Problems with Wormholes

Although most physicists agree, at least in theory, that Gribbin’s astronaut could enter and experience the strange effects of hyperspace inside the black hole, they caution that it is by no means certain that he or she could do so safely. First, they warn, there are serious dangers lurking outside the event horizon. Even before entering the black hole, the astronaut would have to find some way of surviving the extreme tidal forces and searing radiation in the spinning accretion disk.

For the sake of argument, however, assume that the astronaut manages to invent special shielding to protect against these lethal effects. And he or she makes it across the event horizon and into the black hole in one piece. From that point on, it is far from certain that the astronaut will be able to make it back to his or her starting point in space and time. Astronomer Sagan addresses this problem in Contact:

As measured from Earth, it takes an infinite amount of time for us to pass through a black hole, and we could never, never return to Earth. . . . A Kerr-type tunnel can lead to grotesque causality violations [a breakdown of normal cause and effect]. With a modest change of trajectory [its path] inside the tunnel, one could emerge from the other end as early in the history of the universe as you might like a picosecond [a small fraction of a second] after the Big Bang, for example. That would be a very disorderly universe.

Still another challenge for the astronaut to overcome is the instability of the wormhole gateway and tunnel. A wormhole in a Kerr black hole might well remain open a good deal longer than the extremely short-lived version in a Schwarzschild black hole. However, a Kerr wormhole would still be highly fragile. Even the rather small gravitational effects created by the astronaut and his ship entering the tunnel might be enough to cause its collapse, which would simply crush the ship out of existence. According to Sagan:

There is an interior tunnel in the exact Kerr solution of the Einstein field equations, but it’s unstable. The slightest perturbation would seal it off and convert the tunnel into a physical singularity through which nothing can pass. I have tried to imagine a superior civilization that would control the internal structure of a collapsing star to keep the interior tunnel stable.

This is very difficult. The civilization would have to monitor and stabilize the tunnel forever. It would be especially difficult with something as large as [a spacecraft] falling through.

The Matricide Paradox

In his acclaimed book Black Holes and Time Warps, noted physicist Kip Thorne describes the phenomenon called the matricide paradox this way: “If I have a time machine . . . I should be able to use it to go back in time and kill my mother before I was conceived, thereby preventing myself from being born and killing my mother.” Obviously, the paradox lies in the fact that the murder appears to stop itself from happening.

Thorne credits Joe Polchinski, a physicist at the University of Texas in Austin, with supplying the following scientific description of how the paradox might work:

Take a wormhole that has been made into a time machine, and place its two mouths [each located at a black hole] at rest near each other out in interplanetary space. Then, if a billiard ball is launched toward the right mouth . . . with an appropriate initial velocity, the ball will enter the right mouth, travel backward in time, and fly out of the left mouth before it entered the right . . . and it will then hit its younger self, thereby preventing itself from ever entering the right mouth and hitting itself.