Field
Matter
The furniture of the world, from empty atoms to moving plates.
40 entries
Atoms are almost empty
An atom’s mass sits in a tiny nucleus. The rest of its volume is a cloud of electrons; ordinary matter is mostly void.
Entropy does not volunteer to fall
In a closed system, entropy does not decrease. Useful energy dissipates. This is the second law of thermodynamics.
The ground moves
Earth’s lithosphere is broken into plates that move over the mantle. Continents drift, seafloor spreads, and mountains rise at the boundaries.
Crystals can be impossible
Quasicrystals show sharp diffraction and a symmetry — including five-fold — that classical crystallography had proved a repeating lattice cannot have.
Absolute zero cannot be reached
The third law of thermodynamics implies that no finite sequence of steps can cool a system to absolute zero.
Helium refuses to freeze
Helium-4 at ordinary pressure stays liquid down to absolute zero. It does not become a solid unless the pressure exceeds about 25 atmospheres.
Liquid helium climbs out of its cup
Below 2.17 kelvin, helium-4 becomes a superfluid and creeps as a thin film up and over the walls of an open container.
Lakes freeze from the top because water is heaviest at 4 °C
Liquid water reaches its maximum density at about 4 °C. Colder water floats, so a lake loses heat at the surface and ice forms there, insulating the life below.
Lightning is hotter than the surface of the sun
A lightning return stroke heats its channel to about 30,000 kelvin. The Sun’s photosphere is about 5,800 kelvin.
A proton is heavier than its parts
The quarks inside a proton account for only about one percent of its mass. The rest is the energy of the gluon field and the motion of the quarks, via E = mc².
The sun runs on a reaction that is classically forbidden
Proton fusion in the sun requires quantum tunnelling. At solar-core temperatures the protons do not have the energy to cross their mutual electric repulsion.
A phone remembers by breaking a rule
Flash memory stores bits by letting electrons tunnel onto an isolated conductor. They should not be able to cross the insulator. A thin enough insulator is not a veto.
Atoms can be seen
A scanning tunnelling microscope images individual atoms by measuring a tunnelling current between a sharp tip and a surface.
A cloud of atoms can become one wave
In a Bose–Einstein condensate, thousands of atoms occupy the same quantum state and behave as a single matter wave. It was made in a gas in 1995.
The floor holds you up by a rule about electrons
You do not fall through the floor because the Pauli exclusion principle keeps electrons from all occupying the same low-energy state. Electrostatic charge is not the whole story.
A sheet of carbon can be one atom thick
Graphene is a single layer of carbon atoms in a honeycomb. It is strong, an excellent conductor, and it was isolated with adhesive tape.
A magnet will float above a cold metal
A superconductor expels magnetic fields. A magnet placed over it levitates, held by the currents the field itself induces.
Entanglement does not send a message
Entangled particles show correlations that no local classical story can match. Those correlations cannot be used to send information faster than light.
Empty space can pull two plates together
The Casimir effect is an attractive force between two uncharged metal plates in a vacuum, caused by the way the plates change the quantum field between them.
You are radioactive
A typical adult body contains enough potassium-40 and carbon-14 to undergo several thousand nuclear decays every second.
Hospitals use antimatter
A PET scan detects pairs of gamma rays produced when positrons — antielectrons — annihilate with electrons in the body.
Glass is not a slow liquid
Window glass is an amorphous solid. Medieval panes are thicker at the bottom because they were made that way, not because the glass has flowed.
A metal melts in a warm hand
Gallium melts at 29.8 °C. A solid piece of it liquefies on a palm.
Gold is yellow because of relativity
Without special relativity, gold's electron structure would make it white, like silver. Relativistic contraction of the inner orbitals shifts the absorption that gives gold its colour.
Most ordinary matter is plasma
In stars, and in much of the gas between them, atoms are ionised. Plasma is the state of most ordinary matter in the universe.
Water can boil and freeze at the same time
At the triple point — 0.01 °C and a pressure of 611 pascals — ice, liquid water, and vapour coexist. Change the volume and the liquid boils and freezes together.
A solid can be lighter than air and still not float
Silica aerogel can be made with a solid network less dense than air. It does not float away, because its pores are full of air.
The smell of rain is a molecule
The scent of rain on dry earth is largely geosmin, made by soil bacteria and thrown into the air by the impact of drops.
Tin can crumble in a cold winter
Below about 13 °C, white tin is metastable and can transform into grey tin, a brittle form that makes an object crumble.
A diamond will burn
Diamond is carbon. Heated in oxygen to roughly 700 to 900 °C, it oxidises and is gone. It does not melt in any ordinary fire.
A drop of water can skate across a hot pan
Above a critical temperature a drop does not wet the surface. It floats on its own vapour and lasts much longer than it would on a merely hot pan.
A metal can remember a shape
Nitinol, an alloy of nickel and titanium, can be bent while cool and returns to a trained shape when warmed through a phase change.
Gold can be made, and it does not pay
Nuclear reactions transmute other elements into gold. Accelerators have done it. The yield is atoms, often radioactive, and the cost exceeds the metal.
Mercury is the only metal that is liquid in a cool room
At 20 °C and ordinary pressure, mercury is the only metallic element that is a liquid. Every other metal is a solid.
There is an ice hotter than a forge
At the pressures inside a giant planet, water forms a solid oxygen lattice with liquid-like hydrogen flowing through it. Experiments have made this superionic ice at thousands of kelvin.
Squeeze sodium hard enough and it turns clear
Near two million atmospheres, sodium ceases to be a shiny metal and becomes transparent, and an insulator. Release the pressure and the metal returns.
The solid in the funnel is a liquid, and it drips
Pitch poured into a funnel in 1927 has been dripping since the stem was cut in 1930. A drop takes the better part of a decade. The experiment is still running.
In a reactor pool, particles outrun light
Light in water travels at about three quarters of its speed in vacuum. A charged particle from a reactor can go faster than that. The result is a blue glow.
Light has been slowed to the speed of a bicycle
In a cold cloud of sodium atoms, a light pulse crawled at 17 metres a second. That is about sixty kilometres an hour. In vacuum it still does 300,000 kilometres a second.
The iron in blood was made in a star
The Big Bang made hydrogen, helium, and a trace of lithium. Iron was forged later, in massive stars, and scattered when those stars died. The iron in haemoglobin is that debris.