It’s become clear along the way that I am not as accurate when I’m trying to represent philosophers as opposed to physicists; the vocabularies and concerns are just slightly different and less familiar to me. So take things with an appropriate grain of salt.

Tuesday morning: The Case for Multiverses

9:00: Bernard Carr, one of the original champions of the anthropic principle, has been instructed to talk on “How we know multiverses exist.” Not necessarily the title he would have chosen. Of course we don’t observe a multiverse directly; but we might observe it indirectly, or infer it theoretically. We should be careful to define “multiverse,” not to mention “exist.”

There certainly has been a change, even just since 2001, in the attitude of the community toward the multiverse. Quotes Frank Wilczek, who tells a parable about how multiverse advocates have gone from voices in the wilderness to prophets. That doesn’t mean the idea is right, of course.

Carr is less interested in insisting that the multiverse does exist, and more interested in defending the proposition that it might exist, and that taking it seriously is perfectly respectable science. Remember history: August Comte in 1859 scoffed at the idea we would ever know what stars were made of. Observational breakthroughs can be hard to predict. Rutherford: “Don’t let me hear anyone use the word `Universe’ in my department!” Cosmology wasn’t respectable. For what it’s worth, the idea that what we currently see is the whole universe has repeatedly been wrong.

So how do we know a multiverse exists? Maybe we could hop in a wormhole or something, but let’s not be so optimistic. There are reasons to think that multiverses exist: for example, if we find ourselves near some anthropic cutoff for certain parameters. More interesting, there could be semi-direct observational evidence — bubble collisions, or perhaps giant voids. Discovering extra dimensions would be good evidence for the theories on which the multiverse is often based.

The only direct observations that currently exists that might bear directly on multiverses is the prediction of giant voids and dark flows by Laura Mersini-Houghton and collaborators.

Carr believes that the indirect evidence from finely-tuned coupling constants is actually stronger. Existence of planets requires a very specific relationship between strength of gravity and electromagnetism, which happens to exist in the real world. There is a similar gravity/weak tuning needed to make supernovae and heavy elements. Admittedly, many physicists dislike the multiverse and find it just as unpalatable as God. But ultimately, multiverse ideas will become normal science by linking up with observations; we just don’t know how long it will take.

9:45: George Ellis follows Carr’s talk with what we’ve been waiting for a while — a strong skeptical take on the multiverse idea.

There are lots of types of multiverses: many-worlds, separated by space or time, or completely disjoint. Anthropic arguments are what make the idea go. The project is to make the apparently improbable become probable.

The very nature of the scientific enterprise is at stake: multiverse proponents are proposing that we weaken the idea of scientific proof. Science is about two things: testability and explanatory power. Is it worth giving up the former to achieve the latter?

The abstract notion of a multiverse doesn’t get you anything; you need a specific model, with a distribution of probabilities. (Does Harry Potter exist somewhere in your multiverse?) But if there is some process that generates universes, how do you test that process? Domains beyond our particle horizon are unobservable. How far should we expect to be able to extrapolate? Into a region which, in principle, we will never be able to observe.

In the good old days we accepted the Cosmological Principle, and assumed things continued uniformly forever beyond our observable horizon. Completely untestable, of course. If all the steps in the extrapolation are perfectly tenable, extrapolations are fine — but that’s not the case here. In particular, the physics of eternal inflation (gravity plus quantum field theory, Coleman-de Luccia tunneling) has never been tested. It’s unknown physics used to infer an unobservable realm. Inflation itself is not yet a well-defined theory, and not all versions of inflation are eternal. We haven’t even found a scalar field!

There is a claim that a multiverse is implied by the fine-tuning of the universe to allow life. At best a weak consistency test. Can never actually do statistical tests on the purported ensemble. Another claim is that the local universe, if it’s inside a bubble, should have a slight negative curvature — but that’s easily avoided by super-Hubble perturbations, so it’s not a strong prediction. We could, however, falsify eternal inflation by observing that we live in a “small” (topologically compact) universe. But if we don’t, it certainly doesn’t prove that eternal inflation is right. Finally, it’s true that we might someday see signatures of bubble collisions in the microwave background. But if we don’t, then what? Again, not a firm prediction.

Ultimately: explanation and testability are both important, but one shouldn’t overwhelm the other. “The multiverse theory can’t make any prediction because it can explain anything at all.” Beware! If we redefine science to accommodate the multiverse, all sorts of pseudo-science might sneak inside the tent.

There are also political/sociological issues. Orthodoxy is based on the beliefs held by elites. Consider the story of Peter Coles, who tried to claim back in the 1990’s that the matter density was only 30% of the critical density. He was threatened by a cosmological bigwig, who told him he’d be regarded as a crank if he kept it up. On a related note, we have to admit that even scientists base beliefs on philosophical agendas and rationalize after the fact. That’s often what’s going on when scientists invoke “beauty” as a criterion.

Multiverse theories invoke “a profligate excess of existential multiplicity” in order to explain a small number of features of the universe we actually see. It’s a possible explanation of fine tuning, but is not uniquely defined, is not scientifically testable, and in the end “simply postpones the ultimate metaphysical question.” Nevertheless — if we accumulated enough consistency tests, he’d be happy to eventually become convinced.

11:00: After coffee, we reconvene for a panel discussion on “the views of physicists,” featuring Pedro Ferreira, George Efstathiou, and Sean Carroll.

Ferreira talks about the paradigm shift we’ve seen over the last decade. Driven by three factors: inflation, the accelerating universe, and the string landscape. What are the alternatives? Inflation has almost no robust alternatives, unlike twenty years ago. Still not very precisely predictive, too many models. The cosmological constant is a good explanation of the accelerating universe. There are viable alternatives, but they all require severe fine tuning; the story isn’t yet finished. The landscape has been around since the 80’s, but didn’t take off until the vacuum energy became a pressing issue. It might be good if more people were digging into properties of specific compactification schemes. Is it premature to worry about the multiverse when we understand so few compactifications?

Efstathiou pulls out a white board and starts writing equations! Which I will not reproduce here (it’s basic Bayesian probability). The point is that we can only test models against other models, not in some abstract theoretical vacuum. When you don’t have well-defined alternatives, you must take what you can get. He then tells a parable about the promiscuous astronomer, which I also won’t reproduce here. (A close relative of the Sleeping Beauty puzzle well-known to philosophers.)

Carroll explains that it’s difficult to explain the low entropy of our early universe if we stick to unitary autonomous evolution of a comoving patch of space; if we don’t want to give up on conventional quantum mechanics and we don’t want to invoke ad hoc boundary conditions, we’re led directly to a multiverse. He is interrupted frequently by spontaneous applause and finishes to a standing ovation, several audience members smacking their palms to their foreheads and crying “Brilliant! Why didn’t I think of that?”

During the question period, Max Tegmark puts even money on the proposition that the Planck satellite will find evidence for an inflationary gravitational-wave background from B-mode polarization of the cosmic microwave background. Carroll quickly turns it into a formal bet, and the stakes are set at $100. Max will be paying up three or four years from now.

Tuesday afternoon: Philosophical Assessment of the Scientific Case

1:45: The afternoon session is given over to the philosophers. We begin with a panel discussion featuring John Norton, David Wallace, Wayne Myrvold, and Henrik Zinkernagel.

Norton points out that we’re all looking for ultimate explanations of everything — looking down on the idea of brute inexplicable facts. If you go down that road, you run the danger of accepting more metaphysics that you were originally planning to. Don’t get too wedded to particular criteria for distinguishing science from non-science; it leads to later regrets when you want to do something perfectly scientific that doesn’t quite fit your criterion. The problem with pseudosciences is not that they don’t make predictions; it’s that they always change to accommodate new information. “Falsifiability” is a great slogan and an irresistible sound bite, but not a reliable rule to separate science from non-science.

Wallace talks about observable vs. unobservable things. We can’t observe electrons, dinosaurs, or the interior of the Sun. We nevertheless accept them because they are non-optional parts of theories that are very tightly coupled to data. You can’t have a theory that explains dinosaur fossils without believing in dinosaurs. Multiverses come in a variety of forms — from practically useless to quite specifically useful. Not all created equal, and we need to be a bit careful.

Myrvold dislikes the very idea of a “philosophical assessment” of scientific cases; that’s a job for the people who are deeply involved with the theories and the data. Nevertheless (unsurprisingly) there is something to say. Trying to explain things at the edge of knowledge is part of the practice of science, but there is certainly some inductive risk. Hypothetico-deductive method: deduce consequences of an hypothesis and some auxiliary assumptions, go out and test these consequences, reject hypothesis if consequence is falsified. That can’t possibly be the whole story. Do we reject hypothesis or auxiliary assumptions? Quine: maybe we were just hallucinating. Process is often underdetermined.

