We have all been asked the question 'what is the time?', but this entry will be addressing the subtly different question: 'what is time?' Many people, on being asked this question, would probably say that they don't have the time to answer. In this entry, we will explore many theories about the nature of time. No one theory has unquestioning truth about it, so it will be up to you to decide on the one you see is best.

The Classical Approach to Time

There is not very much to say on this theory since it is the one with which we are most familiar. Traditionally, time is simply seen as a measure of the distance between events. It has a past, present and a future. The past is considered to have already happened and to be unchangeable, while the future is considered to be open to many possibilities. Humans measure time using many units, some based on real events like the rotation of the Earth, others that are even more arbitrary.

Sir Isaac Newton's classical description of time in his highly-regarded work Principia is that it 'flows equably of itself', which means that time 'flows' at a constant rate that is the same for everybody - it is independent of the events that take place in it. It would be untrue to say that this idea was unchallenged until the twentieth century - the 18th-Century empiricist philosopher George Berkeley, for example, disagreed with Newton and held that time was 'the succession of ideas in the mind' - but there was no serious evidence to suggest that Newton's elegant and absolute description was wrong until Einstein destroyed it.

Unfortunately, the classical view of time is biased towards the human perception of the 'flow' of time. We see events in one direction, and we assume time to be the same everywhere. The classical approach to time does not explain exactly why we perceive time in this way, and it does not describe how the effect is achieved. The other theories of the nature of time challenge the very roots of this natural point of view.

Relativity

The Theory of Relativity is the celebrated discovery of the physicist Albert Einstein. Originally, it was two theories: the Special Theory of Relativity came first in 1905 and states that the rate at which time passes is not the same all over the universe - it is dependent on the observer (in other words, it is relative). It is not hard to see that different people perceive the passing of time at a different rate to others: as we get older, less information is processed about our surroundings per second, so we perceive time to be going faster.

But Einstein's theory went further than this. The relativity of time is based not on our age, but on our speed of movement through space. The faster we travel through space, the slower we travel through time. Although this sounds crazy at first, it makes sense when thought of in a particular way. The theory of relativity demands that we view space and time not as separate entities but as one concept called space-time. Time becomes a fourth dimension, just like the other three dimensions of space that we are used to (height, width and length). This view of time is crucial to understanding most of the other theories about time's ultimate nature.

Humans only possess two-dimensional retinae (the light-receptive surface at the back of our eyes), which means that we can only see in two dimensions. Our vision of the third dimension is a result of perspective and the existence of our binocular vision. If we had three-dimensional retinae, we would be able to see all of an entire room simultaneously - its walls, its floor and its ceiling at the same time! For this reason, it is very difficult, if not totally impossible, for humans to visualise a fourth dimension.

To overcome this impairment, it is useful to use lower-dimensional analogies when talking about dimensions above three, even if we are talking about time as one of these dimensions. So in this case, let us imagine that the universe is shaped like a cuboid¹, and that humans are two-dimensional and have one-dimensional retinae. Imagine that the spatial dimensions are the width and height of a cross-section of the cuboid, meaning that humans can move up, down, left and right at will within the cuboid. Imagine that the depth of the cuboid is time.

Right, now imagine that you are a two-dimensional human within the cuboid and that you start off being midway up the cuboid. Then you start moving upward (ie, through space, but not time). Eventually you hit the edge of the cuboid. Now imagine that you move downwards, but that you also move through time in a forward direction. This time it will take you longer to get back to being mid-way up the cuboid because you are not taking a direct route downwards - you are also moving through time. As you can see, moving through time slows down your movement in space.

It works the other way around too. If you stay still in space and just move forward in time, then it will take less time to get to a particular point in time than if you move upwards and forwards in time simultaneously. So movement in space slows down your movement in time. This is what relativity states about how time really is. However, the amount by which time is slowed down when you move through space is very small in everyday situations, and you would need to move at a speed of a considerable percentage the speed of light in order for it to make any noticeable difference.

Relativity has been proven too. Atomic clocks² have been placed in aeroplanes moving at high speeds and then compared with clocks that were on Earth. Slight differences that are exactly predicted by the mathematical equations of relativity were indeed detected.

The general theory of relativity goes a step further and was published in 1916. Einstein stated that mass curves the 'fabric' of space-time to create the illusion of the force of gravity. Again, a lower-dimensional analogy is best. Imagine putting bowling balls on a sheet of rubber. They bend the rubber. Any object coming into the vicinity of the curve begins to move around the curve like something spiralling around a sink basin. Einstein's picture of gravity is that simple. And again, this has been proved. Einstein made predictions about how light would be taking technically curved paths around large masses, and this effect was measured during a total eclipse of the sun.

Time and Determinism

You will have noticed that the theory of relativity does not carry any description of a 'flow' of time, and in fact, it describes time in almost exactly the same way that we are used to thinking about space. Relativity unifies space and time. All points in space are in existence simultaneously - this is common sense; so are all points in time in existence simultaneously too? This would suggest that all events in time are already 'here' and that there is no scope for choosing what happens in the future. This view of time is called determinism because events are pre-determined.

