According to Buddhist tradition, the final stage of Prince Siddhartha Gautama's becoming the Buddha came after he had rejected the extremes of both luxury and asceticism in his quest for enlightenment. Sitting beneath a Bodhi tree in meditation, he vowed not to stir from that spot until he had found the answer to the problem of suffering.
While he sat there, he had an encounter with the demon lord Mara, who sought to keep him from enlightenment by breaking his concentration. Mara first threatened the Buddha by firing arrows at him, then summoned horrifying demons and violent storms to assault him, then sent his sensual daughters to tempt him, but the Buddha's serenity revealed all these threats to be illusory. Finally, Mara himself approached the Buddha and demanded to change places with him. Mara claimed that he, and not a human being, had the right to sit where the Buddha was sitting. His legions of demons would vouch for him, he said; who would vouch for the Buddha?
In response, Prince Siddhartha touched the ground with his right hand, calling the earth itself to be his witness. At this moment, Mara was defeated, and the Buddha attained enlightenment and freedom from the endless cycle of suffering and rebirth.
This is a lovely story, and a great metaphor. I think we can apply it in a slightly different sense. Today, after several centuries of patient scientific investigation, we've uncovered a glimpse of planet Earth's vast history and the immense and complex biosphere that it supports. Yet there are still people who deny all this and would reduce our world's multibillion-year past to a paltry 6,000 years, who view all its living things as separate and immutable creations of God rather than interrelated twigs on the tangled tree of life.
When it comes to dealing with these people, we, too, should call the Earth to be our witness. The very ground that the creationists walk on, the stones beneath their feet, testify that this world has a far deeper and grander history than the pitiful scrap of time they would allow it.
Radioactive isotopes decay at a known and measurable rate. For any sample of radioactive atoms, the length of time it takes for half of those atoms to disintegrate is called the halflife, and each isotope has a different one. Generally, after about 20 halflives, the number of radioactive atoms remaining from an original sample is too small to be detectable. Some short-lived isotopes, such as carbon-14, are continually produced by natural nuclear reactions (usually as the intermediate decay product of a longer-lived isotope), but many, so far as we know, are manufactured only in exploding stars.
Below is a chart listing all the isotopes with a half-life of one million years or more that are not constantly being produced by natural reactions, and whether that isotope is known to exist on the Earth.
(in millions of years)
Detected on Earth?
yes (by extreme effort)*
When understood for what it is saying, this table shouts out that the Earth is old. According to the mainstream scientific position, our planet originally condensed from a cloud of gas, dust, and heavy elements manufactured in supernovae. Ever since then, the clock has been ticking, as the radioactive isotopes present in Earth at its formation decay and are not replenished. The long-lived isotopes, which in 4.5 billion years would have had less than 20 halflives, are still around. However, those which have had far more than 20 halflives have decayed to such an extent that they are undetectable.
The line between detected and not detected can be drawn between uranium-235, with a halflife of 703 million years, and samarium-146, with 103 million. Thus, not only does this chart show that the Earth is old, it indicates - even without the use of any more precise dating methods - approximately how old the Earth is. For samarium-146 to be undetectable through ordinary methods, the Earth must be on the order of 2 billion years old or more. (More sophisticated methods show that the real answer is about 4.5 billion.)
Evidence like this shatters, beyond all possibility of reasonable doubt, the religious dogmas of a 6,000-year-old cosmos. The only way a creationist could avoid this evidence would be to assume that God decided to create the world with the long-lived isotopes, but without any of the short-lived isotopes, in a way that perfectly mimics what we'd expect to find on an ancient planet and an ancient universe. In other words, the creationist belief makes God into the Great Deceiver, deliberately fashioning misleading clues to deceive us, and then eternally condemning people who fall for the ruse that he himself perpetrated on them. To such bizarre and deceitful speculations are creationists reduced in their effort to deny the evidence that's right before their eyes.
(Credit to the Don Lindsay Archive for this list.)
* Plutonium-244 is a special case. After 4.5 billion years, this isotope has seen 55 halflives, so it should be undetectable - and it is, by ordinary methods. It was found only by an extremely thorough study that refined 85 kilograms of ore and then literally counted the atoms by running them through a mass spectrometer. The total amount found was approximately one hundred-trillionth of a gram. Samarium-146 could probably be detected if someone went to similar trouble. Curium-247, which has had over 280 halflives in the lifetime of the Earth, should be many orders of magnitude rarer, almost certainly beyond the reach of even our most sensitive tools.