Wednesday, February 15, 2017

Self-Causation, Time, and Quantum Physics

"The future is not what it used to be."
(Paul Valéry)


1. Introduction: Self-causation from Plotinus to Wheeler
A recurrent theme on this blog is the idea that we need some notion of self-causation in order to answer Leibniz's famous question: "Why is there something rather than nothing?" If we define "reality" as the totality of what exists (including past and future existence), then by definition nothing exists outside of reality (not even "nothing"). If we then presuppose the Principle of Sufficient Reason – that there is a sufficient reason for every fact, including the fact that reality exists – then it follows that the reason for reality's existence must lie within reality itself, since there is nothing outside it. And since we generally call the reason why something exists the cause of that something, we must conclude that reality has to be self-causing. In this post I want to investigate the possibility of self-causation in relation to time. 


Can reality bootstrap itself into existence?
1.1 Physicalism and the fig-leaf conception of self-causation
Before going more deeply into the topic of this post, let me first say a few words about the controversial concept of self-causation, which is bound to raise eyebrows. Let me just note that by "self-causation" I mean roughly the same as what contemporary thinkers mean by "explanatory self-subsumption" (Robert Nozick), "self-explanation" (Nicholas Rescher), "cosmic bootstrapping" (Peter Atkins), and "self-excitation / self-synthesis" (John Wheeler). All these thinkers agree with the point made above: that since there is nothing outside of reality as a whole, the reason for its existence must lie within itself, such that reality must ultimately be understood as self-producing. So why don't these thinkers just use the term "self-causation", which is after all the traditional term of art for what is meant here? Premodern and early modern philosophers, from Plotinus to Spinoza, had no qualms in speaking of God as being self-caused (causa sui). So what has changed in the meantime?

What has changed, of course, is the rise of physicalism as the dominant ontology of the modern age, due to the huge experimental successes of mathematical physics and the victory of Neo-Darwinism. As a result, the concept of causation has
become virtually synonymous with "physical causation". And if physical causation is the only form of causation around, then clearly self-causation doesn't make much sense (or does it? see the discussion below about retrocausation in quantum physics). Contemporary thinkers have become so imbibed with physicalism as the dominant ontology that they consciously or unconsciously – even if they explicitly reject physicalism! – adopt the physicalist ban on self-causation and use fig leaf notions in its stead, such as "explanatory self-subsumption", "self-explanation", "cosmic bootstrapping", etc.

At the same time, however, we should note that physicalism is currently going through a deepening crisis, mainly brought on by the troublesome phenomenon of consciousness which refuses complete reduction to a physicalist framework. This crisis of physicalism means that the concept of self-causation becomes somewhat less of taboo: it gets a second chance.
(See e.g. philosopher John Leslie who, as a Neoplatonizing Spinozist, is quite happy to invoke self-causation.) The crucial role of consciousness in bringing on this crisis makes one wonder if perhaps consciousness holds the key to understanding the self-causation of reality... But this is something I will discuss further in my next post.

1.2 Self-causation and the problem of time
In this post I will focus on a somewhat more specialized topic: the possibility of self-causation in relation to time. As a process in time, self-causation is clearly impossible. As I will argue below, the self-causing entity would either have to precede itself in time or instantaneously emerge in time from out of nowhere – two possibilities which are equally absurd. So if we need self-causation in order to explain reality's existence, then it seems we must be dealing with timeless self-causation. This, of course, was one of the reasons why the philosophical tradition from Plotinus to Hegel conceived of God (i.e. "the One", "Substance" or "the Absolute") as existing outside of time; for otherwise God's self-causation would become unintelligible.

On the other hand, however, quantum physics appears to upset this conclusion, because in quantum physics it does seem possible for causality to work backwards in time. I am, of course, referring to the mysterious retrocausality displayed in delayed choice experiments, where an act of observation can collapse the wave function of a quantum state not only in the present but also backwards in time, altering the state's past. This retrocausality, seemingly allowed by quantum physics, has been used by the theoretical physicist John Archibald Wheeler (1911-2008) to explain how reality has brought itself into existence. On Wheeler's hypothesis of the self-observing universe, present-day and future observers retro-actively collapse the wave function of the universe from the big bang onwards, thereby facilitating their own – as well as the universe's – evolution. On Wheeler's scenario, therefore, the self-causation of reality seems to be possible in time after all. In other words: Wheeler seems to contradict the claim that self-causation is only possible as a timeless process.

