from Through the Wormhole; In a way, every man, woman, and child on this earth is a time traveler. Like it or not, we're all being shot relentlessly forward, making the journey from birth to death, and there's no going back. And there's no way of looking into the future. Or is there? What if we could travel back to witness events in the distant past or journey into the far future, see our destiny? Just think what we might learn if we could watch history unfold right before our eyes or what we could change in our own lives if we had the chance? For many, life's greatest sorrow is losing a loved one. The time I spent with my grandmother when I was a child helped make me the man I am today. I often wish I could see my grandmother again, or go back in time and show her who I am and what I've become as an adult. Seems like an impossible dream. But is it? Can science find a way to tear down the walls between now and then? Is time travel possible? To find the answer, we must first understand the nature of time. And that's a lot harder than it sounds. Steve Jefferts is an atomic scientist and master timekeeper. We all, I think, have some innate feeling that we understand time. It flows past us. Time goes on, we get older, things that happened yesterday are not happening today, et cetera. But I don't think that any of us, whether we're physicists who study time or just somebody who lives his life really, truly understands time. Steve works at the National institute of Standards and Technology in Boulder, Colorado. It's one of six labs around the world that calculate coordinated universal time, the official world time. Precision timekeeping like this makes our high-tech, computer-driven lifestyles possible. 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. This means that time travelers walk among us. Cosmonaut Sergei Krikalev is the world's greatest time traveler. Krikalev has spent 803 days moving at 17,000 miles per hour. He traveled fast outside earth's gravity, so time moved more slowly for him than for us. Because time passed at different rates, he has traveled into the future, 1/48 of a second into the future. If he travels for a year, he'll come back and find out that while he has aged 12 months, earth is 10 years older. Europe's Large Hadron Collider, or LHC, the world's biggest and baddest particle accelerator. Steve Nahn is a Professor of physics at M.I.T. Using the LHC, Nahn and thousands of other scientists turn pieces of atoms into time travelers. The time-travelling 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. 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. 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. According to Einstein, time is just like space. Sean Carroll is a physicist at the California institute of Technology. Physicists tend to be eternalists. They think that the whole universe, the whole four-dimensional space-time in which we live, is equally real. Professor Frank Tipler was one of the renegade physicists who followed in Gödel's footsteps. We can't rotate the universe, it either is rotating or not, but we might be able to do something on a smaller scale. An obvious, easy-to-solve model in relativity was a rotating cylinder. And so I was able to show that a rotating cylinder would give rise to these loops in time, being able to go backwards into time. Tipler's gigantic cylinder would hang in space, whirling at nearly the speed of light. Space turns into time and time into space as both become twisted around the cylinder. 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. 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. No one knows more about wormholes than renowned physicist Kip Thorne. For starters, he can tell you why they're called wormholes. If you have an apple, a worm drills a hole through the apple, reaches from one side to the other, you can think of the surface of the apple as being like our universe, and the worm has gone through some higher dimension to reach the other side. If they exist, wormholes are smaller than atoms. If we want to go through them, we need to stretch them out and hold them open. Prying open a wormhole would take a tremendous amount of energy, not just ordinary energy, but something called negative energy. Negative energy is antigravitational. It repels the fabric of space and time and would prevent gravity from crushing a wormhole. One problem, a lot of people don't believe negative energy exists. The kind of energy that would antigravitate is ridiculous. I was not willing to dismiss this possibility out of hand. 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. 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. Cosmic strings are thin strands of energy that may run through the universe. But not understanding something has never stopped people from experimenting. Right now, 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. It's the world deep inside the atom, where the weird laws of quantum mechanics take hold. When you combine the ideas of quantum mechanics with the ideas of time travel, all hell breaks loose. So let's say that someday we develop that better understanding of time. After solving the riddle of quantum gravity, we build a working time machine. What would happen then? Would we be able to change the past? The answers are fantastic, disturbing, and a little. Strange. 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. 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? If backward time travel is possible, then what does nature do about the so-called grandfather paradox? What does nature do about that? The conservative interpretation is that space-time is one four-dimensional thing. It doesn't change. So if time travelers go to the past, they were always part of the past, and they don't change it. In other words, if you had time travelers aboard the Titanic, they might have warned the Captain about the iceberg, but he didn't pay any attention to them, like he didn't pay any attention to the other iceberg warnings, because we know the ship ended up hitting an iceberg.
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