It is worth noting that relativity does not rule out the idea of free will, but does not provide any support for it either. Many people can get upset about the evidence supporting determinism because humans like to think they have a free will to make independent decisions. Such people would not feel better if they heard about the many worlds theory of quantum mechanics.

Time in the Many Worlds Theory of Quantum Mechanics

To understand this theory, we need to go back to our cuboid example. You will notice that each event in time is simply a cross-section of the cuboid (a square). Humans effectively perceive the dimension of time in this cuboid to be a succession of these squares. Like frames in a movie, these create the illusion of a smooth passage of time. But why is it that we see time like this? The answer to this question will be explored later.

If you think about the world around you, you will most likely notice that it seems to have been tailor-made for life. The universe has the precise properties that led to the formation of life on Earth. For example, in the early universe there was a 'battle' between matter and anti-matter³. The particles with the certain quantum properties that we now characterise as 'matter', for a hitherto inexplicable reason, won the battle. If this hadn't happened, we could not exist, or we would not be the same as we are today. Many physicists have speculated that this and other similar events are too much of a coincidence to be regarded as just that: a coincidence.

Martin Rees, the Astronomer Royal of the UK, paints an analogous picture of going into a clothes shop. If you go into a clothes shop that only sells one size of clothing, it would be a big coincidence if you found it was your size. However, we get no surprise when finding our own clothes size in a clothes shop because good clothes shops sell a wide range of clothes sizes. We can now extend this picture to the universe. It is very unlikely that the universe should exist because of how biased it seems to have been towards gravitational stability and the creation of diverse life later on. However, if we see the universe as providing a wide range of 'universes' of different properties, it will come as no surprise if we find one universe that supports life.

You can think of this theory as multiple cuboids in a vast universe of cuboids, all with their own space-time. Each cuboid represents one universe that has a different set of laws of physics, and therefore could be wildly different from all the other universes. There may in fact be a large number of universes that support life but with small differences, just as there might be many shirts of your size in the clothes shop, but perhaps in different colours.

In this view, there are multiple timelines. Some people have likened this view of time to train tracks. We move along a train track in one direction, but there are huge numbers of other train tracks running parallel to ours. Each train track may be different in some way (it might have trains on it, for example). For this reason, the other universes around us in this 'multiverse'⁴ are referred to as parallel universes.

A multiverse of space-times is not just a theory that solves the question of why our environment is so suited to life; it is also a theory of quantum mechanics. In the quantum theory there are many events that take place because of random chance. In electric currents, for example, the electrons that make up the current follow a random path in the wires that is influenced by a fields of electrical forces they pass through, which is why it always seems that the current is split 50:50. Many physicists believe that with each quantum decision like this, every possibility has a separate universe in which it is enacted. Hence, in one universe the electron goes one way; in another, it goes the other way.

In this theory - which is called the many worlds interpretation of quantum mechanics - every possibility gets enacted. Since quantum interactions are the fundamentals of any larger (or 'macroscopic') reaction, we can infer that everything happens in one universe or other. So if you have a decision to make, say whether to take a holiday to Hawaii or not, there is one universe where you go, and one universe where you don't. This also spells trouble for free will. All possibilities get played out, so it is just a matter of which universe you are in to determine which way you go.

There is a variation of this theory. For this variation we will need to think another dimension lower. So, instead of imagining universes as cuboids, we need to imagine them as rectangles. Imagine the length of the rectangle is time; and its other dimension, space. The rectangle has no thickness whatsoever, so if you put multiple rectangles (ie, multiple universes) on top of each other, the whole structure becomes no thicker. This version of the many worlds interpretation is slightly easier to grasp, because otherwise we would have universes branching off from one another to eternity, which is rather difficult to imagine. There is no real evidence for or against either of the theories of the multiverse, unfortunately.

You will have noticed that in all these theories, time has two directions just like all the other dimensions. In theory, there is nothing to stop us from moving in the other direction. There is another slightly different theory of time as being bi-directional, and you might also be interested to see how this could lead to possibilities of time-travel.

Why Do We Perceive Time the Way We Do?

What, then, is time? If no one asks me, I know what it is. If I wish to explain it to him who asks me, I do not know.
- St Augustine.

If time is a dimension just like all the others, why do we experience it so differently? This is the question that interests James Hartle of the University of California in Santa Barbara, along with physicists Stephen Hawking, Murray Gell-Mann and Steven Weinberg. They believe that the passage of time is just an illusion.

Hartle thinks that time's arrow is a product of the way we process information. Gell-Mann gave creatures that process time in this way the name 'information gathering and utilising systems' (IGUSs). Humans are IGUSs. Because of our two-dimensional retinae, we can't take in multiple cross-sections of the 'cuboid' - ie 'frames' of time - simultaneously. We gather information about this frame - our surrounding environment - using our senses, and then we store the information in an input register. This does not have an unlimited capacity, so we have to transfer the information to our memory registers before we can input the information about the next frame. Humans have a short-term and a long-term memory, as well as our cerebellums that store 'unforgettable' information (such as how to swim).

