When time flows backwards

25 January 2016

Interview with

Professor Huw Price, University of Cambridge

Time seems to have a forward direction - eggs don't unscramble for example butequations in physics, that's not the case. Graihagh Jackson spoke to Huw Price about this puzzling feature of physics starting with the conudrum that is the beginning of time...

Huw - It's a very good question.  Many cosmologists now think that there is...

Graihagh - That's Professor Huw Price, Bertrand Russell Philosopher at Cambridge University and his research spans vast areas of science and metaphysics, including the philosophy of time.

Huw - This view goes back to the discovery that the universe is expanding (a discovery made in the 1920's).  The logical implication of that is that there must have been a time when it was at minimum size and that's what we now think of as big bang and, on most views, that's the beginning of time - there's literally nothing before that.  People often find that puzzling and ask the question "Well, what happened before the big bang" but, really, what you have to understand is the answer to that is nothing, because there is no before.  And, interesting, that's and answer which was understood hundreds of years before modern cosmology by the great early philosopher, Augustin, who was a Bishop in North Africa in, I think, the 4th Century AD and he was interested in the theological puzzle of what God was doing before he created the Universe.  And his answer to that was effectively the modern cosmological answer "well there was no before" because one of the things God created was time itself.  He simply couldn't speak of what God was doing before he created the Universe and that's the same answer that you get in the big bang model in modern cosmology.

Graihagh - Basically, with the big bang, time was created. Time did not exist before the big bang because the big bang created time. Armed with the best physics of the 20th century, Albert Einstein came to very similar conclusions with his theory of relativity.

Consider time dilation i.e. the effect of mass on time.

Planet Earth's hefty mass warps time. It's why the clocks on orbiting satellites run a little slower and why astronauts on the International Space Station return having aged slightly less - although not by much: so after 6 months, Tim Peak would be about 0.007 seconds younger than he would had he lived on Earth. You'd don't have to be in space to experience it though - if you stood next to a big building or Ayres Rock, and time would run more slowly than if you stood on the flat plains somewhere, like in Cambridge.

That's an aside but the big picture here is that space and time is warped by mass and because at the big bang all the mass in the universe would have been contained in something smaller than an atom - a singularity - it would have brought time to a standstill.

Why then, did time and indeed this infinitely dense singularity, not stay like this forever? And what caused the universe to be in this state to begin with?

As a child, it's in built in us that there is a cause and an effect. Things just don't happen. Something makes them happen. Even when a magician pulls rabbits out of hats, trickery is suspected. Ergo, there must have been something before the singularity... Right?

Huw - So one answer you could give is that really physics tells us that there really is no such thing as causation.  All we can do is just describe the great pattern as we find it in the world and in that pattern, in the big bang patterns of cosmology, it turns out that the first moment time - a kind of boundary in time - just as we might have a boundary in space.  So that's one possibility; another possibility might be that we look for causes in the future as well as in the past and, if that's the case, then the best answer to the question - what caused the big bang - would be to look to the future and basically run the story backwards.  The big bang has to happen because of how things are at later times.

Graihagh - How could you ever look to the future to discover what happened in the past?

Huw - In the physics, there's really no preference between the past and the future at a fundamental level.  In deterministic models you can easily well run the equations in either direction and infer the past from the future, just as you infer the future from the past.  Now, for creatures like us who happen to have a memory that works backwards, so we know more about the past than we do about the future, it's natural to have a causation that we think of as running forwards.  That's the direction in which we deliberate, in which we act on the world, but physics seems to suggest that that's more a product of our viewpoint on the world than it's a product of anything that's fundamentally there in nature.

Graihagh - If I get this right then... Something I do tomorrow could have caused something that I did yesterday?

Huw - Well, that way of looking at things, from the point of view of physics, is just as valid as the ordinary way of looking at things.  Now for many purposes it's much better to look at things from a human point of view than from the point of view of fundamental physics and, from the human point of view, the useful notion of causation is the one that does work forwards.  That's the one when we're deliberating about, you know, what to have for lunch or something and we're thinking about what the effects in the afternoon might be... But physics doesn't care about that sort of thing.

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