They are powered by massive black holes at the centres of galaxies, into which entire stars are falling - up to several per day for a large quasar - shredded by tidal effects as they spiral in. Intense magnetic fields channel some of the gravitational energy back out in the form of jets of high-energy particles, which illuminate the surrounding gas with the power of a trillion suns.
Conditions are still more extreme in the black hole's interior (within the surface of no return known as the 'event horizon'), where the very fabric of space and time may be being ripped apart. All this is happening in a relentlessly expanding universe that began about fourteen billion years ago with an all-encompassing explosion, the Big Bang, that makes all the other phenomena I have described seem mild and inconsequential by comparison. And that whole universe is just a sliver of an enormously larger entity, the multiverse, which includes vast numbers of such universes.
The physical world is not only much bigger and more violent than it once seemed, it is also immensely richer in detail, diversity and incident. Yet it all proceeds according to elegant laws of physics that we understand in some depth. I do not know which is more awesome: the phenomena themselves or the fact that we know so much about them.
Most of the universes flash by too fast for Lyra to see. But her measuring instruments are not subject to the limitations of human senses - nor to our world's laws of physics. After they are switched on, their displays show a running average of the values from all the universes they have been in, regardless of how much time they spent in each.
So, for instance, if the even-numbered universes have astrophysicists and the odd-numbered ones do not, then at the end of a two-minute journey through all the universes her SETI-like instrument will be displaying 0. 5. So in that multiverse it is meaningful to say that half the universes have astrophysicists.
Using a universe-travelling device that visited the same universes in a different order, one would obtain a different value for that proportion. But, suppose that the laws of physics permit visiting them in only one order (rather as our own laws of physics normally allow us to be at different times only in one particular order).
Since there is now only one way for measuring instruments to respond to averages, typical values and so on, a rational agent in those universes will always get consistent results when reasoning about probabilities - and about how rare or common, typical or untypical, sparse or dense, fine-tuned or not anything is. And so now the anthropic principle can make testable, probabilistic predictions. What has made this possible is that the infinite set of universes with different values of D is no longer merely a set.
It is a single physical entity, a multiverse with internal interactions (as harnessed by Lyra's device) that relate different parts of it to each other and thereby provide a unique meaning, known as a measure, to proportions and averages over different universes. None of the anthropic-reasoning theories that have been proposed to solve the fine-tuning problem provides any such measure. Most are hardly more than speculations of the form 'What if there were universes with different physical constants?' There is, however, one theory in physics that already describes a multiverse for independent reasons.
All its universes have the same constants of physics, and the interactions of these universes do not involve travel to, or measurement of, each other. But it does provide a measure for universes. That theory is quantum theory, which I shall discuss in Chapter 11.
Physics deals in distances, not numbers of points. Similarly, Newton and Leibniz were able to use infinitesimal distances to explain physical quantities like instantaneous velocity, yet there is nothing physically infinitesimal or infinite in, say, the continuous motion of a projectile. To the management of Infinity Hotel, issuing a finite public-address announcement is a finite operation, even though it causes a transformation involving an infinite number of events in the hotel.
On the other hand, most logically possible transformations could be achieved only with an infinite number of such announcements - which the laws of physics in their world do not allow. Remember, no one in Infinity Hotel - neither staff nor guest - ever performs more than a finite number of actions. Similarly in the Lyra multiverse, a measuring instrument can take the average of an infinite number of values during a finite, two- minute expedition.
So that is a physically finite operation in that world. But taking the 'average' of the same infinite set in a different order would require an infinite number of such trips, which, again, would not be possible under those laws of physics. Only the laws of physics determine what is finite in nature.
Countably infinite Infinite, but small enough to be placed in one-to- one correspondence with the natural numbers. Measure A method by which a theory gives meaning to proportions and averages of infinite sets of things, such as universes. Singularity A situation in which something physical becomes unboundedly large, while remaining everywhere finite.
Multiverse A unified physical entity that contains more than one universe. Infinite regress A fallacy in which an argument or explanation depends on a sub-argument of the same form which purports to address essentially the same problem as the original argument. Computation A physical process that instantiates the properties of some abstract entity.
Proof A computation which, given a theory of how the computer on which it runs works, establishes the truth of some abstract proposition. ENCOUNTERED IN THIS CHAPTER - The ending of the ancient aversion to the infinite (and the universal). - Calculus, Cantor's theory and other theories of the infinite and the infinitesimal in mathematics.
- The view along a corridor of Infinity Hotel. - The property of infinite sequences that every element is exceptionally close to the beginning. - The universality of reason.
- The infinite reach of some ideas. - The internal structure of a multiverse which gives meaning to an 'infinity of universes'. - The unpredictability of the content of future knowledge is a necessary condition for the unlimited growth of that knowledge.
We can understand infinity through the infinite reach of some explanations. It makes sense, both in mathematics and in physics. But it has counter-intuitive properties, some of which are illustrated by Hilbert's thought experiment of Infinity Hotel.
Since we are heading towards real physics, let me also retain the speed-of-light limit on communication, and let the laws of physics be universal and symmetrical (i. e. they make no distinction between the universes).
Moreover, they are deterministic: nothing random ever happens, which is why the universes have remained alike - so far. So how can they ever become different? That is a key question in the theory of the multiverse, which I shall answer below. All these basic properties of my fictional world can be thought of as conditions on the flow of information: one cannot send a message to the other universe; nor can one change anything in one's own universe sooner than light could reach that thing.
Nor can one bring new information - even random information - into the world: everything that happens is determined by laws of physics from what has gone before. However, one can, of course, bring new knowledge into the world. Knowledge consists of explanations, and none of those conditions prevents the creation of new explanations.
This is to stress that a universe is not a receptacle containing physical objects: it is those objects. In real physics, even space is a physical object, capable of warping and affecting matter and being affected by it. So now we have two perfectly parallel, identical universes, each including an instance of our starship, its crew and its transporter, and of the whole of space.
Because of the symmetry between them, it is now misleading to call one of them 'the ordinary universe' and the other 'the phantom zone'. So I shall just call them 'universes'. The two of them together (which comprise the whole of physical reality in the story so far) are the multiverse.
Similarly, it is misleading to speak of the 'original' object and its 'doppelgänger': they are simply the two instances of the object. If our science-fiction speculation were to stop there, the two universes would have to remain identical for ever. There is nothing logically impossible about that.
I have already imagined some inexplicable worlds for that very reason in previous chapters, and I shall imagine more in this chapter. But, in the end, I want to tell of an explicable world, and it will be ours. A remark about terminology: The world is the whole of physical reality.
In classical (pre-quantum) physics, the world was thought to consist of one universe - something like a whole three-dimensional space for the whole of time, and all its contents. According to quantum physics, as I shall explain, the world is a much larger and more complicated object, a multiverse, which includes many such universes (among other things). And a history is a sequence of events happening to objects and possibly their identical counterparts.
