How Old Is the Earth Anyway?

What does how old the Earth is have to do with a blog on moving and living in New Zealand?

Well, at least something since the great New Zealand-born physicist, Ernest Rutherford, discovered how to answer this question which had perplexed mankind for years.

In 1650 Archbishop James Usher of the Church of Ireland, after making a careful study of the Bible, concluded that the Earth had been created at midday on October 23, 4004 BC. Pretty accurate, huh? Not on the 22nd or the 24th—no. And not at nine in the morning or very late at night when supposedly an insomniac God had felt the need to do something.

Notwithstanding how specific Usher’s assertion was, by the 1700’s the science of geology had been born and no serious scientist believed the Earth was that young. Edmond Halley, of Halley’s comet fame, proposed dividing the total amount of salt in the ocean by the amount added each year (salt is added to the ocean by the breakdown of rocks) and thus getting a rough estimate of the Earth’s age, but this experiment proved impractical.

It was known that the Earth was giving off heat, supposedly at a constant rate since its inception, and in the 1770s Georges-Louis Leclerc, a Frenchman, experimented with measuring the rate of heat loss from heated spheres and thus extrapolating the age of the Earth. He concluded the Earth was from 75,000 to 168,000 years old, but faced excommunication for expressing these views.

By the middle of the nineteenth century, the finding and study of fossils (paleontology) was in full swing. Fossils were being found by the thousands; people knew that they occurred sequentially in layers but had no way of accurately dating the distances or separation of these layers. Scientists began postulating that the Earth was not thousands of years old, but rather millions of years old.

Charles Darwin in 1859 concluded that geological processes in an area of England had taken 306,662,400 years (another highly specific number) to complete. Lord Kelvin, a superman scientist in the 1800s—remember the Kelvin temperature scale—took up the challenge of dating the Earth by studying the loss of heat by the Earth somewhat similar to Leclerc. In 1862, Lord Kelvin suggested the Earth was 98 million years old, eventually downgrading his number to 24 million years.

The problem was that as more and more fossils were found, it was recognized that a great number of species had come into existence on Earth and then disappeared, and as the number of ages and epochs the Earth had gone through increased, it became less and less scientifically palatable that whole groups of ancient creatures had been created and then disappeared in what amounted to a short geological instant.

But the problem of dating the Earth remained. By the end of the nineteenth century, depending on what book you read, the time span between the dawn of life in the Cambrian period and the present day was 3 million years, 18 million years, 600 million years, 794 million years or 2.4 billion years. This is when Ernest Rutherford showed up.

Ernest Rutherford

Ernest Rutherford

Ernest Rutherford was born in New Zealand in 1871 in the “back blocks”, a New Zealand term for a particularly rural area. After receiving his education here, he received a fellowship to study abroad and spent his entire research career in Canada and the UK. Known as the father of nuclear physics, he was instrumental in delineating the nature of atom, and was the first to “split the atom” in a controlled nuclear reaction.

But it was Rutherford’s earlier work that provided the tools needed to date the Earth. Rutherford, while studying at McGill University in Montreal, discovered the concept of radioactive half-life (t1/2), where one element through the emission of radiation breaks down at a fixed rate into another element. For example, uranium-238, which is an isotope of uranium containing 92 protons and 146 neutrons, breaks down over time transitioning through a number of other elements to eventually become lead, which has 82 protons. Rutherford recognized that radioactive materials always took the same amount of time for half the sample to decay. This is what is now termed a radioactive element’s half-life, and each radioactive element has its own individual half-life. That is, if the half-life of an element were thirty years, then in thirty years half of the amount of radioactive substance in the original sample would remain. In another thirty years, half of that remaining amount would remain, and so forth. Going back to our example, the half-life of uranium-238 is 4.5 billion years.

Radioactive decay showing half-lives

Radioactive decay showing half-lives

Hence, Rutherford by discovering and using radioactive materials was able to show that enough radioactive uranium had broken down in given samples of rocks such that the Earth was not millions of years old, but rather more likely billions of years old.

In 1904, Rutherford travelled to London to present his findings. In his notes, he relates how it was with some trepidation that he presented his findings to an audience in which the old but still terribly distinguished Lord Kelvin was present. As noted, Rutherford’s discoveries were in marked contradiction to famed Lord Kelvin’s, but as Rutherford tells it, fortunately Lord Kelvin appeared to be soundly asleep during his presentation.

To continue the story, over the next decades the techniques used by Rutherford were refined and expanded. Using radioactive isotopes, Arthur Holmes in 1946 was able to say that the Earth was at least 3 billion years old and most likely older.

Part of the problem at this time was finding sufficiently old Earth rocks, actually the oldest of Earth rocks, and also rocks that weren’t contaminated with lead, since when dealing with calculations spanning hundreds of millions of years, even a small contamination of lead could throw the calculations off. Clair Patterson, another scientist and a male despite his name, deduced that by using meteorites he could essentially have access to rocks that existed at the formation of the Earth and also weren’t contaminated with lead. Over a number of years Patterson collected and evaluated sterile samples of meteorites, and in 1953 presented a paper where he announced that the age of the Earth was 4.550 billion years (plus or minus 70 million years), a number which still stands today.

After discovering radioactive decay, for which he received the Nobel Prize in Chemistry in 1908, Rutherford continued his research into the nature of the atom.

His most famous experiment was performed in 1910. Rutherford, who was assisted by Hans Geiger of Geiger Counter fame, fired ionized helium particles, or alpha particles, at a sheet of gold foil. Surprisingly some of the particles bounced back. This was entirely unexpected. Rutherford said this was akin to firing a 15-inch shell at a sheet of paper and having the shell rebound back into one’s lap. Rutherford realized that some of the particles must be striking something incredibly dense, and he postulated they were colliding with the extremely dense nuclei of atoms. This was the start of formulating the exact nature of the atom— a dense central positively charged nucleus surrounded by mostly empty space containing negatively charged electrons.

Representation of Rutherford's gold foil experiment

Representation of Rutherford’s gold foil experiment

To further convey the size and mass differential of the nucleus compared to the atom itself, as one scientist put it, if an atom were expanded to the size of a cathedral, the nucleus of the atom would be the size of a fly in the cathedral but weigh many thousands of times more than the cathedral itself.

As an aside, physics is felt to be the ultimate science by physicists and they tend to look down on all other sciences. In keeping with this, Rutherford once said ”All science is either physics or stamp collecting.”

 All science is either physics or stamp collecting.

Despite his notable success, Rutherford was not felt to be a particularly brilliant—these things are relative—by many of his colleagues and was not even terribly adept at mathematics. The story is told of him once bumbling the equations related to his discoveries and saying to his colleagues something like, “You can figure it out yourselves.” But everyone agreed he was particularly tenacious, hard working and also open to alternative explanations of natural phenomena. Again, according to his colleagues, he would be willing to work far harder and longer than others to solve a problem and this in great part contributed to his success, his lasting contribution to mankind, and exemplifies the Kiwi spirit at its finest.

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