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Earth before life: more skeptico-romantic chitchat

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The early Earth - more cracks than facade?

The early Earth – more cracks than facade?

Canto: So we’ve talked all too briefly about Earth’s probable formation and how its moon was formed some fifty million years later, and I’m not sure whether I want to go back further in time to try to answer some big questions about the solar system in general or the solar nebula, or forward to consider how life emerged from inanimate matter on this seething-hot, volatile planetary surface…

Jacinta: Well since we’re the blind leading the blind, it doesn’t much matter which direction we go. Let’s choose life.

Canto: Okay, but we’ll have a way to travel before we get there.

Jacinta: Well most of us learned at school that the Earth has a crust, a mantle and a core, and that the core is of iron and it’s really hot down there, and the crust is formed of plates that move around and go under each other, and that the atmosphere above the crust consists of layers, like the stratosphere and the ionosphere, and the atmosphere around us is around three-quarters nitrogen and a quarter oxygen with traces of other gases, and if it wasn’t like that we wouldn’t be here. But it wasn’t anything like that when the first life appeared.

Canto: Yes, it was very different, and it seems there’s more that we don’t know about the period between 4.5 and 4 billion BP than there is that we do know, if you know what I mean.

Jacinta: BP?

Canto: Before the Present. I got that from the excellent Stuff You Should Know podcast, and I’m going to use it from now on.

Jacinta: D’accord. So yes, we know that the early Earth was incredibly hot, reaching temperatures of 2000 celsius or more, but there’s also evidence from ancient amphibolite rocks and banded iron formations that there was water on the Earth, and plenty of it, 4.3 billion years ago. Which suggests an extraordinarily fast cooling down period, and where did all that water come from?

Canto: Yes I think we really need to look at this period, or what we know of it, to try and make sense of it, because it doesn’t quite make sense to me. A hot magma world, melted fom the inside out, but also bombarded from the outside by meteorites, then after the bombardment suddenly cooling from the outside in, and flowing with water. All in a couple of hundred million years?

Really?

Really?

Jacinta: That’s a long time actually. We’re hoping to live for a hundred years for some strange reason – a two millionth of our time-frame, if we’re very lucky.

Canto: Well it’s all relative, but where did this water come from? Some say it must’ve come from space, because that’s all that happened, meteors from out there crashing into here. Where else could it come from?

 

Jacinta: How do you trap water here when the surface temperature is so high? Water boils at 100c, right?

Canto: Under ‘normal’ atmospheric pressure. The early Earth was anything but normal.

Jacinta: Anyway it just doesn’t seem possible to get so much water from rocks crashing into us. There’s another alternative – the water was already here. So the original bits and pieces that formed the Earth – carbonaceous chondrites or whatever – contained water and this water somehow made its way to the surface.

Canto: Somehow. Leaving aside the rising-to-the-surface problem, carbon-rich chondrites are found in asteroids today, and they have apparently a similar water-plus-impurities ratio to our oceanic water, and that’s obviously very suggestive.

Jacinta: Yes and the isotopic ratios pretty well match, but they don’t for comets. Scientists have been able to measure the isotopic ratios in comets such as Halley and Hale-Bopp, and they don’t have anything like the proportions found in our oceans. I’m talking heavy water here, deuterium, but also protium which is another isotope of hydrogen.

Canto: NASA also launched a spacecraft, Deep Impact, to probe the constituents of a comet, Tempel1, and the results were negatory for its candidature as feeder of the Earth’s water, had it ever landed here, but of course not nugatory for astronomical research generally. But then, what comet is ever typical? Anyway, there’s a just-so story, sort of, that I watched on video recently, which explained the oceans, sort of. It told us that the planetesimals that created the Earth contained water locked inside, and that years of later volcanic activity released that water to the surface as steam, which condensed in the cool upper atmosphere and fell as rain. And the rain it rainèd every day.

Jacinta: So the Bible was right then?

Canto: More than forty days and nights – thousands of years, they claimed. But that made up only half the world’s oceans. The rest came from comets, they said. Now that seems unlikely, but replace comets with the right sorts of asteroids, and the recipe still works.

Jacinta: Well here’s another story, which is meant to explain how that heat-creating heavy bombardment came to an end.  The Earth’s bombarded surface was extremely hot, melting everything, even the rocks, and in this state the heavier elements such as iron sank to the centre, forming our core, which was vital in protecting us from the notorious solar wind – that incredibly strong force that has blown away the atmosphere of Mars.

Canto: Yeah, they say it kind of magnetised the Earth, and that was like a shield of steel.

Jacinta: Aka the magnetosphere, but I’m afraid that electromagnetism was a subject that transformed me into a gibbering mass of incomprehension at school.

image

Canto: I can’t say I understand it myself, but the magnetosphere works to almost perfectly preserve our atmosphere. We do lose a percentage to the solar wind every year but it’s so tiny that it’s not a problem. Another anthropic circumstance that proves the existence of God.

Jacinta: Hallelujah. So did this magnetosphere form before or after the formation of the moon?

Canto: God knows.

Jacinta: Goddess.

Canto: Sorry princess.

Jacinta: Princess, goddess, actress, countess, diminutives. They diminish.

Canto: Seamstress.

Jacinta: Temptress.

Canto: Watercress. Anyway it probably happened around the same time. The great crash that probably created the moon has been nicely computer-simulated by Robin Canup of the Southwest Research Institute – it’s well worth a look. The theory goes that this great glancing blow tilted the Earth and gave us our seasons, probably vital to life as we know and love it.

Jacinta: Yes but it would’ve heated up the planet even more, so I’m interested in the problem of the shift from this to our amphibolite rocks under water from nearly 4.3 billion years ago. Where the eff did that water come from? It steamed up from beneath the surface? Not likely. And from asteroids? Really?

Canto: Possibly. But according to this excellent Naked Science video, the best-preserved meteorites ever recovered came from a landfall in British Columbia in 2000. And when they investigated this meteorite material they found that it was made up of 20% water by weight, and that’s pretty significant…

Jacinta: Because water isn’t dense like rock is it, so that sounds like a lot of water. We’re learning a lot from this video, such as that meteorites don’t cause great fireballs or anything like that, because they’ve been tumbling about in cold space for eons, and their entry into the Earth’s atmosphere only heats up a few millimetres of the outer surface, and then only for a very brief period, so they pretty well instantly go cold again.

Canto: Right and maybe that explains something else; that a heavy bombardment of these big wet boulders – and apparently they’ve found that the further they are from us, the more water they contain – would’ve cooled the planet.

