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- Herausgeber: Icon Books
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
A mind-warping excursion into the wildly improbable truths of science. Echoing Sherlock Holmes' famous dictum, John Gribbin tells us: 'Once you have eliminated the impossible, whatever is left, however improbable, is certainly possible, in the light of present scientific knowledge.' With that in mind, in his sequel to the hugely popular Six Impossible Things and Seven Pillars of Science, Gribbin turns his attention to some of the mind-bendingly improbable truths of science. For example: We know that the Universe had a beginning, and when it was – and also that the expansion of the Universe is speeding up. We can detect ripples in space that are one ten-thousandth the width of a proton, made by colliding black holes billions of light years from Earth. And, most importantly from our perspective, all complex life on Earth today is descended from a single cell – but without the stabilising influence of the Moon, life forms like us could never have evolved.
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Praise for Six Impossible Things
‘[A]n accessible primer on all things quantum … rigorous and chatty.’
Sunday Times
‘Gribbin has inspired generations with his popular science writing, and this, his latest offering, is a compact and delightful summary of the main contenders for a true interpretation of quantum mechanics. … If you’ve never puzzled over what our most successful scientific theory means, or even if you have and want to know what the latest thinking is, this new book will bring you up to speed faster than a collapsing wave function.’
Jim Al-Khalili
‘Gribbin gives us a feast of precision and clarity, with a phenomenal amount of information for such a compact space. It’s a TARDIS of popular science books, and I loved it. … This could well be the best piece of writing this grand master of British popular science has ever produced, condensing as it does many years of pondering the nature of quantum physics into a compact form.’
Brian Clegg, popularscience.co.uk
‘Elegant and accessible … Highly recommended for students of the sciences and fans of science fiction, as well as for anyone who is curious to understand the strange world of quantum physics.’
Forbes
Praise for Seven Pillars of Science
‘[In] the last couple of years we have seen a string of books that pack bags of science in a digestible form into a small space. John Gribbin has already proved himself a master of this approach with his Six Impossible Things, and he’s done it again … [Seven Pillars of Science is] light, to the point and hugely informative. … It packs in the science, tells an intriguing story and is beautifully packaged.’
Brian Clegg, popularscience.co.uk
EIGHT IMPROBABLE POSSIBILITIES
The Mystery of the Moon, and Other Implausible Scientific Truths
JOHN GRIBBIN
CONTENTS
ABOUT THE AUTHOR
John Gribbin’s numerous bestselling books include In Search of Schrödinger’s Cat, The Universe: A Biography, 13.8: The Quest to Find the True Age of the Universe and the Theory of Everything, and Out of the Shadow of a Giant: How Newton Stood on the Shoulders of Hooke and Halley.
His most recent book is Seven Pillars of Science: The Incredible Lightness of Ice, and Other Scientific Surprises. His earlier title, Six Impossible Things: The ‘Quanta of Solace’ and the Mysteries of the Subatomic World, was shortlisted for the Royal Society Insight Investment Science Book Prize for 2019.
He is an Honorary Senior Research Fellow at the University of Sussex, and was described as ‘one of the finest and most prolific writers of popular science around’ by the Spectator.
For Steve Guest, who appreciates things like this!
ACKNOWLEDGEMENTS
Once again, I am grateful to the Alfred C. Munger Foundation for financial support, and to the University of Sussex for providing a base and research facilities.
As with all my books, Mary Gribbin ensured that I did not stray too far into the thickets of incomprehensibility, and on this occasion Improbability Eight owes a particular debt to her. The mistakes, of course, are all mine.
LIST OF ILLUSTRATIONS
A solar eclipse
The radio telescope that Penzias and Wilson used to discover the CMB
Vera Rubin
The LIGO detection site at Livingston, Louisiana
Ernst Mach
A ‘butterfly’ diagram
Lynn Margulis
Milutin Milankovitch
xv
‘When you have excluded the impossible, whatever remains, however improbable, must be the truth.’
