Hello! This is Theo here for another post, on the nature of time. Does time exist? Why does it go one way? What about time travel?
Time is possibly one of the most baffling aspects of our universe. In the everyday world, it seems to be constant, stable, and relentless, yet as we have found out in the Irregularity of Time posts (1, 2, 3) on relativity, this is not always the case. In this post, I will talk about the question that has challenged scientists and philosophers from Plato to Hawking: what is time?



Firstly, there is an argument as to whether time exists at all. This might sound absurd, but the two titanic theories of modern physics, General Relativity and Quantum Mechanics, treat time in very different ways. In order to unify these theories, they must treat time identically. Bryce DeWitt and John Wheeler came up with the form of an equation that could describe the whole if the universe which works with both theories, and tine was not included in this equation. Essentially, this implies that time does not exist in their description of the universe, but all of history was instead contained within it.
This sounds quite complicated, but it may help to think of the universe as a roll of film for an old projector. When watching the movie, it feels like time is passing, but if you take out the roll of film and stretch it out, you just see a series of frames. In other words, one does not need time to describe the film. So, whilst it feels like time is passing in the universe, DeWitt and Wheeler argue that like the film, it can be described without it. It seems that this is saying that the passage of time is an illusion, in the same way that that time passing in a film is actually the roll moving through the projector.


Time is, however, considered to be a dimension by almost all physicists, although this is slightly misleading. It is much easier to think of the dimension as duration, when we move forward in time we are only moving in one direction in this dimension. Mentioned in a previous post on Neutrinos, Rob Bryaton has done a great video on the fourth dimension:

 

It seems strange then, that we can only travel in one direction in time, but can move freely in other dimensions. However, Stephen Hawking has an answer. He identifies three main ‘arrows’ of time. The first one is the psychological arrow, i.e. we remember the past, but not the future. The second is the cosmological arrow, in which time is defined as passing in the direction in which the universe is expanding (If there is a ‘Big Crunch,’ where the universe begins to contract then this will become more complicated). The final arrow is given to us by the Second Law of Thermodynamics. This states that entropy, or chaos, will always increase in a closed system. This is easily observable in everyday life. Imagine watching a film backwards. Many things would still look reasonable, such as a ball bouncing. One of the main clues that the film is playing backwards would be people and cars running backwards, although it would be physically possible, just psychologically unlikely, so this is of limited interest from a physicist’s perspective. However, if you saw a vase falling off a window ledge and breaking backwards, it would look impossible. The disorderly pieces would suddenly pick themselves up and arrange themselves into an orderly pattern, which the Second Law of Thermodynamics says cannot happen.
To a certain extent though, the cosmological arrow is quite weak, that is to say it becomes irrelevant if the universe begins to contract. Originally, Hawking thought that time would begin to run backwards at this point: that vases would reassemble, and the universe would simply be playing back everything since the Big Bang in reverse, but since then he has adjusted his theory to state that the Thermodynamic arrow would remain the same (humans most likely will have died out before this happens, so the Psychological arrow is of reduced importance). That leaves the Thermodynamic arrow as being the only main constant arrow of time. Incidentally, there are other arrows, but most are derived from the Thermodynamic arrow. It is this, then, that is our most solid definition of time: the constant increase of entropy.
Entropy  can be thought of disorder. Brian Cox described it very well in a TV show he did recently, if you imagine a small pile of sand next to a sand castle containing the same amount of sand grains. There are a huge amount of ways to rearrange the sand in the pile of sand so that it would stay the same, you can pick up some and pour it back on top and the structure would stay the same, this is because it is so disorganized. This means the pile of sand has High Entropy. On the other hand, almost anything you do to the sand castle would change its structure and therefore it has Low Entropy. The Universe is slowly moving towards a high entropy state and this happens over time.
So to round that up, Stephen Hawking’s arrows of time are:

 

 


But what about time travel? A staple of science-fiction for many years, it is a common misconception that the current laws of physics treat it as impossible. However, this is not the case. Due to Special Relativity, time travel happens every time we move (admittedly on a very small scale). However, the closer you get to the speed of light, the more significant this becomes.

Special Relativity states that time passes slower for an object the faster it is travelling. In everyday terms this discrepancy is negligible, but if we consider long missions on extremely fast spaceships there is quite a large effect. For example, if a spaceship from Earth spent five years (as measured by a clock on board) travelling at 80% of the speed of light, 8 years would have passed on Earth when they returned. This effect increases exponentially as one approaches the speed of light, so that if one reached the exact speed of light, their time would stop completely.

This means that it is technically possible to travel to any point in time in the future simply by travelling very fast. However, to reach 99% of the speed of light in a 10 tonne spaceship would take more than a billion tonnes of petrol! The only energy source that has the potential to propel this spaceship would be antimatter. Even using that, 3 tonnes of antimatter would be needed, and considering that humans have not produced more than 1/1000000 of a gram in total, this is a tall order, and even if it could be done, there would need to be extremely strong dust shields to avoid stray particles ripping through the ship and radioactively decaying. If we could do that, humans could travel to the future. Simples!
Travelling back is harder still. Even if such a time machine was built, and you took a trip to the future tomorrow, it would be impossible to travel backwards further than the moment in which the machine first operated, so unfortunately, we can’t go back to Wembley in 1966 and watch England win the world cup! In some ways, that’s probably a relief considering all the potential paradoxes. But even to return to the present would be hard work: one would have to harness anti-gravity to spin at least 10 neutron stars in a circle to open a wormhole through which the ship could travel. Brian Clegg’s upcoming book “Build Your own Time Machine” promises to explain this further.


This post has been a bit of a whistle-stop tour through the mysteries of time, but I hope you have enjoyed it! Time is a subject that has much left to discover, and I’m sure that before long some of the remaining questions will have been definitively answered.


Check out our last two posts: 
The Irregularity of Time [2/2] -  Why is time moving slower for us on earth than for someone is space? Part of a series (the others are linked within this one) looking a relativity and how time is not as constant as we like to think.
What is Energy? – The word energy is used so loosely these days, so what does it actually mean?