EIGHTEENTH-CENTURY ASTRONOMY

While Newton did not make any significant astronomical observations, in 1668 he built the first reflecting (as opposed to refracting) telescope. By using a curved mirror rather than a lens to provide the magnification, the reflecting telescope avoids the chromatic aberration that occurs in a lens because of the dependence of the refractive index of glass on the color of light. The reflecting telescope is the primary type of astronomical optical telescope in use today, in space and on Earth, although it wasn't until the late nineteenth century that reflectors were used at altitude.

With the telescope, astronomy was now able to move beyond the limitation of unaided human vision, and a never-imagined cosmos began to reveal itself. In 1655, the Dutch physicist and astronomer Christiaan Huygens (1629–1695) built a refracting telescope with a magnification of fifty and discovered Saturn's moon Titan, the largest moon in the solar system. From his observations of Saturn, he concluded that the planet is surrounded by a solid ring. Later Huygens was able to resolve the Orion nebula into separate stars and observed the transit of Mercury across the sun.

Huygens was also one of the great physicists of all time. He invented the pendulum clock and the internal combustion engine, derived the formula for centrifugal force, and wrote a book on probability theory.

Among his many achievements, in 1678 Huygens quantitatively described the wave theory of light, which Robert Hooke had proposed in 1672. Newton's corpuscular theory actually came later. Huygens demonstrated how waves are able to bend (“diffract”) around corners. Newtonian's corpuscular theory of light was ostensibly refuted when in 1800 Thomas Young (1772–1829) demonstrated conclusively that it exhibits the interference and diffraction effects associated with waves. As we will see, in the twentieth century, light was shown to be composed of particles called photons and the waves associated with light refer not to individual photons but to the statistical behavior of beams of light containing many photons.

KANT'S COSMOS

In 1755, a young privatdocent at the University of Königsberg named Immanuel Kant (1724–1804) published a book on the structure of the universe titled Allgemeine naturgeschichte und theorie des himmels (Universal Natural History and Theory of the Heavens).1 Unlike Newton but like Leibniz, Kant saw no need for any divine miracles to explain the universe. He wrote, “A constitution of the world which did not maintain itself without a miracle, has not the character of that stability which is the mark of the choice of God.”²

Kant's universe starts with a divinely created chaos of particles in an infinite void. The particles attract one another by Newtonian gravitation and form condensations, which then evolve into orderly structures such as the solar system. The Milky Way is disk-shaped conglomeration of stars, and the observed nebulous bodies are not individual stars but conglomerations like the Milky Way wending indefinitely throughout infinite space.³

The book remained largely unknown until 1854 when the German physicist Hermann von Helmholtz (1821–1894) mentioned it in a lecture. Kant is still better known for his philosophy than his astrophysics, but the latter was not too shabby either.

HERSCHEL'S HEAVENS

Perhaps the most productive astronomer in the eighteenth century was Frederick William Herschel (1738–1822). During the latter part of the century, Herschel built his own reflecting telescopes and made a series of important discoveries. He helped establish that Uranus, which had previously been thought to be a star, was in fact a planet. He discovered two moons of Saturn and two of Uranus. He determined that the Milky Way was in the form of a disk. Herschel observed binary and multiple stars. He showed that the sun emits infrared light. And, by the way, using a microscope he proved that coral is an animal, not a plant.

Between 1782 and 1802, Herschel made a systematic study of nonstellar objects, that is, diffuse bodies called nebulae. He produced a catalogue of over a thousand nebulae, classifying them according to brightness, shape, size, and other features.

Starting in 1785, Herschel wrote a series of papers under the common title “The Construction of the Heavens” that suggested the nebulae were far away. Because of the finite speed of light, by observing them the astronomer should have been able to follow their movements through the heavens. However, no such movement was evident, indicating the nebulae were very distant.4

In 2009, the European Space Agency launched the Herschel Space Observatory, a large infrared telescope that operated until 2013.⁵

OLBERS'S PARADOX

Halley, along with Kepler and the Swiss astronomer Jean-Philippe de Cheseaux (1718–1751), recognized a problem with the idea proposed by de Cusa and Digges that the universe contains an infinite number of stars. After it was formulated by the German astronomer Heinrich Wilhelm Matthias Olbers (1758–1840), the problem became known as Olbers's paradox: if the universe is infinite in size and eternal in age, then the sky should be bright rather than dark at night because of the light from all those stars.

