John Gribbin

MultiverseWhat are the boundaries of our universe?
Could there be other worlds?
Do we actually live in a multiverse?
Will we meet another ‘us’ in a different reality?
Or are alternative worlds parallel but separate?

Is our universe just one of many?

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2 Responses to John Gribbin

  1. shinichi says:

    In Search of the Multiverse: Parallel Worlds, Hidden Dimensions, and the Ultimate Quest for the Frontiers of Reality

    by John Gribbin

  2. shinichi says:

    Introduction: In an Infinite Universe, Anything Is Possible

    Five hundred years ago, the Universe seemed to be a small place. It was widely believed that the Earth, our home, was the most important thing in the Universe, and lay at its centre. The Sun and the five known planets (Mercury, Venus, Mars, Jupiter and Saturn) were thought to be relatively small objects in orbit around the Earth, and the stars were regarded as points of light attached to a spherical shell around the Earth, rotating once a day just beyond the orbits of the planets. Apart from the rhythms of day and night and the seasons, this setup seemed to be unchanging and eternal. The thought of there being other worlds was literally heretical. Right at the end of the sixteenth century, Giordano Bruno was burned at the stake for espousing ideas which ran counter to mainstream Catholic teaching. These included the idea that the stars are other suns, and that there must be other earths, and life elsewhere in the Universe – although this particular belief was not the main reason for his conviction.

    Things began to change with the work of Nicolaus Copernicus, whose famous book Die Revolutionibus Orbium Coelestium was pub – lished in 1543. Even in ancient times there had been philosophers who speculated that the Earth might go around the Sun, but these ideas never gained widespread acceptance before the sixteenth century. It was Copernicus who started the unbroken line of investigation that has led to our present understanding of the Universe. The shock of his ideas was not just the suggestion that the Earth must move in order to orbit the Sun, but the implication that the Earth is just one of the Sun’s retinue of planets. The other planets might be just as important in the cosmic scheme of things as our home in space.

    The next shock was the suggestion that the Sun might not be the most important object in the sky, but just an ordinary star. In England in 1576, Thomas Digges, after looking at the Milky Way through a telescope and seeing a multitude of stars, stated in a book called Prognostication Everlasting that the Universe is infinite, with stars extending in all directions; Bruno picked up these ideas when he was in England in the 1580s. Galileo Galilei and Johannes Kepler built on the work of Copernicus, and in the seventeenth century astronomers began to estimate the distances to the stars by assuming that they are each as bright as our Sun, but only look faint because they are so far away. In 1728, Isaac Newton came up with an estimate that the star Sirius is about a million times farther from us than the Sun is, not hopelessly far from the distance measured by modern techniques. By then, with the true nature of the orbits of the planets understood, the distances to the Sun and the planets had been determined using geometrical techniques, and astronomers knew that the Sun is about 150 million km from Earth (149,597,870 km according to modern measurements). Saturn, the most distant planet from the Sun known to the Ancients, is nearly ten times farther from the Sun than we are. In the span of just two hundred years, what had seemed to be the entire Earth-centred Universe had shrunk in the considerations of astronomers to a small corner of a vast, possibly infinite, Universe.

    It took another two hundred years for these ideas to be assimilated, and for the technologies of telescopes, astronomical photography and spectroscopy to be developed to the point where the next big leap could be made. Along the way, the discovery of more planets in the Solar System beyond the orbit of Saturn (Uranus and Neptune) mattered less for the big picture than the development of accurate techniques for measuring the distances to the stars and the revelation from spectroscopy of what stars are made of. From the 1920s onwards, these techniques led first to a better understanding of our place in the geography of the Universe, and then to a better understanding of our place in the history of the Universe.

    With his small telescope, Thomas Digges had seen the band of light we call the Milky Way to be made up of innumerable stars. Galileo, who didn’t know about Digges’ work, made the same discovery independently a few decades later. Digges thought that the array of stars revealed by the telescope extended to infinity in all directions; but as early as 1750 the Durham astronomer Thomas Wright argued in his book An Original Theory or New Hypothesis of the Universe that the way the Milky Way forms a band of light across the sky implies that it is a disc-shaped system with a finite size, which he described as being like the grinding wheel of a mill. Crucially, he realized that the Sun is not at the centre of this slab of stars. He also suggested that fuzzy patches of light revealed by telescopes and known as nebulae lie outside the Milky Way.

    Wright’s theoretical reasoning was way ahead of its time, and could not be tested by observations with the technology of the eighteenth and nineteenth centuries. His work was largely forgotten until after the observations, made in the twentieth century, which showed that the Milky Way matches the broad outline of his speculations but provided much more precise detail about the nature of the Universe we live in.

