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Reference: Evolution of Physics

This paper presents Chapter III, section 12 from the book THE EVOLUTION OF PHYSICS by A. EINSTEIN and L. INFELD. The contents are from the original publication of this book by Simon and Schuster, New York (1942).

The paragraphs of the original material (in black) are accompanied by brief comments (in color) based on the present understanding.  Feedback on these comments is appreciated.

The heading below is linked to the original materials.

.

Geometry and Experiment

Our next example will be even more fantastic than the one with the falling lift. We have to approach a new problem; that of a connection between the general relativity theory and geometry. Let us begin with the description of a world in which only two-dimensional and not, as in ours, three-dimensional creatures live. The cinema has accustomed us to two-dimensional creatures acting on a two-dimensional screen. Now let us imagine that these shadow figures, that is, the actors on the screen, really do exist, that they have the power of thought, that they can create their own science, that for them a two-dimensional screen stands for geometrical space. These creatures are unable to imagine, in a concrete way, a three-dimensional space just as we are unable to imagine a world of four dimensions. They can deflect a straight line; they know what a circle is, but they are unable to construct a sphere, because this would mean forsaking their two-dimensional screen. We are in a similar position. We are able to deflect and curve lines and surfaces, but we can scarcely picture a deflected and curved three-dimensional space.

By living, thinking, and experimenting, our shadow figures could eventually master the knowledge of the two-dimensional Euclidean geometry. Thus, they could prove, for example, that the sum of the angles in a triangle is 180 degrees. They could construct two circles with a common centre, one very small, the other large. They would find that the ratio of the circumferences of two such circles is equal to the ratio of their radii, a result again characteristic of Euclidean geometry. If the screen were infinitely great, these shadow beings would find that once having started a journey straight ahead, they would never return to their point of departure.

Let us now imagine these two-dimensional creatures living in changed conditions. Let us imagine that someone from the outside, the “third dimension”, transfers them from the screen to the surface of a sphere with a very great radius. If these shadows are very small in relation to the whole surface, if they have no means of distant communication and cannot move very far, then they will not be aware of any change. The sum of angles in small triangles still amounts to 180 degrees. Two small circles with a common centre still show that the ratio of their radii and circumferences are equal. A journey along a straight line never leads them back to the starting-point.

But let these shadow beings, in the course of time, develop their theoretical and technical knowledge. Let them find means of communication which will enable them to cover large distances swiftly. They will then find that starting on a journey straight ahead, they ultimately return to their point of departure. “Straight ahead” means along the great circle of the sphere. They will also find that the ratio of two circles with a common centre is not equal to the ratio of the radii, if one of the radii is small and the other great.

If our two-dimensional creatures are conservative, if they have learned the Euclidean geometry for generations past when they could not travel far and when this geometry fitted the facts observed, they will certainly make every possible effort to hold on to it, despite the evidence of their measurements. They could try to make physics bear the burden of these discrepancies. They could seek some physical reasons, say temperature differences, deforming the lines and causing deviation from Euclidean geometry. But, sooner or later, they must find out that there is a much more logical and convincing way of describing these occurrences. They will eventually understand that their world is a finite one, with different geometrical principles from those they learned. They will understand that, in spite of their inability to imagine it, their world is the two-dimensional surface of a sphere. They will soon learn new principles of geometry, which though differing from the Euclidean can, nevertheless, be formulated in an equally consistent and logical way for their two-dimensional world. For the new generation brought up with a knowledge of the geometry of the sphere, the old Euclidean geometry will seem more complicated and artificial since it does not fit the facts observed.

The spherical surface of earth appears to be a planar surface to us, but the spherical surface becomes obvious as we are able to travel longer distances in a shorter time. We would have to adjust our thinking to the spherical geometry of earth for such large distances.

Let us return to the three-dimensional creatures of our world.

What is meant by the statement that our three-dimensional space has a Euclidean character? The meaning is that all logically proved statements of the Euclidean geometry can also be confirmed by actual experiment. We can, with the help of rigid bodies or light rays, construct objects corresponding to the idealized objects of Euclidean geometry. The edge of a ruler or a light ray corresponds to the line; the sum of the angles of a triangle built of thin rigid rods is 180 degrees; the ratio of the radii of two circles with a common centre constructed from thin unbendable wire is equal to that of their circumference. Interpreted in this way, the Euclidean geometry becomes a chapter of physics, though a very simple one.

But we can imagine that discrepancies have been discovered: for instance, that the sum of the angles of a large triangle constructed from rods, which for many reasons had to be regarded as rigid, is not 180 degrees. Since we are already used to the idea of the concrete representation of the objects of Euclidean geometry by rigid bodies, we should probably seek some physical force as the cause of such unexpected misbehaviour of our rods. We should try to find the physical nature of this force and its influence on other phenomena. To save the Euclidean geometry, we should accuse the objects of not being rigid, of not exactly corresponding to those of Euclidean geometry. We should try to find a better representation of bodies behaving in the way expected by Euclidean geometry. If, however, we should not succeed in combining Euclidean geometry and physics into a simple and consistent picture, we should have to give up the idea of our space being Euclidean and seek a more convincing picture of reality under more general assumptions about the geometrical character of our space.

