Spiral galaxy

From Wikipedia, the free encyclopedia

An example of a spiral galaxy, the Pinwheel Galaxy (also known as Messier 101 or NGC 5457)

A spiral galaxy is a certain kind of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae^[1] and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disc containing stars, gas and dust, and a central concentration of stars known as the bulge. These are surrounded by a much fainter halo of stars, many of which reside in globular clusters.

Spiral galaxies are named for the spiral structures that extend from the center into the disk. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disk because of the young, hot OB stars that inhabit them.

Roughly two-thirds of all spirals are observed to have an additional component in the form of a bar-like structure,^[2] extending from the central bulge, at the ends of which the spiral arms begin. The proportion of barred spirals relative to their barless cousins has changed over the history of the Universe, with only about 10% containing bars about 8 billion years ago, to roughly a quarter 2.5 billion years ago, until present, where over two-thirds of the galaxies in the visible universe (Hubble volume) have bars.^[3]

Our own Milky Way has recently (in the 1990s) been confirmed to be a barred spiral, although the bar itself is difficult to observe from our position within the galactic disk.^[4] The most convincing evidence for its existence comes from a recent survey, performed by the Spitzer Space Telescope, of stars in the galactic center.^[5]

Together with irregular galaxies, spiral galaxies make up approximately 60% of galaxies in the local Universe.^[6] They are mostly found in low-density regions and are rare in the centers of galaxy clusters.^[7]

Structure

Barred spiral galaxy UGC 12158.

Spiral galaxies consist of five distinct components:

• A flat, rotating disc of (mostly newly created) stars and interstellar matter
• A central stellar bulge of mainly older stars, which resembles an elliptical galaxy
• A near-spherical halo of stars, including many in globular clusters
• A supermassive black hole at the very center of the central bulge
• A near-spherical Dark matter halo

The relative importance, in terms of mass, brightness and size, of the different components varies from galaxy to galaxy.

Spiral arms

NGC 1300 in infrared light.

Spiral arms are regions of stars that extend from the center of spiral and barred spiral galaxies. These long, thin regions resemble a spiral and thus give spiral galaxies their name. Naturally, different classifications of spiral galaxies have distinct arm-structures. Sc and SBc galaxies, for instance, have very "loose" arms, whereas Sa and SBa galaxies have tightly wrapped arms (with reference to the Hubble sequence). Either way, spiral arms contain many young, blue stars (due to the high mass density and the high rate of star formation), which make the arms so bright.

Galactic bulge

A bulge is a huge, tightly packed group of stars. The term commonly refers to the central group of stars found in most spiral galaxies.

Using the Hubble classification, the bulge of Sa galaxies is usually composed of Population II stars, that are old, red stars with low metal content. Further, the bulge of Sa and SBa galaxies tends to be large. In contrast, the bulges of Sc and SBc galaxies are much smaller and are composed of young, blue Population I stars. Some bulges have similar properties to those of elliptical galaxies (scaled down to lower mass and luminosity); others simply appear as higher density centers of disks, with properties similar to disk galaxies.

Many bulges are thought to host a supermassive black hole at their centers. Such black holes have never been directly observed, but many indirect proofs exist. In our own galaxy, for instance, the object called Sagittarius A* is believed to be a supermassive black hole. There is a tight correlation between the mass of the black hole and the velocity dispersion of the stars in the bulge, the M-sigma relation.

Galactic spheroid

Spiral galaxy NGC 1345

The bulk of the stars in a spiral galaxy are located either close to a single plane (the galactic plane) in more or less conventional circular orbits around the center of the galaxy (the Galactic Center), or in a spheroidal galactic bulge around the galactic core.

However, some stars inhabit a spheroidal halo or galactic spheroid, a type of galactic halo. The orbital behaviour of these stars is disputed, but they may describe retrograde and/or highly inclined orbits, or not move in regular orbits at all. Halo stars may be acquired from small galaxies which fall into and merge with the spiral galaxy—for example, the Sagittarius Dwarf Spheroidal Galaxy is in the process of merging with the Milky Way and observations show that some stars in the halo of the Milky Way have been acquired from it.

Unlike the galactic disc, the halo seems to be free of dust, and in further contrast, stars in the galactic halo are of Population II, much older and with much lower metallicity than their Population I cousins in the galactic disc (but similar to those in the galactic bulge). The galactic halo also contains many globular clusters.

The motion of halo stars does bring them through the disc on occasion, and a number of small red dwarf stars close to the Sun are thought to belong to the galactic halo, for example Kapteyn's Star and Groombridge 1830. Due to their irregular movement around the center of the galaxy—if they do so at all—these stars often display unusually high proper motion.

In 2013 and 2014 papers were published presenting evidence that the spheroid is actually a planar structure in about half of all galaxies.^[8]

Oldest spiral galaxy

The oldest spiral galaxy on file is BX442. At eleven billion years old, it is more than two billion years older than any previous discovery. Researchers think the galaxy’s shape is caused by the gravitational influence of a companion dwarf galaxy. Computer models based on that assumption indicate that BX442's spiral structure will last about 100 million years.^[9][10]

Origin of the spiral structure

Spiral galaxy NGC 6384 taken by Hubble Space Telescope.

