Science in Space

All is not well in the world of physics. The two fundamental pillars, general relativity, which describes the large-scale workings of the Universe, and quantum mechanics, which provides a framework for understanding the subatomic scale, refuse to stitch neatly together to formulate a theory of everything – the Holy Grail of physics.

The problem arises because general relativity, which explains how gravity works, breaks down at the subatomic level. The subatomic world is a world of uncertainty. It's full of fluctuations that apply not just to the position of subatomic particles but to spacetime as well. The smooth geometric fabric that Einstein described (like the surface of a trampoline) decays into a sea of writhing foam, one that becomes more chaotic the closer you look. Left-right, up-down, and even past, present and future are no longer predictable.

Such a fundamental conflict between general relativity and quantum mechanics has spurred many physicists on a quest to resolve the nagging issue. Others have learnt to live with it, content in the knowledge that as the conflict only arises at such minuscule scales (billions upon billions upon billions of times smaller than a centimetre), it's too small to lose sleep over. But for those who have risen to the challenge to uncover a unifying theory, there may be light at the end of what's been a very long tunnel. It comes in the form of 'string theory', a theory that stems from a revolution in the perception of the minutia.

The essence of string theory is that, unlike previous thinking, the smallest elements of matter are not point particles. There is a much smaller elementary ingredient from which all matter is ultimately made. The definitive smallest elements of everything are called strings. Strings are small, very, very small – inconceivably small. If an atom were scaled up to the size of the solar system, a string would only amount to the size of a house. This of course means we have no chance of observing strings with current technology. String theory then is a conceptual theory.

Brian Greene has spent the best part of his academic career working on string theory:

'It is a very strange research career, in a way. So far, I've spent something like 17 years working on a theory for which there is essentially no direct experimental support. It's a very precarious way to live and to work … In a sense, we're working on something unfalsifiable. But let me state categorically, if the theory is wrong, I'd like to know it today so I wouldn't waste my time on it any longer.'

'The string theorists have a theory that appears to be consistent and is very beautiful, very complex, and I don't understand it. It gives a quantum theory of gravity that appears to be consistent but doesn't make any other predictions. That is to say, there ain't no experiment that could be done nor is there any observation that could be made that would say, "You guys are wrong." The theory is safe, permanently safe. I ask you, is that a theory of physics or a philosophy?'

'I should say that there is a strong circumstantial case already that it's correct, because it puts together general relativity and quantum mechanics, and each of those theories has already received a fantastic amount of experimental support. String theory is the most developed theory with the capacity to unite general relativity and quantum mechanics in a consistent manner … That's what string theory does, and to me, that's pretty convincing.'

The allure of string theory is its ability to unite two apposing theoretical foundations on which modern physics is built – general relativity and quantum mechanics. The key issue in achieving unification is that strings are not point particles, they are loops of vibrating matter or energy (remember that the two are interchangeable through E=mc²). And of fundamental importance, strings differ from point particles in that they vibrate. Much like the strings of a guitar vibrate when they are plucked, these elementary strands of matter are constantly oscillating. And just like a plucked guitar string, each elementary string appears to 'smear' owing to its vibration. So, unlike point particles, their spatial extent is increased by virtue of their vibrating. Imagine taking a close up photo of a vibrating guitar string, the image will look blurred and the spatial extent of the blur will be greater than that of a single string at rest, the same is true of elementary strings. This has incredibly important consequences for uniting general relativity and quantum mechanics.

The problem with the idea that point particles constitute the smallest fundamental elements of matter is that they exist at a scale where quantum chaos rules – where the devastating effects of the uncertainty principle come into play. But the theoretical understanding that strings are vibrating and are smeared means they do not succumb to the weirdness of the uncertainty principle because of their increased spatial extent.

With string theory, the quantum chaos that arises from the uncertainty principle becomes redundant and the quantum foam can be tamed. In fact, with string theory, the quantum foam never exists. The uncertainty principle arose from the necessity to formulate a framework for quantum mechanics and general relativity with elementary particles that were considered to be point particles, of zero spatial extent. Whilst envisioning all particles to be point-like objects with no spatial extent, physicists were obliged to consider the properties of the Universe on arbitrarily short distance scales. And in considering these tiniest of distances they ran into the infamous uncertainty principle. Uncertainty is now seen to have arisen because they were unaware of the limits of scale and were led by a point particle approach to overstep the bounds of physical reality. But, if all matter is composed of strings, the extended nature of the string happily does away with the ultra-short-distance behaviour responsible for the dilemma of uncertainty. Hey-presto!

Although string theory is still a conceptual theory, it offers a powerful paradigm for the workings of the Universe and promises to provide the link needed to bind together general relativity and quantum mechanics. String theory also provides a tangible understanding of the way things actually behave in the Universe.

Experimental and theoretical physics has done an extraordinarily good job of discovering and explaining the workings of the fundamental building blocks of matter. The tools of the trade for the quantum mechanic are huge particle accelerators or atom smashers, which literally slam matter together at high energies, to release the elementary particles that constitute it. Galaxies, stars, you, me and the kitchen sink are all composed of just a handful of types of elementary particles, such as electrons, muons, and quarks. These elementary particles are the building blocks of atoms. Each type of elementary particle has its own characteristic profile, a specific mass and electric charge. These particles interact with each other by the workings of an even smaller number of 'messenger' or 'force particles', which constitute the four fundamental forces of the Universe: weak, strong, electromagnetic and gravity.

