You notice things. That stuff is somewhere in space, and you can tell whether or not it is moving. If something is moving, it has a direction and a speed. Particles would have a position and velocity if this were true. And if you could measure it, you’d be able to predict where the particle would be in the future and where it had been in the past.
Everything would be predictable and deterministic in a mechanical, clockwork universe. In theory, if we knew all of the positions and velocities of all particles in the universe, we could predict the universe’s entire future as well as precisely know its past. This was how the universe was thought to be about a century ago. It was a traditional view of the universe as defined by classical physics. Then came quantum mechanics, which threw cold water on this theory. This altered our perspective on everything. Reality is probabilistic at the most fundamental level and on the smallest scales. Even in theory, the precise location of a particular particle cannot be predicted in advance. Nonetheless, this concept, as defined by quantum mechanics rules, is the best and most accurate theory we have about how the universe really works. It can account for all of nature’s forces…except gravity. It is not consistent with the quantum mechanical model. Why is gravity so unique?
Are we so certain that this is the truth? Why can’t quantum mechanics model gravity? Let’s look into this and find some answers…coming up soon… The best theory of gravity we have is Einstein’s general theory of relativity, which has been proven correct in test after test over the last 100 years. However, it is still a classical theory, not a quantum theory. So, why can’t we just let well enough be? Why do we need to quantify gravity? Let me give you a quick answer. Because quantum mechanics is correct. It makes predictions, and the predictions fit with observable data to one part in a billion in some cases. It is the most accurate reality theory we have ever had. You may object that you just stated that General Relativity has been proven correct in test after test for a century. That is mostly correct. General relativity, you see, breaks down at quantum scales. It cannot, for example, describe the presumed singularity at the Big Bang or the singularity inside a black hole, where gravity theoretically becomes infinitely large at an infinitesimally small scale. These singularities are the intersections of quantum mechanics, the theory of the very small, and gravity, the theory of the very large.
Because general relativity equations cannot describe gravity on quantum scales, they fail here. The double slit experiment is another example of this incompatibility. This is a quantum mechanics test in which single photons or electrons are fired through a double slit one at a time. Behind the slits, they appear as individual photons on a detection screen. For a while, this appears to be fine, but when thousands of these individual particles are fired one at a time over time, an interference pattern forms, as if the particles are waves. This is the classic demonstration of nature’s wave-particle duality. Quantum mechanics equations predict this behavior because they show that these particles are not like little cannon balls but rather like a wave described by a wave function. The wave function tells us the likelihood of finding these particles on any given part of the screen. They claim that particles are waves until they interact, at which point they become localized like particles. In the case of the double slit experiment, the photon or electron wave interacts with the screen when it hits it, and what you see on the screen is a point-like wave that we perceive as an individual particle. However, a photon or electron, like any other quantum particle, must have a gravitational effect. According to general relativity, anything with energy or mass affects the curvature of spacetime.
Gravity is defined as the curvature of the background space and time in which all matter exists and moves. However, General Relativity does not explain how gravity interacts with a quantum wave function. Where is the gravitational effect of a photon, electron, or any other particle located if it is a wave prior to an interaction, that is, it could be anywhere until we measure it? The theory of General Relativity does not explain how gravity can behave like a wave function. Gravity does not have a wave function. In General relativity, the wave-like description of the particle before it hits the screen does not correspond to a gravitational description. However, because the particle has mass and/or energy, it must affect spacetime and thus gravity. We don’t know how this works because there is no quantum description of gravity. So, in conclusion, we know quantum mechanics works, and it works extremely well at the smallest scales. It can also be demonstrated that quantum mechanics at its extremes results in classical mechanics on large scales. However, we know that General Relativity works very well at large scales. However, at the smallest scales, general relativity does not work at all. This is simply not possible because gravity must operate at the smallest scales in order for its cumulative effects to be effective at large scales. This is why most physicists believe General Relativity must be incorporated into quantum mechanics. It is currently unfinished. It is highly accurate at macro scales, but must be described by laws that have yet to be discovered at quantum scales. This is the issue. In quantum mechanics, matter particles are known as fermions, and force particles are known as bosons. The force particles mediate all interactions between matter particles. In other words, the standard model’s bosons are in charge of electromagnetism, the strong force, and the weak force.
