We take time for granted but it is strange, mind-bending, and not well-understood
We think we understand time...
We think of time as the tick of a clock or a watch. It keeps passing or flowing like a river, things keep happening, we keep aging, clouds keep moving, events keep happening, seasons keep changing, and so on. We are like a boat on the river of time.
And it makes total intuitive sense...
We also think we know the bigger picture of time. We have defined units of time to make it measurable. We measure a day by the time it takes for our planet to rotate, and a year by the time it takes our planet to revolve around its star. We have also defined smaller units of time.
We have also divided our planet into time zones because one-half of our planet does not face the sun and the other half faces the sun hence we all have different notions of time depending on where we are located on our planet. So we have figured out Time. Right?
But deeper questions remain
But let's pause for a moment and take a deep breath. The biggest discoveries are an outcome of curiosity and thought experiments. So let's ask ourselves a few fundamental questions :
Why does time flow in one direction ?
Can it flow backwards ?
Can we speed up or slow down time ?
If time flows, then can we travel faster than it flows ?
And can we travel slower than it flows ?
And our mind starts to bend
We don't notice many things because either they are too large or because they are too small. Our understanding of reality is based on what we experience. However, what we experience may not be reality.
In 1971, a very accurate clock was placed in one location and another very accurate clock was placed inside an aircraft. The two clocks were synchronized and perfectly matched each other. Then the plane took off and circled the planet and landed back in the same location as where it started from. Then when they compared the two clocks, they did not match anymore! The clock on the plane was a tiny bit slower than the clock on the ground. They differed by a very small degree but there was a definitive difference.
This experiment was not a thought experiment at that time. It was an experiment done which proved the greatest genius of physics - Albert Einstein. It proved what Einstein had concluded in the early part of the nineteenth century.
Motion in space affects the motion in time !
We conclude that we don't understand much about it
It turns out that time is weird and there is an intricate link of the motion through space and the passage of time. When we have more of one, we have less of the other. Think of it as traveling in a car. If we travel towards the east, all our motion is towards the east and our motion towards the north is zero. Similarly, if we travel north, all our motion is towards the north and our motion towards the east is zero. However, if we travel northeast, part of our motion is toward the north, and part of it is toward the east.
Now replace east and north by space and time! When we are stationary, we move only through time. But when we move through space, we move a bit slower in time as some of our motion is through space! This was the greatest insight of Albert Einstein, that time and space are the same! There is not only space and not only time but the entity is actually "Spacetime". We live in Spacetime.
So we have an intuitive sense that if we are stationary, then we move only through time. But can we move only through space and not through time at all? When is it possible and what happens if we do so? Well - as per our simple framework, if we only move through space, then we don't move through time at all. In other words, time stops. This is what happens when we move at the speed of light.
Time travel is therefore theoretically possible
The profound implication of all this is time travel. The faster we move through space, the slower we move through time and if we move slowly through time then compared to someone stationary, we age lesser. The effects are minute in our daily lives but they do happen as shown in the 1971 experiment. To observe a perceived difference in time or age, we need to travel at much higher speeds than an aircraft. Imagine you and your friend are of the same age. You take a spaceship and move at immense speeds ( closer to the speed of light ) and come back to where you started while your friend stays back on Earth. When you both meet again your friend will be much much older than you! As shown in the movie Interstellar, a man comes back from space to meet his daughter who is frail and old while he is still much younger than her.
The direction of time
One thing we have not covered so is the direction of time. Why does it flow only forward and can it flow backward? The only possible explanation for this is through a concept called Entropy. The world seems to be moving from an ordered state ( The Big Bang ) to a chaotic or disordered state. So seems to be Time and hence it's flowing in the direction of order to disorder. Note that everything is ultimately moving away from each other towards a randomly disordered state. While it is theoretically possible to move back in time, it would need an attraction of everything in the opposite direction from where we started i.e. exactly one scenario of everything moving backward ending up in where we started i.e. a point in time from which the Big Bang originated. The odds of that happening seem extremely small.
But why should you care?
At this point, you may ask but why should you care? Since the effects are so minute, and they are hardly noticeable in our daily lives. Traveling at the speed of light is not yet possible and the odds of going back in time are nearly zero. It turns out that it does affect us in a very real way. Let's get into that in the next part of this series. I promise you one thing. Your perception of the real world will not be the same before and after reading through this mind-altering journey of Spacetime.
1988, Stephen Hawking's A Brief History Of Time has established itself as a landmark volume in scientific writing. It has become an international publishing phenomenon, translated into forty languages and selling over nine million copies. A non-illustrated version of the same book is an alternative read.
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