A wormhole is a hypothetical structure of space-time that is envisioned as a long, thin tunnel connecting points that are separated in space and time.
Some science fiction writers speculate that wormholes are the intergalactic highways of the future.
The simplest possible wormhole solution was discovered by Albert Einstein and Nathan Rosen in 1935, which is why wormholes are sometimes called “Einstein-Rosen bridges.”
These hypothetical white holes also contain a singularity, but they operate in reverse to a black hole. Nothing can enter the event horizon of a white hole, and any material inside the white hole gets ejected immediately. Einstein and Rosen found that, theoretically, every black hole is paired with a white hole. Because the two holes would exist in separate places in space, a tunnel, a wormhole, would bridge the two ends. But a wormhole created from a pair of black and white holes wouldn’t be very useful. The white holes would be unstable. No particles can enter a white hole. So the energy of the system would continue to increase to infinity, eventually blowing up the white hole.
Even if white holes could exist, the only way to enter this kind of wormhole would be to cross the event horizon of the black hole on the other side. But once an object crossed the event horizon, it could never leave. So objects could enter the wormhole but never escape.
Lastly, the wormholes themselves would be unstable. A single photon, or particle of light, passing through the wormhole tunnel would introduce so much energy to the system that the tunnel would snap apart, destroying the wormhole.
In the 1970s, however, physicists worked out the math needed to make a stable, or “traversable,” wormhole. The trick is to move the entrance of the wormhole tunnel beyond the event horizon of the black hole and to stabilize the tunnel itself so that matter passing through doesn’t cause immediate catastrophic collapse.
The key ingredient for stabilizing wormholes is so-called exotic matter, or some form of matter that has negative mass. But there is no evidence for negative mass. If such a wormhole did exist, it would look very strange. If scientists could somehow construct a wormhole, initially the two ends would be synchronized in time. But if one end were then accelerated to nearly the speed of light, that end would start to lag behind the other end. The two entrances could then be brought together, but then one of the entrances would be in the past of the other.