To see this, picture a thin spherical shell of a certain thickness at a distance r from Earth. Its volume will be equal to that thickness multiplied by the surface area of the shell, 4πr². Assuming a uniform density of stars, the luminosity from the shell will be proportional to that volume. However, the intensity of light (power per unit area) reaching Earth falls off as 1/r². Thus, if the average luminosity of stars is independent of their distances from Earth, the contribution from each shell of the same thickness throughout the universe will be the same and add to give the total light observed on Earth from all the stars in the universe. For an infinite universe, that intensity would appear brighter than the sun—indeed have infinite intensity. Clearly this is not what we observe.

There are a number of possible explanations for why the sky is, in fact, dark at night. In an 1848 essay, “Eureka,” Edgar Allan Poe (1809–1849) suggested that light has not yet reached us from the most distant stars. That is, the universe has a finite lifetime so we are only observing the stars out to the distance that light can travel in that time.

This is true, but another factor that contributes is the expansion of the universe, which we will discuss later. The energy of light coming from great distances as light is reduced because it is redshifted to longer wavelengths or lower energies.

SUFFICIENT REASON

In 1710, Leibniz wrote Theodicy: Essays on the Goodness of God, the Freedom of Man, and the Origin of Evil.6 This term theodicy became associated with the still-unsuccessful attempts to justify the undeniable evil and suffering in a world supposedly under the complete control of an omnibenevolent, omnipotent, omniscient God. Leibniz proposed that God had created the “best of all possible worlds.” The evil that exists is just part of that optimization. Without it, the world would be even worse.

Leibniz also proposed the principle of sufficient reason, also known as the cosmological argument, as a proof of the existence of God. Basically, the argument says that everything that is true requires a complete explanation, a sufficient reason. Since the universe does not explain itself, God must exist as its sufficient reason for the existence of the universe.

These arguments fails for a simple, sufficient reason, which I have already emphasized. Like all purely logical arguments that contain no empirical input, it tells you nothing not already embedded it its premises. Applied here, God must exist because he must exist.

THE CENTER OF THE UNIVERSE

In our historical survey so far, we have seen that a conflict has long persisted over the location of what is termed “the center of the universe.” For the most part, we humans have found it easy to position ourselves at the center. All living species are self-centered, if not always the individual organism but what Richard Dawkins termed their “selfish genes.”⁷ Species would not have survived if they weren't selfish at some level.

There is also a good empirical reason for us to think we are the center of the cosmos. When we look at the sky, everything seems to revolve around us. The planets sometimes turn around and go back the other way, but they soon turn back again and resume circling Earth.

Even if we now know that a sun-centered system is the simplest for visualizing the motions of the planets, how often in our everyday lives do we need to worry about where a planet will be tomorrow, a month from now, or where it was on March 28, 585 BCE?

In fact, for most of our purposes, an Earth-centered model of motion does just fine. It would be silly to calculate the route an airliner should take in going from Tokyo to London in a sun-centered coordinate system. And we could still use a geocentric model for predicting planetary motion if we wanted to.

Of course, we know today that the sun is not the center of the universe, as it was thought to be when Copernicus's picture of the cosmos was one of a solar system of seven planets surrounded by a shell of fixed stars. As telescopes improved, astronomers discovered that our sun is just another star. As I have mentioned, the ancient atomists proposed a picture of a cosmos that is limitless in time and space and that no point in space can be designated as that special place we can call the center of the universe. Similarly, no moment in time can be designated as that special moment when the universe began (or will end). As we will see, scientific consensus today is converging on precisely this cosmological model. But, again, I must emphasis it is a human-constructed model.