Why is it easy to break an egg, but impossible to un-break it? More generally, why is the past so different from the future? We can’t travel into the past, but we’re inexorably carried into the future. We can remember the past, but we can’t reliably predict the future. But strangely, the fundamental laws of physics work just as well backwards as forwards – so why do we perceive an arrow of time? That’s the subject of my new feature article for BBC Earth.
The short answer to this question is “entropy,” as I’ve written about before. But there’s a bit more to it than that; entropy does always increase as we go into the future, but since the fundamental laws of physics work backwards as well as forwards, why doesn’t entropy also increase as we go into the past? The best answer we’ve got for that is the past hypothesis: entropy was very low at some point in the distant past. But why is the past hypothesis true? That’s one of the central questions I tackle in the article.
There is one piece that we had to edit out of the article – well, actually, there were many, but one in particular I want to flag up here. The fundamental laws of physics aren’t exactly the same if you flip them in time. Almost all of them are, but the nuclear weak force isn’t. This is just one of several odd things about the weak force. For example, the weak force also cares about the “handedness” of subatomic particles – it doesn’t treat subatomic particles the same way as it would their mirror images. This is related to its strange lack of symmetry in time, and whole books have been written about this, including one by Martin Gardner. But the reason we cut this out of the article is that literally nobody in the physics community thinks that the weak force is the reason the past doesn’t look like the future. The weak force only governs certain kinds of radioactive decay; it’s not really relevant for most everyday processes like breaking eggs. So my editor asked me if we could leave it out, and I said that was fine, as long as we made sure to say that nearly all the laws of physics are symmetric in time.