A spark gap consists of an arrangement of two conducting electrodes separated by a gap filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the potential difference between the conductors exceeds the breakdown voltage of the gas within the gap, a spark forms, ionizing the gas and drastically reducing its electrical resistance. An electric current flows until the path of ionized gas is broken or the current reduces below a minimum value called the "holding current"; this happens when the voltage drops, but in some cases occurs when the heated gas rises, stretching out and breaking the filament of ionized gas. The action of ionizing the gas is violent and disruptive leading to sound and heat. Spark gaps were used in early electrical equipment, such as spark gap radio transmitters, electrostatic machines, X-ray machines, their most widespread use today is in spark plugs to ignite the fuel in internal combustion engines, but they are used in lightning arresters and other devices to protect electrical equipment from high-voltage transients.
The light emitted by a spark does not come from the current of electrons itself, but from the material medium fluorescing in response to collisions from the electrons. When electrons collide with molecules of air in the gap, they excite their orbital electrons to higher energy levels; when these excited electrons fall back to their original energy levels, they emit energy as light. It is impossible for a visible spark to form in a vacuum. Without intervening matter capable of electromagnetic transitions, the spark will be invisible. Spark gaps are essential to the functioning of a number of electronic devices. A spark plug uses a spark gap to initiate combustion; the heat of the ionization trail, but more UV radiation and hot free electrons ignite a fuel-air mixture inside an internal combustion engine, or a burner in a furnace, oven, or stove. The more UV radiation is produced and spread into the combustion chamber, the further the combustion process proceeds. Spark gaps are used to prevent voltage surges from damaging equipment.
Spark gaps are used in high-voltage switches, large power transformers, in power plants and electrical substations. Such switches are constructed with a large, remote-operated switching blade with a hinge as one contact and two leaf springs holding the other end as second contact. If the blade is opened, a spark may keep the connection between spring conducting; the spark ionizes the air, which becomes conductive and allows an arc to form, which sustains ionization and hence conduction. A Jacob's ladder on top of the switch will cause the arc to rise and extinguish. One might find small Jacob's ladders mounted on top of ceramic insulators of high-voltage pylons; these are sometimes called horn gaps. If a spark should manage to jump over the insulator and give rise to an arc, it will be extinguished. Smaller spark gaps are used to protect sensitive electrical or electronic equipment from high-voltage surges. In sophisticated versions of these devices, a small spark gap breaks down during an abnormal voltage surge, safely shunting the surge to ground and thereby protecting the equipment.
These devices are used for telephone lines as they enter a building. Less sophisticated spark gaps are made using modified ceramic capacitors. A voltage surge causes a spark that jumps from lead wire to lead wire across the gap left by the sawing process; these low-cost devices are used to prevent damaging arcs between the elements of the electron gun within a cathode ray tube. Small spark gaps are common in telephone switchboards, as the long phone cables are susceptible to induced surges from lightning strikes. Larger spark gaps are used to protect power lines. Spark gaps are implemented on Printed Circuit Boards in mains power electronics products using two spaced exposed PCB traces; this is an zero cost method of adding crude overload protection to electronics products. Transils and trisils are the solid-state alternatives to spark gaps for lower-power applications. Neon bulbs are used for this purpose. A triggered spark gap in an air-gap flash is used to produce photographic light flashes in the sub-microsecond domain.
A spark radiates energy throughout the electromagnetic spectrum. Nowadays, this is regarded as illegal radio frequency interference and is suppressed, but in the early days of radio communications, this was the means by which radio signals were transmitted, in the unmodulated spark-gap transmitter. Many radio spark gaps include cooling devices, such as the rotary gap and heat sinks, since the spark gap becomes quite hot under continuous use at high power. A calibrated spherical spark gap will break down at a repeatable voltage, when corrected for air pressure and temperature. A gap between two spheres can provide a voltage measurement without any electronics or voltage dividers, to an accuracy of about 3%. A spark gap can be used to measure high voltage AC, DC, or pulses, but for short pulses, an ultraviolet light source or radioactive source may be put on one of the terminals to provide a source of electrons. Spark gaps may be used as electrical switches because they have two states with
Plasma is one of the four fundamental states of matter, was first described by chemist Irving Langmuir in the 1920s. Plasma can be artificially generated by heating or subjecting a neutral gas to a strong electromagnetic field to the point where an ionized gaseous substance becomes electrically conductive, long-range electromagnetic fields dominate the behaviour of the matter. Plasma and ionized gases have properties and display behaviours unlike those of the other states, the transition between them is a matter of nomenclature and subject to interpretation. Based on the surrounding environmental temperature and density ionized or ionized forms of plasma may be produced. Neon signs and lightning are examples of ionized plasma; the Earth's ionosphere is a plasma and the magnetosphere contains plasma in the Earth's surrounding space environment. The interior of the Sun is an example of ionized plasma, along with the solar corona and stars. Positive charges in ions are achieved by stripping away electrons orbiting the atomic nuclei, where the total number of electrons removed is related to either increasing temperature or the local density of other ionized matter.