So, in my story so far, the world is a multiverse that consists of two universes but has only a single history. So our two universes must not stay identical. Something like a transporter malfunction will have to make them different.
In classical (pre-quantum) physics, the world was thought to consist of one universe - something like a whole three-dimensional space for the whole of time, and all its contents. According to quantum physics, as I shall explain, the world is a much larger and more complicated object, a multiverse, which includes many such universes (among other things). And a history is a sequence of events happening to objects and possibly their identical counterparts.
So, in my story so far, the world is a multiverse that consists of two universes but has only a single history. So our two universes must not stay identical. Something like a transporter malfunction will have to make them different.
Yet, as I said, that may seem to have been ruled out by those restrictions on information flow. The laws of physics in the fictional multiverse are deterministic and symmetrical. So what can the transporter possibly do that would make the two universes differ? It may seem that whatever one instance of it does to one universe, its doppelgänger must be doing to the other, so the universes can only remain the same.
So, in my story so far, the world is a multiverse that consists of two universes but has only a single history. So our two universes must not stay identical. Something like a transporter malfunction will have to make them different.
Yet, as I said, that may seem to have been ruled out by those restrictions on information flow. The laws of physics in the fictional multiverse are deterministic and symmetrical. So what can the transporter possibly do that would make the two universes differ? It may seem that whatever one instance of it does to one universe, its doppelgänger must be doing to the other, so the universes can only remain the same.
Surprisingly, that is not so. It is consistent for two identical entities to become different under deterministic and symmetrical laws. But, for that to happen, they must initially be more than just exact images of each other: they must be fungible (the g is pronounced as in 'plunger'), by which I mean identical in literally every way except that there are two of them.
It means being identical, and that is a very different and counter-intuitive property. Leibniz, in his doctrine of 'the identity of indiscernibles', went so far as to rule out its existence on principle. But he was mistaken.
Even aside from the physics of the multiverse, we now know that photons, and under some conditions even atoms, can be fungible. This is achieved in lasers and in devices called 'atomic lasers' respectively. The latter emit bursts of extremely cold, fungible atoms.
For how this is possible without causing transmutation, explosions and so on, see below. You will not find the concept of fungibility discussed or even mentioned in many textbooks or research papers on quantum theory, even the small minority that endorse the many-universes interpretation. Nevertheless, it is everywhere just beneath the conceptual surface, and I believe that making it explicit helps to explain quantum phenomena without fudging.
When two or more such excitations with identical attributes (such as energy and spin) are present in the cavity, there is no such thing as which one was there first, nor which one will be the next to leave. There is only such a thing as the attributes of any one of them, and how many of them there are.
If the two universes of our fictional multiverse are initially fungible, our transporter malfunction can make them acquire different attributes in the same way that a bank's computer can withdraw one of two fungible dollars and not the other from an account containing two dollars. The laws of physics could, for instance, say that, when the transporter malfunctions, then in one of the universes and not the other there will be a small voltage surge in the transported objects. The laws, being symmetrical, could not possibly specify which universe the surge will take place in.
But, precisely because the universes are initially fungible, they do not have to. It is a rather counter-intuitive fact that if objects are merely identical (in the sense of being exact copies), and obey deterministic laws that make no distinction between them, then they can never become different; but fungible objects, which on the face of it are even more alike, can. This is the first of those weird properties of fungibility that Leibniz never thought of, and which I consider to be at the heart of the phenomena of quantum physics.
Could one say that they all have two owners? Perhaps, but only because that is a vague term. Certainly there is no point in saying that one cent of each of the dollars is owned by the tax authority, because that simply runs into the problem that the cents in the account are all fungible too. But, in any case, notice that the problem raised by this 'diversity within fungibility' is one of language only.
It is a problem of how to describe some aspects of the situation in words. No one finds the situation itself paradoxical: the computer has been instructed to execute definite rules, and there will never be any ambiguity about what will happen as a result. Diversity within fungibility is a widespread phenomenon in the multiverse, as I shall explain.
One big difference from the case of fungible money is that in the latter case we never have to wonder about - or predict - what it would be like to be a dollar. That is to say, what it would be like to be fungible, and then to become differentiated. Many applications of quantum theory require us to do exactly that.
) The third - which had never been imagined before quantum theory - is that two or more initially fungible instances of the observer become different. That is what those transporter-induced jolts bring about, and it makes their outcomes strictly unpredictable despite being described by deterministic laws of physics. These remarks about unpredictable phenomena could be expressed without ever referring explicitly to fungibility.
And indeed that is what multiverse researchers usually do. Nevertheless, as I have said, I believe that fungibility is essential to the explanation of quantum randomness and most other quantum phenomena. All three of these radically different causes of unpredictability could in principle feel exactly the same to observers.
But, in an explicable world, there must be a way of finding out which of them (or which combination of them) is the actual source of any apparent randomness in nature. How could one find out that it is fungibility and parallel universes that are responsible for a given phenomenon? In fiction, there is always the temptation to introduce inter-universe communication for this purpose, making the universes no longer 'parallel'. As I have said, that would really make it a single-universe story - but we might try to disguise that fact by saying that such communication is difficult.
Reflecting on the details (about what the doppelgängers breathe and so on) would then let the inhabitants know that the other universe as a whole was a real place with similar structure and complexity to their own. So their world would be explicable. Since there is no inter-universe communication in real quantum physics, we shall not allow it in our story, and so that specific route to explicability is not open.
The history in which our crew members are married and the one in which they still hardly know each other cannot communicate with each other or observe each other. Nevertheless, as we shall see, there are circumstances in which histories can still affect each other in ways that do not amount to communication, and the need to explain those effects provides the main argument that our own multiverse is real. After the universes in our story begin to differ inside one starship, everything else in the world exists in pairs of identical instances.
We must continue to imagine those pairs as being fungible. This is necessary because the universes are not 'receptacles' - there is nothing to them apart from the objects that they contain. If they did have an independent reality, then each of the objects in such a pair would have a property of being in one particular universe and not the other, which would make them non-fungible.
What happens if the second starship runs its transporter immediately after the first one did? One logically possible answer would be that nothing happens - in other words, the laws of physics would say that, once the two universes are different, all transporters just work normally and never produce a voltage surge again. However, that would also provide a way of communicating faster than light, albeit unreliably and only once. You set up a voltmeter in the transporter room and run the transporter.
If the voltage surges, you know that the other starship, however far away, has not yet run its transporter (because, if it had, that would have put a permanent end to such surges everywhere). The laws governing the real multiverse do not allow information to flow in that way. If we want our fictional laws of physics to be universal from the inhabitants' point of view, the second transporter must do exactly what the first one did.
It must cause a voltage surge in one universe and not in the other. But in that case something must determine which universe the second surge will happen in. 'In one universe but not the other' is no longer a deterministic specification.
