Knomi Curiosity

What if?

The answer

You'd speed up to the centre and slow down again, reaching the other side in about 38 minutes, then fall back and forth forever if nothing slowed you.

This tunnel could never be built: Earth's core is about as hot as the surface of the Sun, and the pressure would crush any material. But it's a famous physics puzzle.

As you fall, gravity pulls you toward the centre, so you speed up. At the centre you're going fastest, roughly 8 to 11 km per second depending on how Earth's mass is packed, and gravity has no direction to pull you, so you coast through. Then gravity pulls you back toward the centre, slowing you down until you stop, just as you reach the far side.

If Earth were packed evenly all the way through, the trip would take about 42 minutes. But Earth's middle is much denser than its outer layers, so gravity stays strong almost all the way down. With Earth's real layers, the trip takes about 38 minutes.

With nothing to slow you down, you'd fall back and forth forever. With air in the tunnel, drag would make each trip a little shorter, until you came to rest at the centre.

What we know and what we don't

What physics tells us

  • Without air, you'd oscillate through the centre to the other side and back.
  • The one-way trip takes about 42 minutes for an evenly dense Earth, and about 38 minutes using Earth's real density layers.

Scientists aren't sure

  • Nothing about the physics is uncertain, but no such tunnel could exist: the heat and pressure inside Earth are far too great.

Go deeper

Why? Why is gravity zero at the centre?

At the centre, Earth is all around you, pulling equally in every direction, so the pulls cancel out. You'd be weightless there.

Why? Does it work for a tunnel that doesn't go through the centre?

Yes. For an evenly packed Earth, any straight tunnel between two points on the surface, say from London to Paris, would take the same 42 minutes. It's one of physics' surprising results.

Why? Why would you stop at the centre if there's air?

Air drag takes away a bit of your energy on every trip, so you'd never quite reach the surface again. Each swing would get smaller until you settled at the centre.

Keep wondering

Sources