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.

In 1900, physicist Max Planck presented his quantum theory to the German Physical Society.
Planck had sought to discover the reason that radiation from a glowing body
changes in color from red, to orange, and, finally, to blue as its temperature rises. He found that
by making the assumption that energy existed in individual units in the same way that matter does,
rather than just as a constant electromagnetic
wave - as had been formerly assumed - and was therefore *quantifiable*, he could find the
answer to his question. The existence of these units became the first assumption of quantum
theory.

Planck wrote a mathematical equation involving a figure to represent these individual units of
energy, which he called *quanta*. The equation explained the
phenomenon very well; Planck found that at certain discrete temperature levels (exact multiples of
a basic minimum value), energy from a
glowing body will occupy different areas of the color spectrum. Planck assumed there was a theory
yet to emerge from the discovery of quanta, but, in fact, their very existence implied a completely
new and fundamental understanding of the laws of nature. Planck won the Nobel Prize in Physics for
his theory in 1918, but developments by various scientists over a thirty-year period all
contributed to the modern understanding of quantum theory.

#### The Development of Quantum Theory

- In 1900, Planck made the assumption that energy was made of individual units, or quanta.
- In 1905, Albert Einstein theorized that not just the energy, but the radiation itself was
*quantized*in the same manner. - In 1924, Louis de Broglie proposed that there is no fundamental difference in the makeup and
behavior of energy and matter; on the atomic and subatomic level either may behave as if made of
either particles or waves. This theory became known as the
*principle of wave-particle duality*: elementary particles of both energy and matter behave, depending on the conditions, like either particles or waves. - In 1927, Werner Heisenberg proposed that precise, simultaneous measurement of two complementary values - such as the position and momentum of a subatomic particle - is impossible. Contrary to the principles of classical physics, their simultaneous measurement is inescapably flawed; the more precisely one value is measured, the more flawed will be the measurement of the other value. This theory became known as the uncertainty principle, which prompted Albert Einstein's famous comment, "God does not play dice."

#### The Copenhagen Interpretation and the Many-Worlds Theory

The two major interpretations of quantum theory's implications for the nature of reality are the Copenhagen interpretation and the many-worlds theory. Niels Bohr proposed the Copenhagen interpretation of quantum theory, which asserts that a particle is whatever it is measured to be (for example, a wave or a particle), but that it cannot be assumed to have specific properties, or even to exist, until it is measured. In short, Bohr was saying that objective reality does not exist. This translates to a principle called superposition that claims that while we do not know what the state of any object is, it is actually in all possible states simultaneously, as long as we don't look to check.

To illustrate this theory, we can use the famous and somewhat cruel analogy of Schrodinger's Cat. First, we have a living cat and place it in a thick lead box. At this stage, there is no question that the cat is alive. We then throw in a vial of cyanide and seal the box. We do not know if the cat is alive or if the cyanide capsule has broken and the cat has died. Since we do not know, the cat is both dead and alive, according to quantum law - in a superposition of states. It is only when we break open the box and see what condition the cat is that the superposition is lost, and the cat must be either alive or dead.

The second interpretation of quantum theory is the *many-worlds* (or *multiverse*
theory. It holds that as soon as a potential exists for any object to be in any state, the universe
of that object transmutes into a series of parallel universes equal to the number of possible
states in which that the object can exist, with each universe containing a unique single possible
state of that object. Furthermore, there is a mechanism for interaction between these universes
that somehow permits all states to be accessible in some way and for all possible states to be
affected in some manner. Stephen Hawking and the late Richard Feynman are among the scientists who
have expressed a preference for the many-worlds theory.

#### Quantum Theory's Influence

Although scientists throughout the past century have balked at the implications of quantum theory - Planck and Einstein among them - the theory's principles have repeatedly been supported by experimentation, even when the scientists were trying to disprove them. Quantum theory and Einstein's theory of relativity form the basis for modern physics. The principles of quantum physics are being applied in an increasing number of areas, including quantum optics, quantum chemistry, quantum computing, and quantum cryptography.

See Brian Greene's introduction to quantum theory on Nova:

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