Principle of Equivalence

If an observer sees an object fall to the floor of his or her laboratory, the observer has a dilemma. The observer does not know if the laboratory is stationary, e.g., on the surface of something like the Earth, or if the laboratory is accelerating upward, e.g., it is carried aloft in a rocket. Einstein stated this in the Principle of Equivalence. The fact that the effects of gravity cannot be distinguished from those found in experiments performed in accelerating rockets in space (far away from any massive object) suggests that gravity is not a real force.

Einstein sought a geometric interpretation for gravity. He noted that objects on flat surface which are not subjected to forces roll in straight lines and that objects that roll on curved surfaces follow curved paths even when they are not subjected to external forces. The natural motion of an unforced object could either be a straight line (as Newton envisioned) or a curved line (as would be incomprehensible to Newton) depending upon the shape of the surface upon which it was rolling.

Einstein proposed that mass distorts the shape of space-time so that falling objects are simply rolling into the depression and orbiting bodies are simply rolling around in the depression. The objects are not undergoing forced motion; they are simply following the lay of the land.

Black holes distort space-time in a much more extreme manner (note the cusp or point the depression comes to at its bottom). An interesting discovery was made by Einstein-Rosen in the 1930's, they found that the black holes would close down but then actually re-open into another Universe forming an Einstein-Rosen bridge. An alternative interpretation is that the black hole opened up into another part of the space-time of our Universe. Such a passageway (wormhole ) leads to the possibilities of rapid travel through the Universe (or between universes--through hyperspace or Star Trek-like warp-drives) and time travel!!

Are There Reasons Why Time Travel Is Not Possible?

There are many complaints which can be leveled at the idea of time travel, both physical and philosphical. Some of the physical complaints can be eased by looking at rotating black holes where you don't have to pass through the singularity, but the problem is still severe in that event horizons are one-way membranes.

There are other types of wormholes which do not suffer from some of the physical problems. These are known as Schwarzschild wormholes where you have singularities which are not cloaked by event horizons. These do not suffer from the one-way nature of event horizons but they do suffer from other problems.

As stuff passes through the throat of a wormhole, it is compressed and so its density (mass/volume) goes up and the gravitational force, GM/r², amplifies and the throat pinches itself off. To get around this problem, Thorne, Morris, & Yurtsever postulated that some arbitrarily advanced civilization (AAC) could develop a new form of matter to hold open the throat of the wormhole. In this case, one could travel from place to place in spacetime and, under the right circumstances, could travel in time. That is, wormholes could act as time machines!!

Is Time Travel Possible?

The impossibility of time travel has never been proven, and so it might be possible. However, there is also no evidence that time travel has occurred and if time travel were allowed then, potentially, we would have serious problems as laid out in notions like the grandfather paradox . Time travel is not something with which we want to deal at this time (or at any time really). It is better that it were not possible. Someday if we can show and understand why time travel is not possible then we will have done something quite significant. There is nothing in our current theories which tells us why time is special and why time flows as it does. (There is no reason to believe, based on theory, why cause must precede effect in our Universe--What defines the Arrow of Time?) Since cause always precedes effect, this probably means that there is a physical reason for why time flows as it does. Unfortunately, the reason escapes us at this time. If we could understand why time travel is not possible, it might help us to understand the fundamental nature of the Universe.