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| Is Time Travel Possible |
Precision timekeeping like this makes our high-tech, computer-driven lifestyles possible. But as our timekeeping systems become ever-more accurate, we find that time does not flow the way we think it does. Gravity slows time, and this is the key to one form of time travel. When you leave a gravity field such as the earth's surface, time moves at a different rate for you than for your friends on earth. The time difference is greatest when you move at high speed. The time-traveling protons at CERN show us that we, too, can travel far forward in time. Decades from now, spaceships traveling near the speed of light could fly into the stars on a 10-year mission. For the people on board, it would be 10 years. On earth, a thousand years would pass. The astronauts would return to a far different future world. Time travel into the future is possible. But is it a one-way trip? Can we make our dream of time travel backwards and forwards come true? With the right technology, time-traveling spaceships could take us into the future.
It would be possible to go to any time and place But can we go against the era of time and journey into the past? Well, it might not b be as hard as it sounds. I mean, after all, the past is all around us. Consider this. The speed of light is 186,000 miles per second. Why, that's awfully fast. But when a piece of light travels from here to there, it takes time. And that means that everywhere you look, you're looking back in time. It takes one billionth of a second for light to travel one foot. In the early years of the 20th century, a young patent clerk named Albert Einstein gave us a possible way back. Riding to work on a streetcar, the barely 20-year-old Einstein looked up at a clock tower, and suddenly it all clicked. Einstein realized that time is relative to where you are and how fast you're moving. Time is the fourth dimension, bound tightly together with length, width, and depth - the dimensions of space. A few years later, Einstein used his ideas about gravity's effect on space and time to create a mathematical map of the cosmos. He proved that the fabric of space and time is curved. If the universe is curved, there might be ways to build bridges across it or create loops inside of it, loops that will allow time travel. That was the conclusion reached in 1949 by the mathematical genius Kurt Gödel. Gödel was a close friend of Einstein's, and he decided to see if the great man's equations permitted time travel. He found that they did. If the universe rotates on its axis and you somehow remain perfectly still, it would be possible to go to any time and place in the universe - an exciting discovery, except that we now know that the universe does not rotate. And without the rotation, you cannot have time travel. So I was wondering if it would be possible to have this sort of structure in a much smaller scale, and I discovered, alas, that's not going to be possible.
The time-traveling protons at CERN Because if you tried to speed up a body to generate the time machine, what you would find before the time-machine property was created, you would rip a hole in space and time. You would create a singularity right there in space and time. So, alas, I had to give up my dream of creating a time machine. Tipler's spinning cylinder might not work, but there are massive objects in the universe that are already spinning near the speed of light - black holes. The immense gravity of black holes push the laws of physics to the extremes. Could the secrets of backwards time travel lurk in their Stygian depths? Black holes are small but incredibly massive objects scattered throughout the universe. The intense gravity of a black hole warps the fabric of time and space more than any other celestial object we know of. Can the time-warping properties of black holes be harnessed? Can we use them to travel through time? Black holes are not time machines. You would fall into a singularity, and you'd be crushed, and you would die. Some interesting effect that we don't yet understand about what happens at the center of a black hole, there's no reason to think that it pushes you backward in time. The black hole is more or less a one-way street. You go in. You will never come back out. So black holes won't work. But another cosmic anomaly made famous by science fiction might do the trick - wormholes. Wormholes are magic doorways connecting two remote locations. These cosmic sky bridges would allow us to jump across space and travel in time. Fly into a wormhole, and you can take a shortcut to another place or time. We have no proof that wormholes exist, but there is plenty of solid science behind them. The fundamental question was could a very advanced civilization accumulate enough negative energy and hold it in the interior of the wormhole long enough to keep the wormhole open so that somebody could travel through it.
We're trying to send signals back in time The answer is we don't know. Meanwhile, another renegade physicist worked up a different way to harness the time-warping effects of celestial phenomena. Richard Gott has been studying the problem of time travel for decades. That's what space-time around the two cosmic strings looks like. So then what I can do is if I circle the two cosmic strings with my spaceship, I can arrive back to planet "A" at noon. Now, planet "A" at noon is the same time and the same place. So I can come back and shake hands with myself as I departed. So my older self can come back, and I can see myself off. This is me visiting an event in my own past. That's real time travel to the past. But, once again, there are one or two problems with this. For starters, when you push two cosmic strings together at high speed, it may create a black hole. You may be killed after doing the time travel, or you could be killed before you even complete the time travel. Another group of explorers hunt for answers to the mystery of time travel in perhaps the least likely place - deep inside the heart of the atom. It seems that time travel is next to impossible in Einstein's world of space and time. Maybe it's more likely that you'll see it one place or another, but there's really a spectrum of possibilities for where you will see the particle. So when you combine the ideas of quantum mechanics with the ideas of time travel, all hell breaks loose. One of the strangest properties of quantum mechanics is called "nonlocality." It's when two particles instantly affect each other, even when they're miles or light-years apart. It's a bit like voodoo. We're trying to send signals back in time. And if that works, perhaps one day we can send humans back in time. An exciting idea, but it opens the door to the problem of paradox. A paradox is a situation that contradicts itself - doesn't make any sense. Say you send a cure for cancer from the future to the past. Would the dead now be alive? See? Time travel is filled with such mysteries.
And without the rotation, you cannot have time travel The things we would like to understand about time travel are, one, is it possible, even in principle, that the laws of physics permit backward time travel? We don't know the answer. We need the laws of quantum gravity in order to find out the answer. Second question is, if backward time travel is possible, then what does nature do about the so-called grandfather paradox, that I can go backward in time if it's possible and kill my grandfather before my father was conceived, thereby changing history so that I no longer exist? What does nature do about that? The conservative interpretation is that space-time is one four-dimensional thing. It doesn't change. Even if we someday have the technology to travel back in time, the machine will only work starting at the point we invent it, creating the first loop in time. The technology that would be required to make a time machine that has even a whisper of a hope of success is as far beyond us today as space travel is beyond the capabilities of an amoeba, because our technology is so puny. There's no hope at all. Time travel seems unlikely if we approach it purely as a matter of taking a person or information from the present and transporting it to the past. But there is another way to journey into the past, a way that until recently would have been considered preposterous but is getting closer to reality every day. We could build the past. We've seen that time travel into the distant future is possible. But it's a one-way trip. Time travel into the past might be theoretically possible, but it requires inconceivable amounts of energy and god-like technology. Our best hope may lie in computer re-creations of times past. So it looks like we won't be able to go back in time to visit the people we've lost or correct the mistakes we made when we were young. Our trajectory through time, from birth to death, is the one thing all living things have in common. Every human has to live with the fact that life is short and time is precious. We have our triumphs. We make our mistakes. If we could go back and correct those mistakes, would we ever learn anything from them? Would we be the people we are today? For now, at least, we can't turn back the clock. But... We'll keep trying.
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