M-theory and “The Grand Design”

[This is updated from when I first wrote a review of Hawking and Mlodinow’s book. M-theory seeks to unify all string theories. So far, no practical verification of the predictions of M-theory have been forthcoming, leading Peter Woit and others to describe string theory as “not even wrong.” On the other hand, Nima Arkani-Hamed describes string theories as “like democracy, the worst idea except for everything else.”]

Stephen Hawking’s book, The Grand Design,1 written with physicist and author Leonard Mlodinow, seeks to answer questions that many have asked:

Why is there something, rather than nothing?

Why do we exist?

Hawking and Mlodinow (H&M) also pose a question which potentially answers the first two:

Why this particular set of laws and not some other?

The answer, say H&M, is to be found in M-theory.2

The trivial answer to the last question is that, if the laws were different, we would not exist and would not be asking any questions. But the observed laws seem to be very finely tuned to allow matter to exist in extended forms, like atoms, molecules and us. This has been called the anthropic principle and, in its strongest form, has often been given as circumstantial evidence in favour of design, allowing god to slip back in after being excluded from all other observed processes.

H&M controversially argue for a strong anthropic principle: “the fact that we exist imposes constraints not just on our environment but on the possible form and content of the laws of nature themselves”. However, their argument does not rely on a grand designer but on the possibilities inherent in M-theory.

M-theory is an attempt to unify all of the forces of nature into one overarching explanation, encompassing the very large and the very small. The reason for trying to do this is not just a love of orderly explanations but that previous unifying theories, that which unified the electric and magnetic forces in the 19th century, that which included quantum mechanics (quantum electrodynamics – QED) and that which unified the weak force with the electromagnetic (EM) force (the Standard Model) in the 20th century, led to enormous benefits. Promising attempts to unify the strong force with the EM and weak forces have been made (Grand Unified Theories – GUTs). M-theory is an example of a Theory of Everything (ToE) which aims to include the gravitational force.

Why the urge to unify or to build more inclusive theories? This sounds like the sort of “blue skies” research that politicians scorn, in favour of research with commercial benefits. However, the work of James Clerk Maxwell in the 19th century to uncover the relation between electric and magnetic fields, curiosity-driven, showed that electromagnetic fields spread through space at the speed of … light! Thus, light was an electromagnetic wave, which led to the discovery of radio waves, microwaves, X-rays, gamma rays, and to untold benefits in medicine and communication. It is quite reasonable (though not guaranteed!) that future unifying theories will lead to useful outcomes.

H&M’s approach leans heavily on the work of my favourite scientist, Richard Feynman, a profound thinker but also an engaging and playful character. You would be rewarded if you looked into his life (and perhaps watched clips of interviews with him on the BBC website).

Feynman worked on the science of the very small, where quantum effects rule. One example concerns the behaviour of light when it shines on two vertical narrow slits very close together. This gives rise, not to two vertical bars on a screen, but to a wide horizontal band of dark and light bars.

This has classically been explained (by Thomas ‘Phenomenon’ Young, another fascinating character) as the interference of the peaks and troughs of waves, sometimes reinforcing, sometimes cancelling each other, much as ripples in water do. This fatally wounded the particle theory of light held by Newton.

This common-sense explanation was however shown to be inadequate, not least by the proof by Einstein that light could act as particles, photons, in the photoelectric effect. Newton’s theory rose again Lazarus-like. More oddly, when faint beams of light consisting of single photons are shone on a double slit, the interference pattern supposedly explained by wave behaviour is gradually reproduced, spot by spot!

The “solution” was to associate a probability wave with each photon so that where it ended up was essentially random but over time a distinct pattern emerged. It was as if each photon passed through both slits and the probabilities interfered with each other resulting in the detection of the photon at a particular place.

Theory predicted that matter particles would also have a probability wave associated with them and, sure enough electrons (and larger particles) behave in a similar way with a double slit – even single electrons interfere with themselves (this experiment was voted the most beautiful experiment in physics in 2002)! [see foot of post]

Feynman’s explanation is that the system, in this case the single electron/double slit/screen system, has not just one but every history. The particles take every possible path on their way from the source to the screen – simultaneously! Furthermore, our observations of the particles go back into their past and influence the paths they take.

If, like me, you’re going “What?”, you’re in distinguished company: Feynman himself said “I think I can safely say that nobody understands quantum mechanics”. Nevertheless, the theory has passed every test.

Lots of people are unhappy with the implication that someone has to be looking before a quantum process is “forced” to arrive at a particular outcome – and yet this has been confirmed by many experiments. It actually is the case that the outcome is influenced by the process of measurement or “detection” (though this need not be a conscious process – I prefer the idea of an interaction causing the quantum process to reach a conclusion).

This sort of crazy quantum behaviour obeys strict laws. Laws of nature are not like human laws which seek to encourage certain preferred behaviours. They explain how things behave and how they can behave. The laws of modern physics, including the modern understanding of gravity, explain an incredible range of observations to incredible precision and have made amazing predictions which have almost entirely been borne out. H&M pose more fundamental questions, including “Is there only one set of possible laws?”

The laws are, needless to say, not entirely known. While three of the four forces of nature, the electromagnetic, weak and strong forces, have provisionally been united in the “standard model”, crucially gravity still needs to be integrated into the picture. This what M-theory, incorporating string theory and supergravity, seeks to do. One of its startling predictions is that there are 10 space dimensions and one time dimension, in contrast with our everyday experience of three space dimensions and one time. The unobserved dimensions are rolled up very small, so that particles are actually vibrating strings or membranes.

M-theory does not predict the exact laws observed. These depend on how the extra dimensions are “rolled up”. A great many universes are possible, some 10*500 or 1 followed by 500 zeroes, each with a different combination of fundamental constants, and it is not surprising that we exist in one where the constants are compatible with the evolution of life. The “apparent miracle” is explained.

H&M point out that the law of gravity is not incompatible with the emergence of a universe “from nothing”. In particular, the principle of conservation of energy is not violated (because, while matter energy is positive, gravitational energy is negative) and, at least in quantum mechanics, what is not forbidden is compulsory. Furthermore, with a wide range of possible sets of constants, some (at least one!) universes must come into existence in which life can evolve. And here, without the need for a creator, we are!

1 Transworld Publishers, 2010. ISBN: 9780553819229

2 A fairly(?) simple explanation can be found here: http://www.damtp.cam.ac.uk/research/gr/public/qg_ss.html

Double slit - single electron diffraction experiment
Double slit – single electron diffraction experiment
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