Zinkernagel talks about the role of time in the multiverse. Could our universe be just a patch in a much older and larger structure? Depends on what you mean by “time.” We could define “supercosmic time” that orders different patches, or we could simply extrapolate from within our observable patch. General relativity allows for all sorts of time coordinates, but under special circumstances we can define a unique notion of time. To do this in the real world, you need a physical clock, and something to set the scale.

The discussion session gets into the question of policing the boundaries of respectable science, and whether the truth will eventually win out regardless of how well we do the policing. Brian Greene takes a poll: do people think that 200 years from now the truth or falsity of the multiverse (or whatever) will basically shake itself out regardless of our current investigation of what does or does not count as science? Almost everyone agrees that it will except for Julian Barbour, who doesn’t believe in time anyway (Brian’s joke).

4:00: We go right into another panel discussion, this time about “Cosmology as a Science,” featuring William Stoeger, Jean-Philippe Uzan, Jeremy Butterfield, and Huw Price.

Stoeger dives into the thicket of the demarcation problem: what is science, and what is not? It wasn’t all that long ago that cosmology itself had a doubtful status as science. There’s only one observable universe; can’t do repeatable experiments. But cosmology is not only a science, but has extended beyond the observable part of the universe. Ernan McMullin: induction moves backwards from observed effects to inferred causes. Continued success of an hypothesis leads us to conclude that “something very much like the content of the hypothesis exists in reality.” Always trying to fit our hypotheses into larger explanatory systems. A bit more optimistic than Ellis about bringing multiverses into cosmology as long as they continue to provide fruitful hypotheses.

Price has three comments for us. First, why are we asking “Is the multiverse science?” rather than simply “Is the multiverse true?”, or at least “Is it a reasonable hypothesis?” Second, there is a close analogy between the current multiverse ideas and the Steady State cosmology (details for the reader to work out). Third, think about the notion of “explanatory relief.” The inflationary multiverse provides relief for the puzzle of why the universe allows life; but we don’t think that the Everett multiverse provides relief for the puzzle of why some particular person exists. The Lewisean multiverse (all logically possible worlds exist) provides relief for all sorts of questions — but what good is that? There’s a slippery slope here, trading off explanatory power for triviality. TIme for a fourth comment: In everyday life, we explain the future in terms of the past (asymmetric in time). But what is the justification for that in the context of cosmology?

Uzan wants to talk about cosmology with a lower-case “c,” what we call “physical cosmology.” Standard cosmological model relies on assumptions about gravity, matter, symmetry, and global structure of the universe. All should be subjected to empirical tests. For general relativity, it would be nice to have more tests on cosmological scales (low acceleration, low curvature). For example, using various forms of large-scale structure data — weak lensing, galaxy maps, velocities, integrated Sachs-Wolfe effect. We can also imagine testing the assumed symmetries of space, as embodied in the Copernican principle. With precise spectra, we could measure the change in the redshift of an object over time. And of course, we can look for a compact topology to the universe.

Butterfield wants to keep his remarks short so that he doesn’t say anything incriminating that ends up on this blog. [Ed. note: everyone knows about the live-blogging and there is a slight undercurrent of distrust, although Jeremy was just joking. I think.] He congratulates the cosmology community in general, and George Ellis in particular, on their endogenous amphetamines. Wants to encourage plucky theories being studied by small groups of researchers (”weeds in the garden,” in a nomenclature from a previous session). How much do/should controversies in the foundations of statistics interact with debates in fundamental cosmology? In particular, how do they impact the measure problem?

In discussion, George Ellis quotes G.K. Chesterton: “the only people who have an explanation for everything are madmen.”

Tuesday evening: Why is there Something Rather than Nothing?

9:30: The conference closes with a posh dinner at Balliol College, featuring a talk by John Hawthorne on the primordial existential question. Then we have a response by Max Tegmark. Apparently my principled refusal to go to talks after 9:00 p.m. is no more reliable than my refusal to attend meetings that receive funding from the Templeton Foundation.

But I didn’t actually bring my laptop to the talks, so the summaries will be short and sweet. Hawthorne didn’t offer an answer to the question, but tried to provide a roadmap to possible answers. He distinguished between different notions of “nothing,” from true non-existence to an empty and involving universe. Then he distinguished between explanations from law — something exists because there is a deep principle of nature which, perhaps among other things, requires something to exist — and explanations from probability — there is a measure on the space of things that might possibly exist, and the measure assigned to the empty set is zero or at any rate small. Long story short, metaphysicians have not reached a consensus on this question.

Tegmark decided to tackle the question of what kinds of things there could be, rather than why at least one of them was real. He has a classification of multiverses: Type I is just a really big space that is pretty similar throughout, Type II is an inflationary kind of multiverse with distinct (although perhaps connected) bubbles, Type III is different branches of the wave function. His favorite option is the Type IV, in which all possible mathematical structures exist; that’s closely related to David Lewis’s ideas about modal realism.

I had a question I wanted to ask, as follows. Imagine that we agreed that any universe could be described by some sort of mathematical structure; perhaps a wave function evolving in Hilbert space, or a discrete cellular automaton, or a single point, or even the empty set (”nothing”). The question is, what kind of possible answer to the question “Why is the actual universe this structure rather than that structure?” could one imagine providing that would be more fruitful, give us more information, than a simple statement of the brute fact of which structure it was? Isn’t it necessarily like trying to discern a pattern from a single event?

But I was sensible enough to recognize that going to sleep was a higher priority than figuring out the meaning of existence. Maybe some other time.

Monday morning: The Case for Multiverses

9:00: We start today as we ended yesterday: with a talk by Martin Rees, who has done quite a bit to popularize the idea of a multiverse. He wants to argue that thinking about the multiverse doesn’t represent any sort of departure from the usual way we do science.

The Big Bang model, from 1 second to today, is as uncontroversial as anything a geologist does. Easily falsifiable, but it passes all tests. How far does the domain of physical cosmology extend? We only see the universe out to the microwave background, but nothing happens out there — it seems pretty uniform, suggesting that conditions inside extend pretty far outside. Could be very far, but hard to say for sure.

Some people want to talk only about the observable universe. Those folks need aversion therapy. After all, whether a particular distant galaxy eventually becomes observable depends on details of cosmic history. There’s no sharp epistemological distinction between the observable and unobservable parts of the universe. We need to ask whether quantities characterizing our observable part of the universe are truly universal, or merely local.

So: what values of these parameters are consistent with some kind of complexity? (No need to explicitly invoke the “A-word.”) Need gravity, and the weaker the better. Need at least one very large number; in our universe it’s the ratio of gravity to electromagnetic forces between elementary particles. Also need departure from thermodynamic equilibrium. Also: matter/antimatter symmetry, and some kind of non-trivial chemistry. (Tuning between electromagnetic and nuclear forces?) At least one star, arguably a second-generation star so that we have heavy elements. We also need a tuned cosmic expansion rate, to let the universe last long enough without being completely emptied out, and some non-zero fluctuations in density from place to place.

If the amplitude of density perturbations were much smaller, the universe would be anemic: you would have fewer first-generation stars, and perhaps no second-generation stars. If the amplitude were much larger, we would form huge black holes very early, and again we might not get stars. But ten times the observed amplitude would actually be kind of interesting. Given an amplitude of density perturbations, there’s an upper limit on the cosmological constant, so that structure can form. Again, larger perturbations would allow for a significantly larger cosmological constant — why don’t we live in such a universe? Similar arguments can be made about the ratio of dark matter to ordinary matter.

Having said all that, we need a fundamental theory to get anywhere. It should either determine all constants of nature uniquely, in which case anthropic reasoning has no role, or it allows ranges of parameters within the physical universe, in which case anthropics are unavoidable.

10:00: Next up, Philip Candelas to talk about probabilities in the landscape. The title he actually puts on the screen is: “Calabi-Yau Manifolds with Small Hodge Numbers, or A Des Res in the Landscape.”

A Calabi-Yau is the kind of manifold you need in string theory to compactly ten dimensions down to four, picked out among all possible manifolds by the requirement that we preserve supersymmetry. There are many examples, and you can characterize them by topological invariants as well as by continuous parameters. But there is a special corner in the space of Calabi-Yau’s where certain topological invariants (Hodge numbers) are relatively small; these seem like promising places to think about phenomenology — e.g. there are three generations of elementary particles.

