The Real Problem with Fine-Tuning

You’ve probably heard it said that the universe is fine-tuned.

The Standard Model, our current best understanding of the rules that govern particle physics, is full of lots of fiddly adjustable parameters. The masses of fundamental particles and the strengths of the fundamental forces aren’t the sort of thing we can predict from first principles: we need to go out, do experiments, and find out what they are. And you’ve probably heard it argued that, if these fiddly parameters were even a little different from what they are, life as we know it could not exist.

That’s fine-tuning…or at least, that’s what many people mean when they talk about fine-tuning. It’s not exactly what physicists mean though. The thing is, almost nobody who studies particle physics thinks the parameters of the Standard Model are the full story. In fact, any theory with adjustable parameters probably isn’t the full story.

It all goes back to a point I made a while back: nature abhors a constant. The whole purpose of physics is to explain the natural world, and we have a long history of taking things that look arbitrary and linking them together, showing that reality has fewer parameters than we had thought. This is something physics is very good at. (To indulge in a little extremely amateurish philosophy, it seems to me that this is simply an inherent part of how we understand the world: if we encounter a parameter, we will eventually come up with an explanation for it.)

Moreover, at this point we have a rough idea of what this sort of explanation should look like. We have experience playing with theories that don’t have any adjustable parameters, or that only have a few: M theory is an example, but there are also more traditional quantum field theories that fill this role with no mention of string theory. From our exploration of these theories, we know that they can serve as the kind of explanation we need: in a world governed by one of these theories, people unaware of the full theory would observe what would look at first glance like a world with many fiddly adjustable parameters, parameters that would eventually turn out to be consequences of the broader theory.

So for a physicist, fine-tuning is not about those fiddly parameters themselves. Rather, it’s about the theory that predicts them. Because we have experience playing with these sorts of theories, we know roughly the sorts of worlds they create. What we know is that, while sometimes they give rise to worlds that appear fine-tuned, they tend to only do so in particular ways. Setups that give rise to fine-tuning have consequences: supersymmetry, for example, can give rise to an apparently fine-tuned universe but has to have “partner” particles that show up in powerful enough colliders. In general, a theory that gives rise to apparent fine-tuning will have some detectable consequences.

That’s where physicists start to get worried. So far, we haven’t seen any of these detectable consequences, and it’s getting to the point where we could have, had they been the sort many people expected.

Physicists are worried about fine-tuning, but not because it makes the universe “unlikely”. They’re worried because the more finely-tuned our universe appears, the harder it is to find an explanation for it in terms of the sorts of theories we’re used to working with, and the less likely it becomes that someone will discover a good explanation any time soon. We’re quite confident that there should be some explanation, hundreds of years of scientific progress strongly suggest that to be the case. But the nature of that explanation is becoming increasingly opaque.

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9 thoughts on “The Real Problem with Fine-Tuning

  1. Wyrd Smythe

    Very interesting take on the fine-tuning thing! I have a question: Are you saying we do not live in an improbable universe — that a better theory would make this the only possible universe? Or is it still the case that we apparently live in an improbable universe? I’ve read papers suggesting that if some of those (apparent) constants had different values, atoms wouldn’t hold together or stellar fusion wouldn’t work.

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    1. 4gravitonsandagradstudent Post author

      That’s a bit more complicated of a question. 😉

      I would argue that a final theory should make this the only possible universe…but we know that probability can be a fundamental part of nature’s laws (cf quantum mechanics), so it could be that we live in an improbable universe from that viewpoint. Then if you believe in quantum many-worlds interpretations you can try anthropic arguments, but if you don’t we’d just be straight-up improbable.

      That said, I’d be wary of arguments that specific constants are fine-tuned, because often these things are tied together in nontrivial ways. For stellar fusion in particular, I’ve heard it argued that the constants appear fine-tuned because we’re assuming that stars would function the same way they do in our universe, but if you change the physics behind stellar fusion then stars would end up forming differently, in a way that would end up with fusion regardless. Can’t find the source at the moment, so I don’t know if the debate has evolved past that point.

      Regardless, from a physics perspective the worry is not so much that we’re in an improbable universe, as that we’re in an inexplicable universe, if that makes any sense…fine-tuning to a physicist isn’t worrying because it means we’re unlikely, it’s worrying because coming up with a better theory where this is the only possible universe is much trickier when our universe appears fine-tuned.

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      1. Wyrd Smythe

        No, I suppose it’s really more the task of philosophy to consider anthropic issues and the nature of our existence. Like you, I’ve heard people suggest similar about stellar fusion and other fine-tuning issues, and I’ve been curious ever since about how valid the fine-tuning arguments are (hence my question to you).

