How do you simulate a universe?

How do you simulate a universe? And why would you want to?
16 July 2019

Interview with 

Nick Henden, Cambridge University


Colourful image of the Universe


Could we be living in a computer simulation? And if so, why would that be, and how could this happen? Well first up, to find out how scientists go about simulating a universe, Heather Jameson spoke to Cambridge University's Nick Henden...

Heather - We humans are a curious bunch. Throughout the aeons we've been striving to improve our understanding of the world around us, and over the last few decades we've developed a really powerful tool to help us achieve this: computer simulations. We use computer simulations across all branches of science and engineering; from aerodynamics to design better cars and planes, to climate models to better understand the effects of global warming. Some people even believe that we might actually be living in a computer simulation.

In 2003, philosopher Nick Bostrom proposed the simulation argument. The argument goes like this. Imagine a super-advanced civilization with immense computing power. If we assume that they had the same thirst for knowledge and understanding of their world as us, then they would probably put that computing power to the task of simulating a whole universe inside their supercomputers for the purpose of research. In fact, they might simulate lots of different universes to understand what differences small changes make. Then what if the simulated civilizations also attained the ability to run their own simulations? Well then we've got simulations within simulations, and quickly the number of simulations vastly outnumbers the one actual reality. If that were true then there could potentially be billions of simulated universes and only one real universe. So in that case statistically we are very unlikely to be living in the one real universe!

Wow. It's a lot to get your head round - especially because I just don't feel like I am a simulation. How would you go about simulating a universe anyway? Well to try and answer this question I spoke to Nick Hendon from Cambridge University’s Institute of Astronomy to find out how cosmologists use computer simulations to improve our understanding of the universe. Nick showed me a computer-simulated video of a cluster of galaxies forming. The colours represented temperature, with the red at the cold end of the scale going through blue, to green, and then to white at the hot end.

At the beginning of the video the space is filled by a red gas.

Nick - So what you're seeing at the beginning with all that red material is fairly cold gas, and it's almost uniformly distributed, but it’s not quite.

Heather - The red gas formed into a 3D web - a bit like candy floss - then bright blue spots started appearing in the web.

Nick - There are some areas with more gas than others and the extra gravity of those regions pulls gas from the less dense regions in towards it.

Heather - The bright spots grew larger into clouds of greenish-white gas.

Nick - The regions that are most dense with gas start to grow, and as gas collapses onto those regions, the gas heats up because the pressure is increasing.

Heather - The individual clouds of gas gradually merged together to form larger clouds, until eventually the scene was dominated by large clouds of greenish white gas, with specks of red gas in the dark spaces in between. It looks just like what I’ve seen in science fiction films.

But how is a simulation like this produced? Well first you tell the computer to simulate a very large box - a box that is hundreds of millions of light years across - and you fill the simulated box with particles.

Nick - Those particles represent mainly gas and dark matter. This is mostly the gas that was around in the very early stages of the universe, so that's about three-quarters hydrogen and one-quarter helium. And then you apply the physical laws such as gravity and the laws of motion that describe the fluid motions of the gas, and you also add additional processes such as star formation, black hole physics, and various other physical processes that are important for the formation of realistic galaxies.

Heather - But the universe isn't a box. So what does the computer do at the edges? The way they get around this problem is by setting it up so that when a particle goes out of the right hand side, it re-enters on the left side. In other words you pretend that the box is replicated on all sides. This works pretty well as long as the box is large enough. But how do cosmologists know that their simulations are accurate representations of reality?

Nick - You can mimic observations with your simulation. So you could pretend, “if I took my telescope and looked at my simulation, what would I see?” And then you compare that to what we actually see on the real sky, and if they match then you know that you've done a reasonably good job in creating a realistic simulation.

Heather - If we look at a distant galaxy with a telescope we are actually looking back in time. That is because the light from those galaxies takes time to reach us. The further away we look, the further back in time we are looking. So if we look really far we can see what galaxies looked like shortly after the Big Bang. If we look less far away we can see different galaxies at a different point in time, but we can't watch individual galaxies evolve. But with simulations we can fill in the gaps. How far can we turn back the clock with these simulations? Can we simulate the Big Bang?

Nick - With these sets of simulations we typically start in the early universe, somewhere around 300,000 years after the Big Bang, which is not very long at all in astronomy terms. And in fact if we were to go back all the way to the Big Bang, at some point the laws of physics would essentially break down, in which case the simulations would be no good anyway.

Heather - So it seems that computer simulations are helping cosmologists to answer some of life's most fundamental questions. And finally - as someone who is in the business of simulating the universe - does Nick think that we might be living in the matrix?

Nick  - I think it's definitely possible. I think that with simulations - the sorts of simulations that we do - the computational power has increased hugely over the past two, three decades. And so it's easy to imagine that if that rate continues, one can imagine that eventually it will be possible to simulate a universe that is indistinguishable from our own. And in that case it seems somewhat arrogant to assume that we are not in one of those simulations run by some future race.


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