Entropy—the tendency of ordered systems to fall into disorder—is the only known feature of this universe that distinguishes the past from the future. If no energy exchange is taking place, all other observable properties of physical activity are time-reversible—meaning if you were to play the video of their motion backwards you wouldn’t notice anything strange. Some people call entropy the “arrow of time,” but in a way, entropy is time. If there’s no other evidence that time exists, then entropy is itself the culprit. It is entropy at the root of our sense that something happened yesterday and that things will go on happening tomorrow or a few moments from now, despite the fact that we have only ever experienced “now.”
There’s an antiquated notion, which has had a certain amount of stickiness in people’s minds, that time is a “fourth dimension.” Really this is imprecise, because it conjures up images of a line going in a different direction than up-down, left-right, or forward-back. But time isn’t a line.
As Einstein showed, time is intrinsically entangled with space. We measure time by measuring “how quickly” something went from one place to another. The most accurate clocks do this by measuring the electron transitions of a cesium atom. But if all the places in the universe were on top of each other in a single point and you couldn’t go from one to the other, there would be no way to observe time. In that scenario, time would have no meaning whatsoever.
So there’s something tied up with entropy and distance. Now, if entropy is the progression of ordered systems into disorder, you could view the beginning of the universe (a time where, indeed, all things were jumbled “on top” of each other in the same spot) as either a time of perfect order or perfect disorder. You couldn’t really say that it was one or the other. Because having a system of order or disorder requires, in the first place, a system of discrete objects. And if there’s no difference between one thing and its neighbor, then what is there to call a system?
With all of that in mind, consider for a moment that our distinctions between objects in this moment right now—our identification of systems in the first place—is entirely a construct of the human mind. In reality, there’s no such thing as any separate closed system, like a plant or animal. It’s just sometimes useful to pretend that those things exist in order to talk about the world around us. There simply is no thing that is disconnected from its environment. We breathe our environment in, we digest it, we grow out of it. In fact, there’s no actual boundary between my body and the air around it. There are only forces of repulsion and attraction that happen at an atomic scale.
So when we’re watching energy transfer from one system to another (i.e. from a collection of atoms called a ball to a collection of atoms called a table), it is our mind that constructs those separate systems rather than seeing them as a continuum of vibrating probabilities. It appears as though one object gets slightly warmer from friction and the other object loses that energy from its momentum, but only because we’re calling those things separate objects. When an icicle composed of beautifully symmetrical ice crystals melts into the unstructured flows of water, it appears as though one system has become much more chaotic. But this is only because human minds made the distinction between that order and chaos to begin with.
In this way, entropy can be shown to be an illusion of our human-sized perspective on the world around us. And since entropy—the last stronghold of time—is only a bi-product of our perception, time only exists in the mind.