Today, I was debating my roommate about some quite existential matters such as: where should we go for lunch today?, what should we do tomorrow?, but amongst these, perhaps having the most existential flavor: what’s the nature of our universe??

The Anthropic Principle

No, this isn’t related to Anthropic, the startup.

Instead, it refers to the selection bias of our observations of the universe because we’re the ones observing it. For example, consider the age of the universe. Amongst all intelligent species that could ever have been born on a uniformly random planet, at a uniformly random point in time in the history of the universe, it must necessarily be the case that intelligent life is born in the instant where intelligent life could be supported… otherwise, we wouldn’t be there to observe it! Thus, our observation of the age of the universe (and other intelligent life, perhaps elsewhere, or in another time, observing it) is skewed! In other words, the types of universes we are allowed to observe are precisely the ones that permit our existence. We can’t observe universes where life is unhabitable, precisely because we would not be there to observe it!

Some Controversial Applications

We’ll try to apply the anthropic principle to something more controversial: doomsday

Since the 1947s, the Bulletin of Atomic Scientists have maintained a ‘clock’ which represents how close humanity is to a extinction-level event. Each year, they move the clock forward or backwards, indicating their estimates about how close we are to human extinction. Of course, there is no way to compute this value… right?

Well, actually, we can get kind of close, given that you believe certain assumptions.

As a thought experiment, suppose that you have two jars. One jar is filled with balls with numbers 1-10 written on them, while the second is filled with balls with 1-100,000 written on them. Now, I’ll give you a ball from one of the two jars. You take a look at it and see that the ball has the number ‘7’ written on it. From which jar is it more likely that this ball came from? Well, in the first jar, there’s a 1 in 10 chance of picking out a 7 but in the second jar, there’s a 1 in 100,000 chance of picking out a 7.

Now, replace the argument above but for the number of humans that have lived thus far and the number of humans that will ever live.

If you believe that the human consciousness or “soul” is implanted uniformly randomly amongst all humans that will ever live, there’s an argument to be made that we are ‘close’ (in a cosmological scale) to the extinction of all humans!

There have been about 100 billion humans that have ever lived. Since you’re randomly drawn and implanted uniformly randomly, there’s a 99% chance that you’re NOT in the first 1% of humans to ever live. This means that we’re in the 99% of humans ‘remaning’, and thus we can be about 99% confident that there will only be about 10 trillion humans. Assuming that the population increases by about ~130 million humans a year, that leaves us with only about 76,000 years left for our species.

Relating back to the above example, suppose there is universe A where humanity prospers so much, that there will be 1 quadrillion quadrillion humans. However, there is also universe B where humanity doesn’t make it out and there only ever will be 1 trillion humans. Now since I’m conscious and have observed myself to be the (about) 100 billion’th human ever, which universe should I believe myself to more likely be a part of?

Conclusion

The anthropic principle, or anthropic reasoning, rather, is the methodology of gathering facts and observations about the observer (us), and how these facts and observations about us change the beliefs of what we know to be true in the universe. In this sense, I feel like the anthropic principle / anthropic reasoning is a very interesting idea and I hope you leave this feeling a similar way.

P.S., if you’re now worried about doomsday, you should know that it’s really just a thought experiment to illustrate a strengthening of the anthropic principle. There are other mathematical frameworks such that the doomsday argument doesn’t hold. And of course, the assumptions we made as part of the argument are also in question.