I really do enjoy the subject of time travel. I've always enjoyed Back to the Future, Terminator, Flying House, and other shows that involve time travel. So, for my final philosophy paper, I wrote on the subject of time travel and which time theories do the best job of answering the question of how it might work.
While one will rarely find Back to the Future cited as an authoritative source on the possibility of time travel, it does do a good job of showing some of the problems one might encounter in time traveling. Indeed there are many problems one runs into no matter which of the popular theories is held. There are two predominant theories concerning time as it exists in reality.
The first is the common assumption that time is like a river. We observe different occurrences flowing by us as we live on. Some have passed us by; some have yet to come. This is based on not only our intuitions, but our experiences. We experience this thing we call time in how we interact and how we live. If Bob and I have qualitatively identical watches and he goes to China for ten years, I expect our perfect watches to match when he comes back. We experience uniformity with time which gives us the impression that there’s something out there called time, and it’s moving. Our language as well reinforces this idea. One might say “Time flies when you’re having fun.” The culmination is an every-day belief that time is something that is moving through our universe.
The other side of the debate comes up saying that time is indeed real, but our conception of it is very different from how it really is. Those on this side argue that time does not flow at all. This is space-time theory, based on Einstein’s theory of special relativity. The space-time theorist argues that time is simply another dimension like space. While insufficient to answer all of the problems in its current form, space-time theory offers the best explanation of the concept of time. Space-time theory addresses the problems associated with the belief that time flows like a river. For if time flowed, it would have to flow with respect to something else. We measure length by standards of length, but they don’t move themselves. If time is moving, then it has to move with respect to something else. Thus, we end up with time moving with respect to a type of hyper-time. But then hyper-time would have to move with respect to something else (which we might call “hyper-hyper-time”). Eventually, you have a situation where you must always have another kind of time above that resulting in a theory of an infinite number of hyper-times. In space-time theory, however, you have a static timeline where we perceive individual moments along the timeline.
With time being simply another dimension, it shares essential properties of dimensions of space, notably in three respects of reality, parts, and here and now. First, that just as all areas of space are equally real, whether occupied or not, so is time. We don’t think that just because we are no longer occupying space X that when we move to Y, it disappears. X is still there, just unoccupied. In the same way, every point in time is still in existence. Second, in the same way that various spatial parts occupy space, temporal parts occupy time. My body has various parts like hands and legs that take up space, and the whole of those parts makes up my body. If I had less spatial parts, I would take up less space. Similarly, I take up various temporal parts in time. My five-year old self takes up a temporal slice of time, the twenty-year old self is taking up a slice, and the 70 year old version (assuming I live to be that old). Finally, the sum of those temporal parts makes up the whole. Finally, time is analogous to space in terms of here and now. John can say he is “here”, and Sarah can say she is “here” with both being true because it’s a relative statement. As opposed to a privileged “present”, all points of time are equally present. We just cannot access them.
Space-time theory has received support by scientific research. Einstein’s theory of special relativity supports it, and other scientists and philosophers have supported the theory (Sider, 47). Nevertheless, there have been criticisms as to the ramifications of space-time theory being true. These criticisms usually involve change, motion and cause. Opponents point out that if space-time theory is true and the future is just as real as the past, how do we account for change? The space-time response is that the common concept of change is wrong if it’s understood to be just different qualities at different points in time. Rather, the space-time theorist will say that it’s spatial heterogeneity with different properties at different places like a highway. The highway is not changing though we may experience different bumps and cracks at different places. The second objection applies to motion, saying that we can move back and forth through space, but not time. The space-time theorist, however, contends that since time is simply another dimension, it is possible to travel back and forth through time. It may be a fact that it’s not feasible for us, but he would say that it is not impossible. The final objection is through causation. We can cause events in any part of space, but we can’t cause events in any part of time. The past is fixed, and we couldn’t go back in time and change something. The space-time theorist again would reply that it may be a simple fact about the world that there is no backwards causation but that it’s not impossible. It’s not inconceivable that Marty McFly would experience 1955 after racing to 88 mph in 1985; it’s just something we don’t experience again because it is not feasible for us.
In terms of logic and science, space-time theory does the best job at answering the question of time. However, it is not without problems, and while I think that it is a good start, there are a lot of questions that arise that seem to pose difficulties. Despite these, however, none of them are knock-down arguments for space-time theory. For this reason, I think that space-time theory is the best direction, though it may be insufficient in its current form as proposed by various physicists.
