We can move through space in any direction we like - but time is strictly a one-way street.
Take a few steps forward, turn around and walk backwards. No problem. Now let it pass for a few seconds, then turn around and come back a few seconds in time. There has been no luck? Of course not. As we know very well, time, unlike space, has only one direction - which flows from past to future, and never the other way around. That sounds like the natural order of things, but if you look closely enough at nature, you will find that it is not. An exhaustive search of the laws of physics results in the non-existence of an arrow of time. For example, you can use Newton's laws of motion to study where a ball was thrown in the past as well as where it will land in the future.
And when it comes to particles, the laws and forces that govern their behavior do not change if you change the future by the past. "The really strange thing is that the laws of physics, which should no doubt be responsible for what we see in the world, can function equally well forward and backward in time," says Dean Rickles, a philosopher of science At the University of Sydney in New South Wales, Australia. "There should not be an arrow." If the arrow of time is not in the laws of physics, where does it come from? An important clue arises from the complex interactions of a large number of particles. All the objects you see around you, including you, are made up of a large collection of particles. These particles are not fixed around them - they are constantly moving and reorganizing. For any macroscopic system - for example, a puddle of water or an ice crystal - physicists assign an entropy. The entropy reflects the number of ways in which the constituent particles of a system can be rearranged without changing its general appearance. A puddle of water can be formed by rearranging H2O molecules in a large number of forms, making it a high entropy system.
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Source: Cosmic space
Credits: Minute physics
