An Introduction to Quantum Physics

In the late nineteenth and early twentieth centuries, physicists were forced to look beyond Newtonian mechanics for a more general theory. Quantum theory arose from observations and experiments which could not be explained by an application of classical physics. Basically, quantum physics describes phenomenon that classical physics cannot: Quantization of certain physical quantities, the uncertainty principle, wave-particle duality, and quantum entanglement. These concepts, along with a few others, are defined very briefly below.

This information is not intended to be complete or comprehensive, but to provide the non-physicist with an easy-to-understand introduction to a few of the fundamental concepts of quantum theory. At the end of this post are links to websites which offer more detailed discussions and explanations of these concepts.

Basic Concepts and Definitions

  • Quantum State

A quantum state is the condition in which a quantum system exists, represented by mathematical object that describes the quantum system.

  • Observable

An observable is a property of a system state that can be determined by some sequence of physical operations. Examples of observables include energy, position, momentum, and angular momentum.

  • Measurement

Measurement of a quantum state is generally described by a probability distribution, determined by the quantum state and the observable describing the measurement.

  • Quantization

Quantization is a procedure for constructing a quantum physics theory from a classical physics theory foundation by restricting a variable quantity to discrete values rather than to a continuous set of values. However, quantum physics does not assign definite values to system observables, but rather makes predictions about obtaining each of the possible outcomes from measuring those observables.

  • Wave Function

A wave function is a quantum theory equation that mathematically describes the probability density of an object in space and time. It is used to describe the propagation of the wave associated with a particle or group of particles.

  • Superposition

Superposition is the phenomenon in which a quantum system exists in all possible states simultaneously, for as long as it remains unobserved.

  • Entanglement

Entanglement describes a quantum state that is not entirely independent of other states, whether or not the individual objects are spatially separated. As a result, measurements performed on one system seem to instantaneously influence the other system(s) entangled with it, so that none of the entangled states can be considered to be isolated from one another.

  • Nonlocality

Nonlocality describes the fact that objects created out of some initially mixed (or common) state will remain correlated and instantaneously “communicate” with each other even when separated by large distances. Nonlocality postulates a principle of holistic interconnectedness operating at the quantum level, contradicting the localistic assumptions of classical, Newtonian physics.

  • Decoherence

Decoherence is the mechanism by which the apparent collapse of superposition (all possible states) into a single definite state occurs, via entanglement.

  • Eigenstate

An eigenstate is one of the many possible states which may exist in a quantum system prior to decoherence. When the system is observed, the quantum state appears to “jump” to a particular eigenstate, and that eigenstate is the one which is perceived.

  • The Uncertainty Principle (Heisenberg)

The Uncertainty Principle states that both the position and the momentum of a particle cannot be simultaneously and precisely measured. The more precisely the position (or momentum) of a particle is measured, the less precisely one can measure what its momentum (or position) might be. This principle has profound implications for both the classical concept of cause-and-effect and the determinacy of past and future events.

  • Wave-Particle Duality

A particle is an irreducible constituent of matter in space/time. It can exhibit properties including mass, electric charge, and magnetic moment which determine how it interacts in the universe. A particle travels along a linear path.

A wave is a disturbance in spacetime, which can transfer energy from one point to another. Unlike a particle, a wave can travel through a vacuum (without a medium). Instead of simply following a linear path like a particle, a wave spreads out as it travels.

In quantum physics, wave–particle duality consolidates the particle-based theories of classical physics with the observed behavior of light (apparently dualistic). It refers to the concept that all matter exhibits both wave-like and particle-like properties.

Challenges to wave-particle duality include theories on particle scattering, “metaparticles,” and WSM (Wave Structure of Matter) theory.

  • Complementarity

Complementarity is the concept that the underlying properties of entities may manifest themselves in contradictory forms at different times, depending on the conditions of observation. It states that a quantum object can either behave as a particle or as wave, but never simultaneously as both; that a stronger manifestation of the particle nature (wave nature) leads to a weaker manifestation of the wave nature (particle nature).

  • The Schrodinger Equation

The Schrödinger equation is the fundamental equation of physics for describing quantum mechanical behavior. It is used to find the allowed energy levels of a quantum system and enables us to analytically and precisely predict the behavior of a wave function. There is a time-dependent form of this equation (used for describing progressive waves as applicable to the motion of free particles), as well as the time-independent form of this equation (used for describing standing waves).

The equation has central importance to quantum mechanics similar to that of Hamilton’s equations of motion in classical mechanics.

Next: Fundamental Forces >>


Getting Started with Quantum (Philip Carr)

Introduction to Quantum Theory (Quantiki)

Quantum theory: Weird and Wonderful (Physics World)

Quantum Physics Made Relatively Simple (Hans Bethe Video Lectures)

The Quantum Exchange (Tutorials and Open Source Software)

14 Responses to “An Introduction to Quantum Physics”

  1. Hi Kelly (the observant one)

    Your pages are great – congratulations.

