In physics, mass (from Greek μᾶζα "barley cake, lump (of dough)"), more specifically inertial mass, can be defined as a quantitative measure of an object's resistance to the change of its speed. In addition to this, gravitational mass can be described as a measure of magnitude of the gravitational force which is

1. exerted by an object (active gravitational mass), or
2. experienced by an object (passive gravitational force)

when interacting with a second object. The SI unit of mass is the kilogram (kg).

In everyday usage, mass is often referred to as weight, the units of which are often taken to be kilograms (for instance, a person may state that their weight is 75 kg). In scientific use, however, the term weight refers to a different, yet related, property of matter. Weight is the gravitational force acting on a given body—which differs depending on the gravitational pull of the opposing body (e.g. a person's weight on Earth vs on the Moon) — while mass is an intrinsic property of that body that never changes. In other words, an object's weight depends on its environment, while its mass does not. On the surface of the Earth, an object with a mass of 50 kilograms weighs 491 Newtons; on the surface of the Moon, the same object still has a mass of 50 kilograms but weighs only 81.5 Newtons. Restated in mathematical terms, on the surface of the Earth, the weight W of an object is related to its mass m by W = mg, where g is the Earth's gravitational field strength, equal to about 9.81 m/s².

The inertial mass of an object determines its acceleration in the presence of an applied force. According to Newton's second law of motion, if a body of fixed mass M is subjected to a force F, its acceleration α is given by F/M. A body's mass also determines the degree to which it generates or is affected by a gravitational field. If a first body of mass M_A is placed at a distance r from a second body of mass M_B, each body experiences an attractive force F_G whose magnitude is F_G = GM_AM_B/r², where G is the universal constant of gravitation, equal to 6.67×10⁻¹¹ N·m²·kg⁻². This is sometimes referred to as gravitational mass.^{[note 1]} Repeated experiments since the 17th century have demonstrated that inertial and gravitational mass are equivalent; since 1915, this observation has been entailed a priori in the equivalence principle of general relativity.

Special relativity shows that rest mass (or invariant mass) and rest energy are essentially equivalent, via the well-known relationship (E = mc²). This same equation also connects relativistic mass and "relativistic energy" (total system energy). The latter two "relativistic" mass and energy are concepts that are related to their "rest" counterparts, but they do not have the same value as their rest counterparts in systems where there is a net momentum. In order to deduce any of these four quantities from any of the others, in any system which has a net momentum, an equation that takes momentum into account is needed. Mass (so long as the type and definition of mass is agreed upon) is a conserved quantity over time. From the viewpoint of any single unaccelerated observer, mass can neither be created or destroyed, and special relativity does not change this understanding. All unaccelerated observers agree on the amount of invariant mass in closed systems at all times, and although different observers may not agree with each other on how much relativistic mass is present in any such system, all agree that the amount does not change over time.

Macroscopically, mass is associated with matter— although matter, unlike mass, is poorly defined in science. On the sub-atomic scale, not only fermions, the particles often associated with matter, but also some bosons, the particles that act as force carriers, have rest mass. Another problem for easy definition is that much of the rest mass of ordinary matter derives from the invariant mass contributed to matter by particles and kinetic energies which have no rest mass themselves (only 1% of the rest mass of matter is accounted for by the rest mass of its fermionic quarks and electrons). From a fundamental physics perspective, mass is the number describing under which the representation of the little group of the Poincaré group a particle transforms. In the Standard Model of particle physics, this symmetry is described as arising as a consequence of a coupling of particles with rest mass to a postulated additional field, known as the Higgs field.

The total mass of the observable universe is estimated at between 10⁵² kg and 10⁵³ kg, corresponding to the rest mass of between 10⁷⁹ and 10⁸⁰ protons.

Units of mass[link]

The kilogram is one of the seven SI base units; among these, it is one of three (besides the second and the Kelvin) which is defined ad hoc, without reference to another base unit, since 1889 by means of the international prototype kilogram.^{[note 2]}

Balance scales allow to directly compare gravitational mass within a gravitational field. Such devices have been in use since at least the Middle Bronze Age (shown is a balance for weighing tobacco dating to the mid-19th century).

In the International System of Units (SI), mass is measured in kilograms (kg). The gram (g) is ¹⁄₁₀₀₀ of a kilogram. The gram was first introduced in 1795, with a definition based on the density of water (so that at the temperature of melting ice, one cubic centimeter of water would have a mass of one gram; while the meter at the time was defined as the 10,000,000th part of the distance from the Earth's equator to the North Pole). Since 1889, the kilogram has been defined as the mass of the international prototype kilogram, and as such is independent of the meter, or the properties of water. In October 2011, the 24th General Conference on Weights and Measures resolved to "take note of the intention" to redefine the kilogram in terms of the Planck constant, scheduled for 2014.

Other units are accepted for use in SI:

• The tonne (t) is equal to 1000 kg.
• The electronvolt (eV) is primarily a unit of energy, but because of the mass–energy equivalence it can also function as a unit of mass. In this context it is denoted eV/c², or simply as eV. The electronvolt is common in particle physics.
• The atomic mass unit (u) is defined so that a single carbon-12 atom has a mass of 12 u; 1 u is approximately 1.66×10⁻²⁷ kg.^{[note 3]} The atomic mass unit is convenient for expressing the masses of atoms and molecules.

Outside the SI system, a variety of different mass units are used, depending on context, such as the slug (sl), the pound (lb), the Planck mass (m_P), and the solar mass.

In normal situations, the weight of an object is proportional to its mass, which usually makes it unproblematic to use the same unit for both concepts. However, the distinction between mass and weight becomes important for measurements with a precision better than a few percent (because of slight differences in the strength of the Earth's gravitational field at different places), and for places far from the surface of the Earth, such as in space or on other planets.

A mass can sometimes be expressed in terms of length. The mass of a very small particle may be identified with its inverse Compton wavelength (1 cm⁻¹ ≈ 3.52×10⁻⁴¹ kg). The mass of a very large star or black hole may be identified with its Schwarzschild radius (1 cm ≈ 6.73×10²⁴ kg).

Summary of mass concepts and formalisms[link]

In classical mechanics, mass has a central role in determining the behavior of bodies. Newton's second law relates the force F exerted in a body of mass M to the body's acceleration α:

Failed to parse (Missing texvc executable; please see math/README to configure.): \mathbf{F}=M\boldsymbol{\alpha}\

. Additionally, mass relates a body's momentum p to its velocity v:

Failed to parse (Missing texvc executable; please see math/README to configure.): \mathbf{p}=M\mathbf{v}\

, and the body's kinetic energy K to its velocity:

Failed to parse (Missing texvc executable; please see math/README to configure.): \mathbf{K}=\tfrac{1}{2}Mv^2\

.

In special relativity, relativistic mass is a formalism which accounts for relativistic effects by having the mass increase with velocity.

Failed to parse (Missing texvc executable; please see math/README to configure.): M=\gamma m_0\!

Failed to parse (Missing texvc executable; please see math/README to configure.): E=Mc^2\!

Since energy is dependent on reference frame (upon the observer) it is convenient to formulate the equations of physics in a way such that mass values are invariant (do not change) between observers, and so the equations are independent of the observer. For a single particle, this quantity is the rest mass; for a system of bound or unbound particles, this quantity is the invariant mass. The invariant mass M of a body is related to its energy E and the magnitude of its momentum p by

Failed to parse (Missing texvc executable; please see math/README to configure.): Mc^2=\sqrt{E^2-(pc)^2},\!

where c is the speed of light.