    Starting from observations made in the 1920s, we now know that the Milky Way is indeed a roughly disc-shaped system, containing hundreds of billions of stars each broadly similar to our Sun, held together by gravity and orbiting their common centre. The disc is about 100,000 light years across (roughly 30 kiloparsecs, in the units favoured by astronomers), so that light, travelling at a speed of just under 300,000 km per second, takes 100,000 years to cross the disc. A light year is about 9.5 thousand billion km. The Sun is roughly two thirds of the way out from the centre of the Milky Way, lying in the plane of the disc, which is about a thousand light years (some 300 parsecs) thick in the vicinity of the Sun. But these impressive statistics, far exceeding the pre-Copernican idea of the Universe, pale almost into insignificance compared with the discovery that the entire Milky Way galaxy is just one island in space, as unspectacular and ordinary a member of the class of galaxies as the Sun is an unspectacular and ordinary member of the class of stars.

    Like his ideas about the nature of the Milky Way itself, Wright’s speculation that the nebulae – or at least, some of them – lie beyond the Milky Way has also proved correct. Although some nebulae are simply glowing clouds of gas and dust within the Milky Way, and are still referred to by that term, the ‘external’ nebulae are what we now call galaxies. Galaxies come in different shapes and sizes, but the Milky Way is an almost exactly average-sized member of the class known as disc galaxies. This is the ultimate recognition of our place in the geography of the Universe – we orbit an ordinary star, one among hundreds of billions of stars in an ordinary galaxy, one among hundreds of billions of galaxies. There is nothing special about our place in the Universe.

    It is estimated that there are hundreds of billions of galaxies visible in principle to present-day telescopes, although only a few thousand have been studied systematically. They are distributed in groups known as clusters across the visible Universe, and the most distant yet photographed are seen by light which has spent well over ten billion years on its journey from them to our telescopes. This is not quite the same thing as saying that these galaxies are more than ten billion light years away, because the other great discovery that built from observations made in the 1920s is that clusters of galaxies are moving apart from one another. The Universe is expanding, so after ten billion years the galaxies that emitted that light are no longer at the same distance from us that they were when the light set out on its journey.

    This universal expansion is the key to understanding our place in cosmic history. It was discovered by accident, but actually predicted by Albert Einstein’s general theory of relativity, although he had ignored the prediction. In the late 1920s and early 1930s, the American astronomer Edwin Hubble was interested in measuring the distances to galaxies, and, working with his colleague Milton Humason, he discovered that the distance to a galaxy is proportional to the redshift of features in its spectrum. This redshift is just what its name suggests – a shift in the spectral features towards the red end of the spectrum, compared with the position of those features measured in the laboratory. Hubble didn’t care why the redshift occurred, and didn’t try to explain it – he was only interested in using it to measure distances. But it was soon appreciated by other astronomers that the effect is caused by the space between galaxies (strictly speaking, between clusters of galaxies) stretching as time passes.

    The reason why the cosmological redshift was interpreted in this way so quickly is that just such a space-stretching effect is a natural consequence of the general theory of relativity, which Einstein developed in the second decade of the twentieth century. At that time, most people still thought that the Milky Way was the entire Universe, and the Milky Way is certainly not expanding. So Einstein had introduced an extra term into his equations, denoted by the Greek letter lambda (K) and often referred to as the ‘cosmological constant’. When the constant was removed, the equations of the general theory naturally predicted a universe expanding in exactly the way that the observations of galaxies revealed. It is important to appreciate that this expansion is indeed caused by the stretching of space itself. The cosmological redshift is not caused by the galaxies moving through space, although it is possible to produce redshifts in that way (and blueshifts as well) through the Doppler effect. Red light has longer wavelengths than blue light, and the cosmological redshift is caused by light waves being stretched on their journey to us as the space between us and the distant galaxy stretches.

    A key feature of this kind of expansion is that it has no centre, unlike the way fragments of shrapnel move outwards from the site of an exploding bomb. By analogy with the Doppler effect, astronomers often refer to the ‘recession velocity’ of a galaxy, even though they know the cosmological redshift is not caused by the galaxy moving through space. What they really mean is, the equivalent velocity that would be required to produce the same redshift by the Doppler effect. In that language, the velocity is proportional to the distance from us – but you would see the same thing, velocity proportional to distance, from any galaxy. We are not at the centre of the Universe, and there is no centre.

    A simple analogy makes this clear. Imagine a sphere, like a basketball, dotted with random spots of paint. If the size of the sphere doubles, every spot of paint will seem to move away from its neighbours. From whichever spot you choose to measure, the other spots will seem to be receding. This is another example of the fact that we occupy no special place in the Universe. The Earth, far from being in a privileged position, seems to be located in such an average place that the Russian cosmologist Alex Vilenkin has coined the term ‘terrestrial mediocrity’ to describe our situation.

