To Explain the World: The Discovery of Modern Science

by Steven Weinberg

With all its imperfections, modern science is a technique that is sufficiently well tuned to nature so that it works—it is a practice that allows us to learn reliable things about the world. In this sense, it is a technique that was waiting for people to discover it.

The West borrowed much scientific knowledge from elsewhere—geometry from Egypt, astronomical data from Babylon, the techniques of arithmetic from Babylon and India, the magnetic compass from China, and so on—but as far as I know, it did not import the methods of modern science.

to understand these early Greeks, it is better to think of them not as physicists or scientists or even philosophers, but as poets.

“we must not lose sight of the fact that Aristotle was a man of his time—and for that time he was extraordinarily perspicacious, acute, and advanced.”2 Nevertheless, there were principles running all through Aristotle’s thought that had to be unlearned in the discovery of modern science.

For Aristotle, the presence of air or some other medium was essential in understanding motion. He thought that without any resistance, bodies would move at infinite speed, an absurdity that led him to deny the possibility of empty space.

“The proper measure of a philosophical system or a scientific theory is not the degree to which it anticipated modern thought, but its degree of success in treating the philosophical and scientific problems of its own day.”

The progress of science has been largely a matter of discovering what questions should be asked.

it would be unfair to conclude that Aristotle was stupid. My purpose here in judging the past by the standards of the present is to come to an understanding of how difficult it was for even very intelligent persons like Aristotle to learn how to learn about nature. Nothing about the practice of modern science is obvious to someone who has never seen it done.

It was essential for the discovery of science that religious ideas be divorced from the study of nature. This divorce took many centuries, not being largely complete in physical science until the eighteenth century, nor in biology even then.

the idea is to see how far one can go without supposing supernatural intervention. Only in this way can we do science, because once one invokes the supernatural, anything can be explained, and no explanation can be verified. This is why the “intelligent design” ideology being promoted today is not science—it is rather the abdication of science.

Pagans were not much concerned to police each other’s private beliefs. There were no authoritative written sources of pagan religious doctrine analogous to the Bible or the Koran. The Iliad and Odyssey and Hesiod’s Theogony were understood as literature, not theology. Paganism had plenty of poets and priests, but it had no theologians.

According to Gibbon, the religions of the Roman Empire “were all considered by the people, as equally true, by the philosopher, as equally false, and by the magistrate, as equally useful.”

During the spring and summer the Sun rises somewhat north of east, whereas during the autumn and winter it comes up south of east. When the shadow of the gnomon at dawn points due west, the Sun is rising due east, and the date must be either the vernal equinox, when winter gives way to spring; or the autumnal equinox, when summer ends and autumn begins. The summer and winter solstices are the days in the year when the shadow of the gnomon at noon is respectively shortest or longest.

The Council of Nicaea in AD 325 decreed that Easter should be celebrated on the first Sunday following the first full moon following the vernal equinox.

To avoid the horror of Easter being celebrated on different days in different places, it was necessary to prescribe a definite date for the vernal equinox, and also for the first full moon following it. The Roman church in late antiquity adopted the Metonic cycle for this purpose, but the monastic communities of Ireland adopted an older Jewish 84-year cycle. The struggle in the seventh century between Roman missionaries and Irish monks for control over the English church was largely a conflict over the date of Easter.

For purposes of calculating the date of Easter, the date of the vernal equinox is now fixed to be March 21, but it is March 21 as given by the Gregorian calendar in the West and by the Julian calendar in the Orthodox churches of the East. So Easter is still celebrated on different days in different parts of the world.

What was learned about light and the heavens was important, but even more important was what was learned about the sort of thing that could be learned, and how to learn it.

The historian Philip Hitti has nicely contrasted the state of learning at this time at Baghdad with the illiteracy of Europe in the early Middle Ages: “For while in the East al-Rashid and al-Mamun were delving into Greek and Persian philosophy, their contemporaries in the West, Charlemagne and his lords, were dabbling in the art of writing their names.”

As had been done by Ibn al-Shatir, Copernicus instead introduced more epicycles: six for Mercury; three for the Moon; and four each for Venus, Mars, Jupiter, and Saturn. Here he made no improvement over the Almagest.

a simple and beautiful theory that agrees pretty well with observation is often closer to the truth than a complicated ugly theory that agrees better with observation.

The work of Copernicus and Kepler made a case for a heliocentric solar system based on mathematical simplicity and coherence, not on its better agreement with observation.

the simplest versions of the Copernican and Ptolemaic theories make the same predictions for the apparent motions of the Sun and planets, in pretty good agreement with observation,

It is odd that Descartes thought that a God who allowed earthquakes and plagues would not allow a philosopher to be deceived.

