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are a very energetic form of electromagnetic radiation that can be used to take images of the human body.

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X-rays are a very energetic form of electromagnetic radiation that can be used to take images of the human body.

Credit: Fotokon | Dreamstime

X-rays are types of electromagnetic radiation probably most well-known for their ability to see through a person's skin and reveal images of the bones beneath it. Advances in technology have led to more powerful and focused X-ray beams as well as ever greater applications of these light waves, from imaging teensy biological cells and structural components of materials like cement to killing cancer cells.

X-rays are roughly classified into soft X-rays and hard X-rays. Soft X-rays have relatively short wavelengths of about 10 nanometers (a nanometer is one-billionth of a meter), and so they fall in the range of the electromagnetic (EM) spectrum between ultraviolet (UV) light and gamma-rays. Hard X-rays have wavelengths of about 100 picometers (a picometer is one-trillionth of a meter). These electromagnetic waves occupy the same region of the EM spectrum as gamma-rays. The only difference between them is their source: X-rays are produced by accelerating electrons, whereas gamma-rays are produced by atomic nuclei in one of four nuclear reactions.

X-rays were discovered in 1895 by Wilhelm Conrad Röentgen, a professor at Würzburg University in Germany. According to the Nondestructive Resource Center's " History of Radiography," Röentgen noticed crystals near a high-voltage cathode-ray tube exhibiting a fluorescent glow, even when he shielded them with dark paper. Some form of energy was being produced by the tube that was penetrating the paper and causing the crystals to glow. Röentgen called the unknown energy "X-radiation." Experiments showed that this radiation could penetrate soft tissues but not bone, and would produce shadow images on photographic plates.

For this discovery, Röentgen was awarded the very first Nobel Prize in physics, in 1901.

X-rays can be produced on Earth by sending a high-energy beam of electrons smashing into an atom like copper or gallium, according to Kelly Gaffney, director of the Stanford Synchrotron Radiation Lightsource. When the beam hits the atom, the electrons in the inner shell, called the s-shell, get jostled, and sometimes flung out of their orbit. Without that electron, or electrons, the atom becomes unstable, and so for the atom to "relax" or go back to equilibrium, Gaffney said, an electron in the so-called 1p shell drops in to fill the gap. The result? An X-ray gets released.

"The problem with that is the fluorescence [or X-ray light given off] goes in all directions," Gaffney told Live Science. "They aren't directional and not focusable. It's not a very easy way to make a high-energy, bright source of X-rays."

Enter a synchrotron, a type of particle accelerator that accelerates charged particles like electrons inside a closed, circular path. Basic physics suggests that any time you accelerate a charged particle, it gives off light. The type of light depends on the energy of the electrons (or other charged particles) and the magnetic field that pushes them around the circle, Gaffney said.

Since the synchrotron electrons are pushed to near the speed of light, they give off enormous amounts of energy, particularly X-ray energy. And not just any X-rays, but a very powerful beam of focused X-ray light.

Synchrotron radiation was seen for the first time at General Electric in the United States in 1947, according to the European Synchrotron Radiation Facility. This radiation was considered a nuisance because it caused the particles to lose energy, but it was later recognized in the 1960s as light with exceptional properties that overcame the shortcomings of X-ray tubes. One interesting feature of synchrotron radiation is that it is polarized; that is, the electric and magnetic fields of the photons all oscillate in the same direction, which can be either linear or circular.

"Because the electrons are relativistic [or moving at near light-speed], when they give off light, it ends up being focused in the forward direction," Gaffney said. "This means you get not just the right color of light X-rays and not just a lot of them because you have a lot of electrons stored, they're also preferentially emitted in the forward direction."

Due to their ability to penetrate certain materials, X-rays are used for several nondestructive evaluation and testing applications, particularly for identifying flaws or cracks in structural components. According to the NDT Resource Center, "Radiation is directed through a part and onto [a] film or other detector. The resulting shadowgraph shows the internal features" and whether the part is sound. This is the same technique used in doctors' and dentists' offices to create X-ray images of bones and teeth, respectively.[ Images: Stunning Fish X-rays]

X-rays are also essential for transportation security inspections of cargo, luggage and passengers. Electronic imaging detectors allow for real-time visualization of the content of packages and other passenger items.

The original use of X-rays was for imaging bones, which were easily distinguishable from soft tissues on the film that was available at that time. However, more accurate focusing systems and more sensitive detection methods, such as improved photographic films and electronic imaging sensors, have made it possible to distinguish increasingly fine detail and subtle differences in tissue density, while using much lower exposure levels.

Additionally, computed tomography (CT) combines multiple X-ray images into a 3D model of a region of interest.

Similar to CT, synchrotron tomography can reveal three-dimensional images of interior structures of objects like engineering components, according to the Helmholtz Center for Materials and Energy.

Radiation therapy uses high-energy radiation to kill cancer cells by damaging their DNA. Since the treatment can also damage normal cells, the National Cancer Institute recommends that treatment be carefully planned to minimize side effects.

According to the U.S. Environmental Protection Agency, so-called ionizing radiation from X-rays zaps a focused area with enough energy to completely strip electrons from atoms and molecules, thus altering their properties. In sufficient doses, this can damage or destroy cells. While this cell damage can cause cancer, it can also be used to fight it. By directing X-rays at cancerous tumors, it can demolish those abnormal cells.

According to Robert Patterson, professor of astronomy at Missouri State University, celestial sources of X-rays include close binary systems containing black holes or neutron stars. In these systems, the more massive and compact stellar remnant can strip material from its companion star to form a disk of extremely hot X-ray-emitting gas as it spirals inward. Additionally, supermassive black holes at the centers of spiral galaxies can emit X-rays as they absorb stars and gas clouds that fall within their gravitational reach.

X-ray telescopes use low-angle reflections to focus these high-energy photons (light) that would otherwise pass through normal telescope mirrors. Because Earth's atmosphere blocks most X-rays, observations are typically conducted using high-altitude balloons or orbiting telescopes.

Additional resources

This page was updated on Oct. 5, 2018 by Live Science Managing Editor, Jeanna Bryner.

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X-rays are types of electromagnetic radiation probably most well-known for their ability to see through a person's skin and reveal images of the bones beneath it. Advances in technology have led to more powerful and focused X-ray beams as well as ever greater applications of these light waves, from imaging teensy biological cells and structural components of materials like cement to killing cancer cells.

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is the concept in quantum mechanics that every particle or quantum entity may be partly described in terms not only of particles, but also of waves.

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Wave-particle duality is the concept in quantum mechanics according to which quantum entities exhibit both particle and a wave properties depending on the experimental circumstances. It expresses the inability of the classical concepts "particle" or "wave" to fully describe the behaviour of quantum-scale objects alone. As Albert Einstein wrote:

It seems as though we must use sometimes the one theory and sometimes the other, while at times we may use either. We are faced with a new kind of difficulty. We have two contradictory pictures of reality; separately neither of them fully explains the phenomena of light, but together they do.

Through the work of Max Planck, Albert Einstein, Louis de Broglie, Arthur Compton, Niels Bohr, Erwin Schrödinger and many others, current scientific theory holds that all particles exhibit a wave nature and vice versa. This phenomenon has been verified not only for elementary particles, but also for compound particles like atoms and even molecules. For macroscopic particles, because of their extremely short wavelengths, wave properties usually cannot be detected.