A spiral home to exploding stars^[11]

The pioneer of studies of the rotation of the Galaxy and the formation of the spiral arms was Bertil Lindblad in 1925. He realized that the idea of stars arranged permanently in a spiral shape was untenable. Since the angular speed of rotation of the galactic disk varies with distance from the centre of the galaxy (via a standard solar system type of gravitational model), a radial arm (like a spoke) would quickly become curved as the galaxy rotates. The arm would, after a few galactic rotations, become increasingly curved and wind around the galaxy ever tighter. This is called the winding problem. Measurements in the late 1960s showed that the orbital velocity of stars in spiral galaxies with respect to their distance from the galactic center is indeed higher than expected from Newtonian dynamics but still cannot explain the stability of the spiral structure.

Since the 1960s, there have been two leading hypotheses or models for the spiral structures of galaxies:

• Star formation caused by density waves in the galactic disk of the galaxy.
• The SSPSF model – star formation caused by shock waves in the interstellar medium.

These different hypotheses do not have to be mutually exclusive, as they may explain different types of spiral arms.

Density wave model

Bertil Lindblad proposed that the arms represent regions of enhanced density (density waves) that rotate more slowly than the galaxy’s stars and gas. As gas enters a density wave, it gets squeezed and makes new stars, some of which are short-lived blue stars that light the arms.

Explanation of spiral galaxy arms.

This idea was developed into density wave theory by C. C. Lin and Frank Shu in 1964.^[12]

Historical theory of Lin and Shu

The first acceptable theory for the spiral structure was devised by C. C. Lin and Frank Shu in 1964, attempting to explain the large-scale structure of spirals in terms of a small-amplitude wave propagating with fixed angular velocity, that revolves around the galaxy at a speed different from that of the galaxy's gas and stars. They suggested that the spiral arms were manifestations of spiral density waves - they assumed that the stars travel in slightly elliptical orbits, and that the orientations of their orbits is correlated i.e. the ellipses vary in their orientation (one to another) in a smooth way with increasing distance from the galactic center. This is illustrated in the diagram. It is clear that the elliptical orbits come close together in certain areas to give the effect of arms. Stars therefore do not remain forever in the position that we now see them in, but pass through the arms as they travel in their orbits.^[13]

Star formation caused by density waves

The following hypotheses exist for star formation caused by density waves:

• As gas clouds move into the density wave, the local mass density increases. Since the criteria for cloud collapse (the Jeans instability) depends on density, a higher density makes it more likely for clouds to collapse and form stars.
• As the compression wave goes through, it triggers star formation on the leading edge of the spiral arms.
• As clouds get swept up by the spiral arms, they collide with one another and drive shock waves through the gas, which in turn causes the gas to collapse and form stars.

The bright galaxy NGC 3810 demonstrates classical spiral structure in this very detailed image from Hubble. Credit: ESA/Hubble and NASA.

More young stars in spiral arms

The arms appear brighter because there are more young stars (hence more massive, bright stars). These massive, bright stars also die out quickly, which would leave just the darker background stellar distribution behind the waves, hence making the waves visible.

While stars, therefore, do not remain forever in the position that we now see them in, they also do not follow the arms. The arms simply appear to pass through the stars as the stars travel in their orbits.

Alignment of spin axis with cosmic voids

Spiral galaxy ESO 373-8.^[14]

Recent results suggest that the orientation of the spin axis of spiral galaxies is not a chance result, but instead they are preferentially aligned along the surface of cosmic voids.^[15] That is, spiral galaxies tend to be oriented at a high angle of inclination relative to the large-scale structure of the surroundings. They have been described as lining up like "beads on a string," with their axis of rotation following the filaments around the edges of the voids.^[16]

Gravitationally aligned orbits

Charles Francis and Erik Anderson showed from observations of motions of over 20,000 local stars (within 300 parsecs), that stars do move along spiral arms, and described how mutual gravity between stars causes orbits to align on logarithmic spirals. When the theory is applied to gas, collisions between gas clouds generate the molecular clouds in which new stars form, and evolution towards grand-design bisymmetric spirals is explained.^[17]

Distribution of stars in spirals

The similar distribution of stars in Spirals

The stars in spirals are distributed in thin disks with surface luminosity (Freeman, 1970).^[18]

$I(r)=I_{0}e^{{-r/R_{D}}}$

with $R_{D}$ being the disk scale-length; $I_{0}$ is the central value; it is useful to define: $R_{{opt}}=3.2R_{D}$ as the size of the stellar disk, whose luminosity is
$L_{{tot}}=2\pi I_{0}R_{D}^{2}$.
The spiral's light profiles, in terms of the coordinate $r/R_{D}$, do not depend on galaxy luminosity.

Spiral nebula

Spiral galaxy ESO 499-G37, seen against a backdrop of distant galaxies, scattered with nearby stars.^[19]

"Spiral nebula" was a term used to describe galaxies with a visible spiral structure, such as the Whirlpool Galaxy, before it was understood that these objects existed outside our Milky Way galaxy. The question of whether such objects were separate galaxies independent of the Milky Way, or a type of nebula existing within our own galaxy, was the subject of the Great Debate of 1920, between Heber Curtis of Lick Observatory and Harlow Shapley of Mt. Wilson Observatory. Beginning in 1923, Edwin Hubble^[20][21] observed Cepheid variables in several spiral nebulae, including the so-called "Andromeda Nebula", proving that they are, in fact, entire galaxies outside our own. The term "spiral nebula" has since fallen into disuse.

