Pondering

When first introduced by physicist Albert Einstein in 1905 and later expanded in 1915 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 funda- mental to understanding the theory of relativity. This chapter describes light and builds this foundation, which also provides important insights into the size of the Universe.

Before Einstein, many scientists believed there was an invisible structure that defi ned 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 this illustration, was the medium through which light waves traveled. Today, we know that there is no aether defi ning 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 fl oor and back to the ceiling. As expected, the photon is found to be traveling at speed “c.”

A light-clock can illustrate time slowing on a moving object. This fi gure shows the use of a light-clock to illustrate the Special Relativity effects of a rocket ship moving rela- tive to Earth. The stopwatch and space compression graphics are used to remind us that time is slowed and space compressed (shorter light path) on moving platforms.

How high-speed travel affects the passage of time for the “moving” rocket ship. In this illustration the rocket ship is moving at &%! of the speed of light relative to the Earth and star. Because the Earth / star is the “stationary” frame-of-reference the elapsed time on the moving rocket ship is less as measured from the Earth

Is the bucket spinning, or is the 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 accel- erations are different 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-di- mensional space to four-dimensional space-time.

Einstein’s twin paradox. According to Einstein’s “twin paradox,” two twins are separated at birth. One twin is accelerated to near the speed of light and travels to a nearby star, turns around, and returns back to Earth. Einstein showed that the twin that traveled to the distant star would age less due to the accelerations and high-speed relative motion experi- enced

Einstein’s theories have profound implications about how we understand the Universe. This chapter continues the discussion of important areas where our understanding of the Universe has been shaped and changed by Einstein’s theories of relativity.

When Einstein first proposed his relativity hypotheses, they represented a very new way of looking at the Universe, and met with initial skepticism and even some resistance. At the time, it was difficult to verify his ideas experimentally and so they were not immediately accepted. Eventually, exper- iments were conducted and scientific evidence was developed that supported his ideas.

Einstein offered a new way of understanding the Universe that now stands as the foundation for much of physics. His work illustrates science at its best. It also raises questions, as he did, about the relation between science and religion. This chapter discusses these issues.