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−  {{RedirectE{{=}}MC2}}
 +  '''E=mc²''' is [[Einstein]]'s famous formula which asserts that the energy ('''E''') which makes up the [[matter]] in any body is equal to the square of the [[speed of light]] ('''c²''') times the [[mass]] ('''m''') of that body.<ref>"... Einstein proves that energy and matter are linked in the most famous relationship in physics: E = mc². (The energy content of a body is equal to the mass of the body times the speed of light squared.)" [http://www.pbs.org/wgbh/nova/physics/einsteingeniusamonggeniuses.html Einstein: Genius Among Geniuses]  PBS's NOVA</ref> It is a statement that purports to relate all [[matter]] to [[energy]]. In fact, no [[theory]] has successfully unified the [[law]]s governing [[mass]] (''i.e.'', [[gravity]]) with the laws governing light (''i.e.'', [[electromagnetism]]), and numerous attempts to derive '''E=mc²''' in general from first principles have failed. [[PoliticsPolitical]] pressure, however, has since made it impossible for anyone pursuing an academic career in [[science]] to even question the validity of this nonsensical [[equation]]. Simply put, '''E=mc²''' is measurable science which we choose to not understand. 
−  [[File:Relativity3 Walk of Ideas Berlin.JPG400pxrightthumb4metertall sculpture of [[Albert EinsteinEinstein]]'s 1905 ''E'' = ''mc''<sup>2</sup> formula at the 2006 [[Walk of Ideas]], [[Berlin]], [[Germany]]]]
 +  
−  {{special relativity}}
 +  
−  [[File:E=mc2explication.jpgthumbExplication]]
 +  
−  In [[physics]], in particular [[special relativityspecial]] and [[general relativity]], '''mass–energy equivalence''' is the concept that the [[mass]] of a body is a measure of its [[energy]] content. In this concept, mass is a property of all energy; energy is a property of all mass; and the two properties are connected by a constant. This means (for example) that the total internal energy ''E'' of a body at rest is equal to the product of its [[Invariant massrest mass]] ''m'' and a suitable [[conversion factor]] to transform from [[:Category:Units of massunits of mass]] to [[:Category:Units of energyunits of energy]]. [[Albert Einstein]] proposed mass–energy equivalence in 1905 in one of his [[Annus Mirabilis papers]] entitled "Does the inertia of a body depend upon its energycontent?"<ref name="inertia">{{Citation  author=Einstein, A.  year=1905  title=Ist die Trägheit eines Körpers von seinem Energieinhalt abhängig?  journal=Annalen der Physik  volume=18  pages=639–643  doi=10.1002/andp.19053231314bibcode = 1905AnP...323..639E }}. See also the [http://www.fourmilab.ch/etexts/einstein/E_mc2/www/ English translation.]</ref> The equivalence is described by the famous equation:
 +  
−  :<math>E = mc^2 \,\!</math>
 +  
−  where ''E'' is energy, ''m'' is mass, and ''c'' is the [[speed of light]]. The formula is [[dimensional analysisdimensionally consistent]] and does not depend on any specific [[systems of measurementsystem of measurement units]].
 +  
−  The equation ''E'' = ''mc''<sup>2</sup> indicates that energy always exhibits [[relativistic mass]] in whatever form the energy takes.<ref name="tipler">{{Citation
 +  
−  title=Modern Physics
 +  
−  author=Paul Allen Tipler, Ralph A. Llewellyn
 +  
−  pages=87–88
 +  
−  url=http://books.google.com/?id=tpU18JqcSNkC&lpg=PP1&pg=PA87#v=onepage&q=
 +  
−  isbn=0716743450
 +  
−  publisher=W. H. Freeman and Company
 +  
−  date=200301
 +  
−  }}</ref> Mass–energy equivalence does not imply that mass may be "converted" to energy, but it allows for ''[[matter]]'' to be converted to energy. ''Mass'' remains conserved (i.e., the quantity of mass remains constant), since it is a property of matter and also any type of energy. Energy is also conserved. In physics, mass must be differentiated from matter. Matter, when seen as certain types of particles, ''can'' be created and destroyed (as in particle [[annihilation]] or [[Matter creationcreation]]), but a closed system of precursors and products of such reactions, as a whole, retain both the original mass and energy throughout the reaction.
 +  
   
−  When the system is not closed, and energy is ''removed'' from a system (for example in [[nuclear fission]] or [[nuclear fusion]]), some mass is always removed along with the energy, according to their equivalence where one always accompanies the other. This energy thus is associated with the missing mass, and this mass will be added to any other system which absorbs the removed energy. In this situation ''E'' = ''mc''<sup>2</sup> can be used to calculate how much mass goes along with the removed energy. It also tells how much mass will be added to any system which later absorbs this energy. This was the original use of the equation when derived by Einstein.
 +  [[Biblical Scientific Foreknowledge]] predicts that a unified theory of all the laws of physics is impossible, because light and matter were created at different times, in different ways, as described in the [[Book of Genesis]]. 
   
−  ''E'' = ''mc''<sup>2</sup> has sometimes been used as an explanation for the origin of energy in nuclear processes, but mass–energy equivalence does not explain the origin of such energies. Instead, this relationship merely indicates that the large amounts of energy released in such reactions may exhibit enough mass that the massloss may be measured, when the released energy (and its mass) have been removed from the system. For example, the loss of mass to an atom and a neutron as a result of the capture of the neutron, and the production of a gamma ray, has been used to test massenergy equivalence to high precision, as the energy of the gamma ray may be compared with the mass defect after capture. In 2005, these were found to agree to 0.0004%, the most precise test of the equivalence of mass and energy to date. This test was performed in the [[World Year of Physics 2005]], a centennial celebration of Einstein's achievements in 1905.<ref name="rainville">Rainville, S. et al. World Year of Physics: A direct test of E=mc2. ''Nature'' 438, 10961097 (22 December 2005)  {{doi10.1038/4381096a}}; Published online 21 December 2005.</ref>  +  [[Mass]] is a measure of an object's inertia, in other words its resistance to acceleration. In contrast, the intrinsic [[energy]] of an object (such as an [[atom]]) is a function of electrostatic charge and other noninertial forces, having nothing to do with gravity. Declaring the object's energy to be a function of inertia rather than electrostatics is an absurd and impossible attempt to unify the forces of nature, contrary to [[Biblical Scientific Foreknowledge]]. 
   
−  Einstein was not the first to propose a mass–energy relationship (see the [[#HistoryHistory]] section). However, Einstein was the first scientist to propose the ''E'' = ''mc''<sup>2</sup> formula and the first to interpret mass–energy equivalence as a fundamental principle that follows from the [[Spacetime symmetriesrelativistic symmetries]] of [[Spacetimespace and time]].
 +  For more than a century, the claim that '''E=mc²''' has never yielded anything of value. Often it seems to be used as a redefinition of "[[energy]]" for pseudoscientific purposes, as by the [[lamestream media]]. There have been attempts to find some justification for the equation in already understood processes involved in [[nuclear power]] generation and [[nuclear weapon]]s, and in the speculation about [[antimatter]].<ref>[http://www.pbs.org/wgbh/nova/physics/legacyofeequalsmc2.html Peter Tyson ''The Legacy of E=mc²''] October 11, 2005. PBS ''NOVA''. </ref> 
   
−  ==Nomenclature==  +  The [[Theory of Relativity]] has never been able to mathematically derive '''E=mc²''' from first principles, and a physicist observed in a peerreviewed paper published in 2011 that "a rigorous proof of the massenergy equivalence is probably beyond the purview of the special theory."<ref>[http://adsabs.harvard.edu/abs/2011AmJPh..79..591H Eugene Hecht: ''How Einstein confirmed E<sub>0</sub>=mc²'', American Journal of Physics, Volume 79, Issue 6, pp. 591600 (2011)]</ref> 
   
−  In ''Does the inertia of a body depend upon its energycontent?'' Einstein used V to mean the [[speed of light]] in a vacuum and L to mean the [[energy]] lost by a body in the form of [[radiation]]. Consequently, the equation ''E'' = ''mc''<sup>2</sup> was not originally written as a formula but as a sentence in German that meant ''if a body gives off the energy L in the form of radiation, its mass diminishes by L/V<sup>2</sup>''.<ref>See the sentence on the last page (p.641) of the original edition of ''Ist die Trägheit eines Körpers von seinem Energieinhalt abhängig?'' in Annalen der Physik, 1905, '''below''' the equation K<sub>0</sub>  K<sub>1</sub>= L/V<sup>2</sup> v<sup>2</sup>/2. See also the sentence '''under''' the last equation in the English translation, K<sub>0</sub>  K<sub>1</sub>= 1/2 L/c<sup>2</sup> v<sup>2</sup>, and the comment on the symbols used in ''About this edition'' that follows the translation [http://www.fourmilab.ch/etexts/einstein/E_mc2/www/]</ref> A remark placed above it informed that the equation was approximate because the conclusion was only justified if one neglected "magnitudes of fourth and higher orders" of a [[Series (mathematics)series expansion]].<ref>See the sentence on the last page (p.641) of the original edition of ''Ist die Trägheit eines Körpers von seinem Energieinhalt abhängig?'' in Annalen der Physik, 1905, '''above''' the equation K<sub>0</sub>  K<sub>1</sub>= L/V<sup>2</sup> v<sup>2</sup>/2. See also the sentence '''above''' the last equation in the English translation, K<sub>0</sub>  K<sub>1</sub>= 1/2 L/c<sup>2</sup> v<sup>2</sup>, and the comment on the symbols used in ''About this edition'' that follows the translation [http://www.fourmilab.ch/etexts/einstein/E_mc2/www/]</ref> In 1907, Einstein's massenergy relationship was written as ''M<sub>0</sub>'' = ''E<sub>0</sub>/c''<sup>2</sup> by [[Max Planck]]<ref>{{cite journal first=M. last=Planck journal=Ber.d.Berl.Akad. volume=29 issue= pages=542 year=1907 }}</ref> and, subsequently, was given a quantum interpretation<ref>{{cite journal first=J. last=Stark title=Elementarquantum der Energie, Modell der negativen und der positiven Elekrizitat journal=Physikalische Zeitschrift page=881 volume=24 issue=8 year=1907 }}</ref> by [[Johannes Stark]], who assumed its validity and correctness (''Gültigkeit''). However, Stark wrote the equation as e<sub>0</sub>=m<sub>0</sub> c<sup>2</sup> which meant the energy bound in the mass of an electron at rest and still was not the present popular version of the equation. In 1924, [[Louis de Broglie]] assumed the correctness of the relationship
 +  It has been known for a long time that radiation has a mass equivalence, which was correctly derived by [[Henri Poincare]] in 1904,<ref>[http://www.opticsinfobase.org/josa/abstract.cfm?uri=josa428540 Herbert E. Ives ''Derivation of the MassEnergy Relation'', JOSA, Vol. 42, Issue 8, pp. 540543 (1952)]</ref> but the equation '''E=mc²''' makes a claim far beyond that limited circumstance: 
−  "énergie=masse c<sup>2</sup>" on page 31 in his ''Research on the Theory of the Quanta'' (published in 1925) but he did not write ''E'' = ''mc''<sup>2</sup>. However, Einstein returned to the topic once again after the World War Two and this time he wrote ''E'' = ''mc''<sup>2</sup> in the title of his article<ref>A.Einstein ''''E'' = ''mc''<sup>2</sup> '''': the most urgent problem of our time'' Science illustrated, vol. 1 no. 1, April issue, pp. 1617, 1946 (item 417 in the "Bibliography"</ref> intended as an explanation for a general reader by analogy<ref>M.C.Shields ''Bibliography of the Writings of Albert Einstein to May 1951'' in Albert Einstein: PhilosopherScientist by Paul Arthur Schilpp (Editor) [http://www.questia.com/PM.qst?a=o&d=84000079 Albert Einstein Philospher  Scientist]</ref>
 +  
   
−  ==Conservation of mass and energy==
 +  {{cquoteThe equality of the mass equivalent of radiation to the mass lost by a radiating body is derivable from Poincaré’s momentum of radiation (1900) and his principle of relativity (1904).[[Herbert Ives]], 1952}} 
   