Bohr viewed measurements as the only reality. A quantum object will exhibit either wave or particle characteristics, depending on the physical setting. He saw such duality as one aspect of complementarity. Bohr regarded renunciation of the cause-effect relation, or complementarity, of the space-time picture, as essential to the quantum mechanical account.

Werner Heisenberg considered the question further. He saw the duality as present for all quantic entities, but not quite in the usual quantum mechanical account considered by Bohr. He saw it in what is called second quantization, which generates an entirely new concept of fields that exist in ordinary space-time, causality still being visualizable. Classical field values (e.g. the electric and magnetic field strengths of Maxwell) are replaced by an entirely new kind of field value, as considered in quantum field theory. Turning the reasoning around, ordinary quantum mechanics can be deduced as a specialized consequence of quantum field theory.

The non-classical nature of wave-particle duality means it is not part of all interpretations of quantum mechanics.

Feynman invited us to think about double slit experiments with electrons. A double slit experiment was thought to prove the wave nature of light. When light passes through two slits, the light which passes through one slit interferes with the light which passes through the other. Since it was supposed that a particle cannot pass through two slits at the same time, the conclusion was that light was essentially a wave phenomenon. How could a particle of light, a photon, pass through two slits at the same time? But electrons, which are generally considered as particles, because they are constituents of atoms, and because they can be detected one by one, exhibit the same phenomenon. We can observe an interference pattern with electrons in a double slit experiment. Can an electron, or more generally any particle, pass through both slits at the same time? Such a pattern can't be the result of an interference between several electrons, because each electron can be detected in the interference pattern even if it is very far from any other electron. Can a particle move like a wave, be at many positions and pass through two slits at the same time? This is the mystery of wave-particle duality. Can a particle interfere with itself ?

Feynman reasoned further. If an electron moves like a classical particle, we should be able to observe through which slit it passes. We just have to look at it. But if we do this, the interference pattern disappears. If we observe where the electron is, we find a single answer: it is at only one position at one time, it passes through only one slit. But if we don't observe it, the interference pattern reappears, and it seems that an electron can be at several positions at the same time, that it passes through both slits, just like if it was spreading in space, like a wave.

Wave-particle duality as a physical model derives from Niels Bohr's 1926 presentation in Como, Italy, at a scientific celebration of the work of Alessandro Volta 100 years previous. Bohr's subject was complementarity, the idea that measurements of quantum events provide complementary information through seemingly contradictory results. While Bohr's presentation was not well received, it did crystallize the issues ultimately leading to the modern wave-particle duality concept. The contradictory results that triggered Bohr's ideas had been building up over the previous 20 years.

The contradictory evidence came from both light and from electrons. The wave theory of light, broadly successful for over a hundred years, had been challenged by Planck's 1901 model of blackbody radiation and Einstein's 1905 interpretation of the photoelectric effect. These theoretical models use discrete energy, a quantum, to describe the interaction of light with matter. Despite confirmation by various experimental observations, the photon theory (as it came to be called later) remained controversial until Arthur Compton performed a series of experiments from 1922 to 1924 demonstrating the momentum of light. The experimental evidence of particle-like momentum seemingly contradicted other experiments demonstrating the wave-like interference of light.

The contradictory evidence from electrons arrived in the opposite order. Many experiments by J. J. Thompson, Robert Millikan, and Charles Wilson among others had shown that free electrons had particle properties. However in 1924, Louis de Broglie proposed that electrons had an associated wave and Schrödinger demonstrated that wave equations accurately account for electron properties in atoms. Again some experiments showed particle properties and others wave properties.

Bohr's resolution of these contradictions is to accept them. In his Como lecture he says: "our interpretation of the experimental material rests essentially upon the classical concepts." Direct observation being impossible, observations of quantum effects are necessarily classical. Whatever the nature of quantum events, our only information will arrive via classical results. If experiments sometimes produce wave results and sometimes particle results, that is the nature of light and of the ultimate constituents of matter.

The photoelectric effect. Incoming photons on the left strike a metal plate (bottom), and eject electrons, depicted as flying off to the right.

While Planck had solved the ultraviolet catastrophe by using atoms and a quantized electromagnetic field, most contemporary physicists agreed that Planck's "light quanta" represented only flaws in his model. A more-complete derivation of black-body radiation would yield a fully continuous and "wave-like" electromagnetic field with no quantization. However, in 1905 Einstein took Planck's black body model to produce his solution to another outstanding problem of the day: the photoelectric effect, wherein electrons are emitted from atoms when they absorb energy from light. Since their existence was theorized eight years previously, phenomena had been studied with the electron model in mind in physics laboratories worldwide.

In 1902, Philipp Lenard discovered that the energy of these ejected electrons did not depend on the intensity of the incoming light, but instead on its frequency. So if one shines a little low-frequency light upon a metal, a few low energy electrons are ejected. If one now shines a very intense beam of low-frequency light upon the same metal, a whole slew of electrons are ejected; however, they possess the same low energy, there are merely more of them. The more light there is, the more electrons are ejected. Whereas in order to get high energy electrons, one must illuminate the metal with high-frequency light. Like blackbody radiation, this was at odds with a theory invoking continuous transfer of energy between radiation and matter. However, it can still be explained using a fully classical description of light, as long as matter is quantum mechanical in nature.

If one used Planck's energy quanta, and demanded that electromagnetic radiation at a given frequency could only transfer energy to matter in integer multiples of an energy quantum hf, then the photoelectric effect could be explained very simply. Low-frequency light only ejects low-energy electrons because each electron is excited by the absorption of a single photon. Increasing the intensity of the low-frequency light (increasing the number of photons) only increases the number of excited electrons, not their energy, because the energy of each photon remains low. Only by increasing the frequency of the light, and thus increasing the energy of the photons, can one eject electrons with higher energy. Thus, using the Planck constant h to determine the energy of the photons based upon their frequency, the energy of ejected electrons should also increase linearly with frequency, the gradient of the line being given by the Planck constant. These results were not confirmed until 1915, when Robert Andrews Millikan produced experimental results in perfect accord with Einstein's predictions.

While energy of ejected electrons reflected the Planck constant, the existence of photons was not explicitly proven until the discovery of the photon antibunching effect. This refers to the observation that once a single emitter (an atom, molecule, solid state emitter, etc.) radiates a detectable light signal, it cannot immediately release a second signal until after the emitter has been re-excited. This leads to a statistically quantifiable time delay between light emissions, so detection of multiple signals becomes increasingly unlikely as the observation time dips under the excited-state lifetime of the emitter. The effect can be demonstrated in an undergraduate-level lab.

This phenomenon could only be explained via photons. Einstein's "light quanta" would not be called photons until 1925, but even in 1905 they represented the quintessential example of wave-particle duality. Electromagnetic radiation propagates following linear wave equations, but can only be emitted or absorbed as discrete elements, thus acting as a wave and a particle simultaneously.

Einstein's explanation of photoelectric effect

In 1905, Einstein provided an explanation of the photoelectric effect, an experiment that the wave theory of light failed to explain. He did so by postulating the existence of photons, quanta of light energy with particulate qualities.