Jacinta: Interesting idea, which I’m sure someone’s thought of and maybe even computer modelled. Certainly it would help to explain the apparent speed with which the oceans were formed. So… I’m not really convinced, but in lieu of a better explanation I’ll take it on trust that the oceans were created in little more than a million years or so by a hailstorm of asteroids, together with water steamed up from below the surface. So now we have a somewhat cooler Earth, ready at last for some kind of life, but not as we experience it.

Canto: Right, we’re talking about an atmosphere containing virtually no oxygen. Made up mostly of nitrogen, carbon dioxide and methane.

Jacinta: And how do they know that? I’ve also heard hydrogen sulphide mentioned.

Canto: Yeah, upwellings from volcanic activity I believe.

image

Jacinta: So the stage is set for some sort of proto-life, with RNA or some precursor. And so the fun begins, if it hasn’t already.

Canto: Indeed it does. So that’s what we’ll be exploring next. I’ve even heard some researchers claim that water isn’t necessary for basic life to get started. Now there’s heresy for you.

Jacinta: That’s the fun of heresy these days, you don’t get burned alive for it, no more than a bit of gentle ribbing. I’m looking forward to the next post.

Written by stewart henderson

July 21, 2016 at 9:34 pm

How on earth? A chat about origins.

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one impression of our proto-sun and solar nebula

one impression of our proto-sun and solar nebula

Jacinta: I’d like to know how we got in this position.

Canto: What position?

Jacinta: Here, on Earth.

Canto: We?

Jacinta: Humans.

Canto: That’s a very long story, which I suspect nobody’s really qualified to tell. But maybe we can report on the best speculations. First, in order to understand how we got here we have to understand how the Earth got here.

Jacinta: And so on, infinitely regressing. So let’s just start with the Earth.

Canto: Needless to say we don’t know all the details and there are doubtless competing theories, and new data is being regularly uncovered, but it obviously has to do with how our entire solar system was formed.

Jacinta: I’ve heard that all the heavy metals like iron and whatnot are forged within stars, like when they go supernova, but our star hasn’t done that, all it seems to produce is light, yet Earth is full of heavy elements. I really don’t get it.

Canto: I recall reading years ago a theory that the Earth was formed from an accretion of planetesimals, little planets…

Jacinta: Planettes?

Canto: Yes, but how those little things came into being themselves I’m not sure.

Jacinta: Well we have lots of rocky bits and bobs called asteroids floating about in the solar system…

Canto: Yes, but not randomly. there’s a whole big asteroid belt between Jupiter and Mars, where they’re coralled, sort of.

Jacinta: But comets are different, they seem to have their individual eccentric orbits.

Canto: I suppose the point is that they also have heavy elements, and how were those elements formed?

Jacinta: Heat and pressure, I’m guessing, so things must’ve been hugely different in earlier times.

Canto: Well, this BBC site gives us some of the latest speculations. They reckon that the Earth probably formed from planetesimals, so that’s still the best hypothesis it seems, though it’s very light on details:

The Earth is thought to have been formed about 4.6 billion years ago by collisions in the giant disc-shaped cloud of material that also formed the Sun. Gravity slowly gathered this gas and dust together into clumps that became asteroids and small early planets called planetesimals.

Jacinta: Yes, that’s extremely vague. How do they know there was a disc-shaped cloud here? How can they investigate that far back?

Canto: Well don’t forget that looking out over huge distances means looking back in time.

Jacinta: Yes but a huge distance away isn’t here. Is it?

Canto: Well it might be here then.

Jacinta: Effing Einstein. But they’re also searching for extra data on the past, like checking out meteorites, which might contain material older than anything on Earth. Can they reliably date material that’s say, 5 billion years old? The Earth’s only about 4.5 billion years old, right?

Canto: I think 4.6 billion, give or take a few minutes. About a third of the age of the universe. And here’s the thing, we’ve dated all the meteorites and asteroids we can get to and they’re all round the same age, within a narrow range of a few hundred million years. So our date for the beginnings of the solar system is the oldest date for these floating and landing rocks, which is also our date for the Earth, about 4.6 billion.

Jacinta: So is our dating system completely accurate, and what by the way are carbonaceous chondrites?

Canto: Well, yes, radioactive decay provides a very accurate clock, and these meteorites have radioactive material in them, just as the core of our planet does. All the evidence so far suggests that things happened very quickly, in terms of accretion and formation of planets, once all this heavy and radioactive material was created. Carbonaceous chondrites are a type of meteorite. They’re amongst the oldest meteorites but relatively rare – they make up less than 5% of our meteorites. I mean the ones that land here. Why do you ask?

Jacinta: I’ve heard about them as being somehow important for research, and maybe dating?

Canto: Well there are different types of C chondrites as they’re called, and some of them, most interesting to us of course, are rich in organic compounds and water. This fact apparently shows that they haven’t been subjected to high temperatures, unlike for example the early Earth. But let me return to that BBC quote above. The theory goes that a supernova explosion, or maybe more than one, created all the heavy elements we have now – iron, carbon, silver, gold, uranium and all the rest, heat and pressure as you say, and these elements swirled around but were gravitationally attracted to a centre, which evolved into our sun. This was the spinning disc-shaped cloud mentioned above, known as the solar nebula.

Jacinta: Would you call that a theory, or a hypothesis, or wild desperate speculation?

Canto: I’d call it ‘the best we can do at the present moment’. But be patient, it’s a great time to be young in astronomy today. What we need is data, data, data, and we’re just starting to collect more data than we can rightly deal with on planets within and especially outside our solar system. Kepler’s just the beginning, girlie.

Jacinta: Je suis tout à fait d’accord, boyo. I think many of the astrophysicists are looking forward to having their cherished models swept aside by all the new telescopes and spectroscopes and what else and the data they spew back to Earth.

Canto: Uhh, well anyway let’s get back to our ‘best scenario for the moment’ scenario. So you have all this matter spinning around and the force of gravity causes accretion. It’s a messy scenario actually because everything’s moving at different velocities and angular momentums if that’s a thing, upwards, forwards, sideways down, and sometimes there’s accretion, sometimes fragmentation, but overall the movement is towards coalescence due to gravity. Particles grow to the size of monuments and then different sized planetesimals, fewer and bigger and farther between. And the smaller, gaseous elements are swept out by the solar wind into the great beyond, where they accrete into gas giants.

Jacinta: Right, but isn’t the data from Kepler and elsewhere already starting to play havoc with this scenario? Gas giants within spitting distance of their suns and the like?

Canto: Well, you need liquid to spit, but maybe you have a point, but I think it’s wise not to be too distracted by exoplanets and their systems at this stage. I think we need to find an internally coherent and consistent account of our own system.

Jacinta: What about the Juno probe, will that help?