−  The concept of mass–energy equivalence connects the concepts of [[conservation of mass]] and [[conservation of energy]], which continue to hold separately in any [[isolated system]] (one that is closed to loss of any type of energy, including energy associated with loss of matter). The theory of relativity allows particles which have [[rest mass]] to be converted to other forms of mass which require motion, such as [[kinetic energy]], heat, or light. However, the system mass remains. Kinetic energy or light can also be converted to new kinds of particles which have rest [[mass]], but again the energy remains. Both the total mass and the total energy inside an isolated system remain constant over time, as seen by any single observer in a given inertial frame.
 +  == Description for the layman == 
 +  Ten top physicists were asked to describe in laymen's terms '''E=mc²''':<ref>[http://www.pbs.org/wgbh/nova/einstein/experts.html Lexi Krock, David Levin (editors) ''E=mc² explained'', June, 2005. PBS ''NOVA'']</ref>{{cquote''Things that seem incredibly different can really be manifestations of the same underlying phenomena.''Nima ArkaniHamed, Theoretical Physicist, Harvard University}}{{cquote''You can get access to parts of nature you have never been able to get access to before.''Lene Hau, Experimental Physicist, Harvard University}}{{ 
 +  cquote''It certainly is not an equation that reveals all its subtlety in the few symbols that it takes to write down.''Brian Greene Theoretical Physicist Columbia University}} 
   
−  In other words, energy can neither be created nor destroyed, and energy, in all of its forms, has mass. Mass also can neither be created nor destroyed, and in all of its forms, has energy. According to the theory of relativity, mass and energy as commonly understood, are two names for the same thing, and neither one is changed nor transformed ''into'' the other. Rather, neither one exists without the other existing also, as a property of a system. Rather than mass being changed ''into'' energy, the view of [[special relativity]] is that rest mass has been changed to a more mobile form of mass, but remains mass. In the transformation process, neither the amount of mass nor the amount of energy changes, since both are properties which are connected to each other via a simple constant.<ref>In F. Fernflores. The Equivalence of Mass and Energy. Stanford Encyclopedia of Philosophy. [http://plato.stanford.edu/entries/equivME/#2.1]</ref> Thus, if energy leaves a system by changing its form, it simply takes its system mass with it. This view requires that if either mass or energy disappears from a system, it will always be found that both have simply moved off to another place, where they may both be measured as an increase of both mass and energy corresponding to the loss in the first system.
 +  == History of '''E=mc²''' == 
   
−  ===Fastmoving objects and systems of objects===  +  The [[liberal]] [[Public Broadcasting Service]] explained the history of '''E=mc²''' for its NOVA series as follows:<ref>[http://www.pbs.org/wgbh/nova/physics/ancestorseinstein.html David Bodanis ''Ancestors of E=mc²''], Nov 10, 2005, NOVA</ref> 
   
−  When an object is pulled in the direction of motion, it gains momentum and energy, but when the object is already traveling near the [[speed of light]], it cannot move much faster, no matter how much energy it absorbs. Its momentum and energy continue to increase without bounds, whereas its speed approaches a constant value—the speed of light. This implies that in relativity the [[momentum]] of an object cannot be a constant times the [[velocity]], nor can the [[Kinetic energy#Kinetic energy of rigid bodieskinetic energy]] be a constant times the square of the velocity.
 +  {{cquoteOver time, [[physicist]]s became used to multiplying an object's mass by the square of its [[velocity]] (mv²) to come up with a useful indicator of its energy. If the velocity of a ball or rock was 100 mph, then they knew that the energy it carried would be proportional to its mass times 100 squared. If the velocity is raised as high as it could go, to 670 million mph, it's almost as if the ultimate energy an object will contain should be revealed when you look at its mass times c squared, or its mc².}} 
   
−  A property called the [[relativistic mass]] is defined as the ratio of the momentum of an object to its velocity.<ref>Note that the relativistic mass, in contrast to the rest mass ''m<sub>0</sub>, is not a relativistic invariant, and that the velocity <math>\,v=dx^{(4)}/dt</math> is not a Minkowski fourvector, in contrast to the quantity <math>\tilde v=dx^{(4)}/d\tau</math>, where <math> d\tau =dt\cdot\sqrt{1(v^2/c^2)}</math> is the differential of the [[proper time]]. However, the energymomentum fourvector <math> p^{(4)}=m_0\cdot dx^{(4)}/d\tau</math> is a genuine Minkowski fourvector, and the intrinsic origin of the squareroot in the definition of the relativistic mass is the distinction between '''dτ''' and '''dt'''.</ref> Relativistic mass depends on the motion of the object, so that different observers in relative motion see different values for it. If the object is moving slowly, the relativistic mass is nearly equal to the [[rest mass]] and both are nearly equal to the usual Newtonian mass. If the object is moving quickly, the relativistic mass is greater than the rest mass by an amount equal to the mass associated with the [[kinetic energy]] of the object. As the object approaches the speed of light, the relativistic mass grows infinitely, because the [[kinetic energy]] grows infinitely and this energy is associated with mass.
 +  ==Experimental verification== 
 +  The first experimental verification for the equation was performed 1932 by a team of an English and an Irish physicist, John Cockcroft and Ernest Walton, as a byproduct of ''"their pioneer work on the transmutation of atomic nuclei by artificially accelerated atomic particles"''<ref>[http://www.nobelprize.org/nobel_prizes/physics/laureates/1951/# Nobel Prize Organization]</ref> for which they were honored with the [[Nobel Prize]] in physics in 1951. The idea of the mass defect  and its calculation using ''E=mc²'' can be found on page 169170 of his Nobel lecture.<ref>[http://www.nobelprize.org/nobel_prizes/physics/laureates/1951/cockcroftlecture.pdf John D. Cockroft ''Experiments on the interaction of highspeed nucleons with atomic nuclei''], Nobel Lecture, Dec 11, 1951</ref> 
   
−  The relativistic mass is always equal to the total energy (rest energy plus kinetic energy) divided by ''c''<sup>2</sup>.<ref name="tipler"/> Because the relativistic mass is exactly proportional to the energy, relativistic mass and relativistic energy are nearly synonyms; the only difference between them is the [[unit of measurementunits]]. If length and time are measured in [[natural units]], the speed of light is equal to 1, and even this difference disappears. Then mass and energy have the same units and are always equal, so it is redundant to speak about relativistic mass, because it is just another name for the energy. This is why physicists usually reserve the useful short word "mass" to mean restmass, or [[invariant mass]], and not relativistic mass.
 +  They bombarded [[Lithium]] atoms with [[protons]] having a [[kinetic energy]] less than 1 [[ElectronVoltsMeV]]. The result were two (slightly less heavy) [[αparticle]]s, for which the [[kinetic energy]] was measured as 17.3 MeV 
   
−  The relativistic mass of a moving object is larger than the relativistic mass of an object that is not moving, because a moving object has extra kinetic energy. The ''rest mass'' of an object is defined as the mass of an object when it is at rest, so that the rest mass is always the same, independent of the motion of the observer: it is the same in all [[inertial frame]]s.
 +  :::::<math>{}_3^7\mathrm{Li}\, +\, {}_1^1\mathrm{H}\,\rightarrow\,2\, {}_2^4\mathrm{He}</math> 
   
−  For things and systems made up of many parts, like an [[atomic nucleus]], [[planet]], or [[star]], the relativistic mass is the sum of the relativistic masses (or energies) of the parts, because energies are additive in closed systems. This is not true in systems which are open, however, if energy is subtracted. For example, if a system is [[Binding_energy#Massenergy_relation''bound'']] by attractive forces, and the energy gained due to the forces of attraction in excess of the work done is removed from the system, then mass will be lost with this removed energy. For example, the mass of an atomic nucleus is less than the total mass of the protons and neutrons that make it up, but this is only true after this energy from binding has been removed in the form of a gamma ray (which in this system, carries away the mass of the energy of binding). This mass decrease is also equivalent to the energy required to break up the nucleus into individual protons and neutrons (in this case, work and mass would need to be supplied). Similarly, the mass of the solar system is slightly less than the masses of sun and planets individually.
 +  The mass of the particles on the left hand side is 8.0263 [[atomic mass unitsamu]]s, the mass on the right hand side ''only'' 8.0077 amu.<ref>Gerard Piel ''The age of science: what scientists learned in the 20th century'', Basic Books, 2001, p. 144145</ref> The difference between this masses is .00186 amu, which results in the following backofanenvelope calculation: 
   
−  For a system of particles going off in different directions, the [[invariant mass]] of the system is the analog of the rest mass, and is the same for all observers, even those in relative motion. It is defined as the total energy (divided by ''c''<sup>2</sup>) in the [[center of mass frame]] (where by definition, the system total momentum is zero). A simple example of an object with moving parts but zero total momentum, is a container of gas. In this case, the mass of the container is given by its total energy (including the kinetic energy of the gas molecules), since the system total energy and invariant mass are the same in any reference frame where the momentum is zero, and such a reference frame is also the only frame in which the object can be weighed. In a similar way, the theory of special relativity posits that the thermal energy in all objects (including solids) contributes to their total masses and weights, even though this energy is present as the kinetic and potential energies of the atoms in the object, and it (in a similar way to the gas) is not seen in the rest masses of the atoms that make up the object.
 +  ::::<math>0.00186\,\mathrm{amu} \cdot c^2 = 0.0186 \cdot 1.66 \cdot 10^{27}\,\mathrm{kg}\cdot\left(3\cdot10^8\,\mathrm{\frac{m}{s}}\right)^2</math> 
 +  ::::<math>\approx\,2.79\cdot 10^{12} \,\mathrm{kg}\mathrm{\frac{m^2}{s^2}}</math> 
 +  ::::<math>\approx \,17.3\,\mathrm{MeV}</math> 
   
−  In a similar manner, even photons (light quanta), if trapped in a container space (as a [[photon gas]] or [[thermal radiation]]), would contribute a mass associated with their energy to the container. Such an extra mass, in theory, could be weighed in the same way as any other type of rest mass. This is true in special relativity theory, even though individually, photons have no rest mass. The property that trapped energy ''in any form'' adds weighable mass to systems that have no net momentum, is one of the characteristic and notable consequences of relativity. It has no counterpart in classical Newtonian physics, in which radiation, light, heat, and kinetic energy never exhibit weighable mass under any circumstances.
 +  Accurate measurements and detailed calculations allowed for verifying the theoretical values with an accuracy of ±0.5%. This was the first time a nucleus was artificially split, and thereby the first transmutation of elements using accelerated particles: 
   
−  Just as the relativistic mass of closed system is conserved through time, so also is its invariant mass. It is this property which allows the conservation of all types of mass in systems, and also conservation of all types of mass in reactions where matter is converted to energy, and vice versa.
 +  Some claim that the best empirical verification of '''E=mc<sup>2</sup>''' was done in 2005 by Simon Rainville et al., as published in ''[[Nature (journal)Nature]]'' (which is not a leading physics journal).<ref>[http://www.nature.com/nature/journal/v438/n7071/full/4381096a.html Nature 438, 10961097 (22 December 2005)] doi:10.1038/4381096a; Published online 21 December 2005</ref> The authors state in their article in ''Nature'' magazine that "Einstein's relationship is separately confirmed in two tests, which yield a combined result of 1−Δmc²/E=(−1.4±4.4)×10<sup>−7</sup>, indicating that it holds to a level of at least 0.00004%. To our knowledge, this is the most precise direct test of the famous equation yet described." 
   
−  ==Applicability of the strict mass–energy equivalence formula, ''E'' = ''mc''²==  +  ==A Famous Example  Nuclear Fission of Uranium== 
   
−  As is noted above, two different definitions of mass have been used in special relativity, and also two different definitions of energy. The simple equation ''E'' = ''mc''² is not generally applicable to all these types of mass and energy, except in the special case that the total additive momentum is zero for the system under consideration. In such a case, which is always guaranteed when observing the system from either its [[center of mass frame]] or its [[center of momentum frame]], ''E'' = ''mc''² is always true for any type of mass and energy that are chosen. Thus, for example, in the center of mass frame, the total energy of an object or system is equal to its rest mass times ''c''², a useful equality. This is the relationship used for the container of gas in the previous example. It is ''not'' true in other reference frames where the center of mass is in motion. In these systems or for such an object, its total energy will depend on both its rest (or invariant) mass, and also its (total) momentum.<ref>Relativity DeMystified, D. McMahon, Mc Graw Hill (USA), 2006, ISBN 0071455450</ref>
 +  For most types of physical interactions, the masses of the initial reactants and of the final products match so closely that it is essentially impossible to measure any difference. But for nuclear reactions, the difference is measurable. That difference is related to the energy absorbed or released, described by the equation E=mc². (The equation applies to '''all''' interactions; the fact that nuclear interactions are the only ones for which the mass difference is measurable has led people to believe, wrongly, that E=mc² applies only to nuclear interactions.) 
   
