Will the world end? A physicist's reply.

[This is a transcript.]

Will the world end? Someone asked me that on twitter. You might think I’m not the right person to answer that question, but hear me out.

The world as we know it depends on the laws of nature continuing to do their thing. Gravity, electromagnetism, nuclear forces and so on. If they stopped working tomorrow, the world would quite literally end.

Take for example Pauli’s exclusion principle. It’s a law of nature that says that two fermions can’t be in the exact same state. Electrons are fermions, and the exclusion principle is the reason why they sit on shells around the atomic nucleus like layers of an onion and not all together on one shell. The structure of these electron shells is the reason why chemistry works. It’s also the reason why you don’t fall through the floor. If the exclusion principle stopped working tomorrow, Earth would collapse.

Or think of gravity, if gravity became weaker, Earth would fly away into outer space. Or if the vacuum expectation value of the Higgs changed, empty space would rip apart and release huge amounts of energy. And so on. There are many ways the world would end.

But you might say the laws haven’t done that so far, so it seems kind of unlikely they’ll do it tomorrow, right? Well, it’s not that that simple.

This method is called “induction”. You look at what happened in the past and draw conclusions about what’s likely to happen in the future. Unfortunately, this argument doesn’t work when it comes to the laws of nature themselves. As David Hume pointed out already in the 18th century, the issue is that to use the argument, you need to assume that the laws of nature continue the way they currently work. And you can’t use induction to justify the use of itself, that’s a circular argument.

It’s called the “Problem of Induction,” and Bertrand Russell, in 1912, compared it to the attempt of a chicken to infer the laws of living on a farm. Russel said, image the chicken is fed reliably every morning at 9am, until one day the farmer chops off its head. Induction would have told the chicken that wouldn’t happen, and yet it did.

You might say, but if the chicken had just been smarter and made better observations it could have know. And that’s right, stupid chicken. But that would be missing Russell’s point. His point was that the laws might change despite there not being any evidence for that happening. Because you can’t assume that it doesn’t happen to argue that it won’t happen.

Then, how do we know that the laws of nature will continue to function tomorrow. We don’t know. We can’t know. The only thing we can do is have faith in the laws of nature, that’s what science is ultimately built on. Yes, that’s right, science is ultimately based on our faith that the laws of nature will just continue to work.

So will the world end? There’s no way to exclude the possibility. But I’ll give you my word that the world will not end, not tomorrow, and not in the next 100 billion years. And if that turns out to be wrong, I’ll make up for it with a bottle of whisky. Deal?

Thanks for watching, see you tomorrow.