What is Time?
Overview
Scientists often highlight the following baffling attributes of time:
- It is relative and is NOT absolute.
- Before the Big Bang, it did not exist.
- It dilates while moving.
- It freezes inside a blackhole.
No surprise, these facts may make you puzzled and wonder what time really is.
What Saint Augustine said is true even now. It is difficult to explain time. Does it really have a physical existence? Or is it just an illusion?
Why does it seem to flow from the past into the future? Why do we feel the arrow of time? We will explain it in a moment.
Is time-travel feasible? Can we go back to the past? Is it possible to jump to future?
Let us investigate these questions. Read on.
Mysterious Time
We are all familiar with time. But do we really know what it is? Can we describe it? No, it is not an easy task. Though scientists have learned a lot about the nature of time, they are not able to come up with a clear description of what it is.
Some philosophers and scientists argue that “what we experience as time is just an illusion. It is an artifact of our consciousness.” 4
It is true that we are not able to clearly describe what it really is. However, we know that it has two important aspects.
The first aspect is its continuity. The universe consists of a continuous succession of moments, and it is like a movie reel, with infinite frames, which is continuously running.
The second aspect is the way it progresses – the direction of flow. Time flows from the past into the future through the present. This is known as the arrow of time.
Arrow of Time
One of the strange aspects of time, as we mentioned above, is the way it progresses. It seems to progress from the past into the future through the present.
Space coordinates are not this way. For instance, we can travel from New York to San Francisco and then travel back to New York. Right? Similarly, if you travel north and you can as well go back in south direction. But time seems to behave differently. Why does it always flow in one direction – from the past into the future only?
Scientists believe that the arrow of time is because of laws of thermodynamics, especially the second law of thermodynamics.
Entropy
In order to understand the second law of thermodynamics, we first need to take a look at the concept of entropy.
What is entropy? To put it in simple words, entropy is a measure of disorder in a system.
For instance, consider a new deck of playing cards. The new deck normally has all the 52 cards ordered by suit and each suit is again arranged in the ascending order as in the picture below. Obviously, a new deck has low randomness. To put it in entropy terms, a new deck has low entropy.
Before you start a game with a new deck, you certainly want to shuffle it so that the randomness increases. Once you shuffle it, the disorder increases and so does the entropy.
In 1877, the Austrian physicist Ludwig Boltzmann, one of the giants of thermodynamics, was the first to define entropy mathematically using statistics.
Below is the equation Boltzmann discovered to quantify entropy of a system.
Where s is the entropy of a system, k is Boltzmann constant, and W refers to the number of all possible microstates (arrangements). So, the entropy of a closed system is proportional to the logarithm of the number of possible arrangements.
2nd Law of Thermodynamics
Coming back to the 2nd law of thermodynamics, it states that, in a closed system, the entropy can either increase or remain the same but can never go down on its own.
What does that mean?
Let us consider a scenario. You are holding a glass of water, and it slips and falls on the hard floor. Obviously, the glass gets shattered into pieces. Now, can you unbreak it by any means? No, it is very much irreversible.
The unbroken glass has low entropy (low disorder), and after it is broken, the disorder (entropy) has increased. So, nature seems to tell us an important reality – the higher entropy state cannot go back to a lower entropy state by itself. This is exactly what the 2nd law of thermodynamics says.
Similarly, we can mix milk and coffee. But we cannot unmix them.
It is important to note the common feature in both of these things: irreversibility. It implies that entropy cannot go down from a higher level to a lower level on its own.
You may wonder how the increasing entropy is related to the arrow of time. Increasing entropy is the cause for the arrow of time.
The universe is a closed system. It started with a low entropy at Big Bang. (Why did it start with a low entropy? Nobody knows.) From the Big Bang, the universe has been progressing through events with increasing entropy.
Scientists believe that time is just a manifestation of the increasing entropy in the macroscopic world. We perceive this progress of increasing entropy as the progress of time. Since entropy can only increase, time too progresses only in one direction – from the past into the future.
Now, let us investigate time travel.
Time Travel
A question which every science enthusiast raises often is, is it possible to travel in time as science fiction movies or books portray? Though some physicists argue that it is theoretically possible in certain conditions, most of the physicists believe that travelling to the past is prohibited by nature. It amounts to decreasing entropy, in a closed system, which is prohibited by the second law of thermodynamics. In addition, it violates the law of causality and leads to a paradox, known as the grandfather paradox – what would happen if you went back in time and killed your grandfather?
So, going back to the past is not a reality in nature. It is just fictional.
How about traveling to future?
Jumping to a future time is a possibility provided we travel at extremely high speed, comparable to the speed of light. This happens as result of time dilation. We will have a detailed discussion on it and Einstein‘s Special theory of Relativity in another post.
References
- Mysteries of Modern Physics: Time. A video course by Sean Carroll from The Great Courses.
- The Biggest Ideas in the Universe – by Sean Carroll
- https://www.thoughtco.com/what-is-time-4156799
- https://www.livescience.com/what-is-time