Energy transforms into mass, mass transforms into energy. The relationship produces, and is dependent on, light.

A very small amount of matter is equivalent to a vast amount of energy.

For example...2 lb of matter converted completely into energy would be equivalent to the energy released by exploding 22 megatons of TNT.

In 1915, Einstein formulated a new theory. He proposed that gravitational effects move at the speed of c. An important feature of Einstein's new theory was that no particle could travel faster than the fundamental speed c.  He called this theory "general relativity" to distinguish it from "special relativity", which only holds when there is no force of gravitation.

Electrons travel speed of light, from center elliptical galaxy M87. Particles are ejected as the blackhole vaporizes a mass of 2 billion suns. Credit: NASA

In the general theory of relativity, he proposed that space and time might be united into a single, four-dimensional geometry consisting of 3 space dimensions and 1 time dimension.   If there is no gravity acting, the most natural lines in this geometry are straight lines.  If gravitation is present, it should not be considered a force. Rather, gravitation changes the most natural world lines and thereby curves the geometry of spacetime.

In a curved geometry, such as the two-dimensional surface of the earth, there are no straight lines. Instead, there are special curves called geodesics. In the solar system, for example, the effect of the sun and the earth is to cause the moon to move on a geodesic that winds around the geodesic of the earth 12 times a year.

Einstein's general relativity theory predicts special gravitational conditions like the "Big Bang".  Another predicted gravitational effect is the existence of black holes - a star with a gravitational force so strong that light cannot escape from its surface.  Because light cannot escape from a black hole, for any object – a particle or a spacecraft - to escape, it would have to move past light. But light moves outward at the speed c. According to relativity, c is the highest attainable speed, so nothing can pass it. The black holes that Einstein envisioned, allow no escape whatsoever.

A singularity forms, like the mint with a hole. In principle, it would be possible to dive into such a black hole and through the ring, to emerge in another place and another time  This solution only developed in the 1970s, after astronomers discovered what seem to be real black holes, in our own Milky Way Galaxy and in the hearts of other galaxies.

Another kind of black hole is graphically described as a "wormhole", in which a black hole at one place and time is connected to a black hole in another place and time (or the same place at a different time).  This was once thought impossible...

However - scientists are watching two supermassive black holes spiral towards each other near the center of a galaxy cluster named Abell 400. Shown in this X-ray/radio composite image are the multi-million degree radio jets emanating from the black holes.

Credit: X-ray: NASA/CXC/AIfA/D.Hudson & T.Reiprich et al.; Radio: NRAO/VLA/NRL

TIME - The Fourth Dimension

With a brilliant idea and equations based on Einstein’s relativity theories, Ronald Mallett from the University of Connecticut has devised an experiment to observe a time traveling neutron in a circulating light beam.

“Say you have a cup of coffee and a spoon,” Mallett explains to PhysOrg.com. “The coffee is empty space, and the spoon is the circulating light beam. When you stir the coffee with the spoon, the coffee – or the empty space – gets twisted. Suppose you drop a sugar cube in the coffee. If empty space were twisting, you’d be able to detect it by observing a subatomic particle moving around in the space.”

And according to Einstein, whenever you do something to space, you also affect time. Twisting space causes time to be twisted, meaning you could theoretically walk through time as you walk through space.

“As physicists, our experiments deal with subatomic particles,” said Mallett. “How soon humans will be able to time travel depends largely on the success of these experiments, which will take the better part of a decade. And depending on breakthroughs, technology, and funding, I believe that human time travel could happen this century.”

“Einstein showed that time is affected by motion, and his theories have been demonstrated experimentally by comparing time on an atomic clock that has traveled around the earth on a jet. It’s slower than a clock on earth.”   Although the jet-flying clock regained its normal pace when it landed, it never caught up with earth clocks – which means that we have a time traveler from the past among us already, even though it thinks it’s in the future.

Some people show concern over time traveling, although Mallett – an advocate of the Parallel Universes theory – assures us that time machines will not present any danger.  “The Grandfather Paradox [where you go back in time and kill your grandfather] is not an issue,” said Mallett. “In a sense, time travel means that you’re traveling both in time and into other universes. If you go back into the past, you’ll go into another universe. As soon as you arrive at the past, you’re making a choice and there’ll be a split. Our universe will not be affected by what you do in your visit to the past.”

The author of Hunters of the Cloud agrees with Mallett and adds this analogy -  If you took a jet from New York to Europe, you'd be a few hours ahead of yourself.  But what if the jet could take you there at the speed of light?  You're waving goodbye to your mom at JFK, and saying hello in German.  It would appear as if you were in two places at once.

Now what if you traveled back to JFK at an even faster rate?  Would you meet yourself back in time?  No - you are in a parallel of time.  If the parallels are close enough, your mom arrives to greet you, and time goes on, as it did for the example of the atomic clock.

If the time is farther apart, you will notice subtle changes in the place you once knew.  The Empire State Building might exist, but your mother does not.  She hasn't been born - and might never be born in this timeline, but she will exist in the original.

If the time dimension is even farther apart, everything you know could be changed or gone, as if you went to a new world.  In the original stream - you would have "disappeared". Mind-boggling isn't it?

When you look closely at the general theory of relativity, it turns out that there is nothing in it to forbid time travel. The theory implies that time travel may be difficult...but not impossible.