We all have a pretty good understanding of what time is. We are born, we live our lives, and we die over a span of time. We perceive time as moving at different rates. Time seems to fly as our children grow up. We wish we could slow time down to enjoy our children before they grow up and move away. In other situations, time moves too slow such as when we are waiting for an ambulance to arrive to care for a heart attack victim.

While time may seem to flow at different rates at different times, we know that time flows at a constant rate. This rate is visible in the moving of the second hand around a clock or in the steady marching off the seconds on a digital watch. Time is probably the most constant and expected attribute of our existence.

But, do we really understand time? Is it something we just take for granted, or it there more to it? Surprising still, there may be more than one form of time. In this article, we will journey through the world of time. We will look at how time was perceived in the past, how physics views time in the present, and the possibility that time may exist in multiple forms.

__Historical Development of Time__

Since ancient times, mankind was more interested in how to measure the passage of time than the nature of time itself. In its most primitive form, time was measured as the passage of the sun from east to west across the sky. All life on Earth reacts to this daily cycle of day and night.

As mankind evolved from hunter-gatherers to farmers, there developed a need to know when to plant the crops. Early astronomers noticed the Sun followed a different path each day. The Sun rose and set at different points on the horizon each day. The point of sunrise moved steadily from north to south until it reached a specific point on the horizon. Then, the Sun reversed direction and moved south to north until it reached another specific point.

This cyclic movement of the Sun corresponded to the seasons of the year. In the northern hemisphere, when the Sun reached its most northern point, farmers knew it was time to plant their crops. Counting the number of days it took for the Sun to complete one cycle south and back north led to the development of the 365 day year.

In a similar manner, observations of the Moon led to another cycle from one full moon to another full moon. This helped to predict the tides and became the basis of what we now call months. These concepts of days, months, and years eventually caused ancient people to developed calendars. Some calendars, such as the Mayan calendar, had a year zero based on the creation of the world. These calendars allowed for the recording of events on specific historical dates.

Measuring time within a day took longer to develop. The Sundial provided an early attempt to measure time during the day. However, sundials only worked during the day and only when the Sun was shining. The ancient Egyptians developed a water clock that could measure the passage of time both day and night.

By the 11^{th} Century, the Chinese were building mechanical clocks with an escapement mechanism. In 1657, the first pendulum clock was patented by Christian Huygens. The chronometer, which allowed ships at sea to accurately measure the passage of time, was invented by John Harrison in 1761. By the 19^{th} Century, pocket watches were in common use. World War I led to the use of wristwatches so that soldiers could keep their hands free while checking the time. In the 1960’s, the Japanese discovered how to use vibrating crystals to track time leading to very accurate clocks.

Today, atomic clocks use the properties of the Cesium atom to track time to within seconds over millions of years. Time is now one of seven fundamental physics quantities defined by the International System of Units (SI). The “second” has been defined by SI as 9,192,631,770 cycles of an electron moving between two energy levels of the ground state of the ^{133}Cs (Cesium) atom.

While the measurement of time has become more precise over mankind’s history, the nature of time took longer to develop. Historically, there were two competing philosophies. Gottfried Leibnitz in the 17^{th} Century and Immanuel Kant in the 18^{th} century argued that time was merely an invention of the human mind. It is neither an event or a thing. Time cannot be measured and cannot be travelled.

Isaac Newton, in the early 17^{th} century held an opposite view. According to Newton, time was part of the fundamental structure of the universe. It was a dimension independent of events and along which events occur in sequence. Time figured prominently in Newton’s three Laws of Motion.

In the 19^{th} Century, a series of scientists, concluding with Ludwig Boltzmann, developed the idea of entropy. Entropy is spontaneous changes that occur in our world. For example, it you pour cream into a cup of black coffee, the cream will spontaneously disperse throughout the coffee. Also, the hot coffee will spontaneously cool over time.

Entropy defines something called the Arrow of Time. The Arrow of Time is the name we give to the one-way nature of time. Our universe can only move from low states of entropy to high states of entropy. There is no way we can go backwards in time just like a cold cup of coffee will not spontaneously get hot and cream will not spontaneously separate itself from the black coffee.

__Our Current Understanding of Time__

Today, we have concepts of time from both the microscopic world of quantum mechanics and the macroscopic world of Einstein’s Theory of Relativity. According to quantum mechanics, time is not continuous. Rather, there exists a smallest unit (or quantum) of time based on the Planck Scale. The smallest interval of time, called Planck time, is 5.39 x 10^{-44} s.

Quantum mechanics also recognizes three basic symmetries of particles – charge, parity, and time. Charge involves reversing the charge of an elementary particle to create antimatter. For example, an electron has a negative charge. Its antimatter particle is the positron with a positive charge. Parity involves reversing the spin of a particle (e.g. left-handed spin to a right-handed spin). Time involves reversing the Arrow of Time. Supposedly, time-reversal involves detecting a particle before it is produced in a particle accelerator. However, this form of detecting a time-reversed particle has never occurred.

Theoretically, if you reverse all three basic symmetries of particles, you would create a universe with the exact same properties as our own universe. This universe would follow an opposite Arrow of Time. However, current science has no way of understanding how this reversed universe would exist.

Einstein, and his Theory of Relativity, has a different twist on the concept of time. In his General Theory of Relativity, Einstein argued that time was an integral part of our universe. His “spacetime” model consists of the three spatial dimensions of length, width, and height. However, spacetime also includes the temporal dimension of time to make our universe a four-dimensional system. As a temporal dimension, objects move and change state over time, but time itself has no velocity.

Incorporating time into his gravitational computations, Einstein was able to generate a significantly better calculation of the precession of the perihelion of Mercury’s orbit. The previous best calculation was based on Isaac Newton’s Universal Gravitation formula which presupposes a three-dimensional universe without a time dimension.

The most controversial aspect of Einstein’s view of time, however, is his concept of time dilation. Time dilation was first introduced in Einstein’s Special Theory of Relativity. The concept was expanded to gravitational time dilation in his General Theory of Relativity.

In developing his Special Theory of Relativity, Einstein made the following assumptions:

- The Principle of Relativity,
- The Principle of Invariant Light Speed,

Our universe has no concept of standing still. Everything exists in its own Inertial Frame of Reference and is in relative motion to everything else. The Principle of Relativity states that every experiment will yield the exact same result in every Inertial Frame of Reference. Also, you cannot perform an experiment to determine in which Inertial Frame of Reference you exist.

The Principle of Invariant Light Speed states that photons move in the vacuum of empty space at a constant speed. That constant speed is the universal constant, *c*, which the International System of Units (SI) defined in 1983 to be 299,792,458 m/s. As an aside, the SI redefined the length of the meter at this same time to make the value of *c* an exact measurement.

Einstein was obsessed with the fact that the universe must have some universal constant to make up for the fact that nothing stood still. Einstein assumed that *c* was that universal constant and that photons provided a constant value upon which everything else could be measured. It must be noted that the Principle of Invariant Light Speed is only an assumption and cannot be proven.

One consequence of assuming both the Principle of Relativity and the Principle of Invariant Light Speed is time dilation. Time dilation is the concept that moving clocks run slower. Einstein expresses the idea in a thought experiment called the Travelling Twins Paradox. Supposedly, if one twin went off travelling at high speed through space and the other twin stayed on Earth, then the travelling twin would return to Earth much younger than his twin that stayed behind.

Physicists of today absolutely believe in time dilation. However, the general population has their doubts. It is true that GPS satellites must compensate for time dilation to synch up with clocks in other satellites and ground-based GPS systems. Also, every time dilation experiment ever performed with atomic clocks has confirmed that moving clocks do run slower than stationary clocks.

Clocks, however, are based on a technology that mimics the assumptions made by Einstein. Every clock has an oscillator that mimics Einstein’s assumption of invariant light speed. Clocks maintain this oscillation no matter how fast the clock moves relative to other clocks (i.e. Principle of Relativity). The combined assumptions of Relativity and Invariant Light Speed does cause time dilation to occur. However, this does not prove that time dilation exists in nature. For that to be true, Einstein’s assumption of invariant light speed must be true in the universe.

__The Theory of Time__

There is a new theory being proposed that offers an alternative explanation of our universe that does not involve time dilation. This theory is called the Theory of Time, which makes the following assumptions:

- The Principle of Relativity
- The Principle of Non-Constant Light Speed
- The existence of a Spatial-Time Dimension in the Multiverse

The Principle of Relativity in the Theory of Time is the same as Einstein’s assumption in his Theory of Relativity. The assumption of non-constant light speed is based on two changes to Einstein’s Principle of Invariant Light Speed. First, the Principle of Invariant Light Speed argued that the speed of light was independent of its source and never changed. The Principle of Non-Constant Light Speed argues that the speed of light is dependent on the speed of the light source. The second change is that photons have a mass-equivalence that causes them to accelerate and decelerate in the presence of gravitational forces.

The revolutionary new idea in the Theory of Time is the existence of a Spatial-Time Dimension. Our universe still exists in a four-dimensional system consisting of three spatial dimensions and the temporal dimension of time. However, the Theory of Time postulates a fourth spatial dimension though which our universe is moving in the Multiverse. The Multiverse is a higher order space in which multiple universes are born, live out their lives, and die.

The Spatial-Time Dimension is still an assumption, but there are some serious facts unfolding from our space probes that suggest its existence. First, both the American WMAP probe and the European Planck probe have confirmed that our universe is based on Euclidean geometry. Einstein’s concept of spacetime is heavily dependent on our universe being non-Euclidean in nature.

If our universe is a three spatial dimension Euclidean system (we are excluding the temporal time dimension for now), then the Big-Bang must exist as a point in that system. However, the location of the Big-Bang is not a point in our sky. Rather, the Hubble Space Telescope has revealed through its Deep Field studies that the Big-Bang is at the farthest point in every direction of the sky. The only way this can happen in a Euclidean system is if there exists a fourth spatial dimension.

The Big-Bang is not a single point in the night sky because our three-dimensional universe has moved away from the point of the Big-Bang in the Multiverse. The concept of our universe moving within the Multiverse raises some possibilities. If we combine these four spatial dimensions with the temporal time dimension, we are living in a five-dimensional Multiverse.

Solids only exist in three-dimensions. This new fourth spatial dimension is only a dimension where objects move and change state over time. This definition should sound a lot like how I described the temporal time dimension within Einstein’s spacetime. The difference is that temporal time has no velocity. This new dimension, called the Spatial-Time Dimension, acts like time, but it has a velocity associated with it.

As we look at deep space objects the night sky, we are actually looking back along the Spatial-Time Dimension. For example, the Andromeda Galaxy is a mere 2.5 million light-years from Earth. We know this means we see the galaxy as it existed 2.5 million years ago. However, this also means we see where our moving universe existed 2.5 million years ago back along the Spatial-Time Dimension.

Einstein’s Mass-Energy Equivalence Equation, *E = mc ^{2}*, now has an interesting new interpretation. Current science does not give an explanation why the universal constant,

*c*, is in this equation. However, we do know that it governs how energy,

*E*, was turned into mass,

*m*, at the time of the Big-Bang. Any mathematician will tell you this equation is an acceleration formula where matter was accelerated to speed

*c*at the time of the Big-Bang.

Current scientific beliefs say this cannot happen because *c* is a limiting factor to the speed of mass. However, *c* is only a limiting factor if we believe Einstein’s assumption of the Principle of Invariant Light Speed and that we exist in only a three-dimensional Multiverse.

The Spatial-Time Dimension answers several perplexing issues in modern physics. For example, matter is always created in pairs – a particle of matter and a particle of antimatter. However, our universe consists mainly of matter. Where did the antimatter go?

Recall the earlier dilemma of the time-reversed particle. With our universe moving away from the Big-Bang at speed *c*, there is now room in the Multiverse for the Big-Bang to create two separate universes – our universe, and a second antimatter universe created with time-reversed particles. Both universes are moving along their own respective Arrows of Time in the Multiverse.

__Time in Multiple Forms__

The Arrow of Time is not determined by entropy alone, but mainly by the direction in which a specific universe is moving in the Multiverse. Twin universes are born together in a low-entropy state, and they move away from each other in opposite directions as they move to higher states of entropy.

Each universe evolves separately from its twin universe in its own Arrow of Time. Following the Principle of Relativity, there is no experiment we could perform that would indicate in which universe we exist. If we could see our twin universe, it would appear to be moving backwards in time. However, the two universes are moving away from each other at twice the speed of *c*. There is no way light from one universe would ever reach its twin universe. As a result, we can never see our twin universe.

Following this scenario farther, the Multiverse may continually be giving birth to twin universes. With each universe following its unique Arrow of Time, the Multiverse may be filled with multiple forms of time – each heading off in a different Arrow of Time.

With no gravitational forces opposing them, each universe will continue to move at speed *c* until each universe meets its ultimate demise. Einstein may be wrong in his assumption of the Principle of Invariant Light Speed. However, Einstein’s dream of *c* being a universal constant may be correct. Rather than *c* being the constant speed of light in the vacuum of empty space, *c* could very well be the universal constant for the speed of time.