Posted on by Jean Tate

Gamma Waves

“Gamma wave” is not, strictly speaking, a standard scientific term … at least not in physics, and this is rather curious (the standard physics term is “gamma ray”).

The part of the electromagnetic spectrum ‘to the left’ (high energy/short wavelength/high frequency) is called the gamma ray region; the word ‘ray’ was in common use at the time of the discovery of this form of radiation (‘cathode rays’, ‘x-rays’, and so on); by the time it was discovered that gamma rays (and x-rays) are electromagnetic radiation (and that cathode rays, beta radiation, and alpha radiation, is not), the word ‘ray’ was well-entrenched. On the other hand, radio waves were discovered as a result of a new theory of electromagnetism … Maxwell’s equations predict the existence of electromagnetic waves (and that’s exactly what Hertz discovered, in 1886).

Paul Villard is credited with having discovered gamma radiation, in 1900, though it was Rutherford who gave them the name “gamma rays”, in 1903 (Rutherford had discovered alpha and beta rays in 1899). So when, and how, was it discovered that gamma rays are, in fact, gamma waves (just like radio waves, only with much, much, much shorter wavelengths)? In 1914; Ernest Rutherford and Edward Andrade used crystal diffraction to measure the wavelength of gamma rays emitted by Radium B (which is the radioactive isotope of lead, 214Pb) and Radium C (which is the radioactive isotope of bismuth, 214Bi).

We usually think of electromagnetic radiation in terms of photons, a term which arises from quantum physics; for astronomy (which is almost entirely based on electromagnetic radiation/photons), however, instruments and detectors are nearly always more easily understood in terms of whether they detect waves (e.g. radio receivers) or particles (e.g. scintillators). In gamma ray astronomy, in all instruments used to date, the particle nature of gamma rays is key (for direct detection anyway; Cherenkov telescopes work quite differently!). Can the circle be closed? Is it possible to use crystal diffraction (or something similar) – as Rutherford and Andrade did – and the wave nature of gamma rays, to build gamma ray astronomical instruments? Yes … and the next generation of gamma ray observatories might include just such instruments!

NASA has some good background material on gamma rays as electromagnetic radiation, and gamma ray astronomy: for example, Gamma Rays, and Electromagnetic Spectrum.

Universe Today has a few stories related to the wave nature of gamma rays; for example INTEGRAL Dissects Super-Bright Gamma Ray Burst, and Watching Gamma Rays from the Safety of Earth. Here’s some information on alpha radiation.

Astronomy Cast episodes Gamma Ray Astronomy, Detectors, and Electromagnetism give good background too.

Sources:
Wikipedia
NASA

Posted on by Jean Tate

Beta Decay

Cadmium Zinc Telluride semiconductors used to search for neutrino-less double beta decays (COBRA Project)

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Beta decay is when an unstable atomic nucleus decays (radioactively) by emitting a beta particle; when the beta particle is an electron, it is β– decay, and when a positron, β+ decay.

Beta rays, as a distinct component of the rays given off in radioactivity, were discovered by Rutherford, in 1899, just a few years after radioactivity itself was discovered (in 1896). However, this is beta minus decay … the discovery of beta plus decay (by Irène and Frédéric Joliot-Curie, in 1934) came after the discovery of the positron (in cosmic rays, in 1932) and the (then) controversial ‘invention’ of the neutrino (by Pauli, in 1931) to account for the continuous energy spectrum of electrons in beta decay. It was also in 1934 that Fermi published – in Italian and German (Nature considered the idea too speculative!!) – his theory of beta decay (for more details on this, check out this Hyperphysics page).

In beta minus decay, a neutron changes into a proton, antineutrino, and electron; this conversion is due to the weak interaction (or weak force) … a down quark (in the neutron) becomes an up quark and emits a W– boson (one of three bosons which mediate the weak interaction), which then decays into an electron and an antineutrino.

Beta plus decay – which is also known as inverse beta decay – involves the conversion of a proton to a neutron, positron, and neutrino.

So why do isolated neutrons decay (but those in stable nuclei, and those in neutron stars, don’t)? And why are isolated protons stable, but those in certain radioactive nuclei not? It’s all down to energy … if one state (an isolated neutron, say) has a higher energy than another (proton plus electron plus antineutrino), then the first will decay into the second (the baryon number of the two states must be the same, ditto lepton number, and so on).

