## How Do We Find Interesting Things in Very Large Simulations?

It’s a growing problem in computational astrophysics.  Hydrodynamic simulations (say of giant molecular clouds and star forming regions) are getting very large.  When we want to analyse them and find interesting features to compare to the physical Universe, simply searching them “by eye” is becoming an enormous task.

One simple solution to this is to farm out the problem to citizen scientists, essentially doing the “by eye” hundreds of thousands of times in a few days.  This technique is great if you can break up the simulation into easily viewable chunks for each citizen scientist to look at.  But what if you can’t do this, or you don’t have access to millions of enthusiastic people?

We must rely on algorithms to solve this problem.  Luckily cosmologists came across similar issues in N-Body simulations of dark matter.  These simulations have slightly less physics inside, and hence grew to large sizes much quicker, which was essential to modelling the growth of structure on cosmic scales.  They used something called tensor classification to analyse the mass distribution.

This is a technique which relies on computing a rank 2 tensor, a matrix, which contains information about how the simulation changes with position over all 3 dimensions.

For example, we can compute a tidal tensor, which is two derivatives of the gravitational potential.  This measures how the gravitational force changes as a function of position.  Manipulation of the tensor (finding its eigenvalues and eigenvectors) allows us to say what shapes and geometries the gravitational force is trying to build.  Is it making pancake-like sheets? Rope-like filaments? Or is it squeezing everything into a sphere? Or, is it doing none of this, creating a void?

This technique gives cosmologists useful information about the filamentary structure of dark matter on very large scales.  In a recent paper, I investigated how these N-Body methods (where the only force active is gravity) could be ported into hydrodynamic calculations (where pressure forces, radiation and perhaps magnetic fields also play a role).

As we work with smoothed particle hydrodynamics (SPH), which also simulates a fluid using particles, these methods are easy to apply, with the advantage that there are less free parameters in the calculation.

And it has some stunning uses.  Want to find the spiral arms in a self-gravitating disc? Presto:

Want to trace the blast wave of a supernova as it travels through interstellar gas? Sure:

It is also quite good at detecting filaments in molecular clouds, but the results aren’t quite as impressive – yet.  Most recently, we’ve been using the technique on simulations of entire galaxies, and we’re starting to see how material flows in and out of spiral arms, which helps to produce molecular clouds.

We’ve really only just begun using tensor classification for problems like this, and there are some great possibilities for analysing magnetic field structures in a similar way.

I have some very exciting plans for this technique, so stay tuned for further updates!

## Searching for the Ruins of Alien Civilisations

This concept came out of a workshop we held at the UK Centre for Astrobiology in 2014, to attract young astrobiologists across lots of different disciplines to work on new projects. This project asked a very simple question: can dead civilisations be spotted at interstellar distances?

There is a train of thought that says the reason that we don’t see any signs of intelligent life is because civilisations have a short lifespan. If this theory is true, then there are ruins dotted all over the Galaxy. If we could figure out a way to detect these remains, not only could we prove that other civilisations have existed, but we could also say something about the typical civilisation lifespan.  This knowledge is extremely relevant for understanding the future of human civilisation!

We were motivated by recent advances in exoplanet detection methods, in particular spectroscopy techniques which are allowing us to probe exoplanet atmospheres for the first time.  As we get better at doing this, astrobiologists are hopeful that we will see signs of biological activity in exoplanet atmospheres, and perhaps even the first indications of technology (such as pollution).  If we can begin to see chemicals such as CFCs in exoplanet atmospheres, could we see the signs of a civilisation’s end?

The three of us got together and discussed the possible ways that humanity could end it all, and what traces that would leave for alien astronomers to search for.  We came up with several gruesome ends, and thought about how these would show themselves in exoplanet observations.

We started by thinking about nuclear war, a common source of dread for humans for decades.  If we detonated all our nuclear weapons, how would that change the Earth as viewed by alien astronomers? They would be extremely fortunate to witness the actual detonation events, which would bathe the Earth’s atmosphere in gamma rays and other high energy particles, but these would be pretty weak and difficult to spot unless our alien observers were actually in the Solar System.

The fallout would spray the atmosphere with large amounts of dust and radioactive particles, which would change the planet’s spectrum significantly at infrared wavelengths.  The ionisation of the upper atmosphere would also be quite obvious to astronomers with a particularly good ultraviolet telescope, i.e. one much better than we can currently build.

