In the “Alchemy” issue of Spring Journal Stanton Marlan published his remarkable essay, “From the Black Sun to the Philosopher’s Stone.” The blackness of sol niger brought with it for Marlan “the mortificatio of brokenness, incision, and wound, castration, cut, negation, with an ultimate ‘No’ to the ego, with what felt unassimilable.” Time and again Marlan insists on there being something unassimilable. The blackness of sol niger is his proof and paradigm. He says he encountered a darkness that refuses conscious assimilation. But is this really true? No doubt, this blackness was initially “felt unassimilable”. But the fact that the psychic images have their own internal solidity, integrity, and intactness and don’t go away even when assimilated in no way means that there is an “unassimilable remainder” in the emphatic sense. The blackness of the black sun is by no means unassimilable, although of course not in the sense of assimilation as being incorporated by an ego stance.
People of sense (as they are called) give themselves great and unwarrantable airs over the rest of the world. If we examine the history of mankind, we shall find that the greatest absurdities have been most strenuously maintained by these very persons, who give themselves out as wiser than every body else. The fictions of law, the quibbles of school-divinity, the chicanery of politics, the mysteries of the Cabbala, the doctrine of Divine Right, and the secret of the philosopher&s;s stone, — all the grave impostures that have been acted in the world, have been the contrivance of those who set up for oracles to their neighbours. The learned professions alone have propagated and lent their countenance to as many perverse contradictions and idle fallacies as have puzzled the wits, and set the credulous, thoughtless, unpretending part of mankind together by the ears, ever since the distinction between learning and ignorance subsisted. It is the part of deep investigators / to teach others what they do not know themselves, and to prove by infallible rules the truth of any nonsense they happen to take in their heads, or chuse to give out to amuse the gaping multitude. What every one felt and saw for himself - the obvious dictates of common sense and humanity — such superficial studies as these afforded a very insufficient field for the excercise of reason and abstruse philosophy, in the view of &s;the demure, grave-looking, spring-nailed, velvet-pawed, green-eyed&s; 1 despisers of popular opinion; their object has regularly been, by taking post in the terra incognita of science, to discover what could not be known, and to establish what could be of no use if it were. Hence one age is employed in pulling down what another with infinite pomp and pains has been striving to build up; and our greatest proof of wisdom is to unlearn the follies and prejudices that have been instilled into us by our predecessors. It took ages of ingenuity, of sophistry, and learning, to incorporate the Aristotelian, or scholastic philosophy, into a complete system of absurdity, applicable to all questions, and to all the purposes of life; and it has taken two centuries of metaphysical acuteness and boldness of inquiry, to take to pieces the cumbrous, disproportioned edifice, and to / convert the materials to the construction of the modern French philosophy , by means of verbal logic, self-evident propositions, and undoubted axioms — a philosophy just as remote from truth and nature, and setting them equally at defiance. What a 227 number of parties and schools have we in medicine, — all noisy and dogmatical, and agreeing in nothing but contempt and reprobation of each other! Again, how many sects in religion, — all confident of being in the right, able to bring chapter and verse in support of every doctrine and tittle of belief, all ready to damn and excommunicate one another; yet only one, out of all these pretenders to superior wisdom and infallibility, can be right; the conclusions of all the others, drawn with such laboured accuracy, and supported with such unbending constancy and solemnity, are, and must be, a bundle of heresies and errors! How many idle schemes and intolerant practices have taken their rise from no better a foundation than a mystic garment, a divining-rod, or Pythagoras&s;s golden thigh! 2 — When Baxter, the celebrated controversial divine, and nonconformist minister in the reign of Charles II went to preach at Kidderminster, he regularly every Sunday insisted from the pulpit that baptism was necessary to salvation, and / roundly asserted, that &s;Hell was paved with infants&s; skulls.&s; This roused the indignation of the poor women of Kidderminster so much, that they were inclined to pelt their preacher as he passed along the streets. His zeal, however, was as great as theirs, and his learning and his eloquence greater; and he poured out such torrents of texts upon them, and such authorities from grave councils and pious divines, that the poor women were defeated, and forced with tears in their eyes, to surrender their natural feelings and unenlightened convictions to the proofs from reason and Scripture, which they did not know how to answer. Yet these untutored, unsophisticated dictates of nature and instinctive affection have, in their turn, triumphed over all the pride of casuistry, and merciless bigotry of Calvinism! We hear it said, that the Inquisition would not have been lately restored in Spain, but for the infatuation and