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Speed of light appears the same to everyone and that nothing could be faster than light. Energy is used to accelerate a particle.

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**Special relativity**
In physics, **special relativity** (**SR**, also known as the **special theory of relativity** or **STR**) is an accepted physical theory about the fundamental nature of space and time. It is based on two postulates: (1) that the laws of physics are invariant (i.e., identical) in all inertial systems (non-accelerating frames of reference); and (2) that the speed of light in a vacuum is the same for all observers, regardless of the motion of the observer relative to the light source. It was originally proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies". The inconsistency of Newton’s Laws with Maxwell’s equations of electromagnetism led to the development of special relativity, which expands upon (and corrects) Newton’s Laws to situations involving motions nearing the speed of light. As of today, special relativity is the most accurate model of motion at any speed. Even so, Newton's laws of motion are still useful (due to its sheer simplicity) as an approximation at small velocities relative to the speed of light.

Special relativity implies a wide range of consequences, which have been experimentally verified, including length contraction, time dilation, relativistic mass, mass–energy equivalence, an upper limit on maximum speed, and relativity of simultaneity. It has replaced the classical notion of invariant time interval for two events with the notion of invariant spacetime interval. Combined with other laws of physics, the two postulates of special relativity predict the equivalence of mass and energy, as expressed in the mass–energy equivalence formula *E* = *mc*2, where *c* is the speed of light in vacuum.

**Faster-than-light travel**
**Faster-than-light** (also **superluminal** or **FTL**) communications and travel refer to the propagation of information or matter faster than the speed of light. Under the special theory of relativity, a particle (that has rest mass) with subluminal velocity needs infinite energy to accelerate to the speed of light, although special relativity does not forbid the existence of particles that travel faster than light at all times (tachyons).

On the other hand, what some physicists refer to as "apparent" or "effective" FTL depends on the hypothesis that unusually distorted regions of spacetime might permit matter to reach distant locations in less time than light could in normal or undistorted spacetime. Although according to current theories matter is still required to travel subluminally with respect to the locally distorted spacetime region, *apparent* FTL is not excluded by general relativity.

**Interstellar travel**
**Interstellar space travel** is manned or unmanned travel between stars. The concept of interstellar travel via starships is a staple of science fiction. Interstellar travel is conceptually much more difficult than interplanetary travel. The distance between the planets in the Solar System is typically measured in standard astronomical units, while the distance between the stars is hundreds of thousands of AU and often expressed in light years. Intergalactic travel, or travel between different galaxies, would be even more difficult.

A variety of concepts have been discussed in the literature, since the first astronautical pioneers, such as Konstantin Tsiolkovsky, Robert Esnault-Pelterie and Robert Hutchings Goddard. Electrically powered spacecraft propulsion powered by a portable power-source, say a nuclear reactor, producing only low accelerations, would take centuries to reach near-by stars, thus unsuitable for interstellar flight within a human lifetime; thermal-propulsion engines such as NERVA produce sufficient thrust, but can only achieve relatively low-velocity exhaust jets, so to accelerate to the desired speed would require an enormous amount of fuel. Geoffrey A. Landis proposed for **interstellar travel** future-technology project еlectrically powered with supplying the energy from an external source (laser of base station). Fission-fragment rockets use nuclear fission to create high-speed jets of fission fragments, which are ejected at speeds of up to 12,000 km/s. Interstellar vehicles using electric propulsion, such as an ion rocket or plasma rocket, can also be powered via a laser beamed from a stationary power-supply. Given sufficient travel time and engineering work, both unmanned and sleeper ship interstellar travel requires no break-through physics to be achieved, but considerable technological and economic challenges need to be met. NASA, ESA and other space agencies have been engaging in research into these topics for decades, and have accumulated a number of theoretical approaches.

**Speed of light**
The **speed of light** in vacuum, commonly denoted **c**, is a universal physical constant important in many areas of physics. Its value is exactly **299,792,458 metres per second**, a figure that is exact because the length of the metre is defined from this constant and the international standard for time. This is approximately 186,282.4 miles per second, or about 671 million miles per hour. According to special relativity, *c* is the maximum speed at which all energy, matter, and information in the universe can travel. It is the speed at which all massless particles and associated fields (including electromagnetic radiation such as light) travel in vacuum. It is also the speed of gravity (i.e. of gravitational waves) predicted by current theories. Such particles and waves travel at *c* regardless of the motion of the source or the inertial frame of reference of the observer. In the theory of relativity, *c* interrelates space and time, and also appears in the famous equation of mass–energy equivalence *E* = *mc*2.

The speed at which light propagates through transparent materials, such as glass or air, is less than *c*. The ratio between *c* and the speed *v* at which light travels in a material is called the refractive index *n* of the material (*n* = *c* / *v*). For example, for visible light the refractive index of glass is typically around 1.5, meaning that light in glass travels at *c* / 1.5 ≈ 200,000 km/s; the refractive index of air for visible light is 1.000293, so the speed of light in air is 299,705 km/s or about 88 km/s slower than *c*.

**Albert Einstein**
**Albert Einstein** (/ /; German: [ˈalbɐt ˈaɪnʃtaɪn] ( listen); 14 March 1879 – 18 April 1955) was a German-born theoretical physicist who developed the general theory of relativity, one of the two pillars of modern physics (alongside quantum mechanics). While best known for his mass–energy equivalence formula *E* = *mc*2 (which has been dubbed "the world's most famous equation"), he received the 1921 Nobel Prize in Physics "for his services to theoretical physics, and especially for his discovery of the law of the photoelectric effect". The latter was pivotal in establishing quantum theory.

Near the beginning of his career, Einstein thought that Newtonian mechanics was no longer enough to reconcile the laws of classical mechanics with the laws of the electromagnetic field. This led to the development of his special theory of relativity. He realized, however, that the principle of relativity could also be extended to gravitational fields, and with his subsequent theory of gravitation in 1916, he published a paper on the general theory of relativity. He continued to deal with problems of statistical mechanics and quantum theory, which led to his explanations of particle theory and the motion of molecules. He also investigated the thermal properties of light which laid the foundation of the photon theory of light. In 1917, Einstein applied the general theory of relativity to model the large-scale structure of the universe.

**Introduction to general relativity**
**General relativity** is a theory of gravitation that was developed by Albert Einstein between 1907 and 1915. According to general relativity, the observed gravitational effect between masses results from their warping of spacetime.

By the beginning of the 20th century, Newton's law of universal gravitation had been accepted for more than two hundred years as a valid description of the gravitational force between masses. In Newton's model, gravity is the result of an attractive force between massive objects. Although even Newton was troubled by the unknown nature of that force, the basic framework was extremely successful at describing motion.

**Human Interest**
In journalism, a **human interest story** is a feature story that discusses a person or people in an emotional way. It presents people and their problems, concerns, or achievements in a way that brings about interest, sympathy or motivation in the reader or viewer.

Human interest stories may be "the story behind the story" about an event, organization, or otherwise faceless historical happening, such as about the life of an individual soldier during wartime, an interview with a survivor of a natural disaster, a random act of kindness or profile of someone known for a career achievement.

**Physics**
**Relativity**
**Tachyons**
**Faster-than-light**
**Light**
**Mass**
**Technology Internet**
**Environment**
**energy**