Fact Horizons

LIGO detects ripples smaller than a proton. Gravitational waves from colliding black holes stretch and squeeze its 4 km arms by about a ten-thousandth of a proton's width. The first detection, in 2015, came from 1.3 billion light-years away.

Helium was discovered on the Sun before it was found on Earth. In 1868, astronomers spotted an unexplained yellow line in sunlight during an eclipse. The element was named after Helios, the Greek Sun god, and wasn't isolated on Earth until 1895.

The Eiffel Tower grows in summer. Iron expands as it warms, so on hot days the tower can be up to about 15 cm taller than in winter. Its tip also leans slightly away from the Sun as the sunny side heats up.

Ice floating is one of the strangest things in nature. Most substances get denser when they solidify. Water expands when it freezes, and liquid water is densest at about 4°C. So lakes freeze from the top down, insulating the life below.

The Big Bang didn't happen at a point. It happened everywhere at once. Space itself has been expanding ever since, so the universe has no centre, and from any galaxy, distant galaxies appear to be moving away.

Your phone's storage relies on quantum tunnelling. Flash memory stores bits by trapping electrons behind a thin insulating layer. To write and erase data, it pushes electrons through a barrier they classically shouldn't be able to cross.

Particles from space reach the ground only because time slows down for them. Muons made high in the atmosphere decay in about 2 microseconds, too fast to travel far. But moving near light speed, their clocks slow down, so huge numbers survive to reach the surface.

The coldest places in the universe are in labs on Earth. The coldest known natural place is the Boomerang Nebula, about 1 degree above absolute zero. Physicists have cooled atoms to less than a billionth of a degree above it.

A fridge magnet is beating the entire Earth. The whole planet's gravity is pulling on that magnet, and a tiny piece of magnetised metal holds it up anyway. Gravity is by far the weakest of the fundamental forces.

Sunsets on Mars are blue. Fine Martian dust scatters blue light forward, toward the setting Sun, while other colours spread more widely. So the glow around the Sun at dusk looks cool blue: the reverse of Earth.

The iron in your blood was made inside stars. Elements heavier than helium were forged in stars and scattered by stellar explosions. The iron carrying oxygen around your body right now is older than the Earth itself.

The sky should arguably be violet, not blue. Air scatters short wavelengths most, and violet is shorter than blue. But sunlight has less violet in it and our eyes are far less sensitive to it, so we see the sky as blue.

On Mercury, one day lasts two years. From sunrise to sunrise takes about 176 Earth days, while one orbit of the Sun takes 88. Its slow spin and fast orbit combine into an extremely long day.

All 8 billion humans would fit in a sugar cube. At least if you removed the empty space inside our atoms. Packed to the density of an atomic nucleus, the entire human race would be about that size, and weigh around 500 million tonnes.

Days were shorter when dinosaurs were alive. About 70 million years ago, a day lasted roughly 23.5 hours. The Moon's tides act like a brake on Earth's spin, slowly making each day longer.

Earth's inner core is about as hot as the surface of the Sun. It's roughly 5,000 to 5,500°C. It stays solid anyway, because the pressure there, over 3 million times the air pressure at sea level, locks the iron into a solid.

Atoms are almost entirely empty space. If a hydrogen atom's nucleus were the size of a pea, the whole atom would be about the size of a football stadium. Everything solid you've ever touched is mostly nothing.

If the Sun turned into a black hole, Earth's orbit wouldn't change. A black hole with the Sun's mass pulls on us exactly as hard as the Sun does at our distance. We'd keep orbiting just as before. We'd just freeze in the dark.

Diamonds and pencil lead are the same element. Both are pure carbon. In diamond, each atom is locked to four neighbours in a rigid 3D lattice. In graphite, atoms form sheets that slide over each other, which is why it marks paper.

A lightning bolt is about five times hotter than the surface of the Sun. The channel of air heats to around 30,000°C in a split second. That sudden, violent expansion of air is the thunder you hear.