Under Construction

Gravity, and the Past

The past is never dead. It's not even past - William Faulkner - Requiem for a Nun

The last page ended with a debt. Facts pile up and never disappear; the past only grows; and somehow that ever-growing weight of what has happened still presses on the present, so that moving one thing here disturbs everything, everywhere, however faintly. That pressure of the settled past upon the moving present is gravity. To see why, we have to look again at space, at mass, and at what becomes of a fact once it has fallen out of Now.

Index

Space Is Not a Box

Mass Bends the Web

Why Gravity Moves Everything At Once

Move One, Move All

Where the Past Goes

Space Is Not a Box

It is tempting to picture space as a great empty box the world is set inside - a stage that would still be standing, waiting, even if every object were taken away. Virtualism denies the box. Space is not a container things sit in; it is the set of differences between things, and nothing besides. Where there is no difference there is no distance, because distance just is a measure of difference. Remove the masses and you are not left with empty space; you are left with no space at all. Space, like everything else, is emergent - it is what one kind of difference between objects looks like when you read it as a frame. And this matters, because if space is woven from the relationships between masses, then anything that alters a mass must, in altering it, tug at the very fabric those relationships make.

Mass Bends the Web

Recall the Tardis from the page on paradox: a massive object whose inside amounts to more than its outside can hold, forcing a new direction open to take the strain. That opened-up extra direction is what mass is. And because space is the web of relationships between masses, a thing with mass does not rest neutrally in that web - it bends it, draws it inward, the way a heavy ball sunk into a stretched sheet pulls the whole sheet toward itself. Other things, following the relationships, slide toward the dimple. That bending, and the ceaseless resettling of it, is gravity.

There is a clean way to see the centre of all this, and we have met it before. The Earth, taken as a whole, has a virtual heart - its centre of gravity, a point with no particle in it that nonetheless steers the planet. Every gathering of masses has such a heart, and so, all of them taken together, does the entire universe. A thing's weight is nothing but its standing relationship to that centre. And notice what is absent from the account: any particle of gravity. None is needed. Gravity is not a thing emitted and caught; it is the geometry of relationships resettling itself - emergent, not fundamental, which is exactly what a growing body of physics has begun to suspect.

Why Gravity Moves Everything At Once

Here is a puzzle gravity poses that this picture answers cleanly. Light takes time to cross a room; gravity seems to seize a whole object all at once, every part of it together. Why the difference?

Because there are two things any change can touch, and they behave quite differently. One is what a thing is on the inside - its own content. The other is where it stands - its position among everything else. Position is not a private fact about a part; it is a fact about the whole thing at once, its place in the web. So when positions resettle, they resettle for the whole object together - there is no part-by-part to it, because position was never held part by part. That is why gravity takes an object entire, while a change working through its insides must make its way across it, piece by piece.

The two are not truly separable, mind. Altering what a thing is shifts where everything stands; the resettling of where things stand sets up the next alteration in what they are. They turn in a loop, each driving the other, and asking which comes first is putting a question the loop will not answer.

Move One, Move All

Follow the web far enough and a striking conclusion appears. If every position is a relationship to every other, then moving any one thing must, strictly, resettle the standing of all the rest. Nudge a single mass and the whole universe has to update its account of where everything now lies, relative to that move. This is an old intuition - that the local is held in place by the whole - and Virtualism supplies the mechanism: when one real thing changes, the whole of reality rearranges to stay consistent, because reality is the one fully-settled web in which every thread is tied to every other.

That gives downward causation its exact and unmysterious meaning. The whole does steer its parts - but as a channel, not as a new force. A wheel rolling downhill carries each of its atoms along a path none of them chose; no atom's own physics is broken, yet the shape of the whole routes where every one of them goes. The whole is the conduit through which the constraint of everything-else reaches the part. Read this way, the two great theories of physics stop quarrelling: one describes parts giving rise to wholes, and the other describes wholes bearing back down upon parts, and they are simply the two directions of a single relationship.

QM = emergence; GR = downward causation.

A small marvel falls out as a footnote. Newton's constant of gravitation, the number G, is not a brute fact dropped into the universe; it is an exchange rate - the going conversion between three ways of pricing the same thing: how much mass, how much distance, how much time. Fix the universe's own scales for any two of them and the third is settled, and G with it. G is the bill; the universe is the till.

Where the Past Goes

Now the debt can be paid in full. When the present produces a fact and a new present forms over it, the old reality does not merely sit behind us unchanged. Its mass - the extra dimension that made it real - drains away, and what remains is the bare fact of what was: true for ever, but no longer real, no longer carrying its own weight. The past is property demerged - position without presence, a settled record stripped of the charge that once made it matter. So reality is not heaping up without limit; it is conserved, handed on from each spent moment to the next.

And yet the past is anything but gone, because the present is built upon the constraint of every settled fact behind it. What we feel as gravity is, at bottom, the whole accumulated past holding the present in shape - the universe keeping faith with all that it has already become. Even the light of the night sky is news from the past: by the time it reaches you the star that sent it has moved on, so that you see not where anything is but where it was. We live always at the leading edge of a vast settled history, which we can never quite catch up to, and can never slip the pull of.

All the engines are now on the table - nothing turned into number, number into fact, fact into the paradox that builds and the hypodox that fills, the wheel of four elements, the making of each moment, and the weight of every moment past. One page remains, and it is the deepest of them: the one that shows all of this as a single turning loop - the same move repeating at every scale, closing upon itself and rising as it closes.