by Mike Bennett
The zip drives were soon overtaken by the development of the rewritable CD/DVD. These drives were then fitted to nearly all computers, rendering the previous internal floppy disk drives and the external zip drives obsolete. However with the massive advance in the memory capacity available on silicon chips, it would not be long before all mechanical drives were effectively obsolete.
Today most people back everything up on USB flash drives. The memory capacity of these sticks would have been unthinkable just ten or fifteen years ago. You can now purchase a flash drive the size of your fingertip which has a memory capacity upwards of 1,000 GB. Even five years ago, most new and fairly expensive computers had nothing like this memory capacity available.
I know that today a lot of people complain because when you buy software with your new PC, tablet or whatever you are not supplied with any type of backup should your system crash. I believe that the software manufacturers have taken advantage of the phasing out of CD/DVD data storage. These days, although you buy a nice-looking plastic box which could contain a disc (to try to justify the hundreds of dollars that you have just spent the software), all you get inside the box is a piece of cardboard with an “authorisation code”, which is required in order to make the software work.
The software is now preloaded onto the device prior to purchase, and then you need to get online and enter your authorisation code to activate it. The problems start when your computer completely freezes, and the hard drive within the computer must be reformatted in many cases, or even replaced. I have heard that the term coined by the computer hardware manufacturers for this is “built-in redundancy”. This new phrase is almost as good as “fiscal easing”, aka printing money.
Should you try to download the originally installed software from the web, it will not be activated as you have already used the original code supplied with the software. Helpline phone numbers seem to be permanently busy, and your emails to their website go unanswered. As you have no CD/DVD backup of the programs, you are often forced to repurchase the software that you have already paid for. I can only imagine the size of the bonuses paid to the software engineers who thought up this piece of marketing genius.
CHAPTER 21
Some of the greatest advances during the last few decades have been made in both terrestrial and space telescope technology. This has greatly increased our investigative power in attempting to understand more about gravity generation and interaction, particle physics, and our understanding of the universe in general.
Telescopes are normally classified by the wavelengths they can detect. The main groups are:
1. X-ray telescopes, using shorter wavelengths than ultraviolet light
2. Ultraviolet telescopes, using shorter wavelengths than visible light
3. Optical telescopes, using visible light
4. Infrared telescopes, using longer wavelengths than visible light
5. Sub-millimetre telescopes, using longer wavelengths than infrared light
As wavelengths become longer, it becomes easier to use antenna technology to detect the electromagnetic radiation. The near-infrared can be handled much like visible light, however in the far-infrared and submillimetre range, the telescopes used are known as radio telescopes. When I was an undergraduate in Manchester, the largest UK radio telescope at that time was at Jodrell Bank, and operated by our physics department.
The amount of data that scientists have collected using these various different types of telescopes is huge. When Einstein first postulated his special theory of relativity and subsequently his general theory of relativity, he did not have access to the type of data available today. Einstein postulated that gravity is caused by the bending of space time. Although this is still believed to be one cause of gravity, it does not explain the full picture that we can now see.
Using modern telescopes, we can observe cosmic objects unknown to Newton and Einstein, such as twin neutron stars. These can produce gravity waves that radiate out through space and can be detected on Earth. At the time of writing, physicists are discussing string theory, and the associated gravity waves that strings generate. Also being discussed is the possibility of gravity being transmitted by a particle dubbed the graviton. The jury is still out regarding these theories, but we are gaining more information every year.
There are, however, objects in the cosmos that we still know very little about. Black holes for example are objects with such an enormous density that they suck in anything within their vicinity. Around the periphery of a black hole is a region that is known as an event horizon. This is the boundary beyond which no object cannot escape the gravitational force of the black hole. It is not even possible to observe the events taking place inside the event horizon.
It is believed that a singularity lies at the centre of each black hole. A singularity is a point at which space time is distorted so violently that it is meaningless, and space time is effectively pinched out.
We will now have a brief discussion about space time. Science today accepts that space and time are basically the same thing, hence the phrase “space time”. As an object accelerates faster towards the speed of light, not only does its mass increase, but time as observed by that object slows down. Perception of space time is dependent on the frame of reference of the observer.
To understand this, imagine a passenger on a fast train bouncing a tennis ball up and down against the floor. Another passenger in the same carriage would observe the motion of the ball to be vertical, covering the short distance between the hand and the floor of the carriage. However, if this same motion was observed by another person who was standing on a station platform as the train passed by, from their point of reference the ball would appear to be travelling in a long diagonal motion and covering a much longer path.
