Sciency Words A to Z: The Zero-One-Infinity Rule

Welcome to a special A to Z Challenge edition of Sciency Words!  Sciency Words is an ongoing series here on Planet Pailly about the definitions and etymologies of science or science-related terms.  In today’s post, Z is for:


All month, we’ve been talking about astrobiology, SETI, and the possibility that we are not alone in the universe.  I’d like to end this series with a prediction for the future, and conveniently my prediction is related to a Z-word: the zero-one-infinity rule.

The zero-one-infinity rule was originally created by Dutch computer scientist Willem Louis Van Der Poel.  For the purposes of computer programming, the rule has to do with how many times a user is allowed to do a thing (whatever that thing might be).

It makes sense for a user to never be allowed to do a certain thing (zero), or it makes sense for a user to do a thing only once (one).  But if you’re going to allow a user to do a thing more than once, you may as well let the user do that thing as many times as the user wants.  As a rule of thumb, the zero-one-infinity rule means there’s no reason to impose arbitrary limits on what users can do.

The zero-one-infinity rule has been adapted to many other scientific fields, including the field of astrobiology.  How many places can life exist in the universe?

  • Zero: the universe might not allow life to exist at all.  Of course we already know this isn’t  true, otherwise we wouldn’t be here.
  • One: the universe might only allow life to develop once.  In this view, Earth is a crazy exception, a one-time fluke in a universe that otherwise does not allow life to exist.
  • Infinity: the universe allows life to exist anywhere and everywhere it can.  Life might still be rare in this view, but there are no arbitrary limits imposed on life.

I remember in the 80’s and early 90’s there were a lot of people (including one of my science teachers) who honestly believed our Solar System might be unique.  No other star except our Sun was known to have planets. Maybe that was because there were no other stars with planets.  In short, our Solar System was a “one” in the zero-one-infinity rule.

Then in 1992, astronomers announced the discovery of the first known exoplanets—planets orbiting a star other than our Sun. At the time, we still had no idea just how many exoplanets we might find, but if the universe had allowed two solar systems to form, why not three?  Why not a dozen, or a thousand, or a million?  As soon as the case for “one” crumbled, the possibilities were suddenly limitless.

I predict the same thing will happen when we finally discover alien life.  Maybe it will be microorganisms on Mars, or sea monsters on Europa, or ham radio enthusiasts in the constellation Sagittarius.  It won’t matter which kind of life we find, specifically. Any alien life will do.

In this special edition of Time Magazine, there’s a brief mention of the zero-one-infinity rule.  In that article, NASA scientist Chris McKay sums up the whole field of astrobiology by saying, “So what we’re searching for is two.”  Because once we know that life developed on not one but two worlds… why not three?  Why not a dozen, or a million?  The possibilities will be truly limitless.

14 thoughts on “Sciency Words A to Z: The Zero-One-Infinity Rule

    1. Yeah, there’s a part of me that wants to change it to the zero-or-infinity rule. It seems to me that if nature allows something to happen once, nature will allow it to happen any number of times.

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      1. One seems like it would require extremely special conditions, such as life being made up of some unique particle, or some universe spanning physical principle that only allows it to start once. Given that we’re made of common chemicals following common processes, the probability of special conditions seems so close to zero that it’s barely worth discussing.

        The interesting question is how often does it arise? Often enough for us to find it somewhere else in the solar system? Or rare enough that we have to travel 1000 light years?

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      2. That is the interesting question! And depending on where we find our first aliens—close to home or very far away—that might tell us something about how rare life really is.

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  1. Excellent article. If the answer is one, could that ever be proven? Or would we be on an endless quest for number two? Also, is planet by planet, or moon by moon, a meaningful distinction? Or should consider the possibility that life form number two exists in interstellar space?

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    1. Well… prior to writing these A to Z posts, I would have said we should stick to planets and moons and maybe large asteroids. But then there’s the panspermia hypothesis, which raises a lot of intriguing possibilities about the space between worlds. One article I read said that 30% of the carbon in cosmic dust clouds may be associated with microorganisms, most of them dead but some of them likely still alive. I’d call that an extraordinary claim, and I don’t see the extraordinary evidence necessary to back up that claim. But the possibility is there, I think.


    1. I’m pretty sure New Horizons has an instrument for collecting and analyzing cosmic dust. But I think that instrument can only tell us that we found complex carbon-based molecules. I don’t think it would be able to tell if those molecules came from an organism or not.

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