Rutherford's time bomb

12:00AM Saturday May 15, 2004

Lord Kelvin (left) was challenged by Lord Rutherford.

By KEITH SIRCOMBE

Time, as we know it, began 100 years ago. Scientists today routinely measure time spanning billions of years, but in the early 20th century these ideas would have been scientific heresy.

It was a New Zealander who, among his other more well-known discoveries, made an intuitive leap that gave humanity a way to measure "deep time".

On May 20, 1904, Ernest Rutherford gave a lecture at the Royal Institution of Great Britain about the structure of atoms. That Friday night in London, Rutherford had every reason to be confident. The boy from colonial Brightwater, southwest of Nelson, was on the cutting-edge of a revolution that was shaking traditional scientists to the core and was taking the general public along for a fascinating ride.

But Rutherford had reason to be anxious, because his lecture would touch upon a particularly sensitive issue - the age of the Earth. Worse still, upon entering the lecture hall crammed with 800 people, Rutherford had spotted the chief protagonist of this bitter debate, Lord Kelvin, the pre-eminent scientist of the Victorian era. The scene was set for a classic moment in science - the young brilliant upstart from the colonies facing down a patriarch of the English establishment.

To understand Rutherford's anxiety, we need to look at a chain of intriguing events.

Most cultures have creation myths; in Western culture this is the biblical story of Genesis. The story itself doesn't provide an absolute date, but in 1650 the Irish Archbishop Ussher, following the scholarly methods of the time, calculated a timeline of the Old Testament that included a creation date for the Earth as October 23, 4004BC.

A biblical age of the Earth remained unchallenged among scholars until a Scottish natural philosopher named James Hutton wrote his revolutionary book Theory of the Earth in 1785. Hutton reasoned that the processes shaping the Earth, such as erosion and sedimentation, occur at an extraordinarily slow rate and have always occurred at this slow rate.

He pointed to the stones of the Roman-era Hadrian's Wall between England and Scotland as an example. Despite being exposed for more than 1500 years, the stones in the wall appear untouched by natural processes.

Hutton also examined sandstones in sea cliffs near Edinburgh and concluded that if the processes to create and deposit the sand were extremely slow, then the hundreds of metres of cliffs must represent an enormous amount of time - much longer than the few thousand years provided by strict biblical interpretation.

Hutton concluded that for the Earth there was "no vestige of a beginning, no prospect of an end". His friends described the concept as awe-inspiring yet terrifying, as if they were peering into an abyss.

Although Hutton's ideas were only slowly accepted, the concept that millions of years - possibly even hundreds of millions years - were needed to create the Earth took root.

Acceptance accelerated in the 19th century as Darwin's ideas about evolution also required large amounts of time. However, no one could provide absolute numbers, and the emerging science of geology could only guess at the age of the Earth as being hundreds of millions of years.

Then, much to the chagrin of geologists, who were becoming quite comfortable with an undefined amount of time to play with, the physicist Lord Kelvin entered the picture in 1862.

Applying his famed rigorous logic and precise mathematics, Lord Kelvin deduced that:

1. The Earth was completely molten when it formed and has been cooling ever since.

2. Iron is the predominant material in the Earth. The rate at which a ball of molten iron cools can be measured.

3. This rate can be extrapolated to a ball the size of the Earth, and the age at which the Earth was entirely molten can be calculated.

Thus Lord Kelvin first calculated the age of the Earth as 40 million years. This was an outrage to geologists who believed the age of the Earth had to be much greater to accommodate Hutton's and Darwin's ideas. However they could not counter the seemingly flawless logic - or Kelvin's intellectual reputation.

As bitter arguments raged, he cut the value down to 20 million years and mockingly suggested it could probably go even lower; he even berated the geologists for being "in direct opposition to the principles of natural philosophy". It was into this resentful stand-off that Rutherford walked.

A prodigious student, Rutherford's talent for experimental physics first blossomed at Canterbury College in Christchurch, where he developed devices for transmitting and receiving radio waves years before Marconi.

In 1895, Rutherford won a scholarship to conduct research at the Cavendish Laboratory at Cambridge University in England. Rutherford initially continued work on his radio detectors, but a significant discovery overtook this promising work. In 1896, the French chemist Becquerel left a uranium sample too close to unexposed photographic plates. Upon developing the plates, he discovered some form of otherwise invisible radiation being emitted from the uranium.

Rutherford began investigating these hitherto unsuspected rays and continued this research when he took up the chair of physics at McGill University in Montreal in 1898. In the process, he launched a whole new field of research in physics and chemistry.

It is difficult to look back now and appreciate how astounding the discovery of radioactivity was. A mechanical view of the universe appeared to explain everything and was the scientific basis of the Industrial Revolution.

