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Wednesday, November 24, 2010

Dark Problems

Chapter 5B

Dark Problems


It had been suspected for some time that galaxies had far more matter than we could determine from what was visible. Once we were able to look at galaxies in the full spectrum we realized that perhaps as much as 90% of some galaxies’ mass was unaccounted for (one main reason for this realization is that the outer stars in a galaxy are going so fast that without that much extra mass to keep them in, they would be flung into intergalactic space). But apparently most of that missing mass (now called “dark matter”) cannot be seen, for it does not interact with ordinary matter, except through gravity. In other words, most of the matter in the universe is completely different from anything we have known until now (it is not made up of protons, neutrons, electrons and the like). To study dark matter it is necessary to observe the universe with space telescopes, as will be explained below.

To make matters worse, it was discovered that the expansion of the universe is accelerating, in violation of the sensible belief that gravitational attraction should slow down and perhaps even reverse the expansion initiated in the most famous of all explosions, the Big Bang. A new form of energy, Dark Energy, which we understand even less than we understand Dark Matter, presumably accounts for that perplexing expansion. Since dark energy and dark matter take up most of the universe, our precious Standard Model tries to explain the entire universe on the basis of the less than ten percent of matter that it is acquainted with. Imagine that you come to a new place and get to know ten percent of it. All you know about the rest is that it is completely different from the small portion you know. How confident would you feel about explaining the whole on the basis of the one part you can handle? And then add the complication that Dark Energy make up two thirds of the universe!

Again, to study dark energy we need to go into space, sooner or later. Some of the work of surveying the universe can be done with terrestrial telescopes, but the findings of such surveys will have to be corroborated and supplemented by telescopes in orbit, as we will see below. This means that to have much of a chance to come up with a fundamental explanation of the universe we need to do space science. Some physicists still hope that in the new particle accelerators, which will produce very high energies, violent collisions will yield some dark matter particles. And, who knows, their hopes may perchance be realized, but since we do not know what dark matter is, those physicists sound a bit too optimistic. And even then we would still have the even bigger puzzle of dark energy.

To do away with the problem of Dark Energy, some physicists have proposed to replace the Theory of General Relativity with another theory in which gravity is not a constant. This hypothesis, which is rejected by most physicists, would of course represent a radical transformation of fundamental physics brought about by space astronomy and physics. Either way, space science should receive significant credit for the serendipity that will result from the soon-to-be new physics.


Let us begin our discussion of Claim (1) by remembering that the connection with astrophysics has been a trademark of modern physical science from its inception and throughout most of its history.

Although it is well known that Copernicus proposed that the sun and not the Earth was at the center of the universe, his motivation is not so well understood. It was not that his system could account for the position of the planets clearly better than the Ptolemaic system, for even Copernicus acknowledged that the matter was not settled. Nor was it obvious either, in spite of the claims by Pierre Duhem and others, that his system was vastly simpler. It is true that the Ptolemaic system employed a variety of mechanisms--epicycles, eccentrics, deferents, equants--to account for the paths of the planets, but with the exception of the equant so did the Copernican system. (See figures).[1] The difference was that the Ptolemaic system often had alternative combinations of such mechanisms for different aspects of the behavior of the same planet. This would seem outrageous to someone weaned on the notion that only one such mechanism could be correct. But the mere talk of correctness assumes that we can inquire about the real nature of the heavens.

We feel entitled to make that assumption rather freely today. But that was not the case in Copernicus' day. From the time of Ptolemy (second century AD.), the inquiry about the reality of the heavens had been looked upon with suspicion. The reason was that whereas the progress of mathematical astronomy made it possible to calculate with increasing precision the positions of the planets, the accounts of why the planets moved as they did had broken down not long after Aristotle had proposed the interaction of concentric spheres made of his quintessence (about 350 BC.).

According to Ptolemy himself, mathematics can apply only to "changes in form: i.e. in trajectory, shape, quantity, size, position, time, and the rest."[2] As to the actual nature of things there is little that science can do because they either take "place far above us, among the highest things in the universe, far away from the objects we directly observe with our senses," or else, as the objects of (terrestrial) physics, those "material things...are so unstable and difficult to fathom that one can never hope to get philosophers to agree about them."[3] Questions about the nature of the heavenly objects must lead one back to the ultimate source of all change, and thus they can only be answered by theology. Therefore science gains little to profit from asking them. And they are also distinct in kind from the sorts of questions that physics tries to answer, whose underlying principles, if any, did not seem amenable to mathematical treatment.

In the long run the Copernican revolution accomplished several important changes in points of view. For one thing it insisted in investigating the nature of the behavior of the heavenly objects. And it did so by looking for mutual underlying mathematical principles for both the heavens and the Earth. The success of this gross violation of Ptolemy's methodological rules turned on Copernicus' belief that astrophysics was possible. Eventually Newton succeeded where Aristotle had failed, and astrophysics became the shining example that new branches of physics had to follow.

Confusions about Copernicus' motivation were created mainly by Osiander's preface to Copernicus' masterpiece, On the Revolutions. Fearing a confrontation with theological dogma, Osiander urged the readers to "permit these new hypotheses to become known together with the ancient hypotheses," and to do so because Copernicus' hypotheses are "admirable and also simple, and bring with them a huge treasure of skillful observations." But the Copernicus' reader, Osiander wrote, should not accept as the truth "ideas conceived for another purpose."[4] All these admonitions by Osiander contradict Copernicus' own words and belie his attempt to discover the truth about the heavens by rational means instead of revelation.

In the centuries following Copernicus, astrophysics continued as a driving force of fundamental theory. Newton is, of course, the most prominent example. His laws of dynamics applied equally to terrestrial and heavenly objects, and his law of gravitation was a striking statement of the discovery that the force that kept us glued to the surface of the Earth was the same that made the stars and planets keep their appointed rounds.

We begin to see why this view of science is distorted when we realize that fundamental questions often cannot be asked without the appropriate technology and will not be asked without the right kind of inspiration and motivation. But this realization leads to another: that a whole host of activities are potentially as crucial to scientific progress as work that aims to solve problems within the most prestigious field of the time. Researchers who create new technology or new opportunities may contribute just as much to keep intact the dynamic character of science. And inspiration has often come as much from the planets as from the stars.

[1] T.S. Kuhn. (1957) The Copernican Revolution. Harvard University Press.

[2] S. Toulmin and J. Goodfield (196 ) The Fabric of the Heavens.

[3] Ibid.

[4] Ibid.

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