Understanding Time: Does It Require Two Dimensions for Clarity?
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To arrange a meeting, you must specify both a time and a location. In a bustling place like New York City, this could be as simple as saying a cross street, such as 29th and Broadway, or indicating a specific floor, like the 6th, at a set time, say 7 PM.
Typically, identifying a position in three-dimensional space requires three coordinates: left/right, forward/backward, and up/down. For instance, an aircraft's position on Earth is defined by its latitude, longitude, and altitude. However, when you want to pinpoint a location in both space and time, a fourth coordinate is necessary—a "when."
When you combine all spatial points with all moments in time, you arrive at a four-dimensional continuum known as spacetime.
Einstein was revolutionary in demonstrating that time operates similarly to space. For observers watching someone accelerate, that person's time seems to pass more slowly. Additionally, as you move faster, your length appears to contract in the direction of travel. Time can stretch, while space may compress.
Moreover, gravity has the capacity to warp both space and time. In essence, your rate of time passage can change simply based on your proximity to Earth.
In the 1970s, researchers placed a highly accurate clock on a plane and circled the globe. It revealed that more time elapsed on the plane than on the ground, illustrating that time slows down nearer to Earth due to gravitational effects.
Recently, a more precise clock indicated that even a slight variation in altitude—just a millimeter—affects how we experience time due to gravity.
This information hints that time is malleable like space, yet it fails to clarify why we cannot navigate through time as we do through space. It also does not elucidate why we perceive time as progressing linearly or why we can recall the past, but not foresee the future. It would be rather absurd if our memories were confined to one side of our environment.
In essence, time is not analogous to space.
If we examine those four coordinates, we realize they lack a crucial element. While I can instruct you to meet me at a specific time and location, I must also synchronize that "now" with the designated time and place for the meeting to actually happen. Thus, it's not just a matter of being aware of time and space; I must also be conscious of the current moment.
This observation, while seemingly minor, is vital for grasping why time cannot be treated as a dimension in the same way as space.
Imagine a chessboard where both horizontal and vertical movements represent spatial dimensions. If I want to position two pieces in adjacent squares, I can easily do so. However, if we consider the vertical axis as time and the horizontal as space, it becomes akin to a conveyor belt where pieces can only shift left or right as they progress forward in time. For the pieces to meet, they must occupy adjacent squares at a specific moment.
However, this introduces a dilemma because I've added a third dimension—time—to the original two-dimensional chessboard. So now we have the horizontal, vertical, and a temporal dimension.
Alternatively, picture taking a series of snapshots of this moving board at different times, where "now" shifts location with each moment. Stacking these images creates a cube that incorporates spatial dimensions, the horizontal, vertical, and a vertical axis for the present moment.
While "now" aligns with temporal position, it must be considered an additional dimension—what we truly experience as time flows—while the downward time axis represents the timing of events.
Interestingly, I do not need to remind anyone to ensure they arrive at the correct moment when I ask them to meet me at a certain time. The movement of "now" through time handles that automatically. Though it may seem like a dimension, it is not one we can control.
There are four interpretations of this idea:
- Consider that time is merely an illusion, with only the dimension of now existing.
- Assume only the dimension of time is real, rendering the now dimension an illusion.
- Propose that now and time are aspects of the same dimension.
- Treat both as genuinely distinct dimensions.
Presentism
The first perspective is known as the Presentist viewpoint. Prior to Einstein's theory of relativity, Presentism was quite logical and was endorsed by Isaac Newton. We cannot traverse time in both directions, and we exist only in the present moment, rendering now sufficient for defining time. The past no longer exists, and the future is yet to be formed.
However, relativity demonstrated that observers can disagree on the timing of "now."
To grasp why this is the case, we must define the meaning of "now." If two events occur simultaneously, they happen at the same moment. Thus, "now" encompasses all events transpiring concurrently.
This understanding is adequate in everyday situations. Still, when dealing with high-speed objects or massive bodies, the concept of simultaneous occurrences becomes subjective.
For instance, if Bob is aboard a spaceship traveling at a significant fraction of light speed relative to Alice, he sets up a light gun in the center of the ship to fire light pulses toward detectors at both ends. Bob calibrates the gun to ensure the pulses reach the ends simultaneously.
From Bob's perspective, the light pulses arrive at both detectors at the same time. However, Alice perceives differently. They both observe the light traveling at the speed of light, consistent with relativity. Yet, due to the ship's motion, Alice sees the rear detector approach the pulse that moves against the ship's direction, while the front detector moves away. Consequently, Alice observes the rear pulse hitting the rear detector before the forward pulse reaches the front detector.
Thus, Bob and Alice cannot concur on when "now" is, meaning they disagree on which events are current and which belong to the past.
This is a variant of what's sometimes referred to as the train paradox. In this scenario, lightning strikes either end of a moving train, and while the simultaneous events appear in the "now" for an observer not on the train, they occur at different times for someone aboard.
The complexity increases in general relativity when gravity is introduced. Individuals near different gravitational bodies also struggle to agree on when "now" is. For instance, time can move so slowly near a black hole that what feels like an hour for one person could equate to a year for another located farther away. Time is not a uniform and consistent measure, as Newton once suggested.
Relativity complicates the acceptance of Presentism in its traditional form. A few original Presentists continue to advocate for it, particularly in the Bohmian mechanics interpretation of quantum theory, assuming a universal "now" exists within an unmeasurable quantum realm.
