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Photons & Wave-Particle Duality

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Rebecca
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« on: June 22, 2007, 01:44:04 am »



Photons emitted in a coherent beam from a laser


In modern physics the photon is the elementary particle responsible for electromagnetic phenomena. It is the carrier of electromagnetic radiation of all wavelengths, including gamma rays, X-rays, ultraviolet light, visible light, infrared light, microwaves, and radio waves. The photon differs from many other elementary particles, such as the electron and the quark, in that it has zero mass[3]; therefore, it travels (in vacuum) at the speed of light, c. Like all quanta, the photon has both wave and particle properties (“wave–particle duality”). As a wave, a single photon is distributed over space and shows wave-like phenomena, such as refraction by a lens and destructive interference when reflected waves cancel each other out; however, as a particle, it can only interact with matter by transferring the amount of energy




where h is Planck's constant, c is the speed of light, and λ is its wavelength. For visible light the energy carried by a single photon would be around a tiny  joules; this energy is just sufficient to excite a single molecule in a photoreceptor cell of an eye,[citation needed] thus contributing to vision.

Apart from energy a photon also carries momentum and has a polarization. It follows the laws of quantum mechanics, which means that often these properties do not have a well-defined value for a given photon. Rather, they are defined as a probability to measure a certain polarization, position, or momentum. For example, although a photon can excite a single molecule, it is often impossible to predict beforehand which molecule will be excited.

The above description of a photon as a carrier of electromagnetic radiation is commonly used by physicists. However, in theoretical physics, a photon can be considered as a mediator for any type of electromagnetic interactions, including magnetic fields and electrostatic repulsion between like charges.

The modern concept of the photon was developed gradually (1905–17) by Albert Einstein[4][5][6][7] to explain experimental observations that did not fit the classical wave model of light. In particular, the photon model accounted for the frequency dependence of light's energy, and explained the ability of matter and radiation to be in thermal equilibrium. Other physicists sought to explain these anomalous observations by semiclassical models, in which light is still described by Maxwell's equations, but the material objects that emit and absorb light are quantized. Although these semiclassical models contributed to the development of quantum mechanics, further experiments proved Einstein's hypothesis that light itself is quantized; the quanta of light are photons.

The photon concept has led to momentous advances in experimental and theoretical physics, such as lasers, Bose–Einstein condensation, quantum field theory, and the probabilistic interpretation of quantum mechanics. According to the Standard Model of particle physics, photons are responsible for producing all electric and magnetic fields, and are themselves the product of requiring that physical laws have a certain symmetry at every point in spacetime. The intrinsic properties of photons — such as charge, mass and spin — are determined by the properties of this gauge symmetry.

The concept of photons is applied to many areas such as photochemistry, high-resolution microscopy, and measurements of molecular distances. Recently, photons have been studied as elements of quantum computers and for sophisticated applications in optical communication such as quantum cryptography


http://en.wikipedia.org/wiki/Photons


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Rebecca
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« Reply #1 on: June 22, 2007, 01:45:07 am »

Nomenclature

The photon was originally called a “light quantum” (das Lichtquant) by Albert Einstein.[4] The modern name “photon” derives from the Greek word for light, φῶς, (transliterated phos), and was coined in 1926 by the physical chemist Gilbert N. Lewis, who published a speculative theory[8] in which photons were “uncreatable and indestructible”. Although Lewis' theory was never accepted — being contradicted by many experiments — his new name, photon, was adopted immediately by most physicists.

In physics, a photon is usually denoted by the symbol , the Greek letter gamma. This symbol for the photon probably derives from gamma rays, which were discovered and named in 1900 by Villard[9][10] and shown to be a form of electromagnetic radiation in 1914 by Rutherford and Andrade.[11] In chemistry and optical engineering, photons are usually symbolized by , the energy of a photon, where  is Planck's constant and the Greek letter  (nu) is the photon's frequency. Much less commonly, the photon can be symbolized by hf, where its frequency is denoted by f.

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HKurtRichter
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« Reply #2 on: June 23, 2007, 10:00:40 am »

Good job of explaining the dual nature of photons, but the historical information you gave could be debated here.  For instance, according to The Encyclopedia of Physics by Lerner and Trigg, published in 1991 (VCH Publishing, page 914, 2nd Edition), it was the founder of quantum theory, Max Planck, who originated the term "quantum", relative to the concept of photons as you described them, and it was Einstein who coined the name "photon" for the quanta of light, which name was later applied to all of the quanta of electromagnetic radiation.

By the way, Einstein won the Nobel Prize in physics using the concept of the photon (which Planck had established) to explain the photoelectric effect, not for his theories of relativity, or the equation E = mc2, for which he is most famous.

In any case, photons are understood to have zero rest-mass.  Their mass in motion could be anything, including their mass when in virtual states; i.e, when their momenta are imaginary quantities (as when acting as the mediator of the magnetic field of a permanent magnet, for instance). 