The Adventure of the Beryl Coronet, Arthur Conan Doyle
PREFACE
What Do We Know?
Science deals with the unknown. My non-scientist friends sometimes offer sympathy when what is perceived as the ‘failure’ of a scientific theory makes headline news. This happened recently with the discovery that the expansion of the Universe is speeding up, and that our simple Big Bang model needs modification. ‘You must be very disappointed,’ they say, ‘that your beautiful theory is wrong.’ Not at all! Good scientists are delighted when new evidence hints that new ideas are needed to explain what is going on in the world. New ideas are the lifeblood of science, and if all our theories were perfect descriptions of the world (by which I mean everything there is, not just planet Earth), there would be nothing left for scientists to do.
You might be surprised that there is anything much for science to do at all. Given how much we already know about how the world works, what is there left to discover? But a warning lesson from history cautions against such complacency. Towards the end of the nineteenth century, there was a widespread feeling among physicists that with Isaac Newton’s theory of gravity and James Clerk Maxwell’s theory of electromagnetism they had all the tools they needed to describe the world, and that no new fundamental discoveries remained to be made. In 1894 A.A. Michelson, an American physicist remembered for his work on measuring the speed of light, said:
While it is never safe to affirm that the future of Physical Science has no marvels in store even more astonishing than those of the past, it seems probable that most of the grand underlying principles have been firmly established and that further advances are to be sought chiefly in the rigorous application of these principles to all the phenomena which come under our notice. It is here that the science of measurement shows its importance – where quantitative work is more to be desired than qualitative work. An eminent physicist remarked that the future truths of physical science are to be looked for in the sixth place of decimals.
It was just as well he put in the opening caveat, because hot on the heels of that remark came the discovery of radioactivity, the special and general theories of relativity, and quantum physics. Definitely marvels even more astonishing than those of the past. Scientists have learned never to say that all that remains is to dot the i’s and cross the t’s of their favoured theories.
How can there be more to be discovered when so much is already known? An analogy may help. Pretend that everything we know about the world is represented by the area inside a small circle drawn on a large, flat piece of paper. Everything we know is inside the circle, everything we don’t know is outside. As we discover more about how the world works, the circle gets bigger. But as it does so, the circumference of the circle, the boundary between what we know and what we don’t know, also gets bigger. As the Lovin’ Spoonful song ‘She is Still a Mystery’ puts it, ‘the more I see, the more I see there is to see’. There will be plenty of work for scientists in the foreseeable future. And that work proceeds by setting up hypotheses (or guesses) about how the world works, then carrying out experiments or making observations to eliminate the incorrect guesses.
Are relativists delighted when a new observation of the Universe confirms, as the headline writers like to put it, that ‘Einstein Was Right’? Only up to a point. What would be really exciting for them would be an observation which showed that the general theory of relativity is good as far as it goes, but that it may not be right everywhere and all the time. That is why such experiments are carried out. Not to ‘prove Einstein was right’ but in the hope of finding out the conditions, or places, in the Universe where Einstein’s theory might be wrong.
So in spite of what popular media may tell you, good scientists do not carry out experiments in order to prove their pet theory is right.* They carry out experiments in order to find where the theory fails, which tells them where new discoveries can be made (and, if you care about such things, where Nobel Prizes might be won).
As Richard Feynman famously pointed out:
If it disagrees with experiment, it is wrong. In that simple statement is the key to science. It does not make any difference how beautiful your guess is, it does not make any difference how smart you are, who made the guess, or what his name is – if it disagrees with experiment, it is wrong.
This is the scientific equivalent of Conan Doyle’s dictum. It is by experiment (or observation) that scientists eliminate the impossible. Thomas Henry Huxley called this ‘The great tragedy of science – the slaying of a beautiful hypothesis by an ugly fact.’
