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{"id":6411,"date":"2017-06-18T16:22:38","date_gmt":"2017-06-18T23:22:38","guid":{"rendered":"http:\/\/www.wall.org\/~aron\/blog\/?p=6411"},"modified":"2017-06-21T13:04:36","modified_gmt":"2017-06-21T20:04:36","slug":"interpreting-the-quantum-world-ii-what-does-it-mean","status":"publish","type":"post","link":"https:\/\/www.wall.org\/~aron\/blog\/interpreting-the-quantum-world-ii-what-does-it-mean\/","title":{"rendered":"Interpreting the Quantum World II: What Does It Mean?"},"content":{"rendered":"<p><strong>In <\/strong><a href=\"http:\/\/www.wall.org\/~aron\/blog\/interpreting-the-quantum-world-i-measurement-non-locality\/\">the first installment<\/a> of this series, we immersed ourselves in the quantum realm that lies beneath our everyday experience and discovered a universe that bears little resemblance to it. Instead of the solid, unambiguously well-behaved objects we\u2019re familiar with, we encountered a <em>unitary<\/em> framework ($$\\hat U$$) in which everything (including our own bodies!) is ultimately made of ethereal \u201cwaves of probability\u201d wandering through immense configuration spaces along paths deterministically guided by well-formed differential equations and boundary conditions, and acquiring the properties we find in them as they rattle through a random pinball machine of collisions with \u201cmeasurement\u201d events ($$\\hat M$$). This is all very elegant\u2014even beautiful\u2026 <em>but what does it mean?<\/em> When my fianc\u00e9 falls asleep in my arms, her tender touch, the warmth of her breath on my neck, and the fragrance of her hair hardly seem like mere probabilities being kicked around by dice-playing measurements. The refreshing drink of sparkling citrus water I just took doesn\u2019t taste like one either. What is it that gives fire to this ethereal quantum realm? How does the Lord God breathe life into our probabilistic dust and bring about the classical universe of our daily lives (Gen. 2:7)? We finished by distilling our search for answers down to three fundamental dilemmas:<\/p>\n<p>1) \u00a0What is this thing we call a wave function? Is it ontologically real, or just mathematical scaffolding we use to make sense of things we don\u2019t yet understand?<\/p>\n<p>2) \u00a0What really happens when a deterministic, well-behaved $$\\hat U$$ evolution of the universe runs headlong into a seemingly abrupt, non-deterministic $$\\hat M$$ event? How do we get them to share their toys and play nicely with each other?<\/p>\n<p>3) \u00a0If counterfactual definiteness is an ill-formed concept, why are we always left with only one experienced outcome? Why don\u2019t we <em>experience<\/em> entangled realities?<\/p>\n<p>Physicists, philosophers, and theologians have been tearing their hair out over these questions for almost a century, and numerous interpretations have been suggested (more than you might imagine!). Most attempt to deal with 2), and from there, back out answers to 1) and 3). All deserve their own series of posts, so let me apologize in advance for only having time to do a fly-by of the more important ones here. In what follows I\u2019ll give an overview of the most viable, and well-received interpretations to date, and finish with my own take on all of it. So, without further ado, here are our final contestants\u2026<\/p>\n<p><span style=\"text-decoration: underline\">Copenhagen<\/span><\/p>\n<p>This is the traditionally accepted answer given by the founding fathers of QM. According to Copenhagen, the cutting edge of reality is in $$\\hat M$$. The world we exist in is contained entirely in our observations. Per the Born Rule, these are irreducibly probabilistic and non-local,and result in classically describable measurements. The wave function and its unitary history $$\\hat U$$ are mere mathematical artifices we use to describe the conditions under which such observations are made, and have no ontic reality of their own. In this sense, Copenhagen has been called a <em>subjective, <\/em>or <em>epistemic\u00a0<\/em>interpretation because it makes our observations the measure of all things (pun intended :-) ). Although few physicists and philosophers would agree, some of the more radical takes on it have gone as far as to suggest that consciousness is the ultimate source of the reality we observe. Even so, few Copenhagen advocates believe the world doesn\u2019t exist apart from us. The tree that falls in the woods does exist whether we\u2019re there to see and hear it or not. What they would argue is that <a href=\"https:\/\/en.wikipedia.org\/wiki\/Counterfactual_definiteness\"><em>counterfactuals<\/em><\/a> regarding the tree\u2019s properties and those of whatever caused it to fall don\u2019t instantiate if we don\u2019t observe them. If no one sees the tree fall or experiences any downstream consequence of its having done so, then the question of whether it has or not is irreducibly ambiguous and we\u2019re free to make assumptions about it.