But is this really the case? In fact not, as I will show in the final section of this post. On Wheeler's account of the universe, time (at least as experienced by us ) becomes an illusion, and the self-causation of the universe turns out to be a timeless affair, a closed loop between past and future within the "block universe" which itself exists outside of time as a purely mathematical structure in 'Plato's Heaven'. Thus even on a Wheelerian approach to reality's self-causation, the latter remains a timeless process.

2. Time and the (im)possibility of self-causation
It seems prima facie clear that self-causation is impossible in time, and that the self-causation required to explain reality's existence must therefore be a timeless process. After all, on our normal, intuitive understanding of time and causality, only two kinds of temporal relation can obtain between a cause x and an effect y: either x is earlier than y, or x and y occur simultaneously. The latter happens in instantaneous causation, as when a locomotive starts pulling a train with perfect mechanical rigidity: the motion of the former instantaneously causes the motion of the latter. Instantaneous causation is a controversial concept, a
lthough some philosophers (notably Kant, Richard Taylor, Myles Brand) have reckoned with its possibility. Be that as it may, it is clear that the concept of self-causation is problematic in either case, both when x precedes y and when x and y occur simultaneously.

Kant thought instantaneous
causation was possible
Starting with the first case (the cause preceding the effect), it is clear that self-causation would require that the cause precedes in time its own existence, which would be absurd. The self-causing entity would literally have to travel back in time in order to effectuate its own existence. It seems we can safely dismiss this as impossible (pace quantum retrocausation). And the situation is not much better when we allow instantaneous causation in time. Admittedly, with instantaneous causation the self-causing entity would not have to travel back in time to cause itself, so in that sense the notion of self-causation becomes less problematic. Still, however, a lot of absurdity remains. For with instantaneous self-causation in time, there would first have to be a time when the self-causing entity did not yet exist, and then suddenly it would instantaneously cause itself to exist. Thus the self-causing entity would magically pop into existence out of nothing, like a 'hiccup from the void'. It seems clear that this fantastic scenario violates the principle that from nothing only nothing can come (ex nihilo nihil fit). Self-causation, then, seems impossible in time. But we need self-causation to explain reality's existence. Therefore we must postulate a timelessly existing self-causing cause of reality.

2.2 Platonic existence and the 'something-ness' of time
Some people argue that since self-causation is impossible in time, self-causation must be impossible per se. But then they falsely presuppose that all existence is temporal, thus forgetting two things. Firstly, they forget the possibility of Platonic existence: the non-spatiotemporal existence of ideal objects, paradigmatically mathematics. Thus the fact that self-causation is impossible in time leaves open the possibility that the self-causing cause of reality exists in 'Platonic Heaven' (and this raises the question whether the self-causation of reality could be mathematical in nature).

Secondly, they forget that time itself is something. Time – like space, with which it is intimately connected, as relativity shows – is an entity of sorts, a 'thing' with various properties (e.g. one-dimensionality, directedness, dilatability). Time, in other words, belongs to the 'something' we try to explain when we ask Leibniz's question: "Why is there something rather than nothing?" Since time obviously does not explain its own existence, it must be explained by something else, ultimately by the self-causing cause of all reality. But, as we have seen, self-causation is impossible in time. Thus time itself already presupposes timeless self-causation.

3. The issue of retrocausation in quantum physics
Above I argued that self-causation is "obviously" impossible in time, both in the case of instantaneous causation, where cause x and effect y occur simultaneously, and in the more normal case where x precedes y. But is this correct? How about retrocausality in quantum physics? There, apparently, an act of observation can collapse the wave function of a quantum state not only in the present but also backwards in time, altering the state's past. Doesn't this enable us to make sense of self-causation as a temporal process? The proposal of the self-observing universe, advanced by the physicist Wheeler, might be interpreted in this way. So does Wheeler give the lie to the claim that the self-causation of reality can only take place outside of time? I will finish this post with discussing this question.

John A. Wheeler (1911-2008)
Before discussing Wheeler's proposal in more detail, however, let us first become clearer about how quantum physics creates room for the notion of retrocausation. As already noted, in quantum physics it seems possible for an act of observation to collapse the wave function of a quantum state not only in the present but also backwards in time. That, at least, is what the famous delayed choice experiment seems to show. The delayed choice experiment was originally devised by Wheeler as a thought experiment in the late seventies and early eighties of the previous century, because back then the technology was not yet sufficient in order to realize this experiment in practice. Due to technological process, however, the experiment did become practically possible around 2006. The most rigorous version of the experiment was not done until 2007 by a research team led by French physicist Alain Aspect. The outcome of the experiment was surprising and precisely as originally predicted by Wheeler: observation is able to collapse wave functions in the past and thus to work retro-actively.

