The Big Splatby Dana McKenzie
Wiley 2003
I got it on Audible.com
A really good, entertaining and informative book, I'll give five stars for the pleasure of reading it. It centers on theories about the origin of the Moon, but picks up a wealth of detail besides : beginning in prehistoric times to present day discusses the significance of the Moon to human culture. One could learn alot of the generalities of orbital mechanics by reading it.
The title is a play on the current and best supported theory of lunar formation, the impact hypothesis. A Mars like body slams into the proto-Earth some 5-ish billion years ago and the rubble from that huge collision sorts out into the two bodies we know today. This idea really didn't take hold until the Apollo missions, when the analysis of the returned rocks was highly revealing and discounted previous ideas :
- the fission theory. I hadn't known that Charles Darwin's second son George was a first rate physicist and mathematician in his own right, but he proposed this around the turn of the previous century ( circa 1900 ). A hot viscous blob of proto-Earth is rotating rapidly ( once per three hours ) and gradually distorts due to tidal influences from the Sun. A sequence of shape types ensues, sphere -> oblate spheroid ( discus ) -> prolate spheroid ( cigar ) -> bi-lobed ( pear ) -> then snap and the Moon 'buds off'. Trouble was the numbers and the maths didn't really work out, the timelines were wrong ( especially the Moon's retreat from close-in toward it's current orbital radius from Earth ) and the 'equation of state' ( thus how it deforms dynamically ) of the proto-Earth's substance wasn't really known.
- the capture theory. That is a Moon sized body, pre-formed, scoots in near Earth and then joins it in orbit. Rather more ambitious in that it requires a bleed-off of energy/momentum otherwise it would be a single close encounter and then scoot off never to be seen again. This is the weakness of this option as no plausible/workable mechanism to do it. It had some impressive figures though like : the incoming Moon turning in an orbital configuration in a few short minutes!
- the co-accretion theory. Meaning the Earth and Moon formed together from the same swirling cloud of material. Again the theory trouble was to get the numbers and the timelines right. Mentioned here is the work of Edouard Roche, of Roche Lobe fame, for his work describing how circulating matter is partitioned b/w gravitationally 'competing' bodies. These days that has ample application with the evolution of binary star systems.
So with the Moon landings we find what the Moon is actually made of - pieces of Earth! Mind you there are still persisting questions under study. Also the face of the moon is discussed too - where did the craters come from - impacts ( yes ) or vulcanism ( little or none )?
Did you know that the Babylonians had a Moon based calendar? Some ~ 29.53 days is one moon circuit around Earth. So every twelve of these months gives about 11 days short of one year ( 354 vs 365 ), and over 8 years you have 88 days short which is about 3 lunar months, thus you chuck in an extra 3 ( intercalary ) months per 8 year cycle. If you go to 235 lunar months then that is real close to 19 years ...... if we'd kept that calendar from it's first use til present day we'd only be about 11 days out by now, and wouldn't need to put in an extra month in until the year 6000+ !! That approximately 19 year cycle is also the key to predicting eclipses as an integral number of Moon-around-Earth circuits co-incides with an integral number of Earth-around-Sun circuits. Given that the angular momentum axes ( read: orbital planes ) are static over these timescales then the Moon/Earth/Sun configurations ( ie. eclipses ) recur. Well nearly anyway, as other factors niggle the construct, but one can predict well using perturbations around this basic model. One ancient chap ( ? Aristarchus ? ) made a name for himself by using this, and a helio-centric model ( yes, way before Copernicus ), to predict an eclipse or three - so he got some great kingdom-cred for that work.
Another point of interest is the 'Parish Lantern' effect, that is the Moon illuminates the night. Also it served simple timekeeping - it was generally always viewable by one and all with the phases working much like a clock face - in the days before artificial watches and clocks. How do you time a year back then? Well you have to catch something like a solstice, by the judgment of a peak/trough in the noon angle of the Sun above the horizon ( to be sure that's a multi-day observation ) or the Stonehenge like alignments of sun-rise/set with respect to horizons or other objects. Now given also that days ( meaning daylight length ) varied with the seasons, then the moon formed a good clock midway b/w shorter and longer periods.
Cheers, Mike.
( edit ) 365 days per year is of course actually 365 and about a quarter of a day. I'd like to know who was the clever-dick who sorted that out first! Anyone know?
( edit ) Now without overly hair-splitting too many definitions, take the ( synodic ) month as 29.530589 days and the ( mean tropical ) year as 365.24231 days ( Wikipaedia, present day epoch values ). So 235 of those months = 6939.688415 days and 19 of those years is 6939.60389 days so the residual is 0.084525 days. Hence the lunar calendar is 'behind' the solar ie. the Earth/Sun cycle returns to the same arrangement just before the Earth/Moon cycle does. So if you look at that residual on a yearly basis ( 0.084525/19 ~ 0.00444868 ) then you get ~ 6638 years ( 29.530589/0.00444868 ) before you need to adjust the calendar by a full lunar month! Nice work from the Babylonians! :-) :-)
( edit ) The gradually lengthening synodic month ( timescale of millennia, where the Moon recedes a few inches per year from the Earth into a higher and thus longer orbit ) and ignoring a far milder year length shortening ( 0.5 second shorter per century ) when taken into account means : you have to add up more of those residuals to get an intercalary lunar month. So taking whatever rates from the onset of the Babylonian calendar use, by integrating the ongoing variation, extends the date for that special intercalary month insertion to beyond the year 6000 AD.
( edit ) Some present day Islamic countries do use purely lunar calendars for various purposes.
I have made this letter longer than usual because I lack the time to make it shorter ...
... and my other CPU is a Ryzen 5950X :-) Blaise Pascal
Uranium Warsby Amir D. Aczel
Macmillan 2010
I got it on Audible.com
This is the story of the discovery of spontaneous and induced nuclear fission, spread across many decades starting before the discovery of radio-activity through to the realisation of the ultimate weapon. It is detailed, factual to the point of chilling in places but clearly a thoroughly researched work as is typical of Aczel's writing generally. I'd give it 4.5 stars, subtracting 0.5 because it gave me the willies in some parts.