This can be accompanied by the dissociation of molecular bonds, though this process is distinctly different from chemical processes of ion interactions in liquids or the behaviour of shared ions in metals. The response of plasma to electromagnetic fields is used in many modern technological devices, such as plasma televisions or plasma etching. Plasma may be the most abundant form of ordinary matter in the universe, although this hypothesis is tentative based on the existence and unknown properties of dark matter. Plasma is associated with stars, extending to the rarefied intracluster medium and the intergalactic regions; the word plasma comes from Ancient Greek πλάσμα, meaning'moldable substance' or'jelly', describes the behaviour of the ionized atomic nuclei and the electrons within the surrounding region of the plasma. Each of these nuclei are suspended in a movable sea of electrons. Plasma was first identified in a Crookes tube, so described by Sir William Crookes in 1879; the nature of this "cathode ray" matter was subsequently identified by British physicist Sir J.
J. Thomson in 1897; the term "plasma" was coined by Irving Langmuir in 1928. Lewi Tonks and Harold Mott-Smith, both of whom worked with Irving Langmuir in the 1920s, recall that Langmuir first used the word "plasma" in analogy with blood. Mott-Smith recalls, in particular, that the transport of electrons from thermionic filaments reminded Langmuir of "the way blood plasma carries red and white corpuscles and germs."Langmuir described the plasma he observed as follows: "Except near the electrodes, where there are sheaths containing few electrons, the ionized gas contains ions and electrons in about equal numbers so that the resultant space charge is small. We shall use the name plasma to describe this region containing balanced charges of ions and electrons." Plasma is a state of matter in which an ionized gaseous substance becomes electrically conductive to the point that long-range electric and magnetic fields dominate the behaviour of the matter. The plasma state can be contrasted with the other states: solid and gas.
Plasma is an electrically neutral medium of unbound negative particles. Although these particles are unbound, they are not "free" in the sense of not experiencing forces. Moving charged particles generate an electric current within a magnetic field, any movement of a charged plasma particle affects and is affected by the fields created by the other charges. In turn this governs collective behaviour with many degrees of variation. Three factors define a plasma: The plasma approximation: The plasma approximation applies when the plasma parameter, Λ, representing the number of charge carriers within a sphere surrounding a given charged particle, is sufficiently high as to shield the electrostatic influence of the particle outside of the sphere. Bulk interactions: The Debye screening length is short compared to the physical size of the plasma; this criterion means that interactions in the bulk of the plasma are more important than those at its edges, where boundary effects may take place. When this criterion is satisfied, the plasma is quasineutral.
Plasma frequency: The electron plasma frequency is large compared to the electron-neutral collision frequency. When this condition is valid, electrostatic interactions dominate over the processes of ordinary gas kinetics. Plasma temperature is measured in kelvin or electronvolts and is, informally, a measure of the thermal kinetic energy per particle. High temperatures are needed to sustain ionisation, a defining feature of a plasma; the degree of plasma ionisation is determined by the electron temperature relative to the ionization energy, in a relationship called the Saha equation. At low temperatures and electrons tend to recombine into bound states—atoms—and the plasma will become a gas. In most cases the electrons are close enough to thermal equilibrium that their temperature is well-defined; because of the large difference in ma
Cosmic rays are high-energy radiation originating outside the Solar System and from distant galaxies. Upon impact with the Earth's atmosphere, cosmic rays can produce showers of secondary particles that sometimes reach the surface. Composed of high-energy protons and atomic nuclei, they are originated either from the sun or from outside of our solar system. Data from the Fermi Space Telescope have been interpreted as evidence that a significant fraction of primary cosmic rays originate from the supernova explosions of stars. Active galactic nuclei appear to produce cosmic rays, based on observations of neutrinos and gamma rays from blazar TXS 0506+056 in 2018; the term ray is somewhat of a misnomer due to a historical accident, as cosmic rays were at first, wrongly, thought to be electromagnetic radiation. In common scientific usage, high-energy particles with intrinsic mass are known as "cosmic" rays, while photons, which are quanta of electromagnetic radiation are known by their common names, such as gamma rays or X-rays, depending on their photon energy.
