Econstudentlog

Gravity

“The purpose of this book is to give the reader a very brief introduction to various different aspects of gravity. We start by looking at the way in which the theory of gravity developed historically, before moving on to an outline of how it is understood by scientists today. We will then consider the consequences of gravitational physics on the Earth, in the Solar System, and in the Universe as a whole. The final chapter describes some of the frontiers of current research in theoretical gravitational physics.”

I was not super impressed by this book, mainly because the level of coverage was not quite as high as has been the level of coverage of some of the other physics books in the OUP – A Brief Introduction series. But it’s definitely an okay book about this topic, I was much closer to a three star rating on goodreads than a one star rating, and I did learn some new things from it. I might still change my mind about my two-star rating of the book.

I’ll cover the book the same way I’ve covered some of the other books in the series; I’ll post some quotes with some observations of interest, and then I’ll add some supplementary links towards the end of the post. ‘As usual’ (see e.g. also the introductory remarks to this post) I’ll add links to topics even if I have previously, perhaps on multiple occasions, added the same links when covering other books – the idea behind the links is to remind me – and indicate to you – which kinds of topics are covered in the book.

“[O]ver large distances it is gravity that dominates. This is because gravity is only ever attractive and because it can never be screened. So while most large objects are electrically neutral, they can never be gravitationally neutral. The gravitational force between objects with mass always acts to pull those objects together, and always increases as they become more massive.”

“The challenges involved in testing Newton’s law of gravity in the laboratory arise principally due to the weakness of the gravitational force compared to the other forces of nature. This weakness means that even the smallest residual electric charges on a piece of experimental equipment can totally overwhelm the gravitational force, making it impossible to measure. All experimental equipment therefore needs to be prepared with the greatest of care, and the inevitable electric charges that sneak through have to be screened by introducing metal shields that reduce their influence. This makes the construction of laboratory experiments to test gravity extremely difficult, and explains why we have so far only probed gravity down to scales a little below 1mm (this can be compared to around a billionth of a billionth of a millimetre for the electric force).”

“There are a large number of effects that result from Einstein’s theory. […] [T]he anomalous orbit of the planet Mercury; the bending of starlight around the Sun; the time delay of radio signals as they pass by the Sun; and the behaviour of gyroscopes in orbit around the Earth […] are four of the most prominent relativistic gravitational effects that can be observed in the Solar System.” [As an aside, I only yesterday watched the first ~20 minutes of the first of Nima Arkani-Hamed’s lectures on the topic of ‘Robustness of GR. Attempts to Modify Gravity’, which was recently uploaded on the IAS youtube channel, before I concluded that I was probably not going to be able to follow the lecture – I would have been able to tell Hamed, on account of having read this book, that the name of the ‘American’ astronomer whose name eluded him early on in the lecture (5 minutes in or so) was John Couch Adams (who was in fact British, not American)].

“[T]he overall picture we are left with is very encouraging for Einstein’s theory of gravity. The foundational assumptions of this theory, such as the constancy of mass and the Universality of Free Fall, have been tested to extremely high accuracy. The inverse square law that formed the basis of Newton’s theory, and which is a good first approximation to Einstein’s theory, has been tested from the sub-millimetre scale all the way up to astrophysical scales. […] We […] have very good evidence that Newton’s inverse square law is a good approximation to gravity over a wide range of distance scales. These scales range from a fraction of a millimetre, to hundreds of millions of metres. […] We are also now in possession of a number of accurate experimental results that probe the tiny, subtle effects that result from Einstein’s theory specifically. This data allows us direct experimental insight into the relationship between matter and the curvature of space-time, and all of it is so far in good agreement with Einstein’s predictions.”

“[A]ll of the objects in the Solar System are, relatively speaking, rather slow moving and not very dense. […] If we set our sights a little further though, we can find objects that are much more extreme than anything we have available nearby. […] observations of them have allowed us to explore gravity in ways that are simply impossible in our own Solar System. The extreme nature of these objects amplifies the effects of Einstein’s theory […] Just as the orbit of Mercury precesses around the Sun so too the neutron stars in the Hulse–Taylor binary system precess around each other. To compare with similar effects in our Solar System, the orbit of the Hulse–Taylor pulsar precesses as much in a day as Mercury does in a century.”