<\/p>\n<p>Several objections to Copenhagen have been raised. The idea that ontic reality resides entirely in non-local, phenomenologically discrete \u201ccollapse\u201d events that are immune to further unpacking is unsatisfying. Science is supposed to <em>explain<\/em> things, not explain them <em>away<\/em>. It\u2019s also difficult to see how irreducibly random $$\\hat M$$ events could be prepared by a rational, deterministic $$\\hat U$$ evolution if the wave function has no ontic existence of its own. To many physicists, philosophers, and theologians, this is less a statement about the nature or reality than the universe\u2019s way of telling us that we haven\u2019t turned over enough stones yet, and may not even be on the right path.<\/p>\n<p>For their part, Copenhagen advocates rightly point out that this is precisely what our experiments tell us\u2014no more, no less. If the formalism correctly predicts experimental outcomes, they say, metaphysical questions like these are beside the point, if not flat-out ill-formed, and our physics and philosophy should be strictly instrumentalist\u2014a stance for which physicist David Mermin coined the phrase \u201cshut up and calculate&#8221;.<\/p>\n<p><span style=\"text-decoration: underline\">Many Worlds<\/span><\/p>\n<p>One response to Copenhagen is that if $$\\hat U$$ seems to be as rational and deterministic as the very real classical physics of our experience, perhaps that\u2019s because it is. But that raises another set of questions. As we\u2019ve seen, nothing about $$\\hat U$$ allows us to grant special status to any of the eigenstates associated with observable operators. If not, then we\u2019re left with no reason other than statistical probability to consider any one outcome of an $$\\hat M$$ event to be any more privileged than another. Counterfactuals to what we <em>don\u2019t<\/em> observe should have the same ontic status as those we do. If so, then why do our experiments seem to result in discrete irreducibly random and non-local \u201ccollapse\u201d events with only one outcome?<\/p>\n<p>According to the Many Worlds (MWI) interpretation, they don\u2019t. The universe is comprised of one ontically real, and deterministic wave function described by $$\\hat U$$ that\u2019s local (in the sense of being free of \u201cspooky-action-at-a-distance\u201d) and there\u2019s no need for hidden variables to explain $$\\hat M$$ events. What we experience as wave function \u201ccollapse\u201d is a result of various parts of this universal wave function separating from each other as they evolve. Entangled states within it will be entangled while their superposed components remain in phase with each other. If\/when they interact with some larger environment within it, they eventually lose their coherence with respect to each other and evolve to a state where they can be described by the wave functions of the individual states. When this happens, the entanglement has (for lack of a better term) \u201cbled out\u201d to a larger portion of the wave function containing the previous entanglement, and the environment it interacted with, and states are said to have <a href=\"http:\/\/www.wall.org\/~aron\/blog\/quantum-mechanics-ii-decoherence-states\/\"><em>decohered<\/em><\/a><em>.<\/em> Thus, the wave function of the universe never actually collapses anywhere\u2014it just continues to decohere into the separate histories of previously entangles states that continue with their own $$\\hat U$$ histories, never interacting with each other again. As parts of the same universal wave function, all are <em>equally real, <\/em>and questions of counterfactual definiteness are ill-formed.<\/p>\n<p>The advantages of MWI speak for themselves. From a formal standpoint, a universe grounded on $$\\hat U$$ and decoherence that\u2019s every bit as rational and well-behaved as the classical mechanics it replaced, certainly has advantages over one based on subjective hand grenade $$\\hat M$$ events. It deals nicely with the relativity-violating non-locality and irreducible indeterminacy that plague Copenhagen as well. And for reasons I won\u2019t get into here, it also lends itself nicely to quantum field theory, and Feynmann path integral (\u201csum over histories\u201d) methods that have proven to be very powerful.