General relativity and quantum mechanics are the twin foundations of modern physics, but there's a problem: they're mutually exclusive, at least according to our current understanding. A new model using loop quantum gravity might have the beginnings of a solution.
Relativity holds that reality is uniquely determined and always measurable, whereas quantum mechanics tells us that uncertainty and immeasurable values are woven into the very fabric of nature. Both of these facts have been backed up by countless experiments. So what's the solution?
Physicists are still hard at work at an answer to that question - an answer that might finally lead to the long sought after grand unified theory - but a team from Poland's University of Warsaw just might have gotten us a step closer. They have developed a new model of quantum gravity (more on that in a moment) that appears able to explain how classical space-time emerged from the quantum world, describes the full theory of general relativity, and is completely mathematically consistent.
To understand why this matters, it helps to go back to the beginnings of the universe. In general, from a purely empirical point of view, it doesn't really matter that much that relativity and quantum mechanics are incompatible - the former is used to measure extremely massive things, while the latter is used to measure extremely tiny things. The math only starts to break down when you consider things that are both extremely massive and extremely tiny. Black holes are the best example of this in the modern universe, but our entire universe was once both massive and tiny, in the first few moments after the Big Bang.
That means that we're unable to construct a working mathematical model for the earliest universe. At a certain critical point of density, the relativistic effects of gravity and the quantum effects of subatomic forces start to commingle, making it impossible to calculate the universe's origins any further and giving physicists nothing but nonsensical infinities. Currently, the most sensible assumption is that these forces do somehow merge in a way we can't currently calculate and, if we were to work back far enough, take us back to the Big Bang.
But that isn't the only possibility. For instance, the theory put forward by the University of Warsaw physicists suggests their particular combination of quantum and relativistic forces - quantum gravity, so to speak - would keep matter energy density from going above a certain value. If that sets an upper limit for density, then our universe didn't explode from a singularity, as the Big Bang holds.
Rather, our universe came from another, contracting cosmos that had been collapsing in on itself. Once it reached the critical density, it exploded back out, forming our universe in a Big Bounce. That said, this idea isn't supported by the full theory because the math isn't advanced enough yet to say either way - this is just an idea put forward using a highly simplified version of the theory.
Still, one possible way to knit gravity and the quantum world together is with something called loop quantum gravity (LQG), which is what this new model uses. The basic premise of LQG is that all of space is actually weaved from tiny one-dimensional threads. The scale of these things is unimaginably tiny: a square centimeter's worth of space would contain 10^66 threads.
The Warsaw physicists used this as their starting point and then added two theories. One was a gravitational field, as gravity creates the entire geometry of space-time in general relativity. The other was a scalar field, which is essentially a useful mathematical tool in which coordinate-independent values are assigned to every point in space so that any two observers will be able to agree on the value of a given point, regardless of their own locations.
Their new model suggests that time itself emerges from the interrelation of the gravitational and scalar fields. Professor Jerzy Lewandowski explains:
"We pose the question about the shape of space at a given value of the scalar field and Einstein's quantum equations provide the answer. It is worthy of note that time is nonexistent at the beginning of the model. Nothing happens. Action and dynamics appear as the interrelation between the fields when we begin to pose questions about how one object relates to another."
This is a breakthrough. Earlier models of the evolution of the universe were based on just general relativity, and so assumed the gravitational field at different points of the universe was more or less identical. The introduction of the scalar field, however, allows gravity to be quantized and to vary from point to point, getting us closer to an understanding of quantum gravity. This is the first such model that also manages to be mathematically consistent.
Still, this is only a first, very tentative step. There's still a lot we don't know about the specific values at play here, and so this only really offers the rough skeleton of a framework. That said, this provides scientists a chance to test out ideas and theories of the earliest universe with a model that, while very basic, does manage to incorporate quantum gravity. Lewandowski explains what they want to do next, which includes trying to figure out whether the Big Bounce really is a feature of their model:
"We have developed a certain theoretical machinery. We may begin to ply it with questions and it will provide the answers. In the future, we will try to include in the model further fields of the Standard Model of elementary particles. We are curious ourselves to find out what will happen.