From this point of view the Universe appears to be incredibly finely tuned in the sense that the numerous processes that occur within it (like the thermonuclear reactions within stars, the formation of black holes and the explosive force of supernovae) can be explained in terms of the actions of a handful of elementary particles and the four fundamental forces [click on Elementary Particles]. If these parameters were to be changed by even a miniscule amount, the grand structure of the Universe would be completely different. The burning question is what dictates the precise parameters of these particles and forces? It's possible to measure their physical properties, but no-one knows why they are the way they are.

String theory comes to the rescue here too, encapsulated within it is an explanation for the way these particles and forces behave. Traditionally, elementary particles and the fundamental forces have been classified on the basis of their uniqueness. It seems sensible to suppose that electrons, for example, are made of a different material than quarks because their weight and electric charge are different. Once the connection was made that all particles of all types could be made of strings, string theory revealed that the characteristics of each fundamental particle could be a product of the unique vibration of each string.

In a nutshell, the fundamental particles of the Universe are not made of different material, but the same material. The reason they display different characteristics is because their internal strings are vibrating differently. If string theory is correct, the sum total of the unimaginably small vibrating strings equates to the harmonic symphony of the Universe we see around us.

While particles and forces are identifiable as separate entities today, most theorists believe that at extreme temperatures and pressures, such as those that existed during the Big Bang, all particles and forces would have been united. This might have worked in much the same way as ice cubes unite to form the same pool of water when heated. This idea is known as the 'standard model'.

In order for the standard model to be viable, it must be able to accommodate the workings of all the known particles and forces. This is one reason why many scientists so desperately want unification between general relativity, where the force of gravity dominates, and that of quantum mechanics, where the three other fundamental forces dominate. All four forces must get along, since these four forces are at the heart of everything in the observable Universe – including the Big Bang.

University of Toronto string theorist Amanda Peet explains why physics need strings to understand the Big Bang:

'If you just had point particles as your basic structure of everything, then right at the beginning of the Universe, everything would have been compressed down to some incredibly tiny – essentially infinitely tiny – distance scale, and the temperature would have been essentially infinite, and that's one of the reasons why it was difficult to describe the birth of the Universe within a theory that has only particles and doesn't have strings as the fundamental quantities. Because it seemed like you would have to deal with infinite things, and the trouble with infinity is that it's very difficult to calculate anything. What that stringy minimum distance phenomenon does for you is soften out that infinite behaviour into something that's finite. It would say that everything was crunched down to a very small size, but it wasn't infinitely small. And the temperature would have been very big, but not infinitely big. And so it provides you with a hope of actually calculating what happened at the beginning of the Universe.'

For the first time, the mathematics of vibrating strings, the heart of string theory, successfully describes what the theory of point particles could never do: the behaviour of a force particle for gravity – the graviton. By theorising that gravity is mediated by a force-particle, which itself is the result of a vibration in a string, string theory allows the harmonious merger of general relativity with quantum physics without gross distortions of spacetime. From this point of view string theory is a concept that has the makings of a truly powerful theory of everything.

Jim Gates, Professor of Physics at the University of Maryland explains how gravitons work:

'Einstein's equations describe how space and time are curved. The analogy is that the Universe is like a sheet of rubber, and when you put a piece of mass some place, it dents the rubber and causes things to fall in, and that's analogous to how gravity works. Now, if you were to take that mass that you dropped on a sheet of rubber and jiggle it back and forth, what would happen to the sheet of rubber? Very quickly you would build up ripples on the sheet of rubber as you take the mass point and jiggle it back and forth. Those ripples are in fact the graviton. So it's the waves of gravitational energy in spacetime that are responsible for communicating the gravitational force.'

Having put paid to quantum uncertainty and having succeeded in tying gravity into the world of the miniscule, you'd have thought that would be enough for strings. But of course, nothing's that simple. For a start, if everything is made from string vibrations, why should there only be a set number of particles? Why not an infinite number of string vibrations leading to an infinite number of particles and forces?

The nub of the theory is that the way strings vibrate is dictated by the geometry in which they are contained. Different fundamental particles reflect different string vibration patterns that in turn are dictated by different geometries – or different dimensions [click on Multidimensional Math].

As the notes produced by wind instruments are a reflection of the space in which the air is vibrating, so the vibration of strings is the result of the shape of the space in which they are contained. Another way to understand this is to imagine that you are locked in a box and that you wriggle in an attempt to break free. If the box you are in is a long thin box, you will wriggle in a long thin way, but if you are in a big square box, you will wriggle in a big square way. The wriggling movements that you make are dictated by the shape of the box, just as string vibrations are dictated by the dimensions in which they exist.

Fine, I hear you say, but it requires one more leap of faith to make the maths of string theory work. It turns out that strings operate in a world that contains ten dimensions, three space dimensions, one time dimension and six unknown ones. Technically, extra dimensions are needed to explain the existence of all the particles that have been observed in atom smashers. Having these extra dimensions means that a single string can vibrate in many different directions and therefore can produce the large variety of particles that have already been observed.

In the not too distant past, proposing extra dimensions is something that would have been scoffed at. But anyone who's serious about string theory has to take on board the idea that extra dimensions, parallel Universes even, might be science fact and not science fiction.