## The Gravity Knowledge Eras
Man was aware of up and down, that different objects have different weights. During this period there was no known or prevailing idea of what gravity was or how gravity might work.
About 370 BC in ancient Greece Aristotle performed experiments where he discovered that free fall was accelerative motion. But he was convinced by his thought processes that heavier objects must fall faster than lighter objects. These ideas were held as true for more almost 2000 years. Aristotle’s ideas about gravity were not challenged until about 1585.
In 1585 a Flemish engineer, Simon Stevinus dropped two different weights and noted that they both struck the ground at the same time. During the 1590's Galileo had made an ingenious observation that rolling down inclined planes was just a form of falling. Thus Galileo preformed several experiments using objects rolling down inclined plane to determine that the weight of an object did not affect the rate at which an object fell. No mater the weight they all fell at the same rate. To determine this Galileo realized that movement down an inclined plane was the same as falling just at a slower rate. Thus he was able to control the rate at which objects fell by changing the slope of his test ramps. His discovery indicated that a falling body increases its velocity as the square of time. In 1608 a Dutch scientist Hans Lippershey developed the telescope that Galileo used to make his observations of Jupiter’s moons. In 1609 Johannes Kepler, used the observation of Tycho Brahe, to develop and published his laws of planetary motion. Keplers work was called, “New Astronomy.” In1632 Galileo introduces relativity in physics by stating that physics experiment performed in the closed cabin of a ship could not be used to tell if the ship was moving or stationary in his dialog book, “Concerning the Two Chief World Systems.” Galileo was able to use discoveries made outside of the reach of the inquisition to further his own research. Galileo’s real complication was that he lived within the reach of the inquisition.
In 1674 Robert Hooke puts forward a theory that planetary motion is based on a balance between centrifugal force and the sun’s gravity. In 1679 Hooke confides in Newton that he believes that gravity varies inversely with distance from the sun. Newton decided to see if he could determine what the correct relationship between gravity and the orbit of the planets. In 1680 Newton using Keplers third law of planetary motion, which stated that equal areas are swept out in equal time, develop his equation for gravity based on an inverse square of the radius. By 1684 Hooke has been boosting for five years that he had solved the law of gravity governing the movement of planets with his inverse radius law. Edmund Halley in discussions with Newton ask what equation Newton what equation he thought was needed for gravity would yield an elliptical orbit for the planets. Newton told Halley that he had calculated that it would require the pull of gravity to be proportional to the inverse of the radius squared. Eventually it was also found that the electrical field could also be described by the same inverse radius square equation as had been used to describe gravity.
In 1892 Heaviside proposed that because the Electric field equation had been developed using the gravity field equation that electromagnetism equations developed by Maxwell could be used to describe gravity more fully. In 1906 Einstein published Special Relativity. The two key aspects of Special Relativity are first that the speed of light is a constant and that E = mc^2. In 1915 Einstein published General Relativity. General Relativity was concerned with gravity and basically stated that gravity was a function of curved space. One premise that Einstein used for General Relativity took Galileo’s postulate relating to an observer that is contained in an enclosed ship cabin and expanded it. Galileo stated in 1632 that if physics experiments were performed in the closed cabin of a ship they could not be used to determined if the ship was moving or stationary. Einstein used a similar analogy to state that in a room, elevator or any other enclosed container an observer could not tell if the expression described as gravity was due to the attraction of mass or acceleration of the room by some force. Is this assumption correct? If this assumption is incorrect, does it places doubt on the conclusion called general relativity?
To simplify the wording of this chapter, acceleration that occurs as a result of gravity will be referred to as gravity, and acceleration that occurs as a result of a force other than gravity will be referred to as acceleration. In 1915 Einstein published his General Theory of Relativity stating that measurements made by an observer inside a capsule cannot be used to determine if a capsule is in a gravity field or just accelerating. Aristotle preformed a few experiments with gravity and discovered that gravity was an accelerative motion. Aristotle, without further experiments to check his assumption stated that heavy objects fall faster than light objects. His thought processes seemed logical at the time but lead him to a solution that was partially correct and partially incorrect. The reason was not a faulty thought process but a situation where there were either insufficient experiments or insufficient technology to perform the experiments which were needed. Why go back to Aristotle at this point? Simple, when Einstein made his postulates about gravity and acceleration observations in a closed box the instrumentation was not available to check his assumption. However that fact has changed and current instrumentation could be used to test those assumption about determining the difference between gravity and acceleration. Units of acceleration and gravity are named after Newton and Galileo respectively. One “newton” is one meter per second squared or acceleration. One “gal.” is one centimeter per second squared and is used for gravity. Even though these two units are used for different applications they are basically both terms that describe acceleration. In the 1930's about 20 years after the presentation of General Relativity, gravity meters were developed with a sensitivity of 1 part per 10,000,000. For a gravity acceleration of one “g”, that is 9.78 meters per second (newton) squared, or 9,780 cm per second squared (gal). The 1930 technology could detect changes of one ten millionths of a “g” or a change of ten to the minus four gal or 0.0001 cm per sec squared.. This technology measures changes in gravity with an accuracy of approximately 0.1 milligal. The solar and lunar tides have a combined effect from the Sun and Moon is 0.3 milligal. The contribution of the Sun is about 0.1 milligal and the Moon about 0.2 milligal. Change