    But although the cosmological redshift is not caused by the galaxies fleeing from one another through space as if from the seat of a great explosion, if we imagine reversing the cosmic expansion we see that at face value the discovery of the cosmological redshift implies that long ago everything we can see around us was compressed into a much smaller volume of space. This is confirmed by the same equations which describe the present expansion of the Universe so well. Modern measurements of the expansion of the Universe, combined with those mathematical models, suggest that the entire visible Universe emerged from a hot fireball of energy occupying a volume smaller than an atom 13.7 billion years ago.

    The precision of this number is remarkable. As recently as twenty years ago, cosmologists argued about whether this ‘age of the Universe’ was nearer to 10 billion years or nearer to 20 billion years, and pinning the number down to within a factor of two seemed impressive enough to anyone not involved in the sometimes heated debate. Now, there is very little room for manoeuvre, and the age of the Universe seems definitely to lie between 13.6 and 13.8 billion years. The outburst of the Universe from this tiny origin is known as the Big Bang, a term coined by the British cosmologist Fred Hoyle to poke fun at what he saw as a ludicrous idea, but one which has stuck – even though it was not an explosion and nothing went bang.

    The discovery of the finite age of the Universe, combined with evidence that the Universe changes (‘evolves’) as time passes, puts us in our place in cosmic history. It turns out that in a sense we do live at a special time in history, although that does not conflict with the idea (sometimes called a ‘principle’) of terrestrial mediocrity. As I shall explain later, it took time for stars and galaxies to evolve and for the chemical elements that we are made of to be processed inside stars. The Sun and the Earth are about 4.5 billion years old, so they were born roughly nine billion years after the Big Bang. This was just about as soon as it would have been possible for planets like the Earth, rich in the chemical constituents of life, to have formed. In that sense, the Earth formed at a special time; but there is no reason to think that it was the only such planet to form.

    Which brings us back to Giordano Bruno. Bruno’s vision was of a universe filled with an infinite array of stars, each one like the Sun, and with other planets, many of them bearing life. He wrote:

    In space there are countless constellations, suns and planets; we see only the suns because they give light; the planets remain invisible, for they are small and dark. There are also numberless earths circling around their suns, no worse and no less than this globe of ours.

    It was an early example of a ‘many worlds’ hypothesis, using the term ‘world’ in one of its many alternative meanings as a synonym for ‘planet’. At the beginning of the seventeenth century, there seemed to be no fundamental reason why those other worlds – all of them – could not, in principle, be seen from Earth, if you had a powerful enough telescope, or even visited, if you had the patience for a very long journey. The fact that stars are grouped together in galaxies like the Milky Way does not affect the thrust of Bruno’s argument; but he did not know that the Universe as we know it was born at a finite time in the past, nor that the speed of light is finite. Those facts do alter our perception of what is meant by ‘many worlds’, and whether they might be observable.

    The finite nature of the speed of light was only established in the second half of the seventeenth century, by the Dane Ole Rømer, from observations of eclipses of the moons of Jupiter; the fact that the speed of light is the ultimate speed limit, and nothing can travel faster than light, was only established by Albert Einstein at the beginning of the twentieth century. Since the Universe only came into existence 13.7 billion years ago, there has only been time for light to travel a finite distance through space since the Big Bang. This distance is not 13.7 billion light years, because, as we have seen, space has been expanding while light has been on its journey. For this reason, careful astronomers prefer to use the term ‘look back time’ rather than ‘distance’ to a particular galaxy. But this does not affect the argument. From our perspective, we can only look out into the Universe, even if we had perfect telescopes, to distances corresponding to a look back time of 13.7 billion years. Light from farther away in the Universe has not yet had time to reach us. But it might! If there are any intelligent observers around on Earth in a billion years from now, they will be able
    to see out to distances corresponding to a look back time of 14.7 billion years. The bubble of space which can in principle be known to us, and which can in principle affect us, is growing all the time.

    The same is true for bubbles of space centred on any galaxy in the Universe. If the Universe is indeed infinite, there may be an infinite number of these bubbles, some overlapping one another, others completely separate from one another, but all inhabiting the same Universe of space and time. There may indeed be an infinite number of worlds, in the sense Bruno would have understood, but no single observer would ever be able to know them all.

    And the Universe really could be infinite, even though we see only a finite volume. When cosmologists talk about the origin of the Universe in a fireball of energy smaller than an atom, they mean the entire visible Universe. The original superdense state may itself have been infinitely large, and our visible Universe may represent just one tiny piece of that infinite region that swelled up to a much larger size, as I discuss in Chapter Five.

