In 1905 Albert Einstein published his groundbreaking theory of special relativity. In order to come to an understanding of time, it is necessary to grasp the basic conclusions of the theory of relativity; as it created a fundamental link between space and time.

For our purposes, the two primary conclusions are:

1) The universe can be viewed as having three space dimensions — up/down, left/right, forward/backward — and one time dimension. This 4-dimensional space is referred to as the space-time continuum, or space-time.

2) Motion, and time, are relative to the observer. The theory of relativity explains how to interpret motion/time between reference frames; that is, places that are moving relative to each other.

For Einstein, the importance of reference frames was that there is no such thing as an absolute frame of reference – meaning that there is no place in the universe that is completely stationary. You may think that sitting on the couch in front the TV is stationary. Sorry – you are standing on a planet that is spinning at 1,000 miles per hour; moving around the sun at the rate of 66,600 miles per hour; in a solar system moving at 420,000 miles per hour around our galactic core. So, while we don’t really notice, our reference point in always changing in relation to other celestial bodies.

The relativity of motion and time become evident between two different reference frames.

Back on earth, there is a simple example: ( see youtube http://www.youtube.com/watch?v=wteiuxyqtoM )

As shown below at the top, imagine that you are traveling in a spaceship at one-half the speed of light. You are holding a laser so it shoots a beam of light straight up, striking a mirror you’ve placed on the ceiling. In your reference from, the light beam will then come straight back down and strikes a detector. In your reference frame, no matter what speed you are traveling, you observe the laser beam as absolutely vertical.

At the the top of the image, you see a beam of light go up, bounce off the mirror, and come straight down. Below, astronaut Sharon sees the beam travel along a diagonal path.

However, if astronaut Sharon were spying on you, as in the image, she observes a very different result.

Since you are traveling past Sharon, she sees your beam of light travel upward along a diagonal path, strike the mirror, and then travel downward along a diagonal path before striking the detector. In other words, you and Sharon have different reference frames and would see different paths for the light.

More importantly, according to Einstein, those paths aren’t even the same length. This means that the time the beam takes to go from the laser to the mirror to the detector must also be different for you and Sharon. (the speed of light is always constant)

This phenomenon is known as time dilation, but the critical point here is that two observers can witness the same event, yet perceive different durations of time. The conclusion is that time is not absolute and can vary from one observation to another.

Another example demonstrates the relativity of time: If you and Sharon synchronize your watches and she stays on the ground while you fly around the world a few times; upon landing, your watch and Sharon will no longer reflect the same time. Your watch will reflect a time earlier that Sharon (granted, in this example your watch will be a slower by a billionth of a billionth of a second). Times slows the faster you accelerate.

We will come back to this next time.