Retrocognition is very similar to the experience of seeing a ghost of the dead. Suppose that you are walking down the street when you see the ghost of a neighbor who died a year ago. If you have read much about ghosts, you will know that they are not the scary phantoms that many people think they are. The neighbor's ghost may be walking in her garden or sitting on the front steps of her house or standing on the front lawn, gazing into the distance--something she may have often done. As you see her ghost, you are glimpsing a kind of "video replay" of a past moment in that woman's life. You are in the present time, but somehow and scientists don't know how a part of her life has become visible for a moment.

Instead of just seeing one person's ghost, you have stepped out your door and momentarily entered the past. In a few moments, though, it will all fade from view, replaced by the present.

Travel to the future is called precognition, or "knowledge of future events before they happen." You can travel to the future to gain a glimpse of what will happen to you, usually, or to someone you know. You won't find yourself beaming aboard the Starship Enterprise, though, or even the space shuttle Endeavor. Neither will you find yourself in some future city trying to find or avoid a "terminator."

As an adult, Mrs. Appleton married an optician and lived a happy life. One day, after she had been married fifteen years, her husband brought home a photograph album that contained many pictures from his childhood. Since she hadn't seen many photos of him as a child, she eagerly looked through the book.

As her eyes focused on one picture, "my heart stopped, and I felt physically ill," she wrote. "There in front of me was the little boy from my childhood."

The photograph showed her husband as a young boy: wearing a sailor suit, standing on the roof of an apartment building. Although he was not holding a tomato plant, one was beside him on the roof.

Why had she seen a vision of her future husband as a child? Was this a peek into the future by peering into the past? Or did she have a vision of the day that she would see the childhood picture of her husband?

"For so long I have felt alone with my story, not daring to discuss it fully with anyone for fear of being thought a crank," she wrote author Inglis. "I have never told my husband."

Another typical precognitive experience--and one that came true in a much shorter period of time--happened to an air-conditioning contractor living in Idaho, according to researcher Louisa Rhine. One night he dreamed that he was working all night in a hardware store. As the sun came up, he. picked up his toolbox and walked toward the front of the store where the sunlight was streaming through the plate-glass windows. When he reached the front door, he turned around and looked at the rear of the store. There he saw two large air conditioners hanging from the ceiling. When he woke up, he remembered his dream because, as a person who installed air conditioners, he had never seen any hang from the ceiling.

Three weeks later, his dream began to come true. The owner of a hardware store asked him to install two air conditioners. Because there was no floor space available, they would have to hang from the ceiling. What's more, so that the installation wouldn't disrupt business, they would have to be installed at night. The night that he installed the units, the man finished around 5 A.M. He picked up his toolbox and walked to the front door, where he saw bright sunlight illuminating the store. He glanced back at the store and saw the two air conditioners suspended from the ceiling. He was chilled by the knowledge that three weeks earlier he had somehow visualized everything that happened that night.

Did this really happen to him? The reader must take his word for it, since he didn't follow some easy steps to prove that his dream was precognitive. He would have given good evidence that he visited the future in his dream if he had written down his dream and if he had told it to someone--before he walked into the hardware store and realized that his dream was coming true. But this points out an important fact about precognitive dreams: They often provide a glimpse of an unimportant event, so unimportant in fact that the dreamer may not think that the dream is worth remembering.

The most common type of time travel involves a trip to the future. Getting a glimpse of the future is reported much more often than stepping into the past--even momentarily. Thousands of people have precognitive dreams or visions each year, but only a few claim to have visited the past. Many more experience what is known as déjà vu (which means "already seen" in French), a sense that they have visited a place or experienced an event before it actually occurred.

Is future travel easier? Actually, precognition may not be an easier process than retrocognition. It may simply be that it's easier to get others to believe your trip to the future. If you write down what you see and give it to someone before the future event occurs, people will believe you. On the other hand, tell people that you've visited a time in the past and seen the Battle of Bull Run and they are more likely to laugh or to think that you've gone crazy. Since there's no way to prove that a time trip to the past was real, many people may choose not to tell anyone about it.

Although time travel is usually pictured as a great adventure and although it can be enjoyable to imagine, it is not always fun in reality. In fact, travel to the past is usually a confusing experience. If you see a past time or event, how can you be sure that it really occurred and that you did not imagine it? How can you convince other people to believe you? Trying to talk about your experience can be frustrating if no one believes you.

Travel to the future can be unpleasant, too. H. G. Wells imagined future time travel in his time machine to be similar to riding a "switchback" or roller coaster. The time traveler felt as if he were going to crash at any moment. His eyes hurt as night and day flashed by.

Although real travel to the future has not had the same physical effects, its emotional effects can be upsetting, especially if your dream or vision allows you to have foreknowledge of a tragedy. For example, author Cassandra Eason has written about the experiences of a girl named Pat who saw the future in a dream and was blamed for it. When she was thirteen, she had a dream about someone at the bottom of a nearby lake. The next morning she told her mother about the dream, but her mother wasn't interested in listening to her story. As Pat told Eason, her mother "sent me off to school and told me not to be stupid."

That afternoon, her mother yelled at Pat when she returned home. Angrily, she told Pat that her cousin--an older woman with failing eyesight had drowned that day. She had gone for a walk around the lake and had stepped on some algae growing at the lake's edge, thinking that it was grass.

"Your bloody dream," her mother repeated over and over again, as if Pat's dream had caused the events rather than merely pictured their occurrence.

As you see, time travel appears to be possible, but perhaps not in the ways you have imagined. Rather, to make a time trip, all you'll need is your brain and some luck. But before you pack your bags and say "bon voyage," you need more information about the types of time trips people have taken.

Rhine, Louisa. The Invisible Picture. Jefferson, NC: McFarland, 1981.

Wells, H. G.  Three Prophetic Novels. New York: Dover, 1960.