While The Naked Time was truly the first Star Trek episode to expose viewers to the idea of time travel with its discovery of the Cold Start method, it was the later episode, Tomorrow is Yesterday, that served as the first [of many] to fully utilize the concept as a major plot device. Interestingly, Tomorrow is Yesterday was initially conceived as a follow-up episode to The Naked Time, but was ultimately reworked into a stand-alone story.
The opening shots of Tomorrow is Yesterday find the crew of the Enterprise orbiting Earth sometime during the 1960s as a result of the effects of a high-gravity “black star” (or, black hole, as we know them today). This episode introduced us to the method of time travel known in Star Trek lore as the Slingshot Effect. The Slingshot Effect is described as a specific maneuver “performed by traveling at an extremely high warp factor towards a massive body with a high gravitational attraction, such as a star. After allowing the gravitational pull to accelerate the vessel to even faster speeds, the vessel would then break away from the stellar body, creating a whiplash effect which could transport the vessel through time.”
In the episode, the Enterprise is quickly detected by military radar as an Unidentified Flying Object (UFO). They are then pursued by a U.S Air Force F-104 interceptor piloted by Captain John Christopher (guest star, Roger Perry). After the plane is accidentally damaged via tractor beam, Christopher is transported aboard the Enterprise. From that point on, as anyone with a casual understanding of time travel centered science-fiction stories could predict, time shenanigans and desperate attempts to avoid paradoxes and timeline disruptions soon ensue. A difficult decision to bring Christopher back to the future is disrupted by Spock’s findings that his future son will play an important role in a space mission to Saturn. Kirk is detained by a police officer while attempting to recover recordings of the Enterprise from an airbase. With every effort to preserve the timeline, the Enterprise crew seems to dig themselves further and further into a proverbial hole.
Fortunately for everyone, chief engineer and resident miracle worker, Montgomery Scott, ultimately comes up with a fix – in the form of the aforementioned Slingshot Method – that would effectively reverse time, erasing all incidents of timeline meddling, and send the Enterprise back to the 23rd century.
The Science fiction genre has toyed with the idea of time travel countless times. H.G. Wells popularized its use as a major plot device through The Time Machine, and it has continued as a science fiction mainstay through popular culture giants such as Doctor Who, the Terminator franchise, Back to the Future as well as countless other examples within the various Star Trek incarnations.
Setting popular culture and science fiction aside for a moment, what does actual science say about time travel?
To start, we first have to look at how we understand time as a concept. Isaac Newton, in Philosophiæ Naturalis Principia Mathematica, wrote about the idea of absolute time (in contrast to relative time). Newton believed that absolute time and space exists independently of perception and progresses at a consistent pace throughout the universe, thus the concept can only be measured mathematically. Relative time, on the other hand, can be visually perceived by the movement of objects (i.e. rotation of the Earth and Sun). This philosophy is sometimes referred to as Platonism with Respect to Time. The Stanford Encyclopedia of Philosophy describes the concept using a metaphor of an empty container: “time is like an empty container into which things and events may be placed; but it is a container that exists independently of what (if anything) is placed in it.” The counter point to this, dating back to Aristotle – often referred to as Reductionism with Respect to Time – argues that time does not exist independently of events as we know them, and in fact we can understand time by understanding temporal relations between various things and events.
Philosophies of time range from deeply mathematical (the above ideas from Newton and Aristotle) to deeply philosophical and metaphysical as well. There are the concepts of presentism and eternalism, for example. Presentism asserts that it is only the present that truly exists – the past and the future exist only as alterations that have occurred or will occur to the present. Eternalism, as a counter to presentism, states that the past and the future both exist in a real sense. Eternalism sees time as a dimension similar to that of space. Concepts such as fatalism postulate that the future is already written (i.e. predestination) and nothing we can do will change that, whereas other philosophies, such as the ideas from metaphysician, J.M.E. McTaggart in his book, The Unreality of Time, argue that time does not truly exist and what we see as time is simply an appearance.
As one could imagine, there is no shortage of philosophies that try to examine the nature of time as we know it.
From a scientific standpoint, much of our understanding of how time operates and the potential for time travel comes from Einstein’s theories of General and Special Relativity. As Dr. Michio Kaku puts it, Einstein’s ideas proved that “time was more like a river, which meandered around stars and galaxies, speeding up and slowing down as it passed around massive bodies. One second on the earth was not one second on Mars. Clocks scattered throughout the universe beat to their own distant drummer.” Special Relativity tells us that if someone were to travel near to the speed of light, time would go slower for that individual than time would for people left behind. General Relativity states that greater proximity to larger gravitational fields would cause time to move at a slower pace.
We have seen evidence of these theories play out in the actual physical world, time dilation has been evident in ticking clocks vs. stationary clocks, as well as through NASA astronauts, specifically, twin brothers, Scott and Mark Kelly.
So with all of that, not only is the very concept of time travel possible, but it has already happened! What’s left to determine, however, is the probability of the more extreme examples we see in popular culture. Could we theoretically rewind time, or go back centuries in the past as the crew of the Enterprise did in Tomorrow is Yesterday?
Before delving into the science, it’s interesting to take note of the Novikov self-consistency principle, named for Russian physicist Igor Dmitriyevich Novikov. Novikov asserts that any actions a time traveler might take outside of their own time (in the past) would automatically become part of history. While the traveler may take actions that become the cause of certain events or courses of history, it is effectively impossible to truly change history.
That being said, science generally has not been kind to the concept of time travel as it’s presented to us in most of popular culture. However, every now and again, scientists will find and develop mathematical theories that support the idea in abstract ways.
Recently, physicists Ben Tippet of the University of British Colombia and David Tsang out of the University of Maryland, developed a mathematical formula that they say proves time travel to be theoretically possible. Their paper, “Traversable acausal retrograde domains in spacetime” (yes, the initials do indeed spell out the acronym TARDIS after The Doctor’s eponymous time machine), out of the journal Classical and Quantum Gravity theorizes the possibility of a machine that allows “observers travel within the interior of a ‘bubble’ of geometry which moves along a circular, acausal trajectory through space-time.” Thinking of our four dimensions as connected, Tippet and Tsang’s theory states that time could be bent in a circle effectively moving passengers faster than the speed of light within the aforementioned geometric bubble. In order for this machine to work, however, it would require the invention of what the researchers refer to as “exotic matter,” or material that can bend space-time in a such a way that time slows down (for further details on “exotic matter,” see Time travel is mathematically possible, but don’t expect it anytime soon, from Wired).
As it stands, we have yet to find a way to create “exotic matter,” and according to Novikov, we’ll never truly know if any alterations to the past have occurred. Until then, we can only continue on our own natural courses throughout space-time and the cosmos – moving forward in time at our own pace.