“Time is something that prevents everything from happening simultaneously.” The statement of the physicist John Wheeler rightly sums up what time does, unlike anything else. Especially it 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 before it was clear that the laws of physics work the same, regardless of whether you are moving forward in time or back. And it just does not fit our experience. What is time? Here are five of our best theories at the moment.
Time … just got
Following the general theory of relativity, quantum mechanics quickly arrived and adopted the concept of time that is familiar to us. The buzz of the quantum world corresponds to the authoritarian ticking of the watch, which is beyond the scope of any described particle system. Nevertheless, the quantum-mechanical image of time is not convincing. Take the Wheeler-De Witt equation describing the quantum state of the entire universe. If this system is everything we know, where will the ticking clock be then?
Time … just an illusion
Physicist Julian Barbour believes that we may need to kill time completely . In his opinion, the space and time, united by Einstein’s general theory of relativity, must be separated. The only way to determine space, in his opinion, is to consider it as a geometric relationship between the observed particles, not paying attention to time. He calls each configuration a “snapshot” that exists in the “space of possibilities”. In the concept of Barbour there are only these pictures. 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.
Time … this is the entropy arrow
Only here the scheme of Barbour does not touch upon a more subtle question. All of our physical laws are symmetric in time, which means, mathematically speaking, everything can flow equally forward and backward in time. With one exception. The second law of thermodynamics says that entropy, or the amount of disorder, always increases with time in separate collections of particles and energy. The second law explains why a pot of water can not warm itself up, for example. The unique asymmetry of this law made many physicists think that an exclusively one-sided flow of time is associated with entropy. There is also a quantum version of this “entropic time arrow” developed by physicist Sandu Popescu from the University of Bristol in the UK. Popescu and his colleagues showed that we can consider growing entropyas a result of the growth of quantum entanglement .
Time … absolutely real, in the end
Perhaps the time entropy arrow is not the whole story, says Lee Smolin of Perimeter Institute in Waterloo, Canada. He notes that if the entropy is constantly growing, then the universe at the time of the Big Bang had to be in a state of low entropy (high orderliness). But there is no explanation of why this should be so. This brings us back to the question of why our physical laws are symmetric in time. Perhaps we have just the wrong laws, says Smolin. Together with his colleagues he tries to find alternative fundamental laws, in which the time orientation is built in. The only problem is that his strange approach leads to the fact that laws change over time.
Time … deserves equality
John Vaccaro of the University of Griffith in Australia is experimenting with putting 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 in one time, but not in another, without the need for interactions that create or destroy it.
An attempt to correct the equations taking into account this did not lead to anything, since it violates the cornerstone of physics – the law of conservation of mass. But Vaccaro shows that from the wreckage of these equations it is possible to restore quantum mechanics in a revised form. Simply we need experimental evidence supporting this idea. In 2012, the BaBar experiment at the National SLAC Accelerator Center in California showed that the decay of B-meson particles proceeds differently at different times. Perhaps there is more in Wakkaro’s ideas.