prejudices of the populace. That is, after power and priestcraft have been instilling the poison of superstition and cruelty into the minds of the people for centuries together, hood-winking their understandings, and hardening every feeling of the heart, it is made a taunt and a triumph over this very people (so long the creatures of the government, carefully / moulded by them, like clay in the potter&s;s hands, into vessels, not of honour, but of dishonour) that their prejudices and misguided zeal are the only obstacles that stand in the way of the adoption of more liberal and humane principles. The engines and establishments of tyranny, however, are the work of cool, plotting, specious heads, and not the spontaneous product of the levity and rashness of the multitude. It is a work of time to reconcile them to such abominable and revolting abuses of power and authority, as it is a work of time to wean them from their monstrous infatuation * . We may 228 trace a speculative absurdity or practical enormity of this kind into its tenth or fifteenth century, supported story above story, gloss upon gloss, till it mocks at Heaven, and tramples upon earth, propped up on decrees and councils and synods, and appeals to popes and cardinals and fathers of the church (all grave, reverend men!) with the regular clergy and people at their side battling for it, and others below (schismatics and heretics) / oppugning it; till in the din and commotion and collision of dry rubs and hard blows, it loses ground, as it rose, century by century; is taken to pieces by timid friends and determined foes; totters and falls, and not a fragment of it is left upon another. A text of Scripture, or a passage in ecclesiastical history, is for one whole century &s;torn to tatters, to very rags,&s; 4 and wrangled and fought for, as maintaining the doctrine of the true and Catholic church; in the next century after that, the whole body of the Reformed clergy, Lutherans, Calvinists, Arminians, get hold of it, wrest it out of the hands of their adversaries, and twist and torture it in a thousand different ways, to overturn the abominations of Anti-Christ; in the third a great cabal, a clamour, a noise like the confusion of Babel, jealousies, feuds, heart-burnings, wars in countries, divisions in families, schisms in the church arise, because this text has been thought to favour a lax interpretation of an article of faith, necessary to salvation; and in the fourth century from the time the question began to be agitated with so much heat and fury, it is discovered that no such text existed in the genuine copies. Yet all and each of these, Popes, councils, fathers of the church, reformed leaders, Lutherans, Calvinists, Independents, Presbyterians, / sects, schisms, clergy, people, all believe that their own interpretation is the true sense; that, compared with this fabricated and spuriour faith of theirs, &s;the pillar&s;d firmament is rottenness, and earth&s;s base built on stubble;&s; 5 and are so far from being disposed to treat the matter lightly, or to suppose it possible that they do not proceed on solid and indubitable grounds in every contradiction they run into, that they would hand over to the civil power, to be consigned to a prison, the galleys, or the stake (as it happened), any one who demurred for a single instant to their being people of sense, gravity, and wisdom. Sense (that is, that sort of sense which consists in pretension and a claim to superiority) is shewn, not in things that are plain and clear, but in deciding upon doubts and difficulties; the greater the doubt, therefore, the greater must be the dogmatism and the consequential airs of those who profess to settle points beyond the reach of the vulgar; nay, to increase the authority of such persons, the utmost stress must be laid on the most frivolous as well as ticklish questions, and the most unconscionable absurdities have always had the stoutest sticklers, and the most numerous victims. The affectation of sense so far, / then, has given birth to more folly and done more mischief than any one thing else.
The paper is aimed at a very accurate topic which is problematic of cryptocurrency Bitcoin. The main aim of this paper is to evaluate the potential of this most important cryptocurrency to serve as full-value money. The matter of the first chapter is introducing the Bitcoin concept including its characters and history. In this phase, we are thinking about Bitcoin as a commodity. The analysis of cryptocurrency Bitcoin within currency relations framework is subject for next chapter. Each commodity, which has a role of money, must fulfil three criteria. This commodity must be convertible, relative stable and liquid. This paper discusses the evaluation of these aspects. Based on the analysis, we can contribute to the question whether bitcoin can be used as full-value money in the future. The paper is not concerned on technical aspects like the algorithm of mining, blockchain technology or account safety.