There are 92 naturally occurring elements, and Uranium is the heaviest. It was born in supernovae and amounts are incorporated into subsequent generations of stars and their planetary systems. Man has dug it up and used it. Near a Bohemian town called Joachimstal mining recovered many valuables like silver, but had leftover tailings given the name 'pitchblende' ( black dirt ). A chap called Klaproth ( in English the 'h' is silent, so pronounced clap-rot with a hard 't' ) managed to extract a new element from it, but kindly named it after the recent discovery of a new planet by William Herschel, instead of after himself as he could have ( otherwise we'd be discussing 'Klaprothium Wars' ).
Now radioactivity has a long and rather tortuous path of discovery, but in essence it was found that certain substances can affect distant ones by some sort of 'rays' that travel between. Humans have no senses to detect these. Thus are X-rays/gamma-rays ( photons ), beta-rays ( electrons ) and alpha-rays ( helium nuclei = 2 protons + 2 neutrons ). Elements have long been ordered in the sequence of their atomic number ( number of nuclear protons hence nuclear charge ) in rows and columns by a system developed by Mendeleev. That wasn't how it was originally described, as it was the number of electrons in the outer shells ( of the neutral atom ) that determines chemical behaviour by a property called valency ( roughly how many connections can an atom make with others in a molecule say ).
Enter the Curies who took a tonne or so of pitchblende ( delivered by horse and cart in cloth bags! ) and by some very clever chemistry managed to separate out different substances within ( Nobel prize winning work ). When a nucleus spontaneously changes it does so as per quantum mechanics, which means that for a specific nucleus type ( nuclide ) there is a characteristic chance of it decaying in some given time period. More importantly this is independent of history, so that if there is a 30% chance of decaying in 30 minutes, say, but after 30 minutes the nucleus hasn't then : the chance for the next 30 minutes is unaltered. This leads to a mathematical/statistical model of groups of nuclides called exponential decay, which can be characterised for a given nuclide by the term half-life. We could be talking of third-life or two-fifth-life or reciprocal-of-PI-life if we wanted to, 1/2 life is just a convenient & easy to visualise rate label : how long does it take on average for half of the ( identical ) nuclei in a sample to decay?
Nuclides with longer half-lives don't do much in any given time compared to those with shorter half-lives. Hence the 'hotter' stuff doesn't last as long. This is important for detection as either really short lived or really long lived species may not be detected or recognised as radioactive. So stability is a relative term. Francium for instance has a brief existence, at any given moment only a few dozen actual nuclei will exist on Earth, on the other hand K40 ( potassium ) seems stable but simply has an extremely long half life. It all comes down to the probability of some quantum transition and some energy barrier(s).
Thus it was even more amazing that the Curies and others were able to make sense of any of it from a pile containing a mixture of very many things, each cooking off to it's own tune. So gradually sequences, or decay chains, were discovered by using brutal mind-numbing experimental persistence.
Enter Fermi. What a guy. He was the quintessential mix of theoretician and experimenter which I don't think has an equal. A child prodigy who became a brilliant man. By his arrival on the scene it was known that beta-decay ( emission of an electron ) did not change the mass ( much ) but added a proton thus putting the nuclide one element up. Nowadays we would say that a neutron has turned into a proton while spitting out an electron plus a ( anti- ) neutrino. Also alpha-decay meant that the nuclide went down four ( about ) in mass and down two on the Mendeleev chart. At the time many were firing alpha particles at various things to see what happened, like at aluminium or magnesium or boron or beryllium targets. Fermi noted that beryllium when hit with an alpha particle would perhaps spit out a neutron ( also recently discovered, the 1930's was really a busy decade ) and that neutron could be fired at something else. The advantage being it would penetrate where charged particles wouldn't ( cardboard can stop alphas ) - right into the nucleus. Now transmutations really began to happen ...
Enter Lise Meitner with Otto Hahn and Otto Frisch who found that lumps of Uranium had a tendency to gain barium atoms when bombarded with neutrons. What the what?? They were expecting heavier things not lighter ones. It was Meitner & Frisch on a cold Swedish back road who took Bohr's liquid-drop nucleus model and calculated that Uranium nuclei would split either spontaneously or if tickled by the right sort of neutron. Frisch asked a cell biologist friend : what do you call it when a cell splits? The term nuclear fission was born.
Most of this stuff was either published or transmitted via letters to colleagues around the world. For a short time there was a concept just hovering in the air .... and then the penny dropped - the nuclear chain reaction with it's evident conversion of a little mass to alot of energy. As they say, the rest is history ....
Imagine taking a few bites from a Mars bar. Or holding your mobile phone. Or buying a bunch of pencils. Any of those objects when converted completely into pure energy can wipe out a city and kill or maim hundreds of thousands of people. The nuclear genie. It's been done.
Cheers, Mike.
( edit ) BTW 'nuclide' is not the same as 'isotope'. Nuclide is a noun for 'single nuclear species type' whereas isotope is a word used to describe different nuclides with the same number of protons as in : chlorine has two isotopes with different numbers of neutrons. Also are 'isobars' = same number of nucleons ( total of protons and neutrons ) and 'isotones' = same number of neutrons. Thus beta decay will change one nuclide to another but because that will be to a different element ( the proton number changes ) then that's not isotopic, or isotonic, but it will be isobaric as the total number of nucleons is the still the same if a neutron becomes a proton ( or vice versa ). This terminology doesn't include mass/energy measurements per se ( or changes thereof ) as different nuclides have different binding energies - but that's a whole other topic.
( edit ) If you take exponential decay as indicated thus ( all named variables positive ):
[pre]A = A0 * exp[-a*t][/pre]
then for two times on the decay curve ( x > y ) :
[pre]y - x = (ln[K])/a[/pre]
'a' is specific for a given nuclide ( a bigger 'a' makes it decay faster which is why it divides here ).
If you choose a particular K then y - x is fully specified. y - x is a time interval we call the half-life for K = 0.5. If we change K ( keeping 'a' the same ) you'd get a different (y - x).
The especial thing about the exponential curve is that any particular choice of fixed time interval gives a correspondingly fixed ratio between the heights above the horizontal axis measured at the endpoints of said interval. In calculus terms we say that the derivative is proportional to the ordinate, in everyday terms we say that we lose an amount in strict proportion to what we started with.
