A tunnel through the centre of the Earth

Imagine we drill straight through the Earth — a perfectly straight well through the very centre, pole to pole. We pump out the air, stand on the edge and drop a stone. What happens? The answer is one of the most beautiful in all of physics — and it is best seen in motion.

What is actually happening

The stone starts falling and accelerates — but ever more weakly. The deeper it goes, the less mass stays "below" it and the weaker gravity becomes. At the very centre of the planet gravity is zero: mass surrounds the stone on every side and pulls equally in all directions, so the forces cancel. The stone passes the centre at about 7.9 km/s — as fast as a satellite in low orbit — after which the same gravity begins to brake it. In an ideal, friction-free world it oscillates like this forever.

This is simple harmonic motion — exactly like a mass on a spring. The restoring force grows in direct proportion to the distance from the centre, and that is the mathematical recipe for perfect, even oscillation.

Why exactly 84 minutes

A full swing there and back takes about 84 minutes, and a single crossing to the other side of the globe — half of that, about 42 minutes. The period of these oscillations is given by a surprisingly simple formula:

The magic is in what the formula leaves out: the mass of the stone. A feather and a boulder cross the Earth in the same time. Nor does it depend on how hard you throw the stone — you can see this when you add a second stone from half-depth: both pass the centre in perfect step.

At the centre you are weightless

Against all intuition, at the Earth’s core gravity is not strongest but zero.

There the stone is fastest — and at the same time completely weightless, like an astronaut in orbit. The white arrow in the animation shows it directly: it shrinks to nothing at the exact moment the stone is moving fastest.

And if we really tried?

It is a thought experiment. The Earth is not uniform: it has a dense iron core, so gravity stays nearly constant down to half the radius — the crossing would shorten to about 38 minutes. A real well also has air: resistance slowly damps the oscillation (toggle "air resistance"). And a small detail: the centre sits at several thousand degrees and millions of atmospheres — the stone would simply melt.

So why talk about itThis is exactly what thought experiments are for. We need not drill the planet to see how one simple rule — gravity weakening toward the centre — turns a falling stone into a giant, 84-minute pendulum hidden inside the Earth.