What Is Time Dilation?
In 1916, Albert Einstein one of the greatest scientist of all time came up with his theory of special relativity which changed the perspective of physics. His theory of spacial relativity stated that light is fastest object in the universe and nothing can travel faster than light. Which leads to creation of many interesting phenomenon, one of them is Time Dilation.
Before understanding time dilation let's understand that to calculate motion or rest, one should first have the frame of reference. The frame of reference decides wheather the body is in motion or in rest. Example if we look up in the sky then we will say that we the peoples who live on earth are in rest in relative of the outer space but if a person from the moon watches the earth then he will say that the earth is in the motion. So this tells that motion and rest are dependent on the viewer's frame of reference. Let us know something interesting about light, no matter what the frame of reference is the speed of light always remains constant.
When a body travels at the speed of light then the time for the body slows down in compare with the bodies which are not travelling at the speed of light. When we travel at the speed of light the time ticks slower for us because there is a speed limit in the universe which is known as the Cosmic Limit. The cosmic limit is of 300,000 km/s whereas the speed of light is 299,792,458 m/s. So even light can't travel beyond the cosmic limit and the same thing happens with us when we try to approach the speed of light. Let's us understand it in brief. When we try to approach at the light speed, we are close towards breaking the cosmic limit. we know that time = distance/velocity. So, for satisfying this we need to vary one of them, the speed of light is constant so it can't be varied. The distance is as it is. So here time can be varied that's why the time slows down for the law to be satisfied and to prevent the brokage of the law of physics.
Can We Reverse Time?
Changing past events to change the future is one thing, but what about the reverse? Can we go into the future and change events that are yet to occur? But how can you change future events that have not happened? In any case, you can't go into the future without passing through the present. By going into the future, if you are able to change it, what about present events that caused these future events? Would they change too? In other words, can we change the present by changing the future? Can effect change its cause? This is retrocausality, for they are philosophical paradoxes and thought experiments that are not tested or proven. Physics does not embrace retrocausality, although there are theoretical possibilities based on Einstein's theories and effects observed in Quantum Mechanics.
Let's set aside the topic of cause-effect order and ask the basic question: Can we reverse time? In the classical world, when you drop that glass of milk on the floor, one way to get back the glass and milk would be to reverse the flow of time. If you ask your friend to video record the event of breaking the glass of milk and play this video in reverse order, we will be able to restore our glass and milk. But this is only a video trick. In the classical world, there is no way to reverse time to get all the molecules that make up the glass and milk to come together again. Plus, entropy, that we learnt about earlier, does not allow this. An event or an episode as a whole may not be reversible in time, but if you take individual atoms or sub-atomic particles like electrons or protons, etc., we can actually make them travel back in time. Well, at least this is the mathematical interpretation of time for particles interacting with antiparticles. We still have not figured out the concept of time travelling backwards.
Time can be stretched and contracted; it can even be looped back on its origin, and it can flow back and front. Time can be twisted by speed and gravity. Although time is still an enigma to us, its effects are real. The GPS application covered earlier in the book gave us enough proof of time's existence. Time may or may not be eternal, but it influences us and we just cannot ignore it.
What if time stands still? We know that we can stop the flow of time only if we travel at the speed of light. For light itself, time does not pass. Our sense of time and its speed of flow must have come to us through a long evolutionary process. Our body clock is synchronized to the motions of the sun, moon and the stars. The slightest change in the motions of these celestial objects jeopardizes our sense of time and our body clock. It may even have an adverse effect on our minds and our cognition capacity. We are aware of the effects of time on birds and animals during a total solar eclipse, when they return to their nests and homes earlier than usual, thinking it is the end of the day. This has nothing to do with the speed of time or a change in the Earth's speed of rotation, but a combination of the multiple effects of reduced sunlight and its impact on the body clock, the false perception of time and the abrupt change in the regularity to which we are accustomed to in nature.
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