9 minute read

In a paper in 1980 [1], mathematical physicist Frank Tipler presented his argument that if extraterrestrial intelligent beings exist they would already be in our solar system since they would inevitably develop technology that would explore the galaxy in less than 300 million years. As they are not here it follows that they don’t exist.

This argument had been expressed before. Five years earlier, Michael Hart published his reasons for rejecting the many explanations for why extraterrestrials are not already on Earth [2], in a paper that was the first to formalise what is now famously called the Fermi Paradox (if there are so many extraterrestrial civilisations then where are they all?) Tipler felt that the force of the argument against the existence of extraterrestrial intelligence (ETI) had not been appreciated and wanted to give it more weight in his paper.

Here are my notes on Tipler’s 1980 paper. You can find my notes on other classic SETI papers in my roundup of the technosignatures literature.

The evolution of intelligence is rare

According to Tipler, in 1980, most leading experts in evolutionary biology thought that the evolution of intelligence in our galaxy was unique to Earth since there are many more alternate “pathways” evolution could take that do not lead to intelligence. He claimed that it was primarily astronomers that thought otherwise.

Tipler agreed with the biologists and he put the probability of another technological civilisation emerging after 5 bilion years of evolution at less than \(10^{-10}\) (i.e. 1 planet in 10 billion: he estimated that there are 10 billion planets in our galaxy that host life and only one of these (Earth) has intelligent life).

Any communicative species would also have developed rockets and computers

If we assume another civilisation is capable of interstellar communication via radio waves, then we must also assume they are capable of interstellar space travel and are likely to use sophisticated computers. Tipler went one step further and proposed that they would be capable of exploring and/or colonising the galaxy within 300 million years using machines, with an initial cost less than operating a microwave beacon for several hundred years:

They will also develop a self-replicating universal constructor with intelligence comparable to the human level

Maximum information for the lowest cost

Minimising cost involves using:

  1. “Off-the-shelf” technology to reduce research and development costs
  2. Resources that would not otherwise be used (“useless” materials in other (uninhabited) stellar systems)

Space exporation with von Neumann machines and extrasolar resources

Tipler proposed sending a probe to another stellar system with:

  • An engine for slowing down
  • An engine for travelling within the target system
  • A self-reproducing universal constructor with human-level intelligence (a von Neumann machine) as a payload.

This probe would initiate galaxy exploration or colonisation in the following stages:

  1. The von Neumann machine seeks out construction material (meteors, asteroids, comets etc.) it can use to make several copies of itself and the orignal probe and engines
  2. Copies of the initial probe are launched at nearby stars
  3. The von Neumann machine starts exploring (and, using available resources, conducting scientific research in) the stellar system it is in and relay information back to its system of origin
  4. When new probes reach their target system the process is repeated until all stars in the galaxy have been covered

Once in another system, a von Neumann machine can use resources there to conduct projects that would otherwise be too expensive. Some projects Tipler suggested include:

  • Building an O’Neill colony (a proposed construction for settlement in space comprising two rotating cylinders) in systems with no planets to colonise
  • Creating new inhabitants for the system by synthesising fertilised eggs and artificial wombs, together with robots to raise the children to adulthood (avoiding the problem of damage to living cells during interstellar travel)
  • Receiving updates, via microwave, with instructions for building the latest devices developed on the system of origin (particularly if the journey time is very long)
  • Constructing second generation probes with faster travel times
  • Constructing an artifact in the target system that would be detected by any technological civilisation present in order to initiate communication

Getting a von Neumann machine to another stellar system

Tipler argued that the main problem is getting a von Neumann machine to another stellar system. This was already possible with rocket technology of the time, with journey times of \(10^4\) - \(10^5\) years. Faster travel times require more cost, almost all of which is rocket fuel.

Tipler proposed sending an initial low speed probe with a von Neumann machine that could use resources at the target system to construct and fuel faster second generation probes to send to the next targets. This requires the species at the system of origin to wait a long time before it gets any information back but the cost would be minimised (since von Neumann machines would be developed anyway for other purposes). Tipler estimated the exploration of the galaxy in this way would cost about 3 billion dollars in 1970s money (one tenth the cost of the Apollo program).

Time to explore the galaxy

Tipler estimated a minimum time of 4 million years, given a high speed probe and 100 years for von Neumann machines to self-replicate. For rocket technology of the time he estimated a more conservative 300 million years.

Strategies for exploring the galaxy

Applying the mathematical theory of island colonisation to galactic colonisation, Tipler argued there are two strategies for exploration:

The r-strategy

The net reproductive rate, r, is maximised at the expense of all else. This would be the strategy used in the early stages of colonisation.

The K-strategy

The carrying capacity of the environment, K, is used to secure the ecological niche in the target system. This would be the strategy used once the system had been colonised for some time. Fewer probes would be sent in this phase.

