What’s the Theory of Everything?

What is the Theory of Everything? How Do We Combine General Relativity and Quantum Mechanics? Different Perspectives on the Road to the Theory of Everything..

Einstein’s General Theory of Relativity and Quantum Theory are two basic theories that explain how the Universe works. In his General Theory of Relativity, which explains the large-scale Universe, Einstein likens space-time to a cloth. Just as if you drop a heavy object on a stretched fabric or a cover by holding it from all four sides, the fabric bends due to the weight, so the mass bends the space-time fabric in a similar way. This is called “gravity”. More precisely, the tendency of objects to move towards each other as they pass through this warped space-time fabric is called “gravity”. That is, it says that everything in the Universe falls on one another due to gravity. For example, you know that when you let go of the ball, it will fall to the ground due to Earth’s gravity. And that doesn’t seem strange to you at all.

On the other hand, things do not work like this in Quantum Theory, which explains the behavior of atoms and subatomic particles. To be more clear, consider that the ball in our example is an electron, which is a subatomic particle. You cannot say “here” or “here” for the location of this ball in the subatomic world because it is “all over the place” at the same time. Let’s take this situation out of the subatomic world and move it to our own world. Let’s say you are at home right now. If you were a subatomic particle, you could be at home, at work, at the top of Everest, in short, everywhere.

There is a very critical incompatibility between the General Theory of Relativity, known as the physics of very large objects (for example, galaxies) and the Quantum Theory, known as the physics of very small objects (for example, electrons), as we have just explained: The famous gravitational force is with the General Theory of Relativity. Although it can be explained, it is not compatible with Quantum Theory. However, the General Theory of Relativity is not suitable for describing subatomic particles. This is one of the most important physics problems that modern physicists contemplate.Because the goal is to construct a common and holistic theory that can explain the behavior and properties of the smallest particles of the Universe, as well as the largest galaxy clusters. Physicists even call this type of theory the Theory of Everything.

Here, physicists came up with the idea of ​​”String Theory” in order to combine these two theories, one of which is compatible with our common sense and the other contrary to our common sense, under one roof, that is, to obtain the “Theory of Everything”.

String Theory is based on the assumption that subatomic particles are composed of strings, or one-dimensional filament-like structures, vibrating at different frequencies. These strings vibrate at different frequencies, producing different particles. We can liken this situation to a violin that makes different notes as its strings vibrate. Each note on the violin corresponds to a particle. When the violin string or string vibrates in a certain way (frequency), a certain note or particle (for example, a proton) is formed. It is thought that the particle named “graviton”, which is supposed to transmit the gravitational force in Quantum Mechanics, is formed as a result of a different vibration of the strings.

Therefore, the quest to develop such a theory forces physicists to choose one of three options:

  1. Fitting quantum theory to gravity (“gravitating quantum”),
  2. Quantuming gravity (“quantizing gravity”),
  3. A completely new approach and theory development.

Let’s leave the third of these aside for now, because we may be much closer than we think to the final answer, which may reveal the most fundamental secrets of the Universe. So let’s focus on the first two possibilities.

From Our Perceptions to the Universe: History of Physics
For many years, scientists have been trying to fit (make) the Quantum Theory to gravity, hence the General Theory of Relativity. This is because we can experience gravity very well. In other words, in the evolutionary process, we have become much more attuned to understanding gravity and thinking from its perspective. For this reason, the history of modern physics begins with Classical Physics, which Newton developed by looking at what is happening around us. Although Newton’s theory is too weak and insufficient to solve the mysteries of the Universe today, it is sufficient for us to solve the fundamental problems that directly concern us (for example, to develop automotive or aircraft technology) and to model the general behavior of objects, albeit with higher margins of error. But when it comes to solving the fundamental workings of the Universe, we can’t go very far with Newtonian Physics. For subatomic particles we have to refer to Quantum Physics, for astronomical phenomena and the fabric of space-time we have to refer to the Physics of Relativity. However, this dual approach creates the conflict we mentioned above and the necessity of resolving this conflict.

