Blazar

A blazar is a very compact quasar (quasi-stellar radio source) associated with a presumed super-massive black hole at the center of an active, giant elliptical galaxy. Blazars are among the most energetic phenomena in the universe and are an important topic inextragalactic astronomy.

Blazars are members of a larger group of active galaxies that host active galactic nuclei (AGN). A few rare objects may be "intermediate blazars" that appear to have a mixture of properties from both optically violent variable (OVV) quasars and BL Lac objects. The name "blazar" was originally coined in 1978 by astronomer Edward Spiegel to denote the combination of these two classes.

Blazars are AGN with a relativistic jet that is pointing in the general direction of the Earth. We observe "down" the jet, or nearly so, and this accounts for the rapid variability and compact features of both types of blazars. Many blazars have apparent superluminal features within the first few parsecs of their jets, probably due to relativistic shock fronts.

The generally accepted picture is that OVV quasars are intrinsically powerful radio galaxies while BL Lac objects are intrinsically weak radio galaxies. In both cases the host galaxies are giant ellipticals.

Alternative models, for example, gravitational lensing, may account for a few observations of some blazars which are not consistent with the general properties.

Structure

Blazars, like all AGN, are thought to be ultimately powered by material falling onto a super-massive black hole at the center of the host galaxy. Gas, dust and the occasional star are captured and spiral into this central black hole creating a hot accretion disk which generates enormous amounts of energy in the form of photons, electrons, positrons and other elementary particles. This region is quite small, approximately 10⁻³ parsecs in size.

There is also a larger opaque toroid extending several parsecs from the central black hole, containing a hot gas with embedded regions of higher density. These "clouds" can absorb and then re-emit energy from regions closer to the black hole. On Earth the clouds are detected as emission lines in the blazar spectrum.

Perpendicular to the accretion disk, a pair of relativistic jets carries a highly energetic plasma away from the AGN. The jet is collimated by a combination of intense magnetic fields and powerful winds from the accretion disk and toroid. Inside the jet, high energy photons and particles interact with each other and the strong magnetic field. These relativistic jets can extend as far as many tens of kiloparsecs from the central black hole.

All of these regions can produce a variety of observed energy, mostly in the form of a nonthermal spectrum ranging from very low frequency radio to extremely energetic gamma rays, with a high polarization (typically a few percent) at some frequencies. The nonthermal spectrum consists of synchrotron radiation in the radio to X-ray range, and inverse Compton emission in the X-ray to gamma-ray region. A thermal spectrum peaking in the ultraviolet region and faint optical emission lines are also present in OVV quasars, but faint or non-existent in BL Lac objects.

Relativistic beaming

Viewing angle - 1. at 90 degrees to the jet:: Radio galaxy / Seyfert 2 Galaxy; 2, 3. at an angle to the jet: Quasar/Seyfert 1 Galaxy; 4. down the jet: Blazar.

The observed emission from a blazar is greatly enhanced by relativistic effects in the jet, a process termed relativistic beaming. The bulk speed of the plasma that constitutes the jet can be in the range of 95%–99% of the speed of light. (This bulk velocity is not the speed of a typical electron or proton in the jet. The individual particles move in many directions with the result being that the net speed for the plasma is in the range mentioned.)

The relationship between the luminosity emitted in the rest frame of the jet and the luminosity observed from Earth depends on the characteristics of the jet. These include whether the luminosity arises from a shock front or a series of brighter blobs in the jet, as well as details of the magnetic fields within the jet and their interaction with the moving particles.

A simple model of beaming however, illustrates the basic relativistic effects connecting the luminosity emitted in the rest frame of the jet, S_e and the luminosity observed on Earth, S_o. These are connected by a term referred to in astrophysics as the doppler factor, D, where S_o is proportional to S_e × D².

When looked at in much more detail than shown here, three relativistic effects are involved:

• Relativistic Aberration contributes a factor of D². Aberration is a consequence of special relativity where directions which appear isotropic in the rest frame (in this case, the jet) appear pushed towards the direction of motion in the observer's frame (in this case, the Earth).
• Time Dilation contributes a factor of D⁺¹. This effect speeds up the apparent release of energy. If the jet emits a burst of energy every minute in its own rest frame this may be observed on Earth as being a much faster release, perhaps one burst every ten seconds.
• Windowing can contribute a factor of D⁻¹ and then works to decrease the amount of boosting. This happens for a steady flow, because there are then D fewer elements of fluid within the observed window, as each element has been expanded by factor D. However, for a freely propagating blob of material, the radiation is boosted by the full D⁺³.

An example

Consider a jet with an angle to the lines of sight θ = 5° and a speed of 99.9% of the speed of light. On Earth the observed luminiosity is 70 times that of the emitted luminosity. However if θ is at the minimum value of 0° the jet will appear 600 times brighter from Earth.

