In part I of this series, I left you on a pretty unusual question: what is the present? Here is a seemingly unrelated question: why is there so many particles in the Universe? Here is how these two are related.
In part I, I explained that you only need two things to get the past to be different from the future. First, you need causality. In physics, we describe the exact way into which events are linked to other events with a grouping of physical laws we call dynamics. Dynamics answers questions such as this one. I have this physical system that I prepared, let’s say a ball on the edge of a bowl. What happens after I push it inside? Dynamics will tell you all sorts of things about what happens. It will give you the top horizontal speed as the ball reaches the bottom of the bowl, the height that the ball will reach as it climbs the opposite side of the bowl, how long it will oscillate back and forth, the frequency at which it will do so, the exact pattern it will trace if you also give it a little push sideways, etc. The beauty of physics is that all these things that happen are intimately related to the initial instant you pushed the ball inside the bowl. Dynamics describes exactly how. Dynamics is the realization of causality.
The second thing that you need for the past to be different from the future is that your physical system be large and complex, preferably with a very large number of moving parts. Knowing that the world is made of countless particles dancing with each other, this requirement is obviously met in the real world. We do indeed see that the past is different from the future. The question of why there is an arrow of time, or why the past is different from the future is itself answered, but it leaves us with two deeper questions. Why is there so many fundamental particles such that the Universe has an arrow of time, and why is there a moving boundary between the past and the future? This second question is what I want to focus on now.
Let’s consider for a moment that this boundary which we call the present is an illusion, that it is a subjective thing. What might this imply? It implies that nothing prevents you and I to experience our lives at different times. From my perspective, I may be living the moment you have been living 5 years ago from your perspective. Is your brain still intact?
It’s perfectly fine if you can’t wrap your head around such a possibility because it doesn’t make any sense. In such a Universe, you would still need to explain how individuals experience a present. It is exactly like a Universe in which there really a present, except as many times worse as there are conscious beings in the Universe. Such a Universe would also have to be perfectly deterministic, otherwise you would run into pretty serious inconsistencies. For example, your own future has to be determined because it is part of the fixed past of someone else, like someone who is not already born (sorry, I promise not to make your brain hurt again). You could escape determinism by allowing each conscious being to have its own copy of the Universe where they are the only one who’s future is not determined. But what does that make the other conscious being in your Universe? (Ok, last time I promise!) That’s without saying that you still wouldn’t have answered why there is a boundary between determined and undetermined. The more you try to make a universe into which we are all conscious at our own special times the more you encounter logical absurdities. Occam’s razor must cut this one short. The present must have an objective reality.
In a perfectly deterministic universe such as the one depicted by Newtonian mechanics, there is nothing that can single out a special moving point on the time dimension as the present. Fortunately, we might not live in a perfectly deterministic universe. Quantum mechanics has been screwing with determinism as much as it has been screwing with our minds ever since it has been discovered. But does quantum mechanics offers an explanation for the reality of the present? It cannot even attribute any reality to its central concepts… We’ll get to it.
Quantum mechanics is all in the details. We don’t see any of the weird effects it imply in our daily lives, yet, quantum mechanics is everywhere. It is in every atom that constitutes the Universe. It is the law of the very small. It seriously messes with your head. I will have to break my promise not to hurt your brain. Next post, prepare to scrape it off from the ceiling.