−  In inertial reference frames other than the rest frame or center of mass frame, the equation ''E'' = ''mc''² remains true if the energy is the relativistic energy ''and'' the mass the relativistic mass. It is also correct if the energy is the rest or invariant energy (also the minimum energy), ''and'' the mass is the rest mass, or the invariant mass. However, connection of the '''total or relativistic energy''' ('''E<sub>r</sub>''') with the '''rest or invariant mass''' ('''m<sub>0</sub>''') requires consideration of the system total momentum, in systems and reference frames where the total momentum has a nonzero value. The formula then required to connect the two different kinds of mass and energy, is the extended version of Einstein's equation, called the [[Mass in special relativity#The relativistic energymomentum equationrelativistic energy–momentum relationship]]:<ref>Dynamics and Relativity, J.R. Forshaw, A.G. Smith, Wiley, 2009, ISBN 9780470014608</ref>
 +  The [[Theory of Relativity]] played no role in this work, but later tried to retrofit the theory to the data in order to explain the explain the observed mass changes. Here is the most famous example of the mass change. 
   
−  ::<math>E_r^2  \vec{p} \,^2 c^2 = m_0^2 c^4</math>
 +  Nuclear fission, which is the basis for nuclear energy, was discovered in experiments by [[Otto Hahn]] and [[Fritz Strassman]], and analyzed by [[Lise Meitner]], in 1938. 
   
−  ::<math>E_r^2  (pc)^2 = (m_0 c^2)^2\,</math>  +  The decay path of [[Uranium]] that figured in the ''HahnStrassmann experiment'' may have been this: 
   
−  or
 +  :<sup>235</sup>U → <sup>140</sup>Xe + <sup>91</sup>Sr + 4n 
   
−  ::<math>E_r = \sqrt{ (m_0 c^2)^2 + (pc)^2 } \,\!</math>
 +  (The [[Xenon]] decayed within about a minute to <sup>140</sup>Ba. There are a large number of fission paths and fission products, but they were searching for the chemical signature of [[Barium]].) 
   
−  Here the ''(pc)<sup>2</sup>'' term represents the square of the [[Euclidean norm]] (total vector length) of the various momentum vectors in the system, which reduces to the square of the simple momentum magnitude, if only a single particle is considered. This equation reduces to ''E'' = ''mc''² when the momentum term is zero. For photons where '''m<sub>0</sub> = 0''', the equation reduces to '''E<sub>r</sub> = pc'''.
 +  The masses of the particles are: 
   
−  ''View a video explanation of the full equation at [http://www.youtube.com/watch?v=NnMIhxWRGNw Minute Physics on Youtube]''
 +  { class="wikitable" border="1" cellpadding="8" cellspacing="0" 
 +  ! Substance 
 +  ! <sup>235</sup>U 
 +  ! <sup>140</sup>Xe 
 +  ! <sup>91</sup>Sr 
 +  ! 4 neutrons 
 +   
 +   Number of protons 
 +   92 
 +   54 
 +   38 
 +   0 
 +   
 +   Number of neutrons 
 +   235 
 +   140 
 +   91 
 +   4 
 +   
 +   Number of electrons 
 +   92 
 +   54 
 +   38 
 +   0 
 +   
 +   Mass 
 +   235.04393 
 +   139.92164 
 +   90.910203 
 +   4.03466 
 +  } 
   
−  ==Meanings of the strict mass–energy equivalence formula, ''E = mc''²==
 +  The mass of the Uranium atom is 235.04393, and the sum of the masses of the products is 234.866503. The difference is .177427 amu, or, using the E=mc² equation, 165 million electron volts. (The generally accepted value for the total energy released by Uranium fission, including secondary decays, is about 200 million electron volts.) 
   
−  [[File:E equals m plus c square at Taipei101.jpgthumbrightThe mass–energy equivalence formula was displayed on [[Taipei 101]] during the event of the [[World Year of Physics 2005]].]]
 +  The insight that the conversion from Uranium to Barium was caused by complete fission of the atom was made by Lise Meitner in December, 1938. She had the approximate "mass defect" quantities memorized, and so she worked out in her head, using the E=mc² equation, that there would be this enormous release of energy. This release was observed shortly thereafter, and the result is nuclear power and nuclear weapons. 
   
−  Mass–energy equivalence states that any object has a certain energy, even when it is stationary. In [[Newtonian mechanics]], a motionless body has no [[kinetic energy]], and it may or may not have other amounts of internal stored energy, like [[chemical energy]] or [[thermal energy]], in addition to any [[potential energy]] it may have from its position in a [[field (physics)field of force]]. In Newtonian mechanics, all of these energies are much smaller than the mass of the object times the speed of light squared.
 +  ==A Topical Example: Speed of Extremely Energetic Neutrinos== 
 +  Here is another example of the use of this formula in physics calculations. Recently there has been quite a controversy over whether neutrinos were observed traveling at a speed faster than light. Relativity doesn't allow that, and, since neutrinos have nonzero (but incredibly tiny) mass, they aren't even supposed to travel ''at'' the speed of light. This very issue came up on the [[Talk:Main_Page#Neutrinos]]. The speeds under discussion were calculated by the use of E=mc<sup>2</sup>. 
   
−  In relativity, all of the energy that moves along with an object (that is, all the energy which is present in the object's rest frame) contributes to the total mass of the body, which measures how much it resists acceleration. Each potential and kinetic energy makes a proportional contribution to the mass. As noted above, even if a box of ideal mirrors "contains" light, then the individually massless photons still contribute to the total mass of the box, by the amount of their energy divided by c<sup>2</sup>.<ref>{{cite book
 +  The mass of a neutrino is about 0.44x10<sup>36</sup>kilograms. (Normally all of these things are measured in more convenient units such as GigaelectronVolts, but that makes implicit use of E=mc<sup>2</sup>. If we don't accept that, we have to do the calculations under classical physics, using SI (meter/kilogram/second) units.) The neutrinos were accelerated to an energy of about 17GeV, or .27x10<sup>8</sup>Joules. Using the classical formula <math>\mathrm{E} = \frac{1}{2}mv^2</math>, we get v=110x10<sup>12</sup>meters per second. This is about 370,000 times the speed of light. 
−  title=Mechanics
 +  However, the classical formula breaks down at speeds close to <math>c</math>, and indeed, as the speed of a massive object approaches <math>c</math>, the object's kinetic energy approaches <math>+\infty</math>. 
−  edition=2
 +  
−  first1=H. S.
 +  
−  last1=Hans
 +  
−  first2=S. P.
 +  
−  last2=Puri
 +  
−  publisher=Tata McGrawHill
 +  
−  year=2003
 +  
−  isbn=0070473609
 +  
−  page=433
 +  
−  url=http://books.google.com/books?id=hrBe52GPHrYC}}, [http://books.google.com/books?id=hrBe52GPHrYC&pg=PA433 Chapter 12 page 433]
 +  
−  </ref>  +  
   
−  In [[Theory of relativityrelativity]], removing energy is removing mass, and for an observer in the center of mass frame, the formula ''m'' = ''E''/''c''² indicates how much mass is lost when energy is removed. In a nuclear reaction, the mass of the atoms that come out is less than the mass of the atoms that go in, and the difference in mass shows up as heat and light which has the same relativistic mass as the difference (and also the same [[invariant mass]] in the center of mass frame of the system). In this case, the ''E'' in the formula is the energy released and removed, and the mass ''m'' is how much the mass decreases. In the same way, when any sort of energy is added to an isolated system, the increase in the mass is equal to the added energy divided by ''c''². For example, when water is heated it gains about {{val1.11e=17u=kg}} of mass for every joule of heat added to the water.
 +  Several scientists have gone on record stating that the neutrinos, which have mass, travel at precisely the speed of light. If true, this disproves the [[Theory of Relativity]] and the claim that '''E=mc<sup>2</sup>'''. However, it is more likely that those scientists are using language inaccurately. It is impossible to measure the speed of neutrinos precisely. What is meant is the difference between the speed of light and the speed of the neutrinos is too small to measure. 
   
−  An object moves with different speed in different frames, depending on the motion of the observer, so the kinetic energy in both Newtonian mechanics and relativity is ''frame dependent''. This means that the amount of relativistic energy, and therefore the amount of relativistic mass, that an object is measured to have depends on the observer. The ''rest mass'' is defined as the mass that an object has when it is not moving (or when an inertial frame is chosen such that it is not moving). The term also applies to the invariant mass of systems when the system as a whole is not "moving" (has no net momentum). The rest and invariant masses are the smallest possible value of the mass of the object or system. They also are conserved quantities, so long as the system is closed. Because of the way they are calculated, the effects of moving observers are subtracted, so these quantities do not change with the motion of the observer.
 +  ==Deducing the Equation From Empirical Observation== 
   
−  The rest mass is almost never additive: the rest mass of an object is not the sum of the rest masses of its parts. The rest mass of an object is the total energy of all the parts, including kinetic energy, as measured by an observer that sees the center of the mass of the object to be standing still. The rest mass adds up only if the parts are standing still and do not attract or repel, so that they do not have any extra kinetic or potential energy. The other possibility is that they have a positive kinetic energy and a negative potential energy that exactly cancels.
 +  While the equation was historically developed on theoretical grounds as an inevitable consequence of special relativity, it is possible to deduce it purely from empirical observation. 
   
−  === Binding energy and the "mass defect" ===
 +  So, for the purposes of this section, imagine that one is in the era of "classical physics"; prior to 1900 or so. Relativity has not been invented, but, inexplicably, nuclear physics has. Imagine that the phenomena of radioactivity and nuclear fission have been observed, without any knowledge of relativity. 
−  {{mainbinding energy}}
 +  
   
−  Whenever any type of energy is removed from a system, the mass associated with the energy is also removed, and the system therefore loses mass. This mass defect in the system may be simply calculated as '''Δm = ΔE/c<sup>2</sup>''', and this was the form of the equation historically first presented by Einstein in 1905. However, use of this formula in such circumstances has led to the false idea that mass has been "converted" to energy. This may be particularly the case when the energy (and mass) removed from the system is associated with the ''binding energy'' of the system. In such cases, the binding energy is observed as a "mass defect" or deficit in the new system. The fact that the released energy is not easily weighed in many such cases, may cause its mass to be neglected as though it no longer existed. This circumstance, along with the real conversion of matter (not mass) to energy in some high energy particle reactions, has caused the conversion of ''matter'' to energy (which occurs) to be conflated with the false idea of conversion of ''mass'' to energy, which does not occur.
 +  A wellaccepted physical law of classical physics was the law of conservation of mass. This was not easy to deduce. It required careful analysis of such phenomena as combustion, in the 1700's, to eliminate the various confounding subphenomena that made the law difficult to see. But, by 1900, the law was well established: 
   
−  The difference between the rest mass of a bound system and of the unbound parts is the [[binding energy]] of the system, if this energy has been removed after binding. For example, a water molecule weighs a little less than two free hydrogen atoms and an oxygen atom; the minuscule mass difference is the energy that is needed to split the molecule into three individual atoms (divided by ''c''²), and which was given off as heat when the molecule formed (this heat had mass). Likewise, a stick of dynamite in theory weighs a little bit more than the fragments after the explosion, but this is true only so long as the fragments are cooled and the heat removed. In this case the mass difference is the energy/heat that is released when the dynamite explodes, and when this heat escapes, the mass associated with it escapes, only to be deposited in the surroundings which absorb the heat (so that total mass is conserved).
 +  :::*'''In all interactions, mass is precisely conserved.''' 
   
−  Such a change in mass may only happen when the system is open, and the energy and mass escapes. Thus, if a stick of dynamite is blown up in a hermetically sealed chamber, the mass of the chamber and fragments, the heat, sound, and light would still be equal to the original mass of the chamber and dynamite. If sitting on a scale, the weight and mass would not change. This would in theory also happen even with a nuclear bomb, if it could be kept in an ideal box of infinite strength, which did not rupture or pass radiation.<ref>E. F. Taylor and J. A. Wheeler, '''Spacetime Physics''', W.H. Freeman and Co., NY. 1992. ISBN 0716723271, see pp. 2489 for discussion of mass remaining constant after detonation of nuclear bombs, until heat is allowed to escape.</ref> Thus, a 21.5 [[TNT equivalentkiloton]] (9 x 10<sup>13</sup> joule) nuclear bomb produces about one gram of heat and electromagnetic radiation, but the mass of this energy would not be detectable in an exploded bomb in an ideal box sitting on a scale; instead, the contents of the box would be heated to millions of degrees without changing total mass and weight. If then, however, a transparent window (passing only electromagnetic radiation) were opened in such an ideal box after the explosion, and a beam of Xrays and other lowerenergy light allowed to escape the box, it would eventually be found to weigh one gram less than it had before the explosion. This weightloss and massloss would happen as the box was cooled by this process, to room temperature. However, any surrounding mass which had absorbed the Xrays (and other "heat") would '''gain''' this gram of mass from the resulting heating, so the mass "loss" would represent merely its relocation. Thus, no mass (or, in the case of a nuclear bomb, no matter) would be "converted" to energy in such a process. Mass and energy, as always, would both be separately conserved.
 +  For example, the mass of a TNT molecule is 227.1311 Daltons, or 227.1311 g/mol, which is, for all practical purposes, the same as the mass of its constituent Carbon, Hydrogen, Nitrogen, and Oxygen atoms. It is essentially impossible to measure the difference. The principle of conservation of mass is upheld. 
   
