All objects, such as our computers and mice, are three-dimensional. Everything has length, width, and height. But there is another dimension: time. Traveling through time means traveling through the fourth dimension of spacetime.
A car driving in a straight line moves through one-dimensional space. Turning left or right adds a second spatial dimension. Bouncing along a winding mountain road takes it through three-dimensional space. But how can we travel through time? How can we find a path into the fourth dimension?
Science-fiction movies usually feature a giant, energy-hungry machine—a Time Machine—that opens a path to the past or future. The idea may seem far-fetched, and reality could be very different, but the concept itself isn't so crazy.
Physicists have also racked their brains over time tunnels, but we'll approach the question from another angle. We don't know whether the laws of nature allow a gateway to the past or future. In fact, we think they might—and we've even given them a name: wormholes.
In fact, wormholes are everywhere, but they're so tiny that they're hard to see with the naked eye. They're incredibly small, existing in every corner of the world and of space. You might not believe it, but keep reading.
Nothing is perfectly flat or completely solid. If you look closely, you'll find tiny holes and cracks everywhere—at the microscopic level. This is a basic principle of physics, and it applies to time as well.
A billiard table looks smooth and flat on the surface, but up close, it isn't. Its surface is full of cracks and tiny holes. Even something as smooth as a billiard ball has small cracks, crevices, and holes. That's obviously true of three-dimensional space, but believe it or not, it's true of the fourth dimension, too. Time also has tiny cracks, crevices, and holes. When we keep shrinking down to the smallest scales, even smaller than molecules and atoms, we reach what's known as Quantum Foam. Wormholes exist there.
In the quantum world, tiny tunnels or shortcuts through spacetime continually form, disappear, and form again. They connect two separate regions and two different points in time.
Unfortunately, these real time tunnels are only 10^-33 centimeters wide, far too small for a human to pass through. But the idea of a wormhole Time Machine has inspired new possibilities.
Some scientists believe it might be possible to capture one of these wormholes and enlarge it trillions of times, until it was big enough for humans or even spaceships to enter.
Suppose we had enough energy or the right technology. We might be able to construct a giant wormhole in space. It may not be feasible, but it would be an extraordinary machine, with one end near Earth and the other far away among the stars.
In theory, wormholes could serve an even greater purpose. If both ends opened in the same place, with no distance between them but at different times, a spaceship could take off and land near Earth while traveling back to the distant past. Perhaps dinosaurs would be lucky enough to witness a spaceship landing.
Wormholes are also an incredibly baffling and complex concept.
Hawking once came up with a simple time-travel experiment, but it didn't work. Why not? Hawking believed one reason was the well-known problem of traveling through time: paradoxes.
This is a cosmologist's nightmare. Such a Time Machine would violate a fundamental law governing the entire universe: cause and effect would be reversed, contradicting our common sense at every turn. Otherwise, the universe would inevitably descend into chaos.
So we believe that something will eventually happen to prevent paradoxes.
Therefore, although miniature wormholes do exist and might one day be enlarged, their fleeting existence makes them impossible to use as Time Machines.
That doesn't mean all time travel is impossible.
Satellites orbiting Earth. Every spacecraft has an extremely precise clock. Even so, they gain about one-third of a billionth of a second each day. The system has to correct this discrepancy, or the tiny difference will throw everything off, causing every GPS device on Earth to be off by about six miles a day. The resulting chaos is easy to imagine.
The problem isn't with the clocks. They run fast because time itself passes faster up there than it does on Earth. The reason for this strange phenomenon is Earth's enormous mass...
Einstein realized that matter slows down time, like a slow-moving stretch of a river. The heavier an object is, the more it slows time. This astonishing fact opened a door of hope to traveling into the future.
At the center of the Milky Way, 26,000 light-years away from us, lies the most massive object in the entire galaxy.
A Black Hole, with a mass equal to four million Suns, has been compressed into a point by its own gravity. The closer you get to a Black Hole, the stronger its gravitational pull. Once you get close enough, even light can't escape.
A Black Hole lies hidden within a dark sphere 15 million miles across. A Black Hole like this has a powerful effect on time, slowing it more than anything else in the Milky Way and making it a natural Time Machine.
Imagine astronauts orbiting a Black Hole for five years, while ten years passed on Earth. When they returned, everyone on Earth would be five years older than they were.
Of course, this isn't entirely practical. Its advantage over wormholes is that it doesn't create paradoxes or destroy itself in an instant through feedback. But it's extremely dangerous.
Fortunately, there's another way to travel through time, and it's our final—and most promising—hope of building a real Time Machine.
Traveling through time via the fourth dimension will never be as easy as taking a walk in the park. But there is, in fact, an astonishing shortcut. You just have to move incredibly fast—even faster than the speed needed to avoid being sucked into a supermassive Black Hole. The secret lies in another remarkable fact about the universe: there's a universal speed limit—186,000 miles per second, known as the Speed of Light. It's the fastest speed in the world and one of the most authoritative scientific principles.
Believe it or not, if you move close to the Speed of Light, you can travel into the future.
We've already built a machine that can do something similar.
The world's largest particle accelerator is located at CERN in Geneva, Switzerland. Deep underground, inside a 16-mile circular tunnel, a beam made up of trillions of tiny particles can be accelerated from a standstill to 60,000 miles per hour the instant the machine starts. With more power, the particles can accelerate further, until the beam circles the tunnel 11,000 times per second. That's extremely close to the Speed of Light, but it can never reach it. It can only get to 99.99% of the Speed of Light. At that speed, the particles can travel through time, too.
Under normal circumstances, these particles decay after 25 billionths of a second. But when they're accelerated close to the Speed of Light, their lifespans become 30 times longer. These particles are living time travelers.
It really is that simple. If we want to travel into the future, we just have to run faster—much faster. (I'm speechless after getting this far.)
The above is based on the Discovery Channel program Into the Universe with Stephen Hawking: Time Travel. If you're interested, you can check it out.
But if you enjoy science-fiction novels and movies, don't take them too seriously. There's no fun in that. We read for enjoyment, so why get hung up on the science?
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