What Did They Know, and When Did They Know It? This old question in law, journalism and commerce has decided the outcomes of criminal trials, ended political careers, and settled the fate of companies. But when it comes to digital information, until recently it was quite difficult to answer. Not Before Time (NBT) is a public broadcast facility that connects information to time in three ways that can be trusted without trusting any single person or organisation: ...
The topic of reversible computers and backwards execution is quite different to logical reversibility. My experiences of reversibility were all driven by correctness in software and it didn’t matter that under the bonnet nothing truly executes backwards. Now AI with its hungry datacentres has made energy a top priority problem to solve, and development of reversibile hardware to achieve thermodynamic reversibility seems feasible. Logical and thermodynamic reversibility Large Language Models are subject to the laws of physics in a bad way, because they use so much power and make so much heat. With reversibility, physics might come to our rescue and greatly reduce the amount of power required. ...
Reversible execution creates computers that seem to run backwards, applying time shifting techniques with simulation/virtualisation. They address the problems of software unreliability and complexity, and I believe my excited comments from 2005 still stand: Reversibility is the biggest advance in debugging since source code debugging — Me, on the GDB developers list In 2026, reversibility still isn’t seen as an ubiquitous must-have for software development, but awareness is increasing. At the same time, the equally interesting topics of logical reversibility and thermodynamic reversibility have become very important. They are not really anything to do with reversible execution, they just sound similar. But if you’re interested in the problems AI datacentres present the world, this kind of reversibility looks highly relevant. ...