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## Reply to Maudlin: The Calibrated Cosmos

I recently read philosopher of science Tim Maudlin’s book Philosophy of Physics: Space and Time and thought it was marvellous, so I was expecting good things when I came to read Maudlin’s article for Aeon Magazine titled “The calibrated cosmos: Is our universe fine-tuned for the existence of life – or does it just look that way from where we’re sitting?“. I’ve got a few comments. Indented quotes below are from Maudlin’s article unless otherwise noted.

### In a weekend?

Theories now suggest that the most general structural elements of the universe — the stars and planets, and the galaxies that contain them — are the products of finely calibrated laws and conditions that seem too good to be true. … The details of these sorts of calculations should be taken with a grain of salt. No one could sit down and rigorously work out an entirely new physics in a weekend.

Two few quick things. “Theories” has a ring of “some tentative, fringe ideas” to the lay reader, I suspect. The theories on which one bases fine-tuning calculations are precisely the reigning theories of modern physics. These are not “entirely new physics” but the same equations (general relativity, the standard model of particle physics, stellar structure equations etc.) that have time and again predicted the results of observations, now applied to different scenarios. I think Maudlin has underestimated both the power of order of magnitude calculations in physics,  and the effort that theoretical physicists have put into fine-tuning calculations. For example, Epelbaum and his collaborators, having developed the theory and tools to use supercomputer lattice simulations to investigate the structure of the C12 nucleus, write a few papers (2011, 2012) to describe their methods and show how their cutting-edge model successfully reproduces observations. They then use the same methods to investigate fine-tuning (2013). My review article cites upwards of a hundred papers like this. This is not a back-of-the-envelope operation, not starting from scratch, not entirely new physics, not a weekend hobby. This is theoretical physics.

It can be unsettling to contemplate the unlikely nature of your own existence … Even if your parents made a deliberate decision to have a child, the odds of your particular sperm finding your particular egg are one in several billion. … after just two generations, we are up to one chance in 10^27. Carrying on in this way, your chance of existing, given the general state of the universe even a few centuries ago, was almost infinitesimally small. You and I and every other human being are the products of chance, and came into existence against very long odds.

The slogan I want to invoke here is “don’t treat a likelihood as if it were a posterior”. That’s a bit to jargon-y. The likelihood is the probability of what we know, assuming that some theory is true. The posterior is the reverse – the probability of the theory, given what we know. It is the posterior that we really want, since it reflects our situation: the theory is uncertain, the data is known. The likelihood can help us calculate the posterior (using Bayes theorem), but in and of itself, a small likelihood doesn’t mean anything. The calculation Maudlin alludes to above is a likelihood: what is the probability that I would exist, given that the events that lead to my existence came about by chance? The reason that this small likelihood doesn’t imply that the posterior – the probability of my existence by chance, given my existence – is small is that the theory has no comparable rivals. Brendon has explained this point elsewhere.

I find that this sort of example is a smokescreen.  Don’t sweat the small numbers you get from fine-tuning calculations, we are told, because small probabilities happen all the time. Sometimes a small likelihood does imply a small posterior. Sometimes it doesn’t. Which kind of case is fine-tuning? It depends on what rival theories can be offered.

… if a ‘constant of nature’ really is a fixed value, then it was not the product of any chancey process. It is not at all clear what it means to say, in this context, that the particular values that obtain were ‘improbable’ or ‘unlikely’.

Again, theory and data are backwards. What we want to know is: what is the probability that the value of a physical constant in our universe was set by a process that had life “in mind”, given that life-permitting universes like ours need to be fine-tuned? If the theory “the constants of nature are just fixed and that’s all there is to is” fails to predict a probability for the data, then it fails to have a likelihood, fails to have a posterior and thus fails to be able to be investigated rationally and should be rejected.