Also, photonic tunneling experiments prove that photons can be made to exceed the speed of light in a vacuum, which implies that there is more to photons than just their dual nature and a zero rest-mass. 

Personally, I suspect that photons of electromagnetism, just like all other "particles" we can detect, and those whose existence physicists infer from experimental evidence, have substructures that we cannot detect; namely, superluminal substructures.

In other words, I believe that photons are acually made-up of infinitessimally small tachyons, as are all physically evident particles.  Indeed, it is likely that our entire existence has a superluminal foundation.

For more on tachyons, go to www.TachyonicsSociety.org

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Brooke
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« Reply #3 on: July 01, 2007, 08:34:51 pm »



A commemoration plaque for Max Planck on his discovery of Planck's constant, in front of Humboldt University, Berlin. English translation: "Max Planck, discoverer of the elementary quantum of action h, taught in this building from 1889 to 1928."
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Brooke
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« Reply #4 on: July 01, 2007, 08:36:13 pm »

Planck constant

The Planck constant
(denoted h) is a physical constant that is used to describe the sizes of quanta. It plays a central role in the theory of quantum mechanics, and is named after Max Planck, one of the founders of quantum theory. A closely-related quantity is the reduced Planck constant (also known as Dirac's constant and denoted ħ, pronounced "h-bar"). The Planck constant is also used in measuring energy emitted as photons, such as in the equation E=hν, where E is energy, h is Planck's constant, and ν (Greek letter nu) is frequency.

The Planck constant and the reduced Planck constant are used to describe quantization, a phenomenon occurring in subatomic particles such as electrons and photons in which certain physical properties occur in fixed amounts rather than assuming a continuous range of possible values.
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Brooke
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« Reply #5 on: July 01, 2007, 08:39:50 pm »

Units, value and symbols

The Planck constant has dimensions of energy multiplied by time, which are also the dimensions of action. In SI units, the Planck constant is expressed in joule-seconds. The dimensions may also be written as momentum times distance (N·m·s), which are also the dimensions of angular momentum. Often the unit of choice is eV·s, because of the small energies that are often encountered in quantum physics.

The value of the Planck constant is:



The two digits between the parentheses denote the standard uncertainty in the last two digits of the value.

The value of the Dirac constant is:



The figures cited here are the 2006 CODATA-recommended values for the constants and their uncertainties. The 2006 CODATA results were made available in March 2007 and represent the best-known, internationally-accepted values for these constants, based on all data available as of 31 December 2006. New CODATA figures are scheduled to be published approximately every four years.

Unicode reserves codepoints U+210E (ℎ) for the Planck constant, and U+210F (ℏ) for the Dirac constant.

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Brooke
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« Reply #6 on: July 01, 2007, 08:43:28 pm »

Origins of Planck's constant

The Planck constant, , was proposed in reference to the problem of black-body radiation. The underlying assumption to Planck's law of black body radiation was that the electromagnetic radiation emitted by a black body could be modeled as a set of harmonic oscillators with quantized energy of the form:



is the quantized energy of the photons of radiation having frequency (Hz) of  (nu) or angular frequency (rad/s) of  (omega).

This model proved extremely accurate, but it provided an intellectual stumbling block for theoreticians who did not understand where the quantization of energy arose — Planck himself only considered it "a purely formal assumption". This line of questioning helped lead to the formation of quantum mechanics.

In addition to some assumptions underlying the interpretation of certain values in the quantum mechanical formulation, one of the fundamental corner-stones to the entire theory lies in the commutator relationship between the position operator  and the momentum operator



where δij is the Kronecker delta. For more information, see the mathematical formulation of quantum mechanics.

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« Reply #7 on: July 01, 2007, 08:50:17 pm »

Usage

The Planck constant is used to describe quantization. For instance, the energy (E) carried by a beam of light with constant frequency (ν) can only take on the values



It is sometimes more convenient to use the angular frequency which gives



Many such "quantization conditions" exist. A particularly interesting condition governs the quantization of angular momentum. Let J be the total angular momentum of a system with rotational invariance, and Jz the angular momentum measured along any given direction. These quantities can only take on the values



Thus, may be said to be the "quantum of angular momentum".

The Planck constant also occurs in statements of Heisenberg's uncertainty principle. Given a large number of particles prepared in the same state, the uncertainty in their position, Δx, and the uncertainty in their momentum (in the same direction), Δp, obey
where the uncertainty is given as the standard deviation of the measured value from its expected value.

There are a number of other such pairs of physically measurable values which obey a similar rule.




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« Reply #8 on: July 01, 2007, 08:53:27 pm »

Dirac constant

The Dirac constant or the "reduced Planck constant",  , differs only from the Planck constant by a factor of 2π. The Planck constant is stated in SI units of measurement, joules per hertz, or joules per (cycle per second), while the Dirac constant is the same value stated in joules per (radian per second). Both constants are conversion factors between energy units and frequency units.

