The subject of time travel is briefly discussed in the following articles:
"Time Travel: Q/A"
"Temporal Passage": Endnote #15
"Considering Alternatives to the Spacetime Model of Reality": Time Travel
"Space, Time, Matter and Mind": Time Travel
"Parallel Universes and the Mind": Time Travel
These articles consider the possibility of self-consistent time travel scenarios.
The possibility of time travel greatly disturbs many physicists. This is understandable: it seems that the logical structure of the universe is threatened by travel to the past. Yet, the theory of relativity seems to permit travel into both past and future: by changing our velocity and the strength of the surrounding gravitational field, we can visit the future; and by traversing wormholes we may be able to go back in time.
Physicists are therefore confronted with a profound dilemma: how are the paradoxes in history which come about from time travel to be resolved? Perplexed, some theorists postulate the existence of a mechanism which prevents all attempts to enter the past. There is at present, however, no strong evidence to support this conjecture. Many other physicists choose, quite simply, to sweep the entire issue under the carpet. In this paper, I demonstrate that travel to the physical/geometric past creates no temporal inconsistencies: no paradoxes at all result.
What principle permits us to visit the geometric past? The answer is implicit in our current physical theories--the theory of relativity and the quantum theory--in the linkage between consciousness, time, and matter. I will describe the nature of this linkage and show that an understanding of time and consciousness forces us to draw quite remarkable conclusions about the nature of our universe.
Let us first discuss time. Relativity describes this phenomenon as a sort of vertical block, with all points or locations on that block existing at once, in a static/concurrent fashion. We tend, however, to think of time as something which flows: we have the impression of a present moment in constant motion, constantly moving into the future. These two descriptions seem incompatible. I will show, in fact, that they refer to two distinct aspects of reality.
What we refer to as time (for example, the distance between the years 2000 and 2010) is actually a type of spatial distance. We speak of the distance between New York and San Francisco as a spatial distance; we must realize, however, that the distance between the years 2000 and 2010 is also a type of spatial distance. I will refer to this latter type of space as "A-(annum) space," in contrast to what might be termed "O-space" (ordinary three dimensional space). A-space is clearly different from O-space, yet both are physical spaces--they are both spatial in nature. This point cannot be overemphasized, as it is the failure to recognize the difference between spatial distance and temporal distance (or time) that is at the root of our confusion over the concept "time."
Before defining "temporal distance," I must discuss the nature of consciousness. We often describe consciousness as electrical activity in the brain, but this description runs into difficulties when the inherent quantum uncertainty of all matter and energy is taken into account. According to the quantum theory, matter and energy are not real when they are not observed: the mind, therefore, if it is described as electrical energy in the brain, cannot be real when it is not observed. As it is possible for all minds to remain unobserved at a given moment in time, all mental activity might be eliminated from the universe once and for all. Clearly, then, we cannot define consciousness as matter or energy.
The mind, in fact, is entirely removed from the material world. It does not exist in A- and O-space. If it did, the mind would have had to have both location and volume--it has neither. Some readers might not be convinced that mental life occurs "outside" of A-space, but I will show how it can. (It will be explained later that mind actually remains linked with space.)(1)
This brings me to the definition of existence. Most people do not realize
that in order for something to "exist" it must remain in existence. It
becomes easier to understand this once the relationship between mind, matter and
time is clarified. As shown in Example 1, the mind exists outside A-space. In
this diagram, I depict A-space with a horizontal axis. Each point on the axis
represents the physical universe at a given point in time, the totality of
O-space, matter, and energy in existence in the world at a given moment. Point M
represents a conscious mind and is located outside of everything (it does not
exist inside A-space, O-space, matter or energy).
If the mind is to exist, it must constantly remain in existence. We may, therefore, define existence as a process--the process of remaining in existence, or persistence. The reader can appreciate that existence is dynamic rather than static in its nature: persistence (existence) is a kind of constant motion.
It is essential that the reader understand the concept of existence as persistence. It is a mistake to think of existence as a state. As explained above, what we call "existence" is the quality of remaining in existence. The present moment, for instance, persists, and this is precisely why we say it exists. (As regards the present, it should be pointed out that what we call the "passage of time," or the "flow of the river of time" is actually the persistence [existence] of the present.)
Consider the following two statements: "This book, which exists now, will exist during the next two seconds, as well" and "This book will remain in existence for two seconds." There is absolutely no difference between these two remarks. To exist and to remain in existence mean exactly the same thing. Our intuition has led us astray, leading us to describe existence as "state" and persistence as "process."(2) This is a fundamental error and it is at least partly responsible for the confusion which surrounds the concept "time."(3)
As the mind persists, a certain type of distance is created between the present moment of consciousness and what may be called the initial "location" of the mind--point I in Example 2 (the constant persistence of the mind creates a disparity between its present location and its initial location, I). This distance is what I call "temporal distance" (duration) or "time." It should be clearly evident that time is created by consciousness: the persistence of consciousness--that is, the mind's maintenance of its own existence--creates time. It should be noted as well that time (temporal distance) expands as the mind (point M) persists. In Example 3, I have labeled the expanding temporal distance as the "time line."
