Time's arrow

Time seems to have a clear and irreversible direction; physics doesn't recognise this though. This problem has been a big problem in physics
25 January 2016

Interview with 

Dr Julian Barbour, Independent Physicist


Time seems to have a clear and irreversible direction. This is what astrophysicist arrowArthur Eddington coined as time's arrow. It's a big problem in physics. All laws of physics dictate that time works both ways - backwards and forwards - but in everything we observe, time moves forward. The problem is something phycists have been grappling with for over 100 years, that is until earlier this year when Julian Barbour did some modelling, as he explained to Graihagh Jackson...

Julian - If we are right, it may be that we have succeeded in solving one of the really outstanding problems in physics...

Graihagh - Julian Barbour, Physicist.

Julian - Well, first of all it was a publication with two of my collaborators, Tim Kotsolvsky and Flavia Nakarti and we attached the problem that is called - well it's the origin of the arrow of time.  And this has been a great problem for 120 years or so, since the time of the great Austrian Physicist Ludvic Boltsleim.

Graihagh - And that's when he did some work on The Second Law of Thermodynamics - right?

Julian - That's right. It's a very unsatisfactory...

Graihagh - The Second Law of Thermodynamics refers to entropy and how disordered - the universe is becoming. At the beginning, the universe was extremely ordered - everything smushed into something smaller that the atom - but as the universe expanded, everything became a bit more complex and disorganised with the advent of galaxies, stars, planets, asteroids, and even humans all being added to the mix. The second law of thermodynamics just says that things will just get more chaotic as time moves forward, from low entropy to high...

The problem is this: our laws of physics just can't make low entropy conditions that we need for the thermodynamic arrow of time. They just don't work unless you add a series of messy equations to the end and, actually, scientists don't like this. That's because it links to the idea that one day they'll be a theory of everything - one simple equation that explains, well, everything. And this 'special' initial, messy condition of low entropy just doesn't fit the mould.

And so it's no wonder that Julian and many other physicists find this all just a bit troublesome.

Julian - It's very unsatisfactory that we have a wonderful law of nature, particularly gravity, but we seem to have to add, in addition, very special initial conditions to get what the Universe looks like.  Because the Universe is fairly special; we have these processes which all seem to point in the same direction - this extraordinary uni-directionality - all these arrows, all pointing in the same direction.  It seems very unsatisfactory that we can't get that behaviour without making some ad-hoc, very special assumption.

Graihagh - So you've built a model that could potentially overcome this idea of having very special, initial conditions?

Julian - Yes, that's what we've done.  Now, this is only a first step but it seems to be completely new and the remarkable think is, all we've done is really, actually look at dear old Newton's law of gravity.

 Graihagh - They've got a super computer and repeatedly simulated what a 1000 particles would do under the laws of Newton's gravity, not thermodynamics. And guess what? They evolve into a low complexity state roughly analogous to the Big Bang. Thus the sheer force of gravity - not thermodynamics - is what draws the bowstring of time's arrow...

Julian -   What we have thereby shown is that in every single solution of Newton's theory - this actually happens.  There is an arrow of time and it's nothing whatever to do with a special condition because no special conditions have been imposed.  It generally has the potential to change the whole way we think about this problem of the arrow of time.

That's all very well and good but what's this got to do with the beginning of time, I hear you ask? Well, firstly, it means that two futures evolve from one past...

Julian -   Well, effectively the solution divides into two universes really.  They are qualitatively similar but they are quantitatively different.  So you and I wouldn't be talking to each other in the other universe - you and I are on this side of that central region.  There could be intelligent beings - I hope we're intelligent - on the other side of that central region talking about the mystery of the arrow of time.

Graihagh - That's not all. If this holds true, then the big bang wouldn't be the cosmic beginning of time but rather a phase in a timeless and eternal universe, i.e. there is no beginning of time...

Julian - But, of course, I have to emphasise this is still very much early days.  This is just a simple model.  Basically, if we're going in the right direction, we may be able to explain much more just on the basis of the law without adding any additional ingredients like a special initial condition, followed by inflation, and special types of matter and things like that.  Now this is just a hope at the moment but it's an indication that we might be going in the right direction.  In which case, it could be a very big advance and it could lead to quite new ways of looking and thinking about the universe.


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