Seite 5: Uncommon Sense

Though our eyes and arms effortlessly predict the liftability of a rock, the action of a lever or the flight of an arrow, mechanics was deeply mysterious to those overly thoughtful ancients who pondered why stones fell, smoke rose, or the moon sailed by unperturbably. Newtonian mechanics revolutionized science by precisely formalizing the intelligence of eye and muscle, giving the Victorian era a viscerally satisfying mental grip on the physical world. In the twentieth century, this common sense approach was gradually extended to biology and psychology. Meanwhile, physics moved beyond common sense. It had to be reworked because, it turned out, light did not fit the Newtonian framework.

In a one-two blow, intuitive notions of space, time and reality were shattered, first by relativity, where space and time vary with perspective, then more seriously by quantum mechanics, where events lose their objective existence. Though correctly describing everyday mechanics as well as such important features of the world as the stability of atoms and the finiteness of heat radiation, the new theories were so offensive to common sense, in concept and consequences, that they inspire persistent misunderstandings and bitter attacks to this day. The insult will get worse.

General relativity, superbly accurate at large scales and masses has not yet been reconciled with quantum mechanics, itself superbly accurate at tiny scales and huge energy concentrations. Incomplete attempts to unite them in a single theory hint at possibilities that exceed even their individual strangeness.

The strangeness begins just beyond the edges of the everyday world. When an object travels from one place to another, common sense insists that it does so on a definite, unique, trajectory. Not so, says quantum mechanics. A particle in unobserved transit goes every possible way simultaneously until it is observed again. The indefiniteness of the trajectory manifests itself in the kind of interference pattern created by waves that spread and recombine, adding where they meet in step, and canceling where out of step. A photon, a neutron or even a whole atom sent to a row of detectors via a screen with two slits, will always miss certain of the detectors, because the wave of its possible positions, having passed through both slits, cancels there.

Experimental results forced the quantum view of the world on reluctant physicists piecemeal during the first quarter of the twentieth century, and it still has ragged edges. The theory is neat in describing the unobserved, where, for instance, a particle spreads like a wave. It fails to define or pinpoint the act of observation, when the "wave function" collapses, and the particle appears in exactly one of its possible places, with a probability given by the intensity of its wave there. It may be when the detector responds, or when the instrumentation connected to the detector registers, or when the experimenter notes the instrument readings, or even when the world reads about the result in the physics journals!

In principle, if not practice, the point of collapse can be pinpointed: before collapse, possibilities interfere like waves, creating interference patterns, after collapse, possibilities simply add in a common sense way. Very small objects, like neutrons traveling through slits, make visible interference patterns. Unfortunately, large, messy objects like particle detectors or observing physicists, would produce interference patterns much, much finer than atoms, indistinguishable from common sense probability distributions because they are so easily blurred by thermal jiggling.

Because, for humans, common sense is easier than quantum theory, workaday physicists take collapse to happen as soon as possible, for instance when a particle first encounters its detector. But this early collapse view can have peculiar implications. It implies that the wave function can be repeatedly collapsed and uncollapsed in subtle experiments that allow measurements to be undone through deliberate cancellation at the experimenter's whim.

This wave function yo-yo is eliminated if one assumes the collapse happens further upstream, where there is no hope of undoing the measurement, for instance when the result registers in the experimenter's consciousness. This thinking has led some philosophically inclined physicists to suggest that it is consciousness itself that is the mysterious wave-collapsing process that quantum theory fails to identify.

Consciousness-collapse, which predicts the world behaves quantum mechanically until some human observes it, at which point it becomes common sensical, eliminates philosophical problems for laboratory physicists. It creates problems for cosmologists, whose scope is the entire universe, for it implies the world is peppered with collapsed wave functions surrounding individual conscious observers. These collapses have no theory and cannot be experimentally quantified, so make it impossible to set up equations for the universe overall. Instead, cosmologists assume the entire universe behaves as a giant wave function that evolves according to quantum theory, and never collapses. But how can a universal wave function in which every particle forever spreads like a wave, be reconciled with individual experiences of finding particles in particular positions?

In a 1957 PhD thesis, Hugh Everett addressed that question. Given a universally evolving wave function, where the configuration of a measuring apparatus, no less than of a particle, spreads wavelike through its space of possibilities, he showed that if two instruments recorded the same event, the overall wave function had maximum magnitude for situations where the records concurred, and canceled where they disagreed. Thus, a peak in the combined wave represents a possibility where, for instance, an instrument, an experimenter's memory and the marks in a notebook agree on where a particle alighted - eminent common sense. But the whole wave function contains many such peaks, each representing a consensus on a different outcome.

