Matt Williams, Author at Universe Today

August 21, 2011October 1, 2016 by Matt Williams

What is Galactic Cannibalism?

Seattle, January, 2003. Two prestigious astronomers: Puragra GuhaThakurta of UCSC and David Reitzel of UCLA present some new findings to the American Astronomical Society that would seem to indicate that large spiral galaxies grow by gobbling up smaller satellite galaxies. Their evidence, a faint trail of stars in the nearby Andromeda galaxy that are thought to be a vast trail of debris left over from an ancient merger of Andromeda with another, smaller galaxy. This process, known as Galactic Cannibalism is a process whereby a large galaxy, through tidal gravitational interactions with a companion galaxy, merges with that companion, resulting in a larger galaxy.

The most common result of this process is an irregular galaxy of one form or another, although elliptical galaxies may also result. Several examples of this have been observed with the help of the Hubble telescope, which include the Whirlpool Galaxy, the Mice Galaxies, and the Antennae Galaxies, all of which appear to be in one phase or another of merging and cannibalising. However, this process is not to be confused with Galactic Collision which is a similar process where galaxies collide, but retain much of their original shape. In these cases, a smaller degree of momentum or a considerable discrepancy in the size of the two galaxies is responsible. In the former case, the galaxies cease moving after merging because they have no more momentum to spare; in the latter, the larger galaxies shape overtakes the smaller one and their appears to be little in the way of change.

All of this is consistent with the most current, hierarchical models of galaxy formation used by NASA, other space agencies and astronomers. In this model, galaxies are believed to grow by ingesting smaller, dwarf galaxies and the minihalos of dark matter that envelop them. In the process, some of these dwarf galaxies are shredded by the gravitational tidal forces when they travel too close to the center of the “host” galaxy’s enormous halo. This, in turn, leaves streams of stars behind, relics of the original event and one of the main pieces of evidence for this theory. It has also been suggested that galactic cannibalism is currently occurring between the Milky Way and the Large and Small Magellanic Clouds that exist beyond its borders. Streams of gravitationally-attracted hydrogen arcing from these dwarf galaxies to the Milky Way is taken as evidence for this theory.

As interesting as all of these finds are, they don’t exactly bode well for those of us who call the Milky Way galaxy, or any other galaxy for that matter, home! Given our proximity to the Andromeda Galaxy and its size – the largest galaxy of the Local Group, boasting over a trillion stars to our measly half a trillion – it is likely that our galaxy will someday collide with it. Given the sheer scale of the tidal gravitational forces involved, this process could prove disastrous for any and all life forms and planets that are currently occupy it!

We have written many articles about galactic cannibalism for Universe Today. Here’s an article about ancient galaxies feeding on gas, and here’s an article about an article, Galactic Ghosts Haunt Their Killers.

If you’d like more info on galaxies, check out Hubblesite’s News Releases on Galaxies, and here’s NASA’s Science Page on Galaxies.

We’ve also recorded an episode of Astronomy Cast about galaxies. Listen here, Episode 97: Galaxies.

Sources:
http://en.wikipedia.org/wiki/Interacting_galaxy
http://en.wikipedia.org/wiki/Andromeda_Galaxy
http://www1.ucsc.edu/currents/02-03/01-13/debris.html
http://blogs.physicstoday.org/update/2009/10/galactic-cannibalism.html
http://news.discovery.com/space/hubble-spiesz-aftermath-of-galactic-cannibalism.html

July 16, 2011July 24, 2020 by Matt Williams

What are Active Optics?

For astronomers and physicists alike, the depths of space are a treasure trove that may provide us with the answers to some of the most profound questions of existence. Where we come from, how we came to be, how it all began, etc. However, observing deep space presents its share of challenges, not the least of which is visual accuracy.