“[I]n Einstein’s theory, gravity is due to the curvature of space-time. Massive objects like stars and planets deform the shape of the space-time in which they exist, so that other bodies that move through it appear to have their trajectories bent. It is the mistaken interpretation of the motion of these bodies as occurring in a flat space that leads us to infer that there is a force called gravity. In fact, it is just the curvature of space-time that is at work. […] The relevance of this for gravitational waves is that if a group of massive bodies are in relative motion […], then the curvature of the space-time in which they exist is not usually fixed in time. The curvature of the space-time is set by the massive bodies, so if the bodies are in motion, the curvature of space-time should be expected to be constantly changing. […] in Einstein’s theory, space-time is a dynamical entity. As an example of this, consider the supernovae […] Before their cores collapse, leading to catastrophic explosion, they are relatively stable objects […] After they explode they settle down to a neutron star or a black hole, and once again return to a relatively stable state, with a gravitational field that doesn’t change much with time. During the explosion, however, they eject huge amounts of mass and energy. Their gravitational field changes rapidly throughout this process, and therefore so does the curvature of the space-time around them.

Like any system that is pushed out of equilibrium and made to change rapidly, this causes disturbances in the form of waves. A more down-to-earth example of a wave is what happens when you throw a stone into a previously still pond. The water in the pond was initially in a steady state, but the stone causes a rapid change in the amount of water at one point. The water in the pond tries to return to its tranquil initial state, which results in the propagation of the disturbance, in the form of ripples that move away from the point where the stone landed. Likewise, a loud noise in a previously quiet room originates from a change in air pressure at a point (e.g. a stereo speaker). The disturbance in the air pressure propagates outwards as a pressure wave as the air tries to return to a stable state, and we perceive these pressure waves as sound. So it is with gravity. If the curvature of space-time is pushed out of equilibrium, by the motion of mass or energy, then this disturbance travels outwards as waves. This is exactly what occurs when a star collapses and its outer envelope is ejected by the subsequent explosion. […] The speed with which waves propagate usually depends on the medium through which they travel. […] The medium for gravitational waves is space-time itself, and according to Einstein’s theory, they propagate at exactly the same speed as light. […] [If a gravitational wave passes through a cloud of gas,] the gravitational wave is not a wave in the gas, but rather a propagating disturbance in the space-time in which the gas exists. […] although the atoms in the gas might be closer together (or further apart) than they were before the wave passed through them, it is not because the atoms have moved, but because the amount of space between them has been decreased (or increased) by the wave. The gravitational wave changes the distance between objects by altering how much space there is in between them, not by moving them within a fixed space.”

“If we look at the right galaxies, or collect enough data, […] we can use it to determine the gravitational fields that exist in space. […] we find that there is more gravity than we expected there to be, from the astrophysical bodies that we can see directly. There appears to be a lot of mass, which bends light via its gravitational field, but that does not interact with the light in any other way. […] Moving to even smaller scales, we can look at how individual galaxies behave. It has been known since the 1970s that the rate at which galaxies rotate is too high. What I mean is that if the only source of gravity in a galaxy was the visible matter within it (mostly stars and gas), then any galaxy that rotated as fast as those we see around us would tear itself apart. […] That they do not fly apart, despite their rapid rotation, strongly suggests that the gravitational fields within them are larger than we initially suspected. Again, the logical conclusion is that there appears to be matter in galaxies that we cannot see but which contributes to the gravitational field. […] Many of the different physical processes that occur in the Universe lead to the same surprising conclusion: the gravitational fields we infer, by looking at the Universe around us, require there to be more matter than we can see with our telescopes. Beyond this, in order for the largest structures in the Universe to have evolved into their current state, and in order for the seeds of these structures to look the way they do in the CMB, this new matter cannot be allowed to interact with light at all (or, at most, interact only very weakly). This means that not only do we not see this matter, but that it cannot be seen at all using light, because light is required to pass straight through it. […] The substance that gravitates in this way but cannot be seen is referred to as dark matter. […] There needs to be approximately five times as much dark matter as there is ordinary matter. […] the evidence for the existence of dark matter comes from so many different sources that it is hard to argue with it.”

“[T]here seems to be a type of anti-gravity at work when we look at how the Universe expands. This anti-gravity is required in order to force matter apart, rather than pull it together, so that the expansion of the Universe can accelerate. […] The source of this repulsive gravity is referred to by scientists as dark energy […] our current overall picture of the Universe is as follows: only around 5 per cent of the energy in the Universe is in the form of normal matter; about 25 per cent is thought to be in the form of the gravitationally attractive dark matter; and the remaining 70 per cent is thought to be in the form of the gravitationally repulsive dark energy. These proportions, give or take a few percentage points here and there, seem sufficient to explain all astronomical observations that have been made to date. The total of all three of these types of energy, added together, also seems to be just the right amount to make space flat […] The flat Universe, filled with mostly dark energy and dark matter, is usually referred to as the Concordance Model of the Universe. Among astronomers, it is now the consensus view that this is the model of the Universe that best fits their data.”

