Everyone likes to speculate about the possibility of time travel, but no one seems to have enough information to fend off the countless arguments for every single opinion. Every idea about time travel, whether for it or agin’ it, is laden with assumptions about the way things actually are, such as the manner in which time functions in the universe, and indeed that time exists at all.
What we call “time” in our daily lives is really just a change of bodies of matter with different masses, in relation to each other. Earth time, no matter what artificial devices are in place for measuring it, is ultimately the result of the relationship between the masses of the earth, moon, and sun (or within the masses of one of these). Thus years, seasons, months, and days are demarcated. Seconds are measured by 9,192,631,770 transitions of energy between the magnetic dipoles of a cesium-133 atom at 0 Kelvin – again, just a relation between masses. Einstein showed that even when such “objective” means are used to segment and define time for those of us on earth, such means are nonetheless highly variable. Their variability doesn’t matter all that much to most Earthlings because few of us are affected by it to the degree that it would even be noticeable. The one-quintillionth of a second less that a person experiences while in an airplane, as opposed to someone on the ground, is so insignificant that no one ever knows about it unless they have an atomic clock.
So when time is understood to be a relationship between masses, then time travel would seem likely to involve some sort of mass-relationship adjustment. But the passage of time is not just objective. As much as temporal scientists would like to think they can perform experiments under controlled conditions, the cognitive scientists ultimately have the upper hand because the passage of time is fundamentally more of a perceptual phenomenon than a physical one. Two people in precisely identical physical circumstances may not perceive time to pass in the same way, for reasons that are poorly understood.
It is often reported by people involved in automotive accidents (and I have personally found this to be the case) that during the moment of impact, time creeps to a sluggish crawl. An event that surely lasts no more than 1 or 2 seconds seems to stretch on for many seconds, and the mind is conscious and aware of the slow motion moment, but is unable to change it. This is true also of certain psychotropic drugs, namely hallucinogens. One of the many effects potentially caused by drugs such as hallucinogenic mushrooms and LSD is sometimes referred to in the drug literature as “temporal dilation.” I find this to be an apt term no matter whether it is produced by drug use or sudden violent impact.
It is likely in my opinion that temporal dilation (or constriction, for that matter) is a product of the rate at which the brain processes incoming sensory information. In a person’s everyday life, the brain is accustomed to receiving information from the senses at a rate that produces the most advantageous response from the human, as conditioned by evolutionary means over millions of years. When an event occurs that either causes a sudden surge of adrenaline and other neurochemicals, or that affects neurotransmitter concentrations and binding properties in other ways (like dropping acid), then the rate at which the brain processes incoming sensory information becomes dramatically greater. If the brain is processing more sensory input, then the amount of time that elapses during those moments is perceived as being larger, when compared to the amount of sensory information that the brain processes under normal circumstances. To make this especially clear, consider the following exaggerated example: In normal life, a brain processes, say, 1 million “units” of sensory data per second. If circumstances suddenly furnish conditions for the brain to process 10 million units of sensory data per second, then it seems to the brain that 10 normal second have passed in the space of 1 normal second. Thus, time appears to slow down.
The reason I think it is important to at least mention how time is measured and how it is perceived is because these issues reveal the major philosophical problems inherent in any discussion about time travel. Before the topic of time travel can be talked about in any intelligent (and perhaps realistic) way, it is crucial that the notion of time, and what it actually is, be firmly established and agreed upon. I do not think such agreement will be feasible for a great many count of years – several hundred minimally, and perhaps several thousand, or perhaps never. But this should not be a roadblock to conversation on the matter. Time travel is, after all, quite interesting, regardless of its philosophical detriments.
It is well to begin by addressing the single greatest problem with the manner in which time travel is treated in film and literature. The problem is: Time travel is time travel, not space travel. Let us assume for the sake of film and literary entertainment that time travel is possible, and furthermore that it is possible and safe for human bodies. If a person goes back in time to 1955 (or 40,000 B.C.), there is no way that he or she would find him or herself in the same geographic location unless the time machine is also a space-traveling machine. Every second the earth rotates thousands of miles about its axis, revolves thousands of miles on its orbit around the sun, and follows appropriately the movements of the solar system, galaxy, and greater star systems. The state of California was in an altogether different place in the universe in 1955. Any designer of a time machine who wishes to keep the traveler in the same geographic location, or indeed within the confines of the earth, must incorporate some method of dealing with this problem. This seems to be grossly overlooked in popular media.
The Grandfather Paradox is an almost legitimate dilemma cited often by time travel philosophers. A paradox, in its general sense, is a set of true premises that lead to a contradictory conclusion. For those readers unfamiliar with the Grandfather Paradox, here is a brief summary: A young fellow, let’s call him Phil, travels back in time and meets his grandfather, Earl, as a boy. This is well before Earl has met Phil’s grandmother. For whatever reason, Phil kills his Earl. With Earl dead before producing children, it is impossible for Phil to be born, and it is impossible for Phil to travel back in time to kill Earl. This is seen as a paradox because it is a scenario that, if it occurred, would prevent its occurring.
I am wholly unconvinced that such a scenario is paradoxical at all, but it depends, again, on the assumptions that one makes regarding how the temporal universe actually works. First of all, the entire dilemma can only arise if it turns out that it is actually possible to change any element of the past. Perhaps events of the past are set in stone and cannot be changed. In that case, going back in time might only be a spectator sport - you could watch, but only with the inability to affect the environment. This could also mean that no one in that time period would be aware of your existence, because if they could see you, then this would be an effect on their environment. Thus Phil could travel back in time, but would not be able to change anything, and no one would be aware of his existence. It may also be the case that it is possible to change events of the past, but never at a paradoxical expense. Therefore, maybe Phil could go back in time and attempt to kill Earl, but circumstances would conspire against his effort, making it impossible for him to succeed.