Beaming away

Relativistic beaming also has another critical consequence. The jet which is not approaching Earth will appear dimmer because of the same relativistic effects. Therefore two intrinsically identical jets will appear significantly asymmetric. Indeed, in the example given above any jet where θ < 35° will be observed on Earth as less luminous than it would be from the rest frame of the jet.

A further consequence is that a population of intrinsically identical AGN scattered in space with random jet orientations will look like a very inhomogeneous population on Earth. The few objects where θ is small will have one very bright jet, while the rest will apparently have considerably weaker jets. Those where θ varies from 90° will appear to have asymmetric jets.

This is the essence behind the connection between blazars and radio galaxies. AGN which have jets oriented close to the line of sight with Earth can appear extremely different from other AGN even if they are intrinsically identical.

Discovery

Many of the brighter blazars were first identified, not as powerful distant galaxies, but as irregular variable stars in our own galaxy. These blazars, like genuine irregular variable stars, changed in brightness on periods of days or years, but with no pattern.

The early development of radio astronomy had shown that there are numerous bright radio sources in the sky. By the end of the 1950s the resolution of radio telescopes was sufficient to be able to identify specific radio sources with optical counterparts, leading to the discovery of quasars. Blazars were highly represented among these early quasars, and indeed the first redshift was found for 3C 273 — a highly variable quasar which is also a blazar.

In 1968 a similar connection between the "variable star" BL Lacertae and a powerful radio source VRO 42.22.01 was made. BL Lacertae shows many of the characteristics of quasars, but the optical spectrum was devoid of the spectral lines used to determine redshift. Faint indications of an underlying galaxy — proof that BL Lacertae was not a star — were found in 1974.

The extragalactic nature of BL Lacertae was not a surprise. In 1972 a few variable optical and radio sources were grouped together and proposed as a new class of galaxy: BL Lacertae-type objects. This terminology was soon shortened to "BL Lacertae object", "BL Lac object" or simply "BL Lac". (Note that the latter term can also mean the original blazar and not the entire class.)

As of 2003, a few hundred BL Lac objects are known.

Current vision

Blazars are thought to be active galactic nuclei, with relativistic jets oriented close to the line of sight with the observer.

The special jet orientation explains the general peculiar characteristics: high observed luminosity, very rapid variation, high polarization (when compared with non-blazar quasars), and the apparent superluminal motions detected along the first few parsecs of the jets in most blazars.

A Unified Scheme or Unified Model has become generally accepted where highly variable quasars are related to intrinsically powerful radio galaxies, and BL Lac objects are related to intrinsically weak radio galaxies. The distinction between these two connected populations explains the difference in emission line properties in blazars.

Alternate explanations for the relativistic jet/unified scheme approach which have been proposed include gravitational microlensing and coherent emission from the relativistic jet. Neither of these explain the overall properties of blazars. For example microlensing is achromatic. That is, all parts of a spectrum will rise and fall together. This is very clearly not observed in blazars. However it is possible that these processes, as well as more complex plasma physics can account for specific observations or some details.

Some examples of blazars include 3C 454.3, 3C 273, BL Lacertae, PKS 2155-304, Markarian 421, and Markarian 501. The latter two are also called "TeV Blazars" for their high energy (teraelectron-volt range) gamma-ray emission.

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Quasars

It is a massive and extremely remote celestial object, emitting exceptionally large amounts of energy, which typically has a starlike image in a telescope. It has been suggested that quasars contain massive black holes and may represent a stage in the evolution of some galaxies.

Shining so brightly that they eclipse the ancient galaxies that contain them, quasars are distant objects powered by black holes a billion times as massive as our sun. These powerful dynamos have fascinated astronomers since their discovery half a century ago.

In the 1930s, Karl Jansky, a physicist with Bell Telephone Laboratories, discovered that the static interference on transatlantic phone lines was coming from the Milky Way. By the 1950s, astronomers were using radio telescopes to probe the heavens, and pairing their signals with visible examinations of the heavens.

However, some of the smaller point-source objects didn't have a match. Astronomers called them "quasi-stellar radio sources," or "quasars," because the signals came from one place, like a star.  Naming them didn't help determine what these objects were. It took years of study to realize that these distant specks, which seemed to indicate stars, are created by particles accelerated at velocities approaching the speed of light.

Shining so brightly that they eclipse the ancient galaxies that contain them, quasars are distant objects powered by black holes a billion times as massive as our sun. These powerful dynamos have fascinated astronomers since their discovery half a century ago.

In the 1930s, Karl Jansky, a physicist with Bell Telephone Laboratories, discovered that the static interference on transatlantic phone lines was coming from the Milky Way. By the 1950s, astronomers were using radio telescopes to probe the heavens, and pairing their signals with visible examinations of the heavens.

