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Timecop part 8: Co-location

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The "real" danger in time travel, according to Matuzak, is the danger that the same matter might occupy the same space, and thus that a time traveler might meet his younger self. No one knows what would happen, but we see the result when time traveling Walker throws past McComb into time traveling McComb. It is not pretty, and it is definitely fatal to both of them. It makes for a dramatic climax to the story--but is it at all credible?

Let's face it: matter cannot occupy the same space at the same time. That's a rule that defines matter. If a time traveler materializes in the past, he must displace the matter that is already there--usually air, sometimes water, hopefully nothing solid. When we touch objects, they generally move and our own bodies always compress slightly, precisely because matter cannot occupy the same space as other matter. The fear here, though, seems to be that the reaction would be different if the same molecules came in contact with themselves.

That seems immediately to be a ridiculous concern. It simply could not happen. In a very material sense, you are not the same person you were ten years ago.

Any such contact, assuming it happened to get past our clothing (and seriously, how often do you wear anything you owned ten years ago?), would be epidermis to epidermis--the outer layer of skin. What distinguishes the epidermis is that it is comprised almost entirely of dead dermal tissue, cells which died to create an outer shield for the body, and that that skin is constantly wearing off and being replaced by freshly dead dermal cells. It does not take very long at all for the entire exterior of your body to be replaced completely. We notice it with the slightly different cells that comprise our hair and our nails, but it is happening with our exterior constantly: the old is wearing off and being replaced with new. When McComb touches McComb, it is not the same skin.

Beneath that, there are some parts of the body that remain mostly the same--parts of the bones, the teeth, some cellular membranes--but not only are there many cells being destroyed and replaced (red blood cells, white blood cells), the human body is over half water by weight (estimates range from 45% to 75%, depending on factors such as age and physique), and the water is constantly recycled. Given a decade, if the same water molecule is anywhere in your body, it is likely to be a remarkable coincidence. That means that at least half the mass of your body will have been replaced given a decade.

Besides, if we're talking matter, we have to be talking molecules. Thus even if some part of McComb that does not constantly regenerate--such a a tooth--touches that of his counterpart, the precision necessary for this molecule to contact this molecule is incredible. There is about as much danger in the possibility that a time traveler would drink a glass of water in which there happened to be an atom of hydrogen already present in his saliva.

Apart from all that, there is no particular reason to suppose that molecules are individualized in some way. We know that they pass electrons to each other, and thus just as your body is constantly replacing its outer shell, so too all of the atoms within it are constantly replacing theirs. The notion of "the same matter" is almost devoid of meaning on any level.

There is danger in meeting yourself in the past, but that danger lies in the fact that you alter your own history, and your younger self will now have some knowledge gained from your older self that your older self did not then have. That, though, happens simply by seeing or hearing or otherwise sensing the presence of someone who was not present in the previous history, and is complicated even when the younger self does not know that something is different from what happened to his older self. Consider when Harry Potter saves himself. Those kinds of predestination paradoxes are very unstable. But despite its popularity in time travel stories, there is no reason why touching your past self should be any different from touching any other person in the past.

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