A pion is an unstable particle that has an average lifetime of 26.033 nanoseconds (2.6033×10−8 seconds). This is the time interval between its creation in a nuclear process and its extinction into decay.
a pion (or a pi meson, denoted with the Greek letter pi: π) is any of three subatomic particles: π0, π+, and π−. Each pion consists of a quark and an antiquark and is therefore a meson. Pion are the lightest mesons and, more generally, the lightest hadrons. They are unstable, with the charged pions π+ and π− decaying after a mean lifetime of 26.033 nanoseconds (2.6033×10−8 seconds),
The fundamental reason for merging space and time into spacetime is that space and time are separately not invariant, which is to say that, under the proper conditions, different observers will disagree on the length of time between two events (because of time dilation) or the distance between the two events (because of length contraction). But special relativity provides a new invariant, called the spacetime interval, which combines distances in space and in time. All observers who measure the time and distance between any two events will end up computing the same spacetime interval.
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