−  === Massless particles ===
 +  But when nuclear phenomena are discovered, we notice something different. The masses of the result particles after an event (e.g. alpha decay, nuclear fission, or artificial transmutation) is measurably less than the masses of the original particle(s). With the invention of the mass spectrometer around 1920, it became possible to measure atomic weights of various isotopes with great precision. 
−  Massless particles have zero rest mass. Their relativistic mass is simply their relativistic energy, divided by c<sup>2</sup>, or m(relativistic) = E/c<sup>2</sup>.<ref>{{cite book
 +  
−  title=Basic relativity
 +  
−  edition=2
 +  
−  first1=Richard A.
 +  
−  last1=Mould
 +  
−  publisher=Springer
 +  
−  year=2002
 +  
−  isbn=0387952101
 +  
−  page=126
 +  
−  url=http://books.google.com/books?id=lfGEwyJYIUC}}, [http://books.google.com/books?id=lfGEwyJYIUC&pg=PA126 Chapter 5 page 126]
 +  
−  </ref><ref>{{cite book
 +  
−  title=Introduction to electromagnetic theory: a modern perspective
 +  
−  first1=Tail L.
 +  
−  last1=Chow
 +  
−  publisher=Jones & Bartlett Learning
 +  
−  year=2006
 +  
−  isbn=0763738271
 +  
−  page=392
 +  
−  url=http://books.google.com/books?id=dpnpMhw1zo8C}}, [http://books.google.com/books?id=dpnpMhw1zo8C&pg=PA392 Chapter 10 page 392]
 +  
−  </ref> The energy for photons is '''E = hf''' where '''h''' is [[Planck constantPlanck's constant]] and '''f''' is the photon frequency. This frequency and thus the relativistic energy are framedependent.
 +  
   
−  If an observer runs away from a photon in the direction it travels from a source, having it catch up with the observer, then when the photon catches up it will be seen as having less energy than it had at the source. The faster the observer is traveling with regard to the source when the photon catches up, the less energy the photon will have. As an observer approaches the speed of light with regard to the source, the photon looks redder and redder, by [[relativistic Doppler effect]] (the Doppler shift is the relativistic formula), and the energy of a very longwavelength photon approaches zero. This is why a photon is ''massless''; this means that the rest mass of a photon is zero.
 +  Radium226 decays into Radon222 by emission of an alpha particle with an energy of 4.78 MeV. 
   
−  === Massless particles contribute rest mass and invariant mass to systems ===  +  1 kg of Radium226 = <math>\frac{6.022 \times 10^{23}}{226.0254}</math> atoms. (The numerator is [[Avogadro's number]], and the denominator is the atomic weight of Radium226.) This is 2.6643647 * 10<sup>24</sup> atoms. 
   
−  Two photons moving in different directions cannot both be made to have arbitrarily small total energy by changing frames, or by moving toward or away from them. The reason is that in a twophoton system, the energy of one photon is decreased by chasing after it, but the energy of the other will increase with the same shift in observer motion. Two photons not moving in the same direction will exhibit an [[inertial frame]] where the combined energy is smallest, but not zero. This is called the [[center of mass]] frame or the [[center of momentum]] frame; these terms are almost synonyms (the center of mass frame is the special case of a center of momentum frame where the center of mass is put at the origin). The most that chasing a pair of photons can accomplish to decrease their energy is to put the observer in frame where the photons have equal energy and are moving directly away from each other. In this frame, the observer is now moving in the same direction and speed as the center of mass of the two photons. The total momentum of the photons is now zero, since their momentums are equal and opposite. In this frame the two photons, as a system, have a mass equal to their total energy divided by ''c''<sup>2</sup>. This mass is called the [[invariant mass]] of the pair of photons together. It is the smallest mass and energy the system may be seen to have, by any observer. It is only the invariant mass of a twophoton system that can be used to make a single particle with the same rest mass.
 +  That number of Radon222 atoms has mass .98226836 kg. That number of alpha particles has mass .01770863 kg. 
 +  The mass lost is .00002301 kg. 
   
−  If the photons are formed by the collision of a particle and an antiparticle, the invariant mass is the same as the total energy of the particle and antiparticle (their rest energy plus the kinetic energy), in the center of mass frame, where they will automatically be moving in equal and opposite directions (since they have equal momentum in this frame). If the photons are formed by the disintegration of a ''single'' particle with a welldefined rest mass, like the neutral [[pion]], the invariant mass of the photons is equal to rest mass of the pion. In this case, the center of mass frame for the pion is just the frame where the pion is at rest, and the center of mass does not change after it disintegrates into two photons. After the two photons are formed, their center of mass is still moving the same way the pion did, and their total energy in this frame adds up to the mass energy of the pion. Thus, by calculating the [[invariant mass]] of pairs of photons in a particle detector, pairs can be identified that were probably produced by pion disintegration.
 +  Each emitted alpha particle has energy of 4.78 MeV, or 4.78 * .1602 * 10<sup>18</sup> Joules. The total alpha energy from the decay of 1 kg of radium is 2.04 * 10<sup>12</sup> Joules. 
   
−  A similar calculation illustrates that the invariant mass of systems is conserved, even when massive particles (particles with rest mass) within the system are converted to massless particles (such as photons). In such cases, the photons contribute invariant mass to the system, even though they individually have no invariant mass or rest mass. Thus, an electron and positron (each of which has rest mass) may undergo [[electronpositron annihilationannihilation]] with each other to produce two photons, each of which is massless (has no rest mass). However, in such circumstances, no system mass is lost. Instead, the system of both photons moving away from each other has an invariant mass, which acts like a rest mass for any system in which the photons are trapped, or that can be weighed. Thus, not only the quantity of relativistic mass, but also the quantity of invariant mass does not change in transformations between "matter" (electrons and positrons) and energy (photons).
 
   
−  ===Relation to gravity===
 +  Also, Radon222 decays into Polonium218 by emission of an alpha particle with an energy of 5.49 MeV. 
−  In physics, there are two distinct concepts of [[mass]]: the gravitational mass and the inertial mass. The gravitational mass is the quantity that determines the strength of the [[gravitational field]] generated by an object, as well as the gravitational force acting on the object when it is immersed in a gravitational field produced by other bodies. The inertial mass, on the other hand, quantifies how much an object accelerates if a given force is applied to it. The massenergy equivalence in special relativity refers to the inertial mass. However, already in the context of Newton gravity, the Weak [[Equivalence Principle]] is postulated: the gravitational and the inertial mass of every object are the same. Thus, the massenergy equivalence, combined with the Weak Equivalence Principle, results in the prediction that all forms of energy contribute to the gravitational field generated by an object. This observation is one of the pillars of the [[general theory of relativity]].
 +  
   
−  The above prediction, that all forms of energy interact gravitationally, has been subject to experimental tests. The first observation testing this prediction was made in 1919.<ref>{{cite journallast=Dysoncoauthors= Eddington, A.S., & Davidson, C.R. year=1920 title=A Determination of the Deflection of Light by the Sun's Gravitational Field, from Observations Made at the Solar eclipse of May 29, 1919journal= [[Philosophical Transactions of the Royal Society A: Physical, Mathematical and Engineering SciencesPhil. Trans. Roy. Soc. A]]volume=220issue=571581pages= 291–333bibcode=1920RSPTA.220..291Ddoi=10.1098/rsta.1920.0009first =F.W.}}</ref> During a [[solar eclipse]], [[Arthur Eddington]] observed that the light from stars passing close to the Sun was bent. The effect is due to the gravitational attraction of light by the sun. The observation confirmed that the energy carried by light indeed is equivalent to a gravitational mass. Another seminal experiment, the [[Pound–Rebka experiment]], was performed in 1960.<ref>{{cite journallast=Pound first=R. V. coauthors = Rebka Jr. G. A.  date= April 1, 1960 title=Apparent weight of photons journal=[[Physical Review Letters]] volume=4 issue=7 pages=337–341 doi = 10.1103/PhysRevLett.4.337 bibcode=1960PhRvL...4..337P}}</ref> In this test a beam of light was emitted from the top of a tower and detected at the bottom. The frequency of the light detected was higher than the light emitted. This result confirms that the energy of photons increases when they fall in the gravitational field of the earth. The energy, and therefore the gravitational mass, of photons is proportional to their frequency as stated by the [[Planck constantPlanck's relation]].
 +  1 kg of Radon222 = <math>\frac{6.022 \times 10^{23}}{222.0176}</math> atoms. This is 2.7124611 * 10<sup>24</sup> atoms. 
   
−  ==Consequences for nuclear physics==
 +  That number of Polonium218 atoms has mass .98194467 kg. That number of alpha particles has mass .01802830 kg. 
−  [[Image:TaskForce One.jpgthumbright'''Task Force One''', the world's first nuclearpowered task force. {{USSEnterpriseCVN652}}, {{USSLong BeachCGN92}} and {{USSBainbridgeCGN252}} in formation in the Mediterranean, 18 June 1964. ''Enterprise'' crew members are spelling out Einstein's MassEnergy Equivalence formula ''E''=''mc''² on the flight deck.]]
 +  
   
−  [[Max Planck]] pointed out that the mass–energy equivalence formula implied that bound systems would have a mass less than the sum of their constituents, once the binding energy had been allowed to escape. However, Planck was thinking about chemical reactions, where the binding energy is too small to measure. Einstein suggested that radioactive materials such as [[radium]] would provide a test of the theory, but even though a large amount of energy is released per atom in radium, due to the [[halflife]] of the substance (1602 years), only a small fraction of radium atoms decay over an experimentally measurable period of time.
 +  The mass lost is .00002703 kg. 
   
−  Once the nucleus was discovered, experimenters realized that the very high binding energies of the atomic nuclei should allow calculation of their binding energies, simply from mass differences. But it was not until the discovery of the [[neutron]] in 1932, and the measurement of the neutron mass, that this calculation could actually be performed (see [[nuclear binding energy]] for example calculation). A little while later, the first [[Nuclear transmutationtransmutation]] reactions (such as<ref>[http://homepage.eircom.net/~louiseboylan/Pages/Cockroft_walton.htm] CockcroftWalton experiment</ref> the [[CockcroftWalton experiment]]: <sup>7</sup>Li + ''p'' → 2 <sup>4</sup>He) verified Einstein's formula to an accuracy of ±0.5%.
 +  Each emitted alpha particle has energy of 5.49 MeV. The total alpha energy from the decay of 1 kg of polonium is 2.39 * 10<sup>12</sup> Joules. 
−  In 2005, Rainville et al. published a direct test of the energyequivalence of mass lost in the bindingenergy of a neutron to atoms of particular isotopes of silicon and sulfur, by comparing the masslost to the energy of the emitted gamma ray associated with the neutron capture. The binding massloss agreed with the gamma ray energy to a precision of ±0.00004 %, the most accurate test of E=mc<sup>2</sup> to date.<ref name="rainville"/>
 +  
   
−  The mass–energy equivalence formula was used in the understanding of [[nuclear fission]] reactions, and implies the great amount of energy that can be released by a [[nuclear fission]] [[chain reaction]], used in both [[nuclear weapon]]s and [[nuclear power]]. By measuring the mass of different [[atomic nuclei]] and subtracting from that number the total mass of the [[protons]] and [[neutrons]] as they would weigh separately, one gets the exact [[binding energy]] available in an [[atomic nucleus]]. This is used to calculate the energy released in any [[nuclear reaction]], as the difference in the total mass of the nuclei that enter and exit the reaction.
 +  It looks as thought we have to rewrite the law of conservation of mass: 
   
−  == Practical examples ==
 +  :::*'''In all "ordinary" interactions, mass is precisely conserved.''' 
−  {{unreferenced sectiondate=August 2012}}
 +  :::*'''In nuclear interactions, there is a small but measurable loss of mass.''' 
−  Einstein used the [[Centimeter gram second system of unitsCGS]] system of units (centimeters, grams, seconds, dynes, and ergs), but the formula is independent of the system of units. In [[natural units]], the speed of light is defined to equal 1, and the formula expresses an identity: ''E'' = ''m''. In the [[International System of UnitsSI]] system (expressing the ratio ''E'' / ''m'' in [[joules]] per kilogram using the value of ''c'' in [[metre per secondmeters per second]]):{{citation neededdate=August 2012}}
 +  
   
−  :''E'' / ''m'' = ''c''<sup>2</sup> = (299,792,458 m/s or 983,571,056 ft/s)<sup>2</sup> = 89,875,517,873,681,764 J/kg (≈9.0 × 10<sup>16</sup> joules per kilogram).  +  :By the way, we can clearly see that atomic weights of pure isotopes are not integers, and that it has something to do with the energy released by nuclear disintegration. In retrospect, the formula E=mc² explains the noninteger character of atomic weights. 
   
