New Spacecraft Models Coming in Alpha 19

Universe Sandbox ² - 3 New

New models in Alpha 19. Left to right: New Horizons, Saturn V 3rd Stage, Police Box

Alpha 19 is not yet officially available. You can try an opt-in preview version of Alpha 19 via Steam:

  • Right-click on the game title in your Steam Library
  • Click on ‘Properties’
  • Select the ‘Betas’ tab
  • Set the dropdown menu to ‘alpha19preview‘
  • Close the Properties window
  • Steam will now update Universe Sandbox ² to the preview version
  • Once updated, launch Universe Sandbox ²


In Alpha 19 we’ve added in new models for 3 different kinds of spacecraft: the New Horizons Probe, the Third Stage of the Saturn V rocket, and a police box.

These models were made by Toby Halter for Universe Sandbox ².  Older alpha versions of Universe Sandbox ² had models for both the New Horizons Probe and the Saturn V 3rd Stage, but Toby completely remade these, adding a lot more detail and texture to make them much truer to the real spacecraft. And thanks to Georg, our shader wizard, for working his magic on these models and implementing them in Universe Sandbox ².


1. Third Stage of the Saturn V Rocket

Universe Sandbox ² - Saturn V Third Stage Close-Up

This model is based on the third stage of the Saturn V rocket which was used in the 1969 Apollo 12 mission, the second to land astronauts on the moon.

The simulation in Universe Sandbox ² refers to the discovery of what was suspected to be an asteroid orbiting Earth in 2002. After more observation, however, astronomers concluded the most likely explanation was that this object was not an asteroid, but rather the third stage of the Saturn V rocket used in Apollo 12. It was likely flung out of Earth’s orbit in 2003, but there’s a good chance it’ll show up again in a few decades.

Check it out in the included sim: Home > Open > Third Stage of the Apollo 12 in Orbit of Earth in 2003.

Or you place it in any simulation: Add > Objects tab > Third Stage of the Apollo 12.


2. New Horizons Probe

Universe Sandbox ² - New Horizon Probe

The New Horizons probe is most famous for giving us our first close look at Pluto, which it approached in July, 2015. Now that it’s sent us detailed pictures of Pluto and its moons, it continues on its way toward the very distant Kuiper Belt.

The most notable change between this model and the old one is the shiny, gold foil wrapped around it, which serves as a insulating blanket for the grand piano-sized probe.

Check it out in the included sim: Home > Open > New Horizons Pluto Encounter in 2015.

Or you place it in any simulation: Add > Objects tab > New Horizons.


3. Police Box

Universe Sandbox ² - Police Box

As everyone knows, there’s nothing like a police box for traveling through space… or time.

You can add the police box to any simulation: Add > Objects tab > Police Box.


4. What’s Coming

Universe Sandbox ² - Pyramid Blocks Early Model

Toby is going to continue making models for Universe Sandbox ², but we’ve decided to take a break from spacecraft for our next model. Wouldn’t it be pretty cool to have a gigantic pyramid floating through space? The image above is a selection of work-in-progress bricks which will lay the groundwork for an Egyptian-style pyramid.

And while you may not have the urge to pilot a pyramid, you’re probably wondering if you’ll ever be able to control the other nice looking spacecraft in Universe Sandbox ². Well… we’re talking about it.



Follow us on Twitter and Facebook for the latest Universe Sandbox ² news.

You can buy Universe Sandbox ² on our website or on the Steam Store.

Universe Sandbox ² Alpha 19 Preview

Universe Sandbox ² - Third Stage Saturn V

The new model for the third stage of the Saturn V rocket.

We’re still working hard to get Alpha 19 polished and ready to go. We hope to have it ready in the next few weeks.

Until then, you can read about what’s coming in Alpha 19 below and try out a preview version by manually opting in via Steam:

  1. Right-click on the game title in your Steam Library
  2. Click on ‘Properties’
  3. Select the ‘Betas’ tab
  4. Set the dropdown menu to ‘alpha19preview
  5. Close the Properties window
  6. Steam will now update Universe Sandbox ² to the preview version
  7. Once updated, launch Universe Sandbox ²


1. Roche Limit & Tidal Heating

In this simulation, the Moon is orbiting Earth within the Roche limit and is being torn apart by tidal forces.

In this simulation, the Moon is orbiting Earth within the Roche limit and is being torn apart by tidal forces.

When a small celestial body approaches a larger body at a distance inside its Roche limit, it begins to stretch and disintegrate due to tidal forces. Essentially, the gravitational force acting on the closer side of the smaller body is stronger than the force acting on the far side. Outside of the Roche limit, these tidal forces are still present and can heat bodies in close orbits.