In current usage, the term cosmic ray exclusively refers to massive particles – those that have rest mass – as opposed to photons, which have no rest mass, neutrinos, which have negligible rest mass. Massive particles have additional, mass-energy when they are moving, due to relativistic effects. Through this process, some particles acquire tremendously high mass-energies; these are higher than the photon energy of the highest-energy photons detected to date. The energy of the massless photon depends on frequency, not speed, as photons always travel at the same speed. At the higher end of the energy spectrum, relativistic kinetic energy is the main source of the mass-energy of cosmic rays; the highest-energy fermionic cosmic rays detected to date, such as the Oh-My-God particle, had an energy of about 3×1020 eV, while the highest-energy gamma rays to be observed, very-high-energy gamma rays, are photons with energies of up to 1014 eV, the highest energy neutrinos detected so far have energies of several 1015 eV.
Hence, the highest-energy detected fermionic cosmic rays are about 3×106 times as energetic as the highest-energy detected cosmic photons. Of primary cosmic rays, which originate outside of Earth's atmosphere, about 99% are the nuclei of well-known atoms, about 1% are solitary electrons. Of the nuclei, about 90% are simple protons; these fractions vary over the energy range of cosmic rays. A small fraction are stable particles of antimatter, such as positrons or antiprotons; the precise nature of this remaining fraction is an area of active research. An active search from Earth orbit for anti-alpha particles has failed to detect them. Cosmic rays attract great interest due to the damage they inflict on microelectronics and life outside the protection of an atmosphere and magnetic field, scientifically, because the energies of the most energetic ultra-high-energy cosmic rays have been observed to approach 3 × 1020 eV, about 40 million times the energy of particles accelerated by the Large Hadron Collider.
One can show that such enormous energies might be achieved by means of the centrifugal mechanism of acceleration in active galactic nuclei. At 50 J, the highest-energy ultra-high-energy cosmic rays have energies comparable to the kinetic energy of a 90-kilometre-per-hour baseball; as a result of these discoveries, there has been interest in investigating cosmic rays of greater energies. Most cosmic rays, however, do not have such extreme energies. After the discovery of radioactivity by Henri Becquerel in 1896, it was believed that atmospheric electricity, ionization of the air, was caused only by radiation from radioactive elements in the ground or the radioactive gases or isotopes of radon they produce. Measurements of increasing ionization rates at increasing heights above the ground during the decade from 1900 to 1910 could be explained as due to absorption of the ionizing radiation by the intervening air. In 1909, Theodor Wulf developed an electrometer, a device to measure the rate of ion production inside a hermetically sealed container, used it to show higher levels of radiation at the top of the Eiffel Tower than at its base.
However, his paper published in Physikalische Zeitschrift was not accepted. In 1911, Domenico Pacini observed simultaneous variations of the rate of ionization over a lake, over the sea, at a depth of 3 metres from the surface. Pacini concluded from the decrease of radioactivity underwater that a certain part of the ionization must be due to sources other than the radioactivity of the Earth. In 1912, Victor Hess carried three enhanced-accuracy Wulf electrometers to an altitude of 5,300 metres in a free balloon flight, he found the ionization rate increased fourfold over the rate at ground level. Hess ruled out the Sun as the radiation's source by making a balloon ascent during a near-total eclipse. With the moon blocking much of the Sun's visible radiation, Hess still measured rising radiation at rising altitudes, he concluded that "The results of the observations seem most to be explained by the assumption that radiation of high penetrating power enters from above into our atmosphere." In 1913–1914, Werner Kolhörster confirmed Victor Hess's earlier results by measuring the increased ionization enthalpy rate at an altitude of 9 km.
