What is the Quantum Physics ?

Quantum theory seems to say that light is both wavelike and particlelike. It seems to say that objects can be in two places at once, or even that cats can be both alive and dead, or neither alive nor dead, or—what? There can be no doubt that there is something troubling about quantum theory.

Quantum theory, more or less as it is known at the beginning of the twenty-first century, was developed during the first quarter of the twentieth century in response to several problems that had arisen with classical mechanics.

The first is the problem of blackbody radiation. A blackbody is any physical body that absorbs all incident radiation. As the blackbody continues to absorb radiation, its internal energy increases until, like a bucket full of water, it can hold no more and must re-emit radiation equal in energy to any additional incident radiation.

The problem is, most simply, that the classical prediction for the energy of the emitted radiation as a function of its frequency is wrong.

The problem was well known but unsolved until the German physicist Max Planck (1858–1947) proposed in 1900 the hypothesis that the energy absorbed and emitted by the blackbody could come only in discrete amounts, multiples of some constant, finite, amount of energy.

While Planck himself never felt satisfied with this hypothesis as more than a localized, phenomenological description of the behavior of blackbodies, others eventually accepted Planck's hypothesis as a revolution, a claim that energy itself can come in only discrete amounts, the quanta of quantum theory.

A second problem with classical mechanics was the challenge of describing the spectrum of hydrogen, and eventually, other elements. Atomic spectra are most easily understood in light of a fundamental formula linking the energy of light with its frequency: E = h ν, where E is the energy of light, h is a constant (Planck's constant, as it turns out), and ν is the frequency of the light (which determines the color of the visible light).

The Sun is hot, as the more astute of you will have noticed. It is hot because its enormous weight – about a billion billion billion tons – creates vast gravity, putting its core under colossal pressure. Just as a bicycle pump gets warm when you pump it, the pressure increases the temperature. Enormous pressure leads to enormous temperature.

Suppose, now, that the energy of some atom (for example, an atom of hydrogen) is increased. If the atom is subsequently allowed to relax, it releases the added energy in the form of (electromagnetic) radiation.

The relationship E = h ν reveals that the frequency of the light depends on the amount of energy that the atom emits as it relaxes. Prior to the development of quantum theory, the best classical theory of the atom was Ernest Rutherford's (1871–1937), according to which negatively charged electrons orbit a central positively charged nucleus

The energy of a hydrogen atom (which has only one electron) corresponds to the distance of the electron from the nucleus. (The further the electron is, the higher its energy is.) Rutherford's model predicts that the radiation emitted by a hydrogen atom could have any of a continuous set of possible energies, depending on the distance of its electron from the nucleus.

Hence a large number of hydrogen atoms with energies randomly distributed among them will emit light of many frequencies. However, in the nineteenth century it was well known that hydrogen emits only a few frequencies of visible light.

In 1913, Niels Bohr (1885–1962) introduced the hypothesis that the electrons in an atom can be only certain distances from the nucleus; that is, they can exist in only certain "orbits" around the nucleus

The differences in the energies of these orbits correspond to the possible energies of the radiation emitted by the atom

Quantum theory is the theoretical basis of modern physics that explains the nature and behavior of matter and energy on the atomic and subatomic level. The nature and behavior of matter and energy at that level is sometimes referred to as quantum physics and quantum mechanics.

When an electron with high energy "falls" to a lower orbit, it releases just the amount of energy that is the difference between the energies of the higher and lower orbits.

Because only certain orbits are possible, the atom can emit only certain frequencies of light.

Today, the most precise clocks in the world, atomic clocks, are able to use principles of quantum theory to measure time. They monitor the specific radiation frequency needed to make electrons jump between energy levels.

The crucial part of Bohr's proposal is that electrons cannot occupy the space between the orbits, so that when the electron passes from one orbit to another, it "jumps" between them without passing through the space in between.

Thus, Bohr's model violates the principle of classical mechanics that particles always follow continuous trajectories. In other words, Bohr's model left little doubt that classical mechanics had to be abandoned.

Over the next twelve years, the search was on for a replacement. By 1926, as the result of considerable experimental and theoretical work on the part of numerous physicists, two theories—experimentally equivalent—were introduced, namely, Werner Heisenberg's (1901–1976) matrix mechanics and Erwin Schrödinger's (1887–1961) wave mechanics.

Quantum theory grew from the ideas of Max Planck. He proposed that atoms could only emit (give out) energy in fixed units called quanta. The theory has been developed to explain the behavior of particles and the energy they emit.

Quantum theory grew from the ideas of Max Planck. He proposed that atoms could only emit (give out) energy in fixed units called quanta. The theory has been developed to explain the behavior of particles and the energy they emit.

Quantum theory is the theoretical basis of modern physics that explains the nature and behavior of matter and energy on the atomic and subatomic level. The nature and behavior of matter and energy at that level is sometimes referred to as quantum physics and quantum mechanics.

Planck wrote a mathematical equation involving a figure to represent these individual units of energy, which he called quanta.