It was Isaac Newton who gave us the first concrete picture of time. To him, it was like an arrow: once fired, it went unerringly towards its target, never wavering, never slowing down, never coming back. It was a common sense view that was shared by everyone back then. So one second on earth is one second on Jupiter or anywhere in the universe; clocks tick at the same rate throughout the universe.

But this view has been challenged by the revolution led by Einstein and quantum physicists, who have forced us to stretch the limits of common sense. To Einstein, time was like a river which could slow down and speed up. According to his Special Theory of Relativity of 1905, you would slow down in time the faster you moved. Hence, a clock orbiting in space would slow down relative to a clock on earth. An observer on earth, watching an astronaut's rocket near the speed of light, would see the astronaut moving in slow motion.

This means that, in principle, an astronaut may take over 4 years to reach the nearest star in a super fast rocket ship, but to him, it might only appear to be 4 minutes. Hence, after a round trip, the earth will have aged more than 8 years, but he will have aged only 8 minutes. As incredible as this sounds, this effect is measured every day with our GPS satellites and our atom smashers.

This also means that our astronauts are time travelers, moving a fraction of a second into the future as they orbit the earth. So going forwards in time is something which we physicists measure every day. Contrary to some popular notions, you cannot use this Special Relativistic effect to go backwards in time. For example, in Superman I, the man of steel circles the earth fast enough that he breaks the light barrier, and the earth starts to spin backwards. So Superman, by traveling faster than light, goes back in time. Similarly, in Star Trek IV, the crew of the Enterprise hijack a Klingon ship, whip around the sun and break the light barrier, and wind upin the 1960s in San Francisco. But this is not possible. According to Special Relativity, you also get heavier and flatter the faster you move. When you hit the speed of light, you are infinitely heavy, are infinitesimally flat, and time itself stops. Since this is impossible, you cannot break the light barrier. (The reason you get heavier is because the energy of motion is being converted into mass, making you heavier. In fact, when you calculate how much energy is being converted to making you heavier, you get precisely E = mc squared. In fact, that is how this equation is derived.)

But in 1915, Einstein discovered his General Theory of Relativity, which gave us a much different view of time. Time was still a river, but it couldmeander its way around the universe, speeding up and slowing down near stars and planets. For example, time beats faster on the moon, and slower on Jupiter, than it does on the earth. However, the new wrinkle on all of this which has gained the attention of physicists around the world is that Einstein's General Theory allows for the river of time to have whirlpools, and even fork into two rivers. In this situation, time travel is possible.

Einstein himself was aware that his equations allowed for time travel. His next-door neighbor at Princeton, Kurt Gödel, perhaps the most important mathematical logician of the past millennium, found a new solution of Einstein's equations in 1949 which allowed for time travel into the past. He found that if the universe rotated, and you traveled around the universe fast enough, you could go back in time, and arrive before you left.

In fact, since then, a series of solutions of Einstein's equations have been discovered which allow for time travel back into the past. Time travel is allowed for:

a) traveling around a spinning universe

b) traveling around a spinning cylinder which is infinitely long

c) traveling around two colliding cosmic strings

d) traveling through a spinning black hole

e) stretching or compressing space via negative matter

f) traveling through a wormhole.

Each of these methods has their own advantages and disadvantages, which I discuss in the next blog post, where I answer the following questions:

a) Can you really build a time machine?

b) What does the quantum theory say about time travel?

c) What happens if you meet yourself as a child? (Or kill your parents before you are born?)

To be continued....