11 General quantum talk

From Double Slits to the Big Bang: A Curiosity Trail Through Quantum Physics

Jump to a section: Double-slit · Why we don’t interfere · Gravity & collapse · Spacetime breaking · Two kinds of superposition · Chemical bonding · Water gelling into liquid · Empty atoms · Light & color · Is absorption really deterministic? · What is a photon · Fields, not particles · The Big Bang · Glossary

1. The Double-Slit Experiment: Where It All Starts

Fire particles — photons (light) or electrons — one at a time through a barrier with two narrow slits, toward a detector screen behind it.

If particles were just tiny bullets, you’d expect two clumps on the screen, one behind each slit. Instead, you get an interference pattern: alternating bright and dark bands, the signature of waves overlapping and reinforcing or canceling each other.

Interference: When two waves overlap, their peaks and troughs add together. Two peaks meeting makes the wave bigger (constructive interference — a bright band). A peak meeting a trough cancels out (destructive interference — a dark band).

The strange part: this pattern shows up even when particles go through one at a time, minutes apart. Each one still lands as a single dot, proving it behaves like a particle on arrival — but thousands of dots build up into the same wave-like pattern over time. It’s as if each particle interferes with its own possibilities.

Electrons do this too. So do neutrons, whole atoms, and even large molecules (buckyballs, and molecules with hundreds of atoms). It’s not a quirk of light — it’s a general feature of quantum objects.

The catch: put a detector at the slits to find out which one each particle actually went through, and the interference pattern disappears. Gaining “which-path” information collapses the wave-like behavior.

2. Why Don’t We Show Interference? (Decoherence)

Two separate reasons, and only one of them is really about “size.”

A) Wavelength shrinks with mass.

de Broglie wavelength (λ): Every particle has a wavelength, λ = h/p, where h is Planck’s constant and p is momentum (mass × velocity). Bigger, faster objects get absurdly tiny wavelengths.

A walking human has a wavelength around 10⁻³⁵ meters — smaller than a proton by a wide margin. You’d need physically impossible slit sizes to ever see interference from a person, even in perfect isolation.

B) Decoherence — the bigger reason.

Decoherence: A quantum system loses its superposition because it interacts with its environment (stray photons, air molecules, heat). Each interaction acts like a tiny “measurement” that reveals which path or state the system is in.

A single electron in a vacuum chamber can stay coherent for a while. A buckyball needs cooling and ultra-high vacuum — one stray photon bouncing off it kills the interference pattern. A human is constantly emitting infrared radiation and colliding with air molecules, getting “measured” billions of times a second. Superposition never gets a chance to survive at that scale.

The current experimental frontier is interference with molecules of tens of thousands of atomic mass units — achievable only with extreme cooling and isolation.

3. Does Gravity Cause Its Own Kind of Collapse?

This one’s more speculative — a proposal from physicist Roger Penrose (and separately, Lajos Diósi).

The natural objection: gravity in general relativity isn’t a force like electromagnetism, it’s the curvature of spacetime itself. So how could it “measure” anything the way a stray photon does?

Penrose’s argument: put a massive object into a spatial superposition — here and there — and each branch corresponds to a different curvature of spacetime, since mass curves spacetime and “here” vs. “there” curve it differently. Ordinary quantum mechanics assumes both branches live in the same spacetime background. If they don’t — if they genuinely correspond to different geometries — there’s no clean way to combine them, and the superposition becomes mathematically ill-defined once gravity is factored in.

Penrose’s proposal: nature resolves this by forcing a spontaneous collapse to one branch, faster for larger masses. This isn’t decoherence in the usual sense — nothing is “looking” at the object. It’s a proposed modification to quantum mechanics itself, specific to gravity’s odd status as geometry rather than an ordinary field.

Untested and speculative for now — but labs are actively trying to put increasingly massive objects into superposition to see whether they survive as long as ordinary decoherence predicts, or collapse sooner as Penrose’s math predicts.

