Radioactive molecules lose energy by radiation from its nuclei. This is known as radioactive decay and is aat an atomic level. It is impossible to predict when an atom will decay but for a significant number of identical atoms it is possible to predict the average amount of decay that would occur. The overall decay rate is known as the half life. In other words, half life is the time taken for something to halve its quantity. An element can have molecules with different number of neutrons. These are known as isotopes and different isotopes have its own half life. Carbon-14 has a half life of 5730 years. It is commonly used to organic material.
This concept so far is pertaining to physics. But can beideas as well. Ideas and knowledge have a half life as well. What we learn today might not be true in some years. When a field is in its growing stage, the half life is quiet small. Indicating new theories are being found that disproves existing ones. In the past it used to take longer to disprove something and change the commonly prevailing notion. Now with a low cost to spreading and sharing ideas, the half life has reduced even more. Secrets also have a half life. It is becoming harder and harder to keep secrets with the exposure to a global medium like the internet around.