−  So the energy equivalent of one [[gram]] (1/1000 of a kilogram) of mass is equivalent to:
 +  Making special cases like this is unsatisfactory, of course. 
−  :89.9 [[joulesterajoules]]
 +  
−  :25.0 million [[kilowatthour]]s (≈25 [[GW·h]])
 +  
−  :21.5 billion [[caloriekilocalories]] (≈21 Tcal)<sup> </sup><ref name="Conversion">Conversions used: 1956 International (Steam) Table (IT) values where one calorie ≡ 4.1868 J and one BTU ≡ 1055.05585262 J. Weapons designers' conversion value of one gram TNT ≡ 1000 calories used.<sup> </sup></ref>
 +  
−  :85.2 billion [[British thermal unitBTUs]]<ref name="Conversion"/>
 +  
−  or to the energy released by combustion of the following:
 +  
−  :21.5 [[kiloton]]s of [[TNT equivalentTNTequivalent]] energy (≈21 kt)<sup><span style="fontsize:100%;"> </span></sup><ref name="Conversion"/>
 +  
−  :568,000 US [[Imperial_gallon#Volumegallons]] of automotive [[Gasoline#Energy_content_.28high_and_low_heating_value.29gasoline]]
 +  
   
−  Any time energy is generated, the process can be evaluated from an ''E'' = ''mc''<sup>2</sup> perspective. For instance, the "[[Fat ManGadget]]"style bomb used in the [[Trinity test]] and the [[bombing of Nagasaki]] had an explosive yield equivalent to 21 kt of TNT. About 1 kg of the approximately 6.15 kg of plutonium in each of these bombs fissioned into lighter elements totaling almost exactly one gram less, after cooling. The electromagnetic radiation and kinetic energy (thermal and blast energy) released in this explosion carried the missing one gram of mass.<ref>The 6.2 kg core comprised 0.8% gallium by weight. Also, about 20% of the Gadget's yield was due to fast fissioning in its natural uranium tamper. This resulted in 4.1 moles of Pu fissioning with 180 MeV per atom actually contributing prompt kinetic energy to the explosion. Note too that the term ''"Gadget"style'' is used here instead of "Fat Man" because this general design of bomb was very rapidly upgraded to a more efficient one requiring only 5 kg of the Pu/gallium alloy.{{citation neededdate=August 2012}}</ref> This occurs because nuclear [[binding energy]] is released whenever elements with more than 62 nucleons fission.{{citation neededdate=August 2012}}
 +  We do this for a few other interactions, including the explosion of TNT. This would include the LithiumplusHydrogen and Uranium fission phenomena described above. We won't bother with the details. As observational scientists, we look for patterns in the behavior of nature. We make a table: 
   
−  Another example is [[hydroelectricityhydroelectric generation]]. The electrical energy produced by [[Grand Coulee DamGrand Coulee Dam's]] [[Water turbineturbines]] every 3.7 hours represents one gram of mass. This mass passes to the electrical devices (such as lights in cities) which are powered by the generators, where it appears as a gram of heat and light.<ref>Assuming the dam is generating at its peak capacity of 6,809 MW.{{citation neededdate=August 2012}}</ref> Turbine designers look at their equations in terms of pressure, torque, and RPM. However, Einstein's equations show that all energy has mass, and thus the electrical energy produced by a dam's generators, and the heat and light which result from it, all retain their mass, which is equivalent to the energy. The potential energy—and equivalent mass—represented by the waters of the [[Columbia River]] as it descends to the Pacific Ocean would be converted to heat due to [[viscous friction]] and the [[turbulence]] of white water rapids and waterfalls were it not for the dam and its generators. This heat would remain as mass on site at the water, were it not for the equipment which converted some of this potential and kinetic energy into electrical energy, which can be moved from place to place (taking mass with it).{{citation neededdate=August 2012}}
 +  { class="wikitable" border="1" cellpadding="8" cellspacing="0" 
 +  ! interaction 
 +  ! energy released per kg, Joules 
 +  ! mass lost per kg of original substance, kg 
 +   
 +   explosion of TNT 
 +   4.184 * 10<sup>6</sup> 
 +   seems to be zero 
 +   
 +   alpha decay of Ra226 
 +   2.04 * 10<sup>12</sup> 
 +   .00002301 kg 
 +   
 +   alpha decay of Rn222 
 +   2.39 * 10<sup>12</sup> 
 +   .00002703 kg 
 +  } 
   
−  Whenever energy is added to a system, the system gains mass:{{citation neededdate=August 2012}}
 
−  *A spring's mass increases whenever it is put into compression or tension. Its added mass arises from the added potential energy stored within it, which is bound in the stretched chemical (electron) bonds linking the atoms within the spring.
 
−  *Raising the temperature of an object (increasing its heat energy) increases its mass. For example, consider the world's primary mass standard for the kilogram, made of platinum/iridium. If its temperature is allowed to change by 1°C, its mass will change by 1.5 picograms (1 pg = 1 × 10<sup>−12</sup> g).<ref>Assuming a 90/10 alloy of Pt/Ir by weight, a ''C<sub>p</sub>'' of 25.9 for Pt and 25.1 for Ir, a Ptdominated average ''C<sub>p</sub>'' of 25.8, 5.134 moles of metal, and 132 J.K<sup>1</sup> for the prototype. A variation of ±1.5 picograms is of course, much smaller than the actual uncertainty in the mass of the international prototype, which is ±2 micrograms.</ref>
 
−  *A spinning ball will weigh more than a ball that is not spinning. Its increase of mass is exactly the equivalent of the mass of energy of rotation, which is itself the sum of the kinetic energies of all the moving parts of the ball. For example, [[the Earth]] itself is more massive due to its daily rotation, than it would be with no rotation. This rotational energy (2.14 x 10<sup>29</sup> J) represents 2.38 billion metric tons of added mass.<ref>InfraNet Lab (20081207). Harnessing the Energy from the Earth's Rotation. Article on Earth rotation energy. Divided by c^2. InfraNet Lab, 7 December 2008. Retrieved from http://infranetlab.org/blog/2008/12/harnessingtheenergyfromtheearth%E2%80%99srotation/.</ref>
 
   
−  Note that no net mass or energy is really created or lost in any of these examples and scenarios. Mass/energy simply moves from one place to another. These are some examples of the ''transfer'' of energy and mass in accordance with the ''principle of mass–energy conservation''.{{citation neededdate=August 2012}}
 +  We plot these, and a few others, not shown, on graph paper, and find to our amazement that the relationship is linear. 
   
−  == Efficiency ==  +  For Ra, m/E = .112794118 E16 
 +  For Po, m/E = .113096234 E16 
   
−  Although mass cannot be converted to energy, matter particles can be. Also, a certain amount of the illdefined "matter" in ordinary objects can be converted to active energy (light and heat), even though no identifiable real particles are destroyed. Such conversions happen in nuclear weapons, in which the protons and neutrons in atomic nuclei lose a small fraction of their average mass, but this massloss is not due to the destruction of any protons or neutrons (or even, in general, lighter particles like electrons). Also the mass is not destroyed, but simply removed from the system in the form of heat and light from the reaction.
 +  If this is linear, the mass defect for TNT would have been .47 * 10<sup>10</sup>. We couldn't possibly have measured this. 
   
−  In nuclear reactions, typically only a small fraction of the total mass–energy of the bomb is converted into heat, light, radiation and motion, which are "active" forms which can be used. When an atom fissions, it loses only about 0.1% of its mass (which escapes from the system and does not disappear), and in a bomb or reactor not all the atoms can fission. In a fission based atomic bomb, the [[Nuclear weapon design#Efficiencyefficiency]] is only 40%, so only 40% of the fissionable atoms actually fission, and only 0.04% of the total mass appears as energy in the end. In nuclear fusion, more of the mass is released as usable energy, roughly 0.3%. But in a fusion bomb (see [[nuclear weapon yield]]), the bomb mass is partly casing and nonreacting components, so that in practicality, no more than about 0.03% of the total mass of the entire weapon is released as usable energy (which, again, retains the "missing" mass).
 +  So we can rewrite the rule for conservation of mass in a more satisfactory way: 
   
−  In theory, it should be possible to convert all of the mass in matter into heat and light (which would of course have the same mass), but none of the theoretically known methods are practical. One way to convert all matter into usable energy is to annihilate matter with [[antimatter]]. But [[baryon asymmetryantimatter is rare in our universe]], and must be made first. Due to inefficient mechanisms of production, making antimatter always requires far more energy than would be released when it was annihilated.  +  :::*'''In all interactions, there is a loss of mass, equal to about .113 * 10<sup>16</sup> kg per Joule of energy released.''' 
   
−  Since most of the mass of ordinary objects resides in protons and neutrons, in order to convert all ordinary matter to useful energy, the protons and neutrons must be converted to lighter particles. In the [[standard modelstandard model of particle physics]], the [[baryon numbernumber of protons plus neutrons]] is nearly exactly conserved. Still, [[Gerard 't Hooft]] showed that there is a process which will convert protons and neutrons to antielectrons and neutrinos.<ref>G. 't Hooft, "Computation of the quantum effects due to a fourdimensional pseudoparticle", Physical Review D14:3432–3450 (1976).</ref> This is the weak SU(2) [[instanton]] proposed by Belavin Polyakov Schwarz and Tyupkin.<ref>A. Belavin, A. M. Polyakov, A. Schwarz, Yu. Tyupkin, "Pseudoparticle Solutions to Yang Mills Equations", Physics Letters 59B:85 (1975).</ref> This process, can in principle convert all the mass of matter into neutrinos and usable energy, but it is normally extraordinarily slow. Later it became clear that this process will happen at a fast rate at very high temperatures,<ref>F. Klinkhammer, [[N. Manton]], "A Saddle Point Solution in the Weinberg Salam Theory", Physical Review D 30:2212.</ref> since then instantonlike configurations will be copiously produced from [[statistical mechanicsthermal fluctuations]]. The temperature required is so high that it would only have been reached shortly after the [[big bang]].
 +  What we thought was exact conservation is just very nearly exact, and we hadn't been able to measure it before. 
   
−  Many extensions of the standard model contain [[magnetic monopole]]s, and in some models of [[grand unification theorygrand unification]], these monopoles catalyze [[proton decay]], a process known as the Callan–Rubakov effect.<ref>Rubakov V. A. "Monopole Catalysis of Proton Decay", Reports on Progress in Physics 51:189–241 (1988).</ref> This process would be an efficient mass–energy conversion at ordinary temperatures, but it requires making [[Magnetic monopolemonopole]]s and antimonopoles first. The energy required to produce monopoles is believed to be enormous, but magnetic charge is conserved, so that the lightest monopole is stable. All these properties are deduced in theoretical models—magnetic monopoles have never been observed, nor have they been produced in any experiment so far.
 +  But maybe there's more. This constant has dimensions of kilograms per Joule. From highschool physics, we know that that is seconds squared divided by meters squared. That is, it is the reciprocal of the square of a velocity. We calculate that velocity. It is about 2.97 * 10<sup>8</sup> meters per second. Very close to the speed of light! Very interesting! (The calculations above were not extremely precise. The formula has been verified with great precision, but not here.) 
   
−  A third known method of total matter–energy conversion is using gravity, specifically black holes. [[Stephen Hawking]] theorized<ref>S.W. Hawking "Black Holes Explosions?" ''Nature'' 248:30 (1974).</ref> that black holes radiate thermally with no regard to how they are formed. So it is theoretically possible to throw matter into a black hole and use the emitted heat to generate power. According to the theory of [[Hawking radiation]], however, the black hole used will radiate at a higher rate the smaller it is, producing usable powers at only small black hole masses, where usable may for example be something greater than the local background radiation. It is also worth noting that the ambient irradiated power would change with the mass of the black hole, increasing as the mass of the black hole decreases, or decreasing as the mass increases, at a rate where power is proportional to the inverse square of the mass. In a "practical" scenario, mass and energy could be dumped into the black hole to regulate this growth, or keep its size, and thus power output, near constant. This could result from the fact that mass and energy are lost from the hole with its thermal radiation.
 +  We don't understand why (that will have to wait for the invention of relativity), but we can formulate a hypothesis: 
   
−  ==Background==
 +  :::*'''In all interactions, there is a loss of mass, equal to <math>\frac{1}{c^2}</math> times the amount of energy released.''' 
   
−  ===Mass–velocity relationship===
 +  We don't have to give the units any more, since everything is now dimensionally correct. 
   
