No, it never propagates if I set a gap or prevention.
Some of the prettiest mathematics ever invented was used to explain classical mechanics, where the law of force and action principles are utilized. This gave rise to world-changing ideas, such as E = mc2. The math has a beautiful symmetry: time-reversal invariance. The equations look the same going forward in time, as they do going backwards in time. They describe nature as a perfect palindrome (unlike the imperfection imparted by the punctuation in my title, which serves to give meaning), like a perfect propagating wave, oscillating indefinitely without dissipation.
So pretty is this math, so beautiful are its consequences, that there have been arguments in the scientific community as to whether time goes forward or not. In essence, these are high-level arguments on causality: determinism versus free will. The quaintness of these arguments, and the ability of scientists to focus relentlessly on the consequences of their theories without quarter, has no analog I know of in any other field of human endeavor. If you think I am exaggerating the heat and significance of these arguments, I recommend to you a review of the life and work of Ludwig Boltzmann who, with a world-changing idea, explained the origin of time. Indeed, there are still some scientists who fuss and fret about this. But of course the simple act of arguing argues against the thesis: convincing one means something has changed, and is hence irreversible, not reversible.
I reviewed the origin of time in another post: it is from the second law of thermodynamics – in an isolated system, randomness, or more precisely entropy increases with time. This is not just a good idea, it is the law. No other physical principle explains the arrow of time. Having said that, this is not what people think of as the arrow of time. They think of the march of history, of the size of Cleopatra’s nose, of grandfather-endangering time machines, and of the progress of our species from cave-dwelling brutes to our world of brutish realized dreams: of moving vehicles, color TVs and rocket ships.
And in fact, the second law is often misunderstood as implying everything will get more and more run-down and random as time progresses. For example, it is argued that it is inconsistent with evolution by the natural selection of species. Not so, this is sophistry. I do not wish to monkey around, but I draw your attention to the many natural phenomena with complicated structures, such as snowflakes, turbulent clouds, river trajectories, even a humble scrap of paper, where those complex structures arise from the second law. I will not explain the origin of those phenomena in this post, but the need is hardly necessary, all you have to do is take a look around and you will be convinced. Thus, following Samuel Johnson, we can refute such sophistry in passing, by advancing a correct idea backed up by observational evidence. All these phenomena work the way cars seemingly magically move forwards and backwards and sideways without being pulled by horses: they work in non-isolated environments with, commonly, processes we can idealize as engines obeying the principles of the second law.
Let me give one last example, and in the circular set-up of this note, I will endeavor to make a convincing argument about convincing arguments, and for once I will try to minimize the number of puns in a post. I have already mentioned it: an idea someone finds convincing is irreversible, as it closes the door on other possible ideas. The forceful expression of an idea, an action, is what we usually think of as the arrow of time. The work in the construction of ideas and the active consequences of them are regulated as is the work done by heat engines, through the second law. The irreversibility of correct ideas, originating in the increase in entropy in isolated systems, leads to an increase in knowledge in our world of ideas, as correct ideas build upon correct ideas, leaving incorrect ones behind. Leading to the march of history I described above.
How do we determine a correct idea, especially a world-changing correct idea? The work done does not tell us the quality of ideas or even their legitimacy, as it is comparable for a bad idea, a good idea, or a world-changing idea. The engines regulating inspiration and innovation in our modern world are Universities, where the scientific method serves to separate good ideas from bad, like wheat from chaff, by testing ideas against observational evidence and, every now and then, turning up a world-changing idea like the pretty mathematics invented to explain classical mechanics, or the origin of time.