FORWARD THINKING IN ASTRONOMY [A series of eight lectures specially prepared for Compu-Serve Information Systems (CIS), for presentation in ASTROFORUM. Copyright 1990 by Tom Van Flandern of Washington, DC [CIS ID code 71107,2320]. Please seek the author's permission before reprinting more than two paragraphs. If redistributed in electronic form, must include this statement of source and copyright.] CHAPTER III. GRAVITY, LIGHT, AND FORCE ******** This week and next we will complete our alternative cosmology from our new starting point. This week we will speak of the nature of force in general and gravitation in particular. Next week we will cover applications and implications. In the following week, we'll switch to the solar system. A. The Nature of Gravitation "Every particle of matter in the universe attracts every other particle of matter with a force which is directly proportional to the product of the masses, and inversely proportional to the square of the distance between them." (Newton's Law of Universal Gravitation) Why! Physics has come to understand something of the nature of light and sound, of matter and energy, of force and motion. We know that heat is a phenomenon caused by the motion of molecules. Matter is composed of molecules, which are composed of atoms, which are composed of baryons, etc. Forces are understood as something pushing against something else, or as action and reaction. But our understanding of gravity is at a far more primitive level. We can describe operationally HOW gravity acts, and the gravitational laws of motion must be the most exactly measured and confirmed in physics. But we really know very little about the WHY of gravity. This is primarily because of its remarkable properties, which seem to defy all understanding. "Every particle of matter in the universe" -- regardless of chemical composition, regardless of atomic makeup, regardless of charge or spin or any other property -- surely a remarkable thing. "Attracts" -- other forces of nature have both attractive and repulsive manifestations, but gravity always attracts -- why? "Inversely proportional to the square of the distance" -- this is perhaps the easiest property of gravity to understand, since it is true of so many other physical manifestations. As something (light, sound, energy) propagates, it spreads out in two dimensions as it moves forward in a third dimension and in time. This causes an inverse square weakening, simply because the "area" of the propagating something increases with the square of distance, so its "density" (substance per unit area) must decrease with the square of distance. Although not spelled out explicitly in the universal law of gravitation, there are two other remarkable properties which need elaboration before we can begin any contemplation about why there is universal gravity. The first is the question of "action at a distance". (We will save the second, the "instantaneous" nature of the action, for a bit later in this discussion.) Can one object act upon another at a distance without some agent passing between them? In the case of atomic forces, there certainly are such agents, said to be photons travelling at the speed of light. However in the case of gravity, it is commonly supposed that there is no agent. B. Action at a Distance Isaac Newton is credited as the first to make a remark which I personally find to be compelling: it is surely impossible for one body to influence another at a distance without the action of some agent passing between them. Forces, such as gravity or magnetism, which appear to act at a distance must actually consist of SOMETHING which passes between the source body and the affected body. In other words, "action at a distance" in its purest form, where something far away is affected by something locally without any causative agent passing between the two, must logically be impossible. In the cases of gravity and magnetism, this postulate is reinforced by the observation that the field of force exists continuously at every distance from the source, suggesting an outward propagation from that source. Sometimes physicists talk about the "curvature of space-time" near a massive body, predicted by Einstein's Relativity Theory, as an example of action at a distance. But SOMETHING must act to maintain the so-called "curvature of space-time" at a distance. Moreover, if the massive body is suddenly accelerated, it must take a finite time (however small) for the accompanying "curvature of space-time" to respond and begin accelerating too. If the massive body were to cease to exist, there must likewise be a finite time before the "curvature" also ceased to exist. This is merely restating the first principle, that there must exist some agent which passes between to accomplish any action; and this agent cannot act instantaneously, because its velocity must be finite. (Note, however, that there is nothing in the field of logic which compels us to set any upper limit on the speed of such hypothetical agents; it particular, logic alone does not place any requirement that they travel at the velocity of light or slower.) The agents which produce forces must exist; therefore they must be tangible in some way. Everything existing is classified as either matter or energy; and ultimately, all matter and energy are interchangeable (matter can be converted completely into energy, and vice versa), according to modern-day physics. For purposes of this discussion, it is unimportant whether the "agents" which give rise to forces are matter or energy, or "something else"; but it will be convenient to think of them as having "substance", at least on some infinitesimal scale. Therefore I suppose, and feel no doubt in my supposition, that gravity acts by means of some sort of agents making contact with matter, despite no such agents being as yet known to physicists. Conceptually, the only way that "agents" have to act on bodies is by means of collisions. If an agent does not come into contact with a body to influence it, then we have another "action at a distance" dilemma, once removed. "Collisions" which lead to sticking, or to destruction of the target body, may be thought of