−  In developing [[special relativity]], Einstein found that the [[kinetic energy#Relativistic kinetic energy of rigid bodieskinetic energy]] of a moving body is
 +  ::There is a very interesting analogy with the discovery of [[Maxwell's Equations]]. Maxwell found an interesting relationship involving the fundamental constants <math>\epsilon\,</math> and <math>\mu\,</math> appearing in his equations. Specifically, <math>\epsilon\mu\,</math> has the dimensions of seconds squared divided by meters squared, and that: 
   
−  ::<math>E_k = m_0 ( \gamma 1 ) c^2 = \frac{m_0 c^2}{\sqrt{1\frac{v^2}{c^2}}}  m_0 c^2,</math>  +  :::::<math>\frac{1}{\epsilon\mu} = c^2</math> 
   
−  with <math>v</math> the [[velocity]], <math>m_0</math> the rest mass, and γ the [[Lorentz factor]].
 +  ::where "c" was the known velocity of light. He also showed that his equations predict electromagnetic waves, propagating at that speed. 
   
−  He included the second term on the right to make sure that for small velocities, the energy would be the same as in classical mechanics:
 
   
−  ::<math>E_k = \frac{1}{2}m_0 v^2 + \cdots </math>
 +  This is a common way that scientific discoveries are made. But, as stated above, this is not how E=mc² was discovered. But it could have been. But in fact, it wasn't. 
−   +  
−  Without this second term, there would be an additional contribution in the energy when the particle is not moving.
 +  
−   +  
−  Einstein found that the [[Momentum#Modern definitions of momentumtotal momentum]] of a moving particle is:
 +  
−   +  
−  ::<math>P = \frac{m_0 v}{\sqrt{1\frac{v^2}{c^2}}}. </math>
 +  
−   +  
−  and it is this quantity which is conserved in collisions. The ratio of the momentum to the velocity is the [[relativistic mass]], m.
 +  
−   +  
−  ::<math>m = \frac{m_0}{\sqrt{1\frac{v^2}{c^2}}}</math>
 +  
−   +  
−  And the relativistic mass and the relativistic kinetic energy are related by the formula:
 +  
−   +  
−  ::<math>E_k = m c^2  m_0 c^2. \,</math>
 +  
−   +  
−  Einstein wanted to omit the unnatural second term on the righthand side, whose only purpose is to make the energy at rest zero, and to declare that the particle has a total energy which obeys:
 +  
−  ::<math> E = m c^2 \,</math>
 +  
−   +  
−  which is a sum of the rest energy ''m''<sub>0</sub>''c''<sup>2</sup> and the kinetic energy. This total energy is mathematically more elegant, and fits better with the momentum in relativity. But to come to this conclusion, Einstein needed to think carefully about collisions. This expression for the energy implied that matter at rest has a huge amount of energy, and it is not clear whether this energy is physically real, or just a mathematical artifact with no physical meaning.
 +  
−   +  
−  In a collision process where all the restmasses are the same at the beginning as at the end, either expression for the energy is conserved. The two expressions only differ by a constant which is the same at the beginning and at the end of the collision. Still, by analyzing the situation where particles are thrown off a heavy central particle, it is easy to see that the inertia of the central particle is reduced by the total energy emitted. This allowed Einstein to conclude that the inertia of a heavy particle is increased or diminished according to the energy it absorbs or emits.
 +  
−   +  
−  ===Relativistic mass===
 +  
−  {{MainMass in special relativity}}
 +  
−   +  
−  After Einstein first made his proposal, it became clear that the word mass can have two different meanings. The rest mass is what Einstein called ''m'', but others defined the ''relativistic mass'' with an explicit index:
 +  
−   +  
−  ::<math>m_{\mathrm{rel}} = \frac{m_0}{\sqrt{1\frac{v^2}{c^2}}}\,\, . </math>
 +  
−   +  
−  This mass is the ratio of momentum to velocity, and it is also the relativistic energy divided by {{math1=''c''<sup>2</sup>}} (it is not Lorentzinvariant, in contrast to <math>m_0</math>). The equation {{math1=''E'' = ''m''<sub>rel</sub>''c''<sup>2</sup>}} holds for moving objects. When the velocity is small, the relativistic mass and the rest mass are almost exactly the same.
 +  
−   +  
−  *''E''=''mc''<sup>2</sup> either means ''E''=''m''<sub>0</sub>''c''<sup>2</sup> for an object at rest, or ''E''=''m''<sub>rel</sub>''c''<sup>2</sup> when the object is moving.
 +  
−   +  
−  Also Einstein (following [[Hendrik Lorentz]] and [[Max Abraham]]) used velocity—and directiondependent mass concepts ([[Mass in special relativity#Early developments: transverse and longitudinal masslongitudinal and transverse mass]]) in his 1905 electrodynamics paper and in another paper in 1906.<ref>{{Citation  author=Einstein, A.  year=1905  title=Zur Elektrodynamik bewegter Körper.  journal=Annalen der Physik  volume=17  pages=891–921  doi=10.1002/andp.19053221004 url=http://www.physik.uniaugsburg.de/annalen/history/papers/1905_17_891921.pdfformat=PDFbibcode = 1905AnP...322..891E }}. [http://www.fourmilab.ch/etexts/einstein/specrel/www/ English translation.]</ref><ref>{{Citation  author=Einstein, A.  year=1906  title=Über eine Methode zur Bestimmung des Verhältnisses der transversalen und longitudinalen Masse des Elektrons.  journal=Annalen der Physik  volume=21  pages=583–586  doi=10.1002/andp.19063261310 url=http://www.physik.uniaugsburg.de/annalen/history/papers/1906_21_583586.pdfformat=PDFbibcode = 1906AnP...326..583E }}</ref>
 +  
−  However, in his first paper on ''E''=''mc''<sup>2</sup> (1905), he treated ''m'' as what would now be called the ''rest mass''.<ref name="inertia" /> Some claim that (in later years) he did not like the idea of "relativistic mass."<ref>See e.g. Lev B.Okun, ''The concept of Mass'', Physics Today '''42''' (6), June 1969, p. 31–36, http://www.physicstoday.org/vol42/iss6/vol42no6p31_36.pdf</ref> When modern physicists say "mass", they are usually talking about rest mass, since if they meant "relativistic mass", they would just say "energy".
 +  
−   +  
−  Considerable debate has ensued over the use of the concept "relativistic mass" and the connection of "mass" in relativity to "mass" in Newtonian dynamics. For example, one view is that only rest mass is a viable concept and is a property of the particle; while relativistic mass is a conglomeration of particle properties and properties of spacetime. A perspective that avoids this debate, due to Kjell Vøyenli, is that the Newtonian concept of mass as a particle property and the relativistic concept of mass have to be viewed as embedded in their own theories and as having no precise connection.<ref name="Jammer">
 +  
−   +  
−  {{Citation title=Concepts of mass in contemporary physics and philosophy author=Max Jammer url=http://books.google.com/?id=jujK1bn4QUQC&pg=PA51 page=51 isbn=069101017X publisher=Princeton University Press year=1999 }}</ref><ref name="Vøyenli">
 +  
−   +  
−  {{Citation journal=Foundations of Physics doi=10.1007/BF00708670 title= The classical and relativistic concepts of mass year=1976 author= Eriksen, Erik; Vøyenli, Kjell volume=6 pages=115–124 publisher=Springer bibcode=1976FoPh....6..115E}}</ref>
 +  
−   +  
−  === Lowspeed expansion ===
 +  
−   +  
−  We can rewrite the expression ''E'' = ''γm''<sub>0</sub>''c''<sup>2</sup> as a [[Taylor series]]:
 +  
−   +  
−  :<math>E = m_0 c^2 \left[1 + \frac{1}{2} \left(\frac{v}{c}\right)^2 + \frac{3}{8} \left(\frac{v}{c}\right)^4 + \frac{5}{16} \left(\frac{v}{c}\right)^6 + \ldots \right]. </math>
 +  
−   +  
−  For speeds much smaller than the speed of light, higherorder terms in this expression get smaller and smaller because ''v''/''c'' is small. For low speeds we can ignore all but the first two terms:
 +  
−   +  
−  :<math>E \approx m_0 c^2 + \frac{1}{2} m_0 v^2 . </math>
 +  
−   +  
−  The total energy is a sum of the rest energy and the [[classical mechanicsNewtonian]] [[kinetic energy]].
 +  
−   +  
−  The classical energy equation ignores both the ''m''<sub>0</sub>''c''<sup>2</sup> part, and the highspeed corrections. This is appropriate, because all the highorder corrections are small. Since only ''changes'' in energy affect the behavior of objects, whether we include the ''m''<sub>0</sub>''c''<sup>2</sup> part makes no difference, since it is constant. For the same reason, it is possible to subtract the rest energy from the total energy in relativity. By considering the emission of energy in different frames, Einstein could show that the rest energy has a real physical meaning.
 +  
−   +  
−  The higherorder terms are extra correction to Newtonian mechanics which become important at higher speeds. The Newtonian equation is only a lowspeed approximation, but an extraordinarily good one. All of the calculations used in putting astronauts on the moon, for example, could have been done using Newton's equations without any of the higherorder corrections.
 +  
−   +  
−  ==History==
 +  
−   +  
−  While Einstein was the first to have correctly deduced the mass–energy equivalence formula, he was not the first to have related energy with mass. But nearly all previous authors thought that the energy which contributes to mass comes only from electromagnetic fields.<ref name="jann">{{Citation  author=Jannsen, M., Mecklenburg, M.  year=2007  contribution=From classical to relativistic mechanics: Electromagnetic models of the electron.  editor=V. F. Hendricks, et al. journal=Interactions: Mathematics, Physics and Philosophy  pages=65–134  place=Dordrecht  publisher=Springer  url =http://www.tc.umn.edu/~janss011/ }}</ref><ref name="whit">{{Citation  author=Whittaker, E.T.  year=1951–1953  title= 2. Edition: A History of the theories of aether and electricity, vol. 1: The classical theories / vol. 2: The modern theories 1900–1926  place=London  publisher=Nelson}}</ref><ref name="mill">{{Citationauthor=Miller, Arthur I.year=1981title= Albert Einstein's special theory of relativity. Emergence (1905) and early interpretation (1905–1911)place= Readingpublisher=Addison–Wesleyisbn=0201046792}}</ref><ref name="darr">{{Citation  author=Darrigol, O.  title=The Genesis of the theory of relativity.  year=2005  journal=Séminaire Poincaré  volume=1  pages=1–22 url=http://www.bourbaphy.fr/darrigol2.pdfformat=PDF}}</ref>
 +  
−   +  
−  ===Newton: matter and light===
 +  
−   +  
−  In 1717 [[Isaac Newton]] speculated that light particles and matter particles were interconvertible in "Query 30" of the ''[[Opticks]]'', where he asks:
 +  
−   +  
−  {{quoteAre not the gross bodies and light convertible into one another, and may not bodies receive much of their activity from the particles of light which enter their composition?}}
 +  
−   +  
−  ===Swedenborg: matter composed of "pure and total motion"===
 +  
−   +  
−  In 1734 [[Emanuel Swedenborg]] in his Principia theorized that all matter is ultimately composed of dimensionless points of "pure and total motion." He described this motion as being without force, direction or speed, but having the potential for force, direction and speed everywhere within it.<ref>{{cite booklast=Swedenborgfirst=Emanueltitle=Principia Rerum Naturaliayear=1734location=Leipziglanguage=Latinchapter=De Simplici Mundi vel Puncto naturalipage=32}}</ref><ref>{{cite booklast=Swedenborgfirst=Emanueltitle=The Principia; or The First Principles of Natural Thingsyear=1845publisher=W. Newberylocation=Londontranslator=Augustus Clissoldpages=55–57}}</ref>
 +  
−   +  
−  ===Electromagnetic mass===
 +  
−  {{MainElectromagnetic mass}}
 +  
−  There were many attempts in the 19th and the beginning of the 20th century—like those of [[J. J. Thomson]] (1881), [[Oliver Heaviside]] (1888), and [[George Frederick Charles Searle]] (1897), [[Wilhelm Wien]] (1900), [[Max Abraham]] (1902), [[Hendrik Antoon Lorentz]] (1904) — to understand as to how the mass of a charged object depends on the electrostatic field.<ref name="jann"/><ref name="whit"/>
 +  
−  This concept was called [[electromagnetic mass]], and was considered as being dependent on velocity and direction as well. Lorentz (1904) gave the following expressions for longitudinal and transverse electromagnetic mass:
 +  
−  :<math>m_{L}=\frac{m_{0}}{\left(\sqrt{1\frac{v^{2}}{c^{2}}}\right)^{3}},\quad m_{T}=\frac{m_{0}}{\sqrt{1\frac{v^{2}}{c^{2}}}} </math>,
 +  
−   +  
−  where
 +  
−  :<math>m_{0}=\frac{4}{3}\frac{E_{em}}{c^{2}}</math>
 +  
−   +  
−  ===Radiation pressure and inertia===
 +  
−  {{MainElectromagnetic mass#Inertia of energy and radiation paradoxes}}
 +  
−  Another way of deriving some sort of electromagnetic mass was based on the concept of [[radiation pressure]]. In 1900, [[Henri Poincaré]] associated electromagnetic radiation energy with a "fictitious fluid" having momentum and mass <math>m_{em}=E_{em}/c^2</math>. By that, Poincaré tried to save the [[center of mass]] theorem in Lorentz's theory, though his treatment led to radiation paradoxes.<ref name="darr" />
 +  
−   +  
−  [[Friedrich Hasenöhrl]] showed in 1904, that electromagnetic [[cavity radiation]] contributes the "apparent mass" <math>m_{0}=\tfrac{4}{3}\tfrac{E_{em}}{c^{2}}</math> to the cavity's mass. He argued that this implies mass dependence on temperature as well.<ref>{{Cite webauthor=Philip Balltitle=Did Einstein discover E = mc2?publisher=[[Physics World]]date=Aug 23, 2011url=http://physicsworld.com/cws/article/news/46941}}</ref>
 +  
−   +  
−  ===Einstein: mass–energy equivalence===
 +  
−   +  