When we construct rigid objects in actual three-dimensional space corresponding to the idealized objects of Euclidean geometry, then the principle of physics also apply to them. When we find that such constructed objects do not always exhibit the idealized properties expected of them, then we would have to give up the idea of our space being Euclidean and seek a more convincing picture of reality under more general assumptions about the geometrical character of our space.

The necessity for this can be illustrated by an idealized experiment showing that a really relativistic physics cannot be based upon Euclidean geometry. Our argument will imply results already learned about inertial CS and the special relativity theory.

Imagine a large disc with two circles with a common centre drawn on it, one very small, the other very large. The disc rotates quickly. The disc is rotating relative to an outside observer, and there is an inside observer on the disc. We further assume that the CS of the outside observer is an inertial one. The outside observer may draw, in his inertial CS, the same two circles, small and large, resting in his CS but coinciding with the circles on the rotating disc. Euclidean geometry is valid in his CS since it is inertial, so that he will find the ratio of the circumferences equal to that of the radii. But how about the observer on the disc? From the point of view of classical physics and also the special relativity theory, his CS is a forbidden one. But if we intend to find new forms for physical laws, valid in any CS, then we must treat the observer on the disc and the observer outside with equal seriousness. We, from the outside, are now watching the inside observer in his attempt to find, by measurement, the circumferences and radii on the rotating disc. He uses the same small measuring stick used by the outside observer. “The same” means either really the same, that is, handed by the outside observer to the inside, or, one of two sticks having the same length when at rest in a CS.

The inside observer on the disc begins measuring the radius and circumference of the small circle. His result must be the same as that of the outside observer. The axis on which the disc rotates passes through the centre. Those parts of the disc near the centre have very small velocities. If the circle is small enough, we can safely apply classical mechanics and ignore the special relativity theory. This means that the stick has the same length for the outside and inside observers, and the result of these two measurements will be the same for them both. Now the observer on the disc measures the radius of the large circle. Placed on the radius, the stick moves, for the outside observer. Such a stick, however, does not contract and will have the same length for both observers, since the direction of the motion is perpendicular to the stick. Thus three measurements are the same for both observers: two radii and the small circumference. But it is not so with the fourth measurement! The length of the large circumference will be different for the two observers. The stick placed on the circumference in the direction of the motion will now appear contracted to the outside observer, compared to his resting stick. The velocity is much greater than that of the inner circle, and this contraction should be taken into account. If, therefore, we apply the results of the special relativity theory, our conclusion here is: the length of the great circumference must be different if measured by the two observers. Since only one of the four lengths measured by the two observers is not the same for them both, the ratio of the two radii cannot be equal to the ratio of the two circumferences for the inside observer as it is for the outside one. This means that the observer on the disc cannot confirm the validity of Euclidean geometry in his CS.

After obtaining this result, the observer on the disc could say that he does not wish to consider CS in which Euclidean geometry is not valid. The breakdown of the Euclidean geometry is due to absolute rotation, to the fact that his CS is a bad and forbidden one. But, in arguing in this way, he rejects the principal idea of the general theory of relativity. On the other hand, if we wish to reject absolute motion and to keep up the idea of the general theory of relativity, then physics must all be built on the basis of a geometry more general than the Euclidean. There is no way of escape from this consequence if all CS are permissible.

On a rotating disc a point very far from the center will have much greater velocity compared to a point close to the center. If the greater velocity is close to the velocity of light, relativistic considerations shall come into play and the geometry shall appear to be non-Euclidean. This is the law of nature and it cannot be denied. This is happening because inertia reduces along the radius as velocity increases.

The changes brought about by the general relativity theory cannot be confined to space alone. In the special relativity theory we had clocks resting in every CS, having the same rhythm and synchronized, that is, showing the same time simultaneously. What happens to a clock in a non-inertial CS? The idealized experiment with the disc will again be of use. The outside observer has in his inertial CS perfect clocks all having the same rhythm, all synchronized. The inside observer takes two clocks of the same kind and places one on the small inner circle and the other on the large outer circle. The clock on the inner circle has a very small velocity relative to the outside observer. We can, therefore, safely conclude that its rhythm will be the same as that of the outside clock. But the clock on the large circle has a considerable velocity, changing its rhythm compared to the clocks of the outside observer and, therefore, also compared to the clock placed on the small circle. Thus, the two rotating clocks will have different rhythms and, applying the results of the special relativity theory, we again see that in our rotating CS we can make no arrangements similar to those in an inertial CS.

As inertia reduces along the radius, the rhythm of the clock (time) will also slow down with increasing velocity.