Hess received the Nobel Prize in Physics in 1936
An electric field surrounds an electric charge, exerts force on other charges in the field, attracting or repelling them. Electric field is sometimes abbreviated as E-field. Mathematically the electric field is a vector field that associates to each point in space the force per unit of charge exerted on an infinitesimal positive test charge at rest at that point; the SI unit for electric field strength is volt per meter. Newtons per coulomb is used as a unit of electric field strengh. Electric fields are created by time-varying magnetic fields. Electric fields are important in many areas of physics, are exploited electrical technology. On an atomic scale, the electric field is responsible for the attractive force between the atomic nucleus and electrons that holds atoms together, the forces between atoms that cause chemical bonding. Electric fields and magnetic fields are both manifestations of the electromagnetic force, one of the four fundamental forces of nature. From Coulomb's law a particle with electric charge q 1 at position x 1 exerts a force on a particle with charge q 0 at position x 0 of F = 1 4 π ε 0 q 1 q 0 2 r ^ 1, 0 where r 1, 0 is the unit vector in the direction from point x 1 to point x 0, ε0 is the electric constant in C2 m−2 N−1When the charges q 0 and q 1 have the same sign this force is positive, directed away from the other charge, indicating the particles repel each other.
When the charges have unlike signs the force is negative, indicating the particles attract. To make it easy to calculate the Coulomb force on any charge at position x 0 this expression can be divided by q 0, leaving an expression that only depends on the other charge E = F q 0 = 1 4 π ε 0 q 1 2 r ^ 1, 0 This is the electric field at point x 0 due to the point charge q 1. Since this formula gives the electric field magnitude and direction at any point x 0 in space it defines a vector field. From the above formula it can be seen that the electric field due to a point charge is everywhere directed away from the charge if it is positive, toward the charge if it is negative, its magnitude decreases with the inverse square of the distance from the charge. If there are multiple charges, the resultant Coulomb force on a charge can be found by summing the vectors of the forces due to each charge; this shows the electric field obeys the superposition principle: the total electric field at a point due to a collection of charges is just equal to the vector sum of the electric fields at that point due to the individual charges.
E = E 1 + E 2 + E 3 + ⋯ = 1 4 π ε 0 q 1 2 r ^ 1 + 1 4 π ε 0 q 2 ( x 2 −
Lightning is a violent and sudden electrostatic discharge where two electrically charged regions in the atmosphere temporarily equalize themselves during a thunderstorm. Lightning creates a wide range of electromagnetic radiations from the hot plasma created by the electron flow, including visible light in the form of black-body radiation. Thunder is the sound formed by the shock wave formed as gaseous molecules experience a rapid pressure increase; the three main kinds of lightning are: created either inside one thundercloud, or between two clouds, or between a cloud and the ground. The 15 recognized observational variants include "heat lightning", seen but not heard, dry lightning, which causes many forest fires, ball lightning, observed scientifically. Humans have deified lightning for millennia, lightning inspired expressions like "Bolt from the blue", "Lightning never strikes twice", "blitzkrieg" are common. In some languages, "Love at first sight" translates as "lightning strike"; the details of the charging process are still being studied by scientists, but there is general agreement on some of the basic concepts of thunderstorm electrification.
The main charging area in a thunderstorm occurs in the central part of the storm where air is moving upward and temperatures range from −15 to −25 °C, see figure to the right. At that place, the combination of temperature and rapid upward air movement produces a mixture of super-cooled cloud droplets, small ice crystals, graupel; the updraft carries the super-cooled cloud droplets and small ice crystals upward. At the same time, the graupel, larger and denser, tends to fall or be suspended in the rising air; the differences in the movement of the precipitation cause collisions to occur. When the rising ice crystals collide with graupel, the ice crystals become positively charged and the graupel becomes negatively charged. See figure to the left; the updraft carries. The larger and denser graupel is either suspended in the middle of the thunderstorm cloud or falls toward the lower part of the storm; the result is that the upper part of the thunderstorm cloud becomes positively charged while the middle to lower part of the thunderstorm cloud becomes negatively charged.