3.1 The delayed choice experiment
The delayed choice experiment can be understood as a variation on the classic double-slit experiment which demonstrates the particle / wave duality of quantum states and the curious involvement of the observer in determining which aspect of this duality comes to the fore. In the double-slit experiment, light from a point source falls on a screen with two slits in it; the light bounces off from the screen, except for the two slits through which some of the light passes, thus creating an image on a second screen. The image appears in the form of bright and dark vertical bands (interference fringes) which demonstrate the wave nature of light. The curious role of the observer in quantum mechanics then becomes manifest when the experimenter deliberately looks to see through which slit the photons pass, for in that case no interference pattern emerges and the wave-like nature of light is lost. The act of observation collapses the wave-function of the light and turns it into a stream of point-like particles.

The double-slit experiment
Thus far the double-slit experiment 'merely' demonstrates the wave / particle duality of light and the weird role of the observer in quantum mechanics. But it gets even weirder when the double-slit experiment is extended into a delayed choice experiment. Here the choice to observe through which slit the photons pass is delayed until the light has already passed through both slits and is just about to create an interference pattern on the second screen. Although the light has already passed unobserved through the slits, and therefore as a wave, the choice to observe nevertheless collapses the wave function and prevents the interference pattern from emerging. Again the act of observation has turned the light into a stream of point-like particles. But now a mystery arises: how is this possible given the fact that the light had already passed unobserved through both slits and therefore as a wave? That this is indeed the case follows from the fact that if the choice to observe had not been made, then the interference pattern would have emerged and the wave-like nature of light would have manifested itself. The only possible conclusion seems to be that the delayed choice to observe affects the nature of the light backwards in time, undoing its earlier wave-like nature and turning it into a stream of discrete particles!

3.2 But is this retrocausality?
To repeat: the light originally went through both slits as a wave, but the delayed choice then forced the light 'to change its mind' and 'retrace its steps', now no longer moving as a wave through both slits simultaneously but as a stream of discrete particles going through just one slit at a time. Let us also repeat the crucial point that this is not just quantum theory. The retro-active influence of observation on past quantum states has been demonstrated in real experiments. But is this retrocausality? This question is a hotly debated one. But the debate seems to be a largely verbal one, since it all depends on how you define "causality". In normal cases of causation, some information and energy is transferred from cause to effect. But no such transference takes place in the delayed choice experiments: from the collapse of the wave function in the past no information can be obtained about the future act of observation responsible for this collapse; likewise no energy is transferred from the future act of observation to the quantum state in the past. Thus it has been concluded that causality plays no role in the effect demonstrated by the delayed choice experiment.

But, as said, all this is to a large extent just semantics. Is the transfer of information / energy really essential to the concept of causation? Well, that's a matter of definition, isn't it? And therefore it is arbitrary up to a point. (Remember that the crisis of physicalism forces us to broaden our definition of causality anyway...) If we define "x causes y" broadly as x is the reason why y exists, as we did above in the introduction (a definition that seems reasonable), then clearly quantum physics allows retrocausation, since in the delayed choice experiment the present observation of a quantum state is the reason for the existence of its wave function collapse in the past. Thus, on a very broad definition of causation, quantum physics does allow retrocausation.

Wheeler's U diagram of
the self-observing universe
4. Wheeler's self-observing universe
But does this quantum retrocausation allow us to make sense of the self-causation of reality? The physicist Wheeler certainly thought so. He wanted to know how contemporary physics could explain the self-creation of reality. "How come existence?", Wheeler asked in his own truncated version of Leibniz's question (Wheeler 1999: 310). As we did above, Wheeler argued that, since there is nothing outside of reality as a whole, the latter must have a way of bringing itself into existence, through some kind of closed causal loop: "Existence is not a globe supported by an elephant, supported by a turtle, supported by yet another turtle, and so on. In other words, no infinite regress... To endlessness no alternative is evident but loop [...], such a loop as this: Physics gives rise to observer-participancy; observer-participancy gives rise to information; information gives rise to physics." (Idem: 313-4)