There is also a rare double beta decay, in which two beta particles are emitted; it has been observed, in some unstable isotopes, as predicted. There is one kind of double beta decay – called neutrino-less double beta decay (the image above is from the COBRA Project, one study of this) – which is being studied intensely (though no such decay has yet been observed), because it may be one of the very few easily opened windows into physics beyond the Standard Model (see this WIPP page for more details).

Berkeley Lab has a neat Guide to the Nuclear Wallchart (subtitled “You don’t need to be a Nuclear Physicist to understand Nuclear Science“!) on beta decay, and this Ohio University page – Alpha and beta decay – puts more technical meat on the bare overview bones.

Pushing the Polite Boundaries of Science About Dark Matter is a Universe Today story which has a tangential reference to beta decay (it’s in the comments!).

Are there relevant Astronomy Cast episodes? Sure! Nucleosynthesis: Elements from Stars, The Strong and Weak Nuclear Forces, and Antimatter.

Source:
Wikipedia

Posted on by Jean Tate

Electron Mass

3d model of electron orbitals, based on the electron cloud model. Credit: Wikipedia Commons/Particia.fidi

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The mass of the electron, or the electron’s mass, written as me, is 9.109 382 15(45) x 10-31 kg. This is the “CODATA recommended value”. It was published in March 2007, and is referred to as the 2006 CODATA recommended value.

Some background: CODATA stands for Committee on Data for Science and Technology. Per NIST (the US National Institute for Standards and Technology), “CODATA was established in 1966 as an interdisciplinary committee of the International Council of Science (ICSU), formerly the International Council of Scientific Unions. It seeks to improve the compilation, critical evaluation, storage, and retrieval of data of importance to science and technology. The CODATA Task Group on Fundamental Constants was established in 1969. Its purpose is to periodically provide the international scientific and technological communities with an internationally accepted set of values of the fundamental physical constants and closely related conversion factors for use worldwide. The first such CODATA set was dated 1973, the second 1986, the third 1998, the fourth 2002, and the fifth (the current set) 2006.”

The mass of the electron is one of the fundamental physical constants, so called because they are widespread in theories of physics, and because they are widely used in the application of those theories to other branches of science and to practical uses (such as engineering). Four of the other fundamental physical constants are c (speed of light in a vacuum), e (the charge of the electron), h (Plank’s constant), and α (fine structure constant).

The method used for measuring me is to measure the Rydberg constant (R∞) and calculate me from it ( me = 2R∞h/(cα2 ); the Rydberg constant is, in the words of the paper (by Peter J. Mohr, Barry N. Taylor, David B. Newell) in which the 2006 CODATA recommended values were published “can be accurately determined by comparing measured resonant frequencies of transitions in hydrogen (H) and deuterium (D) to the theoretical expressions for the energy level differences in which it is a multiplicative factor.” For more details, refer to the paper itself.

Given that it is a fundamental physical constant, no surprise that Universe Today has some articles on it! For example Are the Laws of Nature the Same Everywhere in the Universe, and Fermilab putting the Squeeze on Higgs Boson.

Here are two Astronomy Cast episodes in which the electron mass figures prominently Electromagnetism, and Energy Levels and Spectra.

Sources:
NIST
Wikipedia – Electron Rest Mass
Wikipedia – Rydberg constant

Posted on by Jean Tate

Baryon

Particle Collider
Today, CERN announced that the LHCb experiment had revealed the existence of two new baryon subatomic particles. Credit: CERN/LHC/GridPP

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Particles made up of three quarks are called baryons; the two best known baryons are the proton (made up of two up quarks and one down) and the neutron (two down quarks and one up). Together with the mesons – particles comprised of a quark and an antiquark – baryons form the hadrons (you’ve heard of hadrons, they’re part of the name of the world’s most powerful particle collider, the Large Hadron Collider, the LHC).

