Tag Archives: scientific definition of time

A black and white image of a clock face with a spiral effect distorting the numbers and hands.

Philosophy on the Nature of Time – Part 1/2

Categories, Physics, , ,

One question comes up frequently, is time real? In other words, is time a true physical entity or is it a mental construct? Philosophers have been pondering the nature of time for thousands of years. A philosophy of time weaves through almost every ancient culture. For example, the earliest view of the nature of time by a Western philosopher dates back to ancient Egypt and the Egyptian philosopher Ptahhotep (2650–2600 BCE). Indian philosophers and Hindu philosophers also wrote about time dating back to roughly the same period. The ancient Greek philosophers, such as Parmenides, Heraclitus, and Plato, wrote essays about the nature of time roughly around 500 BCE to 350 BCE.

Many early writers questioned the nature of time, the cause of time, and the unidirectional flow of time, often referred to as the “arrow of time.” One of the most interesting aspects when studying the philosophy of time is that some cultures, like the Incas, dating back to about the thirteenth century, considered space and time woven together. Centuries before Einstein published his now-famous special theory of relativity, which scientifically unified space and time (i.e., spacetime), the Incas philosophically unified space and time into a single concept called “pacha.”

As delineated above,  philosophers have debated the nature of time for over 2500 years. The result of this debate has left us with three principal theories, listed below in no particular order

1)   Presentists Theory of Time—The “presentists” philosophers argue that present objects and experiences are real. The past and future do not exist. This would argue that time is an emerging concept, and exists in our minds.

2)   Growing-Universe Theory of Time—The “growing-universe” philosophers argue that the past and present are real, but the future is not. Their reasoning is the future has not occurred. Therefore, they reason the future is indeterminate, and not real.

3)   Eternalism Theory of Time—The “eternalism” philosophers believe that there are no significant differences among present, past, and future because the differences are purely subjective. Observers at vastly different distances from an event would observe it differently because the speed of light is finite and constant. The farthest-away observer may be seeing the birth of a star while the closest observer may be seeing the death of the same star. In effect, the closest observer is seeing what will be the future for the farthest-away observer.

Let look at how the above theories address the question, is time real? The “presentists” philosophers would argue time is not a true physical entity, but rather a concept of our minds. The “growing-universe” philosophers would argue that only the past and present are real. The “eternalism” philosophers would argue that there are no significant differences among present, past, and future because the differences are purely subjective. Their philosophy rests on Einstein’s special theory of relativity. Essentially, the special theory of relativity holds that the past, present, and future are functions of the speed and position of an observer. In effect, this philosophy argues time is real, but subjective. This brings us to the question, what does science have to say about the reality of time?

From a practical standpoint, the science of time started with Isaac Newton (1642–1727) in the seventeenth century. Newton thought of time as an absolute. He believed that time passed uniformly, even in the absence of change. From Newton’s point of view, any event that occurs at a single point in time occurs simultaneously for all observers, regardless of their position or relative motion. Newton’s view of time as an absolute became a cornerstone of classical physics and prevailed until the early part of the twentieth century.

It is important to mention that Newton’s view of time was likely influenced by Galileo, a brilliant Italian physicist, mathematician, astronomer, and philosopher. Galileo and Newton never met in person, since Galileo died the same year Newton was born, 1642. However, there appears little doubt that Newton’s science of time was significantly influenced by Galileo’s 1638 Discorsi e Dimostrazioni Matematiche (Discussions on Uniform Motion), since Newton’s and Galileo’s views of time are essentially identical.

The science of time underwent dramatic changes early in the twentieth century, when a little-known patent examiner published a paper in the Annalen der Physik in 1905. The paper contained no references, quoted no authority, and had relatively little in the way of mathematical formulation. The writing style was unconventional for a scientific paper, relying on thought experiments combined with verbal commentary. No one suspected that the world of science was about to be changed forever. The little-known patent examiner was twenty-six-year-old Albert Einstein. The paper was on the special theory of relativity, which quietly led to the scientific unification of space and time, and the scientific realization that mass is equivalent to energy. The ink of this one paper rewrote the science of time.

This view of time holds to this day. Most of the scientific community agrees that the most accurate definition of time requires integration with the three normal spatial dimensions (i.e., height, width, and length). Therefore, the scientific community talks in terms of spacetime, especially in the context of relativity, where the event or observer may be moving near the speed of light relative to each another.

Based on Einstein’s spacetime integration, we can argue that time is a physically real aspect of reality. Let us consider an example. A clock moving close to the speed of light will appear to run slower to an observer at rest (one frame of reference) relative to the moving clock (another frame of reference). In simple terms, time is not an absolute, but is dependent on the relative motion of the event and observer. It may sound like science fiction that a clock moving at high velocity runs slower, but it is a widely verified science fact. Even the clock on a jet plane flying over an airport will run slightly slower than the clock at rest in the airport terminal. Einstein predicted this time dilation effect in his special theory of relativity, and he derived an equation to calculate the time difference. Other physical factors affect time. For example, another scientific fact is that a clock in a strong gravitational field will run slower than a clock in a weak gravitational field. Einstein predicted this time dilation effect in his general theory of relativity.