 

The universality of free fall.
Galileo’s Leaning Tower of Pisa experiment.
Isaac Newton/Philosophiæ Naturalis Principia Mathematica/Newton’s law of universal gravitation.
Kepler’s laws of planetary motion.
Luminiferous aether.
Special relativity.
Spacetime.
General relativity.
Spacetime curvature.
Pound–Rebka experiment.
Gravitational time dilation.
Gravitational redshift space-probe experiment (Essot & Levine).
Michelson–Morley experiment.
Hughes–Drever experiment.
Tests of special relativity.
Eötvös experiment.
Torsion balance.
Cavendish experiment.
LAGEOS.
Interferometry.
Geodetic precession.
Frame-dragging.
Gravity Probe B.
White dwarf/neutron star/supernova/gravitational collapse/black hole.
Hulse–Taylor binary.
Arecibo Observatory.
PSR J1738+0333.
Gravitational wave.
Square Kilometre Array.
PSR J0337+1715.
LIGO.
Weber bar.
MiniGrail.
Laser Interferometer Space Antenna.
Edwin Hubble/Hubble’s Law.
Physical cosmology.
Alexander Friedmann/Friedmann equations.
Cosmological constant.
Georges Lemaître.
Ralph Asher Alpher/Robert Hermann/CMB/Arno Penzias/Robert Wilson.
Cosmic Background Explorer.
The BOOMERanG experiment.
Millimeter Anisotropy eXperiment IMaging Array.
Wilkinson Microwave Anisotropy Probe.
High-Z Supernova Search Team.
CfA Redshift Survey/CfA2 Great Wall/2dF Galaxy Redshift Survey/Sloan Digital Sky Survey/Sloan Great Wall.
Gravitational lensing.
Inflation (cosmology).
Lambda-CDM model.
BICEP2.
Large Synoptic Survey Telescope.
Grand Unified Theory.
Renormalization (quantum theory).
String theory.
Loop quantum gravity.
Unruh effect.
Hawking radiation.
Anthropic principle.

July 15, 2017 Posted by | Astronomy, Books, cosmology, Physics | Leave a comment

Probing the Early Universe through Observations of the Cosmic Microwave Background

This lecture/talk is a few years old, but it was only made public on the IAS channel last week (…along with a lot of other lectures – the IAS channel has added a lot of stuff recently, including more than 150 lectures within the last week or so; so if you’re interested you should go have a look).

Below the lecture I have added a few links with stuff (wiki-articles and a few papers) related to the topics covered in the lecture. I didn’t read those links, but I skimmed them (and a few others, which I subsequently decided not to include as their coverage did not overlap sufficiently with the stuff covered in the lecture) and decided to add them in order to remind myself what kind of stuff was included in the lecture/allow others to infer what kind of stuff might be included in the lecture. The links naturally go into a lot more detail than does the lecture, but these are the sort of topics discussed/included.

The lecture is long (90 minutes + a short Q&A), but it was interesting enough for me to watch all of it. The lecturer displays a very high level of speech disfluency throughout the lecture, in the sense that I might not be surprised if I were told that the most commonly word encountered during this lecture was ‘um’ or ‘uh’, rather than more commonly encountered mode words like ‘the’, but you get used to it (at least I managed to sort of ‘tune it out’ after a while). I should caution that there’s a short ‘jump’ very early on in the lecture (at the 2 minute mark or so) where a small amount of frames were apparently dropped, but that should not scare you away from watching the lecture; that frame drop is the only one of its kind during the lecture, aside from a similar brief ‘jump’ around the 1 hour 9 minute mark.

Some links:

Astronomical interferometer.
Polarimetry.
Bolometer.
Fourier transform.
Boomerang : A Balloon-borne Millimeter Wave Telescope and Total Power Receiver for Mapping Anisotropy in the Cosmic Microwave Background.
Observations of the Temperature and Polarization Anisotropies with Boomerang 2003.
THE COBE DIFFUSE INFRARED BACKGROUND EXPERIMENT SEARCH FOR THE COSMIC INFRARED BACKGROUND: I. LIMITS AND DETECTIONS.
Detection of the Power Spectrum of Cosmic Microwave Background Lensing by the Atacama Cosmology Telescope.
Secondary anisotropies of the CMB (review article).
Planck early results. VIII. The all-sky early Sunyaev-Zeldovich cluster sample.
Sunyaev–Zel’dovich effect.
A CMB Polarization Primer.
MEASUREMENT OF COSMIC MICROWAVE BACKGROUND POLARIZATION POWER SPECTRA FROM TWO YEARS OF BICEP DATA.
Spider: a balloon-borne CMB polarimeter for large angular scales.

