Sometime prior to 1500 B.C. in ancient Egypt, the sundial clocks were invented. Since then ‘Time’ has been a topic of profound interest and intensive research. As time passed we evolved from using sundials to looking up time on our watch or phone, from using daytime as a unit of time to using seconds on a daily basis.
But what exactly is Time?
In physics, time is treated as a dimension based on which the evolution of all systems takes place.
Another definition suggests that it is an emergent property of the universe for internal observers but absent for the external observers of the universe, exactly as per the predictions of the Wheeler-DeWitt field equation.
Both definitions are correct, but since we haven’t fully found out what exactly is time, both definitions remain incomplete.
But even amidst this incompleteness let’s try to understand some more about time, so that in the future when physicists finally find out what is time and how it really works we can fully apprehend and appreciate this property of our universe.
The Nature of Time
During the late Renaissance period, an idea was established by Sir Isaac Newton that time is absolute, i.e., time runs equally everywhere and for everybody in the universe.
Then Einstein came along and in 1905 he published a scientific paper named “On the electrodynamics of moving bodies”, which is now known as the special theory of relativity, where he explains the relation between space and time and how time works differently for moving bodies.
Here he stated that the speed of light is a constant in the universe and nothing can go faster than the speed of light itself. But the main part is when he also stated that, if a body gains velocity and starts moving fast, time starts slowing down for it the faster it goes, and if it attains the speed of light then time completely stops running relative to that body of reference.
From there he deduced that time is relative, and not absolute. He also explains, in his general theory of relativity, how time works differently around objects with a significantly large amount of mass (objects such as a planet, star, or other astronomical entities).
Basically, a massive object would have a strong gravitational force and the general theory of relativity states that it would bend the curvature of Space-time in such a way that the closer you move toward the object, the slower the will time run for you. This is known as Time Dilation and is the main reason why we, the people living on the surface of the Earth, experience time slower compared to the satellite sent into the Earth’s orbit or the scientists stationed at the International Space Station in the Lower Earth Orbit (LEO).
Arrows of Time
This idea was proposed by Sir Stephen Hawking in his book, which is also ‘the most’ famous science book ever written- “A brief history of time”, where he explains that an arrow of time is something that distinguishes the past from the future, giving a direction to time.
He also postulated that there are three different arrows of time and they are as follows:
1. Thermodynamic Arrow of Time:
This is the direction of time in which disorder or entropy tends to increase, which is based on the 2nd law of thermodynamics.
To get an even better understanding of this we have to keep in mind that the 2nd law of thermodynamics results from the fact that there are always many more disordered states than there are ordered ones.
For example, consider the color combinations of a Rubik’s cube.
There is one, and only one, combination in which all the 6 centerpieces, 12 edge pieces, and 8 corner pieces completely solve a cube. On the other hand, there are more than 43.25 quintillion combinations in which the pieces are disordered and don’t make a solved Rubik’s cube.
Now, suppose a system starts out in an ordered state instead of a disordered one. As time goes by, the system will eventually evolve according to the laws of nature and its state will change.
At a later time, it is more probable that the system will be in a disordered state than an ordered one because the number of possible disordered states is greater than the ordered ones. This disorder will tend to increase with time if the system obeys an initial condition of high order. [This information will be needed again later]
2. Psychological Arrow of Time:
This is the direction of time in which we feel the passage of time, the direction in which we remember the past but not the future. It is actually dependent on the thermodynamic arrow of time.
To understand this, let’s engage in a thought experiment. Suppose the thermodynamic arrow of time of the universe is reversed and it finishes up in a state of high order, i.e., disorder would decrease with time.
You would see broken cups gathering themselves together and jumping back on top of the table. However, any human being observing the cups would be living in a universe in which disorder decreased with time.
Those human beings would have a reversed psychological arrow of time, i.e., they would remember events in their future, and not remember events in their past (Their definition of past and future would be completely opposite to ours in the first place). It is rather difficult to talk about human memory because we don’t know how the human brain works in detail.
