Time is the continuous, irreversible progression of existence from past to present to future. It governs action, age, and causality, serving as a fundamental component in measuring the sequence and duration of events, as well as quantifying rates of change. Often described as the fourth dimension, time complements the three spatial dimensions in defining our reality.
In 1907, Einstein reinterpreted the physical concepts associated with time and space, challenging the Newtonian idea that time is the same everywhere in the universe.
In 1908, J. M. E. McTaggart's article "The Unreality of Time" argues that time is a self-contradictory idea.
In 1955, the invention of the caesium atomic clock led to the replacement of older astronomical time standards with newer time standards based on atomic time using the SI second.
Since 1967, the International System of Measurements (SI) has based its unit of time, the second, on the properties of caesium atoms, defining it as 9,192,631,770 cycles of radiation corresponding to transitions between two electron spin energy levels of the ground state of the Cs atom.
As of May 2010, the smallest time interval uncertainty in direct measurements is on the order of 12 attoseconds, which is about 3.7 × 10 Planck times.
As of 2025, there is no generally accepted theory of quantum general relativity that reconciles quantum mechanics and general relativity.
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