So, What is quantum gravity? Before digging into the discussion, let focus on some basics.
A strange species evolved upon the third planet orbiting an ordinary yellow dwarf star. Humans! So intrigued, they couldn’t stop trying to find out how it all worked. Although their simple brains were evolved for a hunter-gathering life, they were somehow not content with what they could see with their eyes and feel with their hands. So they invented telescopes to peer into the depths of space, and particle colliders to smash matter into its constituent parts. And eventually, they discovered how everything in the universe works. Although they didn’t manage, did they? Not yet, anyway.
Quantum field and General relativity
So here’s the problem. We have two simple theories explaining the universe, but we don’t know how to integrate them. On one end, we describe how the fundamental particles work and interact with each other with quantum field theory, which explains electromagnetism and the nuclear strong and nuclear weak forces. The general theory relativity describes, on a broad scale, how objects affect one another because of the curvature they create in space-time, otherwise known as gravity. These two theories are the complete description we have of how the universe works. Both have been tested and verified to incredible precision. But they’re not moving along.
In quantum field theory, gravity is not mentioned, and general relativity says nothing about the quantic universe. Big deal, you might say. If they are each doing their bits okay, then there’s no problem. But there is a problem. Two of the greatest mysteries of science exist between those theories: black holes and big-bang. In these extreme events, quantum mechanics and general relativity meet. We will never be able to grasp them until we establish a quantum gravity theory, the basic concept of everything.
We made a couple of attempts. String theory, or M-theory, and quantum loop theory are the most common. But neither of these have back-up science evidence. For example, string theory suggests new frontal particles that we haven’t observed, called supersymmetric particles. So, for now, these theories remain hypothetical. I should note that we are not only using quantum field theory and general relativity to model everything in physics, in reality. Often they would be far too cumbersome, so we use a whole suite of other approaches that are perfectly good approximations for those specific situations. But the quantum field theory and general relativity theory are impressive, as they are the most simple theories.
So why have we made little advancement in quantum gravity for long? General relativity was written in 1915, and the theory of quantic fields was completed in the late 1970s. There has very no improvement been made since then. There is only one explanation. Gravitational force is very much weaker than the other forces. But doing an experiment where there is something that feels all the forces substantially at the same time is quite difficult, maybe impossible. Only when you interact with enormous quantities of the matter, then only gravity become strong. It takes the entire Earth, for example, to keep you on the ground. And for stuff this big like humans or planets or stars, quantum effects aren’t noticeable.
The place in which quantum mechanics’ importance lies in the size of atoms and subatomic particles and such objects are so small. The light under which the force they experience as a result of gravity is negligible compared to other forces. For example, two electrons centimeter apart would feel repulsion from their electric charges but attract each other due to their mutual gravity. But the force from the electric charge is ten to the power of twenty-four times higher than the attraction they four times gravity. That is 24 orders of magnitude more significant, so any effect of gravity is lost down in the twenty-fourth decimal place of the electrostatic force, an incredibly small effect.
So the only testbed of quantum gravity are places where you have a tremendous amount of matter squeezed into very small volumes, which are always incredibly high energy situations, like black holes or the big bang. And unfortunately, these are not things we can create experimentally, given our current level of technology. To get to the right energies, we would need a particle accelerator like CERN. Even the size of the solar system measures Jupiter’s size to get to the right kind of energies. And the experiment will be turned into a black hole that will produce ample energy to test for quantum gravity.
Perhaps our best bet at the moment to verify our theories of quantum gravity is to look at those real phenomena that have happened to us: the black holes and the big bang. We do so by using gravitational wave physics and observations of the cosmic microwave background. The history of cosmic microwaves tells us the very earliest light in the universe, released about 380,000 years ago after the big bang, filling the whole sky. But this light isn’t even in every direction. It contains a fuzziness, which you can see here. These small fluctuations were an example of the big bang’s quantum fluctuations when the universe was smaller than the nucleus of an atom.
Think about how wild that is. It is Heisenberg’s uncertainty principle painted across the sky. Large-scale Quantum mechanics had populated the universe’s large-scale structures, galaxy clusters, and interstellar voids. So this is a simple place where gravity and the quantum universe meet. Hence, people research the cosmic microwave history in great detail, trying to find trends in the data to point them in the right direction. But these patterns are incredibly subtle, and we still don’t know if our current measurements have there a solution we need to see a signature. We need to apply the same cautious optimism to our gravitational-wave experiments.
They are remarkable. If there is a principle of quantum gravity, then there could be a particle of gravity called a graviton, a small little bundle of gravity: a gravitational wave quantization. Unfortunately, they are way too small to be seen by our current gravitational wave detectors LIGO and VIRGO, which have done an incredible job of detecting gravitational waves themselves. But to see a graviton, you’d need to detect minuscule changes in that wave. It is similar to how it is easy for us to see light waves but very hard to detect a single photon. And I can’t overstate how hard it would be to detect a graviton.
They may never be observable because we’d need to measure distances smaller than the Planck length, which is impossible according to quantum mechanics. Instead, astrophysicists are looking very closely at the gravitational waves from black hole collisions hoping that there is some small departure from general relativity. And if there is, then it’ll be a tiny clue which we can follow to see what direction to take with quantum gravity. Until we get some new evidence of a departure from our existing very successful theories, we’ll never get closer to having a grand theory of everything.
The best theory of Quantum gravity
Okay, let’s look at our best contenders for a theory of quantum gravity: string theory and loop quantum gravity. However, I’m not going to spend lots of time on them because they are highly theoretical and very complicated. Quantum field theory says that we have got a field for each of the fundamental particles which all lie over each other in space. The particles are excitations of such fields themselves because gravity is the curvature of space-time, in general relativity. Now string theory considers space-time as another quantum field and aims to unify gravity in one context with the other elements. In contrast, loop quantum gravity doesn’t unify the forces. It attempts to work out what the quantum nature of space-time is.
Thus string theory hypothesizes that the fundamental particles and their properties are the products of various vibrational modes of one-dimensional strings that exist in 11-dimensional space. One of these vibrational modes corresponds to the graviton. So it reduces all the particles in particle physics down to a single entity, which is a string. The theory had some scientific impact but was criticized for struggling to explain the real world, Which is an issue.
Loop quantum gravity starts with general relativity but attempts to model space-time quantum at the very short distances of the Planck length. It implies that there is a minimum possible distance, kind of like a space-time pixel. These are the most popular proposals for quantum gravity, but there are others. And it is essential to state that neither theory has an experimental observation where they predict something that isn’t already covered by quantum field theory or general relativity. Let’s zoom out to the big picture again.
What’s the point of discovering a correct theory of quantum gravity? What change would it have to the world? Wellbeing able to understand black holes would be very cool. Currently, general relativity completely breaks down when space-time becomes infinitely curved. And therefore, a quantum gravity theory will potentially solve the many black holes mysteries. Do they contain other universes? Are they the key to wormholes in our world into other worlds or other places? What happens to the quantity of information about objects that drop into them that quantum mechanics tells us is not capable of destruction? And that might give us an understanding of what happened before the big bang, and at the end where the world comes from.
That is all quite fascinating, so how does it affect us? The reality is, I have no clue. We’d never know until we’ve seen a new idea. When looking at scientific history, any single paradigm shift has contributed to new advances in fundamental physics. Comprehension of quantum mechanics, for example, contributed to computer invention and the whole era of learning. And to arrive at a theory of quantum gravity, we know for sure that we would have to compromise at least one of our basic physics laws. So no doubt, when we have a quantum gravity principle, it will open up possibilities we can’t even imagine right now. And most of all, it would be fantastic for me to understand how the world works.
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