To make clear what conclusions can be drawn from this and previously described idealized experiments, let us once more quote a dialogue between the old physicist O, who believes in classical physics, and the modern physicist M, who knows the general relativity theory. O is the outside observer, in the inertial CS, whereas M is on the rotating disc.

O. In your CS, Euclidean geometry is not valid. I watched your measurements and I agree that the ratio of the two circumferences is not, in your CS, equal to the ratio of the two radii. But this shows that your CS is a forbidden one. My CS, however, is of an inertial character, and I can safely apply Euclidean geometry. Your disc is in absolute motion and, from the point of view of classical physics, forms a forbidden CS, in which the laws of mechanics are not valid.

M. I do not want to hear anything about absolute motion. My CS is just as good as yours. What I noticed was your rotation relative to my disc. No one can forbid me to relate all motions to my disc.

O. But did you not feel a strange force trying to keep you away from the centre of the disc? If your disc were not a rapidly rotating merry-go-round, the two things which you observed would certainly not have happened. You would not have noticed the force pushing you toward the outside nor would you have noticed that Euclidean geometry is not applicable in your CS Are not these facts sufficient to convince you that your CS is in absolute motion?

M. Not at all! I certainly noticed the two facts you mention, but I hold a strange gravitational field acting on my disc responsible for them both. The gravitational field, being directed toward the outside of the disc, deforms my rigid rods and changes the rhythm of my clocks. The gravitational field, non-Euclidean geometry, clocks with different rhythms are, for me, all closely connected. Accepting any CS, I must at the same time assume the existence of an appropriate gravitational field with its influence upon rigid rods and clocks.

In a non-inertial CS the laws of mechanics are not valid, because there happens to be a gravitational field, non-Euclidean geometry, and clocks with different rhythms that are all closely connected.

O. But are you aware of the difficulties caused by your general relativity theory? I should like to make my point clear by taking a simple non-physical example. Imagine an idealized American town consisting of parallel streets with parallel avenues running perpendicular to them. The distance between the streets and also between the avenues is always the same. With these assumptions fulfilled, the blocks are of exactly the same size. In this way I can easily characterize the position of any block. But such a construction would be impossible without Euclidean geometry. Thus, for instance, we cannot cover our whole earth with one great ideal American town. One look at the globe will convince you. But neither could we cover your disc with such an “American town construction”. You claim that your rods are deformed by the gravitational field. The fact that you could not confirm Euclid’s theorem about the equality of the ratio of radii and circumferences shows clearly that if you carry such a construction of streets and avenues far enough you will, sooner or later, get into difficulties and find that it is impossible on your disc. Your geometry on your rotating disc resembles that on a curved surface, where, of course, the streets-and-avenues construction is impossible on a great enough part of the surface. For a more physical example take a plane irregularly heated with different temperatures on different parts of the surface. Can you, with small iron sticks expanding in length with temperature, carry out the “parallel-perpendicular” construction which I have drawn below? Of course not! Your “gravitational field” plays the same tricks on your rods as the change of temperature on the small iron sticks.

M. All this does not frighten me. The street-avenue construction is needed to determine positions of points, with the clock to order events. The town need not be American, it could just as well be ancient European. Imagine your idealized town made of plasticine and then deformed. I can still number the blocks and recognize the streets and avenues, though these are no longer straight and equidistant. Similarly, on our earth, longitude and latitude denote the positions of points, although there is no “American town” construction.

O.  But I still see a difficulty. You are forced to use your “European town structure”. I agree that you can order points, or events, but this construction will muddle all measurement of distances. It will not give you the metric properties of space as does my construction. Take an example. I know, in my American town, that to walk ten blocks I have to cover twice the distance of five blocks. Since I know that all blocks are equal, I can immediately determine distances.

M.  That is true. In my “European town” structure, I cannot measure distances immediately by the number of deformed blocks. I must know something more; I must know the geometrical properties of my surface. Just as everyone knows that from 0° to 10° longitude on the Equator is not the same distance as from 0° to 10° longitude near the North Pole. But every navigator knows how to judge the distance between two such points on our earth because he knows its geometrical properties. He can either do it by calculations based on the knowledge of spherical trigonometry, or he can do it experimentally, sailing his ship through the two distances at the same speed. In your case the whole problem is trivial, because all the streets and avenues are the same distance apart. In the case of our earth it is more complicated; the two meridians 0° and 10° meet at the earth’s poles and are farthest apart on the Equator. Similarly, in my “European town structure”, I must know something more than you in your “American town structure”, in order to determine distances. I can gain this additional knowledge by studying the geometrical properties of my continuum in every particular case.

A non-inertial system simply has laws different from the laws of mechanics. One should be able to determine these laws.

O. But all this only goes to show how inconvenient and complicated it is to give up the simple structure of the Euclidean geometry for the intricate scaffolding which you are bound to use. Is this really necessary?

