The 3 main forces incorporated into the Standard Model all have associated bosons – gravity should probably follow in the same way. A massless spin two particle called the graviton is believed to be the mediating particle in the gravitational force. But it has not yet been possible to build a fully self-consistent theory that contains the graviton. As previously stated, the main difficulty in finding a theory that unifies gravity with the other forces is that the theory of gravity – general relativity – is not a quantum theory: it does not take into account the uncertainty principle.
As the other forces depend on quantum mechanics in essential ways, unifying gravity would require the uncertainty principle to be incorporated into general relativity. In terms of particles, as in the Standard Model, the uncertainty principle means that even “empty” space is filled with pairs of both non-detectable virtual particles and antiparticles. If “empty” was a literal description of space, all fields such as the gravitational and electromagnetic fields would have to be exactly zero – but the value of a field and its rate of change through time is similar to the position and velocity (i.e. change in position) of a particle – the uncertainty principal applies and overall accuracy is dependent upon the balance between the accuracy of different measurements – never dropping below Planck’s constant.
If a field in empty space was zero and the rate of change was also zero, which are precise values, this violates the principle by removing quantum fluctuations. The fluctuations can be though of as pairs of virtual and non-detectable particles which appear together at some time, move apart, and then come together and annihilate each other.
Their indirect effects, such as small changes in the energy of electron orbits, can be measured. But these virtual particles (corresponding to those gauge bosons associated with the forces) have energy, and an infinite number of virtual particles means an infinite amount of energy. From Einstein’s equation E=mc2, this requires an infinite amount of mass, which according to the theory of general relativity means the universe would be curved to an infinitely small size.
Other such absurd infinities, known as singularities, occur in the particle theories of the other three forces – but these can be removed through a nifty mathematical process called renormalisation. The drawback with this is that it doesn’t allow prediction – measured values have to be chosen to fit the observations. Attempts to renormalise quantum infinities from general relativity have proved futile – the only adjustments to be made are the strength of gravity and Einstein’s cosmological constant – still leaving singularities in the curvature of space-time, for example.
There were lots of problems to overcome throughout the early development of String Theory – it predicted a particle which travels faster than the speed of light, for example, which is impossible. Scientists began to lose faith in a model which kept unlocking such complexities and by 1973 barely anyone was continuing to work on it. On the verge of scrapping the theory completely, it was noticed that, by re-thinking the size of these strings, there was a possibility of applying it to gravity – revealing an anomalous massless particle which was currently rather in the way – to be a graviton.
10 In 1984 the complexities and mathematical anomalies were removed and the theory had the mathematical depth to encompass all four forces of the Standard Model, nicely merging the quantum mechanics with general relativity. However, String theory has a major problem in that everything is so tiny and irresolvable by machine- if it cannot be observed or tested in the way that normal theories are tested, is it philosophy or science?
String theory also requires the existence of extra dimensions, but due to what we know of a 4- dimensional universe (including time) these extra dimensions must be hidden – otherwise planetary orbits or electron orbits would be unstable and things generally wouldn’t be the way they are. They are therefore said to be curled up with strings in different orientations – unlike the four dimensions we are used to which are fairly big and flat. The dimensions are said to determine the fixed fundamental constants of nature such as particle mass and force strength.
By the mid 1980s the theory seemed unstoppable. However, behind the scenes the theory was tangled. There were 5 different, but apparently equally valid, versions of the theory which shared the basis of strings and extra dimensions, but differed over whether the strings were loops or open ended strands, or how many extra dimensions were required. String theory was being abandoned again due to lack of progress and a theory which seemed to be unravelling into nothing.