The Need for Exotic Matter

This tendency of wormholes to collapse easily seems at first glance to rule out using such tunnels as gateways to other places and times. And this is the problem that Thorne, Morris, and Yurtsever faced when they began tinkering with the mathematics of black holes at Sagan’s request. They were able to overcome the problem because they used a fresh approach. Instead of treating wormholes as hypothetical objects and trying to predict how they would work if they did exist, they began with the assumption that a stable, traversable wormhole could exist. They described the likely geometry of such an entity; and finally, they applied the principles of general relativity to predict what kind of matter would be needed to keep it open and stable.

In the film version of Carl Sagan’s Contact, astronaut Ellie Arroway prepares to enter hyperspace in a specially designed craft.

The result was seen as a major breakthrough in theoretical physics. The Caltech team’s equations showed that some kind of matter would be needed to exert the pressures required to keep the wormhole stable and open long enough for travelers to pass through. But ordinary matter does not exert enough pressure to do the job. Instead, some kind of extraordinary matter would be needed. Thorne called it exotic matter, or material, about which he later wrote:

I learned from the Einstein field equation, that, in order to gravitationally . . . push the wormhole’s walls apart, the exotic material threading the wormhole must have a negative energy density [a state in which the material exerts no internal pressure, as material in the normal universe does]. . . . Because almost all forms of matter that we humans have ever encountered have positive average energy densities in everyone’s reference frame, physicists have long suspected that exotic material cannot exist. . . . Then in 1974, came a great surprise. [Stephen] Hawking [determined] that vacuum fluctuations [random gravitational effects] near a hole’s [event] horizon are exotic. . . . The horizon distorts the vacuum fluctuations away from the shapes they would have on Earth and by this distortion it makes their average energy density negative, that is, it makes them exotic.

Although exotic matter has not been proven categorically to exist, a number of scientists think that measurable quantities of it may have drifted through the early universe. Perhaps, they say, small amounts of it still exist here and there in the present universe. Possibly, an advanced race of beings could find a way to manufacture exotic matter out of ordinary matter.