Milky Way

The Milky Way was once considered an ordinary spiral galaxy. Astronomers first began to suspect that the Milky Way is a barred spiral galaxy in the 1990s.^[22] Their suspicions were confirmed by the Spitzer Space Telescope observations in 2005^[23] which showed the galaxy's central bar to be larger than previously suspected.

Famous examples

• Andromeda Galaxy
• Milky Way
• Pinwheel Galaxy
• Sunflower Galaxy
• Triangulum Galaxy
• Whirlpool Galaxy

Proton

From Wikipedia, the free encyclopedia

Proton

The quark structure of the proton. The color assignment of individual quarks is arbitrary, but all three colors must be present. Forces between quarks are mediated by gluons.
Classification	Baryon
Composition	2 up quarks, 1 down quark
Statistics	Fermionic
Interactions	Gravity, electromagnetic, weak, strong
Symbol	p, p+, N+
Antiparticle	Antiproton
Theorized	William Prout (1815)
Discovered	Ernest Rutherford (1917–1919, named by him, 1920)
Mass	1.672621777(74)×10−27 kg[1] 938.272046(21) MeV/c2[1] 1.007276466812(90) u[1]
Mean lifetime	>2.1×1029 years (stable)
Electric charge	+1 e 1.602176565(35)×10−19 C[1]
Charge radius	0.8775(51) fm[1]
Electric dipole moment	<5.4×10−24 e·cm
Electric polarizability	1.20(6)×10−3 fm3
Magnetic moment	1.410606743(33)×10−26 J·T−1[1] 1.521032210(12)×10−3 μB[1] 2.792847356(23) μN[1]
Magnetic polarizability	1.9(5)×10−4 fm3
Spin	1⁄2
Isospin	1⁄2
Parity	+1
Condensed	I(JP) = 1⁄2(1⁄2+)

The proton is a subatomic particle, symbol p or p+, with a positive electric charge of +1e elementary charge and mass slightly less than that of a neutron. Protons and neutrons, each with mass approximately one atomic mass unit, are collectively referred to as "nucleons". One or more protons are present in the nucleus of an atom. The number of protons in the nucleus is referred to as its atomic number. Since each element has a unique number of protons, each element has its own unique atomic number. The word proton is Greek for "first", and this name was given to the hydrogen nucleus by Ernest Rutherford in 1920. In previous years Rutherford had discovered that the hydrogen nucleus (known to be the lightest nucleus) could be extracted from the nuclei of nitrogen by collision. The proton was therefore a candidate to be a fundamental particle and a building block of nitrogen and all other heavier atomic nuclei.

In the modern Standard Model of particle physics, the proton is a hadron, and like the neutron, the other nucleon (particle present in atomic nuclei), is composed of three quarks. Although the proton was originally considered a fundamental particle, it is composed of three valence quarks: two up quarks and one down quark. The rest masses of the quarks contribute only about 1% of the proton's mass, however.^[2] The remainder of the proton mass is due to the kinetic energy of the quarks and to the energy of the gluon fields that bind the quarks together. Because the proton is not a fundamental particle, it possesses a physical size; the radius of the proton is about 0.84–0.87 fm.^[3]

At sufficiently low temperatures, free protons will bind to electrons. However, the character of such bound protons does not change, and they remain protons. A fast proton moving through matter will slow by interactions with electrons and nuclei, until it is captured by the electron cloud of an atom. The result is a protonated atom, which is a chemical compound of hydrogen. In vacuum, when free electrons are present, a sufficiently slow proton may pick up a single free electron, becoming a neutral hydrogen atom, which is chemically a free radical. Such "free hydrogen atoms" tend to react chemically with many other types of atoms at sufficiently low energies. When free hydrogen atoms react with each other, they form neutral hydrogen molecules (H₂), which are the most common molecular component of molecular clouds in interstellar space. Such molecules of hydrogen on Earth may then serve (among many other uses) as a convenient source of protons for accelerators (as used in proton therapy) and other hadron particle physics experiments that require protons to accelerate, with the most powerful and noted example being the Large Hadron Collider.

Description

Protons are spin-½ fermions and are composed of three valence quarks,^[4] making them baryons (a sub-type of hadrons). The two up quarks and one down quark of the proton are held together by the strong force, mediated by gluons.^[5]:21–22A modern perspective has the proton composed of the valence quarks (up, up, down), the gluons, and transitory pairs of sea quarks. The proton has an approximately exponentially decaying positive charge distribution with a mean square radius of about 0.8 fm.^[6]

Protons and neutrons are both nucleons, which may be bound together by the nuclear force to form atomic nuclei. The nucleus of the most common isotope of the hydrogen atom (with the chemical symbol "H") is a lone proton. The nuclei of the heavy hydrogen isotopes deuterium and tritium contain one proton bound to one and two neutrons, respectively. All other types of atomic nuclei are composed of two or more protons and various numbers of neutrons.

History

The concept of a hydrogen-like particle as a constituent of other atoms was developed over a long period. As early as 1815, William Prout proposed that all atoms are composed of hydrogen atoms (which he called "protyles"), based on a simplistic interpretation of early values of atomic weights (see Prout's hypothesis), which was disproved when more accurate values were measured.^[7]:39–42

Ernest Rutherford at the first Solvay Conference, 1911

In 1886, Eugen Goldstein discovered canal rays (also known as anode rays) and showed that they were positively charged particles (ions) produced from gases. However, since particles from different gases had different values of charge-to-mass ratio (e/m), they could not be identified with a single particle, unlike the negative electrons discovered by J. J. Thomson.

