What you may be surprised to discover is the accepted estimate dates back to the s and has remained pretty much the same since then, even though scientific knowledge has progressed so dramatically since then in other areas. So what's up with that? Efforts to figure out the age of Earth go back many centuries. The classical Greek philosopher Aristotle, who thought time had no beginning or end, also believed that Earth was infinitely old , while religious scholars in ancient India, who envisioned a universe that perpetually exploded, expanded and collapsed only to begin anew, calculated that Earth had existed for 1.
During the medieval era, various Christian theologians scrutinized the Bible for clues, and came up with estimates of between 5, and 7, years, according to G.
Brent Dalrymple's book " The Age of the Earth. Just before the beginning of the 20th century, scientists figured out that they could calculate the age of a rock by measuring radioactive decay, a method called radiometric dating. Patterson, who had worked on the Manhattan Project to develop the atomic bomb during World War II, measured the isotopic composition of lead from the Canyon Diablo meteorite and several other pieces of space rock, which were believed to data back to the disc of material from which Earth also formed.
In , Patterson came up with an estimate of 4. The processes of plate tectonics mean that the Earth is constantly recycling its rock, breaking it down into magma in the interior before pumping it back up to the surface once more.
But old rocks do exist, says Reich, and the oldest rock we know is a tiny piece of zircon found in western Australia. The process of figuring out a rock's age often falls to the scientific techniques of radiometric dating , the most famous of which is radiocarbon dating.
This process focuses on the ratio between the number of carbon and carbon isotopes in any once-living being: that ratio indicates how long it's been since that being was alive. They assumed that current rates—of sediment deposition and of salt transport by rivers—were the same as historical rates, despite the evidence they had that our own age is one of atypically high geologic activity.
Worse, they measured inputs but ignored outputs. The rock cycle, as we now know, is driven by plate tectonics, with sedimentary material vanishing into subduction zones. And the oceans have long since approached something close to a steady state, with chemical sediments removing dissolved minerals as fast as they arrive. Nevertheless, by the late 19th century the geologists included here had reached a consensus for the age of the earth of around million years.
Having come that far, they were initially quite reluctant to accept a further expansion of the geologic timescale by a factor of 10 or more.
And we should resist the temptation to blame them for their resistance. Radioactivity was poorly understood. Different methods of measurement such as the decay of uranium to helium versus its decay to lead sometimes gave discordant values, and almost a decade passed between the first use of radiometric dating and the discovery of isotopes, let alone the working out of the three separate major decay chains in nature.
The constancy of radioactive decay rates was regarded as an independent and questionable assumption because it was not known—and could not be known until the development of modern quantum mechanics—that these rates were fixed by the fundamental constants of physics.
It was not until , when under the influence of Arthur Holmes, whose name recurs throughout this story the National Academy of Sciences adopted the radiometric timescale, that we can regard the controversy as finally resolved.
Critical to this resolution were improved methods of dating, which incorporated advances in mass spectrometry, sampling and laser heating. The resulting knowledge has led to the current understanding that the earth is 4. That takes us to the end of this series of papers but not to the end of the story.
As with so many good scientific puzzles, the question of the age of the earth resolves itself on more rigorous examination into distinct components. Such questions remain under active investigation, using as clues variations in isotopic distribution, or anomalies in mineral composition, that tell the story of the formation and decay of long-vanished short-lived isotopes. For example, some galaxies look much smaller and fainter than other galaxies of the same kind, showing they are much further away.
The Andromeda galaxy, a near neighbor to our own Milky Way galaxy, is 2. That is, we are seeing it as it was 2. But that is just our local neighborhood. In recent decades, astronomers have detected galaxies located several billion light years away.
If the light has been traveling billions of years to reach us, then the universe must be at least that old. This is completely independent of radiometric dating of the solar system, but both methods point to an age of billions of years, not thousands.
Not only can astronomers measure the distance of galaxies, they can measure how galaxies are moving. Galaxies are not holding still in space, nor are they moving randomly.
Some galaxies are moving towards their neighbors, attracted by their mutual gravity. But the biggest pattern we see is that galaxies are moving apart from one another. This motion apart is not all at the same speed; instead it follows a pattern where galaxies that are further apart are moving more quickly.
This particular pattern indicates the whole universe is expanding. To see why, consider a loaf of raisin bread. The raisins are like galaxies and the dough is like the fabric of space in the universe. As the dough rises, it carries the raisins along, pulling them apart from each other. Raisins that started out on opposite sides of the loaf will be a few inches farther apart after the dough rises, while raisins that started out near each other may only move half an inch.
So, the speed of their motion is proportional to the separation between them. In the same way, the space of the universe pulls galaxies further apart as the universe expands.
When a galaxy is carried away by the expansion of space, its light waves are stretched out, making it appear redder. From the measurements of many galaxies, astronomers can accurately measure the expansion rate of the universe as a whole. In the past the galaxies must have been closer together, and in the distant past they would have been packed together in a tiny point. If we assume that the expansion rate is constant over time, the age for the universe as a whole is about 10 billion years.
However, astronomers have been working over the last 20 years to determine how the expansion rate changes with time. We now know that early in the universe the expansion was slowing down, but now it is speeding up.
Using careful measurements of this change in expansion rate, the age of the universe is now known quite precisely to be
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