Summary of mass related phenomena[link]

The relation between properties of mass and their associated physical constants. Every massive object is believed to exhibit all five properties. However, due to extremely large or extremely small constants, it is generally impossible to verify more than two or three properties for any object.

• The Schwarzschild radius (r_s) represents the ability of mass to cause curvature in space and time.
• The standard gravitational parameter (μ) represents the ability of a massive body to exert Newtonian gravitational forces on other bodies.
• Inertial mass (m) represents the Newtonian response of mass to forces.
• Rest energy (E₀) represents the ability of mass to be converted into other forms of energy.
• The Compton wavelength (λ) represents the quantum response of mass to local geometry.

In physical science, one may distinguish conceptually between at least seven attributes of mass, or seven physical phenomena that can be explained using the concept of mass:^[1]

• The amount of matter in certain types of samples can be exactly determined through electrodeposition^{[clarification needed]} or other precise processes. The mass of an exact sample is determined in part by the number and type of atoms or molecules it contains, and in part by the energy involved in binding it together (which contributes a negative "missing mass," or mass deficit).

• Inertial mass is a measure of an object's resistance to changing its state of motion when a force is applied. It is determined by applying a force to an object and measuring the acceleration that results from that force. An object with small inertial mass will accelerate more than an object with large inertial mass when acted upon by the same force. One says the body of greater mass has greater inertia.

• Active gravitational mass is a measure of the strength of an object’s gravitational flux (gravitational flux is equal to the surface integral of gravitational field over an enclosing surface). Gravitational field can be measured by allowing a small ‘test object’ to freely fall and measuring its free-fall acceleration. For example, an object in free-fall near the Moon will experience less gravitational field, and hence accelerate slower than the same object would if it were in free-fall near the Earth. The gravitational field near the Moon is weaker because the Moon has less active gravitational mass.

• Passive gravitational mass is a measure of the strength of an object's interaction with a gravitational field. Passive gravitational mass is determined by dividing an object’s weight by its free-fall acceleration. Two objects within the same gravitational field will experience the same acceleration; however, the object with a smaller passive gravitational mass will experience a smaller force (less weight) than the object with a larger passive gravitational mass.

• Energy also has mass according to the principle of mass–energy equivalence. This equivalence is exemplified in a large number of physical processes including pair production, nuclear fusion, and the gravitational bending of light. Pair production and nuclear fusion are processes through which measurable amounts of mass and energy are converted into each other. In the gravitational bending of light, photons of pure energy are shown to exhibit a behavior similar to passive gravitational mass.

• Curvature of spacetime is a relativistic manifestation of the existence of mass. Curvature is extremely weak and difficult to measure. For this reason, curvature wasn’t discovered until after it was predicted by Einstein’s theory of general relativity. Extremely precise atomic clocks on the surface of the earth, for example, are found to measure less time (run slower) than similar clocks in space. This difference in elapsed time is a form of curvature called gravitational time dilation. Other forms of curvature have been measured using the Gravity Probe B satellite.

• Quantum mass manifests itself as a difference between an object’s quantum frequency and its wave number. The quantum mass of an electron, the Compton wavelength, can be determined through various forms of spectroscopy and is closely related to the Rydberg constant, the Bohr radius, and the classical electron radius. The quantum mass of larger objects can be directly measured using a watt balance.

Inertial mass, gravitational mass, and the various other mass-related phenomena are conceptually distinct. However, every experiment to date has shown these values to be proportional, and this proportionality gives rise to the abstract concept of mass. If, in some future experiment, one of the mass-related phenomena is shown to not be proportional to the others, then that specific phenomenon will no longer be considered a part of the abstract concept of mass.

Weight and amount[link]

A depiction of early balance scales in the Papyrus of Hunefer (dated to the 19th dynasty, ca. 1285 BC). The scene shows Anubis weighing the heart of Hunefer.

Weight, by definition, is a measure of the force which must be applied to support an object (i.e. hold it at rest) in a gravitational field. The Earth’s gravitational field causes items near the Earth to have weight. Typically, gravitational fields change only slightly over short distances, and the Earth’s field is nearly uniform at all locations on the Earth’s surface; therefore, an object’s weight changes only slightly when it is moved from one location to another, and these small changes went unnoticed through much of history. This may have given early humans the impression that weight is an unchanging, fundamental property of objects in the material world.

In the Egyptian religious illustration to the right, Anubis is using a balance scale to weigh the heart of Hunefer. A balance scale balances the force of one object’s weight against the force of another object’s weight. The two sides of a balance scale are close enough that the objects experience similar gravitational fields. Hence, if they have similar masses then their weights will also be similar. This allows the scale, by comparing weights, to also compare masses, and gives it the distinction of being one of the oldest known devices capable of measuring mass.

The concept of amount is very old and predates recorded history, so any description of the early development of this concept is speculative in nature. However, one might reasonably assume that humans, at some early era, realized that the weight of a collection of similar objects was directly proportional to the number of objects in the collection:

Failed to parse (Missing texvc executable; please see math/README to configure.): w_n \propto n

,

where w is the weight of the collection of similar objects and n is the number of objects in the collection. Proportionality, by definition, implies that two values have a constant ratio:

Failed to parse (Missing texvc executable; please see math/README to configure.): \frac{w_n}{n} = \frac{w_m}{m}

, or equivalently Failed to parse (Missing texvc executable; please see math/README to configure.): \frac{w_n}{w_m} = \frac{n}{m} .

Consequently, historical weight standards were often defined in terms of amounts. The Romans, for example, used the carob seed (carat or siliqua) as a measurement standard. If an object’s weight was equivalent to 1728 carob seeds, then the object was said to weigh one Roman pound. If, on the other hand, the object’s weight was equivalent to 144 carob seeds then the object was said to weigh one Roman ounce (uncia). The Roman pound and ounce were both defined in terms of different sized collections of the same common mass standard, the carob seed. The ratio of a Roman ounce (144 carob seeds) to a Roman pound (1728 carob seeds) was:

Failed to parse (Missing texvc executable; please see math/README to configure.): \frac{ounce}{pound} = \frac{w_{144}}{w_{1728}} = \frac{144}{1728} = \frac{1}{12}

.

This example illustrates a common occurrence in physical science: when values are related through simple fractions, there is a good possibility that the values stem from a common source.

Various atoms and molecules as depicted in John Dalton's A New System of Chemical Philosophy (1808).

The name atom comes from the Greek ἄτομος/átomos, α-τεμνω, which means uncuttable, something that cannot be divided further. The philosophical concept that matter might be composed of discrete units that cannot be further divided has been around for millennia. However, empirical proof and the universal acceptance of the existence of atoms didn’t occur until the early 20th century.

As the science of chemistry matured, experimental evidence for the existence of atoms came from the law of multiple proportions. When two or more elements combined to form a compound, their masses are always in a fixed and definite ratio. For example, the mass ratio of nitrogen to oxygen in nitric oxide is seven eights. Ammonia has a hydrogen to nitrogen mass ratio of three fourteenths. The fact that elemental masses combined in simple fractions implies that all elemental mass stems from a common source. In principle, the atomic mass situation is analogous to the above example of Roman mass units. The Roman pound and ounce were both defined in terms of different sized collections of carob seeds, and consequently, the two mass units were related to each other through a simple fraction. Comparatively, since all of the atomic masses are related to each other through simple fractions, then perhaps the atomic masses are just different sized collections of some common fundamental mass unit.