Canto: Well I’m sure it will help us learn more about gas giants, but let’s just focus on the Earth now.

Jacinta: Okay, stay focussed.

Canto: These larger planetesimals became bigger gravitational attractors, each accumulating matter until we had four rocky planets in different, sufficiently distant orbits around their sun.

Jacinta: Oh yes, and what about the moons? Why didn’t they coalesce as neatly as all the other minor rocky bits?

Canto: Mmmm, well there’s nothing neat about all this, but mmmm…

Jacinta: How many moons are there?

Canto: For the inner planets? Only three, ours and two for Mars. So the question is, how come some of those rocks, or at least three, didn’t get stuck to the bigger rocks i.e. planets, via gravity, but instead started circling those planets, also due to gravity.

Jacinta: Yes, which might be the same question as why do the planets orbit around this massive gravitational attractor, the sun, instead of getting sucked into it, like what happens with those supermassive supersucking black holes?

Canto: Well first let me talk about our moon, because the most currently accepted theory about how our moon came into existence might surprise you.

Jacinta: It was a lot closer to the Earth at the beginning, wasn’t it? So it’s slowly spiralling away from us?

schematic of tial forces affecting moon's orbit and earth's rotation

schematic of tidal forces affecting moon’s orbit and earth’s rotation

Canto: Yes. Tidal forces. The moon’s tidally locked to the Earth, it’s the same face she shows us always, but let’s keep on track, it was formed in the very early days, when things were still very chaotic. A pretty large planetesimal, or planetoid, slammed into Earth, which was somewhat smaller then, and it stuck to it and coalesced with it – the Earth was pretty-well molten in those days – and a lot of debris was thrown out into space, but this debris didn’t quite escape Earth’s gravitational field, instead it coalesced to form our moon. This theory was first put forward a few decades ago, after moon rocks brought back from the Apollo missions were found to be younger than the oldest Earth rocks, and composed of much the same stuff, which came as a great surprise. But now the theory is well accepted, as it accounts for a number of other factors in the relationship between the two bodies.

the hypothesised Thea impact, which enlarged the Earth and created the Moon

the hypothesised Thea impact, which enlarged the Earth and created the Moon

Jacinta: Okay, so is that it on how the Earth was formed?

Canto: Well, yes, but the bigger question is your original one – how did we get here. And that means we have to look at how life got started here. Because we’re only up to about 4.5 billion years ago – with the moon being formed about 50 million years after the Earth. And at that point the Earth was like a sea of hot magma, hot from all the collisions on the surface, and hot from the radiation bursting out from its core. Hardly great conditions for life.

Jacinta: Well there might’ve been life, but not as we know it boyo.

Canto: I’m skeptical, but we’ll talk about that next time.

Some sources:

on the relation between moon and earth

formation of the moon

the solar nebula theory and its problems

 

 

Written by stewart henderson

July 19, 2016 at 8:45 am

more on Einstein, black holes and other cosmic stuff

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Einstein in 1931 staring at a wee bit of the universe, with Edwin Hubble

Einstein at Mount Wilson in 1931, staring at a wee bit of the universe, with Edwin Hubble

Jacinta: Well Canto I’d like to get back to Einstein and space and time and the cosmos, just because it’s such a fascinating field to inhabit and explore.

Canto: Rather a big one.

Jacinta: I’ve read, or heard, that Einstein’s theory, or one of them, predicted black holes, though he didn’t necessarily think that such entities really existed, but now black holes are at the centre of everything, it seems.

Canto: Including our own galaxy, and most others.

Jacinta: Yes, and there appears to be a correlation between the mass of these supermassive black holes at the centres of galaxies and the mass of the galaxies themselves, indicating that they appear to be the generators of galaxies. Can you expand on that?

Canto: Well the universe seems to be able to expand on that better than I can, but I’ll try. Black holes were first so named in the 1960s, but Einstein’s theory of general relativity recast gravity as a distortion of space and time rather than as a Newtonian force, with the distortion being caused by massive objects. The greater the mass, the greater the distortion, or the ‘geodetic effect’, as it’s called. The more massive a particular object, given a fixed radius, the greater is the velocity required for an orbiting object to escape its orbit, what we call its escape velocity. That escape velocity will of course, approacher closer and closer to the speed of light, as the object being orbited becomes more massive. So what happens when it reaches the speed of light? Then there’s no escape, and that’s where we enter black hole territory.

Jacinta: So, let me get this. Einstein’s theory is about distortions of space-time (and I’m not going to pretend that I understand this), or geodetic effects, and so it has to account for extreme geodetic effects, where the distortion is so great that nothing, not even light, can escape, and everything kind of gets sucked in… But how do these massive, or super-massive objects come into being, and won’t they eventually swallow all matter, so that all is just one ginormous black hole?

Canto: Okay I don’t really get this either but shortly after Einstein published his theory it was worked out by an ingenious astrophysicist, Karl Schwarzschild – as a result of sorting out Einstein’s complex field equations –  that if matter is severely compressed it will have weird effects on gravity and energy. I was talking a minute ago about increasing the mass, but think instead of decreasing the radius while maintaining the mass as a constant…

Jacinta: The same effect?

Karl Schwarzschild

Karl Schwarzschild

Canto: Well, maybe, but you’ll again reach a point where there’s zero escape, so to speak. In fact, what you have is a singularity. Nothing can escape from the object’s surface, whether matter or radiation, but also you’ll have a kind of internal collapse, in which the forces that keep atoms and sub-atomic particles apart are overcome. It collapses into an infinitesimal point – a singularity. It was Schwarzschild too who described the ‘event horizon’, and provided a formula for it.

Jacinta: That’s a kind of boundary layer, isn’t it? A point of no return?

Canto: Yes, a spherical boundary that sort of defines the black hole.

Jacinta: So why haven’t I heard of this Schwarzschild guy?

Canto: He died in 1916, shortly after writing a paper which solved Einstein’s equations and considered the idea of ‘point mass’ – what we today would call a singularity. But both he and Einstein, together with anyone else in the know, would’ve considered this stuff entirely theoretical. It has only become significant, and very significant, in the last few decades.

Jacinta: And doesnt this pose a problem for Einstein’s theory? I recall reading that this issue of ‘point mass’, or a situation where gravity is kind of absolute, like with black holes and the big bang, or the ‘pre-big bang’ if that makes sense, is where everything breaks down, because it seems to bring in the mathematical impossibility of infinity, something that just can’t be dealt with mathematically. And Einstein wasn’t worried about it in his time because black holes were purely theoretical, and the universe was thought to be constant, not expanding or contracting, just there.

