Bob Kurland is a retired, cranky, old physicist, and convert to Catholicism. He shows that there is no contradiction between what science tells us about the world and our Catholic faith.
One question in science is not “is this hypothetical model true” but “is this model better than the alternatives”…If we believe dogmatically in a particular view, then no amount of contradictory data will convince us otherwise… —John Skilling, “Foundations and Algorithms” in Bayesian Methods in Cosmology.
Ellis gives as an important criteria for a scientific theory that it be empirically testable. My position may be even stronger than that of Ellis: if a theory cannot be confirmed by quantitative measurements then it is not in my view (and that of Fr. Stanley Jaki), science, but something else—mathematical metaphysics?
- What can be confirmed by measurement is limited by the time, distance and physics horizons mentioned in the first post.
Using electromagnetic radiation we cannot see further back in time than when radiation decoupled from matter, about 380,000 years after the origin.
- We cannot see further in space than given by the distance horizon, the distance at which space will be expanding at faster than the speed of light.
- We cannot duplicate the tremendous energies present in the initial, quantum stages of the beginning of the universe (these energies are orders of magnitude greater than even the huge energies that will be available in the SLAC Hadron supper collider), so we cannot test projected theories of particle creation.
What can be measured are inferred consequences of various theories: what the cosmic background radiation (CBR) shows about homogeneity, isotropy, fluctuations, the cosmological constant (lambda, representing expansion pressure), etc. Recent examples are the report of Gurzadyan and Penrose of rings in the CBR representing cataclysmic events pre-Big Bang and B-mode measurements of the CBR from which are inferred gravitational waves in the early universe and thus inflation. One may disagree with the aspects of the theory, but the tie-in with measured data is commendable.
Ellis gives a series of theses for his position. The theses in Issue F, “The explicit philosophic basis”, are presented in detail. As a preliminary and review, here are Ellis’s theses pertinent to the science of cosmology.
- THESIS A1: The universe itself cannot be subjected to physical experimentation. We cannot re-run the universe with the same or altered conditions to see what would happen if they were different , so we cannot carry out scientific experiments on the universe itself.
- THESIS A2: The universe cannot be observationally compared with other universes. We cannot compare the universe with any similar object, nor can we test our hypotheses about it by observations determining statistical properties of a known class of physically existing universes.
- THESIS B3: Establishing a Robertson-Walker geometry for the universe relies on plausible philosophic assumptions. The deduction of spatial homogeneity follows not directly from astronomical data but because we add to the observations a philosophical principle that is plausible but untestable.
In Thesis B3, Ellis refers to the notion that the universe is isotropic and homogeneous (on a large scale). From our vantage point, we can see that the CBR (cosmic background radiation) yields this result; but to show that the inference is valid for the universe as a whole, we would need to make the same observation from at least two other (far removed) vantage points. However, if the Copernican Principle is invoked that we do not occupy a special place in the universe (this is the philosophic principle Ellis refers to in Thesis B3), then what see is equivalent to what would be seen from other positions, and the homogeneity and isotropy is demonstrated.
- THESIS B6: Observational horizons limit our ability to observationally determine the very large scale geometry of the universe. We can only see back to the time of decoupling of matter and radiation and so have no direct information about earlier times; and unless we live in a ‘small universe’, most of the matter in the universe is hidden behind the visual horizon. Conjectures as to its geometry on larger scales cannot be observationally tested. The situation is completely different in the small universe case: then we can see everything there is in the universe, including our own galaxy at earlier times! (emphasis and exclamation point added)
- THESIS C1: The Physics Horizon limits our knowledge of physics relevant to the very early universe. We cannot experimentally test much of the physics that is important in the very early universe because we cannot attain the required energies in accelerators on Earth. We have to extrapolate from known physics to the unknown and then test the implications; to do this, we assume some specific features of known lower energy physics are the true key to how things are at higher energies. We cannot experimentally test if we have got it right.
- THESIS C2: The unknown nature of the inflation means inflationary universe proposals are incomplete. The promise of inflationary theory in terms of relating cosmology to particle physics has not been realized. This will only be the case when the nature of the inflaton (the particle representing the scalar force causing inflation)has been pinned down to a specific field that experiment confirms or particle physics requires to exist.
