All Astronautical Evolution posts in 2015:

Britain Takes the Wrong Approach to Manned Spaceflight (Dec.) (Comments)

“Drowning in Process” (Nov.)

Does Intergalactic SETI Make Any Sense? (Oct.)

SETI and Sanity (Oct.)

SpaceX, SpaceY, SpaceZ (Sept.)

A Letter to Britain’s New Space Minister (June)

Mars: 25 Years After Mars Direct – Discussion (May)

The Astronist Mars Strategy (May)

Mars: Still So Distant, 25 Years After Mars Direct (May)

The Mariner Anniversary Calendar for Mars (April)

Mars: An Awful or an Awesome Place? (April)

Should We Phone ET? (March)

More Pluto Controversy (Feb.)

The Pluto Controversy (Jan.)

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AE posts:

2022: What’s to do on Mars?…

2021: New space company Planetopolis…

2020: Cruising in Space…

2019: The Doomsday Fallacy, SpaceX successes…

2018: I, Starship, atheism versus religion, the Copernican principle…

2017: Mars, Supercivilisations, METI…

2016: Stragegic goal for manned spaceflight…

2015: The Pluto Controversy, Mars, SETI…

2014: Skylon, the Great Space Debate, exponential growth, the Fermi “paradox”…

2013: Manned spaceflight, sustainability, the Singularity, Voyager 1, philosophy, ET…

2012: Bulgakov vs. Clarke, starships, the Doomsday Argument…

2011: Manned spaceflight, evolution, worldships, battle for the future…

2010: Views on progress, the Great Sociology Dust-Up…

Chronological index

Subject index

General essays:

Index to essaysincluding:

Talk presented to students at the International Space University, May 2016

Basic concepts of Astronautical Evolution

Options for Growth and Sustainability

Mars on the Interstellar Roadmap (2015)

The Great Sociology Debate (2011)

Building Selenopolis (2008)


Issue 111, 1 March 2015 – 46th Apollo Anniversary Year

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Should We Phone ET?

They’re at it again! The enthusiasts for alien civilisations, frustrated at the lack of evidence of artificial radio signals being beamed our way after 50 years of searching, are again debating whether to send a wake-up call to the extraterrestrials supposedly lurking in our galactic neighbourhood.

Seth Shostak

Last month’s discussion at the AAAS follows a similar debate at the Royal Society in 2010 (see January 2014 issue of JBIS = first 8 entries on this page). In the red corner: those who believe that getting in touch with E.T. will be the greatest event in human history since the discovery of fire. In the blue corner: those who believe that E.T. is only waiting for an e-mail from us to come over here and wipe us out through any one of a long menu of possible dooms: culture shock, name-calling, bombardment from space, virus infection, or maybe outright invasion as in the 1996 movie Independence Day.

But isn’t this an unfair caricature of the sober arguments being presented by serious scientists? Unfortunately not. Because what none of them seems to be saying is that the chance of our receiving a message from the stars is extremely low. This is not mere opinion: it follows logically from the scale of the Milky Way in time and space, and from the fact that the Solar System shows no indications that spacefaring extraterrestrials have ever visited here.

In 2014 well known SETI astronomer Seth Shostak stated his belief that intelligent alien life will be detected within the next quarter century. Surely a top expert in the field cannot be far wrong?

But look at the logic. In order for SETI to have a reasonable probability of success, there must obviously exist other civilisations out there like ours, with astronomers and radio telescopes (lasers have also been suggested as an interstellar communications medium). Not so obviously, those civilisations must be sufficiently long-lived or sufficiently numerous that at least one exists within hailing distance of the Sun at the present moment in time. So for at least the past one billion years (one tenth of the age of the Galaxy) there need to have been at least one thousand civilisations operating SETI programmes around the Galaxy.

One billion years: because the random emergence of totally independent civilisations could hardly be coordinated any more precisely than that. And one thousand civilisations: because even if they are restricted to a narrow galactic habitable zone that takes up only, say, one fifteenth of the volume of the galactic disk, one thousandth of that zone still has a volume of about 520 million cubic light years. A sphere of that volume surrounding the Sun has a radius of 500 light years and contains in the region of 1.5 million stars (about the sample size that Shostak had in mind).

This is a very generous allocation of the volume within which SETI searches may be realistically pursued. So far as the active SETI or METI debate goes (Messaging to hypothetical ExtraTerrestrial Intelligences), it is even somewhat too large, since so far as I know nobody is seriously proposing to send a message to which we will not receive a reply for up to a full millennium.

The implication is therefore that these searches presume that there has already existed at least one trillion (a billion times a thousand) civilisation-years of activity in the Galaxy. Thus for example a thousand civilisations times a longevity of a billion years each, or a million civilisations times a longevity of a million years each, or a billion civilisations times a thousand years each – where “longevity” means the duration of their interest in and capability for astronomical research.

But any scenario we entertain must be consistent with the fact (so far as is known at present) that no traces have been found that any spacefaring alien civilisation has yet visited the Solar System, let alone established a permanent colony here.

