Fine-Tuning Really Is A Problem In Physics

The latest from Ethan Siegel:


Thanks @Patrick.

Here are some bits that I thought were most interesting.

For example, if you take a look at an enormous rock balanced precariously on a perch, you would assume that something caused it to be that way. It could be because someone carefully placed and balanced it there, or it could be because erosion and weathering happened in such a way that this structure evolved naturally. Fine-tuning doesn’t need to imply a fine-tuner, but rather that there was a physical mechanism underlying why something appears finely-tuned today. The effect may look like an unlikely coincidence, but this may not be the case if there’s a cause responsible for the effect we see.

The language is somewhat confusing here. “doesn’t need to imply” becomes “there was”. Is Ethan politely saying that there is no fine-tuner or merely that we need to investigate deeper? I get the sense that it’s the later.

The whole point of a fine-tuning argument isn’t to declare that we have a weird coincidence, and therefore anything that explains this coincidence is likely to be right. Rather, it points us to the various ways we might think about an otherwise unexplained puzzle, to try and provide a physical explanation for a phenomenon that has no obvious cause.

So fine-tuning becomes a motivator to uncover something new. People like to figure out puzzles, and fine-tuning looks like a puzzle.

In science, our goal is to describe everything we observe or measure in the Universe through natural, physical explanations alone. When we see what appears to be a cosmic coincidence, we owe it to ourselves to examine every possible physical cause of that coincidence, as one of them might lead to the next great breakthrough. That doesn’t mean you should credit (or blame) a particular theory or idea without further evidence, but the possible solutions we can theorize do tell us where it might be smart to look.

I don’t really get the last sentence, anybody else figure it out?

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This is a good one for the @physicists to comment first. Ethan is among my favorite authors.

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I think this is a good summary of what Ethan is trying to say, at least based on my reading his article.

In my reading it seems that he is just reiterating his previous point:

He cautions that just because someone invents a theory that explains a fine tuned parameter, it doesn’t mean that that theory is automatically right - it has to also pass other requirements:

As always, we have strict requirements for any such theory to be accepted, which includes reproducing all the successes of the previous leading theory, explaining these new puzzles, and also making new predictions about observable, measurable quantities that we can test.

Interesting - most astrophysicists I know either never heard of Ethan or dislike Ethan’s articles. Perhaps it’s just the circle I move in.


What do you think of Sabine at Backreaction?

Sabine is great! She is not without her detractors as she is not afraid of saying controversial or difficult-to-swallow statements, but I think that is to her credit.

The biggest different between Sabine and Ethan is that Sabine is an active, and well-respected physicist who blogs in her spare time. Ethan on the other hand has not been an astrophysicist for some years now. Despite this, he attacked articles that were published in peer reviewed journals in his un-peer reviewed blog posts. Unfortunately this nuance is lost in the internet where laypeople think that his blogs are just as scientifically valid as peer-reviewed articles.

He has been called out before by Stacy McGough in this somewhat-unprofessional-but-hilarious twitter chain.


Agreed. I think that this short little blurb from Sabine is relevant to the discussion:

Suppose you just hatched from an egg and don’t know anything about astrophysics. You brush off a loose feather and look at our solar system for the first time. You notice immediately that the planetary orbits almost lie in the same plane.

Now, if you assume a uniform probability distribution for the initial values of the orbits, that’s an incredibly unlikely thing to happen. You would think, well, that needs explaining. Wouldn’t you?

The inflationary approach to solving this problem would be to say the orbits started with random values but then some so-far unobserved field pulled them all into the same plane. Then the field decayed so we can’t measure it. “Problem solved!” you yell and wait for the Nobel Prize.

But the right explanation is that due to the way the solar system formed, the initial values are likely to lie in a plane to begin with! You got the initial probability distribution wrong. There’s no fancy new field.

In the case of the solar system you could learn to distinguish dynamics from initial conditions by observing more solar systems. You’d find that aligned orbits are the rule not the exception. You’d then conclude that you should look for a mechanism that explains the initial probability distribution and not a dynamical mechanism to change the uniform distribution later.

While she does agree that inflation has evidence to support it, we don’t actually know whether various constants of physics are natural or unnatural. Things that are natural generally aren’t seem as something we need to explain but things that are unnatural require an explanation. The solar system example is a nice analogy to that. Either way such results can be invitations to learn more. Like the cosmological constant problem wasn’t a problem for cosmologists (it is natural to add to the equations of GR) but bringing in a calculation from particle physics messes everything up. Why the disagreement? = invitation to learn more.

One problem I think that exists though with Sabine’s approach is that if someone isn’t familiar with how science works, they can use her blog as evidence against science being able to explain much of anything. One could get that impression fairly easily despite the fact that she is not saying that or anything close to that. I think her blog can serve as an inspiration for many to work on new/productive ideas that they might not have otherwise considered. On the other hand, Ethan’s blog is a bit more triumphalistic which focuses more on the things we have done a good job explaining. I think both are needed in today’s world and its hard to balance. Sean Carroll had an interesting post recently:

In that one he called various things ‘true facts’ - which is not really language that most physicists use. But at the same time, the word is kind of appropriate given the large amount of evidence towards many things about the natural world. Such measurements are much closer to a ‘true fact’ than even someone’s memory of what they did just last night. Thoughts?