In Universe Sandbox ² Alpha 19, you can see these effects when a satellite tightly orbits its primary body, or when a star gets too close to a massive black hole, or even if two identically-sized bodies are in a close binary orbit. First, the secondary body will heat up, then if it’s close enough, it will begin to disintegrate and lose mass by producing a stream of particles. Often times these particles will begin orbiting and form a nice glowing ring around the primary body.


2. Models for Saturn V 3rd Stage, New Horizons probe, & Police Box

Universe Sandbox ² - Updated New Horizons Model

We’ve put in improved models for the third stage of the Saturn V rocket, which was used in multiple lunar missions, and the New Horizons probe, which has famously given us a closer look at Pluto. We’ve also added in another very important spacecraft, the police box.  These new and improved models were made for Universe Sandbox ² by Toby Halter, who is continuing to make models for us. It’s not possible to pilot or control these spacecraft… but we’re talking about it.

To check these out, you can load up the following sims:
- Third Stage of the Apollo 12 in Orbit of Earth in 2003
- New Horizons Pluto Encounter in 2015

Or you can place them in any simulation: Add > Objects tab.


3. Improved Explosion Power

Universe Sandbox ² - Earth Moons Explode

We’ve improved both the visuals and the simulated effects for the explosion power.

As you can see in the screenshot above, using Explode on a body will send out a “shock wave” which will affect surrounding bodies as well, causing a chain of destruction. Realistic destruction, of course.


4. Simulation Quality Setting

You can now adjust a Simulation Quality setting in Home>Settings>General tab. This will adjust the maximum number of attracting and non-attracting bodies, which in turn will affect the number of particles/fragments produced and how long they last. Set it to “Auto” to determine the best settings for your hardware.

In the future we hope to have this setting adjust dynamically with what is happening in the simulation, in order to constantly find the best balance between performance and simulation experience.



We still have some bugs to sort through and some polish to add, so hold tight. We’ll let everyone know once Alpha 19 is available.

The laser, which is already part of the VR mode, is still being integrated with the additional surface detail feature. We expect these features to be ready for Alpha 20.

You can buy Universe Sandbox ² on our website or on the Steam Store.

Follow us on Twitter and Facebook for the latest Universe Sandbox ² news.

Celebrating Our Rare and Special Planet on Earth Day

Universe-Sandbox-²-Earth Day

I hear a lot of people say that looking at our universe from a solar system, galactic, or cosmological scale makes them feel small and insignificant. Insignificant to who and what? I look at our universe and I see the incredible complexity that has arisen from a few forces and a few fundamental laws. You see it in the structure of the cosmic background, you see it in the shapes of galaxies, you see it in the heavy elements formed in our stars. Nowhere do you see that complexity more than when you look at life. It was a glorious accident that allowed us to exist, and we exist in an incredibly delicate balance.

99.99999999…% of the universe is empty space. 99.9999999…% of that which is not empty space is lifeless. Why does it matter if somewhere light years away something doesn’t care about your individual existence?  We have this amazing planet and it might not be unique in the universe in its complexity, but it is incredibly rare and special, and I feel very significant — overwhelmed with a sense of purpose and duty, in fact — because it is up to us to try as hard as we can to preserve this rare and special place and all the life on it.

The structure of the universe doesn’t make me feel insignificant. It makes me feel incredibly lucky. By some chance I exist in this rare place. Thus it is my duty to try to keep life existing here, and to try to make that life as pleasant as possible for the other rare and lucky creatures who share it with me.

Jenn Seiler, astrophysicist and Universe Sandbox ² developer.


Learn how we simulate Earth’s climate and how you can explore it in Universe Sandbox ²:


Hiring a Simulation Developer

April 21, 2016 | We are still accepting applications for this position.

Giant Army is looking for a simulation developer to join our team. You will work with scientists and developers to improve and create geophysical and astrophysical simulation code. This code must smoothly integrate with the other systems in Universe Sandbox ², including running in real-time under computational restraints. You must be an experienced programmer with a broad toolkit of numerical techniques, and a collaborative worker.

Join us… we’re making something incredible that’s never been done before.