If a genetic experiment goes out of control and kills all life on Earth, then that might also be visible.  Decaying organisms emit very specific chemical compounds that can only be produced by biological sources, and a global extinction event would release huge amounts into the atmosphere.  However, our alien observers would have to be quick, as this would rapidly disappear from the Earth’s atmosphere over the course of a year or so.

Nanotechnology could be just as devastating.  A self-replicating nanobot could rapidly turn the world’s carbon (i.e. all living organisms) into sand in a couple of months.  That sand would enter the atmosphere as aerosol particles, and the world would be covered in deserts made of perfect sand grains.  There might be some very odd signatures in such a planet’s atmosphere, especially as the planet moves into secondary transit (where the host star comes between us and the planet).

Pollution of course is another possibility.  This could be pollution of the host star, where a civilisation uses their Sun as a hazardous waste trash can, which would show up in spectra, or pollution of the planet‘s atmosphere, or even pollution of the planet’s orbital environment, which humans have become quite good at doing.

Finally, for a bit of fun, we thought about what would happen if a planet was completely destroyed (such as the fate of Alderaan in Star Wars, when it became a target of the terrifying Death Star).  This might sound a bit frivolous, but when you hear SETI scientists talk about Dyson spheres and megastructures around stars, what they neglect to mention is that the raw material required would probably mean the destruction of several terrestrial planets!

So what did we conclude about detecting dead civilisations? For almost every scenario we came up with, we found that current technology was still unable to observe the signatures we were looking for.  It may not even be possible with the next generation of space telescopes and ground based surveys, unless the destruction is vast, and happens to occur within a few years of us looking at the system.

However, it certainly seems true that within the coming century, the trajectory of exoplanet detection science is good enough that we will be able to start looking for the remains of alien technology.  When that happens, we’ll know in much better detail what the fate of our own civilisation could be.

## If aliens exist, they’ll find political union as tough as we do

I’ve thought a great deal about the so called Zoo solution to Fermi’s Paradox over the last five years. This is the idea that intelligent aliens have decided not to reveal themselves to us, and this is why we see no signs of intelligent life.

It’s quite an unscientific solution, as we cannot gather evidence in favour of it.  If we could, then we would have proof of intelligent life, and that would be the end of it!

The question now is: can we prove (or disprove) this solution while still having no evidence for aliens? The best we can do at this stage is challenge the assumptions that we must make to support it.

This is what I did in my latest paper.  The critical assumption about solutions that forbid contact with us (the Zoo Solution, the Prime Directive, the Interdict Hypothesis) is that this requires some form of law or moral tradition. Laws require agreement between multiple parties – in this case, most likely multiple species of intelligent life, from a variety of different backgrounds with very different belief systems.  To use Carl Sagan’s line, these solutions need a “Galactic Club” – underwritten by a host of treaties – to work.

So how can we test if a “uniformity of motive” can be established? I ran some computer simulations that were in effect an experiment in uniformity. In each experiment, I assumed that there N civilisations would appear in the Galaxy over its lifetime.   I placed them at given locations at a given time. I then tested whether a signal from civilisation a) would reach civilisation b) before b) became advanced enough to receive it. In other words, I wanted to measure how much civilisation a) can influence civilisation b).

This is a strict condition for universal interstellar law to emerge. It’s true that we can relax this condition by allowing civilisations to evolve after they make contact, but I wanted to see whether a universal legal system is a natural consequence in a populated Galaxy.

And, surprise surprise, it isn’t. In previous work, I was able to show that Galactic Clubs were very unlikely. In my latest research, I was able to quantify what the Club would be replaced with. More often than not, my simulations showed a large number of small civilisation groupings, which I call “Galactic cliques”.  It’s quite likely that these cliques do not share much in common culturally (at least initially).

So we can conclude that Galactic Clubs are far from guaranteed. In fact, to obtain a single Club the distribution of civilisations in space and time must be quite odd (you can read the paper to find out more), or one clique must dominate the others well after the fact (via political or military means).

This work hardly destroys these solutions to Fermi’s Paradox. What it does show us is the weaknesses in the assumptions we make to propose these solutions.  The Galaxy is big enough that it would be pretty tough for every civilisation to come to an agreement about how to conduct themselves.

This is a consequence of the laws of physics, and it holds true whether there are ten or ten thousand civilisations out there.

It’s certainly true even if there’s only one, as our own civilisation can attest to.