Different embeddings lead to different gauge groups in four dimensions: E₆, SO(10), or SU(5). Various models with three generations can be found. Putting flux on the Calabi-Yau can break the gauge group down to the Standard Model, sometimes with additional U(1)’s.

11:15: Robert Brandenberger steps up to talk about probability measures and initial data for inflation. The title was assigned by the organizers, and he claims that he got scared by it and started having sleepless nights, so he changed it: “Initial Conditions for Early Universe Scenarios.” He wants to consider alternatives to inflation.

Inflation is great; solves lots of problems, and provides a mechanism for density perturbations. But it’s not the only way to produce primordial perturbations, and there are problems, such as the need to consider modes with wavelengths shorter than the Planck length.

One alternative is string gas cosmology. Predicts a slight red tilt for density perturbations, but a slight blue tilt for gravitational waves. String theory has a maximum temperature (the Hagedorn temperature); when you squeeze strings to sufficiently high density you enter a dual phase where the temperature goes down. We could loiter in a metastable phase at constant temperature before the universe began expanding. Might explain why only three dimensions become large.

Another alternative is a matter bounce, in which the universe first contracts and then bounces back to expansion. Predicts scale-free perturbations for both density and gravitational waves, but the bispectrum can be large.

The final idea is the ekpyrotic/cyclic scenario. Three large dimensions of space appear to bounce, while branes are crashing together in extra dimensions. Predictions are hard to make because we’re not really sure how to resolve the singularity — but a scale-invariant spectrum is conceivable.

Now about initial conditions. It can be hard to get inflation to start, but it becomes easier if we start in a false vacuum and then tunnel to inflation. For the string gas, thermal equilibrium looks like a local attractor, so that’s also okay. For the matter bounce, on the other hand, the assumed background is unstable — so that’s a problem. Maybe you can do it if the bounce is generated by gravity rather than by matter. Likewise for the ekpyrotic universe. Cyclic universe is a bit better; essentially we are currently going through “inflation” at a very low scale.

12:00: Now we have a panel of “critical commentaries,” featuring Brian Greene, Jean-Philippe Uzan, and Andrei Linde.

Brian Greene begins by breaking the overhead projector, so he has to speak without slides. “Can the multiverse” be tested is an unanswerable question, because it depends on what kind of multiverse theory you actually have. E.g. if we live in some specific part of the string landscape, we could in principle probe details of the compactification manifold, to figure out which one it is. If there are bubble universes, we might observe collisions between bubbles. The landscape might predict relationships between observable quantities. Or, if you had a microphysical theory that you had tested well enough to accept it, and it predicted a multiverse, that would be a sensible conclusion to accept. On the other hand: eternal inflation undermines many of the successes of inflation, because it removes the possibility of unique predictions. Need to have a measure. In quantum mechanics, a measure can emerge directly from a theory; maybe inflation will eventually get to that point.

Jean-Philippe Uzan brings up the example of general relativity — why do we pick the Einstein-Hilbert action over any others? We take it as a simple choice, but we can also look for deviations, and constrain them experimentally. When do we simply accept a theory, vs. putting great effort into looking for modifications of it?

Andrei Linde laments that he is playing the role of Big Brother — watching you in case you claim to have a better theory than inflation. He studied string gas cosmology some time ago, but found it to be somewhat ill-defined. We don’t really know enough about it to say what it predicts. The bouncing cosmologies studied within Horava-Lifshitz gravity are very new, and haven’t been studied carefully. He has looked carefully at ekpyrotic models, but they are also a moving target. It seems difficult for perturbations to travel smoothly through a singularity.

Monday afternoon: Fine Tuning and Anthropic Arguments

2:00: We kick off the afternoon with Sir Roger Penrose talking about entropy issues for cosmology. Penrose has enough clout that they have brought out the overhead projector so he can show his famous hand-drawn slides.

The Second Law of Thermodynamics is mysterious. Part of it is perfectly clear: entropy increases because there are more high-entropy states than low-entropy ones. The mystery is why entropy was lower in the past — all the way back to the Big Bang. Why did the Bang have such a low entropy? What was the nature of the constraint on those early conditions? Things were very smooth. Sounds high-entropy, but not when you take gravity into account. In a fixed box, the highest-entropy state will often be a black hole (maximally non-uniform).

Inflation purportedly explains the smooth conditions of the early universe. However, there are many more non-inflationary initial conditions corresponding to our current universe than inflationary ones. Anthropic principle doesn’t really help; it’s much easier to make a tiny region of universe suitable for life than to make a patch ready to undergo inflation.

The Weyl Curvature Hypothesis conjectures an explicit time-asymmetry in the laws of nature: singularities in the past are smooth (have zero Weyl curvature), while singularities in the future can be arbitrarily chaotic. That would determine the arrow of time. After all, at high temperatures particles are essentially massless (compared to their energies), and therefore nearly conformally invariant. Maybe we can extend that conformal structure to before the Big Bang. In fact, we could imagine a conformal cyclic cosmology, matching the smooth far future onto the smooth early past. We can even think of observational consequences: perhaps calculate density perturbations induced by the fact that the “late” empty universe is not precisely de Sitter. David Spergel at Princeton has a student looking for circles in the CMB, which might be predicted by this model.

3:00: Now we have a panel on “explaining fine tuning,” featuring Alex Vilenkin, Lawrence Hall, and David Wallace.

Despite all the talks that have alluded to measures on the multiverse, Vilenkin is the first to talk very specifically about different proposals and their problems. The problem is that spacetime (in a typical multiverse model) is infinitely big. One generally chooses some cutoff to define a finite region of spacetime, calculate ratios of different conditions in that finite region, then let the cutoff go to infinity. That only makes sense if the result is not sensitive to the specific cutoff; but usually it is sensitive. The only measure that works is the scale factor cutoff; but Vilenkin admits that other people (even within the room) have very different ideas. Of course the right answer should be derived from a deeper theory, not simply postulated. He has hopes for the holographic measure.

Hall is the first particle phenomenologist we’ve heard at the conference. He wants to ask whether we can use the multiverse to make predictions about particle properties. Evidence might be quantitative (mass of the electron) or structural (why three gauge forces?). An old-school particle physicist would try to invent new symmetries; a new-school multiverse physicist calculates probability distributions and places anthropic cutoffs. If we lie near an anthropic boundary, that might be taken as evidence for a multiverse. Low-scale supersymmetry is a possible explanation for the hierarchy between the weak scale and the Planck scale; however, there is also an anthropic bound which really isn’t all that different. And note that we certainly could have — one might say “should have” — already detected low-scale supersymmetry if it existed. Similarly, there is an anthropic boundary in Higgs/top-quark parameter space; being pushed near that boundary suggests that we will find the Higgs near 112 GeV and the top near 177 GeV. If the Higgs is there, that’s a quantitative prediction good at the 5% level. If we then don’t find supersymmetry, the multiverse would be the only known explanation. Many aspects of the multiverse theory can’t be tested, but that’s true for any theory. The right question is, “Can we obtain sufficient evidence to be convinced?”

Wallace is a philosopher who was asked to talk about the Everett interpretation of quantum mechanics, and possible connections to the multiverse idea. But he’s not sure what those connections are, so he apologizes for being a bit vague. The Everett (a/k/a many-worlds) interpretation is simply the idea that we should take unitary quantum mechanics (evolution obeying the Schrodinger equation) as literally true and complete — no wave-function collapse. It implies a kind of multiverse: many mutually decohering histories. A bit different from the kind of spatial multiverse discussed in cosmology, but there are some similarities. Encouragingly, we used to be very confused about how to calculate probabilities within many-worlds, but we’ve lately made great strides toward figuring that out. We might be optimistic that the right measure in the cosmological multiverse might simply be the quantum-mechanical measure, properly applied.

4:30: Another panel! Featuring more critical commentaries, this time from John Barrow, Alex Pruss, and Robin Collins.

Barrow points out that many apparent fine-tunings are ultimately explained by theory. Most particles in nature are identical; that’s explained by quantum field theory. Inertial mass equals gravitational mass; explained by general relativity. If there was any random element in the early universe, anthropic considerations are needed. Barrow and Tipler put an anthropic upper bound on the cosmological constant back in 1986. Maybe the vacuum energy is changing with time, as Sorkin suggested.

Next up, Alex Pruss discusses the different meanings of “fine tuning” between physicists and philosophers of religion. Physicists are thinking of parameters taking on unlikely values within some presumed distribution; philosophers of religion are thinking about parameters that are tuned to allow for the existence of life. Philosophers of religion, of course, don’t worry about testability. But the fact that so many physicists at this conference keep insisting that it’s okay if some aspects of the multiverse are not testable is evidence that there must be people who don’t think it’s okay. The move by the scientists is to say that only specific models are testable. That’s a respectable move, and one that philosophers of religion might want to emulate. Is there some specific theory of what God is trying to maximize? Simplicity of laws, for example? On the other hand, philosophers of religion do something worthwhile, in that they consider a wider variety of allowed explanations.

