Browse Definitions :

quantum theory

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. Organizations in several countries have devoted significant resources to the development of quantum computing, which uses quantum theory to drastically improve computing capabilities beyond what is possible using today's classical computers.

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:

This was last updated in October 2020

Continue Reading About quantum theory

Join the conversation


Send me notifications when other members comment.

Please create a username to comment.

Ummmm...... Thanks!!! I'm doing a report on this and it was really helpful...... But Einstein was never actually alive, was he? I am sorry but I think he was a fake guy:):)! Other than that, everything SEEMS right so YAAAYY for whoever made this!!!:):)
Einstein was as real as you or me!!!!
You have a good point both of you but in quantum theory there is a universe for both.
Please say this is a joke.
You are sooooo stupid! Albert Enstine was a real person!
Correction Ender360: Albert Einstein. Check yourself before talking about others. 
And AnonymousUser: yes, Einstein is real.. how do you think the theory of general relativity was thought of? And e= mc2? Google him. He's real, alright.
thx really helpful
The guy who thinks Einstein wasn't real is hilarious.

Was he the Robin Hood of the science world? xD
Does this mean that God can be everywhere?
Is interninternet because of quantum theory
now here is a question the quantum theory i get but the one thing that gets me is that when it says that it knows the behavior of every matter would this mean it would cancel out the butterfly effect. the reason is this sense the butterfly effect is the theory of two choices can do massive damage in the future. the thing also that stands out here is that the butter fly effect actually supports how that there is such a thing a multiverse that is separated be decision but then would this means that what ever we do here is the opposite of what we do in other multiverses or something else. plus the butterfly effect and the multiverse also support the multiple dimensions because the thing is i was thinking that there was such the thing, but if all this supported each other then would all three of these things prove the quantum theory wrong because then would it mean that there is and infinite versions of us alive and dead but then time travel would be possible because of the choices we make makes it to where all versions of us would die in totally different places and time so would that mean that everything but the quantum theory was true or backwards?
Quantum mechanics (QM -- also known as quantum physics, or quantum theory) is a branch of physics which deals with physical phenomena at nanoscopic scales where the action is on the order of the Planck constant. It departs from classical mechanics primarily at the quantum realm of atomic and subatomic length scales. Quantum mechanics provides a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. Quantum mechanics provides a substantially useful framework for many features of the modern periodic table of elements including the behavior of atoms during chemical bonding and has played a significant role in the development of many modern technologies.

In advanced topics of quantum mechanics, some of these behaviors are macroscopic (see macroscopic quantum phenomena) and emerge at only extreme (i.e., very low or very high) energies or temperatures (such as in the use of superconducting magnets). For example, the angular momentum of an electron bound to an atom or molecule is quantized. In contrast, the angular momentum of an unbound electron is not quantized. In the context of quantum mechanics, the wave--particle duality of energy and matter and the uncertainty principle provide a unified view of the behavior of photons, electrons, and other atomic-scale objects.

The mathematical formulations of quantum mechanics are abstract. A mathematical function, the wavefunction, provides information about the probability amplitude of position, momentum, and other physical properties of a particle. Mathematical manipulations of the wavefunction usually involve bra--ket notation which requires an understanding of complex numbers and linear functionals. The wavefunction formulation treats the particle as a quantum harmonic oscillator, and the mathematics is akin to that describing acoustic resonance. Many of the results of quantum mechanics are not easily visualized in terms of classical mechanics. For instance, in a quantum mechanical model the lowest energy state of a system, the ground state, is non-zero as opposed to a more "traditional" ground state with zero kinetic energy (all particles at rest). Instead of a traditional static, unchanging zero energy state, quantum mechanics allows for far more dynamic, chaotic possibilities, according to John Wheeler.

