How old is Earth? A word to sceptics on the dating game

Radiometric dating puts paid to some cherished beliefs … kind of. DonkeyHotey

In one respect, science and religion have been largely reconciled since the 19th century, when geologists such as Charles Lyell recognised the evidence for a very old Earth. Within a few decades, most mainstream religious denominations accepted this view as well.

But, much to the consternation of scientists, young-Earth creationism, which holds Earth is only about 6,000 years old, continues to be promoted in some quarters, and remains very popular with the public, especially in the United States.

A 2010 Gallup poll found 40% of Americans believe that “God created humans in their present form within the last 10,000 years”.

A 2009 poll found 39% agreed that “God created the universe, the earth, the sun, moon, stars, plants, animals and the first two people within the past 10,000 years”.

(By contrast, and more representative of OECD countries, only about half as many Canadians espouse such beliefs.)

Such notions, of course, differ vastly to the findings of modern science, which pegs the age of the earth at 4.56 billion years, and the age of the universe at 13.75 billion years.

While there are numerous experimental methods used to determine geologic ages, the most frequently employed technique is radiometric dating, based on measurements of various radioactive isotopes in rocks.

The phenomenon of radioactivity is rooted in the fundamental laws of physics and follows simple mathematical formulae, taught to all calculus students.

Dating schemes based on rates of radioactivity have been refined and scrutinised over several decades, and the latest high-tech equipment permits reliable results to be obtained even with microscopic rock samples.

Radiometric dating is self-checking, because the data (after certain preliminary calculations are made) are fitted to a straight line (called an isochron) by means of standard linear regression methods of statistics.

The slope of the line determines the age of the rock, and the closeness of fit is a measure of the statistical reliability of this conclusion. The graphic below gives the general idea, and more technical detail can be found here.

Samarium/Neodymium isochron of samples from the Great Dyke, Zimbabwe. Wikimedia Commons

Reliability of radiometric dating

So are radiometric methods foolproof? As with any experimental procedure in any field of science, measurements are subject to certain “glitches” and “anomalies”, as noted in the literature.

The overall reliability of radiometric dating was addressed in some detail in a recent book by Brent Dalrymple, an expert in the field.

He argues the few instances in which radiometric dating has produced anomalous results “may be due to laboratory errors (mistakes happen), unrecognised geologic factors (nature sometimes fools us), or misapplication of the techniques (no-one’s perfect)”.

Dalrymple also notes scientists do not rely solely on the self-checking nature of radiometric dating to confirm their results. They repeat their measurements to eliminate laboratory error, and wherever possible they apply multiple dating procedures to the same rock sample.

As he notes: “if two or more radiometric clocks based on different elements and running at different rates give the same age, that’s powerful evidence that the ages are probably correct.”

Along this line, the physicist Roger Wiens asks those who are sceptical of radiometric dating to consider that “all of the different dating methods agree … a great majority of the time” that Earth is billions of years old.

The scientific disagreements highlighted by sceptics are “usually close to the margin of error … a few percent, not orders of magnitude!”

Radioactive isotopes and the age of Earth

Until recently, only large scientific laboratories could afford mass spectrometers, the principal tool used to measure dates of rock samples.

But recently the prices of these devices have dropped to levels that even amateur meteorite hunters and others can afford. Used mass spectrometers are currently available at eBay for as little as US$99.

Some people have suggested the most hardcore flat-Earth believers did not give up their fight until they could hold a GPS receiver in their hand that gave their latitude-longitude position.

Will sceptics of old-Earth geology wait until mass spectrometers are in every home before finally conceding that the earth is more than 6,000 years old?

The burden of proof

Radiometric dating, as with any other experimental discipline, is subject to a variety of errors, ranging from human error to rare anomalies resulting from highly unusual natural circumstances. But while errors and anomalies can occur, the burden of proof is not on scientists to fully account for each and every error.

The burden is on sceptics to explain why tens of thousands of other carefully measured ages are all internally and externally consistent. Indeed, there is no known physical phenomenon that can yield consistent results in many thousands of measurements, year after year, except one: the isotopic decay in these geological specimens, measured by radiometric dating.

As biologist Kenneth Miller observed: “The consistency of [radiometric] data … is nothing short of stunning.”

A version of this article first appeared on Math Drudge.