"Time is what prevents everything from happening at the same time." Physicist John Wheeler's statement rightly summarizes what time does as opposed to anything else. This especially stands out against the background of the fact that our hunt for the most basic ingredients of reality did not bring us anything that could be connected with time. Einstein succeeded more than others: he combined time with space. But even before him, it was clear that the laws of physics work the same, regardless of whether you are moving forward in time or backward. And that just doesn't fit our experience. What is time? Here are five of our best theories so far.
Time … just is
Following the general theory of relativity, quantum mechanics quickly arrived and established the concept of time that we are used to. The buzz of the quantum world corresponds to the authoritarian ticking of a clock that is outside of any described particle system. However, the quantum mechanical representation of time is not convincing. Take the Wheeler-DeWitt equation, which describes the quantum state of the entire universe. If this system is all we know, where would the ticking quantum clock be?
Time … just an illusion
Physicist Julian Barbour thinks we may need to kill time completely. In his opinion, space and time, united by Einstein's general theory of relativity, must be separated. The only way to define space, in his opinion, is to consider it as a geometric relationship between the observed particles, regardless of time. He calls each configuration a "snapshot" that exists in the "space of possibilities." In Barbour's concept, only these images exist. Time is not real, but only a consequence of our perception - an illusion that appears due to the fact that the universe is constantly changing from one picture to another.
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Time … is the arrow of entropy
Only here Barbour's scheme does not touch upon a more subtle issue. All our physical laws are symmetrical in time, which means, mathematically speaking, everything can flow equally forward and backward in time. With one exception. The second law of thermodynamics states that entropy, or the amount of disorder, always increases over time in individual collections of particles and energy. The second law explains why a pot of water cannot heat up on its own, for example. The unique asymmetry of this law has led many physicists to think that the extremely one-sided flow of time is associated with entropy. There is also a quantum version of this "entropic arrow of time" developed by physicist Sandu Popescu of the University of Bristol in the UK. Popescu and his colleagues showedthat we can view the growing entropy as a result of the growth of quantum entanglement.
Time … absolutely real after all
Perhaps the arrow of the entropy of time is not the whole story, says Lee Smolin of the Perimeter Institute in Waterloo, Canada. He notes that if entropy is constantly growing, then the Universe at the time of the Big Bang should have been in a state of low entropy (high order). But there is no explanation why everything has to be this way. This brings us back to the question of why our physical laws are symmetrical in time. Maybe we just have the wrong laws, says Smolin. Together with colleagues, he is trying to find alternative fundamental laws in which the direction of time is embedded. The only problem is that his strange approach leads to the fact that laws change over time.
Time … deserves equality
John Vaccaro of Griffith University in Australia is experimenting to put time and space on an equal footing. Quantum mechanics allows a particle to exist in one place, but not in another. Perhaps, says Vaccaro, it allows a particle to exist at one time, but not another, without the need for interactions that create or destroy it.
An attempt to correct the equations with this in mind did not lead to anything, since it violates the cornerstone of physics - the law of conservation of mass. But Vaccaro shows that from under the debris of these equations, quantum mechanics can be restored in a corrected form. We just need experimental evidence to support this idea. In 2012, the BaBar experiment at the SLAC National Accelerator Center in California showed that B-meson particles decay differently at different times. Perhaps there is more to Vaccaro's ideas.
ILYA KHEL