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SESK: Stuff Everyone Should Know · sesk.info/eng-third-law-absolute-zero

What limit does the third law place on cooling?

Absolute zero cannot be reached in finitely many cooling steps.

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  1. Absolute zero takes finitely many steps
  2. Absolute zero needs infinitely many steps ✔
  3. Cooling stops at water's freezing point

Why: As temperature approaches 0 K, extracting additional thermal energy becomes progressively harder. The third law says no finite sequence of cooling operations reaches exactly absolute zero.

Huh, didn't know that: Another form of the third law says a perfect crystal's entropy approaches zero as temperature approaches absolute zero.

The dad joke
Absolute zero is always close, but never quite reachable.
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References

Answer: Absolute zero needs infinitely many steps. As temperature approaches 0 K, extracting additional thermal energy becomes progressively harder. The third law says no finite sequence of cooling operations reaches exactly absolute zero.

  1. OpenStax University Physics Volume 2, Entropy on a Microscopic Scale openstax.org/books/university-physics-volume-2/pages/4-7-entropy-on-a-microscopic-scale
    The absolute zero temperature cannot be reached through any finite number of cooling steps.
    Checked 2026-10-10.

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