What is the method of replacing string theory with quantum gravity ?
Image sourced from: Electronic Publications Committee (Contemporary Physics Education Project), National Science Foundation/Process Verification/Lawrence Berkeley National Laboratory.
The universe we know and love, using Einstein 's General Relativity as our theory of gravity and the quantum field theory of the other three forces, there is a problem we don't often mention: it is incomplete. It is good for Einstein's theory itself, which describes how matter and energy are associated with the curvature of space and time. Quantum field theory itself is also good, which describes how particles interact and experience their power. The calculation of quantum field theory is usually done in straight space where space and time will not bend. We can also do it in the curved space described by Einstein's theory of gravity (although it will increase in difficulty, it is not impossible), which is the so-called semi-classical gravity. This is how we calculate Hawking radiation and black hole decay.
Image source: NASA
But even the semi-classical approach is only valid near and outside the black hole horizon, not where gravity is really strongest: singularity , which is theoretically considered centered, or mathematically absurd prediction. We have many physical instances that require an quantum gravity theory , because at very small and short quantum distances, all of which are related to strong gravitational physics on the smallest scale. There are important questions here, such as:
- What changes will happen to an electron's gravitational field when it passes through the double slit?
- If the final state of a black hole is thermal radiation, what changes will happen to the particles forming the black hole?
- What behavior will the gravitational field/force near the singularity cause?
Without the theory of quantum gravity, nothing is determined.
To explain what happens in a short distance in the presence of a gravitational source or mass, we need a gravitational theory based on quantum discreteness. The known quantum force is mediated by particles called bosons, or particles with integer spin . photon mediates electromagnetic force, W and Z bosons mediate weak forces, while gluon mediates strong forces. All these types of particles have a spin of 1, and the spin of large-mass particles (such as W and Z bosons) can be -1,0 or 1; for non-mass particles (such as gluons and photons), their spin can only be -1 or +1.
Although the Higgs boson does not mediate any force, it is still a boson with a spin of 0. Because of our understanding of gravity, general relativity is a tensor gravitational theory, which must be mediated by a massless particle with a spin of 2, which means that its spin value can only be -2 or +2.
. This is incredible! This means that we already have a certain understanding of a quantum gravity theory before we establish it. The reason we know this is because no matter what the true theory of quantum gravity is, when we are not very close to a large-mass particle or object, it must conform to the general theory of relativity. Just like 100 years ago, we knew that general relativity needed to be reduced to Newton gravity in weak field regions.
Image sourced from: NASA, an artist's concept of gravitational detector B orbiting the Earth to measure the curvature of space-time.
The biggest problem is of course how to do it? How to quantify gravity in a correct (when describing reality), consistent way (consistent with GR and QFT), and hopefully make computable predictions of new phenomena that may be observed, or tested in some way. Of course, the leading competitor is a string theory you've already heard of.
string theory is an interesting framework, whether it is fermions or bosons, it contains all the annotation model fields and particles. It also contains a ten-dimensional tensor- scalar gravitational theory: it has nine spatial dimensions and a time dimension, and a scalar field parameter.If we eliminate six of these spatial dimensions (through a process that has not been fully defined, which people call compaction) and make the parameter (ω) that defines the interaction of scalars approach infinity, we can restore general relativity.
Image sourced from: NASA/Goddard/Wade Cicell, Brian Green's speech on string theory.
.) But string theory has many phenomenological problems.
One of them is that it predicts a large number of new particles, including all supersymmetric particles that have not been discovered yet. It claims that "free parameters" like the standard model (for the mass of particles) are not needed, but it replaces this with a worse problem. String theory refers to "10^500 possible solutions", which refer to the lack of mechanisms to restore their string field vacuum expectations; if you want string theory to work, you need to give up on dynamics and simply put, "Well, it must be human choice." String theory itself is full of setbacks, flaws, and problems. But the biggest problem may not be these mathematical problems. Instead, there are four other ways to guide us towards quantum gravity, which are completely independent of string theory.
Image is derived from: Wikipedia user Linfoxman, a graphical illustration of quantifying "spatial structure".
.) Circle quantum gravity.
Circle quantum gravity is an interesting problem: one of the central features of circle quantum gravity is not an attempt to quantify particles, but rather that the space itself is discrete. Imagine a common metaphor for gravity: there is a bowling ball in the center of a tightened sheet. We know that the sheet itself is quantized , not a continuous structure, because it is composed of molecules, molecules are composed of atoms , atoms are composed of atoms ( quark and gluons) and electrons. The same may be true for the
space! Maybe it's like a fabric, but maybe it's made up of finite, quantified entities. But perhaps it is composed of "rings", which is the origin of this theory. By woven together these rings, you can get a spin network, which represents the quantum state in the gravitational field. In this picture, not only matter itself, but also space itself is quantized. From the idea of spin networks to a perhaps realistic method of gravity calculation is an active field of research, one that witnessed a huge leap in 2007/8, so this is still being actively promoted.