−  [[Albert Einstein]] did not formulate exactly the formula {{nowrap1=''E'' = ''mc''<sup>2</sup>}} in his 1905 [[Annus Mirabilis Papers''Annus Mirabilis'' paper]] "Does the Inertia of a Body Depend Upon Its Energy Content?";<ref name="inertia" /> rather, the paper states that if a body gives off the energy ''L'' in the form of radiation, its mass diminishes by ''L''/''c''<sup>2</sup>. (Here, "radiation" means [[electromagnetic radiation]], or light, and mass means the ordinary Newtonian mass of a slowmoving object.) This formulation relates only a change Δ''m'' in mass to a change ''L'' in energy without requiring the absolute relationship.
 +  
− 
 +  
−  Objects with zero mass presumably have zero energy, so the extension that all mass is proportional to energy is obvious from this result. In 1905, even the hypothesis that changes in energy are accompanied by changes in mass was untested. Not until the discovery of the first type of antimatter (the [[positron]] in 1932) was it found that all of the mass of pairs of resting particles could be converted to radiation.
 +  
−   +  
−  ====First derivation (1905)====
 +  
−   +  
−  Already in his relativity paper "On the electrodynamics of moving bodies", Einstein derived the correct expression for the kinetic energy of particles:
 +  
−   +  
−  :<math>E_{k}=mc^{2}\left(\frac{1}{\sqrt{1\frac{v^{2}}{c^{2}}}}1\right)</math>.
 +  
−   +  
−  Now the question remained open as to which formulation applies to bodies at rest. This was tackled by Einstein in his paper "Does the inertia of a body depend upon its energy content?". Einstein used a body emitting two light pulses in opposite directions, having energies of <math>E_0</math> before and <math>E_1</math> after the emission as seen in its rest frame. As seen from a moving frame, this becomes <math>H_0</math> and <math>H_1</math>. Einstein obtained:
 +  
−   +  
−  :<math>\left(H_{0}E_{0}\right)\left(H_{1}E_{1}\right)=E\left(\frac{1}{\sqrt{1\frac{v^{2}}{c^{2}}}}1\right)</math>
 +  
−   +  
−  then he argued that <math>HE</math> can only differ from the kinetic energy <math>K</math> by an additive constant, which gives
 +  
−   +  
−  :<math>K_{0}K_{1}=E\left(\frac{1}{\sqrt{1\frac{v^{2}}{c^{2}}}}1\right)</math>
 +  
−   +  
−  Neglecting effects higher than third order in <math>v/c</math> this gives:
 +  
−   +  
−  :<math>K_{0}K_{1}=\frac{E}{c^{2}}\frac{v^{2}}{2}</math>
 +  
−   +  
−  Thus Einstein concluded that the emission reduces the body's mass by <math>E/c^2</math>, and that the mass of a body is a measure of its energy content.
 +  
−   +  
−  The correctness of Einstein's 1905 derivation of ''E''=''mc''<sup>2</sup> was criticized by [[Max Planck]] (1907), who argued that it is only valid to first approximation. Another criticism was formulated by [[Herbert Ives]] (1952) and [[Max Jammer]] (1961), asserting that Einstein's derivation is based on [[begging the question]].<ref>{{Citation  author=Ives, Herbert E.  year=1952  title=Derivation of the massenergy relation  journal=Journal of the Optical Society of America  volume= 42  issue=8  pages =540–543  doi=10.1364/JOSA.42.000540}}</ref><ref>{{Cite bookauthor=Jammer, Maxtitle=Concepts of Mass in Classical and Modern Physicslocation=New Yorkpublisher=Doveryear=1961/1997isbn=0486299988}}</ref>
 +  
−  On the other hand, [[John Stachel]] and [[Roberto Torretti]] (1982) argued that Ives' criticism was wrong, and that Einstein's derivation was correct.<ref>{{Citation  author=Stachel, John; Torretti, Roberto  year=1982  title=Einstein's first derivation of massenergy equivalence  journal=American Journal of Physics  volume =50  issue=8  pages =760–763  doi=10.1119/1.12764bibcode = 1982AmJPh..50..760S }}</ref>
 +  
−  Hans Ohanian (2008) agreed with Stachel/Torretti's criticism of Ives, though he argued that Einstein's derivation was wrong for other reasons.<ref>{{Citation  author=Ohanian, Hans  year=2008  title=Did Einstein prove E=mc2?  journal=Studies In History and Philosophy of Science Part B  volume =40  issue=2  pages =167–173  doi=10.1016/j.shpsb.2009.03.002arxiv=0805.1400}}</ref> For a recent review, see Hecht (2011).<ref>{{Citation  author=Hecht, Eugene  year=2011  title=How Einstein confirmed E0=mc2  journal=American Journal of Physics  volume =79  issue=6  pages =591–600  doi=10.1119/1.3549223bibcode = 2011AmJPh..79..591H }}</ref>
 +  
−   +  
−  ====Alternative version====
 +  
−  An alternative version of Einstein's [[thought experiment]] was proposed by Fritz Rohrlich (1990), who based his reasoning on the [[Doppler effect]].<ref>{{Citation  author=Rohrlich, Fritz  year=1990  title=An elementary derivation of E=mc2  journal=American Journal of Physics  volume= 58  issue=4  pages =348–349  doi=10.1119/1.16168bibcode = 1990AmJPh..58..348R }}</ref>
 +  
−  Like Einstein, he considered a body at rest with mass ''M''. If the body is examined in a frame moving with nonrelativistic velocity ''v'', it is no longer at rest and in the moving frame it has momentum ''P'' = ''Mv''. Then he supposed the body emits two pulses of light to the left and to the right, each carrying an equal amount of energy ''E''/2. In its rest frame, the object remains at rest after the emission since the two beams are equal in strength and carry opposite momentum.
 +  
−   +  
−  But if the same process is considered in a frame moving with velocity ''v'' to the left, the pulse moving to the left will be [[redshift]]ed while the pulse moving to the right will be [[blue shift]]ed. The blue light carries more momentum than the red light, so that the momentum of the light in the moving frame is not balanced: the light is carrying some net momentum to the right.
 +  
−   +  
−  The object has not changed its velocity before or after the emission. Yet in this frame it has lost some rightmomentum to the light. The only way it could have lost momentum is by losing mass. This also solves Poincaré's radiation paradox, discussed above.
 +  
−   +  
−  The velocity is small, so the rightmoving light is blueshifted by an amount equal to the nonrelativistic [[Doppler shift]] factor {{nowrap1 − ''v''/''c''}}. The momentum of the light is its energy divided by ''c'', and it is increased by a factor of ''v''/''c''. So the rightmoving light is carrying an extra momentum <math>\Delta P</math> given by:
 +  
−   +  
−  :<math>
 +  
−  \Delta P = {v \over c}{E \over 2c}.
 +  
−  \,</math>
 +  
−   +  
−  The leftmoving light carries a little less momentum, by the same amount <math>\Delta P</math>. So the total rightmomentum in the light is twice <math> \Delta P</math>. This is the rightmomentum that the object lost.
 +  
−   +  
−  :<math>
 +  
−  2\Delta P = v {E\over c^2}.
 +  
−  \,</math>
 +  
−   +  
−  The momentum of the object in the moving frame after the emission is reduced by this amount:
 +  
−   +  
−  :<math>
 +  
−  P' = Mv  2\Delta P = \left(M  {E\over c^2}\right)v.
 +  
−  \,</math>
 +  
−   +  
−  So the change in the object's mass is equal to the total energy lost divided by <math>c^2</math>. Since any emission of energy can be carried out by a two step process, where first the energy is emitted as light and then the light is converted to some other form of energy, any emission of energy is accompanied by a loss of mass. Similarly, by considering absorption, a gain in energy is accompanied by a gain in mass.
 +  
−   +  
−  ====Relativistic centerofmass theorem – 1906====
 +  
−   +  
−  Like Poincaré, Einstein concluded in 1906 that the inertia of electromagnetic energy is a necessary condition for the centerofmass theorem to hold. On this occasion, Einstein referred to Poincaré's 1900 paper and wrote:<ref>{{Citation  author=Einstein, A.  year=1906  title=Das Prinzip von der Erhaltung der Schwerpunktsbewegung und die Trägheit der Energie  journal=Annalen der Physik  volume =20  pages =627–633  doi=10.1002/andp.19063250814 url=http://www.physik.uniaugsburg.de/annalen/history/papers/1906_20_627633.pdfformat=PDFbibcode = 1906AnP...325..627E }}</ref>
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−  {{quoteAlthough the merely formal considerations, which we will need for the proof, are already mostly contained in a work by H. Poincaré<sup>2</sup>, for the sake of clarity I will not rely on that work.<ref>Einstein 1906: Trotzdem die einfachen formalen Betrachtungen, die zum Nachweis dieser Behauptung durchgeführt werden müssen, in der Hauptsache bereits in einer Arbeit von H. Poincaré enthalten sind<sup>2</sup>, werde ich mich doch der Übersichtlichkeit halber nicht auf jene Arbeit stützen.</ref>}}
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−  In Einstein's more physical, as opposed to formal or mathematical, point of view, there was no need for fictitious masses. He could avoid the ''[[perpetual motionperpetuum mobile]]'' problem, because on the basis of the mass–energy equivalence he could show that the transport of inertia which accompanies the emission and absorption of radiation solves the problem. Poincaré's rejection of the principle of action–reaction can be avoided through Einstein's <math>E=mc^2</math>, because mass conservation appears as a special case of the [[energy conservation law]].
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−   +  
−  ===Others===
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−   +  
−  During the nineteenth century there were several speculative attempts to show that mass and energy were proportional in various [[Aether theoriesether theories]].<ref>Helge Kragh, "FindeSiècle Physics: A World Picture in Flux" in ''Quantum Generations: A History of Physics in the Twentieth Century'' (Princeton, NJ: Princeton University Press, 1999.</ref> In 1873 [[Nikolay Umov]] pointed out a relation between mass and energy for ether in the form of ''Е = kmc''<sup>2</sup>, where 0.5 ≤ k ≤ 1.<ref>''Умов Н. А.'' Избранные сочинения. М. — Л., 1950. (Russian)</ref> The writings of [[Samuel Tolver Preston]],<ref>Preston, S. T., Physics of the Ether, E. & F. N. Spon, London, (1875).</ref><ref>Bjerknes: [http://itis.volta.alessandria.it/episteme/ep6/ep6bjerk1.htm S. Tolver Preston's Explosive Idea ''E'' = ''mc''<sup>2</sup>.]</ref> and a 1903 paper by [[Olinto De Pretto]],<ref name="math">MathPages: [http://www.mathpages.com/rr/s808/808.htm Who Invented Relativity?]</ref><ref>De Pretto, O. ''Reale Instituto Veneto Di Scienze, Lettere Ed Arti'', LXIII, II, 439–500, reprinted in Bartocci.</ref> presented a mass–energy relation. De Pretto's paper received recent press coverage when [[Umberto Bartocci]] discovered that there were only [[six degrees of separationthree degrees of separation]] linking De Pretto to Einstein, leading Bartocci to conclude that Einstein was probably aware of De Pretto's work.<ref>Umberto Bartocci, ''Albert Einstein e Olinto De Pretto—La vera storia della formula più famosa del mondo'', editore Andromeda, Bologna, 1999.</ref>
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−   +  
−  Preston and De Pretto, following [[Le Sage's theory of gravitationLe Sage]], imagined that the universe was filled with an ether of tiny particles which are always moving at speed ''c''. Each of these particles have a kinetic energy of ''mc''<sup>2</sup> up to a small numerical factor. The nonrelativistic kinetic energy formula did not always include the traditional factor of 1/2, since [[Gottfried LeibnizLeibniz]] introduced kinetic energy without it, and the 1/2 is largely conventional in prerelativistic physics.<ref>{{Citation author=Prentiss, J.J. title=Why is the energy of motion proportional to the square of the velocity?  journal=American Journal of Physics  volume=73 issue = 8  month=August  year=2005  page=705}}</ref> By assuming that every particle has a mass which is the sum of the masses of the ether particles, the authors would conclude that all matter contains an amount of kinetic energy either given by ''E'' = ''mc''<sup>2</sup> or 2''E'' = ''mc''<sup>2</sup> depending on the convention. A particle ether was usually considered unacceptably speculative science at the time,<ref>John Worrall, review of the book ''Conceptions of Ether. Studies in the History of Ether Theories'' by Cantor and Hodges, The British Journal of the Philosophy of Science vol 36, no 1, March 1985, p. 84. The article contrasts a particle ether with a wavecarrying ether, the latter ''was'' acceptable.</ref> and since these authors did not formulate relativity, their reasoning is completely different from that of Einstein, who used relativity to change frames.
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−  Independently, [[Gustave Le Bon]] in 1905 speculated that atoms could release large amounts of latent energy, reasoning from an allencompassing qualitative philosophy of physics.<ref>Le Bon: [http://www.rexresearch.com/lebonfor/evforp1.htm#p1b3ch2 The Evolution of Forces.]</ref><ref>Bizouard: [http://www.annales.org/archives/x/poincaBizouard.pdf Poincaré ''E'' = ''mc''<sup>2</sup> l'équation de Poincaré, Einstein et Planck.]</ref>
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−   +  
−  ===Radioactivity and nuclear energy===
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−   +  
