Science Interviews


Mon, 25th Jan 2016

When time flows backwards

Professor Huw Price, University of Cambridge

Listen Now    Download as mp3 from the show Cosmic Quandries: The Origins of Time

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 - Itís a very good question.  Many cosmologists now think that there is...

Graihagh - Thatís 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 1920ís).  The logical implication of that is that there must have been a time when it was at minimum size and thatís what we now think of as big bang and, on most views, thatís the beginning of time Ė thereís literally nothing before that.  People often find that puzzling and ask the question ďWell, what happened before the big bangĒ but, really, what you have to understand is the answer to that is nothing, because there is no before.  And, interesting, thatís 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 ďwell there was no beforeĒ because one of the things God created was time itself.  He simply couldnít speak of what God was doing before he created the Universe and thatís 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 Earthís hefty mass warps time. Itís why the clocks on orbiting satellites run a little slower and why astronauts on the International Space Station return having aged slightly less Ė 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. Youíd donít have to be in space to experience it though Ė 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.

Thatís 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 Ė 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, itís in built in us that there is a cause and an effect. Things just donít 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 Ė a kind of boundary in time - just as we might have a boundary in space.  So thatís one possibility; another possibility might be that we look for causes in the future as well as in the past and, if thatís 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, thereís 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, itís natural to have a causation that we think of as running forwards.  Thatís the direction in which we deliberate, in which we act on the world, but physics seems to suggest that thatís more a product of our viewpoint on the world than itís a product of anything thatís fundamentally there in nature.

Graihagh - If I get this right thenÖ 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 itís 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.  Thatís the one when weíre deliberating about, you know, what to have for lunch or something and weíre thinking about what the effects in the afternoon might be... But physics doesnít care about that sort of thing.


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Did I miss it? Where's the part about time flowing backwards? I thought Dr. Stephen Hawking concluded that was model was too simplistic and time would not flow backwards even if the universe had enough mass to result in a "Big Crunch" after its expansion is finished. I think you are sensationalizing physics at the cost of spreading misinformation. rkt137, Sun, 17th Apr 2016

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