Second off, if it is possible to change events of the past, the mere fact that Phil exists proves that he cannot kill his grandfather. This is obvious if one follows the scenario to its logical conclusion, and any general confusion about it stems from people, as Doc Brown would say, not thinking fourth dimensionally. Let’s say that Phil goes back in time, not to kill Earl, but to deliver him a non-fatal wound. For example, perhaps Phil goes to teenage Earl and drives a piercing blade into his thigh. By the time Earl is an adult and long before Phil has been born, Earl would be able to repeat this strange story of his youth and could proudly display his scar. So Earl would have a scar from being stabbed by Phil before the moment that Phil actually decides to travel through time.
Then, later, when Earl is an old man and Phil is a young fellow, Phil could travel back in time, approach his young grandfather and stab him in the leg. So, if the story starts with Phil going back in time, and ends with Earl getting stabbed, then the story fails to acknowledge that the first instance of time travel actually began many years prior – at the moment Earl receives the wound. This is why the fact that Phil exists can prove he cannot kill his grandfather. If he exists, then clearly his grandfather was not killed by him (or by anyone). Otherwise, of course, Phil would not exist (nor would Earl), and there would be no paradox because everyone involved would either be dead, or would never have been born in the first place.
The notion of causality in time travel has been explored to some degree in popular media, and is treated by theoretical physicists with the study of chaos theory and by mathematicians with the study of fractality (fractals). It seems obvious (though not indisputable) that everything from the neurotransmitter binding in our brains to airplane turbulence is subject at least to some degree to other events happening in the system (the universe). The inter-relatedness of all things is frequently exemplified with the illustration of the Butterfly Effect, in which a butterfly flaps its wings in Central Park and affects the air currents in minute and subtle ways that begin a chain reaction in the atmosphere so as to eventually culminate as a typhoon in Shanghai. This is a simplified example, but it makes the point.
Ultimately, every single event, no matter how apparently insignificant, therefore has an effect (or a potential effect) on every other event in the universe. If chaos theory accurately represents causality, then the implications on time travel are extreme. A person who travels back in time could potentially force a drastically changed future by merely walking down the road. Even standing perfectly still is not immune to affecting events because a person’s body would displace air, or may influence the decisions of animals or other humans to occupy that same space. When the time traveler returns to his or her original time, his or her present would (or could) be so vastly different as to be unrecognizable, and would surely be the source of much consternation. In principle, the further back in time one goes, the more drastic would be the change in the present. So if time travel is possible for humans, and furthermore if chaos theory is right, and furthermore if it is possible to change any event of the past, then traveling back in time would be dangerous business indeed! Traveling into the future would not be quite so risky, since whatever changes are implemented by the time traveler would be met as normal when that time comes around.
Many time travel naysayers (including renowned astrophysicist Stephen Hawking) claim that if time travel were possible, we would be inundated with visitors from the future; therefore it is not possible. What ludicrous rubbish! This has got to be the worst reason to claim that time travel is impossible. Maybe time travel is impossible, but if so, this isn’t the reason. Here’s why: First off, who’s to say we’re not inundated with visitors from the future? I have already mentioned that it may not be possible to change any element of the past, and if this is the case, then no one in the present would be capable of seeing visitors from the future. Just seeing those visitors would mean that our present minds would be affected.
Second, if the visitors are visible, do you think they would make themselves so obvious? Do you think you could tell the difference between a Japanese tourist from 2008 and a Japanese tourist from 3208 pretending to be a Japanese tourist from 2008? Imagine the effect sneaky time travelers could have on their young selves, relatives, loved ones, or even interested strangers. They could pass along advanced technology, divulge lucrative stocks and winning sports teams, or give warnings to avoid personal disaster or death. In light of this possibility, perhaps special training is required and sacred oaths are taken by time travelers not to spoil anything, for good or ill, for citizens of earlier times.
Third, just because time travel may be possible in the future doesn't mean people actually do it - especially as a commercial enterprise. It is true that in this day and age, commercialism of everything – even space travel – is the natural consequence of just about every type of technology as soon as it is economically feasible. But things may be different in five hundred, a thousand, or five hundred thousand years. It is ridiculous to assume that every culture in the future of the world will be as materialistic and money driven as current Western society. These three points should be sufficient to thwart continued attempts to furnish the “visitors from the future scenario” as a counterargument to the possibility of time travel.
Another confounding factor that has rarely been addressed is that the universe is a system with a finite number of atoms (or whatever the ultimately small unit of matter turns out to be, if such a thing exists at all). If we send any object into another point in time, then the universe of that time would contain its original "total" number of atoms plus the additional number of atoms in the object that was sent. Is it possible for the universe to contain more than its total number of atoms? Maybe there is only enough room in the universe for the number of atoms that it already contains. But maybe there is enough room for the existing atoms to move closer together and allow for more matter to fit in. This is not something that can be established experimentally because no one knows how to create or destroy matter. Many other theoretical, conceptual, temporal, and practical issues are surely still rampant with the idea of time travel. I do not wish to feign insight into them all, but the few points I have raised are hopefully adequate to act as discussion starters.
It is important for scientists to remember that philosophy is higher than science because philosophy exposes the assumptions on which science is necessarily based. We are infants in this conglomerate of possibilities. The answer, whatever it is, is no doubt so simple that it will be beyond our comprehension for eons to come.
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