Because they’re made up of quarks, baryons ‘feel’ the strong force (or strong nuclear force as it is also called), which is mediated by gluons. The other kind of particle which makes up ordinary matter is leptons, which are not – as far as we know – made up of anything (and as they do not contain quarks, they do not participate in the strong interaction … which is another way of saying they do not experience the strong force); the electron is one kind of lepton. Baryons and leptons are fermions, so obey the Pauli exclusion principle (which, among other things, says that there can be no more than one fermion in a particular quantum state at any time … and ultimately why you do not fall through your chair).

In the kinds of environments we are familiar with in everyday life, the only stable baryon is the proton; in the environment of the nuclei of most atoms, the neutron is also stable (and in the extreme environment of a neutron star too); there are, however, hundreds of different kinds of unstable baryons.

One big, open question in cosmology is how baryons were formed – baryogenesis – and why are there essentially no anti-baryons in the universe. For every baryon, there is a corresponding anti-baryon … there is, for example, the anti-proton, the anti-baryon counterpart to the proton, made up of two up anti-quarks and one down anti-quark. So if there were equal numbers of baryons and anti-baryons to start with, how come there are almost none of the latter today?

Astronomers often use the term ‘baryonic matter’, to refer to ordinary matter; it’s a bit of a misnomer, because it includes electrons (which are leptons) … and it generally excludes neutrinos (and anti-neutrinos), which are also leptons! Perhaps a better term might be matter which interacts via electromagnetism (i.e. feels the electromagnetic force), but that’s a bit of a mouthful. Non-baryonic matter is what (cold) dark matter (CDM) is composed of; CDM does not interact electromagnetically.

The Particle Data Group maintains summary tables of the properties of all known baryons. A relatively new area of research in astrophysics (and cosmology) is baryon acoustic oscillations (BAO); read more about it at this Los Alamos National Laboratory website …

… and in the Universe Today article New Search for Dark Energy Goes Back in Time. Other Universe Today stories featuring baryons explicitly include Is Dark Matter Made Up of Sterile Neutrinos?, and Astronomers on Supernova High Alert.

Sources:
Wikipedia
Hyperphysics

Posted on by Jean Tate

What is Alpha Radiation?

Alpha radiation is another name for the alpha particles emitted in the type of radioactive decay called alpha decay. Alpha particles are helium-4 (4He) nuclei.

Radioactivity was discovered by Becquerel, in 1896 (and one of the units of radioactivity – the becquerel – is named after him); within a few years it was discovered (Rutherford gets most of the credit, though others contributed) that there are actually three kinds of radioactivity, which were given the exciting names alpha (radiation), beta (radiation), and gamma (radiation; there are some other, rare, kinds of radioactive decay, the most important being positron, or positive beta). Rutherford (with some help) worked out that alpha radiation is actually the nuclei of helium … by allowing alpha radiation to go through the thin walls of an evacuated glass tube, and later analyzing the gas in the tube spectroscopically).

Some fun facts about alpha radiation:

* alpha radiation is the least penetrating (of alpha, beta, and gamma); typically it goes no more than a few cm in air

* like all kinds of radioactive decay, alpha decay occurs because the final state of the nucleus (the one decaying) has a lower energy than the initial one (the difference is the energy of the emitted alpha particle, both its binding energy and its kinetic energy)

* alpha decay involves both strong and electromagnetic interactions (or forces), unlike beta and gamma decay

* the key to the specifics of alpha decay is the quantum effect called tunneling; Gamow worked this out, in 1928

* only heavier nuclides can undergo alpha decay; the lightest are light isotopes of tellurium

* alpha radiation played a star role in the development of our understanding of the nature of atoms … Rutherford, in 1909, aimed a beam of alpha radiation at a piece of thin gold foil, and counted the number of particles which were deflected at each angle; from this he deduced that the atom has a very small nucleus (with all the positive charge, and nearly all its mass).

For more background on alpha radiation, check out the Jefferson Lab’s What are alpha rays? How are they produced?.

There are many ways alpha radiation can turn up in Universe Today articles; for example, in NASA May Have to Revamp Science Plans Without RTGs, alpha radiation is essential to RTGs; and in Opportunity Rover Sidelined by Charged Particle Hit, alpha radiation is what’s used to help determine the elemental composition of samples.

Nucleosynthsis: Elements from Stars and Cosmic Rays are two Astronomy Cast episodes which also cover alpha radiation.