Your Role

  • Build on, improve and optimize, and create new geophysical and astrophysical simulation codes which talk to all the other pieces of the simulation
  • Work with scientists to define a problem and iteratively determine the best numerical solution that meets computational constraints
  • Work with developers to integrate with other systems and the graphical display smoothly
  • Quickly prototype and estimate computational costs
  • Help change the way the world perceives our universe and fragile planet

Ideal Candidate

  • Is a self-driven and talented programmer with experience applying/integrating numerical models in a compiled language using modern developer best-practices
  • Writes clean code that is comprehensible to teammates
  • Loves physics simulations and fantastical what-if scenarios: (note citation #6 on 148)
  • Is excited about developing a tool that allows users to experiment with simulations, pushing the boundaries of what has been done in real time for public consumption

Mandatory Qualifications

  • Solid programmer (object oriented)
  • Familiar with a broad toolkit of numerical techniques
  • Curious about science
  • Enjoys a challenging problem
  • Works collaboratively
  • Familiar with version control, some kind of task-management system, unit tests, code reviews and so forth

Preferred Qualifications

  • (Strong C# && Unity Engine experience) || (Experience with an entity component model && object-oriented programing)
  • M.S. in relevant field (e.g. applied math, computer science)
  • Ability to meet in person at least once a week in Seattle, WA, United States, but will consider outstanding candidates from anywhere in the world

More Information

You don’t need to be a subject-matter expert in any area of geophysical sciences or astronomy, but you must be curious about science, and love the idea of working with scientists who would lay out a computational problem that you could help solve and implement. Everything must run in real time; computational efficiency is key. Sometimes the level of complexity that we can represent is a continual negotiation depending on the efficiencies that prove possible. Which is why this work is super-important to making Universe Sandbox more awesome.

There is an element of iteration with the scientists and developers. We may start from a wish-list or pseudocode that is too computationally expensive, and iterate from there, coming up with new ideas for which simple models of the physical process to suggest next. Your job in this iteration is to quickly prototype, if needed, and provide estimates for whether the current idea has yet achieved feasibility.

Besides the scientists who want to compute things in real time, the graphics programmer is concerned with displaying them smoothly, so you also need to iterate on the output and its interface with the graphical display. And because it’s a sandbox on a broad scale, everything must integrate well with existing systems. Models of specific processes do not exist in isolation of the influence of the rest of the simulation; everything must work together and be designed with this in mind.

This is very much a collaborative position, and so the ability to work well in a team is an important qualification. You must be good at working with other people’s code, and you need to be able to take direction from the creator of Universe Sandbox on design constraints, from the scientists on physical realism, and work with the systems architect to make it all operate smoothly with the core simulation and gravity integration that is already in place. You may work with the numerical physics programmer responsible for the gravity integration on problems like threading and other computational optimizations for the software at large.

Code samples are appreciated. It need not be in C# or use Unity3D. If you’re just accustomed to a different language, you can learn C#, but object-oriented design is something you should already do.

Portfolio-wise, visualizations are appreciated. Visualizations of simulations that did not run in real time may impress us, but in this position everything must run in real time, so things that do will impress us more, perhaps in the form of runnable code (e.g. an executable).

Partial List of Simulation Improvements

  • Improving the climate simulation (calculating temperature, pressure, greenhouse effect, user adjustable materials, dust from impacts affecting the temperature)
  • Improving asteroid collisions (heating, impact size, velocities of fragments)
  • Adding atmospheric drag (based on the properties of the body and the atmosphere, with correct heating of the body and material loss from the friction and heat)
  • Improving our material system (metallic hydrogen in gas giants, simulating subsurface oceans on moons)
  • Improving supernova and stellar evolution (and making that all work smoothly together)
  • Improved transitions of bodies as their properties change (add hydrogen to a rocky body to make it become a gas giant, keep adding to turn it into a star)

Company Overview

Giant Army is the company behind Universe Sandbox, a space simulator that’s sold hundreds of thousands of copies on Steam. Our headquarters are in Seattle, Washington, USA, with team members in Arizona, Missouri, Germany, and Denmark.

Team members enjoy a flexible, collaborative environment. We pursue the features that get us excited about science. We do the work so that we can share that with others. We’re also committed to creating an accessible experience that can’t be found elsewhere.

Science is for everyone, and we welcome all qualified applicants.

Product Overview

Universe Sandbox ² is our core product that lets the user explore and learn about our amazing universe and fragile planet while creating and destroying on a scale they’ve never before imagined. It’s more than a game; it’s a way of experiencing and learning about reality in a way that’s never been done before. Early alpha access now available for Windows, Mac, and Linux.


