Theoretical Physicists like Nobel Prize winner Steven Weinberg (1933-2021) have publicly and correctly stated that General Relativity is an unreliable measure of gravity on the small scale(less than 10^-35 meters). It is great for predicting planetary orbits, or how long you have to live before impact, if you fall off of a tall building.
It is terrific for knowing precisely how a star’s mass will bend the space-time around itself, so we can see the “bent light” of stars that are behind it. One particular formation is named Einstein’s Cross.
Actually there are only 2 objects here. One is a galaxy 400 million light years away, and 20 times further, the one behind it is a quasar, about 8 billion light years distant. The galaxy sits between us and the quasar, so normally we couldn’t see the quasar directly. Instead of being a uniform “halo” of annular light surrounding the galaxy in the middle, they are far enough off-center from each other to create 5 images of the quasar(faint one in the center and 4 duplicates surrounding it). There are more examples of Einstein Crosses that have been found, proving that General Relativity is quite correct on the large scale.
Even our Global Positioning System (GPS) system proves both General and Special Relativity on a daily basis. Due to their high orbit speed the onboard clocks experience time more slowly, and due to being in a lower gravity field because of their height they experience time faster.
The net effect is that they advance over ground clocks by 39 milliseconds (39,000,000 nanoseconds) per day, and this difference means that they would be wrong by about six miles (10 kilometers) per day if we didn’t correct for relativistic effects. By the end of the month GPS could be reporting that your house was in a completely different town, if not a different state, 180 miles away!
What it is not so good at is predicting the precise location and behavior of an electron around a proton—more specifically, the precise location or momentum of any tiny particle. The Heisenberg Uncertainty Principle states that you cannot know both location and momentum at the same time because the simple act of measuring something changes the values. Either, not both, is the rule.
The mathematical formulae for this entire subject are so dense that it would be impossible to outline them here. Simply put, if you have an exact location, the equations to calculate momentum then require “infinite” calculations to achieve a value—and with values high enough to require the presence of a microscopic black hole. We know black holes don’t actually spawn just because we measure something.
Any time you need “infinite anything” to explain something, we know that the answer is wrong. We know that conventional physics is missing something to adequately explain what gravity is, and the explanation lies in the quantum realm.
Quantum physics is much better at explaining why subatomic objects behave the way that they do. The explanations have matched the predictions very closely, so we know we’re on the right track.
Conventional physics explains the observable universe quite well; quantum mechanics explains the subatomic realm quite well. The fact that they contradict each other where they meet isn’t a problem. It simply means that there is more to learn.
Of course that meeting point contains exciting high energy events like the Big Bang and Black Holes, things explained well by neither Conventional nor Quantum physics. We have theories to fill in this gap such as M-Theory (aka String Theory) and Loop Quantum Gravity but there is no experimental evidence for either, so they remain hypothetical at this point.
Gravity is incredibly weak as forces go. A simple magnet can pick up a mass against the force of gravity with ease. Gravity is 10 octillion (10^24) times weaker than electrostatic forces that keep electrons apart, and 10^40 times weaker than the electromagnetic force. Getting single molecules, atoms, or electrons to respond to gravity, when being affected by any of the other forces, is virtually impossible to measure. Gravity’s effect is washed away, buried, or lost in the background.
According to convention physics, gravity is not a “force” like electromagnetism; it is an effect. It is “quick-n-easy” for us to think of gravity similarly to metal filings being attracted to a magnet, but in reality, gravity is a manifestation of mass upon the shape of space-time itself. Every object “attracts” every other object in the Universe by affecting the shape of space around it. By compressing space-time towards itself, it makes the surrounding volume appear “downhill” to everything else in the universe. The closer you are, the steeper the apparent grade.
You’ve probably seen a heavy weight sitting on a rubber sheet to illustrate this. In effect, anything traveling along the A-G path is mostly unaffected because the curve, although not zero, is very shallow. The path is actually asymptotic, never reaching zero, right to the edge of the universe, but because other things exist in the universe its effect is quickly lost amongst the noise generated by every other object in existence.
On the other hand, the H-I path will affect direction causing significant deviation, and J-K could alter its direction all the way up to including trapping the object in orbit around the other object. Paths L-T results in surface collisions, and so on…
As you can see, this describes a very flat interaction, along a plane, but it can occur in any direction, overhead, beneath, or diagonally. The rubber sheet analogy only indicates bent space-time relative to these two objects.
The quantum explanation of gravity will require a particle. We’ve never seen it, of course, and we’ve just barely managed to measure gravity waves with the LIGO and VIRGO gravity wave detectors. Nevertheless, we call it the graviton, and it is the quantization of these gravitational waves. The waves are extremely long, and require two stations (or more) on different parts of the planet to measure them.
We see these waves with difficulty, best measured when two black holes or neutron stars collide and send out massive gravitational events. The graviton is so correspondingly tiny, however, that it is akin to trying to detect an individual photon from a searchlight if you don’t have eyes. Our technology will improve and we may have success one day, but for now our technology is completely incapable of measuring anything as small as a graviton.
As mentioned, the math is mind-boggling, so in simplified form, let’s see what these two ideas have to offer.
String Theory suggests that all of the forces are “vibrations” of fields that can move in x, y, and z directions. How much they vibrate, how strongly, and in which of those directions, gives them their characteristics. It includes space-time as a vibration, too, as a way to include gravity. All of these vibrations overlay each other.
The theory stipulates that all these vibrations occur in a super-symmetrical space containing 11 dimensions, and manifest as one dimensional strings. This suggests that all existing quantum particles and their effects condense into a one-dimensional string, and one of the vibrations of that string constitutes the graviton, and gravity itself. It is so complex that it is difficult to make any simpler than this.
LQG Theory begins with General Relativity, modeling space-time over distances of 1.61 × 10−35 meters, also called the Planck Length. It treats gravity as a geometric quality of space, altering its actual shape. Gravitons may or may not arise in the LQG Theory, but are not required.
The biggest difference is that supersymmetry is not required, and no extra dimensions are called for in order for Loop Theory to function. Some say that Occam’s Razor (simpler is better) makes Loop Theory a better contender as the likely solution.
So, what is Quantum Gravity? In short, we don’t know. We’re making progress on a number of fronts that are increasing our understanding. One day we may be able to reconcile it with either relativity or the quantum realm, and in a perfect world, with both.
If we do get it sorted out, we could very well have what science has sought since we were mature and wise enough to understand what it was: a Grand Unified Field Theory, which includes everything except gravity. That could finally lead to “The Theory of Everything” (TOE), including gravity, and create a framework to understand every interaction possible in the Universe.
Not the “End of Science”, to be sure! Actually, more like the beginning, where we would spend centuries testing out all the possibilities and predictions made possible by a TOE. That would be a universe I’d like to live in… how about you?
