Some notes on time travel

John and Nick wonder, if time travel is possible, why haven't we seen any time travelers from the future?

Although physicists have imagined a couple different designs for a time machine, they all share a basic limitation: you can't travel to any date prior to the time machine's construction.

We tend to think of a time machine as something like the DeLorean in Back to the Future, just vanishing and reappearing at dates specified on a digital readout. Instead, the time machine envisioned by physicists can be thought of as a machine with two doors. If you walk into door A, you'll walk out of door B, say, tomorrow. If someone walks into door B tomorrow, they'll walk out of door A today. But you need to have built this machine first. Before the machine is built, there's no door for anyone to walk out of.

So, we shouldn't expect to see any time travelers right now, because no one's built a time machine yet. Only after we build the machine can we expect people to start walking out of it. And that means we'll never be able to go see the dinosaurs, unless we find a time machine that was built by aliens a hundred million years ago.

Some notes on evolution

Hey all. I just wanted to respond to some of the comments posted on Monday.

Regarding evolution, John asked, "If we as humans did evolve from monkeys, toward what I would presume would be a more advanced being, where are the links in the evolutionary chain? Why have the less primitive primates outlasted these middle species?"

First of all, it's not that humans evolved from monkeys. Humans and monkeys have a common ancestor; you could say they're our distant cousins instead of our great-grandparents. Our common ancestor is gone, but some of its surviving descendants are humans, and others are chimpanzees. And these are the descendants that survived; just as there may be branches of your personal family tree that ended because your great-uncle didn't have any kids, that common ancestor also produced branches that didn't make it to the present.

Also, it's important to realize that evolution does not have a goal. It's not trying to produce a more "advanced" species. That old picture of a line of animals, starting with a fish crawling out of the ocean and ending with a human being, is profoundly misleading. We as humans are (understandably) most interested in humans, and tend to see ourselves as special and important, but evolution does not see us that way.

For a better picture of evolution, go to this page and click on the link labeled "Tree of Life." It's a PDF file, showing an immense circular tree of life on earth. Zoom in close to read the species names at the tips of the branches. The 3000 species on this diagram are only a tiny fraction of all the species on earth, but you can get some sense of how varied life is; humans, mice, birds, frogs, and fish all fit in one tiny sliver of this circle.

Evolution is moving in all directions at once; different kinds of bacteria are evolving in a dozen different ways right now, and they aren't necessarily becoming more "advanced." They're just becoming different. The same is true for funguses, plants, insects, worms, etc. Human evolution is only one tiny part of the picture.

It's true that some species remain essentially unchanged for millions of years, while others do not. That's just because they occupy different environments. One population of fish might occupy a particular region of the ocean where it faces no competitive pressure, while elsewhere, another population finds itself competing against squids for food. It shouldn't be surprising that, after a few million years, those two populations may look very different.