( edit ) Actually Leo Szilard ( a good mate of Albert Einstein ) had come up with the general idea of a nuclear chain reaction for use in a bomb in the early 30's. He kept the idea mainly to himself, being concerned about the implication, though there is an obscure patent he filed on the matter. It was the Europeans as described above who revealed the specific mechanisms.
I have made this letter longer than usual because I lack the time to make it shorter ...
... and my other CPU is a Ryzen 5950X :-) Blaise Pascal
Israel has the Hebrew Calendar and I have used one for the last 30 years and since I got my first pc I have had one in Hebrew to check for certain days each year.
[b][i][=red]
Did you know that the Babylonians had a Moon based calendar? Some ~ 29.53 days is one moon circuit around Earth. So every twelve of these months gives about 11 days short of one year ( 354 vs 365 ), and over 8 years you have 88 days short which is about 3 lunar months, thus you chuck in an extra 3 ( intercalary ) months per 8 year cycle. If you go to 235 lunar months then that is real close to 19 years ...... if we'd kept that calendar from it's first use til present day we'd only be about 11 days out by now, and wouldn't need to put in an extra month in until the year 6000+
( edit ) 365 days per year is of course actually 365 and about a quarter of a day. I'd like to know who was the clever-dick who sorted that out first! Anyone know?
( edit ) Now without overly hair-splitting too many definitions, take the ( synodic ) month as 29.530589 days and the ( mean tropical ) year as 365.24231 days ( Wikipaedia, present day epoch values ). So 235 of those months = 6939.688415 days and 19 of those years is 6939.60389 days so the residual is 0.084525 days. Hence the lunar calendar is 'behind' the solar ie. the Earth/Sun cycle returns to the same arrangement just before the Earth/Moon cycle does. So if you look at that residual on a yearly basis ( 0.084525/19 ~ 0.00444868 ) then you get ~ 6638 years ( 29.530589/0.00444868 ) before you need to adjust the calendar by a full lunar month! Nice work from the Babylonians! :-) :-)
( edit ) The gradually lengthening synodic month ( timescale of millennia, where the Moon recedes a few inches per year from the Earth into a higher and thus longer orbit ) and ignoring a far milder year length shortening ( 0.5 second shorter per century ) when taken into account means : you have to add up more of those residuals to get an intercalary lunar month. So taking whatever rates from the onset of the Babylonian calendar use, by integrating the ongoing variation, extends the date for that special intercalary month insertion to beyond the year 6000 AD.
( edit ) Some present day Islamic countries do use purely lunar calendars for various purposes.
Emilio Segre', whose book "Persons and discoveries in contemporary physics" I have edited while at Mondadori Publishing House, writes that in 1935 a completely unknown chemist, Ida Noddack, had suggested the right interpretation of the Fermi group experiments done in Rome.She proposed that the uranium nucleus could be split by a neutron in two big fragments. Of course, nobody believed her.
In a different source, whose title I cannor remember, Segre' writes "God, for inscrutable reasons, made us all blind". This was confirmed to me by Segre' in a private conversation.
Tullio
See also J.Bernstein, Eyes on the prize, New York Review, March 24 1994.
Spektrum der Wissenschat, Juli 2011 p 48 – 57: Mit Pulsaren auf der Jagd nach Gravitationswellen. (Spektrum der Wissenschaft is the German speaking branch of Scientific American. There I didn´t find such article within the last 6 month, as their view is more towards USA.) It´s a thorough article, showing some basics, alternative theories and the overlapping and extensions of this method comparing to the laser-interferometers.
The Essential Einstein : His Greatest Works edited with commentary by Stephen Hawking. I bought it from Dymocks for $AUD 28.00 as a paperback.
This is one of a handful of compendium type books that Mr Hawking has done in the last few years. It is what it says, a collection of the highlights of Albert Einstein's career, mostly about the Special and General Theories of Relativity but not limited to those. Of ~ 450 pages around 50 contain some pretty dense mathematical sections ( tensors as they relate to General Relativity ). Not being an avid tensor gymnast I skimmed those parts, and really only focused on the conclusions. The remainder can really be read by anyone without a 'physics background' but requires a good careful read and intellectual rigor ( with Mr Einstein as your guide ). As Albert points out :
Quote:
I make no pretence of having withheld from the reader difficulties which are inherent to the subject.
Absolutely! He seems to pretty well retrace for us his own sequence/evolution of thoughts, and what a path it is. You will probably benefit from reading 'Relativity, The Special and General Theory' ( pages 125 - 234 ) first up if you have no familiarity with his work. This was written in 1916 to satisfy a request for a layperson's version and step by step brings one through important distinctions and understandings about the process of physical measurement. Length is that which is measured by rigid rods, and time is that which is measured by clocks. I think this is what baulks many people who read and try to fathom Einstein ie. why does he go on about observers, rods, clocks and whatnot ?
I think an answer to that legitimate furrowed-brow query is : forget about humans and their apparatus for a moment. Pretend you are an electron, say. By what means are you influenced by the rest of the universe, for instance another electron coming into the vicinity ? Answer : the exchange of photons. While that specific viewpoint wasn't fully matured back in 1916, it was known that delays occur in generating an effect ( I, an electron, repel ) from a cause ( the other electron moves closer ). So in that sense everything is 'an observer' which can be affected ( feel a force ) depending upon the reception of force carrying particles. Go up in scale and pretend you are the planet Earth. You 'know' about the Sun and go around it more-or-less as per Newton BUT 'really' your information ( the Sun's mass and distance away ) is around eight minutes old. The speed of light is fast but it is still finite. So General Relativity can be thought of as Newtonian gravity plus time delays. The cheque is in the mail and can be cashed on Monday even though it was written up on Friday! And so much is made of how one viewpoint ( 'reference frame' ) can be aware of goings on in another. Mr Einstein is very helpful by starting with something we think we know pretty well : a train passing by an embankment.