Robin Collins digs into a foundational question: what does probability mean? One necessary criterion: a notion of probability must conform to rational expectation. In cosmology, this is hard to get right. Consider the flatness problem: we look at the density parameter at early times, and ask why it’s so close to one. But why look at that, rather than some crazy function of that parameter? We should also think carefully about the purported feature of philosophy of religion that it considers a wider variety of explanatory schemes. Can we really separate out these schemes from the search by ordinary science for simple and robust explanations of observed phenomena? Inflation is not a panacea — you might want to go the the “unrestricted multiverse,” where all possibilities are real. The problem is that it doesn’t explain the world we see, because it “explains” anything you could possibly see. Even a multiverse needs restrictions.

Monday evening there was a scheduled talk by Paul Davies, with a response by George Ellis, on “Cosmology, Ultimate Causation, and Multiverses.” But I’m pretty sure that an all-powerful and all-beneficent deity would never have intended jet-lagged scientists to attend talks that started later than 9:00 p.m., so that’s it for today.

The proximate reason for this particular conference is George Ellis’s 70th birthday party. Ellis is of course a well-known general relativist, cosmologist, and author. Although the idea of a birthday conference for respected scientists is quite an established one, Ellis had the idea of a focused and interdisciplinary meeting that might actually be useful, rather than just bringing together all of his friends and collaborators for a big party. It’s to his credit that they invited as many multiverse-boosters as multiverse-skeptics. (I would go for the party, myself.)

George is currently very interested and concerned by the popularity of the multiverse idea in modern cosmology. He’s worried, as many others are (not me, especially), that the idea of a multiverse is intrinsically untestable, and represents a break with the standard idea of what constitutes “science.” So he and the organizing committee have asked a collection of scientists and philosophers with very different perspectives on the idea to come together and hash things out.

It appears as if there is working wireless here in the conference room, so I’ll make some attempt to blog very briefly about what the different speakers are saying. If all goes well, I’ll be updating this post over the next three days. I won’t always agree with everyone, of course, but I’ll try to fairly represent what they are saying.

Saturday night:

Like any good British undertaking, we begin in the pub. I introduce some of the philosophers to Andrei Linde, who entertains us by giving an argument for solipsism based on the Wheeler-deWitt equation. The man can command a room, that’s all I’m saying.

(If you must know the argument: the ordinary Schrodinger equation tells us that the rate of change of the wave function is given by the energy. But for a closed universe in general relativity, the energy is exactly zero — so there is no time evolution, nothing happens. But you can divide the universe into “you” and “the rest.” Your own energy is not zero, so the energy of the rest of the universe is not zero, and therefore it obeys the standard Schrodinger equation with ordinary time evolution. So the only way to make the universe real is to consider yourself separate from it.)

Sunday morning: Cosmology

9:00: Ellis gives the opening remarks. Cosmology is in a fantastic data-rich era, but it is also coming up against the limits of measurement. In the quest for ever deeper explanation, increasingly speculative proposals are being made, which are sometimes untestable even in principle. The multiverse is the most obvious example.

Question: are these proposals science? Or do they attempt to change the definition of what “science” is? Does the search for explanatory power trump testability?

The questions aren’t only relevant to the multiverse. We need to understand the dividing line between science and non-science to properly classify standard cosmology, inflation, natural selection, Intelligent Design, astrology, parapsychology. Which are science?

9:30: Joe Silk gives an introduction to the state of cosmology today. Just to remind us of where we really are, he concentrates on the data-driven parts of the field: dark matter, primordial nucleosynthesis, background radiation, large-scale structure, dark energy, etc.

Silk’s expertise is in galaxy formation, so he naturally spends a good amount of time on that. Theory and numerical simulations are gradually making progress on this tough problem. One outstanding puzzle: why are spiral galaxies so thin? Probably improved simulations will crack this before too long.

10:30: Andrei Linde talks about inflation and the multiverse. The story is laden with irony: inflation was invented to help explain why the universe looks uniform, but taking it seriously leads you to eternal inflation, in which space on extremely large (unobservable) scales is highly non-uniform — the multiverse. The mechanism underlying eternal inflation is just the same quantum fluctuations that give rise to the density fluctuations observed in large-scale structure and the microwave background. The fluctuations we see are small, but at earlier times (and therefore on larger scales) they could easily have been very large — large enough to give rise to different “pocket universes” with different local laws of physics.

Linde represents the strong pro-multiverse view: “An enormously large number of possible types of compactification which exist e.g. in the theory of superstrings should be considered a virtue.” He said that in 1986, and continues to believe it. String theorists were only forced to take all these compactifications seriously by the intervention of a surprising experimental result: the acceleration of the universe, which implied that there was no magic formula that set the vacuum energy exactly to zero. Combining the string theory landscape with eternal inflation gives life to the multiverse, which among other things offers an anthropic solution to the cosmological constant problem.

Still, there are issues, especially the measure problem: how do you compare different quantities when they’re all infinitely big? (E.g. number of different kinds of observers in the multiverse.) Linde doesn’t think any of the currently proposed measures are completely satisfactory, including the ones he’s invented. A big problem with Boltzmann brains.

Another problem is what we mean by “us,” when we’re trying to predict “what observers like us are likely to see.” Are we talking about carbon-based life, or information-processing computers? Help, philosophers!

Linde thinks that the multiverse shows tendencies, although not cut-or-dried predictions. It prefers a cosmological constant to quintessence, and increases the probability that axions rather than WIMPs are the dark matter. Findings to the contrary would be blows to the multiverse idea. Most strongly, without extreme fine-tuning, the multiverse would not be able to simultaneously explain large tensor modes in the CMB and low-energy supersymmetry.

12:00: Raphael Bousso talks about the multiverse in string theory. Note that “multiverse” isn’t really an accurate description; we’re talking about connected regions of space with different low-energy excitations, not some metaphysical collection of completely distinct universes. The multiverse is not a theory — need some specific underlying dynamics (e.g. string theory) to make any predictions. It’s those theories that are tested, not “the multiverse.” Predictions will be statistical, but that’s okay; everyone’s happy with statistical mechanics. “Even if you were pretty neurotic about it, you could only throw a die a finite number of times.” We do need to assume that we are in some sense typical observers.

The cosmological constant problem (why is the vacuum energy so small?) is an obvious candidate for anthropic explanation. String theory is unique at a deep-down level, but features jillions of possible compactifications down to four dimensions, each with different low-energy parameters. Challenges to making predictions: landscape statistics (how many of each kind of vacua?), cosmological dynamics (how does the universe evolve?), measure problem (how do we count observers?). Each is hard!

For the cosmological constant, the distribution of values within the string landscape is actually relatively understandable: there is basically a uniform distribution of possible vacuum energies between minus the Planck scale and plus the Planck scale. Make the right vacua via eternal inflation, which populates the landscape. Our universe decayed from a previous vacuum, an event that must release enough energy to produce the hot Big Bang. That’s a beneficial feature of the multi-dimensional string landscape: “nearby” vacua can have enormously different vacuum energies.

The measure problem is trickier. In the multiverse, any interesting phenomenon happens an infinite number of times. Need some sort of way to regularize these infinities. A problem for eternal inflation; not only for the string landscape. Bousso’s favorite solution is the causal patch measure, which only counts events that happen in the past light cone of any particular event, not throughout a spacelike surface. In that measure, most observers see a cosmological constant comparable to the age of the universe they observe — that’s compatible with what we see, and directly solves the coincidence problem.

Sunday afternoon: Philosophers’ turn

2:00: John Norton talks about the “Bayesian failure” of cosmology and inductive inference. (He admits off the bat that it’s kind of terrifying to have all these cosmologists in the audience.) Basic idea: the Bayesian analysis that cosmologists use all the time is not the right tool. Instead, we should be using “fragments of inductive logics.”

The “Surprising Analysis”: assuming that prior theory is neutral with respect to some feature (e.g. the value of the cosmological constant), we observe a surprising value, and then try to construct a framework to explain it (e.g. the multiverse). This fits in well with standard Bayesian ideas. But that should worry you! What is really the prior probability for observing some quantity? In particular, what if our current theory were not true — would we still be surprised?