July 13, 2017 Posted by | Astronomy, cosmology, Lectures, Physics | Leave a comment

Galaxies

I have added some observations from the book below, as well as some links covering people/ideas/stuff discussed/mentioned in the book.

“On average, out of every 100 newly born star systems, 60 are binaries and 40 are triples. Solitary stars like the Sun are later ejected from triple systems formed in this way.”

“…any object will become a black hole if it is sufficiently compressed. For any mass, there is a critical radius, called the Schwarzschild radius, for which this occurs. For the Sun, the Schwarzschild radius is just under 3 km; for the Earth, it is just under 1 cm. In either case, if the entire mass of the object were squeezed within the appropriate Schwarzschild radius it would become a black hole.”

“It only became possible to study the centre of our Galaxy when radio telescopes and other instruments that do not rely on visible light became available. There is a great deal of dust in the plane of the Milky Way […] This blocks out visible light. But longer wavelengths penetrate the dust more easily. That is why sunsets are red – short wavelength (blue) light is scattered out of the line of sight by dust in the atmosphere, while the longer wavelength red light gets through to your eyes. So our understanding of the galactic centre is largely based on infrared and radio observations.”

“there is strong evidence that the Milky Way Galaxy is a completely ordinary disc galaxy, a typical representative of its class. Since that is the case, it means that we can confidently use our inside knowledge of the structure and evolution of our own Galaxy, based on close-up observations, to help our understanding of the origin and nature of disc galaxies in general. We do not occupy a special place in the Universe; but this was only finally established at the end of the 20th century. […] in the decades following Hubble’s first measurements of the cosmological distance scale, the Milky Way still seemed like a special place. Hubble’s calculation of the distance scale implied that other galaxies are relatively close to our Galaxy, and so they would not have to be very big to appear as large as they do on the sky; the Milky Way seemed to be by far the largest galaxy in the Universe. We now know that Hubble was wrong. […] the value he initially found for the Hubble Constant was about seven times bigger than the value accepted today. In other words, all the extragalactic distances Hubble inferred were seven times too small. But this was not realized overnight. The cosmological distance scale was only revised slowly, over many decades, as observations improved and one error after another was corrected. […] The importance of determining the cosmological distance scale accurately, more than half a century after Hubble’s pioneering work, was still so great that it was a primary justification for the existence of the Hubble Space Telescope (HST).”

“The key point to grasp […] is that the expansion described by [Einstein’s] equations is an expansion of space as time passes. The cosmological redshift is not a Doppler effect caused by galaxies moving outward through space, as if fleeing from the site of some great explosion, but occurs because the space between the galaxies is stretching. So the spaces between galaxies increase while light is on its way from one galaxy to another. This stretches the light waves to longer wavelengths, which means shifting them towards the red end of the spectrum. […] The second key point about the universal expansion is that it does not have a centre. There is nothing special about the fact that we observe galaxies receding with redshifts proportional to their distances from the Milky Way. […] whichever galaxy you happen to be sitting in, you will see the same thing – redshift proportional to distance.”

“The age of the Universe is determined by studying some of the largest things in the Universe, clusters of galaxies, and analysing their behaviour using the general theory of relativity. Our understanding of how stars work, from which we calculate their ages, comes from studying some of the smallest things in the Universe, the nuclei of atoms, and using the other great theory of 20th-century physics, quantum mechanics, to calculate how nuclei fuse with one another to release the energy that keeps stars shining. The fact that the two ages agree with one another, and that the ages of the oldest stars are just a little bit less than the age of the Universe, is one of the most compelling reasons to think that the whole of 20th-century physics works and provides a good description of the world around us, from the very small scale to the very large scale.”

“Planets are small objects orbiting a large central mass, and the gravity of the Sun dominates their motion. Because of this, the speed with which a planet moves […] is inversely proportional to the square of its distance from the centre of the Solar System. Jupiter is farther from the Sun than we are, so it moves more slowly in its orbit than the Earth, as well as having a larger orbit. But all the stars in the disc of a galaxy move at the same speed. Stars farther out from the centre still have bigger orbits, so they still take longer to complete one circuit of the galaxy. But they are all travelling at essentially the same orbital speed through space.”