In 1805, the chemist John Dalton published his first table of relative atomic weights, listing six elements, hydrogen, oxygen, nitrogen, carbon, sulfur, and phosphorus, and assigning hydrogen an atomic weight of 1. And in 1815, the chemist William Prout concluded that the hydrogen atom was in fact the fundamental mass unit from which all other atomic masses were derived.

Carbon atoms in graphite (image obtained with a Scanning tunneling microscope)

If Prout's hypothesis had proven accurate, then the abstract concept of mass, as we now know it, might never have evolved, since mass could always be defined in terms of amounts of the hydrogen atomic mass. Prout’s hypothesis; however, was found to be inaccurate in two major respects. First, further scientific advancements revealed the existence of smaller particles, such as electrons and quarks, whose masses are not related through simple fractions. And second, the elemental masses themselves were found to not be exact multiples of the hydrogen atom mass, but rather, they were near multiples. Einstein’s theory of relativity explained that when protons and neutrons come together to form an atomic nucleus, some of the mass of the nucleus is released in the form of binding energy. The more tightly bound the nucleus, the more energy is lost during formation and this binding energy loss causes the elemental masses to not be related through simple fractions.

Hydrogen, for example, with a single proton, has an atomic weight of 1.007825 u. The most abundant isotope of iron has 26 protons and 30 neutrons, so one might expect its atomic weight to be 56 times that of the hydrogen atom, but in fact, its atomic weight is only 55.9383 u, which is clearly not an integer multiple of 1.007825. Prout’s hypothesis was proven inaccurate in many respects, but the abstract concepts of atomic mass and amount continue to play an influential role in chemistry, and the atomic mass unit continues to be the unit of choice for very small mass measurements.

Weights as in a box with weights

When the French invented the metric system in the late 18th century, they used an amount to define their mass unit. The kilogram was originally defined to be equal in mass to the amount of pure water contained in a one-liter container. This definition, however, was inadequate for the precision requirements of modern technology, and the metric kilogram was redefined in terms of a man-made platinum-iridium bar known as the international prototype kilogram.

Gravitational mass[link]

Active gravitational mass is a property of the mass of an object that produces a gravitational field in the space surrounding the object, and these gravitational fields govern large-scale structures in the Universe. Gravitational fields hold the galaxies together. They cause clouds of gas and dust to coalesce into stars and planets. They provide the necessary pressure for nuclear fusion to occur within stars. And they determine the orbits of various objects within the Solar System. Since gravitational effects are all around us, it is impossible to pin down the exact date when humans first discovered gravitational mass. However, it is possible to identify some of the significant steps towards our modern understanding of gravitational mass and its relationship to the other mass phenomena. Some terms associated with gravitational mass and its effects are the Gaussian gravitational constant, the standard gravitational parameter and the Schwarzschild radius.

Keplerian gravitational mass[link]

Johannes Kepler 1610.

Johannes Kepler was the first to give an accurate description of the orbits of the planets, and by doing so; he was the first to describe gravitational mass. In 1600 AD, Kepler sought employment with Tycho Brahe and consequently gained access to astronomical data of a higher precision than any previously available. Using Brahe’s precise observations of the planet Mars, Kepler realized that the traditional astronomical methods were inaccurate in their predictions, and he spent the next five years developing his own method for characterizing planetary motion.

In Kepler’s final planetary model, he successfully described planetary orbits as following elliptical paths with the Sun at a focal point of the ellipse. The concept of active gravitational mass is an immediate consequence of Kepler's third law of planetary motion. Kepler discovered that the square of the orbital period of each planet is directly proportional to the cube of the semi-major axis of its orbit, or equivalently, that the ratio of these two values is constant for all planets in the Solar System. This constant ratio is a direct measure of the Sun's active gravitational mass, it has units of distance cubed per time squared, and is known as the standard gravitational parameter:

Failed to parse (Missing texvc executable; please see math/README to configure.): \mu = 4 \pi^2 \frac{Distance^3}{Time^2} \propto Gravitational \, Mass

In 1609, Johannes Kepler published his three rules known as Kepler's laws of planetary motion, explaining how the planets follow elliptical orbits under the influence of the Sun. On 25 August of that same year, Galileo Galilei demonstrated his first telescope to a group of Venetian merchants, and in early January of 1610, Galileo observed four dim objects near Jupiter, which he mistook for stars. However, after a few days of observation, Galileo realized that these "stars" were in fact orbiting Jupiter. These four objects (later named the Galilean moons in honor of their discoverer) were the first celestial bodies observed to orbit something other than the Earth or Sun. Galileo continued to observe these moons over the next eighteen months, and by the middle of 1611 he had obtained remarkably accurate estimates for their periods. Later, the semi-major axis of each moon was also estimated, thus allowing the gravitational mass of Jupiter to be determined from the orbits of its moons. The gravitational mass of Jupiter was found to be approximately a thousandth of the gravitational mass of the Sun.

Galilean gravitational field[link]

Galileo Galilei 1636.

The distance traveled by a freely falling ball is proportional to the square of the elapsed time.

Sometime prior to 1638, Galileo turned his attention to the phenomenon of objects falling under the influence of Earth’s gravity, and he was actively attempting to characterize these motions. Galileo was not the first to investigate Earth’s gravitational field, nor was he the first to accurately describe its fundamental characteristics. However, Galileo’s reliance on scientific experimentation to establish physical principles would have a profound effect on future generations of scientists. Galileo used a number of scientific experiments to characterize free fall motion. It is unclear if these were just hypothetical experiments used to illustrate a concept, or if they were real experiments performed by Galileo,^[2] but the results obtained from these experiments were both realistic and compelling. A biography by Galileo's pupil Vincenzo Viviani stated that Galileo had dropped balls of the same material, but different masses, from the Leaning Tower of Pisa to demonstrate that their time of descent was independent of their mass.^{[note 4]} In support of this conclusion, Galileo had advanced the following theoretical argument: He asked if two bodies of different masses and different rates of fall are tied by a string, does the combined system fall faster because it is now more massive, or does the lighter body in its slower fall hold back the heavier body? The only convincing resolution to this question is that all bodies must fall at the same rate.^[3]

A later experiment was described in Galileo’s Two New Sciences published in 1638. One of Galileo’s fictional characters, Salviati, describes an experiment using a bronze ball and a wooden ramp. The wooden ramp was "12 cubits long, half a cubit wide and three finger-breadths thick" with a straight, smooth, polished groove. The groove was lined with "parchment, also smooth and polished as possible". And into this groove was placed "a hard, smooth and very round bronze ball". The ramp was inclined at various angles to slow the acceleration enough so that the elapsed time could be measured. The ball was allowed to roll a known distance down the ramp, and the time taken for the ball to move the known distance was measured. The time was measured using a water clock described as follows:

"a large vessel of water placed in an elevated position; to the bottom of this vessel was soldered a pipe of small diameter giving a thin jet of water, which we collected in a small glass during the time of each descent, whether for the whole length of the channel or for a part of its length; the water thus collected was weighed, after each descent, on a very accurate balance; the differences and ratios of these weights gave us the differences and ratios of the times, and this with such accuracy that although the operation was repeated many, many times, there was no appreciable discrepancy in the results.".^[4]

Galileo found that for an object in free fall, the distance that the object has fallen is always proportional to the square of the elapsed time:

Failed to parse (Missing texvc executable; please see math/README to configure.): g = \frac{Distance}{Time^2} \propto Gravitational \, field

Galileo Galilei died in Arcetri, Italy (near Florence), on 8 January 1642. Galileo had shown that objects in free fall under the influence of the Earth’s gravitational field have a constant acceleration, and Galileo’s contemporary, Johannes Kepler, had shown that the planets follow elliptical paths under the influence of the Sun’s gravitational mass. However, the relationship between Galileo’s gravitational field and Kepler’s gravitational mass wasn’t comprehended during Galileo’s lifetime.