<\/p>\n<p>But its disadvantages speak just as loudly. For starters, it\u2019s not at all clear that decoherence can fully account for what we directly experience as wave function <em>collapse<\/em>. Nor is it clear how MWI can make sense of the extremely well-established Born Rule. Does decoherence always lead to separate well-defined histories for every eigenstate associated with every observable that in one way or another participates in the evolution of $$\\hat U$$? If not, then what meaning can be assigned to probabilities when some states decohere and others don\u2019t. Even if it does, what reasons do we have for expecting that it should obey probabilistic constraints?<\/p>\n<p>And of course, we haven\u2019t even gotten to the real elephant in the room yet\u2014the fact that we\u2019re also being asked to believe in the existence of <em>an infinite number of entirely separate universes that we can neither observe, nor verify,<\/em> even though the strict formalism of QM doesn\u2019t require us to. Physics aside, for those of us who are theists this raises a veritable hornet\u2019s nest of theological issues. As a Christian, what am I to make of the cross and God\u2019s redemptive plan for us in a sandstorm of universes where literally everything happens somewhere to infinite copies of us all? It\u2019s worth noting that some prominent Christian physicists like Don Page embrace MWI, and see in it God\u2019s plan to ultimately gather all of us to Him via one history or another, so that eventually \u201cevery knee shall bow, and every tongue confess, and give praise to God (Rom. 14:11). While I understand where they\u2019re coming from, and the belief that God will gather us all to Himself some day is certainly appealing, this strikes me as contrived and poised for Occam\u2019s razor.<\/p>\n<p>In the end, despite its advantages, and with all due respect to Hawking and its other proponents, I don\u2019t accept MWI because, to put it bluntly, it\u2019s more than merely unnecessary\u2014it\u2019s <em>bat-shit crazy<\/em>. According to MWI there is, <em>quite literally,<\/em> a world out there somewhere in which I, Scott Church (peace be upon me), am a cross-dressing, goat worshipping, tantric massage therapist, with 12\u201d Frederick\u2019s of Hollywood stiletto heels (none of that uppity Victoria\u2019s Secret stuff for me!), and D-cup breast implants\u2026<\/p>\n<p>Folks, I am here to tell you\u2026 there isn\u2019t enough vodka or LSD anywhere on this lush, verdant earth to make that believable! Whatever else may be said about this veil of tears we call Life, rest assured that indeterministic hand grenade $$\\hat M$$ events and \u201cspooky action at a distance\u201d are infinitely easier to take seriously. :D<\/p>\n<p><span style=\"text-decoration: underline\">De Broglie\u2013Bohm<\/span><\/p>\n<p>Bat-shit crazy aside, another approach would be to try separating $$\\hat U$$ and $$\\hat M$$ from each other completely. If they aren\u2019t playing together at all, we don\u2019t have to worry about whether they\u2019ll share their toys. Without pressing that analogy too far, this is the basic idea behind the De Broglie-Bohm interpretation (DBB).<\/p>\n<p>According to DBB, particles <em>do<\/em> have definite locations and momentums, and these are subject to hidden variables. $$\\hat U$$ is real and deterministic, and per the Schr\u00f6dinger equation governs the evolution of a <em>guiding,<\/em> or <em>pilot<\/em> wave function that exists separate from particles themselves. This wave function is non-local and does not collapse. For lack of a better word, particles \u201csurf\u201d on it, and $$\\hat M$$ events acting on them are governed by the local hidden variables. In our non-local singlet example from Part I, the two electrons <em>were<\/em> sent off with spin-state box lunches. All of this results in a formalism like that of classical thermodynamics, but with predictions that look much like the Copenhagen interpretation. In DBB the Born Rule is an added hypothesis rather than a consequence of the inherent wave nature of particles. There is no particle\/wave duality issue of course because particles and the wave function remain separate, and Bell\u2019s inequalities are accounted for by the non-locality of the latter.