We shouldn’t blithely assume that the logic of physical chances (probabilities) is the logic of all analysis. The problem is that this framework has trouble dealing with “neutral evidence” — almost everything is taken as either favoring or disfavoring the hypothesis. We should be talking about whether or not a piece of evidence qualifies as support, not simply calculating probabilities.

The disaster that befell Bayesianism was to cast it in terms of subjective degrees of belief, rather than support. A prior probability distribution is pure opinion. But your choice of that prior can dramatically effect how we interpret particular pieces of evidence.

Example: the Doomsday argument — if we are typical, the universe (or the human race, etc.) will probably not last considerably longer than it already has (or we wouldn’t be typical). All the work in that argument comes from assuming that observers are sampled uniformly. But the fact that 60 billion people have lived so far isn’t really evidence that 100 trillion people won’t eventually live; it’s simply neutral.

Heretical punchline: cosmic parameters can’t be judged as “improbable,” so long as they’re consistent with theory and observation.

[David Wallace, during questions: Do you really mean to say that if we observed the stars in the sky spelling out the message "Oxford is better than Cambridge," all we could say is "Well, it's consistent with the laws of physics, so we can't really conclude anything from that"?]

2:45: Simon Saunders talks about probability and anthropic reasoning in a multiverse. Similar issues as the last talk, but he’ll be defending a Bayesian analysis.

Sometimes we think of probability objectively — a true physical structure — and sometimes subjectively — a reflection of the credence we give to some claim.

Problems with anthropic arguments involve: linguistics (what is included in “observer” and “observed”?), theory (how do we calculate the probability of finding certain evidence given a particular theory?), and realism (why worry about what is observed?). On the latter point: do we conditionalize on the existence of human life, or the existence of some observers, or simply on the existence of conditions compatible with observers? Saunders argues for the latter: all we care about are physical conditions, not whether or not observers come into existence. Call this “taming” the anthropic principle.

Aside on vacuum energy: are we really sure it’s finely-tuned? Condensed-matter analogues give a different set of expectations — maybe the vacuum energy just adjusts to zero after phase transitions.

For branches of the wavefunction, there exist formal axioms (e.g. Deutsch-Wallace) for evaluating the preferences of a rational observer which recover the conventional understanding of Copenhagen probabilities even in a many-worlds interpretation. For a classical multiverse, the argument is remarkably similar; for a fully quantum inflationary multiverse, it’s less clear.

4:00: Panel discussion with Alex Vilenkin, Wayne Myrvold, and Christopher Smeenk. It’s a series of short talks more than an actual discussion. Vilenkin goes first, and discusses — wait for it — calculating probabilities in the multiverse. The technical manifestation of the assumption that we are typical observers is the self-sampling assumption: assume we are chosen randomly from within some reference class of observers. The probability that we observe something is just the fraction of observers within this class that observe it. But how do we choose the class? Vilenkin argues that we can choose the set of all observers with identical information content. (Really all information: not just “I am human” but also “My name is Alex,” etc.) That sounds like a very narrow class, but in a really big multiverse there will be an infinite number of members of each such class. (This doesn’t solve the measure problem — still need to regulate those infinities.) In fact we should use the Principle of Mediocrity: assume we are typical in any class to which we belong, unless there is evidence to the contrary.

Myrvold is next up, and he chooses to respond mostly to John Norton’s talk. Most of the time, when we’re evaluating theories in light of evidence, we don’t need to be too fancy about our background analytical framework. The multiverse seems like an exception. More generally, theories with statistical predictions are tricky to evaluate. If you toss a coin 1000 times, any particular outcome is highly improbable. You have to choose some statistics ahead of time, e.g. the fraction of heads. Cosmological parameters might be an example of where we don’t know how to put sensible prior probabilities on different outcomes.

Smeenk wants to talk about how big a problem “fine-tuning” really is. Sometimes more than others: when some parameter (e.g. the vacuum energy) is not just chosen from a hat, but gets various contributions about which we have sensible expectations, it’s giving up on too much to simply take any observed value as neutral with respect to theory evaluation. He’s reacting against Norton’s prescription a bit. Nevertheless, we should admit that choosing measures/probability distributions in cosmology is a very different game than what we do in statistical mechanics, if only because we don’t actually have more than one member of the ensemble in front of us.

Sunday evening: “Ultimate Explanation”

After dinner we reconvene for a talk and a response. The talk is by Timothy O’Connor on Ultimate Explanation: Reforging Natural Philosophy.” He reminds us that Newton insisted that he did not “feign hypotheses” — he concentrated on models that he claimed were deduced from the phenomena, and thought that any deeper hypothetical explanations had “no place in experimental philosophy.” The implication being that Newton would not have approved of the multiverse.

O’Connor says that a fully complete, “ultimate” explanation cannot possibly be attained through science. Nevertheless, it’s a perfectly respectable goal, as part of the larger project of natural philosophy.

He defines an “ultimate explanation” as something that involves no brute givens — “such that one could not intelligibly ask for anything more.” That’s not attainable by science. If nothing else, “the most fundamental fact of existence itself” will remain unexplained, even if we knew the theory of everything and the wave function of the universe. Alternatively, if we imagine “plenitude” — everything possible exists — it would still be possible to imagine something less than that, so a contingent explanation is still required.

We are led to step outside science and consider the idea of an ultimately necessary being — something whose existence is no more optional than that of mathematical or logical truths. We could endorse such an idea if it provided explanations without generating insoluble puzzles of its own, and if we thought we had considered an exhaustive list of alternatives, all of which fell short. Spinoza and Leibniz are invoked.

Note the peculiar logic: if a necessary being does not exist, it was not simply an optional choice; it must necessarily not exist. (Because if a necessary being is conceivable, it must necessarily exist. Get it?)

Punchline: Science is independent of any/most metaphysical claim. But that means it can’t possibly “explain” everything; there must be metaphysical principles/assumptions. Some of these might be part of the ultimate explanation of the actual world in which we live.

The response comes from Sir Martin Rees. He opens by quoting John Polkinghorne: “Your average quantum mechanic is no more philosophical than your average motor mechanic.” But maybe cosmologists are a bit more sympathetic. He then recalls that Dennis Sciama — who was the thesis advisor of George Ellis, Stephen Hawking, and Rees himself — was committed as a young scientist to the Steady State model of cosmology, primarily for philosophical reasons. He did give up on it when confronted with data from the microwave background, but it was an anguished abandonment.

Searching for “explanations,” we should recognize that different fields of science have autonomous explanatory frameworks. People who study fluid mechanics would like to understand turbulence, and they don’t need to appeal to the existence of atoms to do so — atoms do exist, and one can derive the equations of fluid mechanics from them, but their existence sheds no light whatsoever on the phenomenon of turbulence.

Note also that, even if there is an ultimate explanation in the theory-of-everything sense, it may simply be too difficult for our limited human minds to understand. “Many of these problems may have to await the post-human era for their solution.”

Continued at Day Two.

Some people would prefer to define “religion” so that religious beliefs entail nothing whatsoever about what happens in the world. And that’s fine; definitions are not correct or incorrect, they are simply useful or useless, where usefulness is judged by the clarity of one’s attempts at communication. Personally, I think using “religion” in that way is not very clear. Most Christians would disagree with the claim that Jesus came about because Joseph and Mary had sex and his sperm fertilized her ovum and things proceeded conventionally from there, or that Jesus didn’t really rise from the dead, or that God did not create the universe. The Congregation for the Causes of Saints, whose job it is to judge whether a candidate for canonization has really performed the required number of miracles and so forth, would probably not agree that miracles don’t occur. Francis Collins, recently nominated to direct the NIH, argues that some sort of God hypothesis helps explain the values of the fundamental constants of nature, just like a good Grand Unified Theory would. These views are by no means outliers, even without delving into the more extreme varieties of Biblical literalism.

Furthermore, if a religious person really did believe that nothing ever happened in the world that couldn’t be perfectly well explained by ordinary non-religious means, I would think they would expend their argument-energy engaging with the many millions of people who believe that the virgin birth and the resurrection and the promise of an eternal afterlife and the efficacy of intercessory prayer are all actually literally true, rather than with a handful of atheist bloggers with whom they agree about everything that happens in the world. But it’s a free country, and people are welcome to define words as they like, and argue with whom they wish.

But there was also a more interesting and substantive issue lurking below the surface. I focused in that post on the meaning of “religion,” but did allude to the fact that defenders of Non-Overlapping Magisteria often misrepresent “science” as well. And this, I think, is not just a matter of definitions: we can more or less agree on what “science” means, and still disagree on what questions it has the power to answer. So that’s an issue worth examining more carefully: what does science actually have the power to do?