Newtonian gravitational mass[link]

Isaac Newton 1689.

 = Failed to parse (Missing texvc executable; please see math/README to configure.): 398 600 \frac{km^3}{s^2}

Robert Hooke published his concept of gravitational forces in 1674, stating that: “all Cœlestial Bodies whatsoever, have an attraction or gravitating power towards their own Centers" [and] "they do also attract all the other Cœlestial Bodies that are within the sphere of their activity”. He further states that gravitational attraction increases “by how much the nearer the body wrought upon is to their own center.^[5] In a correspondence of 1679–1680 between Robert Hooke and Isaac Newton, Hooke conjectures that gravitational forces might decrease according to the double of the distance between the two bodies.^[6] Hooke urged Newton, who was a pioneer in the development of calculus, to work through the mathematical details of Keplerian orbits to determine if Hooke’s hypothesis was correct. Newton’s own investigations verified that Hooke was correct, but due to personal differences between the two men, Newton chose not to reveal this to Hooke. Isaac Newton kept quiet about his discoveries until 1684, at which time he told a friend, Edmond Halley, that he had solved the problem of gravitational orbits, but had misplaced the solution in his office.^[7] After being encouraged by Halley, Newton decided to develop his ideas about gravity and publish all of his findings. In November 1684, Isaac Newton sent a document to Edmund Halley, now lost but presumed to have been titled De motu corporum in gyrum (Latin: "On the motion of bodies in an orbit").^[8] Halley presented Newton’s findings to the Royal Society of London, with a promise that a fuller presentation would follow. Newton later recorded his ideas in a three book set, entitled Philosophiæ Naturalis Principia Mathematica (Latin: "Mathematical Principles of Natural Philosophy"). The first was received by the Royal Society on 28 April 1685–6, the second on 2 March 1686–7, and the third on 6 April 1686–7. The Royal Society published Newton’s entire collection at their own expense in May 1686–7.^[9]

Isaac Newton had bridged the gap between Kepler’s gravitational mass and Galileo’s gravitational acceleration, and proved the following relationship:

Failed to parse (Missing texvc executable; please see math/README to configure.): g = \frac{\mu}{r^2}

, where g is the apparent acceleration of a body as it passes through a region of space where gravitational fields exist, μ is the gravitational mass (standard gravitational parameter) of the body causing gravitational fields, and r is the radial coordinate (the distance between the centers of the two bodies).

By finding the exact relationship between a body's gravitational mass and its gravitational field, Newton provided a second method for measuring gravitational mass. The mass of the Earth can be determined using Kepler’s method (from the orbit of Earth’s Moon), or it can be determined by measuring the gravitational acceleration on the Earth’s surface, and multiplying that by the square of the Earth’s radius. The mass of the Earth is approximately three millionths of the mass of the Sun. To date, no other accurate method for measuring gravitational mass has been discovered.^[10]

Newton's cannonball[link]

Newton's cannonball was a thought experiment used to bridge the gap between Galileo’s gravitational acceleration and Kepler’s elliptical orbits. It appeared in Newton's 1728 book A Treatise of the System of the World. According to Galileo’s concept of gravitation, a dropped stone falls with constant acceleration down towards the Earth. However, Newton explains that when a stone is thrown horizontally (meaning sideways or perpendicular to Earth’s gravity) it follows a curved path. “For a stone projected is by the pressure of its own weight forced out of the rectilinear path, which by the projection alone it should have pursued, and made to describe a curve line in the air; and through that crooked way is at last brought down to the ground. And the greater the velocity is with which it is projected, the farther it goes before it falls to the Earth.”^[11]

Newton further reasons that if an object were “projected in an horizontal direction from the top of a high mountain” with sufficient velocity, “it would reach at last quite beyond the circumference of the Earth, and return to the mountain from which it was projected.” Newton’s thought experiment is illustrated in the image to the right. A cannon on top of a very high mountain shoots a cannon ball in a horizontal direction. If the speed is low, it simply falls back on Earth (paths A and B). However, if the speed is equal to or higher than some threshold (orbital velocity), but not high enough to leave Earth altogether (escape velocity), it will continue revolving around Earth along an elliptical orbit (C and D).

Universal gravitational mass and amount[link]

Newton's cannonball illustrated the relationship between the Earth’s gravitational mass and its gravitational field; however, a number of other ambiguities still remained. Robert Hooke had asserted in 1674 that: "all Celestial Bodies whatsoever, have an attraction or gravitating power towards their own Centers", but Hooke had neither explained why this gravitating attraction was unique to celestial bodies, nor had he explained why the attraction was directed towards the center of a celestial body.

An apple experiences gravitational fields directed towards every part of the Earth; however, the sum total of these many fields produces a single powerful gravitational field directed towards the Earth’s center.

To answer these questions, Newton introduced the entirely new concept that gravitational mass is “universal”: meaning that every object has gravitational mass, and therefore, every object generates a gravitational field. Newton further assumed that the strength of each object’s gravitational field would decrease according to the square of the distance to that object. With these assumptions in mind, Newton calculated what the overall gravitational field would be if a large collection of small objects were formed into a giant spherical body. Newton found that a giant spherical body (like the Earth or Sun, with roughly uniform density at each given radius), would have a gravitational field which was proportional to the total mass of the body,^[12] and inversely proportional to the square of the distance to the body’s center.^[13]

Newton's concept of universal gravitational mass is illustrated in the image to the left. Every piece of the Earth has gravitational mass and every piece creates a gravitational field directed towards that piece. However, the overall effect of these many fields is equivalent to a single powerful field directed towards the center of the Earth. The apple behaves as if a single powerful gravitational field were accelerating it towards the Earth’s center.

Newton’s concept of universal gravitational mass puts gravitational mass on an equal footing with the traditional concepts of weight and amount. For example, the ancient Romans had used the carob seed as a weight standard. The Romans could place an object with an unknown weight on one side of a balance scale and place carob seeds on the other side of the scale, increasing the number of seeds until the scale was balanced. If an object’s weight was equivalent to 1728 carob seeds, then the object was said to weigh one Roman pound.

According to Newton’s theory of universal gravitation, each carob seed produces gravitational fields. Therefore, if one were to gather an immense number of carob seeds and form them into an enormous sphere, then the gravitational field of the sphere would be proportional to the number of carob seeds in the sphere. Hence, it should be theoretically possible to determine the exact number of carob seeds that would be required to produce a gravitational field similar to that of the Earth or Sun. And since the Roman weight units were all defined in terms of carob seeds, then knowing the Earth’s, or Sun's “carob seed mass” would allow one to calculate the mass in Roman pounds, or Roman ounces, or any other Roman unit.

