Physical concept · Physics
Entropy & the Arrow of Time
Cosmology & UniverseEstablished Science
Definition
A measure of disorder that, on cosmic scales, only increases — giving time its direction.
Scientific overview
The second law of thermodynamics says total entropy never decreases. The Universe began in a remarkably low-entropy state, and its steady rise defines the arrow of time. Black holes carry enormous entropy proportional to their horizon area — a clue linking gravity, quantum mechanics, and information.
Open questions
- Why did the Universe begin in such a low-entropy state?
Related topics
Knowledge connections
- Required byHawking Radiation
- Related toHawking Radiation
- Related toThe Big Bang
Sources & references
The primary and reference sources this topic draws on.
- NASANational Aeronautics and Space Administration
Mission data, planetary science, space telescopes, and public-domain imagery.
Most NASA-produced imagery is in the public domain; individual items are checked for usage terms before publication.