matter meets antimatter, they annihilate — converting entirely to energy (photons). The Big Bang should have created equal amounts of matter and antimatter. They should have annihilated each other completely. But here we are.

The slight asymmetry (roughly one extra matter particle per billion) that allowed the universe to exist is called CP violation. We know it exists. We don't fully understand why it's large enough to produce a universe full of matter. This is one of the deepest unsolved problems in physics.

Below a critical temperature, some materials lose all electrical resistance — current flows forever without energy loss. This is superconductivity, explained by BCS theory: electrons pair up (Cooper pairs) and behave as bosons, condensing into a single quantum state. The whole macroscopic current flows as one coherent quantum object.

Superfluids (like liquid helium below 2.17K) flow with zero viscosity — they can climb up and over the walls of a container. Both phenomena are quantum mechanics operating at scales you can see and touch.

The strong nuclear force binds quarks into protons and neutrons, and protons and neutrons into nuclei — it's the strongest force in nature but operates only at subatomic distances. The weak force mediates radioactive decay, allowing one type of quark to change into another (converting a neutron to a proton in beta decay).

Radioactive decay is quantum mechanical tunneling — a particle escaping a potential barrier it classically shouldn't be able to cross. Half-life is the time for half a sample to decay, determined purely by quantum probability. You cannot predict when any individual atom decays.

When two light nuclei fuse, the product is slightly lighter than the sum of its parts. The missing mass converts to energy via E = mc². The sun fuses roughly 600 million tons of hydrogen per second, converting about 4 million tons of that to pure energy.

Fusion requires overcoming the electromagnetic repulsion between positively charged nuclei — which requires extreme temperatures and pressures. In stars, quantum tunneling does much of the work: protons tunnel through the Coulomb barrier at temperatures lower than classical physics would require. Without tunneling, the sun wouldn't shine.

A classical bit is 0 or 1. A qubit can be in superposition — both 0 and 1 simultaneously — until measured. Two entangled qubits share a correlated quantum state: measuring one instantly determines the other regardless of distance.

Quantum computers don't just try all solutions simultaneously. They use quantum interference to amplify correct answers and cancel wrong ones. They're not faster at everything — they excel at specific problems: factoring large numbers (Shor's algorithm), searching unsorted databases (Grover's algorithm), simulating quantum systems.

To see small things, you need short wavelengths. Short wavelengths mean high energies. The LHC accelerates protons to 99.9999991% of the speed of light and collides them — producing temperatures 100,000 times hotter than the center of the sun for a fraction of a second.

The collision energy briefly recreates conditions from the early universe, allowing new particles to materialize from pure energy (E = mc² in reverse). Detectors the size of cathedrals capture the particle showers that follow. The Higgs boson was found this way in 2012 — a particle predicted in 1964, confirmed 48 years later.

Entangled particles share a quantum state: measuring one instantly affects the other no matter the distance. Einstein called this "spooky action at a distance" and thought it proved quantum mechanics was incomplete. Bell's theorem (1964) and subsequent experiments proved Einstein wrong — the correlations are real and cannot be explained by hidden local variables.

Quantum teleportation transfers the quantum state of a particle to another location — but requires a classical communication channel alongside it. It cannot transmit information faster than light. What's teleported is the configuration, not the matter. It's more like faxing than Star Trek.

String theory proposes that fundamental particles are not points but one-dimensional vibrating strings. Different vibrational modes produce different particles — the "notes" of the string correspond to particle types. It naturally incorporates gravity and could unify quantum mechanics with general relativity.

The theory requires 10 or 11 spacetime dimensions, with the extra 6–7 compactified at the Planck scale. The landscape of possible string vacua is estimated at 10^500 — a number so large it suggests the multiverse may be the natural setting for the theory.

The Copenhagen interpretation says the wave function collapses upon measurement — but what counts as a measurement? The Many-Worlds interpretation (Everett, 1957) avoids this by saying the wave function never collapses: instead, every quantum event causes the universe to branch. All outcomes occur, in separate branches.

There are at least four distinct multiverse concepts in modern physics: Level I (beyond our cosmic horizon), Level II (inflation bubbles), Level III (Many-Worlds), Level IV (mathematical structures). They're not all the same idea and shouldn't be conflated.

The cosmic microwave background — thermal radiation left over from 380,000 years after the Big Bang — shows tiny temperature fluctuations. These fluctuations originated as quantum fluctuations in the inflaton field, stretched to cosmic scales by inflation. The large-scale structure of the universe (galaxies, clusters, voids) grew from quantum noise.

The first fraction of a second after the Big Bang is a quantum gravity regime — both general relativity and quantum mechanics are needed, and we don't have a working theory. The singularity at t=0 is not a prediction of physics; it's where our current theories break down. What "before" the Big Bang means, if anything, is an open question.