As Jinnah defined it, the Punjab represented the cornerstone of Pakistan. Here his party, the Muslim League, fought one of its most interesting political battles in order to defeat its local competitor, the Unionist Party, in the key 1945–6 elections. Taken as a proof of the League’s power and its declared aspiration to be the sole representative of Indian Muslims, the outcome of this electoral campaign shook the local political arena. It indeed challenged the traditional affiliations that had consolidated in the province over the previous decades. Ideas of religious identity and a lethal overlapping of politics and religion provided the raw ingredients for both the 1945–6 League victory and the different bouts of violence that rocked the Punjab after the withdrawal of Khizr Tiwana’s ministry in February 1947. Partition-related communal clashes relied heavily on the value of the symbols ‘handled’ by social actors during the spring and the summer of 1947. The circumstances involved in the early fluxes of migrants and then in later journeys towards what refugees perceived to be safer places or their new home emphasised the relevance of the meanings that local society attached to the political ideas at stake and violence.
From a historical perspective, the multiverse is just one more step in our progress from geocentric to heliocentric to galactocentric to cosmocentric worldview. Indeed, several lessons of relevance to the multiverse debate can be gleaned from considering the history of this progression. To the ancient Greeks, the heavenly spheres were the unchanging domain of the divine and therefore outside science by definition. It required Tycho de Brahe's observation of a supernova in 1572 and the realization that its apparent position did not change as the Earth moved around the Sun to dash that view. Because this contradicted the Aristotelian view that the heavens cannot change, the claim was at first received sceptically. Frustrated by those who had eyes but would not see, Brahe wrote: “O crassa ingenia. O coecos coeli spectators.” (Oh thick wits. Oh blind watchers of the sky.) Lesson 1: Theoretical prejudice should not blind one to the evidence. Of course, we will never see the other universes themselves — in that sense we are necessarily blind, so this point might seem irrelevant to the multiverse. However, I would claim that the analogue of Tycho's supernova is the fine-tunings. Long after Galileo had speculated that the Milky Way consists of stars like the Sun and Newton had shown the laws of Nature could be extended beyond the solar system, there was still a prejudice that the investigation of this region was beyond the domain of science. In 1842 August Comte said of the study of stars: “Never, by any means, will we be able to study their chemical compositions. The field of positive philosophy lies entirely within the Solar System, the study of the universe being inaccessible in any possible science.” Comte had not foreseen the advent of spectroscopy, which identified absorption features in stellar spectra with chemical elements. Lesson 2: New observational developments are hard to anticipate. Perhaps we will find extra dimensions at the Large Hadron Collider or even create baby universes in the laboratory one day. Cosmology attained the status of a proper science in 1915, when the advent of general relativity gave it a secure mathematical basis. Nevertheless, for a further decade there was resistance to the idea that science could be extended beyond our galaxy. Indeed many astronomers refused to believe that there was anything beyond. Although Kant had speculated as early as 1755 that some nebulae are “island universes” similar to the Milky Way, most astronomers continued to adopt a galactocentric view until the 1920s. Indeed, the most popular model of the galaxy at the start of the 20th century — Kapteyn's Universe — even had the Sun at its centre! The controversy came to a head in 1920 when Heber Curtis defended the island universe theory in a famous debate with Harlow Shapley. The issue was finally resolved in 1924, when Edwin Hubble measured the distance to M31 using Cepheid variable stars. In many ways this parallels the current debate about whether anything exists beyond our horizon. Lesson 3: More conservative cosmologists might prefer to maintain the cosmocentric view but perhaps the tide of history is against them. The evidence for other universes can never be as decisive as that for extragalactic nebulae but the transformation of worldview required may be just as necessary. A few years later Hubble obtained radial velocities and distance estimates for several dozen nearby galaxies, thereby discovering that all galaxies are moving away from us with a speed proportional to their distance. The most natural interpretation of this is that space itself is expanding, as indeed had been predicted by Alexander Friedmann in 1920 on the basis of general relativity. Einstein rejected this model at the time because he believed the universe (i.e. the Milky Way) was static and he even introduced an extra repulsive term into his equations — the cosmological constant — to allow this possibility. After Hubble's discovery, he described this as his “biggest blunder”. Lesson 4: One should not necessarily reject theoretical predictions because they have no observational support. In fact, Einstein continued to uphold the static model even after the evidence was against it — he only accepted the Friedmann model in 1931, several years after Hubble published his data — so knowing how much weight to attach to theory and observation can be tricky. Let me now address George's specific issues. There are plausibly galaxies just beyond the visual horizon, where we cannot see them, so we can extend this argument, step by step, to way beyond the horizon and infer there are many different universes that we cannot see. Even though we can never prove what happens