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The most fascinating thing about nature is not how the human body is constructed by laws of physics. But it is how the physical reality is structured by biology of human mind.

Time is the most intriguing concept constructed by human mind. It is the framework over which we experience change and motion in reality. Does it truly exist or is it just a creation of our consciousness? Perhaps it is an outcome of interplay between something in the reality and human mind. Everyone can intuitively understand Time and yet, it is the most difficult thing to describe or define. Reflecting on the nature of Time is a fascinating experience that is unparalleled by any other aspects of nature. Ideas that are used to gather insight into Physical Time like fabric of spacetime, invariance of speed of light and thermodynamic arrow of time are pinnacles of human imagination and curiosity.

Time is non punctual

Space-time tells matter how to move; Matter tells space-time how to curve

John Wheeler

Would you believe if I said that a tall person’s brain ages faster than a shorter person?

It is a conceptual leap to imagine and understand that the passage of time is not a constant throughout the universe. Our experience of reality gives us the impression that Time is an universally valid framework. If my clock ticks 10 seconds then my friend’s clock also ticks exactly 10 seconds even if he is on the other side of the globe. But as our insight into gravity and light expanded, the notion of absolute Time disappeared. Imagine a thought experiment in which you are a triplet and all three of you have decided to challenge the understanding of time. You chose to stay on earth while one of your triplet has gone to moon and the last one has boarded a spaceship that moves at a velocity close to light. All three of you have decided to meet on earth exactly a year later. When your moon brother arrives for the get together, you would not have completed a year and your sister on the spaceship would still have a plenty of time to go. By the end of the experiment, you all would have aged differently! The ticking of Time is not the same everywhere. It varies depending on your speed and the closeness to a gravitating mass.

Invariance of Speed of Light

299,792,458 meter per second – This magical number is the speed at which light travels through empty space. But the knowledge unraveled by this numerical value is lot profounder than what appears superficially. Rather than informing us about the velocity of light, this number actually redefined our concepts of meter and second!

It is natural to perceive light as something instantaneous. We don’t normally think of it as having a finite travel speed. Within the limits of our living world, the starting point and ending point of light occurs simultaneously. In fact the calculated speed also confirms our simple perception. At this speed, light can travel around the earth 7 times in one second. So it is natural to believe and accept that such a thing has infinite speed. But 350 years ago, an astute observer, who was looking at one of the moons of Jupiter from an observatory noticed an interesting phenomenon. He observed that the time taken by the moon to be in an eclipse behind the Jupiter was not a constant but changed repeatedly with every eclipse. He concluded that the effect was due to the change in the distance between him and the moon, as earth and Jupiter were revolving around the sun. This confirmed that light actually travels at a finite speed. A even more startling discovery that changed our understanding of space and time happened when our experiments revealed that the speed of light is a constant irrespective of our relative motion! This finding baffled human minds. Imagine that you are traveling in a train with a goal to beat the speed of light. At the starting point you measure the light speed as 300,000km/s. You begin the race and reach a speed of about 200,000 km/s. Then you look through the window to measure the speed of the flash of light that you are trying defeat. You will be stumped to find out that light is still traveling at 300,000 km/s ahead of you instead of being only 100,000 km/s. You had accelerated further and reached a speed of 290,000km/s and then done the measurement. It stumps you again. No matter how fast you travel, light always seems to be speeding away from you at the same speed. Its not just that you couldn’t defeat it but, you weren’t even able to begin a race against it!

If speed of light stays the same irrespective of the motion of observer, then the reality of space and time itself must be changing with the frame of reference! This is absolutely a marvelous insight into the nature of reality around us. There is no single universally valid frame of reference! Everything about space and time is relative! We just have a tiny sliver of space called the ‘surface of earth’ where time and space appear to be absolute, and it is just a buffoonery that human societies argue about a single absolute point of view in everything.

Arrow of Time

We look at the world once, in childhood. The rest is memory.

Louis Elisabeth Gluck

Nature has an undeviating loyalty towards arrow of time. It always proceeds spontaneously in one direction. A direction in which a physical quantity called entropy of the system increases or stays the same with time. It is impossible to walk backwards against this insurmountable barrier set forth by nature. A drop of ink put in a glass of water, always spontaneously goes to a state of uniform mixing. No matter how many times you repeat that experiment, it will never change the direction of evolution. The end point is the same – a state of thermodynamic equilibrium. The drop will never become a clump in the bottom of the glass or break into three exact droplets and stay still. Also the ink solution will never spontaneously walk back in time to yield that ink drop no matter how many centuries we wait. It is these irreversible events in nature that construct Time and make it unidirectional. Nature is full of such irreversible events in which matter and energy progresses from a low entropy point to an uniform state of thermodynamic equilibrium. As time unwinds, heat flows from hot to cold objects, water becomes vapor, seeds become trees, single cell divides into two cells and life goes on… but never backwards.

