The annual "X" column by Ctein
Seasonal greetings to you all. As is my tradition, my X-mas present to you is another "X" column that addresses one of the mysteries and wonders of the physical universe.
This time I'm going to take on the preeminent culture war of modern physics. No, not string theory—although all the spleen that's being spewed about over that topic does have a certain entertainment value. We even have a famous physicist raging that it has ruined an entire generation of scientists. Really, dude? I haven't read such pointlessly fiery rhetoric since the debates of the mid-1800s. I'm staying away from that one.
The question that splits the community and interests me is considerably more subtle and profound. I can state it in five words:
Is time a "process" or "content?"
I'll spend the rest of this column explaining just what the hell that sentence means.
The thing is, while we all experience time, we don't actually understand what we're experiencing. There is no physics, no experiment that really addresses this. We can measure time. Well, actually, spans of time...but we don't know what we're measuring them in.
The common idea is that time is content, a real, physical thing. That's embodied in a lot of the way we talk about time. Einstein brought us the concept of space-time, which no one really disputes (at least, no one who should be taken seriously), conceptualizing it as another dimension, the so-called "fourth dimension." Bodies move not just through the three spatial dimensions but the fourth time dimension as well. A standard way of visualizing this is a "world line." Think of a planet in a circular orbit about a star, viz. figure 1. The star's at the center, and the planet goes round and round. But in this flatland world, if you draw in the "third dimension" of time, as in figure 2, you can describe that same path as a spiral, where the planet moves ever forward in time, around the star.
The concept of time as a real physicality is embodied in a lot of real physics and, more fancifully, the whole idea of time machines and traveling through time.
So, why don't we just accept this notion of time? It lets us make excellent physical predictions about the world, and, as I often say, data trumps theory. When the data seems to say that's the way it is, why even consider that it might not be?
Because there's one big problem with this notion of time as content. We can't observe the content! So far it has proven utterly impossible to experience any point in time except the present. We cannot move freely in time, like we can in the three spatial dimensions. Every attempt to come up with a real physical model that results in "travel through time" fails. It doesn't matter whether it's forward or backwards. We are stuck in time, moving forward at the rate of one second per second.
Now, the fact that every time machine theory has been disproven (or is, at least, unverified), so far, doesn't make it impossible. Some of them have come close. But that doesn't actually prove anything. The folks who designed perpetual motion machines came close. That doesn't mean that they are actually possible (hint: they're not). Close-but-no-cigar still means no cigar. So far, so far as we can tell, time travel is not possible. But we can't prove that.
(I should mention at this point that the kind of time travel that comes up in relativistic time dilation is a peculiar special case. It really involves the transformation of coordinates, it is not a free motion through time as a physical entity. It doesn't come into play in this debate, and I won't be discussing it further.)
And that's why there's a really, really big problem with the idea of time as content, because we have absolutely no idea, no physical model that can explain why we can't access other parts of the content than "now." The simplest and most satisfying explanation for that is that the reason we can't is because they aren't there. There are no other parts to time than now. Time is a process, the evolution of the universe second by second. It's a little like the universe is a great big grandfather clock, and the pendulum swings and the gears go tick tick tick, marking the passage of time. But that's just what the clock does; the clock is not time.
Much of modern physics incorporates this notion of time, too. A lot of quantum mechanics doesn't include time explicitly—it's something you apply to see how a wave state changes. Time is that second-by-second progression of physicality. there isn't really anything before now or after now. It is entirely now.
So, why is the debate so hot and heavy and laden with florid rhetoric. Because, not to put a fine point on it, this is philosophy, not really physics. Not yet. Both perspectives are equally attractive (or unattractive); the appeal of one or the other defaults back to an unstated sense of what "obviously makes sense" about the nature of reality. Once you buy into the axiom, the rest follows logically. But you have to buy into the axiom, and if two different philosophers buy into two different axioms, well, then there's no meeting of minds.
It is a wonderful puzzle and a profound question: What is the nature of time? Everyone's got an opinion, and none of us really have a clue.
Physicist Ctein dabbles in photography and/or photographer Ctein dabbles in physics. We are not quite sure which is the correct philosophical model; but it's always fun to go with the flow column (tick) by column (tock).
©2013 by Ctein, all rights reserved
Links in this post may be to our affiliates; sales through affiliate links may benefit this site.
(To see all the comments, click on the "Comments" link below.)
Featured Comments from:
Erik Petersson: "This is an old problem. In the fifth century Augustine of Hippo said: 'For what is time? Who can easily and briefly explain it? Who even in thought can comprehend it, even to the pronouncing of a word concerning it? But what in speaking do we refer to more familiarly and knowingly than time? And certainly we understand when we speak of it; we understand also when we hear it spoken of by another. What, then, is time? If no one ask of me, I know; if I wish to explain to him who asks, I know not. Yet I say with confidence, that I know that if nothing passed away, there would not be past time; and if nothing were coming, there would not be future time; and if nothing were, there would not be present time. Those two times, therefore, past and future, how are they, when even the past now is not; and the future is not as yet? But should the present be always present, and should it not pass into time past, time truly it could not be, but eternity. If, then, time present—if it be time—only comes into existence because it passes into time past, how do we say that even this is, whose cause of being is that it shall not be—namely, so that we cannot truly say that time is, unless because it tends not to be?'"