Discover How We Came to Know the Cosmos

Chapter 30. Where are all the Aliens?

18th December 2017 by Dr Helen Klus

30.1 The Fermi paradox

Given that there may be half a billion habitable planets in the Galaxy, that the first life may have evolved about 8 billion years ago, and that it would only take about half a million years for self-replicating spacecraft to travel across the Galaxy (all discussed in Chapter 29), it seems that we should have come across direct evidence of intelligent alien life forms by now.

If the number of habitable worlds is so large, and the universe has existed for so long, then where are all the aliens?

This question is known as the Fermi paradox after Italian physicist Enrico Fermi, who first discussed the idea in 1950, long before any exoplanets had been found.[1] In 1961, American astronomer and SETI (Search for Extraterrestrial Intelligence) founder Frank Drake reformed this query as the Drake equation[2] (discussed in Chapter 29).

If we assume that habitable worlds are common, we can conclude from the Fermi paradox that either:

  • Habitable worlds are common, but life is not.
  • Life is common, but intelligent life is not.
  • Intelligent life is common, but they do not often make the technological advances required for space travel.
  • Intelligent life is common, but they do not often desire space travel.
  • Intelligent life has travelled across most of the Galaxy, but they are still unaware of us.
  • Intelligent life forms are hiding from us, either intentionally or unintentionally.
  • Intelligent life forms are hiding from everyone, either intentionally or unintentionally.
  • None of the above.

30.1.1 Habitable worlds are common, but life is not

There is currently no way of knowing how often life arises from inorganic matter, even in the ‘perfect’ environment. As far as we know, this has only happened once on Earth in the last four billion years.

30.1.2 Life is common, but intelligent life is not

Although life has been present on Earth for almost four billion years,[3] only single-celled organisms existed for much of this time. Multi-cellular life may have evolved about 2 billion years ago.[4] Mammals evolved over 200 million years ago,[5] and ‘intelligent’ life has only existed for about 200,000 years.[6] We have only been able to read and write for about 5000 years,[7] and have only been industrialised for about 200 years.[8] This means that the vast majority of life on Earth has been ‘un-intelligent’.

Intelligence is not an ‘end point’ of evolution, our intelligence is likely to be an evolutionary accident, and we have no way of knowing how likely it is that any given life form in the Galaxy will be intelligent.

30.1.3 Intelligent life is common, but they do not often make the technological advances required for space travel

Even if intelligent life forms are common, they may not be able develop the technology needed for long distance space travel. We have not yet achieved this ourselves, and so do not know exactly what it will entail. Alien civilisations may not even reach our level of technology if their planet lacks resources, such as fossil fuels, or if they lack the ability to manipulate the external world.

Aliens like these are described as ‘The Handicapped’ in American science-fiction writer Larry Niven’s Known Space series, and may include whales and dolphins on Earth. Aquatic life may have the added problem of not being able to create fire.

It’s also possible that civilisations do not often reach the technological level needed for long distance space travel because life is usually destroyed before it can progress to this stage. Many have worried that it’s in the nature of technologically advanced civilisations to destroy themselves through incompetence or aggression.[9]

Life may also be periodically destroyed by naturally occurring events. There have been numerous mass extinctions on Earth,[10] and we face threats from asteroids, gamma ray bursts,[11] super-volcanoes,[12] and even viruses and bacteria.[13] Perhaps a more disturbing possibility is that life is periodically destroyed by other spacefaring life forms.

30.1.4 Intelligent life is common, but they do not often desire space travel

It may be the case that technologically advanced alien civilisations are common, but most species simply don’t desire space travel. This may be because it’s common for life to evolve in a climate of competition, which gives rise to a fear of the unknown. Although this is evident in humans, it is also a fear we strive to overcome, and this is why we have explored the whole of our planet and travelled to the Moon.

Intelligent life forms may also avoid long distance space travel because it is not economical or sustainable. In 1983, astronomers Carl Sagan and William Newman suggested that self-replicating spacecraft would destroy most of the Galaxy, and so intelligent species would not expand so rapidly.[14] They may also destroy any other self-replicating probes they come across. This may be good for humanity, as any unsustainable rapidly expanding alien species would most likely destroy us before its civilisation collapsed.

It could also be the case that intelligent, technologically advanced aliens may just have different desires to our own. They may not feel a need to leave their planet, or they may spend their time exploring the universe in other ways, perhaps through simulated realities.[15] Civilisations like these may be well defended and/or well hidden.