There is a way - I think it is the only way - to meet simultaneously the requirements that our fictional laws of physics be universal and deterministic, and forbid faster-than-light and inter-universe communication: more universes. Imagine an uncountably infinite number of them, initially all fungible. The transporter causes previously fungible ones to become different, as before; but now the relevant law of physics says, 'The voltage surges in half the universes in which the transporter is used.
' So, if the two starships both run their transporters, then, after the two spheres of differentiation have overlapped, there will be universes of four different kinds: those in which a surge happened only in the first starship, only in the second, in neither, and in both. In other words, in the overlap region there are four different histories, each taking place in one quarter of the universes. Our fictional theory has not provided enough structure in its multiverse to give a meaning to 'half the universes', but the real quantum theory does.
As I explained in Chapter 8, the method that a theory provides for giving a meaning to proportions and averages for infinite sets is called a measure. A familiar example is that classical physics assigns lengths to infinite sets of points arranged in a line. Let us suppose that our theory provides a measure for universes.
Now we are allowed storylines such as the following. In the universes in which the couple married, they spend their honeymoon on a human- colonized planet that the starship is visiting. As they are teleporting back up, the voltage surge in half those universes causes someone's electronic notepad to play a voice message suggesting that one of the newlyweds has already been unfaithful.
This sets off a chain of events that ends in divorce. So now our original collection of fungible universes contains three different histories: in one, comprising half the original set of universes, the couple in question are still single; in the second, comprising a quarter of the original set, they are married; and in the third, comprising the remaining quarter, they are divorced. Thus the three histories do not occupy equal proportions of the multiverse.
There are twice as many universes in which the couple never married as there are universes in which they divorced. Now suppose that scientists on the starship know about the multiverse and understand the physics of the transporter. (Though note that we have not yet given them a way of discovering those things.
This sets off a chain of events that ends in divorce. So now our original collection of fungible universes contains three different histories: in one, comprising half the original set of universes, the couple in question are still single; in the second, comprising a quarter of the original set, they are married; and in the third, comprising the remaining quarter, they are divorced. Thus the three histories do not occupy equal proportions of the multiverse.
There are twice as many universes in which the couple never married as there are universes in which they divorced. Now suppose that scientists on the starship know about the multiverse and understand the physics of the transporter. (Though note that we have not yet given them a way of discovering those things.
) Then they know that, when they run the transporter, an infinite number of fungible instances of themselves, all sharing the same history, are doing so at the same time. They know that a voltage surge will occur in half the universes in that history, which means that it will split into two histories of equal measure. Hence they know that, if they use a voltmeter capable of detecting the surge, half of the instances of themselves are going to find that it has recorded one, and the other half are not.
But they also know that it is meaningless to ask (not merely impossible to know) which event they will experience. Consequently they can make two closely related predictions. One is that, despite the perfect determinism of everything that is happening, nothing can reliably predict for them whether the voltmeter will detect a surge.
The other prediction is simply that the voltmeter will record a surge with probability one-half. Thus the outcomes of such experiments are subjectively random (from the perspective of any observer) even though everything that is happening is completely determined objectively. This is also the origin of quantum-mechanical randomness and probability in real physics: it is due to the measure that the theory provides for the multiverse, which is in turn due to what kinds of physical processes the theory allows and forbids.
Notice that when a random outcome (in this sense) is about to happen, it is a situation of diversity within fungibility: the diversity is in the variable 'what outcome they are going to see'. The logic of the situation is the same as in cases like that of the bank account I discussed above, except that this time the fungible entities are people. They are fungible, yet half of them are going to see the surge and the other half not.
Most locations on board are packed with people, some of them on very unusual errands, and all unable to perceive each other. The spaceship itself is on many slightly different courses, due to slightly different behaviours of the crew. Of course we are 'seeing' this only in our mind's eye.
Our fictional laws of physics ensure that no observer in the multiverse itself would see anything like that. Consequently, on closer inspection (in our mind's eye), we also see that there is great order and regularity in that apparent chaos. For instance, although there is a flurry of human figures in the Captain's chair, we see that most of them are the Captain; and although there is a flurry of human figures in the Navigator's chair, we see that few of them are the Captain.
Regularities of that kind are ultimately due to the fact that all the universes, despite their differences, obey the same laws of physics (including their initial conditions). We also see that any particular instance of the Captain only ever interacts with one instance of the Navigator, and one instance of the First Officer; and those instances of the Navigator and First Officer are precisely the ones that interact with each other.
* So far in the story we have set up a vast, complex world which looks very unfamiliar in our mind's eye, but to the overwhelming majority of the inhabitants looks almost exactly like the single universe of our everyday experience and of classical physics, plus some apparently random jiggling whenever the transporter operates. A tiny minority of the histories have been significantly affected by very 'unlikely' events, but even in those the information flow - what affects what - is still very tame and familiar.
For instance, a version of the ship's log that contains records of bizarre coincidences will be perceptible to people who remember those coincidences, but not to other instances of those people. Thus the information in the fictional multiverse flows along a branching tree, whose branches - histories - have different thicknesses (measures) and never rejoin once they have separated. Each behaves exactly as if the others did not exist.
If that were the whole story, that multiverse's imaginary laws of physics would still be fatally flawed as explanations in the same way that they have been all along: there would be no difference between their predictions and those of much more straightforward laws saying that there is only one universe - one history - in which the transporter randomly introduces a change in the objects that it teleports. Under those laws, instead of branching into two autonomous histories on such occasions, the single history randomly does or does not undergo such a change.
For instance, a version of the ship's log that contains records of bizarre coincidences will be perceptible to people who remember those coincidences, but not to other instances of those people. Thus the information in the fictional multiverse flows along a branching tree, whose branches - histories - have different thicknesses (measures) and never rejoin once they have separated. Each behaves exactly as if the others did not exist.
If that were the whole story, that multiverse's imaginary laws of physics would still be fatally flawed as explanations in the same way that they have been all along: there would be no difference between their predictions and those of much more straightforward laws saying that there is only one universe - one history - in which the transporter randomly introduces a change in the objects that it teleports. Under those laws, instead of branching into two autonomous histories on such occasions, the single history randomly does or does not undergo such a change.
Thus the entire stupendously complicated multiverse that we have imagined - with its multiplicity of entities including people walking through each other and its bizarre occurrences and its entanglement information - would collapse into nothing, like the galaxy in Chapter 2 that became an emulsion flaw. The multiverse explanation of the same events would be a bad explanation, and so the world would be inexplicable to the inhabitants if it were true.
If that were the whole story, that multiverse's imaginary laws of physics would still be fatally flawed as explanations in the same way that they have been all along: there would be no difference between their predictions and those of much more straightforward laws saying that there is only one universe - one history - in which the transporter randomly introduces a change in the objects that it teleports. Under those laws, instead of branching into two autonomous histories on such occasions, the single history randomly does or does not undergo such a change.