In this case, scientists use what is known as Active Optics in order to compensate for external influences. The technique was first developed during the 1980s and relied on actively shaping a telescope’s mirrors to prevent deformation. This is necessary with telescopes that are in excess of 8 meters in diameter and have segmented mirrors.

Definition:

The name Active Optics refers to a system that keeps a mirror (usually the primary) in its optimal shape against all environmental factors. The technique corrects for distortion factors, such as gravity (at different telescope inclinations), wind, temperature changes, telescope axis deformation, and others.

The twin Keck telescopes shooting their laser guide stars into the heart of the Milky Way on a beautifully clear night on the summit on Mauna Kea. Credit: keckobservatory.org/Ethan Tweedie — *The twin Keck telescopes shooting their laser guide stars into the heart of the Milky Way on a beautifully clear night on the summit on Mauna Kea. Credit: keckobservatory.org/Ethan*

Adaptive Optics actively shapes a telescope’s mirrors to prevent deformation due to external influences (like wind, temperature, and mechanical stress) while keeping the telescope actively still and in its optimal shape. The technique has allowed for the construction of 8-meter telescopes and those with segmented mirrors.

Use in Astronomy:

Historically, a telescope’s mirrors have had to be very thick to hold their shape and to ensure accurate observations as they searched across the sky. However, this soon became unfeasible as the size and weight requirements became impractical. New generations of telescopes built since the 1980s have relied on very thin mirrors instead.

But since these were too thin to keep themselves in the correct shape, two methods were introduced to compensate. One was the use of actuators which would hold the mirrors rigid and in an optimal shape, the other was the use of small, segmented mirrors which would prevent most of the gravitational distortion that occur in large, thick mirrors.

This technique is used by the largest telescopes that have been built in the last decade. This includes the Keck Telescopes (Hawaii), the Nordic Optical Telescope (Canary Islands), the New Technology Telescope (Chile), and the Telescopio Nazionale Galileo (Canary Islands), among others.

*The New Technology Telescope (NTT) pioneered the Active Optics. Credit: ESO/C.Madsen. Bacon*

Other Applications:

In addition to astronomy, Active Optics is used for a number of other purposes as well. These include laser set-ups, where lenses and mirrors are used to steer the course of a focused beam. Interferometers, devices which are used to emit interfering electromagnetic waves, also relies on Active Optics.

These interferometers are used for the purposes of astronomy, quantum mechanics, nuclear physics, fiber optics, and other fields of scientific research. Active optics are also being investigated for use in X-ray imaging, where actively deformable grazing incidence mirrors would be employed.

Adaptive Optics:

Active Optics are not to be confused with Adaptive Optics, a technique that operates on a much shorter timescale to compensate for atmospheric effects. The influences that active optics compensate for (temperature, gravity) are intrinsically slower and have a larger amplitude in aberration.

*Artist’s impression of the European Extremly Large Telescope deploying lasers for adaptive optics. Credit: ESO/L. Calçada/N. Risinger*

On the other hand, Adaptive Optics corrects for atmospheric distortions that affect the image. These corrections need to be much faster, but also have smaller amplitude. Because of this, adaptive optics uses smaller corrective mirrors (often the second, third or fourth mirror in a telescope).

We have written many articles about optics for Universe Today. Here’s The Photon Sieve Could Revolutionize Optics, What did Galileo Invent?, What did Isaac Newton Invent?, What are the Biggest Telescopes in the World?

We’ve also recorded an entire episode of Astronomy Cast all about Adaptive Optics. Listen here, Episode 89: Adaptive Optics, Episode 133: Optical Astronomy, and Episode 380: The Limits of Optics.