<\/p>\n<p>There\u2019s a naturalness to DBB that resolves much of the \u201cweirdness\u201d that has plagued other interpretations of QM. But it hasn\u2019t been well-received. The non-locality of its pilot wave $$\\hat U$$ still raises the whole \u201cspooky action at a distance\u201d issue that physicists and philosophers alike are fundamentally averse to. Separating $$\\hat U$$ from $$\\hat M$$ and duct-taping them together with hidden variables adds layers of complexity not present in other interpretations, and runs afoul of all the issues raised by the <a href=\"https:\/\/en.wikipedia.org\/wiki\/Kochen%E2%80%93Specker_theorem\">Kochen-Specker Theorem<\/a>. We have to wonder whether our good friend Occam and his trusty razor shouldn\u2019t be invited to this party. And like MWI, it\u2019s brutally deterministic, and as such, subject to all the philosophical and theological nightmares that go along with that, not to mention our direct existential experience as freely choosing people. Even so, for a variety of reasons (including theories of a \u201csub-quantum realm\u201d where hidden variables can also hide from Kochen-Specker) it\u2019s enjoying a bit of a revival and does have its rightful place among the contenders.<\/p>\n<p><span style=\"text-decoration: underline\">Consistent Histories<\/span><\/p>\n<p>As we\u2019ve seen, the biggest challenge QM presents is getting $$\\hat U$$ and $$\\hat M$$ to play together nicely. Most interpretations try to achieve this by denying the ontological reality of one, and somehow rolling it up into the other. What if we denied the individual reality of <em>both,<\/em> and rolled them up into a larger ontic reality described by an expanded QM formalism? Loosely speaking, Consistent Histories (or <em>Decoherent Histories<\/em>) attempts to do this by generalizing Copenhagen to a quantum cosmology framework in which the universe evolves along the most internally consistent and probable histories available to it.<\/p>\n<p>Like Copenhagen, CH asserts that the wave function is just a mathematical construct that has no ontic reality of its own. Where it parts company is in its assertion that $$\\hat U$$ represents the wave function of the entire universe, and it never collapses. What we refer to as \u201ccollapse\u201d occurs when some parts of it decohere with respect to larger parts leading, it is said, to macroscopically irreversible outcomes that are subject to the ordinary additive rules of classical probability. In CH, the potential outcomes of any observation (and thus, the possible histories the universe might follow) are classified by how <em>homogeneous<\/em> and <em>consistent<\/em> they are. This, it\u2019s said, is what makes some of them more probable than others. A homogeneous history is one that can be described by a <em>unique<\/em> temporal sequence of single-outcome propositions, such as, \u201cI woke up\u201d &gt; \u201cI got out of bed\u201d &gt; \u201cI showered\u201d \u2026 Those that cannot be, such as ones that include statements like \u201cI walked to the grocery store <em>or<\/em> drove there\u201d are not. These events can be represented by a <em>projection operator<\/em> $$\\hat P$$ from which histories can be built, and the more internally consistent they are (per criteria contained in a class operator $$\\hat P$$), the more probable they are.<\/p>\n<p>Thus, in CH $$\\hat M$$ is not a fundamental QM concept. The evolution of the universe is described by a mathematical construct, $$\\hat U$$ that can be interpreted as decohering into the most internally consistent (and therefore probable) homogeneous histories possible for it to. The paths these histories take give us a framework in which some sets of classical questions can be meaningfully asked, and other can\u2019t. Returning to our <a href=\"http:\/\/www.wall.org\/~aron\/blog\/interpreting-the-quantum-world-i-measurement-non-locality\/\">electron singlet example<\/a>, CH advocates would maintain that the wave function wasn\u2019t entangled in any real physical sense. Rather, there are two internally consistent histories for the prepared electrons that could have emerged a spin measurement: Down\/Up, and Up\/Down. Down\/Up\/Up\/Down isn\u2019t a meaningful state, so it\u2019s meaningless to say that the universe was \u201cin\u201d it. Rather, when the entire state of us\/laboratory\/observation is accounted for, we will find that the universe followed the history that was most consistent for that. There is no need to discriminate between observer and observed. Decoherence is enough to account for the whole history, so $$\\hat M$$ is a superfluous construct.