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:
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.
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?