outside our visual horizon, the standard FRW model has been well tested within it, so there is surely some probabilistic sense in which one can extrapolate models at least some way beyond it. Also the smooth dependence of the CMB fluctuations on angular separation (whatever the source of those fluctuations) gives no reason to suppose that anything strange happens just beyond the horizon. George himself seems to accept this, which illustrates the problem of regarding speculations as non-scientific just because they involve the unobservable. Admittedly one's confidence in any proposed model must decrease as one extrapolates ever further beyond the horizon, but one should beware of using Rees's slippery slope argument in reverse: we cannot extrapolate to scales much larger than the horizon, so we should not extrapolate to scales only slightly outside it. The problem comes when one makes the jump from the Level I to Level II multiverse (which is where George's argument that the FRW solution extends everywhere must fail). In fact, the inflationary scenario does provide an answer to this. For if the amplitude of the density fluctuations increases slightly with scale (as appears to be the case), one can predict the scale at which the FRW approximation breaks down. Current data suggest that this happens at around 10100 horizon scales. The existence of a multiverse is implied by inflation, which is verified by the CMB anisotropy observations. In particular, known physics leads to chaotic inflation and this implies a multiverse. There are two distinct issues here: does one believe in inflation and does inflation lead to a multiverse? Inflation is attractive because it resolves several cosmological conundra. Quantum fluctuations of the scalar field can also generate the small density perturbations that eventually give rise to galaxies and large-scale structure and it is impressive that the predicted dependence of the CMB fluctuations on angular separation is almost exactly as observed by the WMAP satellite (Spergel et al. 2003). Of course, the evidence for inflation is not conclusive — there is still no evidence for any scalar field in Nature!— but the Level I multiverse is still a good bet. As regards the second issue, I agree with George that the evidence for the sort of chaotic inflation that leads to a Level II multiverse is more equivocal, and certainly one cannot infer this from the form of the CMB anisotropies. There are now around 100 models of inflation and, while Linde (1990) claims that the existence of other domains with different coupling constants is generic, this is debatable. The multiverse idea is testable, because it can be disproved if we determine there are closed spatial sections in the universe (for example, if the curvature is positive). This is really a straw man argument because we have seen that inflation is only one of several multiverse proposals — for example, quantum cosmology models give closed spatial sections — and not all inflationary models require that the spatial sections be open anyway. However, George is surely right to stress the importance of looking for circles in the CMB. The idea of small universes is not mainstream but it has the advantage that it can be tested. The existence of a multiverse is the only physical explanation for the fine-tuning of parameters that leads to our existence. In the absence of direct evidence for other universes, I regard the anthropic fine-tunings as the best indirect evidence. (A multiverse in which the constants were the same everywhere would have no explanatory value.) I agree with George that the fine-tunings do not constitute proof, but they still carry weight. One can argue about how impressive the fine-tunings are (could we really exclude life if the constants changed a lot?), but I still think the number and precision of the tunings is remarkable. Nearly 30 years ago I wrote a review with Martin Rees about these fine-tunings (Carr and Rees 1979). In the intervening period a few of them have gone away (e.g. inflation may explain the value of the cosmological density parameter) but most of them have got stronger. Without a multiverse one may be forced to adopt a non-physical explanation like a fine-tuner, which is why Neil Manson (2003) claims that “the multiverse is the last resort of the desperate atheist”. This is not necessarily true — Paul Davies (2006) advocates a “third way” in which the laws of Nature evolve in a single universe in such a way that life can arise — but if you reject the multiverse, you certainly lower the scientific status of the anthropic arguments. I agree with George's argument against physical infinities. However, we do not need an infinity to validate the anthropic principle — just a large number. The existence of a multiverse is implied by a probability argument: the universe is no more special than it need be to create life. In particular, the small value of the cosmological constant shows that other universes exist. George argues that multiverse theories are not useful because they cannot be disproved: if all possibilities exist somewhere, then they can explain all conceivable observations. However, the fact that we only observe one sample of the multiverse still allows the proposal to be refuted at a given confidence level. Statistical predictions still qualify as science and that is why Rees has stressed the importance of calculating the probability distribution for various parameters across the