“The importance of studying objects at great distances across the Universe is that when we look at an object that is, say, 10 billion light years away, we see it by light which left it 10 billion years ago. This is the ‘look back time’, and it means that telescopes are in a sense time machines, showing us what the Universe was like when it was younger. The light from a distant galaxy is old, in the sense that it has been a long time on its journey; but the galaxy we see using that light is a young galaxy. […] For distant objects, because light has taken a long time on its journey to us, the Universe has expanded significantly while the light was on its way. […] This raises problems defining exactly what you mean by the ‘present distance’ to a remote galaxy”

“Among the many advantages that photographic and electronic recording methods have over the human eye, the most fundamental is that the longer they look, the more they see. Human eyes essentially give us a real-time view of our surroundings, and allow us to see things – such as stars – that are brighter than a certain limit. If an object is too faint to see, once your eyes have adapted to the dark no amount of staring in its direction will make it visible. But the detectors attached to modern telescopes keep on adding up the light from faint sources as long as they are pointing at them. A longer exposure will reveal fainter objects than a short exposure does, as the photons (particles of light) from the source fall on the detector one by one and the total gradually grows.”

“Nobody can be quite sure where the supermassive black holes at the hearts of galaxies today came from, but it seems at least possible that […] merging of black holes left over from the first generation of stars [in the universe] began the process by which supermassive black holes, feeding off the matter surrounding them, formed. […] It seems very unlikely that supermassive black holes formed first and then galaxies grew around them; they must have formed together, in a process sometimes referred to as co-evolution, from the seeds provided by the original black holes of a few hundred solar masses and the raw materials of the dense clouds of baryons in the knots in the filamentary structure. […] About one in a hundred of the galaxies seen at low redshifts are actively involved in the late stages of mergers, but these processes take so little time, compared with the age of the Universe, that the statistics imply that about half of all the galaxies visible nearby are the result of mergers between similarly sized galaxies in the past seven or eight billion years. Disc galaxies like the Milky Way seem themselves to have been built up from smaller sub-units, starting out with the spheroid and adding bits and pieces as time passed. […] there were many more small galaxies when the Universe was young than we see around us today. This is exactly what we would expect if many of the small galaxies have either grown larger through mergers or been swallowed up by larger galaxies.”

Links of interest:

Galaxy (‘featured article’).
Leonard Digges.
Thomas Wright.
William Herschel.
William Parsons.
The Great Debate.
Parallax.
Extinction (astronomy).
Henrietta Swan Leavitt (‘good article’).
Cepheid variable.
Ejnar Hertzsprung. (Before reading this book, I had no idea one of the people behind the famous Hertzsprung–Russell diagram was a Dane. I blame my physics teachers. I was probably told this by one of them, but if the guy in question had been a better teacher, I’d have listened, and I’d have known this.).
Globular cluster (‘featured article’).
Vesto Slipher.
Redshift (‘featured article’).
Refracting telescope/Reflecting telescope.
Disc galaxy.
Edwin Hubble.
Milton Humason.
Doppler effect.
Milky Way.
Orion Arm.
Stellar population.
Sagittarius A*.
Minkowski space.
General relativity (featured).
The Big Bang theory (featured).
Age of the universe.
Malmquist bias.
Type Ia supernova.
Dark energy.
Baryons/leptons.
Cosmic microwave background.
Cold dark matter.
Lambda-CDM model.
Lenticular galaxy.
Active galactic nucleus.
Quasar.
Hubble Ultra-Deep Field.
Stellar evolution.
Velocity dispersion.
Hawking radiation.
Ultimate fate of the universe.

 

February 5, 2017 Posted by | Astronomy, Books, cosmology, Physics | Leave a comment

Bill Bryson (II)

More quotes from his wonderful book:

1. “Before [Richard] Owen, museums were designed primarily for the use and edification of the elite, and even they found it difficult to gain access. In the early days of the British Museum, prospective visitors had to make a written application and undergo a brief interview to determine if they were fit to be admitted at all. They then had to return a second time to pick up a ticket – that is, assuming they had passed the interview – and finally come back a third time to view the museum’s treasures. Even then they were whisked through in groups and not allowed to linger. Owen’s plan was to welcome everyone, even to the point of encouraging working men to visit in the evening, and to devote most of the museum’s space to public displays. He even proposed, very radically, to put informative labels on each display so that people could appreciate what they were viewing.”

2. “At the turn of the twentieth century, palaeontologists had literally tons of old bones to pick over. The problem was that they still didn’t have any idea how old any of these bones were. Worse, the agreed ages for the Earth couldn’t comfortably support the numbers of aeons and ages and epochs that the past obviously contained. If Earth were really only twenty million years old or so, as the great Lord Kelvin insisted, then whole orders of ancient creatures must have come into being and gone out again practically in the same geological instant. It just made no sense. […] Such was the confusion that by the close of the nineteenth century, depending on which text you consulted, you could learn that the number of years that stood between us and the dawn of complex life in the Cambrian period was 3 million, 18 million, 600 million, 794 million, or 2,4 billion – or som other number within that range. As late as 1910 [five years after Einstein’s Annus Mirabilis papers], one of the most respected estimates, by the American George Becker, put the Earth’s age at perhaps as little as 55 million years.”

