Gravity, the Mass(if) Mystery
When the average person talks about gravity, the first thing that comes to mind are objects falling down, Newton’s apple or Galileo’s leaning tower of Pisa with balls rolling down an incline or perhaps the swing of a pendulum. Or at least that was what my generation was brought up on. “Every particle of matter in the universe attracts every other particle with a force whose direction is along the line joining the two, and whose magnitude is directly proportional to the product of the masses divided by the square of their distance from each other.” that is Newton’s brilliant summary of two thousand years of enlightened learning. With a stroke of genius he combined heaven and Earth with the one natural force. The motion of the Moon and other heavenly movements could be explained by the same law that applied to a falling apple or the trajectory of a cannonball here on Earth. These days when you mention Gravity the images that come to mind are more likely to be black holes, neutron stars, missing mass or meteors colliding with Earth. We really don’t appreciate clockwork universes, we much prefer the ultimate answer to be an enigma. Such is the power of the popular science press that it changed something mundane, an everyday occurrence, back into an exotic mystery.
What Newton did to philosophy was that he opened the heavens to our mind. ‘If what is up there is the same as what is down here then there is no reason why we could not understand it as well’. With the help of his invention of the calculus(he called it fluxion at the time) he mathematically proved that the geometric centre of mass is also the centre of gravitational attraction, and that gravity cancels itself at every point within a hollow shell of matter. People began to study the stars. And, as happens so often in science, when you widen your horizons it helps you to understand your own environment better. We started to compere and to learn from the other planets and the stars and science on Earth went ahead in leaps and bounds.
But our fascination with Gravity did not start with Newton. Newton published his Principia in 1687. It is considered the most important scientific book ever written, and influenced the course of science over the next three centuries. But some 80 years before, Galileo wrote his controversial Starry Messenger, and Kepler his Solemnium (the Dream) (arguably the first science fiction book) which tells the story of a man who travels to the moon. The monk Copernicus, who turned the world on its head with his sun-centred universe, was not really an astronomer. He changed the arrangements of the solar system more for aesthetical reasons. He did not like the epicycle system of the Greek. To him our Lord in making the world surely would not have had to resort to such devious tricks as epicycles on epicycles to arrange the planets. As far as accuracy of calculations of planetary motion went the Copernican system was no improvement on the system of Ptolemaeus. In fact in most cases it was worse. It was Kepler and his ellipses who finally proved the case for the sun-centred solar system. But the mystery of Gravity, action at a distance, remained.
My personal fascination with the subject of Gravity started when they taught us in school the similarity of the structure of an atom to a planetary system. It was as though nature repeated itself on a different scale. Yes, that is going back a long way. But to me that picture brought home the connection that seems to exists between the infinitely small and the infinitely large. I came to realise the way to understand the world is to assume the universe is built up of a limited number of parts, or modules, or programs, which are used again and again at different levels of structure. Mysteries we do not understand can sometimes be solved by building a scaffold or a bridge of assumptions, or hypotheses from what we do know to the new frontier. Or by just stepping back a few scholarly paces from a problem. Someone’s mystery may be a common occurrence, in a different guise, somewhere else. No other tool has been as successful in solving mysteries than the cross-fertilisation between different disciplines. Subjects as far apart as chemistry and electronics, physics and biology, yes even cosmology and consciousness have at times been linked successfully to further our understanding.
Astronomy more that anything else seems to tie the very large to the very small. Perhaps this is because nothing is bigger than astronomy (or cosmology) and our mind needs to have a familiar (man size) picture in which to insert the variables. Somehow we feel that brings the problem down to Earth. Newton brought Gravity down to Earth. It became common knowledge. Anybody with a few simple calculations or a slide rule could now put the planets and the stars in their place. Whole Galaxies were forced to follow Newton’s reasoning. Or were they? Einstein did not set out to shake Newton’s apple tree. He just wondered what the world would look like riding on a beam of light. Then he realised time would actually stand still, distance would disappear and mass become meaningless. And it took him ten years to sort this mess out.
Having earlier declared Newton’s hypothesis law, scientists (now wary of absolutes) called Einstein’s conclusions a theory. And a Theory of Relativity it remains to this day, despite the fact its findings have been proven beyond doubt by science, industry and astronomy. The reluctance of the scientific establishment to award l-a-w status to Relativity is not based on insufficient proof, no, it is just a reflection of our greater understanding these days that all so-called laws are really limited in scope to a particular domain. Today the question is no longer whether Relativity or Quantum theory is correct, because both are needed in the respective domains where they are applicable. And these domains are strangely enough not even differentiated by size or mass, rather by the question you ask, by the answer you seek, by what you want to test for. Scientist now wonder if there exists a deeper, more fundamental level that ties the two together.
No longer is the strict separation valid that relegated quantum physics to the very small and gravitation to the very large. These days we talk of quantum cosmology and molecular gravity every day. For instance, do you know how a tree knows how to grow straight up? The part of the sap that makes cells stretch and grow is ever so slightly heavier than the rest of the sap and settles at the lower part of the tree-trunk. Consequently the cells at that spot in the tree grow bigger and tilt the trunk the other way. In the absence of other influences in its growth, such as shade or dominance by other branches the tree cannot do other than grow vertically.- Or how do you think a fertilised egg knows where to place the various organs of the developing embryo? The heart, the head, the limbs? You guessed it, by gravitational separation of the various growth hormones in the egg.