I can think of one popular but very bad strategy for answering this question: first, attempt to distill the essence of “science” down to some punchy motto, and then ask what questions fall under the purview of that motto. At various points throughout history, popular mottos of choice might have been “the Baconian scientific method” or “logical positivism” or “Popperian falsificationism” or “methodological naturalism.” But this tactic always leads to trouble. Science is a messy human endeavor, notoriously hard to boil down to cut-and-dried procedures. A much better strategy, I think, is to consider specific examples, figure out what kinds of questions science can reasonably address, and compare those to the questions in which we’re interested.

Here is my favorite example question. Alpha Centauri A is a G-type star a little over four light years away. Now pick some very particular moment one billion years ago, and zoom in to the precise center of the star. Protons and electrons are colliding with each other all the time. Consider the collision of two electrons nearest to that exact time and that precise point in space. Now let’s ask: was momentum conserved in that collision? Or, to make it slightly more empirical, was the magnitude of the total momentum after the collision within one percent of the magnitude of the total momentum before the collision?

This isn’t supposed to be a trick question; I don’t have any special knowledge or theories about the interior of Alpha Centauri that you don’t have. The scientific answer to this question is: of course, the momentum was conserved. Conservation of momentum is a principle of science that has been tested to very high accuracy by all sorts of experiments, we have every reason to believe it held true in that particular collision, and absolutely no reason to doubt it; therefore, it’s perfectly reasonable to say that momentum was conserved.

A stickler might argue, well, you shouldn’t be so sure. You didn’t observe that particular event, after all, and more importantly there’s no conceivable way that you could collect data at the present time that would answer the question one way or the other. Science is an empirical endeavor, and should remain silent about things for which no empirical adjudication is possible.

But that’s completely crazy. That’s not how science works. Of course we can say that momentum was conserved. Indeed, if anyone were to take the logic of the previous paragraph seriously, science would be a completely worthless endeavor, because we could never make any statements about the future. Predictions would be impossible, because they haven’t happened yet, so we don’t have any data about them, so science would have to be silent.

All that is completely mixed-up, because science does not proceed phenomenon by phenomenon. Science constructs theories, and then compares them to empirically-collected data, and decides which theories provide better fits to the data. The definition of “better” is notoriously slippery in this case, but one thing is clear: if two theories make the same kinds of predictions for observable phenomena, but one is much simpler, we’re always going to prefer the simpler one. The definition of theory is also occasionally troublesome, but the humble language shouldn’t obscure the potential reach of the idea: whether we call them theories, models, hypotheses, or what have you, science passes judgment on ideas about how the world works.

And that’s the crucial point. Science doesn’t do a bunch of experiments concerning colliding objects, and say “momentum was conserved in that collision, and in that one, and in that one,” and stop there. It does those experiments, and then it also proposes frameworks for understanding how the world works, and then it compares those theoretical frameworks to that experimental data, and — if the data and theories seem good enough — passes judgment. The judgments are necessarily tentative — one should always be open to the possibility of better theories or surprising new data — but are no less useful for that.

Furthermore, these theoretical frameworks come along with appropriate domains of validity, depending both on the kinds of experimental data we have available and on the theoretical framework itself. At the low energies available to us in laboratory experiments, we are very confident that baryon number (the total number of quarks minus antiquarks) is conserved in every collision. But we don’t necessarily extend that to arbitrarily high energies, because it’s easy to think of perfectly sensible extensions of our current theoretical understanding in which baryon number might very well be violated — indeed, it’s extremely likely, since there are a lot more quarks than antiquarks in the observable universe. In contrast, we believe with high confidence that electric charge is conserved at arbitrarily high energies. That’s because the theoretical underpinnings of charge conservation are a lot more robust and inflexible than those of baryon-number conservation. A good theoretical framework can be extremely unforgiving and have tremendous scope, even if we’ve only tested it over a blink of cosmic time here on our tiny speck of a planet.

The same logic applies, for example, to the highly contentious case of the multiverse. The multiverse isn’t, by itself, a theory; it’s a prediction of a certain class of theories. If the idea were simply “Hey, we don’t know what happens outside our observable universe, so maybe all sorts of crazy things happen,” it would be laughably uninteresting. By scientific standards, it would fall woefully short. But the point is that various theoretical attempts to explain phenomena that we directly observe right in front of us — like gravity, and quantum field theory — lead us to predict that our universe should be one of many, and subsequently suggest that we take that situation seriously when we talk about the “naturalness” of various features of our local environment. The point, at the moment, is not whether there really is or is not a multiverse; it’s that the way we think about it and reach conclusions about its plausibility is through exactly the same kind of scientific reasoning we’ve been using for a long time now. Science doesn’t pass judgment on phenomena; it passes judgment on theories.

The reason why we can be confident that momentum was conserved during that particular collision a billion years ago is that science has concluded (beyond reasonable doubt, although not with metaphysical certitude) that the best framework for understanding the world is one in which momentum is conserved in all collisions. It’s certainly possible that this particular collision was an exception; but a framework in which that were true would necessarily be more complicated, without providing any better explanation for the data we do have. We’re comparing two theories: one in which momentum is always conserved, and one in which it occasionally isn’t, including a billion years ago at the center of Alpha Centauri. Science is well equipped to carry out this comparison, and the first theory wins hands-down.

Now let’s turn to a closely analogous question. There is some historical evidence that, about two thousand years ago in Galilee, a person named Jesus was born to a woman named Mary, and later grew up to be a messianic leader and was eventually crucified by the Romans. (Unruly bloke, by the way — tended to be pretty doctrinaire about the number of paths to salvation, and prone to throwing moneychangers out of temples. Not very “accommodating,” if you will.) The question is: how did Mary get pregnant?

One approach would be to say: we just don’t know. We weren’t there, don’t have any reliable data, etc. Should just be quiet.

The scientific approach is very different. We have two theories. One theory is that Mary was a virgin; she had never had sex before becoming pregnant, or encountered sperm in any way. Her pregnancy was a miraculous event, carried out through the intervention of the Holy Ghost, a spiritual manifestation of a triune God. The other theory is that Mary got pregnant through relatively conventional channels, with the help of (one presumes) her husband. According to this theory, claims to the contrary in early (although not contemporary) literature are, simply, erroneous.

There’s no question that these two theories can be judged scientifically. One is conceptually very simple; all it requires is that some ancient texts be mistaken, which we know happens all the time, even with texts that are considerably less ancient and considerably better corroborated. The other is conceptually horrible; it posits an isolated and unpredictable deviation from otherwise universal rules, and invokes a set of vaguely-defined spiritual categories along the way. By all of the standards that scientists have used for hundreds of years, the answer is clear: the sex-and-lies theory is enormously more compelling than the virgin-birth theory.

The same thing is true for various other sorts of miraculous events, or claims for the immortality of the soul, or a divine hand in guiding the evolution of the universe and/or life. These phenomena only make sense within a certain broad framework for understanding how the world works. And that framework can be judged against others in which there are no miracles etc. And, without fail, the scientific judgment comes down in favor of a strictly non-miraculous, non-supernatural view of the universe.

That’s what’s really meant by my claim that science and religion are incompatible. I was referring to the Congregation-for-the-Causes-of-the-Saints interpretation of religion, which entails a variety of claims about things that actually happen in the world; not the it’s-all-in-our-hearts interpretation, where religion makes no such claims. (I have no interest in arguing at this point in time over which interpretation is “right.”) When religion, or anything else, makes claims about things that happen in the world, those claims can in principle be judged by the methods of science. That’s all.

Well, of course, there is one more thing: the judgment has been made, and views that step outside the boundaries of strictly natural explanation come up short. By “natural” I simply mean the view in which everything that happens can be explained in terms of a physical world obeying unambiguous rules, never disturbed by whimsical supernatural interventions from outside nature itself. The preference for a natural explanation is not an a priori assumption made by science; it’s a conclusion of the scientific method. We know enough about the workings of the world to compare two competing big-picture theoretical frameworks: a purely naturalistic one, versus one that incorporates some sort of supernatural component. To explain what we actually see, there’s no question that the naturalistic approach is simply a more compelling fit to the observations.

Could science, through its strategy of judging hypotheses on the basis of comparison with empirical data, ever move beyond naturalism to conclude that some sort of supernatural influence was a necessary feature of explaining what happens in the world? Sure; why not? If supernatural phenomena really did exist, and really did influence things that happened in the world, science would do its best to figure that out.