How to Apply

Email us: jobs at

In the email please include:

  • Relevant email subject
  • Link to or copy of your resume or CV
  • Link to examples of your work, including visualizations of numerical simulations
  • Answer to these two questions:
    • What is the most appealing part of this opportunity?
    • What feature would you be most interested in adding or helping make better?
  • Where you originally heard about this job posting

Hiring a Producer

Giant Army is looking for a producer to help in our continued development on Universe Sandbox ².  You will work closely with the project lead with the overall goal of making Universe Sandbox ² even more awesome. You will also work with the team as a whole to organize and prioritize tasks and projects, while helping ensure goals are met within established time-frames. You must be self-directed, organized, and an excellent communicator.

Join us… we’re making something incredible that’s never been done before.

Universe Sandbox ² - 20160401-003218

Your Role

  • Work with the team to help change the way people think about our amazing universe and fragile planet
  • Work closely with Dan, the project director, to discuss, articulate, and execute on the project goals
  • Maintain a broad view of the project to ensure it is oriented toward these long-term goals each step of the way
  • Work with each team member to maintain a clear vision of their individual progress and stay on track towards goals in established time-frames
  • Maintain, prioritize, and coordinate plans for projects which involve multiple team members
  • Help make decisions and provide possible solutions to problems

Ideal Candidate

  • Interested by physics simulations and fantastical what-if scenarios: (note citation #6 on 148)
  • Experience with software/video game/educational development, having worked with both artists and programmers
  • Has educational, business, and/or marketing acumen
  • Has experience as a lead or in some role managing others
  • Has ideas for improving production and an understanding of agile methodologies
  • Ability to meet in person at least once a week in Seattle, WA, United States (or willing to relocate), but we will consider outstanding candidates from anywhere in the world (without relocation)

Mandatory Qualifications

  • Excited about science
  • Self-directed, comes up with good ideas and executes them
  • Enthusiasm for the project’s science focused vision
  • Energized by meetings, personal interactions, and problem solving
  • Personable and energized by working with a wide-variety of personalities

Company Overview

Giant Army is the company behind Universe Sandbox, a space simulator that’s sold hundreds of thousands of copies on Steam. Our headquarters are in Seattle, Washington, USA, with team members in Arizona, Missouri, Germany, and Denmark.

Team members enjoy a flexible, collaborative environment. We pursue the features that get us excited about science. We do the work so that we can share that with others. We’re also committed to creating an accessible experience that can’t be found elsewhere.

Science is for everyone, and we welcome all qualified applicants.

Product Overview

Universe Sandbox ² is our core product that lets the user explore and learn about our amazing universe and fragile planet while creating and destroying on a scale they’ve never before imagined. It’s more than a game; it’s a way of experiencing and learning about reality in a way that’s never been done before. Early alpha access now available for Windows, Mac, and Linux.

How to Apply

Email us: jobs at

In the email please include:

  • Relevant email subject
  • Link to or copy of your resume or CV
  • Answer to the following two questions:
    • What is the most appealing part of this opportunity?
    • Why would you be a good producer on this project?
  • Where you originally heard about this job posting


VR is Now Available for Universe Sandbox ²

Universe Sandbox ² - Earth Explosion VR

For the past few months our highest priority has been adding in HTC Vive/SteamVR support for Universe Sandbox ². The team’s hard work has paid off: we now have a fully-functional virtual reality version which allows you to walk and fly around your simulations, melt planets with a giant laser, then grab them and hurl them off into nothingness with a flick of the wrist.

The best way to experience the amazing sense of scale and immersion in Universe Sandbox ² VR is to put on a headset and check it out for yourself. But if that’s not possible, then here’s a trailer for Universe Sandbox ² VR, featuring user-created VR gameplay videos courtesy of Brad Lynch and Count Lorek of BMcD Gaming.

VR is Free for Owners of Universe Sandbox ²

Universe Sandbox ² now includes both the desktop version and the new VR mode. So anyone who buys Universe Sandbox ² will also receive Universe Sandbox ² VR, and anyone who already owns Universe Sandbox ² can access the VR mode at no extra charge.

If you own Universe Sandbox ² on Steam and have access to a Vive, just run Steam to update. Then launch Universe Sandbox ² and select “Launch Universe Sandbox ² in Steam VR Mode” in the popup dialog box.

Universe Sandbox ² - Uranus VR

Lots of Love for Universe Sandbox ² VR at GDC

Thanks to Valve, we were able to demo our VR version of Universe Sandbox ² in the Steam booth two weeks ago at GDC, the world’s largest professionals-only video game conference.

We received great feedback from everyone who stopped by for a demo, with many agreeing that it was one of the most compelling VR experiences they had tried.