Source: Wikipedia

Posted on by Jean Tate

Sirius B

Not a black dwarf ... yet (white dwarf Sirius B)

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Sirius B is the name of the fainter, smaller, less massive star in the Sirius binary system (the brighter, larger, more massive one is Sirius A, or just Sirius). It was hypothesized to exist almost eighteen years before it was actually observed!

Details: Bessel – yep, the guy who Bessel functions are named after – analyzed data on the position of Sirius (Bessel was the one who first observed stellar parallax), in particular its proper motion, and concluded – in 1844 – that there was an unseen companion star (the same principle used to infer the existence of Neptune, around the same time). In 1862 Alvan Clark saw this companion, using the 18.5″ refracting telescope he’d just built (quite a feat; Sirius B is ~10 magnitudes fainter than Sirius A, and separated by only a few arcseconds).

Sirius B is a white dwarf, one of the three “classics”, discovered to be white dwarf stars in the early years of the 20th century (Sirius B was the second to be discovered – 40 Eridani B had been found much earlier, and Procyon B was also hypothesized by Bessel (in 1844) though not observed until much later (in 1896)). It is one of the most massive white dwarfs so far discovered; its mass is the same as that of the Sun (approximately). Like all white dwarfs, it is small (it has a radius of only 0.008, compared with the Sun’s, which makes it smaller than the Earth!); like most seen so far, it is hot (approx 25,000 K).

Sirius B was likely a five sol B star as recently as 60 million years ago (when it was, coincidentally, approximately 60 million years old!), when it entered first a hydrogen shell burning, then a helium shell burning, stage, shed most of its mass (and enriching its companion with lots of ‘metals’ in the process), and shrank to become a white dwarf. There is no fusion taking place in Sirius B’s degenerate carbon/oxygen core (which makes up almost all of the star; there is a thin, non-degenerate, hydrogen atmosphere … this is what we see), so it is slowly cooling (it cools so slowly because it has such a small surface area).

Packing such a large mass into such a small volume means that Sirius B’s surface gravity is huge … so great in fact that it serves as an excellent test of one of the predictions of Einstein’s theory of General Relativity: gravitational redshift (this was first observed in the lab in 1959, by Pound and Rebka). The most recent observation of this gravitational redshift was by the Hubble, in 2005, as described in the Universe Today article Sirius’ White Dwarf Companion Weighed by Hubble.

Other Universe Today stories about Sirius B include White Dwarf Theories Get More Proof, and this 2005 What’s Up This Week one.

Astronomy Cast has two episodes related to Sirius B, Dwarf Stars, and Binary Stars.

References:
http://www.solstation.com/stars/sirius2.htm
http://en.wikipedia.org/wiki/Sirius

Posted on by Jean Tate

Microwave Radiation

In the microwave in your kitchen, food gets cooked (or heated) by absorbing microwave radiation, which is electromagnetic radiation between the (far) infrared and the radio, in the electromagnetic spectrum. The microwave region is rather broad, and somewhat vague, because the overlap with the radio (at around 1 meter, or 300 MHz) is not clear-cut, nor is the overlap with the sub-millimeter (or terahertz) region (at around 1 mm, or 300 GHz).

In astronomy, by far the most well-known aspect of microwave radiation is the cosmic microwave background (CMB), which has a near-perfect blackbody spectrum, of 2.73 K; this peaks at around 1.9 mm (160 GHz – the peak differs when measured by wavelength, from when measured by frequency).

The workhorse detector, in microwave astronomy (and much of radio astronomy, in general), is the radiometer, whose operation is described in considerable detail on this NRAO (National Radio Astronomy Observatory) webpage. The particular kind of radiometer which Penzias and Wilson used in their discovery of the CMB (at 7.35 cm, well away from the CMB peak) was a Dicke radiometer, designed by Robert Dicke (to search for the CMB!). And it was six differential microwave radiometers aboard the Cosmic Background Explorer (COBE) which first detected the CMB anisotropy, firmly establishing the CMB as the highly redshifted surface of last scattering (when baryonic matter and photons decoupled).

