Field events

For a few seconds at a time, the world can change its own gravity, when the director, the Keeper (see Story), chooses. The first candidate is a sweep, a band of push that crosses the sphere and carries whatever it passes. Every number here uses the game's gravity in the 0.65× Hollow.

The sweep, in a slice through the centre

0.0 s

Push strength Band width Band speed Surfer's speed along the wave

Crosses the world in
Kick to anything still
Thrown ahead if moving with it faster than
The surfer leaves at
A crossing lands off by
A skyshell lands off by
Weight on the wall it starts from · heads for

BodiesSkyshellSame flight with no wave

The band crosses from left to right. A player jumps off a spire deck at the bottom, a surfer drifts through the upper void, and a skyshell goes up from the lower left. The jump and the skyshell are timed to meet the wave when it bends them most.

Reading the sketch

The sketch shows one event in three frames: a band of arrows enters the sphere from one side, crosses it and leaves from the other. Inside the band everything is pushed the way it travels. Behind it, the field goes back to normal.

In the game this is one extra term in gravity: a push of strength A along a fixed direction, felt only inside a band w metres wide whose centre moves at speed c. It fades smoothly to nothing at both edges, so it never starts or stops suddenly.

push = A · cos²(π · x / w), along the band's direction x is the distance from the band's centre; zero outside the band the band's centre moves at c m/s from outside one wall to outside the other

Ground, void and shots

The same push means very different things in different places, because skin gravity is 22 m/s² at the ground and nearly zero in the middle.

On the ground it is mild. Friction stops a standing player from sliding, so the push mostly changes your weight. With a 6 m/s² push, the gravity you feel on the wall the band starts from drops from 22 to 16 m/s², so jumps go 1.4× as high, and on the wall it heads for, the gravity you feel rises to 28 m/s².
In the void it is strong. Almost nothing else pulls there, so anything floating at rest gets a kick of a few metres per second, and a player halfway across can land tens of metres from where they aimed.
Slow shots bend the most. A skyshell (30 m/s) that meets the band can land 10–20 m off. A Skylance round (85 m/s) moves 2–4 m, because it crosses the band in a fraction of a second.

The table measures four versions of the wave; each worst case tries every launch time and keeps the largest miss.

Version	Push · width · speed	Crosses in	Kick when still	Thrown ahead above	Crossing off by	Skyshell off by	Skylance off by	Weight, start wall · far wall

The wave is a moving hill

To someone riding with the band, the push is a still hill that slows anything moving backwards through it. Something the band meets slowly either climbs over the hill and keeps a small forward kick, or rolls back down the front and is thrown ahead at about twice the band's speed minus its own. That second case is a gravity slingshot, the way a spacecraft gains speed from a moving planet: a moving field can add energy, and a still one, like the Heart, can only turn things.

kick to something still Δv = c − √(c² − A·w) about A·w / 2c when small thrown ahead if moving at v > c − √(A·w) and it leaves at about 2c − v most it can ever add 2 · √(A·w)

Two numbers decide how the wave feels. A·w is the height of the hill, which sets the most a surfer can gain. c decides who gets thrown: when c is well above √(A·w), only a surfer already moving with the wave gets carried, so there is a skill to learn and nobody is punished for being in the way.

The flights in the figure differ a little from these formulas, which leave out skin gravity and the Heart.

Keeping it graceful

Never move the camera. A body's "up" follows gravity, so a wave added to gravity would tilt the view by 15° on the ground and swing it around in the void. Adding the wave to velocity after "up" is worked out changes where you go and leaves "down" alone.
Always beatable. Keep the push below the void's swimming thrust (9 m/s²), so anyone in the air can hold their line by using their movement on it.
Nobody gets thrown by accident. Keep c at least 10 m/s above √(A·w), so only a body already moving with the wave faster than 10 m/s is thrown.
No damage. The wave only moves things inside a closed sphere, so the worst case is a harmless landing somewhere else.
Previews include it. The fall arc, skyshell dots and Skylance path step through the same field, so they show the wave's exact bend before you jump or fire.
Rare and announced. One event at a time, one or two a match, with 8–10 seconds of warning.

Seeing it coming

Inside a concave world everyone can see the whole sky, so everyone sees the wave coming.

The band itself: a faint sheet across the sky with streaks flowing in its direction. The void line's grid ripples where it passes.
Things it carries: dust in the void gets swept along, and trees and lift beams lean as it passes.
Your own body: inside it, a small arrow at the crosshair shows the push and your fall arc slides.
The map: the band on the round map, with seconds until it reaches you.
The director's voice: one line when it starts, such as "The shell exhales from the Antipode."
Sound: a low rumble that rises as the band gets close and drops in pitch as it passes.

More events from the same idea

Event	What happens	What you notice	Risk	Build cost	Verdict
Sweep	A band crosses the world, pushing the way it travels.	Crossings and skyshells drift, surfers ride it, the wall it starts from gets lighter.	Low, if the rules above hold.	One field term, one effect mesh, one HUD line.	first
Heartbeat	A shell of push races between the Heart and the ground, outward or inward.	A ring sweeping the whole sky. Outward it clears the void, and inward it makes everyone light for a moment as it passes the ground.	Low. It is short and the same for everyone.	Low: the same as the sweep with a radius in place of a plane.	second
Tide	Gravity depth goes up and down over 20–30 s, so the void line rises and sinks.	The void line grid moving; spire decks getting heavier or lighter; crossings easier or harder.	Very low. It is slow.	Very low: the Gravity depth dial already exists.	cheap test
Flare	The Heart's well grows to three times its reach for 20 s.	A corona around the Heart; crossings and shots near the centre whip around it.	Low, local to the centre.	Very low: one multiplier.	cheap test
Lull	A ring along the ground where skin gravity drops to a fifth travels from one pole to the other.	A ripple across the ground; people and loose things float up a few metres as it passes.	Medium. Losing your footing mid-fight can feel unfair.	Medium.	later
Spin	A swirl around an axis through the centre; everything in the void drifts around it.	Long curved crossings, and debris circling.	Medium. It can disorient.	Low.	maybe

Building it

One function per event. Each event is a fixed function of position and time, set by six numbers: kind, start time, direction, strength, width and speed. The server sends them once and every client computes the same field, so the event costs no bandwidth while it runs.
Where it plugs in. The body step adds the push to velocity after "up" is worked out, and the projectile step adds it to rounds, skyshells and their previews, which then need each step's absolute time as well as its count, a small change to the solver.
Bots. They share the body step, so the wave pushes them too; their brains would learn to avoid it, then to surf it.
The director. At first, a pause-menu button and a timer that fires one sweep per match from a random direction.
Tests. Check with the existing tools that crossings and skyshells land where this page predicts, the camera never rolls, previews still match flights to the metre, and bots don't get stuck.

Build the sweep first, with the "Start here" values: a 6 m/s² push, 30 m wide, moving at 24 m/s. It crosses the 0.65× world in about 9 seconds, kicks anything at rest by about 4 m/s, and throws ahead only someone moving with it faster than about 11 m/s. These three numbers are a starting point, and the right ones are still open, so the sweep ships with dials for all three and a button to trigger it. After playing it, decide on the Heartbeat.

Open questions:

Should a wave also push people standing on the ground, like wind, or only change their weight as it does now?
Should the director aim waves, at the safe zone or across the busiest sky to break a stalemate, or pick directions at random?
Should the Skylance stay almost immune, as the physics makes it, or should events bend it on purpose?

The band's look is decided: whatever runs the holograms paints it, with refraction and colour fringes that follow the real push (details on Inside the Shell).