Anyone who has watched Steven Spielberg’s movie Back To The Future is left with a curiosity of ‘what if’. What if you could travel back in time, as Michael J. Fox did in his DeLorean car — turned — time machine? What would you do? Where would you go? What would you change? Who would you meet? If you could travel to the future, what would you want to know?
The idea of time travel has long fascinated people. In 1895, H. G. Wells’ “The Time Machine” spawned a pop culture of time travel enthusiasts. A decade later the world came closer to time travel when Albert Einstein released his paper on special relativity. Previously, science had taught that time is absolute. It was believed that every event could be labeled with a unique “time”. In discovering that the speed of light appeared the same to every observer, no matter how fast she was moving, the notion of absolute time was abolished and replaced with a theory that space and time are intimately related.
One result of space-time is that the closer to the speed of light one travels, the greater the difference in measured time between the traveler and someone at home becomes. This is the classic Twins Paradox. Suppose there are twins Alice and Bob, both living on earth. Assume that Bob decides to take a 25 year trip on his spaceship at close to light speed. On his return, Bob will find that when he meets Alice, she aged by about 80 years while he was gone.
But theories of time travel are not based on moving faster than light. Instead they use the deep interconnectedness of space and time described by relativity. If you could warp space, a resulting change in time would occur. This warping can be thought of as a shortcut between two points in space-time. Physicists theorize that this warping occurs at wormholes.
In 1935, Einstein and Nathan Rosen showed that general relativity allows the formation of space-time bridges now commonly known as wormholes. What is required to build a wormhole? Unfortunately, the flux capacitor of Back to the Future won’t do it. We need negative energy. Quantum Theory and the Casimir effect describe how the universe can have areas of negative energy densities, though areas of positive energy densities must compensate for them elsewhere. These varying energy densities result in a distortion and possible warping of the universe.
Using sophisticated experiments with metal plates at varying distances that reflect particles of light, it has been shown experimentally that we can create this warping effect in space-time. By observing spinning particle and antiparticle reactions, it has been shown that a positive particle can be annihilated by an antiparticle moving backwards in time. World renowned physicist Stephen Hawking states in his book A Brief History Of Time; “We thus have experimental evidence both that space-time can be warped and that it can be curved in the way necessary to allow time travel. One might hope therefore that as we advance in science and technology, we would eventually manage to build a time machine.”
How do we build a time machine? The engineering required exceeds our capabilities by a long shot; a food-processor-powered DeLorean just wont cut it, as it did in Back To The Future 2. Machines have been built that actually demonstrate time travel on a microscopic scale, sending certain test particles into the future.
But it would require a structure of galactic proportions and tremendous amounts of energy to create wormholes allowing for time travel on a macroscopic level.
The “how” of time travel is only the first step in a long list of questions. We still must decide how to control such a machine, the repercussions of time travel, and our moral obligations. In theory then, it is possible that one day time travel will be a reality. Which brings us to one final question. Why haven’t we been visited by future generations?
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