Put another way : if the speed of light were not so fast as compared with everyday experience then we would have no need to be taught relativity. We would know it as a fact of experience from a young age. Human scales - distances, speeds, times - are generally such that we don't sense any propagation delay for light. Try this : wave a flag here on Earth while watching yourself doing that via a TV camera sited on Mars ( attached to our recently landed rover ) aimed at you. You will have a multi-minute wait before you see that flag being waved. With further patience you will even see yourself tapping your fingers after waving the flag while you waited for the round trip to give you that image. Etc. This is the essence of his method of synchronising separated clocks, or what is really meant when we say that events happen 'simultaneously' :-) :-)
As for 'why' is the speed of light constant ( in a vacuum, regardless of reference frame ), who can say? That is not explained, it is simply taken as a true fact derived from experience ( Michelson and Morley, Fizeau etc ) and taken to it's logical conclusion. A moving object can appear to shorten along the line of relative motion and moving clocks appear to run slower. Again experiment decides eg. muons take longer to decay the faster they are going, and do so by precisely the amount as predicted. Note the use of 'appear' as it depends upon what reference frame is doing the looking.
This is not really a book that you could/should read in a straight push. Even if one is a seasoned fully fledged relativist I still think it would teach something. There are subtleties to relativity because it is mostly anti-intuitive. I would also add that you can think of the Relativities in daily life if you want. It is not as arcane as you might at first think. Try the next time you step into a lift : then ponder upon the Principle of Equivalence or how one can substitute ( at least for short periods of time and within limited regions ) an 'acceleration' for 'gravity'. Or what would happen if you were on a tram and tried to catch up to a light ray. That's what Einstein did ......
Cheers, Mike.
I have made this letter longer than usual because I lack the time to make it shorter ...
... and my other CPU is a Ryzen 5950X :-) Blaise Pascal
The best scientific biography of Einstein I know is "Subtle is the Lord", by Abraham Pais.On a more human plane "Albert Einstein Creator and Rebel", by Banesh Hoffman and Helen Dukas, who was Einstein's secretary.Since August 6 1945 was the date a nuclear bomb was dropped on Hiroshima,this was Einstein's reaction to the news as recounted by Dukas: Oh Weh!
Tullio
Cosmic Catastrophes by J. Craig Wheeler,
Cambridge University Press; January 2007 ISBN 9780511267161, I downloaded from eBooks
What a gem of a book! A terrific rundown of things that go bump in the sky, or why do stars blow up? With an easy, almost conversational, style the author approaches the subject beginning with the basic physical principles required to understand the behaviours of these huge balls of gas called stars. Great sketches and diagrams and barely an equation in sight ( neat for those who are shy of them ).
Stars are presented as a negotiated balance between competing forces and processes : gravity which always wins in the end, the nuclear forces which are responsible for fusion/fission of elements plus conversion of protons to and from neutrons, the electromagnetic force which carries away energy and shapes magnetic fields in dense environs, with special attention upon those elusive neutrinos that we often forget but play a crucial role in the end stages of stellar evolution.
Then there is the really interesting bit : most stars are not alone! They can develop and mature in the presence of another. If they orbit at some distance then there is no other interaction of note, but if they are close then all manner of phenomena come in to play. Enter Mr. Roche who showed there are zones of influence in binary systems that may dynamically compete for material. So mass may transfer from one to the other. A 'flat star' or accretion disk may form around one - stuff spirals inwards while angular momentum moves outwards - being fed by the other through the junction b/w their 'Roche lobes'. All manner of evolutionary sequences arise. A zoo of variants.
But regardless, gravity has the last say. Always attractive and never sleeping. What happens then depends on mass. Below a certain amount white dwarves form ( BTW the terms 'dwarf' and 'giant' originally referred to intrinsic luminosity, but as this generally correlates with size so well this distinction is now lost ). Our Sun will end as one, but not before going through a red giant phase consisting of an inner core still fusing upwards towards iron and an outer shell of hydrogen that expands to gobble & fry the planets. If there's more mass at end of life then a neutron star might form. Here the density is such that ordinary matter - distinct atoms comprising nuclei and a swirl of orbiting electrons - collapses. The protons in the nuclei combine with the electrons and neutrons are hence produced along with prodigious quantities of neutrinos. This happens in less time than it takes to read this sentence. A key point becomes what small fraction of these outbound neutrinos - 1% or 0.1% say - hits the outer layers thus blowing them off or, if not, do those layers fall inwards leading to the next possibility. Black holes. The ultimate trophy of gravity.
So supernovae are covered as being the distant sign of all these goings on. The categories - and their difficulties - are discussed. Where appropriate he indicates uncertainties and/or controversies in thinking. Neutron stars in the general and specific are discussed, ditto for black holes. You will note that here at E@H we avidly study the signal emissions - across modes and spectra thereof - from all the beasts mentioned within.
He tops off an excellent read with the view on supernovae as cosmological indicators, plus some 'bonus' topics like wormholes or highly speculative directions for future physics research.
I give it the full five stars. I found it hard to put it down, even at 11pm ... :-)
Cheers, Mike.
( edit ) Also I think I finally understand quantum/degenerative pressure. Fermions like electrons and neutrons will only occupy the same volume if their momenta/energies are different, so if you keep stacking material onto a white dwarf say, then higher and higher energy levels must be occupied. The electrons approach light speed, a necessary limit, and so per Chandrasekhar this dictates the top mass for the white dwarf mode. Similiarly for neutron stars but the exact limit is not as certain. We don't usually think of the Exclusion Principle as dictating/defining a 'force' but it certainly acts as one!
( edit ) I think 'Wolf-Rayet' stars are now my favorite un-condensed stars. They start cooking with upwards of about 30 solar masses and with the core temperature so high - fusion rates being proportional to some high power ( ? fifth ? sixth ) of temperature - that they really churn through the material. Some may only last a million years from go to whoa. Unless otherwise disturbed ( ? companion ) then you are pretty well going to get a black hole at the end. The biggest ones are close to what is known as the Eddington Limit, where the outgoing radiation from the core may well exceed the 'normal' gas pressure that keeps a star in equilibrium with gravity, leading to massive loss of material spewing into nearby space and thus no longer partaking in the subsequent behaviour. These are really hot stars - surface temperatures estimated at up to 200,000 K !!
I have made this letter longer than usual because I lack the time to make it shorter ...
... and my other CPU is a Ryzen 5950X :-) Blaise Pascal
I've just noticed the existence of the first issue of LIGO Magazine. I've had a browse thru and I think it is a cracking read centered on all things inferometer-ish and gravity wave-ish! Lovely layout and cool graphics/photos, with names to faces and roles. Later I'll settle back with it for more time and a deeper soak ... :-)
Cheers, Mike.