Most Presentists have abandoned the notion of a universal present and instead propose that while the universe operates under Presentism, each observer has their localized "now." This necessitates that the universe be divided into slices, akin to a loaf of bread, referred to as a foliation. Italian physicist and author Carlo Rovelli has published several academic papers on this concept.
Eternalism
The second perspective is the Eternalist or Block time theory. Einstein favored this interpretation, which became central to his life philosophy. In this view, "now" is an illusion. We are conscious entities experiencing a fixed four-dimensional reality, much like a person scanning a painting. Our attention flits across time, absorbing all there is to perceive.
The sense of time flowing linearly is a construct created by our brains. Our minds can only understand what our brains relay, which convinces us that time is in motion, though it is merely a sequence of snapshots presented to our consciousness, complete with memories to support the notion that we existed just a moment prior.
This theory, sometimes referred to as the B-theory of time, dates back to J.M.E. McTaggart’s 1908 book, The Unreality of Time. It is intrinsically dualist, as it assumes a non-physical mind responsible for experiencing time. The physical brain generates the illusion of time's passage; however, the mind must be capable of experiencing distinct moments, without any two moments overlapping.
Kurt Vonnegut incorporated the B-theory into his acclaimed World War II novel, Slaughterhouse-Five. The narrative highlights the absurdity of such a notion.
Eternalism poses significant challenges. It presupposes a mind that exists separately from the body and can traverse time akin to a specter, potentially through supernatural means. This creates more complications than it resolves and lacks the foundation of a scientific theory.
Interestingly, many scientists identify as Eternalists while rejecting non-physical explanations, placing them in a philosophical quandary that could be alleviated by the introduction of a second temporal dimension.
Half-Block Time
The third interpretation posits that time consists of the past, while "now" represents the leading edge of that dimension, akin to a plant's growth toward the sun. The half-block time theory encounters similar issues as Presentism, necessitating a multi-time interpretation to align with relativity. Therefore, I will not delve deeper into this notion.
Eternalism with Additional Time Dimension
The fourth perspective is Eternalism combined with two temporal dimensions. One dimension relates to events and history, while the other represents the present moment traversing through history.
Since our position in the now aligns perfectly with our current moment in history, we effectively lose what physicists term a "degree of freedom," explaining why the universe appears four-dimensional rather than five. We cannot navigate through history without simultaneously moving through the current moment in the same manner. The only way to distinguish between these two dimensions is through the lens of relativity.
Thus far, the scientific community has been slow to embrace this philosophical conclusion. Theories of time often overlook the experience of time itself—the fact that every instant is "now" at some specific point, necessitating an additional dimension for explanation, is frequently omitted from equations.
Those who do consider this matter tend to focus on the Presentist multi-time approach. However, it remains uncertain whether we can entirely foliate the universe, including all types of black holes. A specific foliation called "York time" can accommodate them; yet, for Presentism to hold, it would require universality from cosmic scales down to the Planck length.
For an Eternalist view of time to be valid, it must incorporate two time dimensions, unequivocally. There is no interpretation of motion within a block time framework that lacks another dimension to explain this motion, unless one resorts to supernatural explanations.
The advantage of the two-time dimension Eternalism is that it allows for the existence of spacetime geometries that cannot be divided into slices, including Closed Timelike Curves (CTCs) and wormholes leading back into the past—scenarios excluded by foliation. While we do not currently observe phenomena traversing back in time, at the quantum level, CTCs could potentially manifest.
Moreover, quantum physics inherently encompasses correlations across all points in spacetime simultaneously. In fact, quantum field theory suggests that relativity becomes increasingly violated with distance. This can produce a peculiar effect known as retrocausality, where the future influences the past—a phenomenon not yet observed but predicted by standard quantum theory.
This prediction arises because all quantum theories are symmetric in time, failing to differentiate between past and future.
A Presentist viewpoint inherently excludes retrocausality due to its time-asymmetric nature. However, CTCs present at the quantum foam level within a theory of quantum gravity may allow for short-distance retrocausality to become a common characteristic of our universe.
In contrast, the two-time dimension Eternalist perspective aligns with quantum theory, being time-symmetric because it possesses an additional dimension that does not appear in current equations.
Retrocausality also aligns philosophically with the concept of free will when incorporating two time dimensions, as opposed to one. In the scenario with two time dimensions, we can envision a timeline that includes all of history, allowing for an event, A, to occur before a later event, B, while B causes A. For example, if B represents a time traveler going back to eliminate his grandfather, we could observe that A resulted from a later occurrence, B.
If we can confirm that A was caused by a subsequent event B, we hold the agency to prevent B from happening—such as destroying the time machine beforehand—leading to a time paradox. However, because we navigate through a second time dimension, we can avert B and eliminate the retrocausal link from history, as all of history is permitted to shift and adjust within that second dimension.
If quantum experiments were to confirm the occurrence of retrocausality, it would categorically disprove the Presentist viewpoint. For all the reasons detailed above, such a finding would also dismiss the notion of a singular time dimension, irrespective of free will.
Another advantage of the dual time approach is that it provides insight into the quantum measurement problem. While I have extensively discussed this topic elsewhere, I will not elaborate on it here.
Thus, a five-dimensional universe could potentially clarify two significant mysteries of modern physics: time and quantum theory, all within one framework. This certainly warrants further investigation.