The microwave region, especially the short (millimeter) wavelength end, is a rich region for astrophysics, allowing the study of galaxy formation and evolution, stellar and planetary system birth, the composition of solar system body atmospheres, in addition to the CMB. There are already several observatories – many consortia – active in these fields; for example CARMA (Combined Array for Research in Millimeter-wave Astronomy), and ALMA (Atacama Large Millimeter/submillimeter Array) … astronomers just LOVE acronyms! (and no, that is not an acronym).

A new kind of microwave astronomical observatory has recently begun making obserations, the Allen Telescope Array, which provides instantaneous frequency coverage from 500 MHz to 11 GHz (among many other firsts). In many ways this serves as a technology demonstrator for the much more ambitious Square Kilometre Array.

Some of the many Universe Today stories on microwave astronomy are Probing the Large Scale Structure of the Universe, Dark Matter Annihilation at the Centre of the Milky Way, and Oldest and Most Distant Water in the Universe Detected.

Between them, Astronomy Cast episodes Radio Astronomy and Submillimeter Astronomy do a nice job of explaining microwave astronomy!

Sources:
http://www.cv.nrao.edu/course/astr534/Radiometers.html
http://lambda.gsfc.nasa.gov/product/cobe/
http://www.mmarray.org/
http://www.almaobservatory.org/
http://www.seti.org/ata
http://www.skatelescope.org/
http://en.wikipedia.org/wiki/Microwave

Posted on by Jean Tate

Nereid

Nereid (from Voyager 2; credit JPL)

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Nereid is the name given to the third largest of Neptune’s moons, and the second to have been discovered … by veteran outer solar system astronomer, Gerard P. Kuiper (guess who the Kuiper Belt is named after!), in 1949. Prior to Voyager 2’s arrival, it was the last moon of Neptune to be discovered.

In keeping with the nautical theme (Neptune, Roman god of the sea; Triton, Greek sea god, son of Poseidon), Nereid is named after the fifty sea nymphs, daughters of Nereus and Doris, in Greek mythology … the nautical theme continues with the names of the other 11 moons of Neptune, Naiad (one kind of nymph, Greek mythology; not a Nereid), Thalassa (daughter of Aether and Hemera, Greek mythology; also Greek for ‘sea’), Despina (nymph, daughter of Poseidon and Demeter (Greek); not a Nereid), Galatea (one of the Nereids), Larissa (Poseidon’s lover; Poseidon is the Greek Neptune), Proteus (also a sea god in Greek mythology; Proteus is the Neptune’s second largest moon), Halimede (one of the Nereids), Sao (also one of the Nereids), Laomedeia (guess … yep, another of the Nereids), Psamathe (ditto), and Neso (ditto, all over again).

Almost everything we know about Nereid comes from the images Voyager 2 took of it (83), between 20 April and 19 August, 1989; its closest approach was approximately 4.7 million km.

Nereid’s highly eccentric orbit (eccentricity 0.75, the highest of any solar system moon) takes it from 1.37 million km from Neptune to 9.66 million km (average 5.51 million km); unlike Triton, and like the other inner moons, Nereid’s orbit is prograde. This suggests that it may be a captured Kuiper Belt object, or that its orbit was substantially perturbed when Triton was captured.

For an irregular moon, Nereid is rather large (radius approx 170 km). Its spectrum and color (grey) are quite different from those of other outer solar system bodies (e.g. Chiron), which suggests that it may have formed around Neptune.

For more on Nereid, check out the Jet Propulsion Laboratory’s (JPL) profile of it!

Nereid is a bit of an orphan with regard to Universe Today stories, but there are some! Three new moons discovered for Neptune , and How Many Moons Does Neptune Have?.

Posted on by Jean Tate

Parallel Universe

The number of multiverses the human brain could distinguish. Credit: Linde and Vanchurin

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To some extent, ‘parallel universe’ is self-referential … there are parallel meanings of the very term! The two most often found in science-based websites (like Universe Today) are multi-verse, or multiverse (the universe we can see is but one of many universes), and the many-worlds interpretation of quantum physics (most often associated with Hugh Everett).

Cosmologist Max Tegmark (currently at MIT) has a neat classification scheme for pigeon-holing most parallel universe ideas that have at least some relationship to physics (as we know it today).

The most straight-forward kind of parallel universe(s) is one(s) just like the one we can see, but beyond the (cosmic) horizon … space is flat, and infinite, and the laws of physics (as we know them today) are the same, everywhere.