In the photoelectric effect, it was observed that shining a light on certain metals would lead to an electric current in a circuit. Presumably, the light was knocking electrons out of the metal, causing current to flow. However, using the case of potassium as an example, it was also observed that while a dim blue light was enough to cause a current, even the strongest, brightest red light available with the technology of the time caused no current at all. According to the classical theory of light and matter, the strength or amplitude of a light wave was in proportion to its brightness: a bright light should have been easily strong enough to create a large current. Yet, oddly, this was not so.

Einstein explained this enigma by postulating that electrons can receive energy from an electromagnetic field only in discrete units (quanta or photons): an amount of energy E that was related to the frequency f of the light by

where h is the Planck constant (). Only photons of a high enough frequency (above a certain threshold value) could knock an electron free. For example, photons of blue light had sufficient energy to free an electron from the metal, but photons of red light did not. One photon of light above the threshold frequency could release only one electron; the higher the frequency of a photon, the higher the kinetic energy of the emitted electron, but no amount of light below the threshold frequency could release an electron. To violate this law would require extremely high-intensity lasers that had not yet been invented. Intensity-dependent phenomena have now been studied in detail with such lasers.

Einstein was awarded the Nobel Prize in Physics in 1921 for his discovery of the law of the photoelectric effect.

de Broglie's hypothesis

Propagation of de Broglie waves in 1d-real part of the complex amplitude is blue, imaginary part is green. The probability (shown as the colour opacity) of finding the particle at a given point x is spread out like a waveform; there is no definite position of the particle. As the amplitude increases above zero the curvature decreases, so the amplitude decreases again, and vice versa-the result is an alternating amplitude: a wave. Top: Plane wave. Bottom: Wave packet.

In 1924, Louis-Victor de Broglie formulated the de Broglie hypothesis, claiming that all matter has a wave-like nature. He related wavelength and momentum:

This is a generalization of Einstein's equation above, since the momentum of a photon is given by and the wavelength (in a vacuum) by , where c is the speed of light in vacuum.

De Broglie's formula was confirmed three years later for electrons with the observation of electron diffraction, as it had been observed with X-rays, in two independent experiments. At the University of Aberdeen, George Paget Thomson and Alexander Reid passed a beam of electrons through a thin celluloid film, then later metal films, and observed the predicted interference patterns. (Alexander Reid, who was Thomson's graduate student, performed the first experiments but he died soon after in a motorcycle accident and is rarely mentioned.) At Bell Labs, Clinton Joseph Davisson and Lester Halbert Germer reflected an electron beam from a nickel sample in their experiment, popularly known as the Davisson-Germer experiment.

De Broglie was awarded the Nobel Prize for Physics in 1929 for his hypothesis. Thomson and Davisson shared the Nobel Prize for Physics in 1937 for their experimental work.

Heisenberg's uncertainty principle

In his work on formulating quantum mechanics, Werner Heisenberg postulated his uncertainty principle, which states:

where

here indicates standard deviation, a measure of spread or uncertainty;

x and p are a particle's position and linear momentum respectively.

is the reduced Planck constant (the Planck constant divided by 2 ).

Heisenberg originally explained this as a consequence of the process of measuring: Measuring position accurately would disturb momentum and vice versa, offering an example (the "gamma-ray microscope") that depended crucially on the de Broglie hypothesis. The thought is now, however, that this only partly explains the phenomenon, but that the uncertainty also exists in the particle itself, even before the measurement is made.

In fact, the modern explanation of the uncertainty principle, extending the Copenhagen interpretation first put forward by Bohr and Heisenberg, depends even more centrally on the wave nature of a particle. Just as it is nonsensical to discuss the precise location of a wave on a string, particles do not have perfectly precise positions; likewise, just as it is nonsensical to discuss the wavelength of a "pulse" wave traveling down a string, particles do not have perfectly precise momenta that correspond to the inverse of wavelength. Moreover, when position is relatively well-defined, the wave is pulse-like and has a very ill-defined wavelength, and thus momentum. And conversely, when momentum, and thus wavelength, is relatively well-defined, the wave looks long and (complex-)sinusoidal, and therefore it has a very ill-defined position.

Real double-slit experiment with electrons. Individual particles appear on the detector, slowly filling in the interference pattern.

Simulated double-slit experiment.

Single-particle interferometry has become a classic for its clarity in expressing the central puzzles of quantum mechanics. Because it demonstrates the fundamental limitation of the ability of the observer to predict experimental results, Richard Feynman called it "a phenomenon which is impossible [...] to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery [of quantum mechanics].

In 1974, the Italian physicists Pier Giorgio Merli, Gian Franco Missiroli, and Giulio Pozzi performed the first experiment of single particle interferometry using electrons and a biprism (instead of slits), showing that the statistics of electron detection leads to interference patterns as predicted by quantum theory.

In 2018, single particle interference has been first demonstrated for antimatter in the Positron Laboratory (L-NESS) of Rafael Ferragut in Como ( Italy), by a group led by Marco Giammarchi.

Since the demonstrations of wave-like properties in photons and electrons, similar experiments have been conducted with neutrons and protons. Interference of atom matter waves was first observed by Immanuel Estermann and Otto Stern in 1930, when a Na beam was diffracted off a surface of NaCl. Authors of similar recent experiments with atoms and molecules, described below, claim that these larger particles also act like waves.

A dramatic series of experiments emphasizing the action of gravity in relation to wave-particle duality was conducted in the 1970s using the neutron interferometer. Neutrons, one of the components of the atomic nucleus, provide much of the mass of a nucleus and thus of ordinary matter. In the neutron interferometer, they act as quantum-mechanical waves directly subject to the force of gravity. While the results were not surprising since gravity was known to act on everything, including light (see tests of general relativity and the Pound-Rebka falling photon experiment), the self-interference of the quantum mechanical wave of a massive fermion in a gravitational field had never been experimentally confirmed before.

In 1999, the diffraction of C ₆₀ fullerenes by researchers from the University of Vienna was reported. Fullerenes are comparatively large and massive objects, having an atomic mass of about . The de Broglie wavelength of the incident beam was about , whereas the diameter of the molecule is about , about 400 times larger. In 2012, these far-field diffraction experiments could be extended to phthalocyanine molecules and their heavier derivatives, which are composed of 58 and 114 atoms respectively. In these experiments the build-up of such interference patterns could be recorded in real time and with single molecule sensitivity.

In 2003, the Vienna group also demonstrated the wave nature of tetraphenylporphyrin-a flat biodye with an extension of about 2 nm and a mass of 614 u. For this demonstration they employed a near-field Talbot Lau interferometer. In the same interferometer they also found interference fringes for C ₆₀F ₄₈, a fluorinated buckyball with a mass of about 1600 u, composed of 108 atoms. Large molecules are already so complex that they give experimental access to some aspects of the quantum-classical interface, i.e., to certain decoherence mechanisms. In 2011, the interference of molecules as heavy as 6910 u could be demonstrated in a Kapitza-Dirac-Talbot-Lau interferometer. In 2013, the interference of molecules beyond 10,000 u has been demonstrated.

Whether objects heavier than the Planck mass (about the weight of a large bacterium) have a de Broglie wavelength is theoretically unclear and experimentally unreachable; above the Planck mass a particle's Compton wavelength would be smaller than the Planck length and its own Schwarzschild radius, a scale at which current theories of physics may break down or need to be replaced by more general ones.