Sources:

July 4, 2011December 24, 2015 by Matt Williams

Absorption of Light

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Those who can remember sitting through elementary science class might recall learning that with all matter, light is absorbed and converted into energy. In the case of plants, this process is known as photosynthesis. However, they are by no means the only species or objects that do this. In truth, all objects, living or inorganic are capable of absorbing light. In all cases, absorption depends on the electromagnetic frequency of the light being transmitted (i.e. the color) and the nature of the atoms of the object. If they are complementary, light will be absorbed; if they are not, then the light will be reflected or transmitted. In most cases, these processes occur simultaneously and to varying degrees, since light is usually transmitted at various frequencies. Therefore most objects will selectively absorb light while also transmitting and/or reflecting some of it. Wherever absorption occurs, heat energy is generated.

As already noted, absorption depends upon the state of an objects electrons. All electrons are known to vibrate at specific frequencies, what is commonly known as their natural frequency. When light, in the form of photons, interacts with an atom with the same natural frequency, the electrons of that atom will become excited and set into a natural vibrational motion. During this vibration, the electrons of the atom interact with neighboring atoms in such a way as to convert this vibrational energy into thermal energy. Subsequently, the light energy is not to be seen again, hence why absorption is differentiated from reflection and transmission. And since different atoms and molecules have different natural frequencies of vibration, they will selectively absorb different frequencies of visible light.

By relying on this method, physicists are able to determine the properties and material composition of an object by seeing which frequencies of light it is able to absorb. Whereas some materials are opaque to some wavelengths of light, they transparent to others. Wood, for example, is opaque to all forms of visible light. Glass and water, on the other hand, are opaque to ultraviolet light, but transparent to visible light.

Ultimately, absorption of electromagnetic radiation requires the generation of the opposite field, in other words, the field which has the opposite coefficient in the same mode. A good demonstration of this is color. If a material or matter absorbs light of certain wavelengths (or colors) of the spectrum, an observer will not see these colors in the reflected light. On the other hand if certain wavelengths of colors are reflected from the material, an observer will see them and see the material in those colors. For example, the leaves of green plants contain a pigment called chlorophyll, which absorbs the blue and red colors of the spectrum and reflects the green. Leaves therefore appear green, whereas reflected light often appears to the naked eye to be refracted into several colors of the spectrum (i.e. a rainbow effect).

We have written many articles about the absorption of light for Universe Today. Here’s an article about absorption spectra, and here’s an article about absorption spectroscopy.

If you’d like more info on light absorption, check out an article about Light Absorption, Reflection, and Transmission. Also, here’s an article about reflection and absorption of light.

We’ve also recorded an entire episode of Astronomy Cast all about Energy Levels and Spectra. Listen here, Episode 139: Energy Levels and Spectra.

Sources:
http://en.wikipedia.org/wiki/Absorption_%28electromagnetic_radiation%29
http://hyperphysics.phy-astr.gsu.edu/hbase/biology/ligabs.html
http://www.physicsclassroom.com/class/light/u12l2c.cfm
http://www.andor.com/learning/light/?docid=333
http://www.chemicool.com/definition/absorption_of_light.html
http://hyperphysics.phy-astr.gsu.edu/hbase/biology/photosyn.html#c1

July 4, 2011October 18, 2016 by Matt Williams

What is Absolute Space?

The explosion in the sciences that took place in the 17th and 18th centuries revolutionized not only the way we think of our world, but of time and space itself. Much of this is owed to individuals like Sir Isaac Newton, a man whose theories came to form the basis of modern physics. Though much of his theories would later come to be challenged with the discovery of relativity and quantum mechanics, they were nonetheless extremely influential because they gave later generations a framework. It is to him, for example, that we are indebted for the notions of Absolute Time and Absolute Space, and how the two were thought to be separate aspects of objective reality.
In his magnum opus, PhilosophiæNaturalis Principia Mathematica (Mathematical Principles for Natural Philosophy), Newton laid the groundwork for the concept of Absolute Space thusly:

“Absolute space, in its own nature, without regard to anything external, remains always similar and immovable. Relative space is some movable dimension or measure of the absolute spaces; which our senses determine by its position to bodies: and which is vulgarly taken for immovable space … Absolute motion is the translation of a body from one absolute place into another: and relative motion, the translation from one relative place into another.”

