Looking up at the night sky we sometimes can’t help but wonder, where did it all come from? What really happened at the big bang? To address this question, scientists need a quantum theory of gravity. Einstein had famously tried and failed to make his “theory of everything” consistent with quantum mechanics but has failed. String Theorists have also tried to fit relativity with QM with relatively little success. However, in the recent years the ability of Loop Quantum Gravity (LQG) to confront this question has been gaining increasing attention.
WE NEED A THEORY OF QUANTUM GRAVITY
To describe the phenomena of everyday life classical (objects “large and light”) physics is sufficient. General relativity (objects “large and heavy”) explains the gravitation force between massive objects, but only if they’re large enough so the quantum properties are negligible, quantum theory (objects “small and light”) accurately describes what happens at small length scales but only if the material is light enough so the gravitational interaction can be detected. But about situations in which objects are very heavy and very small at the same time?
To analyze Black Holes which are “small and heavy”, we need Quantum Field Theory. “Quantum fuzzyness” of particles and the geometry spacetime itself must be deconstructed. To describe such a situation we need a theory of Quantum Gravity which combines both Quantum Mechanics and General Relativity. It would help us explain black holes, stars collapsed to less than an atom, as well as the origin of the big bang, where the entire mass of the universe was concentrated (both objects “small and heavy”, see image above).
Loop Quantum Gravity is an attempt at treating gravity quantum mechanically. Instead of a continuous spacetime which can be divided indefinitely (as in General relativity). LQG tells us that space time has a discrete structure (a smallest/minimum “quanta” of spacetime, solving Xeno’s paradox), that it’s made of individual units which cannot be subdivided and work through the mathematics of “spin networks”. This has dramatic implications for the nature of our universe.
Einstein gave us his theory of relativity, in which gravity is coded in the very geometry of spacetime. That theory is very geometrical and precise, but Quantum Theory is a different animal, wrought with probability and uncertainty where nothing is definite. They are very different worlds, one uses algebraic tools so they can even vary mathematically. Uniting the two may be more important than you’d think. Once you have such a theory it has applications, predictions, technology, and so on. We, human civilization, may need this more than we know, which this essay will seek to explain…
LOOP QUANTUM COSMOLOGY
What if space isn’t as empty as we thought it was? LQG proposes that all space is woven together, creating an endless span of connections. LQG suggests that all matter, including space, has an atomic structure. EVerything regardless of size or location is woven together to allow the universe to function. This means space can have and house reactions just like any other object, eventually allowing big bang, big bounce, and big crunch to even occur. To sum it up “there is no space in space”.
We know in nature that many fields, like the electric and magnetic fields for example, are quantum mechanical in nature. Therefore, we expect that the gravitational field is also quantum mechanical in nature. On the other hand, Einstein told us that through general relativity that gravity is encoded in geometry. Therefore, in fact, the gravitational field is quantum mechanical, that even the very geometry of spacetime is subject to quantum mechanics. The geometry itself is quantum mechanical just as particles are and the drama that unfolds is the equations of Loop Quantum Gravity.
The main goal of any theory of quantum gravity is to tell us what happens when general relativity fails, when the gravitational field, the curvature of spacetime, and the density of matter is very high (close to plank scale). We just need to extend general relativity to include the nature of quantum gravity.
In the classical model, the universe begins with an infinite density of curvature, but according to Loop Quantum Cosmology, the density of the universe has an upper limit and it cannot become infinite. If we were to rewind the clock back in time we would not arrive at an infinite singularity. Instead, as it approaches maximum density it just runs out of room and bounces back and forth, expanding and contracting. A “quantum bridge” connects our universe to a previous contracting phase.
In LQG we get “The Big Bounce” rather than the Big Bang Singularity. To be clear, it does not predict that the universe will collapse (the discovery of Dark Energy indicates expansion), it says that IF the universe were to collapse in the future, then the event of that collapse would not be a singularity, just as it wasn’t for the birth of ours. LQG predicts that the universe WAS initially infinitely big, then contracted until it reached the minimum size (the effects of loop quantum gravity being important in avoiding that collapse), then produces a “bounce” where the universe can again expand forever (but doesn’t imply the universe will recollapse).