    As you would know, I work with the Wave Structure of Matter. And after ten years of thinking from this foundation it just seems so obvious to me that all problems of modern physics relate to the foundations first established by Newton. Of an absolute Space and Time to which he added discrete mass particles and instantly acting gravity forces, where F=m.a

    The error was to put space in the background and work from the foundation of many things (matter particles) so then we have to add more things (forces / fields) to connect the particles in space and time.
    Einstein was the closest to solving this problem by rejecting the particle and describing reality in terms of continuous fields in space-time.
    However, his solution does not work as it does not explain discrete aspects of reality found in quantum theory.

    The most simple solution, and the one that I am convinced is correct, is to describe reality in terms of space alone, and that matter is formed from spherical standing waves in Space.

    We then can understand how matter is connected to other matter (by the spherical in and out waves / Einstein’s locality) and also understand the non-local aspects of QT / EPR, as with relative motion of two spherical standing waves you get Doppler shifts that form a phase wave (the de Broglie wave) which has a very high velocity of c^2 / relative velocity. And in the same wave equations you get the relativistic mass increase where E=hf.

    The particle wave duality is simply explained.
    The wave center of the spherical standing wave forms the particle.
    Light is due to resonant coupling where the standing wave interactions are discrete (eigenstates). This gives the effect of discrete photons.

    I think that physics is moving in this direction, to better understand that matter is a structure of the universe, the discrete ‘particle’ like world that we see is an illusion of our limited senses. (and this is important as it affects our view of the world, how we treat things like Nature!)

    Hope this helps a bit.
    Cosmic Cheers,
    Geoff Haselhurst

  2. Thank you Geoff.
    I appreciate your insights, and I am intrigued by WSM.


  3. There’s a book whose title presently escapes me written by a semi-retired University of Montréal faculty member (whose name also escapes me) that makes a very similar argument. I’m almost positive the guy’s name was Phillip somebody-or-other, not that it helps much. I’m sure it will come to me in the middle of the night when I’m nowhere near a computer…

  4. I think this new way to “Observe” all the Universe we have is .. I really don t find the right words to describe the possibilities we have.. Anyway, Hi! I am Sandra from Italy, I am looking for informations about the observer effect and more about it.
    thanks a lot for this website! I like it!

  5. Thank you for all of this insight into the world of Quantum Physics. It was a great introduction and I intend to continue furthering my knowledge on this topic. I was wondering if you possibly had any sources to gain information on The Holographic Universe. Thank you agin, Christopher Irwin

  6. im new to this but so far from what i understand reality only exsists in our mind ever thing we expeirience is nothing more than perceptions of the mind for example me typing this or what seems to me typing is nothing more than an illusion thats going on in the external theres no blue sky or tables just energy our eyes capture the reflectled light off objects send them via electrical impulses and our brain creates a mental pictureis this true

    • That’s a good question. We have to start by first acknowledging that everything we know about everything is a result of our sensory data and subsequent interpretations of that data. 😉

  7. what if we were able to see the external world without our eyes or brains how would it look?

  8. the difference between a live body and a dead body is sensation which guides perception which guides beliefs and influence our health and evolution(overall biology). the power to sense is the power to evolve. its a gift that a spirit without a body does not have, feeling. we must take in to account thought, emotions and the feelings that derive from them. stress weakens the immune system, slows down growth and even effect brain function. thats why consciousness can heal, it can contribute to the positive thoughts emotions and feelings that battle stress and influence our health.

  9. ok ive read some things about quantum theory and the double slit experiment and can someone clear this up for me? Is everything a wave function until we look at it?

  10. “A wise man is truly wise when he accepts that he knows nothing.”
    Samuel Aviles

  11. I almost never drop comments, however i did a
    few searching and wound up here An Introduction to Quantum Physics | the
    observer effect. And I actually do have a couple of questions for you if it’s allright.
    Could it be just me or does it look like a few of
    these remarks appear as if they are written
    by brain dead visitors? 😛 And, if you are posting at additional sites, I’d like to follow everything new you have to post.
    Would you make a list of every one of all your community sites like
    your linkedin profile, Facebook page or twitter feed?

    • I am not posting on additional websites, but if and when I do, I’ll drop any links here. Thanks! 🙂

  12. I propose that like string theory the vibration of strings that particles are formed by vibrating waves, the quantum particles get their function from the frequency the wave vibrates, this gives theory to vibrations in waves similar to string theory yet proven in quantum physics waves exist, yet the strings do not, this would allow for waves to be the make up of the universe. The shock wave from the big bang may have created waves that are tiny and appear to be subatomic particles when in fact they are tiny waves that are the after effect of the big bang.

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