M. I am afraid it is, if we want to apply our physics to any CS, without the mysterious inertial CS. I agree that my mathematical tool is more complicated than yours, but my physical assumptions are simpler and more natural.

The physical assumptions for non-inertial CS are simpler and more natural, even though the mathematical tooi it uses appears to be more complicated.

The discussion has been restricted to two-dimensional continua. The point at issue in the general relativity theory is still more complicated, since it is not the two-dimensional, but the four-dimensional time-space continuum. But the ideas are the same as those sketched in the two-dimensional case. We cannot use in the general relativity theory the mechanical scaffolding of parallel, perpendicular rods and synchronized clocks, as in the special relativity theory. In an arbitrary CS we cannot determine the point and the instant at which an event happens by the use of rigid rods, rhythmical and synchronized clocks, as in the inertial CS of the special relativity theory. We can still order the events with our non-Euclidean rods and our clocks out of rhythm. But actual measurements requiring rigid rods and perfect rhythmical and synchronized clocks can be performed only in the local inertial CS For this the whole special relativity theory is valid; but our “good” CS is only local, its inertial character being limited in space and time. Even in our arbitrary CS we can foresee the results of measurements made in the local inertial CS But for this we must know the geometrical character of our time-space continuum.

Our idealized experiments indicate only the general character of the new relativistic physics. They show us that our fundamental problem is that of gravitation. They also show us that the general relativity theory leads to further generalization of time and space concepts.

The inertial CS is only local, its inertial character being limited in space and time. In the larger non-inertial landscape we must know the geometrical character of our time-space continuum. Our fundamental problem is that of gravitation. The general relativity theory leads to further generalization of time and space concepts.

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Final Comment

Einstein is basically concerned with the role of thoughts and ideas in the adventurous search for knowledge of the physical world. The laws of mechanics apply only to matter. New laws now explain the phenomena of electromagnetism and light. The phenomenon of gravitation still needs to be explained fully. 

Are there more fundamental laws of the physical world that explain the connection among the physical phenomena such as, the motion of matter, light and gravitation?

Einstein correctly identifies that the solution requires further generalization of time and space concepts.

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Reference: Evolution of Physics

This paper presents Chapter III, section 11 from the book THE EVOLUTION OF PHYSICS by A. EINSTEIN and L. INFELD. The contents are from the original publication of this book by Simon and Schuster, New York (1942).

The paragraphs of the original material (in black) are accompanied by brief comments (in color) based on the present understanding.  Feedback on these comments is appreciated.

The heading below is linked to the original materials.

.

Outside and Inside the Lift

The law of inertia marks the first great advance in physics; in fact, its real beginning. It was gained by the contemplation of an idealized experiment, a body moving forever with no friction nor any other external forces acting. From this example, and later from many others, we recognized the importance of the idealized experiment created by thought. Here again, idealized experiments will be discussed. Although these may sound very fantastic, they will, nevertheless, help us to understand as much about relativity as is possible by our simple methods.

We had previously the idealized experiments with a uniformly moving room. Here, for a change, we shall have a falling lift.

Thought experiments assist us in understanding many things in Physics, such as, inertia.

Imagine a great lift at the top of a skyscraper much higher than any real one. Suddenly the cable supporting the lift breaks, and the lift falls freely toward the ground. Observers in the lift are performing experiments during the fall. In describing them, we need not bother about air resistance or friction, for we may disregard their existence under our idealized conditions. One of the observers takes a handkerchief and a watch from his pocket and drops them. What happens to these two bodies? For the outside observer, who is looking through the window of the lift, both handkerchief and watch fall toward the ground in exactly the same way, with the same acceleration. We remember that the acceleration of a falling body is quite independent of its mass and that it was this fact which revealed the equality of gravitational and inertial mass (p. 37). We also remember that the equality of the two masses, gravitational and inertial, was quite accidental from the point of view of classical mechanics and played no role in its structure. Here, however, this equality reflected in the equal acceleration of all falling bodies is essential and forms the basis of our whole argument.

The acceleration of a falling body is quite independent of its mass. It is based on the gravitational field. This reveals the equality of gravitational and inertial mass.

Let us return to our falling handkerchief and watch; for the outside observer they are both falling with the same acceleration. But so is the lift, with its walls, ceiling, and floor. Therefore: the distance between the two bodies and the floor will not change. For the inside observer the two bodies remain exactly where they were when he let them go. The inside observer may ignore the gravitational field, since its source lies outside his CS. He finds that no forces inside the lift act upon the two bodies, and so they are at rest, just as if they were in an inertial CS. Strange things happen in the lift! If the observer pushes a body in any direction, up or down for instance, it always moves uniformly so long as it does not collide with the ceiling or the floor of the lift. Briefly speaking, the laws of classical mechanics are valid for the observer inside the lift. All bodies behave in the way expected by the law of inertia. Our new CS rigidly connected with the freely falling lift differs from the inertial CS in only one respect. In an inertial CS, a moving body on which no forces are acting will move uniformly for ever. The inertial CS as represented in classical physics is neither limited in space nor time. The case of the observer in our lift is, however, different. The inertial character of his CS is limited in space and time. Sooner or later the uniformly moving body will collide with the wall of the lift, destroying the uniform motion. Sooner or later the whole lift will collide with the earth, destroying the observers and their experiments. The CS is only a “pocket edition” of a real inertial CS.

