December 18, 2009

The age of the Earth-12: The final synthesis

(My latest book God vs. Darwin: The War Between Evolution and Creationism in the Classroom has just been released and is now available through the usual outlets. You can order it from Amazon, Barnes and Noble, the publishers Rowman & Littlefield, and also through your local bookstores. For more on the book, see here. You can also listen to the podcast of the interview on WCPN 90.3 about the book.)

For previous posts in this series on the age of the Earth, see here.

The minimum age of the Earth kept getting pushed back as older rocks kept being found and methods of analysis improved with the invention of new instruments such as the mass spectrometer. The minimum age was raised to 1.90 billion years in 1935, 3.35 billion years in 1947, to 3.45 billion years in 1956.

But there arose a new problem. Astronomers had discovered that the universe was expanding and Edwin Hubble's (1889-1953) discovery of the law now associated with his name enabled scientists to estimate the time when the universe would have begun, and they initially arrived at an age of 1.80 billion years (Jackson, p. 251). It was absurd to suppose that the universe was younger than the Earth and this caused some consternation. But as the reach of telescopes increased and greater and greater expanses of the vastness of the universe came under observation, the calculated age of the universe kept increasing, to 10 billion years by the early 1950s, to 13 billion years by 1958, and finally to the present value of 13.7 billion years. Thus the potential paradox of the universe being younger than the Earth was resolved.

To obtain the age of the Earth we would need to find rocks that were formed at the time of the Earth's formation. But the problem with finding those rocks is that plate tectonics would likely have crushed most of them and so they are not easy to obtain. Despite this, determined efforts have steadily unearthed older and older rocks. According to the US Geological Survey:

The oldest rocks on Earth found so far are the Acasta Gneisses in northwestern Canada near Great Slave Lake (4.03 Ga) and the Isua Supracrustal rocks in West Greenland (3.7 to 3.8 Ga), but well-studied rocks nearly as old are also found in the Minnesota River Valley and northern Michigan (3.5-3.7 billion years), in Swaziland (3.4-3.5 billion years), and in Western Australia (3.4-3.6 billion years)… In Western Australia, single zircon crystals found in younger sedimentary rocks have radiometric ages of as much as 4.3 billion years, making these tiny crystals the oldest materials to be found on Earth so far. The source rocks for these zircon crystals have not yet been found.

The final major shift in the age of the Earth came when Clair Patterson (1922-1995) studied the age of meteorites that had crashed to the Earth. It was assumed that these meteorites were formed at the same time as the Earth and the solar system but since they arrived here much later were less likely to be contaminated by the ebbs and flows of Earth's geological history. Patterson studied two samples from the Canyon Diablo meteorite that fell in Arizona about 50,000 years ago and reported in 1953 that their ages were 4.510 and 4.570 billion years. More and more meteorites from all over the globe started to be analyzed and the ages were all consistent and converged to 4.550 billion years. The oldest moon rocks have been measured to be between 4.4 and 4.5 billion years old, which is consistent with the other dates.

So that is where we are today, with the age of the Earth determined to be 4.55 billion years, with an uncertainty of about 1%.

The search for the age of the Earth is truly a remarkable story. There are many lessons that the search can teach us but the one that I want to pick out to end this series of posts is how the desire for consistency among the fields of geology, paleontology, chemistry, physics, biology, and astronomy all played a role. One of the consequences of this search has been the intermeshing of these fields, theories in one being interwoven with theories in the others. It is this interconnectedness that gives strength to the conclusions.

Those people who think the Earth is 6,000 years old are living intellectually in a time before the Enlightenment. It is not simply that the value they have for the age of the earth is wrong, even absurdly so. It is that they think they are living in a time when facts were largely isolated things that could be accepted or rejected individually. This is no longer true. Nowadays a scientific 'fact', although identified with one particular field, is the product of a large network of theories encompassing many fields that were once separate. To get an authoritative answer to the question of the age of the Earth you would probably go first to someone identified as a geologist, but her answer will not be the product of just geological research but of a whole complex of theories that spread far and wide. To reject such a fact without exploring the consequences it has for all the other elements that went into its production is to reject science altogether.

Pierre Duhem, one of the earliest people who spoke about the interconnectedness of science, captured this beautifully in a metaphor comparing how a watchmaker and doctor operate:

People generally think that each one of the hypotheses employed in physics can be taken in isolation, checked by experiment, and then, when many varied tests have established its validity, given a definitive place in the system of physics. In reality, this is not the case. Physics is not a machine which lets itself be taken apart; we cannot try each piece in isolation and, in order to adjust it, wait until its solidity has been carefully checked. Physical science is a system that must be taken as a whole; it is an organism in which one part cannot be made to function except when the parts that are most remote from it are called into play, some more so than others, but all to some degree. If something goes wrong, if some discomfort is felt in the functioning of the organism, the physicist will have to ferret out through its effect on the entire system which organ needs to be remedied or modified without the possibility of isolating this organ and examining it apart. The watchmaker to whom you give a watch that has stopped separates all the wheelworks and examines them one by one until he finds the part that is defective or broken. The doctor to whom a patient appears cannot dissect him in order to establish his diagnosis; he has to guess the seat and cause of the ailment solely by inspecting disorders affecting the whole body. Now, the physicist concerned with remedying a limping theory resembles the doctor and not the watchmaker. (my italics)

Duhem wrote this in 1906 in his book The Aim and Structure of Physical Theory. I do not think it a coincidence that this was the very period when convergence of scientific theories was occurring.

(Main sources for this series of posts are The Chronologers' Quest: The Search for the Age of the Earth (2006) by Patrick Wyse Jackson and Lord Kelvin and the age of the Earth by Joe D. Burchfield (1975).)

POST SCRIPT: There's a Jesus on a spring?

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