Image source: Wikipedia user reasNink, generated by Wolfram Mathematica 8.0
.) Progressively safe gravity.
This is my favorite attempt at quantum gravity theory. Progressive freedom developed in the 1970s to explain the unusual properties of strong interactions: in very short distances it is a very weak force, and it becomes stronger and stronger as the distance between charged particles gets further and further. Unlike electromagnetics, which has a very small coupled constant, the strong coupling constant is very large. Due to some interesting properties of quantum chromodynamics, if you end up with a (color) neutral system, the intensity of the interaction drops rapidly. This can explain properties like baryons (such as protons and neutrons), physical dimensions of meons (such as π meons).
On the other hand, asymptotic security seems to solve a fundamental problem related to this: you don't need small coupling (or coupling tends to zero), but just coupling is limited at the high energy limit. All coupling constants vary with energy, so it is a stepwise safe approach to select a high-energy fixed point for the constant (technically, for the reordination group, the coupling constant is obtained from the reordination group) and then calculate everything else at a lower energy.
At least, this is a solution! We already understand that doing this in 1+1 dimensions (one space and one time), but not in 3+1 dimensions. Still, some progress has been made, with the most notable of which is Christopher Wetridge, who published two groundbreaking papers in the 1990s.Wetridge used asymptotic safety six years ago to calculate mass predictions before the discovery of the Higgs boson by the Large Hadron Collider. What is the result?
Image from: Mikhail Shaposhnikov Christov Vetrich
Surprisingly, what it shows is exactly in line with what the LHC collider finally found. This would be a surprising prediction if the asymptotic security is correct. When the error bars are further suppressed to the pole quark, the mass of the W boson and the Higgs boson is finalized, it may not even be necessary to make physics stable all the way to the Planck scale. Not only is it promising, it has many attractive properties like string theory: it can successfully quantify gravity, reduce to GR under low energy limits, and is UV-limited. Plus, it outperforms string theory in at least one aspect: it does not require adding new particles or parameters that we have not proven! Of all the alternatives to string theory, this one is my favorite.
.) Causal dynamic triangulation.
The idea was first developed by Renat Lore in 2000, and later expanded by others, so it is considered a new role. Since space itself is separated, it is similar to Gaussian linear quadratic equations, but the main focus is on how space itself evolves. There is an interesting nature about this idea, that time must also be discrete. As an interesting feature, it gives us a four-dimensional space-time (not even derived from a priori, but something theory gives us), but in cases of extremely high energy and extremely short distances (like the Planck scale), it shows a two-dimensional structure. It is based on a mathematical structure called simplex, a multidimensional model of triangles.
Image source: Wikipedia Simplex Page Screenshot
2-Simple is a triangle, 3-Simple is a tetrahedron, and so on. One of the advantages of this choice is causality—a concept that most people regard as sacred is clearly preserved in the triangulation of causal dynamics. (Sabin has some insights on causal dynamic triangulation here, which may be related to progressively safe gravity.) It may be able to explain gravity, but it cannot be 100% sure whether the standard model of elementary particles is suitable for this framework. Only significant progress in computing can make it a fairly well-researched alternative approach, so this is underway and relatively young.
.) Emergency gravity.
Finally, let's take a look at the most speculative possibility of quantum gravity. In 2009, Eric Ferlind proposed the entropy force hypothesis of gravity. In this model, gravity is no longer a basic force, but as an entropy-related phenomenon, emergency gravity is only prominent at this time. In fact, the origin of emergency gravity can be traced back to Andrei Sakharov, the discoverer of the asymmetric condition of matter-antimatter, who proposed the concept as early as 1967.
Image sourced from: flickr gallery of J. Gabas Esteban.
We are sure we need a quantum gravity theory to ensure the universe works at a basic level, but we are not sure what this theory looks like, and we do not know whether any of these five pathways (including string theory) will prove to be valid. String theory is the best studied of all options, but compared to other theories that have been carefully considered for a long time, circle quantum gravity is the second on the rise. They say that the answer is always dark and bright again, appearing in the last place you are looking for, which may inspire you to start seriously looking for new places and new answers.
Reference
1. Wikipedia Encyclopedia
2. Astronomical noun
3. forbes- Ethan Siegel-Qingzhou
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