In a freely falling lift, bodies remain exactly where they are, when they are let go. They act as if they are in an inertial frame.
Within a freely falling lift the acceleration cancels out the gravitational field.

This local character of the CS is quite essential. If our imaginary lift were to reach from the North Pole to the Equator, with the handkerchief placed over the North Pole and the watch over the Equator, then, for the outside observer, the two bodies would not have the same acceleration; they would not be at rest relative to each other. Our whole argument would fail! The dimensions of the lift must be limited so that the equality of acceleration of all bodies relative to the outside observer may be assumed.

With this restriction, the CS takes on an inertial character for the inside observer. We can at least indicate a CS in which all the physical laws are valid, even though it is limited in time and space. If we imagine another CS, another lift moving uniformly, relative to the one falling freely, then both these CS will be locally inertial. All laws are exactly the same in both. The transition from one to the other is given by the Lorentz transformation.

Let us see in what way both the observers, outside and inside, describe what takes place in the lift.

The outside observer notices the motion of the lift and of all bodies in the lift, and finds them in agreement with Newton’s gravitational law. For him, the motion is not uniform, but accelerated, because of the action of the gravitational field of the earth.

However, a generation of physicists born and brought up in the lift would reason quite differently. They would believe themselves in possession of an inertial system and would refer all laws of nature to their lift, stating with justification that the laws take on a specially simple form in their CS. It would be natural for them to assume their lift at rest and their CS the inertial one.

It is impossible to settle the differences between the outside and the inside observers. Each of them could claim the right to refer all events to his CS Both descriptions of events could be made equally consistent.

We see from this example that a consistent description of physical phenomena in two different CS is possible, even if they are not moving uniformly, relative to each other. But for such a description we must take into account gravitation, building, so to speak, the “bridge” which effects a transition from one CS to the other. The gravitational field exists for the outside observer; it does not for the inside observer. Accelerated motion of the lift in the gravitational field exists for the outside observer, rest and absence of the gravitational field for the inside observer. But the “bridge”, the gravitational field, making the description in both CS possible, rests on one very important pillar: the equivalence of gravitational and inertial mass. Without this clue, unnoticed in classical mechanics, our present argument would fail completely.

We see from this example that a consistent description of physical phenomena in two different CS is possible, even if they are not moving uniformly, relative to each other. But for such a description we must take into account gravitation, building the “bridge” which effects a transition from one CS to the other. This rests on the equivalence of gravitational and inertial mass.

Now for a somewhat different idealized experiment. There is, let us assume, an inertial CS, in which the law of inertia is valid. We have already described what happens in a lift resting in such an inertial CS. But we now change our picture. Someone outside has fastened a rope to the lift and is pulling, with a constant force, in the direction indicated in our drawing. It is immaterial how this is done. Since the laws of mechanics are valid in this CS, the whole lift moves with a constant acceleration in the direction of the motion. Again we shall listen to the explanation of phenomena going on in the lift and given by both the outside and inside observers.

The outside observer: My CS is an inertial one. The lift moves with constant acceleration, because a constant force is acting. The observers inside are in absolute motion, for them the laws of mechanics are invalid. They do not find that bodies, on which no forces are acting, are at rest. If a body is left free, it soon collides with the floor of the lift, since the floor moves upward toward the body. This happens exactly in the same way for a watch and for a handkerchief. It seems very strange to me that the observer inside the lift must always be on the “floor”, because as soon as he jumps the floor will reach him again.

The inside observer: I do not see any reason for believing that my lift is in absolute motion. I agree that my CS, rigidly connected with my lift, is not really inertial, but I do not believe that it has anything to do with absolute motion. My watch, my handkerchief, and all bodies are falling because the whole lift is in a gravitational field. I notice exactly the same kinds of motion as the man on the earth. He explains them very simply by the action of a gravitational field. The same holds good for me.

These two descriptions, one by the outside, the other by the inside, observer, are quite consistent, and there is no possibility of deciding which of them is right. We may assume either one of them for the description of phenomena in the lift: either non-uniform motion and absence of a gravitational field with the outside observer, or rest and the presence of a gravitational field with the inside observer.

This experiment may be described as either non-uniform motion and absence of a gravitational field, or rest and the presence of a gravitational field.

The outside observer may assume that the lift is in “absolute” non-uniform motion. But a motion which is wiped out by the assumption of an acting gravitational field cannot be regarded as absolute motion.

