Archive for October, 2010

Once more unto the breach, dear friends. (Another long fine-tuning post, I’m afraid …)

An oft-cited article on the fine-tuning of the universe for intelligent life was written by Michael Ikeda and Bill Jefferys, and goes by the title: “”The Anthropic Principle Does Not Support Supernaturalism”. It appears online here, and to the best of my knowledge has not been published anywhere has been published in “The Improbability of God“, edited by Michael Martin and Ricki Monnier (edit: 3/11/2010).

IJ’s Argument

Unless otherwise noted, quotes are from Ikeda and Jefferys (hereafter IJ). Their central argument is as follows. Let:

L = The universe exists and contains Life.
F = The conditions in the universe are ‘life-Friendly,’ that is, the conditions in our universe permit or are compatible with life existing naturalistically.
N = “The universe is governed solely by Naturalistic law.” The negation, ~N, is that it is not governed solely by naturalistic law, that is, some non-naturalistic (supernaturalistic) principle or entity is involved. N and ~N are not assumptions; they are hypotheses to be tested.

L is, of course, true of our universe. For the sake of argument, IJ assume that F is true. N and ~N are taken to have an a priori non-zero probability of being true. Now, the anthropic principle roughly states that living observers must observe conditions that permit the existence of observers. IJ formulate this as:

P(F|N\&L) = 1.         (1)

N appears in the expression just in case a supernatural agent decides to miraculously sustain life in a non-life-friendly universe.

Now, after dealing with the fallacious1 argument P(F|N) \ll 1 \Rightarrow P(N|F) \ll 1, IJ reach their Bayesian climax:

P(N|F\&L) = \frac{P(F|N\&L) P(N|L)} {P(F|L)}        (Bayes Theorem)
= \frac{P(N|L)} {P(F|L)}                                                    (using 1)
\ge P(N|L)                                                 (since P(F|L) \le 1)

Thus, the fine-tuning of the universe for intelligent life is at best irrelevant to the truth of naturalism, and could actually make it more likely. The fine-tuning of the universe, even if it is true, cannot support supernaturalism. Notice that all probabilities are conditioned on L. As IJ say:

… for an inference to be valid, it is necessary to take into account all known information that may be relevant to the conclusion. In the present case, we happen to know that life exists in our universe (i.e., that L is true). Therefore, it is invalid to make inferences about N if we fail to take into account the fact that L, as well as F, are already known to be true. It follows that any inferences about N must be conditioned upon both F and L … In inferring the probability that N is true, it is entirely irrelevant whether P(F|N) is large or small. It is entirely irrelevant whether the universe is “fine-tuned” or not. Only probabilities conditioned upon L are relevant to our inquiry.

I have two responses. Here I will contend that IJ’s formulation of the argument is incomplete. In the second part, I’ll raise a few issues with this “conditioning on everything” idea.

My Formulation (more…)

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The opening lines from R. Brent Tully’s “Lighthouses in the Shoals of Dark Halos“:

In the imaginary world of simulations, researchers have a well developed picture of the collapse of matter into halos. Overtime, small halos are absorbed into larger units. Collapsed regions filled with substructure can be defined by hundreds or thousands of particles. Halos can be identified with precision within those simulations

In the real world, most galaxies are observed to lie in groups or clusters. However, membership may be so limited that the structure is ill defined.

The first line is certainly arresting, especially for a theoretician! It sounds like we’re off with the fairies, making our own universes, while the real world lies waiting. But the point Tully is making is actually more subtle, and more interesting. By page 3 he cites an N-body paper to reinforce a conclusion drawn from the simple analytic model of Bertschinger (1985), so He obviously believes that simulations aren’t complete fiction.

The point he is making is that simulators are perfect observers. We know where every particle is and where it is going. So if we want to group galaxies into clusters and calculate the virial radius, then we can. We can follow a galaxy for a few billion years and see if it escapes the group or collides with another galaxy.

The real universe, unfortunately, isn’t that easy. We can only see things that, for whatever reason, have been lit up bright enough to be visible from astronomical distances. We cannot watch a galaxy for a few billion years to see how it turns out.

So we need to be smart. Instead of the virial radius, Tully considers the radius of 2nd turnaround. Think of a perfectly smooth, expanding universe. Then make a little bit of it a bit denser, and watch gravity make it collapse. At some outer point, the inwards pull of gravity will have caused the outwards expansion of the universe to stall – this is the first turnaround. The matter inside falls into the centre of the overdense lump, overshoots and sloshes outwards. The maximum radius reached by this material marks the second turnaround. This radius we can observe – the density will step down as we move out past this radius. With this radius in hand, we can test (and confirm) a prediction of self-similar collapse models: the mass within this radius, divided by the radius cubed, will be constant across the population of clusters.

The general moral of the story, I think, is that there is more to astronomy than just theory and observation. It sometimes takes considerable cleverness to think of a way to test a model. Often the most direct ideas are swamped by complications. This missing link between theory and observation must be bridged from both sides. Theoreticians will be tempted to idealise the telescope. Observers can (and have) oversimplify models, testing non sequiturs.

But that, in the end, is the fun of astrophysics. You didn’t think that the universe would be easy, did you?