3. “Soon after taking up his position [in the beginning of the nineteenth century], [Humphry] Davy began to bang out new elements one after the other – potassium, sodium, magnesium, calcium, strontium, and aluminum or aluminium […] He discovered so many elements not so much because he was serially astute as because he developed an ingenious technique of applying electricity to a molten substance – electrolysis, as it is known. Altogether he discovered a dozen elements, a fifth of the known totals of his day.”

4. “They [Ernest Rutherford and Frederick Soddy] also discovered that radioactive elements decayed into other elements – that one day you had an atom of uranium, say, and the next you had an atom of lead. This was truly extraordinary. It was alchemy pure and simple; no-one had ever imagined that such a thing could happen naturally and spontaneously. […] For a long time it was assumed that anything so miraculously energetic as radioactivity must be beneficial. For years, manufacturers of toothpaste and laxatives put radioactive thorium in their products, and at least until the late 1920s the Glen Springs Hotel in the Finger Lakes region of New York (and doubtless others as well) featured with pride the therapeutic effects of its ‘Radio-active mineral springs’. It wasn’t banned in consumer products until 1938. By this time it was much too late for Mme Curie, who died of leukaemia in 1934.”

5. “In 1875, when a young German in Kiel named Max Planck was deciding whether to devote his life to mathematics or to physics, he was urged most heartily not to choose physics because the breakthroughs had all been made there. The coming century, he was assured, would be one of consolidation and refinement, not revolution.”

6. “You may not feel outstandingly robust, but if you are an average-sized adult you will contain within your modest frame no less than 7 x 10^18 joules of potential energy – enough to explode with the force of thirty very large hydrogen bombs, assuming you knew how to liberate it and really wished to make a point. Everything has this kind of energy trapped within it. We’re just not very good at getting it out. Even a uranium bomb – the most energetic thing we have produced yet – releases less than 1 per cent of the energy it could release if only we were more cunning.”

7. “It is worth pausing for a moment to consider just how little was known of the cosmos at the this time. Astronomers today believe there are perhaps 140 billion galaxies in the visible universe. […] In 1919, when Hubble first put his head to the eyepiece, the number of these galaxies known to us was exactly one: the Milky Way. Everything else was thought to be either part of the Milky Way itself or one of many distant, peripheral puffs of gas. […] at the time Leavitt and Cannon were inferring fundamental properties of the cosmos from dim smudges of distant stars on photographic plates, the Harvard astronomer William H. Pickering, who could of course peer into a first-class telescope as often as he wanted, was developing his seminal theory that dark patches on the Moon were caused by swarms of seasonally migrating insects.”

8. “Atoms, in short, are very abundant. They are also fantastically durable. Because they are so long-lived, atoms really get around. Every atom you possess has almost certainly passed through several stars and been part of millions of organisms on its way to becoming you. We are each so atomically numerous and so vigorously recycled at death that a significant number of our atoms – up to a billion for each of us, it has been suggested – probably once belonged to Shakespeare.”

From the wiki correction page: “Jupiter Scientific has done an analysis of this problem and the figure in Bryon’s book is probably low: It is likely that each of us has about 200 billion atoms that were once in Shakespeare’s body.”

9. “Even though lead was widely known to be dangerous, by the early years of the twentieth century it could be found in all manner of consumer products. Food came in cans sealed with lead solder. Water was often stored in lead-lined tanks. Lead arsenate was sprayed onto fruits as a pesticide. Lead even came as part of the composition of toothpaste tubes. […] Americans alive today each have about 625 times more lead in their blood than people did a century ago.”

In this chapter we also learn that we did not arrive at the current best estimate of the age of the earth until little over 50 years ago – I won’t quote from the book, but wikipedia has the short version: “An age of 4.55 ± 1.5% billion years, very close to today’s accepted age, was determined by C.C. Patterson using uranium-lead isotope dating (specifically lead-lead dating) on several meteorites including the Canyon Diablo meteorite and published in 1956.” At this point, the age of the universe was still very uncertain, from the book: “In 1956, astronomers discovered that Cepheid variables were more variable than they had thought; they came in two varieties, not one. This allowed them to rework their calculations and come up with a new age for the universe of between seven billion and twenty billion years” – as Bryson puts it, that estimate was “not terribly precise”. Our knowledge about the age of the universe is quite new.

