In the 19th century, an outstanding physicist emerged in Austria, Louis Boltzmann. To commemorate his outstanding contributions, a monument was erected after his death, and an ingenious formula was engraved on the top of the monument:
This is an equation that describes entropy, a concept that symbolizes disorder and irregularity. At the heart of this concept is that everything in the universe seems to follow a law that gradually evolves from order to disorder. In the case of a 569-page book, the pages are numbered in order to appear orderly, but if entropy is allowed to intervene, the pages will be scattered. How so? Because there is only one way to arrange the pages of a single drop, and there are countless ways to mess up. As we see in our daily experience, things spontaneously move from order to disorder.
Looking around, we see disorder everywhere: eggs are broken, scattered and splashed; The ice cubes melt and the shape is no longer regular; Smoke rises, and order is increasingly lacking. Order fades into disorder, as if the arrow of time is flying along this path. What we are witnessing is the spread of chaos, often expanding in the direction of events.
Boltzmann argues that it is this tendency that drives the formation of the arrow of time, and this may be the answer: in the course of nature, everything tends to develop towards a more chaotic and disorderly development. This seems to be the case, but there is a slight doubt with this theory: the laws of physics do not give a difference between the future and the past, so the growth of entropy should not only be towards the future, but also towards the past, but this does not seem reasonable.
Why is there such a huge gap between our experience and the laws of physics? There must be some kind of missing link. If we affirm that the past was more orderly, and that the evolution of everything towards disorder is the general trend, then according to the formula of entropy, the book should have been disorganized in the past, and then it would have been assembled into an orderly book! Such an assumption is clearly counterintuitive.
In the case of baseball, for example, the laws of physics may help us predict where the ball will land, but to accurately predict it, we also need to know the initial conditions, such as the strength of the shot. Similarly, if the laws of physics are not sufficient to explain the arrow of time, we may need to know more about the initial state of the universe, and for this we have to go back to the time of the Big Bang.
Suppose that the history of the universe is like a movie played backwards, and we are witnessing the retrograde evolution of everything from chaos to order. The universe, which is dotted with billions of galaxies today, will eventually degenerate into nebulae and dust, and everything will shrink until all things are one. In the earlier past, these clouds and dust gathered more and more, shrinking in size until we reached a fixed point, the singularity. At this moment, all space and time cease to exist, and the source of order and low entropy is self-evident – the Big Bang.
The Big Bang was an extremely orderly moment, probably the most orderly in the universe. Since then, everything has gradually fallen apart, and the Big Bang has shown us why the universe is different if we look back and look back at history, and if we go against the current, we will find that the universe is not only very different from the present, but also orderly.
We don't yet understand why the entropy was so low at the beginning of the universe, but we at least know that at that moment, entropy was at a very low level. Therefore, we can only be sure that it was the Big Bang that injected power into the arrow of time. Like a clockwork, once the winding spring gradually relaxes, the stored energy is released, and the universe has continued to relax and evolve in a more disordered direction since the Big Bang.
Every scene we see is advancing a process that began billions of years ago. For example, when a glass shatters, it cannot be restored because it follows the natural trend of the Big Bang from order to disorder. As we move from the past to the future, everything we see, whether it is the formation of the stars or the course of life, is a small incidental phenomenon in this chaotic journey, and in this undulating universe, the boundary between the past and the future is established.