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.)

New in 2015:

Short story The Marchioness

AE posts:

2017: Mars…

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 118, 14 October 2015 – 46th Apollo Anniversary Year

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Does Intergalactic SETI Make Any Sense?

The 100 least remote galaxies

The Breakthrough Listen project is proposing to extend its search for artificial radio and optical signals to include the 100 nearest galaxies.

But should we expect to find a beacon from a technological civilisation that is bright enough for us to detect over intergalactic distances?

Firstly, what might those distances be? A helpful Wikipedia page shows the 126 nearest galaxies. About 50 galaxies make up the Local Group of which the Milky Way is a member, the rest are part of neighbouring galaxy groups. Most of the galaxies shown are relatively small dwarf galaxies, with a few large spirals like the Milky Way (which is the second largest in the Local Group). A sampling:

Name Characteristics Distance from Earth (light years)
Canis Major Dwarf Irregular dwarf galaxy in the process of merging with the Milky Way; closest identified external galaxy 25,000 LY
Large Magellanic Cloud Dwarf spiral; fourth largest galaxy in the Local Group; closest substantial external galaxy and believed to be a satellite of the Milky Way 160,000 LY
Small Magellanic Cloud Irregular dwarf galaxy in the Local Group; believed to be a satellite of the Milky Way 200,000 LY
Andromeda Galaxy (M31) Giant spiral; largest in the Local Group; best known to the public and easiest to spot in the night sky 2.5 million LY
Triangulum Galaxy (M33) Large spiral; third largest in the Local Group ~2.7 million LY
Maffei 1 and 2 Giant galaxies outside Local Group; hard to observe because they are in the plane of the Milky Way ~10 million LY
NGC 404, “Mirach’s Ghost” Isolated dwarf galaxy, no.100 in the Wikipedia list ~10 million LY

The distances to all these galaxies are hard to imagine. The light which we see today left even the nearest well before our ancestors developed agriculture; we are seeing the most distant with light which set out on its journey across space well before our ancestors split away from the apes.

A for Andromeda

M31, the Andromeda Galaxy

The Andromeda Galaxy, number 31 in Messier’s catalogue, will be a good test case. It is the brightest of those shown within the 100 nearest galaxies, hence is presumably the largest, has a good line of sight from the Solar System, and is only one quarter of the distance of the most distant of the 100 to be observed in the Breakthrough Listen programme. It is closely similar to our own galaxy. If we can’t find extragalactic aliens in M31, we’re not likely to find them anywhere else.

Let’s ask what sort of transmitter power levels are expected of our alien pen-friends. A recent article by John Billingham and James Benford sets out some of the technicalities of radio signalling (reference given below).

In order to estimate the power, we shall need to make some assumptions. We shall take a radio frequency of 5 GHz, the same as used for Alexander Zaitsev’s Cosmic Call 1 message. This is near the middle of the 1–10 GHz “microwave window” favoured by SETI astronomers.

For simplicity we assume a radio transmitter the same as Zaitsev’s RT-70 in Evpatoria, with a dish diameter of 70 metres and hence a gain at that frequency (a narrowing of the beam) of 7.05 × 106, which at a range of 2.5 million light years then produces a beam spot size of diameter 3,800 light years.

Since M31 is fairly close to the plane of the Milky Way (though not close enough to obscure it from our view), the Milky Way will appear from a point of view in M31 to be roughly as close to edge-on as M31 appears to us (see photo). A single conical beam whose diameter was large enough to include the entire galaxy would waste a lot of power over its largely starless polar regions, so it makes more sense to narrow down the beam to address only a fraction of the tilted disk. While it does mean that a number of dishes are then required to cover the whole of the Milky Way, it will soon become clear that an array of dishes will be needed in any case.

Billingham and Benford state that such a transmitter, at its power level of 150 kW, could be detected by the forthcoming Square Kilometre Array (SKA) up to a distance of about 650 light years. The SKA is the largest detector likely to be used by Breakthrough Listen or its immediate successor programmes. But at this range only the bare fact that a signal of some sort is present can be distinguished, because it must be observed for a period of 4 hours (the duration of the longest Cosmic Call 1 transmission) in order to distinguish it from background noise. In other words, only one binary digit of data can be sent every 4 hours, because that amount of time is needed to detect the signal at all.

If bits are sent at a faster rate, they cannot be distinguished. Thus in order to receive a message coded at 100 bits/second, the maximum range falls to only 19 light years (the range is inversely proportional to the fourth root of the data rate).

How fast do aliens talk?

The issue of the speed of transmitting the bits of data is then critical: for a given receiver, picking up a high data rate requires a stronger signal at any given range, a low rate can get away with a weaker one.

The RT-70 radio telescope

Speaking of time, let’s think about the fact that, if the Andromedans send us a message through any sort of electromagnetic waves, they cannot possibly expect to receive any kind of reply for at least 5 million years (unless their message contains instructions for building a tachyon radio, in which case 2.5 million years). Is it reasonable to imagine any kind of intelligent being which embarks on a project whose (uncertain) payoff cannot arrive until a time millions of years in the future?