universes. Indeed, a core difference between the Bayesian and frequentist views is the former's willingness to make inferences from single, and possibly unrepeatable, pieces of data. George rejects the Λ argument but there is no doubt that this has been very influential in attracting many physicists to the multiverse cause. It used to be thought that Λ was exactly zero and it was then plausible that there might be some physical (non-anthropic) explanation for this. However, the fact that Λ is non-zero but very tiny is a profound mystery that completely changes the situation. Critics say that we cannot know what distribution for Λ is predicted across the multiverse and that is correct. It may be simplistic to assume that the distribution is uniform, but postulating that there is a spike in precisely the observed region is just as improbable as what we are trying to explain. Even if one does not accept inflation, multi-verses are predicted by many theories of particle physics. It is still legitimate to invoke the existence of other universes for which there can be no direct evidence if one has a theory (like M-theory) that predicts this. It is not necessary to check all predictions of the theory for it to be considered scientific (e.g. we cannot probe inside black holes and we cannot see quarks but we still regard these as subjects for scientific discourse); it is only necessary to test some of them. Does M-theory qualify in this respect? George claims no; it does not come under the purview of science because our confidence in it is based on faith and aesthetic considerations (mathematical beauty etc) rather than experimental data. Certainly he is not alone in this attitude. For example, Woit (2006) and Smolin (2007) dismiss M-theory as mathematics rather than physics because it has not made contact with observations after 20 years. However, I feel this rejection is premature. It may take 200 years to solve the equations of M-theory and test them, but the definition of what constitutes a scientific question should not depend on how difficult it is. The nature of science changes, so what is illegitimate science today may be legitimate tomorrow. The fundamental issue in the dispute between myself and George concerns which features of science are to be regarded as sacrosanct. Experimentation used to be regarded as sacrosanct but by that criterion all of astronomy would be excluded since one cannot experiment with stars and galaxies. Fortunately, one can still make observations and — since there are billions of these objects — Nature effectively performs experiments for us. Cosmologists are in worse shape because there is only one universe to observe and speculations about processes at very early and very late times have to be viewed as ultra-speculative. For this reason, more conservative physicists regard even relatively standard cosmological speculations as trespassing into metaphysics. George places a lot of emphasis on falsifiability, but not everybody in the philosophy of science agrees with Popper on this and it is surely dangerous to impose a philosophical prescription that prevents scientists changing the border of their field. As Susskind cautions, it would be a pity to miss out on some fundamental truth because of an over-restrictive definition of science. Of course, one needs some degree of falsifiability, but the question is, how much? It is certainly not fair to put M-theory in the same class as astrology. On the other hand, I share George's scepticism of the Level IV multiverse, which corresponds to universes governed by different mathematical structures. The view that any mathematically possible universe must exist somewhere seems untestable in a deeper sense than Levels I to III. The notion of a multiverse entails a new perspective of the nature of science and it is not surprising that this causes intellectual discomfort. But this situation has often occurred before and one should not be surprised if it happens again. The Cosmic Uroborus in figure 2 shows that the history of physics might be regarded as the extension of knowledge into ever smaller and ever larger scales. The ideas encountered at the two frontiers have often been viewed as part of philosophy rather than science, so in a sense the debate is nothing new. However, there is another sense in which the current situation is very special. This is because — for the first time — the boundaries at the largest and smallest scales have connected, as indicated by the top of the Cosmic Uroborus, so the two science/philosophy frontiers have merged. Does this merging represent the completion of science or merely the sort of transformation in the perceived nature of science that accompanies every paradigm shift? This is a contentious issue and clearly we do not yet know the answer. I accept that there may eventually be a limit to the sort of questions that science can address; George and I merely disagree on whether we have reached that limit with the multiverse. In any case, we are surely behoven to try to take science as far as possible. I will end with a comment by Steven Weinberg (2007) in his contribution to Universe or Multiverse?: “We usually mark advances in the history of science by what we learn about Nature, but at certain critical moments the most important thing is what we discover about science itself. These discoveries lead to changes in how we score our work, in what we consider to be an acceptable theory.”