( edit ) You know, I wonder if I ought print out a copy or three and put it in my waiting room!! :-)
I have made this letter longer than usual because I lack the time to make it shorter ...
... and my other CPU is a Ryzen 5950X :-) Blaise Pascal
The Big Splat by Dana
)
The Big Splat by Dana McKenzie
Wiley 2003
I got it on Audible.com
A really good, entertaining and informative book, I'll give five stars for the pleasure of reading it. It centers on theories about the origin of the Moon, but picks up a wealth of detail besides : beginning in prehistoric times to present day discusses the significance of the Moon to human culture. One could learn alot of the generalities of orbital mechanics by reading it.
The title is a play on the current and best supported theory of lunar formation, the impact hypothesis. A Mars like body slams into the proto-Earth some 5-ish billion years ago and the rubble from that huge collision sorts out into the two bodies we know today. This idea really didn't take hold until the Apollo missions, when the analysis of the returned rocks was highly revealing and discounted previous ideas :
- the fission theory. I hadn't known that Charles Darwin's second son George was a first rate physicist and mathematician in his own right, but he proposed this around the turn of the previous century ( circa 1900 ). A hot viscous blob of proto-Earth is rotating rapidly ( once per three hours ) and gradually distorts due to tidal influences from the Sun. A sequence of shape types ensues, sphere -> oblate spheroid ( discus ) -> prolate spheroid ( cigar ) -> bi-lobed ( pear ) -> then snap and the Moon 'buds off'. Trouble was the numbers and the maths didn't really work out, the timelines were wrong ( especially the Moon's retreat from close-in toward it's current orbital radius from Earth ) and the 'equation of state' ( thus how it deforms dynamically ) of the proto-Earth's substance wasn't really known.
- the capture theory. That is a Moon sized body, pre-formed, scoots in near Earth and then joins it in orbit. Rather more ambitious in that it requires a bleed-off of energy/momentum otherwise it would be a single close encounter and then scoot off never to be seen again. This is the weakness of this option as no plausible/workable mechanism to do it. It had some impressive figures though like : the incoming Moon turning in an orbital configuration in a few short minutes!
- the co-accretion theory. Meaning the Earth and Moon formed together from the same swirling cloud of material. Again the theory trouble was to get the numbers and the timelines right. Mentioned here is the work of Edouard Roche, of Roche Lobe fame, for his work describing how circulating matter is partitioned b/w gravitationally 'competing' bodies. These days that has ample application with the evolution of binary star systems.
So with the Moon landings we find what the Moon is actually made of - pieces of Earth! Mind you there are still persisting questions under study. Also the face of the moon is discussed too - where did the craters come from - impacts ( yes ) or vulcanism ( little or none )?
Did you know that the Babylonians had a Moon based calendar? Some ~ 29.53 days is one moon circuit around Earth. So every twelve of these months gives about 11 days short of one year ( 354 vs 365 ), and over 8 years you have 88 days short which is about 3 lunar months, thus you chuck in an extra 3 ( intercalary ) months per 8 year cycle. If you go to 235 lunar months then that is real close to 19 years ...... if we'd kept that calendar from it's first use til present day we'd only be about 11 days out by now, and wouldn't need to put in an extra month in until the year 6000+ !! That approximately 19 year cycle is also the key to predicting eclipses as an integral number of Moon-around-Earth circuits co-incides with an integral number of Earth-around-Sun circuits. Given that the angular momentum axes ( read: orbital planes ) are static over these timescales then the Moon/Earth/Sun configurations ( ie. eclipses ) recur. Well nearly anyway, as other factors niggle the construct, but one can predict well using perturbations around this basic model. One ancient chap ( ? Aristarchus ? ) made a name for himself by using this, and a helio-centric model ( yes, way before Copernicus ), to predict an eclipse or three - so he got some great kingdom-cred for that work.
Another point of interest is the 'Parish Lantern' effect, that is the Moon illuminates the night. Also it served simple timekeeping - it was generally always viewable by one and all with the phases working much like a clock face - in the days before artificial watches and clocks. How do you time a year back then? Well you have to catch something like a solstice, by the judgment of a peak/trough in the noon angle of the Sun above the horizon ( to be sure that's a multi-day observation ) or the Stonehenge like alignments of sun-rise/set with respect to horizons or other objects. Now given also that days ( meaning daylight length ) varied with the seasons, then the moon formed a good clock midway b/w shorter and longer periods.
Cheers, Mike.
( edit ) 365 days per year is of course actually 365 and about a quarter of a day. I'd like to know who was the clever-dick who sorted that out first! Anyone know?
( edit ) Now without overly hair-splitting too many definitions, take the ( synodic ) month as 29.530589 days and the ( mean tropical ) year as 365.24231 days ( Wikipaedia, present day epoch values ). So 235 of those months = 6939.688415 days and 19 of those years is 6939.60389 days so the residual is 0.084525 days. Hence the lunar calendar is 'behind' the solar ie. the Earth/Sun cycle returns to the same arrangement just before the Earth/Moon cycle does. So if you look at that residual on a yearly basis ( 0.084525/19 ~ 0.00444868 ) then you get ~ 6638 years ( 29.530589/0.00444868 ) before you need to adjust the calendar by a full lunar month! Nice work from the Babylonians! :-) :-)
( edit ) The gradually lengthening synodic month ( timescale of millennia, where the Moon recedes a few inches per year from the Earth into a higher and thus longer orbit ) and ignoring a far milder year length shortening ( 0.5 second shorter per century ) when taken into account means : you have to add up more of those residuals to get an intercalary lunar month. So taking whatever rates from the onset of the Babylonian calendar use, by integrating the ongoing variation, extends the date for that special intercalary month insertion to beyond the year 6000 AD.
( edit ) Some present day Islamic countries do use purely lunar calendars for various purposes.
I have made this letter longer than usual because I lack the time to make it shorter ...
... and my other CPU is a Ryzen 5950X :-) Blaise Pascal
Uranium Wars by Amir D.