We cannot discuss the speed of this expansion, however, for speed is defined exactly as distance per unit of time. To speak of the "speed of time" is to treat time as though time were space (i.e., O-space), which it is not. We must realize that speed is a concept that we have invented to address the specific relationship between spatial (O) distances and time. The concept "speed" has no applicability to the flow of time itself.(4)
I wish to caution the reader against confusing time with A-space (e.g., the space between the years 2000 and 2010). We commonly mistake temporal expansion, which results from the persistence of the mind, for motion through A-space. This mistaken impression stems from our deeply rooted belief that we exist in A-space and move through successive moments which already exist in this space. Those moments which we pass, we label "past moments"; those moments toward which we are heading, we label "future moments." The particular moment at which we happen to be located, we label "the present."
This entire conception is wrong. We do not exist in A-space; our minds, depicted as Point M in Example 2, are dynamic and necessarily exist outside the static framework of A-space. In addition, our understanding of the past, present and future is flawed. Only one moment exists--the present moment--and this one moment endures: it remains in existence. This moment is the Point M to which I have already referred. (I have already defined Point M as consciousness; later in this paper I will demonstrate that it also constitutes "the present moment.") Although there are many points in A-space, there is only one Point M, i.e., there is only one moment of time, and this one moment exists outside A-space.
Time differs from A-space in one fundamental respect: time passes, whereas A-space remains immobile. The reason for this passage of time becomes apparent upon closer examination. Clearly:
I must clarify one point that might be confusing to the reader. Unlike A- and O-space, the distance between point M and I (durational distance or the "time line") is not a physical thing. Rather, the distance (disparity) between M and I is a relation. This durational distance comes about through the persistence of M: as M persists, the disparity between M and I increases. Note, however, that although it is possible to speak of the relationship between M and I, only Point M (the present moment) actually exists.
When we say that M remains in existence (say at M=2 on the time line: Example 3), we imply that M existed before (earlier)--i.e., M existed at lower points on the time line.(5) However, it may be argued that when M=I, it is not proper to say that M remains in existence, since I is the first point on the time line (the first "location" of M) at which M exists: M does not exist before I. Therefore, I propose that we regard Point I as a "singularity" from which the present emerges--from which the present comes into existence.(6) In the big bang description of the origin of time, time does not actually exist at the big bang singularity. Rather, time emerges, or comes into existence, after the big bang singularity. There is no first moment of time in this description; every moment of time, consequently, is preceded by an infinite number of other moments. For every moment in time, there is a before; i.e., there are "earlier" moments closer still to the big bang singularity. The coming into existence of the present, in my description of time, may be conceived in a manner analogous to the big bang description of the origin of time. We may treat "I" as a singularity from which the present (consciousness) comes into existence. M, therefore, exists at all points on the time line to the right of ("after") "I," but not at I itself. Similarly to the big bang description, every point on the time line is preceded by other points (infinite in number)--points which are even closer to the "singularity" I. Regardless of where M happens to be on the time line, it is true that M existed earlier. It will always be correct, therefore, to describe M as remaining in existence since it will always have existed earlier (at lower points on the time line--points closer [earlier] still to I). Therefore, we may always correctly refer to the present moment as being in a state of persistence because it always will have existed earlier (beforehand).(7)
Quantum theory tells us that A-space--as well as matter and energy--has to be observed in order to be real: A-space (commonly and mistakenly called "time") is affected by the same quantum fuzziness which permeates the world of the atom. This space, therefore, ordinarily remains in a "potential state of existence," to use the terminology of the Copenhagen interpretation.(8) At each point in A-space we find regular three-dimensional space, matter, and energy which also exist in a potential state.
This brings me to the question of the mind and its relationship to the human body--an issue which is much clarified when we take into account the "potential" nature of all physical existence. We must conclude that a person is just mind, not mind and body. I need only call attention to the fact that the human body, itself being material, does not exist concretely when unobserved. If humans were material they would not truly exist when unobserved. However, since we know that humans do exist, concretely, even when unobserved, we know that they cannot be material.
We tend to think of ourselves as material beings. The fact that we feel ourselves as physical beings, however, does not mean that we are physical beings. Our physical/sensory experiences (the sensation of physicality) are part of our general conscious experience; to feel one has a body is simply to feel one has a body. However, there is an undeniable association between states of mind and the physical states of the human body. Changes in our states of mind are correlated with changes in the physical states of the body. The specific nature of this relationship is not yet thoroughly understood.
