My fascination with the nature of time was sparked by a chapter in Stephen Hawking’s pop-science bestseller The Brief History of Time. The chapter was named “The Arrow of Time” and it dealt with the linear or unidirectional nature of time. The Second Law of Thermodynamics—which states that the entropy of the universe increases with time—produces an asymmetry in time. For example, if you drop your phone on the ground and the screen shatters into a thousand pieces, the entropy (or randomness) increases. But if you reverse the process, you get a new screen from a smashed mess — which would be great for a disgruntled iPhone user. Too bad it decreases entropy and violates the Second Law. Now you know why time moves in one direction, scientifically speaking.
Since science disallows non-linear time, we will have to to delve into the rabbit hole of science fiction to understand more about it. Amongst a plethora of examples in sci-fi books and movies, three vivid examples come to mind when I think of non-linear time.
First, Billy Pilgrim from Slaughterhouse Five who mysteriously comes unstuck in time and meets 4-D aliens called Trafalmadorians. Second, Cooper from Interstellar who finds himself in a tesseract constructed by unknown beings from the future. And finally, Abbott and Costello from Arrival who are 4-D aliens that perceive time as non-linear.
If you haven’t had the pleasure of reading about or watching the escapades of these fictional characters, perhaps you are scratching your head wondering what non-linear time is. Let me explain.
Picture a cylinder like this.
Do you see the cylinder as a solid 3-D object? Do you see it exist completely at all times? Good. Let us name the nearer face of the cylinder as Face 1 and the farther face as Face 2. Let the length of the cylinder be “L”.
Now imagine FlatMan. This is a 2-D guy that lives in a 2-D world.
For FlatMan, the cylinder does not exist because he cannot imagine anything in 3-D. He can only see a circular plane (or cross-section) of the cylinder.
Now imagine that FlatMan is at Face 1 of the cylinder. For us, the length of the cylinder acts as a spatial dimension but it exists as a temporal dimension for FlatMan. Consequently, what is L units of length for us is L units of time (or say FlatTime) for FlatMan.
So after L units of FlatTime, FlatMan reaches Face 2. For every plane that you cut in the cylinder, you will observe FlatMan performing some particular action. That is what FlatMan is doing in that instant of FlatTime.
What is your perspective of all this though? Even though for FlatMan, only one plane exists at one FlatTime instant, you can see his progress from Face 1 to Face 2 at all times because you can see the complete cylinder at all times. So if FlatMan asks you what you were doing when FlatTime was L/2, you would give him a confused look. Because you don’t understand what FlatTime is, you only understand length.
Similarly, if a 4-D alien named Max were to appear in front of you and you were to ask him the time, he would give you the same confused look. He can see all time instants of your 3-D life from start to end —that is from birth to death — at all times.
For Max, accessing any time period of your life is like accessing a plane of the cylinder. Your birth is Face 1 of a 4-D cylinder and death is Face 2 of the same 4-D cylinder. Max can see your whole life because he can see the whole 4-D cylinder all at once.
“All moments, past, present and future, always have existed, always will exist.”
-Kurt Vonnegut, Slaughterhouse Five