Vertical section drawing of Cavendish's torsion balance instrument including the building in which it was housed. The large balls were hung from a frame so they could be rotated into position next to the small balls by a pulley from outside. Figure 1 of Cavendish's paper.

This possibility extends beyond Roman units and the carob seed. The British avoirdupois pound, for example, was originally defined to be equal to 7,000 barley grains. Therefore, if one could determine the Earth’s “barley grain mass” (the number of barley grains required to produce a gravitational field similar to that of the Earth), then this would allow one to calculate the Earth’s mass in avoirdupois pounds. Also, the original kilogram was defined to be equal in mass to a liter of pure water (the modern kilogram is defined by the man-made international prototype kilogram). Thus, the mass of the Earth in kilograms could theoretically be determined by ascertaining how many liters of pure water (or international prototype kilograms) would be required to produce gravitational fields similar to those of the Earth. In fact, it is a simple matter of abstraction to realize that any traditional mass unit can theoretically be used to measure gravitational mass.

Measuring gravitational mass in terms of traditional mass units is simple in principle, but extremely difficult in practice. According to Newton’s theory all objects produce gravitational fields and it is theoretically possible to collect an immense number of small objects and form them into an enormous gravitating sphere. However, from a practical standpoint, the gravitational fields of small objects are extremely weak and difficult to measure. And if one were to collect an immense number of objects, the resulting sphere would probably be too large to construct on the surface of the Earth, and too expensive to construct in space. Newton’s books on universal gravitation were published in the 1680s, but the first successful measurement of the Earth’s mass in terms of traditional mass units, the Cavendish experiment, didn’t occur until 1797, over a hundred years later. Cavendish found that the Earth's density was 5.448 ± 0.033 times that of water. As of 2009, the Earth’s mass in kilograms is only known to around five digits of accuracy, whereas its gravitational mass is known to over nine significant figures.

Inertial and gravitational mass[link]

Although inertial mass, passive gravitational mass and active gravitational mass are conceptually distinct, no experiment has ever unambiguously demonstrated any difference between them. In classical mechanics, Newton's third law implies that active and passive gravitational mass must always be identical (or at least proportional), but the classical theory offers no compelling reason why the gravitational mass has to equal the inertial mass. That it does is merely an empirical fact.

Albert Einstein developed his general theory of relativity starting from the assumption that this correspondence between inertial and (passive) gravitational mass is not accidental: that no experiment will ever detect a difference between them (the weak version of the equivalence principle). However, in the resulting theory, gravitation is not a force and thus not subject to Newton's third law, so "the equality of inertial and active gravitational mass [...] remains as puzzling as ever".^[14]

Inertial mass[link]

Inertial mass is the mass of an object measured by its resistance to acceleration.

To understand what the inertial mass of a body is, one begins with classical mechanics and Newton's Laws of Motion. Later on, we will see how our classical definition of mass must be altered if we take into consideration the theory of special relativity, which is more accurate than classical mechanics. However, the implications of special relativity will not change the meaning of "mass" in any essential way.

According to Newton's second law, we say that a body has a mass m if, at any instant of time, it obeys the equation of motion

Failed to parse (Missing texvc executable; please see math/README to configure.): F=m\alpha\!

,

where F is the force acting on the body and α is the acceleration of the body.^{[note 5]} For the moment, we will put aside the question of what "force acting on the body" actually means.

This equation illustrates how mass relates to the inertia of a body. Consider two objects with different masses. If we apply an identical force to each, the object with a bigger mass will experience a smaller acceleration, and the object with a smaller mass will experience a bigger acceleration. We might say that the larger mass exerts a greater "resistance" to changing its state of motion in response to the force.

However, this notion of applying "identical" forces to different objects brings us back to the fact that we have not really defined what a force is. We can sidestep this difficulty with the help of Newton's third law, which states that if one object exerts a force on a second object, it will experience an equal and opposite force. To be precise, suppose we have two objects A and B, with constant inertial masses m_X and m_Y. We isolate the two objects from all other physical influences, so that the only forces present are the force exerted on X by Y, which we denote F_XY, and the force exerted on Y by X, which we denote F_YX. Newton's second law states that

Failed to parse (Missing texvc executable; please see math/README to configure.): \!F_{XY}=m_X\alpha_X

,

Failed to parse (Missing texvc executable; please see math/README to configure.): \!F_{YX}=m_Y\alpha_Y

, where α_X and α_Y are the accelerations of X and Y, respectively. Suppose that these accelerations are non-zero, so that the forces between the two objects are non-zero. This occurs, for example, if the two objects are in the process of colliding with one another. Newton's third law then states that

Failed to parse (Missing texvc executable; please see math/README to configure.): F_{XY}=-F_{YX}\!

and thus

Failed to parse (Missing texvc executable; please see math/README to configure.): \frac{m_X}{m_Y}=-\frac{\alpha_Y}{\alpha_X}\!

.

Note that our requirement that α_X be non-zero ensures that the fraction is well-defined.

This is, in principle, how we would measure the inertial mass of an object. We choose a "reference" object and define its mass m_Y as (say) 1 kilogram. Then we can measure the mass of any other object in the universe by colliding it with the reference object and measuring the accelerations.

Newtonian gravitational mass[link]

The Newtonian concept of gravitational mass rests on Newton's law of gravitation. Let us suppose we have two objects A and B, separated by a distance r_AB. The law of gravitation states that if A and B have gravitational masses M_A and M_B respectively, then each object exerts a gravitational force on the other, of magnitude

Failed to parse (Missing texvc executable; please see math/README to configure.): F=\frac{GM_AM_B}{r_{\mathrm{AB}}^2}\!

, where G is the universal gravitational constant. The above statement may be reformulated in the following way: if g is the magnitude at a given location in a gravitational field, then the gravitational force on an object with gravitational mass M is

Failed to parse (Missing texvc executable; please see math/README to configure.): F=Mg\!

. This is the basis by which masses are determined by weighing. In simple spring scales, for example, the force F is proportional to the displacement of the spring beneath the weighing pan, as per Hooke's law, and the scales are calibrated to take g into account, allowing the mass M to be read off. A balance measures gravitational mass; only the spring scale measures weight.

Equivalence of inertial and gravitational masses[link]

The equivalence of inertial and gravitational masses is sometimes referred to as the Galilean equivalence principle or weak equivalence principle. The most important consequence of this equivalence principle applies to freely falling objects. Suppose we have an object with inertial and gravitational masses m and M respectively. If the only force acting on the object comes from a gravitational field g, combining Newton's second law and the gravitational law yields the acceleration

Failed to parse (Missing texvc executable; please see math/README to configure.): a = \frac{M}{m} g.

This says that the ratio of gravitational to inertial mass of any object is equal to some constant K if and only if all objects fall at the same rate in a given gravitational field. This phenomenon is referred to as the 'universality of free-fall'. (In addition, the constant K can be taken to be 1 by defining our units appropriately.)

