Updated: Dec 11, 2020
Time is something that everyone is familiar with
60 seconds is one minute, 60 minutes is one hour. We know this as linear time and is something that everyone is acquainted with and agrees upon.
Time crumbles things; everything grows old under the power of time and is forgotten through the lapse of time.
Understanding time according to physics packs a different story. Let's jump right in.
What is 'time'?
What is the best way you can define time? What is time in a general perspective? If someone asks you to draw time, what would you draw? You would probably sketch a regular old clock or perhaps an hourglass. But hourglasses, pendulum clocks, very recent atomic clocks are all only ways to measure time.
We understand from this time is not a physical object, but a mental concept. Time is one concept we are very familiar with, but why is it so hard to define it? Because time is far more complicated than you realize. For now, let's stick to the definition that time is something that we measure using clocks; it represents a flow of events from past, to present, to future.
How did the concept of time evolve?
According to the legends of classical physics such as to Newton or Gallileo, time was something absolute. In their world, there was something called 'absolute time'; it is not subject to any kind of change. There is one flow time for the entire universe, events take place in its grand, absolute order.
One could with little irregularities measure time, and it would be the same for everyone irrespective of their distance or state of motion. It was an elegant and complete picture to imagine.
But this is physics, it never allows us to have a perfect uniform picture of the world. When everything looks near perfect, someone comes in and messes it up; then we rewrite everything we know.
This view of absoluteness of time changed when Einstein (Yes, it is him again!) published his equations of special and general relativity. It views time as a phenomenon inexplicably tied to space.
General relativity regards time as the fourth dimension and ties it into a mathematical framework called space-time.
Hold on tight, it gets real spooky from here.
Changing your perspective of time:
Before stepping into the bizarre world of moving clocks and gravitational impact, you need to let go of some of your previous understandings of time. Let's take this one step at a time.
Does absolute time really exist?
General relativity put an end to absolute time. Time does not flow at a constant rate throughout the universe, but it dilates. Time dilation is the slowing down of time that occurs under the effects of gravity or motion.
Hence, if you are moving through space, your passage of time is slower than a stationary person.
Also, if you are nearer to a massive object, like the earth, the sun or a black hole; the immense gravitational pull from that object causes your time to slow down.
Your one second is a bit longer than the one second of the person sitting on the top floor of Burj Khalifa.
The time difference is minimal, like a billionth of a second but at enormous distances and faster velocities, the difference is profound.
Read our other post, The 5 step guide to understanding space-time to understand time dilation effects in detail.
What is now?
If you and your friend sitting at the top of Burj Khalifa, don't experience the same flow of time, it automatically takes us to the next question; What does 'now' even mean for us? We now know that your one second and her one second is not the same. Well, now means nothing in the universe's scale.
Let's perform a brief thought experiment. Let's borrow your friend sitting at the top of Burj Khalifa and put her near the star Proxima Centauri; that is approximately 4 light-years away from us. It takes light, four years to reach us from Proxima Centauri.
Imagine somehow you can see her just by looking at the sky and you see her cooking in her kitchen. What you are seeing is not what she is doing right now, but what she was doing 4 years ago. Anything you see now has already happened. This bizarre event is because light takes four years to reach you from Proxima Centauri.
If you are looking at the moon, you do not see how it is now; you are seeing how it was 1.5 seconds ago, the sun 8 minutes ago and some stars ten thousand to many billions of years ago.
What does all of this bizarreness of reality tell us?
The notion of our present is our own, and it does not correspond with the universe. 'Present' exists only in our little bubble; not extended and same throughout the universe. What is 'now' is a meaningless question in the universe's scale. The universe is simply a collection of events not ordered in time.
What is time according to the laws of physics?
The laws of physics do not differentiate between the past, present and the future. Even if we take everything in our universe and change its fundamental constituents into anti-matter particles, the laws of physics would remain the same.
Even if we reverse the order of events, the laws of physics do not change. The physics describing the splattering of an egg can be, with little change, be used to describe un-splattering of an egg.
The laws of physics do not discriminate between the past, present and the future.
The fundamental equations of Newton, Maxwell or Schrodinger do not have the time variable. This brings us to the conclusion that we do not need time to describe how the world works on a fundamental level.
What does that even mean? If physics treats time indifferently, why is our experience different? Why do we continuously experience the flow of time in daily life? Let's learn why.
Is time an illusion?
We know quantum physics explains the tiniest constituents of nature. Quantum mechanics and general relativity treat time in different ways.