In other words, Absolute Space is the study of space as an absolute, unmoving reference point for what inertial systems (i.e. planets and other objects) exist within it. Thus, every object has an absolute state of motion relative to absolute space, so that an object must be either in a state of absolute rest, or moving at some absolute speed.

These views were controversial even in Newton’s own time. However, it was with the advent of modern physics and the Theory of Special Relativity, that much of the basis for Newtonian physics would come to be shattered. In essence, special relativity proposed that time and space are not independent realities but different expressions of the same thing. In this model, time and motion are dependent on the observer and there is no fixed point of reference, only relative forms of motion which are determined by comparing them to other points of reference.

However, it would be fair to say that it was Newton’s own definitions of space and time as independent phenomena that allowed for the development of physics as we know it today. By giving physicists clear definitions to work with and challenge, later generations of scientists like Einstein were able to express clearly how space was not absolute since it itself was always in motion, and how one could not divorce space from time.

We have written many articles about absolute space for Universe Today. Here’s an article about what is space, and here’s an article about how cold space is.

If you’d like more info on absolute space, check out an article about Isaac Newton’s “Absolute Space”. Also, here’s an article about Absolute Time and Space.

We’ve also recorded an entire episode of Astronomy Cast all about Space Elevators. Listen here, Episode 144: Space Elevators.

Sources:
http://en.wikipedia.org/wiki/Absolute_time_and_space
http://en.wikipedia.org/wiki/Sir_Isaac_Newton
http://plato.stanford.edu/entries/newton-stm/
http://en.wikipedia.org/wiki/Special_relativity
http://novan.com/spcenrgy.htm

July 4, 2011April 26, 2016 by Matt Williams

What is Absolute Pressure?

When it comes to measurements, the everyday kind that deal with things like air pressure, tire pressure, blood pressure, etc., there is no such thing as an absolute accuracy. And yet, as with most things, scientists are able to come up with a relatively accurate way of gauging these things by measuring them relative to other things. When it comes to air pressure (say for example, inside a tire), this takes the form of measuring it relative to ambient air temperature, or a perfect vacuum. The latter case, where zero pressure is referred against a total vacuum, is known as Absolute Pressure. The name may seem slightly ironic, but since the comparison is against an environment in which there is no air pressure to speak of.

In the larger context of pressure measurement, Absolute Pressure is part of the “zero reference” trinity. This includes Absolute Pressure (AP), Gauge Pressure, and Differential Pressure. As already noted, AP is zero referenced against a perfect vacuum. This is the method of choice when measuring quantities where absolute values must be determined. Gauge Pressure, on the other hand, is referenced against ambient air pressure, and is used for conventional purposes such as measuring tire and blood pressure. Differential Pressure is quite simply the difference between the two points.

Cases where AP are used include atmospheric pressures readings: where one is trying to determine air pressure (expressed in units of atm’s, where one is equal to 101,325 Pa), Mean Sea Level pressure (the air pressure at sea level; on average: 101.325 kPa), or the boiling point of water (which varies based on elevation and differences in air pressure). Another instance of AP being the method of choice is with the measurement of deep vacuum pressures (aka. outer space) where absolute readings are needed since scientists are dealing with a near-total vacuum. Altimeter pressure is another instance, where air pressure is used to determine the altitude of an aircraft and absolute values are needed to ensure both accuracy and safety.

To produce an absolute pressure sensor, manufacturer will seal a high vacuum behind the sensing diaphragm. If the connection of an absolute pressure transmitter is open to the air, it will read the actual barometric pressure (which is roughly 14.7 PSI). This is different from most gauges, such as those used to measure tire pressure, in that such gauges are calibrated to take into account ambient air pressure (i.e. registering 14.7 PSI as zero).

We have written many articles about absolute pressure for Universe Today. Here’s an article about Boyle’s Law, and here’s an article about air density.