Thus the entire stupendously complicated multiverse that we have imagined - with its multiplicity of entities including people walking through each other and its bizarre occurrences and its entanglement information - would collapse into nothing, like the galaxy in Chapter 2 that became an emulsion flaw. The multiverse explanation of the same events would be a bad explanation, and so the world would be inexplicable to the inhabitants if it were true.
It may seem that, by imposing all those conditions on information flow, we have gone to a lot of trouble to achieve that very attribute - to hide, from the inhabitants, the Byzantine intricacies of their world. In the words of Lewis Carroll's White Knight in Through the Looking Glass, it is as if we were . .
. thinking of a plan To dye one's whiskers green, And always use so large a fan That they could not be seen. Now it is time to start removing the fan.
In quantum physics, information flow in the multiverse is not as tame as in that branching tree of histories I have described. That is because of one further quantum phenomenon: under certain circumstances, the laws of motion allow histories to rejoin (becoming fungible again). This is the time-reverse of the splitting (differentiation of history into two or more histories) that I have already described, so a natural way to implement it in our fictional multiverse is for the transporter to be capable of undoing its own history-splitting.
If we represent the original splitting like this where X is the normal voltage and Y is the anomalous one introduced by the transporter, then the rejoining of histories can be represented as In an interference phenomenon, differentiated histories rejoin. This phenomenon is known as interference: the presence of the Y- history interferes with what the transporter usually does to an X-history. Instead, the X and Y histories merge.
If we represent the original splitting like this where X is the normal voltage and Y is the anomalous one introduced by the transporter, then the rejoining of histories can be represented as In an interference phenomenon, differentiated histories rejoin. This phenomenon is known as interference: the presence of the Y- history interferes with what the transporter usually does to an X-history. Instead, the X and Y histories merge.
This is rather like the doppelgängers merging with their originals in some phantom-zone stories, except that here we do not need to repeal the principle of the conservation of mass or any other conservation law: the total measure of all the histories remains constant. Interference is the phenomenon that can provide the inhabitants of the multiverse with evidence of the existence of multiple histories in their world without allowing the histories to communicate.
For example, suppose that they run the transporter twice in quick succession (I shall explain in a moment what 'quick' means): An interference experiment If they did this repeatedly (with, say, different copies of the transporter on each occasion), they could soon infer that the intermediate result could not be just randomly X or Y, because if it were then the final outcome would sometimes be Y (because of ), while in fact it is always X.
If the differential effects can all be undone, then interference between those original values becomes possible again; but the laws of quantum mechanics dictate that undoing them requires fine control of all the affected objects, and that rapidly becomes infeasible. The process of its becoming infeasible is known as decoherence. In most situations, decoherence is very rapid, which is why splitting typically predominates over interference, and why interference - though ubiquitous on microscopic scales - is quite hard to demonstrate unambiguously in the laboratory.
Nevertheless, it can be done, and quantum interference phenomena constitute our main evidence of the existence of the multiverse, and of what its laws are. A real-life analogue of the above experiment is standard in quantum optics laboratories. Instead of experimenting on voltmeters (whose many interactions with their environment quickly cause decoherence), one uses individual photons, and the variable being acted upon is not voltage but which of two possible paths the photon is on.
Instead of the transporter, one uses a simple device called a semi-silvered mirror (represented by the grey sloping bars in the diagrams below). When a photon strikes such a mirror, it bounces off in half the universes, and passes straight through in the other half, as shown on next page: Semi-silvered mirror The attributes of travelling in the X or Y directions behave analogously to the two voltages X and Y in our fictitious multiverse. So passing through the semi-silvered mirror is the analogue of the transformation above.
Nevertheless, it can be done, and quantum interference phenomena constitute our main evidence of the existence of the multiverse, and of what its laws are. A real-life analogue of the above experiment is standard in quantum optics laboratories. Instead of experimenting on voltmeters (whose many interactions with their environment quickly cause decoherence), one uses individual photons, and the variable being acted upon is not voltage but which of two possible paths the photon is on.
Instead of the transporter, one uses a simple device called a semi-silvered mirror (represented by the grey sloping bars in the diagrams below). When a photon strikes such a mirror, it bounces off in half the universes, and passes straight through in the other half, as shown on next page: Semi-silvered mirror The attributes of travelling in the X or Y directions behave analogously to the two voltages X and Y in our fictitious multiverse. So passing through the semi-silvered mirror is the analogue of the transformation above.
And when the two instances of a single photon, travelling in directions X and Y, strike the second semi-silvered mirror at the same time, they undergo the transformation , which means that both instances emerge in the direction X: the two histories rejoin. To demonstrate this, one can use a set-up known as a 'Mach-Zehnder interferometer', which performs those two transformations (splitting and interference) in quick succession: Mach-Zehnder interferometer The two ordinary mirrors (the black sloping bars) are merely there to steer the photon from the first to the second semi-silvered mirror.
If a photon is introduced travelling rightwards (X) after the first mirror instead of before as shown, then it appears to emerge randomly, rightwards or downwards, from the last mirror (because then, happens there). The same is true of a photon introduced travelling downwards (Y) after the first mirror. But a photon introduced as shown in the diagram invariably emerges rightwards, never downwards.
By doing the experiment repeatedly with and without detectors on the paths, one can verify that only one photon is ever present per history, because only one of those detectors is ever observed to fire during such an experiment. Then, the fact that the intermediate histories X and Y both contribute to the deterministic final outcome X makes it inescapable that both are happening at the intermediate time. In the real multiverse, there is no need for the transporter or any other special apparatus to cause histories to differentiate and to rejoin.
Under the laws of quantum physics, elementary particles are undergoing such processes of their own accord, all the time. Moreover, histories may split into more than two - often into many trillions - each characterized by a slightly different direction of motion or difference in other physical variables of the elementary particle concerned. Also, in general the resulting histories have unequal measures.
Under the laws of quantum physics, elementary particles are undergoing such processes of their own accord, all the time. Moreover, histories may split into more than two - often into many trillions - each characterized by a slightly different direction of motion or difference in other physical variables of the elementary particle concerned. Also, in general the resulting histories have unequal measures.
So let us now dispense with the transporter in the fictional multiverse too. The rate of growth in the number of distinct histories is quite mind- boggling - even though, thanks to interference, there is now a certain amount of spontaneous rejoining as well. Because of this rejoining, the flow of information in the real multiverse is not divided into strictly autonomous subflows - branching, autonomous histories.
Although there is still no communication between histories (in the sense of message-sending), they are intimately affecting each other, because the effect of interference on a history depends on what other histories are present. Not only is the multiverse no longer perfectly partitioned into histories, individual particles are not perfectly partitioned into instances. For example, consider the following interference phenomenon, where X and Y now represent different values of the position of a single particle: How instances of a particle lose their identity during interference.
So let us now dispense with the transporter in the fictional multiverse too. The rate of growth in the number of distinct histories is quite mind- boggling - even though, thanks to interference, there is now a certain amount of spontaneous rejoining as well. Because of this rejoining, the flow of information in the real multiverse is not divided into strictly autonomous subflows - branching, autonomous histories.