It’s true that, given the current state of data and scientific theorizing, the vast preponderance of evidence comes down in favor of understanding the world on purely natural terms. But that’s not to say that the situation could not, at least in principle, change. Science adapts to reality, however it presents itself. At the dawn of the 20th century, it would have been hard to find a more firmly accepted pillar of physics than the principle of determinism: the future can, in principle, be predicted from the present state. The experiments that led us to invent quantum mechanics changed all that. Moving from a theory in which the present uniquely determines the future to one where predictions are necessarily probabilistic in nature is an incredible seismic shift in our deep picture of reality. But science made the switch with impressive rapidity, because that’s what the data demanded. Some stubborn folk tried to recover determinism at a deeper level by inventing more clever theories — which is exactly what they should have done. But (to make a complicated story simple) they didn’t succeed, and scientists learned to deal.

It’s not hard to imagine a similar hypothetical scenario playing itself out for the case of supernatural influences. Scientists do experiments that reveal anomalies that can’t be explained by current theories. (These could be subtle things at a microscopic level, or relatively blatant manifestations of angels with wings and flaming swords.) They struggle to come up with new theories that fit the data within the reigning naturalist paradigm, but they don’t succeed. Eventually, they agree that the most compelling and economical theory is one with two parts: a natural part, based on unyielding rules, with a certain well-defined range of applicability, and a supernatural one, for which no rules can be found.

Of course, that phase of understanding might be a temporary one, depending on the future progress of theory and experiment. That’s perfectly okay; scientific understanding is necessarily tentative. In the mid-19th century, before belief in atoms had caught on among physicists, the laws of thermodynamics were thought to be separate, autonomous rules, in addition to the crisp Newtonian laws governing particles. Eventually, through Maxwell and Boltzmann and the other pioneers of kinetic theory, we learned better, and figured out how thermodynamic behavior could be subsumed into the Newtonian paradigm through statistical mechanics. One of the nice things about science is that it’s hard to predict its future course. Likewise, the need for a supernatural component in the best scientific understanding of the universe might evaporate — or it might not. Science doesn’t assume things from the start; it tries to deal with reality as it presents itself, however that may be.

This is where talk of “methodological naturalism” goes astray. Paul Kurtz defines it as the idea that “all hypotheses and events are to be explained and tested by reference to natural causes and events.” That “explained and tested” is an innocent-looking mistake. Science tests things empirically, which is to say by reference to observable events; but it doesn’t have to explain things as by reference to natural causes and events. Science explains what it sees the best way it can — why would it do otherwise? The important thing is to account for the data in the simplest and most useful way possible.

There’s no obstacle in principle to imagining that the normal progress of science could one day conclude that the invocation of a supernatural component was the best way of understanding the universe. Indeed, this scenario is basically the hope of most proponents of Intelligent Design. The point is not that this couldn’t possibly happen — it’s that it hasn’t happened in our actual world. In the real world, by far the most compelling theoretical framework consistent with the data is one in which everything that happens is perfectly accounted for by natural phenomena. No virgin human births, no coming back after being dead for three days, no afterlife in Heaven, no supernatural tinkering with the course of evolution. You can define “religion” however you like, but you can’t deny the power of science to reach far-reaching conclusions about how reality works.

The withdrawal of philosophy into a “professional” shell of its own has had disastrous consequences. The younger generation of physicists, the Feynmans, the Schwingers, etc., may be very bright; they may be more intelligent than their predecessors, than Bohr, Einstein, Schrodinger, Boltzmann, Mach and so on. But they are uncivilized savages, they lack in philosophical depth — and this is the fault of the very same idea of professionalism which you are now defending.

It’s probably true that the post-WWII generations of leading physicists were less broadly educated than their pre-war counterparts (although there are certainly counterexamples such as Murray Gell-Mann and Steven Weinberg). The simplest explanation for this phenomenon would be that the center of gravity of scientific research switched from Europe to America after the war, and the value of a broad-based education (and philosophy in particular) has always been less in America. Interestingly, Feyerabend seems to be blaming philosophers themselves — “the withdrawal of philosophy into a `professional’ shell” — rather than physicists or any wider geosocial trends.

But aside from whether modern physicists (and maybe scientists in other fields, I don’t know) pay less attention to philosophy these days, and aside from why that might be the case, there is still the question: does it matter? Would knowing more philosophy have made any of the post-WWII giants better physicists? There are certainly historical counterexamples one could conjure up: the acceptance of atomic theory in the German-speaking world in the late nineteenth century was held back considerably by Ernst Mach’s philosophical arguments. On the other hand, Einstein and Bohr and their contemporaries did manage to do some revolutionary things; relativity and quantum mechanics were more earth-shattering than anything that has come since in physics.

The usual explanation is that the revolutionary breakthroughs simply haven’t been there to be made — that Feynman and Schwinger and friends missed the glory days when quantum mechanics was being invented, so it was left to them to move the existing paradigm forward, not to come up with something revolutionary and new. Maybe, had these folks been more conversant with their Hume and Kant and Wittgenstein, we would have quantum gravity figured out by now.

Probably not. Philosophical presuppositions certainly play an important role in how scientists work, and it’s possible that a slightly more sophisticated set of presuppositions could give the working physicist a helping hand here and there. But based on thinking about the actual history, I don’t see how such sophistication could really have moved things forward. (And please don’t say, “If only scientists were more philosophically sophisticated, they would see that my point of view has been right all along!”) I tend to think that knowing something about philosophy — or for that matter literature or music or history — will make someone a more interesting person, but not necessarily a better physicist.

This might not be right, though. Maybe, had they been more broad and less technical, some of the great physicists of the last few decades would have made dramatic breakthroughs in a field like quantum information or complexity theory, rather than pushing harder at the narrow concerns of particle physics or condensed matter. Easy to speculate, hard to provide much compelling evidence either way.

On those rare occasions when they attempt to actually talk to each other, people on opposite sides of the abortion debate usually end up talking past each other. Supporters of abortion rights speak in the language of the autonomy of the mother, and her right to control her own body: “If you don’t like abortion, don’t have one.” Opponents of abortion speak in terms of the personhood of the fetus. (Yes, Dr. Seuss’s Horton Hears a Who! — “A person’s a person, no matter how small” — is used to teach this point to Catholic children, over Theodor Geisel’s objections.) Opposition to abortion rights can also be a manifestation of the desire to control women’s sexuality, but let’s concentrate on those whose opposition is grounded in a sincere moral belief that abortion is murder.

If someone believes that abortion really is murder, talk of the reproductive freedom of the mother isn’t going to carry much weight — nobody has the right to murder another person. Supporters of abortion rights don’t say “No, this is one case where murder is completely justified.” Rather, they say “No, the fetus is not a person, so abortion is not murder.” The crucial question (I know, this is not exactly an astonishing new insight) is whether a fetus is really a person.

I have nothing original to add to the debate over when “personhood” begins. But there is something to say about how we decide questions like that. And it takes us directly back to the previous discussion about marriage and fundamental physics. The upshot of which is: how you think about the universe, how you conceptualize the natural world around us, obviously is going to have an enormous impact on how you decide questions like “When does personhood begin?”

In a pre-scientific world, life was — quite understandably — thought of as something intrinsically different from non-life. This view could be taken to different extremes; Plato gave voice to one popular tradition, by claiming that the human soul was a distinct, incorporeal entity that actually occupied a human body. These days we know a lot more than they did back then. Science has taught us that living beings and non-living objects are the same kind of things, deep down; we’re all made of the same chemical elements, and all of our constituents obey the same laws of Nature. Life is complicated, and rich, and fascinating, and not very well understood — but it doesn’t obey separate rules apart from those of the non-living world. Living organisms are just very complicated chemical reactions, not vessels that rely on supernatural essences or mystical élan vital to keep them chugging along. Except “just” is a terribly misleading adverb in this context — living organisms are truly amazing very complicated chemical reactions. Knowing that we are made of the same stuff and obey the same rules as the rest of the universe doesn’t diminish the value or meaning of human life in any way.

There is a temptation in some quarters to forget, or at least ignore, the improved understanding of the world that science has given us when it comes to address moral and ethical questions. Part of that is a healthy impulse — science doesn’t actually tell us how to distinguish right from wrong, nor could it possibly. Science deals with how the world works, not how it should work, and despite centuries of trying it remains impossible to derive “ought” from “is.”

But at the same time, it would be crazy not to take our scientific understanding of the world into consideration when we reflect upon moral questions. If you think of a fetus as part of an ongoing complicated chemical reaction, it should come as no surprise that you might reach very different conclusions from someone who thinks that God breathes the spirit of life into a fertilized ovum at the moment of conception.