Yve Couder and his team showed that macroscopic oil droplets on a vibrating fluid bath can be used as an analogue model of wave-particle duality; a localized droplet creates periodical wave field around itself. Resonant interaction between the droplet and its own wave field exhibits behaviour analogous to quantum particles: interference in double-slit experiment, unpredictable tunneling (depending in complicated way on practically hidden state of field), orbit quantization (that particle has to 'find a resonance' with field perturbations it creates-after one orbit, its internal phase has to return to the initial state) and Zeeman effect. Other single and double slit experiments have shown that wall-droplet interactions rather than diffraction or interference of the pilot wave may be responsible for the observed hydrodynamic patterns, which are different from slit-induced interference patterns exhibited by quantum particles.

Wave-particle duality is deeply embedded into the foundations of quantum mechanics. In the formalism of the theory, all the information about a particle is encoded in its wave function, a complex-valued function roughly analogous to the amplitude of a wave at each point in space. This function evolves according to Schrödinger equation. For particles with mass this equation has solutions that follow the form of the wave equation. Propagation of such waves leads to wave-like phenomena such as interference and diffraction. Particles without mass, like photons, have no solutions of the Schrödinger equation. Instead of a particle wave function that localizes mass in space, a photon wave function can be constructed from Einstein kinematics to localize energy in spatial coordinates.

The particle-like behaviour is most evident due to phenomena associated with measurement in quantum mechanics. Upon measuring the location of the particle, the particle will be forced into a more localized state as given by the uncertainty principle. When viewed through this formalism, the measurement of the wave function will randomly lead to wave function collapse to a sharply peaked function at some location. For particles with mass, the likelihood of detecting the particle at any particular location is equal to the squared amplitude of the wave function there. The measurement will return a well-defined position, and is subject to Heisenberg's uncertainty principle.

Following the development of quantum field theory the ambiguity disappeared. The field permits solutions that follow the wave equation, which are referred to as the wave functions. The term particle is used to label the irreducible representations of the Lorentz group that are permitted by the field. An interaction as in a Feynman diagram is accepted as a calculationally convenient approximation where the outgoing legs are known to be simplifications of the propagation and the internal lines are for some order in an expansion of the field interaction. Since the field is non-local and quantized, the phenomena that previously were thought of as paradoxes are explained. Within the limits of the wave-particle duality the quantum field theory gives the same results.

Most scientists accept wave-particle duality as the best explanation for a broad range of observed phenomena. Alternative views are also presented here. These views are not generally accepted by mainstream physics.

The pilot wave model, was originally developed by de Broglie and further developed by David Bohm into the hidden variable theory. The phrase "hidden variable" is misleading since the variable in question is the positions of the particles. Instead of duality, the pilot wave model proposes that both wave and particle are present with the wave guiding the particle in a deterministic fashion. The wave in question is the wavefunction obeying Schrödinger's equation. Bohm's formulation is intended to be classical, but it has to incorporate a distinctly non-classical feature: a nonlocal force (" quantum potential") acting on the particles.

Bohm's original purpose (1952) "was to show that an alternative to the Copenhagen interpretation is at least logically possible. Soon after he set the project aside and did not revive it until he met Basil Hiley in 1961 when both were at Birbeck College (University of London). Bohm and Hiley then wrote extensively on the theory and it gained a wider audience. While the model is in many ways similar to Schrödinger equation, it is known to fail for relativistic cases and does not account for features such as particle creation or annihilation in quantum field theory. Many authors such as nobel laureates Werner Heisenberg, Sir Anthony James Leggett and Sir Roger Penrose have criticized it for not adding anything new.

The many-worlds interpretation (MWI) is sometimes presented as a waves-only theory, including by its originator, Hugh Everett who referred to MWI as "the wave interpretation".

The of Ryszard Horodecki relates the particle to wave. The hypothesis implies that a massive particle is an intrinsically spatially, as well as temporally extended, wave phenomenon by a nonlinear law.

The deterministic collapse theory considers collapse and measurement as two independent physical processes. Collapse occurs when two wavepackets spatially overlap and satisfy a mathematical criterion, which depends on the parameters of both wavepackets. It is a contraction to the overlap volume. In a measurement apparatus one of the two wavepackets is one of the atomic clusters, which constitute the apparatus, and the wavepackets collapse to at most the volume of such a cluster. This mimics the action of a point particle.

Hegerfeldt's theorem, which appears to demonstrate the incompatibility of the existence of localized discrete particles with the combination of the principles of quantum mechanics and special relativity, has also been used to support the conclusion that reality must be described solely in terms of field-based formulations.

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Wave-particle duality is the concept in quantum mechanics according to which quantum entities exhibit both particle and a wave properties depending on the experimental circumstances. It expresses the inability of the classical concepts "particle" or "wave" to fully describe the behaviour of quantum-scale objects alone.

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A prism can break white light up into the visible light spectrum. Visible light is a form of electromagnetic (EM) radiation, as are radio waves, infrared radiation, ultraviolet radiation, X-rays and microwaves.

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Visible light is a form of electromagnetic (EM) radiation, as are radio waves, infrared radiation, ultraviolet radiation, X-rays and microwaves. Generally, visible light is defined as the wavelengths that are visible to most human eyes.

EM radiation is transmitted in waves or particles at different wavelengths and frequencies. This broad range of wavelengths is known as the electromagnetic spectrum. That spectrum is typically divided into seven regions in order of decreasing wavelength and increasing energy and frequency. The common designations are radio waves, microwaves, infrared (IR), visible light, ultraviolet (UV), X-rays and gamma-rays.

Visible light falls in the range of the EM spectrum between infrared (IR) and ultraviolet (UV). It has frequencies of about 4 × 10 ¹⁴ to 8 × 10 ¹⁴ cycles per second, or hertz (Hz) and wavelengths of about 740 nanometers (nm) or 2.9 × 10 −5 inches, to 380 nm (1.5 × 10 −5 inches).

Color

Perhaps the most important characteristic of visible light is color. Color is both an inherent property of light and an artifact of the human eye. Objects don't "have" color, according to Glenn Elert, author of the website The Physics Hypertextbook. Rather, they give off light that "appears" to be a color. In other words, Elert writes, color exists only in the mind of the beholder.

Our eyes contain specialized cells, called cones, that act as receivers tuned to the wavelengths of this narrow band of the EM spectrum, according to NASA's Mission Science website. Light at the lower end of the visible spectrum, having a longer wavelength, about 740 nm, is seen as red; light in the middle of the spectrum is seen as green; and light at the upper end of the spectrum, with a wavelength of about 380 nm, is seen as violet. All other colors that we perceive are mixtures of these colors.

For instance, yellow contains both red and green; cyan is a mixture of green and blue, and magenta is blend of red and blue. White light contains all colors in combination. Black is a total absence of light. The first person to realize that white light was made up of the colors of the rainbow was Isaac Newton, who in 1666 passed sunlight through a narrow slit and then a prism to project the colored spectrum onto a wall, according to Michael Fowler, a physics professor at the University of Virginia.

Color and temperature

As objects grow hotter, they radiate energy dominated by shorter wavelengths, which we perceive as changing colors, according to NASA. For example, the flame of a blowtorch changes from reddish to blue as it is adjusted to burn hotter. This process of turning heat energy into light energy is called incandescence, according to the Institute for Dynamic Educational Advancement's website, WebExhibits.org.