Following the discovery of the atomic nucleus by Ernest Rutherford in 1911, Antonius van den Broek proposed that the place of each element in the periodic table (its atomic number) is equal to its nuclear charge. This was confirmed experimentally by Henry Moseley in 1913 using X-ray spectra.

In 1917, (in experiments reported in 1919) Rutherford proved that the hydrogen nucleus is present in other nuclei, a result usually described as the discovery of the proton.^[8] Rutherford had earlier learned to produce hydrogen nuclei as a type of radiation produced as a product of the impact of alpha particles on nitrogen gas, and recognize them by their unique penetration signature in air and their appearance in scintillation detectors. These experiments were begun when Rutherford had noticed that, when alpha particles were shot into air (mostly nitrogen), his scintillation detectors showed the signatures of typical hydrogen nuclei as a product. After experimentation Rutherford traced the reaction to the nitrogen in air, and found that when alphas were produced into pure nitrogen gas, the effect was larger. Rutherford determined that this hydrogen could have come only from the nitrogen, and therefore nitrogen must contain hydrogen nuclei. One hydrogen nucleus was being knocked off by the impact of the alpha particle, producing oxygen-17 in the process. This was the first reported nuclear reaction, ¹⁴N + α → ¹⁷O + p. (This reaction would later be observed happening directly in a cloud chamber in 1925).

Rutherford knew hydrogen to be the simplest and lightest element and was influenced by Prout's hypothesis that hydrogen was the building block of all elements. Discovery that the hydrogen nucleus is present in all other nuclei as an elementary particle, led Rutherford to give the hydrogen nucleus a special name as a particle, since he suspected that hydrogen, the lightest element, contained only one of these particles. He named this new fundamental building block of the nucleus the proton, after the neuter singular of the Greek word for "first", πρῶτον. However, Rutherford also had in mind the word protyle as used by Prout. Rutherford spoke at the British Association for the Advancement of Science at its Cardiff meeting beginning 24 August 1920.^[9] Rutherford was asked by Oliver Lodge for a new name for the positive hydrogen nucleus to avoid confusion with the neutral hydrogen atom. He initially suggested both proton and prouton (after Prout).^[10] Rutherford later reported that the meeting had accepted his suggestion that the hydrogen nucleus be named the "proton", following Prout's word "protyle".^[11] The first use of the word "proton" in the scientific literature appeared in 1920.^[12]

Stability

The free proton (a proton not bound to nucleons or electrons) is a stable particle that has not been observed to break down spontaneously to other particles. Free protons are found naturally in a number of situations in which energies or temperatures are high enough to separate them from electrons, for which they have some affinity. Free protons exist in plasmas in which temperatures are too high to allow them to combine with electrons. Free protons of high energy and velocity make up 90% of cosmic rays, which propagate in vacuum for interstellar distances. Free protons are emitted directly from atomic nuclei in some rare types of radioactive decay. Protons also result (along with electrons and antineutrinos) from the radioactive decay of free neutrons, which are unstable.
The spontaneous decay of free protons has never been observed, and the proton is therefore considered a stable particle. However, some grand unified theories of particle physics predict that proton decay should take place with lifetimes of the order of 10³⁶ years, and experimental searches have established lower bounds on the mean lifetime of the proton for various assumed decay products.^[13][14][15]

Experiments at the Super-Kamiokande detector in Japan gave lower limits for proton mean lifetime of 6.6×10³³ years for decay to an antimuon and a neutral pion, and 8.2×10³³ years for decay to a positron and a neutral pion.^[16] Another experiment at the Sudbury Neutrino Observatory in Canada searched for gamma rays resulting from residual nuclei resulting from the decay of a proton from oxygen-16. This experiment was designed to detect decay to any product, and established a lower limit to the proton lifetime of 2.1×10²⁹ years.^[17]

However, protons are known to transform into neutrons through the process of electron capture (also called inverse beta decay). For free protons, this process does not occur spontaneously but only when energy is supplied. The equation is:

p+ + e− → n + ν
e

The process is reversible; neutrons can convert back to protons through beta decay, a common form of radioactive decay. In fact, a free neutron decays this way, with a mean lifetime of about 15 minutes.

Quarks and the mass of the proton

In quantum chromodynamics, the modern theory of the nuclear force, most of the mass of the proton and the neutron is explained by special relativity. The mass of the proton is about 80–100 times greater than the sum of the rest masses of the quarks that make it up, while the gluons have zero rest mass. The extra energy of the quarks and gluons in a region within a proton, as compared to the rest energy of the quarks alone in the QCD vacuum, accounts for almost 99% of the mass. The rest mass of the proton is, thus, the invariant mass of the system of moving quarks and gluons that make up the particle, and, in such systems, even the energy of massless particles is still measured as part of the rest mass of the system.

