Superstring theory, in popular language, represents the universe as a collection of vibrating strands of energy - strings. They are the basis of nature. The hypothesis also describes other elements - branes. All matter in our world is made up of vibrations of strings and branes. A natural consequence of the theory is the description of gravity. That is why scientists believe it holds the key to unifying gravity with other forces.
Concept evolving
The unified field theory, superstring theory, is purely mathematical. Like all physical concepts, it is based on equations that can be interpreted in certain ways.
Today no one knows exactly what the final version of this theory will be. Scientists have a rather vague idea of its general elements, but no one has yet come up with a definitive equation that would cover all superstring theories, and experimentally it has not yet been able to confirm it (although not to disprove it either). Physicists have created simplified versions of the equation, but so far it doesn't quite describe our universe.
Superstring Theory for Beginners
The hypothesis is based on five key ideas.
- Superstring theory predicts that all objects in our world are made up of vibrating filaments and membranes of energy.
- She tries to combine general relativity (gravity) with quantum physics.
- Superstring theory will unify all the fundamental forces of the universe.
- This hypothesis predicts a new connection, supersymmetry, between two fundamentally different types of particles, bosons and fermions.
- The concept describes a number of additional, usually unobservable dimensions of the Universe.
Strings and branes
When the theory arose in the 1970s, the threads of energy in it were considered 1-dimensional objects - strings. The word "one-dimensional" means that the string has only 1 dimension, the length, unlike, for example, a square, which has both a length and a height.
The theory divides these superstrings into two types - closed and open. An open string has ends that do not touch each other, while a closed string is a loop with no open ends. As a result, it was found that these strings, called strings of the first type, are subject to 5 main types of interactions.
Interactions are based on the ability of a string to connect and separate its ends. Since the ends of open strings can combine to form closed strings, it is impossible to construct a superstring theory that does not include looped strings.
This turned out to be important, as closed strings have properties, physicists believe, that could describe gravity. In other words, scientistsrealized that superstring theory, instead of explaining the particles of matter, can describe their behavior and gravity.
After many years, it was discovered that, in addition to strings, other elements are necessary for the theory. They can be thought of as sheets, or branes. Strings can be attached to one side or both sides.
Quantum gravity
Modern physics has two basic scientific laws: general relativity (GR) and quantum. They represent completely different fields of science. Quantum physics studies the smallest natural particles, and GR, as a rule, describes nature on the scale of planets, galaxies and the universe as a whole. The hypotheses that attempt to unify them are called quantum gravity theories. The most promising of them today is the string.
Closed threads correspond to the behavior of gravity. In particular, they have the properties of a graviton, a particle that carries gravity between objects.
Joining Forces
String theory tries to combine the four forces - electromagnetic, strong and weak nuclear forces, and gravity - into one. In our world, they manifest themselves as four different phenomena, but string theorists believe that in the early universe, when there were incredibly high levels of energy, all these forces are described by strings interacting with each other.
Supersymmetry
All particles in the universe can be divided into two types: bosons and fermions. String theorypredicts that there is a relationship between them, called supersymmetry. In supersymmetry, there must be a fermion for every boson and a boson for every fermion. Unfortunately, the existence of such particles has not been experimentally confirmed.
Supersymmetry is a mathematical relationship between elements of physical equations. It was discovered in another area of physics, and its application led to the renaming of supersymmetric string theory (or superstring theory, in popular parlance) in the mid-1970s.
One of the benefits of supersymmetry is that it greatly simplifies the equations by allowing some variables to be eliminated. Without supersymmetry, the equations lead to physical contradictions such as infinite values and imaginary energy levels.
Because scientists have not observed the particles predicted by supersymmetry, it is still a hypothesis. Many physicists believe that the reason for this is the need for a significant amount of energy, which is related to mass by Einstein's famous equation E=mc2. These particles could have existed in the early universe, but as it cooled and energy expanded after the Big Bang, these particles moved to low energy levels.
In other words, strings that vibrated as high-energy particles lost their energy, turning them into lower vibration elements.
Scientists hope that astronomical observations or experiments with particle accelerators will confirm the theory by revealing some of the supersymmetric elements with a higherenergy.
Additional measurements
Another mathematical consequence of string theory is that it makes sense in a world with more than three dimensions. There are currently two explanations for this:
- The extra dimensions (six of them) have collapsed, or, in string theory terminology, compactified to an incredibly small size that will never be perceived.
- We are stuck in the 3D brane, and the other dimensions extend beyond it and are inaccessible to us.
An important line of research among theorists is the mathematical modeling of how these extra coordinates might be related to ours. The latest results predict that scientists will soon be able to detect these extra dimensions (if they exist) in upcoming experiments, as they may be larger than previously expected.
Understanding the purpose
The goal that scientists strive for when exploring superstrings is a "theory of everything", that is, a single physical hypothesis that describes the entire physical reality at a fundamental level. If successful, it could clarify many questions about the structure of our universe.
Explanation of matter and mass
One of the main tasks of modern research is to find solutions for real particles.