The upward motions within the storm and winds at higher levels in the atmosphere tend to cause the small ice crystals in the upper part of the thunderstorm cloud to spread out horizontally some distance from thunderstorm cloud base. This part of the thunderstorm cloud is called the anvil. While this is the main charging process for the thunderstorm cloud, some of these charges can be redistributed by air movements within the storm. In addition, there is a small but important positive charge buildup near the bottom of the thunderstorm cloud due to the precipitation and warmer temperatures. A typical cloud-to-ground lightning flash culminates in the formation of an electrically conducting plasma channel through the air in excess of 5 km tall, from within the cloud to the ground's surface; the actual discharge is the final stage of a complex process. At its peak, a typical thunderstorm produces three or more strikes to the Earth per minute. Lightning occurs when warm air is mixed with colder air masses, resulting in atmospheric disturbances necessary for polarizing the atmosphere.
However, it can occur during dust storms, forest fires, volcanic eruptions, in the cold of winter, where the lightning is known as thundersnow. Hurricanes generate some lightning in the rainbands as much as 160 km from the center; the science of lightning is called fulminology, the fear of lightning is called astraphobia. Lightning is not distributed evenly around the planet. On Earth, the lightning frequency is 44 times per second, or nearly 1.4 billion flashes per year and the average duration is 0.2 seconds made up from a number of much shorter flashes of around 60 to 70 microseconds. Many factors affect the frequency, distribution and physical properties of a typical lightning flash in a particular region of the world; these factors include ground elevation, prevailing wind currents, relative humidity, proximity to warm and cold bodies of water, etc. To a certain degree, the ratio between IC, CC and CG lightning may vary by season in middle latitudes; because human beings are terrestrial and most of their possessions are on the Earth where lightning can damage or destroy them, CG lightning is the most studied and best understood of the three types though IC and CC are more common types of lightning.
Lightning's relative unpredictability limits a complete explanation of how or why it occurs after hundreds of years of scientific investigation. About 70 % of lightning occurs over land in the tropics; this occurs from both the mixture of warmer and colder air masses, as well as differences in moisture concentrations, it happens at the boundaries between them. The flow of warm ocean currents past drier land masses, such as the Gulf Stream explains the elevated frequency of lightning in the Southeast United States; because the influence of small or absent land masses in the vast stretches of the world's oceans limits the differences between these variants in the atmosphere, lightning is notably less frequent there than over larger landforms. The North and South Poles are limited in their coverage of thunderstorms and theref
Relativistic runaway electron avalanche
A relativistic runaway electron avalanche is an avalanche growth of a population of relativistic electrons driven through a material by an electric field. RREA has been hypothesized to be related to lightning initiation, terrestrial gamma-ray flashes, sprite lightning, spark development. RREA is unique as it can occur at electric fields an order of magnitude lower than the dielectric strength of the material; when an electric field is applied to a material, free electrons will drift through the material as described by the electron mobility. For low-energy electrons, faster drift velocities result in more interactions with surrounding particles; these interactions create a form of friction. Thus, for low-energy cases, the electron velocities tend to stabilize. At higher energies, above about 100 keV, these collisional events become less common as the mean free path of the electron rises; these higher-energy electrons thus see less frictional force as their velocity increases. In the presence of the same electric field, these electrons will continue accelerating, "running away".
As runaway electrons gain energy from an electric field, they collide with atoms in the material, knocking off secondary electrons. If the secondary electrons have high enough energy to run away, they too accelerate to high energies, produce further secondary electrons, etc; as such, the total number of energetic electrons grows exponentially in an avalanche. The RREA mechanism above only describes the growth of the avalanche. An initial energetic electron is needed to start the process. In ambient air, such energetic electrons come from cosmic rays. In strong electric fields, stronger than the maximum frictional force experienced by electrons low-energy electrons can accelerate to relativistic energies, a process dubbed "thermal runaway." RREA avalanches move opposite the direction of the electric field. As such, after the avalanches leave the electric field region, frictional forces dominate, the electrons lose energy, the process stops. There is the possibility, that photons or positrons produced by the avalanche will wander back to where the avalanche began and can produce new seeds for a second generation of avalanches.
