Where the method can't reach

Everything that falls into a black hole stays inside it, so all of its mass is still in there. Scientists can measure that mass from outside: the more mass inside, the faster nearby stars swing around the black hole. By timing those stars, astronomers worked out that the black hole at the centre of our galaxy holds about 4 million times the mass of the Sun. They can also measure how fast a black hole spins.

According to Einstein's theory, that is all the outside can ever show: how much mass is inside, how fast it spins, and its electric charge. Two black holes made from completely different things, one from a collapsed star and one from a cloud of gas, would look exactly the same from outside if those numbers matched. Physicists sum this up as “black holes have no hair.”

The nearest black hole we know of is more than 1,500 light-years away. Even travelling at the speed of light, a trip there would take more than 1,500 years.

According to Einstein's theory of gravity, an astronaut falling into a very large black hole would cross the event horizon without feeling anything special at that moment. For a while, she could look around inside. But nothing she sends, not even light, could ever get back out to anyone else.

Earthquake waves are how scientists found the core in the first place. In 1906, Richard Oldham noticed that waves travelling deep through the Earth arrived late, as if they had passed through something different. One kind of earthquake wave can't travel through liquid, and it never comes out the far side of the core, so scientists worked out that the outer core is liquid. In 1936, Inge Lehmann spotted faint waves arriving where none should be, and worked out that there was a separate inner core inside it.

Waves that pass through the very centre of the Earth come out on the opposite side from the quake. Scientists agree on what these waves show: some arrive a few seconds earlier or later, depending on which direction they travelled through the centre.

What scientists disagree about is what those few seconds mean. Some teams say the times show a separate innermost core (interior B). Other scientists say the same times can be explained another way, without a separate centre (interior A). They are looking at the same evidence and interpreting it differently.

The argument is hard to settle because very few waves cross the very centre where sensors can catch them: most earthquakes happen in a few zones, and most sensors are on land. In 2023, a team in Australia tried a new trick: they used waves that bounced back and forth through the whole Earth up to five times. They say this supports interior B. Other scientists are not convinced yet.

In 1952, a student named Stanley Miller filled a sealed flask with water and the gases he thought were in the Earth's early air: methane, ammonia and hydrogen. He sent electric sparks through it to copy lightning. After about a week, the water held amino acids, the building blocks of the proteins in every living thing. The flask in this tool is based on his experiment.

Since then, labs have made some of the building blocks of RNA (in 2009) and joined building blocks into short chains. But no experiment has gone all the way from simple chemicals to something alive.

Nobody recorded how life began, but the early Earth left some clues. Tiny crystals called zircons suggest there was liquid water on Earth about 4.4 billion years ago. Layered rocks built up by microbes are about 3.5 billion years old.

Clues in very old rocks also suggest what the early air was like. Most scientists now think it was mostly carbon dioxide and nitrogen, not the methane, ammonia and hydrogen that Miller used in his flask (Card 2). When the experiment is run with carbon dioxide and nitrogen, it makes far fewer building blocks. So the rock clues made Miller's version of how life began less likely, even though nobody can go back and check.