as consisting of numerous elastic, non-destructive collisions occurring at a more microscopic level. It is also intrinsic to any collision that it requires a finite time interval; it cannot occur instantaneously. C. The Sea of Agents We might then imagine two possibilities: the agents come from within the matter which attracts; or the agents originate outside the matter. Our first impulse is to assume an origin from within; but it is not easy to see intuitively how agents coming from within one body, then travelling to and making contact with a second body, can give rise to a force of attraction between the two bodies. Such a force would logically be "pushing", or repulsive. On the other hand, a sea of rapidly moving agents everywhere outside a body would tend to push down on the surface of the body, giving rise to an apparent force of attraction toward the center of the body. Moreover, two bodies would shadow each other from some of the agents, giving rise to an apparent force of attraction between the bodies (since fewer agents would be available to "push" from the shadowed side than from the opposite side of each body). Such a force would be always attractive, as gravity is. If the universe is filled with these "agents", the force would be universal. The force would be inverse square, as would any force from agents which diverge in two dimensions while moving in a third. And lastly, assume that the agents are sufficiently small that most of them can easily pass entirely through large solid bodies without contact. (Recall that, at the atomic level, ordinary matter is mostly empty space anyway.) Then every atom of matter, even the ones near the center of the body, will contribute its share to the net "gravity" force exerted by the body, because the occasional collision between an agent and an atom of matter in the body would be equally likely to occur for every unit of mass. (The agents which pass through without contact contribute no force at all. Matter which deflects agents then shields matter behind it from the possibility of collision with the same agents, which results in an imbalance of collisions on one side.) To summarize, consider a spherical body of ponderable mass. Although the hypothetical agents are flying through it all the time, some of them are always colliding with the atomic particles making up the body, producing push-like forces. When a collision occurs, the agent rebounds, and therefore is not available to collide with other atomic particles deeper in the spherical body. This means that more agents strike every atom from above than from below, because of the shielding effect of the nearby matter. The resultant of all such collisions must therefore be that every particle of the spherical body is "pushed" in the net direction toward the center of the mass, where the most shielding from push-collisions occurs. If the picture is not yet clear, take it to the logical extreme: suppose the matter in the spherical body was so dense that no agent could penetrate, and every agent reflected off the surface of the body. Then all collisions of agents with the spherical body would serve to cause a downward force at the surface of the body pushing toward the center. The body seems to have "gravity", and would be held together, even though its atoms do not, in this concept, actually attract one another. Now suppose we have two spherical bodies of ponderable mass some distance apart. Each shields the other from some collisions by the omni-present agents. The result is that each body experiences more "pushes" from one side than from the side toward the other body. It is just as if the two bodies somehow magically attracted one another from a distance -- but all of the action is produced by the pushing collisional forces of the supposed universal agents. Since the spherical bodies are nearly transparent to the rapidly-moving agents, every particle of matter within them helps contribute to the "force" experienced by the remote body. Moreover, since the angle subtended by the remote body decreases with the square of the distance from the shielding body, the force it experiences is inversely proportional to the square of the distance between the bodies. In other words, the hypothetical force we have just constructed exactly mimics Newton's Universal Law of Gravitation. D. The Role of Time Delay If the basic model is clear, let me now turn to a potential problem with it. The objection has been raised that the hypothetical "sea of agents" should act like a "perfect gas". This means that it should apply pressures equally on all sides of every particle. There should then be no more of a tendency for two bodies in space to attract each other because of collisions with agents, than there is for two bodies in air to be pushed together by collisions from air molecules. To see this point from a different perspective suggested by Hal Finney of CIS's ASTROFORUM, consider an infinite wall of infinite density, through which no agents can pass. If an ordinary body approaches the wall, agents strike it from the outside, producing a force toward the wall. However those agents which get through fill the space between the body and the wall, eventually to the same density as those outside; and provide an equal number of strikes from the inside, which should produce an equal force away from the wall. So the body should experience no net force. Since the objection is important, let me provide one last example of it. Consider a linear chain of simple particles. At first glance it appears that the particle on the end of the chain will be struck by agents more often from above than below, and so will experience a force toward the rest of the chain. But on further inspection it may be seen that each of the other particles in the chain reflects some agents back toward the first particle, exactly compensating for the agent collisions which were otherwise missing from below. Again, there would be no net force. This objection must be answered to preserve our starting assumption that some such agents must exist to produce the action of gravity over finite distances. If the agents were absorbed instead of reflected, a net force