People can raise many objections to space-time, although the most problematic routes for space-time includes those of motion and causation specifically in relation to the issue of time travel. Here, one can bring back good old Marty McFly. Marty understands some of the problems posed by traveling through time. He goes back to the past and ends up messing things up for his parents getting together, eventually fixes it, but not after he’s changed things, and comes back to the future to find a different future than what he left. Here one faces the question of changing the past.
A person might ask a space-time theorist how it would be possible to travel back in time if you know that no one was there. It’s all well and good saying that we’re going to travel back before any of us were born, but what about traveling back to when I know I didn’t travel back. The space-time theorist would respond that it’s not that you couldn’t, it’s that you didn’t. Say that at 12:00 you decided to travel back through time and visit yourself in the past. You would not set up your time machine, check your watch, go back at 12:00 to 11:55 and surprise your past self. No. You would set up your time machine, check your watch, and see yourself come out of nowhere at 11:55. Five minutes later, you would travel back yourself and continue the path into the future.
People have speculated on whether you could kill your ancestor or even your past self. In his work,Paradoxes of Time Travel, David Lewis addresses this issue and uses the example of Tim going back in time to kill his own grandfather at the time before his father was conceived. Tim could not kill grandfather in one sense in that he is capable, but if his grandfather died, then one would have a contradiction. Tim murders grandfather before the victim is able to marry, but Grandfather also lives to marry and produce Tim’s father who in turn produced Tim so he could travel back in time. Because there is a contradiction, and all contradictions cannot exist, Tim is also unable to do it because it is a fact that grandfather lived to produce Tim’s father (Lewis, 77). A person might respond that this is not true because the future is not yet “written” as we say. However, according to the theory of space-time, all points in a timeline are equally true. Grandfather is just as much a young man as he is an old man, but the two temporal slices cannot access each other. Say that another man, Tom, also tries to kill grandfather. Tom is equally unable to murder him because it was true at that time that grandfather would live a long life and have children.
In effect, space-time theory contends that one can possibly travel through time. Yet because all of time is already in existence and there is no privileged “now”, one cannot change the past or the future. The past and future just are, and if we learn in 2035 how to travel back in time to 250 B.C., then we will not change history. It would be a simple fact that back in 250 B.C. some people came back from the future. So, sadly for Marty McFly, he would not travel back in time, change events, and come back to 1985 to find his parents hip. No, he would travel back in time, get his parents together, and come back to find that his parents were exactly the same as they had always been because back in 1955 before Marty was born a guy named Marty got them together. Now, a final objection in this scenario is to ask “Well, what happened first?” One would seem to have a chicken and the egg situation where the McFly’s had Marty so he could travel back in time to get them together so they could have Marty. But remember that in space-time, all points are equally real. We can have backwards or even circular causation though we don’t experience it ourselves. The space-time theorist will say that it is certainly possible to travel back in time, but it’s not possible to go back and change the past.
As one can see, space-time theory offers good answers to problematic questions brought up in issues of time travel. Yet space-time theory does not necessarily provide enough answers to all of the questions. For instance, it doesn’t directly address the question of how we experience movement through time, and if future and past are just as real, how is it we only experience it in a linear fashion? Yet because we cannot travel through time as of yet, many of our ideas are educated guesses. Perhaps time travel would simply move someone’s consciousness through time at a quicker and slower rate, and Marty could not actually go back to see a double of himself like he did.
That is a serious question. But with the flux capacitor still waiting to be invented, we can only guess at how time travel might work. The realm of time and its mysteries is still an area ripe for great exploration. We’ve learned a lot, and although it seems counterintuitive, research seems to suggest that time, in fact, does not flow. The theory of space-time answers a lot of questions, but not all including the big question of how we perceive motion and a flowing of time. Briane Greene writes that “It is possible that some insightful person will one day devise a new way of looking at time and reveal a bona fide physical foundation for a time that flows” (Greene, 141). That day has not come, however, and while it is true space-time theory doesn’t answer all of our questions, it is doing a better job than other theories, and that’s probably a sign we’re heading in the right direction.
Works Cited
Conee, Earl, Theodore Sider. Riddles of Existence. Oxford: Clarendon Press, 2005.
Greene, Brian. The Fabric of the Cosmos: Space, Time, and the Texture of Reality. New York.
Lewis, David. Philosophical Papers Volume II. Oxford: Oxford UP, 1986.
