I don't know what children play with today, but two of my early and most pleasing toys were a telescope and a microscope. Of course I had no idea at the time that they were probably invented close together, the microscope in 1590 and the telescope in 1608.

I say "probably" because of suggestions that the telescope was invented much earlier, but was kept under wraps as a secret weapon. Galileo, who more or less originated modern astronomy when he turned his homemade telescope to the heavens, was aware of the military potential of his instruments. He presented one to the Senate in Venice, and wrote that from the highest towers you could see with it "sails and shipping that were so far off that it was two hours before they were seen steering full sail into the harbor."

Despite their close origins, and the optical theory common to both, the use of telescopes and microscopes quickly diverged. Astronomers were interested in the heavens, with the ultimate objective of observing, measuring, and understanding the nature and large-scale structure of the universe. To that end they made bigger and bigger instruments, studying first planets, then stars, and then galaxies. The distances involved increased from hundreds of millions of miles (to the planets), to light-years (to the stars - one light-year is about six trillion miles), to millions and then billions of light-years (to the nearest and farthest galaxies).

The times of interest increased to match the distances. By the beginning of the twentieth century, astronomers confronted a universe billions of light-years in size and billions of years old. The latter prospect gave them fits. The stars, including our Sun, seemed to be billions of years old, but no one could think of any way they could keep shining for so long. The best nineteenth century theory used gravitational contraction as the energy source. Unfortunately that provided a maximum age for the stars of about twenty million years. Just as bad, some stars seemed impossibly dense. Sirius is a binary star system, and calculations indicated that Sirius B was so dense that one cubic inch weighed several tons.

By 1900, astronomy faced a major crisis. The microscopists didn't care. They were too busy burrowing down in the opposite direction. For three hundred years they had been devising instruments that allowed them to study tiny insects, then microfilarial worms, then protozoa and single cells and the contents of cells. The final objective was to see and understand the individual atoms from which everything was built.

In 1896, Henri Becquerel discovered radioactivity and made life more complicated for both physicists and microscopists. The energy of radioactivity seemed to come from nowhere. Also, atoms could no longer be an absolute end point. They must have an internal structure. Microscopes could not probe that structure, but other tools could. If you fired at an atom other small particles, such as the nuclei of hydrogen or helium atoms, then the way those tiny bullets were scattered by or combined with the original atom told you a lot about its interior. Unfortunately, there was a limit to how much energy you could put into the bullets.

The subatomic world presented problems of both observation and theory. Classical physics could not explain radioactivity, with its energy-from-nowhere paradox. By 1900, researchers of the world of the very small had a crisis of their own to match that of astronomers interested in the world of the very large.

At this point an odd thing happened. First, Einstein in 1905 showed that there is an equivalency between mass and energy. Radioactivity was explained, since a minute part of the mass of an atomic nucleus is enough to provide a large amount of energy. This, of course, is exactly what astronomers needed to keep the stars shining. Eddington pointed out that the conversion of a modest amount of the Sun's mass to energy would be enough to light the solar system for billions of years. Also, because atoms proved to have lots of space inside them, the astronomers now understood how some stars could be so enormously dense.

At the same time as Einstein's work was published, researchers on gases found that an unknown kind of radiation seemed able to ionize those gases. This happened no matter how thick the walls of the vessel containing them might be. It was assumed at first that this radiation was a by-product of radioactivity on Earth; however, the effect was more at great heights in the atmosphere. It also came from all directions in space. The radiation was named "cosmic rays," although in the 1920s scientists learned that these "rays" were mostly charged particles, coming from far beyond the Sun and moving faster than anything that could be generated in earthly laboratories.

Here was a convergence of two formerly diverging fields. Research at the subatomic level explained how on the large scale the stars could shine for billions of years and how some could be so dense. At the same time, from way off in space came particles with energies enough to probe the smallest-scale structure of atomic nuclei.

A happy ending, you might think? Not quite. The universe on the largest scale can only be explained using ideas of cosmology that depend on a theory of gravity and of curved space-time. The underpinning for those is the theory of general relativity. At the same time, the universe on the smallest scale leads to a quite different picture of space-time. At a level far below anything our instruments can probe today, the universe must be a sea of discontinuity with minuscule particles appearing from nowhere and then as quickly vanishing. The underpinning for these ideas is quantum theory.

General relativity and quantum theory have each been hugely successful at explaining aspects of our world. Together, they would seem to offer a unified picture of the nature of the universe. Unfortunately, in their present forms they cannot both be right, and all attempts over the past seventy years to make the two ends meet by merging them into a single theoretical framework have been unsuccessful.

However, recently there is new hope of progress. An esoteric combination of mathematics and physics known as "brane theory" (or "M-brane theory") still offers formidable problems in its development, but proponents suggest that it may be able to contain within it all the elements of quantum theory, gravity, and general relativity.

Are they right? I wish I could answer that question. The most that I can state with certainty about brane theory is that my brain doesn't understand it. However, next week I propose to write about it anyway.

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