If the Universe is Teeming with Aliens ... Where is Everybody?

Book cover - Where is Everybody?

My book If the Universe is Teeming with Aliens…Where is Everybody? is about the Fermi paradox, so there are posts here on astrobiology, the search for extraterrestrial intelligence and so on. The subtitle to the second edition is “Seventy-five solutions to the Fermi paradox and the problem of extraterrestrial life”. I’m always on the lookout for new solutions to the paradox, so if you have a good one that isn’t already covered here – let me know! I might include it in a future edition. Available from Springer Science.

FAST completed

Time rolls by. When I wrote New Eyes on the Universe I pictured FAST – the Five-hundred-meter Aperture Spherical radio Telescope – as an instrument for the mid-term future. Construction of the telescope began in March 2011 but I thought its remote location in a karst sinkhole in Guizhou Province, combined with the technical difficulty of operating such a large device, would cause innumerable delays. In September 2016, however, the telescope saw first light. Chinese scientists are now calibrating the telescope, and by 2019 it should be doing astronomy.

FAST is a staggering telescope. It consists of 4450 triangular reflecting panels that combine to form a collection area more than twice as large as the famous 300m Arecibo telescope in Puerto Rico. If it works to its design specifications, FAST will be twice as sensitive as Arecibo, observe three times more sky than Arecibo, and be able to survey the sky 5–10 times faster than Arecibo.

Such a vast instrument has the potential to make a number of discoveries in pure astronomy, but a further intriguing possibility lies in its use for SETI: scientists interested in the search for extraterrestrial intelligence will be able to “piggyback” on pure science projects and comb the data for signals.


FAST lies in a natural basin in the Guizhou Province of China. The telescope is undoubtedly an impressive astronomical instrument, but its name is a slight misnomer: although the reflector diameter is 500m only a 300m diameter circle is used at any one time. (Credit: Asianewsphoto)

A SETI simulation

In a former life, back when computers were powered by steam and the Web was only a twinkle in Tim Berners-Lee’s eye, I helped develop simulations for teaching and learning physics. Years have gone by since I last coded a physics simulation, but I still find enjoy using them and I’m convinced that the best ones can really improve a student’s understanding of physical theory. I’m not alone in that belief. For example the Nobel laureate Carl Wieman, who has long argued that metrics for assessing the quality of an academic’s teaching should sit alongside metrics for assessing the quality of an academic’s research, founded PhET Interactive Simulations to improve the way physics is taught and learned. (PhET stands for “Physics Education Technology”, but since Wieman founded it in 2002 the project has expanded to include biology, chemistry and earth sciences.) The simulations that PhET release allow students to see in real time the effect of changing the value of a physical variable; these software-based experiments allow students to play safely, quickly and cheaply with systems, and can help inculcate a deeper understanding of the underlying physics. The simulations don’t have to be particularly complicated to be of value. A basic simulation of a wave on a string, for example, will allow students to change the values for frequency, wavelength and amplitude – and the students will immediately be able to visualise and understand what those terms mean. Simulations don’t necessarily replace textual descriptions – there has to be a pedagogic scaffold surrounding the simulations – but they certainly bring texts to life.

In writing Where is Everybody? I toyed with the idea of developing simulations to illustrate some of the points I wanted to make. When discussing the challenges faced by SETI scientists I couldn’t help but think that readers would get the point more readily if they were able to visualise the search space. What is the effect on likely detection if we double the range over which we believe radio signals from an extraterrestrial civilisation can be detected? What happens if we treble the range, or half it? What is the effect on likely detection if we increase the lifespan over which a civilisation is likely to transmit, or decrease the time over which a civilisation is motivated to listen? How important to the SETI endeavour is the rate at which radio civilisations come into existence? A reader could of course answer these questions by working through the mathematics. But not all readers have a mathematical background. A simulation would handle the mathematics for the reader, and allow everyone – even the math-phobic – to explore these questions.

I no longer have the time to devote to coding simulations. However I recently discovered that Roger Guay has already produced a gorgeous-looking simulation, called Alien Civilization Detection, that allows us all to play with various factors associated with SETI.