The earliest versions of quantum mechanics were formulated in the first decade of the 20th century. About this time, the atomic theory and the corpuscular theory of light (as updated by Einstein)[1] first came to be widely accepted as scientific fact; these latter theories can be viewed as quantum theories of matter and electromagnetic radiation, respectively. Early quantum theory was significantly reformulated in the mid-1920s by Werner Heisenberg, Max Born and Pascual Jordan, (matrix mechanics); Louis de Broglie and Erwin Schrödinger (wave mechanics); and Wolfgang Pauli and Satyendra Nath Bose (statistics of subatomic particles). Moreover, the Copenhagen interpretation of Niels Bohr became widely accepted. By 1930, quantum mechanics had been further unified and formalized by the work of David Hilbert, Paul Dirac and John von Neumann[2] with a greater emphasis placed on measurement in quantum mechanics, the statistical nature of our knowledge of reality, and philosophical speculation about the role of the observer. Quantum mechanics has since permeated throughout many aspects of 20th-century physics and other disciplines including quantum chemistry, quantum electronics, quantum optics, and quantum information science. Much 19th-century physics has been re-evaluated as the "classical limit" of quantum mechanics and its more advanced developments in terms of quantum field theory, string theory, and speculative quantum gravity theorie
Anonymous, I am engaging in a line of thought that suggests that everything we 'see' is a result of an interpretation. That QM is stuck inside it's own interpretation, which is deeply flawed, and thus results in these 'self-evidently anomalous' conclusions. Thoughts?
it too much for a man to comprehend but yet too simple to see. Lets just say that it is the Breath of God, in everything. Look at how far we come to identify all matter. Light is also a matter that has space and time, so as it is for dark. We will never identify or explain quantum physics unless we identify and truly understand ourselves as one. Only then will quantum physics be of no relevance than to live our life in harmony will all matter /creation.
According to Quantum Physics whatever is there in our Universe is nothing but rays of light, the different angles of rays make things visible. But in our universe is there not a single place where light is not there but matter is prevailing. We have not explored the whole universe yet! 
We probably never will explore the entire universe since the Doppler effect shows that every large body in the universe is moving away form each other at exponential speeds. 
Has quantum physics influenced modern day living? 
That was so helpful. Thank you to the person who wrote it.
if we got a vast knowledge about it we can travel in many dimensions
This is incredible and yes Brian this is right "I am not the one I was before watching it and understanding it" Woahh..
please, do not curse.
hey guys i had a theory according to einstein all light travels at the same speed so what if electricity was as fast as light, and i have proof, lightning has a flash of light followed up by thunder , i know i know this is where you are like where is your evidence but think about it if light is followed up by sound and lightning is electric then are the people who are shocked by lightning shocked first or at the same time as they see light i would love to do some reserch on that so help me out
hi I dont know but when you watch lightning in slow motion it is not a ball or pulse of light that travels but a stream so you may not see it when it hits you but you will have seen it before it stops. i think. also even though the stream is defined by light not all electricity is visible. our eyes which have to have the gift of sight cannot see all the different light waves there are, but i do have a question if all light travels at the same speed why are the waves in the wave lengths different? is that not determined by the speed in which they travel?  that's the problem with theory's they will always have questions and those who are paid to answer them will always have work. 
The Qauntum Theory is  Like =

"Where was you going, when I saw you coming back"

                      Pip.  (Phillip Daniels)
Not only was Albert Einstein known for the theory of relativity and the equation E=mc^2, he is also known for the development of the Atomic bombs, Little Boy and Fat Man which were dropped on Japan Hiroshima and Nagasaki on August 6th and 9th, 1945 during World War II.
Quantum Computing opens a whole new universe of possibilities. In Probability outcomes, an either/or option means several universes can exist. It is only when we identify a true algorithm for an outcome that the alternative path becomes false. Truth is one!
over my life i have had instances that i have seen events  days and weeks ahead.....tsuani being an example.....i am starting to wonder during these happenings that i may cross over into a para world and not be at 75 i am starting to investigate this closer

I am happy to see someone is taking the time to "speak the language" and translate for the multitudes before its too late - but it already may be judging by the 2014 comments.  But, well its a new decade and life goes on, so keep up the language translations at least so the level of knowledge doesn't slip below the 8th grade level of communication.  By now 2020, is there a group of members under your following that pledge to support the Einstein theory of quantum mechanics and would allow me to join, only being at best a "shade tree" quantum mechanic with real time life experience in the field of nature?
What do you mean by "shade tree" quantum mechanic?  
Loves Physics and to have a more clear understanding of Quantum Physics.
Because unknown mysteries of universe could only be discovered with the help of Quantum Mechanics.

In Devs on Hulu, a quantum computer is used to setlle the determinism vs free will debate.  New episodes every week.

Quantum computing is central in Origin, a Dan Brown novel.

Want more? Give Singularity by Bill DeSmedt a try. The cataclysmic Tunguska explosion of 1908 destroyed an estimated 770 square miles of forest cover in the sparsely populated East Siberian taiga region.  For decades, its cause has remained unsettled. Was it a comet, or was it an asteroid? Neither explanation holds up to scrutiny. In this novel, a 3rd theory is explored--a quantum blackhole passed through Earth and leveled the forest.


So I clicked on the question of "Why are you interested in quantum computing?" while perusing quantum theory after reading about tetraquarks this morning. Actually I wasn't interested at all yet I wanted to see where the conversations would go. Obviously no one actually answered the question as far as I can see. Hmm Must be a theory there as to why not?
because im majoring in science chemistry which the quantum theory is in my course,after reading and doing some research in seems like I have some interest in this topic.tq for this clear explanation.its so detailed
Because it is likely to significantly improve AI.


  • risk assessment

    Risk assessment is the identification of hazards that could negatively impact an organization's ability to conduct business.

  • PCI DSS (Payment Card Industry Data Security Standard)

    The Payment Card Industry Data Security Standard (PCI DSS) is a widely accepted set of policies and procedures intended to ...

  • risk management

    Risk management is the process of identifying, assessing and controlling threats to an organization's capital and earnings.




  • call tree

    A call tree is a layered hierarchical communication model that is used to notify specific individuals of an event and coordinate ...

  • Disaster Recovery as a Service (DRaaS)

    Disaster recovery as a service (DRaaS) is the replication and hosting of physical or virtual servers by a third party to provide ...

  • cloud disaster recovery (cloud DR)

    Cloud disaster recovery (cloud DR) is a combination of strategies and services intended to back up data, applications and other ...