There is, possibly, a way out of the ambiguity of two such different descriptions, and a decision in favour of one against the other could perhaps be made. Imagine that a light ray enters the lift horizontally through a side window and reaches the opposite wall after a very short time. Again let us see how the path of the light would be predicted by the two observers.

The outside observer, believing in accelerated motion of the lift, would argue: The light ray enters the window and moves horizontally, along a straight line and with a constant velocity, toward the opposite wall. But the lift moves upward, and during the time in which the light travels toward the wall the lift changes its position. Therefore, the ray will meet at a point not exactly opposite its point of entrance, but a little below. The difference will be very slight, but it exists nevertheless, and the light ray travels, relative to the lift, not along a straight, but along a slightly curved line. The difference is due to the distance covered by the lift during the time the ray is crossing the interior.

The inside observer, who believes in the gravitational field acting on all objects in his lift, would say: there is no accelerated motion of the lift, but only the action of the gravitational field. A beam of light is weightless and, therefore, will not be affected by the gravitational field. If sent in a horizontal direction, it will meet the wall at a point exactly opposite to that at which it entered.

It seems from this discussion that there is a possibility of deciding between these two opposite points of view as the phenomenon would be different for the two observers. If there is nothing illogical in either of the explanations just quoted, then our whole previous argument is destroyed, and we cannot describe all phenomena in two consistent ways, with and without a gravitational field.

But there is, fortunately, a grave fault in the reasoning of the inside observer, which saves our previous conclusion. He said: “A beam of light is weightless and, therefore, will not be affected by the gravitational field.” This cannot be right! A beam of light carries energy and energy has mass. But every inertial mass is attracted by the gravitational field, as inertial and gravitational masses are equivalent. A beam of light will bend in a gravitational field exactly as a body would if thrown horizontally with a velocity equal to that of light. If the inside observer had reasoned correctly and had taken into account the bending of light rays in a gravitational field, then his results would have been exactly the same as those of an outside observer.

The gravitational field of the earth is, of course, too weak for the bending of light rays in it to be proved directly, by experiment. But the famous experiments performed during the solar eclipses show, conclusively though indirectly, the influence of a gravitational field on the path of a light ray.

It follows from these examples that there is a well-founded hope of formulating a relativistic physics. But for this we must first tackle the problem of gravitation.

We saw from the example of the lift the consistency of the two descriptions. Non-uniform motion may, or may not, be assumed. We can eliminate “absolute” motion from our examples by a gravitational field. But then there is nothing absolute in the non-uniform motion. The gravitational field is able to wipe it out completely.

In this experiment the light may appear to bend with respect to the lift either due to the non-uniform motion, or due to the presence of a gravitational field. A famous experiment performed during the solar eclipses show, conclusively though indirectly, the influence of a gravitational field on the path of a light ray. The light bends because it has some inertia. This shows that all external forces may be replaced by equivalent gravitational fields.

The ghosts of absolute motion and inertial CS can be expelled from physics and a new relativistic physics built. Our idealized experiments show how the problem of the general relativity theory is closely connected with that of gravitation and why the equivalence of gravitational and inertial mass is so essential for this connection. It is clear that the solution of the gravitational problem in the general theory of relativity must differ from the Newtonian one. The laws of gravitation must, just as all laws of nature, be formulated for all possible CS, whereas the laws of classical mechanics, as formulated by Newton, are valid only in inertial CS.

The problem of the general relativity theory is closely connected with that of gravitation because both motion and gravitation are the result of equilibrium of inertia for the system.

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Final Comment

When the motion of a “body” is balanced by its inertia we have uniform motion of the body in a straight line manifested as constant velocity.

When the motion of a “system of bodies” is balanced by the inertia of the bodies we have a dynamic equilibrium in which, we have uniform motion of the bodies along curved paths in space. This is seen as the manifestation of gravitational forces.

The greater is the inertia of a body the greater is its centeredness in space and the lesser is its forwrd motion. The body with greater inertia acts as a center around which bodies with lesser inertia move. 

The inertia appears as mass in the material region, quantum wave-particles in the atomic region, and as electromagnetic frequency in the field region.

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Reference: Evolution of Physics

This paper presents Chapter III, section 10 from the book THE EVOLUTION OF PHYSICS by A. EINSTEIN and L. INFELD. The contents are from the original publication of this book by Simon and Schuster, New York (1942).

The paragraphs of the original material (in black) are accompanied by brief comments (in color) based on the present understanding.  Feedback on these comments is appreciated.

The heading below is linked to the original materials.

.

General Relativity

There still remains one point to be cleared up. One of the most fundamental questions has not been settled as yet: does an inertial system exist? We have learned something about the laws of nature, their invariance with respect to the Lorentz transformation, and their validity for all inertial systems moving uniformly, relative to each other. We have the laws but do not know the frame to which to refer them.