4. Is Spacetime Always Smooth? Where It “Breaks”

Spacetime is usually treated as smooth and continuous, curving under mass and energy. But there’s one place this is known to fail:

Singularity: A point (or ring) where spacetime curvature becomes infinite and general relativity’s equations stop making sense. Found at the center of black holes. Considered a sign GR is incomplete — physicists expect a future theory of quantum gravity to describe what actually happens there.

Event horizons are a related but different kind of boundary — not a break in smoothness (a falling observer wouldn’t feel anything special crossing one), but a break in causal connection: past this point, nothing, not even light, can escape back out.

Whether new mass can spawn a genuinely separate spacetime (“baby universes,” or bubble universes in inflationary cosmology) is speculative and unconfirmed — but it shows physicists take seriously the idea that spacetime has limits to how much curvature it can smoothly represent. That’s the same theme Penrose is invoking, just at everyday-object scale instead of black-hole scale.

5. Superposition Inside an Atom vs. Superposition in the Double Slit

If double-slit superposition is so fragile, why do electrons inside atoms stay “spread out” reliably?

Because there are two different kinds:

Path/double-slit superposition — a superposition of two distinguishable, divergent histories (left slit vs. right slit). Fragile: one environmental nudge destroys the interference between branches.
Orbital superposition — an electron in an orbital is a superposition of position states, but a stable, unchanging one, called a stationary state. Its probability cloud doesn’t evolve over time — more like a standing wave on a guitar string than two racing alternatives.

Stationary state: A quantum state whose overall probability distribution stays constant over time, even though it’s mathematically a superposition of position states.

There’s no single crisp textbook term drawing this line — both are formally “superpositions.” Physicist Anthony Leggett proposed a concept called disconnectivity: a measure of how many particles are involved in a superposition, and how distinguishably far apart the branches are — meant to capture exactly the gap between a trivial microscopic superposition (an orbital) and a genuinely macroscopic one (a cat, or a mass in two places).

The practical point: an electron’s delocalization within a stable orbital survives constantly, unaffected by being part of you. What decoheres almost instantly in a warm, dense system like your body is any large-scale coherence between distant, distinguishable branches — the double-slit kind.

6. Chemical Bonding: How Atoms Actually “Lock” Together

Take water, H₂O — two hydrogens, one oxygen.

“Shells” are bookkeeping, not real structures. Electron shells (K, L, M, or 1s, 2p labels) are a naming convention for grouping orbitals by energy level, not literal spherical containers. What’s physically real is a nucleus and a probability cloud (the orbital) describing where an electron is likely to be found.

What happens when atoms bond covalently:

Atomic orbitals from separate atoms overlap and mathematically combine.
This creates a new molecular orbital — a single quantum state spanning both nuclei.
Electrons in that shared orbital stop being “hydrogen’s” or “oxygen’s” — they’re delocalized across the whole bond. Asking which atom “owns” an electron stops being meaningful.
The negative charge concentrated between the two positive nuclei is literally the glue — both nuclei are electrically drawn to that shared electron density.

Covalent bond: A chemical bond formed when two atoms share electrons through a merged molecular orbital, with the shared density attracting both nuclei at once.

The influence goes both ways. Once bonded, an electron feels the pull of every nearby nucleus, not just the one it “started” on. Hydrogen’s electrons get pulled by oxygen’s nucleus; oxygen’s electrons get pulled (more weakly) by the hydrogen nuclei.

That pull isn’t even — and that’s what makes water polar. Oxygen is more electronegative — it pulls harder on shared electron density than hydrogen does. So:

Oxygen ends up slightly negative (δ⁻)
Each hydrogen ends up slightly positive (δ⁺)

Polar molecule: A molecule with an uneven distribution of electrical charge, caused by atoms pulling unevenly on shared electrons. Water is the classic example.

7. How Individual Water Molecules “Gel” Into Liquid Water

A single H₂O molecule is a fixed covalent unit. Liquid water is a separate, higher-level phenomenon, built from hydrogen bonds between molecules.

