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.