As Henry Reich describes in the video above , the process of scientifically estimating the age of the Earth revolves around, essentially, finding the oldest piece of the planet we can, then figuring out how old that piece is. Finding super old rocks is conceptually straightforward, but practically difficult. 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. Our planet was pegged at a youthful few thousand years old by Bible readers by counting all the "begats" since Adam as late as the end of the 19th century, with physicist Lord Kelvin providing another nascent estimate of million years.
Kelvin defended this calculation throughout his life, even disputing Darwin's explanations of evolution as impossible in that time period. In , Marie Curie discovered the phenomenon of radioactivity, in which unstable atoms lose energy, or decay, by emitting radiation in the form of particles or electromagnetic waves.
By physicist Ernest Rutherford showed how this decay process could act as a clock for dating old rocks. Meanwhile, Arthur Holmes was finishing up a geology degree at the Imperial College of Science in London where he developed the technique of dating rocks using the uranium-lead method.
Light travels incredibly fast — , kilometers per second, or , miles per second. On Earth, the delay due to light travel time is a tiny fraction of a second. But in space, the distances are so vast that the light takes a substantial amount of time to travel to us: 8.
The calculation of the light travel time is simple once you know the speed of light and have a measurement of the distance. The speed of light is well known from experiments on Earth, and various astronomical observations confirm that the speed of light has not changed over the history of the universe.
Instead, astronomers use several interlocking methods to determine the distances, such as geometric calculations and brightness measurements. 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. 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. Isotopic ratios between stable isotopes both on the earth and in meteorites are coming under increasingly close scrutiny, to see what they can tell us about the ultimate sources of the very atoms that make up our planet.
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