One hundred years of Einstein�s theory of general relativity

This year marks an important milestone in physics: 100 years of the most elegant scientific theory ever created – Albert Einstein’s theory of general relativity. First published in November 1915, general relativity was Einstein’s tour de force. The theory gave us an extraordinary new insight into the nature of the Universe, particularly on the complex interplay between gravity, space and time.

General relativity is intimidatingly difficult to comprehend. Nevertheless, many of its basic ideas can be understood without digging into its complexity by going back to 1687, the year Isaac Newton presented the concept of gravity. According to Newton, gravity is a force that causes every mass to attract every other mass, regardless of how far away it is.

Newtonian gravity is a beautiful synthesis between terrestrial and celestial phenomenon reaching across the vast expanse of the Universe. It works with clockwork precession for slow moving, low density masses, such as the planets and their moons. For example, the gravitational force exerted by the Sun on the planets keep them on their elliptic orbits.

However, there are many phenomena Newtonian gravity cannot explain satisfactorily. One of them is the mystery of “action-at-a-distance,” a concept involving two bodies interacting with each other without being in physical contact. Specifically, how does a planet feel the Sun’s attraction and know to orbit it? To Newton, it was inconceivable how inanimate matter could affect other matter without the mediation of something else which is not matter.

Another major shortcoming of Newtonian gravity is its failure to account accurately for motion of enormously massive objects. It also cannot explain gravitational lensing _ the observed bending of light as it passes the Sun. More importantly, Newtonian gravity failed to account for the discrepancy, albeit ever so small, between the calculated results and experimental observation of the shift in Mercury's orbit.

Theory of general relativity was Einstein’s answer to the deficiencies of Newtonian gravity. Einstein didn't believe that gravity was a force at all. His depiction of gravity was stunningly different from the orthodox view that had prevailed since the time of Newton.

According to Einstein, gravity is a manifestation of the curving of spacetime _ time and three-dimensional space fused in a four-dimensional continuum _ by massive objects. Space becomes curved and time slows down in the presence of matter. The more the mass, the greater is the curving, stronger is the gravity and slower the clocks will tick. And everything, including light, is affected by the curving of spacetime.  

Thus, the falling apple that inspired Newton to postulate the law of gravity fell to Earth not because there was a mutual force of gravitational attraction. It fell into the deep hollow in spacetime caused by the mass of the Earth.

General relativity calls for a new geometry: geometry of curved space and time as well. We can get a glimpse of this curving by placing a heavy ball on the middle of a trampoline. The more massive the ball, the greater it curves the surface. A marble rolled across the trampoline, but far away from the ball, will roll in a relatively straight line path, whereas a marble rolled near the ball will curve as it rolls across the indented surface. The planets similarly orbit the Sun in the space warped by the Sun.  

The above example clearly demonstrates that the curvature of spacetime creates attraction between all the pieces of matter in the Universe. Clearly, the mediation of gravitational attraction by the deformation of spacetime geometry resolves the ambiguity of action-at-a-distance. The distortions reach their limit in the case of a star that collapses into a black hole, where spacetime completely folds over itself. Only Einstein’s gravity reaches into this domain.

In physics, a theory is considered to be a piece of mathematics arising from purely abstract thinking until its predictions are experimentally verifiable. General relativity has passed all its observational tests with flying colors even though its effects on Earth and in the solar system—the places where we can most easily perform experiments—are very small.
For a simple terrestrial test, climb to the top of Mount Everest and stand on a weighing scale. You will weigh 0.30 percent less. Why? The spacetime near the top of Mount Everest, being farther away from the sea level, is less warped by the Earth. Hence, gravitational attraction on you is a tad weaker.

One of the predictions of general relativity – bending of light as it passes through the Sun’s gravitational field – was confirmed in 1919 by a team of astronomers led by the British Astrophysicist Arthur Eddington. They successfully measured the deflection of starlight due to the warping of space caused by the Sun during a solar eclipse. The experiment was done during an eclipse because otherwise the brightness of the Sun would make it difficult to see stars close to it.

