There is a long tradition of time travel in science fiction, going back to H.G.Wells's The Time Machine in 1895. As a kid, I was fascinated by these stories. Let me tell you about a fantasy experiment I thought about back then involving a machine that could send something (a marble, say) backward in time. I imagined a desk with two buttons and a place where the marble would appear after being sent back fifteen minutes; the other would blow up both the machine and the marble. I tried to imagine what it would be like to sit at that desk and see the marble from the future pop into existence. It would be like a game of "gotcha" with the universe. If the marble didn't show up from the future, you could press the button to send it, and if it did show up you could blow up the machine and not send it. Either way, it would be a clear victory of human free will over the impersonal laws of the universe.
You run into all sorts of paradoxes when you talk about time travel. Perhaps the most famous is the "grandfather paradox," which involves someone who goes back in time and prevents the marriage of his grandfather. If he did this, of course, he couldn't have been born, in which case he couldn't have gone back in time, in which case his grandfather would have married, in which case he would have been born and… you got the idea. It was, perhaps, thinking about such paradoxes that let Stephen Hawking to propose something called the "Chronology Protection Conjecture," which says, in essence, that there will always be a reason why time travel cannot take place. As a scientific idea, time travel is a little like parallel universes. It occasionally surfaces as an unexpected consequence of some theory and is announced with great fanfare, but (so far, at least) when people look into it more closely they find a factor that hasn't been considered that will keep it from happening.
I was fortunate, I suppose, because I got involved in one of these episodes early in my career and thereby acquired a kind of immunity. In the early 1970s, physicists were talking about a hypothetical partical, called the tachyon, that could travel faster than light. It turns out that if such particles exist (and so far no one has any evidence that they do), then it would be possibleto play some games with tachyons moving near the speed of light (you really don't want to know all the details) that would have the effect of sending messages into the past. This would lead to the grandfather paradox as easily as sending a person back in time--after all, the message "My God, don't marry that woman!" would start the paradox in motion as well as anything else. In this case, a student and I were able to show that the problem of distinguishing between tachyons sent on purpose and background cosmic ray tachyons would actually prevent any meaningful message from being sent by this scheme.
More recently, conjectures about time travel have concerned the massive objects called black holes, which have the effect of warping the fabric of space in their vicinity. General relativity tells us that space and time are linked (hence the term "space-time"), so distorting space also distorts time. In some cases, it is possible to find a path along which a spaceship could travel that would, in effect, bring it back to its starting point before it left.
One popular recent version of this scheme involved something called a "wormhole." Familiar too Star Trek aficionados, a wormhole is a hypothetical connection between two black holes--think of it as a tunnel through another dimension. The scheme involved bending the wormhole around so that the two ends were near each other, then accelerating one black hole to near the speed of light. This would have the effect of distorting time in the region and would allow a spaceship to travel "downwhen" (to use a wonderful term coined by Isaac Asimov into its own past).
Unfortunately, when people began looking into this idea in more detail, it turned out that the situation wasn't so simple (although accelerating black holes to the speed of light isn't something most people would call "simple"). It turns out that the massive energy necessary to distort space around an accelerating black hole would disrupt the vacuum in the region and, in effect, destroy the wormhole. In other words, the "tunnel" from one black hole to another would be destroyed as soon as it was built.
Can I guarantee that every time-travel scheme will meet a similar fate? Of course not--you can never tell what future scientifuc theories will look like. I harbor a fond hope, for example, that human beings will find some way to circumvent the speed-of-light barrier and break out into the galaxy. I harbor no such hopes for time travel, however, despite the hours of pleasure I have derived from reading fiction devoted to it. Perhaps the most convincing argument supporting this point of view comes from Stephen Hawking. If time travel is possible, he argues, then surely a future civilization will figure out how to accomplish it.
So, he asks, "When are all the tourists from the future?"
You run into all sorts of paradoxes when you talk about time travel. Perhaps the most famous is the "grandfather paradox," which involves someone who goes back in time and prevents the marriage of his grandfather. If he did this, of course, he couldn't have been born, in which case he couldn't have gone back in time, in which case his grandfather would have married, in which case he would have been born and… you got the idea. It was, perhaps, thinking about such paradoxes that let Stephen Hawking to propose something called the "Chronology Protection Conjecture," which says, in essence, that there will always be a reason why time travel cannot take place. As a scientific idea, time travel is a little like parallel universes. It occasionally surfaces as an unexpected consequence of some theory and is announced with great fanfare, but (so far, at least) when people look into it more closely they find a factor that hasn't been considered that will keep it from happening.
I was fortunate, I suppose, because I got involved in one of these episodes early in my career and thereby acquired a kind of immunity. In the early 1970s, physicists were talking about a hypothetical partical, called the tachyon, that could travel faster than light. It turns out that if such particles exist (and so far no one has any evidence that they do), then it would be possibleto play some games with tachyons moving near the speed of light (you really don't want to know all the details) that would have the effect of sending messages into the past. This would lead to the grandfather paradox as easily as sending a person back in time--after all, the message "My God, don't marry that woman!" would start the paradox in motion as well as anything else. In this case, a student and I were able to show that the problem of distinguishing between tachyons sent on purpose and background cosmic ray tachyons would actually prevent any meaningful message from being sent by this scheme.
More recently, conjectures about time travel have concerned the massive objects called black holes, which have the effect of warping the fabric of space in their vicinity. General relativity tells us that space and time are linked (hence the term "space-time"), so distorting space also distorts time. In some cases, it is possible to find a path along which a spaceship could travel that would, in effect, bring it back to its starting point before it left.
One popular recent version of this scheme involved something called a "wormhole." Familiar too Star Trek aficionados, a wormhole is a hypothetical connection between two black holes--think of it as a tunnel through another dimension. The scheme involved bending the wormhole around so that the two ends were near each other, then accelerating one black hole to near the speed of light. This would have the effect of distorting time in the region and would allow a spaceship to travel "downwhen" (to use a wonderful term coined by Isaac Asimov into its own past).
Unfortunately, when people began looking into this idea in more detail, it turned out that the situation wasn't so simple (although accelerating black holes to the speed of light isn't something most people would call "simple"). It turns out that the massive energy necessary to distort space around an accelerating black hole would disrupt the vacuum in the region and, in effect, destroy the wormhole. In other words, the "tunnel" from one black hole to another would be destroyed as soon as it was built.
Can I guarantee that every time-travel scheme will meet a similar fate? Of course not--you can never tell what future scientifuc theories will look like. I harbor a fond hope, for example, that human beings will find some way to circumvent the speed-of-light barrier and break out into the galaxy. I harbor no such hopes for time travel, however, despite the hours of pleasure I have derived from reading fiction devoted to it. Perhaps the most convincing argument supporting this point of view comes from Stephen Hawking. If time travel is possible, he argues, then surely a future civilization will figure out how to accomplish it.
So, he asks, "When are all the tourists from the future?"