Now, think for a moment – is there really no trace? Well, of course YOU know that you DID it, so the trace is inside your head! This information seems safe, but in principle there might exist a sci-fi machinery capable of getting this information from inside your brain, or simply a pair of gentlemen in black suits playing amateur dentists can get it out of you using less sophisticated methods…

However, if you suffered from amnesia, brain damage, or some other unpleasant accident, this information seems to be permanently lost. In this case the question “did it really happened?” seems to be unanswerable.

Recently I rediscovered a paper [PRL 103, 080401 (2009)]  (clearly the original event where I first read the paper did exist even if I had forgotten it) that addresses the above question. In particular, it is suggested that events in which entropy is decreasing might actually happen in our universe, but they necessarily leave no trace. Therefore, one cannot observe them and hence we conclude from observable data that entropy can never decrease. Since the job of a physicist is to study things that we do observe, it seems that these strange events are beyond the scope of physics…