4.1 "How come the quantum?"
Wheeler looked in particular at quantum mechanics as allowing such a closed causal loop. In fact he explained the existence and nature of quantum reality ("How come the quantum?") by arguing that it is the universe's means for self-creation: "The strange necessity of the quantum as we see it everywhere in the scheme of physics comes from the requirement that – via observer-participancy – the Universe should have a way to come into being." (Wheeler 1983: 206) On Wheeler's account, then, the classical universe – i.e. the universe whose wave function has been collapsed – brings itself into existence by evolving the very observers whose acts of observation retro-actively collapse that wave function: "Beginning with the big bang, the universe expands and cools. After eons of dynamic development it gave rise to observership. Acts of observer-participancy ‒ via the mechanism of the delayed-choice experiment ‒ in turn gave tangible "reality" to the universe not only now but back to the beginning." (Wheeler 1983: 209) To illustrate this idea, Wheeler came up with the U diagram of the universe as "self-excited circuit": "Starting small (thin U at upper right), it grows (loop of U) and in time gives rise (upper left) to observer-participancy – which in turn imparts "tangible reality" [...] to even the earliest days of the universe." (Wheeler 1983: 209)

4.2 "It from Bit"
It should be stressed, however, that this appeal to quantum retrocausation on a cosmic scale forms only one half of Wheeler's hypothesis of the self-observing universe. As noted above, quantum retrocausation can only explain the classical universe, i.e. the universe whose wave function has been collapsed. This still leaves unexplained the universe at the quantum level, i.e. the universal wave function and the Schrödinger equation which describes its evolution. Where do they come from? If Wheeler's idea of the self-observing universe is to answer Leibniz's question, then Wheeler must also explain their existence. In order to do this, Wheeler left quantum theory behind and generalized his idea by making critical use of information theory. Wheeler argued – as one of the first – that physical reality ultimately consists of bits of information, a point of view encapsulated by his famous dictum "It from Bit". On this view, physical reality exists only for the observers who pose the yes-no questions to which the bits are the answers. As Wheeler puts it: "It from bit. Otherwise put, every it – every particle, every field of force, even the spacetime continuum itself – derives its function, its meaning, its very existence entirely – even if in some contexts indirectly – from the apparatus-elicited answers to yes or no questions, binary choices, bits." (Wheeler 1999: 310-11) Since the observers posing the yes-no questions are part of the very same information space that emerges through their questions, we should conclude that on Wheeler's account these observers ultimately bring themselves along with all of reality into existence.

Is reality nothing but information?
This is how Wheeler explains the existence of the universal wave function and the Schrödinger equation which describes its evolution: they emerge as special substructures in the information space created by the posers of the yes-no questions. Once the universal wave function exists in information space, and the evolution of its myriad superposed states is dictated by the Schrödinger equation, we find in one of its superposed branches the biological evolution of intelligent observers. These observers then retroactively collapse the universal wave function, resulting in their possible universe becoming the universe – the tangible, classical universe we observe around us. In this way concrete reality bootstraps itself into existence out of the abstract information space created by the observers who pose the yes-no questions – observers who, remember, are themselves part of that concrete reality.

On Wheeler's scenario, then, the universe must have emerged in such a way that conscious observers exist within it, since it is only in relation to them that the universe can exist. This also explains why according to Wheeler the information space contained the Schrödinger equation: because the latter facilitates the evolution of a universe containing intelligent observers. Wheeler pointed out that this is one way to explain the remarkable role played by the Anthropic Principle in cosmology. According to Wheeler, the bio-friendliness of the universe is just what one should expect for a participatory universe; he therefore spoke of the "Participatory Anthropic Principle" (PAP).

5. The timelessness of self-causation on Wheeler's scenario
Clearly Wheeler's hypothesis of the self-observing universe is indeed just that: a hypothesis – or rather, as Wheeler himself humbly admitted, an "idea for an idea". It is by no means yet a well-established scientific theory. This holds in particular for the information-theoretic side of Wheeler's hypothesis: the idea that posers of yes-no questions bring themselves into existence by creating the very information space in which they exist. This idea, clearly, is wildly speculative and incredibly vague. At least with the quantum-theoretical side of Wheeler's hypothesis we have some kind of theoretical and experimental underpinning (respectively, quantum mechanics and the delayed choice experiment). But even here we have no empirical evidence whatsoever for the claim that observers now and in the future retro-actively collapse the wave function of the universe all the way back to the big bang. Wheeler's hypothesis of the self-observing universe is therefore to a large extent pure speculation (as he himself was the first to admit). Nevertheless, the fact that this hypothesis presents a distinct scientific possibility, worthy of further investigation, is acknowledged by many contemporary philosophers and scientists. We should therefore take it seriously. So let us ask: is Wheeler's scenario at odds with the claim that self-causation is impossible in time?