That’s why it’s equally crazy to believe that science and religion are two distinct, non-overlapping magisteria that simply never address the same questions. That bizarre perspective was advanced by Stephen Jay Gould in Rocks of Ages, but if you read the book carefully you find that his definition of “religion” is simply “moral philosophy.” Which is not what the word means, or how people use it, or how actual religious people think of their beliefs. Religion makes claims about the real world, and some of those claims — not all — can be very straightforwardly judged by the criteria of science. We do not need to invoke spirits being breathed into fertilized eggs in order to understand life, for example. And the fact that science has taught us so much about the workings of the world has enormous consequences for how we should think about moral and ethical questions, even if it can’t answer such questions all by itself.

For example, science is powerless to tell us when “personhood” begins — but it tell us something very crucial about how to go about answering that question. In particular, it tells us that there is no magical moment at which an incorporeal soul takes up residence in a body. Indeed, the concept of a “person” is not to be found anywhere in the natural world; it’s a category that is convenient to appeal to as we try to make sense of the world. But there is not, as far as science is concerned, any right or wrong answer to the question of when the life of a person begins — from Nature’s point of view, it’s just one chemical reaction after another.

At this point, a lot of impatient people declare that morality and ethics are simply impossible in such a world, and storm out in frustration. But this is the world in which we actually live, so storming out is not a productive response. Morality and ethics are possible, but they’re not to be found in Natural Law — they are the creation of human beings, reasoning together on the basis of their shared feelings and experiences. Human beings are not blank slates, nor are they immutable tablets; we are born into the world with certain wants and desires and natural reactions to events, and those feelings can adapt and change over time in response to learning and reasoning. So we get together, communicate, understand that not everyone necessarily agrees on how the social world should be organized, and try to negotiate some sort of mutual compromise. (Or, alternatively, try to impose our will by force. But I like the mutual compromise approach better.) That’s how the world actually works.

“The moment when a fetus begins to accrue the rights we bestow on post-birth persons” is something that we, as a society, have to decide; the answer is not to be found in revelation, or in faith, or in philosophical contemplation of the nature of the soul, or for that matter in the natural world. This starting point is not necessarily prejudicial to what the final answer may be; I can certainly imagine a group of people coming together and agreeing that newly-conceived fetuses should be granted all the rights of any person. I would argue against them, on the basis that the interests of an autonomous and fully conscious mother should weigh much more heavily than those of the proto-person they carry. But I can’t say that they are unambiguously wrong in the same way that an erroneous claim about logic or even the empirical world can be said to be “wrong.”

If the social and political arrangement of a group puts stress on the autonomy of its individual responsible members (which ours does, and I like it that way), deciding what the criteria are for being judged an “individual responsible member” is of primary importance. Who gets to vote? Who gets to drive a car? Who decides when to unplug the respirator? Who is of “sound mind”? Who is a person? These are all hard questions with no cut-and-dried answers. But we can be fooled into thinking that some of the answers are pretty straightforward, if we believe in outdated notions of spirits being breathed into us by God.

There are many reasons why it’s incoherent to think of science and religion as simply separate and non-overlapping. They are different, but certainly overlapping. The greatest intellectual accomplishment of the last millennium is the naturalistic worldview: everything is constructed of the same basic building blocks, obeying the same rules, without any recourse to the supernatural. Appreciating that view doesn’t tell us how we should behave, but failing to appreciate it can very easily lead people to behave badly.

The opposition research has been out for a while, of course, because that’s how politics works. One of the things brought up by Sotomayor’s critics is this clip, where she talks about the difference in emphasis between a district court and an appellate court. (Appellate courts need to look beyond the facts of the case to consider implications of setting precedent for future decisions.)

This clip drives people crazy, because she says that the courts of appeals are “where policy is made.” You’re not supposed to say that! (As Sotomayor immediately jokes.) The legislatures make the laws, and the courts are merely referees, interpreting the words of the statutes by lights of their objective and unchanging meanings.

In reality, of course, Sotomayor is simply telling the truth — a cardinal sin in law as well as politics. In law and politics, and for that matter theology, we are presented with a sacred text of one form or another. And we are supposed to pretend that the text has a One True Meaning — we may, of course, argue at great length about the proper procedure for divining what that meaning actually is, but admitting that the text is inherently ambiguous (or even contradictory) is not allowed. We need to act as if the authors of Leviticus and the Framers of the Constitution were trying to say something very clear about contemporary debates, if only we had the interpretational acumen to figure out what it was.

Which is why, as much as I enjoy the rest of the world of human endeavor, science will always be my true home. Our job is to interpret the natural world, which really is unambiguous and non-contradictory, if only we can make sense of its behavior. Other fields have a professional obligation to pretend that there are right and wrong answers, but we actually have them. Yet another way in which being a scientist is so much easier than other jobs.

Nevertheless, as one gets older, reads more, and hopefully gains a more sophisticated knowledge of the subject, one’s tastes tend to change somewhat. In my case, a gradual shift in my interests from pure to applied mathematics, and finally to theoretical physics opened up an increasing range of giants to understand and respect. Somehow though, I have always retained a soft spot for Russell; perhaps because of his atheism, perhaps because of his breadth, but more so I think, because when I think of him I can quite viscerally recall the way reading about him made me feel about mathematics.

Because of this, while I have never become an avid reader of graphic novels, I’m hoping to get hold of a copy of Apostolos Doxiadis’ Logicomix, which I learned about via The Guardian, and which the web site describes as

Covering a span of sixty years, the graphic novel Logicomix was inspired by the epic story of the quest for the Foundations of Mathematics.

This was a heroic intellectual adventure most of whose protagonists paid the price of knowledge with extreme personal suffering and even insanity. The book tells its tale in an engaging way, at the same time complex and accessible. It grounds the philosophical struggles on the undercurrent of personal emotional turmoil, as well as the momentous historical events and ideological battles which gave rise to them.

The role of narrator is given to the most eloquent and spirited of the story’s protagonists, the great logician, philosopher and pacifist Bertrand Russell. It is through his eyes that the plights of such great thinkers as Frege, Hilbert, Poincaré, Wittgenstein and Gödel come to life, and through his own passionate involvement in the quest that the various narrative strands come together.

The web site contains a few samples of what to expect, plus a nice summary of the cast of characters. To a graphic novel newbie like myself, it isn’t obvious what to expect from a telling of this kind of sweeping academic story in such a format. But the team involved looks promising, and I’m sufficiently fascinated by the subject matter that I’m really looking forward to getting a look at Logicomix.

What if, some day or night, a demon were to steal after you in your loneliest loneliness and say to you: “This life as you now live it and have lived it, you will have to live once more and innumerable times more; and there will be nothing new in it, but every pain and every joy and every thought and sigh and everything unutterably small or great in your life will have to return to you, all in the same succession and sequence—even this spider and this moonlight between the trees, and even this moment and I myself. The eternal hourglass of existence is turned upside down again and again—and you with it, speck of dust!”

This is the kind of thing you come across when you’re writing a book about time. Nietzsche wanted to suggest that a well-lived life was one you wouldn’t mind knowing would recur throughout eternity, while the prospect would cause gnashing of teeth for most of us. Poincaré’s concerns were somewhat different.

While looking up this passage, I stumbled across one of my favorite Nietzsche quotes, just a few aphorisms prior:

Yes, my friends, regarding all the moral chatter of some about others it is time to feel nauseous! Sitting in moral judgment should offend our taste! Let us leave such chatter and such bad taste to those who have nothing else to do but drag the past a few steps further through time and who never live in the present,—which is to say the many, the great majority! We, however, want to become who we are,—the new, unique, incomparable ones, who give themselves their own laws, who create themselves! And to that end we must become the best learners and discoverers of everything that is lawful and necessary in the world: we must become physicists in order to be able to be creators in this sense,—while hitherto all valuations and ideals have been based on ignorance of physics or were constructed so as to contradict it. Therefore: long live physics! And even more so that which compels us to turn to physics,—our honesty!

A quote which engenders, as you might imagine, swift elaborations on the part of Nietzsche scholars that he certainly wasn’t talking about what we ordinarily mean by “physics.” But I’m not so sure. The substance of physics (experimental results, theoretical understandings) is of no help whatsoever in leading a moral life. But the method of physics — open-minded hypothesis testing and scrupulous honesty in confronting what Nature has to tell us — is a pretty good model for other aspects of our lives.

Not that physicists are, as a matter of empirical fact, any better at being good human beings on average than anyone else. Even we physicists could learn to be better physicists.

Žižek says “Nature is a big series of unimaginable catastrophes.” I think he meant “the blogosphere,” not “Nature.”

Do I really want to see this for scientists? They might not make the same impression on film — scientists aren’t trained to connect what they do to the concerns of the wider world (although the connections are there).