What does a heavy thing actually do to the space around it?

Ask what gravity is doing and most pictures answer with motion. Arrows sweeping inward. Little streams of something pouring towards the heavy thing. A rubber sheet with a bowling ball on it, everything rolling down into the dip.

All of those are wrong in the same way, and the rubber sheet is the worst of them, because it explains gravity by quietly using gravity — the ball only makes a dip because something is already pulling it down. It is a picture children remember for thirty years and have to unlearn.

Here is what is actually happening, which is stranger and much quieter.

Nothing is going in and nothing is coming out

Put a stone on a table. Nothing leaves it. Nothing arrives. It does not emit anything, use anything up, or reach out and take hold of the room.

What it does is change what would happen to anything else that was there. Every point of space near the stone now has a property it did not have before: a strength and a direction, describing how something released at that exact spot would begin to move. That is all a field is — a value at every point, saying what would happen there.

The stone did not do anything to make this so. It simply is somewhere, and being somewhere is enough.

This is the part that resists intuition, so it is worth sitting with. There is no ongoing effort. Nothing is being spent. A stone that has sat in a field for a billion years is not running low. The change is not an action it takes; it is a fact about where it is.

It falls off fast, and it never stops

Double your distance from something and its pull drops to a quarter. Triple it and you are down to a ninth. That is the inverse square law, and the reason for it is almost embarrassingly simple: the influence spreads out over the surface of an imaginary sphere, and a sphere's area grows as the square of its radius. Same amount, spread thinner.

Which means the effect drops away very quickly indeed. Step back from a person and their gravity becomes unmeasurably small almost at once.

But look at that arithmetic again. A quarter. A ninth. A sixteenth. A hundredth. A millionth. Not one of those is zero.

There is no distance at which the sum comes out as nothing. Not across a room, not across a continent, not across the gulf between galaxies. The number gets small beyond any instrument's ability to notice, and it keeps being a number. Every object with mass is, right now, changing every other object's field everywhere in the universe by some absurdly tiny amount.

Your body is doing it. So is your chair. So is a boot.

The consequence worth carrying away is that the halo around a heavy thing does not have an edge. If you draw it, you have to choose where to stop drawing, and wherever you choose you have drawn a lie. Turn the sensitivity up and there is always more.

Two of them, and the arithmetic that makes fields worth having

Now put two heavy things in the room, some distance apart.

Each has its own field. The question is what happens where the two overlap — and the answer is the most useful thing in this entire subject: they simply add.

Not interfere. Not compete. Not partly cancel through some rule about which is stronger. At every point in the room, you work out what the first one contributes, work out what the second one contributes, and add them together, arrows and all. That is called superposition, and it is why a field is worth drawing at all: it turns a problem about many objects into arithmetic you can do one object at a time.

Bring the two together and the space between them brightens, because both are being counted there. Nothing flowed. Nothing moved between them. The two contributions were always both there; you have only changed where they overlap.

The whole of orbital mechanics, every spacecraft trajectory ever flown, is that idea applied carefully.

Now let go of something

A field says what would happen to a thing released at a point. So release one, and watch.

Suppose you could mark where a falling stone was at perfectly even intervals — a mark every fraction of a second, all the way down. What would the marks look like?

Most people expect them evenly spaced, which is what you would get if the stone had been given one push at the start and then left alone. That is not what happens. The first two marks nearly touch. The next pair is a little further apart. And the last two are a stride apart.

The gaps grow because the push never stops. The field is not a shove at the beginning; it is a condition at every point along the way, still acting on the stone at the bottom exactly as it acted at the top. Every instant adds a little more speed to the speed already there, and the marks record it.

Growing gaps are what a constant, continuous influence looks like when you write down positions instead of speeds. If you ever see evenly spaced marks, nothing is accelerating.

And there is no "down"

Here is the last piece, and it is the one that changes how the rest of it reads.

We say things fall down. But "down" is not a direction the universe knows about. It is the direction of the nearest very large object, which for everyone reading this happens to be the same object, which is why it feels like a fact about space rather than a fact about a planet.

Put a stone in a room with a heavy thing at one end and let it go, and it will not go down. It will go towards. Move the heavy thing to the other end and the stone goes that way instead. Put the heavy thing above and the stone falls upward, and nothing whatsoever is odd about that.

The marks trace a line to the mass, wherever the mass is. There is no down. There is only towards — and the only reason it ever looks like down is that we all happen to be standing on the same very large stone.

Which is worth remembering the next time someone calls a planet enormous. On this scale, a planet is a pebble that got away with it.