would occur; but all bodies would be continually increasing the total amount of their substance. If agents were absorbed from one direction and re-emitted in a completely random direction, they would still behave like a perfect gas, producing no net force. But even a perfect gas produces pressure. The resolution of the dilemma appears to be that the compensating forces occur after a transmission time delay, so that they do not balance. For example, in the chain of particles, those reflected back from inside the chain arrive later than those which strike the first particle from the outside, resulting in a net force toward the center of the chain. For the infinite wall, the agents between the body and the wall apply their back pressure slightly later than the agents striking from the front. These agents differ from the perfect gas because they penetrate matter, and because the back-scattered agents apply their back force slightly later than the arriving agents apply the forward force. It might seem that, after a time, the number of "old" agents applying back forces would equal the number of "new" agents applying forward forces, giving no net force. But the imbalance my be seen to be present for any single agent. So no matter how many agents are present, the imbalance cannot be nullified; rather, it is multiplied agent-by-agent. It might also seem that this time-delay property was invoked to save the model: always a risky proposition. But on reflection it may be seen that the delay property for the sea of agents is required, not arbitrary. Moreover, it expands the model in some additional, remarkable ways. We reasoned earlier that, at the most minute levels, space-time must be everywhere filled densely with substance. In order to change the motion state of a body (for example, to give it a small impulse, so that it moves slightly faster), it is necessary to push the substance immediately in front of the body out of the way, which in turn pushes other substance, and so on. In other words, a wave is sent out from the body through the substance which fills space. And the body itself feels resistance to this change in its state of motion; that is, it exhibits "inertia" (resistance to changes of motion). At the same time, the impulse tends to create a vacuum behind the body. But as already noted, the substance of the universe will immediately rush in to fill any vacuum, followed by other substance further out, and so on. So we have a wave propagating outward behind the body, also. The velocity of propagation of waves in this medium is the speed of light. Once the state of the body's motion has been altered, and the substance of the universe has adjusted to accommodate that change, the body will feel no further resistance to motion (of the sort it would feel if it moved relative to a medium) because the state of the entire medium has changed to accommodate the body. Only if the body tried to move faster than the speed of light would the medium be unable to adjust fast enough to accommodate the change of the body's speed. To elaborate this point a bit, in air the molecules are widely-spaced; and there is plenty of room to accommodate molecules with get pushed out of the way. But the substance of the universe is supposed to be a continuum, with entities filling all space at all times on all scales. So in order for a body to accelerate it must displace a potentially infinite column of such entities. In practice, the pressure of the displacement is a wave, and propagates away at the velocity of light. So only a finite number of entities appear to resist the acceleration. Then once the body has attained a new velocity, the entities establish an accommodating flow condition on both sides such that only a new acceleration (or deceleration) would be resisted, but not the body's velocity. Just as ocean currents do not damp out from resistance, neither would these presumably frictionless entity currents. In addition to giving us an understanding of the origin of inertia, our sea of agents go one step further. They show us WHY a uniform acceleration of a body is just like immersing that body in a gravitational field: in either case, agents pile up on one side of the body, producing a net force on it. This property is Einstein's famous "Equivalence Principle". A uniform acceleration and a gravitational field are indistinguishable observationally. And so they must be, in this model. We conclude that such a force produced by the action of a sea of agents would have all the properties which gravity has. Since the existence of some such agents seems required to avoid "action at a distance" paradoxes, we conclude that this is an operational description of the "how" and "why" of gravity. It requires the existence of a universal sea of minute substances, so small that large solid bodies are virtually transparent to them. The advantages of the underlying concept we have just described, which has been discussed by numerous authors since the eighteenth century, is that it provides an intuitively understandable construct to explain why there is universal gravitation, how bodies can act on one another at a distance, why the force is always attractive, why there is resistance to acceleration, and why accelerations are equivalent to gravitational fields. The equality of inertial and gravitational masses for bodies also follows in a natural way from the model. E. The Speed of Gravity The other remarkable property of gravity not explicitly stated in Newton's Law of Universal Gravitation is that gravity is assumed to act instantaneously over all distances. In the equations of motion of Celestial Mechanics, for example, each body in the solar system affects each other body from its instantaneous true position. By contrast, solar system bodies are not SEEN to be in their instantaneous true positions; but rather, they appear in the positions they occupied when the light just reaching the observer left them. Take as an example the case of the Sun and Earth. It takes light nearly 500 seconds, or 8.3 minutes, to travel the 150,000,000 kilometers between the two bodies. When we look at the Sun in the sky, we do not see it where it is now, but