Einstein’s Mass-Energy Equivalence Equation


Einstein discovered that mass and energy were essentially two aspects of the same thing. He showed, in this special theory of relativity, that mass and energy are related by the equation:


MASS-ENERGY EQUIVALENCE EQUATION


E  mc2 E  energy,m  mass, c  the speed of light.6


Einstein understood that the energy ‘produced’ in a nuclear reaction is due to the loss in mass of the reactants. He also theorised that energy can be converted to mass in accordance with the same relationship. The total mass plus energy of a system is conserved in all reactions.


Analogy: Imagine studying two different planets. One planet was cold—the temper- ature never rose above 0 C and ice never melted. The other planet was hot—the


temperature never dropped below 0 C and water never froze. Looking at H2O in these two environments you may never realise that ice and water are essentially the same thing. Solid ice and liquid water have very different properties.The inhabitants of each


planet would never have seen how easily H2O can change state, so those aliens may have difficulty comprehending the equivalence of ice and water.We know that H2O can easily be converted between ice and water—the same is true of mass and energy.The


conversion between mass and energy is less familiar to us, but it is still a fact that mass and energy are essentially the same thing.


Fission FISSION


Nuclear fission is the splitting of a large nucleus into two smaller nuclei, of roughly equal mass, with emission of neutrons and release of energy.


One example of the fission of uranium-235 is the emission of xenon-143 and strontium- 90 plus three neutrons when struck by a slow neutron (Fig 22.19).The equation for the reaction is


235 92U  1


0n:143 54Xe  90 38 Sr  3(1 0n) .


 rays (energy)


Released neutrons


Neutron


Nuclear collision


143 54Xe Fig 22.20: Fission of U-235


6When referring to the ‘speed of light’, we are referring to ‘speed of light in a vacuum’. As discussed in Chapter 12, the speed of light is different in different media.The speed of light in a vacuum is actually 299 792 458 m s1, but this number


is generally rounded off to 3  108 m s1. 7Particle accelerators, and hadrons, will be dealt with in greater detail in Chapter 23.


38Sr 90


ALBERT EINSTEIN, 1879–1955 (GERMAN) Arguably the greatest theoretical physi- cist of all time, he explained the photo- electric effect and formulated the pho- toelectric law, making him the founder of quantum mechanics. He formulated the mass-energy equation E  mc2; predicted nuclear fission; explained the nature of space and time in his special theory of relativity; explained gravity in his general theory of relativity, predict- ing that light would ‘bend’ near massive bodies.The year 1905 is often referred to as ‘annus mirabilis’, as Albert Einstein made equally revolutionary discoveries concerning the photoelectric effect, Brownian motion (helping to prove the existence of atoms) and the special theory of relativity (uniting the con- cepts of space and time) and produced science’s best-known equation, E  mc2 (showing that mass and energy are essentially the same thing). He intro- duced his general theory of relativity (which explains gravity) in 1916.He spent the last 30 years of his life work- ing on a theory that would explain all other forces and matter in one unified theory, now called ‘the Theory of Everything’. He failed to solve this problem, and 50 years later physicists are still in search of a cohesive theory. It is hoped that the large hadron collider (LHC) in CERN, which was launched in September 2008, will provide evidence for ‘the Theory of Everything’.7 1932 is also considered an ‘annus mirabilis’: it was the miraculous year of nuclear physics; the discoveries of that year will be dealt with in Chapter 23. In 1666, Isaac Newton had the origi- nal ‘annus mirabilis’, making revolu- tionary inventions and discoveries in calculus, motion, optics and gravita- tion that year.


THE NUCLEUS 409


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