Time Reversal Symmetry Is Unnatural
Many of the fundamental laws of physics have time reversal symmetry. They run equally well forwards or backwards in time. This is mathematically elegant but perverse when it comes to the real world of nature. In the real world there is a blatant arrow of time that is ubiquitous and cannot be reversed. Ever seen an egg in a pan jump out and reassemble with its many former pieces of eggshell into an intact egg? Ever seen a supernova run backwards? Of course not.
This disconnect between physics and nature was often explained as a statistical interaction of a very large number of particles. But not so fast! A Dutch team of astronomers have shown that if even as few as 3 objects move around each other, then their history cannot be reversed.
Bottom line: “So not being able to turn back time is no longer just a statistical argument. It is already hidden in the basic laws of nature. Not a single system of three moving objects, big or small, planets or black holes, can escape the direction of time.”
To those who would say but Quantum Mechanics and the physics of the microcosm require time reversal symmetry, I would direct you to a paper posted to arxiv.org today.
Here is the bottom lines from that paper: “The point of this paper is to document the way that the laws of quantum physics encodes an arrow of causality. This is contained in the factor i associated with the quantization procedure. The fundamental action or Hamiltonian may be time symmetric, but the quantization rules are not. This is most clear in the path integral framework, where the causal direction is associated with the phase in exp(±iS). This causal arrow is what gives a direction to scattering and decay processes. … These considerations have important implications for discussions of the arrow of time. The basic statement saying that the fundamental laws of physics do not differentiate an arrow of time is not correct. At the microscopic level, reactions run in one direction but not the other. The causal direction of these reactions determines the direction of increasing entropy, and hence determines the arrow of thermodynamics.”
Causality rules in nature, and that determines the arrow of time.