While we can argue that time is real, not a mental construct, there is no consensus on the scientific definition of time. Instead, science describe how time behaves during an interval, a change in time. Science is unable to point to an entity and say “that is time.” The reason for this is that time is not a single entity, but scientifically an interval. We cannot slice time down to a shadowlike sliver, a dimensionless interval. In fact, scientifically speaking, the smallest interval of time that science can theoretically define, based on the fundamental invariant aspects of the universe, is Planck time.

In part 2 of this post, we will discuss Planck time and a new concept, the time uncertainty principle.

Close-up of a large clock face with Roman numerals illuminated by warm golden light.

The Science of Time – Part 3/3

Categories, Time Travel, , , , ,

The Science of Time: This is taken from appendix 5 of my new book, How to Time Travel, which will be available on Amazon in early September 2013.

Approaching a scientific definition of time

How is coordinate time related to proper time? Einstein’s special theory of relativity relates coordinate time and proper time by the following convention. For an observer with a clock in an inertial frame of reference, the coordinate time at the event is equal to proper time at the event when measured by a clock that is stationary relative to the observer and at the same location as the event. This convention assumes synchronization of the clock at the event with the observer’s clock. Unfortunately, there have been numerous methods suggested for accurately synchronizing clocks and defining synchronization. For our purposes in defining coordinate time and proper time, it is only important to assume the hands of both clocks move in unison, independent of the method of synchronization.
Most of the scientific community agrees that the most accurate definition of time requires integration with the three normal spatial dimensions (i.e., height, width, and length). Therefore, the scientific community talks in terms of spacetime, especially in the context of relativity, where the event or observer may be moving near the speed of light relative to each another.

Let us consider an example. A clock moving close to the speed of light will appear to run slower to an observer at rest (one frame of reference) relative to the moving clock (another frame of reference). In simple terms, time is not an absolute, but is dependent on the relative motion of the event and observer. It may sound like science fiction that a clock moving at high velocity runs slower, but it is a widely verified science fact. Even the clock on a jet plane flying over an airport will run slightly slower than the clock at rest in the airport terminal. Einstein predicted this time dilation effect in his special theory of relativity, and he derived an equation to calculate the time difference.

Other physical factors affect time. For example, another scientific fact is that a clock in a strong gravitational field will run slower than a clock in a weak gravitational field. Einstein predicted this time dilation effect in his general theory of relativity.
Lastly, the time dilation effects of high velocity and strong gravitational fields are additive. That means a clock moving near the speed of light will move slower if it enters a strong gravitational field.
I termed this section “Approaching a scientific definition of time” for a reason. There is no consensus on the scientific definition of time. However, we can help ourselves conceptualize time by summarizing the salient points discussed above:

  • In our everyday existence, time appears to be an absolute. Time is the same for everyone. When an event occurs, we believe it to occur simultaneously regardless of its relative position or velocity to us, or our relative position or velocity to the event. This is our everyday reality. We typically do not worry about accurately synchronizing our watches, since a minute one way or the other does not matter for most real-life applications. The simple fact is that treating time as an absolute works in most real-life applications. However, it is an illusion and breaks down as the relative velocity of any constituent approaches the speed of light, or when the distances from the event become extremely large and different for the observers. On this last point, regarding relative distances, consider the following example. An observer close to an event will record its occurrence a thousand years sooner than an observer a thousand light-years from the event. The reason for this is that it takes the light a thousand years to reach the furthest observer. This means that the stars we view at night may no longer exist. The light has traveled thousands to millions of light-years to reach our eyes. The stars may have died long ago, but we will not know it for thousands to millions of years.
  • In the world of relativity, where frames of reference can move near the speed of light or gravitational fields can play a factor, time becomes relative. Here are four examples.

1. A clock moving near the speed of light will appear to run slower to an observer at rest, relative to the clock.

2. A clock in a strong gravitational field will appear to run slower to an observer a distance (as little as one meter) farther away from the gravitational field.

3. A clock will run even slower when moving near the speed of light when it enters a strong gravitational field (i.e., the affects are additive).

4. An event will appear to occur first to the observer closest to the event. The farther away an observer is from an event, the longer it will take the light to travel to the observer, and for the observer to become aware of the event.

Notice that all our attempts to define time fail. Instead, we describe how time behaves during an interval, a change in time. We are unable to point to an entity and say “that is time.” The reason for this is that time is not a single entity, but scientifically an interval. We cannot slice time down to a shadow-like sliver, a dimensionless interval. In fact, scientifically speaking, the smallest interval of time that science can theoretically define, based on the fundamental invariant aspects of the universe, is Planck time.

Planck time is the smallest interval of time that science is able to define. The theoretical formulation of Planck time comes from dimensional analysis, which studies units of measurement, physical constants, and the relationship between units of measurement and physical constants. In simpler terms, one Planck interval is approximately equal to 10^-44 seconds (i.e., 1 divided by 1 with 44 zeros after it). As far as the science community is concerned, there is a consensus that we would not be able to measure anything smaller than a Planck interval. In fact, the smallest interval science is able to measure as of this writing is trillions of times larger than a Planck interval. It is also widely believed that we would not be able to measure a change smaller than a Planck interval. From this standpoint, we can assert that time is only reducible to an interval, not a dimensionless sliver, and that interval is the Planck interval. Therefore, our scientific definition of time forces us to acknowledge that time is only definable as an interval, the Planck interval.