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100 years of Relativity                                                                                     Print this essay

Posted at: Apr/28/2015 : Posted by: mel

Related Category: Historical Insights, Science,

Way back when I was in high school I was introduced to physics for the first time. I loved the class, but in retrospect believe I must have tormented my teacher. While he was teaching about blocks sliding on an inclined plain, I was interrupting with questions about surface tension on air foil surfaces. When the lecture moved to centripetal force, I started asking about relativity. The teacher grabbed a book off a shelf in the classroom titled “Relativity and Common Sense” (Hermann Bondi 1964) and told me to figure it out and tell the class. It has been over 40 years and I never did return the book. Driven by a love of physics I still open the book periodically. It has been 100 years since Albert Einstein published his theory of General Relativity and I thought I would take another stab at explaining how it works and why it is so important.

Isaac Newton is widely considered the father of most of the laws of motion we now collectively call physics. One of his most important laws pertains to how masses react to each other and the distances between them. When applied to our world these laws do a good job of explaining how things work. The motion of the moon, the forces on a bridge, how far a cannon ball will go all are easily quantified with Newton’s laws. Effectively, one of Isaac Newton’s laws says that any object with mass exerts an attractive force on any other object with mass; the bigger the masses, and the closer the two objects, the stronger the attraction. We call this “Newton’s Law of Universal Gravitation.”

While gravity was always there, we attribute Newton with its discovery and quantification. I have no idea if he was hit on the head with an apple or not, but it makes for a good children’s story. One of the greatest triumphs for Newton’s law of gravity came in the mid-1800s with the discovery of the planet Neptune. In 1846 precision telescopes were being used by scientist to make very exacting planetary motion observations. French mathematician Urbain Le Verrier crunched the numbers on his observations of the planet Uranus’ weird orbit concluding that there must be another massive body affecting its motion. Just a few months later German astronomers spotted Neptune lurking right where Newton’s laws predicted and exactly accounting for the subtle gyrations of Uranus.

Science moves forward on the intelligent observations and the questions inspired by new or anomalous data. Ironically, it was another orbital discrepancy observed by Le Verrier in 1859 that turned out to be the first dent in Newton’s gravitational theory. Le Verrier pointed out that the planet Mercury was arriving at its perihelion or closest orbital position to the sun a half-arcsecond behind schedule. An arcseond is 1/3600th of a degree. At the time it was believed that the behavior of Mercury could only be accounted for under Newton’s law of gravitation if there was another yet to be discovered planet in the vicinity. Due to its presumed close proximity to the sun, this conjectured planet was named Vulcan. Despite decades of searching, Vulcan was never found and the errant gravitational behavior of the planet Mercury persisted, thwarting contemporary Newtonian explanation.

In 1915 Albert Einstein stepped in with his brand-new theory of “General Relativity” that precisely accounted for the weirdness observed in Mercury’s orbital behavior. Einstein had labored for nearly a decade to formulate the mathematical relationships that are behind his revolutionary theory. According to General Relativity, gravitational attraction was due to the warping of the cosmos. Effectively, a massive object like the sun literally bends the three dimensional fabric of the universe around it taking small objects in the vicinity along for the ride.

The warping of space around large objects is best explained by referencing a mattress. When a collection of tennis balls and ping pong balls are placed on a mattress, there is very little impact of one ball on another. If something very massive, such as a bowling ball is placed in the middle of the mattress, the mattress warps affecting the position, rolling path, and behavior of the other balls. Since the balls are actually in motion in various orbits, this warping effect is actually referred to as warping “space-time.” As previously mentioned, the math to explain this warping took Einstein nearly a decade to generate so there was nothing simplistic about the general relativity relationships mathematically.