)
Uranium Wars by Amir D. Aczel
Macmillan 2010
I got it on Audible.com
This is the story of the discovery of spontaneous and induced nuclear fission, spread across many decades starting before the discovery of radio-activity through to the realisation of the ultimate weapon. It is detailed, factual to the point of chilling in places but clearly a thoroughly researched work as is typical of Aczel's writing generally. I'd give it 4.5 stars, subtracting 0.5 because it gave me the willies in some parts.
There are 92 naturally occurring elements, and Uranium is the heaviest. It was born in supernovae and amounts are incorporated into subsequent generations of stars and their planetary systems. Man has dug it up and used it. Near a Bohemian town called Joachimstal mining recovered many valuables like silver, but had leftover tailings given the name 'pitchblende' ( black dirt ). A chap called Klaproth ( in English the 'h' is silent, so pronounced clap-rot with a hard 't' ) managed to extract a new element from it, but kindly named it after the recent discovery of a new planet by William Herschel, instead of after himself as he could have ( otherwise we'd be discussing 'Klaprothium Wars' ).
Now radioactivity has a long and rather tortuous path of discovery, but in essence it was found that certain substances can affect distant ones by some sort of 'rays' that travel between. Humans have no senses to detect these. Thus are X-rays/gamma-rays ( photons ), beta-rays ( electrons ) and alpha-rays ( helium nuclei = 2 protons + 2 neutrons ). Elements have long been ordered in the sequence of their atomic number ( number of nuclear protons hence nuclear charge ) in rows and columns by a system developed by Mendeleev. That wasn't how it was originally described, as it was the number of electrons in the outer shells ( of the neutral atom ) that determines chemical behaviour by a property called valency ( roughly how many connections can an atom make with others in a molecule say ).
Enter the Curies who took a tonne or so of pitchblende ( delivered by horse and cart in cloth bags! ) and by some very clever chemistry managed to separate out different substances within ( Nobel prize winning work ). When a nucleus spontaneously changes it does so as per quantum mechanics, which means that for a specific nucleus type ( nuclide ) there is a characteristic chance of it decaying in some given time period. More importantly this is independent of history, so that if there is a 30% chance of decaying in 30 minutes, say, but after 30 minutes the nucleus hasn't then : the chance for the next 30 minutes is unaltered. This leads to a mathematical/statistical model of groups of nuclides called exponential decay, which can be characterised for a given nuclide by the term half-life. We could be talking of third-life or two-fifth-life or reciprocal-of-PI-life if we wanted to, 1/2 life is just a convenient & easy to visualise rate label : how long does it take on average for half of the ( identical ) nuclei in a sample to decay?
Nuclides with longer half-lives don't do much in any given time compared to those with shorter half-lives. Hence the 'hotter' stuff doesn't last as long. This is important for detection as either really short lived or really long lived species may not be detected or recognised as radioactive. So stability is a relative term. Francium for instance has a brief existence, at any given moment only a few dozen actual nuclei will exist on Earth, on the other hand K40 ( potassium ) seems stable but simply has an extremely long half life. It all comes down to the probability of some quantum transition and some energy barrier(s).
Thus it was even more amazing that the Curies and others were able to make sense of any of it from a pile containing a mixture of very many things, each cooking off to it's own tune. So gradually sequences, or decay chains, were discovered by using brutal mind-numbing experimental persistence.
Enter Fermi. What a guy. He was the quintessential mix of theoretician and experimenter which I don't think has an equal. A child prodigy who became a brilliant man. By his arrival on the scene it was known that beta-decay ( emission of an electron ) did not change the mass ( much ) but added a proton thus putting the nuclide one element up. Nowadays we would say that a neutron has turned into a proton while spitting out an electron plus a ( anti- ) neutrino. Also alpha-decay meant that the nuclide went down four ( about ) in mass and down two on the Mendeleev chart. At the time many were firing alpha particles at various things to see what happened, like at aluminium or magnesium or boron or beryllium targets. Fermi noted that beryllium when hit with an alpha particle would perhaps spit out a neutron ( also recently discovered, the 1930's was really a busy decade ) and that neutron could be fired at something else. The advantage being it would penetrate where charged particles wouldn't ( cardboard can stop alphas ) - right into the nucleus. Now transmutations really began to happen ...
Enter Lise Meitner with Otto Hahn and Otto Frisch who found that lumps of Uranium had a tendency to gain barium atoms when bombarded with neutrons. What the what?? They were expecting heavier things not lighter ones. It was Meitner & Frisch on a cold Swedish back road who took Bohr's liquid-drop nucleus model and calculated that Uranium nuclei would split either spontaneously or if tickled by the right sort of neutron. Frisch asked a cell biologist friend : what do you call it when a cell splits? The term nuclear fission was born.
Most of this stuff was either published or transmitted via letters to colleagues around the world. For a short time there was a concept just hovering in the air .... and then the penny dropped - the nuclear chain reaction with it's evident conversion of a little mass to alot of energy. As they say, the rest is history ....
Imagine taking a few bites from a Mars bar. Or holding your mobile phone. Or buying a bunch of pencils. Any of those objects when converted completely into pure energy can wipe out a city and kill or maim hundreds of thousands of people. The nuclear genie. It's been done.
Cheers, Mike.
( edit ) BTW 'nuclide' is not the same as 'isotope'. Nuclide is a noun for 'single nuclear species type' whereas isotope is a word used to describe different nuclides with the same number of protons as in : chlorine has two isotopes with different numbers of neutrons. Also are 'isobars' = same number of nucleons ( total of protons and neutrons ) and 'isotones' = same number of neutrons. Thus beta decay will change one nuclide to another but because that will be to a different element ( the proton number changes ) then that's not isotopic, or isotonic, but it will be isobaric as the total number of nucleons is the still the same if a neutron becomes a proton ( or vice versa ). This terminology doesn't include mass/energy measurements per se ( or changes thereof ) as different nuclides have different binding energies - but that's a whole other topic.
( edit ) If you take exponential decay as indicated thus ( all named variables positive ):
[pre]A = A0 * exp[-a*t][/pre]
then for two times on the decay curve ( x > y ) :
[pre]A(y) = A0 * exp[-a*y]
A(x) = A0 * exp[-a*x][/pre]
so
[pre]K = A(x)/A(y) = exp[-a*x]/exp[-a*y]
= exp[a(y-x)][/pre]
or
[pre]y - x = (ln[K])/a[/pre]
'a' is specific for a given nuclide ( a bigger 'a' makes it decay faster which is why it divides here ).
