Gravity: Quantization ca. 1990
String theory does not do away with the need for some kind of background spacetime. The equations describing the movement of strings do so from the standpoint of a string moving in a flat, Minkowski space of up to 26-D. The 1-D strings trace-out 2-D surfaces. The geometry of these surfaces have a set of internal symmetries associated with them which allow the fields to appear as different kinds of particles. There are literally millions of different topologies for these compact spaces. The number of holes in these spaces, the so-called topological genus, may have something to do with the number of different kinds of lepton types; electron, muon, tauon. There may exist regions in the universe where these spaces have chosen slightly different ways to curl-up so that differences in the fundamental particle types mayexist in our universe.
How do string theorists actually think about spacetime after the 1982 revolution? About the initial claim that spacetime had to be enlarged to 26-D, some physicists five years after the revolution didn't necessarily see that this was inevitable, or the only conclusion that can be drawn from the mathematical structure of the theory. Steven Weinberg, for example, was of theopinion that,
"...I talked about the extra 6 dimensions wrapping thenselves up, but that's not necessarily the way one thinks about it now. One thinks about the theory formulated in four dimensions but with some extravariables which can, in some cases, be interpreted as coordinates ofextra dimensions, but needn't be. In fact, in somecases, CANNOT be...The original picture of ten dimensions, of which six get curled up, is just seen as a special case..."
These extra variables may not even have a direct physical meaning, merely serving the bookkeeping needs of the theory as the coordinates of a fictitious internal space. When you divide one number by another in long division, all of the intermediate numbers you generate are merely means to an end, and are disgarded once you have arrived at the answer to the problem. Computers don't even generate these numbers but electronically jump to the answer in virtually a single step. As for the topology of spacetime itself, the implications of string theory are less clear.
The string-like character of all the fundamental particles does not become apparent until you reach a scale of 10-33 centimeters, or so the belief seems to be. Only then do strings stop looking like point particles. But this difference is enough to cure all string field theories of the infinities that plagues point field theories such as QED. It is at this scale that it may not be correct to think about the strings as moving through what we normally consider to be continuous spacetime. According to Michael Green,
"...In a theory of gravity, you can't really separate thestructure of space and time from the particles which are associated withthe force of gravity...The notion of a string is inseparable from thespace and time in which it is moving, and therefore if one has radically modified one's notion of the particle responsiblefor gravity, so that it is now string-like, one is also forced to abandon at some level the conventional notions ofthe structure of space and time...at these incredably short scales associated with the Planck distance...A lot of the present research is focussed on trying to understand precisely how [this] works..."
By 1987, Steven Weinberg also viewed spacetime in the post-string world as a very differentanimal from what it used to be,
"...I think that in these theories space and time may not turnout to have overwhelming importance. Space and time coordinates are just four out of the many degrees of freedom that have to be put together to make a consistent theory, and it's only we human beings who give them thatpeculiar geometric significance which is so important to us...What we are going to have is not so much a new view of space and time, but a deemphasis of space and time. I don't think that everyone should work on superstring theory, and I don't think that everyone should work on phenomenology and low energy physics. I think people ought to do what they can...I think it's also worth trying to look ahead, jump over 17 orders of magnitude in energy and look up to the Planck scale where the final answer may lie."
How much of string theory should we allow to sway our understanding of the vacuum? Perhaps none at all. Sheldon Glashow has been a very outspoken critic of superstring theory almost from its very first year in the theoretical lime light.
"... I know that they are not going to say anything about the physicalwourld that I know and love. Mainly, that's the reason that I don't like these theories...some of us at Harvard are still trying to follow the upward path, to go from experiment to theory, rather than pursuing the superstring vision, which requires the highest inaccessible dream-like energies to build a theory that deals with the down-to-earth world under our feet."
Richard Feynman has also offered his own skeptical opinions similar in spirit to Glashow's,
"...I have noticed when I was younger, that lots of old men in the field [of physics] couldn't understand new ideas very well...I'm an old man now, and these are new ideas, and they look crazy to me, and they look like they'reon the wrong track. Now I know that other old men have been very foolish in saying things like this...[but] I think all this superstring stuff is crazy and is in the wrong direction. I don't like that they're not calculating anything, I don't like that they don't check their ideas..."
In 1988, Feynman went onto criticize the idea that superstrings work only in 10 dimensions so an explanation has to seemingly be artificially created that shows why only 4 of the dimensions don't get compressed, instead of throwing the entire theory out as violating a very basic observational fact of our existence.
Related EoC Articles
- Gravity: Canonical Quantization
- Gravity: Covariant Quantization
- Gravity: Dimensionally-Extended
- Gravity: Gauge Field
- Gravity: Graviton
- Gravity: Kaluza-Kline Theory
- Gravity: Quantization ca. 1990
- Gravity: Quantum
- Gravity: Renormalization
- Gravity: String Theory
- Gravity: Superspace Quantization
- Gravity: Supersymmetry
- Gravity: What is Gravity?
Gravitational waves are propagating gravitational fields, "ripples" in the curvature of space-time, generated by the motion of massive particles, such as two stars or two black holes orbiting each other. Gravitational waves cause a variable strain of space-time, which result in changes in the distance between points, with the size of the changes proportional to the distance between the points. Gravitational waves can be detected by devices which measure the induced length changes. Waves of different frequencies are caused by different motions of mass, and difference in the phases of the waves allow us to perceive the direction to the source and the shape of the matter that generated them. (Source: NASA-The Laser Interferometer Space Antenna (LISA).)