Faster than Light Travel Part 2: Traversable Wormholes

Warp drives, as previously discussed here, are not the only candidates for future faster than light travel. Another common topic in both modern science and science fiction is the wormhole, which creates a throat (i.e. tunnel) of spacetime to connect two points in spacetime. A wormhole connecting two distant locations would allow near-instant travel between them.

The typical depiction of a wormhole is the Schwarzschild wormhole, shown below in Figure 1. This type of wormhole is symmetric and typically very unstable due to its geometry.

Figure 1. A n-1 (2nd dimensional space) depiction of a Schwarzschild wormhole; Image source

Visser has proposed a different class of wormholes with a different geometry that leads to stable wormholes in some states (Visser 2008).

Even this stable geometry would not automatically allow the wormhole to be traversed. For a human to be able to travel through a wormhole, the throat would need to be expanded and upheld by a matter with a negative energy density. This matter is called “exotic matter” due to how little we know about it (Morris et al. 1998). The existence of this matter directly violates the weak energy condition, yet Visser states that “It cannot be strongly enough emphasised that the weak energy hypothesis has been experimentally tested in the laboratory, and has been experimentally shown to be false” (Visser 2008). Further, the weak energy condition implies the neutral energy condition, which is known to be violated in many small instances in quantum physics (Visser et al. 2008).

If this exotic matter exists, it is in our best interests to minimize how much of it we need to stabilize a wormhole. Visser (et al.) showed that the amount of exotic matter required can be made “infinitesimally small” by altering the geometry of the wormhole and therefore the wormhole throat (Visser et al. 2008).

One of the biggest issues with the usage of FTL travel is that it may inadvertently invent time travel by letting objects travel faster than the information (e.g. light) they carry. Krasnikov suggests that wormholes cannot cause the proper time of a journey to decrease, although the journey can be near instantaneous when viewed by an observer (Krasnikov 1998). Crawford details that wormholes do not inherently violate causality, and Hawking goes as far as to propose that “the laws of physics do not allow the appearance of closed timelike curves,” i.e. that spacetime cannot be bent into a way as to allow time travel (Crawford 1995, Hawking 1991).

The current scientific community does not know enough about wormholes and their relevant physics to properly speculate on any future usage of them. Our understanding so far is that wormholes may be traversable with some arbitrarily advanced technology, but we are still many years away from detailing exactly how.

References:

Morris MS, Thorne KS, Yurtsever U. 1988. Wormholes, Time Machines, and the Weak Energy Condition 1446:1449

Visser M. 2008. Traversable wormholes from surgically modified Schwarzschild spacetimes 1:12

Visser M, Kar S, Dadhich N. 2008. Traversable wormholes with arbitrarily small energy condition violations 1:4

Krasnikov SV. 1998. Hyperfast interstellar travel in general relativity 1:18

Crawford IA. 1995. Some Thoughts on the Implications of Faster-Than-Light Interstellar Space Travel 205:206, 209:210

Hawking SW. 1991. Chronology projection conjecture 603:611