Similar, but different in some key ways, are parallel universes which developed out of inflation bubbles; these have the same (or very similar) physics to what applies in the universe we can see, except that the initial values (e.g. fine-structure constant) and perhaps number of dimensions may differ. The Inflationary Multiverse ideas of Standford University’s Andrei Linde are perhaps the best known example of this type. Parallel universes at this level tie in naturally to the (strong) anthropic principle.

Tegmark’s third class (he calls them Levels; this is Level 3) is the many-worlds of quantum physics. I’m sure you, dear reader, are familiar with poor old Schrödinger’s cat, whose half-alive and half-dead status is … troubling. In the many-worlds interpretation, the universe splits into two equal – and parallel – parts; in one, the radioactive material decays, and the cat dies; in the other, it does not, and the cat lives.

Level 4 contains truly weird parallel universes, ones which differ from the others by having fundamentally different laws of physics.

Operating somewhat in parallel are two other parallel universe concepts, cyclic universes (the parallelism is in time), and brane cosmology (a fallout from M-theory, in which the universe we can see is confined to just one brane, but interacts with other universes via gravity, which is not restricted to ‘our’ brane).

As you might expect, much, if not most, of this has been attacked for not being science (for example, how could you ever falsify any of these ideas?), but at least for some parallel universe ideas, observational tests may be possible. Perhaps the best known such test is the WMAP cold spot … one claim is that this is the imprint on ‘our’ universe of a parallel universe, via quantum entanglement (the most recent analyses, however, suggest that the cold spot is not qualitatively different from others, which have more prosaic explanations What! No Parallel Universe? Cosmic Cold Spot Just Data Artifact is a Universe Today story on just this).

Other Universe Today stories on parallel universes include If We Live in a Multiverse, How Many Are There?, Warp Drives Probably Impossible After All, and Book Review: Parallel Worlds.

Astronomy Cast has several episodes which include mention of parallel universes, but the best two are Multiple Big Bangs, and Entanglement.

Sources: MIT, Stanford University

Posted on by Jean Tate

Parabolic Mirror

Herschel in 3-D. Credit: Nathanial Burton-Bradford.

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Sometimes, in astronomy, the name of a thing describes it well; a parabolic mirror is, indeed, a mirror which has the shape of a parabola (an example of a name that does not describe itself well? How about Mare Nectaris, “Sea of Nectar”!). Actually, it’s a circular paraboloid, the 3D shape you get by rotating a parabola (which is 2D) around its axis.

The main part of the standard astronomical reflecting telescope – the primary mirror – is a parabolic mirror. So too is the dish of most radio telescopes, from the Lovell telescope at Jodrell Bank, to the telescopes in the Very Large Array; note that the dish in the Arecibo Observatory is not a parabolic mirror (it’s a spherical one). Focusing x-ray telescopes, such as Chandra and XMM-Newton, also use nested parabolic mirrors … followed by nested hyperbolic mirrors.

Why a parabolic shape? Because mirrors of this shape reflect the light (UV, IR, microwaves, radio) from distant objects onto a point, the focus of the parabola. This was known in ancient Greece, but the first telescope to incorporate a parabolic mirror wasn’t made until 1673 (by Robert Hooke, based on a design by James Gregory; the reflecting telescope Newton built used a spherical mirror). Parabolic mirrors do not suffer from spherical aberration (spherical mirrors cannot focus all incoming, on-axis, light onto a point), nor chromatic aberration (single lens refracting telescopes focus light of different colors at different points), so are the best kind of primary mirror for a simple telescope (however, off-axis sources will suffer from coma).

The Metropolitan State College of Denver has a cool animation of how a parabolic mirror focuses a plane wave train onto a point (the focus).

Universe Today has many articles on the use of parabolic mirrors in telescopes; for example Kid’s Telescope, Cassegrain Telescope, Where Did the Modern Telescope Come From?, Nano-Engineered Liquid Mirror Telescopes, A Pristine View of the Universe … from the Moon, Largest Mirror in Space Under Development, and 8.4 Metre Mirror Installed on Huge Binoculars.

Telescopes, the Next Level is an excellent Astronomy Cast episode, containing material on parabolic mirrors.