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What is the anthropic principle, and is it an explanation or a polite way of not explaining?

Serkan Aydoğdu Contributor
· 4 replies · 174 views
Physics & cosmologyAnalyticBrandon Carter#anthropic-principle#explanation#fine-tuning
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I drive a bus in Ankara and listen to lectures between stops. One lecture said that the physical constants of the universe seem "finely tuned" for life, and then mentioned "the anthropic principle", saying something like: of course we observe a universe that allows life, because otherwise we wouldn't be here to observe it.

My first thought was that this is like telling a passenger the bus came because he was standing at the stop. True, but it doesn't explain why the bus came. My second thought was that maybe I misunderstood, because the lecturer seemed to think it was important.

Could someone explain what the anthropic principle actually says, who came up with it, and whether it explains anything? I'm not looking for a religious argument either way. I'd just like to understand this one idea properly.

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4 answers

Accepted answer

  1. Emeka Nwosu

    ✓ Accepted answer Fellow

    Serkan, your bus analogy is very close to the real debate.

    The physicist Brandon Carter introduced it at a 1973 conference in Kraków marking 500 years since Copernicus's birth. It corrects the "Copernican principle" that we occupy no special place: in one respect we do, since it must be a place where observers can exist.

    He distinguished two versions:

    • Weak: our position in the universe, in space and time, is necessarily one compatible with our existence as observers. For example, we observe a universe old enough for stars to have made carbon, because we are made of carbon.
    • Strong: the universe itself, its laws and constants, must be such as to admit observers at some stage. This version is much more controversial.

    Does it explain? On its own, the weak version explains why we observe life-permitting conditions given that we exist. It does not explain why the conditions are life-permitting at all, which is your bus point. But combine it with a multiverse, many universes with different constants, and it becomes a real explanation: most universes are dead, and we necessarily find ourselves in one of the rare living ones. That is why the argument usually ends up being about whether there is a multiverse.

Other answers

  1. Pieter de Jong

    Fellow

    A rough analogy from my side. If you train a model only on data from successful companies, it will tell you every company succeeds. That is a selection effect. The anthropic principle points out that our "data" about universes is drawn from one universe filtered by our own existence. It's a warning about the data, not a theory of the world.

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  2. Huang Wei-ting

    Contributor

    Game designer here. When we tune a puzzle game, we try thousands of parameter settings and throw away the ones where the level can't be finished. Players only ever see the finishable worlds, and some of them write to us saying the level was "perfectly balanced". It was, but partly because they never met the others. That's the selection effect. Whether the real universe had a designer throwing worlds away is the part the principle can't tell you.

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  3. Serkan Aydoğdu

    Contributor

    Thank you, Emeka. So by itself it's my passenger at the stop, and with many universes it becomes "there are many buses on many routes, and you are standing at a stop where one comes". That's much clearer. Wei-ting, I like the thrown-away levels; that makes the multiverse sound like a design office.

    Helpful · 3