Landauer's principle

A conventional computer computes by shuttling electric charges along wires. But can you compute with heat? A network of pipes carrying hot and cold water can perform simple logic operations if the pipes are connected in the right order.

Alice prepares a quantum particle in one of several states and sends it to Bob. The channel the particle travels through isn't perfect: it's noisy, scatters, interacts with the environment. Bob receives the signal and tries to figure out what Alice actually sent. This is the classic "prepare-send-measure" scheme, and there's always a risk of losing something along the way.

In the quantum world, a fact becomes objective when its value is imprinted on many independent fragments of the environment, like a notice posted on hundreds of bulletin boards across the city. Each copy alone adds no new information, but it changes the rules of the game for anyone trying to erase that information.

A fussy thermometer shows different temperatures depending on who is looking at it. A black hole behaves much the same way: its thermal properties, entropy and temperature, depend on how we describe the information hidden inside.

A hunter sets snares in the woods. They know the animal's habits but not whether it will ever show up. In much the same way, physicists set up detectors in hopes of catching particles of dark matter — the invisible substance thought to make up most of the Universe's mass. A new study offers hunters an unusual tool: not a trap, but a measuring tape.

Erasing even a single bit means releasing heat. This discovery by Rolf Landauer hinted that information is neither material nor illusory. Imagine that every event casts a shadow: that is information, the collection of all unrealized possibilities.

Every quantum system (whether an atom, a crystal, or the entire Universe) has its own set of allowed energies, like the steps of a ladder. The hotter the system, the more readily particles climb to higher rungs.

Spacetime isn't a flat sheet, but a crumpled fabric: stars and planets leave creases on it. Information is written into every crease, and to erase it, heat would have to be released — that's how Landauer's principle works. Thus, the 'crumpledness' of space can be measured through entropy — the universal measure of disorder.

A voice recorder captures a city square. You hear the hum of a crowd, footsteps, snatches of conversation, the rumble of traffic. Somewhere in that sonic stew, a street musician plays a simple tune on a flute. Hearing it isn't easy: there's just too much mixed together.

In 1867, James Clerk Maxwell proposed an image that still stirs physicists today. Two compartments of gas are separated by a door. Molecules dart about chaotically: fast and slow. A tiny gatekeeper opens the door only for fast ones, letting them through to one side, and slow ones to the other. The hot compartment gets hotter, the cold one colder.

If you write down a protein recipe on a piece of paper and send it to a print shop, a typo in a single letter can ruin the whole batch.

When a bright spot flares up on a star, astronomers want to know how much energy was released into space. They used to picture a flare as a growing campfire: the area of the fire increases, but the heat remains the same. That made calculations simpler, but reality turned out to be different.

Erasing a single bit of information isn't free. In 1961, Rolf Landauer showed that at room temperature, it takes at least about 3 zeptojoules of energy — a tiny but nonzero amount. This limit was considered as ironclad as the conservation of energy. But what if you could give the system a little nudge?