But it is the weakness of the gravitational attraction that makes Gravity the least exact of all the sciences. G, at 6.67 x 10-11 N is by far the weakest of all the known natural forces. In fact it is over 30 magnitudes smaller than the electromagnetic force. Cavendish, in 1797 with an ingenious device established the value to one decimal place. Today, with all the scientific tools we have, this has only been improved to an accuracy of four decimal places. Compared to other natural constants, which routinely are measured to fifteen decimal places this is an appalling lack of certainty. For in spite of its feeble nature Gravity is the force that shapes our world. It pervades everything. It shapes the Universe. I’ll come back to that with a proviso.
One option open to us in further exploring the nature of this elusive, massive mystery, is to look for other aspects of gravity. Look and see if there are other measurable ways in which gravity affects the environment. An incidental side effect of Einstein’s General Theory of Relativity is that gravitational disturbances should produce Gravity Waves that travel through space at the speed of light and should theoretically be measurable, if you had equipment sensitive enough to detect them. Einstein himself is known to have said these gravity ripples in space from ordinary celestial disturbances will never be detected for by the time the reach Earth they will be weaker than the thermal noise in the equipment used to detect them. But then people did some calculations on disturbances caused by extra-ordinary celestial events, such as colliding stars, collapsing black holes and came up with figures which come in the realm of modern day detection equipment.
In the 1960s Joseph Weber build elaborate detectors consisting of two huge bars of very pure aluminium, cooled to within a whisker of absolute zero, and placed ultra-sensitive distortion detectors between them. Some initial success in 1969 prompted the sceptics to ask for a second detector unit to be built to eliminate spurious noise results by simultaneous validation. There are still several of these “Bar Detectors” place in locations around the world, but no detection passed the co-incident test unambiguously in the intervening years. The exercise proved that if gravity waves do exist, they must be below the level Weber’s equipment was capable of detecting.
Some thirty years later a new generation of physicists, refugees from the demised Super Collider Project in the US, among them Kip Thorne from Caltech, are about to start test-tuning a new venture called LIGO, or Laser Interferometer Gravitational-Wave Observatory. In this project laser beams are sent down two 4 km tunnels at right angles and reflected repeatedly by end-mirrors. It is hoped gravity waves will stretch and squeeze the space between mirrors and result in interference fringes when the two light beams are recombined. The challenge is daunting: the equipment must detect reliably mirror displacements of less than one thousandth the diameter of a single proton. The project is expected to take two years of fine tuning, and even then success is not assured.
An indication of the importance placed on a better understanding of Gravity, the massive mystery, is that here 365 million dollars are invested in a project with such feeble promises of success, and an upgrade of the facility is already planned for 2007. But then, as one of the physicists in the project put it, even a negative result will be of considerable value, it will have deep implications as far as the nature of Gravity is concerned.
Here I would like to leave the domain of facts and figures and venture for a few minutes into speculation and hypotheses. Build a bridge into the unknown, so to speak. Apply known patterns to unknown quantities. For example look at our understanding of the expression Temperature. What do we measure by it? Movement, of course. We compare the speed of molecular movement in various substances, or movement variations over time. Temperature is a convenient way of expressing in simple form the definition of speed: that is distance over time. But we do not express the speed of an automobile in degrees of temperature. Or does hotrod qualify in that direction? Could you quantify a traffic jam by its temperature? Density versus movement? You get my drift. - Returning to gravity it seems odd to me that we have three so called fundamental units: inertia, the unit of time (second) and the unit of distance (meter) and yet all three are dependent on each other in some strict (if mysterious) sense, and this interdependence we call Gravity. Does convention here too put blinkers on us to see only their differences in other applications? Mass is of course the common factor between the three of them. Mass makes itself known to us through inertia and through gravitational attraction. Einstein has shown gravity to be a product of distance and time, so we are left with the three fundamentals: Inertia, distance and time. Remember, Gravity, despite its seemingly powerful omnipresence, is not a fundamental. It is an artificially derived product, like temperature above. So what happens if we eliminate the mass factor, what remains of the other units? Voila, they disappear! (I include energy here in the definition of mass)
Have you ever played with an alarm clock and disassembled it by taking the escapement off? For those of you born in the last thirty years, that’s the thing that went tick - tock - tick - tock in the days before batteries replaced the springs in clocks. You know of course what will happen. The thing will go brrrr and the spring is run down instantly. Now that is exactly what would happen to a world with no mass. Without mass there is no inertia and if in such a world you took a trip, the start of your journey would at the same time be the finish. In other words in the absence of mass there is no gravity, time does not exist, nor does distance. If you think this is stranger than fiction I tell you it is the only prediction that makes sense in the paradoxes of spooky quantum action at a distance. Such as the behaviour of single photons going multiple ways at the same time, or the notion that entangled particles can affect one another instantly across vast distances.
There is a report in a recent Science magazine of an experiment that showed two photons reacting to each other, communicating over a distance of 10km at a speed which must have been in excess of 107 times the speed of light. 10 million times faster than light. Think of it! And that limit was only set by the limitations in the switching speed of their pico-second apparatus.
The title of my talk was Gravity, the massive mystery. I tell you there is a massive mystery yet to be discovered. Understanding Gravity may just be the key that opens the door to a new world. A world not in another dimension, but Infinity, a world without dimensions.