Pondering

When first introduced by physicist Albert Einstein in ıeO5 and later expanded in ıeı5, the theories of relativity were controversial and not universally accepted. Since then, many scientific studies have confirmed the theories’ predictions. Today, relativity is central to our understanding of the Universe.

The nature of light and how we experience and measure it are fundamental to understanding the theory of relativity. This chapter describes light and buildsthisfoundation, which also providesimportant insightsinto the size of the Universe. It’s also important to appreciate that light is just a part of the larger electromagnetic spectrum, which includes, for example, radio waves and x-rays.

Before Einstein, many scientists believed there was an invisible structure that defined space called the aether. It was believed that, once found, the aether would serve as a common reference for measuring all motion. It was also believed that the aether,represented as a grid in thisillustration, wasthe medium through which light waves traveled. Today, we know that there is no aether defining space.

The speed of light is measured within a train car. This is done using a special (imaginary) stopwatch capable of measuring extremely short periods of time. The scientist measures the speed of the photon as it goes from the light bulb on the ceiling to the mirror on the floor and back to the ceiling. As expected, the photon is found to be traveling at speed “c.”

Hypothetical clocks that use the movement of light as a measure of time provide significant insight into how relativity works. In light-clocks, the movement of light inside the clock serves as the oscillator.

First how long doesit take the rocketship to travel from the Earth to the star as measured from the Earth / star frame-of-reference (the lower stopwatch)? This is easy just like figuring out how long it will take to drive somewhere when the distance and driving speed are known. For example to figure out how long it will take to drive ı20 miles at 60 miles per hour you would divide ı20 miles by 60 miles per hour to figure out that it would take 2 hours

Is the bucket spinning, or isthe Universe spinning around the bucket? Could a “spinning” bucket just as easily be considered to be stationary with the Universe spinning around it in the opposite direction? If it is the Universe spinning around the bucket, then to explain the centrifugal forces experienced, Einstein concluded that motions involving accelerations are diWerent from uniform motion and cannot be described by Special Relativity. He developed General Relativity to explain the centrifugal forces.

Adding dimensions affects geometry in unexpected ways. Going from two to three dimensions creates geometric anomalies that cannot be easily predicted or understood when only considering two dimensions. This is similar to going from our accustomed three-dimensional space to four-dimensional space-time.

Time and aging. Relative motion, acceleration and gravity affect the rate of time passage and can even affect how fast we age. This section describes the effects of relativity on aging using Einstein’s twin paradox. It illustrates how relativity affects the rate of aging.

Locations are defined by a spatial coordinate and a time coordinate. Time and space exist together, so locations and events must have a spatial component and a time component. This is shown here on a space-time diagram. We can control our movements through space, but not time

In spite of hislowly position as a patent clerk, Einstein published hisinitial proposals about Special Relativity in the respected German journal “Annalen der Physik” in ı905. These ideas elicited some discussion in Germany, but were all but ignored elsewhere.

The early stages of Universe evolution. The evolution of the Universe began with the Big Bang and evolved over the subsequent billions of years to create many forms of matter and energy, and ultimately self-replicating molecules that formed the basisfor early life