We are the only communicative intelligent life in the galaxy

Together with his argument for the inevitability of galactic exploration using von Neumann machines, Tipler used the Drake equation to argue that we are the only technologically advanced civilisation in the galaxy.

He used the following assumptions:

  1. Any intelligent species that attempts interstellar communication will begin galactic exploration within 100 years after developing the technology required
  2. It takes 300 million years to explore the galaxy
  3. No probes from another civilisation are present in our solar system
  4. It takes 5 billion years to evolve intelligence and develop technology for interstellar travel (Earth is average)
  5. The probabilities of the Drake equation (that a star has planets, that the planets are habitable, that life evolves, that intelligence evolves and that communicative civilisations evolve) do not vary with time / age of the galaxy
  6. The galaxy is 11 - 18 billion years old and contains \(10^{11}\) stars
  7. The rate of star formation has been decreasing exponentially since the initial burst of heavy element formation
  8. The rate of earth-like planet formation is constant

From the above, Tipler argued, it follows that:

  1. All stellar systems older than 5.3 billion years are candidates for hosting technological civilisations, which is around the number of stars in the galaxy (\(10^{11}\))
  2. Of these \(10^{11}\) candidates, none have succeeded in colonising the galaxy
  3. The probability of an intelligent civilisation existing in our galaxy is less than 1/\(10^{11}\)
  4. The number of existing communicating civilisations in our galaxy is less than or equal to this probability multiplied by the number of stars in our galaxy = \(10^{-11} \times 10^{11} = 1\) (i.e. us)

Longevity is irrelevant

As long as a civilisation has time to construct and send its initial probe containing a von Neumann machine, Tipler argued, the exploration of the galaxy would happen automatically, regardless of the longevity of the original civilisation.

There is no good reason for choosing not to explore

Tipler argued that there is no good reason to believe an intelligent civilisation, interested in interstellar communication, would not explore the galaxy. Any argument for interstellar communication is an even stronger argument for galactic exploration, given the initial investment is for only one probe carrying a von Neumann machine.

Exploration improves long-term survival of the species

All life on Earth tends towards dispersal into new environments. For an intelligent species, this dispersal is limited only by the level of technology.

Colonising the stars increases the probability that a species will survive the death of its star and other existential risks.

Keeping control of the machines

Tipler proposed three ways of ensuring the machines don’t become independent from the species that created them:

  1. Coding the control program such that its failure triggers failure of the entire program
  2. Probes are programmed to form colonies of the species that created them, colonies which could destroy any machines that slipped out of control
  3. The machines are an extension of the biological species and heir to their civilisation: losing control over them is accepted

Are ETI already here?

Tipler considered the possibility that probes from extraterrestrial intelligence(s) are already in our solar system: we just haven’t detected them. He argued that if this was the case, it is most likely that they arrived a billion years ago and, since there was no intelligent life on Earth at that time, they would have no reason to hide and would already have used up the materials in the asteroid belt.

Tipler equated a belief in the existence of extraterrestrial intelligence anywhere in the galaxy with the belief that UFOs are extraterrestrial spaceships.

The Anthropic Principle

He ended his paper by referring to the Anthropic Principle. Original reasoning based on this principle went that because we are here, the Universe must be compatible with the existence of (intelligent) life. Tipler defined it as:

Many aspects of the Universe are determined by the requirement that intelligent life exists in it.

He then argued that it follows that the Universe must contain \(10^{20}\) stars in order to contain a single intelligent species and that we should not therefore be surprised that it contains only one (us).

This kind of argument for the fine-tuning of the Universe for intelligent life to exist is controversial because it reverses cause and effect. The Universe seems fine-tuned for life from our perspective only because we have evolved and adapted to (a very tiny part of) it (via natural selection). Tipler published more on this, together with John Barrow, in a book called The Anthropic Cosmological Principle.


Tipler’s conclusion that intelligent extraterrestrials don’t exist is based on the assumption that any intelligent species interested in interstellar communication will also develop self-replicating machines that would be sent out to the nearest stars, at low cost, to rapidly and autonomously explore the entire galaxy. Given this assumption, these machines would already be in our solar system. His paper complemented the paper by Hart, published in 1975, that rejected a series of explanations for why technologically extraterrestrial civilisations aren’t already here.

This was another paper expressing a pessimistic view of SETI and was an interesting read, especially given the current acceleration of progress in (and interest in risks from) artificial intelligence. Tipler perhaps underestimated the concern a biological species might have in sending von Neumann machines with human-level intelligence out into the Universe with the goal of colonisation.


  1. 1. Tipler FJ: Extraterrestrial Intelligent Beings Do Not Exist. Quarterly Journal of the Royal Astronomical Society 1980, 21:267–281.
  2. 2. Hart MH: Explanation for the Absence of Extraterrestrials on Earth. Quarterly Journal of the Royal Astronomical Society 1975, 16:128.