There are a couple of other little items about gravity that need to be discussed before moving on.
Newton was a genius for realizing that the force of gravity was proportional to the square of the distance from the source, but he never gave us a reason why he thought that it was. And to be sure that the gravity calculations would require the input of that distance squared, he had to build those units into his constant so that everything in the equations would work out right. So what he ended up with was a constant that is just so confusing and, well, limited in its ability to describe the situation. The units that describe it,
G = 6.674 X 10-11 Nm2/kg2 = 6. 674 X 10-11 m3/(kg * s2)
are such dimensional gobbledegook. Cubic meters per kilogram-seconds squared? What kind of constant is that anyway? But it gets even worse when you use the general force calculation:
Fg = G(M1m2)
and you realize that when ‘r’ becomes very small, the force, Fg, goes to infinity. No wonder they can’t get it to work in the quantum world. Utilizing Newton’s constant in the formulation for General Relativity means those little problems are still in there.
The second topic has to do with a specific prediction from the General Theory itself. General Relativity supposes that the force of gravity has a finite velocity, the speed of light, which gives it, unlike the Newtonian definition, a time function. And if it does have a time function, then it can change over time. In Dr. Einstein’s picture, the curved geometry of the space surrounding any object is constantly and consistently evolving.
In our universe, there are many binary star systems, that is, systems in which two stars are so close together that they orbit each other. Although this relationship does not have the overall stability of an isolated star, if each of the stars within the binary system has a sufficient velocity, the centrifugal force created by that velocity will keep the two stars apart for a long, long time. Sometimes these stars are massive, and sometimes one or both of them explode. We call this a nova. And what is left over after a star goes nova in many cases is a small, extremely dense core, which in some cases is so dense that it becomes a neutron star, that is, a ‘star’ of sorts that is made from only neutrons. Such a star can no longer radiate large amounts of energy, but they have extremely powerful gravitational fields. A time-evolving gravitational field.
And if such a star were orbiting a sufficiently massive partner close enough that the velocity is high (near light speed not required, but very fast) this system would be stable for a long time. Given what we know about the cosmos, it is very likely that systems like this exist, although, since a neutron star does not give off much energy that we are prepared to observe, they are difficult to detect.
A system like this is a classic oscillator, since the dark orbiting partner’s motion around must create an oscillating gravitational field around its more massive partner. Since gravity is evolving from the sources at a finite speed, this oscillation of the field should create a time evolving pattern of gravitational pulses. That is, waves. For what is a wave but a series of alternating pulses, transmitted through the fabric that is its medium?
In this case, the medium is space itself, bent to the form required by the gravitational fields.
So if this is the case, there must be gravity waves, variations in the field created by the oscillation of one massive object around another. And if gravity waves exist, we should be able to detect them. Gravity waves can also be created by other sufficiently massive events such as a supernova or the collision of two black holes.
In order to do this we have created a series of ‘gravity interferometers’ the most famous of which has been dubbed LIGO, which stands for Large Interferometer Gravitational wave Observatory and includes two nearly identical devices, one located in Louisiana and the other in Hanford, Washington. These interferometers work in tandem in order to eliminate the possibility that detector noise or a spurious event would be mistakenly identified as a gravitational pulse. They work very much like the interferometer employed by Michelson and Morley, but have the advantage of utilizing lasers rather than light that is only coherent. Each one has two arms that are perpendicular to each other, one being four kilometers long, and the other, two.
The theory that these devices are based on is this: that the passing of a sufficiently energetic gravitational wave should alter the fabric of space in and around the interferometer, it should alter the length of the path of the lasers in the devices and be revealed as in interference pattern exhibited by the rejoining of the split laser beams.
These devices have been operated and tuned over the years and have been demonstrated to have the sensitivity to detect a change in the path length of the lasers to within one thousandth of the diameter of a proton (a proton has a diameter of about 10-18 meters, so the accuracy of the measurement is in the range of 10-21meters), which gives the device, based on some estimations of the frequency of the waves expected, a detection range of about 26 million light years. Of course, that’s still 2½ orders of magnitude below our optical observations, but it’s still a very respectable portion of space. This accuracy was achieved in 2005.
Their findings so far: zero.
Actually, it’s a little less than zero because in February of 2007, during an experimental run, a gamma ray burst event was optically detected coming from the Andromeda galaxy. These events release so much energy in such a short period of time that it is thought that only a large gravitational disturbance could be at the source of their power. So, what did LIGO detect in February, 2007?
Currently, scientists are blaming this lack of detection on the instruments that they are using, and have proposed enhancements that will create a “LIGO 2” within the current LIGO facility and have also proposed a system called LISA (Laser Interferometer Space Antenna) which would involve three spacecraft serving as the endpoints for the right angle legs of the system.
However, the lack of findings by LIGO is a serious discrepancy in the science. Gravity waves are a direct prediction of General Relativity, and to date, none have been detected.
It is ironic, don’t you think, that the same device that gave the same sort of readings that started Dr. Einstein down the path that led to so much insight into the true nature of the universe, might provide the insight that something may be wrong with his ultimate theory.