Incandescent light is produced when hot matter releases a portion of its thermal vibration energy as photons. At about 800 degrees Celsius (1,472 degrees Fahrenheit), the energy radiated by an object reaches the infrared. As the temperature increases, the energy moves into the visible spectrum and the object appears to have a reddish glow. As the object gets hotter, the color changes to "white hot" and eventually to blue.

Visible light astronomy

The color of hot objects, such as stars, can be used to estimate their temperatures, according to IDEA. For example, the sun's surface temperature is about 5,800 Kelvin (9,980 F or 5,527 C). The light emitted has a peak wavelength of about 550 nm, which we perceive as visible white light (or slightly yellowish).

According to NASA, if the sun's surface temperature were cooler, about 3,000 C, it would look reddish, like the star Betelgeuse. If it were hotter, about 12,000 C, it would look blue, like the star Rigel.

Astronomers can also determine what objects are made of because each element absorbs light at specific wavelengths, called an absorption spectrum. By knowing the absorption spectra of elements, astronomers can use spectroscopes to determine the chemical composition of stars, dust clouds and other distant objects.
Additional resources

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Visible light is a form of electromagnetic (EM) radiation, as are radio waves, infrared radiation, ultraviolet radiation, X-rays and microwaves. Generally, visible light is defined as the wavelengths that are visible to most human eyes. EM radiation is transmitted in waves or particles at different wavelengths and frequencies.

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is a major risk factor for most skin cancers. Sunlight is the main source of UV rays.

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Exposure to ultraviolet (UV) radiation is a major risk factor for most skin cancers. Sunlight is the main source of UV rays. Tanning lamps and beds are also sources of UV rays. People who get a lot of UV exposure from these sources are at greater risk for skin cancer.

Even though UV rays make up only a very small portion of the sun's rays, they are the main cause of the sun's damaging effects on the skin. UV rays damage the DNA of skin cells. Skin cancers start when this damage affects the DNA of genes that control skin cell growth.

There are 3 main types of UV rays:

UVA rays age skin cells and can damage their DNA. These rays are linked to long-term skin damage such as wrinkles, but they are also thought to play a role in some skin cancers. Most tanning beds give off large amounts of UVA, which has been found to increase skin cancer risk.

UVB rays have slightly more energy than UVA rays. They can damage skin cells' DNA directly, and are the main rays that cause sunburns. They are also thought to cause most skin cancers.

UVC rays have more energy than the other types of UV rays, but they don't get through our atmosphere and are not in sunlight. They are not normally a cause of skin cancer.

Both UVA and UVB rays can damage skin and cause skin cancer. UVB rays are a more potent cause of at least some skin cancers, but based on what's known today, there are no safe UV rays.

The strength of the UV rays reaching the ground depends on a number of factors, such as:

Time of day: UV rays are strongest between 10 am and 4 pm.

Season of the year: UV rays are stronger during spring and summer months. This is less of a factor near the equator.

Distance from the equator (latitude): UV exposure goes down as you get further from the equator.

Altitude: More UV rays reach the ground at higher elevations.

Cloud cover: The effect of clouds can vary. Sometimes cloud cover blocks some UV from the sun and lowers UV exposure, while some types of clouds can reflect UV and can increase UV exposure. What is important to know is that UV rays can get through, even on a cloudy day.

Reflection off surfaces: UV rays can bounce off surfaces like water, sand, snow, pavement, or grass, leading to an increase in UV exposure.

The amount of UV exposure a person gets depends on the strength of the rays, the length of time the skin is exposed, and whether the skin is protected with clothing or sunscreen.

People who live in areas with year-round, bright sunlight have a higher risk of skin cancer. Spending a lot of time outdoors for work or recreation without protective clothing and sunscreen increases your risk.

The pattern of exposure may also be important. For example, frequent sunburns in childhood may increase the risk for some types of skin cancer many years or even decades later.

Skin cancers are one result of getting too much sun, but there are other effects as well. Sunburn and tanning are the short-term results of too much exposure to UV rays, and are signs of skin damage. Long-term exposure can cause early skin aging, wrinkles, loss of skin elasticity, dark patches (lentigos, sometimes called age spots or liver spots), and pre-cancerous skin changes (such as dry, scaly, rough patches called actinic keratoses).

The sun's UV rays increase a person's risk of cataracts and certain other eye problems, too. They can also suppress the skin's immune system. Darker-skinned people are generally less likely to get skin cancer than light-skinned people, but they can still get cataracts and immune suppression.

The UV Index

As noted above, the amount of UV light reaching the ground in any given place depends on a number of factors, including the time of day, time of year, elevation, and cloud cover. To help people better understand the strength of UV light in their area on a given day, the National Weather Service and the Environmental Protection Agency (EPA) have developed the UV Index. It gives people an idea of how strong the UV light is in their area, on a scale from 1 to 11+. A higher number means greater risk of exposure to UV rays and a higher chance of sunburn and skin damage that could ultimately lead to skin cancer.

The UV Index is given daily for regions throughout the country. Many television, online, and smartphone weather forecasts include the projected UV Index. Further information about the UV Index, as well as your local UV Index forecast, can be found on the EPA's website at www.epa.gov/sunsafety/uv-index-1. Smartphone apps are available from the EPA at www.epa.gov/enviro/uv-index-mobile-app. As with any forecast, local changes in cloud cover and other factors could change the actual UV levels experienced.

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Exposure to ultraviolet (UV) radiation is a major risk factor for most skin cancers. Sunlight is the main source of UV rays. Tanning lamps and beds are also sources of UV rays. People who get a lot of UV exposure from these sources are at greater risk for skin cancer.

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are a type of electromagnetic radiation, as are microwaves, infrared radiation, X-rays and gamma-rays.

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A radio tuner receives radio waves and converts them to mechanical vibrations in the speaker to create sound waves that can be heard.

Credit: Ensuper | Shutterstock

Radio waves are a type of electromagnetic radiation best-known for their use in communication technologies, such as television, mobile phones and radios. These devices receive radio waves and convert them to mechanical vibrations in the speaker to create sound waves.

The radio-frequency spectrum is a relatively small part of the electromagnetic (EM) spectrum. The EM spectrum is generally divided into seven regions in order of decreasing wavelength and increasing energy and frequency, according to the University of Rochester. The common designations are radio waves, microwaves, infrared (IR), visible light, ultraviolet (UV), X-rays and gamma-rays.

Radio waves have the longest wavelengths in the EM spectrum, according to NASA, ranging from about 0.04 inches (1 millimeter) to more than 62 miles (100 kilometers). They also have the lowest frequencies, from about 3,000 cycles per second, or 3 kilohertz, up to about 300 billion hertz, or 300 gigahertz.

The radio spectrum is a limited resource and is often compared to farmland. Just as farmers must organize their land to achieve the best harvest regarding quantity and variety, the radio spectrum must be split among users in the most efficient way, according to the British Broadcasting Corp. (BBC). In the U.S., the National Telecommunications and Information Administration within the United States Department of Commerce manages the frequency allocations along the radio spectrum.