Although we are only minds (not minds and bodies), our kinesthetic/physical sensory experiences form a significant part of our over-all conscious experience. The physical states of matter (our bodies) reflect this complex activity of kinesthetic experiences. Nevertheless, our bodies are not "part of us." We are minds--and minds alone.
Given this framework of understanding, we may make a few observations about the nature of conscious experience. Let us start with a definition of a thought, an emotion or mental sensation of any kind as a mental state which occurs at a fixed point on one's time line (Point M in Example 2). It follows, then, that "human life" (mental life) is simply a succession of these mental states. As we do not experience an infinite number of thoughts, emotions, etc. in a given period, we must conclude that each mental state is maintained for some finite duration, at which time it is replaced by another state, which in turn persists for some time; a third mental state then replaces the second, and so on.(9) The time line (time) expands as mental life, or consciousness, persists.
The persistence of consciousness is closely associated with the concept of the present and the "passage of time." Physicists recognize that the present has no objective existence. The present is a subjective phenomenon which owes its origin--its existence--to consciousness. There can be no present without consciousness. I believe, in fact, that the terms "present" and "consciousness" (or "conscious experience") are entirely synonymous: the present is nothing other than consciousness itself.
I define the present as conscious experience. This means, of course, that the present is a strictly subjective and individual matter: there are as many presents as there are individual consciousnesses. This type of present may be called a "subjective" present: each person's present applies only to that one person. In contrast, we may speak of an "objective" present--the universal present which we usually have in mind when we discuss time. This universal present would encompass two or more individuals (it would apply to several individuals rather than just one). This present would enable us to coordinate events in one person's consciousness with events in another person's consciousness: we would be able to say that a particular event in one individual's (Tom's) conscious experience happens simultaneously with a particular event in another individual's (John's) conscious experience. We would also say that a particular experience in Tom's life happens before (or after) a particular experience in John's life. This "objective/universal" present would apply to both Tom and John, enabling us to coordinate their conscious experiences temporally.
If we define the present as "conscious experience," however (i.e., as subjective), this automatically rules out the possibility of the existence of a universal present in the universe.(10) Consequently, we cannot coordinate Tom's and John's experiences as above.(11)
No temporal (or for that matter, spatial) relationship exists between consciousnesses.(12) Each person, nevertheless, maintains contact with the physical world (A- and O-space, matter, and energy). Before describing the nature of this contact, however, let me discuss one more aspect of conscious experience--the perception of motion.
I must emphasize that conscious experience is represented by a point (Point M) in the diagrams, and not by a line segment. If conscious experience were represented as a line segment, it would mean that consciousness is a phenomenon which "occurs" over successive moments in time. It is, however, represented as a single point (M), which means that it is a phenomenon which takes place instantaneously, at a single moment (the present moment).(13) In Example 2, Point M is in constant motion to the right; this motion visually depicts the persistence of conscious experience.(14) Remember Point M can exist only if it is moving; if it stops moving to the right, it ceases to exist. (Existence is defined as persistence. Persistence is represented in the diagram as rightward motion. Therefore, if Point M's rightward motion ceases, the existence of consciousness ceases.) The fact that consciousness persists or endures over time should not confuse us into thinking that consciousness is a phenomenon which takes place over a succession of moments in time. To reiterate, consciousness occurs instantaneously, at one moment.
It is particularly important to bear this in mind when discussing the mental experience of motion. Let us consider the consequences of defining the sensation of motion as an experience which takes place over a period of time (i.e., let us assume that motion cannot occur instantaneously).
If this is the case, the perception of motion must be described as a succession of mental states(15)--with each mental state in the succession corresponding to the object located at a particular location (point) between A and J (see Example 4); that is to say, each mental state must be a frozen, static mental image of the object at a particular point on line AJ. Let us assume that Tom is watching an object move from 10 A.M. to 10 seconds after 10 A.M. Let us also assume that his experience of this motion consists of 10 mental states, and that each individual mental state persists for 1 second. Tom must see a frozen image of the object at 10 A.M. (the object will be seen as located at point A), and he must keep seeing this same frozen image until 1 second after 10 A.M. At 1 second after 10 A.M., Tom sees a new frozen image of the object--an image of the object located at point B. Tom keeps seeing this frozen image until 2 seconds after 10 A.M. Thus, throughout the time period (10 A.M. to 10 seconds after 10 A.M.), Tom sees nothing but frozen, static images--each image enduring for 1 second.
