You can EXIT this exhibit to a page on James Hutton, who made the key discoveries, and a short page on the history of the geologic time scale

You can EXIT this exhibit to read a newspaper article on 2008 measurements.

Brought to you by the
Center for History of Physics, a Division of the
American Institute of Physics

Christians in Isaac Newton's day believed that the universe was a few thousand years old — created in six days in nearly its present condition. Cosmic history and human history seemed one and the same. Some clerics, and even Newton himself, made calculations based on their study of Bible passages, and announced an age of about 6000 years (they disagreed on the exact number). Other cultural and philosophical traditions thought the universe was much older, perhaps eternal. But most Europeans believed the universe was created recently.

By the mid-19th century, scientists had discovered that time was far deeper. They worked out, for example, how mountain ranges erode and rivers carry the sediments down to the sea. The process is so slow that it must have taken many millions of years to lay down the great layers of shale and sandstone, adding up to kilometers in thickness. Geologists also found several other processes that must have taken millions of years. For all science could tell, the Earth and the starry universe were eternal. Controversy went on for decades, but by the end of the nineteenth century, all scientists who had studied the evidence were convinced that the age of the Earth must be at least many tens of million of years. Biblical fundamentalists continued to vehemently deny it all. But mainstream Christian theologians showed how the words of their Bible could be reconciled with the facts of geology.

One way to find the age of the Earth was to look at the source of the Sun's energy. The most energetic chemical reactions known could keep the Sun burning for only a few thousand years. But there was another possible energy source, the conversion of gravitational energy into heat energy as the Sun contracted. A hundred million years' worth of such heat might be possible. By the end of the nineteenth century, scientifically informed people thought that was a plausible age for our entire solar system. Elsewhere in the universe, such systems of stars and planets might be perpetually coelescing from gas clouds and gradually dying away.

The discovery of radioactive elements around the start of the twentieth century brought new and startling facts. Laboratory work showed that when an atom of a mineral element like uranium gives off radioactivity, it decays into another element. This starts a regular chain of decay that ends with inert lead. (See the page on radioactive decay in our Marie Curie exhibit). When physicists measured how much of various types of radioactive elements ("isotopes") remained in, say, a lump of lead ore, they could work out when the rock had first formed. The rate of decay for some elements in the chain is very slow, and the measurements showed that some rocks were not just hundreds of millions, but billions of years old. Similar measurements of isotopes in asteroids (rocks fallen from elsewhere in the solar system) also gave ages in billions of years.

Meanwhile the discovery of radioactivity led to the science of nuclear physics, which pointed to the true source of the energy of the Sun and other stars. In the 1960s, astrophysicists began to use digital computers to calculate how stars evolve over time. They found, for example, that a star like our Sun must eventually expand into a "red giant" and later dwindle into a "white dwarf." The calculations matched neatly with the types of stars that astronomers saw in star clusters (groups that had all formed at the same time from one enormous cloud of gas). Eventually astrophysicists could even calculate precisely how interior nuclear processes made certain types of stars pulsate regularly. All this gave scientists confidence that their calculations truly worked. Turning to the evolution of our own Sun, they got a present age of roughly five billion years. This was so close to the age of the Earth and asteroids, as measured in an entirely different way from radioactivity, that few could doubt that our solar system had formed around that time — the number now accepted for the age of the Earth, Sun and solar system is 4.5 billion years.

Finding the age of the entire universe, if indeed it was not eternal, was still harder. One idea was to find the distance to the farthest visible objects, as measured in light-years, since that would tell how many years ago they had emitted the light that was finally reaching us. When Edwin Hubble made such measurements, he discovered a linear relation between the distance and velocity of galaxies — the farther away a galaxy, the faster it was moving away. That suggested an expansion outward from a single original point. The idea of such a cosmic explosion had already been suggested to follow from Einstein’s equations of General Relativity (with all galaxies moving apart from one another as space itself expanded). So the velocity-distance relation could be interpreted to give the age of the entire universe. Because Hubble’s distance estimates during the 1930s were only half what astronomers later worked out, the time scale, as he remarked, "seems suspiciously short — a small fraction of the estimated age of some stars." After other astronomers fixed the error in his distances, they became confident that the universe is indeed expanding. For the corrected timescale fitted quite well with the timescales that others were finding in the evolution of stars, in radioactivity, in geology, and even in biology from estimates of the rate of evolution of species.

To get a more precise answer, astronomers continued to improve measurements of distances to the farthest visible objects, not only galaxies but later supernovae and quasars. They devised several independent ways to determine these distances. Some of the ways gave only a general idea ("billions of light-years") while others gave rough numbers. Another possible way to find the age of the universe was to calculate how long it had taken the oldest visible stars, white dwarfs, to evolve. Still more possibilities came from the discovery in the 1950s of faint radiation remaining from the explosive "Big Bang" at the origin of our universe. Applying fundamental physics to characteristics of this primal radiation, physicists could estimate an age in several different ways.

At first the many independent ways of measuring the age of the universe gave a wide variety of estimates, ranging up to twenty billion years or more, and astronomers argued over whose results were best. But around the end of the twentieth century, the answers began to converge — agreeing on an age in the range of twelve to fifteen billion years. That still left room for vigorous debate. As old tools were refined and yet more new ones were devised, the answer was narrowed down to 13.7 billion years, give or take a few hundred million.

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BACK to Cosmic Journey-a History of Scientific Cosmology:
Expanding Universe - Big Bang - New Tools - Journey Continues

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