30.1.5 Intelligent life has travelled across most of the Galaxy, but they are still unaware of us

It’s possible that intelligent, spacefaring species have travelled throughout the Galaxy but are still unaware of us. This could be because life is extremely common, and we are not of great interest, or it may be due to the scale of the universe, and the fact that intelligent life has only existed on Earth for a relatively short time.

If aliens did once come to Earth and leave a Bracewell probe in the Solar System that is waiting to be activated (discussed in Chapter 29), then it unlikely that we would have found it yet. A self-replicating probe would most likely stay somewhere with plenty of resources, like the asteroid belt, Kuiper Belt, or Oort Cloud, and we have scarcely explored any of these places.

Even if a probe was left on Earth, we may not yet have the ability to recognise it for what it is. Microscopic bio-engineered life forms, or molecular nanotechnology, for example, could go completely unnoticed and - to paraphrase British author Arthur C. Clarke - extremely advanced technology may seem like magic to us.[16]

30.1.6 Intelligent life forms are hiding from us, either intentionally or unintentionally

Perhaps aliens are aware of us, we’re just not aware of them. It’s possible that most intelligent spacefaring species are so different from us that we cannot detect them - any more than a tree knows when someone walks straight past it - and they cannot communicate with us.

It’s also possible that aliens have directly communicated with humans before, during the 195,000 or so years when we did not keep written records.

In 1966, Sagan and Russian physicist Iosif Shklovsky suggested that the superhuman beings discussed in early myths and religions could really be aliens.[17] They highlighted the example of the Babylonian fish-god Oannes, who is said to have come out of the sea during the day to teach humans about writing, art, and science. The idea that aliens visited Earth in our early history has been extensively explored in science fiction.

Aliens may also be intentionally invisible, which means they could make contact with us at any time. Aliens may hide by intentionally cloaking evidence of their planet.[18] This would be possible if they shone a light from the dark side of their planet in order to fill its light curve. This method could also be used to block evidence of molecules created naturally by life, or even to block evidence of the whole atmosphere.

There are many reasons why aliens may be intentionally hiding from us. We may be the subject of a scientific experiment, or being used for entertainment. Aliens may be treating the Earth like a wildlife preserve, or they may just be waiting until we reach a certain epoch. One reason for this is that it might help preserve our culture. They may also worry that people would become very afraid, and paranoid, if they revealed themselves, especially if their physical appearance is very different to our own. They are equally likely to avoid us because they find us disturbing to look at, or to communicate with.

Since it would only take one species, or fraction, to blow the cover of the others, these scenarios are more likely if there’s only one dominant species of spacefaring aliens, or if different alien intelligences are united by tradition. This is possible; the first spacefaring life forms could be billions of years more advanced than subsequence civilisations, and so may affect their behaviour even after their demise. This is not dissimilar to life on Earth, where we are all born into set political and religious traditions that we have very little way of controlling.

It may be relatively easy for aliens to hide from us. If there were an alien spacecraft in the Solar System, then we are very unlikely to detect it unless it makes direct contact with us.

It’s possible that we could detect an alien spacecraft in the Kuiper Belt if it happened to pass the field-of-view of one of the telescopes we have in orbit around the Earth, or if it passed close to the New Horizons space probe, but it is very unlikely to do this by accident. This is because objects in the Kuiper Belt are small and far away, and so the Kuiper Belt is largely unexplored and unmonitored. We have only observed a few hundred of the 1000 billion objects it’s thought to contain.[19] The chances of an alien spacecraft passing the field-of-view of the Hubble Space Telescope while it is at the same distance as the Kuiper Belt is about 1 in 20 million.

If an alien spacecraft did happen to pass the field-of-view of a telescope on Earth, then it would have to be extremely large and extremely luminous for it to be visible from the Kuiper Belt. In order to get a rough idea of its shape, it would have to be comparable in size to Pluto (which is over 2000 km wide).