Although there is still no communication between histories (in the sense of message-sending), they are intimately affecting each other, because the effect of interference on a history depends on what other histories are present. Not only is the multiverse no longer perfectly partitioned into histories, individual particles are not perfectly partitioned into instances. For example, consider the following interference phenomenon, where X and Y now represent different values of the position of a single particle: How instances of a particle lose their identity during interference.
Has the instance of the particle at X stayed at X or moved to Y? Has the instance of the particle at Y returned to Y or moved to X? Because these two groups of instances of the particle, initially at different positions, have gone through a moment of being fungible, there is no such thing as which of them has ended up at which final position. This sort of interference is going on all the time, even for a single particle in a region of otherwise empty space.
Even different electrons do not have completely separate identities. So the reality is an electron field throughout the whole of space, and disturbances spread through this field as waves, at the speed of light or below. This is what gave rise to the often- quoted misconception among the pioneers of quantum theory that electrons (and likewise all other particles) are 'particles and waves at the same time'.
There is a field (or 'waves') in the multiverse for every individual particle that we observe in a particular universe. Although quantum theory is expressed in mathematical language, I have now given an account in English of the main features of the reality that it describes. So at this point the fictional multiverse that I have been describing is more or less the real one.
But there is one thing left to tidy up. My 'succession of speculations' was based on universes, and on instances of objects, and then on corrections to those ideas in order to describe the multiverse. But the real multiverse is not 'based on' anything, nor is it a correction to anything.
There is a field (or 'waves') in the multiverse for every individual particle that we observe in a particular universe. Although quantum theory is expressed in mathematical language, I have now given an account in English of the main features of the reality that it describes. So at this point the fictional multiverse that I have been describing is more or less the real one.
But there is one thing left to tidy up. My 'succession of speculations' was based on universes, and on instances of objects, and then on corrections to those ideas in order to describe the multiverse. But the real multiverse is not 'based on' anything, nor is it a correction to anything.
Universes, histories, particles and their instances are not referred to by quantum theory at all - any more than are planets, and human beings and their lives and loves. Those are all approximate, emergent phenomena in the multiverse. A history is part of the multiverse in the same sense that a geological stratum is part of the Earth's crust.
But there is one thing left to tidy up. My 'succession of speculations' was based on universes, and on instances of objects, and then on corrections to those ideas in order to describe the multiverse. But the real multiverse is not 'based on' anything, nor is it a correction to anything.
Universes, histories, particles and their instances are not referred to by quantum theory at all - any more than are planets, and human beings and their lives and loves. Those are all approximate, emergent phenomena in the multiverse. A history is part of the multiverse in the same sense that a geological stratum is part of the Earth's crust.
One history is distinguished from the others by the values of physical variables, just as a stratum is distinguished from others by its chemical composition and by the types of fossils found in it and so on. A stratum and a history are both channels of information flow. They preserve information because, although their contents change over time, they are approximately autonomous - that is to say, the changes in a particular stratum or history depend almost entirely on conditions inside it and not elsewhere.
Nor does a stratum have well- defined edges. Also, there are regions of the Earth - for instance, near volcanoes - where strata have merged (though I think there are no geological processes that split and remerge strata in the way that histories split and remerge). There are regions of the Earth - such as the core - where there have never been strata.
And there are regions - such as the atmosphere - where strata do form but their contents interact and mix on much shorter timescales than in the crust. Similarly, there are regions of the multiverse that contain short-lived histories, and others that do not even approximately contain histories. However, there is one big difference between the ways in which strata and histories emerge from their respective underlying phenomena.
Although not every atom in the Earth's crust can be unambiguously assigned to a particular stratum, most of the atoms that form a stratum can. In contrast, every atom in an everyday object is a multiversal object, not partitioned into nearly autonomous instances and nearly autonomous histories, yet everyday objects such as starships and betrothed couples, which are made of such particles, are partitioned very accurately into nearly autonomous histories with exactly one instance, one position, one speed of each object in each history. That is because of the suppression of interference by entanglement.
Although not every atom in the Earth's crust can be unambiguously assigned to a particular stratum, most of the atoms that form a stratum can. In contrast, every atom in an everyday object is a multiversal object, not partitioned into nearly autonomous instances and nearly autonomous histories, yet everyday objects such as starships and betrothed couples, which are made of such particles, are partitioned very accurately into nearly autonomous histories with exactly one instance, one position, one speed of each object in each history. That is because of the suppression of interference by entanglement.
As I explained, interference almost always happens either very soon after splitting or not at all. That is why the larger and more complex an object or process is, the less its gross behaviour is affected by interference. At that 'coarse-grained' level of emergence, events in the multiverse consist of autonomous histories, with each coarse-grained history consisting of a swathe of many histories differing only in microscopic details but affecting each other through interference.
Spheres of differentiation tend to grow at nearly the speed of light, so, on the scale of everyday life and above, those coarse-grained histories can justly be called 'universes' in the ordinary sense of the word. Each of them somewhat resembles the universe of classical physics. And they can usefully be called 'parallel' because they are nearly autonomous.
Spheres of differentiation tend to grow at nearly the speed of light, so, on the scale of everyday life and above, those coarse-grained histories can justly be called 'universes' in the ordinary sense of the word. Each of them somewhat resembles the universe of classical physics. And they can usefully be called 'parallel' because they are nearly autonomous.
To the inhabitants, each looks very like a single- universe world. Microscopic events which are accidentally amplified to that coarse- grained level (like the voltage surge in our story) are rare in any one coarse-grained history, but common in the multiverse as a whole. For example, consider a single cosmic-ray particle travelling in the direction of Earth from deep space.
That particle must be travelling in a range of slightly different directions, because the uncertainty principle implies that in the multiverse it must spread sideways like an ink blot as it travels. By the time it arrives, this ink blot may well be wider than the whole Earth - so most of it misses and the rest strikes everywhere on the exposed surface. Remember, this is just a single particle, which may consist of fungible instances.
To the inhabitants, each looks very like a single- universe world. Microscopic events which are accidentally amplified to that coarse- grained level (like the voltage surge in our story) are rare in any one coarse-grained history, but common in the multiverse as a whole. For example, consider a single cosmic-ray particle travelling in the direction of Earth from deep space.
That particle must be travelling in a range of slightly different directions, because the uncertainty principle implies that in the multiverse it must spread sideways like an ink blot as it travels. By the time it arrives, this ink blot may well be wider than the whole Earth - so most of it misses and the rest strikes everywhere on the exposed surface. Remember, this is just a single particle, which may consist of fungible instances.
The next thing that happens is that they cease to be fungible, splitting through their interaction with atoms at their points of arrival into a finite but huge number of instances, each of which is the origin of a separate history. In each such history, there is an autonomous instance of the cosmic- ray particle, which will dissipate its energy in creating a 'cosmic-ray shower' of electrically charged particles. Thus, in different histories, such a shower will occur at different locations.