If the electric field region is large enough, the number of second-generation avalanches will exceed the number of first-generation avalanches and the number of avalanches itself grows exponentially. This avalanche of avalanches can produce large populations of energetic electrons; this process leads to the decay of the electric field below the level at which feedback is possible and therefore acts as a limit to the large-scale electric field strength. The large population of energetic electrons produced in RREA will produce a correspondingly large population of energetic photons by bremsstrahlung; these photons are proposed as the source of terrestrial gamma-ray flashes. Large RREA events in thunderstorms may contribute rare but large radiation doses to commercial airline flights; the American physicist Joseph Dwyer coined the term "dark lightning" for this phenomenon, still the subject of research
Theory of relativity
The theory of relativity encompasses two interrelated theories by Albert Einstein: special relativity and general relativity. Special relativity applies to elementary particles and their interactions, describing all their physical phenomena except gravity. General relativity explains the law of its relation to other forces of nature, it applies to the astrophysical realm, including astronomy. The theory transformed theoretical physics and astronomy during the 20th century, superseding a 200-year-old theory of mechanics created by Isaac Newton, it introduced concepts including spacetime as a unified entity of space and time, relativity of simultaneity and gravitational time dilation, length contraction. In the field of physics, relativity improved the science of elementary particles and their fundamental interactions, along with ushering in the nuclear age. With relativity and astrophysics predicted extraordinary astronomical phenomena such as neutron stars, black holes, gravitational waves. Albert Einstein published the theory of special relativity in 1905, building on many theoretical results and empirical findings obtained by Albert A. Michelson, Hendrik Lorentz, Henri Poincaré and others.
Max Planck, Hermann Minkowski and others did subsequent work. Einstein developed general relativity between 1907 and 1915, with contributions by many others after 1915; the final form of general relativity was published in 1916. The term "theory of relativity" was based on the expression "relative theory" used in 1906 by Planck, who emphasized how the theory uses the principle of relativity. In the discussion section of the same paper, Alfred Bucherer used for the first time the expression "theory of relativity". By the 1920s, the physics community accepted special relativity, it became a significant and necessary tool for theorists and experimentalists in the new fields of atomic physics, nuclear physics, quantum mechanics. By comparison, general relativity did not appear to be as useful, beyond making minor corrections to predictions of Newtonian gravitation theory, it seemed to offer little potential for experimental test, as most of its assertions were on an astronomical scale. Its mathematics seemed difficult and understandable only by a small number of people.
Around 1960, general relativity became central to astronomy. New mathematical techniques to apply to general relativity streamlined calculations and made its concepts more visualized; as astronomical phenomena were discovered, such as quasars, the 3-kelvin microwave background radiation and the first black hole candidates, the theory explained their attributes, measurement of them further confirmed the theory. Special relativity is a theory of the structure of spacetime, it was introduced in Einstein's 1905 paper "On the Electrodynamics of Moving Bodies". Special relativity is based on two postulates which are contradictory in classical mechanics: The laws of physics are the same for all observers in uniform motion relative to one another; the speed of light in a vacuum is the same for all observers, regardless of their relative motion or of the motion of the light source. The resultant theory copes with experiment better than classical mechanics. For instance, postulate 2 explains the results of the Michelson–Morley experiment.
Moreover, the theory has many counterintuitive consequences. Some of these are: Relativity of simultaneity: Two events, simultaneous for one observer, may not be simultaneous for another observer if the observers are in relative motion. Time dilation: Moving clocks are measured to tick more than an observer's "stationary" clock. Length contraction: Objects are measured to be shortened in the direction that they are moving with respect to the observer. Maximum speed is finite: No physical object, message or field line can travel faster than the speed of light in a vacuum; the effect of Gravity can only travel through space at the speed of light, not faster or instantaneously. Mass -- energy equivalence: E = mc2, energy and mass are transmutable. Relativistic mass, idea used by some researchers; the defining feature of special relativity is the replacement of the Galilean transformations of classical mechanics by the Lorentz transformations.. General relativity is a theory of gravitation developed by Einstein in the years 1907–1915.
The development of general relativity began with the equivalence principle, under which the states of accelerated motion and being at rest in a gravitational field are physically identical. The upshot of this is that free fall is inertial motion: an object in free fall is falling because, how objects move when there is no force being exerted on them, instead of this being due to the force of gravity as is the case in classical mechanics; this is incompatible with classical mechanics and special relativity because in those theories inertially moving objects cannot accelerate with respect to each other, but objects in free fall do so. To resolve this difficulty Einstein first proposed. In 1915, he devised the Einstein field equations which relate the curvature of spacetime with the mass and any momentum within it; some of the consequences of general relativity are: Gravitational time dilation: Clocks run slower in deeper gravitational wells. Precession: Orbits precess in a way unexpected in Newton's theory of gravity