Over the past decade, a field of physics has developed that postulates the existence of mysterious algebraic entities called spin networks. These networks—proposed as the constituent stuff of space and time—condensed to produce the Universe as we know it. That condensation resulted in the event that we currently call the Big Bang, giving the field its name: condensate cosmology.
It may sound like an odd idea, but we already know that the Universe works in very strange ways.
The idea, technically termed “Group Field Theory (GFT) condensate cosmology,” is a branch of quantum gravity, a field of physics that aims to establish the fundamentals of what everything from light and matter to space and time is made of. It is an idea based completely in theoretical calculations—and it’s totally untested for now. Condensate cosmology requires a great deal of abstract reasoning to even try to understand it.
Despite these challenges, quantum gravity has drawn a lot of attention from some of the sharpest minds in all of physics. Its ideas are bold and daring, highly creative, and extraordinarily imaginative.
Why quantum gravity?
Quantum gravity has been formulated to tackle one of the greatest problems in all of physics: the need to unite the two great theories of the 20th century—general relativity and quantum mechanics.
The former presents a framework for understanding the world in terms of space and time, and it covers behavior over large distances. General relativity introduces the notion that time is relative and that gravity itself exists because of a curved space-time. As Einstein first realized, a ball does not fall to the Earth because it is attracted to its mass, as Newton told us; it falls because of the existence of a space-time field that permeates the Universe and curves around large objects.
Quantum mechanics is a mysterious yet incredibly accurate theory that describes the world of the very small. It tells us that both particles and fields exist in discrete units that, because of uncertainty, can only be described probabilistically. The theory also describes entanglement, the bewildering phenomenon in which physical systems can be so intertwined with one another that they lose their independent, individual reality and start obeying rules that apply to a collective.
As far as we can tell, these two theories are both right—and in conflict. Their simultaneous existence generates a paradox, meaning physics is, in a sense, in disarray. While quantum mechanics deals with reality in discrete, granular fashion, relativity tells us that space-time, and therefore gravity, is continuous and non-discrete.
One way to deal with this is to give one of the theories precedence. Since we know the world is quantum, general relativity must be an approximation of an underlying quantum description of space-time itself. And this suggests that any unification of the theories requires that gravity become discrete.
Development of LQG
Over the past few decades, a branch of quantum gravity called Loop Quantum Gravity (LQG) has shown some potential in solving the challenge of making gravity discrete. LQG begins with Einstein’s field equations, but it takes a closer look at what might be hiding beneath the surface of space-time. The mathematics produced myriad discrete geometric objects, including loops, lattices, and polygons, arranged in various constructions called spin-networks and spin foams. Together, they can describe the structure of reality itself—these geometric oddities of LQG do not exist in space and time, but rather they are space and time and therefore the very constituents of gravity itself.
While recent progress has greatly elaborated LQG, the idea has a long history. The dichotomy between general relativity and quantum mechanics became apparent during the 20th century’s interwar period. This spawned approaches to quantum gravity that were developed by taking general relativity and using different methods to quantize it. But the approach to this problem changed during the 1970s and 1980s when physicists began to learn new things from semi-classical physics, according to Daniele Oriti, Heisenberg Group Leader at the Arnold Sommerfeld Centre for Theoretical Physics, Ludwig-Maximilians-Universität in Munich.
Oriti told Ars that, at that time, emerging ideas about black holes focused on using quantum mechanics to describe the matter fields around them. This work suggested that theorists might need another, more radical, approach to quantum gravity—rather than simply quantizing general relativity, a new way to understand the nature of space-time at a microscopic level might be needed.
These ideas derived from black holes suggested that the gravitational field itself is not really fundamental, regardless of whether it is classical or quantum. Instead, it became apparent that the gravitational field might be a manifestation of something more fundamental, something that does not look like a field at all and so cannot be described in standard spatio-temporal ways.
New approaches to quantum gravity, like LQG, began to emerge during this period. In the 1990s through to the 2000s, LQG gained a lot of credibility amongst a growing population of theoretical physicists. “One thing LQG achieved was a precise suggestion of how space-time could look at a more fundamental level,” says Oriti. “It was discovered that, at least according to the theory, the basic entities of space and time do not look at all like the gravitational field as we know it. In LQG we call these basic entities spin-networks, which are discrete, algebraic objects.”