Time you were asked to define time,

Time is one of the most fundamental and ubiquitous of the many things we encounter daily. Despite this, it remains elusive, and what exactly it is, remains a mystery to almost everyone. In this essay, the question, “what is time” will be addressed, with theories ranging from the historical Newtonian time, through Relativistic time, Quantum time and a number of philosophical ideas about time, culminating with theories about how time is perceived. 

If you were asked to define time, there is no doubt that your answers would leave much to be desired. This is because no one really understands time. A major issue is that time cannot be seen, it is simply there all around us. Similarly, we are not necessarily aware of the passage of time – only of its effects on us and our surroundings.

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The earliest widely accepted theory about time is Absolute time (sometimes called Newtonian Time after its most famous proponent). Absolute time is the idea that time exists independently of any perceiver, and continues at a constant unchanging pace. It was considered fundamental, and Newton believed that time would exist even if the universe had no matter or energy.
Absolute time is ‘non-relativistic’ time, meaning that it is from a time before Einstein’s theories of relativity. When Einstein’s Theory of Relativity was published, our understanding of time was changed, and the outdated idea of Absolute time was replaced with the idea of time as one dimension in a four-dimensional space-time. 

As a result of this, time is now considered as not merely a sequence of events in chronological order, but past present and future exist simultaneously in four-dimensional space-time. Time does not flow, but ‘just is’. This is in line with Eternalism; the idea that past and future events do exist – just as much as present ones do – and the linear passage of time is merely an illusion of our consciousness. This is illustrated by the diagram below. 

An interesting – though seemingly absurd to our day to day existence – consequence of Relativistic time is time dilation. This is the (proven) theory that time passes different at different rates of relative motion. For example, a spaceship traveling at 99% of the speed of light would appear to an external observer to experience time at half the speed that we do. The onboard clocks would move at half speed, and the astronauts would appear to move around in slow motion. Similarly, at 99.5% the speed of light, time would pass 10 times slower than normal. This continues to increase exponentially until at the speed of light, time stands still. Of course, this property of space-time is only seen at Relativistic speeds (those close to the speed of light). As a result, the effect of time dilation is negligible in our lives. Despite this, an astronaut traveling at close to the speed of light would take 100,000 years to reach a star 100,000 light years away (as gauged by people on Earth). He would, however, appear to age very little, and would still be alive when he reached his destination. This consequence of time dilation has engendered ideas about the possibility of time travel.

Time travel is not possible within the bounds of Absolute time, but could exist within Relativistic time. A basic form of time travel is for the aforementioned astronaut to travel to his star and back. When he returns, more time has elapsed on Earth than it has for the astronaut. As a result, he has essentially traveled into the future. This is, however, not what is really meant by time travel. Rather an irreversible fast forwarding of time. 

General Relativity does, however, allow for time travel through wormholes. The idea here is that space-time is so warped by gravity and travel at relativistic speeds that it bends back on itself (imagine a piece of paper bent round until it touches itself). If this were the case, a short-cut or wormhole could provide a quick route to another period in time. This has been questioned, however by Steven Hawking who suggests that radiation feedback would destroy the wormhole, preventing it for lasting long enough to be used for time travel. Controlling the positioning of the wormhole in space and time is another issue with this idea, and as a result, it can be considered nullified.

Hawking has also suggested that the fundamental laws of nature – chiefly the idea that an effect is always proceeded by a cause – would prevent any form of time travel. Many sceptics also argue that the apparent lack of ‘time tourists’ from the future means that time travel cannot exist. (Arguably an unsubstantiated argument based on the assumption that a future civilisation of humanity will be capable and willing enough to realise time travel.)