The first experiments demonstrating the universality of free-fall were conducted by Galileo. It is commonly stated that Galileo obtained his results by dropping objects from the Leaning Tower of Pisa, but this is most likely apocryphal; actually, he performed his experiments with balls rolling down inclined planes. Increasingly precise experiments have been performed, such as those performed by Loránd Eötvös,^[15] using the torsion balance pendulum, in 1889. As of 2008^[update], no deviation from universality, and thus from Galilean equivalence, has ever been found, at least to the precision 10⁻¹². More precise experimental efforts are still being carried out.

The universality of free-fall only applies to systems in which gravity is the only acting force. All other forces, especially friction and air resistance, must be absent or at least negligible. For example, if a hammer and a feather are dropped from the same height through the air on Earth, the feather will take much longer to reach the ground; the feather is not really in free-fall because the force of air resistance upwards against the feather is comparable to the downward force of gravity. On the other hand, if the experiment is performed in a vacuum, in which there is no air resistance, the hammer and the feather should hit the ground at exactly the same time (assuming the acceleration of both objects towards each other, and of the ground towards both objects, for its own part, is negligible). This can easily be done in a high school laboratory by dropping the objects in transparent tubes that have the air removed with a vacuum pump. It is even more dramatic when done in an environment that naturally has a vacuum, as David Scott did on the surface of the Moon during Apollo 15.

A stronger version of the equivalence principle, known as the Einstein equivalence principle or the strong equivalence principle, lies at the heart of the general theory of relativity. Einstein's equivalence principle states that within sufficiently small regions of space-time, it is impossible to distinguish between a uniform acceleration and a uniform gravitational field. Thus, the theory postulates that the force acting on a massive object caused by a gravitational field is a result of the object's tendency to move in a straight line (in other words its inertia) and should therefore be a function of its inertial mass and the strength of the gravitational field.

Mass and energy in special relativity[link]

The term mass in special relativity usually refers to the rest mass of the object, which is the Newtonian mass as measured by an observer moving along with the object. The invariant mass is another name for the rest mass of single particles. However, the more general invariant mass (calculated with a more complicated formula) may also be applied to systems of particles in relative motion, and because of this, is usually reserved for systems which consist of widely separated high-energy particles. The invariant mass of systems is the same for all observers and inertial frames, and cannot be destroyed, and is thus conserved, so long as the system is closed. In this case, "closure" implies that an idealized boundary is drawn around the system, and no mass/energy is allowed across it.

In as much as energy is conserved in closed systems in relativity, the mass of a system is also a quantity which is conserved: this means it does not change over time, even as some types of particles are converted to others. For any given observer, the mass of any system is separately conserved and cannot change over time, just as energy is separately conserved and cannot change over time. The incorrect popular idea that mass may be converted to (massless) energy in relativity is because some matter particles may in some cases be converted to types of energy which are not matter (such as light, kinetic energy, and the potential energy in magnetic, electric, and other fields). However, this confuses "matter" (a non-conserved and ill-defined thing) with mass (which is well-defined and is conserved). Even if not considered "matter," all types of energy still continue to exhibit mass in relativity. Thus, mass and energy do not change into one another in relativity; rather, both are names for the same thing, and neither mass nor energy appear without the other. "Matter" particles may not be conserved in reactions in relativity, but closed-system mass always is.

For example, a nuclear bomb in an idealized super-strong box, sitting on a scale, would in theory show no change in mass when detonated (although the inside of the box would become much hotter). In such a system, the mass of the box would change only if energy were allowed to escape from the box as light or heat. However, in that case, the removed energy would take its associated mass with it. Letting heat out of such a system is simply a way to remove mass. Thus, mass, like energy, cannot be destroyed, but only moved from one place to another.^[16]

In bound systems, the binding energy must (often) be subtracted from the mass of the unbound system, simply because this energy has mass, and this mass is subtracted from the system when it is given off, at the time it is bound. Mass is not conserved in this process because the system is not closed during the binding process. A familiar example is the binding energy of atomic nuclei, which appears as other types of energy (such as gamma rays) when the nuclei are formed, and (after being given off) results in nuclides which have less mass than the free particles (nucleons) of which they are composed.

The term relativistic mass is also used, and this is the total quantity of energy in a body or system (divided by c²). The relativistic mass (of a body or system of bodies) includes a contribution from the kinetic energy of the body, and is larger the faster the body moves, so unlike the invariant mass, the relativistic mass depends on the observer's frame of reference. However, for given single frames of reference and for closed systems, the relativistic mass is also a conserved quantity.

Because the relativistic mass is proportional to the energy, it has gradually fallen into disuse among physicists.^[17] There is disagreement over whether the concept remains pedagogically useful.^[18][19][20]

Mass in general relativity[link]

In general relativity, the equivalence principle is any of several related concepts dealing with the equivalence of gravitational and inertial mass. At the core of this assertion is Albert Einstein's idea that the gravitational force as experienced locally while standing on a massive body (such as the Earth) is the same as the pseudo-force experienced by an observer in a non-inertial (accelerated) frame of reference.

However, it turns out that it is impossible to find an objective general definition for the concept of invariant mass in general relativity. At the core of the problem is the non-linearity of the Einstein field equations, which makes it impossible to write the gravitational field energy as part of the Stress–energy tensor in a way that is invariant for all observers. For a given observer, this can be achieved by the Stress–energy–momentum pseudotensor.^[21]

Mass in quantum physics[link]

In classical mechanics, the inert mass of a particle appears in the Euler–Lagrange equation as a parameter m,

Failed to parse (Missing texvc executable; please see math/README to configure.): \frac{\mathrm{d}}{\mathrm{d}t} \ \left( \, \frac{\partial L}{\partial \dot{x}_i} \, \right) \ = \ m \, \ddot{x}_i

. After quantization, replacing the position vector x with a wave function, the parameter m appears in the kinetic energy operator,

Failed to parse (Missing texvc executable; please see math/README to configure.): i\hbar\frac{\partial}{\partial t} \Psi(\mathbf{r},\,t) = \left(-\frac{\hbar^2}{2m}\nabla^2 + V(\mathbf{r})\right)\Psi(\mathbf{r},\,t)

. In the ostensibly covariant^{[disambiguation needed ]} (relativistically invariant) Dirac equation, and in natural units, this becomes

Failed to parse (Missing texvc executable; please see math/README to configure.): (-i\gamma^\mu\partial_\mu + m) \psi = 0\,

Where the "mass" parameter m is now simply a constant associated with the quantum described by the wave function ψ.

In the Standard Model of particle physics as developed in the 1960s, there is the proposal that this term arises from the coupling of the field ψ to an additional field Φ, the so-called Higgs field. In the case of fermions, the Higgs mechanism results in the replacement of the term mψ in the Lagrangian with Failed to parse (Missing texvc executable; please see math/README to configure.): G_{\psi} \overline{\psi} \phi \psi . This shifts the explanandum of the value for the mass of each elementary particle to the value of the unknown couplings G_ψ. The discovery of a massive Higgs boson would be regarded as a strong confirmation of this theory. But there is indirect evidence for the reality of the Electroweak symmetry breaking as described by the Higgs mechanism, and the non-existence of the Higgs boson would indicate a "Higgsless" description of this mechanism.

Notes[link]

References[link]

Bibliography[link]

• Sir Isaac Newton; N. W. Chittenden (1848). Newton's Principia: The mathematical principles of natural philosophy. D. Adee. http://books.google.com/books?id=KaAIAAAAIAAJ&pg=PA31. Retrieved 12 March 2011. 

