Space is what we measure with a ruler, and time is what we measure with a clock.

- Albert Einstein

Humans live in the present and can travel into the past by the use of memory, but why can’t we travel into the future? It is this obstacle that writers of fiction have attempted to overcome in ingenious ways in an effort to throw off the restraining shackles of this complex property that everyone assumes we have under our control. We appear to control time by a mere glance at a watch; it enables us to put our lives and actions into context. But in reality we are ruled by time, and its relentless march onward governs our endeavors, purpose and existence.

Human perception of time then, strictly speaking, always occurs in the present. An individual cannot reach forward or backward in time (by means other than his memories) to perceive future or past events but can only perceive what is happening now. For the purposes of measuring time, the present is supposed to have no duration. However, the present as experienced in perception does have a brief but definite duration. That is, one can take in some change in an event in one act of perception, without having to refer to memory. The length of this perceptual moment of the present can vary, depending on the nature of, and organization between the events involved.

Nevertheless, we are prisoners of time. Just like anyone bound in captivity, we are desperately seeking a way out of the boundaries that contain us. Before Einstein published his Special Theory of Relativity in 1905, we were bound in a scientific sense by Newton’s ideal of absolute time and space. Einstein opened the door of the prison, not only in a perceptual way, but perhaps also in a literal one, since many scientists today are actively seeking ways of manipulating time. However, so far it would appear that we are out of the cell, but still within the prison – relativity deals with time relative to outside observers, so whilst time dilation effects are interesting, time still moves relentlessly forward despite our frame of reference. We cannot turn the clock back, or make it go forward more quickly. It is this inability to control time that frustrates many people and adds pathos to our attempts to gain release from our incarceration.

However, the imagination is no prisoner of time, as we can conceive of future events and write or converse about them as if they have already happened. In a similar way we can alter the past by our interpretation of events or the importance we attach to specific episodes in time. It is this creative ability that has been used throughout history to break the bonds and bounds of time.

The temporal prison

The concept of time underwent few revisions between the period of the Greek philosophers and our modern day. Isaac Newton considered time to be an absolute – it was irreversible and unchanging – but this rather harsh opinion was modified somewhat by physicists in the 19th century who produced a branch of science we call thermodynamics. One of the tenets of this new wave was the application of time as a measurable and reducible feature or order of the system under examination. This can be seen when the property of entropy is considered. Entropy is a measure of the increasing disorder of a system over a period of time and is encapsulated as the second law of thermodynamics. The point of this measurement can be seen when we consider a steam engine. Fuel has to be burned to boil water, which produces steam, which drives an engine – but along the way the system produces waste and becomes more inefficient with increasing time. You cannot wind back the clock to put all the waste heat and energy back into the system, nor can one cannot collect the steam, force it back into the boiler or force the fuel ash to reconstitute itself into fuel. The examination of this entropic process gave rise to the concept of the Arrow of Time. Time is one directional with reference to all closed systems – including the universe – and absurdities and paradoxes result if you try to break the second law of thermodynamics.

This situation prevailed until the early years of the 20th century when Albert Einstein considered what it would be like to travel on a beam of light. Pondering this, he redefined time with his special theory of relativity and introduced the concept that time is a phenomena that can change, depending upon your frame of reference. Einstein’s friend Hermann Minkowski reworked his equations and discovered that they had surprising implications that could one day lead to time travel. Kurt Gödel, a contemporary of Minkowski, proposed solutions to the equations of general relativity whereby time-like world lines in a universe built upon the finite but unbounded principles of Einstein’s general theory of relativity could bend back on themselves. If you accept this philosophy and think of the flow of time as going around and around such closed time-like world lines and if space near such timelines can be warped until the lines intersect, a form of time travel may occur.

Such exotic considerations were mostly well beyond the knowledge of the common man, and of course our engineering is not up to the task of constructing a craft that would discover, let alone survive a journey into a wormhole. However, a device enabling the exploration of time has been placed at our disposal by fiction writers, who are not bound by the considerations of engineering. Dreams, determined efforts to reach into the past by concentration of the mind or the use of some kind of memory of the future have been used in literature, but perhaps the most poignant invention of this genre was the idea of the “time machine” a piece of hardware or technology that could be used to explore the past and the future. The imagination had found a way out of our temporal prison.

In next week’s concluding essay, I will consider the author Kurt Vonnegut’s exploration of time travel.