The Anthropic Principle: Five Short Examples

by Optimization Process 4 min read26th Sep 201713 comments


(content warning: nuclear war, hypothetical guns, profanity, philosophy, one grammatically-incorrect comma for readability’s sake)

This is a very special time of year, when my whole social bubble starts murmuring about nuclear war, and sometimes, some of those murmurers, urging their listeners to worry more, will state: "Anthropic principle."

To teach people about the anthropic principle and how to use it in an epistemically virtuous way, I wrote a short dialogue featuring five examples.

1. Life and death and love and birth

Avery: But how can you not believe the universe was designed for life? It's in this cosmic Goldilocks zone, where if you tweaked the speed of light or Planck's Constant or the electron mass by just a few percent, you couldn't have atoms, you couldn't have any patterns complex enough to replicate and evolve, the universe would just be this entropic soup!

Brook: I'm not sure I buy the "no complex patterns at all" bit, but even granting it for the sake of argument -- have you heard of the anthropic principle?

Avery: No. Explain?

Brook: We're alive. Life can only exist in a universe where life exists, tautologically. If the cosmos couldn't support life, we wouldn't be having this conversation.

Avery: And so, a universe... created... ngh, I vaguely see what you're getting at. Elaborate?

Brook: And so, a universe created by some kind of deity, and tuned for life, is indistinguishable from a universe that happens to have the right parameters by coincidence. "We exist" can't be evidence for our living in one or the other, because that fact doesn't correlate with design-or-lack-of-design -- unless you think that, from inside a single universe, you can derive sensible priors for the frequency with which all universes, both designed and undesigned, can support life?

Avery: I... oof. That argument feels vaguely like cheating, but... I'll think about it.

2. ...and peace and war on the planet Earth

Avery: In any case, it's interesting that the topic of Earth's hospitality comes up today of all days, given that we so nearly made it inhospitable.

Brook: What do you mean, "today of all days"?

Avery: Oh! You don't know!? September 26 is Petrov Day! It's the anniversary of when some Soviet early-warning system sounded an alarm, and the officer in charge noticed something funny about it and wrote it off -- correctly, as it turned out -- as a false alarm. If he hadn't, Russia might have "retaliated," starting nuclear war and destroying the world.

Brook: Yikes.

Avery: Yeah! And Wikipedia has a list of similar incidents. We need to be extremely careful to never get into a Cold-War-like situation again: it was incredibly lucky that we survived, and if we get there again, we'll almost certainly melt the planet into a radioactive slag heap.

Brook: Hmm. Nuclear war is definitely bad, and we should try hard to prevent it. But I suspect things aren't as bad as they appear, and that those reports of near-disaster have been exaggerated due to people's credulity for "cool" shiver-inducing things. Theories should get punished for assigning low probabilities to true things: if your model claims that the odds of surviving the Cold War were only 1:1000, it takes a thousandfold probability hit. Any model that predicts Cold-War-survival better, is correspondingly more plausible.

Avery: Not so! Anthropic principle, remember? If the world had ended, we wouldn't be standing here to talk about it. Just as the fact that intelligent life exists shouldn't surprise us (because we can only exist in a universe with intelligent life), the fact that the world didn't end in 1983 shouldn't surprise us (because we can only exist in a world that didn't dissolve into flames).

Brook: I... see what you're saying...

3. Improbability upon improbability

Avery: Oh! And that's not all! According to this article, of the officers who took shifts monitoring that station, Petrov was the only one to have had a civilian education; the others were just taught "to issue and obey orders." It's really lucky that the false alarm went off when it did, instead of twelve hours later when somebody else was at the helm.

Brook: That... rings false to me. I expect there was a miscommunication somewhere between Petrov's lips and your eyes: if there were six officers taking shifts watching the early-warning system, and five of them would've pressed the button, you just declared the probability of surviving this false alarm to be six times smaller: your model takes another 6x hit, just like it would if it also claimed that Petrov rolled a die and decided he'd only ignore the warning if it came up 1.