There are counter arguments against the whole idea of Relativistic time. Most notably the twins paradox. Imagine that the astronaut previously mentioned returns from his trip to find that his twin sister has aged ten times as much as him as a result of the time dilation he experienced by traveling at relativistic speeds. Of course, there appears to be a paradox in this simple fact, but the true issue lies in the fact that we could just as easily think of the traveler in the spaceship as the one who remains stationary, whilst the Earth (and his twin) accelerate off at relativistic speeds, before returning back. We would therefore expect the twin sister to have aged only one year, whilst the astronaut aged ten – which conflicts with the earlier fact that he aged less. 

This is in fact disproved by Mach’s Principle that the spaceship is accelerating away from the bulk of the universe, whilst the Earth is not. Nevertheless, it is good food for thought.
Another source of criticism for the theory of relativity was from the Soviet Union. Philosophers from the time considered it to be anti-materialistic, and it was banned by the government. Similar scepticism occurred in the People’s Republic of China during the Cultural Revolution.

Despite the criticisms of the time, both General and Special relativity are considered as truth today.

There have recently been large discoveries in the field of quantum mechanics. Quantum mechanics is based around the idea that everything is split into small units of quanta, for example, light is split into photons. As such, many things are not continuous, but discrete. This raises the question of whether time is split into small quanta – that is to say, does it move in small jumps – or is it continuous and infinitely divisible? The general consensus in the scientific community is that time is continuous, and a number of experiments have been carried out to try to observe any small units of time. These have been unsuccessful – with time appearing continuous. A sceptic would argue that our sensory equipment is not precise enough to accurately measure such small instances of time, and that time could still be made up of small units.

Another big question surrounding the physics of time is whether it has a direction. It seems obvious in our everyday macroscopic world that time does have a direction. It is easy to break an egg, but hard to fix it. Indeed, the second law of thermodynamics states that systems will always move towards states of increased entropy (disorder). Examples include that if a building is neglected, it will crumble down into disorder, whereas a pile of bricks and rubble will never form itself into a building. It seems therefore that the direction or ‘arrow’ of time must point in the direction of increasing entropy.

 A counter argument, however, is that there are instances where a system can become more ordered, for example through the development of intelligent life, or the formation of stars from clouds of cosmic dust. Similarly, when an ordered deck of cards is shuffled, it is more than likely that it will end up in a different order, however, a very small chance that it will end up in the correct order. As such, the second law of thermodynamics can be thought of more as an observation of probability than a fundamental law of nature, and the arrow of time does not follow it.

Some scientists argue that the arrow of time could point in different directions, or based on different things in different universes (whether these other universes are real or not is of little consequence). Thinkers such as Steven Hawking say that an arrow of time pointing towards increased entropy is necessary for us to exist (an example is that we must eat food – a relatively ordered form of energy – and convert it to heat – a disordered form of energy – to survive). As a result, we can only observe a thermodynamic arrow of time, as it is the only one that will allow us to exist. This is known as the weak anthropic principle.

Perhaps even more interesting than the physics of time is the philosophical ideas about it. Philosophers have been fascinated by time from the time of Aristotle and no doubt before, and many have speculated about the true nature of time. Questions such as, ‘what is time made of?’, ‘does time have a direction?’ And ‘does time really exist?’ have been debated for centuries. Despite this, there is no solid conclusions to any of them, and so they are still debated to this day.

In ancient Indian philosophy, time was thought of as being in cycles. As such, the universe goes through an endlessly repeated cycle of creation, destruction and rebirth. This was known as ‘Kalachakra’ or the ‘wheel of time’. This belief is still held by some Hindu and Buddhist people today, and is coupled with a belief in a cycle of birth, death and rebirth for individuals. 

This view does, however, rely of the assumption that there is an overall linear ordering of these cycles, with one after the other in something that could be called a ‘hypertime’. In this way, it is impossibly for a purely circular time system to exist.

Early Ancient Greek philosophers generally believed that time, much like space, is infinite. Philosopher Antiphon believed that time was an illusion, rather than a reality, and others at the time saw existence as being only what exists in the present moment, with the past and future being an illusion. Of course, many would argue that the past and future are very much real, and indeed it seems so in our everyday lives. (As a side note, due to the processing of our brains, we experience everything in the world around us a fraction of a second after it happens. As a result, we are arguably always living in the past, and so the past must arguably be real!) Heraclitus – another early Greek philosopher – strongly believed that the flow of time from past, through present, to future is real, and the essence of reality.