If you’d like more info on absolute pressure, check out an article about pressure from Wikipedia. Also, here’s another article from Engineering Toolbox.

We’ve also recorded an entire episode of Astronomy Cast all about Temperature. Listen here, Episode 204: Temperature.

Sources:
http://en.wikipedia.org/wiki/Pressure_measurement
http://www.pumpworld.com/absolute%20pressure.htm
http://www.sensorsone.co.uk/pressure-measurement-glossary/absolute-pressure.html
http://en.wikipedia.org/wiki/Atmospheric_pressure
http://en.wikipedia.org/wiki/Altimeter

March 26, 2011April 26, 2016 by Matt Williams

Southern Cross Constellation

[/caption]For the lucky residents of the Southern Hemisphere, or those fortunate enough to enjoy a vacation in Hawaii or Cancun, there’s a stellar delight that few Northerners know about. It’s called the Southern Cross, a small but beautiful constellation located in the southern sky, very close to the neighboring constellation of Centaurus. Originally known by the Latin name Crux, which is due to its cross shape, this constellation is one of the easiest to identify in the night sky. For centuries, it has served as a navigational beacon for sailors, an important symbol to the Egyptians, and played an important role in the spiritual beliefs of the Aborigines and many other cultures in the Southern Hemisphere.

The first recorded example of Crux’s discovery was around 1000 BC during the time of the Ancient Greeks. At the latitude of Athens, Crux was clearly visible, though low in the night sky. At the time, the Greeks identified it as being part of the constellation Centaurus. However, the precession of the equinoxes gradually lowered its stars below the European horizon, and they were eventually forgotten by the inhabitants of northern latitudes. Crux fell into anonymity for northerners until the Age of Discovery (from the early 15th to early 17th centuries) when it was rediscovered by Europeans. The first to do so were the Portuguese, who mapped it for navigation uses while rounding the southern tip of Africa. During this time, Crux was also separated from Centaurus, though it is not altogether clear who was responsible. Some attribute it to the French astronomer Augustin Royer who did it in 1679 while others believe it was Dutch astronomer PetrusPlancius who did the deed in 1613. Regardless, it is believed to have taken place in the 17th century, placing it within the context of European expansion and the revolution that was taking place in the sciences at the time.

In terms of cultural significance, the Crux, like all constellations, played an important role in the belief system of many cultures. In the ancient mountaintop village of Machu Picchu, a stone engraving exists which depicts the constellation. In addition, in Quechua (the language of the Incas) Crux is known as “Chakana”, which literally means “stair”, and holds deep symbolic value in Incan mysticism (the cross represented the three tiers of the world: the underworld, world of the living, and the heavens). To the Aborigines and the Maori, Crux is representative of animist spirits who play a central role in their ancestral beliefs. To the ancient Egyptians, Crux was the place where the Sun Goddess Horus was crucified, and marked the passage of the winter season. The Southern Cross is also featured prominently on the flags of several southern nations, including Australia, Brazil, New Zealand, Papua New Guinea, and Samoa.

We have written many articles about the Southern Cross constellation for Universe Today. Here’s an article about Crux, and here’s an article about constellations.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We’ve done many episodes of Astronomy Cast about stars. Listen here, Episode 12: Where Do Baby Stars Come From?

References:
http://en.wikipedia.org/wiki/Crux
http://en.wikipedia.org/wiki/Age_of_Discovery
http://library.thinkquest.org/C005462/scross.html
http://www.windows2universe.org/the_universe/crux.html
http://www.ancientworlds.net/aw/Article/941062

March 26, 2011December 24, 2015 by Matt Williams

Radioisotope

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It was just over a century ago that a little known French scientist named Henri Becquerel came across something new and immensely startling. At the time, while working with phosphorescent materials (i.e. materials that glow in the dark after being subjected to light), he discovered naturally occurring rays that he couldn’t account for. In time, these rays were discovered to be present in several naturally occurring elements, and were dubbed radioactivity. Those metals that exhibited them also came to be known as Radioactive Isotopes.