Some non-negligible proportion of all cancers are caused in this way. As a result, there exist histories in which any given person, alive in our history at any time, is killed soon afterwards by cancer. There exist other histories in which the course of a battle, or a war, is changed by such an event, or by a lightning bolt at exactly the right place and time, or by any of countless other unlikely, 'random' events.
This makes it highly plausible that there exist histories in which events have played out more or less as in alternative-history stories such as Fatherland and Roma Eterna - or in which events in your own life played out very differently, for better or worse. A great deal of fiction is therefore close to a fact somewhere in the multiverse. But not all fiction.
For instance, there are no histories in which my stories of the transporter malfunction are true, because they require different laws of physics. Nor are there histories in which the fundamental constants of nature such as the speed of light or the charge on an electron are different. There is, however, a sense in which different laws of physics appear to be true for a period in some histories, because of a sequence of 'unlikely accidents'.
For instance, there are no histories in which my stories of the transporter malfunction are true, because they require different laws of physics. Nor are there histories in which the fundamental constants of nature such as the speed of light or the charge on an electron are different. There is, however, a sense in which different laws of physics appear to be true for a period in some histories, because of a sequence of 'unlikely accidents'.
(There may also be universes in which there are different laws of physics, as required in anthropic explanations of fine-tuning. But as yet there is no viable theory of such a multiverse. ) Imagine a single photon from a starship's communication laser, heading towards Earth.
Like the cosmic ray, it arrives all over the surface, in different histories. In each history, only one atom will absorb the photon and the rest will initially be completely unaffected. A receiver for such communications would then detect the relatively large, discrete change undergone by such an atom.
. Ymany are intermediate results that depend on the input X. All of them are needed to compute the output f(X) efficiently.
Just as the starship crew members could achieve the effect of large amounts of computation by sharing information with their doppelgängers computing the same function on different inputs, so an algorithm that makes use of quantum parallelism does the same. But, while the fictional effect is limited only by starship regulations that we may invent to suit the plot, quantum computers are limited by the laws of physics that govern quantum interference. Only certain types of parallel computation can be performed with the help of the multiverse in this way.
They are the ones for which the mathematics of quantum interference happens to be just right for combining into a single history the information that is needed for the final result. In such computations, a quantum computer with only a few hundred qubits could perform far more computations in parallel than there are atoms in the visible universe. At the time of writing, quantum computers with about ten qubits have been constructed.
In this discussion I am assuming that it is. However, the quantum mechanics of time is not yet fully understood, and will not be until we have a quantum theory of gravity (the unification of quantum theory with the general theory of relativity), so it may turn out that things are not as simple as that. One thing we can be fairly sure of, though, is that, in that theory, different times are a special case of different universes.
In other words, time is an entanglement phenomenon, which places all equal clock readings (of correctly prepared clocks - or of any objects usable as clocks) into the same history. This was first understood by the physicists Don Page and William Wooters, in 1983. In this full version of the quantum multiverse, how is our science-fiction story to continue? Almost all the attention that the quantum theory has attracted, from physicists, philosophers and science-fiction authors alike, has focused on its parallel-universes aspect.
That is ironic, because it is in the parallel-universe approximation that the world most resembles that of classical physics, yet that is the very aspect of quantum theory that many people seem to find viscerally unacceptable. Fiction can explore the possibilities opened up by parallel universes. For instance, since our story is a romance, the characters may well wonder about their counterparts in other histories.
In that one respect, they are unlucky: they were indeed selected by coincidence. Another way of putting that is that they were selected by the very story that I am now telling about them. All fiction that does not violate the laws of physics is fact.
Some fiction in which the laws of physics appear to be violated is also fact, somewhere in the multiverse. This involves a subtle issue about how the multiverse is structured - how histories emerge. A history is approximately autonomous.
If I boil some water in a kettle and make tea, I am in a history in which I switched on the kettle and the water became gradually hotter because of the energy being poured into it by the kettle, causing bubbles to form and so on, and eventually hot tea forms.
If I boil some water in a kettle and make tea, I am in a history in which I switched on the kettle and the water became gradually hotter because of the energy being poured into it by the kettle, causing bubbles to form and so on, and eventually hot tea forms.
That is a history because one can give explanations and make predictions about it without ever mentioning either that there are other histories in the multiverse where I chose to make coffee instead or that the microscopic motion of the water molecules is slightly affected by parts of the multiverse that are outside that history. It is irrelevant to that explanation that a small measure of that history differentiates itself during that process and does other things.
In some tiny sliver of it, the kettle transforms itself into a top hat, and the water into a rabbit which then hops away, and I get neither tea nor coffee but am very surprised. That is a history too, after that transformation. But there is no way of correctly explaining what was happening during it, or predicting the probabilities, without referring to other parts of the multiverse - enormously larger parts (i.
That is a history because one can give explanations and make predictions about it without ever mentioning either that there are other histories in the multiverse where I chose to make coffee instead or that the microscopic motion of the water molecules is slightly affected by parts of the multiverse that are outside that history. It is irrelevant to that explanation that a small measure of that history differentiates itself during that process and does other things.
In some tiny sliver of it, the kettle transforms itself into a top hat, and the water into a rabbit which then hops away, and I get neither tea nor coffee but am very surprised. That is a history too, after that transformation. But there is no way of correctly explaining what was happening during it, or predicting the probabilities, without referring to other parts of the multiverse - enormously larger parts (i.
e. with larger measures) - in which there was no rabbit. So that history began at the transformation, and its causal connection with what happened before that cannot be expressed in history terms but only in multiverse terms.
In some tiny sliver of it, the kettle transforms itself into a top hat, and the water into a rabbit which then hops away, and I get neither tea nor coffee but am very surprised. That is a history too, after that transformation. But there is no way of correctly explaining what was happening during it, or predicting the probabilities, without referring to other parts of the multiverse - enormously larger parts (i.
e. with larger measures) - in which there was no rabbit. So that history began at the transformation, and its causal connection with what happened before that cannot be expressed in history terms but only in multiverse terms.
In simple cases like that, there is a ready-made approximative language in which we can minimize mention of the rest of the multiverse: the language of random events. This allows us to acknowledge that most of the high-level objects concerned still behaved autonomously except for being affected by something outside themselves - as when I am affected by the rabbit. This constitutes some continuity between a history and a previous history from which it split, and we can refer to the former as a 'history that has been affected by random events'.
e. with larger measures) - in which there was no rabbit. So that history began at the transformation, and its causal connection with what happened before that cannot be expressed in history terms but only in multiverse terms.
In simple cases like that, there is a ready-made approximative language in which we can minimize mention of the rest of the multiverse: the language of random events. This allows us to acknowledge that most of the high-level objects concerned still behaved autonomously except for being affected by something outside themselves - as when I am affected by the rabbit. This constitutes some continuity between a history and a previous history from which it split, and we can refer to the former as a 'history that has been affected by random events'.
