Wormholes

Thorne believes there could be another type of tunnel-like structure existing in the universe that could be used for a time travel portal. Wormholes, also called Einstein-Rosen Bridges, are considered to have the most potential for time travel if they do exist. Not only could they allow us to travel through time, they could allow us to travel many light-years from Earth in only a fraction of the amount of time that it would take us with conventional space travel methods.

Wormholes are considered possible based on Einstein's theory of relativity, which states that any mass curves spacetime. To understand this curvature, think about two people holding a bed sheet up and stretching that sheet tight. If one person were to place a baseball on the bed sheet, the weight of the baseball would roll to the middle of the sheet and cause the sheet to curve at that point. Now, if a marble were placed on the edge of the same bed sheet it would travel toward the baseball because of the curve.

Imagining space as a curved, two-dimensional plane, wormholes like this could be formed by two masses applying enough force on spacetime to create a tunnel connecting distant points in the universe.

In this example, space is depicted as a two-dimensional plane rather than the four dimensions that actually make up spacetime. Imagine that this sheet is folded over, leaving a space between the top and bottom. Placing the baseball on the top side will cause a curvature to form. If an equal mass were placed on the bottom part of the sheet at a point that corresponds with the location of the baseball on the top, the second mass would eventually meet with the baseball. This is similar to how wormholes might form.

In space, masses that place pressure on different parts of the universe could eventually come together to form a tunnel -- this is a wormhole. We could then travel from Earth to another galaxy and back relatively quickly (within a lifetime). For instance, let's picture a scenario in which we would want to travel to Sirius, a star that's seen in the Canis Major constellation just below Orion. Sirius is about 9 light-years from Earth, which is about 54 trillion miles (90 trillion km). Obviously, this distance would be far too great for space travelers to traverse and return in time to tell us about what they saw there. So far, the farthest people have traveled into space is to the moon, which is only about 248,548 miles (about 400,000 km) away from Earth. If we could find a wormhole that connected us to the space around Sirius, then we could cut the time considerably by avoiding the trillions of miles that we would have to cross with traditional space travel.

So how does all of this relate to time travel? We'll find out in the next section.