The probability of detecting an alien spacecraft goes up the closer it gets to the Earth, but it’s still very unlikely that it will be detected until it’s at least within the orbit of Mars. The asteroid belt between Mars and Jupiter, for example, is thought to contain at least 3 million asteroids,[20] yet only around 700,000 of these have been identified.[21]

Once it passed the orbit of Mars, a spacecraft sized object would be large enough to be detected from Earth. It could then be detected by an amateur astronomer who happened to be looking in the right direction, or in a sky survey, like the Catalina Sky Survey or Pan-STARRS (the Panoramic Survey Telescope and Rapid Response System). Sky surveys are designed to detect near-Earth objects such as asteroids, comets, and space debris. They work by taking photographs of the sky every night, and then comparing the photos to see if anything has moved relative to the background stars.[22]

Near-Earth objects are objects that come within about 45 million km of the Earth.[23] This is over 100 times the distance to the Moon. There are thought to be about 15,000 near-Earth objects larger than 140 metres in diameter, and about 1000 larger than 1 km.[22] About 90% of these have been identified.[24] This leaves about 100 objects over 1 km that have yet to be discovered, and over 1000 smaller objects. Sky Surveys tend to focus on the plane of the Solar System that the planets and asteroids orbit within, and so an alien spacecraft could also evade detection by bypassing this plane completely.

Diagram showing the distances that different objects would have to be to be visible from Earth. A 3 km object could be seen from about twice the distance to the Moon.

Figure 30.1
Image credit

The Earth and Moon to scale, showing the distances various objects would have to be from Earth for the Hubble Space Telescope to produce a 20 pixel wide image of them.

Spacecraft and the Hubble Space Telescope

Hubble’s Wide Field Camera has a resolution of 0.04’’ (where 1’’=1/3600th of a degree). This means an object that takes up 0.04’’ of the sky would produce 1 pixel in an image produced by Hubble.[25]

In order to take up this much of the sky, an object needs to either be really big, or really close. The full moon, for example, takes up 1800’’ of the sky when viewed from Earth, and Mars takes up about 25’’.[26]

The relationship between an object’s angular size and its distance is found using:

Angular size of object (θ) = Diameter of object (D)/Distance to object (d) (30.1)
Diagram showing how the angular size of an object is related to its diameter divided by its distance.

Figure 30.2
Image credit

Angular size.

Here, θ is measured in radians. There are 206,265’’ in 1 radian, and so:

Angular size of object in ’’ (θ)/206,265 = Diameter of object (D)/Distance to object (d) (30.2)

This means a spacecraft that is about 642 metres across (about twice the width of the International Space Station), would not be visible until it is about 3 million km away. This is over 8 times the distance to the Moon, and about 1/36th of the distance to the orbit of Mars.

To discern the shape of an object, it needs to cover enough space to produce more than 1 pixel. To cover a width of 20 pixels, an object would have to take up 0.8’’ of the sky, and to cover a width of 100 pixels, an object would have to take up 4’’ of the sky.

A 20 pixel wide image of a 642 metre spacecraft could be made when it is about 165,000 km from Earth. This is just under half the distance to the Moon. For a 100 pixel wide image, it would have to be within 33,000 km of the Earth. This is about 1/12th of the distance to the Moon.

If an alien spacecraft does travel at the same speed as an asteroid, then it may be detected a few weeks before it arrives at Earth. The faster the spacecraft is, the less likely it is to be identified, and if it travels close to the speed of light, then we would probably not be able to detect it until it arrived.

We might be able to detect an alien spacecraft if it were to decelerate as it approaches Earth. This is because an object cannot decelerate without expelling energy, which we may be able to detect. It may do this with chemical rockets, nuclear power, or solar sails, all of which would produce excess radiation.[27] If an unusual object were detected this close to the Earth, then it may be possible to construct an image of it using the Hubble Space Telescope.

Once it entered the atmosphere, an alien spacecraft could be detected over military or commercial airspace using radar, assuming it is not using stealth technology. Although, only a minority of the Earth has radar coverage, and there is no coverage at all over the oceans.[28,29] It’s quite possible that an alien spacecraft could land on Earth without ever being detected at all.

30.1.7 Intelligent life forms are hiding from everyone, either intentionally or unintentionally

Perhaps the most disturbing possibility is that we have not communicated with aliens yet because something is systematically destroying life in the Galaxy, either intentionally or unintentionally. In this scenario, any life forms that do remain may not want to make their presence known.

Aliens may be intentionally destroying life for selfish reasons. There is no definitive link between intelligence and ethics, and so no reason to believe that an intelligent species would be benevolent. Selfish spacefaring aliens may want to eat us, enslave us for labour or entertainment, or destroy cultures through evangelism. They might attack us out of aggression, or because they consider us a threat. Many have criticised attempts to contact aliens for these reasons.

