Title: Quantum mechanics Content: Quantum physics explains how the universe works at a scale smaller than atoms. It is also called quantum mechanics or quantum theory. Mechanics is the part of physics that explains how things move and quantum is the Latin word for 'how much'. A quantum of energy is the least amount possible (or the least extra amount), and quantum mechanics describes how that energy moves or interacts. Atoms were long considered the smallest pieces of matter, but modern science has shown that there are even smaller subatomic particles, like protons, neutrons and electrons. Quantum mechanics describes how the particles that make up atoms work. Quantum physics also tells us how electromagnetic waves (like light) work. Wave–particle duality means that particles behave like waves and waves behave like particles. (They are not two kinds of things, they are something like both: this is their duality.) Much of modern physics and chemistry can be described and understood using the mathematical rules of quantum mechanics. The mathematics used to study subatomic particles and electromagnetic waves is very complex because they act in very strange ways. thumb|250px|The wavelength of a wave of light == Waves and photons == Photons are particles that are point-sized, tinier than atoms. Photons are like "packets" or packages of energy. Light sources such as candles or lasers produce light in bits called photons. The more photons a lamp produces, the brighter the light. Light is a form of energy that behaves like the waves in water or radio waves. The distance between the top of one wave and the top of the next wave is called a 'wavelength'. Each photon carries a certain amount, or 'quantum', of energy depending on its wavelength. thumb|300px|Black at left is ultraviolet (high frequency); black at right is infrared (low frequency). A light's color depends on its wavelength. The color violet (the bottom or innermost color of the rainbow) has a wavelength of about 400 nm ("nanometers") which is 0.00004 centimeters or 0.000016 inches. Photons with wavelengths of 10-400 nm are called ultraviolet (or UV) light. Such light cannot be seen by the human eye. On the other end of the spectrum, red light is about 700 nm. Infrared light is about 700 nm to 300,000 nm. Human eyes are not sensitive to infrared light either. Wavelengths are not always so small. Radio waves have longer wavelengths. The wavelengths for an FM radio can be several meters in length (for example, stations transmitting on 99.5 FM are emitting radio energy with a wavelength of about 3 meters, which is about 10 feet). Each photon has a certain amount of energy related to its wavelength. The shorter the wavelength of a photon, the greater its energy. For example, an ultraviolet photon has more energy than an infrared photon. thumb|250px|Pictorial description of frequency Wavelength and frequency (the number of times the wave crests per second) are inversely proportional, which means a longer wavelength will have a lower frequency, and vice versa. If the color of the light is infrared (lower in frequency than red light), each photon can heat up what it hits. So, if a strong infrared lamp (a heat lamp) is pointed at a person, that person will feel warm, or even hot, because of the energy stored in the many photons. The surface of the infrared lamp may even get hot enough to burn someone who may touch it. Humans cannot see infrared light, but we can feel the radiation in the form of heat. For example, a person walking by a brick building that has been heated by the sun will feel heat from the building without having to touch it. The mathematical equations of quantum mechanics are abstract, which means it is impossible to know the exact physical properties of a particle (like its position or momentum) for sure. Instead, a mathematical function called the wavefunction provides information about the probability with which a particle has a given property. For example, the wavefunction can tell you what the probability is that a particle can be found in a certain location, but it can't tell you where it is for sure. Because of this uncertainty and other factors, you cannot use classical mechanics (the physics that describe how large objects move) to predict the motion of quantum particles. Ultraviolet light is higher in frequency than violet light, such that it is not even in the visible light range. Each photon in the ultraviolet range has a lot of energy, enough to hurt skin cells and cause a sunburn. In fact, most forms of sunburn are not caused by heat; they are caused by the high energy of the sun's UV rays damaging your skin cells. Even higher frequencies of light (or electromagnetic radiation) can penetrate deeper into the body and cause even more damage. X-rays have so much energy that they can go deep into the human body and kill cells. Humans cannot see or feel ultraviolet light or x-rays. They may only know they have been under such high frequency light when they get a radiation burn. Areas where it is important to kill