Two terms are used in referring to the mass of the quarks that make up protons: current quark mass refers to the mass of a quark by itself, while constituent quark mass refers to the current quark mass plus the mass of the gluon particle field surrounding the quark.^{[18]:285–286 [19]:150–151} These masses typically have very different values. As noted, most of a proton's mass comes from the gluons that bind the current quarks together, rather than from the quarks themselves. While gluons are inherently massless, they possess energy—to be more specific, quantum chromodynamics binding energy (QCBE)—and it is this that contributes so greatly to the overall mass of the proton (see mass in special relativity). A proton has a mass of approximately 938 MeV/c², of which the rest mass of its three valence quarks contributes only about 9.4 MeV/c²; much of the remainder can be attributed to the gluons' QCBE.^[20][21][22]

The internal dynamics of the proton are complicated, because they are determined by the quarks' exchanging gluons, and interacting with various vacuum condensates. Lattice QCD provides a way of calculating the mass of the proton directly from the theory to any accuracy, in principle. The most recent calculations^[23][24] claim that the mass is determined to better than 4% accuracy, even to 1% accuracy (see Figure S5 in Dürr et al.^[24]). These claims are still controversial, because the calculations cannot yet be done with quarks as light as they are in the real world. This means that the predictions are found by a process of extrapolation, which can introduce systematic errors.^[25] It is hard to tell whether these errors are controlled properly, because the quantities that are compared to experiment are the masses of the hadrons, which are known in advance.

These recent calculations are performed by massive supercomputers, and, as noted by Boffi and Pasquini: "a detailed description of the nucleon structure is still missing because ... long-distance behavior requires a nonperturbative and/or numerical treatment..."^[26] More conceptual approaches to the structure of the proton are: the topological soliton approach originally due to Tony Skyrme and the more accurate AdS/QCD approach that extends it to include a string theory of gluons,^[27] various QCD-inspired models like the bag model and the constituent quark model, which were popular in the 1980s, and the SVZ sum rules, which allow for rough approximate mass calculations.^[28] These methods do not have the same accuracy as the more brute-force lattice QCD methods, at least not yet.

Charge radius

The internationally accepted value of the proton's charge radius is 0.8768 fm (see orders of magnitude for comparison to other sizes). This value is based on measurements involving a proton and an electron.
However, since 5 July 2010, an international research team has been able to make measurements involving an exotic atom made of a proton and a negatively-charged muon. After a long and careful analysis of those measurements, the team concluded that the root-mean-square charge radius of a proton is "0.84184(67) fm, which differs by 5.0 standard deviations from the CODATA value of 0.8768(69) fm".^[29] In January 2013, an updated value for the charge radius of a proton—0.84087(39) fm—was published. The precision was improved by 1.7 times, but the difference with CODATA value persisted at 7σ significance.^[30]

The international research team that obtained this result at the Paul Scherrer Institut (PSI) in Villigen (Switzerland) includes scientists from the Max Planck Institute of Quantum Optics (MPQ) in Garching, the Ludwig-Maximilians-Universität (LMU) Munich and the Institut für Strahlwerkzeuge (IFWS) of the Universität Stuttgart (both from Germany), and the University of Coimbra, Portugal.^[31][32] They are now attempting to explain the discrepancy, and re-examining the results of both previous high-precision measurements and complicated calculations. If no errors are found in the measurements or calculations, it could be necessary to re-examine the world's most precise and best-tested fundamental theory: quantum electrodynamics.^[31]

Interaction of free protons with ordinary matter

Although protons have affinity for oppositely-charged electrons, free protons must lose sufficient velocity (and kinetic energy) in order to become closely associated and bound to electrons, since this is a relatively low-energy interaction. High energy protons, in traversing ordinary matter, lose energy by collisions with atomic nuclei, and by ionization of atoms (removing electrons) until they are slowed sufficiently to be captured by the electron cloud in a normal atom.
However, in such an association with an electron, the character of the bound proton is not changed, and it remains a proton. The attraction of low-energy free protons to any electrons present in normal matter (such as the electrons in normal atoms) causes free protons to stop and to form a new chemical bond with an atom. Such a bond happens at any sufficiently "cold" temperature (i.e., comparable to temperatures at the surface of the Sun) and with any type of atom. Thus, in interaction with any type of normal (non-plasma) matter, low-velocity free protons are attracted to electrons in any atom or molecule with which they come in contact, causing the proton and molecule to combine. Such molecules are then said to be "protonated", and chemically they often, as a result, become so-called Bronsted acids.

Proton in chemistry

Atomic number

In chemistry, the number of protons in the nucleus of an atom is known as the atomic number, which determines the chemical element to which the atom belongs. For example, the atomic number of chlorine is 17; this means that each chlorine atom has 17 protons and that all atoms with 17 protons are chlorine atoms. The chemical properties of each atom are determined by the number of (negatively charged) electrons, which for neutral atoms is equal to the number of (positive) protons so that the total charge is zero. For example, a neutral chlorine atom has 17 protons and 17 electrons, whereas a Cl⁻ anion has 17 protons and 18 electrons for a total charge of −1.

All atoms of a given element are not necessarily identical, however, as the number of neutrons may vary to form different isotopes, and energy levels may differ forming different nuclear isomers. For example, there are two stable isotopes of chlorine: 35
17Cl with 35 − 17 = 18 neutrons and 37
17Cl with 37 − 17 = 20 neutrons.