PC Gamer assistant editor Tom Marks said Universe Sandbox ² “was one of the coolest thing [he's] tried on Vive so far.” Check out the video below for his discussion of Universe Sandbox ² VR:

Scott Manley, a popular YouTuber whose videos combine science and games, was one of the first to stop by for a demo:

Here’s a small write-up from Rock Paper Shotgun editor Graham Smith: Universe Sandbox ² Now Has a Virtual Reality Mode

In another article about VR, Graham mentions Universe Sandbox ² as one of the few upcoming VR games which he thinks “look spectacular.”

And here’s an older article from UploadVR, based on an interview with Dan, creator and project lead on Universe Sandbox ²: Literally Play Pool with the Planets in Universe Sandbox ² VR

Universe Sandbox ² - Planets VR

What’s Next for VR?

We still have work to do, but we’ve already got an enjoyable and impressive VR experience. Beyond adding polish to the interface, we also have to work on getting the best performance possible in order to keep up with VR requirements of 90 frames per second. With VR it’s crucial to meet this requirement in order to keep the risk of motion sickness at a minimum.

We’ll continue to work on the VR version as well as the desktop version, and we’ll release updates when they’re ready. We don’t support Oculus Rift yet, but we’re looking into adding this in once the Touch is available.

You can buy Universe Sandbox ² on our website or on the Steam Store.

Follow us on Twitter and Facebook.

Did We Say VR? We Meant RV! Cruise Around Space with RV Mode


RV photo credit: Greg Gjerdingen (CC License)

RV Mode is Coming Soon to Universe Sandbox ² (sorry, it’s RV, not VR mode)

First, we must apologize for this typo which was made many, many times in many of the posts here and on our promotional material. While we’re not sure who was responsible for editing these posts, we’re in the process of hiring someone now who can find the culprit and immediately fire them. We will then immediately fire this new employee as well.

So to be clear, we meant to say RV mode, not VR mode.  We’re talking recreational vehicles here, not virtual reality. Yep, RVs, those huge clunky things you can use to take the whole family on the greatest ever cross-country road trip, with no need for rest stops because inside of that RV you’ve got a bathroom, plenty of room to pace back and forth, and, if you’re lucky, even a satellite TV.

Step Inside the RV of Your Dreams… In Space

While a VR mode for Universe Sandbox ² does sound pretty cool (we’ll look into this), the RV mode which we’ve actually been focusing on for the past few months is something which we think you’ll agree is way cooler.

Just imagine: You load up a simulation of our solar system and engage RV Mode. Suddenly you’re sitting in the cushy front seat of your very own recreational vehicle, feeling like royalty on the Intergalactic Highway. You kick it into high gear and blaze through space at 60 miles per hour (that’s about 96 km/h if you’re in Europe), looking out the huge, panoramic windshield as you pass the gas giants of our solar system. What’s that, not interested in Jupiter’s moons? No problem, take a break and check out the kitchenette. Explore the mini-fridge. Explore the contents of the pot on the hot-plate. Are you picking up a signal on the satellite TV? If so, explore the 200+ channels of premium content. 

Of course, while making this RV Mode we haven’t forgotten about the spirit of the sandbox. So yes you can still experiment and get answers to fantastical what-if questions like, “What if I put the hot tub jets [yes you can unlock a hot tub] on full blast? How much sooner will I run out of gas?” or “What if I open the windows?” or “What if I drive this thing right into the sun?”

Why RV Mode for Universe Sandbox ²?

Good question. But have you considered “Why not?”

Truth be told, this may seem like it’s coming out of left field for a space and gravity simulator, but since day one it’s been part of the dream for Universe Sandbox ². Dan Dixon, creator and project lead on Universe Sandbox ², pointed out the obvious reason: “The driving force behind this project has always been about giving the player the freedom and tools to do whatever they want. And what embodies freedom more than cruising around the universe in an RV?”

So forget about exploring your Universe Sandbox ² simulations in full 3D and waving your hands around to make planets collide. The real frontier of next-generation gaming lies in the powerful and inspiring potential of the RV, the cutting edge of unwieldy, carbon-friendly (and family-friendly) automobiles. 

Again, sorry for all those typos.

Exploring Some Lesser-Known Features

Maybe you’ve noticed that there are a lot of features in Universe Sandbox ².  As we continue development, we work on ways to make all of these features discoverable and easy-to-use.

But some remain a little hidden right now. So here’s a list of some of the features we see commonly requested, and which are actually already in Universe Sandbox ².


1. You can fly the camera around your simulation using the WASD keys.

  • Use the WASD keys on your keyboard to freely move the camera around
  • Click and drag the mouse to point the camera in a different direction
  • You can use this to focus the camera on a point in your simulation instead of a specific body


2. Want to see moons and rings in our solar system? The default sim doesn’t have these because they tend to slow things down, but you can load sims which do.