Scottish physicist James Clerk Maxwell, who developed a unified theory of electromagnetism in the 1870s, predicted the existence of radio waves, according to the National Library of Scotland. In 1886, Heinrich Hertz, a German physicist, applied Maxwell's theories to the production and reception of radio waves. Hertz used simple homemade tools, including an induction coil and a Leyden jar (an early type of capacitor consisting of a glass jar with foil layers both inside and out) to create electromagnetic waves. Hertz became the first person to transmit and receive controlled radio waves. The unit of frequency of an EM wave - one cycle per second - is called a hertz, in his honor, according to the American Association for the Advancement of Science.

The National Telecommunications and Information Administration generally divides the radio spectrum into nine bands:

ELF radio waves, the lowest of all radio frequencies, have a long range and are useful in penetrating water and rock for communication with submarines and inside mines and caves. The most powerful natural source of ELF/VLF waves is lightning, according to the Stanford VLF Group. Waves produced by lightning strikes can bounce back and forth between Earth and the ionosphere (the atmosphere layer with a high concentration of ions and free electrons), according to Phys.org. These lightning disturbances can distort important radio signals traveling to satellites.

LF and MF radio bands include marine and aviation radio, as well as commercial AM (amplitude modulation) radio, according to RF Page. AM radio frequency bands fall between 535 kilohertz to 1.7 megahertz, according to How Stuff Works. AM radio has a long range, particularly at night when the ionosphere is better at refracting the waves back to earth, but it is subject to interference that affects the sound quality. When a signal is partially blocked - for example, by a metal-walled building such as a skyscraper - the volume of the sound is reduced accordingly.

HF, VHF and UHF bands include FM radio, broadcast television sound, public service radio, cellphones and GPS (global positioning system). These bands typically use "frequency modulation" (FM) to encode, or impress, an audio or data signal onto the carrier wave. In frequency modulation, the amplitude (maximum extent) of the signal remains constant while the frequency is varied higher or lower at a rate and magnitude corresponding to the audio or data signal.

FM results in better signal quality than AM because environmental factors do not affect the frequency the way they affect amplitude, and the receiver ignores variations in amplitude as long as the signal remains above a minimum threshold. FM radio frequencies fall between 88 megahertz and 108 megahertz, according to How Stuff Works.

Shortwave radio uses frequencies in the HF band, from about 1.7 megahertz to 30 megahertz, according to the National Association of Shortwave Broadcasters (NASB). Within that range, the shortwave spectrum is divided into several segments, some of which are dedicated to regular broadcasting stations, such as the Voice of America, the British Broadcasting Corp. and the Voice of Russia. Throughout the world, there are hundreds of shortwave stations, according to the NASB. Shortwave stations can be heard for thousands of miles because the signals bounce off the ionosphere, and rebound back hundreds or thousands of miles from their point of origin.

SHF and EHF represent the highest frequencies in the radio band and are sometimes considered to be part of the microwave band. Molecules in the air tend to absorb these frequencies, which limits their range and applications. However, their short wavelengths allow signals to be directed in narrow beams by parabolic dish antennas (satellite dish antennas). This allows for short-range high-bandwidth communications to occur between fixed locations.

SHF, which is affected less by the air than EHF, is used for short-range applications such as Wi-Fi, Bluetooth and wireless USB (universal serial bus). SHF can work only in line-of-sight paths as the waves tend to bounce off objects like cars, boats and aircraft, according to the RF Page. And because the waves bounce off objects, SHF can also be used for radar.

Outer space is teeming with sources of radio waves: planets, stars, gas and dust clouds, galaxies, pulsars and even black holes. By studying these, astronomers can learn about the motion and chemical composition of these cosmic sources as well as the processes that cause these emissions.

A radio telescope "sees" the sky very differently than it appears in visible light. Instead of seeing point-like stars, a radio telescope picks up distant pulsars, star-forming regions and supernova remnants. Radio telescopes can also detect quasars, which is short for quasi-stellar radio source. A quasar is an incredibly bright galactic core powered by a supermassive black hole. Quasars radiate energy broadly across the EM spectrum, but the name comes from the fact that the first quasars to be identified emit mostly radio energy. Quasars are highly energetic; some emit 1,000 times as much energy as the entire Milky Way.

Radio astronomers often combine several smaller telescopes, or receiving dishes, into an array in order to make a clearer, or higher-resolution, radio image, according to the University of Vienna. For example, the Very Large Array (VLA) radio telescope in New Mexico consists of 27 antennas arranged in a huge "Y" pattern that's 22 miles (36 kilometers) across.

Additional resources:

This article was updated on Feb. 27, 2019 by Live Science contributor Traci Pedersen.

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Radio waves are a type of electromagnetic radiation best-known for their use in communication technologies, such as television, mobile phones and radios. These devices receive radio waves and convert them to mechanical vibrations in the speaker to create sound waves. The radio-frequency spectrum is a relatively small part of the electromagnetic (EM) spectrum.

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What is a picometer (pm), a measurement unit of length

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The picometer (pm) definition and its relation to other length measurement units. Online unit conversions and unit conversion tables

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refers to the emission, or ejection, of electrons from the surface of, generally, a metal in response to incident light.

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is a spectroscopic method that uses the near infrared region of the electromagnetic spectrum.

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are a type of electromagnetic radiation, and are useful in communications, radar and cooking.

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(Image credit: ESA and the Planck Collaboration)

Microwaves are a type of electromagnetic radiation, as are radio waves, ultraviolet radiation, X-rays and gamma-rays. Microwaves have a range of applications, including communications, radar and, perhaps best known by most people, cooking.

Electromagnetic radiation is transmitted in waves or particles at different wavelengths and frequencies. This broad range of wavelengths is known as the electromagnetic spectrum EM spectrum). The spectrum is generally divided into seven regions in order of decreasing wavelength and increasing energy and frequency. The common designations are radio waves, microwaves, infrared (IR), visible light, ultraviolet (UV), X-rays and gamma-rays. Microwaves fall in the range of the EM spectrum between radio and infrared light.

(Image credit: Shutterstock)

Microwaves have frequencies ranging from about 1 billion cycles per second, or 1 gigahertz (GHz), up to about 300 gigahertz and wavelengths of about 30 centimeters (12 inches) to 1 millimeter (0.04 inches), according to the Encyclopedia Britannica. This region is further divided into a number of bands, with designations such as L, S, C, X and K, according to Ginger Butcher's book "Tour of the Electromagnetic Spectrum."

Communications & radar

Microwaves are used mostly for point-to-point communications systems to convey all types of information, including voice, data and video in both analog and digital formats, according to the Federal Communications Commission (FCC). They are also used for supervisory control and data acquisition (SCADA) for remote machinery, switches, valves and signals.

Another important application of microwaves is radar. The word "radar" was originally an acronym for RAdio Detection And Ranging. Prior to World War II, British radio engineers found that short-wavelength radio waves could be bounced off distant objects like ships and aircraft, and the returning signal could be detected with highly sensitive directional antennas so the presence and locations of those objects could be determined. Usage of the term "radar" has become so common that it is now a word in and of itself, and can refer to systems that use microwaves or radio waves.

A little-known historical fact is that an early radar installation was built atop Kahuku Point on Oahu's northernmost tip. According to the state of Hawaii's website, the station actually detected the first wave of Japanese aircraft on their way to attack Pearl Harbor when the planes were 132 miles (212 kilometers) out. However, because the system had been in operation for only two weeks, it was considered unreliable, and the warning was ignored. Over the course of the war, radar was improved and refined, and it has since become an essential element of national defense and civilian air-traffic control.