Canto: Well I’ve read – and I dont know if it’s true – that Einstein believed, at least for a time, that black holes couldn’t actually exist because of an upper limit imposed on the gravitational energy any mass can produce – preventing any kind of ‘infinity’ or singularity.

Jacinta: Well if that’s true he was surely wrong, as the existence of black holes has been thoroughly confirmed, as has the big bang, right?

Canto: Well of course knowledge was building about that in Einstein’s lifetime, as Edwin Hubble and others provided conclusive evidence that the universe was expanding in 1929, so if this expansion was uniform and extended back in time, it points to an early much-contracted universe, and ultimately a singularity. And in fact Einstein’s general relativity equations were telling him that the universe wasn’t static, but he chose to ignore them, apparently being influenced by the overwhelming thinking of the time – this was 1917 – and he introduced his infamous or famous cosmological constant, aka lambda.

Jacinta: And of course 1917 was an early day in the history of modern astronomy, we hardly knew anything beyond our own galaxy.

Canto: Or within it. One of the great astrophysicists of the era, Sir Arthur Eddington, believed at the time that the sun was at the centre of the universe, while admitting his calculations were ‘subject to large uncertainties’.

Jacinta: Reminds me of Lord Kelvin on the age of the Earth only a few generations before.

Canto: Yes, how quickly our best speculations can become obsolete, but that’s one of the thrills of science. And it’s worth noting that the work of Hubble and others on the expansion of the universe depended entirely on improved technology, namely the 100-inch Hooker telescope at Mount Wilson, California.

Jacinta: Just as the age of the Earth problem was solved through radiometric dating, which depended on all the early twentieth century work on molecular structure and isotopes and such.

Canto: Right, but now this lambda (λ) – which Einstein saw as a description of some binding force in gravity to counteract the expansion predicted by his equations – is very much back in the astrophysical frame. The surprising discovery made in 1998 that the universe’s expansion is accelerating rather than slowing has, for reasons I can’t possibly explain, brought Einstein’s lambda in from the cold as an explanatory factor in that discovery, which is also somehow linked to dark energy.

Jacinta: So his concept, which he simply invented as a ‘fix-it’ sort of thing, might’ve had more utility than he knew?

Canto: Well the argument goes, among some, that Einstein was a scientist of such uncanny insight that even his mistakes have proved more fruitful than others’ discoveries. Maybe that’s hero worship, maybe not.

Jacinta: So how does lambda relate to dark energy, and how does dark energy relate to dark matter, if you please?

Canto: Well the standard model of cosmology (which is currently under great pressure, but let’s leave that aside) has been unsuccessful in trying to iron out inconsistent observations and finding with regard to the energy density of the universe, and so dark energy and what they call cold dark matter (CDM) have been posited as intellectual placeholders, so to speak, to make the observations and equations come out right.

Jacinta: Sounds a bit dodgy.

Canto: Well, time will tell how dodgy it is but I don’t think anyone’s trying to be dodgy, there’s a great deal of intense calculation and measurement involved, with so many astrophysicists looking at the issue from many angles and with different methods. Anyway, to quickly summarise CDM and dark energy, they together make up some 96% of the mass-energy density of our universe according to the most currently accepted calculations, with dark energy accounting for some 69% and CDM accounting for about 27%.

Jacinta: Duhh, that does sound like a headachey problem for the standard model. I mean, I know I’m only a dilettanty lay-person, but a model of universal mass-energy that only accounts for about 4% of the stuff, that doesn’t sound like much of a model.

Canto: Well I can assure they’re working on it…

Jacinta: Or working to replace it.

Canto: That too, but let me try to explain the difference between CDM and dark energy. Dark energy is associated with lambda, because it’s the ‘missing energy’ that accounts for the expansion of the universe, against the binding effects of gravity. As it happens, Einstein’s cosmological constant pretty well perfectly counters this expansive energy, so that if he hadn’t added it to his equations he would’ve been found to have predicted an expanding universe decades before this was confirmed by observation. That’s why it was only in the thirties that he came to regret what he called the greatest mistake of his career. Cold dark matter, on the other hand, has been introduced to account for a range of gravitational effects which require lots more matter than we find in the observed (rather than observable) universe. These effects include the flat shapes of galaxies, gravitational lensing and the tight clustering of galaxies. It’s described as cold because its velocity is considerably less than light-speed.

Lambda-Cold_Dark_Matter,_Accelerated_Expansion_of_the_Universe,_Big_Bang-Inflation

the lambda- cold dark matter model

Jacinta: Okay, so far so bad, but let’s get back to black holes. Why are they so central?

Canto: Well, that’s perhaps the story of supermassive black holes in particular, but I suppose I should try to tell the story of how astronomers found black holes to be real. As I’ve said, the term was first used in the sixties, 1967 to be precise, by John Wheeler, at a time when their actual existence was being considered increasingly likely, and the first more or less confirmed discovery was made in 1971 with Cygnus x-1. You can read all about it here. It’s very much a story of developing technology leading to increasingly precise observational data, largely in the detecting and measuring of X-ray emissions, stuff that was undetectable to us with just optical instruments.

Jacinta: Okay, go no further, I accept that there’s been a lot of data from a variety of sources that have pretty well thoroughly confirmed their existence, but what about these supermassive black holes? Could they actually be the creators of matter in the galaxies they’re central to? That’s what I’ve heard, but my reception was likely faulty.

Canto: Well astrophysicists have been struggling with the question of this relationship – there clearly is a relationship between supermassive black holes and their galaxies, but which came first? Now supermassive black holes can vary a lot – our own ‘local’ one is about 4 million solar masses, but we’ve discovered some with billions of solar masses. But it was found almost a decade ago that there is correlation between the mass of these beasties and the mass of the inner part of the galaxies that host them – what they call the galactic bulge. The ratio is always about 1 to 700. Obviously this is highly suggestive, but it requires more research. There are some very interesting examples of active super-feeding black holes emitting vast amounts of energy and radiation, which is both destructive and productive in a sense, creating an active galaxy. Our own Milky Way, or the black hole at its centre, is currently quiescent, which is just as well.

Jacinta: You mean if it starts suddenly feeding, we’re all gonna die?

Canto: No probably not, the hole’s effects are quite localised, relatively speaking, and we’re a long way from the centre.

Jacinta: Okay thanks for that, that’s about as much about black holes as I can stand for now.

Canto: Well I’m hoping that in some future posts we can focus on the technology, the ground-based and space-based telescopes and instruments like Hubble and Kepler and James Webb and so many others that have been enhancing our knowledge of black holes, other galaxies, exoplanets, all the stuff that makes astrophysics so rewarding these days.