At the big bounce, it was an unimaginably tiny and repulsive force of Loop Quantum Cosmology that drove it to the new homogenous low entropy state.
CONTRAST WITH STRING THEORY (M-THEORY)
The standard model of particle physics covers most of the fundamental forces; the strong force, weak force, electromagnetism, and the Higgs field but essentially nothing about Gravity, the force that binds the universe together. The reason is simple. Gravity is incredibly more weak in the quantum world than the other known forces. This means there’s no chance we’ll see any effect of gravity in a particle physics experiment. Since our best theory of gravity is General Relativity, LQG proponants suggest applying it to the subatomic realm.
There should exist a theory of Quantum Gravity, otherwise we’ll not be able to describe everything in the world of the very small. We predict that a particle called a “graviton” should exist, we haven’t seen one but if it exist, it must be subject to gravity’s infinite range and be massless. Unfortunately, since gravity is such a weak force, it’s impossible to find the graviton in a particle physics supercollider experiment.
M-Theory proponents suggest that the standard model of particle physics is an inelegant theory altogether. 36 quarks and antiquarks, 19 or more free parameters, an overwhelming number of various subatomic particles, and only accounts for 4% of the matter-energy content in the universe (it doesn’t account for dark energy and dark matter). String theory proposes a higher theory that includes gravity, not included in the static model, and a whole new set of particles called “sparticles” (superparticles), a new “octave” of the string. This is called “Supersymmetry” and offers a better description of the interactions we find in nature. This next “String” octave is believed to be the source of dark matter and dark energy, they breaking of symmetry for this new level of “sparticles”.
M-Theory has been popular for many years although some have criticized it for not making testable predictions. String/M-theorists suggest this theory better unites General Relativity with Quantum Mechanics, but LQG proponents argue Loop Quantum Gravity offers more testable predictions. LQG could potentially see if Quanti spacetime may manifest itself as minute differences in the speed of light for different colors, or account for the entropy of black holes.
In LQG, researchers really focus with spacetime structure extensively. LQG researchers generally have a relativity background while those who work on String Theory have more of a particle physics background, where the emphasis is more on unifying the interactions of nature and not so much about a particular part of spacetime itself. In string theory, supersymmetry and higher dimensions are essential, but not so much in LQG.
This doesn’t mean that the two aren’t compatible, because they very well could be. But whatever the final theory is, it will assimilate some of the ideas from loop quantum gravity, particularly to do with the geometry of spacetime. But it would pick up some ideas of string theory, having to do with the unifications of interactions and some of the very powerful mathematical techniques.
In this sense, in fact, it is very likely that the final theory will have signatures coming from both these camps.
WHAT DOES THIS MEAN FOR US?
Human beings have long sought our cosmic origins and with modern cosmology and the big bang, it seems like we were close to discovering them. But if Loop Quantum Cosmology is right, the Big Bang is not the ultimate beginning, but rather a transition with a much longer history as big crunches and bounces end cycle after cycle. We might just be the single iteration in a long line of Earths.
We understand very little about the big bang because we don’t have a theory of quantum gravity, and that’s why physicists are so interested in it. They hope that they’ll be able to solve the last questions about nature.
What happens at the center of a black hole, what exactly is dark matter, why is the Cosmic Microwave Background Radiation so evenly distributed? And most importantly, what came before the big bang? Was it a bounce? Has all this happened before? So the search for our cosmic origins is not at an end. But perhaps we’ve opened the door on an exciting new Vista.
Through science, we can explore this vast expanse around us and LQG is just one more way to explore it. The universe is large and mysterious and there’s a lot we don’t know, but there’s a good chance that finding a theory of quantum gravity would help us understand a bit more of it…