In the past chapter, time was like a railway line where you could only travel forward. But maybe there are loops and branches, so even if you can only go forward, you could retrace your steps or double back, meaning you could time travel. Like many things that were once science fiction, it could become a reality.
With a better understanding of how time and space works, humans can indulge in imaginary scenarios that could potentially not be all that imaginary after all. Science fiction is increasingly becoming reality.
Mathematician Kurt Gödel suggested a new model of space-time in 1949 under general relativity. He said the whole universe was rotating in the direction spinning tops point. As a side effect, you could set out in a spaceship and return before you left. This annoyed Einstein, who didn’t want time travel as part of his theory of relativity. This also doesn’t match observation, as the universe does not rotate.
While Einstein could humble himself over his erroneous cosmological constant proposition, he did not like other people messing with his theories, which Hawking mocks with ironic language in his description. Nevertheless, Gödel’s model was unviable anyway, somewhat sparing Einstein’s reputation.
Other space-times allowed by the rules of general relativity do allow for time travel, and fit what one can observe in the universe. For example, in the interior of a rotating black hole, or a space-time where two cosmic strings move past each other really fast. These cosmic strings could have formed in the early universe as a result of symmetry-breaking, and hold such high tension they can propel vast objects at high speeds in milliseconds when they straighten out.
Gödel’s defunct model did not kill off the idea of time travel altogether, perhaps because humans found the idea simply too good to give up. There are other models that allow time travel, of varying dubiousness.The cosmic strings mentioned here are different from the string theories mentioned in the next chapter.
In the Gödel solution and the cosmic string space-time, the universe was so distorted in the beginning that travel into the past was allowed. But there is no reason to believe God created such a chaotic reality. The uniform microwave energy and the abundance of light suggest the universe was not so chaotic and curved near its beginning to permit time travel. This would also be true in the no boundary condition. The question follows whether we could warp space-time enough to permit time travel.
Hawking remains skeptical about the theories mentioned so far. He invokes God in a casual way, equating the creator’s will with the order apparent across the universe. Thoughts like these appear often throughout the text, in an attempt to not write theist approaches out of science altogether.
When it comes to long-distance space travel, because time is not absolute, an interstellar or intergalactic journey would appear to take much less time to the travelers compared to those back home on earth. But this form of travel into the future is joyless, as everyone the astronauts knew would be long dead. This is only true if you cannot travel faster than the speed of light. If you could, then you could arrive back before you left.
Hawking shows what could be considered future time travel would be possible by traveling long distances. But without the possibility of return, very few would want to attempt it. The story is different if those interstellar travelers are traveling faster than the speed of light.
If a space ship travels from event A to event B below the speed of light, all observers will say event A happened before event B. But if the ship traveled above the speed of light, different observers moving at different speeds would have different measurements, and disagree which happened first. It could even be possible to travel back from B, faster than the speed of light, before A happened.
If a space ship could travel from point A faster than the speed of light, then it would arrive at B long before any other information reached B, even information about events at point A before the ship departed.
Exponentially more energy is needed to achieve the speed of light, however, and rockets cannot get enough power to achieve this speed. Perhaps space-time could be warped to allow a shortcut, allowing a wormhole between two regions; this would allow information from B to pass back to A faster than light took to get from A to B the normal way, effectively allowing time travel to the past.
While this possibility of time travel might excite imaginative humans, in reality, no object with mass can reach the speed of light. As such, another approach is required. Shortcuts through space-time would be another good option, Hawking argues. Humans’ dreams of time travel can live on.
Einstein and Nathan Rosen were the first to suggest wormholes could exist, hence their other name, Einstein-Rosen bridges. These bridges are unstable and do not stay open long, but an advanced civilization might be able to stabilize one. Matter has positive energy and curves space time like a sphere; negative energy would curve it like a saddle, meaning to create wormhole, one would need negative energy density.
While Einstein opposed the idea of direct time travel, as his theory of relativity stated nothing could travel faster than light, he proposed the idea of wormholes. Such bridges would connect portions of the universe to other distant regions, and, crucially, can be manipulated. If one arrived in a distant region before light had arrived there from one’s origin, the effect is the same as time travel.