Zeno, another Greek philosopher devised a number of paradoxes to argue for the established ideas that time in infinite. One of these is known as ‘Achilles and the Tortoise’. In this paradox, Achilles challenges a tortoise to a running race. Knowing the tortoise to be slower than him, Achilles gives it a head start of 100m. After a certain – finite – amount of time, Achilles has run 100m, and reached the tortoise’s start point. However, the tortoise has now moved on – say 10m. Achilles then runs this 10m, by which point the tortoise has moved on another 1m. This continues on forever, with Zeno arguing that Achilles can never overtake the tortoise, as there will always be an infinite number of positions that the tortoise is ahead of him. Crucially, there are an infinite number of positions, and it is impossible to traverse an infinite number of positions in a finite amount of time, so time in infinite.

Some philosophers argue that this isn’t really a paradox, and that Achilles will obviously overtake the tortoise. Indeed, there would seem to be much anecdotal and experimental evidence to support this, as it is not unheard of for someone to be overtaken in a running race.

Plato believed that time was finite, and was created along with matter by ‘The Creator’. For many today, this is not a viable reason, as belief in ‘The Creator’ has dwindled somewhat since Plato’s time. There was, however, opposition at the time by Plato’s own student, Aristotle. Aristotle believed that time was – unlike the finite universe – infinite, and that the universe has always existed and always will exist. He believed that time was an attribute of movement, and and exists relative to the movement of things. As such, he believed it to be rather like a measure of movement. Despite this, he stressed that is is not the same as movement, as movement can occur at different speeds, whilst, he said, time could not.

Aristotle was also the first to consider a famous paradox about the existence of time. That is that if time consists of two ‘non-things’ – the past or no-longer and the future or not-yet, separated by an instant and infinitely small moment called the present, then it seems stupid to consider time as existing at all. A criticism to this paradox is that the infinitely many present moments add up to a great deal of time, and so time must exist. Also, when you add an infinite amount of anything together, you get something infinite – regardless of how small that something is – meaning that time must be infinite. 

Another counterargument came much later in the theories about the ‘specious present’ of the early 19th century. E. Robert Kelley wrote in his book that that present is merely the most recent part of the past, and argued that the present moment is not instant and infinitely small, but rather has a duration of a certain period of time. William James further developed this idea, defining the specious present as, “The short duration of which we are immediately and incessantly sensible.” HE argued that at any one time, and observer sees a form of motion, and each moment has a distinct earlier and later part to it. As motion must take place during a period, he argued that the present moment must have a duration. This does, however, raise questions such as whether these moments can overlap, and if they are perceived the same by different people. 

After the demise of the Ancient Greeks, there was not much progress in the philosophy of time until the time of the early Christian theologist and philosopher St.Augustine. He said, “What then is time? If no one asks me, I know; if I wish to explain it to one that asks, I know not”, and this captures effectively the inability to define time that I mentioned at the start of the essay. St.Augustine adopted a subjective view of time (contrary to the largely objective views of the Ancient Greeks). He argued that time was a “distension” of the mind which allows us to view the past by memory, the present by attention and the future by speculation.

In the Middle Ages, Christianity and other Abrahamic Faiths were forced to adopt a view known as ‘Temporal Finitism’. This is the view that time is necessarily finite in nature, and came about as a result of their belief in God. Since God created time and space at a defined moment, the past must be finite. They also believed that time will come to an end when the world ends. 
With all the major world religions enforcing this view of finite time, there was not much opposition to their views during the rest of the Middle Ages. The only change was in the exact details of these views. To this day, Young-Earth Creationists believe that God created the Earth, sometime in the last ten thousand years, complete with fossils and rock formations to ‘make it seem old’. Old Earth Creationists, have updated their believes to account for the scientifically proven age of the Earth, saying that the six days of creation were ‘God days’ and much longer than human days. This does not, however, account for the fact that Genesis states that humans were created before animals – including the dinosaurs. Perhaps the most extreme version of Temporal Finitism comes from the 17th Century bishop James Ussher, who decisively concluded that God created the Earth on Sunday the 23rd of October 4004BCE at exactly 6:00pm!