When Einstein unveiled his general theory of relativity, he wasn’t exactly met with applause. Almost no one else could or would accept his abstract ideas. Still worse, there was no direct experiment available at the time to back up his revolutionary theory. Further, space was considered a void and therefore the notion of warping a void was contradictory to perceived common sense. In 1919 Arthur Eddington and his collaborators made very precise measurement during a solar eclipse. If Einstein’s theory was correct, during the eclipse, light from a distant star passing near the alignment of our suns mass would warp and show a subtle, but measurable apparent position shift for the star. Eddington’s measurements on May 29 performed at Sobral Brazil, and Principe (an island off the West African coast) did show a quantitative shift in the star's perceived position during the eclipse event and became the first validation of general relativity.

The news of this discovery made headlines worldwide, with the Nov. 7 London Times proclaiming: “Revolution in Science/New Theory of the Universe/Newtonian Ideas Overthrown.” Scientists such as Einstein are renowned for accumulating recognition in journals and text books. That Einstein became noted in the regular press and rapidly evolved to a household name is remarkable for a physicist of any era.

With the passage of time, other experiments and the advancing of technology have continued to validate Einstein’s general relativity. In another validation experiment, NASA launched its Gravity Probe-A rocket in 1976. Researchers looked for a change in the frequency of waves — with shorter wavelengths meaning a higher frequency, and vice versa — in a type of onboard atomic clock driven laser signal compared to a similar clock on earth. At a peak altitude of 6,200 miles, the clock aboard Gravity Probe-A ran ever so slightly faster than its brother on the ground. The difference between the two clocks was a mere 70 parts per million, but matched Einstein’s math with unprecedented precision being warped by the mass of the earth.

Using even more advanced technologies scientists at the National Institute of Standards and Technology in 2010 went even further, showing that at just 1 foot higher in elevation, a clock ticks four-hundred-quadrillionths faster per second. The more interesting takeaway from this experiment is that your head ages ever so slightly faster than your feet as space-time is warped around you.

As shown, since it was proposed, Einstein’s theory has continued to pass ever more stringent tests. It remains our best explanation of the phenomenon of gravity. The theory bears out all sorts of wild predictions, the bulk of which boil down to this: Gravity behaves the same for all observers, resulting from curving “space-time,” the fabric of the universe. This curving or warping of space by large gravitational masses has since been used to explain many stranger effects including cosmic ripples and the apparent behavior of black holes.

One of the key components to general relativity is the equivalence principle. This principle states that bodies “fall” at the same rate through a gravitational field regardless of their mass. Therefore, the laws of nature must be the same everywhere and throughout time, stretching all the way back to the Big Bang.

The first test of the equivalence principle came in 1589 when Italian astronomer Galileo Galilei is storied to have released two balls from atop the Leaning Tower of Pisa. The balls, though made of different materials, met little air resistance and landed at the same time. In 1971, a more distinct demonstration took place on the moon. During the Apollo 15 mission, astronaut Dave Scott simultaneously let go of a hammer and a feather. In the airless lunar environment, the objects fell together and struck the lunar surface simultaneously, mirroring Galileo’s experiment and demonstrating that gravity acted uniformly on them despite their overt differences.

Time has a way of expanding our view of the world and thereby challenging what we thought we knew. Newton’s theory of universal gravitation was not wrong, but began to break down with large distances and extreme masses proving it to be an incomplete solution. After a century of testing by physicists and the abuses of Moore’s Law, Einstein’s theory of general relativity continues to remain pertinent. With our expanding vision of the universe using radio telescopes and super colliders, the tests for general relativity are not over. As scientists explore the behaviors in the realm of monstrous gravity wells and on the event horizons of black holes, general relativity will again be tested. Maybe, much like Newton, in these extreme environments general relativity may well be insufficient and another theory will yet be needed…maybe not.

It is difficult to know what inspired Einstein’s unique insight into the inner workings of the universe. Whether he truly possessed a unique brain, or merely had a unique way of looking at the world is unclear. Einstein’s work, while highly disputed when first published has helped to expand and inspire our understanding of the world and the universe around us for 100 year.

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