WHAT IS THEORITICAL PHYSICS??

Theoretical physicists use mathematics to describe certain aspects of Nature. Sir Isaac Newton was the first theoretical physicist, although in his own time his profession was called "natural philosophy".
By Newton's era people had already used algebra and geometry to build marvelous works of architecture, including the great cathedrals of Europe, but algebra and geometry only describe things that are sitting still. In order to describe things that are moving or changing in some way, Newton invented calculus.
The most puzzling and intriguing moving things visible to humans have always been been the sun, the moon, the planets and the stars we can see in the night sky. Newton's new calculus, combined with his "Laws of Motion", made a mathematical model for the force of gravity that not only described the observed motions of planets and stars in the night sky, but also of swinging weights and flying cannonballs in England.
Today's theoretical physicists are often working on the boundaries of known mathematics, sometimes inventing new mathematics as they need it, like Newton did with calculus.
Newton was both a theorist and an experimentalist. He spent many many long hours, to the point of neglecting his health, observing the way Nature behaved so that he might describe it better. The so-called "Newton's Laws of Motion" are not abstract laws that Nature is somehow forced to obey, but the observed behavior of Nature that is described in the language of mathematics. In Newton's time, theory and experiment went together.
Today the functions of theory and observation are divided into two distinct communities in physics. Both experiments and theories are much more complex than back in Newton's time. Theorists are exploring areas of Nature in mathematics that technology so far does not allow us to observe in experiments. Many of the theoretical physicists who are alive today may not live to see how the real Nature compares with her mathematical description in their work. Today's theorists have to learn to live with ambiguity and uncertainty in their mission to describe Nature using math.

why did string enter the theory??

this image is showing string theory ...this image can be enlarged by just clicking on itRelativistic quantum field theory has worked very well to describe the observed behaviors and properties of elementary particles. But the theory itself only works well when gravity is so weak that it can be neglected. Particle theory only works when we pretend gravity doesn't exist.
General relativity has yielded a wealth of insight into the Universe, the orbits of planets, the evolution of stars and galaxies, the Big Bang and recently observed black holes and gravitational lenses. However, the theory itself only works when we pretend that the Universe is purely classical and that quantum mechanics is not needed in our description of Nature.
String theory is believed to close this gap.
Originally, string theory was proposed as an explanation for the observed relationship between mass and spin for certain particles called hadrons, which include the proton and neutron. Things didn't work out, though, and Quantum Chromodynamics eventually proved a better theory for hadrons.
But particles in string theory arise as excitations of the string, and included in the excitations of a string in string theory is a particle with zero mass and two units of spin.
If there were a good quantum theory of gravity, then the particle that would carry the gravitational force would have zero mass and two units of spin. This has been known by theoretical physicists for a long time. This theorized particle is called the graviton.
This led early string theorists to propose that string theory be applied not as a theory of hadronic particles, but as a theory of quantum gravity, the unfulfilled fantasy of theoretical physics in the particle and gravity communities for decades.
But it wasn't enough that there be a graviton predicted by string theory. One can add a graviton to quantum field theory by hand, but the calculations that are supposed to describe Nature become useless. This is because, as illustrated in the diagram above, particle interactions occur at a single point of spacetime, at zero distance between the interacting particles. For gravitons, the mathematics behaves so badly at zero distance that the answers just don't make sense. In string theory, the strings collide over a small but finite distance, and the answers do make sense.
This doesn't mean that string theory is not without its deficiencies. But the zero distance behavior is such that we can combine quantum mechanics and gravity, and we can talk sensibly about a string excitation that carries the gravitational force.
This was a very great hurdle that was overcome for late 20th century physics, which is why so many young people are willing to learn the grueling complex and abstract mathematics that is necessary to study a quantum theory of interacting strings.

AIR PLANT

Common name: Air Plant, Donkey Ears, Life Plant, Leaf of Life, Resurrection Plant, Canterbury Bells, Cathedral Bells, Mexican Love Plant, Floppers • Hindi: Amar poi अमर पोई • Malayalam: Elamarunna • Tamil: ), Runakkalli • Bengali: Kop pata • Urdu: Zakhmhaiyat ज़ख़्महयात • Manipuri: , মনাহিদাক Manahidak
Botanical name: Kalanchoe pinnata/ Bryophyllum pinnatum Family: Crassulaceae (sedum family)

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Native Hawaiian plant. Easy to grow just from one leaf set on top of moist soil. Very fast growing, drought tolerant small shrub. Tolerates almost any conditions. Spectacular bloomer. Air Plant grows to about 3-6 feet tall. The erect, thick, succulent stems bear large, fleshy leaves, each with 3 or 5 oval leaflets with round-toothed edges. Young plantlets develop along the margins of the mature leaves. The attractive, drooping blooms are borne on large panicles. The flowers have purple or yellowish-white tinged calyxes and reddish corollas. Kalanchoe is a genus of about 125 species of tropical, succulent flowering plants in the Family Crassulaceae, mainly native to the Old World but with a few species in the New World. These plants are cultivated as ornamental houseplants and rock or "cactus" garden plants. They are popular because of their ease of propagation, low water requirements, and wide variety of flower colors typically borne in clusters well above the vegetative growth. The "Air plant" Kalanchoe pinnata is a curiosity because new individuals develop vegetatively at indents along the leaf, usually after the leaf has broken off the plant and is laying on the ground, where the new plant can take root.
Medicinal uses: Bahamians call it Life Leaf or Ploppers. In the Bahamas it is mostly used for Asthma or shortness in breath.
Photographed in New Delhi