PC Gamer - UK (2021-12)

Gravity

FEATURE

themysteriousandinvisiblesubstancethatfillsour

universe,addingtoitsgravityandkeepingapparently

‘underweight’ galaxies in shape. “Part

of the problem with dark matter is

that it doesn’t really come in chunks,”

says Erika Nesvold, Giant Army’s

in-house astrophysicist. “There was a

theory that it was big, compact

objects, but it seems more likely now

that it’s sort of spread throughout a

galaxy. And that’s just really hard to

handle in an n-body simulation. And

it’s also hard, of course, to show dark

matter to a user. But we’d love to get

back into it someday.”

As a sim with no objectives

besides meddling with the cosmos, UniverseSandbox 2

doesn’t have to worry as much about the design and

playability pressures that shape gravity in Kerbal and

Outer Wilds. But in adopting n-body physics, it has to

wrangle with the question of mounting errors dependent

on the speed at which the simulation progresses.

The bigger the ‘step’ between each set of n-body

calculations, the less precise things become. “As a rule of

thumb, picking a time step that’s bigger than the time it

takes something to orbit in a circle is a bad idea,” Nesvold

says. The game performs additional ‘sub-step’ calculations

for finesse in the event of objects making sharp turns, but

“there’s only so many you can do”. Increase the time

of hundreds of bodies on each other’s movements, though

it economises in places by, for instance, treating close-knit

asteroids as a single mass. “It can become very, very slow,”

Dixon acknowledges. “But in many ways, gravity is the

simplest thing we do now.”

Giant Army has even looked at recreating dark matter,

iteration can recreate the motions of entire galaxies, each

with its central supermassive black hole.

Unlike Outer Wilds and Kerbal, Universe Sandbox 2

‘true’ n-body physics simulation. It calculates the effects is a

simulator in high school, it’s been in development for ten

years and has swollen to the point that the current

gravity is Giant Army’s

initially by Dan Dixon, who wrote his first gravity Universe Sandbox 2. Created

PLAYING WITH STARS

Way out at the deep end of videogame simulations of

our limit for stability.”

says. “But also, the 20-ish minute mark was originally

everything reset is an easy way to handle that,” Ver Hoef

reversible, like Brittle Hollow falling apart, and having

mass–thesun.ThereasonissimplythatOuterWilds’

solarsystemistiny,withplanetsthesizeofislands

separatedbytensofkilometres–assuch,thepullof

gravitydropsawaytooquickly.“Earlyon,Itriedthe

inversesquarelawonplanets,”saysAlexBeachum,

creativedirector.“Butbecausethey’resosmall,itjust

becomestooeasytoachieveescapevelocity–it doesn’t

feel right.” Instead, gravity decreases in a more ‘linear’

fashion, and the developer can set alignment fields so that

players orient towards masses automatically.

Outer Wilds also exercises a bit of licence with regard

to interactions between planets, adds tech artist and

programmer Logan Ver Hoef. “We have this sphere of

influence for the sun and we’ve got spheres of influence

for each planet. But because planets end up passing so

close by each other, if they actually affected each other at

that scale, they’d form these wild patterns and probably

just escape orbit.” The game’s equivalent of our Moon, the

Attlerock, responds only to the gravity of one planet, but

with extra acceleration dolloped on top to reflect that

planet’s orbit – otherwise, says Beachum, the Attlerock

might be left behind or dragged into the sun.

Elsewhere, however, the simulation is more stringent.

Your spacecraft may tumble off the back of a comet when

it nears the sun, for example, because in keeping with

Newton’s law, the star’s gravity acts differently on the

ship’s mass versus the comet. Moments like these were

often discovered during play, and left in for kicks. For all

its resemblance to a cosmic stage production, with planets

shape-shifting on schedule, Outer Wilds is on some level a

‘pure simulation’ with plenty of potential for chaos.

It’s possible to wreak havoc simply by venturing too

far. “If you disable autopilot and fly away from the solar

system, and open your star map, everything’s lost its

mind,” Beachum says. “Because floating point errors build

up, and the planets are too far away. You can actually

break the simulation if you try hard enough, because

they’re not on rails.” The game’s 22 minute timeloop was

introduced partly as a kind of safety valve. “We wanted

the solar system to change in ways that aren’t easily

Outer Wilds is similarly selective in its simulation. For one

thing, Newton’s inverse square law only applies to one

Mobius Digital’s planet-hopping timeloop adventure

SUM OF ALL SPHERES

masses, where the pull from each mass is equal.

• tactically advantageous locations in space between

astrophysical ‘terrain variables’, such as Lagrangian points

trade-off is that

Kerbal can’t simulate certain fancier

OUTER WILDS

IS ON

SOME LEVEL

A ‘PURE

SIMULATION’