rather where it was 8.3 minutes ago. This amounts to a displacement of about 20 arc seconds on average -- an astronomically large angle, impossible to mistake. Despite the almost unimaginably fast speed of light, it is not fast enough to allow the Sun, planets, or stars to appear in their present locations. In the case of some distant stars, we see them where they were thousands of years ago. Astronomical observations are accurate enough to permit us to measure the direction of the force acting on the Earth caused by the Sun. Do you suppose that direction corresponds with the Sun's apparent position in the sky (which it really occupied 8.3 minutes ago, and is the position from which light now appears to come), or with the Sun's true instantaneous position now (which we won't be able to see until 8.3 minutes in the future)? While the astronomers who calculate the motions of solar system bodies use equations which assume instantaneous action of gravity, NOT gravity acting at the speed of light, what would be the observable consequences of assuming a finite propagation speed for gravity? In fact, the Sun's gravity emanates from its instantaneous true position, as opposed to the direction from which its light seems to come. If gravity propagated at the speed of light, it would act to accelerate the orbital speed of bodies. By observation, no such acceleration exists down to the level of about one arc second per century squared. The absence of the acceleration implies that the gravitational lines of force arriving at the Earth from the Sun are not parallel to the paths of its arriving photons, but rather have directions which differ by about 20 arc seconds. [Understanding why the Sun would accelerate the Earth if gravity propagated at the speed of light is of no importance to our discussion. But for those who would like to develop a feel for this, consider the analogy of a vertically-falling rain encountered by a rapidly-moving train. The faster the train moves, the more slanted from the forward direction the rainfall appears. The faster the Earth's motion, the more slanted in the forward direction the sunlight (and by extension, its gravity, if it travelled at light speed) would appear to be. The Sun would then always have a forward-pulling component to its force which would accelerate the Earth.] Relativists have postulated that the speed of light is the upper limit for the speed of anything with substance in nature. There are a variety of supporting arguments for this postulate, some of which seem quite strong. We will consider these arguments next week. For now, it is enough that the existence of the postulate should not hinder us from looking at the evidence objectively. The absence of an observed orbital acceleration of the Earth about the Sun places a lower limit to the speed of propagation of hypothetical gravitational agents between the Sun and Earth. This lower limit is about 1E8 times the speed of light! Relativists argue that the existence of the Sun's mass produces a curve in space-time which bends the motion of bodies near it, producing what appears to be a gravitational force. Since the space-time field at the Earth's distance is already pre-curved by the Sun's mass, the Earth simply encounters the already curved field and responds to it instantaneously. In this view, the gravitational force is produced by the Earth's encounter with the space-time curvature, not by its encounter with gravitational agents from the Sun which produce the curvature; so the propagation velocity of the agents, which is assumed to be the speed of light, is said to be irrelevant. In point of fact, I believe the reasoning in such a construction is defective. If the agents maintaining the field propagate with the velocity of light, then the directions of lines of force in the field must still suffer "aberration", just as light does. If the Earth moves through the field with a velocity of 30 km/s or 1E-4 the speed of light, then the field lines must seem to stream somewhat toward the Earth, bent by 1E-4 radians (about 20 arc seconds). Just as for light, this effect arises from the RELATIVE velocity of the Sun and Earth, and is not dependent upon which is thought to be "moving", and which "stationary". So the ratio of the speed of a moving body to the speed of regeneration of the local space-time curvature determines the resultant direction of the force lines. To visualize aberration at work, consider the flight of an arrow from a source (analogous to the Sun) toward a train passing in a perpendicular direction (analogous to the Earth). The arrow at all instants moves radially away from the Sun, both before, during, and after its encounter with the moving train. Now visualize the path of the arrow as it passes through an open window, through the train, and out another window on the other side, without meeting any obstruction. As seen by passengers on the train, the flight path through the passenger cabin has a rearward component due to the forward motion of the train. Indeed, if the arrow interacted with the train by striking it, it would apply a slight decelerating force to the train. (A pulling force like gravity would have a slight accelerating component acting on the Earth.) The same point may be extended to the mutual interactions of three or more bodies. Consider the Sun-Earth-Moon system. The Moon's orbit around the Earth is approximately an ellipse, but one which is quite distorted by the Sun's gravitation. I myself have analyzed observations of the two bodies to solve for the direction of the force exerted by the Sun on the Moon's orbit. The solution showed that the Sun's force comes from its true, instantaneous position rather than its apparent, aberrated position, to a precision of one arc second. (The two positions differ by 20 arc seconds). This solution alone constrains the speed of propagation of the Sun's force to be at least 20 times that of light. No relativist has yet, to my knowledge, devised a theory to explain how it can be that the direction of the Sun's gravitational force on the Earth and the direction of the photons arriving from the Sun are not parallel. Perhaps contact binary star