Is it possible to time travel before the Big Bang?

James - Thanks for this one, Daniel. Now, as far as we know, the laws of physics don't allow us to travel backwards in time, only forwards, but we're going to indulge the hypothetical and for that I'm going to need some help. Here's Toby Wiseman, theoretical physicist at Imperial College London.

Toby - The question really is, was there time before the Big Bang? And the answer is unclear, but there are various proposals. So one of them, one that I personally quite like, is one due to Hawking. And Hawking says that actually both time, space and all the matter in the universe was created in a quantum event. And really you can't think of time existing before that.

James - Can I ask you to define a quantum event as best you can?

Toby - By quantum event here, we really mean the universe, if you like, was measured <laugh>. So something somehow came along and measured the universe and forced it into a state. So we know in quantum mechanics you have to measure things and then they decide what they want to do. And the idea here would be that the state it was in is such that when we look at it today and we look back, it appears that it was just created at this instant time along with space and matter. So time was born with the Big Bang, with the whole universe and there was no before.

James - For this theory to be true, we kind of have to be comfortable with the idea that something came from nothing.

Toby - There are indeed other ideas out there. One such as the multiverse, which is quite popular, definitely suggests there was a before. So in that theory, the whole of our universe emerged from a tiny, tiny patch of some rather strange primordial space and time that was cold and possibly very quantum. But then for whatever reason, something caused our tiny patch to suddenly inflate in size incredibly. And then after that, it then heated up and formed what we call now the hot Big Bang. And that's a popular idea as well. The honest truth is we don't really know which, if any of these is the correct proposal at the moment.

James - We tried our best. Daniel <laugh>. I suppose the sticking point here is that this is where we are at in physics, so to speak. We've got our theory of general relativity from Einstein and how we understand spacetime and the physical world at the macro level. And that doesn't quite match up with our understanding of the very small things, the quantum world, which is what we need to get into if we want to understand the Big Bang. And until we find a way of kind of joining up these theories, that question is going to remain a mystery, isn't it?

Toby - That's exactly right. And there are many proposals now, but it's still early days for all of them. And perhaps one day we'll know the answer.