An inertial system is a coordinate system where objects not subject to forces move in straight lines at constant speed. This aligns with Newton’s first law and forms the foundation of special relativity. In the theory of relativity, the laws of nature are consistent with Lorentz transformation. This means that neither space nor time are absolute. And that points to changes in both velocity and inertia according to some law that we need to discover.

In order to be more aware of this difficulty, let us interview the classical physicist and ask him some simple questions:

“What is an inertial system?”

“It is a CS in which the laws of mechanics are valid. A body on which no external forces are acting moves uniformly in such a CS This property thus enables us to distinguish an inertial CS from any other.”

“But what does it mean to say that no forces are acting on a body?”

“It simply means that the body moves uniformly in an inertial CS“

The body moves uniformly only when its velocity is balanced by its inertia. In a system of bodies, the velocity and inertia of all bodies are in a dynamic equilibrium. This further expands the definition of inertial system. The only forces acting on the bodies are inertial forces. The gravitational forces come into play only when the dynamic equilibrium of a system is disturbed. Such gravitational forces act only to restore the equilibrium.

Here we could once more put the question: “What is an inertial CS?” But since there is little hope of obtaining an answer differing from the above, let us try to gain some concrete information by changing the question:

“Is a CS rigidly connected with the earth an inertial one?”

“No, because the laws of mechanics are not rigorously valid on the earth, due to its rotation. A CS rigidly connected with the sun can be regarded for many problems as an inertial CS; but when we speak of the rotating sun, we again understand that a CS connected with it cannot be regarded as strictly inertial.”

If a CS is rigidly connected with the earth, then it is part of earth’s inertial system. Earth is part of the larger inertial system of the solar system. As long as every particle is in equilibrium it is part of some inertial system.

“Then what, concretely, is your inertial CS, and how is its state of motion to be chosen?”

“It is merely a useful fiction and I have no idea how to realize it. If I could only get far away from all material bodies and free myself from all external influences, my CS would then be inertial.”

“But what do you mean by a CS free from all external influences?”

“I mean that the CS is inertial.”

Once more we are back at our initial question!

Our interview reveals a grave difficulty in classical physics. We have laws, but do not know what frame to refer them to, and our whole physical structure seems to be built on sand.

This interview reveals that the definition of inertial system needs to be expanded for a system of bodies, because no body is totally isolated.

We can approach this same difficulty from a different point of view. Try to imagine that there is only one body, forming our CS, in the entire universe. This body begins to rotate. According to classical mechanics, the physical laws for a rotating body are different from those for a non-rotating body. If the inertial principle is valid in one case, it is not valid in the other. But all this sounds very suspicious. Is it permissible to consider the motion of only one body in the entire universe? By the motion of a body we always mean its change of position in relation to a second body. It is, therefore, contrary to common sense to speak about the motion of only one body. Classical mechanics and common sense disagree violently on this point. Newton’s recipe is: if the inertial principle is valid, then the CS is either at rest or in uniform motion. If the inertial principle is invalid, then the body is in non-uniform motion. Thus, our verdict of motion or rest depends upon whether or not all the physical laws are applicable to a given CS.

The rotation of a single body in the entire universe shall contribute to its inertia (centeredness). The degree of centeredness shall determine its forward velocity on an absolute basis; because the forward velocity of a body with infinite inertia would be zero. Einstein thinks that it is not permissible to consider the motion of only one body in the entire universe; but the motion of a body can be considered relative to itself as in acceleration.

Take two bodies, the sun and the earth, for instance. The motion we observe is again relative. It can be described by connecting the CS with either the earth or the sun. From this point of view, Copernicus’ great achievement lies in transferring the CS from the earth to the sun. But as motion is relative and any frame of reference can be used, there seems to be no reason for favouring one CS rather than the other.

We can differentiate the CS of sun from the CS of earth on the basis of their respective inertia. The forward velocity of earth shall be greater than the forward velocity of sun on an absolute basis because the inertia of sun is obviously greater than the inertia of earth. Einstein falsely believes, “But as motion is relative and any frame of reference can be used, there seems to be no reason for favouring one CS rather than the other.”

Physics again intervenes and changes our commonsense point of view. The CS connected with the sun resembles an inertial system more than that connected with the earth. The physical laws should be applied to Copernicus’ CS rather than to Ptolemy’s. The greatness of Copernicus’ discovery can be appreciated only from the physical point of view. It illustrates the great advantage of using a CS connected rigidly with the sun for describing the motion of planets.

No absolute uniform motion exists in classical physics. If two CS are moving uniformly, relative to each other, then there is no sense in saying, “This CS is at rest and the other is moving”. But if two CS are moving non-uniformly, relative to each other, then there is very good reason for saying, “This body moves and the other is at rest (or moves uniformly).” Absolute motion has here a very definite meaning. There is, at this point, a wide gulf between common sense and classical physics. The difficulties mentioned, that of an inertial system and that of absolute motion, are strictly connected with each other. Absolute motion is made possible only by the idea of an inertial system, for which the laws of nature are valid.