So, instead of relying on Quantum Mechanics, where far more extraordinary events take place and which is completely against our common sense, scientists have chosen to base gravity, which largely measures with our common sense, and the Physics of Relativity, which manages to explain it. What does this mean? On the way to the Theory of Everything, scientists chose to make the following basic assumption: “There is gravity as a part of the space-time fabric, that is, the phenomenon of massive objects bending the space-time fabric around them, and it is real. This is a fundamental principle of the Universe. ”

But every time we set out from this assumption, we got stuck somewhere. The biggest problem is that this approach creates physically impossible deadlocks in the physical models of the Universe, such as infinities and singularities. Physicists such as Richard Feynman, Shin’ichirō Tomonaga, and Julian Seymour Schwinger have sought to normalize these singularities in order to solve these problems. From one point of view it is like attempting to fit the fabric of the Universe into our theories. However, the opposite should be true: Our theories must be updated and developed to be compatible with the fabric of the Universe.

At this point, some physicists decided to try new approaches. Sean M. Carroll is one of them.


Untie the Knot Reversely…
Caltech University professor Sean M. Carroll and his colleagues preferred to take quantum mechanics as the fundamental law and truth of the Universe, rather than taking gravity as a fundamental fact of nature. Thus, they tried to “quantize gravity”. This approach has very serious physical and philosophical implications. Let’s take a look at these.

First, let’s listen to the reason behind the idea that prompted Sean Carroll to think like this:

Whether we like it or not, we humans tend to think of the Universe in classical physics terms. Objects, positions, velocities, places, space, time zones… So what we call an “event” in the Universe is, “Where?” and when?” shaped by questions. It is very clear to us what a singular event is. According to quantum mechanics, concepts such as the “position” or “velocity” of particles are not real. Instead, what actually exists is a wave function. This function extends everywhere, and what it represents is the probability that concepts such as position and velocity will be found in a given state. From this perspective, the concept of “event” (something that happened or happened) in the Universe becomes problematic. It is no longer just a local and singular situation. Especially if you go to the concept of quantum gravity, even what we call the “space-time fabric” becomes part of this wave function. So it is nothing but a wave function! It is extremely difficult for us humans to think this way.

So, in this human-centered model of the Universe, whatever we want to examine (whether it’s gravity or a structure like the Hydrogen atom), we start with a classical physics theory and try to “quantize” that structure to understand its behavior. In other words, we apply the rules of Quantum Mechanics to the classical physics equivalent of this structure we are examining.

Seeing Everything Through the Quantum Window…
So how do we do this? Imagine that we are interested in the position of a particle. Instead of considering the singular position of that particle, we identify all the positional probabilities it can be in and assign them a probabilistic value (a number). We call this the wave function.

Here’s what we do when we want to fit Quantum Physics to gravity and the General Theory of Relativity (or when we try to gravitize the quantum). Gravity, Einstein said, is a product of the curvature of the fabric of space-time. So there is a field that stretches across the Universe. We assign a number to every possible curvature of this field, and what we get is the Quantum Theory of Gravity.


This is exactly the approach that Carroll opposes. Nature did not start from classical physics and extend to the quantum. On the contrary, nature itself is quantum in nature. Therefore, if we want to understand its nature, we must start with the wave functions analyzed within the framework of Quantum Mechanics and reach other elements in the Universe starting from this point.

So every particle, field, etc. in the Universe. The structure is approximate models of Quantum Mechanics developed by us humans. The power of this view of the Universe becomes all the more striking when we realize that there are no “particles” or “fields” in the Universe itself, but are the product of the wave function collapsing into certain states during certain observations. Using this approach, Carroll states the following on the subject:

Let’s start with a wave function. Then ask the question: Under what conditions would this quantum mechanical wave function look like the fabric of space-time, that is, a 3-dimensional manifold?
To achieve this, Carroll and his team meet the concept of “distance” with the concept of quantum entanglement. The interbody distance is actually just quantum entanglement between different parts of the wave function.

When you poke different parts of a quantum mechanical infrastructure that you set up in this way, you can get extremely interesting products. To do this, physicists modify the quantum entanglement by adding energy to this quantum basis, and thus the geometry resulting from this wave function also changes. That’s how it’s possible to generate gravity entirely from quantum mechanics!

