Based on our observations, time seems to flow. Not only does it seem to flow, it seems to flow in one direction; the future. But, contrary to popular opinion, the distinction between past, present, and future is only an illusion, however persistent. Time itself is relative, and events we think can unfold in one direction, can also unfold in reverse.
Because our perception of everything in the universe is based on what our mind tells us this is a difficult idea to grasp. Our mind is so obtuse that some smartly drawn geometric patterns are enough to completely mess it up; dependance on such a machine to understand the cosmos is the biggest blunder, and that’s why we put to test Einstein’s theory of relative spacetime through extremely advanced machines and not biological networks of our body.
Isaac Newton gave us the first mathematical model for time and space in his ‘Principia Mathematica’, published in 1687, and essentially stated that time is the same for everyone, and there is nothing we can do about it. This aligns with our intuition; however, German thinker Immanuel Kant said that Newton’s theory meant that time existed forever and if infinite time had passed, it raises a question of why the universe had not reached thermal equilibrium. Einstein obliterates Newton’s idea of space and time being independent; there is no grand cosmic clock that is the ultimate source of understanding of what time really is. In fact, time is in the eye of the beholder. He studied Maxwell’s work on electromagnetic waves and derived a conclusion that if speed of time is constant then space and time must adjust with each other to let light be constant to the eyes of the observer. Einstein found that space and time are perpendicular to each other and any object at rest is moving in time, but, every object that’s in motion, is moving through space, and, time, and thus, slower in time as compared to his stationary self; the slowing down can be briefly understood through vectors, however, Einstein’s math is much more complicated and includes geodesics and metric tensors which are not the topic of this blog.
This theory – relativity – has been tried and tested countless times. A muon accelerated to a speed close to the speed of light, exists for a few seconds as compared to its stationary self, that self destructs in a millionth of a second. Time slows down for the muon because even though it is close to the speed of light, if the muon looks at light – obviously it won’t literally look at light, but hypothetically, if it did – it would see it move at the speed of light. Imagine how slow time must be flowing for the muon for it to observe such a phenomenon. If you were put in such an accelerator for a few seconds of ‘your time’, you would come out a few million years later.
Just how can you move in space, and, time. The fact is we do it all the time. Einstein unified space and time, and proved that they are profoundly connected; if you can move in space, why is it so difficult to digest the fact, that you can, and are, moving in time.
Past, present, and future coexist; it is unimaginably wrong to believe that the past has occurred and the future will occur, and we live in the present. We see a clock ticking, a meteor hitting the moon, a man running, at the same instant and define this as the present. However, motion affects time, and hence for every particle in the entire universe, time passes differently, and the present is different.
To understand this bizarre concept imagine an alien ten million light years away from you that is at rest just as you are – this is not possible since galaxies spin, planets orbit stars or black holes, and planets themselves spin, and all the velocity adds up to make your velocity at a point in spacetime, but assume that you are at rest with respect to each other and the universe for some time – then both of you are existing in the present. But, if the alien gets in motion then time will passes differently for it than it does for you, and hence it will no longer exist in your present. (Einstein explains this by asking the reader to think of spacetime as a loaf of bread. If you take a knife and cut it parallel to the square side you will have your ’now’ slice – your present slice – however if you get in motion, your now slice’s angle with respect to the alien’s now slice where both of you existed, will change.) Even though the angle made by the alien with our now slice is small, the distance between us is so enormous that in the alien’s present, Beethoven is finishing his fifth symphony. If the alien moves towards Earth, his now slice will point towards our future instead of the past.
Just the way you can move your telescope by a millimetre and see another star or galaxy, by gaining the smallest velocity, you can change your present slice by a copious amount and the time difference between your now slice when you were at rest and your now slice when you were in motion for an object far away from you will differ by the distance of the object from you.
Hawking proposed that if one follows our past light cone back in time, it will be bent by the matter in the early universe. The whole universe we observe is contained in a region whose boundaries shrink to zero at the Big Bang. This would be a singularity, a place where the density of matter would be infinite and classical relativity would break down, and since space had a beginning, time had a beginning.