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> Objects and Identity
Let's go back to what I said just a minute ago, that a physical object progresses through a series of versions, and look at an example. Suppose I have a rock, maybe a pet rock. Mostly it just sits there, unchanging, a single instance of a single version; but maybe one day it falls on the floor and gets chipped (a new version); and maybe another day a happy face gets painted on (a new version) and then removed (yet another version).
That leads to an interesting philosophical question: if the happy face is completely and perfectly removed, do we really have yet another version, or do we just have the previous version back again? I vote for the former. I don't see anything wrong with having two different versions that happen to be identical in content. If I make a true copy of a file, I think of myself as having two different objects that happen to be identical in content; why then wouldn't I allow the same thing for versions?
In practice, though, you just can't get paint off a rock … and that leads to another interesting point. We tend not to think about it, but at the atomic level, the surface of the rock is always changing. I don't really know the details, but I imagine that the rock loses atoms here and there, and gets oxidized, and so on. In any case, the point is, underneath the discrete sequence of macroscopic versions there is a continuous development of microscopic versions.
The same, only more so, is true of living organisms, a fact that is nicely illustrated in The Twenty-Third Voyage.
Surely you are cognizant, my dear alien friend, of the findings of modern physiology, which say that all the atoms of our body are constantly being replaced with new; some bonds break, others form; the loss is made up thanks to the assimilation of foods and liquids, and thanks also to the respiratory process—all of which, taken together, we call metabolism. So then, the atoms that composed your body a year ago have long since left it and now are wending their way through regions far removed; it is only the general structure of the organism that remains unchanged, the interlocking system of its material pieces.
The idea of microscopic versions doesn't require persistent objects, but I thought it was important to have in mind anyway. The continuous development of microscopic versions is what I was thinking of when I said that for physical objects, the agent of growth is time, or physics.
Now, returning to the pet rock, what if one day it falls on the floor again and breaks right in two? What then? I wouldn't say that one of the pieces is the pet rock and the other just a big chip; and I wouldn't say that the two, together, constitute a new version, unless perhaps I later glued them back together. No, I'd probably say that the pet rock, as an object, had been destroyed, and that the pieces had been created; and I might also try to capture the connection between the rock and the pieces in some kind of composite object framework.
(Amusingly, the exact same thing happens with information objects when I break an essay into subessays, as I've done here.)
It's interesting, however, that the breaking of the rock is not unlike the branching of a persistent object. The pieces aren't the same size and shape as the original, of course, but if the original was homogeneous, one could argue that the pieces have the same pattern, the same general structure. Before, there was one instance of the pattern; afterward, there are two, each free to develop independent of the other.
Or, as a slightly more convincing example, you can imagine a bacteria fissioning, i.e., splitting in two.
The following paragraph from Life-Line is clearly not about the branching of persistent objects, but I still thought it was interesting here.
He stepped up to one of the reporters. “Suppose we take you as an example. Your name is Rogers, is it not? Very well, Rogers, you are a space-time event having duration four ways. You are not quite six feet tall, you are about twenty inches wide and perhaps ten inches thick. In time, there stretches behind you more of this space-time event reaching to perhaps nineteen-sixteen, of which we see a cross-section here at right angles to the time axis, and as thick as the present. At the far end is a baby, smelling of sour milk and drooling its breakfast on its bib. At the other end lies, perhaps, an old man someplace in the nineteen-eighties. Imagine this space-time event which we call Rogers as a long pink worm, continuous through the years, one end at his mother's womb, the other at the grave. It stretches past us here and the cross-section we see appears as a single discrete body. But that is illusion. There is physical continuity to this pink worm, enduring through the years. As a matter of fact there is physical continuity in this concept to the entire race, for these pink worms branch off from other pink worms. In this fashion the race is like a vine whose branches intertwine and send out shoots. Only by taking a cross-section of the vine would we fall into the error of believing that the shootlets were discrete individuals.”
Now let me step back for a second. What I'm doing, here, is writing about certain abnormal situations involving physical objects, situations that require thinking in terms of persistent objects. Physical objects don't normally branch; what we've seen above, apart from some digressions, is a few weak examples that look like branching. Physical objects don't normally have multiple instances, either; and that's what I want to discuss next.
It's not completely unreasonable to say that elementary particles really can have multiple instances. Suppose that an energetic photon decays into an electron-positron pair, and that the positron later collides with another electron, so that the two annihilate each other and produce another photon. The whole event looks something like this. (The initial photon is on the lower right.)
It turns out that the way one describes such an event mathematically makes it easy to think of the positron as being an electron traveling backward in time—and not just any electron, but the same electron. So, at the marked instant, there are two instances of one electron, or maybe three if you count the positron too.
On the other hand, if we're willing to think of things as traveling backward in time, maybe we should think that there's still only one instance of the electron, developing through a (linear) series of versions, and that it just so happens that the series sometimes goes backward in time.
And, in any case, elementary particles aren't exactly normal physical objects. The thing that makes them elementary is that they don't have any internal properties that can change, so they don't really develop at all. And, they're all exactly the same, so much so that mathematically there's no difference (except maybe a sign change) between saying “this one is here, that one is over there”, and vice versa. So, it is also not unreasonable to say that there is really just one electron, with lots of instances. (The instances aren't all connected together, though—there are more electrons than positrons, and there can be loops as well.)
Now, for a change of subject, consider consumer goods. Suppose we have some cans of green beans, all of the same brand. The cans, of course, are all independent physical objects, capable of independent change, so they aren't multiple instances of a single version, but they might as well be multiple branches of a single object, each with one instance. Whether they actually came from an assembly line, from a matter duplicator (think photocopier) or from some hypothetical branching device (think beam splitter) is not of much interest.
The same thought holds not just for consumer goods but for almost any kind of physical object. It's true there are objects that happen to have only a single branch, but it isn't difficult or troublesome to imagine running any such object through a matter duplicator … unless the object happens to be conscious! That's when the trouble begins. Look at how people are fascinated by identical twins, and at how they get all stirred up by the possibility of creating clones … and that's just the tip of the iceberg.
@ September (2002)