systems place the tightest constraints on a lower limit to the speed of propagation of gravity. Unless the speed of gravity exceeds 1E10 times the speed of light, such systems would fly apart within a few hundred years. Moreover, since the centers of mass of the two nearby massive bodies are accelerating, not merely moving linearly at high speed, one could show by such examples that even the response of a gravitational field to an ACCELERATION (not merely a motion) of its source body must propagate faster than light. The conclusion of the preceding considerations is that whatever agents propagate the force of gravity from the source body to its field must travel at least 1E10 times faster than light. It might seem that the thrust of the argument is that the action must be instantaneous. But this would be another form of action at a distance: action which propagates at infinite speed. I do not at all propose that the velocity of propagation is infinite; far from it. A velocity of a MERE 1E10 times light speed, or across the observable universe in 1.5 years, is a very far cry indeed from infinite velocity if the universe is truly infinite. This disbelief was manifested when the velocity of light was itself first measured. It took a very long time to accept that a velocity of 300,000 km/s, or seven times around the Earth in one second, was real; it had always been assumed up to that time that light propagated instantaneously throughout the universe. It was likewise very difficult to accept the initial discovery that stars were, after all, at a finite distance, although the nearest of them was 25 trillion miles away! Later it was difficult to accept the dimensions of our galaxy, or the Hubble age of the universe. Each time our knowledge of the size and age of the universe was extended, the initial reaction was disbelief. Then, when these new limits were finally accepted, each time there was a tendency to believe that the new limits were truly LIMITS, not merely the latest extension of our growing knowledge of the universe. So the picture of gravity we have arrived at here demands a universe filled with gravitational agents moving at velocities much faster than light, in order to explain the nearly instantaneous action of gravity on the local scale. F. Some Corollaries It is logical to ask if the other three fundamental forces in nature can be modeled in similar ways to gravity. Our knowledge about the structure of matter below the atomic level is evolving rapidly, and in my opinion suffers from a lack of structured models with which to interpret new results. What is badly needed is a collection of experimental results which must be adhered to in forming new models. What is made available instead is the interpretation of those experimental results ("spin", "mass", "charge", etc.). How can it be possible for protons to repel protons, electrons to repel electrons, and yet protons and electrons to attract? We can construct an analog in the meta world of light and gravity to illustrate one way in which such a thing can happen. We can make a balloon satellite large enough that the repulsive force of light from the Sun would be greater than the gravitational force of attraction between the two. Consider, then, a set of such balloon satellites and a set of Sun-like stars. All balloon satellites would gravitationally attract one another, because they emit no light. All stars gravitationally attract one another because light pressure is negligible in comparison with their gravitation. And yet all stars would repel all balloon satellites because light pressure exceeds gravitation. Light, because it originates from within, produces repulsion; gravity is attractive because it results from the shadowing of external agents. If matter exhibits gravitation because of the shadowing of other matter from the action of a sea of agents, it follows that at some density, the shielding is complete, and no gravitational agents can penetrate at all. If a sphere of matter were to collapse to such a high density that no gravitational agents could penetrate, then only the surface layers would reflect the gravitational agents. None of the matter in the interior of the body would make any contribution to the strength of its gravitational field. It follows that mathematical black holes would not exist in physics, since the gravitational force exerted by a finite body cannot become indefinitely large at its surface as the body collapses. Escape velocities could never exceed the velocity of light, since that is the limiting velocity of propagation in the sea of agents which produce gravity. Direct observational tests for the existence of shielded mass within large bodies would not be easy, because large bodies reveal their mass only through their gravitation. The easiest method would seem to arise in a three-body case. For example, suppose the Earth were massive enough so as to block some gravitons from passing through its core regions. Then at the times of a total eclipse of the Moon, some of the Sun's gravitation should also be blocked from reaching the Moon. For a brief period, the net gravitational force on the Moon would decrease, allowing the Moon to move slightly farther away, and lengthen its period of revolution around the Earth. Such an effect is observed, but it is attributed entirely to the effects of tidal friction. Perhaps observational accuracy will eventually improve to the point where we can learn if some of the "tidal acceleration" in fact occurs in discrete increments at times of lunar eclipse, rather than continuously, as presently assumed. In any event, a similar test involving more massive bodies, such as any third body in orbit around a binary pulsar, could provide the opportunity for a definitive test of this idea. ******** Before we can put this model into proper perspective, we must complete it; and that requires us to deal with Einstein's General and Special Theories of Relativity, and the nature of waves, which we will do next week. We will also see how this model compares to the real universe, and how it contrasts with the "Big Bang" theory of the universe. 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