If you choose a particular K then y - x is fully specified. y - x is a time interval we call the half-life for K = 0.5. If we change K ( keeping 'a' the same ) you'd get a different (y - x).
The especial thing about the exponential curve is that any particular choice of fixed time interval gives a correspondingly fixed ratio between the heights above the horizontal axis measured at the endpoints of said interval. In calculus terms we say that the derivative is proportional to the ordinate, in everyday terms we say that we lose an amount in strict proportion to what we started with.
( edit ) Actually Leo Szilard ( a good mate of Albert Einstein ) had come up with the general idea of a nuclear chain reaction for use in a bomb in the early 30's. He kept the idea mainly to himself, being concerned about the implication, though there is an obscure patent he filed on the matter. It was the Europeans as described above who revealed the specific mechanisms.
I have made this letter longer than usual because I lack the time to make it shorter ...
... and my other CPU is a Ryzen 5950X :-) Blaise Pascal
Israel has the Hebrew
)
Israel has the Hebrew Calendar and I have used one for the last 30 years and since I got my first pc I have had one in Hebrew to check for certain days each year.
It is the 10th of Shevat the year 5771 today.
Hebrew Calendar
http://en.wikipedia.org/wiki/Hebrew_calendar
Emilio Segre', whose book
)
Emilio Segre', whose book "Persons and discoveries in contemporary physics" I have edited while at Mondadori Publishing House, writes that in 1935 a completely unknown chemist, Ida Noddack, had suggested the right interpretation of the Fermi group experiments done in Rome.She proposed that the uranium nucleus could be split by a neutron in two big fragments. Of course, nobody believed her.
In a different source, whose title I cannor remember, Segre' writes "God, for inscrutable reasons, made us all blind". This was confirmed to me by Segre' in a private conversation.
Tullio
See also J.Bernstein, Eyes on the prize, New York Review, March 24 1994.
I have a pile of books I am
)
I have a pile of books I am working off:-)
I added these ones to the pile:
Entropy Demystified
Physics and Chance: Philosophical Issues in the Foundations of Statistical Mechanics
I added these to my wishlist..
Self-Organized Criticality: Emergent Complex Behaviour in Physical and Complex Systems
Uncertainity and Information: Foundations on Generalized Information Theory
Other books on my pile..
Sound Like water Dripping: in Search of the Boreal Owl
A Mountain Year: Diary of a Wilderness Dweller
I hope I live long enough to get through the pile and my Dynamic Wishlist:-)
There are some who can live without wild things and some who cannot. - Aldo Leopold
Searching for GW by means of
)
Searching for GW by means of Pulsars
Spektrum der Wissenschat, Juli 2011 p 48 – 57: Mit Pulsaren auf der Jagd nach Gravitationswellen. (Spektrum der Wissenschaft is the German speaking branch of Scientific American. There I didn´t find such article within the last 6 month, as their view is more towards USA.) It´s a thorough article, showing some basics, alternative theories and the overlapping and extensions of this method comparing to the laser-interferometers.
Kind regards
Martin
The Essential Einstein : His
)
The Essential Einstein : His Greatest Works edited with commentary by Stephen Hawking. I bought it from Dymocks for $AUD 28.00 as a paperback.
This is one of a handful of compendium type books that Mr Hawking has done in the last few years. It is what it says, a collection of the highlights of Albert Einstein's career, mostly about the Special and General Theories of Relativity but not limited to those. Of ~ 450 pages around 50 contain some pretty dense mathematical sections ( tensors as they relate to General Relativity ). Not being an avid tensor gymnast I skimmed those parts, and really only focused on the conclusions. The remainder can really be read by anyone without a 'physics background' but requires a good careful read and intellectual rigor ( with Mr Einstein as your guide ). As Albert points out :
Absolutely! He seems to pretty well retrace for us his own sequence/evolution of thoughts, and what a path it is. You will probably benefit from reading 'Relativity, The Special and General Theory' ( pages 125 - 234 ) first up if you have no familiarity with his work. This was written in 1916 to satisfy a request for a layperson's version and step by step brings one through important distinctions and understandings about the process of physical measurement. Length is that which is measured by rigid rods, and time is that which is measured by clocks. I think this is what baulks many people who read and try to fathom Einstein ie. why does he go on about observers, rods, clocks and whatnot ?
I think an answer to that legitimate furrowed-brow query is : forget about humans and their apparatus for a moment. Pretend you are an electron, say. By what means are you influenced by the rest of the universe, for instance another electron coming into the vicinity ? Answer : the exchange of photons. While that specific viewpoint wasn't fully matured back in 1916, it was known that delays occur in generating an effect ( I, an electron, repel ) from a cause ( the other electron moves closer ). So in that sense everything is 'an observer' which can be affected ( feel a force ) depending upon the reception of force carrying particles. Go up in scale and pretend you are the planet Earth. You 'know' about the Sun and go around it more-or-less as per Newton BUT 'really' your information ( the Sun's mass and distance away ) is around eight minutes old. The speed of light is fast but it is still finite. So General Relativity can be thought of as Newtonian gravity plus time delays. The cheque is in the mail and can be cashed on Monday even though it was written up on Friday! And so much is made of how one viewpoint ( 'reference frame' ) can be aware of goings on in another. Mr Einstein is very helpful by starting with something we think we know pretty well : a train passing by an embankment.
Put another way : if the speed of light were not so fast as compared with everyday experience then we would have no need to be taught relativity. We would know it as a fact of experience from a young age. Human scales - distances, speeds, times - are generally such that we don't sense any propagation delay for light. Try this : wave a flag here on Earth while watching yourself doing that via a TV camera sited on Mars ( attached to our recently landed rover ) aimed at you. You will have a multi-minute wait before you see that flag being waved. With further patience you will even see yourself tapping your fingers after waving the flag while you waited for the round trip to give you that image. Etc. This is the essence of his method of synchronising separated clocks, or what is really meant when we say that events happen 'simultaneously' :-) :-)
As for 'why' is the speed of light constant ( in a vacuum, regardless of reference frame ), who can say? That is not explained, it is simply taken as a true fact derived from experience ( Michelson and Morley, Fizeau etc ) and taken to it's logical conclusion. A moving object can appear to shorten along the line of relative motion and moving clocks appear to run slower. Again experiment decides eg. muons take longer to decay the faster they are going, and do so by precisely the amount as predicted. Note the use of 'appear' as it depends upon what reference frame is doing the looking.