However, this is never literally what has happened: the part of that 'history' prior to the 'random event' is fungible with the rest of the broader history and therefore has no separate identity from it: it is not separately explicable. But the broader of those two histories still is. That is to say, the rabbit history is fundamentally different from the tea history, in that the latter remains very accurately autonomous throughout the period.
In the rabbit history I end up with memories that are identical to what they would be in a history in which water became a rabbit. But those are misleading memories. There was no such history; the history containing those memories began only after the rabbit had formed.
For that matter, there are also places in the multiverse - of far larger measure than that one - in which only my brain was affected, producing exactly those memories. In effect, I had a hallucination, caused by random motion of the atoms in my brain. Some philosophers make a big issue of that sort of thing, claiming that it casts doubt on the scientific status of quantum theory, but of course they are empiricists.
In reality, misleading observations, misleading memories and false interpretations are common even in the mainstreams of history. We have to work hard to avoid fooling ourselves with them. So it is not quite true that, for instance, there are histories in which magic appears to work.
There are only histories in which magic appears to have worked, but will never work again. There are histories in which I appear to have walked through a wall, because all the atoms of my body happened to resume their original courses after being deflected by atoms in the wall. But those histories began at the wall: the true explanation of what happened involves many other instances of me and it - or we can roughly explain it in terms of random events of very low probability.
It is a bit like winning a lottery: the winner cannot properly explain what has just happened without invoking the existence of many losers. In the multiverse, the losers are other instances of oneself. The 'history' approximation breaks down completely only when histories not only split but merge - that is to say, in interference phenomena.
For example, there are certain molecules that exist in two or more structures at once (a 'structure' being an arrangement of atoms, held together by chemical bonds). Chemists call this phenomenon 'resonance' between the two structures, but the molecule is not alternating between them: it has them simultaneously. There is no way of explaining the chemical properties of such molecules in terms of a single structure, because when a 'resonant' molecule participates in a chemical reaction with other molecules, there is quantum interference.
Whenever we observe anything - a scientific instrument or a galaxy or a human being - what we are actually seeing is a single-universe perspective on a larger object that extends some way into other universes. In some of those universes, the object looks exactly as it does to us, in others it looks different, or is absent altogether. What an observer sees as a married couple is actually just a sliver of a vast entity that includes many fungible instances of such a couple, together with other instances of them who are divorced, and others who have never married.
We are channels of information flow. So are histories, and so are all relatively autonomous objects within histories; but we sentient beings are extremely unusual channels, along which (sometimes) knowledge grows. This can have dramatic effects, not only within a history (where it can, for instance, have effects that do not diminish with distance), but also across the multiverse.
Since the growth of knowledge is a process of error-correction, and since there are many more ways of being wrong than right, knowledge-creating entities rapidly become more alike in different histories than other entities. As far as is known, knowledge-creating processes are unique in both these respects: all other effects diminish with distance in space, and become increasingly different across the multiverse, in the long run. But that is only as far as is known.
We are channels of information flow. So are histories, and so are all relatively autonomous objects within histories; but we sentient beings are extremely unusual channels, along which (sometimes) knowledge grows. This can have dramatic effects, not only within a history (where it can, for instance, have effects that do not diminish with distance), but also across the multiverse.
Since the growth of knowledge is a process of error-correction, and since there are many more ways of being wrong than right, knowledge-creating entities rapidly become more alike in different histories than other entities. As far as is known, knowledge-creating processes are unique in both these respects: all other effects diminish with distance in space, and become increasingly different across the multiverse, in the long run. But that is only as far as is known.
Here is an opportunity for some wild speculations that could inform a science-fiction story.
Here is an opportunity for some wild speculations that could inform a science-fiction story.
What if there is something other than information flow that can cause coherent, emergent phenomena in the multiverse? What if knowledge, or something other than knowledge, could emerge from that, and begin to have purposes of its own, and to conform the multiverse to those purposes, as we do? Could we communicate with it? Presumably not in the usual sense of the term, because that would be information flow; but perhaps the story could propose some novel analogue of communication which, like quantum inference, did not involve sending messages.
Would we be trapped in a war of mutual extermination with such an entity? Or is it possible that we could nevertheless have something in common with it? Let us shun parochial resolutions of the issue - such as a discovery that what bridges the barrier is love, or trust. But let us remember that, just as we are at the top rank of significance in the great scheme of things, anything else that could create explanations would be too. And there is always room at the top.
Would we be trapped in a war of mutual extermination with such an entity? Or is it possible that we could nevertheless have something in common with it? Let us shun parochial resolutions of the issue - such as a discovery that what bridges the barrier is love, or trust. But let us remember that, just as we are at the top rank of significance in the great scheme of things, anything else that could create explanations would be too. And there is always room at the top.
Fungible Identical in every respect. The world The whole of physical reality. Multiverse The world, according to quantum theory.
Universe Universes are quasi-autonomous regions of the multiverse. History A set of fungible universes, over time. One can also speak of the history of parts of a universe.
Fungible Identical in every respect. The world The whole of physical reality. Multiverse The world, according to quantum theory.
Universe Universes are quasi-autonomous regions of the multiverse. History A set of fungible universes, over time. One can also speak of the history of parts of a universe.
Parallel universes A somewhat misleading way of referring to the multiverse. Misleading because the universes are not perfectly 'parallel' (autonomous), and because the multiverse has much more structure - especially fungibility, entanglement and the measures of histories. Instances In parts of the multiverse that contain universes, each multiversal object consists approximately of 'instances', some identical, some not, one in each of the universes.
Universe Universes are quasi-autonomous regions of the multiverse. History A set of fungible universes, over time. One can also speak of the history of parts of a universe.
Parallel universes A somewhat misleading way of referring to the multiverse. Misleading because the universes are not perfectly 'parallel' (autonomous), and because the multiverse has much more structure - especially fungibility, entanglement and the measures of histories. Instances In parts of the multiverse that contain universes, each multiversal object consists approximately of 'instances', some identical, some not, one in each of the universes.
Quantum The smallest possible change in a discrete physical variable. Entanglement Information in each multiversal object that determines which parts (instances) of it can affect which parts of other multiversal objects. Decoherence The process of its becoming infeasible to undo the effect of a wave of differentiation between universes.
Quantum interference Phenomena caused by non-fungible instances of a multiversal object becoming fungible. Uncertainty principle The (badly misnamed) implication of quantum theory that, for any fungible collection of instances of a physical object, some of their attributes must be diverse. Quantum computation Computation in which the flow of information is not confined to a single history.
The physical world is a multiverse, and its structure is determined by how information flows in it. In many regions of the multiverse, information flows in quasi-autonomous streams called histories, one of which we call our 'universe'. Universes approximately obey the laws of classical (pre-quantum) physics.