Clearly, the definition of a sensation of motion as an event which takes
place over time is wrong. If we accept this definition, we must conclude that we
experience motion as persisting individual frozen images, separated by intervals
of time. We do not, however, experience motion this way. We must, therefore,
redefine the perception of motion. There is only one logical alternative(16):
the perception of motion must be an instantaneous experience. Like all other
mental states, an individual sensation of motion "occurs" instantaneously, at
one moment, and persists for a short finite duration (a small fraction of a
Diagrams for Part
The Theory of Persistence: Part Two
2. It is a mistake to regard the fact that a thing exists as established and to treat the fact that it persists as an attribute or incidental feature. We refer to the entire process of persistence when we speak of the existence of a thing; without persistence, the existence of a thing cannot be defined. Therefore, we cannot distinguish conceptually between "existing" and "persisting."
3. Greek philosophers have emphasized the distinction between "being" and "becoming." We ought not, however, think of the passage of time as a process of "becoming." To reiterate, "being" (existence) is a process, not a "state." A thing which persists (endures) over time is; it does not "become."
4. In the future, we may discover a second time with which to gauge the passage of ordinary time; it will then be meaningful to talk of the "rate at which time passes." In the absence of such a second time, however, any reference to the "speed at which time passes" must be regarded as a misuse of speech--a meaningless remark. If only one time exists, the concept "rate" can in no sense be considered applicable to time. (See Paul Davies, About Time: Einstein's Unfinished Revolution (New York: Simon & Schuster, 1995), 253-276.)
5. I have chosen to say "earlier" or "beforehand" rather than "at an earlier time" or "at an earlier moment in time." This is because the last two expressions imply that time is made up of many moments. As I have established, however, time has only one moment. This one moment, the present, in a sense "changes its location" constantly--each location corresponding to a different point on the time line. It is, therefore, correct to say "earlier" and incorrect to say "at an earlier moment in time" when discussing lower (earlier) points on the time line (see Example 3).
6. A "singularity" in physics is a state of infinite compression. However, when I refer to "I" as a singularity, I am not referring to this aspect of infinite compression. In using the term "singularity," I mean simply "a past temporal edge to time." The singularity "I," therefore, is a "past temporal edge to the present." (The present, as we know, has a lifetime; i.e., it endures for a length of time which we call durational distance, or the time line--bounded by M and I.)
7. If we treat "I" as a singularity, it follows that, if the present exists now, it must have existed earlier--i.e., it must have existed before now. (In terms of the diagrams, this means that, for any location of M on the time line, M must have been located at points closer still to I before arriving at its present location on the line.)
8. Werner Heisenberg, Physics and Philosophy: The Revolution in Modern Science (New York: Harper & Row, 1958), 53.
9. It is noticeable that each mental state includes an awareness or "memory" of the mental states which precede it. As a result, we experience mental life as a coherent succession of mental states.
10. An "objective present" is, by definition, not consciousness; since it is not consciousness, it obviously cannot qualify as "the present." If the present is conscious experience the "universal present" becomes a superfluous concept: we cannot accommodate its existence.
11. It is only the universal present that allows such temporal coordination.
12. We might conjecture that some sort of universal present exists in addition to the many subjective presents which exist (i.e., in addition to the many individual minds that exist). I must point out, however, that there is nothing to suggest that any sort of objective present exists. In the absence of such a present, events in one person's consciousness, i.e., events in one person's mental life, cannot be coordinated with events in another person's consciousness.
13. The reader must appreciate the distinction between an event which "happens" "over a period of time," or with the "passage" of time, and an event which happens at a single point, but endures for a period of time.
14. Point M's motion rightward should not be understood to mean that the present (Point M) "moves into the future." Time has only one aspect--the present. There is no past and there is no future. The fact that the present persists (as it must for it to exist) leads us to feel that we "move into the future." This, of course, is a misconception. We always remain at the present, and the present endures.
15. Remember, mental states themselves are instantaneous experiences.
16. Motion is either an instantaneous experience or an experience which occurs over a period of time.
17. Let us consider an individual sensation of motion--i.e., motion which is sustained for a short period of time. In Example 5, the sensation we experience that a marble moves between points a and b is sustained for a short duration. The sensation itself, however, is an instantaneous experience (it is a single impression, not a succession of impressions). We must not be misled by intuition which suggests the opposite--that the sensation of motion is a succession of experiences "over a period of time." The impression (sensation) of motion that we experience (between a and b) endures, like all mental states (although for a short duration of time). Later in this paper, I will take up the more complicated matter of the perception of motion over an extended period of time.
18. We must not let the fact that the sensation of motion endures over time confuse us into thinking that the sensation actually occurs "over an interval of time": it does not require an interval of time in order to "happen." To reiterate, all mental sensations, including the impression (sensation) of motion, take place instantaneously at point M.