Brief Answers to the Big Questions

A plane is two-dimensional and flat. On a plane, a triangle always has internal angles that add up to 180 degrees. A sphere is two-dimensional but curved; on a sphere, a triangle adds up to more than 180 degrees. Tiny, two-dimensional beings on a large sphere eventually would discover that large triangles on their sphere had inner angles that add up to more than 180 degrees. Likewise, our three-dimensional universe seems to be “flat,” and standard Euclidean geometry works fine everywhere. If, though, our three-dimensional universe is located on a four-dimensional sphere, we’d eventually find that our universe is curved.

Einstein, in his 1905 paper on special relativity, showed that space and time can be considered a four-dimensional unity—space-time—but that the time portion varies depending on how fast various observers are traveling relative to one another. His 1915 paper on general relativity proved that gravity warps space-time. For example, near the sun, light from nearby stars gets bent, so those stars’ positions appear to shift very slightly.

In addition, Einstein proved that, to travel as fast as light, a mass would need to be accelerated by an infinite force. This seems to rule out space travel at light speed or greater. It might, however, be possible to generate a “wormhole” that goes from one side of our galaxy to the other. Returning via a second wormhole might allow people to get home before they left.

Wormholes require immense negative mass and energy, which are hard to come by, though the uncertainty principle of quantum mechanics allows for this via virtual particles, which arise spontaneously in matched pairs and then disappear. Two metal plates, when sufficiently close, limit the type of virtual particles that can form between them. A greater number of virtual particles around those plates exerts a pressure on the plates. The energy density of the universe around the plates averages out to zero, so the energy density between the plates must be negative. This phenomenon, called the Casimir effect, has been demonstrated by scientists. Someday, people may be able to harness such energy to create wormholes.

If time travel is, indeed, possible, then perhaps someone should by now have come back to tell about it: “[I]t is difficult to believe that someone wouldn’t show off and tell us poor benighted peasants the secret of time travel” (137). Possibly, space-time hasn’t been warped enough to permit time travel but might become able to do so in the future. Folks from that time would be able to visit the past—but not so far back as to visit us. Any travel into the past could create paradoxes. For example, people might be able to kill their own ancestors and prevent themselves from being born.

One solution is that time travel to the past is possible only if it’s self-consistent; another is that quantum mechanics permits any number of possible timelines and that time travelers move along some of these lines but not others. M-theory, which contains 11 dimensions instead of four, might allow for time travel. The odds for any of these possibilities, though, seem low.