Some physicists had even gone so far as to announce that all the physical laws and constants of Nature had been found, and the role of physics in the future was merely to refine those values.

Radioactive substances profoundly challenged this cosy view. Here were materials that could generate a large amount of energy without any apparent change; standard chemical parameters such as temperature and pressure didn't change the rate of activity; radioactive "emanations" or gases were produced, then mysteriously disappeared again; and although Rutherford readily identified three types of radiation, the activity of each increased and decreased in complex ways over time.

While at McGill University, Rutherford and the chemist Soddy experimented with samples of radium, and over 1902 and 1903 developed a coherent theory of radioactivity that astounded the world of classical physics and chemistry. They discovered that radioactivity was a product of a radioactive element (the parent) spontaneously disintegrating into another element (the daughter).

Once you sorted through the various types of radiation, this change occurred at predictable and measurable rates. Rutherford later won a Nobel Prize for this research, but ironically for the physicist who had endured much scepticism from chemists, the prize was in chemistry.

During this research, Rutherford made his intuitive leap into deep time. He realised that the predicability of the radioactive processes offered a way of measuring the age of rocks and minerals containing radioactive elements.

An hourglass makes a good analogy. Sand in the top is the amount of parent element left, sand in the bottom is the amount of daughter element created. If you know how fast the sand is falling (the decay rate), measuring the amounts of parent and daughter sand will tell you how long the hourglass has been running. In the case of radioactive elements, that can be hundreds of millions of years.

So in 1904, Rutherford stood on the podium at the Royal Institution with the formidable Lord Kelvin in the audience.

Rutherford knew of rock ages measured in the hundreds of millions of years, but scientists like Lord Kelvin were already deeply sceptical about radioactivity, let alone using it to measure the age of the Earth.

So Rutherford took another tack. In his own words: "I came into the room which was half-dark and presently spotted Lord Kelvin in the audience, and realised that I was in for trouble at the last part of my speech dealing with the age of the Earth, where my views conflicted with his.

"To my relief, Kelvin fell fast asleep, but as I came to the important point, I saw the old bird sit up, open an eye and cock a baleful glance at me.

"Then a sudden inspiration came, and I said Lord Kelvin had limited the age of the Earth, provided no new source [of heat] was discovered. That prophetic utterance referred to what we are now considering tonight, radium! Behold! The old boy beamed upon me."

Building on the work of others such as Marie Curie, Rutherford had noted that the radioactive substances such as radium produced enormous amounts of heat when decaying. Thus, even if only trace amounts of these elements were present inside the Earth, their decay and heat would mean the Earth was cooling much slower - and therefore was much older.

By implying that Lord Kelvin had wisely predicted such a new heat source, Rutherford gained the scientist's favour and the idea was accepted.

However, the absolute age of the Earth was still an unsolved question. Geologists came to realise that because the Earth was constantly recycling old rocks, it was unlikely a pristine rock from the primordial Earth would be found that could be dated directly. Another method had to be found. And here Rutherford made another contribution.

After helping to develop sonar for submarine detection in World War I, Rutherford returned to Cambridge University as the new director of the Cavendish Laboratory.

In the ground-floor of the laboratory, F.W. Aston was developing the first mass spectrometer - an instrument that measures a sample according to atomic weight. Aston coined the term "isotope" to describe those atoms that - although they were, in effect, the same element - had slightly different atomic weights.

In 1929, Aston published a brief paper outlining the discovery of three isotopes of lead at atomic weights 206, 207 and 208. Rutherford used this discovery to deduce that the lead isotope 207 came from uranium isotope 235 and lead isotope 206 came from uranium isotope 238.

Based on the two uranium isotopes decaying at different rates, he was able to calculate the Earth's age at 3.4 billion years. Hutton's vision of vast amounts of geological time had been confirmed.

The lead-uranium method is now a major part of the science of geochronology, or dating earth materials. Rutherford moved on to other projects and became a widely respected and greatly honoured scientist. He was made a peer - the Baron of Nelson - and died in 1937.

After World War II, researchers further refined dating methods and pushed back the age of the Earth to 4.6 billion years by dating meteorites - the primordial building material of the Earth and solar system.

In comparison, a human lifespan is the merest of motes. If the history of the Earth could be compressed into a single year starting at midnight on January 1, then our first human ancestors didn't show up until late in the afternoon on December 31; all recorded history happens in the last 15 seconds before midnight. There is a profound aspect to deep time. Rutherford's research toppled the last pillar of a dogmatic human-centric universe.

To some, these revolutions have demoted humanity to transient apes, adrift in an endless and indifferent universe. Yet we apparently insignificant creatures can look into the unknown, explore it, measure it and, ultimately, understand it. One hundred years ago Rutherford beamed a light into the abyss of time.