### What place us?

It can be emotionally difficult to absorb the radical contingency of humanity. … Nicolaus Copernicus upended this picture in the 16th century by relocating the Earth to a slightly off-centre position, and every subsequent advance in our knowledge of cosmic geography has bolstered this view — that the Earth holds no special position in the grand scheme of things. The idea that the billions of visible galaxies, to say nothing of the expanses we can’t see, exist for our sake alone is patently absurd. Scientific cosmology has consigned that notion to the dustbin of history. … But our modern understanding of cosmology does demote many facts of central importance to humans to mere cosmic accident … , and none of the methods for overcoming fine-tuning hold out any prospect for reversing that realisation.

Remind me: what button do you press on a telescope to measure specialness? Where is the term for relevance in the Friedmann equations? How do I build an importance filter, or an it’s-just-an-accident-ometer? The question of whether the universe exists for some purpose is not a scientific question because purpose is not a scientific concept. I can’t measure it, and I can’t infer it from measurements alone. The argument requires a philosophical premise.

1. If the universe is really big, then we aren’t special.
2. The universe is really big.
3. Thus, we aren’t special.

Science will give you 2, but it won’t tell you anything about 1. And clearly, 1 is dubious. Why measure specialness by physical size? I’ve said this before: my daughter contributes nothing to the structural integrity of my house, but in the event of a fire I’m not going back for the load-bearing beams. My considerable height and weight advantage over Martin Rees does not make me more important. Earth is obviously special, since its the only known place in the universe where beings exist that know what special means and can appreciate special things. Purpose, special, relevant, and important are moral terms, and we are moral agents.

To rule out purpose using modern cosmology and physics, one must add the philosophical assertion that “no universe built for a purpose would look like this one”. But we can glimpse reasons why a life-permitting universe needs to be large. As John Barrow has noted,

Chemical complexity requires basic atomic building blocks which are heavier than the elements of hydrogen and helium which emerge from the hot early stages of the universe. Heavier elements, like carbon, nitrogen, and oxygen, are made in the stars, as a result of nuclear reactions that take billions of years to complete. Then, they are dispersed through space by supernovae after which they find their way into grains, planets, and ultimately, into people. This process takes billions of years to complete and allows the expansion to produce a universe that is billions of light years in size. Thus we see why it is inevitable that the universe is seen to be so large. A universe that is billions of years old and hence billions of light years in size is a necessary pre-requisite for observers based upon chemical complexity.

I think “inevitable” is too strong, but the point remains. There is a link between life and the size of the universe, and thus a reason for a universe made to support life to be big. If the lurking presence of the divine bothers you, then consider a natural designer. If I were designing a universe for the purpose of evolving and supporting moral agents, this universe would be worthy of my consideration. It’s not uniquely life-permitting, but it has its charms. In particular, the universe and its contents develop in a way that can be investigated by the life it contains, so science (and the moral good of acquiring knowledge by rational investigation) is possible.

### “Fine-tuning” is a technical, physics term

This is an example of cosmological fine-tuning. In order for the standard Big Bang model to yield a universe even vaguely like ours now, this particular initial condition [W = 1] had to be just right at the beginning. [Another example …] The uniformity of temperature would therefore already have had to exist in the initial state of the Big Bang and, while this initial condition was certainly possible, many cosmologists feel this would be highly implausible.

(I assume that by “W = 1” Maudlin means $\Omega = 1$, which is the standard cosmological notation. The same substitution crept into a powerpoint presentation for a lab report I gave as an undergrad. Curses, Microsoft!)

This section confuses (or at least could lead the reader to confuse) two senses of the term “fine-tuning” in physics literature. Both are nicely explained by John Donoghue here. The standard use of “finely-tuned” in physics simply refers to an extreme sensitivity of a certain outcome to a certain input. For example, if a model can explain the data only if a free parameter (or initial condition) is assumed to fall within a very small range, then this worries physicists. One would prefer a model in which the data falls out more robustly. The fine-tuning of the universe for intelligent life is analogous to this, but instead of the data it is the suitability of the universe for life that is the outcome we are concerned with. Some early anthropic articles (most notably Collins and Hawking) argued that the isotropy (uniformity of temperature) of the universe is required for life. This would make the physical fine-tuning problem into a life-permitting fine-tuning problem. However, the article assumes that only critical density universes would develop isotropy, and only critical density universes would evolve galaxies and life, and both of these assumptions appear to be false. More modern fine-tuning for life articles discuss instead the fine-tuning of the level of cosmic inhomogeneity Q. The horizon problem, discussed by Maudlin, would still be a problem for cosmology even if there were no connection to life.