<\/p>\n<p>CH advocates claim that it offers a cleaner, and less paradoxical interpretation of QM and classical effects than its competitors, and a logical framework for discriminating boundaries between classical and quantum phenomena. But it too has its issues. It\u2019s not at all clear that decoherence is as macroscopically irreversible as it\u2019s claimed to be, or that by itself it can fully account for our experience of $$\\hat M$$. It also requires additional projection and class operator constructs not required by other interpretations, and these cannot be formulated to any degree practical enough to yield a complete theory.<\/p>\n<p><span style=\"text-decoration: underline\">Objective Collapse Theories<\/span><\/p>\n<p>Of course, we could just make our peace with $$\\hat U$$ and $$\\hat M$$. Objective collapse, or quantum mechanical spontaneous localization (QMSL) models maintain that the universe reflects both because the wave function <em>is<\/em> ontologically real, and \u201cmeasurements\u201d (perhaps interactions is a better term here) really <em>do<\/em> collapse it. According to QMSL theories, the wave function is non-local, but collapses locally in a random manner (hence, the \u201cspontaneous localization\u201d), or when some physical threshold is crossed. Either way, observers play no special role in the collapse itself. There are several variations on this theme. The <a href=\"https:\/\/en.wikipedia.org\/wiki\/Ghirardi%E2%80%93Rimini%E2%80%93Weber_theory\">Ghirardi\u2013Rimini\u2013Weber theory<\/a> for instance, emphasizes random collapse of the wave function to highly probably stable states. Roger Penrose has proposed another theory based on energy thresholds. Particles have mass-energy that, per general relativity, will make tiny &#8220;dents&#8221; in the fabric of space-time. According to Penrose, in the entangled states of their wave function these will superpose as well, and there will be an associated energy difference that entangled states can only sustain up to a critical threshold energy difference (which he theorizes to be on the order of one Planck mass). When they decohere to a point where this threshold is exceeded, the wave function collapses per the Born Rule in the usual manner (Penrose, 2016).<\/p>\n<p>For our purposes, this interpretation pretty much speaks for itself and so do its advantages. Its disadvantages lie chiefly in how we understand and formally handle the collapse itself. For instance, it\u2019s not clear this can be done mathematically without violating conservation of energy or bringing new, as-yet undiscovered physics to the game. In the QMSL theories that have been presented to date, if energy is conserved the collapse doesn\u2019t happen completely, and we end up with left-over \u201ctails\u201d in the final wave function state that are difficult to make sense of with respect to the Born Rule. It has also proven difficult to render the collapse compliant with special relativity without creating divergences in probability densities (in other words, blowing up the wave function). Various QMSL theories have handled issues like this in differing ways, some more successfully than others, and research in his area continues. But to date, none of the theories on the table offers a slam-dunk.<\/p>\n<p>The other problem QMSL theories face is a lack of experimental verification. Random collapse theories like Ghirardi\u2013Rimini\u2013Weber could be verified if the spontaneous collapse of a single particle could be detected. But these are thought to be extremely rare, and to date, none have been observed. However, several tests for QMSL theories have been proposed (e.g. Marshall et al., 2003; Pepper et al., 2012; or Weaver et al., 2016 to name a few), and with luck, we\u2019ll know more about them in the next decade or so (Penrose, 2016).