Jacinta: You’re never out of work if you’re an astrophysicist nowadays, so I’ve heard. Halcyon days.

an x-ray burst from a supermassive black hole - artist's impression

an x-ray burst from a supermassive black hole – artist’s impression

Written by stewart henderson

December 19, 2015 at 9:02 pm

exoplanets – an introduction of sorts

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future_habitable_exoplanets

Jacinta: So do you think we’ve hauled ourselves out of ignorance sufficiently to have a halfway stimulating discussion on exoplanets?

Canto: I think we should try, since it’s one of the most exciting and rapidly developing fields of inquiry at the moment.

Jacinta: And that’s saying something, what with microbiomes, homo naledi, nanobots and quantum biology…

Canto: Yes, enough to keep us chatting semi-ignorantly to the end of days. But let’s try to enlighten each other on exoplanets…

Jacinta: Extra solar planets, planets orbiting other stars, the first of which was discovered just over 20 years ago, and now, thanks largely to the Kepler Space Observatory, we’ve discovered thousands, and future missions, using TESS and the James Webb telescope, will uncover megatonnes more.

Canto: Yes, and you know, about the Kepler scope, l was blown away – this might be veering off topic a bit, but I was blown away in researching this by the fact that Kepler orbits the sun. I mean, I knew it was a space telescope, but I just assumed it was in orbit around earth, probably at a great distance to avoid interference from our atmosphere, but that we can position satellites in orbit around the sun, that really sort of stunned me, more I think than the exoplanet discoveries. Am I being naive?

Jacinta: No, not at all. Well, yes and no. Everything is stunning if you haven’t followed the incremental steps along the knowledge pathway. I mean, if you think, hey the sun’s a way away, and it’s really big and dangerous, best not go there, or something like that, you might be shocked, but think about it, we’ve been sending satellites around the earth for a long time now, and we know how to do it because we know about earth’s gravitational field and can calculate precisely how to harness it for satellite navigation. We’ve currently got a couple of thousand human-made satellites orbiting the earth and trying more or less successfully to avoid colliding with each other. So the sun also has a gravitational field and we’ve known the mathematics of gravitational fields since Newton. It’s the same formula for a star, a planet or whatever, all you need to know is its mass and its radius. And look at all the natural objects orbiting the sun without a problem. Can’t be that hard.

Canto: Okay… so how do we know the mass of the sun? Okay, forget it, let’s get back to exoplanets. What’s the big fuss here? Why are we so dead keen on exploring exoplanets?

Jacinta: Well the most obvious reason for the fuss is SETI, the search for extra-terrestrial intelligence, but to me it’s just satisfying a general curiosity, or you might say a many-faceted curiosity. And it’s all about us mostly. For example, is the solar system that we inhabit typical? We’ve mostly thought it was but we didn’t have anything to compare it with, but now we’re discovering all sorts of weird and wonderful planetary systems, and star systems, with gas giants like Jupiter orbiting incredibly close to their stars – it’s completely overturned our understanding of how planets exist and are formed, and that’s fantastically exciting.

Canto: So you say we discovered the first exoplanet about 20 years ago, and now we know about thousands – that’s a pretty huge expansion of our knowledge, so how come things have changed so fast? You’ve mentioned new technologies, new space probes, why have they suddenly become so successful?

Jacinta: Well I suppose it’s been a convergence of developments, but let’s go back to that first discovery, back in 1992. Two planets, the first ever discovered, were found orbiting a pulsar – a rapidly rotating neutron star. First discovery, first surprise. Pulsars with planets orbiting them, who would’ve thought? Pulsars are the remnants of supernovae – how could planets have survived that? But that first discovery was largely a consequence of our ability to measure, and the fact that pulsars pulse with extreme regularity. Any anomaly in the pulsing would be cause for further investigation, and that’s how the planets were found, and later independently confirmed. Now this was big news, in a field that was already becoming alert to the possibility of exoplanets, so you could say it opened the floodgates.

Canto: Really? But they didn’t discover any more for two or three years.

Jacinta: Well, okay it opened the gates but it didn’t start the flood, that really happened with the second discovery, the first discovery of a planet orbiting a main-sequence star like ours.

Canto: Main sequence? Please explain?

Jacinta: Well these are stars in a stable state, a state of balance or equilibrium, fusioning hydrogen – basically stars not too different from our own, within much the same range in terms of mass and luminosity. So 51 pegasus b was the first planet to be discovered by the radial velocity method, and radial velocity means the speed at which a star is moving towards or away from us. We can measure this, and whether the star is accelerating or decelerating in its movement, by means of the Doppler effect – waves bunch up when the object emitting them is moving towards us, they spread out when the object is receding from us, and the degree of the bunching or the spreading is a measure of their speed and whether it’s accelerating or decelerating. Now we can measure this with extreme accuracy using spectrometers, and that includes any perturbations in the star’s movement caused by orbiting bodies. That’s how 51 pegasus b was discovered.

Canto: So… how long have we had these spectrometers? Were they first developed in the nineties, or to the level of accuracy that they could detect these perturbations?

Jacinta: Well I don’t have a precise answer to that apart from the general observation that spectroscopes are getting better, and more carefully targeted for specific purposes. The French ELODIE spectrograph, for example, which was used to find 51 pegasus b, was first deployed in 1993 specifically for exoplanet searching, and since then it’s been replaced by another improved instrument, but of the same type. So it’s a kind of non-vicious circle, research leads to new technology which leads to new research and so on.

doppler1

Canto: So – we’ve gotten very good at measuring perturbations in a star’s regular movements…

Jacinta: Regular perturbations.

Canto: And we know somehow that these are caused by planets orbiting around them? How do we know this?

Jacinta: Well we will know from the size of the perturbation and its regularity that it’s an orbiting body, and we know it’s not a star because it’s not emitting any light (though it may be a low-mass star whose light isn’t easily separated from its parent star). We also know – we knew from the results that it was a massive planet orbiting very close to its star – a hot Jupiter as they  call it. And that was another surprise, but we’ve developed different techniques for discovering these things and we often use them to back each other up, to confirm or disconfirm previous findings. The ELODIE observation of 51 pegasus b was confirmed within a week of its announcement by another instrument, the Hamilton spectrograph in California. So there’s a lot of confirmation going on to weed out false positives.

Canto: So radial velocity is one technique, and obviously a very successful one as it got everyone excited about exoplanets, but what others are there, and which are the most successful and promising?