Deduction simply cannot carry the weight that Descartes placed on it.

This very important use of letters of the alphabet to represent unknown distances or other numbers originated in the sixteenth century with the French mathematician, courtier, and cryptanalyst François Viète, but Viète still wrote out equations in words. The modern formalism of algebra and its application to analytic geometry are due to Descartes.

The best-known statement along these lines is that of John Maynard Keynes (who had bought some of the Newton papers in the 1936 auction at Sotheby’s): “Newton was not the first of the age of reason. He was the last of the magicians, the last of the Babylonians and Sumerians, the last great mind which looked out on the visible and intellectual world with the same eyes as those who began to build our intellectual inheritance rather less than 10,000 years ago.”

suppose we know the distance D(t) traveled in any time t; how do we find the velocity at any time? He reasoned that in nonuniform motion, the velocity at any instant is the ratio of the distance traveled to the time elapsed in an infinitesimal interval of time at that instant. Introducing the symbol o for an infinitesimal interval of time,

Copernicus had placed the Earth among the planets, Tycho had shown that there is change in the heavens, and Galileo had seen that the Moon’s surface is rough, like the Earth’s, but none of this related the motion of planets to forces that could be observed on Earth.

Gram for gram, the Moon attracts the ocean beneath it more strongly than it attracts the solid Earth, whose center is farther away, and it attracts the solid Earth more strongly than it attracts the ocean on the side of the Earth away from the Moon.

Thus there is a tidal bulge in the ocean both below the Moon, where the Moon’s gravity pulls water away from the Earth, and on the opposite side of the Earth, where the Moon’s gravity pulls the Earth away from the water.

Though Newton knew very well that gravitation was not the only physical force, as far as it went his theory was universal—every particle in the universe attracts every other particle with a force proportional to the product of their masses and inversely proportional to the square of their separation.

Despite Newton’s own commitment to Unitarian Christianity, some in England, like the theologian John Hutchinson and Bishop Berkeley, were appalled by the impersonal naturalism of Newton’s theory.

The rejection of quantum mechanical theories by these physicists meant that they were unable to participate in the great progress in the physics of solids, atomic nuclei, and elementary particles of the 1930s and 1940s.

When Einstein’s theory was confirmed in 1919 by the observation of a predicted bending of rays of light by the gravitational field of the Sun, the Times of London declared that Newton had been shown to be wrong. This was a mistake. Newton’s theory can be regarded as an approximation to Einstein’s, one that becomes increasingly valid for objects moving at velocities much less than that of light. Not only does Einstein’s theory not disprove Newton’s; relativity explains why Newton’s theory works, when it does work. General relativity itself is doubtless an approximation to a more satisfactory theory.

The conceptual basis of general relativity is thus different from that of Newton. The notion of gravitational force is largely replaced in general relativity with the concept of curved space-time.

The difference between Einstein’s and Newton’s theories is far less than the difference between Newton’s theories and anything that had gone before.

The sociologist Robert Merton supposed that Protestantism created social attitudes favorable to science and promoted a combination of rationalism and empiricism and a belief in an understandable order in nature—attitudes and beliefs that he found in the actual behavior of Protestant scientists.

even if our predecessors could have imagined science as it is today, they might not have liked it very much. Modern science is impersonal, without room for supernatural intervention or (outside the behavioral sciences) for human values; it has no sense of purpose; and it offers no hope for certainty.

Quantum mechanics not only solved the problem of the stability of atoms and the nature of spectral lines; it also brought chemistry into the framework of physics. With the electrical forces among electrons and atomic nuclei already known, the Schrödinger equation could be applied to molecules as well as to atoms, and allowed the calculation of the energies of their various states.

from the 1920s on, it would be understood that any general principle of chemistry, such as the rule that metals form stable compounds with halogen elements like chlorine, is what it is because of the quantum mechanics of nuclei and electrons acted on by electromagnetic forces.

The Standard Model is not the end of the story. It leaves out gravitation; it does not account for the “dark matter” that astronomers tell us makes up five-sixths of the mass of the universe; and it involves far too many unexplained numerical quantities, like the ratios of the masses of the various quarks and electron-like particles.

The Standard Model would have seemed unsatisfying to many natural philosophers from Thales to Newton. It is impersonal; there is no hint in it of human concerns like love or justice. No one who studies the Standard Model will be helped to be a better person, as Plato expected would follow from the study of astronomy.

What we mean by the unification of biology with the rest of science is only that there can be no freestanding principles of biology, any more than of geology. Any general principle of biology is what it is because of the fundamental principles of physics together with historical accidents, which by definition can never be explained.