But a good scientist doesn’t go quite as far as Doyle does. Once you have eliminated the impossible, whatever is left is certainly possible, in the light of present knowledge, but may not be the ultimate truth. It may yet, in its turn, be slain by an ugly fact. It is with that in mind that we should turn our attention to some of the improbable (in the light of present knowledge) truths of science.
John Gribbin May 2020
* There are, of course, bad scientists who do just that, but they have no place here.
IMPROBABILITY1
The Mystery of the Moon
A total eclipse of the Sun is one of the most spectacular and beautiful sights visible from the surface of the Earth. It is so spectacular because the Moon and Sun look the same size to us. So when the Moon passes in front of the Sun, it can exactly cover the bright solar disc, plunging the region affected by the eclipse into darkness, but allowing the glowing outer layer of the Sun, its corona, to become visible like a glorious halo. But why are we lucky enough to see this sight? Why are the apparent sizes of the Sun and Moon just right to produce it? The question is more profound than it seems at first, because the coincidence has not always held. Our human civilisation exists at a rare moment of astronomical time when the Moon is perfectly placed to make this kind of eclipse. In the not too distant geological past, it was too close to Earth and would have blotted out the corona as well; in the astronomical future it will be too far away and will look like a small dark blob passing across the solar disc. Improbably, it is ‘just right’ just at the time we are here to notice it.
A solar eclipse Science Photo Library
But the effect only happens at all because the Moon is so large. As a fraction of the size of its parent planet (Earth), it is by far the largest moon in the Solar System. Indeed, many astronomers think that the Earth–Moon system should better be regarded as a double planet than as a planet plus a moon. And that is all down to the way the double planet formed.
The Sun and Solar System formed when a cloud of gas and dust in space collapsed under the pull of its own gravity. Most of the material went in to the central star, the Sun. Some of the dust, and icy particles, was left in a disc around the star, and particles of that dust collided and stuck together until some were big enough to tug other particles towards them by gravity, so that bigger and bigger objects built up. This eventually made the planets, but some material was left over to make smaller objects, asteroids and comets. The late stages of this process were far from gentle, as proto-planets were bombarded with debris as they swept their orbits around the Sun clear. Just a hint of what this bombardment was like can be gleaned from the battered face of the Moon; but this tells less than the full story, because the Moon itself only formed after most of the process of planet building had taken place.
It is straightforward to account for the moons that we see orbiting around other planets in the Solar System, such as Mars, Jupiter and Saturn. The moons of Mars are clearly small pieces of debris – asteroids – left over from the planet-building process and captured by Mars. The moons of giant planets like Jupiter and Saturn are much bigger than asteroids – but 4the giant planets are much bigger than Mars. Their families of moons formed around the parent planets in the same way that the planets formed around the Sun, making miniature ‘solar systems’. But the Moon is 25 per cent as big as the Earth, in terms of its diameter, and clearly formed in a different way. The best explanation is that within a few million years of the Earth forming, the planet was involved in a collision with another young planet, an object the size of Mars, which struck it a glancing blow. In the heat generated by this violent event, the incoming object would have been destroyed, and the proto-Earth’s newly formed crust would have melted. The heavy metallic core of the incomer would have sunk to the centre of the Earth, mixing with Earth’s own metallic heart to make a planet with a very dense core and a relatively thin crust. The crust would be thin because molten material from the impact, a mixture of stuff from the proto-Earth and the incomer – graphically referred to by astronomers as the Big Splash – would have been flung off into space, some escaping entirely but some staying to form a ring around the Earth from which the Moon coalesced. It is easy to remember how long this process took; computer simulations tell us that something resembling the Moon would have formed within a presentday month of the impact. Dating of lunar rock samples tells us that all this happened about 4.4 billion years ago. Among other things, the impact set the Earth spinning rapidly on its axis, and knocked it out of the vertical, causing the tilt which is responsible for the cycle of the seasons.5
All of this explains many oddities about the Earth. The planet Venus, just sunward of the Earth, is roughly the same size as the Earth, but has a thick crust, a small metallic core, and as a result a negligible magnetic field. It rotates only once every 243 of our days. The Earth has a thin crust, a large metallic core that is responsible for a strong magnetic field, relatively rapid rotation, and a large Moon. These features go together like a hand in a glove. Our planet is the odd one out in the Solar System, produced by a highly improbable sequence of events, all linked to the Moon. And the consequences of those events are far-reaching.
