**Supernova remnant SNR 0454 67.2**

Astronomy
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String theory is a fascinating physical model in which all particles are replaced by one-dimensional objects known as *strings*. This theory says that we live in more than four dimensions, but we can not perceive them.

String theory, is a complete theory and unites quantum physics with Einstein’s general relativity.

On distance scales larger than the string scale, a string looks just like an ordinary particle, with its mass, charge, and other properties determined by the vibrational state of the string. In string theory, one of the many vibrational states of the string corresponds to the graviton, a quantum mechanical particle that carries gravitational force. Thus string theory is a theory of quantum gravity.

According to string theory, the reason we can not observe these dimensions is because they are very small and compact (smaller than the plank length 10 ^{−35})

Compactification is one way of modifying the number of dimensions in a physical theory. In compactification, some of the extra dimensions are assumed to “close up” on themselves to form circles. In the limit where these curled up dimensions become very small, one obtains a theory in which spacetime has effectively a lower number of dimensions. A standard analogy for this is to consider a multidimensional object such as a garden hose. If the hose is viewed from a sufficient distance, it appears to have only one dimension, its length. However, as one approaches the hose, one discovers that it contains a second dimension, its circumference. Thus, an ant crawling on the surface of the hose would move in two dimensions.

Compactification can be used to construct models in which spacetime is effectively four-dimensional. However, not every way of compactifying the extra dimensions produces a model with the right properties to describe nature. In a viable model of particle physics, the compact extra dimensions must be shaped like a Calabi–Yau manifold

Another approach to reducing the number of dimensions is the so-called brane-world scenario. In this approach, physicists assume that the observable universe is a four-dimensional subspace of a higher dimensional space. In such models, the force-carrying bosons of particle physics arise from open strings with endpoints attached to the four-dimensional subspace, while gravity arises from closed strings propagating through the larger ambient space. This idea plays an important role in attempts to develop models of real world physics based on string theory, and it provides a natural explanation for the weakness of gravity compared to the other fundamental forces

One notable feature of string theories is that these theories require extra dimensions of spacetime for their mathematical consistency. In bosonic string theory, spacetime is 26-dimensional, while in superstring theory it is 10-dimensional, and in M-theory it is 11-dimensional. In order to describe real physical phenomena using string theory, one must therefore imagine scenarios in which these extra dimensions would not be observed in experiments.

The original version of string theory was bosonic string theory, but this version described only bosons, a class of particles which transmit forces between the matter particles, or fermions. Bosonic string theory was eventually superseded by theories called superstring theories. These theories describe both bosons and fermions, and they incorporate a theoretical idea called supersymmetry.

This is a mathematical relation that exists in certain physical theories between the bosons and fermions. In theories with supersymmetry, each boson has a counterpart which is a fermion, and vice versa.

There are several versions of superstring theory: type I, type IIA, type IIB, and two flavors of heterotic string theory (*SO*(32) and *E*_{8}×*E*_{8}). The different theories allow different types of strings, and the particles that arise at low energies exhibit different symmetries. For example, the type I theory includes both open strings (which are segments with endpoints) and closed strings (which form closed loops), while types IIA, IIB and heterotic include only closed strings.

Branes

In string theory and other related theories, a brane is a physical object that generalizes the notion of a point particle to higher dimensions. For instance, a point particle can be viewed as a brane of dimension zero, while a string can be viewed as a brane of dimension one. It is also possible to consider higher-dimensional branes. In dimension *p*, these are called *p*-branes. The word brane comes from the word “membrane” which refers to a two-dimensional brane

In string theory, D-branes are an important class of branes that arise when one considers open strings

D-branes are typically classified by their spatial dimension, which is indicated by a number written after the *D.* A D0-brane is a single point, a D1-brane is a line (sometimes called a “D-string”), a D2-brane is a plane, and a D25-brane fills the highest-dimensional space considered in bosonic string theory. There are also instantonic D(–1)-branes, which are localized in both space and time.

