One of the most important theories in the history of Astrophysics which has changed our views to see the universe entirely. One of the most brilliant minds the world has ever seen, Albert Einstein has proposed this theory of general relativity. The General theory of relativity and Time Dilation go hand in hand. This theory further expands the views of time dilation and combines it with space expansion. Let’s get into it. Again, this might shatter some of our preoccupied notions and might be thought as sci-fi but, it has been accepted by many scientists and its time we do too.
Let’s again start with a hypothetical experiment. We are sitting in the car which is at rest and the headlights are blinked. The speed of light emitted from headlights is measured and is recorded accurately as speed of light. Now, if the car is moved at a speed of say 80 and the headlights are again blinked, and the speed is recorded, the speed remains constant. Yes, as the speed of car is negligible as compared to that of light but still there is not even negligible increase in the speed of light when measured accurately enough. The reason for this as we know now is Time Dilation. The time moves slowly so as to keep the speed of light constant. But, Einstein says that it’s not only time that moves slowly but, the space (or in case of car ‘Distance’) expands. So, basically, we know Speed= distance(Space)/Time. Einstein says not only time slows down but also the space expands and both of them thus keep resultant speed of light constant. This led to a new revolutionary theory that space and time are not different entities at all. They are related and linked to each other. This new entity, Einstein called it as Spacetime.
One of the most ground-breaking theories in physics which led us to the understanding of universe that we know of now is Time dilation. But, before we move on to that, lets brush up with some of our basic concepts. So, we all have studied in physics about Newtonian relativity which states that the speed of any body is measured relative to the observer’s frame of reference. In simple words, If you are sitting in a moving car with your friend, your friend appears to be stationary to you as you both are moving with same speed in same direction viz, you both are in same frame of reference. But, if a guy who is standing on the road sees you he will record your speed as the speed of the car. Also, if you throw a ball from this moving car, at a speed , say 20 and car is at speed say 40. Then in ideal case, the ball will have total speed of 60 when thrown.
Now, lets apply similar logic with a slightly different scenario. You are in a stationery car, and you start its headlights and the exact speed of light is measured and recorded. Now, the car is moved at a speed of say, 80 km/hr and the headlights are blinked and again the speed is measured. Now, Newton’s theory says that the speed of light that would be measured must be addition of speed of light and speed of car (Which is of course very minute as compared to speed of light) But, the results say otherwise. The speed of light remains constant and not even minute increase in speed is recorded. This concludes that speed of light is constant irrespective of frame of reference in which it moves.
The question is, does Time depend on the frame of reference of the observer? Is time different for people in different frames of references? And surprisingly enough, the answer is Yes. The Time passes differently depending upon your frame of reference. If you are sitting at rest. The time passes normally as your speed is zero. But, if you are moving at a certain speed, the time will pass slower than it will at rest. This thing that time passing differently for each person depending on their frame of reference is called Time Dilation. So, why don’t we observe it? Because it is minute when compared to the human speeds. To observe measurable time dilation, one would have to go close to the speed of light. This theory was put forth by Albert Einstein and then was worked upon by many other physicist all of them leading to prove it right. Continue reading