Humanity's Understanding of Time Through Science

Approximately 13.7 billion years ago at a single point in space, our universe exploded into existence and has since expanded into the universe we recognise today, bringing with it forces, matter and energy. Our planet Earth formed 9.1 billion years later, with human life emerging around 200,000 years ago. The very earliest humans would have used the cycles of day and night to keep track of passing time, however since Ancient Egyptian era, civilisations have gradually progressed thus resulting in the discovery of more accurate time measurement devices. The sundial was the first type of clock, the earliest dating back to 1500BC in Egypt, which utilised the varying angles of shadows cast by the Sun throughout the day to effectively track time. These were followed by the Ancient Greek and Roman water clocks, where time was measured by the draining of water at a slow, constant speed into a time-marked vessel. As humanity strived to better its timekeeping, other types of clocks such as candles, incense and hourglasses came into and out of fashion until the era of precision mechanical clocks emerged during the 17th Century. This pioneering invention regulated time using the periodic oscillations of pendulums and springs. Today, physicists have improved upon this method of using natural periodic oscillations to measure time, by applying the same concept to a type of oscillation that is unaffected by the changing world around it; atomic oscillations.

The most precise means of time measurement in the 21st century are atomic clocks, usually made using caesium-133. This system of timekeeping is exceptionally precise and reliable due to the nature of the caesium-133 atom. Caesium-133 has one electron in its outer shell, which can exist in two different quantum states depending on whether the spin of the electron is aligned or anti-aligned with the spin of the nucleus. These two states are known as ‘hyperfine’ owing to their very close spacing, being only 0.000038 eV apart. When the outer electron is excited, it emits a very precise microwave frequency of 9,192,631,770 Hz.

By passing the atom through a microwave field of equal frequency, the atoms resonate. This resonance is then detected by the atomic clock and for every 9,192,631,770 cycles of this microwave frequency, one second will have elapsed. An atomic clock can precisely match the natural oscillation frequency of the caesium atom by passing the atoms through a microwave field with a varying frequency and counting the number of excited atoms that reach the detector. Consequently, the microwave field is tuned to the frequency that causes the highest number of atoms to become excited, which is the resonant frequency used to count a second. This method of timekeeping is accurate to within one second per 138 million yearі and is currently used as the standard unit of time in the International System of Units (SI). The 21st century has seen the greatest advancements since the days of the sundials, due to technological innovation and pioneers of the field who continue to push the limits of how accurately one can measure time, however an important question still remains unanswered; what is time?

In the late 17th Century, Newton proposed that “absolute, true and mathematical time, of itself, and from its own nature, flows equably without relation to anything external”. Such a perception of time would mean that it is completely independent of the space around it and if all matter and forces in the universe were to be removed, time would continue on, unchanged. This ‘absolute’ time is perhaps the simplest explanation, in that it is forever unchanging no matter the observer or location it is observed. Despite its simplicity, absolute time is a good explanation for what occurs in our everyday lives. The field of classical mechanics is centered on this concept of time, giving us descriptions of the movements of everything from planets to projectiles. If the objects in question are macroscopic, not moving at speeds close to the speed of light and not in an extremely strong gravitational field, this description of time remains a good approximation, hence being a functional description for most things we are likely to encounter in our everyday life. However, when those conditions are not met, absolute time breaks down.

The fundamental law of physics, that the speed of light is the same in any inertial reference frame, is the reason why absolute time can only be used as an approximation of what really occurs in the physical world. If a laser were to be bounced off a mirror vertically, the time taken for the light to return to its original position, a ‘tick’, while the frame is at rest would be, where is the speed of light and is the distance between the laser and the mirror. Rearranging this expression to calculate the distance in respect to time gives. If this entire system were to begin moving horizontally at a constant speed v and it were to take a time t’ for a full tick to be completed, by the time the light had travelled a distance d vertically to the mirror, it will have travelled a distance of horizontally. As these are orthogonal, Pythagorus’ theorem states that the total distance travelled by the light to the mirror is. Therefore, the time taken for one full tick to occur in this moving reference frame is. Substituting in the expression found in the rest frame gives t. This equation can be simplified to leave the expression for time dilation, which arose from Einstein’s theory of special relativity and describes the difference in elapsed time for two observers in different reference frames. If this equation approximates to which is why absolute time is an adequate explanation for everyday life, being the same in all reference frames and independent of the space around it. In actuality, time can be warped by gravitational fields, as shown by Einstein’s theory of general relativity and is dependent upon the reference frame of the observer.