Jacinta: Well the radial velocity method was initially the most successful as you say, and hundreds of exoplanets have been discovered that way, but this method actually led to a kind of bias in the findings, because it was only able to detect perturbations above a certain level, so it was best for finding large planets close to their stars. But of course that was good too because we had never imagined that large gassy planets could exist so close to their stars. It’s opened up the whole field of planet formation. Then again, if the main aim is to find earth-like planets, this method is less effective than other methods. So let’s move on to the Kepler project. Kepler was launched in 2009, and since then you could say it has blitzed the field in terms of exoplanet detection. It uses transit photometry, which means that it measures the dimming of the light from a star when an orbiting planet passes between it and the Kepler detector.

Canto: So I get the idea of transit, as in the transit of venus, which we can see pretty clearly, but it’s amazing that we can detect transiting planets attached to stars so many light years away.

Jacinta: Well this is how we’ve expanded our world, from the infinitesimally small to the unfathomably large, from multiple billions of years to femtoseconds and beyond, through continuously refining technology, but let’s get back to Kepler. It orbits around the sun, and has collected data from around 145,000 main sequence stars in a fixed field of view – stars that are generally around the same distance from that dirty big black hole at the centre of our galaxy as our sun is.

Canto: Is that significant – that we’re focusing on stars in that range?

Jacinta: Apparently so, at least according to the Rare Earth Hypothesis, which puts all sorts of unimaginative limits on the likelihood of earth-like planets, IMHO, but no matter, it’s still a vast selection of stars, and we’ve reaped a grand harvest of planets from them – some 3000-odd, with over 1000 confirmed.

Canto: So… promising earth-like planets?

Jacinta: Yes, but I must point out that earth-like planets are difficult to detect. You see, Kepler was a kind of experiment, and we’ve learned from it, so that our next project will be much improved. For various reasons due to the photometric precision of the instrument, and inaccurate estimates of the variability of the stars in the field of view, we found that we needed to observe more transits to be sure we’d detected something. In other words we needed a longer mission than we’d planned for. And of course, Kepler has suffered serious technical problems, especially the failure of two reaction wheels, which have affected our ability to repoint the instrument. Having said that, we’ve been more than happy with its success.

Canto: Okay I just want to talk about these exoplanets. Can you summarise the most interesting discoveries?

Jacinta: Well, Kepler has certainly corrected the view we might’ve gotten from the earlier radial velocity method that large Jupiter-like planets are more common than smaller ones. We’ve had a number of reports from the Kepler group over the years, and over time they’ve adjusted downwards the average mass of the planets detected. And yes, they’ve discovered a number of planets in the ‘habzone’ as they call it. But that’s not all – only this year NASA confirmed the existence of five rocky planets, smaller than earth, orbiting a star that’s over 11 billion years old. I’m just trying to give you an idea of the explosion of findings, whether or not these planets contain life. And we’ve only just begun our hunt, and the refinement of instruments. It’s surely a great time to study astrophysics. It’s not just SETI, it’s about the incredible diversity of star systems, and working out where we fit into all this diversity.

ExoplanetDiscoveries-Histogram-20140226

Canto: Okay, I can see this an appropriately massive subject. Maybe we can revisit it from time to time?

Jacinta: Absolutely.

Some very useful sites:

http://www.planetary.org/explore/space-topics/exoplanets/

http://www.smithsonianmag.com/science-nature/how-do-astronomers-actually-find-exoplanets-180950105/?no-ist

https://en.wikipedia.org/wiki/Kepler_(spacecraft)

 

Written by stewart henderson

October 30, 2015 at 10:05 pm

disassembling Kevin Vandergriff’s gish gallop, part 3

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IfGodMadeUs

Argument 7: God is the best explanation of the connection between the flourishing of the kinds of moral agents there are, and the necessary moral truths that apply to them.

Here we move more and more into the field of the preposterous, IMHO. He quotes a Christian philosopher, Gregory Ganssle, as saying:

Not only do we have beings to which necessary moral truths apply, but we have beings that are made up in such a way that doing what is right turns out to be good for them, it contributes to their flourishing rather than their languishing. Maybe only one in ten universes that are moral, in that they have the right sorts of beings that are such that moral goodness, and the flourishing of those beings involved, converge.

The last sentence is virtually meaningless, but the emphasis here on necessary moral truths is bizarre. I’m not sure what they are, but I’m certain that I haven’t the slightest interest in the concept. Vandergriff spoke earlier of the prohibition against murder as being a necessary moral truth, but many Christians are in favour of capital punishment, which is murder by the state. The murder of Bin Laden a couple of years ago raised very little moral outrage, nor does the murder of hostage-takers and other terrorists today. So these are apparently moral prohibitions that are on some occasions more ‘necessary’ than others.

I find the pretence of surprise that acting on ‘necessary moral truths’ seems coincidentally to promote human flourishing to be ridiculous and thoroughly disingenuous. The fact is that we’ve promoted human flourishing through social evolution. One of the most comprehensive explanations of how this has been achieved is presented by Steven Pinker in The better angels of our nature, a work of empiricism, not philosophy. Pinker has no more interest in ‘necessary moral truths’ than I do, he is concerned to explain how some human populations, and an increasing proportion of them, have been able to learn from the destructive errors of the past and to build better legal, economic, political, social, health and education systems, to better balance co-operation and competition, and individual and social goods. His analysis owes nothing to ‘necessity’, everything to the lessons learned through bitter and often traumatic experience. There are no perfect systems, but polities can be improved grindingly through continued analysis and experiment based on hard-won knowledge. Vandergriff and Ganssle put the cart before the horse. We flourish because the systems we put in place are designed for our flourishing. Yes, horror of horrors, our morality is all about enlightened self-interest, not ‘necessary goodness’. The horrors of the Great War drove us to attempt, for the first time in history, an organisation of international co-operation. Its dissolution was a setback rather than a complete failure. The later United Nations, with all its failings, has gradually grown in strength and will continue to be a force for peace, together with other international and intergovernmental organisations. The success of Medicins sans frontieres has spawned similar organisations ‘without borders’, and the trend is likely to continue. People get enormous satisfaction from helping others. Selfish satisfaction? Yes, but that vastly oversimplifies the matter. It is above all the satisfaction of being connected, which is so important for perhaps the most social species on the planet. And our increasing knowledge of our connections with other species is expanding our circle of sympathy, as philosopher Peter Singer has eloquently pointed out.

But as you might be able to detect, my sympathy with these arguments is starting to run out, and it gets worse.

Argument 8: God is the best explanation of why there are self-aware beings.

It should be pointed out that supernatural beings of any kind (let alone the mass-murdering war-god of the Old Testament) are always massively problematic ‘explanations’ because they have no empirical foundation. These are abstract objects, in spite of their variously imagined ‘histories’ in innumerable sacred texts. The development of self-awareness in many species on our planet is a contingent empirical fact.