  • Search for “moon” or “ring” in Home > Open to find simulations which have the moons and rings of our solar system
  • These sims require a lot of accuracy to keep moons and rings in tight orbits, so they tend to run slower than a standard simulation

Universe Sandbox ² - Jupiter Moons


3. Or you can add rings yourself to any body in a simulation.

  • Select a Body > Add > Rings tab > Select a preset > Add Ring

Universe Sandbox ² - Mars Rings


4. Universe Sandbox ² has a database of over 50 thousand known objects.

  • Use the search bar in the Add panel to find less common objects
  • Add > Search bar (top right)

Universe Sandbox ² - Search


5. You can land the camera on the surface of a body.

  • Click twice to focus on a body > Press C on your keyboard
  • Press C again to return to the normal camera view
  • Getting dizzy? Properties > Motion tab > Rotation Lock (bottom of list)


6. You can create randomly-generated solar systems.

  • Home > Create > Systems

Universe Sandbox ² - Random Solar System


7. Earth and Mars both have climate simulation.

  • Select Earth or Mars > Properties > Climate tab > Enable Climate
  • Learn more about how we simulate climate in our Climate post


8. Accidentally delete the sun? No worries, you can Undo!

  • Press Ctrl+Z on your keyboard to undo your last action
  • This is still a work-in-progress feature and does not undo every action


9. You can change the frame of reference for trails.

  • Select a body > Properties > Actions tab > Set as Trail Center
  • Return to default: Actions tab > Clear Trail Center
  • For example, instead of seeing how bodies in our solar system move in relation to the sun, you can see how they move in relation to Earth, as shown in the screenshot below

Universe Sandbox ² - Trail Centers


10. You can view and customize keyboard shortcuts for many commands.

  • Home > Settings > Input


11. You can hide the interface at any time for a more cinematic view.

  • Press Tab on your keyboard to hide the interface
  • Press Tab again to make it return


12. You can change a simulation’s starting date, have it start paused, and adjust other advanced starting parameters.

  • Home > [Open or Create] > Hover over a preset and click the pencil icon

Universe Sandbox ² - Starting Parameters


13. Bodies and trails can be colored according to their velocity, mass, or acceleration.

  • View > Colors

Universe Sandbox ² - Colors

Working Through the N-Body Problem in Universe Sandbox ²

Orbits of major planets and all possible dwarf planets in our solar system. (from Universe Sandbox ²)

Orbits of major planets and all possible dwarf planets in our solar system. (From Universe Sandbox ²)

What is the n-body problem?

The n-body problem can be defined as “the problem of predicting the individual motions of a group of celestial objects interacting with each other gravitationally.” Or, in a gravitational system of n bodies (where n can be any number), where will they all be after one year?

It’s helpful to frame this in contrast to the two-body problem, which looks at the motion of just two objects interacting with each other. For example, you can look at the Earth and Moon as a two-body problem. The Earth pulls on the Moon quite a bit, keeping it in orbit, and the Moon pulls on the Earth just a little bit.

The issue here is that the Moon is not affected gravitationally by just the Earth; it is also being pulled by the Sun, and Jupiter, and every other object in space. The same is true when looking at the Sun and Earth: the Sun is not the only object pulling on Earth. So to account for all of these gravitational forces, you need to use an n-body solution.

The problem of the n-body problem in Universe Sandbox ²

In Universe Sandbox ², every object is simulated as part of an n-body problem. Unfortunately, when solving for many objects, or n objects, you can’t just jump forward in time without getting massive errors. There’s simply no way around this. Solving an n-body problem requires calculating how each object affects each other object every step of the way. Errors will still happen, but taking smaller steps reduces them.

By default, the simulations in Universe Sandbox ² try to set an accuracy which prevents orbits from falling apart due to error. This means setting a maximum error tolerance for each step and also making sure the total error doesn’t reach an upper limit.

If you crank up the time step, the simulation then has to take fewer, larger steps. This means the potential for greater error. And the greater the error, the more likely it is that an orbit, which otherwise would be stable, falls apart. Moons crash into planets, Mercury gets thrown out of the solar system — things like that.

This isn’t what most people want in their simulations. But at the same time, most people also don’t want a limit on how fast they can run their simulation. This is a problem.

An imperfect solution

So how can we get around this problem? How can we accurately simulate thousands of objects while still allowing for large steps forward in time? For example, what if you wanted to simulate our solar system on a time scale of millions of years per second so that you could see the evolution of our Sun?

