Gauge theories are important as the successful field theories explaining the dynamics of elementary particles . Quantum electrodynamics is an abelian gauge theory with the symmetry group U(1) and has one gauge field, the electromagnetic four-potential , with the photon being the gauge boson. The Standard Model is a non-abelian gauge theory with the symmetry group U(1) × SU(2) × SU(3) and has a total of twelve gauge bosons: the photon , three weak bosons and eight gluons .
In string theory , D-branes are a class of extended objects upon which open strings can end with Dirichlet boundary conditions , after which they are named. D-branes were discovered by Dai, Leigh and Polchinski , and independently by Hořava in 1989. In 1995, Polchinski identified D-branes with black p-brane solutions of supergravity , a discovery that triggered the Second Superstring Revolution and led to both holographic and M-theory dualities.
Can you imagine our world without drinking water?We use water every day for cooking and washing.We use water without even thinking that there are places where people have no clear water to drink.However,there are water shortages all over the world.We need water not only for our homes,gardens and factories.We need water to survive as a species.Water pollution is one of the burning problems of modern society.Plants and factories pour chemicals into the lakes and rivers.Contaminated water goes to the seas and oceans.We must take measures before it is too late.First of all,we can save water at our homes.We should take showers instead of baths.We should repair the taps and pipes in our houses to avoid leaking.Factories and plants should use recycled water to stop polluting rivers and lakes.It's extremely important to persuade people to save water.People must understand that maintaining clean and healthy environment is in their own best interests.Too much depends on our decisions and our actions.We must do everything we can before the world without water has become a horrible reality.