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Seven theories of everything that pretend to describe the fundamental nature of the universe


vv009We still don't have a theory that describes the fundamental nature of the universe, but there are plenty of candidates. The "theory of everything" is one of the most cherished dreams of science. If it is ever discovered, it will describe the workings of the universe at the most fundamental level and thus encompass our entire understanding of nature. It would also answer such enduring puzzles as what dark matter is, the reason time flows in only one direction and how gravity works. Small wonder that Stephen Hawking famously said that such a theory would be "the ultimate triumph of human reason – for then we should know the mind of God". But theologians needn't lose too much sleep just yet. Despite decades of effort, progress has been slow. Rather than one or two rival theories whose merits can be judged against the evidence, there is a profusion of candidates and precious few clues as to which (if any) might turn out to be correct.

Here's a brief guide to some of the front runners.


String theory

This is probably the best known theory of everything, and the most heavily studied. It suggests that the fundamental particles we observe are not actually particles at all, but tiny strings that only "look" like particles to scientific instruments because they are so small.

What's more, the mathematics of string theory also rely on extra spatial dimensions, which humans could not experience directly.

These are radical suggestions, but many theorists find the string approach elegant and have proposed numerous variations on the basic theme that seem to solve assorted cosmological conundrums. However, they have two major challenges to overcome if they are to persuade the rest of the scientific community that string theory is the best candidate for a ToE.

First, string theorists have so far struggled to make new predictions that can be tested. So string theory remains just that: a theory.

Secondly, there are just too many variants of the theory, any one of which could be correct – and little to choose between them. To resolve this, some physicists have proposed a more general framework called M-theory, which unifies many string theories.

But this has its own problems. Depending how you set it up, M-theory can describe any of 10500 universes. Some physicists argue that this is evidence that there are multiple universes, but others think it just means the theory is untestable.


Loop quantum gravity

Although it hasn't had the same media exposure, loop quantum gravity is so far the only real rival to string theory.

The basic idea is that space is not continuous, as we usually think, but is instead broken up into tiny chunks 10-35 metres across. These are then connected by links to make the space we experience. When these links are tangled up into braids and knots, they produce elementary particles. Loop quantum gravity has produced some tentative predictions of real-world effects, and has also shed some light on the birth of the universe. But its proponents have so far struggled to incorporate gravity into their theories. And as with string theory, a true experimental test is still some way off.

A lecture from the CERN website by Carlo Rovelli (who initially introduced the theory of loop quantum



Causal dynamical triangulations looks pretty similar to loop quantum gravity at first glance. Just as loop quantum gravity breaks up space into tiny "building blocks", CDT assumes that space-time is split into tiny building blocks – this time, four-dimensional chunks called pentachorons. The pentachorons can then be glued together to produce a large-scale universe – which turns out to have three space dimensions and one time dimension, just as the real one does. So far, so good, but there's a major drawback: CDT as it currently stands cannot explain the existence of matter.

A little excerp of Renate Loll's vivid talk explaining Causal Dynamic Triangulation. JOUAL 2009 in CNR/PISA


Parallel Universes is a 2001 documentary produced by the BBC's Horizon series. The documentary has to do with parallel universes, string theory, M theory, supergravity, and other theoretical physics concepts. Participants include Michio Kaku, Paul Steinhardt, and other physicists


Quantum Einstein gravity

This idea, proposed by Martin Reuter of the University of Mainz, Germany, takes a rather different tack.

Part of the problem with unifying gravity and quantum mechanics is what happens to gravity at small scales. The closer two objects are to each other, the stronger the gravitational attraction between them; but gravity also acts on itself, and as a result, at very small distances a feedback loop starts. According to conventional theories the force should then become ridiculously strong – which means there's something wrong with the conventional theories.

However, Reuter has come up with a way to generate a "fixed point": a distance below which gravity stops getting stronger. This could help solve the problem, and lead to a quantum theory of gravity.

Modern physics is composed of two theories that are ferociously incompatible—reached its schizophrenic impasse: one theory, known as general relativity, is fantastically successful in describing big things like stars and galaxies, and another, called quantum mechanics, is equally successful in describing small things like atoms and subatomic particles.
Albert Einstein, the inventor of general relativity, dreamed of finding a single theory that would embrace all of nature's laws. But in this quest for the so-called unified theory, Einstein came up empty-handed, and the conflict between general relativity and quantum mechanics has stymied all who've followed. That is, until the discovery of string theory


This movie illustrates how excitations of geometry change as dictated by the Quantum Einstein Equations. The Max Planck Institute for Gravitational Physics made the accompanying movie of Quantum Spin Dynamics in Loop Quantum Gravity depicting the quantum evolution of geometry in Loop Quantum Gravity, with the colors of the faces of the tetrahedrals indicating where and how much area exists at any given moment.