ACD introductory screen

The introductory screen from Roger Guay’s app Alien Civilization Detection (Credit: Roger Guay)

Roger’s simulation features a concise introduction that first outlines the problem of detection and then explains why his simulation is of only a “small” (10000 x 8000 light year) region of the Galaxy. Interactive tabs at the bottom of the simulation present useful information on the Drake equation, a slider that illustrates timescales, and example analyses. Screen tools at the side of the simulation allow you to change its appearance (and toggle some “fun facts”; did you know, for example, that firelight from the last of the Stone Age people is now just a quarter of the way to the centre of the Galaxy?).

Instructions for ACD

Some of the features in the Alien Civilization Detection simulation. (Credit: Roger Guay)

The simulation itself presents the viewer with a number of dials. We can set the typical maximum distance for radio detection, from 600 light years to 2000 light years. We can set the timespan over which a civilization will broadcast, from 160 years up to 2000 years; similarly, we can set the timespan over which a civilisation will be actively listening. The view can be either “Earth-centric” or “Galaxy-centric”. Press the green “Start” button, or the space bar, and watch as this small part of the Galaxy fills with radio signals. We see civilizations appear; we see civilisations that are currently both listening and transmitting; we see civilisations that are listening but no longer transmitting; and we see the ever-weakening signals from civilisations that are no longer listening, no longer transmitting. A counter keeps track of the elapsed time along with the number of potential detections that could have been made during that time.

ACD-typical run showing detection potential

Screenshot of a typical run of the Alien Civilization Detection app. Note the detection potential! (Credit: Roger Guay)

It’s rather hypnotic to watch civilisations pop into existence, transmit radio waves into the cosmos, and then cease transmitting or listening (perhaps they die?) before those waves cross the path of another civilisation. Occasionally, though, there is the chance for detection. Those rare events are what SETI scientists hope to see.

I viewed the simulation on a Mac, but I believe that a Windows version is in the pipeline. If you would like to contact Roger to discuss the simulation, his email address is irog@icloud.com. And if you would like to download the app, you can do so via this Dropbox link.


Alien megastructures

For the past five decades the search for extraterrestrial intelligence has been dominated by the search for radio signals. There are good reasons why this search strategy makes sense, but the available search space is so vast (our dishes have to be pointing in the right direction at the right time, and tuned to the right frequency) that the phrase “radio SETI” is an excellent synonym for the phrase “looking for a needle in a haystack”. Are there any other options for the search?

Personally, I believe that we need to adopt a Stapledonian approach to the problem.

Olaf Stapledon, a British philosopher and science fiction author, considered what might happen to intelligence in the distant future. For example in one of his novels, Star Maker, published in 1937, he described what we now call Dyson spheres: structures that orbit a star and enable a civilisation to utilise most of the energy output of its parent star. The creation of a Dyson sphere is far beyond our present technical capabilities, but who knows what we (or our mind-children) will be able to achieve a thousand years from now, or ten thousand years from now, or a hundred thousand years from now. And 100,000 years is an eye blink in cosmic terms; if there are extraterrestrial intelligences out there then they might be millions of years in advance of us. Thus in a Stapledonian approach to SETI we would look for examples of astroengineering, or megastructures that could only have been developed by technologically advanced intelligences. The detection of such a megastructure wouldn’t open up the possibility of communication, as a traditional radio SETI detection might do, but it would at least tell us that we were not alone. That in itself would be a terrifically important discovery.