SETI researchers are at liberty to imagine such an intelligence if they wish. Their sponsor may even think that checking the idea is worth the outlay of a few million dollars. I find it highly implausible. The only intelligent beings of whose existence we have any knowledge think (and allocate funding) on vastly shorter timescales.

A beacon which merely indicated to us that there was once a transmitter, apparently of artificial origin, millions of years ago in another galaxy, would surely not be worth anybody’s while to build. But it might be that the aliens believe that they are under a religious duty to spread the Good News, whatever that may be, to the universe, and are therefore willing to transmit a specific message with no expectation of hearing within any reasonable period of time whether any other beings have intercepted the message, or decoded it. So I think the information content of such a signal will be important.

On the other hand, given the immense length of time before the signal reaches the Milky Way and the aliens’ absence of any practical interest in making it easy to decode, they might not be too fussy about a high data rate. One bit every four hours does seem awfully slow to transmit a message, though. A reasonable compromise might be to send one bit every 5 minutes. This would allow up to about 100 bits to be received per observing session on a rotating planet such as our own. The maximum range for detection by the SKA would then be about 250 light years.

How much power are they willing to transmit?

But M31 is 10,000 times further away, and therefore the power requirement must be 108 times greater (as power in a conical beam dissipates according to the square of the distance). So the 150 kW transmitter needs to become a 15 TW (15 million megawatt) installation, using power at about the same rate as Earth’s entire present-day global industrial civilisation.

Clearly, no single transmitting dish could do this. It would melt. An array of millions of linked dishes would be needed, perhaps covering the surface (together with their power supply systems) of an airless dwarf planet.

But so far we are only covering a spot 3,800 light years in diameter. Around 300 such arrays would be needed to cover the whole of the Milky Way (presenting an ellipse to the aliens’ view of say 100,000 × 40,000 light years), boosting the power requirement to 4,500 TW (4.5 billion megawatts).

The aliens might of course just use the one array, targeting first one spot, then another in turn. So instead of our receiving their signal when we turn our own telescopes to the sky, we have only a one in 300 chance of receiving it at any given time.

Either way, it is important to realise that a civilisation devoting such large resources to a project with little or no payoff will necessarily be using much larger resources for more practical purposes. As James Benford said in a postscript, “Larger-scale civilizations will leave larger footprints.” Its large-scale economic activities in space should be detectable in the form of an excess of waste heat, or spill-over from activities such as terraforming or starship propulsion.

Hunting for fabulous monsters

NGC 55 galaxy

In conclusion, the search for artificial signals deliberately directed to us by extragalactic intelligence is chasing an implausible beast.

The aliens cannot possibly know of our existence. If they have somehow identified another intelligent species in the Milky Way they will certainly use a much narrower beam to communicate, and it is unlikely that the Solar System would fall by chance into that beam. A dish in the Andromeda Galaxy with diameter one kilometre and using the same 5 GHz frequency, for example, would have a footprint at our distance whose diameter was only 264 light years. We would be better off searching for the other species in the Milky Way. But in any case the 2.5-million-year signal travel time each way makes this scenario hardly credible.

If they are broadcasting at the Milky Way for ideological reasons such as suggested above, then they would need to apply power at levels of tens to thousands of terawatts, and to continue doing so for geological periods of time (hundreds of millions to billions of years) if there was to be any realistic chance of our intercepting the signal. They would need to continue this programme (and maintain all the power and transmitter arrays in good repair) despite receiving no tangible return on all this effort.

One might, of course, argue that their own galaxy is teeming with intelligence, and so they are merely assuming that the Milky Way galaxy is like their own in that regard. But then the Milky Way would have to be well populated too. As I have argued before, if this were the case then at least one of those civilisations would have embarked on interstellar spaceflight at some point over the past several billion years, vastly expanding the amount of activity and surely colonising our own Solar System. The fact that they have clearly not done so disproves this hypothesis.

Extragalactic SETI is a fascinating, thought-provoking subject. Infra-red searches for signs of large-scale civil space engineering projects in nearby galaxies are worth doing, but have not turned up anything definite so far. A radio and optical search for hypothetical beacons which are signalling us with messages is not worth the effort, because it is not reasonable to suppose that any such exist.

Here is an example of how SETI should best be done: not by pinning one’s hopes on finding specific things like beacons, but simply by exploring to see what’s out there and, when something new turns up, keeping an open mind as to possible explanations.


John Billingham and James Benford, “Costs and Difficulties of Interstellar ‘Messaging’ and the Need for International Debate on Potential Risks”, JBIS, vol.67 no.1 (January 2014), p.17-23.

James Benford, “Rebuttal to the Advocates of Messaging”, ibid., p.40-41.

I am grateful to James Benford for his comments on this post. Any errors or infelicities that may remain are, however, my own.

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