And all of these interactions take place in the context of space and time. Gravity does not fit in with this picture because, according to general relativity, gravity is not a force. In general relativity, gravity is caused by a disturbance, warping, or curving of the background spacetime itself. In general relativity, there is no force-carrying particle that mediates gravity between matter particles. Gravity is embedded in the fabric or canvas of the background, which contains matter and energy. This is very different from our current picture of the quantum mechanical world, which can be compared to layers of paint on top of a background canvas. The waves of matter particles would be analogous to the various paint colors. And the interactions of these paint colors would result in a lovely painting, one that represented our lovely universe. However, gravity is the canvas. And it’s not a static background monolith like in a typical painting. Is it undulating, warping, and curving in such a way that it affects the overall appearance of the painting, similar to how gravity affects our universe? Now, I’d like to emphasize that just because General Relativity, as we know it today, does not fit quantum mechanics does not mean it’s incorrect. This isn’t incorrect. It is simply insufficient. This is analogous to how Isaac Newton’s laws of gravity are not invalid simply because Albert Einstein discovered a more fundamental theory. In fact, Newton’s laws were used to launch astronauts to the moon. Newton’s laws are correct within their limits, as demonstrated by general relativity. General Relativity provides a more complete picture only when Newton’s laws do not apply at extremely high levels of gravity, such as very close to the sun or near black holes. General relativity, for example, correctly described Mercury’s precession, whereas Newton’s laws did not. Similarly, any quantum gravity theory must demonstrate that General Relativity is correct at its classical limits. It would simply describe a more comprehensive picture of gravity at quantum scales. So, despite the fact that this has been one of the most active areas of research for decades, we do not have a quantum theory of gravity. Why is it so difficult to quantify gravity?
I’ll explain it in the simplest way I can think of, but this is still a very difficult area to grasp, so don’t be discouraged if you don’t get it after just one viewing. Even Einstein, one of the world’s most brilliant physicists, couldn’t figure it out. Here’s the fundamental issue. You’ve probably heard that when physicists try to quantify gravity, infinities arise. Where do these infinitesimals come from? Let us use a Feynman diagram to demonstrate this. This diagram depicts an electron and a positron annihilating to form an energetic photon, which then decays to an electron and a positron. However, due to quantum uncertainty, the photon in the middle on its way to becoming an electron and positron can convert to any of a different number of particles, such as a top quark and anti top quark, which annihilates, or it can become an electron and positron and then back into a photon. It can also become a muon particle, a bottom quark, or even a W particle before becoming an electron or positron. It is able to do so because the uncertainty principle allows virtual particles to be created and destroyed. This means that particles can be created by temporarily borrowing energy from the vacuum of space, as long as that energy is returned to the vacuum almost immediately. Because matter and energy are interchangeable, this energy could take the form of any fundamental particle. And it can do this ten, hundred, thousand, or even an infinite number of times before reverting to electron and positron. In other words, it can transform into a plethora of particles and do so indefinitely. When we describe this mathematically, we must account for all of the particles’ momentums as well as all potential interactions between the particles. There appear to be an infinite number of interaction combinations.
This is where infinities appear in quantum mechanics equations. However, a few brilliant scientists, most notably Richard Feynman, discovered a way to eliminate these infinities through the use of clever mathematical tools, such as demonstrating that the infinite quantities themselves do not matter, but only how those quantities change. As a result, even if the equations contain meaningless infinities, useful information can be extracted from them. This is known as renormalization. And voilà, we have a working quantum theory, and the standard model is safe. However, when gravity is considered, this renormalization process fails. Gravity, according to general relativity, is not a force. It all comes down to the curvature of space-time. It is not about particles, as in quantum field theory, but about the environment in which these particles or fields exist. So you may understand the issue here. We can’t get rid of the infinities if we use the same Feynman diagram as before, but instead of just the math representing the myriad of particle interactions, we also have to take into account every possible configuration of spacetime underneath every one of those possible interactions. The math is impossible to solve. As a result, we’re stuck. Now, just because we can’t solve it doesn’t mean that there aren’t any solutions. They most likely do, but a completely new approach is required. Some commendable efforts have been made. Loop quantum gravity and string theory are two of the most well-known attempts. If you want to learn more, I have a video on this.
Essentially, loop quantum gravity seeks to quantize the background spacetime itself, describing reality as a pixilated landscape akin to the pixels on your television screen. Rather than looking for a particle that mediates a gravitational force, it quantizes the entire spacetime background. String theory, on the other hand, is an attempt to describe reality as a completely new paradigm in which everything is a string vibrating in multiple unseen small dimensions. It considers gravity to be a force that is mediated by a new particle known as a graviton. However, the problems with both of these theories thus far are that they largely fail to explain observed phenomena and fail to make any NEW testable predictions. As a result, we’re still stuck. Even Einstein, who lived until his death, tried and failed to solve the problem of reconciling gravity and quantum mechanics. But there is almost certainly a solution out there waiting to be discovered. It will most likely take the right person with a daring, perhaps radical, new perspective to see the true reality. And if you want to learn more about the reality we live in, Wondrium has a fantastic course called “Redefining Reality: The Intellectual Implications of Modern Science” – I adore this 36-video course taught by award-winning educator Professor Steven Gimbel of Gettysburg College because he takes you on a kind of curiosity excursion where he explains concepts from the perspective of their historical origins. It’s a diverse course that covers everything from metaphysics to quantum field theory, dark energy, quantum consciousness, psychology, life’s origins, artificial intelligence, and more.