Argument 8 and all the other ‘best explanation’ arguments given by Vandergriff, William Lane Craig and other theists are usually  accompanied by claims that ‘this situation/these events are extremely improbable under naturalism but entirely consistent/to be expected under theism’. That’s to say, they’re all ‘cart before the horse’ arguments. You define your supernatural agent as the repository of necessary truths, the generator of all value, the seat of ‘infinite consciousness’ (as Vandergriff quotes J P Moreland, another theist philosopher and theologian, as claiming), and the source of all meaning and ‘worthwhileness’ (argument 10), and then you say ‘hey look, we are value-seeking, meaning-requiring, self-aware, necessary-truth-understanding beings, so surely the whole kit-and caboodle was made by a god who made us as close to him as anything else, because he cares so much for us. Otherwise, all our amazing attributes are meaningless.’ I should point out that the amazing attributes of non-human species are constantly downplayed by theists, as they are in Vandergriff’s spiel, because they don’t contribute anything to this unique god-human relationship. They were downplayed throughout the Christian era too, of course, before it was challenged by the theory of natural selection. Stephen Jay Gould has cited many cases in his essays: for example the early 19th century German embryologist Lorenz Oken wrote that

The animal kingdom is only a dismemberment of the highest animal, that is, of Man

and in an 1835 work, naturalist William Swainson reflected thus:

When we discover evident indications of a definite plan, upon which all these modifications have been regulated by a few simple and universal laws, our wonder is as much excited at the inconceivable wisdom and goodness of the SUPREME by whom these myriads of beings have been created and are now preserved, as at the mental blindness and perverted understanding of those philosophers, falsely so called, who would persuade us, that even Man, the last and best of created things, is too insignificant for the special care of Omnipotence.

We readily forgive these dated claims, partly because they don’t directly challenge us any more, bit it seems clear that many theists have learned nothing and forgotten nothing over the centuries. There are many obvious problems with this way of thinking, but the one I find most indigestible is that in order to avoid the horrors of contingency, which, certainly in the case of William Lane Craig, is the greatest and most unacceptable horror of all, theists are still forced to conclude that everything – the possibly-infinite multiverse, the big bang, quasars, black holes, dark matter and dark energy, gravity, quantum mechanics, the laws of nature, the elements and their proportions etc etc – was created by their god for us. We, containing so many of the god’s qualities, albeit in infinitesimal proportions, are the fulfilment of his purpose. We are what he created it all for. Not a geocentric universe perhaps, but an anthropocentric one for sure, with a complexity that the god gradually reveals to us as our privilege to work out.

So theism here presents us with a choice, or so it believes: total meaninglessness, or the humbling knowledge that we are central to a god’s plan, the pinnacle of his creation, created in his image, fumbling caretakers of his multiverse. As fantasies go, it’s a whopper. From an empiricist perspective however, it’s a non-starter, except in psychological terms. It has helped our forebears to get through many dark nights of history.

I’ll dispense quickly with Vandergriff’s last two arguments. Argument 10, God is the best explanation for the worthwhileness of life, is just more of the same and requires no further analysis. Argument 9, probably the most preposterous of all the arguments, is that ‘God is the best explanation of the historical facts about Jesus of Nazareth’. There are no historically established facts about Jesus of Nazareth, even of his birth, his preaching, his trial and his death, let alone of his putative miracles and resurrection. Scholars may argue to and fro about these matters, but their arguments are entirely textual and have no serious empirical value.

Okay, I’m done with this. Never again, I hope.

spirituality issues, encore

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a mob of didges, right way up

a mob of didges, right way up

To me – and I’ve written about this before – the invocation of the supernatural, the ‘call’ of the supernatural, if you will, is something deeply psychological, and so not to be sniffed at, though sniff at it I often do.

I’m prompted to write about this because of a program I saw recently on Heath Ledger (Australia’s own), an understandably romantic, mildly hagiographic presentation, in which a few film directors and friends fondly remembered him as wise beyond his years, with hidden depths, a kind of inner force, a certain je ne sais quoi, that sort of thing. As both a romantic and a skeptic, I was torn as usual. The word ‘spiritual’ was given an airing, unsurprisingly, though mercifully it wasn’t dwelt on. I once came up with my own definition of spirituality: ‘To be spiritual is to believe there’s more to this world than this world, and to know that by believing this you’re a better person than those who don’t believe it’. This might sound a mite cynical but I didn’t mean it to be, or maybe I did.

Anyway one of Ledger’s associates, a film director I think, told this story of the young Heath. A number of friends were partying in his apartment when he, the director, picked up a didgeridoo, which obviously Ledger had brought with him from Australia, and attempted to play it, but not knowing much about the instrument, held it upside-down. Heath gently took it from him and corrected him, saying ‘no, no, if you hold it that way it will lose its power, the power of the instrument and its maker,’ or some such thing. And the seriousness and respectfulness with which this young actor spoke of his didge impressed the director, who considered this a favourite memory, something which caught an ‘essence’ of Ledger that he wanted to preserve.

I’ve been bothered by this tale, and by my ambivalent response to it, ever since. It would be superfluous, I suppose, to say that I don’t believe that briefly holding a didge upside-down has any permanent effect on its musical power.

It’s quite likely that Ledger didn’t believe this either, though you never know. What I’m fairly sure of, though, was that his respectfulness was genuine, and that there was something very likeable, to me at least, in this.

All of this takes me back to a piece I wrote some years ago, since lost, about big and small religions. I was contrasting the ‘big’ religions, like Catholicism and the two main strands of Islam, with their political power in the big world, often horrific in its impact, with the ‘small’ religions or spiritual belief systems, such as those found among Australian Aboriginal or some African societies, who have no political power in the big world but provide their adherents with identity and a kind of social energy that’s marvelous to contemplate. My piece focused on the art work of Emily Kame Kngwarreye, whose prolific and astonishing oeuvre, with its characteristic energy and vitality, clearly owed so much to the beliefs and practices of her ‘mob’, the so-called Utopian Community in Central Australia, between Alice Springs and Tenant Creek to the north.

Those beliefs and practices include dreaming stories and totemic identifications that many western skeptics, such as myself, might find difficult to swallow, in spite of a certain romantic appeal. The fact is, though, that the Utopian Community has been remarkably successful, in terms of the usual measures of well-being, and particularly in the area of health and mortality, compared to other Aboriginal groups, and its success has been put down to tighter community living, an outdoor outstation life, the use of traditional foods and medicines, and a greater resistance to the more destructive western products, such as alcohol.