Interest in the nature of time was rekindled in the Age of Enlightenment of the 17th and 18th centuries. Rene Descartes had the unorthodox idea that things have no capacity for constant temporal endurance, but God constantly sustains – or rather continually re-creates – them in every instance of the present. Despite this, there were two main and opposing ideas around this time. They were ‘realism’ and ‘anti-realism’. The first was the Absolute or Newtonian Time that was discussed earlier. This was – of course – championed by Sir Isaac Newton, and asserted that time is a part of the fundamental structure of the universe, and exists independently of the perceiver – understandable only through mathematics. 

The opposition came from Newton’s biggest rival; Gottfried Leibniz. He believed that time is not an actual dimension that ‘flows’, but rather is a convenient intellectual concept that allows us to sequence and compare events in chronology or otherwise. This is the ‘Anti-realist’ view of time, and states that time has no meaning unless it has objects to relate to and interact with. As a result of this, the anti-realist would argue that the events that take place actually are time. Indeed, when we recall a moment in time, all that we can think of is the events that happened in that instant, and this seems to support the anti-realist views on time.

It is in the nature of science that scientists try to see the world in an objective way, and it is as a result of such that almost all of the scientific theories about time have their main oppositions in philosophy. Phenomenalism is a philosophical movement that began in the late 19th and early 20th centuries. It’s founder, Edmund Husserl, said that we cannot have any perception of the immediate present without knowledge of the past, and speculation about the future to put it into context. Indeed, it is very hard (if not impossible) to picture one single moment in time isolated from all others. 

French phenomenalist Henri Bergson asserted that time is composed purely of subjective experience, and cannot be experienced directly – only through our perception. He even went as far to say that the established scientific view of time as a dimension misrepresents reality. The views of phenomenalism are, however, widely refuted. They are inherently solipsistic, and many people find it hard to imagine the universe existing only through their perception. 

One more recent philosophical theory about time is known as the tense-less theory of time. This calls for the abolition of the idea of pst present and future in favour of ordering events with a system where one would say, ‘Earlier than’ or ‘later than’. This comes from the idea that tensed terminology can be easily replaced with tense less terminology (for example, “we will win the game” can be replaced with, “we do win the game at time t, where time t happens after the time of this utterance” Tense-less theorists argue that because this is possible, there is no difference between the past, present and future, all of which are equally real. 

The other side to this argument comes from the tensed theory of time, which denies that this theory is valid, saying that the use of tenses in grammar is for a reason, and reflects the true nature of events. This is the more conventional and widely accepted of the two views (evidenced by the fact that we still use tenses in language).

Finally, the most modern theories in physics and philosophy have engendered a number of more unusual and complex theories about the nature of time. One such idea comes from the concept of alternate universes . In an infinite number of parallel universes, some could have linear time, whilst some have circular time. Time could constantly fork, or different branches of time could merge. Some scientists even argue that there could be more than one dimension of time, with ideas of a six dimensional universe consisting of three space dimensions and three time dimensions considered.

Really, there is no simple answer to the question of, ‘What is time?’. It seems that the more we learn about the nature of the universe through scientific and philosophical thought, the more questions are raised. In my opinion, time is very much real, and likely infinite. Despite this, every person has a different view on the subject. Regardless of its true nature, however, time is still essential to our lives, and everything from our inevitable ageing, to the chronological structure of this essay is centered around it. Most likely, whether time is tensed or tenseless, finite or infinite, continuous or discrete or even just a trick of the mind, nothing will change.