You may want to sit down to read this feature. When considering time, it’s easy to quickly get lost in the complexity of the topic. Time is all around us, ever present, and is the basis of how we record life on Earth. It’s the constant that keeps the World, the solar system and even the universe ticking. Civilisations have risen and fallen, stars have been born and extinguished, and our one method of keeping track of every event in the universe and on earth has been comparing them to the present day with the regular passing of time. But is it really a constant? is time really as simple as a movement from one second to the next? We’re about to find out.

13.7 billion years ago the universe was born, and since then time has flown by to the present day, overseeing the creation of galaxies and the expansion of space. But when it comes to comparing time, it’s daunting to realise just how little of time we’ve actually experienced. The Earth might be 4.7 billion years old, but  we modern humans have inhabited it for no more than 400,000 years, just 0.003% the age of the universe. Feeling small yet? it gets worse. You’ve experienced so little time on Earth that in astronomical terms you’re entirely negligible. you would have to relieve your life 150,0000 times just to match the age of the youngest known star in the universe.

In the 17th Century Newton saw time as an arrow fired from a bow, travelling in a direct straight line and never deviating from its path. To Newton, one second on Earth was the
same length of time as that same second on Mars, or Jupiter, or in deep space. He believed that absolute motion could not be detected, which meant that nothing in the Universe had a  constant speed, even light. By applying this theory he was able to assume that, if the speed of light could vary, then time must be constant. time must tick from one second to the next, with no difference between the length of any two seconds. This is something that you probably think to be true. Every day has roughly 24 hours; you don’t have one day
with 26 and another with 23 hours.

However on 1905, Einstein asserted that the speed of light doesn’t vary, but rather it was a constant (roughly 299,792,458 metres per second). He postulated that time was more like a river, ebbing and flowing depending on the effects of gravity and space-time. Time would speed up and slow down around cosmological bodies with differing masses and velocities, and therefore one second on Earth was not the same length of time everywhere in the universe. This posed a problem. If the speed of light was really a constant, then there had to be some variable that altered over  large distances in the universe. With the Universe expanding and planets and galaxies moving on a galactically humongous scale, something had to give to allow for small fluctuations. And this variables had to be time.

It was ultimately Einstein’s theory that was not only believed to be the truth, but also proved to be entirely accurate. In October 1971, two physicists named Hafale and Keating set about proving its validity. To do this, they flew four Caesium atomic clocks on planes around the world, eastwards and then westwards. According to Einstein’s theory, when compared clocks (in this instance at the US Naval Observatory in Washington DC),
Hafale and Keating’s airbone clocks would be about 40 nanoseconds slower after their eastward trip and about 275 nanoseconds faster after travelling west, due to the gravitational effects of the Earth on the velocity of the planes. Incredibly, the clocks did indeed register a difference when travelling east and west around the World, about 59 nanoseconds slower and 237 nanoseconds faster respectively when compared to the US Naval observatory. This proved that Einstein’s was correct, specifically with his theory of time dilation and the time did indeed fluctuate throughout the universe.

Newton and Einstein did agree on one thing, though-that time moves forward. So far there’s no evidence of anything in the universe that is able to dodge time and move forwards and backwards at will. Everything ultimately moves forward in time, be it at a regular pace or slightly warped if approaching the speed of light. Can we answer why time ticks forward, though? Not quite, although there are several theories as to why it does. One of these brings in the laws of thermodynamics, specifically the second law. This states that everything in the universe wants to move from low to high entropy, or from uniformity to disorder, beginning with simplicity at the Big Bang and moving to the almost random arrangement of galaxies and their inhabitants in the present day. this is known as the ‘arrow of time’, coined by British astronomer Arthur Eddington in 1927. He suggested
that time was not symmetrical, stating: If as we follow the arrow we find more and more of the random element in the state of the World, then the arrow is pointing towards the future; if the random element decrease, the arrow points towards the  past. For example, if you were to observe a star in almost uniformity, but later saw it explode as a supernova and become a scattered nebula, you would know that time had moved forwards from equity to chaos.

Another theory suggests that the passage of time is due to the expansion of the universe. As the universe expands it pulls time with it, as space and time are linked as one, but this would mean that if the universe were to reach a theoretical limit of expansion and begin to contract then time would reverse, a slight paradox for scientists and astronomers. Would time really move backwards, with everything coming back to an era of simplicity and ending with a ‘ Big Crunch’ (as opposed to the Big Bang)? it’s unlikely we’ll be around to find out, but we can postulate on what we think might happen.

It’s incredible to think of the progress we have made in our understanding of time over the past century. From ancient sundials to modern atomic clocks, we can even track the passing of a second more closely than ever before.

Time remains a complex topic, but thanks to scientific visionaries, we are getting closer to unlocking the secrets of this not-so-constant universal constant.

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