But we know of the rest of the multiverse, and can test the laws of quantum physics, because of the phenomenon of quantum interference. Thus a universe is not an exact but an emergent feature of the multiverse. One of the most unfamiliar and counter-intuitive things about the multiverse is fungibility.
The physical world is a multiverse, and its structure is determined by how information flows in it. In many regions of the multiverse, information flows in quasi-autonomous streams called histories, one of which we call our 'universe'. Universes approximately obey the laws of classical (pre-quantum) physics.
But we know of the rest of the multiverse, and can test the laws of quantum physics, because of the phenomenon of quantum interference. Thus a universe is not an exact but an emergent feature of the multiverse. One of the most unfamiliar and counter-intuitive things about the multiverse is fungibility.
The laws of motion of the multiverse are deterministic, and apparent randomness is due to initially fungible instances of objects becoming different. In quantum physics, variables are typically discrete, and how they change from one value to another is a multiversal process involving interference and fungibility. .
But we know of the rest of the multiverse, and can test the laws of quantum physics, because of the phenomenon of quantum interference. Thus a universe is not an exact but an emergent feature of the multiverse. One of the most unfamiliar and counter-intuitive things about the multiverse is fungibility.
The laws of motion of the multiverse are deterministic, and apparent randomness is due to initially fungible instances of objects becoming different. In quantum physics, variables are typically discrete, and how they change from one value to another is a multiversal process involving interference and fungibility. .
Philosophy With Some Comments on Bad Science By the way, what I have just outlined is what I call a 'physicist's history of physics', which is never correct . . .
Richard Feynman, QED: The Strange Theory of Light and Matter (1985) READER: So, I am an emergent, quasi-autonomous flow of information in the multiverse. DAVID: You are. READER: And I exist in multiple instances, some of them different from each other, some not.
And those are the least weird things about the world according to quantum theory. DAVID: Yes. READER: But your argument is that we have no option but to accept the theory's implications, because it is the only known explanation of many phenomena and has survived all known experimental tests.
Both versions of the theory were formulated between 1925 and 1927, and both explained motion, especially within atoms, in new and astonishingly counter-intuitive ways. Heisenberg's theory said that the physical variables of a particle do not have numerical values. Instead, they are matrices: large arrays of numbers which are related in complicated, probabilistic ways to the outcomes of observations of those variables.
With hindsight, we now know that that multiplicity of information exists because a variable has different values for different instances of the object in the multiverse. But, at the time, neither Heisenberg nor anyone else believed that his matrix-valued quantities literally described what Einstein called 'elements of reality'. The Schrödinger equation, when applied to the case of an individual particle, described a wave moving through space.
But Schrödinger soon realized that for two or more particles it did not. It did not represent a wave with multiple crests, nor could it be resolved into two or more waves; mathematically, it was a single wave in a higher- dimensional space. With hindsight, we now know that such waves describe what proportion of the instances of each particle are in each region of space, and also the entanglement information among the particles.
Its combination of vagueness, immunity from criticism, and the prestige and perceived authority of fundamental physics opened the door to countless systems of pseudo-science and quackery supposedly based on quantum theory. Its disparagement of plain criticism and reason as being 'classical', and therefore illegitimate, has given endless comfort to those who want to defy reason and embrace any number of irrational modes of thought. Thus quantum theory - the deepest discovery of the physical sciences - has acquired a reputation for endorsing practically every mystical and occult doctrine ever proposed.
Not every physicist accepted the Copenhagen interpretation or its descendants. Einstein never did. The physicist David Bohm struggled to construct an alternative that was compatible with realism, and produced a rather complicated theory which I regard as the multiverse theory in heavy disguise - though he was strongly opposed to thinking of it in that way.
And in Dublin in 1952 Schrödinger gave a lecture in which at one point he jocularly warned his audience that what he was about to say might 'seem lunatic'. It was that, when his equation seems to be describing several different histories, they are 'not alternatives but all really happen simultaneously'. This is the earliest known reference to the multiverse.
Not every physicist accepted the Copenhagen interpretation or its descendants. Einstein never did. The physicist David Bohm struggled to construct an alternative that was compatible with realism, and produced a rather complicated theory which I regard as the multiverse theory in heavy disguise - though he was strongly opposed to thinking of it in that way.
And in Dublin in 1952 Schrödinger gave a lecture in which at one point he jocularly warned his audience that what he was about to say might 'seem lunatic'. It was that, when his equation seems to be describing several different histories, they are 'not alternatives but all really happen simultaneously'. This is the earliest known reference to the multiverse.
Here was an eminent physicist joking that he might be considered mad. Why? For claiming that his own equation - the very one for which he had won the Nobel prize - might be true. Schrödinger never published that lecture, and seems never to have taken the idea further.
Here was an eminent physicist joking that he might be considered mad. Why? For claiming that his own equation - the very one for which he had won the Nobel prize - might be true. Schrödinger never published that lecture, and seems never to have taken the idea further.
Five years later, and independently, the physicist Hugh Everett published a comprehensive theory of the multiverse, now known as the Everett interpretation of quantum theory. Yet it took several more decades before Everett's work was even noticed by more than a handful of physicists. Even now that it has become well known, it is endorsed by only a small minority.
I have often been asked to explain this unusual phenomenon. Unfortunately I know of no entirely satisfactory explanation. But, to understand why it is perhaps not quite as bizarre and isolated an event as it may appear, one has to consider the broader context of bad philosophy.
Nor, therefore, can it solve the Fermi problem, 'Where are they?' It may turn out to be a necessary part of the explanation, but it can never explain anything by itself. Also, as I explained in Chapter 8, any theory involving an anthropic argument must provide a measure for defining probabilities in an infinite set of things. It is unknown how to do that in the spatially infinite universe that cosmologists currently believe we live in.
That issue has a wider scope. For example, there is the so-called 'quantum suicide argument' in regard to the multiverse. Suppose you want to win the lottery.
You buy a ticket and set up a machine that will automatically kill you in your sleep if you lose. Then, in all the histories in which you do wake up, you are a winner. If you do not have loved ones to mourn you, or other reasons to prefer that most histories not be affected by your premature death, you have arranged to get something for nothing with what proponents of this argument call 'subjective certainty'.
Here is a related but starker moral question. Take a powerful computer and set each bit randomly to 0 or 1 using a quantum randomizer. (That means that 0 and 1 occur in histories of equal measure.
) At that point all possible contents of the computer's memory exist in the multiverse. So there are necessarily histories present in which the computer contains an AI program - indeed, all possible AI programs in all possible states, up to the size that the computer's memory can hold. Some of them are fairly accurate representations of you, living in a virtual-reality environment crudely resembling your actual environment.
(Present-day computers do not have enough memory to simulate a realistic environment accurately, but, as I said in Chapter 7, I am sure that they have more than enough to simulate a person. ) There are also people in every possible state of suffering.