What was needed to make E mc2

What does E = m c2 mean?

Einstein discovered in 1905 that energy E and mass m are two sides of the same coin because they are equivalent over the square of the speed of light c. A few months later he added a three-page addendum to his article “On the electrodynamics of moving bodies”, which was published in the Annalen der Physik and which founded the special theory of relativity, which was only named in 1915, the headline of which he cautiously formulated as a question: “Is inertia one Body depends on its energy content? ”In this, Einstein showed that an object that radiates energy also loses mass. E = mc2 - he used this notation later - only indicates the rest mass of the body. If it moves with momentum p, the equation is: E2 = (mc2) 2 + (pc) 2. (By the way, the c stands for “constant” - or for “celeritas”, Latin for “speed”). The astonishing consequence: mass is nothing more than a certain form of energy.


Einstein did not come to this result through experimental data, but through a mathematical derivation. “The thought is funny and captivating; but whether the Lord is laughing about it and fooling me, I cannot know ”, he wrote in autumn 1905 to his friend Conrad Habicht. Einstein hoped, however, that the validity of the formula could be tested when measuring radioactive decay. "It cannot be ruled out that a test of the theory will succeed in bodies whose energy content is highly variable (e.g. with the radium salts)," he wrote at the end of his article.

Confirmation came in 1932 when John Cockcroft and Ernest Walton at the Cavendish Laboratory in Cambridge used the world's first particle accelerator to shoot protons at lithium atoms, creating alpha particles. The balance was only correct if, in addition to the starting and end product masses, the energy was also included. Shortly afterwards, Irène and Frédéric Joliot-Curie observed in Paris that particles can arise from high-energy radiation. So Einstein was right: energy and mass can transform into one another and are not essentially different at all.

This also makes it understandable that a body in motion has energy - and cannot be accelerated to the speed of light, because this would require an infinite amount of energy and it would be infinitely heavy. The mass of an aircraft that flies at almost 1000 kilometers per hour is, for example, 0.0000000001 percent greater than when standing at the gate. But bodies at rest also contain energy. The mass of a brick weighing one kilogram, for example, could theoretically supply a 100 watt lightbulb with electricity for 30 million years. However, this energy can never be extracted in practice.

That E = mc² still has a very real meaning - this is also a confirmation of the special theory of relativity - became evident at the latest in 1945 with the detonation of the first atomic bombs. And nuclear power plants demonstrate it every day: The fission of heavy atomic nuclei releases large amounts of energy. In the case of the bombs that killed over 100,000 people in Hiroshima and Nagasaki, only about one gram of uranium or plutonium was used. The reverse process, the merging of light atomic nuclei, is also an enormous source of energy. This was first used destructively in the form of the hydrogen bomb in 1952, but has not yet been implemented in a constructive manner in the form of nuclear fusion reactors.

Nature is still there: our sun has been shining for 4.6 billion years due to the fusion of hydrogen into helium. 1038 nuclear fusion processes take place in its 15.7 million degree hot center every second. In the process, 500 million tons of hydrogen are converted - and around 4 million tons of this are converted into energy, 0.7 percent of the total mass involved. That would meet mankind's current energy needs for a million years. Within 45 million years the sun will be “lightened” by the mass of the earth due to nuclear fusion and E = mc². Of this wasteful annihilation - the luminosity of the sun is 3.8 · 1026 watts - on average only 1367 joules of energy per second and square meter arrive on earth - but that is enough to drive all life processes here. In this respect, even our existence cannot be understood without the theory of relativity. ■

January 18, 2011

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