Hydrogen bond: A relatively weak electrostatic attraction (not a shared-electron covalent bond) between a slightly positive hydrogen on one molecule and a slightly negative atom, like oxygen, on a neighboring molecule.

Water is bent, not linear (~104.5°), and polar — so the δ⁺ hydrogens on one molecule are drawn toward the δ⁻ oxygen on a nearby molecule. Each molecule can form up to 4 hydrogen bonds (2 as donor via its hydrogens, 2 as acceptor via oxygen’s spare electron pairs). These bonds are 10-20x weaker than the covalent O-H bond inside a molecule, but strong enough to matter, and they’re constantly breaking and reforming — on a timescale of picoseconds — creating a fluid, ever-shifting network rather than a rigid one.

That network explains water’s well-known quirks:

High boiling point for such a small molecule (100°C, vs. -60°C for the similarly-shaped but less polar H₂S) — breaking all those hydrogen bonds takes real energy.
Ice floats — hydrogen bonds lock into a hexagonal lattice in ice that holds molecules farther apart than in liquid water, making solid water less dense.
Surface tension and solvency — both driven by the same polarity and hydrogen-bonding.

8. Atoms Are (Almost) Entirely Empty Space

A hydrogen nucleus (a proton) has a radius of about 10⁻¹⁵ meters. The electron cloud around it extends to roughly 5 × 10⁻¹¹ meters — a ratio of about 50,000 to 1. If the nucleus were the size of a marble, the electron cloud’s edge would sit about 500 meters away.

Even the “occupied” region isn’t literally filled — the electron is a probability distribution, not a tiny ball smeared thin. Quantum field theory treats the electron as a point particle with no measured size at all.

Nearly all of an atom’s mass (99.97%+) is in the nucleus; nearly all of its volume is empty space threaded by electromagnetic fields and probability clouds. What we experience as “solid” objects touching each other isn’t literal contact between little balls of stuff — it’s electromagnetic repulsion between charged fields, plus quantum pressure effects (the Pauli exclusion principle keeping electron clouds from fully overlapping), acting across mostly empty space. If you removed all the empty space from every atom in every person on Earth, humanity would fit inside a sugar cube.

9. How Light (Photons) Interacts With Matter

Light doesn’t interact with truly empty space — a photon traveling through a genuine vacuum does nothing. What we call “empty space” inside an atom is permeated by the electron’s probability field, so light interacts with that field, not with nothingness.

Why electrons, not the nucleus, dominate light interactions:

A photon interacts with electric charge, and the interaction strength depends heavily on the mass of the charged particle — lighter charges respond more readily to a photon’s oscillating electric field.
Electrons are ~1836 times lighter than protons and sit at the outer edge of the atom — the first thing an incoming photon meets.
The nucleus, heavier and buried deep inside the electron cloud, interacts with visible-light-energy photons far more weakly.

What happens on interaction:

Absorption — if a photon’s energy exactly matches the gap between two of a molecule’s electron energy levels, the electron absorbs it whole and jumps to a higher level. All-or-nothing, because energy levels are discrete (quantized) — no partial absorption is possible.
Scattering — if the photon’s energy doesn’t match any available gap, it can’t be absorbed. The electron cloud’s field briefly jostles, and a photon of the same energy re-emerges in a new direction, without ever being captured. This is why water and glass are transparent to most visible light.
Re-emission — after absorption, the electron typically re-emits a photon (fluorescence) or converts the energy into molecular vibration (heat).

Color: A map of which photon energies a material absorbs. What you see as an object’s color is the light left over after certain wavelengths have been absorbed by its electrons’ available energy transitions.

For a water molecule specifically: the photon overwhelmingly interacts with the electron cloud, not the nucleus. The nucleus only becomes a significant player at vastly higher energies (gamma rays) — well outside the range relevant to everyday light and color.

10. Is Photon Absorption Actually Deterministic?

It’s tempting to picture the electron as the only “fuzzy,” probabilistic piece of this story, and the photon-meets-atom interaction itself as a clean yes/no event once the energy matches. That’s not quite right — the interaction is probabilistic too.