A striking fact about string theory is that the different versions of the theory prove to be highly non-trivial in relation. One of the relationships that exist between different theories is called S-duality. This is a relationship that says that a collection of interacting particles in a theory may in some cases be viewed as a collection of weak interacting particles in a completely different theory. Approximately, a collection of particles is said to interact strongly if they combine and deteriorate frequently and interact poorly if they do so infrequently. The type I string theory turns out to be equivalent by S-duality to the heterotic string theory SO (32). Likewise, type IIB string theory is related to itself in a non-trivial way by S-duality

Another relationship between different string theories is T-duality. Here one considers strings propagating around a circular extra dimension. T-duality states that a string propagating around a circle of radius *R* is equivalent to a string propagating around a circle of radius 1/*R* in the sense that all observable quantities in one description are identified with quantities in the dual description. For example, a string has momentum as it propagates around a circle, and it can also wind around the circle one or more times. The number of times the string winds around a circle is called the winding number. If a string has momentum *p* and winding number *n* in one description, it will have momentum *n* and winding number *p* in the dual description. For example, type IIA string theory is equivalent to type IIB string theory via T-duality, and the two versions of heterotic string theory are also related by T-duality.

In general relativity, a black hole is defined as a region of spacetime in which the gravitational field is so strong that no particle or radiation can escape. In the currently accepted models of stellar evolution, black holes are thought to arise when massive stars undergo gravitational collapse, and many galaxies are thought to contain supermassive black holes at their centers.

Black holes are also important for theoretical reasons, as they present profound challenges for theorists attempting to understand the quantum aspects of gravity. String theory has proved to be an important tool for investigating the theoretical properties of black holes because it provides a framework in which theorists can study their thermodynamics.

The big bang theory doesn’t offer any explanation for what started the original expansion of the universe. This is a major theoretical question for cosmologists, and many are applying the concepts of string theory in attempts to answer it. One controversial conjecture is a cyclic universe model called the ekpyrotic universe theory, which suggests that our own universe is the result of branes colliding with each other.

Some things that string theory could explain: Neutrinos would have to have mass (minimum), Decay of Proton, New fields of force (short and long range) defined by some forms of calabi-yau, Explanations for Dark Matter.

*String theory is a very complex and broad area, so this post is only a summary. To better understand, I suggest you read Brian Greene’s books: The Elegant Universe and The Fabric of the Cosmo.*

Drake Equation

What do we need to know about to discover life in space? How can we estimate the number of technological civilizations that might exist among the stars? While working as a radio astronomer at the National Radio Astronomy Observatory in Green Bank, West Virginia, Dr. Frank Drake conceived an approach to bound the terms involved in estimating the number of technological civilizations that may exist in our galaxy. The Drake Equation, as it has become known, was first presented by Drake in 1961 and identifies specific factors thought to play a role in the development of such civilizations. Although there is no unique solution to this equation, it is a generally accepted tool used by the scientific community to examine these factors.

- N = The number of civilizations in the Milky Way Galaxy whose electromagnetic emissions are detectable.
*R*_{∗}= The rate of formation of stars suitable for the development of intelligent life.

*f*_{p}= The fraction of those stars with planetary systems.- n
_{e}= The number of planets, per solar system, with an environment suitable for life. *f*_{l}= The fraction of suitable planets on which life actually appears.*f*_{i}= The fraction of life bearing planets on which intelligent life emerges.*f*_{c}= The fraction of civilizations that develop a technology that releases detectable signs of their existence into space.*L*= The length of time such civilizations release detectable signals into space.

Original estimates

*R*_{∗}= 1 yr^{−1}(1 star formed per year, on the average over the life of the galaxy; this was regarded as conservative)*f*_{p}= 0.2 to 0.5 (one fifth to one half of all stars formed will have planets)*n*_{e}= 1 to 5 (stars with planets will have between 1 and 5 planets capable of developing life)*f*_{l}= 1 (100% of these planets will develop life)*f*_{i}= 1 (100% of which will develop intelligent life)*f*_{c}= 0.1 to 0.2 (10–20% of which will be able to communicate)*L*= 1000 to 100,000,000 years (which will last somewhere between 1000 and 100,000,000 years)

Within the limits of our existing technology, any practical search for distant intelligent life must necessarily be a search for some manifestation of a distant technology. In each of its last four decadal reviews, the National Research Council has emphasized the relevance and importance of searching for evidence of the electromagnetic signature of distant civilizations.

Besides illuminating the factors involved in such a search, the Drake Equation is a simple, effective tool for stimulating intellectual curiosity about the universe around us, for helping us to understand that life as we know it is the end product of a natural, cosmic evolution, and for making us realize how much we are a part of that universe. A key goal of the SETI Institute is to further high quality research that will yield additional information related to any of the factors of this fascinating equation. *seti.org (read more)*