From the atom to the galaxy, with everything in between, there is unambiguous evidence of design in the universe (some call it order) – there are distinct laws (physical, mental, and spiritual). From Isaac Newton (1643-1727) onwards – the era of modern science – scientists have increasingly seen the universe as a complex machine – a well-planned, mechanical contrivance. They saw design, or order, in the world around them.
Explanations which introduce physical, or seemingly material, ideas – forces, pressures, and tensions – are of course dynamical or mechanical in their nature. It is not surprising that such explanations also should have been attempted from ancient times onward. Plato (428/427-348/347 BCE) informs us that Anaxagoras (circa 500-428 BCE) claimed to be able to explain the workings of nature as a machine – he obviously was extremely deluded.
In more recent times Isaac Newton (1643-1727), Christiaan Huygens (1629-1695), and others thought that the only possible explanation of nature was mechanical.
However, Newton engaged an ‘outside’ force (software as opposed to mechanical hardware) called gravitation, although fully conscious of the metaphysical consequences of this idea. Newton argued that the movements of celestial bodies and the free fall of objects on Earth are determined by the same attracting force. The classical Greek philosophers did not consider the celestial bodies to be affected by gravity, because the bodies were observed to follow perpetually repeating ascending trajectories in the sky.
Newton discovered the relationship between the motion of the moon and the motion of a body falling freely on the Earth. By his dynamical and gravitational theories, he explained Johannes Kepler’s (1571-1630) three laws of planetary motion and established the modern quantitative science of gravitation.
Newton assumed the existence of an attractive force between all massive bodies, one that does not require bodily contact and that acts at a distance.
By invoking his law of inertia(bodies not acted upon by a force move at constant speed in a straight line), Newton concluded that a force exerted by the Earth on the Moon is needed to keep it in a circular motion about the Earth rather than moving in a straight line. He realised that this force could be, at long range, the same as the force with which the Earth pulls objects on its surface downward.
When Gottfried Wilhelm Leibniz (1646-1716) attacked Newton for introducing these ‘occult’ qualities and miracles into natural science, Newton replied that, “… to understand the motions of the planets under the influence of gravity, without knowing the cause of gravity, is as good a progress in philosophy as to understand the frame of a clock, and the dependence of the wheels upon one another, without knowing the cause of gravity of the weight which moves the machine, is in the philosophy of clockwork.”
There was a strong undercurrent of resistance to Newton’s gravitational force concept when it was introduced, since it seemed to represent an almost ‘magical force’ at a time when concrete rational thought was finally beginning to prevail over the superstition of the Middle Ages.
Today, largely as a result of the scientific acceptance of Newtonian gravity, we have grown accustomed to the idea of unexplained forces reaching across empty space to affect objects at a distance in someequally unexplained manner, because we still do not know what gravity, or magnetism is!
In Newton’s time such concepts were only known in stories of myth and magic. To scientists and philosophers such as Rene Descartes (1596-1650), it had been a long journey for society to shake of the superstition of the past and finally enter a welcome era of rational thought and debate.
Newton realised this fundamental problem with his theory of a gravitational force, and never claimed to be able to explain it. However, the compelling and rational nature of his accompanying mathematical model soon solidified the force of gravity as a ‘physical reality’ and a scientific fact that continued to grow in acceptance for centuries.
It is currently an accepted scientific law that the speed of light in a vacuum (c = 299 792 458 metre per second) represents an absolute upper speed limit for all objects and also on the speed of the propagation of all fields and all forms of energy through space. The only possible exception is the tachyon!
The tachyon is a hypothetical subatomic particle whose velocity always exceeds that of light. The existence of the tachyon, though not experimentally established, appears consistent with Albert Einstein’s (1879-1955) Theory of Special Relativity (1905), which was originally thought to apply only to particles travelling at or less than the speed of light. Just as an ordinary particle such as an electron can exist only at speeds less than that of light, so a tachyon could exist only at speeds above that of light, at which point its mass would be real and positive. Upon losing energy, a tachyon would accelerate; the faster it travelled the less energy it would have.
Newtonian Gravitational Theory comes with no speed limit. A common example of this is to imagine our Sun suddenly vanishing. While it would still appear as if the Sun were present for roughly eight minutes as the last rays of light eventually made their way to Earth at the speed of light, the gravitational field of the Sun would vanish instantly. The loss of gravity from the Sun would be immediately felt at any distance throughout our Solar System, and indeed throughout the entire Universe.
Although this ‘violation’ lacks any logical justification, a resolution to this conundrum can be found in Einstein’s General Relativity Theory (1916), since one of the key differences with this alternate theory of gravity is that the element of time is built into the equations of the model. However, this is only a proposed solution since the actual speed of gravity is unknown – no direct tests have ever been done to determine it.
Special Relativity Theory, for which Einstein is perhaps best known, is actually a special case of the broader Relativity Theory, which was put forth by Galileo Galilei (1564-1642) and further developed by Henri Poincare (1854-1912) and Hendrik Lorentz (1853-1928).
The work of Poincare and Lorentz on Relativity Theory is essentially a formal mathematical description of the fact that objects do not possess absolute motion, but only have motion relative to each other, but the propagation of light through space was still unresolved.