Absorption isn’t guaranteed, even with a perfect energy match. When a photon’s energy exactly matches the gap between two of an atom’s electron energy levels, there’s a specific probability that absorption happens — governed by what’s called a transition rate.

Transition rate: The probability per unit time that a quantum system (like an electron in an atom) jumps from one state to another, given some interaction (like a passing photon). It depends on how strongly the initial and final electron states overlap, and how well the photon’s properties match the transition. Formally described by Fermi’s Golden Rule.

So a resonant photon — one with exactly the right energy — can still sail straight through an atom unabsorbed. It’s a matter of odds, not certainty.

Where the illusion of determinism comes from:

Large numbers. Real light isn’t one photon meeting one atom — it’s a beam of trillions of photons hitting a chunk of material with trillions of atoms. The macroscopic absorption rate (how much light a piece of glass or water blocks) is an average over an astronomical number of individual probabilistic events. Enough coin flips and the fraction of heads becomes very predictable, even though each flip isn’t. That’s what a law like Beer-Lambert (how absorption scales with material thickness) is really describing — statistics, not certainty at the single-photon level.
Language shortcut. Saying “if the photon’s energy matches the gap, it gets absorbed” is really shorthand for “the probability of absorption becomes nonzero and often significant” — not “absorption is guaranteed.”

So there isn’t one deterministic side and one probabilistic side here. Both are probabilistic, and they combine:

The electron’s position is described by a probability distribution (the orbital).
The interaction itself — whether absorption happens at all, even under perfect energy matching — is also described by a probability (the transition rate).

The overall likelihood that this specific photon gets absorbed by this specific atom comes from multiplying these probabilities together, not from a certain electron position triggering a certain outcome.

Transition matrix element: A precise measure of the “overlap” between the photon’s field and the electron’s wavefunction at the relevant energy. Its square gives the actual absorption probability — related to what’s called the atom’s absorption cross-section. Always a number between 0 and 1 (or a rate), never a hard guarantee, even in the most favorable case of exact energy matching.

What we perceive as reliable, predictable behavior — light gets absorbed by water, glass is transparent, a leaf looks green — is the statistical regularity of enormous numbers of genuinely probabilistic photon-electron encounters, averaging out into results precise enough to look deterministic from a distance.

11. What Is a Photon, Really? (“A Quantized Ripple in the Electromagnetic Field”)

Classical picture first. Before quantum mechanics, physicists already knew light was an electromagnetic wave. Picture a field as a value (strength and direction) assigned to every point in space — like a weather map assigning wind speed to every location. This field can oscillate, and that oscillation propagating outward is a light wave. Energy in this picture is continuous — turn up the brightness a bit, energy goes up a bit, smoothly.

Then the problem. Experiments — notably the photoelectric effect, explained by Einstein in 1905 — showed light actually exchanges energy in discrete chunks, never fractional amounts. Energy = h × frequency (h being Planck’s constant), and you only ever get whole-number multiples of that minimum packet.

Quantized: Restricted to specific, discrete values rather than a smooth continuous range. “Quantized energy” means energy can only be exchanged in fixed-size packets.

So what’s a photon? The smallest possible excitation of the electromagnetic field at a given frequency. The field still ripples and behaves wave-like (which is why light diffracts and interferes) — but when it actually deposits or absorbs energy, it can only do so in these fixed units. That dual behavior — wave-like propagation, particle-like exchange — is what “wave-particle duality” means.

The modern picture: Quantum Field Theory (QFT). The electromagnetic field is the truly fundamental thing, existing everywhere, always — even in a vacuum. A photon isn’t a separate object moving through the field; it is a quantum excitation of the field, the same way a ripple on a pond isn’t separate “ripple-stuff” moving through water — it’s the water itself, moving in a particular pattern.