American geographer Jared Diamond argued that people who send signals to space in the hopes of communicating with aliens are both naïve and dangerous.[30] American physicist David Brin agreed, comparing us to children “in a strange and uncertain cosmos” and stating that we should:

“...listen quietly for a long time, patiently learning about the universe and comparing notes, before shouting into an unknown jungle that we do not understand”.[31]

If we’re going to send information about ourselves to habitable worlds, then we should consider how much information we want to give away. If we tell a hostile alien species too much about our anatomy they may be better equipped to target us with biological weapons, or invasive species. If they know too much about our technology, they could attack us from a distance through electromagnetic transmissions.[32]

Aliens may also be destroying life in the Galaxy unintentionally. This could be due to the unforeseen result of a scientific experiment, or because of the creation of Berserker probes (discussed in Chapter 29). A rapidly expanding civilisation may also inadvertently destroy other species through disease, or the introduction of invasive species, just as humans have done on Earth.

It’s impossible to know how an alien disease, or species, would affect us. Unless they were specifically bio-engineered, they may have no affect at all. On the other hand, if they are able to interact with us, then an invasive species could wipe out many native ones, and any disease could be extremely lethal and contagious, as our immune systems will not be equipped to deal with it. We may also suffer if species we rely on are destroyed, such as certain bacteria or grass.

30.1.8 None of the above

It’s possible that we cannot deduce anything from the Fermi paradox. When we imagine what intelligent aliens might be like, we are heavily influenced by what we think future humans will be like, and we are most probably wrong.

It’s highly unlikely that anyone could have successfully predicted what life is like now as little as 50 years ago. The further you go back in time, the less likely it is that any predictions would be correct. It would be almost impossible for anyone born in the middle ages to successfully guess what life is like now, and we are even less likely to successfully predict what will happen to an advanced alien species.

30.2 How could future alien communication affect us?

Any aliens that communicate with us will almost certainly be more advanced, and so contact will most likely proceed on their terms. They could have the power to completely destroy life on Earth, they might help us considerably, or provide a combination of good and bad effects.[32]

The idea that alien life forms are destroying all other life in the Galaxy has been discussed above, but it’s also possible that we could be specifically targeted for an attack. This may be for reasons beyond our control, perhaps we have a specific talent or resource that they wish to exploit, or are just in their way, but we may also appear to be a threat to other life forms. We have changed the atmosphere in ways that are visible from space, and will be recognised as the product of an intelligent species that is both rapidly expanding and unsustainable.[32]

Aliens may wish to destroy us before we destroy more species on Earth, or before we discover how to build self-replicating ships of our own and become a threat to them. In 1981, British physicist Edward Harrison suggested that advanced aliens might view us in an analogous way to how we view viruses.[33]

Aliens may also damage life on Earth by accident. They may lead us to destroy ourselves if they provide us with information that we misunderstand, they may physically crash into us, or they may not realise the affect that an experiment or brief visit to Earth will have on us.

Russian science fiction writers Arkady and Boris Strugatsky explored this idea in the novel Roadside Picnic, which describes the aftermath of an alien visitation. The aliens do not appear to notice us, and do not stay long, but they leave behind a number of strange artefacts that appear magical to us, and can be dangerous. The visit is described as akin to a human picnic on the side of the road while on a long car journey, the devices are like the rubbish we sometimes leave behind.

Although the knowledge that we are not alone in the universe will have scientific, philosophical, and religious implications that could be both good and bad, alien contact may not have much overall effect on the future of humanity. This could happen if the aliens do not want to communicate further with us, if communication is too difficult, or if we’re unable to learn much from them because they are less advanced than us, or too different. This is more likely to happen if we discover aliens before they discover us. It’s also possible that if alien contact is not direct, many will not accept that it has happened.[32]

Alien contact may also be highly beneficial for humanity. A benevolent advanced race may be able to solve many of Earth’s problems at little cost. They could teach us their philosophy, mathematics, and sciences, and provide us with new art, music, and literature.

30.3 References

  1. Jones, E. M., NASA STI/Recon Technical Report N 1985, 85, 30988.

  2. SETI Institute, The Drake Equation, SETI Institute.

  3. Moorbath, S., Nature 2005, 434, 155.

  4. El Albani, A., Bengtson, S., Canfield, D. E., Bekker, A., Macchiarelli, R., Mazurier, A., Hammarlund, E. U., Boulvais, P., Dupuy, J. J., Fontaine, C., Fürsich, F. T., Nature 2010, 466, 100–104.