This might put a red-blooded but reflective skeptic in something of a quandary, and the response might be something like – ‘well, the downside of their vitality and health, derived from spiritual beliefs which have served them well for thousands of years, is that, in order to preserve it, they must live in this bubble of tribal thinking, unpierced by modern evolutionary or cosmological knowledge, and this bubble must inevitably burst.’ Must it? Is there a pathway from tribalism to modern globalism that isn’t entirely destructive? Is the preservation of tribal spiritual beliefs a good thing in itself? Can we take the statement, that holding a didgery-doo upside-down affects its spirit, as a truth over and above, or alongside, the contrasting truths of physical laws?

I don’t know the answer to these questions, of course. Groping my way through these issues, I would say that we should respect and acknowledge those beliefs that give a people their dignity, and which have served them for so long, but perhaps that’s because we’re feeling the generosity of someone outside that system who’s unlikely to be affected or to feel diminished by it. These are, after all, small religions, from our perspective, not the big, profoundly ambitious religions intent on global domination, with their missionaries and their jihadists and their historical trampling of other belief systems, as in Mexico and South America and Africa and here in Australia.

Of course there’s the question – what if those small religions grew bigger and more ambitious? Highly unlikely – but what if?

Written by stewart henderson

February 16, 2014 at 10:22 am

why are our days getting longer?

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TIDESLOW

I’ve just finished reading a book by the Welsh biologist and science communicator Steve Jones entitled Coral; a pessimist in paradise, which covers a helluva lot of ground and makes me feel inadequate as most science writers do, but one of the many things he has taught me about – something I didn’t know that I didn’t know – is that the days are getting longer, in an inexorable process of rotational slowing. This fact, and the reasons behind it, were further confirmed for me today in an episode of an elegant little podcast out of the University of Houston, called The engines of our ingenuity. I just happened to be browsing through the science and scepticism podcasts on my TV, and I sampled a few curiously titled ones…

Let me backtrack a bit. I’m very very poor (from an affluent western perspective of course) but I received a HD TV from my neighbour recently as part of a complicated deal, and now I can watch free-to-air channels I didn’t have access to before, and what’s more I’ve managed to buy a device which I’m sure many people out there know all about, called an Apple TV, which is so cheap that even I can afford it without too much suffering (what’s a few days without food? it’ll probably extend my lifespan). So now I can explore an almost endless variety of podcasts, vodcasts and classic film noir movies on youtube. That reminds me, one of the podcasts I’ve listened to, the Brain Science Podcast, was all about brain fitness – at least the episode I tuned into was – and inter alia the interviewee informed us that just about the worst thing for the brain was sitting around all day watching TV – Apple or no Apple, presumably…

Anyway I listened to this informative and also charmingly poetic three-minute episode of The engines of our ingenuity, entitled ‘How far the moon?’, narrated and presumably written by Dr John Lienhard. So I’ll share the info, if not the poetry, here.

Our earth spins at a pretty well constant rate because of the forces that set it in motion in the first place and because of Newton’s first law of motion which, put simply, states that an object will stay in the same state (resting or in motion) unless an external force acts on it. A ball spinning in the air will slow down because of air friction, but the earth is spinning in a vacuum, essentially – there’s nothing to slow it down.

Well, not quite. The earth is slowing down, and all in accordance with Newtonian physics. And it’s all due to the moon. Each day is about a twelfth of a second longer than it was when the Egyptians built the pyramids. Doesn’t sound that much, but 4000 years is a mere blip in geological and cosmological time. The moon drags at the earth gravitationally, creating high tides and low tides at a regular rate, and slowing our rate of rotation. But our earth has a much greater influence on the moon than vice versa, the moon having only an eightieth of earth’s mass. This gravitational effect slowed down the moon’s spin until it was in synch with the earth, and locked into the earth’s movement like a dancer being swung around by its partner. And so the moon faces us always. The slowing down of the earth due to the moon’s influence had the effect of loosening the embrace – the moon is slowly moving away from us. Just as a spinning dancer or skater extends her arms out to slow down or pulls her limbs in to speed up.  The moon moves away from us so that our combined rotational inertia remains constant. The distance between earth and moon, and the speed at which the moon moves away from us, is being measured thanks to an instrument, placed on the moon by Apollo astronauts, which reflects laser beams from earth. Through measuring the time taken for the beam to return, we know that the moon is moving away from us at a little under 4 cms a year. Back in the dim distant past, days lasted only 12 hours, and the moon was half of today’s distance from us. This has affected the shape of the earth, which is gradually becoming more spherical. The earth’s diameter is at its greatest at the equator and at its smallest at the poles, because of centrifugal forces operating against the force of gravity…

Okay, let me get clearer on this, with the help of this source, among others. Isaac Newton accepted the mathematics and the accuracy of Kepler’s laws of planetary motion, but the great unanswered question was why planets – and moons – traced out these orbits. Newton’s own first law stated that an object will continue in its trajectory (that is, in a straight line) or in its resting state, unless some external force acted upon it to speed it up or slow it down. This state is called a state of inertia. Clearly planets and moons were being acted upon by some force, which could only be exerted by the object being orbited. This force might be called a centripetal force, though that doesn’t explain it in this case. If you swing a stone around on the end of a string, you apply a force to the stone to keep it going, but the string, and your hand holding the string, exerts a force on the string to keep it ‘in orbit’. Its motion will be circular, providing you keep your hand still, because the length of the string is constant. But there’s nothing obvious attaching the moon to the earth. Newton pondered this for some time, until one day the apple dropped.

I’m thinking that, if the moon is moving away from us, its orbit can’t be entirely circular, it must be spiralling outwards, ever so slightly. In any case, the moon pulls the earth out of shape, and that is due to a centrifugal force that balances the centripetal force exerted by the earth on the moon. The moon is moving away due to a reduction in both these forces, and a slowing of the earth’s rotation, and hence of the moon’s orbit.

But sadly, it gets more complicated than that! This is the Newtonian explanation of how these forces operate, but it doesn’t really answer the why question. I’m not going to go deeply into that here – as if I could – but I’ll end with a quote from an astronomer’s explanation, not so much about the earth’s slowing, but about the moon’s behaviour, in terms of Newtonian and then Einsteinian physics:

First case: – Why does the Moon orbit the Earth? It just does. And you can understand how it does by analyzing the forces on the Moon caused by its orbit and finding the forces pushing in and out are equal.

Second case: – Why does the Moon orbit the Earth? Because the Earth distorts spacetime in the vicinity of the Moon, and causes it to orbit the Earth the way it does and the balance of forces to come out the way it does.

So why do massive objects distort space-time? Apparently they just do?

Written by stewart henderson

September 28, 2013 at 8:25 am