        I’ve heard the argument that, based on all the physical evidence, the idea that we live in a simulated virtual reality is a valid — even likely — answer to all our questions about physics. It’s a weird idea — and not a little unlike “intelligent design” — but it does answer a lot of questions rather simply. Entanglement, especially, stops being spooky! 😀

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          1. Wyrd Smythe

            Ha, indeed! I suppose one could appeal to a “multi-verse” idea that we’re just one of many simulations designed to see what happens if the parameters are set thus.

            One of the arguments for the Simulated Reality idea is that, if such a thing were possible at all, any civilization having the capability would be likely to create many, many simulations (for political studies, economic studies, marketing studies, let alone physics studies). Given that there are many, many, the odds that we are in one seem to increase.

            As I said, it’s a weird but rather compelling argument. I was moved to write a post on it a while ago:
            http://logosconcarne.com/2014/12/12/reality-is-virtual-probably/

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            1. 4gravitonsandagradstudent Post author

              I tend to suspect that we don’t live in a simulated reality, but that’s due to a few weird philosophical commitments of mine. Mostly, I tend to think that simulating a bunch of suffering consciousnesses is wildly unethical, and that morality is the sort of thing that actually has some objective truths that civilizations eventually discover.

              On the other hand, if we did, it wouldn’t necessarily be indistinguishable from a “real” world. Have you heard of the AI-Box Experiment? The gist is that a sufficiently advanced AI could, given just the ability to communicate with the outside world, conceivably convince someone to remove any other limits on its freedom. If we ourselves are artificial intelligences that are being observed by some external beings, then it’s at least plausible that we could convince them to “let us out”, i.e. give us tools to affect the “real world”.

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              1. Wyrd Smythe

                I agree with both your philosophical points (I don’t believe reality is simulated, either, but I’m a spiritual dualist). I very much agree morality has objective foundations, and an advanced civilization may very well prohibit casual creation of sapient entities!

                The AI-Box experiment is new to me. I’d love to try the test, since I believe I have a will of iron — that no amount of talk could convince me if I viewed not releasing the AI as my primary mission. I’d be fun to find out if that’s true. 😀

                The idea of a simulated civilization finding out it is simulated and then petitioning their creators for their own freedom… sounds like an excellent basis for an SF novel!

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              2. Tyron

                Hi Matt. Thank you for your thoughtful posts. If I understand you correctly, some of your philosophical problems with any sort of simulator or designer have to do with your belief in an objective morality and how difficult it would be to reconcile a good simulator or designer with the suffering we are aware of. If you believe in objective morality, is it, therefore, possible that your understanding of what is moral is different than what is actually moral much like how our understanding of physics (though pretty good) is different than the actual physical world? Furthermore would limited knowledge of the complete set of elements in an all encompassing moral equation limit us, to a certain extent, from being able to fully evaluate the morality of such a simulator or designer? (we know from experience that some suffering can produce some good especially in terms of training or discipline – e.g. no pain no gain) If we assume that the simulator or designer is actually objectively good and just (ultimately the source of objective morality we experience), wouldn’t the unjust suffering experienced within this existence suggest that this existence isn’t the end of the story? -that a just simulator (who obviously has a long view of things) would ultimate make everything right? Perhaps this simulation is really a moral training and evaluation program by which the simulator will determine the next phase in our existence.
                Very Respectfully
                – Lay philosopher and Bible believer who doesn’t believe in a six 24-hour day creation – before anyone asks.

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                1. 4gravitonsandagradstudent Post author

                  Eh, the standards are a bit different for “a civilization simulating people” and “a deity”. High-tech civilizations have a lot of limits to contend with, both technological and social, so it makes sense for them to choose a “safe” route. In some sense, that’s all I mean by objective morality here: that a civilization that creates a bunch of simulated people and could prevent them from suffering isn’t going to get away with letting them suffer for long, in much the same way that a civilization that has slaves won’t get away with that forever.

                  The thing with the simulation argument is that it rests on some guess about the psychology of the simulators: that they would likely simulate not just one universe, but many, and at high fidelity. The argument doesn’t work if you relax those assumptions, or if those assumptions are wrong.

                  That’s very different from the argument for a deific designer. You can’t say “well, deities exist, and really love to make worlds, so probably we’re in one of the worlds made by a deity and not the original ones”.

                  In the end, the best argument against a deific designer is the same as the best argument against being in a simulation: every time someone has tried to use these ideas to make predictions, those predictions haven’t panned out. Until one of these proposals makes a sufficiently good prediction they’re intellectual curiosities: something perhaps worth working on, but not dedicated, active belief.

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