Many years ago, I used to get into some interesting discussions with one of my wife’s best friends regarding science. She was often sitting in our lounge drinking coffee when I returned from an overseas trip. When I joined their conversations, I sometimes found it quite amusing. She often tried to impress me with her almost non-existent knowledge of science, and her peer group with her supposedly green credentials.
On one occasion, she had just purchased an electric-powered car, a vehicle which none of her friends at that time owned. She would bleat on about how environmentally friendly she was, as her car produced zero emissions. I said, “Okay, but where do you think the electricity to recharge the batteries comes from?” She said, “Oh, for green cars it comes from solar power, wind power and only renewable resources.”
“Okay, but when do you normally recharge the batteries in your car?” I said. “Oh, at nighttime of course,” she replied, so I suggested that solar power was not available at nighttime. She then replied that it obviously used wind power then. She really did need to be led by the nose. I then asked her where the power came from on a windless night. At that point she realised that she was painting herself into a corner. Surprisingly she did not like me very much.
In physics, string theory is a theoretical framework in which the point-like particle is replaced by one-dimensional objects called strings. String theory aims to explain all types of observed elementary particles using quantum states of these strings. In addition to the particles postulated by the standard model of particle physics, string theory naturally incorporates gravity, and so is a candidate for a theory of everything, a self-contained mathematical model that describes all fundamental forces and forms of matter. Besides this hypothesised role in particle physics, string theory is now widely used as a theoretical tool in physics, and has shed light on many aspects of quantum field theory and quantum gravity.
The earliest version of string theory, called bosonic string theory, incorporated only the class of particles known as bosons, although this theory developed into superstring theory, which postulates that a connection a “super symmetry” exists between bosons and the class of particles called fermions. String theory requires the existence of extra spatial
dimensions for its mathematical consistency. In realistic physical models constructed from string theory, these extra dimensions are typically compacted to extremely small scales.
String theory was first studied in the 1970s as a theory of the strong nuclear force before being abandoned in favour of the theory of quantum chromo-dynamics. Subsequently, it was realised that the very properties that made string theory unsuitable as a theory of nuclear physics made it an outstanding candidate for a quantum theory of gravity.
After five consistent versions of string theory were developed, it was realised in the mid-1990s that these theories could be obtained as different limits of a conjectured eleven-dimensional theory called M-theory. Many theoretical physicists believe that string theory is a step towards the correct fundamental description of nature. This is because string theory allows for the consistent combination of quantum field theory and general relativity, agrees with general insights in quantum gravity such as the holographic principle and black hole thermodynamics, and has passed many non-trivial checks of its internal consistency.
According to Hawking, M-theory is the only candidate for a complete theory of the universe. Other physicists, such as Richard Feynman, Roger Penrose and Sheldon Lee Glashow, have criticised string theory for not providing novel experimental predictions at accessible energy scales and say that it is a failure as a theory of everything.
The starting point for string theory is the idea that the point-like particles of elementary particle physics can also be modelled as one-dimensional objects called strings. According to string theory, strings can oscillate in many ways. On distance scales larger than the string radius, each oscillation mode gives rise to a different species of particle, with its mass, charge and other properties determined by the string’s dynamics. Splitting and recombination of strings correspond to particle emission and absorption, giving rise to the interactions between particles. An analogy for strings’ modes of vibration is a guitar string’s production of multiple distinct musical notes. In this analogy, different notes correspond to different particles.
In string theory, one of the modes of oscillation of the string corresponds to a massless, spin-2 particle. Such a particle is called a graviton since it mediates a force which has the properties of gravity. Since string theory is believed to be a mathematically consistent quantum mechanical theory, the existence of this graviton state implies that string theory is a theory of quantum gravity.
String theory includes both open strings, which have two distinct endpoints, and closed strings, which form a complete loop. The two types of string behave in slightly different ways, yielding different particle types. For example, all string theories have closed string graviton modes, but only open strings can correspond to the particles known as photons. Because the two ends of an open string can always meet and connect, forming a closed string, all string theories contain closed strings.
The earliest string model, the bosonic string, incorporated only the class of particles known as bosons. This model describes, at low enough energies, a quantum gravity theory, which also includes (if open strings are incorporated as well) gauge bosons such as the photon. However, this model has problems. What is most significant is that the theory has a fundamental instability, believed to result in the decay (at least partially) of space time itself. In addition, as the name implies, the spectrum of particles contains only bosons, particles which, like the photon, obey particular rules of behaviour.
Roughly speaking, bosons are the constituents of radiation, but not of matter, which is made of fermions. Investigating how a string theory may include fermions led to the invention of super symmetry, a mathematical relation between bosons and fermions. String theories that include fermionic vibrations are now known as superstring theories. Several kinds have been described, but all are now thought to be different limits of a theory called M-theory.