Hydrogen ion

Protium, the most common isotope of hydrogen, consists of one proton and one electron (it has no neutrons). The term "hydrogen ion" (H+) implies that that H-atom has lost its one electron, causing only a proton to remain. Thus, in chemistry, the terms "proton" and "hydrogen ion" (for the protium isotope) are used synonymously

In chemistry, the term proton refers to the hydrogen ion, H+. Since the atomic number of hydrogen is 1, a hydrogen ion has no electrons and corresponds to a bare nucleus, consisting of a proton (and 0 neutrons for the most abundant isotope protium 1
1H). The proton is a "bare charge" with only about 1/64,000 of the radius of a hydrogen atom, and so is extremely reactive chemically. The free proton, thus, has an extremely short lifetime in chemical systems such as liquids and it reacts immediately with the electron cloud of any available molecule. In aqueous solution, it forms the hydronium ion, H₃O⁺, which in turn is further solvated by water molecules in clusters such as [H₅O₂]⁺ and [H₉O₄]⁺.^[33]

The transfer of H+ in an acid–base reaction is usually referred to as "proton transfer". The acid is referred to as a proton donor and the base as a proton acceptor. Likewise, biochemical terms such as proton pump and proton channel refer to the movement of hydrated H+ ions.

The ion produced by removing the electron from a deuterium atom is known as a deuteron, not a proton. Likewise, removing an electron from a tritium atom produces a triton.

Proton nuclear magnetic resonance (NMR)

Also in chemistry, the term "proton NMR" refers to the observation of hydrogen-1 nuclei in (mostly organic) molecules by nuclear magnetic resonance. This method uses the spin of the proton, which has the value one-half. The name refers to examination of protons as they occur in protium (hydrogen-1 atoms) in compounds, and does not imply that free protons exist in the compound being studied.

Human exposure

The Apollo Lunar Surface Experiments Packages (ALSEP) determined that more than 95% of the particles in the solar wind are electrons and protons, in approximately equal numbers.^[34][35]

Because the Solar Wind Spectrometer made continuous measurements, it was possible to measure how the Earth's magnetic field affects arriving solar wind particles. For about two-thirds of each orbit, the Moon is outside of the Earth's magnetic field. At these times, a typical proton density was 10 to 20 per cubic centimeter, with most protons having velocities between 400 and 650 kilometers per second. For about five days of each month, the Moon is inside the Earth's geomagnetic tail, and typically no solar wind particles were detectable. For the remainder of each lunar orbit, the Moon is in a transitional region known as the magnetosheath, where the Earth's magnetic field affects the solar wind but does not completely exclude it. In this region, the particle flux is reduced, with typical proton velocities of 250 to 450 kilometers per second. During the lunar night, the spectrometer was shielded from the solar wind by the Moon and no solar wind particles were measured.^[34]

Protons also occur in from extrasolar origin in space, from galactic cosmic rays, where they make up about 90% of the total particle flux. These protons often have higher energy than solar wind protons, but their intensity is far more uniform and less variable than protons coming from the Sun, the production of which is heavily affected by solar proton events such as coronal mass ejections.

Research has been performed on the dose-rate effects of protons, as typically found in space travel, on human health.^[35][36] To be more specific, there are hopes to identify what specific chromosomes are damaged, and to define the damage, during cancer development from proton exposure.^[35] Another study looks into determining "the effects of exposure to proton irradiation on neurochemical and behavioral endpoints, including dopaminergic functioning, amphetamine-induced conditioned taste aversion learning, and spatial learning and memory as measured by the Morris water maze.^[36] Electrical charging of a spacecraft due to interplanetary proton bombardment has also been proposed for study.^[37] There are many more studies that pertain to space travel, including galactic cosmic rays and their possible health effects, and solar proton event exposure.

The American Biostack and Soviet Biorack space travel experiments have demonstrated the severity of molecular damage induced by heavy ions on micro organisms including Artemia cysts.^[38]

Antiproton

CPT-symmetry puts strong constraints on the relative properties of particles and antiparticles and, therefore, is open to stringent tests. For example, the charges of the proton and antiproton must sum to exactly zero. This equality has been tested to one part in 10⁸. The equality of their masses has also been tested to better than one part in 10⁸. By holding antiprotons in a Penning trap, the equality of the charge to mass ratio of the proton and the antiproton has been tested to one part in 6×10⁹.^[39] The magnetic moment of the antiproton has been measured with error of 8×10⁻³ nuclear Bohr magnetons, and is found to be equal and opposite to that of the proton.

Consciousness Explained

From Wikipedia, the free encyclopedia

Consciousness Explained

Author	Daniel C. Dennett
Subject	Consciousness
Publisher	Little, Brown and Co.
Publication date	1991
Pages	511
ISBN	0-316-18065-3
OCLC	23648691
Dewey Decimal	126 20
LC Class	B105.C477 D45 1991
Preceded by	The Intentional Stance
Followed by	Darwin's Dangerous Idea

Consciousness Explained is a 1991 book by the American philosopher Daniel Dennett which offers an account of how consciousness arises from interaction of physical and cognitive processes in the brain.

Synopsis

The book puts forward a "multiple drafts" model of consciousness, suggesting that there is no single central place (a "Cartesian Theater") where conscious experience occurs; instead there are "various events of content-fixation occurring in various places at various times in the brain".^[1] The brain consists of a "bundle of semi-independent agencies";^[2] when "content-fixation" takes place in one of these, its effects may propagate so that it leads to the utterance of one of the sentences that make up the story in which the central character is one's "self". Dennett's view of consciousness is that it is the apparently serial account for the brain's underlying parallelism.