What Does All This Mean?
Einstein Sharing Newton’s Fate…
This original perspective, developed by Carroll, opposes the idea that gravity is a force/structure that already exists and precedes the quantum structure. This means that somewhere Einstein’s General Theory of Relativity cannot be used to explain the texture and structure of the Universe. In other words, he condemns Einstein to the same fate as Newton. While this may sound unusual, it has long been known that Einstein’s theory cannot be used to explain the foundations of the Universe.

Of course, this does not mean that Einstein’s theory will no longer be completely useless. Again, just like Newton’s theory, using Einstein’s theory on certain topics and situations is very functional, practical, and quick. But these applications cannot give us the texture and nature of the Universe, that’s all.


As Einstein (and most physicists) admitted, elements such as electrons or fields exist and are real. Quantum Mechanics is used to explain the quantum basis of these independently existing elements. When we do, however, we encounter numerous consequences that go against our common sense. For example, we cannot know exactly where electrons are when we look at them; hence we are compelled to develop explanations that try to fit our observations into our theories.

However, if we were to base quantum mechanics, countless questions such as why electrons are not in a certain position disappear. The only question left is, “How do I nudge the only real wave function so that I can find an electron in this position or that speed?”

Destroying Infinities…
According to Einstein’s General Theory of Relativity, gravity is a field. Therefore, the way to understand gravity is to examine it within the framework of Field Theory. However, this situation raises many problems that we have explained before. The first of these is the concept of “infinity”, which is not physically possible.

The number of degrees of freedom a physical field has to produce an effect is infinite. But nothing in the Universe can be infinite – so much so that even the Universe itself is not thought to be infinite. In that case, there must be a way to destroy the infinities that appear in our theories.

Here, in Carroll’s approach that quantizes gravity, there is no need for an infinite number of degrees of freedom to produce gravity. In fact, it is possible to create the fabric of space-time (hence gravity) with only a certain amount of degrees of freedom. One of the biggest proofs of this comes from Stephen Hawking’s studies on black hole entropy. These studies show that there are a number of (limited) ways to produce a black hole in a particular region of space. From this, it is thought that at any point in space-time there may be a finite number of ways (degrees of freedom) to produce anything. This eliminates the concept of infinity. This enables us to single-handedly solve many of the most challenging problems facing Quantum Gravity and Quantum Field Theory.

The reason why these infinities appear in other theories is that they assume that the space-time texture must exist even at infinitely small space-time distances. If space-time is a truly existing texture, this texture must have a counterpart at infinitely small distances. However, calculations made within the framework of field theories give rise to infinities and contradict the basic assumptions of physics. But since space-time corresponds to a wave function in Carroll’s approach, there is no need to have a real and concrete counterpart at infinitely small distances.

The Nature of Reality…
What we have told up to this point points to this: The only thing that is real is the wave function, and everything in the Universe is a result or product of this function.

You might think that this gives the wave function an almost “divine” dimension. But Carroll denies this:

This is not deifying the wave function. It’s just to make sure it gets the value it deserves. The concept of god in mythology is a much more “active personality”. The wave function, on the other hand, is a phenomenon that has to obey Schrödinger’s Equation.
In other words, a wave function is not a function with a specific consciousness, purpose, goal or intention. Schrodinger’s Equation, and thus perhaps the only “true” and “ultimate” phenomenon in the Universe, can be expressed very simply as follows:

This view is not yet a generally accepted approach in the physics community. However, Carroll states that this is an approach that physicists avoid for the sake of ease of expression. He explains:

Even Earth’s best physicists, quantum gravity experts, inevitably resort to a classical physics-based space-time approach when working on quantum cosmology and trying to explain the evolution of the Universe. In fact, he often says in his lectures or lectures, “I know I shouldn’t do this, but I will do it anyway.” They say things like Because accepting that the wave function is the ultimate truth and everything else must be derived from it is a difficult intellectual step. Moreover, doing this mathematically is quite a challenge. Identifying the questions we should ask the wave function also rests on difficult choices. But it took 80 years for us to even begin to take wave function seriously; So I think it will be accepted after a while.

  1. Derivative Content Source: Closer to Truth | Arşiv Bağlantısı

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