Take the thinness of the crust. It might not sound like a big deal, but it is. The crust is so thin that it can crack like an eggshell, with the pieces of the shell being moved about by convection currents in the fluid layers beneath, in the process known as plate tectonics. Thanks to the thinness of the crust, around the edges of these pieces of shell (the plates) there is constant volcanic activity, releasing gases like carbon dioxide and water vapour into the atmosphere. Where the crust is cracked, usually under the oceans, new crust can be made as molten material wells up and sets, spreading out on either side of the crack, pushing the plates away on each side. But the Earth does not get any bigger, because in other parts of the world, especially along the edges of some continents, crust is being pushed down into the interior. This carries carbonates and water back down where they get fed into volcanoes and are released into the air again in an endless cycle.6
But the cycle does not run at a constant speed. The process which takes gases like carbon dioxide out of the atmosphere is called weathering. Carbon dioxide dissolves in water, and then reacts with minerals in the rocks to make calcium carbonate (limestone). Carbon dioxide in the atmosphere is, of course, a greenhouse gas – it traps heat and keeps the surface of the Earth warmer than it would otherwise be. As it happens, weathering proceeds faster when the world is warmer, so that tends to draw carbon dioxide out of the air efficiently, allowing the planet to cool. But when it cools, weathering is less efficient, and carbon dioxide builds up in the air again. The world warms, and the weathering process speeds up, drawing more carbon dioxide out of the air. There is a negative feedback which, thanks to plate tectonics, helps to keep the temperature at the surface of the Earth in the range where liquid water can exist (although, unfortunately, these natural processes are too slow to compensate for the buildup of carbon dioxide now being caused by human activities quickly enough to save us from the consequence of our own folly). Without this process – without the thin crust produced by the impact that made the Moon – the Earth would probably have become a scorching desert with a thick carbon dioxide atmosphere, like our neighbour Venus.
This isn’t the only thing we have to thank the Moon for. Analysis of seismic waves produced by earthquakes and travelling through the interior of our planet shows just how large the central core is. It is a solid lump of iron and nickel with a 7diameter of about 2,400 km, the top of which is about 5,200 km below the surface of the Earth. But it is surrounded by a layer of liquid material, extending a further 2,500 km upward, roughly halfway to the surface of the Earth from the top of the inner core. Together, the inner and outer core contain a third of the mass of our planet, part of it donated by the impacting object which produced the Moon. It is the outer core that is important to us, and to all life on Earth. The temperature in this iron–nickel liquid layer is about 5,000°C, only a little less than the temperature at the surface of the Sun, maintained by the radioactive decay of elements such as thorium and uranium, left over from the formation of the Solar System. Swirling currents in this layer generate the magnetic field of the Earth.
The Earth’s magnetic field is literally a force field, which protects our planet from a major threat from space. The Sun produces a blast of electrically charged particles, blandly called the ‘solar wind’, which reaches out from its source across space and past the Earth and the other planets. These particles travel at speeds of several hundred kilometres per second most of the time, and up to 1,500 kilometres per second during outbursts known as solar storms. Without the shielding effect of the magnetic field which forms a protective layer around the Earth, these ‘solar cosmic rays’, essentially the same as the particle radiation from a nuclear bomb, could strip away the outer layers of the atmosphere and penetrate to the ground where they would cause considerable damage to life, possibly even sterilising the land surface of the planet. 8
