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Tests of General Relativity |
Numerous tests of general relativity have been proposed and conducted during the decades since general relativity was first proposed. Several examples of such tests are listed below:
Orbit of Mercury
One of the recognized failures of Newtonian physics at the beginning of the
twentieth century was its inability to accurately predict the orbit of the
planet mercury. According to Newtonian gravitation and classical physics, the
orbit of mercury should be an ellipse whose major axis is fixed in space. But careful
measurements of mercury’s actual orbit showed that its major axis slowly
precessed forward with time. This orbital precession, also called the
perihelion shift of mercury, was measured in 1947 to be 42.56±0.94
arc-seconds/century. General relativity predicts a value of 43.03
century/century.
Deflection Of Light
According to classical physics, light is a wave traveling at the speed of light. Since there is no index of refraction in the vacuum of space, light waves should not be deflected by gravity. Or if photons of light are treated as particles traveling at the speed of light, because such particles have zero mass, classical physics again predicts that they will not be deflected by gravity.
But general relativity asserts that light rays must be bent by gravity. Consider, for example, a light ray beamed across an accelerating spaceship. From the point of view of an inertial frame, the beam must be a straight line. But from the point of view of the accelerating frame, the beam must be curved because the spaceship accelerates forward while the beam is crossing the spaceship. Since the principle of equivalence equates gravity with an accelerated frame in free space, a light ray in a gravitational field must behave the same as in an accelerating frame. Therefore, gravity must bend light.
Examples of this can be observed during an eclipse as the light from distant stars pass near the sun. Such light rays are deflected toward the sun, causing the stars to appear displaced outward from the sun. The closer the ray passes to the sun, the greater the deflection. If we let R = (observed deflection)/(predicted deflection), then experimental measurements performed in 1970 using radio waves from quasar 3C279 rendered a value R = 1.07±0.17.
Perhaps a more dramatic example of the deflection of light occurs in the observed gravitational lensing of distant galaxies. Many recent astronomical photographs of distant galaxies lined up with even more distant galaxies show this effect. As the light from the more distant galaxy passes by the intervening galaxy, the gravity of the intervening galaxy acts like a lens, focusing the rays back upon the earth. The result is similar to looking at an object under a poorly constructed magnifying glass or lens. The object looks enlarged and distorted. In the case of distant galaxies, they look like curved arcs circling to the side of the nearer galaxy. Photographs of these arcs demonstrate dramatically that gravity does indeed deflect light as predicted by general relativity.
Gravitational Redshift
According to classical physics, light is a wave emitted with a certain wavelength, frequency, and velocity. Since its velocity in a vacuum is a constant c, the frequency and wavelength should not change as it moves upward through a gravitational field.
But general relativity predicts that the frequency of light rays should decrease as they move to higher elevations – their wavelength should experience a redshift. Again, the principle of equivalence can be used to demonstrate this phenomenon.
Consider the case of a spaceship accelerating in free space viewed from an inertial frame. As the spaceship travels faster and faster, its length gets shorter and shorter. Therefore, the back end of the spaceship is always traveling faster than the front end. As a result, a clock in the back end will run slower than a clock in the front end. Using the principle of equivalence to equate this accelerating spaceship with one sitting in a gravitational field shows that time runs slower at the bottom of a gravitational field than at the top. Therefore, light rays emitted at a normal frequency from a low elevation will be observed to be vibrating slower than normal when viewed from a higher elevation. Since low frequency means long wavelength, the light emitted from a lower elevation will be shifted toward the red as it moves upward. Conversely, starlight viewed from the ground will exhibit a gravitational blueshift.
Experiments performed in 1959-1965 using the Mössbauer effect to accurately measure the gravitational redshift wavelengths of gamma rays demonstrated that the ratio R = (observed redshift)/(predicted redshift) = 0.9990±0.0076.
Gravitational Radiation
Newtonian gravitation predicts that two stars will orbit one another forever without losing any energy due to gravitational radiation. But general relativity predicts that gravitational information will travel outward at the speed of light in the form of gravitational waves. As energy is radiated outward from the gravitational system, the two stars will gradually spiral in together. Close binary pulsars have been observed in recent years whose orbits have been decaying at rates consistent with that of gravitational radiation predicted by general relativity.
Other sources of gravitational waves have been suggested, including supernova explosions and other cataclysmic events. Gravitational telescopes have even been built. But so far I know of no confirmed detection of gravitational waves passing through the vicinity of the earth. Nevertheless, I suspect it will only be a matter of time before gravitational waves are detected.
Black Holes
As far back as 1795 Pierre-Simon Laplace predicted that
according to Newtonian gravity and
But general relativity is quite clear on the subject. It predicts that if an object is sufficiently massive, it will produce a gravitational field so strong that nothing, not even light, can escape the object. The curvature of space is so great that a hole is created that seems to link our universe with something that looks mathematically like another universe. Because no light can escape from such a hole, it is called a black hole.
No one has ever sent a probe into a black hole to see where it goes. But nature has provided numerous examples of material falling into black holes. Super massive black holes, millions of times more massive than the sun, have been seen billions of light years away as they looked billions of years ago when galaxies were being created and matter was accumulating at their centers. Other, smaller black holes have been seen within our own galaxy in recent times when the upper atmosphere of an expanding star is stripped off and falls into a companion black hole. As the matter approaches the black hole, it forms an accretion disk that spirals inward at ever-increasing speeds until it approaches the event horizon of the black hole.
This event horizon is the point of no return. Anything that crosses it can never get back to our universe. In fact no event that occurs inside the event horizon can be observed from our universe. No information inside the event horizon can ever escape the black hole. In essence, the black hole has warped space so much it has cut itself off from the rest of the universe. Things can fall in, but nothing can get back out.
Although the black hole itself cannot be seen, the high-energy accretion disk of infalling material can be and has been seen. The experimental observation of such accretion disks confirms the existence of black holes as predicted by general relativity.
Cosmological Constant
Other tests of general relativity have been proposed and performed. Most of these tests confirm its predictions to the limits of observational accuracy. But some experiments suggest that general relativity may need to be modified. In fact, Einstein himself modified his basic theory after applying it to the cosmos and discovering that his simple cosmological model required the universe to begin as a big bang, expand outward to a maximum size, and then contract to end with a big crunch. To keep the universe from expanding, he introduced a cosmological constant into his equation. Then when Hubble discovered that the universe really was expanding, he discarded the cosmological constant, calling its introduction the biggest mistake of his life.
But today there is evidence to suggest that the introduction of the cosmological constant was not a mistake. In order to be consistent with the recently discovered accelerated expansion rate of the universe, the cosmological constant apparently must again be inserted into Einstein’s gravitational field equation.
Modified Field Equation
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L = cosmological constant
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Therefore, the general theory of relativity is on extremely good footing as far as experimental confirmation is concerned. But the exact form of the theory that is truly applicable to the universe as a whole is still somewhat uncertain.
Questions:
Q1. Which of the following tests
of general relativity were available at the time Einstein introduced the
theory? (A) Orbit of Mercury (B) Gravitational Redshift (C) Gravitational
Radiation (D) Black Holes (E) Accelerating Universe.
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