What is time?
In layman’s terms we can define it as the measurement of events in our day-to-day lives. In physics, we can define it as the progression of events from the past to the present into the future.
At first glance, the answer seems obvious. We use the word ‘time’ on such a daily basis that we do not look deep into what time actually is.
Since ancient times, humans have practised time-keeping. We have measured time as days, weeks, months, seasons and years. So we didn’t have a complicated image about time. Even Isaac Newton said that time is absolute. But then, Einstein came into the picture. Up until then time was thought to be independent of the state of rest or motion, position etc. of a body. Thus, time was thought to be absolute for everyone.
In Albert Einstein’s ‘Special Theory Of Relativity’, he proposes a thought experiment: A train is travelling alongside an embankment. There are two points on top of the railway carriage- A and B. Two lightning bolts strike at these two points. To an observer standing on the embankment, the bolts appear to have occurred simultaneously. But for an observer standing on top of the railway carriage, one bolt occurs earlier. It is because the train is in motion. As the lightning occurs, the train moves in the direction of one point and so the bolt that strikes at say point ‘A’ seemed to occur earlier as viewed by the person on the moving train.
In fact, there is nothing like two simultaneous events. If two people claim to hear a sound simultaneously, it cannot be true due to the finite speed of sound i.e., 330m/s in air. So is the case for light. Hence, nothing can be simultaneous because nothing has a speed of infinity. Now, let us take another example, an easier one. Now, one of our observers is standing on the roof of a skyscraper. The other is standing on the ground. A lightning bolt strikes a little above the skyscraper. For our first observer, the lightning bolt and the clap of thunder occur almost together (we have already disproved simultaneity) but for the second observer the clap of thunder is heard after a certain amount of time [since, speed of light=3*100000 km/s and speed of sound 330 m/s]. So although, the lightning may be seen with a minute gap but there is a certain gap between the observers perceiving the sound of thunder. From the above two considerations, it is clear that time is not absolute for everyone. So if time is not absolute, what role does it play in our universe?
According to Minkowski, we live in a four-dimensional ‘space-time’, where space has three dimensions( length, breadth and height) and time has one. If the world is a four-dimensional coordinate system, then we can specify every event with four values- x,y,z(specifying position) and t(specifying time). This four-dimensional space-time is well proven in Einstein’s General Theory Of Relativity. Now, we have a quite clear picture of what time is( or what it isn’t). So, there arises another question. Does time run at the same rate all across the universe? No. Time is affected by a number of factors including speed of an object and gravity.
According to Special Relativity, the faster a thing moves, the shorter it becomes (to an observer, the object in reality doesn’t lose mass). As it approaches light’s speed its shape becomes like a point. Now since things move in ‘space-time’ and not only space, the object that approaches the speed of light will also have time slowed down around itself. Let us take an example- there are two twins. One of them lives on earth and the other is travelling in a space shuttle at the speed of light. When the astronaut returns to earth, he/she finds that their twin is older than them. This happens because for the astronaut, time slows down near the speed of light. This is called Einstein’s twin paradox. Now a question arises- what determines the pace of time for the twin on earth? The answer is gravity.
Let us now imagine our universe to be a four-dimensional ‘plane’- let’s say. Every object on it curves it on account of its mass and gravitational field. Since space-time includes ‘time’ it bends time too. The greater the curvature, the slower time passes around that body. But light isn’t affected by this. If we want to escape an object’s gravitational pull, we need to travel with what is called the ‘escape velocity’. For earth it is about 7 miles per second. Light’s speed is 186,000 miles per second. It is much much higher. So when you are travelling at light’s speed in interstellar space time slows down as you gain speed. Your journey in space must really be a happy one only if you don’t cross paths with a ‘black hole’. Black Hole’s gravitational pull is so strong that nothing can escape it- not even light! The curvature of space inside a black hole is so great that at the black hole’s singularity, time comes to a halt (assumption by scientists). The velocity of light is smaller than its escape velocity. Since the black hole curves space-time, time around it runs slower. Now for some readers, they might get a little confused. At the speed of light too time slows down and also near a black hole. How? You have to imagine this. We know that speed is distance divided by time taken[v=s/t]. So as the quantity of speed increases, the quantity of time decreases. When you are travelling at the speed of light, things around you appear slower than usual because you are at a very fast speed. So, in this way you also run into the future of things around you like your twin on earth. But you are travelling freely, so no gravity acts on you but near a black hole it does. As we have discussed, the curvature of space-time is so great that time runs slow for it and so when you fall into a black hole time slows down for you. Consider this- at light’s speed time around you runs slow down due to speed and in the case of a black hole, you are inside an object for which time runs slow because of gravity. Hope I am making sense. So here the picture should be clear.