5.1 The paradoxes of retrocausation as a temporal process
At first sight, this does seem to be the case, particularly in light of the quantum-theoretic side of Wheeler's scenario. As we have seen, Wheeler speculates that observers in the present and future retro-actively collapse the wave function of the past universe all the way back to the big bang, thereby facilitating their own and the classical universe's evolution. Thus the arrow of (self-)causation clearly points backwards in time. However, on closer inspection it becomes obvious that Wheeler does not describe the self-causation of the universe as a temporal process at all. Rather, on Wheeler's account, time – at least as experienced by us – becomes an illusion, and the self-causation of the universe turns out to be a timeless affair, a closed loop between past and future within the whole of spacetime which itself exists outside of time (the so-called "block universe"; see below). To see why this should be so, note first of all that the paradoxes surrounding self-causation as a temporal process still stand. On the intuitive conception of time (i.e. time as we experience it), only the present is fully real: the past exists no longer and the future exists not yet. On this intuitive conception, the only thing that fully exists is the "flowing now", this paradoxical limit which separates past from future and constantly moves forward, turning the future into the past. On this conception of time, self-causation by means of retrocausation is absurd: the self-causing entity would literally have to exist before it existed, it would have to travel backwards in time to cause its own existence. But how is this possible if only the present is real and both past and future are inexistent?

5.2 Retrocausation only possible in the "block universe"
In order for self-causation by means of retrocausation to be possible, therefore, this intuitive time must be an illusion. Only if past and future exist together does it make sense to see the future as having a causal effect on the past. That is: only if the "arrow of time" is an illusion (or at least a superficial phenomenon that does not characterize ultimate reality) is it possible for the arrow of causation to point in both directions, i.e. from the past to the future as well as from the future to the past. This "unreality of time" is a familiar view in physics, known as the "block universe". The block universe is a four-dimensional spacetime which represents all the places and all the times that ever have existed and will exist together as a single unchanging entity. There is no essential difference between the past and the future, because there is no present time defined to separate them; they cannot be distinguished from each other, so there is no meaningful present. 


Without an objective present, however, time cannot be said to flow in any real sense: the passage of time must be an illusion. The universe just is and contains the whole of spacetime. Only on such a picture of the universe, where past and future are equally real, is it possible for the future to have some kind of causal effect on the past. Only with a block universe, therefore, does retrocausation make sense. But as we have seen, the delayed choice experiment demonstrates the reality of a form of retrocausation (namely, a present observation collapsing a quantum states' wave function in the past). Thus we must conclude that the delayed choice experiment also demonstrates the unreality of intuitive time and the correctness of the block universe.  
 
5.3 The absence of time in the Wheeler-De Witt equation
That this is also Wheeler's own opinion becomes apparent when we take into account the fact that he is one of the co-inventers of the famous Wheeler-De Witt equation, which attempts to combine mathematically the ideas of quantum mechanics and general relativity. As is well-known, the parameter of time is conspicuously absent in general relativity. As such it constitutes the classic argument for the block universe. Since the Wheeler-De Witt equation attempts to combine general relativity with quantum mechanics, it imports this timelessness from general relativity into quantum theory. In a way, therefore, the Wheeler-De Witt equation is simply the universal wave function without the time parameter: it describes a timeless superposition of quantum states for the whole of spacetime. As such, the Wheeler-De Witt equation is one of the purest examples of the block universe in physics.

On Wheeler's account, then, the self-causation of the universe turns out to be a timeless affair, a closed loop between past and future within the block universe described by the Wheeler-De Witt equation. This timelessness of reality's self-causation, as conceived by Wheeler, is further confirmed by the information-theoretic side of his hypothesis of the self-observing universe. As we have seen, Wheeler speculates that reality starts of as an abstract information space created by the observers who pose the yes-no questions to which the bits are the answers – observers who are themselves inhabitants of that information space! But an information space is an abstract mathematical construct, existing timelessly in Plato's heaven. Thus, for Wheeler, the self-causation of reality comes down to a closed loop within a timeless mathematical structure.

References
-Wheeler, John A. (1983), "Law Without Law", in: J.A. Wheeler & W.H. Zurek (eds.), Quantum Theory and Measurement, pp. 182-213. Princeton University Press.
-Wheeler, John A. (1990), "Information, physics, quantum: The search for links", in: W.H. Zurek (ed), Complexity, Entropy, and the Physics of Information. Redwood City, California: Addison-Wesley.

No comments:

Post a Comment