The Wheeler-Dewitt equation of Quantum mechanics does not include time at all. Would that mean that time does not exist, at the most fundamental level? Below the value called Plank time (10-44 seconds) quantum effects of time begin to manifest.
Among particles that make up the universe, time seems to work and flow in every direction; both forward and backwards. There is no telling which event happens before or after.
At quantum scales, our understanding of time breaks down.
If time does not exist among the fundamental particles, how does it come to be on the macro scale? Is time real or could time just be some sort of illusion generated by the limitations of the way we perceive the universe? We simply do not know.
Do we really experience the flow of time?
What if we put you in a dark room without windows and ask you the same question? Will you know how much time has passed? You probably don't know, you can only speculate.
We human beings, experience time as change. Changes do not happen in a second, they happen gradually through time.
Imagine we leave you in the same dark room with an apple or a candle. You now know how much time has passed by looking at the candle or the condition of that apple.
Change marks the passage of time.
The question is not whether or not time passes; the real challenge is, why it always moves in a single direction. You are constantly travelling through time, one moment to the next, whether or not you like it.
What is the arrow of time?
In our universe, the flow of time clearly is a unidirectional arrow. Apples rot, but don't go back to their initial fresh state, candles melt but do not un-melt, eggs break but don't un-break. These events, even though it is theoretically possible, do not go in the reversed direction. How to understand this?
However, in the set of fundamental equations, there is one equation that differentiates past from the present. It is the second law of thermodynamics that indicates time in fundamental physics.
According to the second law of thermodynamics, the sum of entropy in a system always increases. You might ask, what the heck is entropy? The answer is simple;
Entropy measures the amount of disorder in a system. High entropy means the system is highly disordered and low entropy means the system is highly ordered.
The direction entropy that always flows from low to high marks the direction of 'the arrow of time'. In our universe entropy of a system always increases. Eggs do not un-break, people don't un-age, apples do not un-rot.
When we apply this low to high entropy to the universe, there arises an even bigger problem. What is the source of all this entropy?
What entropy has to do with the big bang?
The big bang is one of the most plausible theories we have regarding the beginning of the universe. Time began at the Big Bang. What it tells us is this, By the time the universe was 2 minutes old, it was in a highly orderly state, filled with 75% hydrogen, 23% helium and small traces of deuterium and lithium. Conditions that were present at the beginning of the universe play a crucial role in determining the arrow of time.
The universe from the big bang began in a state of low entropy (extreme order), this state is the source of the order we currently witness.
Here's how to understand this view:
If the universe began in a state of high entropy, the levels would go down from there or, it will remain constant. Instead, our universe started in a regular, very smooth and ordered state; which leaves us with a condition of increasing entropy since the beginning of time.
Future is not only what's about to happen next; it is the direction of increasing entropy. The arrow of time starts only at low entropy and flows towards increasing entropy.
How this highly ordered state came to existence is still a question that is left to be answered by modern cosmology. Let's wrap up!
We have learnt that time has a different rhythm in every other place and passes accordingly, and there is no single absolute time. The intrinsic difference between past and future does not exist at the fundamental level. The notion of now does not work in the vast universe, there is nothing that we can reasonably call present. Entropy never decreases in the universe. And the direction of the increase of entropy determines the direction of flow of time. What we don't know is the conditions that made big bang itself possible. We don't know how time, with the characteristics we witness now, came into being.
There's no law of physics yet that states that time did not exist before the Big Bang but it is the beginning of 'time' in our universe. Conditions similar to the Big Bang could explain the arrow of time and the origin of our universe.
However, to answer these questions, we need to unite quantum mechanics with Einstein's theory of general relativity; which we haven't figured out yet. This would provide a precise link between the quantum world of atoms with the macro-world we live in, including the stars, galaxies and black holes in the universe. We call this "the theory of everything" and it is the holy grail modern physics.
Thank you for taking this journey with us. Stay tuned until we explore some mind-bending concept next week. If you want to get notified there is always a subscribe button somewhere nearby.
Research and references:
1. A BRIEF HISTORY OF TIME From the Big Bang to Black Holes. By Stephen W. Hawking. Illustrated by Ron Miller. 198 pp. New York: Bantam Books.
2. THE ORDER OF TIME. By Carlo Rovelli, Penguin; 1st edition (26 April 2018)
3. The Fabric of the Cosmos: Space, Time, and the Texture of Reality (2004) Book by Brian Greene
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