This is not really a book that you could/should read in a straight push. Even if one is a seasoned fully fledged relativist I still think it would teach something. There are subtleties to relativity because it is mostly anti-intuitive. I would also add that you can think of the Relativities in daily life if you want. It is not as arcane as you might at first think. Try the next time you step into a lift : then ponder upon the Principle of Equivalence or how one can substitute ( at least for short periods of time and within limited regions ) an 'acceleration' for 'gravity'. Or what would happen if you were on a tram and tried to catch up to a light ray. That's what Einstein did ......
Cheers, Mike.
I have made this letter longer than usual because I lack the time to make it shorter ...
... and my other CPU is a Ryzen 5950X :-) Blaise Pascal
The best scientific biography
)
The best scientific biography of Einstein I know is "Subtle is the Lord", by Abraham Pais.On a more human plane "Albert Einstein Creator and Rebel", by Banesh Hoffman and Helen Dukas, who was Einstein's secretary.Since August 6 1945 was the date a nuclear bomb was dropped on Hiroshima,this was Einstein's reaction to the news as recounted by Dukas: Oh Weh!
Tullio
Cosmic Catastrophes by J.
)
Cosmic Catastrophes by J. Craig Wheeler,
Cambridge University Press; January 2007 ISBN 9780511267161, I downloaded from eBooks
What a gem of a book! A terrific rundown of things that go bump in the sky, or why do stars blow up? With an easy, almost conversational, style the author approaches the subject beginning with the basic physical principles required to understand the behaviours of these huge balls of gas called stars. Great sketches and diagrams and barely an equation in sight ( neat for those who are shy of them ).
Stars are presented as a negotiated balance between competing forces and processes : gravity which always wins in the end, the nuclear forces which are responsible for fusion/fission of elements plus conversion of protons to and from neutrons, the electromagnetic force which carries away energy and shapes magnetic fields in dense environs, with special attention upon those elusive neutrinos that we often forget but play a crucial role in the end stages of stellar evolution.
Then there is the really interesting bit : most stars are not alone! They can develop and mature in the presence of another. If they orbit at some distance then there is no other interaction of note, but if they are close then all manner of phenomena come in to play. Enter Mr. Roche who showed there are zones of influence in binary systems that may dynamically compete for material. So mass may transfer from one to the other. A 'flat star' or accretion disk may form around one - stuff spirals inwards while angular momentum moves outwards - being fed by the other through the junction b/w their 'Roche lobes'. All manner of evolutionary sequences arise. A zoo of variants.
But regardless, gravity has the last say. Always attractive and never sleeping. What happens then depends on mass. Below a certain amount white dwarves form ( BTW the terms 'dwarf' and 'giant' originally referred to intrinsic luminosity, but as this generally correlates with size so well this distinction is now lost ). Our Sun will end as one, but not before going through a red giant phase consisting of an inner core still fusing upwards towards iron and an outer shell of hydrogen that expands to gobble & fry the planets. If there's more mass at end of life then a neutron star might form. Here the density is such that ordinary matter - distinct atoms comprising nuclei and a swirl of orbiting electrons - collapses. The protons in the nuclei combine with the electrons and neutrons are hence produced along with prodigious quantities of neutrinos. This happens in less time than it takes to read this sentence. A key point becomes what small fraction of these outbound neutrinos - 1% or 0.1% say - hits the outer layers thus blowing them off or, if not, do those layers fall inwards leading to the next possibility. Black holes. The ultimate trophy of gravity.
So supernovae are covered as being the distant sign of all these goings on. The categories - and their difficulties - are discussed. Where appropriate he indicates uncertainties and/or controversies in thinking. Neutron stars in the general and specific are discussed, ditto for black holes. You will note that here at E@H we avidly study the signal emissions - across modes and spectra thereof - from all the beasts mentioned within.
He tops off an excellent read with the view on supernovae as cosmological indicators, plus some 'bonus' topics like wormholes or highly speculative directions for future physics research.
I give it the full five stars. I found it hard to put it down, even at 11pm ... :-)
Cheers, Mike.
( edit ) Also I think I finally understand quantum/degenerative pressure. Fermions like electrons and neutrons will only occupy the same volume if their momenta/energies are different, so if you keep stacking material onto a white dwarf say, then higher and higher energy levels must be occupied. The electrons approach light speed, a necessary limit, and so per Chandrasekhar this dictates the top mass for the white dwarf mode. Similiarly for neutron stars but the exact limit is not as certain. We don't usually think of the Exclusion Principle as dictating/defining a 'force' but it certainly acts as one!
( edit ) I think 'Wolf-Rayet' stars are now my favorite un-condensed stars. They start cooking with upwards of about 30 solar masses and with the core temperature so high - fusion rates being proportional to some high power ( ? fifth ? sixth ) of temperature - that they really churn through the material. Some may only last a million years from go to whoa. Unless otherwise disturbed ( ? companion ) then you are pretty well going to get a black hole at the end. The biggest ones are close to what is known as the Eddington Limit, where the outgoing radiation from the core may well exceed the 'normal' gas pressure that keeps a star in equilibrium with gravity, leading to massive loss of material spewing into nearby space and thus no longer partaking in the subsequent behaviour. These are really hot stars - surface temperatures estimated at up to 200,000 K !!
I have made this letter longer than usual because I lack the time to make it shorter ...
... and my other CPU is a Ryzen 5950X :-) Blaise Pascal
I've just noticed the
)
I've just noticed the existence of the first issue of LIGO Magazine. I've had a browse thru and I think it is a cracking read centered on all things inferometer-ish and gravity wave-ish! Lovely layout and cool graphics/photos, with names to faces and roles. Later I'll settle back with it for more time and a deeper soak ... :-)
Cheers, Mike.
( edit ) You know, I wonder if I ought print out a copy or three and put it in my waiting room!! :-)
I have made this letter longer than usual because I lack the time to make it shorter ...
... and my other CPU is a Ryzen 5950X :-) Blaise Pascal