In essence, the Dirac constant is a conversion factor between phase (in radians) and action (in joule-seconds) as seen in the Schrödinger equation. All other uses of Planck's constant and Dirac's constant follow from that.

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« Reply #9 on: July 01, 2007, 08:54:21 pm »

Significance of the size of Planck's constant

Expressed in the SI units of J·s, the Planck constant is one of the smallest constants used in physics. The significance of this is that it reflects the extremely small scales at which quantum mechanical effects are observed, and hence why we are not familiar with quantum physics in our everyday lives in the way that we are with classical physics. Indeed, classical physics can essentially be defined as the limit of quantum mechanics as the Planck constant tends to zero. However, in the natural units describing physics at the atomic scale, the Planck constant is taken as 1, reflecting the fact that physics at the atomic scale is dominated by quantum effects.
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HKurtRichter
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« Reply #10 on: July 04, 2007, 08:36:07 am »

Very good!  Readers not already familiar with this information will find your presentation enlightening.  Now, in keeping with the main heading (Superluminality), you should move on to the deeper details of the wave nature of electromagnetic radiation (EM waves).  Include at least an explanation of Maxwell's Equations and of Sommerfeld-Brillouin Theory, then get into the superluminal propagation of EM waves in dispersive and nondispersive media (including transparent media, and a vacuum), and, of course, evanescent waves. 

From there you would go over to particle-based theory, with reports on superluminal photonic tunneling, quantum nonlocality (i.e., entanglement), and quantum information science (including teleportation); making sure to mention the most recent experiments and the controversies surrounding these topics. 

For a good print reference (though it is somewhat dated), I recommend S.C. Tiwari's book Superluminal Phenomena in Modern Perspective, from Rinton Press (www.rintonpress.com).  But it is rather expensive.  To save money, borrow it through the Interlibrary Loan program at your nearest library.  And supplement that information with more up-to-date online data using an advanced Google search with a time limit (say, not earlier than 2004 or 2005; though you will find that very few listings show up for the current year). 

The most advanced papers on these subjects, and which are often also available online, usually cost something.  So, unless you live close to a university that keeps the latest print versions of the appropriate journals on the shelf (as in those published by the American Physical Society), or unless you are a physics student at a major university engaged in the forefront of research on these subjects, then the very latest accounts of experimental and theoretical efforts in these particular areas will be quite difficult to obtain.

Anyway, good work.  And please continue.
 Smiley
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Jake
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« Reply #11 on: August 08, 2007, 02:30:49 am »

Quote
...quantum nonlocality (i.e., entanglement)...

Another very good book, that is geared more for the layperson:

'Entanglement; The Unlikely Story of How Scientists, Mathematicians, and Philosophers Proved Einstiens Spookiest Theory', by Amir D. Aezel
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Volitzer
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« Reply #12 on: August 08, 2007, 11:53:00 am »

Usage

The Planck constant is used to describe quantization. For instance, the energy (E) carried by a beam of light with constant frequency (ν) can only take on the values



It is sometimes more convenient to use the angular frequency which gives



Many such "quantization conditions" exist. A particularly interesting condition governs the quantization of angular momentum. Let J be the total angular momentum of a system with rotational invariance, and Jz the angular momentum measured along any given direction. These quantities can only take on the values



Thus, may be said to be the "quantum of angular momentum".

The Planck constant also occurs in statements of Heisenberg's uncertainty principle. Given a large number of particles prepared in the same state, the uncertainty in their position, Δx, and the uncertainty in their momentum (in the same direction), Δp, obey
where the uncertainty is given as the standard deviation of the measured value from its expected value.

There are a number of other such pairs of physically measurable values which obey a similar rule.






 Shocked Shocked Shocked Shocked Shocked Shocked Shocked

Brooke !!!!!!!!!!!!!!!!!!!!  I had no idea you were physics minded.

Can we clone say 100 of you and send our cuntras here in NYS to the bottom of the ocean with some cement shoes and create an artificial reef ?? ?? ??   Grin Grin Grin
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Volitzer
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« Reply #13 on: August 08, 2007, 11:58:37 am »

Photons are particles that behave like waves or they can be waves that behave like particles.

It's the old Taste great, less filling type of argument here.   Cheesy Cheesy
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Brooke
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« Reply #14 on: August 09, 2007, 12:08:06 am »


 Shocked Shocked Shocked Shocked Shocked Shocked Shocked

Brooke !!!!!!!!!!!!!!!!!!!!  I had no idea you were physics minded.

Can we clone say 100 of you and send our cuntras here in NYS to the bottom of the ocean with some cement shoes and create an artificial reef ?? ?? ??   Grin Grin Grin

Aww, Volitzer, that is the nicest thing you have ever said to me!

Do you know who Max Planck is?

Brooke
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"The most incomprehensible thing about our universe is that it can be comprehended." - Albert Einstein
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