The conservation laws show that matter and energy can be neither produced from nothing nor reduced to nothing, whether it be through a physical reaction (such as a collision), a chemical reaction (such as combustion) or a nuclear reaction (such as an atomic explosion). The laws govern the actions of momentum, mass, energy, and subatomic particles. All of the conservation laws are based upon hundreds of years of empirical evidence and have as their authors some of the most respected scientists of all time.
The first conservation law to gain universal acceptance was the Law of Conservation of Momentum, which relates to the product of an object's mass and its velocity. This law was proposed by the English mathematician John Wallis in 1668, and was later clarified by Christiaan Huygens. Isaac Newton had already shown in his first law of motion that the momentum of a single body remains unchanged as long as no other force acts upon it. What Wallis did was apply this law to systems containing more than one body as well; he did so by making momentum a vector quantity, with both a magnitude and a direction.
Using vector quantities, the momenta of all bodies within a system can be added together to get the total momentum; while the momenta of the individual bodies may fluctuate, the total momentum remains constant. For example, when two balls roll toward each other at an angle, each has a mass, velocity, and a direction of travel. They collide. After the collision they have the same mass but new velocities and directions. Invariably, the sum of the new velocities and directions will equal the sum of the velocities and directions at the beginning--never more, never less.
Several years after Wallis's announcement, Huygens showed that the total kinetic energy of a system is also conserved in an elastic collision. While related closely to momentum, Huygens' discovery would serve as a stronger proof for the law of conservation of energy almost 150 years later.
The next conservation law to be introduced was the Law of the Conservation of Matter. Its author was the French chemist Antoine-Laurent Lavoisier, who developed the law in an attempt to explain the apparent destruction of mass in chemical reactions. Scientists had observed for years that following certain reactions, particularly burning, less matter appeared to be present than at the beginning.
Lavoisier suggested that the matter was merely transformed during the reaction--probably into gas and particulates. He proved his theory by utilizing a sealed glass container for his experiments. He weighed the container before and after the reaction. Afterwards, even though it looked like there was less material inside, the container weighed the same as it did before the reaction. With this as support, the law of conservation of matter the guiding principle of chemistry during the nineteenth century.
The conservation law that saw the greatest opposition was the Law of the Conservation of Energy. It was reluctantly received by the scientific community in 1847 only after the independent research of Joseph Black, Julius Mayer, William Thomson (Lord Kelvin), James Joule and Hermann von Helmholtz.
The Law of the Conservation of Energy states that the energy at the beginning of a reaction must equal the energy at the end. During that time it can be transformed into a number of forms including heat, electricity, light, sound, kinetic energy, chemical energy and nuclear energy, among others.
Because it was difficult for early scientists to detect (let alone measure) many of these forms of energy it was easier for them to assume that a certain amount of energy was "lost" during a reaction--vanished into thin air--than it was to visualize an unknown energy form.
Ultimately it was Helmholtz who proclaimed that all energy within the human body came from food energy, which itself came from the Sun. This model was extrapolated to include all of the energy in the universe changing from one form into another indefinitely. The law of conservation of energy thus governs all reactions--physical, chemical, and nuclear.
The last great conservation law was proven in the 1920s during the revolution of quantum physics. It was noticed by the German-American physicist Albert Einstein that mass and energy were interrelated--in other words, mass could become energy, and vice versa. He discussed this relationship in his theory of relativity and expressed it using the equation E=mc2, wherein "E" represents energy, "m" represents matter, and "c" represents the speed of light. Thus, according to Einstein, the amount of energy within a substance is equivalent to that substance's mass multiplied by the speed of light squared (3.46 x 10 to the tenth power miles/second).
Because Einstein showed that mass was simply another form of energy, the law of conservation of energy also subsumes the second conservation law (that of matter), while the theory of relativity, sometimes called the law of conservation of mass-energy, governs them both.
The most recent additions to the conservation laws have been used to predict the viability of particle decay reactions. In these reactions subatomic particles (such as electrons, baryons, muons, and leptons) are created or liberated; these particles can be positive or negative "anti-" particles. According to the conservation laws, the particle number (the sum of all the pluses and minuses) must be the same before the decay reaction as after. Since these subatomic particles are combinations of mass and energy, the particle conservation laws are simply specialized versions of those more general laws.
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