Everett had shown that quantum mechanics, stripped of problematical collapsing wave functions, still predicts common sense worlds - only many, many of them, all slightly different. The no collapse view became known as the many-worlds interpretation of quantum mechanics. Its implication that each observation branched the world into something like 10^100 separate experiences seemed so extravagantly insulting to common sense that it was passionately rejected by many. Though cosmologists worked with the universal wave function, its connection to the everyday world was ignored for another twenty years.

Recent subtle experiments confirming the most mind-bending predictions of quantum mechanics lifted many-worlds' stock relative to traditional interpretations, which require influences to leap wildly across time and space to explain the observed correlations. The theoretical trail pioneered by Everett is becoming traveled, and extended. Since the late 1980s James Hartle and Murray Gell-Mann have investigated its underlying notions of measurement and probability.

Everett had demonstrated that the conventional rules for collapsing the wave function to measurement outcome probabilities from outside a system were consistent with what would be reported by (each version of) the uncollapsed observer inside, thus removing the requirement for an outside or a collapse, and raising our consciousness to existence of many worlds. He made no attempt to show how those peculiar measurement rules arose in the first place. Gell-Mann and Hartle are asking this difficult question. They are far from a final resolution, but their work so far shows just how special - or illusory - the common sense world really is.

Hartle and Gell-Mann note that if we were to try to observe and remember events at the finest possible detail - around 10^-30 centimeters, far smaller than anything reachable today - the interference of all possible worlds would present a seething chaos with no permanent structures, no quiet place to store memories, effectively no consistent time. At a coarser viewing scale - 10^-15 centimeters, the submicroscopic world touched by today's high-energy physics - much of the chaos goes unobserved, and multiple worlds merge together, canceling the wildest possibilities, leaving those where particles can exhibit a consistent existence and motion, if still jaggedly unpredictable, through a vacuum that boils with ephemeral virtual energy. Everyday objects have the smooth, predictable trajectories of common sense only because our dim senses are coarser still, registering nothing finer than 10^-5 centimeters. At scales larger than the everyday (or the Hartle Gell-Mann analysis), the events we consider interesting are blurred to invisibility, and the universe is increasingly boring and predictable. At the largest possible scale, the universe's matter is canceled by the negative energy in its gravitational fields (which strengthen while releasing energy, as matter falls together), and in sum there is nothing at all.

No complete theory yet explains our existence and experiences, but there are hints. Tiny universes simulated in today's computers are often characterized by adjustable rules governing the interaction of neighboring regions. If the interactions are made very weak, the simulations quickly freeze to bland uniformity, if they are very strong, the simulated space may seethe intensely in a chaotic boil. Between the extremes is a narrow edge of chaos with enough action to form interesting structures, and enough peace to let them persist and interact. Often such borderline universes can contain structures that use stored information to construct other things, including perfect or imperfect copies of themselves, thus supporting Darwinian evolution of complexity. If physics itself offers a spectrum of interaction intensities, it is no surprise that we find ourselves operating at the liquid boundary of chaos, for we could not function, nor have evolved, in motionless ice nor formless fire.

The odd thing about the Gell-Mann Hartle spectrum is that it is not some external knob that controls the interaction intensity, but varying interpretations of a single underlying reality made by observers who are part of the interpretation. It is, in fact, the same kind of self-interpretation loop we encountered when considering observers inside simulations. We are who we are, in the world we experience, because we see ourselves that way. There are almost certainly other observers in exactly the same regions of the wave function who see things entirely differently, to whom we are simply meaningless noise.

The similarity between Everett's many worlds and the philosophical possible worlds may become stronger yet. In many worlds quantum mechanics, physical constants, among other things, have fixed values. Gravity, in objects like black holes, loosens the rules, and a full quantum theory of gravity may predict possible worlds far exceeding Everett's range - and who knows what potent subtleties lie even further on? It may turn out, as we claw our way out through onion layers of interpretation, that physics places fewer and fewer constraints on the nature of things. The regularities we observe may be merely a self-reflection: we must perceive the world as compatible with our own existence - with a strong arrow of time, dependable probabilities, where complexity can evolve and persist, where experiences can accumulate in reliable memories, and the results of actions are predictable. Our mind children, able to manipulate their own substance and structure at the finest levels, will probably greatly transcend our narrow notions of what is.