Time Travel Paradox

We did not come to fear the future. We came here to shape it.

Barack Obama

Our notion of time includes two distinct properties of nature and its experience. All confusion and paradoxes occur when these two entities are mixed up. Time is a ‘formless thing’ that separates events in the world that could be distinctly perceived by our mind and/or measured by our instruments. This formless thing can be quantified by relating it to cyclical events in nature, like dawn and dusk or waves of light. The other notion of time is “past, present and future”. This concept of “past, present and future” encompasses a series of events in the states of matter and energy in the world around us. In this context Time is only used as a label to identify such events. So Past is not just Time. It is a collection of many different states of matter that are merely Time stamped. It has nothing to do with the ‘formless thing’ that separates sun rise and the sun set. The expression ‘going back in Time’ has no meaning in reality. It is merely an illusion created by memory and our metaphorical language. I hate to digress, but it is important to highlight here that metaphors are not merely a language tool. It is an aspect of our conceptual system that we use to understand reality. However metaphors can deceive as much as it inform. I metaphorically understand ‘Time’ as a path along which ‘I’ travel, and ‘past, present and future’ are stations along this journey. This is nothing more than an elaborate illusion created by our mind. A more meaningful way of describing time travel is to say, “Recreate a past state of matter at present”. It is impossible to go ‘back in time’ to when you were 5 years old. But it is possible to recreate with sufficient knowledge and energy the exact state of matter of you and your world when you were 5 years old. But the caveat is that it would require all the energy in the universe and an inordinate amount of information to perform that task. A simple analogy to demonstrate the difficulty in recreating a past state of matter is the act of mixing a drop of ink in a glass of water. As time passes, the trillions of molecules in the ink will disperse and mix thoroughly with water molecules. In a matter of few seconds, an equilibrium will be reached with uniform distribution of molecules. It is almost impossible to recreate the drop that existed a few seconds ago from the mixture even though the act of mixing is purely spontaneous and effortless. It is perplexing to realize that it takes much lesser amount of energy and knowledge to send an object to Jupiter in the Future than to create a drop of ink that existed in the Past a few seconds ago. If this is what Past means to a drop of ink, then imagine what relevance it has for our body or life. I am made of trillions of molecules that is in a constant exchange with the environment. What belonged to me a few weeks ago is now a part of air or it’s in the clouds or has been built into a worm. What I have with me today was past of a tree a couple of days ago. The only thing that has any connection to the past is this concept of “I” which nobody knows what it means.

Time and knowledge

Tomorrow, and tomorrow, and tomorrow, Creeps in this petty pace from day to day, To the last syllable of recorded time; And all our yesterdays have lighted fools the way to dusty death. Out, out, brief candle!


What is the connection between time and knowledge? Knowledge is the outcome of human brains prodigious desire to understand the ‘Past’ and ‘Present’ in order to predict ‘Future’.

Change is the only constant in the world around us. Imagine a world where there are no humans but just a single timeless observer with free will and intelligence watching over the happenings of the world. Events will continue to unfold spontaneously in the human-less world. Depending on the intelligence of the observer, the changes in the world would appear to have regions of regular pattern and areas of imperceptible chaos. As he watches the ‘Future’ evolve in front of him, he would also notice a pattern in which present things decay continuously as new elements emerge. Soon enough he will appreciate that there is a direction to this madness – The Arrow of Time. Depending on the depth and breadth of the observer’s capacity to appreciate pattern in evolution (knowledge), he gains the ability to predict a fraction of how the future might look like. Now let’s put the observer inside this world of changing things. The observer becomes an integral part of the ticking time with the elements in him caught in the same arena of creation and destruction. The observer’s knowledge about the natural flow of things grants him the ability to alter the ‘Present’ in a predictable fashion. With knowledge and free will, the observer might strive to prolong the existence of his form into the future by influencing the flow of events. However as time flows, the observer would realize that the form itself undergoes changes. The body takes up a new form at every moment in time. The only thing that goes uninterrupted into future is an imaginary line of history that exists in the mind of observer stretching from past to future.

The butterfly counts not months but moments, and has time enough

Rabindranath Tagore

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