### Cosmic Inflation

At first glance, the inflationary scenario seems to solve the fine-tuning problem … The constraints are so severe that some cosmologists fear one form of fine-tuning (exact initial conditions in the original Big Bang theory) has just been traded for another form (the precise details of the inflaton field).

The section on inflation is spot on, and well worth a read. Inflation is a very good illustration of how finding a theory that explains the data doesn’t necessarily solve the fine-tuning problem.

### Homeostasis

In this scenario [homeostasis], the universe’s dynamics do not ‘aim’ at any particular outcome, nor does the universe randomly try out all possibilities, and yet it still tends to produce worlds in which physical quantities might appear to have been adjusted to one another. … We can imagine discovering that some of the quantities we regard as constants are not just variable between bubbles but variable within bubbles. Given the right set of opposing forces, these variables could naturally evolve to stasis, and hence appear later as constants of nature. And stasis would be a condition in which various independent quantities have become ‘fine-tuned’ to one another.

This is no answer to fine-tuning at all, since one still has the question: why is the stable state towards which the universe is evolving one which permits the existence of intelligent beings? Maudlin’s use of “fine-tuned” in the last sentence is quite misleading. The variables would be related to each other, but it would still require fine-tuning for these relations to produce a life-permitting universe. Homeostasis amounts to positing as-yet-unknown connections between the cosmic dials, so that they do not all move independently. But this merely exchanges the fine-tuning of the dial for the fine-tuning of the connections.

### Conclusion

Overall, it’s an interesting article from a thoughtful philosopher and well worth a read.

### 5 Responses

1. Thanks again for an extremely helpful commentary. With reference to humanity being contingent, it might be worth noting Simon Conway Morris Life’s Solution: Inevitable Humans in a Lonely Universe for a different perspective

2. I liked MAudlin article but I feel his treatment of inflation was rather incomplete. There have been quote a few papers on the issue of if inflation is fine in Gr is it also fine tuned in quantum gravity. There was a paper that generated some discussion here:Ashekar and Sloan published on this here:
http://arxiv.org/pdf/1103.2475.pdf
Onme of the authors (David Sloan) you may note is one of the organisers for Cambridge Universities philosophy of cosmology project (with Barrow) so Im surprised Maudlin was not familiar with it. There have been some follow up papers here:

http://prd.aps.org/abstract/PRD/v83/i10/e104006

and here:
http://arxiv.org/abs/1301.1264

If you watched the challenges for the early universe conference lectures at the PI you will se Turok being asked about this and he just said well those calculations have to be wrong. He didn’t explain what was wrong with them . In my opinion Maudlin should have mentioned them. I’m not saying these results are conclusive but they deserve discussion.

3. I have an easy argument to dispel the absurdity of Size or anything else people demand upon someone other than themselves.

1 If Mike were God he would have made the Universe small
2 The universe is not small
3 Therefore, Mike is not God

all people are doing with these arguments is showing how poorly thought out their reasons are.

4. […] seems to have taken an interest in the fine-tuning of the universe for intelligent life. (I have a few reservations about his opinions on that […]

5. […] Cosmologist Luke Barnes says: “The theories on which one bases fine-tuning calculations are precisely the reigning theories of modern physics. These are not “entirely new physics” but the same equations (general relativity, the standard model of particle physics, stellar structure equations etc.) that have time and again predicted the results of observations, now applied to different scenarios.” […]