12. The Universe as a Set of Overlapping Quantum Fields

Worth correcting an intuitive assumption here: it’s not that particles exist everywhere and therefore create a field. It’s the reverse — fields are the fundamental “stuff,” existing at every point in space and time, always, even where nothing is detectably happening. Particles are what appear when a field gets excited above its lowest-energy (ground) state at some location.

There isn’t just one field — there are many, layered over the same spacetime:

Electromagnetic field → excitations are photons
Electron field → excitations are electrons (and positrons, their antiparticle)
Quark fields (six types) → excitations are quarks, which combine into protons/neutrons
Gluon field → carries the strong nuclear force binding quarks together
Higgs field → gives many particles their mass, depending on how strongly they interact with it
Several others (weak-force fields, neutrino fields, and so on)

“Empty space” really means: all these fields present everywhere, sitting in their lowest-energy, unexcited state — not truly “nothing,” but a rich underlying structure with no particles currently excited in it.

13. The Big Bang: How Fields Became Particles

The fields likely weren’t “created” as a step after the universe began — they appear to be part of the fabric of spacetime itself, present from essentially the beginning. What changed dramatically over time was temperature and energy density, and that’s what drove the interesting physics.

Rough sequence, as best understood:

Extremely hot, extremely dense early universe. All the fundamental fields existed, but under conditions so extreme that particles didn’t yet have the distinct properties they have today.
Symmetry breaking (roughly 10⁻¹² seconds in). As the universe expanded and cooled, the Higgs field underwent a phase transition — mathematically similar to water freezing into ice. At extreme temperatures its value was zero everywhere and particles were effectively massless; once the universe cooled past a critical threshold, the Higgs field settled into a nonzero value everywhere, and particles that couple to it began acquiring mass.
Particle-antiparticle pair production. In the hot early universe, ambient energy was high enough that photons routinely converted into particle-antiparticle pairs (electron + positron, for instance), with the reverse — annihilation back into photons — happening just as often. A boiling, constantly-recombining soup of field excitations.
“Freeze-out.” As the universe kept cooling, average photon energies dropped below the threshold needed to keep creating certain particle pairs. Almost all matter and antimatter annihilated back into radiation, but a tiny leftover excess of matter over antimatter (a famous unsolved puzzle) survived. That small excess is, essentially, everything we see in the universe today — including you.

Honest boundary of current knowledge. This picture is well-tested back to roughly 10⁻¹² seconds after the Big Bang (verified in particle colliders) and reasonably well-modeled back to around 10⁻³⁶ seconds (the inflationary era — more speculative, but well-motivated). Closer to the Planck time (10⁻⁴³ seconds), current physics genuinely breaks down: quantum field theory assumes a smooth classical spacetime background to define fields “on top of,” but at Planck-scale densities spacetime itself likely needs a quantum description we don’t yet have. What happened right at t=0 remains a genuinely open question — not settled science.

Quick Glossary

Term	Meaning
Interference	Waves overlapping to reinforce (bright) or cancel (dark) each other
de Broglie wavelength	The wavelength associated with any particle, λ = h/p
Decoherence	Loss of superposition due to interaction with the environment
Stationary state	A stable, unchanging quantum superposition (e.g., an orbital)
Disconnectivity	Leggett’s proposed measure of how “macroscopic” a superposition is
Singularity	A point where spacetime curvature becomes infinite; GR breaks down
Covalent bond	Atoms bonded via a shared, merged molecular orbital
Polar molecule	A molecule with unevenly distributed electrical charge
Hydrogen bond	A weak electrostatic attraction between polar molecules
Transition rate	The probability per unit time that an electron jumps between energy states given an interaction
Transition matrix element	A measure of overlap between a photon’s field and an electron’s wavefunction; its square gives absorption probability
Quantized	Restricted to discrete values, not a continuous range
Photon	The smallest excitation of the electromagnetic field at a given frequency
Quantum field	The fundamental entity existing at every point in spacetime; particles are its excitations
Higgs field	The field responsible for giving many particles their mass

Where physics is still unsettled — gravity-induced collapse, what happened at t=0 — that uncertainty is called out explicitly rather than smoothed over.