in elevation of about one foot reflect a change in gravity of about 0.1 milligal. Thus the 1930's technology could measure changes as a result of the sun and moon and even for a change in elevation of less than one foot. The current technology is three orders of magnitude better or can detect changes of ten to the minus seven or 0.0000001 cm per sec squared (gal). Measurement with the new instruments are of the order of 0.1 microgal, or they can measure a change in gravity for a change in elevation of one thousandth of a foot or about 0.3 millimeters. A second instrument that has the sensitivity to measure changes of this sensitivity is a three directional ring laser gyro. The ring laser gyros currently available are capable of detecting rotation rates as small as 0.00001 degrees per hour or 2 seconds of rotation per minute. The ring laser gyro is sensitive enough that if it is moved a few feet on the surface of the earth it can sense the move. Start a ring laser gyro anyplace on a rotating planet and the instrument can determine the radius of the planet, the rate of rotation, and the latitude where the measurement is being made. These gyros are used in tanks, submarines, and automated mining and tunneling. The channel tunnel under the English channel was mined from both side using ring laser gyros for guidance and imagine the surprise when the two machine meet head on missing by less that one foot in twenty six miles. The accuracy of the ring laser gyro is about 1:250,000. To put this all in perspective here on earth the gravity meters can detect elevation change for less than one thousandth of a foot and the ring laser gyro can measure its location laterally by about one foot. Let us return to that closed room and again ask the question is it possible to tell the difference between gravity and acceleration? Is it possible to make measurements in the closed room to determine if it is under the influence of gravity or acceleration by a constant force? If it is possible to determine the difference the postulate that there is no difference between gravity and acceleration is incorrect. Let us examine each of the possibilities and what measurement within the closed room will indicate. Subsequently let describe these differences mathematically and then compare them to determine the effect. So how can measurements made inside a closed capsule be used by an observer can tell the difference between gravity and acceleration. The answers lay in the measurement of are gradients.
What happens inside a room when under the effects of acceleration do to a force such as a rocket. Everything in the room is accelerating at the same amount. It does not mater where you measure acceleration in the room it will be the same. Thus no mater where acceleration is measured in the room with a modern gravity meter the reading would be exactly the same. Because there is no circular motion the ring laser gyro will also show no changes anywhere in the room. 1. In a capsule under constant linear acceleration there are no gradients. Acceleration by centripetal acceleration or force. What happens inside the same room if it is on the end of a tether and is under centripetal acceleration. The path of the room would be describing a cylinder in space. When acceleration is measured from floor to ceiling it would be discovered that gravity is stronger at the floor and weaker toward the ceiling. This gradient would be directly proportional to the length of the tether. Measurements made on the floor of the room when contoured would look like a bar code. The values would be equal when measured perpendicular to the direction of rotation. However the floor is flat and thus the reading at the edge of the room along the direction of rotation would be slightly farther from the center of rotation than the center line of the room. Thus gravity will be stronger at the edge of the room than in the middle. The ring laser gyro will measure the change in direction just like a stabilized platform thus it will detect the circular motion around the center of rotation. 1. In a capsule that is under radial acceleration there will be a vertical gradient. 2. Particles moving across the capsule will follow a different path in a gravity field than a field cause by radial motion. 3. The gravity gradient pattern on the floor will form a series of strips or bands. 4. Gravity at the edge of the room will be stronger than at the center of the room. 5. Centripetal acceleration will be abundantly evident from gyro measurements.
For the same room on the surface of the earth the measurement is the vertical or from floor to ceiling would be similar to room swung on a tether. Acceleration would be stronger at the floor and weaker at the ceiling. Measurements made on the floor of the room when contoured would make a bulls eye pattern. Again the floor is flat and thus readings at the edge of the room are further from the center of the earth thus they are weaker. Thus gravity will be stronger in the center of the room and weaker at the edges. This is just the opposite as when the room was swinging on a tether. If the body is not rotating the ring laser gyros will indicate that there is no rotational movement just as it did in room accelerated by a rocket. 1. If the capsule is in a gravity field of a body that is not rotating then there will be a vertical gradient. 2. The gravity gradient pattern on the floor will be a series of circles, because as you move out from the center the floor is progressively further from the center of the body. 3. Gravity at the edge of the room will be weaker than at the center of the room. If the capsule is in a gravity field of a rotating body the vertical gradient still exist. The gravity gradient pattern on the floor will be modified and a combination the two effects. Using the three component laser gyros it would be possible by moving along the edge of the room to determine the exact latitude, and the radius of the rotating body that the room is located on. By using modern instrumentation, a gravity meter and three component ring laser gyros, it is possible to conduct physics experiments and know if an enclosed room is under the influence of gravity, centripetal force, or being accelerated by a force such as a rocket. Let us return to that closed room and again ask the question is it possible to tell the difference between gravity and acceleration? The anser is a resounding yes! Thus, it is possible to tell the difference between acceleration and gravity. Yes, it requires very sensitive instrumentation, but it is possible to determine the conditions outside the room from inside using physics experiments. This provokes an even larger question what about relativity first expounded by Galileo and ultimately established by Einstein? What does this imply about general relativity? |