Written by the Fiveable Content Team • Last updated September 2025
Written by the Fiveable Content Team • Last updated September 2025
Definition
Planck time is the smallest meaningful unit of time in the universe, approximately 5.39 × 10^-44 seconds. It represents the time it takes for light to travel one Planck length in a vacuum.
5 Must Know Facts For Your Next Test
Planck time is derived from fundamental constants: Planck's constant, the gravitational constant, and the speed of light.
It marks the earliest moment in the history of the universe that we can theoretically describe.
During Planck time, all four fundamental forces (gravity, electromagnetism, weak nuclear force, and strong nuclear force) were unified.
The concept of Planck time is critical for understanding quantum gravity and string theory.
Events before Planck time (approximately 10^-43 seconds after the Big Bang) are speculative due to limitations in current physical theories.
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Related terms
Planck length: The smallest measurable length in the universe, approximately 1.616 × 10^-35 meters.
Quantum gravity: A field of theoretical physics that seeks to describe gravity according to quantum mechanics principles.
Inflationary epoch: A period shortly after the Big Bang where the universe expanded exponentially within a fraction of a second.