In 1935, Albert Einstein and his postdoc Nathan Rosen found a hypothetical mathematical spacetime structure that bridges two separate regions of space, potentially allowing for a shortcut between them.
Traveling through the throat could be much faster than traveling in the familiar external spacetime that connects them. This is an example of a general class of space-time structures with two mouths connected by a throat, known as ‘wormholes’. In 1962, Robert Fuller and John Wheeler demonstrated that the Einstein-Rosen wormhole is unstable and would pinch off before a particle could pass through.
Then Stephen Hawking, Kip Thorne and associates showed that exotic matter with a negative mass density (energy density) could stabilize a wormhole and hypothetically make it passable. Such a substance is known to exist as the ‘dark energy’ whose repulsive gravity causes the accelerated expansion of the universe. However, to construct a wormhole you would have to dig up dark energy from the cosmic reservoir and mold it into the shape of a wormhole. We don’t know if that’s possible, because the nature of dark energy is unknown.
A traversable wormhole would allow an advanced civilization to do so travel back in time. This is because time passes differently inside and outside the wormhole. As a result, synchronized clocks at both ends of the wormhole remain synchronized for an observer passing through the wormhole. Imagine observers at one end of the wormhole experiencing time dilation and decreasing their age by moving or temporarily visiting a gravitational potential. Such observers could simultaneously connect to the older end of the wormhole and allow the older version of themselves to meet their younger self. This constitutes a time machine from the point of view of an outside observer.
In 1992, Stephen Hawking suggested the ‘chronological protection conjecture’, which argues that the laws of physics prevent time travel, making the universe safe for historians. Since we don’t have a predictive theory that unifies quantum mechanics and gravity, we don’t know if Hawking’s conjecture is true. If so, wormholes cannot be built by advanced alien civilizations.
This series of arguments suggests that if we ever discovered alien visitors near Earth using wormholes to travel faster than light, we would know that Hawking’s conjecture is wrong and that time travel is possible. Their scientific achievement would have major consequences for the theory we develop for quantum gravity. It will also raise ethical questions, such as whether we should request access to their time machine and go back in time to kill Adolf Hitler before the Holocaust. By such an act I could bring back to life 65 members of my father’s family who died in the Nazi concentration camps.
Having access to a time machine as a tool for correcting human history is one possible benefit of encountering alien quantum gravity gadgets. Another would involve travel to distant locations in the cosmos within a lifetime through a wormhole. Which one should we choose first? My personal priority would be to record human history before embarking on interstellar travel. This is because we know what needs to be resolved in our past, but we don’t know which interstellar destination is worth pursuing.
Encountering the technological products of advanced alien scientists could be very useful for our own scientific progress. We only discovered quantum mechanics and general relativity a century ago, and we still have a lot to learn.
Last week I had a wonderful dinner in the company of string theorists from Harvard Black Hole Initiative. Three of them were sitting next to me, and so I couldn’t resist asking them: “In the event that we discover a mini-black hole in the solar system and experimentalists can access it and discover quantum gravity theories such as string theory testing, Would you encourage full funding for the experimentalists to undertake this task, even if the results could potentially invalidate your lifelong work on string theory? They all replied, “Absolutely, yes.” So I commented, “Well, that shows that deep down you are real physicists and not mathematical physicists.”