The very nature of the scientific enterprise is at stake in the multiverse debate. Its advocates propose weakening the nature of scientific proof in order to claim that the multiverse hypothesis provides a scientific explanation. This is a dangerous tactic. Two central scientific virtues are testability and explanatory power. In the cosmological context, these are often in conflict with each other and there has been an increasing tendency in theoretical physics and cosmology to say it does not matter whether a proposal is testable: if it fits into our other theories in a convincing way, with great explanatory power, then testing is superfluous. The extreme case is the multiverse proposal, where no direct observational test of the hypothesis is possible. Despite this, many articles and books dogmatically proclaim that the multiverse is an established scientific fact. In this context one must re-evaluate what the core of science is: can one maintain one has a genuine scientific theory when direct and indeed indirect tests of the theory are impossible? If one claims this, one is altering the meaning of science. One should be very careful before so doing. There are many other theories waiting in the wings, hoping for a weakening of what is meant by “science”. Those proposing this weakening in the case of cosmology should be aware of the flood of alternative scientific theories whose advocates will then state that they too can claim the mantle of scientific respectability. The key observational point is that the domains considered are beyond the visual horizon and are therefore unobservable. You cannot receive signals of any kind from beyond the horizon, as there has not been time for messages to reach us from there since the universe began. Hence no object out there is detectable by any kind of astronomical observation. To see this clearly one should look at the space-time diagrams of our past light cone (figure 5). The assumption made in justifying the multiverse is that we can extrapolate to 10100 times the horizon distance or even more (the word “infinity” is casually used in these writings). The extraordinary pretentiousness of this attempt should be clear. This shows the space-time diagrams of our past light cone in both the usual form (a) and conformal form (b), in which one expands the spatial distances in order to see the causal structure. The light cones are then at ±45°, making clear the observational and causal limits; any observation beyond the visual horizon is impossible. The Hubble distance is where galaxies recede at the speed of light (v=c). The redshift measures the expansion factor and goes to infinity at the horizon itself. (c) shows more clearly the vast extrapolation envisaged in justifications of the multiverse. Note that there is a distinction betweeen the visual horizon and the event horizon. The latter relates to causality in the far distant future and is irrelevant to present-day observations, but becomes relevant if the universe accelerates. (Mark Whittle, University of Virginia) The often claimed existence of physical infinities in the multiverse context — of either universes or spatial sections of universes (Vilenkin 2006) — is dubious. What has been forgotten here is that infinity is an unattainable state rather than a large number — its character is totally different from any finite number and it is a mathematical rather than physical entity. According to David Hilbert (1964): “The infinite is nowhere to be found in reality, no matter what experiences, observations, and knowledge are appealed to.” Even if there were an infinite number of galaxies, and we could see them all (which we could not), we could not count them in a finite time. So there is no way the existence of an infinity can ever be proven correct by observation or any other test. The concept of physical infinities is not a scientific one if science involves testability by either observation or experiment. The claim of infinites in the multiverse context emphasizes how tenuously scientific that idea is. It is a huge act of hubris to extrapolate from one small domain to infinity when infinity is never attainable. Seven different kinds of justification have been proposed for the existence of a multiverse and I will now consider these in turn. There are plausibly galaxies just beyond the visual horizon, where we cannot see them, so we can extend this argument, step by step, to way beyond the horizon and infer there are many different universes that we cannot see. This is the “slippery slope” argument and Rees (2001) uses it to defend both Level I and Level II multiverses. The argument is fine as regards extrapolation in the vicinity of the visual horizon, but the assumption that it can be continued to very distant domains or other universes is an untestable major extrapolation, which assumes a continuity that may or may not be true. If each link in a chain of evidence is well understood and tenable, then indirect evidence such as this carries nearly as much weight as direct evidence. But not all the links in the chain are tenable. If employed to its logical conclusion it seems a priori to lead to the old idea of spatial homogeneity extending forever (“The Cosmological Principle”) rather than the multiverse of chaotic cosmology with domain walls separating different phases. For if the universe within the horizon is almost exactly Friedmann-Robertson-Walker (FRW) — a statistically spatially homogenous and isotropic space-time — it is plausible that it is also FRW just outside the horizon, and a simple extrapolation suggests that it is spatially homogeneous without limit. But supporters of chaotic inflation claim that there are completely different domains out there with different values of the constants, so which is the case? You can say what you like and nobody can prove it right or wrong. The existence of a multiverse is implied by inflation, which is verified by the Cosmic Microwave Background anisotropy observations. In particular, known physics leads to chaotic inflation and this implies a multiverse. A multiverse is implied by some forms of inflation but not others. Inflation is not yet a well defined theory and chaotic inflation is just one variant of it. For example, inflation in a small closed universe fits all the observations, without requiring a multiverse. In any case, the key physics involved in chaotic inflation (Coleman-de Luccia tunnelling) is extrapolated from known and tested physics to quite different regimes; that extrapolation is unverified and indeed unverifiable. The physics is hypothetical rather than tested. We are being told that what we have is “known physics → multiverse”. But the real situation is “known physics → hypothetical physics → multiverse” and the first step involves a major extrapolation which may or may not be correct. The multiverse idea is testable, because it can be disproved if we determine there are closed spatial sections in the universe (for example, if the curvature is positive). The claim is that only negatively curved FRW models can exist in a multiverse based on chaotic inflation, either because Coleman-de Luccia tunnelling only gives negative curvature or because a closed spatial section necessarily implies a single universe. But the first argument is disputed (there are already papers suggesting that tunnelling to positively curved universes is possible) and the second argument would not apply if we lived in a high-density lump imbedded in a low-density universe (i.e. the extrapolation of positive curvature to very large scales may not be valid). Neither argument is conclusive. Certainly observational confirmation of negatively curved space sections would not constitute proof of a multiverse, for that can occur in a single universe. However, chaotic inflation versions of the multiverse can be disproved if we observationally prove that we live in a universe with closed spatial sections that are so small that we have already seen round the universe. We can test this possibility by searching for identical circles in the CMB, together with low anisotropy power at large angular scales (which is indeed observed). This is an important test as it would disprove the chaotic inflation variety of multiverse. But not seeing the circles would not prove a multiverse exists: their non-detection is a necessary but not sufficient condition for multiverses. The existence of a multiverse is the only physical explanation for the fine-tuning of parameters that leads to our existence. The multiverse is a reasonable theoretical explanation of the fine-tunings, but this does not help in observationally confirming the hypothesis. The issue here is, which is more important in cosmology: theory (explanation) or observations (tests against reality)? The essential proposal is that one should downgrade observational testing in favour of theory — a dangerous road to take. In any case, the major problem with this proposal is that it can explain anything at all, because in a multiverse with an infinite or extremely large variety of universe properties — for example, the 10500 possibilities allowed by the landscape of string theory — virtually anything can happen. In that case, the hypothesis does not predict any specific testable fact. The existence of universes with giraffes is certainly predicted by many multiverse proposals, but universes where giraffes do not exist are also predicted. Observing a giraffe neither confirms nor disproves the multiverse. The existence of a multiverse is implied by a probability argument: the universe is no more special than it need be to create life. In particular, the small value of the cosmological constant shows that other universes exist. But the statistical argument invoked here only applies if a multiverse exists; it is simply inapplicable (because the probability distribution has no meaning) if there is no multiverse, so it cannot prove a multiverse exists. It is a calculation that assumes the answer (that a multiverse exists) before it begins. If we only have one object to observe, we can make many observations of that object, but it is still only one object (one universe), and you cannot do statistical tests on its nature. This is a consistency test if there is indeed a multiverse, but it says nothing if there is not. It is not a sufficient condition for its existence. The argument that the actual value of Λ is extremely different from the “natural” one predicted by theoretical physics (120 orders of magnitude smaller!) makes very clear the nature of the multiverse project: it is an attempt to make the extremely improbable appear probable. Even if one does not accept inflation, multiverses are predicted by many theories of particle physics. One example of this is the string landscape of M-theory, but that is a hypothetical proposal with no solid evidence in its favour. Indeed, even many string theorists are sceptical about the landscape, despite the