Radioisotopes, (also known as radioactive isotopes or radionuclides), are atoms with a different number of neutrons than a usual atom. Due to this imbalance, these isotopes have an unstable nucleus that decays, and in the process emitting alpha, beta and gamma rays until the isotope reaches stability. Once it’s stable, the isotope has transformed into another element entirely. Every chemical element has one or more radioisotopes, with over 1,000 isotopes accounted for in total. Approximately 50 of these are found in nature; the rest are produced artificially as the direct result of nuclear reactions or indirectly as the radioactive descendants of these products.

Of the naturally occurring radioisotopes, there are three categories that are used to group them. The first is primordial radionuclides, which originate mainly within the interior of stars and like uranium and thorium, are still present because their half-lives are so long that they have not yet completely decayed. The second group, secondary radionuclides, are radiogenic isotopes derived from the decay of primordial radionuclides and are characterized by their shorter half-lives. The third and final group is known cosmogenic radionuclides, which consists of isotopes like Carbon 14 which are constantly produced in the atmosphere due to cosmic rays. Artificially produced radionuclides, on the other hand, are produced by nuclear reactors, particle accelerators or by radionuclide generators (where a parent isotope, usually produced in a nuclear reactor, is allowed to decay to produce a radioisotope). In addition, nuclear explosions are known to produce artificial radioisotopes as well.

Radioisotopes are used today for a variety of purposes. When it comes to the field of nuclear medicine, radioactive isotopes are used in MRI’s and X-rays for diagnostic purposes, for targeted radiation therapy, and to sterilize medical equipment. In biochemistry and genetics, radionuclides are used in molecular and DNA research in order to “label” molecules and trace chemical and physiological processes. Carbon-14, a naturally occurring cosmogenic isotope, is used for carbon dating by archeologists, paleontologists, and geologists. In agriculture, radiation is used to stop the sprouting of root crops, kill parasites and pests, and in veterinary medicine. And when it comes to industry, radionuclides are used to study the rate of wear and corrosion of metals, to test for leaks and seams, analyze pollutants, study the movement of surface water, measure water runoffs from rain and snow, and the flow rates of streams and rivers.

We have written many articles about radioisotopes for Universe Today. Here’s an article about isotopes, and here’s an article about radioactive decay.

If you’d like more info on radioisotopes, check out these articles from NDT Resource Center and Science Courseware.

We’ve also recorded an entire episode of Astronomy Cast all about the Age of the Universe. Listen here, Episode 122: How Old is the Universe?.

References:
http://en.wikipedia.org/wiki/Radionuclide
http://en.wikipedia.org/wiki/Radioactive_decay
http://www.britannica.com/EBchecked/topic/489027/radioactive-isotope
http://en.wikipedia.org/wiki/Radiocarbon_dating
http://www.ehow.com/about_5095610_radioactive-isotopes.html

March 26, 2011December 24, 2015 by Matt Williams

Law of Inertia

[/caption]In the world of physics, there are few people who have been more influential than Sir Isaac Newton. In addition to his contributions to astronomy, mathematics, and empirical philosophy, he is also the man who pioneered classical physics with his laws of motion. Of these, the first, otherwise known as the Law of Inertia, is the most famous and arguably the most important. In the language of science, this law states that: Every body remains in a state of constant velocity unless acted upon by an external unbalanced force. This means that in the absence of a non-zero net force, the center of mass of a body either remains at rest, or moves at a constant velocity. Put simply, it states that a body will remain at rest or in motion unless acted upon by an external and unbalanced force.