The common sense fourth coordinate of a CS shall be it rigidity or inertia by which the CS is centered in space. This is represented by time. Absolute uniform motion shall be based on zero motion for infinite inertia. Unfortunately, this is not directly recognized by the theory of relativity. It may be hidden under the math of relativity.

It may seem as though there is no way out of these difficulties, as though no physical theory can avoid them. Their root lies in the validity of the laws of nature for a special class of CS only, the inertial. The possibility of solving these difficulties depends on the answer to the following question. Can we formulate physical laws so that they are valid for all CS, not only those moving uniformly, but also those moving quite arbitrarily, relative to each other? If this can be done, our difficulties will be over. We shall then be able to apply the laws of nature to any CS. The struggle, so violent in the early days of science, between the views of Ptolemy and Copernicus would then be quite meaningless. Either CS could be used with equal justification. The two sentences, “the sun is at rest and the earth moves”, or “the sun moves and the earth is at rest”, would simply mean two different conventions concerning two different CS.

Could we build a real relativistic physics valid in all CS; a physics in which there would be no place for absolute, but only for relative, motion? This is indeed possible!

Einstein seems to think that an absolute measure of velocity is not possible; but, the theory of relativity does make it possible by using the velocity of light as an absolute reference point in reverse. A more straightforward absolute reference point shall be zero velocity at infinite inertia. Such a reference point shall be a black hole at the center of a galaxy.

We have at least one indication, though a very weak one, of how to build the new physics. Really relativistic physics must apply to all CS and, therefore, also to the special case of the inertial CS. We already know the laws for this inertial CS. The new general laws valid for all CS must, in the special case of the inertial system, reduce to the old, known laws.

The problem of formulating physical laws for every CS was solved by the so-called general relativity theory; the previous theory, applying only to inertial systems, is called the special relativity theory. The two theories cannot, of course, contradict each other, since we must always include the old laws of the special relativity theory in the general laws for an inertial system. But just as the inertial CS was previously the only one for which physical laws were formulated, so now it will form the special limiting case, as all CS moving arbitrarily, relative to each other, are permissible.

The special theory of relativity applies to inertial systems only; but the general theory of relativity formulates physical laws for all CS moving arbitrarily, relative to each other.

This is the programme for the general theory of relativity. But in sketching the way in which it was accomplished we must be even vaguer than we have been so far. New difficulties arising in the development of science force our theory to become more and more abstract. Unexpected adventures still await us. But our final aim is always a better understanding of reality. Links are added to the chain of logic connecting theory and observation. To clear the way leading from theory to experiment of unnecessary and artificial assumptions, to embrace an ever-wider region of facts, we must make the chain longer and longer. The simpler and more fundamental our assumptions become, the more intricate is our mathematical tool of reasoning; the way from theory to observation becomes longer, more subtle, and more complicated. Although it sounds paradoxical, we could say: Modern physics is simpler than the old physics and seems, therefore, more difficult and intricate. The simpler our picture of the external world and the more facts it embraces, the more strongly it reflects in our minds the harmony of the universe.

The way the general theory of relativity is accomplished is quite vague and abstract. Einstein says, “The simpler and more fundamental our assumptions become, the more intricate is our mathematical tool of reasoning.” But our final aim is always a better understanding of reality.

Our new idea is simple: to build a physics valid for all CS. Its fulfilment brings formal complications and forces us to use mathematical tools different from those so far employed in physics. We shall show here only the connection between the fulfilment of this programme and two principal problems: gravitation and geometry.

The new idea in general theory of relativity is to build a physics valid for all CS. But the way it is achieved is very abstract at the moment.

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Final Comment

Classical mechanics applies to inertial systems that are limited to the familiar material domain. Inertial systems comply with Galileo’s law of inertia that treats inertia as constant and velocity as uniform in all inertial systems. Motion in different inertial systems is related by addition of velocities according to the Galilean transformations. Such motion is perceived in a relative sense only. It has no absolute basis.

In his Special theory of relativity, Einstein introduces Lorentz transformations. His inertial frames are now constrained by the limit of a constant velocity of light. The special relativity gives somewhat better results; but it is still limited to inertial frames that deal only with non-varying inertia and uniform velocity. Inertial frames do not account for rotation and acceleration. Non-inertial frames shall include rotation that will add to inertia, and acceleration that will overcome inertia. The proper physics that applies to all CS, shall quantify inertia and provide a relationship between velocity and inertia. 

The general theory of relativity appears to develop an absolute law that accounts for varying inertia and velocity. That law appears to be based on a series of assumptions and logic that is hidden under abstract mathematical reasoning. The law is described only through a complex mathematical expression.

Any presence of “external force” would imply a system of at least two bodies that influence each other through a field that occupies the space between them. General relativity deals with this in terms of  gravitation and geometry.

Postulate Mechanics looks at it as a dynamic equilibrium of inertia among the bodies that results in an ensemble of motion. This is our universe.

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