One solution proposed by Thomas, our physics programmer, is to allow for a special mode within simulations running at high time steps. This mode (which of course could be toggled) would collapse the existing n-body simulation into a series of 2-body problems: Moon & Earth, Earth & Sun, Europa & Jupiter, Jupiter & Sun, etc.

Solving a 2-body problem is much easier than solving an n-body problem. Not only is it faster computationally, but there is also a relatively arbitrary difference between figuring out where the two objects will be in one year and where they’ll be in a million years — it still requires just one calculation. So if you collapse an n-body simulation into a series of two-body problems, the simulation could take one big step forward, instead of taking the small steps needed for calculating it as an n-body problem.

The results won’t be entirely accurate, as this method would effectively ignore all gravitational influences outside of the main attractor. As mentioned before, calculating Earth’s orbit by looking at how it interacts with just the Sun is not accurate, as Earth is also affected by every other body. The Sun, however, is the most significant factor by far, because it is much more massive than any other object in our solar system. The other, much smaller forces tend to have little effect overall in non-chaotic systems. So while it’s not correct, it’s close enough when simulating something relatively stable like our solar system.

This isn’t a perfect solution. But we think it could be an improvement over the current system and its limitations, which leave you with the choice of either destabilizing the orbits with massive errors, or waiting days for the simulation to advance the millions of years needed for the Sun to evolve. Neither is particularly interesting.

When is this coming to Universe Sandbox ²?

Not anytime soon.

This solution is just a proposed idea right now, and is not a high priority for us, as we already have a big list of exciting features planned. But we think it is useful to understand the complexity of accurately simulating the motions of hundreds to thousands of objects interacting gravitationally.

This is especially a challenge when attempting to do this in real-time on a home computer, which is why researchers run numerical simulations on supercomputers which take days to complete.  With Universe Sandbox ², we’re exploring new territory and working through problems which haven’t been solved before. And this is a big part of why we love making it.

Gravitational Waves & Universe Sandbox ²

A black hole in Universe Sandbox ². Researchers have concluded the gravitational waves they detected were the result of two black holes colliding.

A black hole in Universe Sandbox ². Researchers concluded the detected gravitational waves resulted from two black holes colliding.

What’s the significance of discovering gravitational waves?

This announcement is a huge deal.  It is on par with the discovery of the Higgs Boson particle which provided the missing evidence for a prediction of the Standard Model of particle physics. Gravitational waves are a century-old (almost exactly) prediction now confirmed by a huge number of relentless, and brilliant people after many years of hard work. It is the first direct confirmation of the prediction from Einstein’s General Relativity that matter and energy determine the motion of bodies by warping the fabric of spacetime itself, and in so doing, emanate ripples when massive bodies are accelerated through that space.

It is not only confirmation of general relativity, though. It is also the first of many future observations that will look at the universe in a completely new way. Up until now we’ve used only photons (telescopes all along the electromagnetic spectrum) and sometimes neutrinos. Now we can add listening to the fabric of space to our list of tools. This will allow us to see the dark and the obscured parts of the universe: the early universe, centers of galaxies, things blocked by dust clouds, and so on, by listening for changes in space itself. It is the start of a new age in astronomy.

In addition to this detection being the first direct proof that the predictions of general relativity that matter and energy warp space time are true, and some of the strongest evidence for the reality of black holes, this is also a new kind of astronomy.  Though gravity is the weakest force and gravitational waves are very hard to detect, they do have a few advantages over observations of photons.

  • First, gravitational waves are practically impervious to matter in their path. This means we can see into regions of space that are blocked to optical observatories, such as inside dense clouds of dust, the centers of galaxies, behind large or close bodies.
  • Second, this is an observation of the warping of space itself, meaning we can detect things that have mass but might not produce observable light, such as black holes, dense sources of dark matter (if such were to exist), cosmic string breaks, etc. 
  • Third, gravitational waves fall off in amplitude much more slowly than light. This means that we can receive signals from very far away that we might not notice optically.
  •  And fourth, because gravitational waves also travel at the speed of light and don’t have to bounce off intervening matter, and begin to be potentially detectable from bodies getting close rather than just after the moment of collision, this means that we can work with other telescopes and tell them “Look over there! You’re probably going to see something exciting!”

This all of means that this detection means the beginning of a new kind of observational astronomy, as well as a better understanding of of of the fundamental forces of the universe, gravity.


What role did Jenn, astrophysicist and Universe Sandbox ² developer, play in the discovery?