A resounding seal of approval to general relativity was given by the American astronomer Edwin Hubble after his discovery in 1929 that the Universe is expanding. Solutions of the general relativity equations do indicate a dynamic cosmos.
It has also been possible to detect the predicted gravitational redshift  shift in the frequency of light towards smaller value due to distortion of space. Since the frequency—number of oscillations per second—of a light wave can be thought of as the steady number of ticks of a “clock,” gravitational redshift is an indirect evidence of the prediction that gravity causes time to slow down.

Furthermore, general relativity was able to precisely account for the observed anomalous changes in the orientation of Mercury’s orbit. In particular, Einstein showed that the anomaly is an unavoidable consequence of the warping of space around Mercury, which is large because of its proximity to the Sun.

Perhaps the most impressive ramification of curved spacetime can be experienced in cosmology, in which general relativity determines the temporal evolution of the Universe. When massive objects move, the curvature of spacetime must change to follow their new positions. This produces ripples in spacetime, called gravitational waves, which travel outward from the gravitational sources at the speed of light. It is believed that these yet to be discovered ripples from the earliest moments after the Big Bang and their carrier _ the graviton, is a "smoking gun" for the Universe's rapid expansion.

Today, general relativity theory is almost universally accepted as the most successful theory of gravitation, and its fundamental equations remained unchanged for a century. This does not necessarily mean that Newtonian gravity is wrong. When the curvature of spacetime is small, Einstein and Newton see eye to eye. What goes up must come down can be explained by both. As the gravitating masses increase, Einstein dethrones Newton and begins to rule the Universe. There are no indications that Einstein’s century-long reign is going to end soon.

Thanks to Einstein, we live in a Universe of curved spaces and altered time filled mostly with things we cannot see: dark energy, dark matter and black holes through which we can leave the Universe, never ever to return.

The writer is Professor of Physics at Fordham University, New York.
Photos: Google Image

