About the misrepresentation of Albert Einstein's theory of gravity.
The way many physicists explain Albert Einstein's theory of gravity in TV and DVD documentaries, the way they write about it in books for the general public raises many questions and requires clarification.
In summary, according to these physicists Einstein stated that gravity is the warping of space - time by a massive object having considerable mass such as the sun. The planets orbit the sun because they have to follow the warping ( curvature ) of space-time around the sun. Other stars in the universe which have planets also warp space in their vicinity. By the same reasoning the moon and artificial satellites orbit the Earth, and many planets in the Solar system make their moons orbit them by warping the space-time around themselves with their great masses. Smaller objects such as asteroids generate very weak gravity because their masses are small and therefore warp the space-time around themselves very little. In fact some physicists even say that according to Einstein gravity was not even a force like Isaac Newton claimed it was. To demonstrate this, these physicists stretch a table cloth or a nylon sheet with gridlines, put a ball in the middle of the cloth / nylon stretch. The weight of the ball creates a dent in the strethed cloth. In this demonstration the stretched cloth represents the fabric of space-time, the ball in the middle represents the sun and the dent in the cloth is supposed to be the warping of space-time by the mass of the sun. Then the demonstrator rolls some smaller balls which rotate several times on the cloth around the ball placed in the middle. This is supposed to represent the orbiting planets following the curvature of space - time around the sun. And this is gravity. You can see this demonstration on many documentaries about Einstein and even in the BBC movie named Einstein & Eddington. In the movie, the famous British Astronomer Arthur Eddington does the demonstration with a tablecloth, a loaf of bread as the sun and an apple as an orbiting planet. As is known Arthur Eddington proved Einstein's theory that gravity bends light by photographing the shift in the visible position of the stars aligned behind the sun in the sky during a solar eclipse in 1920.
This interpretation of Einstein's General Theory of Relativity about Gravity raises many questions & objections in the mind of the careful analyst / thinker. Moreover, the demonstration of gravity with the stretched tablecloth representing space-time and the balls representing the sun and planets and the dent in the cloth representing the warp of space-time by a massive body and saying that according to Einstein this is what gravity is all about has a major flaw and is a poor analogy with the Solar System.
The objections / questions below are not to Einstein nor to his theory of gravity, but rather to the way it is explained by some physicists to the general public.
1) How can space-time be warped ? You can warp a malleable substance such as a sheet of aluminum or plastic or any flexible matter. But how can the sun or any massive object warp the space-time around it ? Space-time is not matter, it is emptiness, nothingness. How can nothingness be warped ? There is nothing to warp. Space is full of many particles at sub-atomic scale such as cosmic radiation, solar wind etc. But they are not space-time, they exist in space-time like bigger objects such as the stars and planets do. Space itself is emptiness, nothingness, it is not a substance. How can nothingness be bent or warped by a massive object ?
2) What makes the planets orbit the sun ? According to this interpretation of Einstein's theory of gravity planets orbit the sun because they have to follow the curvature in space-time created by the sun's mass around itself. OK then if gravity is not the force of attraction between objects having a mass like Isaac Newton said it was, what made the planets start to orbit around the sun in the curved fabric of space-time in the first place? Before the planets and the sun formed they were a giant cloud of gas rotating around its axis and held together with the force of gravity.
3) What about Newton's apple ? When Newton's apple falls from the tree onto the ground is it because the Earth's mass has warped the space-time around itself and the apple has to follow that curvature towards the Earth ? If gravity is not a force of attraction but merely the warping of space-time what pulls the apple towards the Earth ? What holds us humans on the surface of the Earth and prevents us from flying off into space if not the attractive force of gravity of the Earth's mass ?