Radar has found many other uses, some of which exploit the Doppler effect. An example of the Doppler effect can be demonstrated by an approaching ambulance: As it nears, the sound of the siren seems to rise in pitch, until it wails by. Then, as it recedes into the distance, the siren seems to lower in pitch.

Robert Mayanovic, a professor of physics at Missouri State University, said that Doppler radar, which often employs microwaves, is used for air-traffic control and vehicular speed-limit enforcement. When an object is approaching the antenna, the returning microwaves are compressed and thus have a shorter wavelength and higher frequency. Conversely, return waves from objects moving away are elongated and have a longer wavelength and lower frequency. By measuring this frequency shift, the speed of an object toward or away from the antenna can be determined.

Common applications of this principle include simple motion detectors, radar guns for speed-limit enforcement, radar altimeters and weather radar that can track the 3D motion of water droplets in the atmosphere. These applications are called active sensing, because microwaves are transmitted, and the reflected signals are received and analyzed. In passive sensing, natural sources of microwaves are observed and analyzed. Many of these observations are conducted

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Microwaves are a type of electromagnetic radiation, and are useful in communications, radar and cooking.

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is a region of the electromagnetic radiation spectrum where wavelengths range from about 700 nano meters (nm) to 1 millimeter (mm).

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Infrared radiation (IR), sometimes referred to simply as infrared, is a region of the electromagnetic radiation spectrum where wavelengths range from about 700 nanometers (nm) to 1 millimeter (mm). Infrared waves are longer than those of visible light, but shorter than those of radio waves. Correspondingly, the frequencies of IR are higher than those of microwaves, but lower than those of visible light, ranging from about 300 GHz to 400 THz.

Infrared light is invisible to the human eye, although longer infrared waves can be sensed as heat. It does, however, share some characteristics with visible light -- namely, infrared light can be focused, reflected and polarized.

Wavelength and frequency

Infrared can be subdivided into multiple spectral regions, or bands, based on wavelength; however, there is no uniform definition of each band's exact boundaries. Infrared is commonly separated into near-, mid- and far-infrared. It can also be divided into five categories: near-, short-wavelength, mid-, long-wavelength and far-infrared.

The near-IR band contains the range of wavelengths closest to the red end of the visible light spectrum. It is generally considered to consist of wavelengths measuring from 750 nm to 1,300 nm -- or 0.75 to 1.3 microns. Its frequency ranges from about 215 THz to 400 THz. This group consists of the longest wavelengths and shortest frequencies, and it produces the least heat.

Visible and invisible light

The intermediate IR band, also called the mid-IR band, covers wavelengths ranging from 1,300 nm to 3,000 nm -- or 1.3 to 3 microns. Frequencies range from 20 THz to 215 THz.

Wavelengths in the far-IR band, which are closest to microwaves, extend from 3,000 nm to 1 mm -- or 3 to 1,000 microns. Frequencies range from 0.3 THz to 20 THz. This group consists of the shortest wavelengths and longest frequencies, and it produces the most heat.

Infrared radiation uses

Infrared is used in a variety of applications. Among the most well-known are heat sensors, thermal imaging and night vision equipment.

In communications and networking, infrared light is used in wired and wireless operations. Remote controls use near-infrared light, transmitted with light-emitting diodes ( LEDs), to send focused signals to home-entertainment devices, such as televisions. Infrared light is also used in fiber optic cables to transmit data.

Electromagnetic spectrum and visible light

In addition, infrared is used extensively in astronomy to observe objects in space that can't be detected in whole or part by the human eye, including molecular clouds, stars, planets and active galaxies.

History of infrared radiation technology

Infrared was discovered by British astronomer Sir William Herschel in 1800. Herschel knew sunlight could be separated into separate components, a step accomplished by refracting the light through a glass prism. He then measured the temperatures of the different colors that were created. He found the temperature increased as the colors progressed from violet, blue, green, yellow, orange and red light. Herschel then went a step further, measuring the temperature in the portion beyond the red area. There, in the infrared area, he found the temperature to be the highest of all.

Continue Reading About infrared radiation (IR)

Dig Deeper on Mobile and wireless network technology

Margaret Rouse asks:

How is infrared used in your networking architecture?

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What is infrared radiation (IR)? - Definition from WhatIs.com

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Infrared radiation (IR), sometimes referred to simply as infrared, is a region of the electromagnetic radiation spectrum where wavelengths range from about 700 nanometers (nm) to 1 millimeter (mm). Infrared waves are longer than those of visible light, but shorter than those of radio waves.

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can be used to treat cancer, and gamma-rye bursts are studied by astronomers.

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This all-sky image, constructed from two years of observations by NASA's Fermi Gamma-ray Space Telescope, shows how the sky appears in gamma-ray light.

Credit: NASA/DOE/Fermi LAT Collaboration

Gamma-rays are a form of electromagnetic radiation, as are radio waves, infrared radiation, ultraviolet radiation, X-rays and microwaves. Gamma-rays can be used to treat cancer, and gamma-ray bursts are studied by astronomers.

Electromagnetic (EM) radiation is transmitted in waves or particles at different wavelengths and frequencies. This broad range of wavelengths is known as the electromagnetic spectrum. The spectrum is generally divided into seven regions in order of decreasing wavelength and increasing energy and frequency. The common designations are radio waves, microwaves, infrared (IR), visible light, ultraviolet (UV), X-rays and gamma-rays.

Gamma-rays fall in the range of the EM spectrum above soft X-rays. Gamma-rays have frequencies greater than about 1,018 cycles per second, or hertz (Hz), and wavelengths of less than 100 picometers (pm), or 4 x 10^9 inches. (A picometer is one-trillionth of a meter.)

Gamma-rays and hard X-rays overlap in the EM spectrum, which can make it hard to differentiate them. In some fields, such as astrophysics, an arbitrary line is drawn in the spectrum where rays above a certain wavelength are classified as X-rays and rays with shorter wavelengths are classified as gamma-rays. Both gamma-rays and X-rays have enough energy to cause damage to living tissue, but almost all cosmic gamma-rays are blocked by Earth's atmosphere.

Gamma-rays were first observed in 1900 by French chemist Paul Villard when he was investigating radiation from radium, according to the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA). A few years later, New Zealand-born chemist and physicist Ernest Rutherford proposed the name "gamma-rays," following the order of alpha rays and beta rays - names given to other particles that are created during a nuclear reaction - and the name stuck.

Gamma-rays are produced primarily by four different nuclear reactions: fusion, fission, alpha decay and gamma decay.

Nuclear fusion is the reaction that powers the sun and stars. It occurs in a multistep process in which four protons, or hydrogen nuclei, are forced under extreme temperature and pressure to fuse into a helium nucleus, which comprises two protons and two neutrons. The resulting helium nucleus is about 0.7 percent less massive than the four protons that went into the reaction. That mass difference is converted into energy, according to Einstein's famous equation E=mc^2, with about two-thirds of that energy emitted as gamma-rays. (The rest is in the form of neutrinos, which are extremely weakly interacting particles with nearly zero mass.) In the later stages of a star's lifetime, when it runs out of hydrogen fuel, it can form increasingly more massive elements through fusion, up to and including iron, but these reactions produce a decreasing amount of energy at each stage.

