“The sun was almost over the horizon. I was looking directly into it and then I saw the lines of the world. I actually perceived the most extraordinary profusion of fluorescent white lines which crisscrossed everything around me.”
– Carlos Castaneda, Journey to Ixtlan
One of the deepest and thorniest unsolved problems of physics is that of Quantum Gravity. As we have learned, both the theories of Relativity and those of Quantum Mechanics are astonishingly accurate when it comes to describing what they describe. The catch is when one tries to reconcile them with one another. That just doesn’t seem to work. What is needed is a singular and self-consistent theory that describes all aspects of reality (or at least that of our physical world)
Enter String Theory. It is promoted as just that; a theory of Quantum Gravity. One where the particles of particle physics are not really particles at all but one dimensional objects called strings. Bear in mind however that the length of these strings is very small indeed. On the order of the Planck length; which is 10-35 metres. At lengths above this the strings are not visible, and only seen as particles. And of course we have no way of ‘seeing’ at lengths that small.
Another catch, if you want to call it such, is that for String Theory to work, it requires the existence of multiple dimensions. There are differing versions of the theory that need from 11 to 26 dimensions. Although much of the current thinking is that the various versions are merely different aspects of a single theory requiring 11 dimensions. If, like me, your brain hurts trying to visualize commonplace everyday 4 dimensional space-time, then 11 is really gonna tax your grey matter.
There is yet one more caveat to String Theory in that it supposedly postulates a staggering number of possible universes. Something in the range of 10500 . I don’t know where you come from but where I come from that’s a pretty big number.
Back to the strings… The theory allows for them to come in both open and closed varieties. The open ones have their ends attached to something called a Brane; of which there are various types. M-Branes, D-Branes, and of course with typical physicist wit… P-Branes. Strings also vibrate. The vibration state of a string is what endows what we see as particles with their various properties such as mass and charge. The crux of the theory I suppose, is that there is a particular vibration of an open string that is responsible for the elusive, speculative, or apocryphal graviton. The particle that would carry the gravitational force. An open string is not attached to any Brane.
We have come a very long way in particle physics and have unearthed a veritable zoo of particles along the way, but as of yet no gravitons. Gravity waves…yes. Gravitons… no. Not even one. Apparently there are no fundamental law limitations to experientially detecting them (if they do exist). But rather the difficulty is purely in the physical enormity and time scales of the effort. Using current thinking, detectors would have to be built that would be planetary in scale… think large planet.
At one point in time not so many years ago there was great interest in and hope that the various string theories would provide unification of the relativistic and quantum realms. And while string theory is still probably the top contender out there, it is by no means accepted by all. Some theorists have started to turn their attention elsewhere…
So stay tuned for… Loop Quantum Gravity, Asymptotical Gravity, Causal Dynamical Triangulations, and Emergent Gravity