The recent furore surrounding KIC 8462852 is an example of how a Stapledonian approach is starting to appear in the ongoing search for extraterrestrial intelligence. KIC 8462852 is an F-type main-sequence star about 1480 light years away from Earth in the constellation Cygnus. The Kepler space telescope recorded fluctuations in the light from the star – but a recent paper demonstrated that the fluctuations were so bizarre that they could not come from a transiting exoplanet. For one thing, the dimming of the starlight is not periodic; for another, the dimming is extreme (15% in one episode, 22% in another; for comparison, a Jupiter-sized planet blocks about 1% of its star’s light). So what is causing this weird behaviour? We don’t know. The authors of the paper suggest that the dimming might be caused by a series of comets, surrounded by clouds of gas, perturbed from their usual orbits by the gravitational influence of a nearby star; a small red dwarf close to KIC 8462852 might be the culprit. It’s possible. But it’s far from certain that this story can explain all the features that are seen. Any other explanations? Well … could it be that we have caught an advanced alien civilisation in the act of building a Dyson sphere? I doubt it. I REALLY doubt it. Just because we observe something we can’t immediately explain we shouldn’t immediately attribute it to alien intelligence (remember that, for a short while, the radio signals from pulsars were thought to be evidence for ETI; astronomers soon figured out the true explanation for the radio emissions). Nevertheless, it surely can’t harm to follow up these observations of KIC 8462852 with traditional radio-based SETI observations.

Several astronomers have already searched for the infrared emission that would accompany a Dyson sphere. But the search for Dyson spheres would form only a small part of a Stapledonian approach to SETI. We need to use our imagination and try to envisage the sorts of technology that a truly advanced civilisation might develop. In a previous post I looked at how John Smart’s transcension hypothesis argues that black holes are an attractor for intelligence. The philosopher Clément Vidal adopts a related approach. (Incidentally, in my book I wrote that is Belgian. He works in Belgium, but is in fact French. Apologies, Clément!)

Clément uses a two-dimensional metric, first proposed by John Barrow, to describe advanced civilisations. The Kardashev metric is well known: K1 civilisations control the energy output of their home planet, K2 civilisations control the energy output of their home star, K3 civilisations control the energy output of their home galaxy. But Barrow pointed out that there is a scale of inward manipulation that might be just as applicable to extraterrestrial civilisations: a B1 civilisation can manipulate the universe at the 1m level; a B2 civilisation can work at the 10–7m scale; a B3 civilisation manipulates at the nanoscale; and a BΩ civilisation can manipulate spacetime at the Planck level. In a 2011 paper Clément talks about the possibility of K2-BΩ civilisations; he has since switched to a more memorable appellation “stellivore”.

If we accept that stellivores could exist, an obvious question is: what might stellivores be doing that our telescopes and instruments might pick up?

Well, a stellivore might possess a technology involving black holes. (I won’t go here into the many reasons they might want to use black holes. Suffice it to say that a Stapledonian mindset would consider black holes to be a natural endpoint for many technologies.) And we know that in principle it is possible to detect activity around black holes; we know this because astronomers have already investigated X-ray binaries (XRBs). In an XRB a donor object (typically a star) loses material to a compact accretor (typically a black hole). The infalling matter releases huge amounts of gravitational potential energy. So could XRBs provide evidence for stellivores? In my book I write that we could “look for evidence for the regulation of energy flow within XRBs”. As Clément points out, there’s already evidence for such regulation; the key question – just as it is with KIC 8462852 – is whether the observations are best interpreted in terms of astrophysics or astrobiology?

Since Clément’s 2011 paper he has extended his vision to include a wider range of XRBs: the stellivore family could include cataclysmic variable X-ray pulsars, for example, with black holes being the end stage.

To my mind, the great thing about this Stapledonian sort of approach is that it broadens the range of techniques we can apply when searching for signs of extraterrestrial intelligence. Traditional radio-based SETI has its place. But the ideas of John Smart and Clément Vidal tell us that we could also profitably search at the highest energies.

The transcension hypothesis

Of the new “solutions” to the Fermi paradox that I discuss in the new edition of Where Is Everybody?, John Smart’s transcension hypothesis is one of the most intriguing.

There are several aspects to John’s hypothesis, so it was quite a challenge to condense the argument into just three pages of the book (and even more of a challenge to use only simple words to describe some rather abstruse concepts; I lack Roberto Trotta‘s ability to illustrate rarefied ideas with monosyllables).

