scene from Back to the Future

Figure 1. The time-traveling car in Back to the Future.

time travel diagram

Figure 2. According to one theory, if you went back to try to save Lincoln's life, you would create two different futures. In one, Lincoln would be shot and killed. In another, containing you and your time machine, he would live on.


Figure 3. Photo of a UFO.

Any sensible explorer will prepare well before setting out on an adventure into the unknown. So, before looking at how real time travel might be achieved, we should consider the risks and problems that might be involved. Like Marty McFly in Back to the Future, we might find that trips into the past can affect the future in unexpected ways.


Imagine, for instance, you have just invented a vehicle that can transport you back to any date you wish. You dial a time 160 years ago, sit back in the pilot's seat, and pull back the starting lever. There is a blurred motion outside the cabin window. In just a few seconds, as measured by your own watch, you have hurtled back more than a century and a half.


Unfortunately, your time machine has landed on and squashed one of your ancestors! Had the accident not happened, this person would have become the father of your great-great-grandfather. But now history has been changed. A whole branch of you family has been wiped out. Worst of all, without that part of your family tree, you will not be born! Yet, here you are standing outside your time machine in the 1840s. How can this be?


Hurriedly climbing back into your machine, you reset the time-dial to April 14, 1865. The destination you choose is Ford's Theater, Washington, D.C. You arrive just minutes before John Wilkes Booth is due to slip into the private box of Abraham Lincoln and his wife to shoot the president. At least now you have a chance to make up for your earlier mistake. Quickly, you warn the authorities of the assassination attempt on Lincoln. Because of your action, the gunman is arrested, and Lincoln is not killed. But what will be the affect on world history?


Changing the Past

Even sending back objects rather than people into the past could cause some strange problems. In one short story, a professor invents a machine that can transport a small brass cube back through time. He tells his friend that at exactly three o'clock, he will place a cube on the time machine's platform and send it back five minutes into the past.


"Therefore," says the professor, "the cube should, at five minutes before three, vanish from my hand and appear on the platform." And, sure enough, it does.


"See!" exclaims the professor. "Five minutes before I shall place it there, it is there!"


But now comes the interesting part of the story. One of the professor's friends has a question. What would happen, now that the box has made its journey back though time, if the professor fails to put the box on the time machine at three o'clock? Fascinated, the professor decides to find out. It proves to be a terrible mistake. In the story, at exactly three o'clock, the professor, his friends, and the whole Universe – except for the brass box – disappear!


When we start talking about time travel into the past, we encounter many logical problems such as the ones just mentioned. For this reason, some scientists believe that going back in time is not possible. Others argue that, if a time traveler did go into the past, it would cause a split in time. One branch of time would carry on as before. The other, with the time traveler in it, would contain differences because of the time traveler's actions.


According to this theory, if you went back to try to save Lincoln's life, you would create two different futures. In one, Lincoln would be shot and killed. In another, containing you and your time machine, he would live on. Look at the diagram above to understand how this might work (see Figure 2).


Where Are All the Time Machines?

One of the strongest arguments that has been made against time travel could be called "the case of the missing time machines." In other words, if in the future time machines are invented, then why has no one seen any of them?


If time travel did become possible, it seems likely that thousands of people would want to take advantage of it. Time cruises might be offered to the age when wooly mammoths and saber-toothed tigers still roamed the Earth. Other tourists might be more interested in seeing ancient Rome, or Columbus setting sail for the New World. Surely, if time machines from the future had visited key moments in history, including our own time, someone would have spotted them by now?


Well, perhaps they have. It is possible, for instance, that some UFOs, or Unidentified Flying Objects, are time machines from the future. Of course, the chance of this being true is very, very small indeed. It is also possible that we cannot see these future time travelers or their machines. They may choose to keep themselves invisible to avoid changing history. In spite of the problems that it seems to raise, time travel may be taking place.


Seeing Into the Future

Can some people see events before they actually happen? Is there a connection between the present and the future?


A large number of men, women, and children claim to have experienced precognition. This often takes the form of a dream in which a disaster is seen taking place, such as the crash of an aircraft. Later, the accident my occur just as in the dream. As with all such claims, it is very difficult to supply hard proof.


A second type of experience, which many of us have from time to time, is called deja vu. This is a sensation, lasting just a few seconds, that what is happening has occurred in exactly the same way before. Could it be that a future situation has been seen in a dream, forgotten, and then remembered only when it actually takes place? No one knows.


Finally, there is the mystery of premonition. This is the feeling many people say they have that something, perhaps very sad, happy, or important, is about to take place. More often than not, it involves members of the same family, and especially identical twins.


If precognition, deja vu, and premonition are real, rather than imagined, then scientists have a difficult problem. They must explain why human beings appear to be able to see into the future. Is there some "sixth sense," as yet undetected, that can reach back and forth in time? Or do future events send ripples into the past that we can pick up, especially when we are asleep? Much more research is needed to find the answers.


But whether or not we can see future events, it is possible that someday people will be able to travel into the future. There are at least two ways, in theory, that human beings could use to journey far into the world of tomorrow.