The arrow of time and irreversibility
Why do physical processes in our universe overwhelmingly run in one time direction (entropy increasing), even though the fundamental microscopic laws of physics are very nearly symmetric under time reversal?
History and references
Still open in the literature as of 2026.
In a section titled 'Three more great problems,' beyond his numbered list of 30, Ginzburg raised the arrow of time, connecting it to the 1964 discovery of CP violation (and, via the CPT theorem, T-violation) in kaon decay. He noted that his teacher Landau considered the law of entropy increase to remain genuinely open, and Ginzburg agreed the question was 'still unclear.' As of 2026 there is still no full, consensus derivation of macroscopic irreversibility purely from microscopically near-reversible dynamics plus the universe's very low-entropy initial (Big Bang) condition; it remains a foundational open topic bridging statistical mechanics and cosmology.
Every fundamental equation of motion works exactly the same played backward or forward in time, yet eggs never unscramble themselves.
References
- What problems of physics and astrophysics seem now to be especially important and interesting (thirty years later, already on the verge of the XXI century)?, V L Ginzburg, Physics-Uspekhi 42 (4) 353-373, 1999
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