Week 3: constructor theory · constructor-theory
Irreversibility from constructor theory
Derive the second law of thermodynamics as a fact about which tasks are reversible.
In this lesson
- Read the core idea (2 min).
- Walk through one worked example.
- Take the 4-question quiz; pass with 4/4 or better.
Hook
Walk into any first-year physics course and the second law of thermodynamics is presented as a fact about probabilities: entropy almost always increases because the macrostate with more microstates is overwhelmingly more likely. That story works, but it leaves something unsatisfying — the law looks like a statistical accident, parasitic on the underlying time-reversible micro-laws. Constructor theory offers a different starting point.
In Marletto and Deutsch's 2014 reformulation, irreversibility is a statement about which tasks are possible. The second law is downstream of a more fundamental fact: that a certain class of tasks ("reverse this process") is forbidden by the laws, in a sharp, on/off sense — not as a probability but as an impossibility. Statistical-mechanical entropy increase is a consequence, not the source.