but no lead atoms fit in the crystalline
structure.
At this point the uranium is sealed into the crystal.
Uranium-235, decaying to lead-207,
has a half-life of 700 million years
and uranium-238, decaying to lead-206,
has a half-life of 4.5 billion years,
so each sample provides two clocks,
a built-in cross-check to correct for loss of lead.
A beginning
For those whose dislike of infinity
is unquestioned, yes, the earth had a beginning;
therefore, and why not, let’s say
the universe had a beginning, too,
without being too precise
about what we mean by universe.
If we cannot see, if we cannot date
anything farther away, or further back in time,
look how easily our “at least”
starts to look like an absolute beginning.
The Jhomon Sugi tree of Yakushima in Japan, dated by counting tree
rings or by radiocarbon dating, could be 7,200 years old. This is
1/625,000th of the duration of the earth.
The Jhomon Sugi tree of Yakushima in Japan, dated by counting tree rings or by radiocarbon dating, could be 7,200 years old. This is 1/625,000th of the duration of the earth.
See also in The book of science:
Readings on wikipedia: