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Australian writer and physicist Paul Davis has called time Einstein’s unfinished revolution. Einstein was first to introduce the concept of slowing of time with motion and in gravity. He was also a proponent of block universe view of time in which past present and future all coexist together laid out as a dimension on a time line. The Greek philosopher Aristotle had speculated that time may be related to motion; he however added that motion could be slower or faster but not time.

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Aristotle did not have the privilege of knowing about Einstein’s relativity in which time also becomes amenable to change. Similarly when Einstein published his theory of General Relativity and proposed the revolutionary idea relating time to curvature in space he did not know that the universe was expanding. This discovery by astronomer Edwin Hubble came thirteen years later. One of the most dramatic aspects of the universe is that it is expanding and the presence of motion, forces and curved space-time happens in the expanding space.

The two main concepts of time are presentism and the block universe view of time. In presentism view of time only present is real while past and future do not exist and passage of time is just an illusion. Presentism may appear more intuitive however the block universe view is more popular among physicists and is supported by heavy weights in physics like Einstein. In block universe view time is laid out as a time-scape. This is based on theory of special relativity and an interpretation of Lorentz transformation equations called the Rietdijk– Putnam argument. Physicist Roger Penrose advanced another variation of this argument called the Andromeda paradox.

These paradoxes and anomalies arise from lack of simultaneity due to Einstein’s postulate of constancy of the speed of light in theory of special relativity. There is no free will in the block universe of time. All times in past and future already exist. There is however no explanation of lack of visitors from a futuristic technologically advanced civilization. Block universe also cannot explain what causes us to move through time towards the future and why we cannot willfully move back and forth through this time-scape. Neither of the two views on time can explain what time really is and what the cause of time is.

Slowing of time, changes of mass with high velocity, and length contraction are the commonly discussed predictions of special relativity. Theory of relativity is backed by large experimental data in particle accelerators. Length contraction and increase in mass with increase in velocity are reversible changes which normalize when moving object is brought to rest. However time dilation effects lead to permanent deficit in the time of the moving object. Lorentz transformation equations also lead to very interesting predictions based on changes in line of simultaneity the so called Rietdijk–Putnam argument or the Andromeda paradox.

Length contraction as well as time dilation is explained by rotation in Minkowski’s space however time dilation is a permanent effect that is measurable afterwards leading to the so called twin paradox. The changes in line of simultaneity for a moving object with its consequences the Andromeda paradox is considered a strong argument for the block universe view of time.

Twin Paradox cannot be explained by acceleration is well known although this argument is still peddled by some relativity books and articles. The best explanation seems to be shift in line of simultaneity of the twins based on Lorentz transformation and Minkowski’s space-time diagrams. However the length contraction which also is similarly explained is not permanent while time dilation is. Another important point to keep in mind is that if time dilation is not caused by accelerated motion then why are we using acceleration in gravity to explain time dilation in gravity. Time dilation in gravity is also a permanent change that can be measured even after clocks ticking at different heights are brought together. The time dilation and changes in mass with motion are precisely tested predictions of special relativity and any explanation which denies these changes cannot be correct. As a consequence of SR time is laid out as a block universe.

John Ellis McTaggart (3 September 1866 – 18 January 1925) was a fellow and lecturer in philosophy at Trinity College, Cambridge UK. He is best known today for his work “The Unreality of Time” (1908). McTaggart’s expressed the view that the way we perceive time is an illusion. He described events as A and B series in time. The A series consisted of events in future present and past moving along the time line toward the past a constantly changing position. This series basically is the so called flow of time.

In his B series he talks of fixed relationship of events as earlier and later. Death of a person is always later then their birth. The B series can also be called the arrow or direction of time. He argued that the changing A series is more essential to time than a fixed relationship of earlier and later therefore our perception of time is an illusion. His most interesting observation however may be that historical events have the same time characteristic as fictional stories. For example fictional events written by various authors, as well as past historical events have in them, the earlier and the later as well as the past the present and the future, thus suggesting that past really is more like memory of events and does not exist anymore than the imagination of a writer. The above point becomes clearer when we compare past present and future to events that are recorded on devices like tapes, CDs, DVDs, and computer hard drives.

Present is the most real perception of time however almost all of what we perceive as the present is already past. The present is a fleeting moment; whatever is happening now (present) is confined to an infinitesimally narrow point on the time line which is being encroached upon by what we think of as the past and the future. Present may be mental awareness of the recording of memory into our brain. A person can go to an event but fall asleep and miss the event completely, so that event basically does not exist in his past. Unless we are consciously aware of an event it does not enter our past memory. Events which we do not directly perceive are part of other people’s memories or other recording devices like books videos. Weathering effect on aging buildings or rocks is another form of natural recording.

Present resembles the sharp point of a recording laser or needle, unlike the present, the past and future are measurable durations of time. Past historical events are in our memory just like recorded material is on tape, CD or DVD. This similarity suggests that past is just like a record in memory, while future is like an unrecorded tape. Future may just be a projection created by our mind from our past experiences stored in our memory. The fact that the most real feeling of time “the present” is not measurable while the past and the future which are not accessible can be measured strongly suggests that the way we perceive time is an illusion.

Present cannot be measured on time-line as it has no duration although it gives us the real feeling of time. We can measure passage of time through the present when we use a stopwatch. This may appear similar to measuring distance (space) when walking or in a moving vehicle. Similarity however ends there as we can willfully move back and forth in space but not in time. So what is similar in measuring passage of time or passage of space? I think the answer is that in both cases we are measuring motion. When we use a tape to measure a distance we are comparing two distances directly. When we measure passage of a distance with a stopwatch we are comparing an unknown motion (velocity) with a standard motion of the clock. In measuring time we are possibly also dealing with some standard of motion as we will see below.

Measurement of time in the past was related to the celestial motions. Morning, afternoon, evening, and night are related to the presence or absence of the sun in the heavens. The measurements of years, months, weeks, are based on earth’s yearly orbit around the sun. The use of units like seconds and minutes which are radial angle measurements in geometry points toward the original connection of time measurements to radial motion of astronomical objects across the sky.

When we measure the speed of a car, we are comparing its motion to the motion of the hands of the clock and indirectly to the fractional motion of sun across the sky. We are comparing a known motion (of the sun) with an unknown motion of the car. The most accurate clocks of NIST use vibration of cesium atoms as a measure of time. All these different ways to measure time suggest that time may be related to motion.

In those moments of stillness when there is no apparent motion we know that there is inexorable passage of time in the aging process and at molecular-atomic-quark level. Passage of time is also evident in the motion of photons which are giving our eyes the snapshot of the moment.

In an article in Scientific American Craig Callender (physicist) compares time with a unit of motion. Time is a way to compare or describe different kinds of motions like speed of light, how fast heart beats or how frequently earth spins around its axis. But these processes could be compared directly without making reference to time. Time may just be common currency or unit of motion against which all other motions are measured making the world easier to describe but having no independent existence. Measuring processes (of motion) with time is like using money rather than barter transactions. This again suggests that underlying mechanism of time is just presence of motion.

Time however is not just motion; forces are also part of time. This is a very important point as this aspect of time may lead to understanding of the arrow of time. Imagine two objects one moving in an orbit around the other. Now suppose from our distant observation point of a fixed time we observe time to get slower in the area where these two objects are moving. We expect to see slower motion. We also should observe proportionally weaker gravitational force, otherwise the objects will get pulled together.

If we observed faster time, we expect to see faster motion and stronger gravity to keep the objects from flying apart. While with zero time motion will freeze and gravity will become zero. The increase or decrease in strength of gravity in this thought experiment is only in relation to our fixed time from where we are making the observation. From the point of view (time) of the orbiting objects neither motion nor gravity has changed. As this thought experiment also can be extended to particles held together by electromagnetic forces we can say that time involves both motion and forces.