One of the book's more controversial claims is that qualia do not (and cannot) exist. Dennett's main argument is that the various properties attributed to qualia by philosophers—qualia are supposed to be incorrigible, ineffable, private, directly accessible and so on—are incompatible, so the notion of qualia is incoherent. The non-existence of qualia would mean that there is no hard problem of consciousness, and "philosophical zombies", which are supposed to act like a human in every way while somehow lacking qualia, cannot exist. So, as Dennett wryly notes, he is committed to the belief that we are all p-zombies (if you define the term p-zombie as functionally identical to a human being without any additional non material aspects)—adding that his remark is very much open to misinterpretation.^[3]

Dennett claims that our brains hold only a few salient details about the world, and that this is the only reason we are able to function at all. Thus, we don't store elaborate pictures in short-term memory, as this is not necessary and would consume valuable computing power. Rather, we log what has changed and assume the rest has stayed the same, with the result that we miss some details, as demonstrated in various experiments and illusions, some of which Dennett outlines.^[4][5] Research subsequent to Dennett's book indicates that some of his postulations were more conservative than expected. A year after Consciousness Explained was published, Dennett noted "I wish in retrospect that I'd been more daring, since the effects are stronger than I claimed". And since then examples continue to accumulate of the illusory nature of our visual world.^[6]

A key philosophical method is heterophenomenology, in which the verbal or written reports of subjects are treated as akin to a theorist's fiction—the subject's report is not questioned, but it is not assumed to be an incorrigible report about that subject's inner state. This approach allows the reports of the subject to be a datum in psychological research, thus circumventing the limits of classical behaviorism.

Also Dennett says that only a theory that explained conscious events in terms of unconscious events could explain consciousness at all: «To explain is to explain away».

Reactions

Critics of Dennett's approach, such as David Chalmers and Thomas Nagel, argue that Dennett's argument misses the point of the inquiry by merely redefining consciousness as an external property and ignoring the subjective aspect completely. This has led detractors to nickname the book Consciousness Ignored and Consciousness Explained Away.^[7][8] Dennett and his eliminative materialist supporters, however, respond that the aforementioned "subjective aspect" of conscious minds is nonexistent, an unscientific remnant of commonsense "folk psychology," and that his alleged redefinition is the only coherent description of consciousness.

However, John Searle argues^[9] that Dennett, who insists that discussing subjectivity is nonsense because it is unscientific and science presupposes objectivity, is making a category error. Searle argues that the goal of science is to establish and validate statements which are epistemically objective, (i.e., whose truth can be discovered and evaluated by any interested party), but are not necessarily ontologically objective. Searle calls any value judgment epistemically subjective. Thus, "McKinley is prettier than Everest" is epistemically subjective, whereas "McKinley is higher than Everest" is epistemically objective. In other words, the latter statement is evaluable (in fact, falsifiable) by an understood ('background') criterion for mountain height, like 'the summit is so many meters above sea level'. No such criteria exist for prettiness. Searle says that in Dennett's view, there is no consciousness in addition to the computational features, because that is all that consciousness amounts to for him: mere effects of a von Neumann(esque) virtual machine implemented in a parallel architecture and therefore implies that conscious states are illusory, but Searle asserts: "where consciousness is concerned, the existence of the appearance is the reality."

Searle said further: "To put it as clearly as I can: in his book, Consciousness Explained, Dennett denies the existence of consciousness. He continues to use the word, but he means something different by it. For him, it refers only to third-person phenomena, not to the first-person conscious feelings and experiences we all have. For Dennett there is no difference between us humans and complex zombies who lack any inner feelings, because we are all just complex zombies. ...I regard his view as self-refuting because it denies the existence of the data which a theory of consciousness is supposed to explain...Here is the paradox of this exchange: I am a conscious reviewer consciously answering the objections of an author who gives every indication of being consciously and puzzlingly angry. I do this for a readership that I assume is conscious. How then can I take seriously his claim that consciousness does not really exist?"^[10]

Gödel, Escher, Bach

From Wikipedia, the free encyclopedia

Gödel, Escher, Bach: an Eternal Golden Braid

Front cover design, 20th Anniversary Edition
Author	Douglas Hofstadter
Country	United States
Language	English
Subject	Consciousness, intelligence
Published	1979 (Basic Books)
Pages	777
ISBN	ISBN 978-0-465-02656-2, ISBN 0-14-017997-6
OCLC	40724766
Dewey Decimal	510/.1 21
LC Class	QA9.8 .H63 1999
Followed by	I Am a Strange Loop

Gödel, Escher, Bach: An Eternal Golden Braid, also known as GEB, is a 1979 book by Douglas Hofstadter. The tagline "a metaphorical fugue on minds and machines in the spirit of Lewis Carroll" was used by the publisher to describe the book.^[1]

By exploring common themes in the lives and works of logician Kurt Gödel, artist M. C. Escher and composer Johann Sebastian Bach, GEB expounds concepts fundamental to mathematics, symmetry, and intelligence. Through illustration and analysis, the book discusses how self-reference and formal rules allow systems to acquire meaning despite being made of "meaningless" elements. It also discusses what it means to communicate, how knowledge can be represented and stored, the methods and limitations of symbolic representation, and even the fundamental notion of "meaning" itself.