  5. Fraser, N. C., Walkden, G. M., Stewart, V., Nature 1985, 314, 161–163.

  6. McDougall, I., Brown, F. H., Fleagle, J. G., Nature 2005, 433, 733–736.

  7. Schmandt-Besserat, D., From Counting to Cuneiform, University of Texas Press, 1992.

  8. McNeese, T., The Industrial Revolution Era: 1760-1830, Milliken Publishing Company, 2000.

  9. Sagan, C., Carl Sagan’s Cosmic Connection: An Extraterrestrial Perspective, Cambridge University Press, 1973.

  10. Barnosky, A. D., Matzke, N., Tomiya, S., Wogan, G. O., Swartz, B., Quental, T. B., Marshall, C., McGuire, J. L., Lindsey, E. L., Maguire, K. C., Mersey, B., Nature 2011, 471, 51–57.

  11. Melott, A. L., Lieberman, B. S., Laird, C. M., Martin, L. D., Medvedev, M. V., Thomas, B. C., Cannizzo, J. K., Gehrels, N., Jackman, C. H., International Journal of Astrobiology 2004, 3, 55–61.

  12. Timmreck, C., Graf, H. F., Lorenz, S. J., Niemeier, U., Zanchettin, D., Matei, D., Jungclaus, J. H., Crowley, T. J., Geophysical Research Letters 2010, 37, 24705.

  13. Pedersen, A. B., Jones, K. E., Nunn, C. L., Altizer, S., Conservation Biology 2007, 21, 1269–1279.

  14. Sagan, C., Newman, W. I., Quarterly Journal of the Royal Astronomical Society 1983, 24, 113.

  15. Bostrom, N., The Philosophical Quarterly 2003, 53, 243–255.

  16. Clarke, A. C., Profiles Of The Future: An Inquiry into the Limits of the Possible, Phoenix, 2000.

  17. Shklovskiĭ, I. S., Sagan, C., Intelligent Life in the Universe, Emerson-Adams Press, 1998 (1966).

  18. Kipping, D. M., Teachey, A., Monthly Notices of the Royal Astronomical Society 2016, 459, 1233–1241.

  19. Brown, M. E., How many dwarf planets are there in the outer solar system?, Caltech.

  20. NASA, Asteroids: In Depth, NASA Solar System Exploration.

  21. IAU, Minor Planet Center, Minor Planet Center.

  22. NASA, NASA Near-Earth Object Search Program, Near-Earth Object Program - NASA.

  23. ESA, A Chronology of Milestones, NEO Coordination Centre - ESA.

  24. NASA, NEO Earth Close Approaches, Near-Earth Object Program - NASA.

  25. ESA, Hubble’s Instruments: WFC3 - Wide Field Camera 3, ESA Hubble Space Telescope.

  26. University of Iowa, Small-Angle Formula, Imaging the Universe.

  27. NASA, Propulsion systems, History Home - NASA.

  28. BBC News, How do you track a plane?, BBC News, 2014.

  29. Cranky Flier, How Often Do Airlines Fly Into Areas Without Radar Coverage?, Cranky Flier, 2009.

  30. Diamond, J., To Whom It May Concern, The New York Times, 1999.

  31. Brin, D., Shouting at the Cosmos, Lifeboat Foundation, 2006.

  32. Baum, S. D., Haqq-Misra, J. D., Domagal-Goldman, S. D., Acta Astronautica 2011, 68, 2114–2129.

  33. Soter, S., Astrobiology Magazine 2005, 17.

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How We Came to Know the Cosmos: Space & Time

I Pre 20th Century theories

1. Constellations

2. Latitude and Longitude

3. Models of the Universe

4. Force, Momentum, and Energy

5. Newton’s theory of Gravity

6. The Age of the Universe

II 20th Century discoveries

7. Einstein’s theory of Special Relativity

8. Einstein’s theory of General Relativity

9. The Origin of the Universe

10. Galaxies

11. Stars

12. Red Giants and White Dwarfs

13. Supergiants, Supernova, and Neutron Stars

14. Black Holes

III Missions to planets

15. The planet Mercury

16. The planet Venus

17. The planet Earth

18. The Earth’s Moon

19. The planet Mars

20. The Asteroid Belt

21. The planet Jupiter

22. The planet Saturn

23. The planet Uranus

24. The planet Neptune

IV Beyond the planets

25. Comets

26. The Kuiper Belt and the Oort Cloud

27. The Pioneer and Voyager Missions

28. Discovering Exoplanets

29. The Search for Alien Life in the Universe

30. Where are all the Aliens?

V List of symbols

31. List of symbols

32. Image Copyright