IGUSs also carry something called a 'schema', which is a generalised model of our perception of our environment. It holds several rules about what is best to do and what is not a good idea to do. The information we receive from our surroundings is passed to the schema to determine how we react in certain situations. The decision is conscious, but we also do unconscious computation of information: the schema is updated unconsciously. The conscious part of the IGUS in humans focuses on the input register, which we call the present. The unconscious part focuses on information in the memories, and we call that the past. This is why we consciously experience the present and remember the past.

The movement of the information through the IGUSs registers creates the illusion of the flow of time. It is not time itself that flows. Each IGUS has a different speed for the flow of its information between registers. This corresponds to differences between the perception of the speed of the flow of time. Flies, for example, need to process more information per second in order to fly so quickly but still avoid common obstacles; therefore, they perceive time as going slower. To us, a fly's perception of time would look like slow motion. Flies only live for a few days, or a few weeks as a maximum, and to a human, this is a very short lifetime. But to a fly, this feels a lot longer.

So the reason that we experience a 'flow' of time could just be because of how we process information. It is a competitive advantage to us as a species to process information bits at a time. It wouldn't make sense for us to have evolved with the capability to see all time simultaneously.

Digital Time, or, Is Time Like a Movie?

You may have noticed a reference to 'frames' of time in the explanations above. We usually think of time as continuous - a smooth passage of events. However, most physical theories define space and time as being the opposite of a continuous passage of events. M-theory and Loop Quantum Gravity, for example, are both serious scientific theories (not proven theories, though) that state that space and time have minimum units. There was even a theory of quantum mechanics to suggest that time was made of particles called 'chronons'!

The theorised minimum length of time possible is called the Planck time and is equivalent to 10^-43 seconds. When space or time is 'digital' like this, we say that it is 'discrete'.

If this theory is true, then our perception of time could be like a movie. Movies are not continuous: if you slow them down enough, you see that they are just collections of still photographs played in quick succession. We process information about our surroundings and obtain a picture just like one frame of a movie or animation. When 'played' in quick succession, this creates the illusion of smooth, continuous movement.

Is Time Really That Much Like Space?

So far, time has been seen as very much like a dimension of space, and its passage in one direction has been seen as an illusion. But there are some counter-arguments; there are still some big differences between time and space that cannot easily be explained as illusions.

One way of supporting the idea that an 'arrow of time' is irrelevant is by proving that all processes are the same if done forwards or backwards. In quantum mechanics, most interactions between particles are 'time-symmetric' - it doesn't matter whether you look at them from past to future or future to past because they look the same. But this is not true of macroscopic objects. Wine glasses shatter, but you rarely see shards of glass assemble themselves into wine glasses. Physicists can explain why shards of glass do not form wine glasses by postulating the existence of 'the thermodynamic arrow of time'.

There are four laws. The third of them, the Second Law, was recognised first; the first, the Zeroth law, was formulated last; the First Law was second; the Third Law might not even be a law in the same sense as the others.

The gist of it is that the universe is always becoming more disordered. The disorder of the universe is called 'entropy', so we say that entropy is always increasing. Nobody really knows why this is the case, but we see it all the time in experiments. This is why heat always flows into colder areas, but never the other way round. Heat is simply the result of giving particles in a given system more energy; they begin to move and vibrate randomly, which is a disordered state. Colder things are more ordered because their constituent particles tend to be harder to move.

This in-built arrow explains why macroscopic objects have irreversible interactions. This is a clear difference from space. If you think of the spatial manifestation of a table, it does not follow that one end of the table is more disordered than the other, but it does follow that the table will end up more disordered in the future than when it has just been made. Hence, there is a very distinct difference between time and space.

Can Time Be Reversed?

If time's 'flow' in one direction really is an illusion, what is there stopping us from reversing it? In theory, nothing! Lawrence Schulman of Clarkson University in New York thoroughly believes that time can run backwards. In other words, shards of glass can turn into wine glasses, people grow younger and younger and the universe gets smaller and smaller.

In fact, Schulman goes as far as to say that such reversed-time zones can exist as spaces within our own universe. A computer simulation has shown that regions with opposite time arrows do not cancel each other out and do not disturb each other at all. The great thing about this theory is that if a civilisation in a reversed-time region kept records of events that occur in our future, the records might have survived to our past (which is their future). Finding these records could tell us the future. This is, of course, a long shot, but still a physical possibility.

Another possibility is that the universe's arrow of time (as far as thermodynamics is concerned) will naturally reverse itself at a crucial point in the history of the universe. At this point, the universe would start to get smaller and everybody would get younger until there was a big crunch analogous to the big bang. This creates a perfect symmetry to the universe.

Again, there is little evidence that shows us that reversed-time regions exist, and there is no evidence that the universe's thermodynamic arrow of time will naturally reverse itself. Equally, there is little evidence against these theories either.

So what is time? Is it a dimension just like space? Does it flow, or is that just an illusion? Is time digital like the frames of a movie, or does it flow continuously? And can time really be reversed or manipulated? None of these questions can be answered with definite confidence, but next time somebody asks you what the time is, perhaps you'll think of the answer differently.