External links[link]

• Stanford Encyclopedia of Philosophy: "The Equivalence of Mass and Energy" by Francisco Flores.
• "The Mysteries of Mass," Scientific American, July 2005.
• Usenet Physics FAQ by John Baez:
  • "Does mass change with velocity?"
  • "What is the mass of a photon?"
• "The Origin of Mass and the Feebleness of Gravity." Video of lecture by the Nobel Laureate Frank Wilczek.
• Okun, L. B., "Photons, Clocks, Gravity and the Concept of Mass." Slides for a talk.
• The Apollo 15 Hammer-Feather Drop.
• Online mass units conversion.

The Sacred Heart of Jesus
Il Sacro Cuore by Pompeo Batoni, 1740, now in Church of the Gesu, Rome
Honored in	Roman Catholic Church, Anglican Church, Lutheran Church
Feast	19 days after Pentecost (Friday)
Attributes	Burning bloodied heart, surmounted with cross and thorns
Patronage	Apostleship of Prayer

The Sacred Heart (also known as Most Sacred Heart of Jesus) is one of the most famous religious devotions to Jesus' physical heart as the representation of his divine love for humanity.

This devotion is predominantly used in the Catholic Church and among some high-church Anglicans and Lutherans. The devotion especially emphasizes the unmitigated love, compassion, and long-suffering of the heart of Christ towards humanity. The origin of this devotion in its modern form is derived from a French Roman Catholic nun, Marguerite Marie Alacoque, who said she learned the devotion from Jesus during a mystical experience. Predecessors to the modern devotion arose unmistakably in the Middle Ages in various facets of Catholic mysticism.^[1]

In the Roman Catholic tradition, the Sacred Heart has been closely associated with Acts of Reparation to Jesus Christ. In his encyclical Miserentissimus Redemptor, Pope Pius XI stated: "the spirit of expiation or reparation has always had the first and foremost place in the worship given to the Most Sacred Heart of Jesus".^[2] The Golden Arrow Prayer directly refers to the Sacred Heart.

Devotion to the Sacred Heart is sometimes seen in the Eastern Catholic Churches, where it remains a point of controversy and is seen as an example of Liturgical Latinisation.

The Sacred Heart is often depicted in Christian art as a flaming heart shining with divine light, pierced by the lance-wound, surrounded by the crown of thorns, surmounted by a cross and bleeding. Sometimes the image shown shining within the bosom of Christ with his wounded hands pointing at the heart. The wounds and crown of thorns allude to the manner of Jesus' death, while the fire represents the transformative power of divine love.

The Feast of the Sacred Heart has been in the Roman Catholic liturgical calendar since 1856, and is celebrated 19 days after Pentecost. As Pentecost is always celebrated on Sunday, the Feast of the Sacred Heart always falls on a Friday.

History of Devotion[link]

Early devotion[link]

Sacred Heart of Jesus by José María Ibarrarán y Ponce, 1896

From the time of John the Evangelist and Paul of Tarsus there has always been in the Church something like devotion to the love of God, but there is nothing to indicate that, during the first ten centuries of Christianity, any worship was rendered to the wounded Heart of Jesus.^[3] It is in the eleventh and twelfth centuries that the first indications of devotion to the Sacred Heart are found. It was in the fervent atmosphere of the Benedictine or Cistercian monasteries, in the world of Anselmian or Bernardine thought, that the devotion arose, although it is impossible to say positively what were its first texts or who were its first devotees. It was already well known to St. Gertrude, St. Mechtilde, and the author of the Vitis mystica (previously ascribed to St. Bernard, now attributed to St. Bonaventure).

From the thirteenth to the sixteenth centuries, the devotion was propagated but it did not seem to have developed in itself. It was everywhere practised by individuals and by different religious congregations, such as the Franciscans, Dominicans, Carthusians, etc. It was, nevertheless, a private, individual devotion of the mystical order. Nothing of a general movement had been inaugurated, except for similarities found in the devotion to the Five Wounds by the Franciscans, in which the wound in Jesus's heart figured most prominently.

In the sixteenth century, the devotion passed from the domain of mysticism into that of Christian asceticism. It was established as a devotion with prayers already formulated and special exercises, found in the writings of Lanspergius (d. 1539) of the Carthusians of Cologne, the Louis of Blois (Blosius; 1566), a Benedictine and Abbot of Liessies in Hainaut, John of Avila (d. 1569) and St. Francis de Sales, the latter belonging to the seventeenth century.

The historical record from that time shows an early bringing to light of the devotion. Ascetic writers spoke of it, especially those of the Society of Jesus. The image of the Sacred Heart of Jesus was everywhere in evidence, largely due to the Franciscan devotion to the Five Wounds and to the habit formed by the Jesuits of placing the image on their title-page of their books and the walls of their churches.

Nevertheless, the devotion remained an individual, or at least a private, devotion. Jean Eudes (1602–1680) made it public, gave it an Office, and established a feast for it. Père Eudes was the apostle of the Heart of Mary; but in his devotion to the Immaculate Heart there was a share for the Heart of Jesus. Little by little, the devotion to the Sacred Heart became a separate one, and on August 31, 1670, the first feast of the Sacred Heart was celebrated in the Grand Seminary of Rennes. Coutances followed suit on October 20, a day with which the Eudist feast was from then on to be connected. The feast soon spread to other dioceses, and the devotion was likewise adopted in various religious communities. It gradually came into contact with the devotion begun at Paray, and resulting in a fusion of the two.

Visions of Saint Margaret Mary Alacoque[link]

Painting representing the famous apparition of the Sacred Heart of Jesus to Saint Margaret Mary Alacoque.

The most significant source for the devotion to the Sacred Heart in the form it is known today was Visitandine Saint Margaret Mary Alacoque (1647–1690), who claimed to have received visions of Jesus Christ. There is nothing to indicate that she had known the devotion prior to the revelations, or at least that she had paid any attention to it. The revelations were numerous, and the following apparitions are especially remarkable:

• On December 27, probably 1673, the feast of St. John, Margaret Mary reported that Jesus permitted her, as he had formerly allowed St. Gertrude, to rest her head upon his heart, and then disclosed to her the wonders of his love, telling her that he desired to make them known to all mankind and to diffuse the treasures of his goodness, and that he had chosen her for this work.
• In probably June or July, 1674, Margaret Mary claimed that Jesus requested to be honored under the figure of his heart, also claiming that, when he appeared radiant with love, he asked for a devotion of expiatory love: frequent reception of Communion, especially Communion on the First Friday of the month, and the observance of the Holy Hour.
• During the octave of Corpus Christi, 1675, probably on June 16, the vision known as the "great apparition" reportedly took place, where Jesus said, "Behold the Heart that has so loved men ... instead of gratitude I receive from the greater part (of mankind) only ingratitude ...", and asked Margaret Mary for a feast of reparation of the Friday after the octave of Corpus Christi, bidding her consult her confessor Father Claude de la Colombière, then superior of the small Jesuit house at Paray. Solemn homage was asked on the part of the king, and the mission of propagating the new devotion was especially confided to the religious of the Visitation and to the priests of the Society of Jesus.