We do, however, know everything about how computer memories work, therefore, we have to discuss the psychological arrow of time for computers. I say it is pretty logical to assume that the psychological arrow of time for computers is the same as that for humans since it was human psychology itself that gave birth to computers in the first place.
A computer uses binary language to store information in its memory that can exist in either of two states (i.e., 0 or 1 in a particular sequence). Before an item is recorded in a computer’s memory, the memory itself is in a disordered state, with equal probabilities for the two possible states.
After the memory interacts with the system to be remembered, it will definitely be in one state or the other. So the memory has passed from a disordered state to an ordered one. But for that to happen we have to necessarily use a certain amount of energy (to power the computer, for example).
This energy is dissipated as heat and increases the overall amount of disorder in the universe. Thus when a computer records an item in its memory and heat is expelled by the computer’s cooling fan, the total amount of disorder in the universe still goes up. This makes the 2nd law of thermodynamics nearly trivial. Disorder increases with time because we measure time in the direction in which disorder increases. It is the safest statement we can make.
3. Cosmological Arrow of Time:
This is the direction of time in which the universe expands. [Bear with me, this one’s going to be long and complex, but interesting]. In Einstein’s general theory of relativity, one can predict how the universe could’ve begun because all the known laws of science would’ve broken down at the big bang singularity. The universe could’ve started out in a very smooth and ordered state.
This would’ve led to a well-defined thermodynamic arrow as well as a cosmological arrow of time, as we observe. But could equally well have started out in a very lumpy and disordered state. In that case, the universe would already be in a state of complete disorder, so disorder could not increase with time.
It would either stay constant, in which case there would be no well-defined thermodynamic arrow of time, or it would decrease, in which case the thermodynamic arrow of time would point in the opposite direction to that of the cosmological arrow. Neither of these possibilities agrees with what we observe.
The classical theory fails to be a good description of the universe when the curvature of Space-Time becomes large enough for quantum gravitational effects to show it’s important. One then has to use a quantum theory of gravity to understand how the universe began. A quantum theory of gravity incorporates Richard Feynman’s proposal to formulate quantum theory in terms of a sum over history.
In this approach, a particle going from A to B does not have just a single history (or a single path in Space-Time) as it would in a classical theory; instead, it is supposed to follow every possible path in Space-Time. A couple of numbers are associated with each history, one represents the size of a wave and the other the phase of the wave, i.e., whether the wave is a crest or a trough.
The probability of the universe having a particular property is given by adding up the waves for all the histories with that property. The histories would be curved spaces that would represent the evolution of the universe in time. But how would the possible histories of the universe behave at the boundary of Space-Time in the past? We do not and cannot know the boundary conditions of the universe in the past.
However, one could avoid this difficulty if the histories satisfy the no-boundary condition, i.e., all the possible histories are finite in extent but have no boundaries, edges or singularities. They are like the surface of the Earth, but with two extra dimensions. This means that the universe would have begun its expansion in a very smooth and ordered state.
Though there had to be small quantum fluctuations in the density and velocity of particles in order to not violate the uncertainty principle of quantum mechanics. The no-boundary condition, however, would imply that these fluctuations were as small as they could be, to maintain consistency with the uncertainty principle. This would explain the existence of the thermodynamic arrow of time.
The universe would start in a state of high order and would become more disordered with time. The psychological arrow of time points in the same direction as the thermodynamic arrow as shown earlier.
Now it is also established that both the cosmological arrow and the thermodynamic arrow of time point towards the same direction as well, i.e., the direction in which the universe is expanding, or the direction of increasing entropy/disorder.
Essentially we are all time travelers because time passes for all of us every second and we travel forward in time into the future as each moment goes by. Aging is proof that we have traveled forward in time.