Quantum  gravity

All the theories above assume that space and time exist, and then try to build up the rest of the universe. Quantum graphity – the brainchild of Fotini Markopoulou of the Perimeter Institute for Theoretical Physics in Waterloo, Ontario, Canada, and colleagues – tries to do away with them. When the universe formed in the big bang, Markopoulou says, there was no such thing as space as we know it. Instead, there was an abstract network of "nodes" of space, in which each node was connected to every other. Very soon afterwards, this network collapsed and some of the nodes broke away from each other, forming the large universe we see today.

Fotini Markopoulou tells us that our best theories ulimately fail. How is quantum graviity different? Find out in this talk from Perimeter Institute's Quantum to Cosmos: Ideas for the Future Festival in Waterloo, ON.

The Danish physicist Niels Bohr, who worked in Rutherford's lab, was the first to describe orbits of fixed size and energy in which electrons are free to travel without losing energy and falling toward the nucleus. According to this model, published in 1913, electrons can only occupy or jump between fixed energy levels and cannot reside in between these levels. In addition, once in their "ground state," electrons maintain the energy they contain. This energy keeps them in perpetual motion, allowing them to resist the attractive force of the nucleus.


Internal relativity

Developed by Olaf Dreyer of the Massachusetts Institute of Technology, internal relativity sets out to explain how general relativity could arise in a quantum world.

Every particle in the universe has a property called "spin", which can be loosely thought of as what happens to the particle when it is rotated. Dreyer's model imagines a system of spins existing independently of matter and arranged randomly. When the system reaches a critical temperature, the spins align, forming an ordered pattern.

Anyone actually living in the system of spins will not see them. All they see are their effects, which Dreyer has shown will include space-time and matter. He has also managed to derive Newtonian gravity from the model: however, general relativity has not yet emerged.

The Massachusetts Institute of Technology in the IdeasLab to discover the latest insights and perspectives on the nature of intelligence



In 2007 the physicist (and sometime surfer) Garrett Lisi made headlines with a possible theory of everythingMovie Camera.

The fuss was triggered by a paper discussing E8, a complex eight-dimensional mathematical pattern with 248 points. Lisi showed that the various fundamental particles and forces known to physics could be placed on the points of the E8 pattern, and that many of their interactions then emerged naturally.

Some physicists heavily criticised the paper, while others gave it a cautious welcome. In late 2008, Lisi was given a grant to continue his studies of E8.

Physicist and surfer Garrett Lisi presents a controversial new model of the universe that -- just maybe -- answers all the big questions. If nothing else, it's the most beautiful 8-dimensional model of elementary particles and forces you've ever seen.


GARRETT LISI is an unlikely individual to be staking a claim for a theory of everything. He has no university affiliation and spends most of the year surfing in Hawaii. In winter, he heads to the mountains near Lake Tahoe, California, to teach snowboarding. Until recently, physics was not much more than a hobby.

That hasn't stopped some leading physicists sitting up and taking notice after Lisi made his theory public on the physics pre-print archive this week ( By analysing the most elegant and intricate pattern known to mathematics, Lisi has uncovered a relationship underlying all the universe's particles and forces, including gravity - or so he hopes. Lee Smolin at the Perimeter Institute for Theoretical Physics (PI) in Waterloo, Ontario, Canada, describes Lisi's work as "fabulous". "It is one of the most compelling unification models I've seen in many, many years," he says.

That's some achievement, as physicists have been trying to find a uniform framework for the fundamental forces and particles ever since they developed the standard model more than 30 years ago. The standard model successfully weaves together three of the four fundamental forces of nature: the electromagnetic force; the strong force, which binds quarks together in atomic nuclei; and the weak force, which controls radioactive decay. The problem has been that gravity has so far refused to join the party.

Most attempts to bring gravity into the picture have been based on string theory, which proposes that particles are ultimately composed of minuscule strings. Lisi has never been a fan of string theory and says that it's because of pressure to step into line that he abandoned academia after his PhD. "I've never been much of a follower, so I walked off to search for my own theory," he says. Last year, he won a research grant from the charitably funded Foundational Questions Institute to pursue his ideas.

He had been tinkering with "weird" equations for years and getting nowhere, but six months ago he stumbled on a research paper analysing E8 - a complex, eight-dimensional mathematical pattern with 248 points. He noticed that some of the equations describing its structure matched his own. "The moment this happened my brain exploded with the implications and the beauty of the thing," says Lisi. "I thought: 'Holy crap, that's it!'"

What Lisi had realised was that if he could find a way to place the various elementary particles and forces on E8's 248 points, it might explain, for example, how the forces make particles decay, as seen in particle accelerators








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