Jeffrey Andrews-Hanna

I have a gravity field that, in theory, could be measurable.

This is what allows us to study the interior planets.

I've got a gravity field, you've got a gravity field, this table has a gravity field, everything with mass has a gravity field and exerts a pole.

And so, we can measure gravity very precisely and use that to examine the distribution of mass.

Now, gravity as a field is incredibly old.

The earliest gravity studies I'm completely in awe of, because how do they measure variations in the Earth's gravity field?

They use something called a plumb bob.

This is a weight on a string.

Basic idea is if you go up next to a very large, massive object, like a big mountain range,

Well, that plumb bob, that weight on the string, will be deflected a little bit towards that massive object.

This took incredibly, incredibly precise measurements, but people were doing this in the 1850s.

One of the very first gravity studies of the Earth using a weight on a string revealed something just completely fundamentally new and important about the structure of the Earth.

And that was looking at the Himalaya Mountains.

So this was Sir George Everest and Archbishop Pratt of Calcutta.

They decided to do a gravity survey of the Himalaya.

So their idea was, well, we can see this mountain range, we can measure the height of the mountain range, we can guess at how much mass of rock is in that mountain range, and we can calculate what the gravitational pull of the Himalaya should be.

Then they went out with their plumb bob and measured the gravitational field of the Himalaya, and it was not what they expected.

In fact, it was only a small fraction of what they expected to see.

And that's not because they did their calculations wrong.

It's not because it was a windy day and the plumb bob was blowing all over the place, although I really don't know how they do this.