Now, let’s get into some of the Sci-fi and theoretical types of time traveling. Time travel is a staple of science fiction ever since H.G. Wells ‘The Time Machine’. We can move freely in three dimensions, so perhaps there was a way to move in the fourth dimension, i.e., time.
Wells imagined entering a time machine, spinning a dial, and then traveling hundreds of thousands of years into the future to the year 802,701 A.D. Since then, scientists have studied the possibility of time travel.
A breakthrough was made in 1949 by the great mathematician Kurt Gödel who, in his interpretation of Einstein’s general relativity, found that if the universe as a whole rotated and one could travel around the spinning universe fast enough, then one could enter the past, i.e., you could return before you left. This unorthodox solution befuddled Einstein.
To Newton, time was like an arrow (not the arrows of time explained earlier) which, upon firing, would irrefutably proceed with uniform speed throughout the universe. To Einstein, however, time was more like a river. It could speed up or slow down as it meanders its way across the stars and galaxies, as a result of which it ticks at different rates across the universe.
New theories suggest that the river of time could have whirlpools that might sweep you to the past (these are called closed time-like curves). Or the river of time might fork into two rivers, so the timeline splits, creating two parallel universes in the process.
Sir Stephen Hawking believed that time-traveling to the past was not possible. He believed there must be a hidden law of physics, not yet found, which ruled out time traveling once and for all, which he called the Chronology Protection Conjecture. But since he was unable to prove this hypothesis, chances are, that time travel to the past might still be consistent with the laws of physics (which, by the way, is the case).
Discussions on time travel are incomplete unless we invoke the logical paradoxes associated with it, such as:
- The Grandfather Paradox: If you go back in time to meet your grandfather and kill him, or prevent him from ever meeting your grandmother, then that means your father would never be born in the first place. Then how can you even exist to go back in time and kill your grandfather?
- The Unknown Time Machine Paradox: Suppose someone from the future gives you the secret of time travel. Years later, you build a time machine using that secret, go back in time and give the secret of time travel to your younger self. This turns into an endless loop of you receiving the secret from your future version and going back in time to give it to your younger version again. Then where did the secret of time travel come from?
- Becoming Your Own Mother: This was introduced by science fiction writer Robert A. Heinlein in his novel ”All you zombies’, where he wrote about maneuvering time in a certain way to become your own family tree. Assume that an orphan girl grows up, but changes into a man. The man then goes back in time, meets herself, and has a baby girl with her. The man then takes the baby further back in time and drops off that baby at the same orphanage, and then repeats the cycle. In this way, she becomes her own mother, daughter, grandmother, granddaughter, etc.
The answer to all these paradoxes will ultimately be delivered once the complete theory of quantum gravity is formulated. Even so, the present-day quantum gravitational approach towards time traveling yields a pretty solid hypothesis which is the best answer we got as of today.
It says that whenever you enter a time machine and travel to the past, your timeline splits in two, and you create a parallel quantum universe.
Let’s say you go back in time and save Abraham Lincoln from being assassinated at the Ford Theatre. Then perhaps you’ve saved Abraham Lincoln but in a parallel universe.
Hence, President Lincoln in your original universe did die in the same place at the same time, and nothing will or can change that. But the universe has split into two separate universes, and you have saved President Lincoln in a parallel universe.
In physics, time is mathematically treated as a dimension and as a whole like a stubborn illusion, just as it was thought of by Einstein. But, as mentioned in the introduction, any definitions of time will remain incomplete until we find out what exactly is time and how it came to be (May be due to the big bang, maybe not. Who knows?).
If this article piqued your interest in time then I highly recommend reading Sir Stephen Hawking’s book-“A brief history of Time”, which will definitely be an adventurous journey as he guides his readers through the Cosmos while giving a very brief description of reality, as said in the title.
And who knows, maybe you can deduce something absolutely different and new from it and contribute to our knowledge and understanding of this mysterious property of the Universe.
We will be waiting for that day. Until then, keep on questioning and contemplating the Reality.