Prior to Aristotle’s theories on inertia, the most generally accepted theory of motion was based on Aristotelian philosophy. This ancient theory stated that, in the absence of an external motivating power, all objects on Earth would come to rest and that moving objects only continue to move so long as long there is a power inducing them to do so. In a void, no motion would be possible since Aristotle’s theory claimed that the motion of objects was dependent on the surrounding medium, that it was responsible for moving the object forward in some way. By the Renaissance, however, this theory was coming to be rejected as scientists began to postulate that both air resistance and the weight of an object would play a role in arresting the motion of that object.

Further advances in astronomy were another nail in this coffin. The Aristotelian division of motion into “mundane” and “celestial” became increasingly problematic in the face of Copernicus’ model in the 16th century, who argued that the earth (and everything on it) was in fact never “at rest”, but was actually in constant motion around the sun.Galileo, in his further development of the Copernican model, recognized these problems and would later go on to conclude that based on this initial premise of inertia, it is impossible to tell the difference between a moving object and a stationary one without some outside point of comparison.

Thus, though Newton was not the first to express the concept of inertia, he would later refine and codify them as the first law of motion in his seminal work PhilosophiaeNaturalis Principia Mathematica (Mathematical Principals of Natural Philosophy) in 1687, in which he stated that: unless acted upon by a net unbalanced force, an object will maintain a constant velocity. Interestingly enough, the term “interia” was not used in the study. It was in fact JohanneKepler who first used it in his Epitome AstronomiaeCopernicanae (Epitome of Copernican Astronomy) published from 1618–1621. Nevertheless, the term would later come to be used and Newton recognized as being the man most directly responsible for its articulation as a theory.

We have written many articles about the law of inertia for Universe Today. Here’s an article about Newton’s Laws of Motion, and here’s an article about Newton’s first law.

If you’d like more info on the law of inertia, check out these articles from How Stuff Works and NASA.

We’ve also recorded an entire episode of Astronomy Cast all about Gravity. Listen here, Episode 102: Gravity.

References:
http://en.wikipedia.org/wiki/Inertia
http://en.wikipedia.org/wiki/Isaac_Newton
http://en.wikipedia.org/wiki/Newton%27s_laws_of_motion
http://science.howstuffworks.com/science-vs-myth/everyday-myths/newton-law-of-motion1.htm

March 26, 2011April 26, 2016 by Matt Williams

Emissivity of Materials

[/caption]In the last few centuries, in which time we have had several scientific revolutions, our understanding of heat, energy and the exchange thereof has grown exponentially. In particular has been the increasing ability to gauge the amounts of energy involved in particular processes and in turn create theoretical frameworks, units, and even tools with which to measure them. One such concept is the measurement known as Emissivity. Essentially, this is the relative ability of a material’s surface (usually written ? or e) to emit energy as radiation. It is expressed as the ratio of the emissivity of the material in question to the radiation emitted by a blackbody (an idealized physical body that absorbs all incident electromagnetic radiation) at the same temperature. This means that while a true black body would have an emissivity value of 1 (? = 1), any other object, known as a “grey body”, would have an emissivity value of less than 1 (? < 1). In general, the duller and blacker a material is, the closer its emissivity is to 1. The more reflective a material is, the lower its emissivity. Emissivity also depends on such factors as temperature, emission angle, and wavelength of the radiation. At the opposite end of the spectrum is the material’s absorptivity (or absorptance), which is the measure of radiation absorbed by a material at a particular wavelength. When dealing with non-black surfaces, the relative emissivity follows Kirchhoff's law of thermal radiation which states that emissivity is equal to absorptivity. Essentially an object that does not absorb all incident light will also emit less radiation than an ideal black body. An important function for emissivity has to do with the Earth’s atmosphere. Like all other “grey bodies”, the Earth’s atmosphere is able to absorb and emit radiation. The overall emissivity of Earth's atmosphere varies according to cloud cover and the concentration of gases that absorb and emit energy in the thermal infrared (i.e. heat energy). In this way, and by using the same criteria by which they are able to calculate the emissivity of “grey bodies”, scientists are able to calculate the amount of thermal radiation emitted by the atmosphere, thereby gaining a better understanding of the Greenhouse Effect. Every known material has an emissivity coefficient. Those that have a higher coefficient tend to be polished metals, such as aluminum and anodized metals. However, certain materials that are not metals and are non-reflective, such as red bricks, asbestos, concrete and pressed carbon, have equally high coefficients. In addition, naturally occurring materials such as ice, marble, and lime also have high emissivity coefficients. We have written many articles about emissivity of materials for Universe Today. Here's an article about heat rejection systems, and here's an article about absorptivity. If you'd like more info on emissivity, check out these articles from Engineering Toolbox and Science World.