10. “Well into the 1970s, one of the most popular and influential geological textbooks, The Earth by the venerable Harold Jefferys, strenuously insisted that plate tectonics was a physical impossibility, just as it had in the first edition way back in 1924. It was equally dismissive of convection and sea-floor spreading. And in Basin and Range, published in 1980, John McPhee noted that even then one American geologist in eight still didn’t believe in plate tectonics.”

11. “By the time Shoemaker came along, a common view was that Meteor Crater had been formed by an underground steam explosion. Shoemaker knew nothing about underground steam explosions – he couldn’t; they don’t exist…”

July 30, 2011 Posted by | Astronomy, Books, Chemistry, cosmology, Geology, Paleontology, Physics | Leave a comment

Wikipedia articles of interest

1. Milankovitch cycles.

“Milankovitch theory describes the collective effects of changes in the Earth’s movements upon its climate, named after Serbian civil engineer and mathematician Milutin Milanković, who worked on it during First World War internment. Milanković mathematically theorized that variations in eccentricity, axial tilt, and precession of the Earth’s orbit determined climatic patterns on Earth through orbital forcing.

The Earth’s axis completes one full cycle of precession approximately every 26,000 years. At the same time the elliptical orbit rotates more slowly. The combined effect of the two precessions leads to a 21,000-year period between the seasons and the orbit. In addition, the angle between Earth’s rotational axis and the normal to the plane of its orbit (obliquity) oscillates between 22.1 and 24.5 degrees on a 41,000-year cycle. It is currently 23.44 degrees and decreasing.”

2. Gravity of Earth. Here’s more, somewhat related.

3. Histone.

4. Lake Victoria.

“With a surface area of 68,800 square kilometres (26,600 sq mi), Lake Victoria is Africa’s largest lake by area, and it is the largest tropical lake in the world. Lake Victoria is the world’s second largest freshwater lake by surface area (only Lake Superior in North America is larger). In terms of its volume, Lake Victoria is the world’s eighth largest continental lake, and it contains about 2,750 cubic kilometers (2.2 billion acre-feet) of water.

[…]

“The introduction of the Nile perch had a decisive impact on Haplochromis stocks which it favoured as its prey, affecting both their abundance and diversity. It is believed that the contribution of this species flock to the fish biomass of the lake has decreased from 80% to less than 1% since the introduction of the Nile perch,[18][33] and that some 65% of the Haplochromis species were driven to extinction in the process, an event which may well represent the largest extinction event amongst vertebrates in the 20th century.[17]

Freed from their evolutionary predators, populations of the diminutive endemic silver cyprinid Rastrineobola argentea (omena in Luo, mukene in Luganda and dagaa in Swahili), flourished, developing into huge shoals. In turn, Pied Kingfisher (Ceryle rudis) populations, that had hitherto fed on haplochromines, exploded in response to this new food source.[34] Similar and other impacts have propagated throughout the ecosystem.

[…]

The Nile perch ‘boom’ was to accelerate and massively expand this process. It coincided with an emerging European market for high-quality white fish meat, prompting the development of industrial fish processing capacity along the lake’s shores in Kisumu, Musoma, Mwanza, Entebbe and Jinja.[40] The export of Nile perch has since expanded away from the European Union (EU) to the Middle East, the United States and Australia, and now represents large foreign exchange earnings to the lake’s riparian states. In Uganda, indeed, its export is second only to coffee in the rankings of export earnings. In 2006, the total value of Nile perch exports from the lake was estimated to be US$ 250 million. […]

With such high demands for Nile perch, the value of the fishery has risen considerably. Labour inflows into the fishery have increased along with growing demand. In 1983, there were an estimated 12,041 boats on the lake. By 2004, there were 51,712, and 153,066 fishermen.[38] The fishery also generates indirect employment for additional multitudes of fish processors, transporters, factory employees and others. All along the lakeshore, ‘boom towns’ have developed in response to the demands of fishing crews with money to spend from a day’s fishing.[Note 4] These towns resemble shanties, and have little in the way of services. Of the 1,433 landing sites identified in the 2004 frame survey, just 20% had communal lavatory facilities, 4% were served by electricity and 6% were served by a potable water supply.[38]”

5. Flesh-fly.

“Flies of the Diptera family Sarcophagidae (from the Greek sarco- = flesh, phage = eating; the same roots as the word “sarcophagus”) are commonly known as flesh flies. Most flesh flies breed in carrion, dung, or decaying material, but a few species lay their eggs in the open wounds of mammals;[1] hence their common name. Some flesh fly larvae are internal parasites of other insects. These larvae, commonly known as maggots, live for about 5–10 days, before descending into the soil and maturing into adulthood. At that stage, they live for 5–7 days. […]

The family contains three subfamilies, the Miltogramminae, the Paramacronychiinae and the Sarcophaginae, containing between them 108 genera. Flesh-flies are quite closely related to the family Calliphoridae, which belongs to the same (large) infraorder, the Muscomorpha, and includes species such as the blowfly that have similar habits to the flesh-flies. [genus/genera is the taxonomic level one step above species. There are a lot of different kinds of flies like this and they are quite successful.]

