When Did “The Present” Begin?

It's a harder question than it sounds. Does the present refer to right this second?
Today? This year? The past few years? Fortunately, science has the answer.

Different scientific disciplines, cultures, and religions base their calendars on different "zero" moments. In geology, as well as other scientific disciplines, the epoch of time referred to as "the present" began on January 1, 1950. It's a somewhat arbitrary, compromise of a date, but a fixed moment in the study of things that takes a really, really long time to change or move.

Here's how that date was selected. In the earth's atmosphere, there are carbon isotopes in relatively equal proportion to each other, including the stable carbon-12 and the unstable carbon-14. Plants absorb both kinds, in the form of carbon dioxide, from the atmosphere. When a living thing dies, it dies with inherent carbon-12 and carbon-14, but the carbon-14 immediately begins decaying at a known, constant rate. Scientists can then measure the amount of carbon-14 left in the dead thing to determine its approximate age. This is called "radiocarbon dating," and it's been used to determine the age of naturally occurring materials, like plants, rocks, and human remains, as well as things made from naturally occurring materials, including the Dead Sea Scrolls and the Shroud of Turin.

Radiocarbon dating was introduced in the 1940s, but it became workable, with standardized methods and widely adopted in the 1950s. Around that same time, governments began widespread atmospheric testing of nuclear weapons. Scientists soon realized that radiocarbon dating could no longer be relied upon to accurate date objects. Why? Nuclear weapons so massively alter an environment, and an atmosphere, that they threw off the ratio of carbon-14 and carbon-12, even turning a lot of carbon-14 into carbon-12.

Basically, any radiocarbon testing done after January 1, 1950—a date agreed upon because it fell in the midst of the beginnings of nuclear testing—is somewhat unreliable. The period after is called "the present." Any time before may be referred to, scientifically, as "B.P." or "before present."

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