4) Double standard about what gravity is . After having dismissed Newtonian Theory of gravity as proven wrong by Einstein, that it is not a force but a mere warping of space-time, the same physicists talk about gravity as a force elsewhere contradicting themselves. For example when they talk about the holy grail of physics, the quest for the Grand Unification Theory of Forces of Nature. That is a theory of everything that will hopefully someday unite the 4 fundamental forces ; nuclear strong force, nuclear weak force, electromagnetic force and the force of gravity under one unified theory. But while explaining Einstein's theory of gravity they had dismissed gravity from being a force. Now they say it is a force, the weakest of the 4 forces.
5) The table cloth demonstration is flawed In the table cloth and balls demonstration of the alleged Einstein's theory of gravity, what creates the dent in the middle of the stretched cloth ? It is the weight of the ball placed in the middle of the cloth. In other words the force of Earth's gravity pulls the ball down towards the center of the Earth. The ball's mass does not mysteriously warp the cloth under it. The sun is not pulled down in space-time like the ball on the table cloth is. Therefore the ball on the cloth can not be an anology for the sun in space-time.
6) Trying to reconcile unreconcilable interpretations Many times we hear / read that Einstein did not prove Newton's theory of gravity wrong, that they are both right in their own domains. To justify this it is said that satellite / space technology uses Newtonian theories of force, mass, acceleration and gravity to calculate rocket trajectories, satellite orbits etc. with great accuracy. And that this demonstrates that Newton's theory of gravity is still valid. On the other hand as Einstein's theory of gravity is explained gravity is dismissed from being a force ( force defined as an influence on an object to set the object into motion or change its motion ) and redefined as the warping of space-time by a massive object in it. Perhaps Newton's theory of Gravity can be reconciled with Einstein's but not with this kind of interpretation. Gravity is either a force or it is not. It can not both be a force and not be a force . At best one of the interpretations is true the other false. They are contradictory statements and therefore can not both be true.
7) Einstein's theory of gravity is not taught in middle and high schools
After almost 100 years after Einstein published his General Theory of Relativity on gravity, most middle school and high school science and physics lessons still only slightly mention if at all Einstein's theory of gravity. They devote the gravity subject almost entirely to Isaac Newton's theory of gravity. Even though Einstein's discovery that gravity is the warping of space - time has been widely accepted for almost a century it is amazing that schools around the world still stubbornly limit their lessons to Newton's theory.
Einstein most certainly was right in his theories including that of gravity and he replaced some misconceptions in Newtonian theories but the way these are explained by some physicists to the general public suffer from the flaws summarized above. Any physicist aiming to explain these to the general public should make sure that these points are dealt with effectively. Otherwise confusion on these subjects will continue.
Long post for what purpose? How would you describe the theory to a lay reader/viewer? It is easily to criticise but much harder to help the scientist to make a clear view.
Having studied relativity in university as a mathematician (albeit many years ago) I could easy describe Einstein's theory as a mathematical model based on geodesic description and measurement caused by the distortion of gravitational fields in space-time - but who would be the wiser?
The whole point of the rubber sheet is that it is an allegory for something that is very difficult to describe in simple terms and was used by my lecturers in university as an allegory. The other thing to remember is that the rubber sheet allegory is describing a hypothetical 2 dimensional version of space - not usual three dimensional space - the depression in the sheet caused by the presence of a heavy object illustrating the deviation from uniformity of the gravitational field. You could describe it in these terms but it all gets very verbose and pompous very quicky. The shorthand of 'mass warps spacetime' is a much cleaner description and much more easily undertood.
You are correct that space is NEARLY empty (it isn't a total vacuum) but from Einstein's point of view space is far from Empty - it is full of force fields caused by the presence of matter/energy and it is these force fields and their effects that relativity describes. The warping of space is a good desrciption and is easily understood. It is also the idea behind stuff like gravitational lensing used in astronomy.
To say that space is not warped is plainly wrong - you can't get much more warped than space when you enter a black hole. As far as we understand it space is so warped at singularities that it ceases to exist (and so does time) as a concept! Even Einstein himself uses the term 'curvature of space' in his book "Relativity - The Special and General Theory" appendix III (in my edition p. 127)