The Physical Possibility of Time Travel and its Mechanisms

Key Insight

The concept of time travel, explored in science fiction, may have a basis in physical laws. In 1949, Kurt Gödel discovered a space-time allowed by general relativity with a rotating universe, where travel to the past was theoretically possible, upsetting Einstein. While this specific solution doesn't match our non-rotating universe, other general relativity-allowed space-times permitting past travel have been found, such as the interior of a rotating black hole or space-time containing two cosmic strings moving at high speed. Cosmic strings are under enormous tension, comparable to a million million million million tons, and could accelerate Earth from 0 to 60 mph in 1/30th of a second, possibly having formed in the early universe.

While initial universe observations suggest it lacked the curvature for time travel, the possibility of warping local space-time regions remains. Rapid interstellar travel is constrained by the speed of light; a trip to Alpha Centauri (about four light-years away) would take at least eight years, and to the galactic center, at least one hundred thousand years. Relativity allows for the 'twins paradox,' where space travelers age less, but this offers little solace if loved ones on Earth have long perished. Significantly, if faster-than-light (FTL) travel were possible, relativity implies time travel to the past would also be feasible, as illustrated by the limerick about the 'young lady of Wight.' Experimental evidence from particle accelerators like Fermilab and CERN shows particles can reach 99.99 percent of light speed but cannot exceed it, suggesting FTL and thus direct time travel, is impossible.

A potential workaround involves warping space-time to create shortcuts, such as 'wormholes,' which are thin tubes connecting distant regions. An Einstein-Rosen bridge (wormhole) could theoretically link the Solar System to Alpha Centauri, reducing a 20 million million mile journey to a few million miles. Such a shortcut would enable FTL travel and, consequently, travel into the past. Maintaining a wormhole, or any space-time warp for time travel, requires a region of space-time with negative curvature, which in turn needs matter with negative energy density. While classical physics forbids negative energy, quantum laws, based on the uncertainty principle, allow it locally, provided the total energy remains positive. The Casimir effect provides experimental evidence for negative energy density, where fewer virtual photons between two parallel metal plates result in a negative energy density relative to the zero energy density of 'empty' space outside, validating the theoretical requirements for time travel.