While I was in the field I ran super-computer simulations to make predictions about the gravitational wave signals that would be produced by binary black hole mergers. Those waveforms are used as templates in the detector pipeline. The detector matches the template banks against the incoming data to find real signals amidst the noise of the detector, while also doing searches for large burst signals (how this one was found). Those waveforms are then used again to determine where the signal came from, what it was (two black holes, a neutron star and a black hole, two neutron stars, etc), and the properties of the bodies that created the signal (spins, masses, separation, etc.). I also worked on developing the analytical formulas to determine those spins and masses from those signals.

Here’s one of the scientific papers on the process of determining the properties of the source of the signal, with three papers cited on which Jenn Seiler was an author:

The Einstein equations for general relativity are ten highly non-linear partial differential equations. This means that it is only possible to obtain exact solutions for astrophysical situations for some very idealized conditions (such as spherical symmetry and a single body). In order to predict the gravitational waveforms produced by compact multi-body systems, or stellar collapse, it is necessary to solve the equations numerically (computationally). This means formulating initial data for spacetimes of interest (such as two in-spiralling black holes of various spins and mass ratios) and evolving them by integrating the solutions of the Einstein equations stepping forward in time by discrete steps. To prove that these computer simulations approximate reality more than just by equations on paper we would run these simulations at multiple resolutions for our discrete spacetimes and show that our solutions converged to a single solution as we approach infinite resolution (that would represent real continuous space) at the rate we expect for the method we were using. 

There were many obstacles in creating these simulations: vast amounts of computational power required for accuracy; the fact that we needed to run tons of these large, slow, computationally intensive simulations in order to cover the parameter space (spins, masses, orientations, etc) of potential sources of gravitational waves; and so on. For black holes, one major challenge was the fact that they contain a singularity. A singularity means an infinity, and computers don’t like to simulate infinities. Numerical relativity researchers had to find a way to simulate black holes without having the singularity point in the slicing of the spacetime integrated in the simulation. The first successful simulation of this kind didn’t happen until 2005 (

Once we had working simulations, groups around the world set to work on simulating the gamut of major potential gravitational wave signal sources. These simulation results were not just useful to the detectors to help identify signals, but also to the theorists to help formulate predictions about the results of such astrophysical events. Predictions such as: the resulting velocity of merged black holes from binaries of various spins, the amount of energy released by black hole mergers, the effect that black hole spins have on the spins and orbits of other bodies, etc.

When will you add gravitational waves into Universe Sandbox ²?

We really can’t do gravitational waves in an n-body simulation, which is the method Universe Sandbox ² uses to simulate gravity. N-body simulations look at the effect that each body has on each other body in a system at small discrete time steps.

General relativity requires simulating the spacetime itself. That is, taking your simulation space, discretizing it to a hi-res 3-D grid and checking the effect that each and every point in that grid has on all neighboring points at every timestep. Instead of simulating N number of bodies, you are simulating a huge number of points. You start with some initial data of the shape of your spacetime and then see how it evolves according to the Einstein equations, which are 10 highly non-linear partial differential equations. Accurate general relativity simulations require supercomputers.

There are some effects and features related to relativity that would be possible to add to Universe Sandbox ², however. Here are a few we are discussing:

  • Gravity travelling at the speed of light.

    • Currently if you delete a body in a simulation, the paths of all other bodies instantly respond to the change. The reality is that it would not be instantaneous; it would take time for that information about the altered gravitational landscape to reach a distant object.

  • Spinning black holes.

    • Most black holes are very highly spinning. If you imagine a spinning star collapsing it is easy to understand why. This is the same effect as when a spinning figure skater pulls in their arms; because of conservation of angular momentum, they spin faster. A consequence of this spin is that, while the event horizon would remain spherical, there would be an oblate spheroid (squished ball) around the black hole called an ergosphere. This ergosphere twists up the spacetime contained within it and accelerates bodies that enter this region (as well as affecting their spins). Because it is outside of the event horizon, this means one can slingshot away from this region and even steal energy from the rotation of that black hole.

  • Corrections to the motions of bodies to approximate general relativity.

    • Loss of momentum due to the emission of gravitational waves causes close massive bodies to inspiral. With this you could recreate the decaying orbits of binary pulsars.

    • Spins of close bodies affect each other’s motion and spins (see above). This would give you things like spun up accretion disks around black holes.

    • These corrections would be made by adding post-newtonian corrections to body velocities.


Learn more

Bad Astronomy article: LIGO Sees First Ever Gravitational Waves as Two Black Holes Eat Each Other

Video and Comic Explaining Gravitational Waves

Reddit AMA (Ask Me Anything) by LIGO Scientists

Paper by LIGO Researchers: Observation of Gravitational Waves from a Binary Black Hole Merger