One aspect is relatively easy to describe and understand: John argues that advanced civilisations will collapse. Note that this is not collapse in the sense of Gibbon’s Decline and Fall! Rather, civilisations will develop in an inward direction rather than outward into space. Qualities that might be used to characterise civilisations – their use of space, time, energy, matter – will all exhibit an increase in density. In John’s words, civilisations will undergo STEM compression.

You can argue that STEM compression is happening to our own civilisation. Let’s consider just one of those four STEM dimensions. In the past, the majority of humans lived in small settlements and the density of information was low. Many people now live in cities, and anyone visiting London or Berlin or New York will immediately appreciate the greater information density to be found in these places compared to that in hamlets or villages. Increasingly, people in developed nations work for knowledge-based companies that possess levels of information density exceeding those of cities. The suggestion is that the density of all four quantities will increase as civilisations become more advanced. And the logical end of this STEM compression? Well, the physical limit is set by the Planck scale. A black hole is thus the natural end of an advanced civilisation. It will be the natural end of our own civilisation.

One commendable aspect of the transcension hypothesis, at least to my mind, is that it offers specific and potentially falsifiable predictions – effects that astrophysicists could look for. (Read the book, or even better read John’s original paper, to learn more about those predictions.) However, is it reasonable to suppose that civilisations inevitably take a developmental path that leads to transcension? Well, John provides support for the idea by arguing from “evo-devo”, or evolutionary developmental biology. Evolution is a random process; development, though, is directed and constrained – a cat embryo gives rise to a cat, a dog embryo gives rise to a dog. Both evolution and development play important roles in life. What if the universe is engaged in a life cycle? (It was born in a Big Bang, it has grown as it aged, and there are processes relating to black holes that might allow it to spawn new universes.) If we live in an evo-devo universe then perhaps transcension is inevitable – just as an embryo gives rise to grown animal.

There are too many “if’s” in the argument for me to buy thetranscension hypothesis. However, in criticising the hypothesis I wrote that it requires that “all individual elements in all civilizations in all neighbouring galaxies develop in the same way”. This was one of those occasions where trying to compress information into a few lines (itself a symptom of STEM compression?) distorts the meaning. John sent a rebuttal to this, and rather than paraphrase it I’ll simply present it here:              

Developmental processes are a very small subset of living processes. They certainly don’t comprise anything like “all individual elements” in an organism, and they ensure that evolutionary diversity grows over time within that organism, both within and across life cycles. If we live in an analogously evo devo universe, only a small subset of average observable changes in any civilization would be developmental. And those changes would be there to ensure that evolutionary diversity grows among civilizations over time, which is perhaps the main reason why we might have multiple civilizations and intelligent planets in our universe, if each is an incomplete and finite computational system.

I believe it will be worthwhile to examine the transcension hypothesis in more detail. Unlike so many “solutions” to the Fermi paradox, this one offers avenues for further research.

A flag for the presence of intelligent life-forms

In the second edition of Where is Everybody? I discuss what Gerard Foschini calls the canonical artefact (TCA) — a flag for the presence of an advanced intelligent life-form.

I don’t propose to discuss the details of TCA in this post — you can read the book if you’re interested — but I do want to provide an update. I was fairly sure that no-one had constructed an example of TCA, but yesterday Foschini emailed me a photo of it: he built it out of coin stacks with black rubber test tube stoppers as separators. Below, shown with gratitude, is Foschini’s TCA.

The canonical artefact, constructed by Jerry Foschini from coins and rubber stoppers. (Credit: Gerald Foschini)

The canonical artefact, constructed by Jerry Foschini from coins and rubber stoppers. (Credit: Gerard Foschini)

The glow from alien technology

The search for extraterrestrial intelligence (SETI) has traditionally focused on looking for messages, encoded in radio waves or laser pulses, that aliens might have sent our way. There is, however, another approach to SETI: we could look for evidence of activity that would allow us to deduce the presence of intelligence. The detection of an alien message aimed directly at Earth’s inhabitants would undoubtedly be more exciting than the discovery of, say, exhaust from a distant antimatter rocket – but the key conclusion would be the same: we would know that we were not alone.

