Does SETI Make Sense? Part I: Numbers

Harry SETI header

Understanding SETI (the Search for Extraterrestrial Intelligence) requires that you become involved in a great many different fields and comprehend some rather difficult concepts. For most, it becomes a matter of faith, just what science is not all about. This series of articles attempts to make sense of it all, to put you in a position of deciding on a rational, not faith, basis whether SETI is worthwhile or a waste of time and money. They also provide the basis for some interesting class discussions. Enjoy.

Carl Sagan

Carl Sagan

Carl Sagan famously was a strong supporter of SETI and even wrote a novel that put the best possible face on it. For many like Sagan, the benefits of simply knowing that other intelligent life exists out there overwhelms the negatives of cost and time. What do you think? Will you have a different opinion when you have finished reading these articles? Read on.

The first problem with addressing SETI and similar issues revolves around the huge numbers involved. They truly are astronomical. For SETI, we have to have an idea of how many planets in the universe may be capable of harboring life. Our galaxy, the Milky Way, has over 100 billion stars, possibly much more. The universe also has over 100 billion galaxies. These are huge numbers indeed, but the total number of stars in the universe is their product, greater than ten sextillion.

The Allen Telescope Array (ATA).

The Allen Telescope Array (ATA).

In case you haven’t heard of a sextillion, it’s a one followed by 21 zeroes. To get an idea of how big that number really is, consider a few examples. 

Beans vary widely in length. One centimeter (cm) is a good estimate for the length of one bean. How far would ten sextillion beans stretch from Earth into space? It would be ten sextillion cm or 100 quintillion meters (m) or 100 trillion kilometers (km). It’s only about 150 million km (one astronomical unit or AU) to the Sun. Pluto’s orbit is around 5.9 billion km. The nearest star out there is Proxima Centauri at about 40 trillion km. Those humble beans would stretch far beyond that distance!

Beans have a mass of around one gram, depending on the bean. Ten sextillion beans would have a mass of ten quintillion kilograms. While that’s not as much as the Earth’s six septillion kilograms, it does match up with about 3,000 Mount Everests.

Hubble image of star cluster Messier 15.

Hubble image of star cluster Messier 15. Click image to enlarge.

Were you to count very fast, maybe five numbers per second, you could reach 100 in 20 seconds and 1,000 in 200 seconds or a bit over three minutes. Getting to a million would require 3,333 minutes or just under an hour. To be fair, you probably couldn’t keep up that fast counting for an hour. Round it off to one hour to simplify things. Reaching a billion would mean that you had been counting for 1,000 hours, about 40 days and nights. At this point, you should hire some help.

Have three shifts counting, and you could reach a trillion on 40,000 days, over 100 years. That’s just impractical. Instead, hire a bunch of 20-year olds and have them work until they’re 80 years of age. Each group of three could count to about 600 million in that time. To reach a quadrillion, you’d have to have nearly 5,000 people (maybe more because of weekends and holidays). A quintillion requires five million people, and ten sextillion means you’d have to have 50 billion counters altogether. That’s nearly ten times the Earth’s population today, especially if you exclude children.

I hope you get the point. These are really large numbers and can have a meaning that their mere figures do not show on first look. What does this all mean for SETI? Even if the chances of intelligent life are incredibly small, the outrageously large number of stars in the universe means that intelligent life must be out there somewhere.

What really are the chances of finding life, complex life, and intelligent life on other planets? Those are our next topics. We’ll then move onto the likelihood of detecting signals from a civilization on another planet somewhere in the universe.

Does SETI Make Sense? Part II: Life
Does SETI Make Sense? Part III: Evolution
Does SETI Make Sense? Part IV: Communicating

5 Responses

  1. The opportunities for teaching are many in this series. I hope a few educators out there take advantage of them.

  2. 3333 minutes is just under an hour ???

    • I seem to have decimal point dyslexia. Thank you for the correction.

      If you can reach 1,000 in three minutes, then it will take 3,000 minutes to reach one million. That’s five hours.

      Please multiply the rest of the numbers by five. It would take fifty times the Earth’s population instead of ten times. I doubt that anyone cares about that difference. It’s huge.

    • Bob, thanks for the correction.

      Harry, email me the revisions, and I’ll post them as updates. Thanks.

  3. I said I had decimal point dyslexia. There it goes again!

    3,000 minutes = 50 hours, not five. So, it takes 500 times the Earth’s population.

    Those pesky decimal points!

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