It must also be consistent with the fact that, since the report on Project Daedalus was published in 1978, it has been known that interstellar travel to nearest neighbour stars on the timescale of a single human lifetime is possible with realistic near-future technologies. The propulsive power comes from nuclear fusion, a technology which was first demonstrated in 1952.

The latest iteration of the Daedalus nuclear pulse rocket, by Robert M. Freeland, frees the design from its original dependence on the scarce helium-3 isotope, and introduces other practical improvements, by reinventing it as a combined fission/fusion engine. The practicality of a long-lived civilisation spreading among the stars, in a manner analogous to the global spread of humans out of Africa 80,000 years ago or the subsequent spread of globalising industrial civilisation out of Europe from 500 years ago, can therefore no longer be doubted.

Given the emergence of a single interstellar supercivilisation based on nuclear fusion technology, the time it would take for it to spread colonies around the entire Galaxy would, given reasonable assumptions, be around 10 to 20 million years – at most 0.002 of the age of the Galaxy. Even if civilisations have only existed for the past one billion years, as we assumed above, this is still only 2% of that period of time. Either way, on a galactic timescale the change from rare civilisation to ubiquitous civilisation would be essentially instantaneous.

A trillion civilisation-years of activity is infeasibly large for civilisations to exist at the level of interstellar communications via radio or laser, yet for not a single one to go on to master interstellar space travel.

Therefore it must be logically concluded that humanity is very likely to possess the only viable technological civilisation in the Galaxy at the present time.

We are likely also the first to emerge, barring any others which may have collapsed after a period of time too short for sustainable space settlement, and which therefore maintained technological capabilities (and SETI research) for no longer than about 500 years maximum.

The only way around this is to assume that an alien civilisation has emerged which is closely coordinated in time with our own evolution, and which now has bases within about 250 light years of the Sun but has not yet arrived and occupied our system. Given the magnitude of galactic time and space, the probability that this is the case is extremely low, on the order of one in a million, or one in a billion after the fact that they do not know of our existence is taken into account, and therefore have no reason to continuously scan our system for signals.

The modern SETI industry is therefore a gamble on a low-probability circumstance. Furthermore, it is a flawed experiment. While receipt of a signal would be a success, proving that we do have cosmic company, non-receipt of any confirmable signal (the situation so far) neither proves nor disproves that technological aliens are out there. The same with METI: non-receipt of a reply would neither prove nor disprove that our signal had been detected, and would neither prove nor disprove that there had been anyone there with the wherewithal to make that detection, if they had been listening at the right moment.

A well designed scientific experiment yields valuable data regardless of whether those data corroborate or contradict the hypothesis under which that experiment had been designed. But SETI experiments unfortunately do not meet that criterion.

Yes, we should promote research in radio and optical astronomy. Yes, it is good to keep an open mind when confronted by puzzling phenomena, and be open to the possibility that an alien civilisation may after all be active out there. But devoting one’s whole research career to an attempt to find a particular thing which is rather unlikely to exist is, well, (looking on the bright side) quixotic.


Me, I blame Mr Spock. Obviously, Leonard Nimoy was one of the cinematic greats, and the most famous character he played an outstanding cultural icon of a peaceful, progressive, humanitarian way to the future. But never forget that Star Trek is fiction! Intelligent aliens are fictional characters. The fact that they provide entertaining storytelling in movie after movie does not prove that they exist in reality!

I think that some astronomers have become caught up in this cultural myth to such an extent that they have come to believe that Mr Spock’s real-life equivalents must exist somewhere out there in the real Galaxy. This (to coin a phrase) is not logical (see above). But is certainly answers to the all-too-human emotional need, which has driven visions, revelations and religions down the ages, to see somewhere out there super-powerful beings, whether angels who will come to save us, or else devils who will come to judge and destroy us!

It is of course powerfully buttressed by the explosion in discoveries of exoplanets, which began in 1995. Every week there is news about more Earth-analogue planets being discovered: similar size to Earth, orbiting in the Earth-analogue temperate (“habitable”) zone around their star. With so many opportunities for life to flourish and evolve intelligence, how could there not be aliens on our doorstep? Which Professor Richard Dawkins would call the “argument from personal incredulity”.

But we know so little about these worlds! Consider the Earth-analogue planet in our own backyard: Mars. After half a century of close-up study (from Mariner 4 in 1965), we are still unable to say exactly that living organisms do or that they do not exist somewhere on Mars or underneath its surface.

Finding whether any of the exoplanets in our galactic neighbourhood really do have microbial or multicellular life, or even intelligent life that we can talk to, will be a long project, and barring a miracle will not reach any definitive conclusions in our lifetimes.

In the long run there is really no alternative to pursuing technologies for nuclear rocket propulsion, autonomous robotics, human life-support and sustainable economic growth in space, in pursuit of creating a human and post-human supercivilisation among the stars. SETI offers a tempting short cut, but only if another sort of miracle has happened: a highly improbable coincidence in both time and space between the emergence of the first and of the second independently evolved technological civilisations in the Galaxy.

If a miracle comes along, that would be nice, but I personally would not stake my career (if I had one) on waiting for one.

P.S. My paper setting out these arguments in detail was submitted to JBIS last October and is currently under review.

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