A few days after the "great apparition", Margaret Mary reported everything she saw to Father de la Colombière, and he, acknowledging the vision as an action of the Spirit of God, consecrated himself to the Sacred Heart and directed her to write an account of the apparition. He also made use of every available opportunity to circulate this account, discreetly, through France and England. Upon his death on February 15, 1682, there was found in his journal of spiritual retreats a copy in his own handwriting of the account that he had requested of Margaret Mary, together with a few reflections on the usefulness of the devotion. This journal, including the account and an "offering" to the Sacred Heart, in which the devotion was well explained, was published at Lyons in 1684. The little book was widely read, especially at Paray. Margaret Mary reported feeling "dreadful confusion" over the book's contents, but resolved to make the best of it, approving of the book for the spreading of her cherished devotion. Outside of the Visitandines, priests, religious, and laymen espoused the devotion, particularly the Capuchins, Margaret Mary's two brothers, and some Jesuits. The Jesuit Father Croiset wrote a book called The Devotion to the Sacred Heart of Jesus, a book which Jesus is said to have told Margaret to tell Fr. Croiset to write, and Fr. Joseph de Gallifet, also a Jesuit, promoted the devotion.

Papal Approvals[link]

The Blessed Mary of the Divine Heart was a nun from Sisters of the Good Shepherd Congregation who requested, in the name of Christ Himself, to Pope Leo XIII that he consecrate the entire World to the Sacred Heart of Jesus.^[4]

The death of Margaret Mary Alacoque on October 17, 1690, did not dampen the zeal of those interested; on the contrary, a short account of her life published by Father Croiset in 1691, as an appendix to his book "De la Dévotion au Sacré Cœur", served only to increase it. In spite of all sorts of obstacles, and of the slowness of the Holy See, which in 1693 imparted indulgences to the Confraternities of the Sacred Heart and, in 1697, granted the feast to the Visitandines with the Mass of the Five Wounds, but refused a feast common to all, with special Mass and Office. The devotion spread, particularly in religious communities. The Marseilles plague, 1720, furnished perhaps the first occasion for a solemn consecration and public worship outside of religious communities. Other cities of the South followed the example of Marseilles, and thus the devotion became a popular one. In 1726 it was deemed advisable once more to importune Rome for a feast with a Mass and Office of its own, but, in 1729, Rome again refused. However, in 1765, it finally yielded and that same year, at the request of the queen, the feast was received quasi-officially by the episcopate of France. On all sides it was asked for and obtained, and finally, in 1856, at the urgent entreaties of the French bishops, Pope Pius IX extended the feast to the Roman Catholic Church under the rite of double major. In 1889 it was raised by the Roman Catholic Church to the double rite of first class.

After the letters of Mother Mary of the Divine Heart (1863–1899) requesting, in the name of Christ Himself, to Pope Leo XIII consecrate the entire World to the Sacred Heart of Jesus, the Holy Father commissions a group of theologians to examine the petition on the basis of revelation and sacred tradition. This investigation was positive. And so in the encyclical letter Annum Sacrum (on May 25, 1899) this same pope decreed that the consecration of the entire human race to the Sacred Heart of Jesus should take place on June 11, 1899. In this encyclical letter the Pope attached Later Pope Leo XIII encouraged the entire Roman Catholic episcopate to promote the devotion of the Nine First Fridays and he established June as the Month of the Sacred Heart. Leo XIII also composed the Prayer of Consecration to the Sacred Heart and included it in Annum Sacrum.^[5]

Pope Pius X decreed that the consecration of the human race, performed by Pope Leo XIII be renewed each year. Pope Pius XI in his encyclical letter Miserentissimus Redemptor (on May 8, 1928) affirmed the Church's position with respect to Saint Margaret Mary's visions of Jesus Christ by stating that Jesus had "manifested Himself" to Saint Margaret and had "promised her that all those who rendered this honor to His Heart would be endowed with an abundance of heavenly graces." The encyclical refers to the conversation between Jesus and Saint Margaret several times^[2] and reaffirmed the importance of consecration and reparation to the Sacred Heart of Jesus.

Finally, Venerable Pope Pius XII, on the occasion of the 100th anniversary of Pope Pius IX's institution of the Feast, instructed the entire Roman Catholic Church at length on the devotion to the Sacred Heart in his encyclical letter Haurietis aquas (on May 15, 1956). On May 15, 2006, also Pope Benedict XVI sent a letter to Father Peter Hans Kolvenbach, the Superior General of the Society of Jesus, on the 50th Anniversary of the encyclical Haurietis Aquas, about the Sacred Heart, by Pope Pius XII. In his letter to Father Kolvenbach, Pope Benedict XVI reaffirmed the importance of the devotion to the Sacred Heart of Jesus.

Worship and Devotion[link]

Stained glass depiction of Jesus and His Most Sacred Heart, in Germany.

The Roman Catholic acts of consecration, reparation and devotion were introduced when the feast of the Sacred Heart was declared. In his Papal Bull Auctorem Fidei, Pope Pius VI praised devotion to the Sacred Heart. Finally, by order of Leo XIII, in his encyclical Annum Sacrum (May 25, 1899), as well as on June 11, he consecrated every human to the Sacred Heart. The idea of this act, which Leo XIII called "the great act" of his pontificate, had been proposed to him by a religious woman of the Good Shepherd from Oporto (Portugal) who said that she had supernaturally received it from Jesus. Since c. 1850, groups, congregations, and States have consecrated themselves to the Sacred Heart. In 1873, by petition of president Gabriel García Moreno, Ecuador was the first country in the world to be consecrated to the Sacred Heart, fulfilling God's petition to Saint Margaret Mary over two hundred years later.

Peter Coudrin of France founded the Congregation of the Sacred Hearts of Jesus and Mary on December 24, 1800. A religious order of the Roman Catholic Church, the order is best known for its missionary work in Hawaii.

Mother Clelia Merloni from Forlì (Italy) founded the Congregation of the Apostles of the Sacred Heart of Jesus in Viareggio, Italy, May 30, 1894.

Worship of the Sacred Heart mainly consists of several hymns, the Salutation of the Sacred Heart, and the Litany of the Sacred Heart. It is common in Roman Catholic services and occasionally is to be found in Anglican services.

The Feast of the Sacred Heart is a solemnity in the Roman Catholic liturgical calendar, and is celebrated 19 days after Pentecost. As Pentecost is always celebrated on Sunday, the Feast of the Sacred Heart always falls on a Friday.

The Enthronement of the Sacred Heart is a Roman Catholic ceremony in which a priest or head of a household consecrates the members of the household to the Sacred Heart. A blessed image of the Sacred Heart, either a statue or a picture, is then "enthroned" in the home to serve as a constant reminder to those who dwell in the house of their consecration to the Sacred Heart. The practice of the Enthronement is based upon Pope Pius XII's declaration that devotion to the Sacred of Jesus is "the foundation on which to build the kingdom of God in the hearts of individuals, families, and nations..."^[6]

Alliance with the Immaculate Heart[link]

The Alliance of the Hearts of Jesus and Mary is based on the historical, theological and spiritual links in Catholic devotions to the Sacred Heart of Jesus and the Immaculate Heart of Mary.^[7][8][9] The joint devotion to the hearts was first formalized in the 17th century by Saint Jean Eudes who organized the scriptural, theological and liturgical sources relating to the devotions and obtained the approbation of the Church, prior to the visions of Saint Marguerite Marie Alacoque.^[10][11][12]