Every civilization, every culture, every religion has its stories about how the Universe came to be – for most of history, this has meant how the Earth came to be. Interestingly, however, while most cultures found a solution to the how part, very few even thought to ask when this creation took place. In Christianity, those who were interested simply added up the generations listed in the Bible – in 1650 this careful examination of Biblical ages led Bishop James Ussher to conclude that the Universe (including the Earth) was created on or about October 22, 4004 BC – an answer that was more precise, but in the same ballpark of the calculations of John Lightfoot a decade earlier.
For the next few centuries such calculations continued to fall into the realm of religious interpretation and authority but, in the 19th century, with science becoming increasingly powerful, religious authority began giving way to scientific explanation and calculation. And for about a half-century – from the mid-1800s through the first decade of the 20th century – these attempts became ever-more sophisticated, pushing the birth of our planet back ever further using a variety of techniques, including looking at sediments accumulating in the oceans, the oceans’ salinity, changes in the Moon’s orbit, the cooling of the Earth, and more. By the 1890s these calculations were putting the age of the Earth at a few billion years – half as old as we now know the Earth to be, but considerably closer to our current understanding than was poor Bishop Ussher, using the best techniques available in his time. And then the physicists got involved.
In the 1850s William Thomson (whose scientific work earned him the title of Lord Kelvin in later years) earned a reputation as one of the foremost physicists of the age, making important contributions in electrical theory and then going on to help develop the field of thermodynamics. Turning his thermodynamics expertise to the problem of how quickly the Earth would cool off, Kelvin realized that the Earth would cool from an initial molten state to solidifying completely in no more than a half-billion years, and most likely in only about 100 million years or less. Such was Lord Kelvin’s reputation and the esteem with which he was held that his pronouncement was taken at face value, leaving science with a dilemma – the ample evidence of an ancient Earth versus the conclusion of the most respected scientist of the day.
Like any good scientist (and be assured, Kelvin was a great scientist who deserved the accolades he was given), Lord Kelvin recognized that his knowledge was not perfect, so he added “…unless source now unknown to us are prepared in the great storehouse of creation” to a statement he made on this subject in 1862.
In 1895 Henri Becquerel discovered radioactivity when a uranium sample wrapped in photographic paper caused the paper to be exposed as though it were exposed to light. Over the next decade humanity’s understanding of radioactivity exploded, leading to “new physics” that brought us to (among so much else), quantum theory and an entirely new understanding of the atom, none of which was even close to being known in 1862. But Becquerel, the Curies, New Zealand physicist Ernest Rutherford, and many others were learning about radiation and radioactivity and they were discovering a huge amount – including the fact that radiation carried energy from the nucleus of unstable atoms and deposited it in whatever was around them, raising the temperature of the absorber.
It doesn’t take much radioactivity to produce a LOT of alpha, beta, and/or gamma radiation; one millionth of a Curie (the American unit of radioactivity) will undergo 2.22 million radioactive decays every minute. That’s an awful lot of decays and it’s reasonable to wonder why the radioactive sources I use to help test radiation instruments aren’t at least warm to the touch (which they aren’t – I was working with some just a few days ago). Here’s the thing – a high-energy beta or gamma has an energy of about 1 million electron volts (MeV) and it takes 26 trillion of them to produce a single calorie of energy. So it takes a 10 milliCurie source to produce enough energy to heat one gram (1 ml) of water by one degree Celsius.
So how (you might ask), do higher-activity sources produce thermal energy? I mean, there are photos of plutonium glowing red from the energy of the radioactive decay so clearly that can happen. Well, this is partly due to the fact that a single kilogram of weapons-grade plutonium emits about more than 2 trillion high-energy alpha particles every second, and most of that energy is trapped inside a disk-shaped lump of metal that is compact enough to emit that energy more slowly than it accumulates. In comparison, the sources that are more likely to be used undergo only a fraction the number of radioactive decays and are only a small fraction of the size, so they can radiate the energy away as quickly as it’s produced. That being said, there are a number of spacecraft in the outer Solar System that are kept running by the heat generated through the radioactive decay of lumps of plutonium onboard (radio-isotopic thermal generators, or RTGs) – with enough decay energy, sources will get hot enough to glow.
What this has to do with the age of the Earth is that the Earth contains a lot of radioactivity, albeit at low concentrations. This means that a mound of dirt or even a fairly rich uranium ore isn’t going to have enough radioactivity to cause it to glow red-hot; but when we sum up all of the energy given off by all of these elements (and, in the case of uranium and thorium, their decay progeny as well) the total amount of energy they give off is enough to keep the Earth warm for billions of years longer than Lord Kelvin calculated.
Rutherford pointed this out to Lord Kelvin at a scientific meeting. Repeating Kelvin’s comment that the Earth would be long-ago cold in the absence of “a source now unknown to us” and pointing out that radioactivity was a phenomenon unknown at the time of that statement, Rutherford noted that the discovery of radioactivity, and of its ubiquity in the rocks of our planet, could well explain how the Earth could be both ancient and still-warm. If I recall properly (I can’t find the book in which I first read this account), Kelvin was not entirely convinced – but at that time, radioactivity was still being studied and there was as much (or more) conjecture as there was established fact. In any event, history has proved Rutherford correct – calculations show that the decay of natural radionuclides in the Earth produces more than 20 TW (one TW is one million megawatts) of energy, which is enough to stretch out the cooling time of the Earth into the billions of years, rather than the millions calculated by Kelvin.
Coincidentally, the same uranium and thorium that keeps our planet’s innards hot and molten also make it possible for us to determine our planet’s age. In other words, they pose the problem and also provide a solution. How cool is that?