Einstein consequently decided that neither Newton’s First Law of Motion nor existing Relativity Theory could explain how light travels through space, and so, he proposed a modification of Relativity Theory.
However, the question arises why it was necessary to develop a new theory of gravity. The answer is that Newton’s theory violates Special Relativity, for it requires an unspecified ‘action at a distance’ through which any two objects – such as the Sun and the Earth – instantaneously pull each other, no matter how far apart. However, instantaneous response would require the gravitational interaction to propagate at infinite speed, which is precluded by Special Relativity.
In practice, this is no great problem for describing our Solar System, for Newton’s law gives valid answers for objects moving slowly compared with light. Nevertheless, since Newton’s theory cannot be conceptually reconciled with Special Relativity, Einstein turned to the development of General Relativity as a new way to understand gravitation.
In order to begin building his theory, Einstein seized on an insight that came to him in 1907. As he explained in a lecture in 1922, “I was sitting on a chair in my patent office in Bern. Suddenly a thought struck me: If a man falls freely, he would not feel his weight. I was taken aback. This simple thought experiment made a deep impression on me. This led me to the theory of gravity.”
Einstein was alluding to a curious fact known in Newton’s time: no matter what the mass of an object, it falls toward the Earth with the same acceleration (ignoring air resistance) of 9.8 metres per second squared. It was Galileo who first provided this very useful constant-acceleration equation for falling bodies.
Newton explained this apparent anomaly by postulating two types of mass: inertial mass, which resists motion and enters into his general laws of motion, and gravitational mass, which enters into his equation for the force of gravity. He showed that, if the two masses were equal, then all objects would fall with that same gravitational acceleration.
Einstein, however, realised something more profound. A person standing in an elevator with a broken cable feels weightless as the enclosure falls freely toward the Earth. The reason is that both he and the elevator accelerate downward at the same rate and so fall at exactly the same speed; hence, short of looking outside the elevator at his surroundings, he cannot determine whether he is being pulled downward.
In fact, there is no experiment he can do within a sealed falling elevator to determine that he is within a Newtonian gravitational field. If he releases a ball from his hand, it will fall at the same rate, simply remaining where he releases it. And if he were to see the ball sink toward the floor, he could not tell if that was because he was at rest within a gravitational field that pulled the ball down or because a cable was yanking the elevator up so that its floor rose toward the ball.
Einstein expressed these ideas in his deceptively simple principle of equivalence, which is the basis of General Relativity: on a local scale – meaning within a given system, without looking at other systems – it is impossible to distinguish between physical effects due to gravity and those due to acceleration.
However, then Einstein continued and lost his link with reality completely with the introduction of his concept of ‘warped four-dimensional space-time’.
The singular feature of Einstein’s view of gravity is its geometric nature. Whereas Newton thought that gravity was a force.
Einstein postulated that gravity arises from the shape of ‘space-time’. Einstein’s space-time can be visualised as a ‘rubber sheet’ that can be deformed (‘warped’). In any region distant from massive cosmic objects such as stars, space-time is absolutely flat. However, the presence of a massive body curves space-time, as if a bowling ball were placed on the rubber sheet to create a cuplike depression.
In this analogy, a marble placed near the depression rolls down the slope toward the bowling ball as if pulled by a force. In addition, if the marble is given a sideways push, it will describe an orbit around the bowling ball, as if a steady pull toward the ball is swinging the marble into a closed path.
In this way, the curvature of space-time near a star defines the shortest natural paths. In Einstein’s theory, space-time geodesics define the deflection of light and the orbits of planets. Clearly note that according to Einstein space-time, that is space and time, warps under gravity!
Even weirder still is the Quantum Theoretical concept of the graviton; where a graviton is a postulated quantum that is thought to be the carrier of the gravitational field. This so-called graviton is proposed to be analogous to the photon of the electromagnetic field.
Gravitons, it is then proposed, like photons, would be massless, electrically uncharged particles travelling at the speed of light and would be emitted only by highly accelerating, extremely massive objects such as stars. Since gravitons would apparently be identical to their ‘antiparticles’, the notion of antigravity is extremely questionable.
So, although we are familiar with the everyday effects of gravity and have ‘scientific’ models of these observed effects, we still do not know what gravity is at all; its ‘physical nature’ is still completely foreign and mysterious to us – even today. Therefore, your pitiable, local, friendly physicist/cosmologist still has some very serious black holes in his/her crown. I say this out of pity, not malice.
That’s life, we do not know everything (in fact we know very little), and although we have very many theories we still do not even understand the most basic facts about our physical realm. Yet, we are extremely arrogant.
I love reading your articles! Question: Could gravity be the opposite of force or repulsion? Sort of like, that which we cannot pull or keep away, it falls or bumps into us? Please pardon my ignorance, I'm just getting started reading about gravity/repulsion/anti-gravity, etc. - it is so darn interesting all these theories.
'Thatâ€™s life, we do not know everything (in fact we know very little), and although we have very many theories we still do not even understand the most basic facts about our physical realm. Yet, we are extremely arrogant.'