Read more: 鈥�About time: Adventures in the fourth dimension鈥�
TIME might be mysterious, but there is one thing we can say with safety: there is an awful lot of it about. Modern cosmology dates the origin of the universe to some 13.75 billion years ago 鈥� that鈥檚 a lifetime of 4.3 脳 1017 seconds, give or take a few million billion.
How do we know this? Figuring out the age of the universe involves a complex series of assumptions about its geometry, expansion rate and composition. Yet it is only fairly recently that we have had an estimate to be happy with. Until a few years ago, cosmological models suggested the universe was younger than its oldest stars.
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Now though, says S酶ren Meibom of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, estimates and observations have largely come together. NASA researchers have used the Wilkinson Microwave Anisotropy Probe to study the cosmic background radiation 鈥� considered to be the big bang鈥檚 afterglow 鈥� to produce the most accurate measures yet of used to date the universe. This gave them the figure of 13.75 billion years ().
Meanwhile, methods for estimating stars鈥� ages have improved 鈥� although it is still a notoriously tricky business. Stellar dating involves measuring properties such as mass, chemical composition and temperature, and comparing them with models of how those properties should change over time for a particular type of star. One problem is that many of these models are calibrated by reference to the one star whose age and characteristics we can measure independently 鈥� our sun. 鈥淭hat can make you a little uneasy,鈥� says Meibom.
Closer to Earth, we can feel more confident in our dating skills. We think we know roughly how old our sun and its surrounds are from tracking radioactive decays in lumps of the solar system鈥檚 original material that rain down from the sky: meteorites. The ratio of lead isotopes in the Allende meteorite, which fell in Mexico in 1969, gives it an age of 4.57 billion years 鈥� and, by extension, the solar system is not much older ().
Such radiometric dating stands us in good stead for dating objects from Earth鈥檚 beginnings almost to the present day. In a large collection of radioactive atoms, a set number will have decayed by a set time; by measuring how much of a particular atom is sealed into a rock compared with the products of its decay, we get an idea of how long ago a rock or artefact formed.
For most rocks, the favoured method is uranium-lead dating. Zircons are silicate minerals found in igneous rocks, and they often incorporate small impurities of uranium into their crystal structures. Two isotopes 鈥� uranium-238, with a half-life of some 4.5 billion years, and uranium-235 with a half-life of 704 million years 鈥� decay through two independent pathways, but both end on a stable isotope of lead. This produces a high level of accuracy. 鈥淚t is widely regarded as the gold standard of dating,鈥� says Alan Dickin, a geologist at McMaster University in Hamilton, Ontario, Canada.
For younger rocks and prehistoric human artefacts, other isotopes provide more accurate results. The decay of potassium into argon has been used to date the first appearance of human tools in the Olduvai gorge in Tanzania to around 2 million years ago. The more substantial parts of human history since about 60,000 years ago, meanwhile, are written largely in the language of carbon isotopes. While the standard carbon-12 isotope is stable, a mutant form with two extra neutrons, carbon-14, has a half-life of 5730 years. Plants incorporate both forms through photosynthesis, and from there the isotopes proceed up the food chain. When an organism dies, the carbon-14 slowly decays away. It鈥檚 useful for dating everything from desiccated plant seeds to ice-bound mammoths.
鈥淭he more substantial parts of human history, since about 60,000 years ago, are written largely in the language of carbon isotopes鈥�
It鈥檚 reassuring to know that, once we鈥檙e six feet under, our bodies will keep the passage of time ticking.
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