In 1960, Freeman Dyson pointed out that the development of technologically advanced civilisations would be limited by energy considerations: growing civilisations require access to increasing amounts of energy. Dyson suggested that a sufficiently advanced civilisation would eventually need to harvest the entire energy output of its home star – which it could achieve by dismantling a planet and using the resulting material to create a host of solar collectors. Such a star-enshrouding swarm came to be known as a Dyson sphere. A galaxy-spanning civilisation – a so-called Kardashev type-3 civilisation – would do this for all the stars in its galaxy.

Dyson sphere

A Dyson sphere around the Sun. The sphere would not need to be a solid object; a “Dyson swarm” is perhaps more realistic. (Public domain)

It would be difficult to hide such astroengineering activity. As Dyson himself pointed out, the laws of thermodynamics mean that a Dyson sphere could be detected by the glow of its radiated waste heat. Visible starlight would be replaced by radiation that peaked in the mid-infrared. If the creation of Dyson spheres is indeed a common activity then the signature of a type-3 civilisation would be a galaxy dim in the visible part of the spectrum but bright in the mid-infrared part of the spectrum. In other words, we can do SETI by looking for the absence of light combined with the presence of waste heat.

A Dyson swarm

A Dyson swarm. (Public domain)

Jason Wright, an astronomer at Pennsylvania State Universe, and his colleagues have recently taken precisely this approach to SETI. They wrote software to assess the 100 million objects contained within the catalogue generated by NASA’s WISE (Wide-field Infrared Survey Explorer) mission. Catalogue objects that were merely mission artefacts, or were clearly not galaxies, were rejected by hand. The team then began looking for optically dim and infrared bright objects. They ended up with a target group of about 100,000 galaxies emitting large amounts of radiation in the mid-infrared. None of those 100,000 galaxies contained a civilisation that was reprocessing more than 85% of its starlight into the mid-infrared: there are no galaxy-spanning type-3 civilisations in our cosmic neighbourhood. Only 50 of those 100,000 galaxies possessed a spectrum that was consistent with the reprocessing of 50% of its starlight into the mid-infrared; but of those few galaxies the infrared brightness can almost certainly be explained by natural processes. So although Wright and his colleagues found a few objects that might be of interest to SETI scientists (and are certainly of interest to traditional astronomers), in essence they registered a null result.

Now, you could argue that we have no idea of the choices that technologically advanced civilisations might make. Perhaps they won’t build Dyson spheres. Well, that’s surely the case. Perhaps they use rotating black holes or antimatter annihilation or even something we don’t yet understand for power generation. Nevertheless, any astroengineering that takes place on a galaxy-wide scale is going to generate waste heat. Wright and his team were looking for signs of that waste heat and they found nothing.

Traditional SETI has so far failed to find a civilisation in our Galaxy at the type-1 or type-2 stage. This recent result from Wright and his colleagues suggests that type-3 civilisations anywhere are rare or non-existent. What are we to make of these null results? Well, perhaps our understanding of the development of technological civilisation is flawed; perhaps aliens are “green” and their long-term survival requires a more sustainable approach to energy use. Perhaps. But there’s another, more obvious explanation: we are alone in the universe.

Lucky reader

Early on in his book Lucky Planet, David Waltham writes about the planet-detecting capabilities of the Kepler space mission. When discussing the chances of Kepler finding a truly Earth-like planet – a body that’s the same size as our Earth and orbiting in the habitable zone of its stellar system – he states that the “announcement of such a world could well come between me finishing this book and its publication (an occupational hazard of discussing topical subjects)”. He was spot on. At about the same time Lucky Planet hit the bookshops, NASA announced the discovery of Kepler-186f, a planet whose radius is about 10% greater than Earth’s and that sits towards the outer edge of the habitable zone of its dwarf star.

Let’s face it. It’s likely that there are billions, perhaps trillions, of Earth-like stars in the cosmos. Can we plausibly argue that our Earth is somehow special, that it alone of all similar worlds possesses complex (which is to say multicellular) life?