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From the Sydney Morning Herald:

Alcohol plays a role in 50 to 60 per cent of the nearly 300,000 criminal cases that come before the state’s Local Courts each year, [New South Wales] Chief Magistrate Graeme Henson said.

That’s about twice as high as I’d have guessed. I tried to track down the source of this statistic, but the closest I could find was a report called “Alcohol related crime for each NSW Local Government Area: Numbers, proportions, rates, trends and ratios” from the NSW Bureau of Crime Statistics and Research. The report gives the percentage of “incidents of non-domestic violence related assault recorded by NSW Police” that are alcohol related as 45%.

I’d love to know what that number is for the United Kingdom, as well as European countries like France or Germany who seem to have an alcohol culture without having as much of a binge drinking culture. I’d expect that the percentage of alcohol related crime was lower for the UK and even lower for most of Europe. I’ll try to track those down.

As to what should be done about the problem, I have no idea. Perhaps nothing – it may be a correlation without causation. Perhaps its an alpha male thing: put too many young men in a nightclub with available women and testosterone will cause friction. The alcohol just happened to be there as well. On the other hand, the anecdotal evidence that certain people are more likely to “kick off after having a few” is well known.

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Every now and then, a paper appears on astroph that has us thinking – is this serious? Or is this an imagination gone a tad wild? (Or is it April 1st?) For example, a friend at the IoA once wrote a proposal (for a graduate course) aiming to find extrasolar planets through bioluminescence. Rarely has an observing proposal been so entertaining.

So it was this week when this paper appeared on astroph. The proposal is as follows: “Genomic complexity” estimates that life on earth began 10 billion years ago (give or take three billion years). This is older than the solar system. A slight problem, perhaps. Suppose, perchance, that life didn’t develop on Earth. Suppose that, long before the solar system was formed, a planet was ejected from another planetary system. En route, this wandering planet (or rogue planet) forms life deep underground, shielded from cosmic rays, lethal radiation and freezing temperatures on the planet surface and warmed by radioactivity of the planet core.

Now suppose that the planet wanders into our solar system. The planet is most likely on an unbound orbit, and thus this will not be a long visit. (The paper estimates this number at around 43 years). But, bathed in the radiation of the sun and the solar wind, the surface of the planet may be stirred up enough to throw rocky shards off the guest planet. Should one of these shards reach a newly-formed Earth, and carry with it life-forms, then we would receive a population of organisms with a few billions of years of evolution behind them – they would be older than the Earth itself. (more…)

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Over at commonsenseatheism, Luke Muehlhauser has posted an interesting review of Sam Harris’ latest book “The Moral Landscape: How Science Can Determine Human Values“. Here are some highlights:

Harris’ central claim is that: ” …questions about values – about meaning, morality, and life’s larger purpose – are really questions about the well-being of conscious creatures …”

But why doesn’t the well-being of non-conscious creatures matter? What is well-being, and why should it be maximized instead of preference satisfaction or desire fulfillment or happiness or pleasure? In fact, why should well-being be maximized at all? Harris denies that anything has intrinsic value, which means well-being has no intrinsic value. So why should we maximize well-being? Is it because well-being is what we care about? If so, what about the other things we care about besides well-being? Or does Harris define well-being so that it, by definition, encompasses everything we care about? What are the primary objects of moral evaluation? Acts? Rules? Desires? Institutions? Are one of these central, such that the others derive their valence from it? If not, what if they come into conflict?

The point about the definition of “well-being” is an important one. I read an article in New Scientist a while back which complained that politicians were making policy decisions on the basis of ideology rather than on the basis of scientific facts. To illustrate the point, the article produced statistics indicating that conservative, abstinence-based programs of sex education were less effective than more liberal programs at lowering the occurrence of teenage pregnancy. “There you go”, they’d say, “ideology says abstinence but science says give them condoms!”

The problem with this argument is that it assumes a definition of “well-being” that isn’t agreed to by all sides. Christians, for example, would not subscribe to a view of teenage sexuality that states that the all that matters is that our daughters don’t get pregnant before they’re 20. Christians don’t think that teenagers should be having sex at all, even with a condom, because they argue that teenagers aren’t mature enough to handle the emotional, psychological ramifications, that sex is best in a loving, committed relationship, that girls looking for love will be used by boys looking for anything, that condoms fail more than 10% of the time are not failsafe.

I’m not going to wade into that debate, but my point should be obvious. Science tells us how the world is, not how it should be.

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I recently shocked a group of non-physicists with what I thought was a well known fact. If you are in space, in “zero-gravity”, it doesn’t feel like floating. It feels like falling.

When you are floating in water, gravity is pulling you down and the buoyancy of the water is holding you up. In particular, the water pressure pushes on your skin, and your innards are held up by the internal sinews and stringy bits inside you. But when you are falling, your organs don’t need to be held in anymore – all your bits fall together. Thus all your insides are free to move around, giving you that “stomach in your mouth” sensation.

If you are orbiting Earth, you are literally falling. It’s just that you’re moving fast enough that the force of gravity, instead of pulling you down towards Earth, pulls you around in a circle. But it feels exactly the same – your innards are orbiting with you, and your stomach is in your mouth.

What surprises me is that human beings can ever get used to that sensation. Just imagine feeling like you’ve just hit the top of a roller-coaster for months on end?

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This is a relatively free-form reflection on the comment left at a previous post.


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