July 26, 2011 Posted by | Astronomy, Biology, cosmology, Ecology, Genetics, Geography, Physics, Wikipedia, Zoology | Leave a comment

Cosmology revisited

So first of all, I know a handful readers or two came by after I commented over at William’s blog – if one or more of you decided to come back to read this: Welcome!

If you didn’t read this post (that is: looked closely at the images) back when I posted that, I suggest you start there. Now Salman Khan has made a series of videos where he starts at Earth, then moves on outwards. I notice in one of the videos he mistakenly uses light year as a measure of time, not distance, but he was pretty excited at that point, for good reason. I’ve posted the first video in the series below – when I watched it on youtube, it automatically started the next video once the previous one had finished, which was both good and bad as I probably sat there for over an hour watching that stuff, but I don’t know if it’ll do the same when embedded here. If not, you should really watch the series on youtube if you think the first part was ok – it gets even better and far more mind boggling as he proceeds.

I love what Sal is doing. If you felt the need to follow the link to Salman Khan’s wikipedia article because you don’t know who he is or what he’s doing, here’s another good video you should watch:

And here’s the link to the site.

In other news, here’s a chess game I played earlier this evening (I was white and it was a 5 minute game so presumably lots of mistakes if you let the silicon monster have a look at it). I haven’t run it through a computer, but I still think my decision to exchange on g7 and move 20.f5 instead of taking on e6 was the right one. I really liked that 20.f5 move when I played it. If black wants to survive, he can’t defend that e6 pawn anyway, i.e. 20…Nf8, 21.f6+ Kg8, 22.Qd2 Nbd7, 23.Qh6 Nf6 (…Ne6, 24.dxe6 Nxf6(□), 25.Nxf6+ Qxf6(□), 26.Rxf6 and white has the same win as in the game with the Nf3 and Ng5-manoeuvre), 24.Nxf6+ Kg8, 25.Nh5! Ne6 (…gxh5 and after 26.Rxf7 black is mated), 26.dxe6 Rg8(□), 27.exf7 and game over). I think 20.f6 was a better defence than Ne5, Ne5 was a bad move. Black needs all the support he can get of the black squares around his king after he’s allowed the exchange of the g7-bishop. That said, the position after f6 is still losing for black.

June 12, 2011 Posted by | Astronomy, Chess, cosmology, education | Leave a comment

Interesting

In astrophysics and physical cosmology, Olbers’ paradox, is the argument that the darkness of the night sky conflicts with the assumption of an infinite and eternal static universe. It is one of the pieces of evidence for a non-static universe such as the current Big Bang model. The argument is also referred to as the “dark night sky paradox” (see physical paradox).

Much more here. I love wikipedia!

I remember that this really puzzled me when I was a kid, that is, why the night sky looked the way it did (Olbers’ paradox actually isn’t a good place to start if you want to know that, but anyway…). I didn’t get why, if there was a lot of stars around in the sky, lighting up everything, then even if the universe was very large and the stars were far away, why was everything so dark at night? If there was a star in almost every direction I could see, then why wasn’t the sky much brigther? Also, if the universe was infinitely large (I had a really difficult time accepting that statement), then there would be an infinite number of stars too, rigth (no, that’s not right, see point a in the link. But I didn’t get that back then of course)? And if there was an infinite number of stars, they would light up the whole sky, not just bits and pieces here and there, rigth? Asking my parents didn’t really help… (I’m pretty sure it was during my “asking age” I wondered about this, that period of your life where you drive your parents insane by asking them so many questions that no human being can possibly answer all of them, but I’m also quite sure it took me a long time to pass that age, so that doesn’t really help me pinpoint the exact time).

Btw: If you were wondering, the number of stars visible with the naked eye at night-time at any given location on earth, given ideal viewing conditions, is in the neighbourhood of 3.000, as Asimov expressed it in his book Facts and Fancy (recommended): The faintest star that can be seen with the naked eye, under the best conditions, is of magnitude 6.5 and the number of stars that exist in the entire circuit of the skies that bright or brighter is just about 6000. That’s all. That’s the hard fact of it. Six thousand.

For comparison, it is estimated that the total number of stars in the observable universe is 7×10^22.

May 25, 2009 Posted by | Astronomy, cosmology, Random stuff | Leave a comment