Light-speed jets

Scientists now suspect that the tiny, point-like glimmers are actually signals from galactic nuclei outshining their host galaxies. Quasars live only in galaxies with super-massive black holes — black holes that contain billions of times the mass of the sun. Although light cannot escape from the black hole itself, some signals can break free around its edges. While some dust and gas fall into the black hole, other particles are accelerated away from it at near the speed of light. The particles stream away from the black hole in jets above and below it, transported by one of the most powerful particle accelerators in the universe.

Most quasars have been found billions of light-years away. Because it takes light time to travel, studying objects in space functions much like a time machine; we see the object as it was when light left it, billions of years ago. Thus, the farther away scientists look, the farther back in time they can see. Most of the more than 2,000 known quasars existed in the early life of the galaxy. Galaxies like the Milky Way may once have hosted a quasar that has long been silent.

Quasars emit energies of millions, billions, or even trillions of electron volts. This energy exceeds the total of the light of all the stars within a galaxy. The brightest objects in the universe, they shine anywhere from 10 to 100,000 times brighter than the Milky Way.

Family tree

Quasars are part of a class of objects known as active galactic nuclei (AGN). Other classes include Seyfert galaxies and blazars. All three require supermassive black holes to power them.

Seyfert galaxies are the lowest energy AGN, putting out only about 100 kiloelectronvolts (KeV). Blazars, like their quasar cousins, put out significantly more energy.

Many scientists think that the three types of AGNs are the same objects, but with different perspectives. While the jets of quasars seem to stream at an angle generally in the direction of Earth, blazars may point their jets directly toward the planet. Although no jets are seen in Seyfert galaxies, scientists think this may be because we view them from the side, so all of the emission is pointed away from us and thus goes undetected.

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Many Universes Hypothesis

In 1954, a young Princeton University doctoral candidate named Hugh Everett III came up with a radical idea: That there exist parallel universes, exactly like our ­universe. These universes are all related to ours; indeed, they branch off from ours, and our universe is branched off of others. Within these parallel universes, our wars have had different outcomes than the ones we know. Species that are extinct in our universe have evolved and adapted in others. In other universes, we humans may have become extinct.

This thought boggles the mind and yet, it is still comprehensible. Notions of parallel universes or dimensions that resemble our own have appeared in works of science fiction and have been used as explanations for metaphysics. But why would a young up-and-coming physicist possibly risk his future career by posing a theory about parallel universes?

With his Many-Worlds theory, Everett was attempting to answer a rather sticky question related to quantum physics: Why does quantum matter behave erratically? The quantum level is the smallest one science has detected so far. The study of quantum physics began in 1900, when the physicist Max Planck first introduced the concept to the scientific world. Planck's study of radiation yielded some unusual findings that contradicted classical physical laws. These findings suggested that there are other laws at work in the universe, operating on a deeper level than the one we know
.

Heisenberg Uncertainty Principle

In fairly short order, physicists studying the quantum level noticed some peculiar things about this tiny world. For one, the particles that exist on this level have a way of taking different forms arbitrarily. For example, scientists have observed photons -- tiny packets of light -- acting as particles and waves. Even a single photon exhibits this shape-shifting [source: Brown University]. Imagine if you looked and acted like a solid human being when a friend glanced at you, but when he looked back again, you'd taken a gaseous form.

This has come to be known as the Heisenberg Uncertainty Principle. The physicist Werner Heisenberg suggested that just by observing quantum matter, we affect the behavior of that matter. Thus, we can never be fully certain of the nature of a quantum object or its attributes, like velocity and location.

This idea is supported by the Copenhagen interpretation of quantum mechanics. Posed by the Danish physicist Niels Bohr, this interpretation says that all quantum particles don't exist in one state or the other, but in all of its possible states at once. The sum total of possible states of a quantum object is called its wave function. The state of an object existing in all of its possible states at once is called its superposition.

According to Bohr, when we observe a quantum object, we affect its behavior. Observation breaks an object's superposition and essentially forces the object to choose one state from its wave function. This theory accounts for why physicists have taken opposite measurements from the same quantum object: The object "chose" different states during different measurements.

Bohr's interpretation was widely accepted, and still is by much of the quantum community. But lately, Everett's Many-Worlds theory has been getting some serious attention. Read the next page to find out how the Many-Worlds interpretation works.

Many Worlds Theory

Young Hugh Everett agreed with much of what the highly respected physicist Niels Bohr had suggested about the quantum world. He agreed with the idea of superposition, as well as with the notion of wave functions. But Everett disagreed with Bohr in one vital respect.

To Everett, measuring a quantum object does not force it into one comprehensible state or another. Instead, a measurement taken of a quantum object causes an actual split in the universe. The universe is literally duplicated, splitting into one universe for each possible outcome from the measurement. For example, say an object's wave function is both a particle and a wave. When a physicist measures the particle, there are two possible outcomes: It will either be measured as a particle or a wave. This distinction makes Everett's Many-Worlds theory a competitor of the Copenhagen interpretation as an explanation for quantum mechanics.

