Loop quantum gravity: Does space-time come in tiny chunks?

 

Are there fundamental units of space-time at some unfathomably tiny scale?

The ultimate ideal — or horror — of physics is to combine quantum mechanics and general relativity. It would be a means to finally characterize gravity using quantum mechanics tools, revealing how gravity works when it is extremely strong and at extremely small scales.

According to Einstein's general theory of relativity, the force of gravity is caused by the warping of space and time. According to quantum mechanics, what we perceive as natural forces are made up of discrete, microscopic bits called as quanta.

If gravity is the bending of space-time, gravity is a force, and all forces are quantized, it's possible that space-time itself is made up of discrete tiny blocks. Perhaps fundamental units of space-time exist on an unfathomably small size.

Fading into the background

One of the most vexing points of contention between general relativity and quantum mechanics is the function of space-time in physics. Space-time is only a backdrop, a stage, a floor, and a container for all the fascinating interactions that make up the physics of the cosmos in quantum mechanics. Yes, that stage may bend and warp, and that bending and warping affects particle pathways — but that's all there is to it. Everything in physics takes place "on top" of that underlying space-time.

Even string theory, which claims that all particles and forces are made up of small fragments of vibrating strings, relies on the presence of a backdrop space-time to function with. As a result, it's a theory of... basically everything.

However, in general relativity, space-time is not merely a backdrop for the actors; it is the actor. General relativity does not presume, but rather creates, a background. General relativity is the language of space-time warping, and it is this warping that generates gravitational physics.

So, in our search to reconcile quantum physics with gravity, we might as well accept Einstein's theory on its face. If gravity is simply the mechanics of space-time, then we actually need to look for a quantum theory of space-time to find a quantum theory of gravity. If we crack that quantization, we'll have a quantum theory of gravity by default, and the problem will be solved.

Going for a loop

Loop quantum gravity is a method for doing this. Because the theory's foundation is built on a rewrite of Einstein's general relativity in terms of lines (rather than points, as is commonly done), the word "loop" appears in the name. It doesn't affect the physics, but it does make some computations easier, particularly when quantizing space-time.

What does "quantize space-time" imply? It indicates that at some imperceptibly small scale, there is a fundamental unit, a discrete piece, of space-timey-ness. The smooth curves and neat edges of the letters would be exposed as a massive number of small squares — pixels — if you zoomed in on this screen.

In much the same way, if you were to zoom in to space-time, you would see that time doesn't advance into the future continuously but in quick little tick-tick-ticks of a discrete clock. When you move, it wouldn't be a smooth motion; instead, it would be a series of stuttering steps from one space-time pixel to the next.

The most significant advantage of this quantization of space-time is that singularities are eliminated. Singularities are areas where densities become endlessly high and gravity becomes infinitely strong, according to Einstein's general relativity. We recognise that this means that our understanding of physics is completely out of whack, and that we have no idea what's going on deep inside a black hole or at the beginning of the Big Bang, when singularities first arise.

However, in loop quantum gravity, such singularities are replaced by incredibly small bits of ultradense (and, presumably, ultra-exotic) matter. We'd just expel the singularity monsters from our reality and replace them with something more comprehensible.

Not a perfect circle

What are those ultradense matter pieces like? We're not sure, to be honest. Loop quantum gravity, you see, isn't quite complete. The largest challenge is that loop quantum gravity is a theory of strong gravity at tiny scales, which should naturally be a theory of weak gravity at normal scales. We've managed to build some of the mathematics of "pixelated" space-time using a mathematical tool called spin networks. That means that in non-crazy situations like Earth orbiting the sun, the equations of loop quantum gravity should provide the same conclusions as general relativity or old-fashioned Newtonian physics.

To put it another way, loop quantum gravity should include Einstein's general relativity, but we don't know if it does. At the smallest scales, you should be able to zoom out from the pixelated, quantum space-time vision and restore the smooth, undulating fabric of general relativity's space-time — but no one knows how.

There are also other issues to consider. According to special relativity, our perceptions of time and space are affected by our velocity, but our perceptions of fundamental physics should be unaffected. However, various observers will have varied perspectives on the sizes of quantized pixels in space-time, fundamentally altering their understanding of physics. As a result, there's a problem.

It's possible that loop quantum gravity won't work out. The equations of loop quantum gravity, like its cousin string theory, which also purports to be a quantum theory of gravity, aren't yielding any feasible solutions. The code could be cracked by a future scientist, paving the way for a complete knowledge of gravity. Alternatively, we could be circling in circles.