What if you jumped into a hole through the Earth?
You'd pop out on the other side in under an hour. Then fall back.
The Gravity Train
2150. A company promises to fall you from Delhi to Chennai in 42 minutes, free.
1. The Lottery
The adverts were everywhere: GRAVRAIL. NO FUEL. NO TICKETS. JUST FALL. A company called Downward Ltd claimed it had drilled straight, airless tunnels through the Earth between India's great cities. A capsule simply fell in at one end, sped up, slowed down and stopped exactly at the other. Gravity did all the work.
Meera's father was a tunnel inspector. When their family won a seat in the GravRail Lottery, everyone in their Delhi block cheered. Meera's father didn't cheer. He went very quiet and started reading the engineering reports at night.
2. Falling
The capsule doors closed and the floor seemed to vanish. Meera floated up from her seat. She was weightless the whole way, her father explained, because she and the capsule were falling together, like astronauts in orbit.
The speed display climbed past a thousand, then ten thousand kilometres an hour. At the deepest point it read nearly 28,000. Then it began to fall again as the capsule climbed towards Chennai. Exactly 42 minutes after leaving, it slowed to a gentle stop at the far mouth of the tunnel.
3. Any Tunnel, Same Time
Back home, Meera found the strangest line in the brochure. 'Delhi to Chennai: 42 minutes. Delhi to London: 42 minutes. Delhi to the far side of the world: 42 minutes.' It sounded like a lie, so she worked it out herself.
In a planet of even density, the pull along any straight tunnel grows in proportion to how far you are from the tunnel's middle. That makes every capsule swing like a pendulum with exactly the same period. A short tunnel gives a gentle push, a long tunnel a big one, and the times come out equal. The maths was right.
4. The Heat
Her father's reports said something else. The centre of the real Earth is around 5,200 °C, at a pressure more than three million times the air's. No material could line a tunnel there. And the real Earth isn't evenly dense: its iron core would make the trip nearer 38 minutes, not 42.
'Then where did we go?' Meera asked. Her father turned his screen to show her: a shallow track, a centrifuge and a dome of screens. A very good simulation, and a very good business. Meera thought about all the families still waiting in the queue, and opened her laptop to write it all down.
The end. Now see the science that the story is built on.
See it happen
What would happen, step by step
Imagine a tunnel straight through the centre of the Earth, with all the air pumped out so there's no drag and no heat (impossible, but this is a thought experiment). You step in and start falling.
The science behind it
Falling through a planet
Uniform Earth: a = −(g ÷ R) x ⟹ one-way time = π √(R ÷ g) ≈ 42 minutes
Outside a planet, gravity weakens the further away you are. Inside a uniform planet it does the opposite: only the mass closer to the centre than you pulls you overall, so gravity grows in proportion to your distance from the centre. A force that grows with distance and points back to the middle gives simple harmonic motion, the motion of a pendulum or a spring.
Why every straight tunnel takes the same time
For a tunnel that misses the centre, only part of gravity points along the tunnel, and that part is proportional to your distance from the tunnel's midpoint. So every straight tunnel behaves like the same pendulum, with the same 42-minute trip in a uniform Earth. The real Earth's dense core shortens the straight-through trip to about 38 minutes.
Why nobody can build one
The deepest hole ever drilled, the Kola Superdeep Borehole in Russia, reached about 12 km, around 0.2% of the way to the centre, before rock temperatures of about 180 °C stopped it. At the centre, the temperature is about 5,200 °C and the pressure is around 3.6 million times that of the air.
In one line: Inside a planet, only the mass closer to the centre than you pulls you overall. For a uniform Earth this gives simple harmonic motion with a one-way trip of about 42 minutes; Earth's dense core makes the real figure closer to 38 minutes.
Gravity on the Physics path →Think about it
In the gravity train you are weightless the whole journey. Why does that mean a ball you let go of inside the capsule would float beside you?