String theory began as a concept describing particles such as hadrons in various higher vibrational states of a string. In most modern formulations, the matter observed in ouruniverse, is the result of vibrations of strings and branes with the lowest energy. Higher vibrations generate high-energy particles that currently do not exist in our world.
The mass of these elementary particles is a manifestation of how strings and branes are wrapped in compactified extra dimensions. For example, in a simplified case where they are folded into a donut shape, called a torus by mathematicians and physicists, a string can wrap this shape in two ways:
- short loop through the middle of the torus;
- a long loop around the entire outer circumference of the torus.
A short loop will be a light particle, and a large loop will be a heavy one. Wrapping strings around toroidal compactified dimensions produces new elements with different masses.
Superstring theory concisely and clearly, simply and elegantly explains the transition of length into mass. The folded dimensions here are much more complicated than the torus, but in principle they work the same way.
It's even possible, though hard to imagine, that the string wraps around the torus in two directions at the same time, resulting in a different particle with a different mass. Branes can also wrap extra dimensions, creating even more possibilities.
Determining space and time
In many versions of superstring theory, the dimensions collapse, making them unobservable at the current level of technology development.
Currently it is not clear whether string theory can explain the fundamental nature of space and timemore than Einstein did. In it, measurements are the background for the interaction of strings and have no independent real meaning.
Explanations have been offered, not fully developed, regarding the representation of space-time as a derivative of the total sum of all string interactions.
This approach does not meet the ideas of some physicists, which led to criticism of the hypothesis. The competitive theory of loop quantum gravity uses the quantization of space and time as a starting point. Some believe it will end up being just a different approach to the same basic hypothesis.
Quantization of gravity
The main achievement of this hypothesis, if it is confirmed, will be the quantum theory of gravity. The current description of gravity in general relativity is inconsistent with quantum physics. The latter, by imposing restrictions on the behavior of small particles, leads to contradictions when trying to explore the Universe on an extremely small scale.
Unification of forces
Currently, physicists know four fundamental forces: gravity, electromagnetic, weak and strong nuclear interactions. It follows from string theory that they were all once manifestations of one.
According to this hypothesis, since the early universe cooled after the big bang, this single interaction began to break up into different ones that are active today.
High energy experiments will one day allow us to discover the unification of these forces, although such experiments are far beyond the current development of technology.
Five options
After the superstring revolution in 1984, development was carried out at a feverish pace. As a result, instead of one concept, there were five, named types I, IIA, IIB, HO, HE, each of which almost completely described our world, but not completely.
Physicists, sorting through versions of string theory in the hope of finding a universal true formula, have created 5 different self-sufficient versions. Some of their properties reflected the physical reality of the world, others did not correspond to reality.
M-theory
At a conference in 1995, physicist Edward Witten proposed a bold solution to the problem of five hypotheses. Based on the newly discovered duality, they all became special cases of a single overarching concept, called Witten's M-theory of superstrings. One of its key concepts was branes (short for membrane), fundamental objects with more than 1 dimension. Although the author did not offer a complete version, which is not yet available, the M-theory of superstrings briefly consists of the following features:
- 11-dimension (10 spatial plus 1 time dimension);
- dualities that lead to five theories explaining the same physical reality;
- branes are strings with more than 1 dimension.
Consequences
As a result, instead of one, there were 10500 solutions. For some physicists, this caused a crisis, while others accepted the anthropic principle, which explains the properties of the universe by our presence in it. It remains to be seen when theorists will find anotherway of orientation in superstring theory.
Some interpretations suggest that our world is not the only one. The most radical versions allow the existence of an infinite number of universes, some of which contain exact copies of our own.
Einstein's theory predicts the existence of a coiled space, which is called a wormhole or an Einstein-Rosen bridge. In this case, two distant sites are connected by a short passage. Superstring theory allows not only this, but also the connection of distant points of parallel worlds. It is even possible to transition between universes with different laws of physics. However, it is likely that the quantum theory of gravity will make their existence impossible.
Many physicists believe that the holographic principle, when all the information contained in the volume of space corresponds to the information recorded on its surface, will allow a deeper understanding of the concept of energy threads.
Some believe that superstring theory allows for multiple dimensions of time, which could result in travel through them.
In addition, within the framework of the hypothesis, there is an alternative to the big bang model, according to which our universe appeared as a result of a collision of two branes and goes through repeated cycles of creation and destruction.
The ultimate fate of the universe has always occupied physicists, and the final version of string theory will help determine the density of matter and the cosmological constant. Knowing these values, cosmologists can determine whether the universe willshrink until it explodes to start it all over again.
No one knows where a scientific theory can lead until it is developed and tested. Einstein, writing the equation E=mc2, did not expect that it would lead to the appearance of nuclear weapons. The creators of quantum physics did not know that it would become the basis for creating a laser and a transistor. And although it is not yet known what such a purely theoretical concept will lead to, history shows that something outstanding will surely turn out.
For more on this conjecture, see Andrew Zimmerman's Superstring Theory for Dummies.