We’ve also recorded an entire episode of Astronomy Cast all about Electromagnetism. Listen here, Episode 103: Electromagnetism.

References:
http://en.wikipedia.org/wiki/Emissivity
http://en.wikipedia.org/wiki/Absorptance
http://en.wikipedia.org/wiki/Black_body
http://www.thefreedictionary.com/emissivity
http://www.monarchserver.com/TableofEmissivity.pdf

February 24, 2011April 26, 2016 by Matt Williams

Tachyon

[/caption]Ever since Einstein unveiled his theory of relativity, the speed of light has been considered to be the physical constant of the universe, interrelating space and time. In short, it was the speed at which light and all other forms of electromagnetic radiation were believed to travel at all times in empty space, regardless of the motion of the source or the inertial frame of reference of the observer. But suppose for a second that there was a particle that defied this law, that could exist within the framework of a relativistic universe, but at the same time defy the foundations on which its built? Sounds impossible, but the existence of such a particle may very well be necessary from a quantum standpoint, resolving key issues that arise in that chaotic theory. It is known as the Tachyon Particle, a hypothetical subatomic particle that can move faster than light and poses a number intriguing problems and possibilities to the field of physics.

In the language of special relativity, a tachyon would be a particle with space-like four-momentum and imaginary proper time. Their existence was first attributed to German physicist Arnold Sommerfeld; even though it was Gerald Feinberg who first coined the term in the 1960s, and several other scientists helped to advance the theoretical framework within which tachyons were believed to exist. They were originally proposed within the framework of quantum field theory as a way of explaining the instability of the system, but have nevertheless posed problems for the theory of special relativity.

For example, if tachyons were conventional, localizable particles that could be used to send signals faster than light, this would lead to violations of causality in special relativity. But in the framework of quantum field theory, tachyons are understood as signifying an instability of the system and treated using a theory known as tachyon condensation, a process that attempts to resolve their existence by explaining them in terms of better understood phenomena, rather than as real faster-than-light particles. Tachyonic fields have appeared theoretically in a variety of contexts, such as the bosonic string theory. In general, string theory states that what we see as “particles” —electrons, photons, gravitons and so forth—are actually different vibrational states of the same underlying string. In this framework, a tachyon would appear as either indication of instability in the D-brane system or within spacetime itself.

Despite the theoretical arguments against the existence of tachyon particles, experimental searches have been conducted to test the assumption against their existence; however, no experimental evidence for the existence of tachyon particles has been found.

We have written many articles about tachyon for Universe Today. Here’s an article about elementary particles, and here’s an article about Einstein’s Theory of Relativity.

If you’d like more info on tachyon, check out these articles from Science World. Also, you may want to browse through a forum discussion about tachyons.

We’ve also recorded an entire episode of Astronomy Cast all about the Theory of Special Relativity. Listen here, Episode 9: Einstein’s Theory of Special Relativity.

Sources:
http://en.wikipedia.org/wiki/Tachyon
http://en.wikipedia.org/wiki/Speed_of_light
http://scienceworld.wolfram.com/physics/Tachyon.html
http://en.wikipedia.org/wiki/D-brane
http://www.nasa.gov/centers/glenn/technology/warp/warp.html