Back to the Future

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.

Image credit: aussiegall [CC BY 2.0], via Wikimedia Commons

3 thoughts on “Back to the Future

    1. Hi Monica! Glad you enjoyed the article. And no, that’s not a ridiculous question to ask.

      Honestly, I haven’t had a chance to take a close look at that paper. But I can tell you that “no Big Bang” is not really a good way to represent what that paper is saying. There’s some confusion over the meaning of the term “big bang,” and its meaning has changed over the years…I should probably write a post about this. Basically, the Big Bang is a point 13.8 billion years ago when the universe was very hot and very dense and very small, and expanding very quickly. Nobody disputes that the universe was in such a state, and nobody disputes that this state was low-entropy. The question is how it got into this low-entropy state. That paper referenced in that news article is one proposal; another is called “inflation,” and that’s probably the most popular one among physicists, though it has its problems; and there are others as well. (Ethan Siegel has a good post on the different meanings of Bg Bang in the context of inflation, and he also mentions that same paper you’re talking about.) But they all try to address the same fundamental question of how the Big Bang came about – and many of them attempt to explain more generally how the universe started in such a low-entropy state.

      So, in short: that paper is not suggesting that there was no Big Bang, and the Big Bang was definitely a low-entropy state. That paper is merely another attempt to explain how the Big Bang happened. I don’t know if that paper is right, but then again, nobody knows that, including the authors of that paper.

      1. Oh! This makes far more sense now–and goes to show how confusing news reports are on scientific discoveries (and why it’s important to have folks like you around to explain these things!). Many, many thanks!

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