![]() However, special relativity does allow for the possibility of time travel to the future.Ĭreatures living in a two dimensional flat world will find it difficult to imagine what a three dimensional world would look like. As a consequence, traveling back in time impossible, as it would violate causality and the law of entropy. Thus it is no coincidence that the thermodynamical arrow of time and the arrow of time flow, point in the same direction, as they both preserve causality. Entropy increase is unidirectional, just as the unidirectionality of time. The irreversibility implied by entropy could be described by the popular line from the Humpty Dumpty nursery rhyme, which says: All the King’s horses and all the King’s men, couldn’t put Humpty together again! For every system, disorder always increases. ![]() That is, a cup falling down and breaking, can never be restored to the same condition, with every atom in place, as it was. You can understand the law of entropy, by just observing the irreversible nature of natural phenomena. If you study thermodynamics, you will come across the law that entropy or disorder in the universe always increases, never can it decrease. This irreversibility of time is inbuilt through the concept of entropy. That is, cause should precede effect and it should not be the other way round. It protects causality, that is the law of cause and effect. The uniqueness of time dimension is that you can travel only forward in it, not backward. This is a bit difficult to understand, but if you give it adequate time to sink in, it’s simple. Therefore, the faster you move in the three space dimensions, the slower you go in the time dimension. Now when they start moving, their velocity increases in the three space dimensions, and therefore it slows down in the time dimension. When the objects are at rest, they are moving only in the time dimension. Only the velocity is distributed over dimensions and most of it is in the time dimension. In a similar way, all objects in the real world are moving in a four dimensional spacetime, at a constant velocity as that of light. Its velocity in the original one dimension has reduced. Its velocity is now distributed over two dimensions and therefore it’s taking longer for the car to cover the same distance. Now consider that it’s trying to reach the finish line, but on an oblique path. It is moving in one dimension and takes some time to reach the finish line. ![]() Consider a racing car moving on an absolutely straight race track at a constant velocity. We are all continuously moving not just in three dimensional space but in four dimensional spacetime. Let us try to understand what time dilation is. If you see, it literally means ‘one space time’ just a lucky coincidence one would say! Time was realized as the 4th dimension. ![]() Split up his name as ‘ EIN+ST+EIN’, ST meaning space time. This forced the world to abandon the concept of separate ideas of space and time and a single unified concept of spacetime came into existence. In fact the faster you move in space, the slower you move in time. The necessity that the speed of light should be constant forces us to abandon the absoluteness of time! That is, different observers in different reference frames show different times in their watches, but the laws of physics will remain the same. Special relativity changed this perception. That is, no matter which reference frame people are using, their clocks if compared show the same time. Newton’s mechanics had the concept of absolute time. ![]() Any other co-ordinate system moving with constant velocity with respect to a co-ordinate system at rest is also an inertial reference frame. By inertial reference frame, we mean a co-ordinate system of reference moving at a constant velocity or which is stationary. The second postulate is that all laws of the physical world should remain the same in any inertial reference frame. The basic postulate of special relativity is that no information can travel faster than the velocity of light in vacuum and it is constant. ![]()
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