enthusiasm with which some propose it. A major problem arises when one has a theory where no verification is possible. It is not necessary to check all the predictions of a theory for it to be considered scientific — one can check some predictions to help the theory gain credence, but that does not prove the theory because it has to be the only one that makes the prediction for it to carry weight; it cannot be conclusive unless no other explanation is possible Let me illustrate with an example. I can propose that there are leopards hidden in the mountains of Scotland. They are very shy, so they hide away and no-one ever sees them. But you can tell they are there because sheep vanish without trace every year. I can put together an exciting research project that will look at statistics of lost sheep in Scotland for the past 50 years, and hence prove the existence of these rare mountain leopards. This seems to me to be analogous to the argument for proof of the existence of a multiverse through any specific property (e.g. the smallness of Λ) that might possibly exist somewhere in the landscape. This may be something that is predicted by some multiverse theory, but it hardly proves it true. I am all for the exercise of applying known physics in more extreme conditions: do it and see what happens. But admit that it is an untested extrapolation and that different extrapolations are possible. You can extrapolate different aspects of known physics to the unknown and different predictions will result. For example, if one extrapolates classical physics to the quantum domain, the answers will be wrong. But in that case you can show this is so by experimental tests. That is what is missing in the multiverse case: you can make the extrapolation but cannot then determine if it is right or not. The nature of science changes, so what is illegitimate science today may be legitimate tomorrow. This is true, but the foundations must be respected if one is to preserve the core features of science that have led to its phenomenal success: that is the feedback from reality to theory provided by experiment and observational testing. One abandons that at one's peril. For example, today's philosophical definition of science excludes astrology, despite all the claimed theory and data supporting it. But now astrologers can take hope from the arguments of string theorists and multiverse enthusiasts: with the weakened kinds of criteria proposed, astrology too will soon be a strong candidate for recognition as a genuine science. The Popperazi (a derogatory term used by Susskind for those who believe testing scientific theories is an indispensible aspect of science) will no longer be able to deny astrology its place as a proper scientific theory. Is that what we really want? At the very least, we must be given a clear statement as to what broader definition of the nature of science is being proposed, and in particular what criteria of testing will be taken to be adequate (Ellis 2006); this then needs to be assessed in relation to cases such as astrology and “intelligent design”, as well as multiverses and string theory, in order to see what its implications are. The multiverse idea is provable neither by observation, nor as an implication of well established physics. It may be true, but it cannot be shown to be true. It does have great explanatory power — it provides an empirically based rationalization for fine tuning, developed from known physical principles — but one must distinguish between explanation and prediction. Successful scientific theories make predictions that can be tested. The multiverse theory cannot make any testable predictions because it can explain anything at all. Even though multiverse proposals are good empirically based philosophical proposals for the nature of what exists, they are not strictly within the domain of science. There is nothing wrong with empirically based philosophical explanation — indeed it is of great value provided it is labelled for what it is — but I suggest that cosmologists should be very careful not to make methodological proposals that erode the essential nature of science in their enthusiasm to support specific theories. For if they do so, there will very likely be unintended consequences in other areas where the boundaries of science are in dispute. Let me state it more strongly: it is dangerous to weaken the grounds of scientific proof in order to include multiverses under the mantle of “tested science”. It is a retrograde step towards the claim that we can establish the nature of the universe by pure thought without having to confirm our theories by observational or experimental tests. This abandons the key principle that has led to the extraordinary success of science. The claim that multiverses exist is a belief rather than an established scientific fact. It is a reasonable belief with strong explanatory nature, but a belief nonetheless. The appropriate statement we can make is not “multiverses exist” or “multiverses have been proved to exist” or even “multiverses can be proved to exist”, but rather “multiverses are a useful explanatory hypothesis”. We should not state more. Martin Gardner (2003) puts it this way: “There is not the slightest shred of reliable evidence that there is any universe other than the one we are in. No multiverse theory has so far provided a prediction that can be tested. As far as we can tell, universes are not as plentiful as even two blackberries.”