- THESIS D2: Testable physics cannot explain the initial state and hence specific nature of the universe. (emphasis added)
Ellis expands on Thesis D2 as follows:
A choice between different contingent possibilities has somehow occurred; the fundamental issue is what underlies this choice. Why does the universe have one specific form rather than another, when other forms consistent with physical laws seem perfectly possible? The reason underlying the choice between different contingent possibilities for the universe (why one occurred rather than another) cannot be explained scientifically. It is an issue to be examined through philosophy or metaphysics. (emphasis added).
This last proposition is, I believe, the most important of those Ellis sets forth.
- THESIS E1: Physical laws may depend on the nature of the universe.
- THESIS F1: Philosophic choices necessarily underlie cosmological theory.Unavoidable metaphysical issues inevitably arise, in both observational and physical cosmology. Philosophical choices are needed in order to shape the theory.
- THESIS F2: Criteria of satisfactoriness for theories cannot be scientifically chosen or validated. Criteria of satisfactoriness are necessary for choosing good cosmological theories; these criteria have to be chosen on the basis of philosophical considerations. They should include criteria for satisfactory structure of the theory, intrinsic explanatory power, and observational and experimental support. These criteria are listed below:
- Satisfactory structure: a) internal consistency, b) simplicity (Ockham’s razor), and c) aesthetic appeal (‘beauty’ or ‘elegance’)
- Intrinsic explanatory power: a) logical tightness, b) scope of the theory—the ability to unify otherwise separate phenomena, and c) probability of the theory or model with respect to some well-defined measure.
- Extrinsic explanatory power, or relatedness: a) connectedness to the rest of science, b) extendability providing a basis for further development;
- Observational and experimental support, in terms of a) testability: the ability to make quantitative as well as qualitative predictions that can be tested; and b) confirmation: the extent to which the theory is supported by such tests as have been made. (emphasis added)
The last criterion in my view (and that of many other scientists and philosophers of science) is critical. If a theory cannot in principle be confirmed quantitatively it is not science, but belongs to other disciplines.
- THESIS F3: Conflicts will inevitably arise in applying criteria for satisfactory cosmological theories. Philosophical criteria for satisfactory cosmological theories will in general come into conflict with each other, so that one will have to choose between them to some degree; this choice will shape the resulting theory.
Ellis elaborates on this last thesis:
The thrust of much recent development has been away from observational tests towards strongly theoretical based proposals, indeed sometimes almost discounting observational tests. At present this is being corrected by a healthy move to detailed observational analysis of the proposed theories, marking a maturity of the subject. (emphasis added)
- THESIS F4: The physical reason for believing in inflation is its explanatory power as regards structure growth in the universe. … This theory has been vindicated spectacularly through observations of the CBR and matter power spectra. It is this explanatory power that makes it so acceptable to physicists, even though the underlying physics is neither well-defined nor tested, and its major large-scale observational predictions are untestable. (emphasis added).
Expanding on Thesis F4, Ellis adds:
Inflation provides a causal model that brings a wider range of phenomena into what can be explained by cosmology (Criterion 2b), rather than just assuming the initial data had a specific restricted form. Explaining flatness (omega0 approximately 1, as predicted by inflation) and homogeneity reinforces the case, even though these are philosophical rather than physical problems (they [the initial restricted conditions] do not contradict any physical law; things could just have been that way). However claims on the basis of this model as to what happens very far outside the visual horizon (as in the chaotic inflationary theory) results from prioritizing theory over the possibility of observational and experimental testing. It will never be possible to prove these claims are correct. (emphasis added)
Ellis asks, “how much should we try to explain” with cosmology? What should the scope of cosmology include?
- THESIS F5:Cosmological theory can have a wide or narrow scope of enquiry. The scope we envisage for our cosmological theory shapes the questions we seek to answer. The cosmological philosophical base becomes more or less dominant in shaping our theory according to the degree that we pursue a theory with more or less ambitious explanatory aims in terms of all of physics, geometry and underlying fundamental causation.