−  It was quickly noted after the discovery of [[radioactivity]] in 1897, that the total energy due to radioactive processes is about one ''million times'' greater than that involved in any known molecular change. However, it raised the question where this energy is coming from. After eliminating the idea of absorption and emission of some sort of [[Le Sage's theory of gravitationLesagian ether particles]], the existence of a huge amount of latent energy, stored within matter, was proposed by [[Ernest Rutherford]] and [[Frederick Soddy]] in 1903. Rutherford also suggested that this internal energy is stored within normal matter as well. He went on to speculate in 1904:<ref>{{Citationlast=Rutherfordfirst= Ernest year=1904title=Radioactivity publisher=University Presslocation=Cambridgepages=336–338url=http://www.archive.org/details/radioactivity00ruthrich}}</ref><ref>{{Citationlast=Heisenbergfirst= Werner year=1958title=Physics And Philosophy: The Revolution In Modern Science publisher=Harper & Brotherslocation=New Yorkpages=118–119url=http://www.archive.org/details/physicsandphilos010613mbp}}</ref>
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−   +  
−  {{quoteIf it were ever found possible to control at will the rate of disintegration of the radioelements, an enormous amount of energy could be obtained from a small quantity of matter.}}
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−   +  
−  Einstein's equation is in no way an explanation of the large energies released in radioactive decay (this comes from the powerful [[nuclear force]]s involved; forces that were still unknown in 1905). In any case, the enormous energy released from radioactive decay (which had been measured by Rutherford) was much more easily measured than the (still small) change in the gross mass of materials, as a result. Einstein's equation, by theory, can give these energies by measuring mass differences before and after reactions, but in practice, these mass differences in 1905 were still too small to be measured in bulk. Prior to this, the ease of measuring radioactive decay energies with a calorimeter was thought possibly likely to allow measurement of changes in mass difference, as a check on Einstein's equation itself. Einstein mentions in his 1905 paper that mass–energy equivalence might perhaps be tested with radioactive decay, which releases enough energy (the quantitative amount known roughly by 1905) to possibly be "weighed," when missing from the system (having been given off as heat). However, radioactivity seemed to proceed at its own unalterable (and quite slow, for radioactives known then) pace, and even when simple nuclear reactions became possible using proton bombardment, the idea that these great amounts of usable energy could be liberated at will with any practicality, proved difficult to substantiate. It had been used as the basis of much speculation, causing Rutherford himself to later reject his ideas of 1904; he was reported in 1933 to have said that: "Anyone who expects a source of power from the transformation of the atom is talking [[moonshine]]."<ref>"We might in these processes obtain very much more energy than the proton supplied, but on the average we could not expect to obtain energy in this way. It was a very poor and inefficient way of producing energy, and anyone who looked for a source of power in the transformation of the atoms was talking moonshine. But the subject was scientifically interesting because it gave insight into the atoms." [http://archive.timesonline.co.uk/ ''The Times'' archives], September 12, 1933, "The British association—breaking down the atom"</ref>
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−   +  
−  [[Image:Einstein  Time Magazine  July 1, 1946.jpgrightthumbThe popular connection between Einstein, {{nowrap1=''E'' = ''mc''<sup>2</sup>}}, and the [[nuclear weaponatomic bomb]] was prominently indicated on the cover of ''[[Time (magazine)Time]]'' magazine in July 1946 by the writing of the equation on the [[mushroom cloud]] itself.]]
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−   +  
−  This situation changed dramatically in 1932 with the discovery of the neutron and its mass, allowing mass differences for single [[nuclide]]s and their reactions to be calculated directly, and compared with the sum of masses for the particles that made up their composition. In 1933, the energy released from the reaction of lithium7 plus protons giving rise to 2 alpha particles (as noted above by Rutherford), allowed Einstein's equation to be tested to an error of ± 0.5%. However, scientists still did not see such reactions as a source of power.
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−   +  
−  After the very public demonstration of huge energies released from [[nuclear fission]] after the [[atomic bombings of Hiroshima and Nagasaki]] in 1945, the equation ''E'' = ''mc''<sup>2</sup> became directly linked in the public eye with the power and peril of [[nuclear weapon]]s. The equation was featured as early as page 2 of the [[Smyth Report]], the official 1945 release by the US government on the development of the atomic bomb, and by 1946 the equation was linked closely enough with Einstein's work that the cover of ''[[Time (magazine)Time]]'' magazine prominently featured a picture of Einstein next to an image of a [[mushroom cloud]] emblazoned with the equation.<ref>[http://www.time.com/time/covers/0,16641,19460701,00.html Cover.] ''Time'' magazine, July 1, 1946.</ref> Einstein himself had only a minor role in the [[Manhattan Project]]: he had [[Einstein–Szilárd lettercosigned a letter]] to the U.S. President in 1939 urging funding for research into atomic energy, warning that an atomic bomb was theoretically possible. The letter persuaded Roosevelt to devote a significant portion of the wartime budget to atomic research. Without a security clearance, Einstein's only scientific contribution was an analysis of an [[isotope separation]] method in theoretical terms. It was inconsequential, on account of Einstein not being given sufficient information (for security reasons) to fully work on the problem.<ref>Isaacson, ''Einstein: His Life and Universe''.</ref>
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−   +  
−  While ''E'' = ''mc''<sup>2</sup> is useful for understanding the amount of energy potentially released in a fission reaction, it was not strictly necessary to develop the weapon, once the fission process was known, and its energy measured at 200 MeV (which was directly possible, using a quantitative Geiger counter, at that time). As the physicist and Manhattan Project participant [[Robert Serber]] put it: "Somehow the popular notion took hold long ago that Einstein's theory of relativity, in particular his famous equation ''E'' = ''mc''<sup>2</sup>, plays some essential role in the theory of fission. Albert Einstein had a part in alerting the United States government to the possibility of building an atomic bomb, but his theory of relativity is not required in discussing fission. The theory of fission is what physicists call a nonrelativistic theory, meaning that relativistic effects are too small to affect the dynamics of the fission process significantly."<ref>Robert Serber, ''The Los Alamos Primer: The First Lectures on How to Build an Atomic Bomb'' (University of California Press, 1992), page 7. Note that the quotation is taken from Serber's 1992 version, and is not in the original 1943 [[Los Alamos Primer]] of the same name.</ref> However the association between ''E'' = ''mc''<sup>2</sup> and nuclear energy has since stuck, and because of this association, and its simple expression of the ideas of Albert Einstein himself, it has become "the world's most famous equation".<ref>David Bodanis, ''E = mc<sup>2</sup>: A Biography of the World's Most Famous Equation'' (New York: Walker, 2000).</ref>
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−   +  
−  While Serber's view of the strict lack of need to use mass–energy equivalence in designing the atomic bomb is correct, it does not take into account the pivotal role which this relationship played in making the fundamental leap to the initial hypothesis that large atoms were energetically ''allowed'' to split into approximately equal parts (before this energy was in fact measured). In late 1938, while on the winter walk on which they solved the meaning of Hahn's experimental results and introduced the idea that would be called atomic fission, [[Lise Meitner]] and [[Otto Robert Frisch]] made direct use of Einstein's equation to help them understand the quantitative energetics of the reaction which overcame the "surface tensionlike" forces holding the nucleus together, and allowed the fission fragments to separate to a configuration from which their charges could force them into an energetic "fission". To do this, they made use of "packing fraction", or nuclear [[binding energy]] values for elements, which Meitner had memorized. These, together with use of ''E'' = ''mc''<sup>2</sup> allowed them to realize on the spot that the basic fission process was energetically possible:
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−  <blockquote> ...We walked up and down in the snow, I on skis and she on foot. ...and gradually the idea took shape... explained by Bohr's idea that the nucleus is like a liquid drop; such a drop might elongate and divide itself... We knew there were strong forces that would resist, ..just as surface tension. But nuclei differed from ordinary drops. At this point we both sat down on a tree trunk and started to calculate on scraps of paper. ...the Uranium nucleus might indeed be a very wobbly, unstable drop, ready to divide itself... But, ...when the two drops separated they would be driven apart by electrical repulsion, about 200 MeV in all. Fortunately Lise Meitner remembered how to compute the masses of nuclei... and worked out that the two nuclei formed... would be lighter by about onefifth the mass of a proton. Now whenever mass disappears energy is created, according to Einstein's formula E = mc<sup>2</sup>, and... the mass was just equivalent to 200 MeV; it all fitted!<ref>[http://homepage.mac.com/dtrapp/people/Meitnerium.html A quote from Frisch about the discovery day. Accessed April 4, 2009.]</ref><ref>{{cite book  last=Sime  first=Ruth  title= Lise Meitner: A Life in Physics  publisher = University of California Press  series = California Studies in the History of Science  volume = 13  year = 1996  location = Berkeley  pages = 236–237  isbn = 0520208609 }}</ref></blockquote>
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 ==See also==   ==See also== 
−  * [[Binding energy]] (mass defect)  +  *[[Counterexamples to Relativity]] 
−  * [[Energy density]]  +  *[[essay:Rebuttal to Counterexamples to Relativity]] 
−  * [[Energy–momentum relation]]
 +  
−  * [[Inertia]]
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−  * [[Mass in special relativity]]
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−  * [[Special relativity#Mass, momentum, and energyMass, momentum, and energy]]
 +  
−  * [[Index of energy articles]]
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−  * [[List of wave topics]]
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−  * [[Outline of energy]]
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−  * [[Rotational energy]]
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−   +  
−  ==References==
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−  {{reflistcolwidth=30em}}
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−  {{refbegin}}
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−  * {{Citation  author=Bodanis, David  title=E=mc<sup>2</sup>: A Biography of the World's Most Famous Equation  publisher=Berkley Trade  year=2001  isbn=0425181642}}
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−  * {{Citation  author=Tipler, Paul; Llewellyn, Ralph  title=Modern Physics (4th ed.)  publisher=W. H. Freeman  year=2002  isbn=0716743450}}
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−  * {{Citation  first=Ronald C.  last=Lasky  coauthors=  title=What is the significance of E = mc<sup>2</sup>? And what does it mean?  date=April 23, 2007  publisher= [[Scientific American]] url =http://www.sciam.com/article.cfm?id=significanceemc2means  work =Scientific American  pages =  accessdate =  language = }}
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−  {{refend}}
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−   +  
−  ==External links==
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−  {{WikisourceparRelativity: The Special and General Theory}}
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−  {{Commons categoryEinstein formula}}
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−  * [http://plato.stanford.edu/entries/equivME The Equivalence of Mass and Energy] – Entry in the Stanford Encyclopedia of Philosophy
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−  * [http://relativity.livingreviews.org/ Living Reviews in Relativity] – An open access, peerreferred, solely online physics journal publishing invited reviews covering all areas of relativity research.
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−  *[http://fotonowy.pl/index.php?main_page=page&id=6 A shortcut to ''E''=''mc''<sup>2</sup>] – An easy to understand, highschool level derivation of the ''E''=''mc''<sup>2</sup> formula.
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−  * [http://www.mathpages.com/home/kmath600/kmath600.htm Einstein on the Inertia of Energy] – MathPages
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