One hundred years of Einstein�s theory of general relativity

This year marks an important milestone in physics: 100 years of the most elegant scientific theory ever created Albert Einsteins theory of general relativity. First published in November 1915, general relativity was Einsteins tour de force. The theory gave us an extraordinary new insight into the nature of the Universe, particularly on the complex interplay between gravity, space and time. General relativity is intimidatingly difficult to comprehend. Nevertheless, many of its basic ideas can be understood without digging into its complexity by going back to 1687, the year Isaac Newton presented the concept of gravity. According to Newton, gravity is a force that causes every mass to attract every other mass, regardless of how far away it is. Newtonian gravity is a beautiful synthesis between terrestrial and celestial phenomenon reaching across the vast expanse of the Universe. It works with clockwork precession for slow moving, low density masses, such as the planets and their moons. For example, the gravitational force exerted by the Sun on the planets keep them on their elliptic orbits. However, there are many phenomena Newtonian gravity cannot explain satisfactorily. One of them is the mystery of action-at-a-distance, a concept involving two bodies interacting with each other without being in physical contact. Specifically, how does a planet feel the Suns attraction and know to orbit it? To Newton, it was inconceivable how inanimate matter could affect other matter without the mediation of something else which is not matter. Another major shortcoming of Newtonian gravity is its failure to account accurately for motion of enormously massive objects. It also cannot explain gravitational lensing _ the observed bending of light as it passes the Sun. More importantly, Newtonian gravity failed to account for the discrepancy, albeit ever so small, between the calculated results and experimental observation of the shift in Mercurys orbit. Theory of general relativity was Einsteins answer to the deficiencies of Newtonian gravity. Einstein didnt believe that gravity was a force at all. His depiction of gravity was stunningly different from the orthodox view that had prevailed since the time of Newton. According to Einstein, gravity is a manifestation of the curving of spacetime _ time and three-dimensional space fused in a four-dimensional continuum _ by massive objects. Space becomes curved and time slows down in the presence of matter. The more the mass, the greater is the curving, stronger is the gravity and slower the clocks will tick. And everything, including light, is affected by the curving of spacetime. Thus, the falling apple that inspired Newton to postulate the law of gravity fell to Earth not because there was a mutual force of gravitational attraction. It fell into the deep hollow in spacetime caused by the mass of the Earth. General relativity calls for a new geometry: geometry of curved space and time as well. We can get a glimpse of this curving by placing a heavy ball on the middle of a trampoline. The more massive the ball, the greater it curves the surface. A marble rolled across the trampoline, but far away from the ball, will roll in a relatively straight line path, whereas a marble rolled near the ball will curve as it rolls across the indented surface. The planets similarly orbit the Sun in the space warped by the Sun. The above example clearly demonstrates that the curvature of spacetime creates attraction between all the pieces of matter in the Universe. Clearly, the mediation of gravitational attraction by the deformation of spacetime geometry resolves the ambiguity of action-at-a-distance. The distortions reach their limit in the case of a star that collapses into a black hole, where spacetime completely folds over itself. Only Einsteins gravity reaches into this domain. In physics, a theory is considered to be a piece of mathematics arising from purely abstract thinking until its predictions are experimentally verifiable. General relativity has passed all its observational tests with flying colors even though its effects on Earth and in the solar systemthe places where we can most easily perform experimentsare very small. For a simple terrestrial test, climb to the top of Mount Everest and stand on a weighing scale. You will weigh 0.30 percent less. Why? The spacetime near the top of Mount Everest, being farther away from the sea level, is less warped by the Earth. Hence, gravitational attraction on you is a tad weaker. One of the predictions of general relativity bending of light as it passes through the Suns gravitational field was confirmed in 1919 by a team of astronomers led by the British Astrophysicist Arthur Eddington. They successfully measured the deflection of starlight due to the warping of space caused by the Sun during a solar eclipse. The experiment was done during an eclipse because otherwise the brightness of the Sun would make it difficult to see stars close to it. A resounding seal of approval to general relativity was given by the American astronomer Edwin Hubble after his discovery in 1929 that the Universe is expanding. Solutions of the general relativity equations do indicate a dynamic cosmos. It has also been possible to detect the predicted gravitational redshift shift in the frequency of light towards smaller value due to distortion of space. Since the frequencynumber of oscillations per secondof a light wave can be thought of as the steady number of ticks of a clock, gravitational redshift is an indirect evidence of the prediction that gravity causes time to slow down. Furthermore, general relativity was able to precisely account for the observed anomalous changes in the orientation of Mercurys orbit. In particular, Einstein showed that the anomaly is an unavoidable consequence of the warping of space around Mercury, which is large because of its proximity to the Sun. Perhaps the most impressive ramification of curved spacetime can be experienced in cosmology, in which general relativity determines the temporal evolution of the Universe. When massive objects move, the curvature of spacetime must change to follow their new positions. This produces ripples in spacetime, called gravitational waves, which travel outward from the gravitational sources at the speed of light. It is believed that these yet to be discovered ripples from the earliest moments after the Big Bang and their carrier _ the graviton, is a smoking gun for the Universes rapid expansion. Today, general relativity theory is almost universally accepted as the most successful theory of gravitation, and its fundamental equations remained unchanged for a century. This does not necessarily mean that Newtonian gravity is wrong. When the curvature of spacetime is small, Einstein and Newton see eye to eye. What goes up must come down can be explained by both. As the gravitating masses increase, Einstein dethrones Newton and begins to rule the Universe. There are no indications that Einsteins century-long reign is going to end soon. Thanks to Einstein, we live in a Universe of curved spaces and altered time filled mostly with things we cannot see: dark energy, dark matter and black holes through which we can leave the Universe, never ever to return. The writer is Professor of Physics at Fordham University, New York. Photos: Google Image