Not so much a theory of the universe as a simple picture of the planet we call home, the flat-earth model proposed that Earth’s surface was level. Although everyday experience makes this seem a reasonable assumption, direct observation of nature shows the real world isn’t that simple. For instance, when a sailing ship heads into port, the first part that becomes visible is the crow’s-nest, followed by the sails, and then the bow of the ship.
If the Earth were flat, the entire ship would come into view at once as soon as it came close enough to shore. The Greek philosopher Aristotle provided two more reasons why the Earth was round. First, he noted that Earth’s shadow always took a circular bite out of the moon during a lunar eclipse, which would only be possible with a spherical Earth.
(If the Earth were a disk, its shadow would appear as an elongated ellipse at least during part of the eclipse.) Second, Aristotle knew that people who journeyed north saw the North Star ascend higher in the sky, while those heading south saw the North Star sink. On a flat Earth, the positions of the stars wouldn’t vary with a person’s location.
Despite these arguments, which won over most of the world’s educated citizens, belief in a flat Earth persisted among many others. Not until explorers first circumnavigated the globe in the 16th century did those beliefs begin to die out. Ptolemy, the last of the great Greek astronomers of antiquity, developed an effective system for mapping the universe. Basing much of his theory on the work of his predecessor, Hipparchus, Ptolemy designed a geocentric, or Earth-centered, model that held sway for 1400 years.
That Ptolemy could place Earth at the center of the universe and still predict the planets’ positions adequately was a testament to his ability as a mathematician. That he could do so while maintaining the Greek belief that the heavens were perfect—and thus that each planet moved along a circular orbit at a constant speed—is nothing short of remarkable. Copernicus made a great leap forward by realizing that the motions of the planets could be explained by placing the Sun at the center of the universe instead of Earth.
In his view, Earth was simply one of many planets orbiting the Sun, and the daily motion of the stars and planets was just a reflection of Earth spinning on its axis. Although the Greek astronomer Aristarchus developed the same hypothesis more than 1500 years earlier, Copernicus was the first person to argue its merits in modern times. Despite the basic truth of his model, Copernicus did not prove that Earth moved around the Sun.
That was left for later astronomers. The first direct evidence came from Newton’s laws of motion, which say that when objects orbit one another, the lighter object moves more than the heavier one. Because the Sun has about 330,000 times more mass than Earth, our planet must be doing almost all the moving. A direct observation of Earth’s motion came in 1838 when the German astronomer Friedrich Bessel measured the tiny displacement, or parallax, of a nearby star relative to the more distant stars.
This minuscule displacement reflects our planet’s changing vantage point as we orbit the Sun during the year. How did the universe really begin? Most astronomers would say that the debate is now over: The universe started with a giant explosion, called the Big Bang. The big-bang theory got its start with the observations by Edwin Hubble that showed the universe to be expanding.
If you imagine the history of the universe as a long-running movie, what happens when you show the movie in reverse? All the galaxies would move closer and closer together until eventually they all get crushed together into one massive yet tiny sphere. It was just this sort of thinking that led to the concept of the Big Bang. The Big Bang marks the instant at which the universe began when space and time came into existence and all the matter in the cosmos started to expand.
Amazingly, theorists have deduced the history of the universe dates back to just 1043 seconds (10 million trillion trillionths of a second) after the Big Bang. Before this time all four fundamental forces—gravity, electromagnetism, and the strong and weak nuclear forces—were unified, but physicists have yet to develop a workable theory that can describe these conditions. During the first second or so of the universe, protons, neutrons, and electrons—the building blocks of atoms—formed when photons collided and converted their energy into mass, and the four forces split into their separate identities.
The temperature of the universe also cooled during this time, from about 1032 (100 million trillion) degrees to 10 billion degrees. Approximately three minutes after the Big Bang, when the temperature fell to a cool one billion degrees, protons and neutrons combined to form the nuclei of a few heavier elements, most notably helium.
The next major step didn’t take place until roughly 300,000 years after the Big Bang when the universe had cooled to a not-quite comfortable 3000 degrees. At this temperature, electrons could combine with atomic nuclei to form neutral atoms. With no free electrons left to scatter photons of light, the universe became transparent to radiation. (It is this light that we see today as cosmic background radiation.) Stars and galaxies began to form about one billion years following the Big Bang, and since then the universe has simply continued to grow larger and cooler, creating conditions conducive to life.
Three excellent reasons exist for believing in the big-bang theory. First, and most obvious, the universe is expanding. Second, the theory predicts that 25 percent of the total mass of the universe should be the helium that formed during the first few minutes, an amount that agrees with observations. Finally, and most convincing, is the presence of cosmic background radiation. The big-bang theory predicted this remnant radiation, which now glows at a temperature just 3 degrees above absolute zero, well before radio astronomers chanced upon it.
Friedmann made two simple assumptions about the universe: that when viewed at large enough scales, it appears the same both in every direction and from every location. From these assumptions (called the cosmological principle) and Einstein’s equations, he developed the first model of a universe in motion. The Friedmann universe begins with a Big Bang and continues expanding for untold billions of years—that’s the stage we’re in now. But after a long enough period of time, the mutual gravitational attraction of all the matter slows the expansion to a stop.
The universe then starts to fall in on itself, replaying the expansion in reverse. Eventually, all the matter collapses back into a singularity, in what physicist John Wheeler likes to call the “Big Crunch.” Gravitational attraction is a fundamental property of matter that exists throughout the known universe. Physicists identify gravity as one of the four types of forces in the universe. The others are the strong and weak nuclear forces and the electromagnetic force.
More than 300 years ago, the great English scientist Sir Isaac Newton published an important generalization that mathematically describes this universal force of gravity. Newton was the first to realize that gravity extends well beyond the boundaries of Earth. Newton’s realization was based on the first of three laws he had formulated to describe the motion of objects. Part of Newton’s first law, the Law of Inertia, states that objects in motion travel in a straight line at a constant velocity unless they are acted upon by a net force. According to this law, the planets in space should travel in straight lines.
However, as early as the time of Aristotle, the planets were known to travel on curved paths. Newton reasoned that the circular motions of the planets are the result of a net force acting upon each of them. That force, he concluded, is the same force that causes an apple to fall to the ground–gravity. Newton’s experimental research into the force of gravity resulted in his elegant mathematical statement that is known today as the Law of Universal Gravitation. According to Newton, every mass in the universe attracts every other mass.
The attractive force between any two objects is directly proportional to the product of the two masses being measured and inversely proportional to the square of the distance separating them.
If we let F represent this force, r the distance between the centers of the masses, and m1 and m2 the magnitude of the two masses, the relationship stated can be written symbolically as: is defined mathematically to mean is proportional to.) From this relationship, we can see that the greater the masses of the attracting objects, the greater the force of attraction between them.
We can also see that the farther apart the objects are from each other, the less the attraction. It is important to note the inverse square relationship with respect to distance. In other words, if the distance between the objects is doubled, the attraction between them is diminished by a factor of four, and if the distance is tripled, the attraction is only one-ninth as much.
Newton’s Law of Universal Gravitation was later quantified by eighteenth-century English physicist Henry Cavendish who actually measured the gravitational force between two one-kilogram masses separated by a distance of one meter. This attraction was an extremely weak force, but its determination permitted the proportional relationship of Newton’s law to be converted into an equation. This measurement yielded the universal gravitational constant or G.