So, now let us go back to Minkowski’s four-dimensional space-time. Space has three and time has one dimension. So time is woven into the fabric of space. So why is it that we can move freely in any direction in space but only in one direction in time? In our universe, time runs in the forward direction, so maybe in a parallel universe it runs backwards! But will it ever be possible to run in any direction in time? For now, it is ‘no’ [ for in future we may build time machines or discover+create wormholes]. This is stated by the thermodynamic arrow of time and the second law of thermodynamics. The thermodynamic arrow of time states that time always runs from the past to the present into the future (as stated at the beginning of the article). The second law of thermodynamics states that disorder(entropy) increases with time. For example, you tore a page from a notebook so the notebook is now in a more disordered state than before. In accordance with the law, the page won’t rejoin and lead to order.
The second law of thermodynamics disrupts one of our theories about the universe’s future: The Big Crunch. It is like the Big Bang but in reverse. We know that our universe is expanding. Due to this expansion, the gravitational attraction between galaxies is depleting due to the repulsive force of dark energy which is causing our universe to expand at an accelerating rate. The Big Crunch theory is that in the course of this cosmic tug of war, gravity wins and repulsion(expansion) stops. So, the universe starts to recollapse. Since the universe is a part of space-time, time too will reverse. But this will violate the second law of thermodynamics so we need to abandon this theory. But we cannot for sure know what the universe’s fate will be. Only, the universe knows. (Also the rate of expansion of our universe is high above the critical rate, so it is very unlikely that the universe will recollapse)
Earlier, I had mentioned a term- wormholes. Their existence is also related to ‘time’. Before venturing deep into it, let us learn the basic concept behind them. Let us take a piece of paper and draw two points on it- A and B. At first sight, the shortest distance from A to B seems like a straight line drawn between them. But if we bend the paper and create a tunnel between the two points then the distance gets smaller. Now, the piece of paper is space-time, A and B are, let’s say, two distant galaxies and the straight line is the distance between them in light years. The tunnel between them is a ‘wormhole’. In this way, if we can bend space-time, we would be able to travel stellar distances within very short periods of time without reaching the speed of light! So, time travel is possible but hypothetically. Another theory of wormholes is that they can open up other hidden dimensions for us. This is explained very simply by Sir Kip Thorne: suppose an ant lives on the surface of an apple hanging from a tree. The ant cannot climb up nor can it go down. So, the apple’s surface is its entire universe. Now, suppose a worm eats the apple from side to side and creates a hole in it so that the ant can travel through it. It does not have to go round the apple. So this is the ant’s wormhole! Furthermore, the surface of the apple is 2-dimensional but the hole is 3-dimensional! So, similarly, if we are intelligent enough in the future to create wormholes, we can open new dimensions and travel through time! For future adventures, I hope human intelligence realizes this vision and we can in a way defy the thermodynamic arrow of time.
Time is a tricky concept, ain’t it? It is a key-element of some of our most significant theories regarding the universe. Physicists are today busy formulating a grand-unified theory of everything that explains the entire working of the universe. And with time being such a fundamental concept of physics, it must also be included in our most fundamental theories right? But certain theories exclude time altogether. In fact, some say that time doesn’t even exist! It is an illusion that is all in our heads. Mind blown?
Nobody for sure knows the secrets of our stellar cosmos. For this, physicists formulate theories and engage in a never-ending process of proving and disproving them. Time is one such disputed concept of theoretical physics. In the 20th century, Einstein revolutionized the understanding of time, tomorrow someone else will come up with a groundbreaking idea. We don’t know now and we may never know about the ultimate secrets of the universe. But I just wish for human knowledge not to cease the journey. What is in store for us will only come up within ‘time’.
Written by: Puja Krishnatreya
If you have any queries please feel free to ask! Thank you.