    An infinite number of worlds allows for an infinite number of variations and, indeed, an infinite number of identical copies. In that sense, in an infinite Universe, anything is possible, including an infinite number of other Earths where there are people identical to you and me going about their lives exactly as we do; and an infinite number of other Earths where you are Prime Minister and I am King. And so on. But the chances of any of these similar Earths occupying ‘our’ bubble are vanishingly small. The nearest ‘other you’ is likely to live in a bubble so far away that, according to a calculation made by the American cosmologist Max Tegmark, to express it in metres you would need a number with 1029 zeroes – not 29 zeroes, but 10 raised to the power of 29 zeroes. For comparison, the total number of atoms in all the stars and galaxies of the visible Universe is estimated to be merely10 raised to the power of 80 – a 1 followed by 80 zeroes.

    If that was all there was to the idea of the multiverse, there would be no point in writing this book. But there is more – much more. Arguably, other bubbles within the same expanding space and time do not even count as other universes; they are simply inaccessible parts of our Universe. The true multiverse idea strikes at the heart of our understanding of science, addressing puzzles such as the reason why the laws of physics are the way they are, and why the Universe is a comfortable home for life. The second of those questions, in particular, stimulated a debate in which the term ‘multiverse’ was first used in an astronomical context, just over a hundred years ago. But the word has since been used with different meanings, and it is important to clear up what I mean by it before proceeding further.

    According to the Oxford English Dictionary, the word ‘multiverse’ was first used by the American psychologist William James (the brother of novelist Henry James) in 1895. But he was interested in mysticism and religious experiences, not the nature of the physical Universe. Similarly, although the word appears in the writings of G. K. Chesterton, John Cowper Powys and Michael Moorcock, none of this has any relevance to its use in a scientific context. From our point of view, the first intriguing scientific use of the word followed from an argument put forward by Alfred Russel Wallace, the man who came up with the idea of evolution by natural selection independently of Charles Darwin, that ‘our earth is the only inhabited planet, not only in the Solar System but in the whole stellar universe.’ Wallace wrote those words in his book Man’s Place in the Universe, published late in 1903, which developed ideas that he had previously aired in two newspaper articles. Unlike Darwin, Wallace was of a religious persuasion, and this may have coloured his judgement when discussing ‘the supposed Plurality of Worlds’.* But as we shall see, there is something very modern about his approach to the investigation of the puzzle of our existence. ‘For many years,’ he wrote:

    I had paid special attention to the problem of the measurement of geological time, and also that of the mild climates and generally uniform conditions that had prevailed throughout all geological epochs, and on considering the number of concurrent causes and the delicate balance of conditions required to maintain such uniformity, I became still more convinced that the evidence was exceedingly strong against the probability or possibility of any other planet being inhabited.

    This was the first formal, scientific appreciation of the string of coincidences necessary for our existence; in that sense, Alfred Russel Wallace should be regarded as the father of what is now called ‘anthropic cosmology’.

    Wallace’s book stirred up a flurry of controversy, and among the people who disagreed publicly with his conclusions were H. G. Wells, William Ramsay (co-discoverer of the inert gas argon), and Oliver Lodge, a physicist who made pioneering contributions to the development of radio. It was Lodge who used the term ‘multiverse’, but referring to a multitude of planets, not a multitude of universes.

    In scientific circles, the word was forgotten for more than half a century, then invented yet again by a Scottish amateur astronomer, Andy Nimmo. In December 1960, Nimmo was the Vice-Chairman of the Scottish branch of the British Interplanetary Society, and was preparing a talk for the branch about a relatively new version of quantum theory, which had been developed by the American Hugh Everett. This has become known as the ‘many worlds interpretation’ of quantum physics, with ‘world’ now being used (as it will be from now on throughout this book) as a synonym for ‘universe’. But Nimmo objected to the idea of many universes on etymological grounds. The literal meaning of the word universe is ‘all that there is’, so, he reasoned, you can’t have more than one of them. For the purposes of his talk, delivered in Edinburgh in February 1961, he invented the word ‘multiverse’ – by which he meant one of the many worlds. In his own words, he intended it to mean ‘an apparent Universe, a multiplicity of which go to make up the whole . . . you may live in a Universe full of multiverses, but you may not etymologically live in a Multiverse of ‘‘universes’’.’

    Alas for etymology, the term was picked up and used from time to time in exactly the opposite way to the one Nimmo had intended. The modern usage of the word received a big boost in 1997, when David Deutsch published his book The Fabric of Reality, in which he said that the word Multiverse ‘has been coined to denote physical reality as a whole’. He says that ‘I didn’t actually invent the word. My recollection is that I simply picked up a term that was already in common use, informally, among Everett proponents.’ In this book, the word ‘Multiverse’ is used in the way Deutsch defines it, which is now the way it is used by all scientists interested in the idea of other worlds. The Multiverse is everything that there is; a universe is a portion of the Multiverse accessible to a particular set of observers. ‘The’ Universe is the one we see all around us. And where better to start our search for the Multiverse than with Everett himself?

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