Stephen Hawking: Aliens Are Probably Out There

This article is an excerpt from the Shortform book guide to "Brief Answers to the Big Questions" by Stephen Hawking. Shortform has the world's best summaries and analyses of books you should be reading.

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What did Stephen Hawking say about aliens in his book Brief Answers to the Big Questions? Did Hawking believe in the existence of extraterrestrials?

According to famed theoretical physicist Stephen Hawking, aliens most likely exist, but there’s a good chance humanity will never encounter them. This is because Hawking believed that extraterrestrial life is likely unintelligent and lacks the technology to communicate with others.

Here’s an in-depth look at Stephen Hawking’s theories about aliens.

Are There Other Civilizations in Space?

As we start colonizing space, will we have any competition? Are there indigenous peoples on other planets or extraterrestrial races already spreading their empires across the galaxy?

According to Stephen Hawking, aliens most likely exist, but he doesn’t think we have any direct evidence one way or the other to prove their existence. He dismisses reported UFO sightings and alien abductions as hoaxes, hallucinations, or misunderstandings—investigators have shown that some of the alleged sightings are indeed hallucinations, and he thinks that is the most reasonable explanation for all of them. Hawking expects that if mankind ever does make contact with technologically advanced extraterrestrials, it will be widely publicized and obvious to everyone.

(Shortform note: Hawking doesn’t elaborate on his reasons for believing that if extraterrestrial spacecraft arrived on Earth, everyone would know, but one reason they probably would know is that the United States maintains a space surveillance network. This network of radar stations, satellites, and other instruments is capable of detecting any object 10 centimeters across or larger entering or leaving low earth orbit. If Earth were ever visited by alien spacecraft, this network would provide official documentation of the craft’s arrival, but no such arrivals have been reported. This also corroborates the idea that reported UFO sightings are most likely to be hallucinations.)

Hawking discusses three possible reasons that we haven’t been contacted by extraterrestrials:

1) Maybe there are intelligent extraterrestrials out there, but we just haven’t made contact with them. The distance between stars and galaxies in our universe is so vast that it would be easy for multiple races to exist in different parts of the universe without ever bumping into each other.

(Shortform note: If there are intelligent extraterrestrials out there, how would we detect them? Even the nearest stars are much too far away for our telescopes to make out details like orbital space stations, cities on their planets, or other evidence of intelligent life. Our best bet might be picking up their radio communications signals. However, distance reduces radio signal strength, limiting the distance at which we can detect signals. Radio telescopes can pick up radio waves from distant stars, but even the dimmest dwarf stars are quadrillions of times more powerful than the most powerful radio transmitters, so it’s unlikely we’d be able to detect the signals any alien life forms might transmit.)

2) Maybe extraterrestrial life is common in the universe, but intelligent life is not. Hawking questions whether intelligence is really beneficial from the standpoint of Darwinian evolution. Maybe species that evolve sufficient intelligence to develop advanced weapons technology tend to bring about their own extinction through war. If intelligence tends to be an evolutionary dead end, there may not be very many intelligent races in the universe at any given time.

(Shortform note: If intelligent life is rare, this would greatly compound the distance problem, because, as we said before, our best shot at detecting extraterrestrial life is picking up their radio signals. But if the extraterrestrials aren’t intelligent enough to build radio transmitters, then we won’t be able to detect them that way.)

3) Maybe the probability of life forming is actually so low that it only happened once in the entire history of the universe—here on Earth. The idea that life is highly improbable seems far-fetched based on our observations of life on Earth, but observers only evolve in places where life actually forms, so our observations may not be representative. As an illustration, a child growing up in a wealthy neighborhood may assume that every kid gets a new car for her sixteenth birthday, just because everyone she knows did. Perhaps we would be similarly naive to assume that life forms on every planet, just because it formed on ours.

Estimating the Probability of Extraterrestrial Life

Various scientists have put forth estimates of the likelihood of extraterrestrial life. Probably the best-known is the Drake equation, which attempts to estimate the number of intelligent races in our galaxy based on the rate of star formation, the fraction of stars that form suitable planets, and the probability of life forming on a habitable planet. Typical assessments based on the Drake equation predict that there should be at least a thousand intelligent races in the galaxy.

However, scientists concede that some of the parameters in the Drake equation, such as the probability of life forming, are just assumptions. Some have also pointed out that there are a large number of physical and astronomical factors that can affect the habitability of a planet, which the Drake equation doesn’t include.
When these factors are included, the expected number of races typically drops to much less than 1, even if the probability of life forming on a habitable planet is assumed to be 100%, and the scope of the equation is broadened from just our galaxy to the entire observable universe.

Admittedly, there is some uncertainty inherent in all estimates of the probability of extraterrestrial life, but if more exhaustive models tend to drive the probability down, that tends to support the third possibility that Hawking presents, namely that we really are alone in the universe.

Stephen Hawking: Aliens Are Probably Out There