When a physicist measures the object, the universe splits into two distinct universes to accommodate each of the possible outcomes. So a scientist in one universe finds that the object has been measured in wave form. The same scientist in the other universe measures the object as a particle. This also explains how one particle can be measured in more than one state.

As unsettling as it may sound, Everett's Many-Worlds interpretation has implications beyond the quantum level. If an action has more than one possible outcome, then -- if Everett's theory is correct -- the universe splits when that action is taken. This holds true even when a person chooses not to take an action.

This means that if you have ever found yourself in a situation where death was a possible outcome, then in a universe parallel to ours, you are dead. This is just one reason that some find the Many-Worlds interpretation disturbing.

Another disturbing aspect of the Many-Worlds interpretation is that it undermines our concept of time as linear. Imagine a time line showing the history of the Vietnam War. Rather than a straight line showing noteworthy events progressing onward, a time line based on the Many-Worlds interpretation would show each possible outcome of each action taken. From there, each possible outcome of the actions taken (as a result of the original outcome) would be further chronicled.

But a person cannot be aware of his other selves -- or even his death -- that exist in parallel universes. So how could we ever know if the Many-Worlds theory is correct? Assurance that the interpretation is theoretically possible came in the late 1990s from a thought experiment -- an imagined experiment used to theoretically prove or disprove an idea -- called quantum suicide. (You can learn more about it in How Quantum Suicide Works.)

This thought experiment renewed interest in Everett's theory, which was for many years considered rubbish. Since Many-Worlds was proven possible, physicists and mathematicians have aimed to investigate the implications of the theory in depth. But the Many-Worlds interpretation is not the only theory that seeks to explain the universe. Nor is it the only one that suggests there are universes parallel to our own. Read the next page to lean about string theory.

Parallel Universes: Split or String?

The Many-Worlds theory and the Copenhagen interpretation aren't the only competitors ­trying to explain the basic level of the universe. In fact, quantum mechanics isn't even the only field within physics searching for such an explanation. The theories that have emerged from the study of subatomic physics still remain theories. This has caused the field of study to be divided in much the same way as the world of psychology. Theories have adherents and critics, as do the psychological frameworks proposed by Carl Jung, Albert Ellis and Sigmund Freud.

Since their science was developed, physicists have been engaged inreverse engineering the universe -- they have studied what they could observe and worked backward toward smaller and smaller levels of the physical world. By doing this, physicists are attempting to reach the final and most basic level. It is this level, they hope, that will serve as the foundation for understanding everything else.

Following his famous Theory of Relativity, Albert Einstein spent the rest of his life looking for the one final level that would answer all physical questions. Physicists refer to this phantom theory as the Theory of Everything. Quantum physicists believe that they are on the trail of finding that final theory. But another field of physics believes that the quantum level is not the smallest level, so it therefore could not provide the Theory of Everything.

These physicists turn instead to a theoretical sub-quantum level called string theory for the answers to all of life. What's amazing is that through their theoretical investigation, these physicists, like Everett, have also concluded that there are parallel universes.

String theory was originated by the Japanese-American physicist Michio Kaku. His theory says that the essential building blocks of all matter as well as all of the physical forces in the universe -- like gravity -- exist on a sub-quantum level. These building blocks resemble tiny rubber bands -- or strings -- that make up quarks (quantum particles), and in turn electrons, and atoms, and cells and so on. Exactly what kind of matter is created by the strings and how that matter behaves depends on the vibration of these strings. It is in this manner that our entire universe is composed. And according to string theory, this composition takes place across 11 separate dimensions.

Like the Many-Worlds theory, string theory demonstrates that parallel universes exist. According to the theory, our own universe is like a bubble that exists alongside similar parallel universes. Unlike the Many-Worlds theory, string theory supposes that these universes can come into contact with one another. String theory says that gravity can flow between these parallel universes. When these universes interact, a Big Bang like the one that created our universe occurs.

While physicists have managed to create machines that can detect quantum matter, the sub-quantum strings are yet to be observed, which makes them -- and the theory on which they're built -- entirely theoretical. It has been discredited by some, although others believe it is correct.

So do parallel universes really exist? According to the Many-Worlds theory, we can't truly be certain, since we cannot be aware of them. The string theory has already been tested at least once -- with negative results. Dr. Kaku still believes parallel dimensions do exist, however [source: The Guardian].

Einstein didn't live long enough to see his quest for the Theory of Everything taken up by others. Then again, if Many-Worlds is correct, Einstein's still alive in a parallel universe. Perhaps in that universe, physicists have already found the Theory of Everything.

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