Avery: Anth--

Brook: Don't you dare.

Avery: *coughthropic principlecough*

Brook: Shut your mouth.

4. Supercritical

Avery: Fine, fine, sorry. Change of subject: I have a friend who works at the DoE, and they gave me a neat little trinket last week. Here, hold onto this. Careful: it's small, but super heavy.

Brook: Oka-- oh, jeez, wow, yeah. What is it?

Avery: A supercritical ball of enriched uranium.

Brook: Gyaah! That's not safe-- wait, supercritical? That can't be, it would detonate in less than a microsecond.

Avery: And kill us, yes. But we're still alive! Therefore, we must be on the unfathomably tiny branch of possible universes where, so far, when an atom of U-235 in this ball has fissioned, the resulting neutrons tended to miss all the other atoms. Thus, the chain reaction hasn't yet occurred, and we survive.

Brook: But Av--

Avery: And you might be tempted to say, "But Avery, that's so improbable I can't even express numbers that small in standard mathematical notation. Clearly this ball is merely platinum or osmium or some other dense metal." But remember! Anthropic principle! You're not allowed to use the fact that you're still alive as evidence! The fact that this ball hasn't detonated is not evidence against its being supercritical uranium!

Brook: I-- um. Okay, that is definitely one hundred percent nonsensical sophistry, I just need to put my finger on--

5. Evidence kills

Avery: Sophistry!? I'm insulted. In fact, I'm so insulted that I pulled out a gun and shot you.

Brook: ...what?

Avery: Clearly we're living in the infinitesimally tiny Everett branch where the bullet quantum-tunnelled through your body! Amazing! How improbable-seeming! But, you know, anthropic principle and all.

Brook: NO. I have you now, sucker: even in the branches where the bullet tunneled through me, I would have seen you draw the gun, I'd have heard the shot, I'd see the bullet hole in the wall behind me.

Avery: Well, that all assumes that the photons from my arm and the wall reached your eye, which is a purely probabilistic quantum phenomenon.

Brook: Yes, but still: of the universes where the bullet tunneled through me, in ninety-nine-point-so-many-nines percent of those universes, there is a bullet hole in the wall. Even ignoring the universes where I'm dead, the lack of a bullet hole is overwhelming evidence against your having shot me.

Avery: Is it, though? When you looked at the wall just now, you saw no bullet hole, yes?

Brook: Yes...

Avery: But you, Brook-who-saw-no-bullet-hole, can basically only exist in a universe where there's no bullet hole to be seen. If there were a bullet hole, you wouldn't exist -- a Brook-who-did-see-a-bullet-hole would stand in your place. Just as the fact that intelligent life exists shouldn't cause you to update (because you can only exist in a universe with intelligent life), the fact that there's no bullet hole shouldn't cause you to update (because you can only exist in a universe without a bullet hole).

Brook: But seeing a bullet hole doesn't kill me.

Avery: There's nothing fundamentally different about death. You wouldn't exist if the world had been destroyed in 1983, but you also wouldn't exist if there were a bullet hole: I'd be talking to Brook-who-saw-a-bullet-hole instead. And the fact that you exist can't be used as evidence.

Brook: Are you high on something!? Are you fucking with me!? You're asking me to throw out the whole notion of evidence!

Avery: Oh, yeah, I'm totally messing with you. Absolutely. Sorry. When did I start, though?

Brook: ...sometime between describing Petrov Day -- that part was true, right? Good. -- and telling me that that ball-of-some-dense-metal was made of uranium.

Avery: Correct. But can you be more specific?

Brook: ...tungsten?

Avery: Heh. Yeah, good guess. But I meant about--

Brook: --yeah. I'm not really sure when you started messing with me. And I'm not really sure when you stopped applying the anthropic principle correctly.

Avery: Hmm. That's too bad. Neither am I.


I have no idea whether the anthropic principle is legit or how to use it, or even whether it has any valid uses.

[cross-posted to a blog; comments here preferred]