Well, yes we can. With the limited data available to scientists we can’t say with any degree of certainty that Earth is the only planet that hosts complex life. But we can certainly make the argument that Earth is exceptional – and Waltham does so beautifully.

Lucky Planet gives the most accessible treatment I’ve yet read of the anthropic selection effect. Our planet has enjoyed four billion years of clement weather, and this climate stability has surely been key to life’s continued existence. Earth’s climate could easily have followed a trajectory towards ice or fire, turning the planet into a snowball or a boiling hell; the climate could have done rapid flip-flops between periods of frigidity and periods of heat. Instead, the average surface temperature of our planet has seen a gentle cooling trend on top of which are relatively minor fluctuations, measured in tens of degrees. With this climate life has been able to thrive.

We can explain life’s continuing existence by saying that life is robust, resilient, capable of withstanding any of the shocks that Nature throws its way. Or we can go even further and argue that various feedback mechanisms allow life itself to create, maintain and develop the conditions needed for life to survive and thrive. This is the Gaia hypothesis. Combine this approach with the vast number of Earth-like planets that exist and one surely must conclude that the Galaxy is teeming with life.

But we can just as well explain life’s continuing existence by attributing it to luck. As Waltham points out, the anthropic selection effect means that intelligent observers must find themselves on planets on which the past climate was such that life could evolve. They can hardly find themselves on a planet on which the past climate was such that their ancestors died. A planet capable of hosting complex life might be a one-in-a-trillion fluke; but if there are trillions of planets then that fluke is going to happen. Goldilocks not Gaia; luck not life.

In Lucky Planet Waltham discusses a number of ways in which Earth might be special, but the  most interesting discussion (to me, at least) was the influence of the Moon. Like most people with an astronomy background I believed that the Moon played a large role in stabilising the Earth’s spin axis. Waltham shows that the true story is more complicated than that. It turns out that … but, no, you should get the book and find out for yourself.

The only criticism I have of the book is that, at two pages, the Further Reading section is rather sparse. The book is aimed at a popular audience, so I can understand the reasons for having only a short section bibliography, but I would have appreciated more detailed references. (The author does link to his website, however, and you can find more technical references there.) That minor quibble aside – thoroughly recommended!

Book review: Five Billion Years of Solitude

When you look up at the stars on a clear night sky it’s difficult not to wonder whether there are conscious, sentient, intelligent beings somewhere out there. And if you accept for a moment their existence, a host of questions follow. What do they look like? Do they create art or make music? Do they believe in god? What is their understanding of space and time? Then another question intrudes, or at least it does whenever I find myself pondering these things: what if they aren’t there? What if humanity is the only species in the universe capable of wondering whether it is alone?

In Five Billion Years of Solitude: the Search for Life Among the Stars, Lee Billings records his discussions with a number of luminaries in the nascent field of astrobiology. Along the way Billings provides crystal-clear descriptions of the science behind the quest for exoplanets, the difficulties of SETI research, the technology with which we might find biosignatures on distant worlds, and much else besides. He gives a masterclass in how to make complex ideas accessible.

But the book is much more than that. The scientists he interviews are driven by competition, ego, rivalry – as are many achievers in various activities; science is not unique in this regard – but they are also united by an overwhelming desire to know whether we are alone in the universe. This makes their accounts both exciting and bitter-sweet. The final chapter in particular, which tells the story of how Sara Seager, a professor of planetary science and physics at MIT, came to find herself searching for the first truly Earthlike planet, is achingly beautiful.

Seager, Marcy or Kasting, or one of the many other scientists interviewed by Billings, might one day find life out there. But it’s entirely possible that they’ll fail (in my view it’s almost certain that they will fail). Earth, this precious blue marble, might be home to the only intelligent life in the universe. Reading Five Billion Years of Solitude will give you an appreciation of the true poignancy of that thought.



Asimov’s humans-only galaxy

I attended the European Planetary Sciences Congress 2013 in London a couple of weeks ago, and gave a talk to a session on the societal implications of astrobiology.

