The mathematical formulas which make up this general theory are much more difficult than those which are concerned with special relativity. The general relativity theory changes the old ideas about gravitation that have dominated physics since the days of Isaac Newton. According to Newton, two bodies attract each other with a force depending upon their mass and their distance apart. The gravitational influence of a star is felt at the same moment throughout the entire universe, even though it decreases with the distance from the star.
But for electromagnetic waves, action spreads through space with great but perfectly definite velocity, that of light. Because of our knowledge of electromagnetic radiation, we tend to reject ideas that disturbances and actions that travel through space have infinite speed. We tend to believe that though they may travel at a very high speed, that speed is not limitless. Einstein illustrated the basic idea of general relativity with an imaginary experiment. Suppose a lift is at rest in space. If a ball is released within the lift, it will float in space and not fall.
If the lift accelerates upward, an observer within the lift will see the ball fall to the floor exactly as it would under the pull of gravity. The ball appears to fall because the floor of the lift–as seen from outside the lift–accelerates upward toward the ball. All the effects we associate with gravity would be seen by the observer in the lift. Einstein called the phenomenon shown in this experiment the Principle of Equivalence. This principle states that it makes no difference whether an object is acted on by a gravitational force or is in an accelerated frame of reference. The result in both cases will be the same.
From this principle, Einstein reasoned that matter in space distorts or “curves” the frame of reference of space. The result of this curvature is what we experience as gravity. Euclidian or “flat” geometry cannot describe curved space. Thus, Einstein used geometries called Riemannian geometries to describe the effects of gravitation. According to Newton’s theory, a planet moves around the sun because of the gravitational force exerted by the sun. According to the theory of general relativity, the planet chooses the shortest possible path throughout the four-dimensional world, which is deformed by the presence of the sun.
This may be compared to the fact that a ship or an aeroplane crossing the ocean follows the section of a circle, rather than a straight line, in order to travel the shortest route between two points. In the same way, a planet or light ray moves along the “shortest” line in its four-dimensional world. So far, three things have been discovered in which Einstein’s theory of general relativity receives experimental proof as opposed to the theories of Newton. These differences are not great, but are measurable.
In the first place, according to Newton’s theory, the planet Mercury moves in an ellipse about the sun. According to Einstein’s theory, Mercury moves along an ellipse, but at the same time the ellipse rotates very slowly in the direction of the planet’s motion. The ellipse will turn about forty-three seconds of an arc per century (a complete rotation contains 360 degrees of an arc and 360 X 60 X 60 seconds of an arc). This effect is rather small, but it has been observed. Mercury is nearest to the sun and the relativistic effect would be still smaller for other planets.
If we take a picture of part of the heavens during an eclipse of the sun and near the eclipsed sun, and then take another picture of the same part of the heavens a little later, the two photographs will not show identical positions for all the stars. This is so because, according to general relativity, a light ray sent by a star and passing near the rim of the sun is deflected from its original path because the sun’s gravity curves space. The effect of gravity on light is also the reason why black holes are invisible.
The gravitation in a black hole is so strong that light cannot escape from it. Physicists have known for more than a hundred years that when some elements are heated to incandescence they give off a pattern of spectral lines (coloured lines) which can be examined through a spectroscope. According to the Einstein theory, the wavelength of light emitted from a massive object will become longer because of gravitation. This results in a shift of the spectral lines towards the red end of the spectrum; this type of red shift is called gravitational red shift.
If we examine the spectral lines of an element on our earth with the spectral lines given off by the same element on the sun or on a star, the spectral lines of the element on the sun or star should be very slightly shifted toward the red end of the spectrum, compared with the spectral lines of the same element on our earth. Experiment has confirmed this shift. In 1960, two American physicists, R. V. Pound and G. A. Rebka, Jr. , detected the red shift resulting from the earth’s gravitational field.
They measured the effect of altitude on the frequency of gamma rays. Many scientists are doing research in general relativity and studying possible improvements on Einstein’s theory. For example, the general theory predicts the existence of waves that “carry” the force of gravity, just as electromagnetic waves carry light. Experimenters have not yet been able to detect these gravitational waves. Scientists are also trying to combine electromagnetic and gravitational forces in a theory called the unified field theory. Relativity and other ideas
The ideas of relativity form a framework which can embrace all laws of nature. Relativity has changed the whole philosophical and physical notions of space and time. It has influenced our views and speculation of the distant worlds and stars and of the tiny world of the atom. Some of this speculation is still going on. Does our universe, regarded as a whole, resemble a plane surface or a sphere? It is not possible to answer this question, because there are many different theories and much uncertainty about the distribution of matter in the universe.
All the theories try to describe the universe as a whole and are based upon the mathematical principles of general relativity. According to some theories, a light ray sent from an arbitrary point in space returns, after a very long time interval, to the point of departure, like a traveller in a journey around our earth. Thus, if you were to start from your home and travel into space along a straight line, you would eventually return to the point from which you started. According to other theories, however, a light ray or a traveller would continue an endless journey through space.
In spite of all these successes of the relativity theory, it is not right to say that Newtonian physics is wrong. Newtonian physics holds true if the velocities of the objects being studied are small compared with the velocity of light. Such objects are found every day in our own experience, and therefore classical physics can still be applied to our daily problems. Astronomers have found that Newton’s theory of gravitation still holds true in their calculations. But the relativity theory does limit the area to which the Newtonian physics can be successfully applied.