Since string theory incorporates all of the fundamental interactions, including gravity, many physicists hope that it fully describes our universe, making it a theory of everything. One of the goals of current research in string theory is to find a solution of the theory that is quantitatively identical with the standard model, with a small cosmological constant, containing dark matter and a plausible mechanism for cosmic inflation. It is not yet known whether string theory has such a solution, nor is it known how much freedom the theory allows to choose the details.
One of the challenges of string theory is that the full theory does not yet have a satisfactory definition in all circumstances. The scattering of strings is most straightforwardly defined using the techniques of perturbation theory, but it is not known in general how to define string theory non-perturbativley. It is also not clear as to whether there is any principle by which string theory selects its vacuum state, the space time configuration that determines the properties of our universe.
The motion of a point-like particle can be described by drawing a graph of its position with respect to time. The resulting picture depicts the world line of the particle in space time. In an analogous way, one can draw a graph depicting the progress of a string as time passes. The string, which looks like a small line by itself, will sweep out a two-dimensional surface known as the world sheet. The different string modes (giving rise to different particles, such as the photon or graviton) appear as waves on this surface.
A closed string looks like a small loop, so its world sheet will look like a pipe. An open string looks like a segment with two endpoints, so its world sheet will look like a strip. In a more mathematical language, these are both Riemann surfaces, the strip having a boundary and the pipe none.
The interactions in the subatomic world can be described in two ways, either as world lines of point-like particles in the standard model, or as world sheets swept up by closed strings in string theory.
Strings can join and split. This is reflected by the form of their world sheet, or more precisely, by its topology. For example, if a closed string splits, its world sheet will look like a single pipe splitting into two pipes. This topology is often referred to as a pair of pants. If a closed string splits and its two parts later reconnect, its world sheet will look like a single pipe splitting in two and then reconnecting, which also looks like a torus connected to two pipes (one representing the incoming string, and the other representing the outgoing one). An open string doing the same thing will have a world sheet that looks like an annulus connected to two strips.
In quantum mechanics, one computes the probability for a point particle to propagate from one point to another by summing certain quantities called probability amplitudes. Each amplitude is associated with a different world line of the particle. This process of summing amplitudes over all possible world lines is called path integration. In string theory, one computes probabilities in a similar way, by summing quantities associated with the world sheets joining an initial string configuration to a final configuration. It is in this sense that string theory extends quantum field theory, replacing point particles by strings. As in quantum field theory, the classical behaviour of fields is determined by an action functional, which in string theory can be either the Nambu–Goto action or the Polyakov action.
An intriguing feature of string theory is that it predicts extra dimensions. In classical string theory the number of dimensions is not fixed by any consistency criterion. However, to make a consistent quantum theory, string theory is required to live in a space time of the so-called “critical dimension”: we must have twenty-six space time dimensions for the bosonic string and ten for the superstring. This is necessary to ensure the vanishing of the conformal anomaly of the world sheet conformal field theory.
Modern understanding indicates that there exist less trivial ways of satisfying this criterion. Cosmological solutions exist in a wider variety of dimensionalities, and these different dimensions are related by dynamical transitions. The dimensions are more precisely different values of the “effective central charge”, a count of degrees of freedom t
hat reduces to dimensionality in weakly curved regimes.
One such theory is the eleven-dimensional M-theory, which requires space time to have eleven dimensions, as opposed to the usual three spatial dimensions and the fourth dimension of time. The original string theories from the 1980s describe special cases of M-theory where the eleventh dimension is a very small circle or a line, and if these formulations are considered as fundamental, then string theory requires ten dimensions.
But the theory also describes universes like ours, with four observable space time dimensions, as well as universes with up to ten flat space dimensions, and also cases where the position in some of the dimensions is described by a complex number rather than a real number. The notion of space time dimension is not fixed in string theory: it is best thought of as different in different circumstances.
Nothing in Maxwell’s theory of electromagnetism or Einstein’s theory of relativity makes this kind of prediction. These theories require physicists to insert the number of dimensions manually and arbitrarily, and this number is fixed and independent of potential energy.
String theory allows one to relate the number of dimensions to scalar potential energy. In technical terms, this happens because a gauge anomaly exists for every separate number of predicted dimensions, and the gauge anomaly can be counteracted by including nontrivial potential energy into equations to solve motion. Furthermore, the absence of potential energy in the “critical dimension” explains why flat space time solutions are possible. This can be better understood by noting that a photon included in a consistent theory (technically, a particle carrying a force related to an unbroken gauge symmetry) must be massless. The mass of the photon that is predicted by string theory depends on the energy of the string mode that represents the photon.
