History of Time in Timeline

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Time

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.

1907: Einstein's reinterpretation of time and space

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.

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1908: J. M. E. McTaggart's article The Unreality of Time

In 1908, J. M. E. McTaggart's article "The Unreality of Time" argues that time is a self-contradictory idea.

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1955: Invention of the caesium atomic clock

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.

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1967: SI second based on caesium atoms

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.

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May 2010: Smallest time interval uncertainty in direct measurements

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.

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2025: No generally accepted theory of quantum general relativity

As of 2025, there is no generally accepted theory of quantum general relativity that reconciles quantum mechanics and general relativity.

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