It seems that time travel has captured the imaginations of people since time began. Einstein’s Special Theory of Relativity speculated that traveling close to the speed of light would physically alter time by dilating it. This means that there should be places where time slows down, and others where time speeds up. Discovery News reports that Hawking recently expanded on this theory, outlining several theoretically realistic ideas for traveling through time.

Hawking almost perfectly explains his theories in a letter to the Daily Mail in a way that non-cosmologists can understand. He encourages people to think of time as dimension just like width, height and length. Just as when you travel in a car, you can go forward, right or left. The fourth dimension would be time. And all around us are wormholes that could act as a time tunnel.

As Hawking writes, "Down at the smallest of scales, smaller even than molecules, smaller than atoms, we get to a place called the quantum foam. This is where wormholes exist. Tiny tunnels or shortcuts through space and time constantly form, disappear, and reform within this quantum world. And they actually link two separate places and two different times." The time tunnels are too small for people to travel through, but Hawking and others believe that someday a wormhole could be widened for person or ship to travel through to the future.

Hawking points out that travel backward in time may be impossible due to the cause and effect theory. (For example, if you travel backward and prevent your birth, how could you have ever been born?) Further, Hawking suspects that radiation might collapse the wormholes, rendering them useless anyway.

Another way to time travel rides on the “time as a river” theory. As Einstein proposed before him, there are places where time moves faster and where time moves slower. It depends if there are things that drag on space, much like rocks in a moving river. The Earth itself drags on space, meaning time moves slower on Earth than it does in space. Hawking points out that the Global Positioning System satellite network in space must be adjusted because of this.

Further, Hawking claims black holes may be the key to time travel. He asks us to imagine a spaceship orbiting a super-massive black hole some 26,000 miles away.  To us, it would just look like the ship makes one orbit every 16 minutes. As Hawking writes, "A black hole ... has a dramatic effect on time, slowing it down far more than anything else in the galaxy. That makes it a natural time machine. … But for the brave people on board, close to this massive object, time would be slowed down. For every 16-minute orbit, they'd only experience eight minutes of time." 

Hawking reminds us that nothing can travel faster than the speed of light, and that time will always slow down right before reaching this speed. Therefore, if we had a ship that could travel near the speed of light, we could also travel in time.

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