Another familiar source of gamma-rays is nuclear fission. Lawrence Berkeley National Laboratory defines nuclear fission as the splitting of a heavy nucleus into two roughly equal parts, which are then nuclei of lighter elements. In this process, which involves collisions with other particles, heavy nuclei, such as uranium and plutonium, are broken into smaller elements, such as xenon and strontium. The resulting particles from these collisions can then impact other heavy nuclei, setting up a nuclear chain reaction. Energy is released because the combined mass of the resulting particles is less than the mass of the original heavy nucleus. That mass difference is converted to energy, according to E=mc^2, in the form of kinetic energy of the smaller nuclei, neutrinos and gamma-rays.

Other sources of gamma-rays are alpha decay and gamma decay. Alpha decay occurs when a heavy nucleus gives off a helium-4 nucleus, reducing its atomic number by 2 and its atomic weight by 4. This process can leave the nucleus with excess energy, which is emitted in the form of a gamma-ray. Gamma decay occurs when there is too much energy in the nucleus of an atom, causing it to emit a gamma-ray without changing its charge or mass composition.

Gamma-rays are sometimes used to treat cancerous tumors in the body by damaging the DNA of the tumor cells. However, great care must be taken, because gamma-rays can also damage the DNA of surrounding healthy tissue cells.

One way to maximize the dosage to cancer cells while minimizing the exposure to healthy tissues is to direct multiple gamma-ray beams from a linear accelerator, or linac, onto the target region from many different directions. This is the operating principle of CyberKnife and Gamma Knife therapies.

Gamma Knife radiosurgery uses specialized equipment to focus close to 200 tiny beams of radiation on a tumor or other target in the brain. Each individual beam has very little effect on the brain tissue it passes through, but a strong dose of radiation is delivered at the point where the beams meet, according to Mayo Clinic.

One of the more interesting sources of gamma-rays are gamma-ray bursts (GRBs). These are extremely high-energy events that last from a few milliseconds to several minutes. They were first observed in the 1960s, and they are now observed somewhere in the sky about once a day.

Gamma-ray bursts are "the most energetic form of light," according to NASA. They shine hundreds of times brighter than a typical supernova and about a million-trillion times as bright as the sun.

According to Robert Patterson, a professor of astronomy at Missouri State University, GRBs were once thought to come from the last stages of evaporating mini black holes. They are now believed to originate in collisions of compact objects such as neutron stars. Other theories attribute these events to the collapse of supermassive stars to form black holes.

In either case, GRBs can produce enough energy that, for a few seconds, they can outshine an entire galaxy. Because Earth's atmosphere blocks most gamma-rays, they're seen only with high-altitude balloons and orbiting telescopes.

Further reading:

This article was updated on Nov. 29, 2018, by Live Science Contributor, Meredith Fore.

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Gamma-rays are a form of electromagnetic radiation, as are radio waves, infrared radiation, ultraviolet radiation, X-rays and microwaves. Gamma-rays can be used to treat cancer, and gamma-ray bursts are studied by astronomers. Electromagnetic (EM) radiation is transmitted in waves or particles at different wavelengths and frequencies.

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is all the wavelengths of light and reveals an otherwise invisible universe.

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Like science? Please donate to help EarthSky keep going. Rather donate via PayPal or send a check? Click here.
When you think of light, you probably think of what your eyes can see. But the light to which our eyes are sensitive is just the beginning; it is a sliver of the total amount of light that surrounds us. The electromagnetic spectrum is the term used by scientists to describe the entire range of light that exists. From radio waves to gamma rays, most of the light in the universe is, in fact, invisible to us!

Light is a wave of alternating electric and magnetic fields. The propagation of light isn't much different than waves crossing an ocean. Like any other wave, light has a few fundamental properties that describe it. One is its frequency, measured in Hertz, which counts the number of waves that pass by a point in one second. Another closely related property is wavelength: the distance from the peak of one wave to the peak of the next. These two attributes are inversely related. The larger the frequency, the smaller the wavelength - and vice versa.

The electromagnetic waves your eyes detect - visible light - oscillates between 400 and 790 terahertz (THz). That's several hundred trillion times a second. The wavelengths are roughly the size of a large virus: 390 - 750 nanometers (1 nanometer = 1 billionth of a meter). Our brain interprets the various wavelengths of light as different colors. Red has the longest wavelength, and violet the shortest. When we pass sunlight through a prism, we see that it's actually composed of many wavelengths of light. The prism creates a rainbow by redirecting each wavelength out a slightly different angle.

But light doesn't stop at red or violet. Just like there are sounds we can't hear (but other animals can), there is also an enormous range of light that our eyes can't detect. In general, the longer wavelengths come from the coolest and darkest regions of space. Meanwhile, the shorter wavelengths measure extremely energetic phenomena.

Astronomers use the entire electromagnetic spectrum to observe a variety of things. Radio waves and microwaves - the longest wavelengths and lowest energies of light - are used to peer inside dense interstellar clouds and track the motion of cold, dark gas. Radio telescopes have been used to map the structure of our galaxy while microwave telescopes are sensitive to the remnant glow of the Big Bang.

Infrared telescopes excel at finding cool, dim stars, slicing through interstellar dust bands, and even measuring the temperatures of planets in other solar systems. The wavelengths of infrared light are long enough to navigate through clouds that would otherwise block our view. By using large infrared telescopes, astronomers have been able to peer through the dust lanes of the Milky Way into the core of our galaxy.

The majority of stars emit most of their electromagnetic energy as visible light, the tiny portion of the spectrum to which our eyes are sensitive. Because wavelength correlates with energy, the color of a star tells us how hot it is: red stars are coolest, blue are hottest. The coldest of stars emit hardly any visible light at all; they can only be seen with infrared telescopes.

At wavelengths shorter than violet, we find the ultraviolet, or UV, light. You may be familiar with UV from its ability to give you a sunburn. Astronomers use it to hunt out the most energetic of stars and identify regions of star birth. When viewing distant galaxies with UV telescopes, most of the stars and gas disappear, and all the stellar nurseries flare into view.

Beyond UV, comes the highest energies in the electromagnetic spectrum: X-rays and gamma rays. Our atmosphere blocks this light, so astronomers must rely on telescopes in space to see the x-ray and gamma ray universe. X-rays come from exotic neutron stars, the vortex of superheated material spiraling around a black hole, or diffuse clouds of gas in galactic clusters that are heated to many millions of degrees. Meanwhile, gamma rays - the shortest wavelength of light and deadly to humans - unveil violent supernova explosions, cosmic radioactive decay, and even the destruction of antimatter. Gamma ray bursts - the brief flickering of gamma ray light from distant galaxies when a star explodes and creates a black hole - are among the most energetic singular events in the universe.

Bottom line: the electromagnetic spectrum describes all the wavelengths of light - both seen and unseen. The shorter the wavelength, the more energetic the light. By using telescopes sensitive to different wavelength ranges of the spectrum, astronomers get a glimpse into a wide variety of objects and phenomena in the universe.

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What is the electromagnetic spectrum? | EarthSky.org

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Like science? Please donate to help EarthSky keep going. Rather donate via PayPal or send a check? Click here. When you think of light, you probably think of what your eyes can see. But the light to which our eyes are sensitive is just the beginning; it is a sliver of the total amount of light that surrounds us.

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is a region in the infrared spectrum of electromagnetic radiation.

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