germs often use ultraviolet lamps to destroy bacteria, fungi, etc. X-rays are sometimes used to kill cancer cells. Quantum mechanics started when it was discovered that if a particle has a certain frequency, it must also have a certain amount of energy. Energy is proportional to frequency (E ∝ f). The higher the frequency, the more energy a photon has, and the more damage it can do. Quantum mechanics later grew to explain the internal structure of atoms. Quantum mechanics also explains the way that a photon can interfere with itself, and many other things never imagined in classical physics. == Quantization == Max Planck discovered the relationship between frequency and energy. Nobody before had ever guessed that frequency is directly proportional to energy (this means that as one of them doubles, the other does, too). Under what are called natural units, then the number representing the frequency of a photon would also represent its energy. The equation would then be: :E = f meaning energy equals frequency. But the way physics grew, there was no natural connection between the units that were used to measure energy and the units commonly used to measure time (and therefore frequency). So the formula that Planck worked out to make the numbers all come out right was: :E = h \times f or, energy equals h times frequency. This h is a number called Planck's constant after its discoverer. Quantum mechanics is based on the knowledge that a photon of a certain frequency means a photon of a certain amount of energy. Besides that relationship, a specific kind of atom can only give off certain frequencies of radiation, so it can also only give off photons that have certain amounts of energy. 150px|thumb|Double-slit experiment: light goes from the light source at left to fringes (marked in the black edge) at the right. 150px|thumb|Photoelectric effect: photons hit metal and electrons are pushed away. == History == Isaac Newton thought that light was made of very small things that we would now call particles (he referred to them as "Corpuscles"). Christiaan Huygens thought that light was made of waves. Scientists thought that a thing cannot be a particle and a wave at the same time. Scientists did experiments to find out whether light was made of particles or waves. They found out that both ideas were right — light was somehow both waves and particles. The Double-slit experiment performed by Thomas Young showed that light must act like a wave. The Photoelectric effect discovered by Albert Einstein proved that light had to act like particles that carried specific amounts of energy, and that the energies were linked to their frequencies. This experimental result is called the "wave-particle duality" in quantum mechanics. Later, physicists found out that everything behaves both like a wave and like a particle, not just light. However, this effect is much smaller in large objects. Here are some of the people who discovered the basic parts of quantum mechanics: Max Planck, Albert Einstein, Satyendra Nath Bose, Niels Bohr, Louis de Broglie, Max Born, Paul Dirac, Werner Heisenberg, Wolfgang Pauli, Erwin Schrödinger, John von Neumann, and Richard Feynman. They did their work in the first half of the 20th century. == Beyond Planck == centre|thumb|Visible light given off by glowing hydrogen. (Wavelengths in nanometers.)|358x358px Quantum mechanics formulae and ideas were made to explain the light that comes from glowing hydrogen. The quantum theory of the atom also had to explain why the electron stays in its orbit, which other ideas were not able to explain. It followed from the older ideas that the electron would have to fall in to the center of the atom because it starts out being kept in orbit by its own energy, but it would quickly lose its energy as it revolves in its orbit. (This is because electrons and other charged particles were known to emit light and lose energy when they changed speed or turned.) Hydrogen lamps work like neon lamps, but neon lamps have their own unique group of colors (and frequencies) of light. Scientists learned that they could identify all elements by the light colors they produce. They just could not figure out how the frequencies were determined. Then, a Swiss mathematician named Johann Balmer figured out an equation that told what λ (lambda, for wave length) would be: :\lambda = B\left(\frac \;\; x = 0,\; x = L Consider x = 0 * sin 0 = 0, cos 0 = 1. To satisfy \psi = 0 \; the cos term has to be removed. Hence D = 0 Now consider: \psi = C \sin kx\; * at x = L, \psi = C \sin kL =0\; * If C = 0 then \psi =0 \; for all x. This solution is not useful. * therefore \sin kL = 0 must be true, giving us :: kL = n \pi \;\;\;\; n = 1,2,3,4,5,... \; We can see that n must be an integer. This means that the particle can only have special energy values and cannot have the energy values in between. This is an example of energy "quantization." == Related pages == * Electromagnetic wave * Electron * Photon * Quantum entanglement * Quantum computer * Quantum state * Schrödinger equation