While classical mechanics and relativity have their own appropriate applications of time, the concept of time is still disputed in quantum mechanics. Both absolute time and relative time are generally accepted to be continuous and may therefore be divided into infinitely small segments. Quantum theory, however, proves that certain aspects of our universe exist only in quantised states. For example, despite travelling as a wave, light is made of discrete ‘packets’ called photons, giving rise to its dual wave-particle properties. Electrons are also known to have quantised orbits around nuclei, only being able to exist in discrete energy levels.

With many aspects of the universe behaving in a similar quantised manner, one must wonder if time itself could in fact be a discrete, rather than continuous, quantity. Experiments using pulsating lasers to probe chemical reactions have shown that time is continuous at the scale of seconds, however if time were quantised it is likely that the smallest possible division would be on the scale of Planck time, seconds. Planck time is the smallest meaningful quantity of time; it is the time light would take to travel one Planck length in a vacuum.

This length of time is so short that a unit of Planck time fits into a second approximately more times than the number of seconds elapsed since the Big Bang. As a result, it is improbable that an experiment could be realistically devised to measure if time is continuous at this interval. Several theories of quantum gravity, which attempt to combine quantum theory and general relativity, predict the existence of quantum time, while others implicate completely different interpretations of time.

When trying to combine quantum theory and general relativity, time becomes problematic. This is because quantum theory uses absolute time as a basis while, alternatively, general relativity uses relative time. John Wheeler and Bryce DeWitt encountered this problem when they formulated the Wheeler-DeWitt equation in the 1970s, which was an attempt to discover a working theory of quantum gravity. The result of combining the two different ‘types’ of time was that time was altogether removed from their equation. This implied that time was not a real quantity and merely a human construct which had been confused for other physical measurements. The ongoing search for a quantum gravity theory that successfully describes the true nature of our complex universe still challenges today’s physicists.

Furthermore, attempts have been made to combine quantum mechanics with statistical mechanics (thermodynamics). Professor Stephen Hawking investigated this particular field and consequently introduced imaginary time. This concept of time is not imaginary in the sense that it does not exist, but rather that it is expressed in imaginary numbers. It is useful as a tool in theoretical physics because it eradicates certain singularities such as black holes and the Big Bang that have arisen from general relativity, while still agreeing with experimental evidence. When applying imaginary time to quantum mechanics it becomes thermodynamics, which provides a useful means of translating one field of physics into another. Unfortunately, imaginary time cannot be applied to the physical world as it permits movement in any direction, in the sense that one can move backwards in time as easily as moving forwards.

In our perception of the universe, one observes that time is asymmetric and therefore, only the future is accessible. This so called ‘arrow of time’ is described by the second law of thermodynamics; total entropy can only increase over time for a closed system. The implications of this law are that events in our universe are statistically more likely to result in a system becoming more disordered, for example it is easy to scramble an egg, but impossible to return the scrambled egg to its original raw state. This is because a scrambled egg is less ordered than the raw egg and therefore statistically, it is highly unlikely to return to its original, more ordered form. If the universe was not subject to this law, human life could not successfully be sustained. For example, after eating an apple it would be equally likely to reform into its original state as to be digested and fuel the body. According to the second law of thermodynamics, one possible end to the universe is that everything will be absorbed into one supermassive black hole, which would eventually evaporate to the point where there is no possible increase in entropy, and hence time itself will cease to exist. A less probable event would be that the universe eventually stops expanding and starts to contract in on itself. Entropy would then decrease until the point where the universe once again becomes a singularity, as it was at the time of the Big Bang. The arrow of time would reverse, and the universe would expand again in a ‘new’ Big Bang, where the cycle would continue infinitely, causing the arrow of time to periodically switch direction. If this were the case, it would be impossible to perceive which way the arrow of time is ‘pointing’ at the position in the cycle in which one currently exists, making this theory hard to validate.

Overall, man-kind’s knowledge of time has vastly increased since the early musings of the Ancient Egyptian and Greek philosophers, starting with the practical applications of measuring time to aid everyday activity and expanding into investigations of the nature of time itself. If the rate of accumulation of such knowledge continues into the future, it seems likely that the discovery of a universal application of time that is valid in all areas of physics is not far away, bringing with it a unified theory of quantum gravity, which will unlock the truth of the workings of the universe.