In response to confusion over the book's theme, Hofstadter has emphasized that GEB is not about mathematics, art, and music but rather about how cognition and thinking emerge from well-hidden neurological mechanisms. In the book, he presents an analogy about how the individual neurons of the brain coordinate to create a unified sense of a coherent mind by comparing it to the social organization displayed in a colony of ants.^[2][3]

Structure

GEB takes the form of an interweaving of various narratives. The main chapters alternate with dialogues between imaginary characters, usually Achilles and the tortoise, first used by Zeno of Elea and later by Lewis Carroll in "What the Tortoise Said to Achilles". These origins are related in the first two dialogues, and later ones introduce new characters such as the Crab. These narratives frequently dip into self-reference and metafiction.

Word play also features prominently in the work. Puns are occasionally used to connect ideas, such as "the Magnificrab, Indeed" with Bach's Magnificat in D; "SHRDLU, Toy of Man's Designing" with Bach's Jesu, Joy of Man's Desiring; and "Typographical Number Theory", or "TNT", which inevitably reacts explosively when it attempts to make statements about itself. One dialogue contains a story about a genie (from the Arabic "Djinn") and various "tonics" (of both the liquid and musical varieties), which is titled "Djinn and Tonic".

One dialogue in the book is written in the form of a crab canon, in which every line before the midpoint corresponds to an identical line past the midpoint. The conversation still makes sense due to uses of common phrases that can be used as either greetings or farewells ("Good day") and the positioning of lines which double as an answer to a question in the next line. Another is a sloth canon, where one character repeats the lines of another, but slower and negated.

Themes

GEB contains many instances of recursion and self-reference, where objects and ideas speak about or refer back to themselves. For instance, there is a phonograph that destroys itself by playing a record titled "I Cannot Be Played on Record Player X" (an analogy to Gödel's incompleteness theorems), an examination of canon form in music, and a discussion of Escher's lithograph of two hands drawing each other. To describe such self-referencing objects, Hofstadter coins the term "strange loop", a concept he examines in more depth in his follow-up book I Am a Strange Loop. To escape many of the logical contradictions brought about by these self-referencing objects, Hofstadter discusses Zen koans. He attempts to show readers how to perceive reality outside their own experience and embrace such paradoxical questions by rejecting the premise—a strategy also called "unasking".

Call stacks are also discussed in GEB, as one dialogue describes the adventures of Achilles and the Tortoise as they make use of "pushing potion" and "popping tonic" involving entering and leaving different layers of reality. Subsequent sections discuss the basic tenets of logic, self-referring statements, ("typeless") systems, and even programming.

Puzzles

The book is filled with puzzles. An example of this is the chapter titled "Contracrostipunctus", which combines the words acrostic and contrapunctus (counterpoint). In a dialogue between Achilles and the Tortoise, the author hints that there is a contrapunctal acrostic in the chapter that refers both to the author (Hofstadter) and Bach. This can be found by taking the first word of each paragraph, to reveal: Hofstadter's Contracrostipunctus Acrostically Backwards Spells 'J. S. Bach'. The second acrostic is found by taking the first letters of the first (in bold) and reading them backwards to get "J. S. Bach" (just as the first acrostic claims).

Impact

Gödel, Escher, Bach won the Pulitzer Prize for general non-fiction^[4] and the National Book Award for Science.^[5][a] Martin Gardner's July 1979 column in Scientific American stated, “Every few decades, an unknown author brings out a book of such depth, clarity, range, wit, beauty and originality that it is recognized at once as a major literary event.”^[6]

For Summer 2007, the Massachusetts Institute of Technology created an online course for high school students^[7] built around the book.

In its February 19, 2010 investigative summary on the 2001 anthrax attacks, the Federal Bureau of Investigation suggested that Bruce Edwards Ivins was inspired by GEB to hide secret codes based upon nucleotide sequences in the anthrax-laced letters he allegedly sent in September and October 2001,^[8] using bold letters, as suggested on page 404 of the book.^[9][10] It was also suggested that he attempted to hide the book from investigators by throwing it in the trash.

Translation

Although Hofstadter claims the idea of translating his book "never crossed [his] mind" when he was writing it, when approached with the idea by his publisher he was "very excited about seeing [the] book in other languages, especially… French". He knew, however, that "there were a million issues to consider" when translating,^[11] since the book relies not only on word-play but "structural puns" as well—writing where the form and content of the work mirror each other (such as the "Crab Canon" dialogue, which reads almost exactly the same forwards as backwards).

Hofstadter gives one example of translation trouble in the paragraph "Mr. Tortoise, Meet Madame Tortue", saying translators "instantly ran headlong into the conflict between the feminine gender of the French noun tortue and the masculinity of my character, the Tortoise".^[11] Hofstadter agreed to the translators' suggestions of naming the French character "Madame Tortue", and the Italian version "Signorina Tartaruga".^[12] Because of other troubles translators might have retaining the meaning of the book, Hofstadter "painstakingly went through every last sentence of GEB, annotating a copy for translators into any language that might be targeted".^[11]

Translation also gave Hofstadter a way to add new meaning and puns. For instance, in Chinese, the subtitle is not a translation of an Eternal Golden Braid, but a seemingly unrelated phrase Jí Yì Bì (集异璧, literally "collection of exotic jades"), which is homophonic to GEB in Chinese. Some material regarding this interplay is to be found in Hofstadter's later book Le Ton beau de Marot, which is mainly about translation.