Quantum theory allows a negative energy balance in certain areas as long as the universe’s overall balance is positive. Scientists have detected virtual particles from observing the different pressures applied to metal plates created by discrepancies of the density of virtual photons between and outside the plates. Within the plates, the photons would only occur in the space if their wavelengths matched the width of the space between the plates in terms of whole numbers, otherwise a wave crest at some point would hit a trough and cancel out. So fewer photons occur within the plates, and the higher density of photons outside the cavity between the plates creates inward pressure. The cavity within the plates can be said to have negative energy, while the normal conditions outside have zero energy. This is called the Casimir effect. Along with light bending during eclipses, these observations show space-time could be warped.
Similar to the earlier examples of electron orbits requiring a whole number of wavelengths, and how light wave crests and troughs cancel each other out to create patterns on a wall, scientists can create pressure from manipulating this interaction of wavelengths between photons. In this experiment, negative energy is created between the two metal plates due to the difference in photon density between and outside the plates.As a result, physicists have learned that they can create negative energy, that in turn could create wormholes. Technology will have to improve some way before the process is stabilized, however.
If such time travel is possible, then one might well ask why we haven’t met anyone from the future. It could be that the past did not have the curvature required for time travel, but this might not be true of the future. So time travel might be confined to the future, explaining why we haven’t seen time tourists yet.
Having discovered that time travel is theoretically possible, the next question is why time travelers have never been uncovered in reality. Hawking offers a possible answer, but only time will tell.
This doesn’t explain away the many paradoxes of time travel in to the past, which involve contradictions if you could change the past. One explanation is called the consistent histories approach, which says everything that happens in space-time must be consistent according to the laws of physics. This means you would only travel into the past if history already showed that you had done so. The idea comes down to free will, which is a debatable concept itself if there really is a unifying theory. Such a theory could well determine human behavior, negating the idea of free will.
The reason time travel in science fiction films is always problematic is the issue of paradoxes. Hawking argues that the problem centers around the question of free will, which hangs in the balance on the basis of whether or not scientists are able to find a unifying theory of everything. Such a theory might truly explain everything, showing choice has never truly been a factor. Thus, there would be no paradoxes as the laws of physics would rule everything.
Another explanation is called the alternative histories hypothesis. This involves time travelers going back into alternative pasts, with total free will. This sounds like Feynman’s sum over histories, which states the universe had every possible history. But, each history would be self contained, so the time traveler would have to travel back to his or her own space-time’s past.
Hawking describes a possible time travel scenario where free will is a factor, but he is skeptical traveling into other histories is a possibility. Instead, it seems more likely each time traveler would remain in their own space-time, which would have to remain consistent.
Feynman’s sum over histories allows time travel on a miniscule scale. As particles follow the C, P, and T symmetries, a particle going backward in time could be considered an anti-particle going forward in time. For example, black holes “emit” particles, where one component of a particle and anti-particle pair escapes as its partner falls in. The former appears to be created by the black hole. It could also be described as an anti-particle traveling back in time out of the black hole.
When seen from the symmetries approach, the question of time travel becomes one of perspective. Just as imaginary time could be considered the real time if it gives us a better pictures of how the universe works, so too could a particle going forward in time be considered an anti-particle going back in time.
One idea, called the chronology protection conjecture, works in a similar way to the cosmic censorship idea, and suggests the laws of science purposefully prevent large-scale time travel. But this has not been proven. The idea goes, when space-time is warped enough to allow time travel, virtual particles moving in closed loops become real particles moving forward in time, adding to the overall energy density of the universe multiple times, and creating much more positive energy density to outweigh the negative energy created to curve space-time. It is not known what curvature these particles would create, or if it would differ between kinds of particles. The question thus remains open.
Scientists have found inherent balance in the universe. Energy always adds up to zero given the overall balance of positive and negative charges. This can fluctuate in isolated regions, but the overall balance will remain across the universe. Warping space time, however, could unleash new energy density into the universe, thus canceling out any excess negative energy created to try to form wormholes. The matter remains one of humanity’s innumerable open questions.