It was an extremely interesting session. Although everyone there was interested in the possibility of extraterrestrial life, and indeed many participants are actively searching for signs of life beyond Earth, I felt there was a recognition that it’s entirely possible that complex, multicellular life might be rare in the universe. (One of the talkers, David Waltham, has a book out next year entitled Lucky Planet. He argues that the four billion years of good weather enjoyed by planet Earth was an unlikely, although necessary, precondition for the emergence of intelligent life. He’s as gloomy about the prospect of SETI success as I am – although both of us would dearly like to be proven wrong! Be sure to read the book when it appears.)

I was reminded of the session yesterday when I read a yet another criticism of the “humans only” universe portrayed by Isaac Asimov in his Foundation series. Such criticism was often levelled at the Good Doctor: his fictional universe contains only humans and robots, the argument goes, because he lacked the imagination to create convincing aliens. The criticism is unfair. Asimov limited his fictional universe because his editor at the time, John W. Campbell, insisted on a human chauvinism that Asimov simply did not share; better not to write about alien intelligence at all than be forced to write about human superiority in order to guarantee a sale. Besides, Asimov’s novel The Gods Themselves disproves the allegation: Odeen, Dua and Tritt are among the most convincing aliens in all of science fiction.

Even though Asimov the science writer wrote about the likely prevalence of alien intelligences, the humans-only Galaxy of his science fiction writing might turn out to be a better description of reality. That was certainly the tenor of my discussions during EPSC13.


Can the simulated discover the simulator?

One of the more outlandish proposals I discussed in Where is Everybody? was the “planetarium hypothesis” – an idea put forward by Stephen Baxter, one of the world’s foremost science fiction writers. Baxter argued that one popular idea in science fiction – that we live inside an artificial reality constructed by beings far in advance of ourselves – could actually be tested by experiment: even the most advanced civilisation would have constraints placed upon it by the laws of physics, and these constraints could be tested by experiment. If it turned out that we were living inside an artificial reality, that we were mere simulations in a computer program constructed by some mighty alien intelligence … well, that would be the Fermi paradox solved!

I didn’t take the planetarium hypothesis seriously, of course, but some people do. Nick Bostrom, for example, has taken the idea further. Bostrom is a professor of philosophy at Oxford University, and he has done some serious analysis of the notion that we are living in a computer simulation. Bostrom’s work is real philosophy, and mind-bending stuff. I urge you to visit his website, and think about his ideas: they are fascinating.

Three physicists who have clearly been influenced by Bostrom’s work are Silas Beane, Zohreh Davoudi and Martin Savage and last week they published a paper on arXiv (entitled “Constraints on the universe as a numerical simulation“) that examines some aspects of this seemingly bizarre notion. Beane, Davoudi and Savage are lattice gauge theorists, which means that they create a simulated toy “universe” in order to study quantum chromodynamics, the fundamental force that governs the interactions of quarks and gluons. They’ve extrapolated the  current trends in computational requirements for lattice QCD, and examined the notion that our own universe is a numerical simulation on a spacetime lattice. They then ask the question: would there be any observable consequences if we were living in such a simulation?

Read their paper for the details. But their final sentence sums it up: “assuming that the universe is finite and therefore the resources of potential simulators are finite, then a volume containing a simulation will be finite and a lattice spacing must be non-zero, and therefore in principle there always remains the possibility for the simulated to discover the simulators.”

Stephen Baxter showed how a K3 civilisation could create a perfect simulation with a radius of about 100AU. Well, Voyager 1 has already passed beyond that distance (as I write, it’s at a distance of 122.64AU) and it didn’t bump into a metal wall painted black. So we know – by experiment! – that we don’t live in a K3 civilisation’s perfect simulation (though of course the Voyager result hasn’t ruled out the possibility that we live in an imperfect simulation; the boundary of an imperfect simulation can be much further away). The work by Beane, Davoudi and Savage provides us with other tools for testing whether we inhabit a simulation.

And just for the record: no, I don’t think we live in a simulation!