<\/p>\n<p><span style=\"text-decoration: underline\">Conclusion<\/span><\/p>\n<p>There are <a href=\"https:\/\/en.wikipedia.org\/wiki\/Interpretations_of_quantum_mechanics\">many other interpretations of QM<\/a>, some of which are more far-fetched than others. But the ones we\u2019ve covered today are arguably the most viable, and as such, the most researched. As we\u2019ve seen, all have their strengths and weaknesses. Personally, I lean toward Objective Collapse scenarios. It\u2019s hard to believe that something as well-constrained and mathematically coherent as $$\\hat U$$ isn\u2019t ontologically real. Especially when the alternative bedrock reality being offered is $$\\hat M$$, which is haphazard and difficult to separate from our own subjective consciousness (the latter in particular smacks of solipsism, which has never been a very compelling, or widely-accepted point of view). Of the competing alternatives that would agree about $$\\hat U$$, MWI is probably the strongest contender. But for reasons that by now should be disturbingly clear, it\u2019s far easier for me to accept a non-local wave function collapse than its take on $$\\hat M$$. Call me unscientific if you will, but ivory towers alone will never be enough to convince me that I have a cross-dressing, goat-worshipping, voluptuous doppelganger somewhere that no one can ever observe. Other interpretations don\u2019t fare much better. Most complicate matters unnecessarily and\/or deal with the collapse in ways that render $$\\hat M$$ deterministic.<\/p>\n<p>It\u2019s been said that if your only tool is a hammer, eventually everything is going to look like a nail. It seems to me that such interpretations are compelling to many because they\u2019re <em>tidy<\/em>. Physicists and philosophers <em>adore<\/em> tidy! Simple, deterministic models with well-defined differential equations and boundary conditions give them a fulcrum point where they feel safe, and from which they think they can move the world. This is fine for what it\u2019s worth of course. Few would dispute the successes our tidy, well-formed theories have given us. But if the history of science has taught us anything, it\u2019s that nature isn\u2019t as enamored with tidiness as we are. Virtually all our investigations of QM tell us that indeterminism cannot be fully exorcized from $$\\hat M$$, and the term \u201ccollapse\u201d fits it perfectly. Outside the laboratory, everything we know about the world tells us we are conscious beings made in the image of our Creator. We are self-aware, intentional, and capable of making free choices\u2014none of which is consistent with tidy determinism. Anyone who disputes that is welcome to come up with a differential equation and a self-contained set of data and boundary conditions that <em>required<\/em> me to decide on a breakfast sandwich rather than oatmeal this morning\u2026 and then collect their Nobel and Templeton prizes and retire to the lecture circuit.<\/p>\n<p>The bottom line is that we live in a universe that presents us with $$\\hat U$$ and $$\\hat M$$. As far as I\u2019m concerned, if the shoe fits I see no reason not to wear it. Yes, QMSL theories have their issues. But compared to other interpretations, its problems are formalistic ones of the sort I suspect will be dealt with when we\u2019re closer to a viable theory of quantum gravity. When we as students are ready, our teacher will come. Until then, as Einstein once said, the world should be made as simple as possible, <em>but no simpler.<\/em><\/p>\n<p>When I was in graduate school my thesis advisor used to say that when people can\u2019t agree on the answer to some question one of two things is always true: Either there isn\u2019t enough evidence to answer the question definitively, or we\u2019re asking the wrong question. Perhaps many of our QM headaches have proven as stubborn as they are because we\u2019re doing exactly that\u2026 asking the wrong questions. One possible case in point\u2026 physicists have traditionally considered $$\\hat U$$ to be sacrosanct\u2014the one thing that above all others, only the worst apostates would ever dare to question. Atheist physicist Sean Carroll has gone so far as to claim that it proves the universe is past-eternal, and God couldn\u2019t have created it! [There are <a href=\"http:\/\/www.wall.org\/~aron\/blog\/did-the-universe-begin-iv-quantum-eternity-theorem\/\">numerous problems with that<\/a> of course, but they\u2019re beyond the scope of this discussion.] However, Roger Penrose is now arguing that we need to do exactly that (fortunately, he\u2019s respected enough in the physics community that he can get away with such challenges to orthodoxy without being dismissed as a crank or heretic). He suggests that if we started with the equivalence principle of general relativity instead, we could formulate a QMSL theory of $$\\hat U$$ and $$\\hat M$$ that would resolve many, if not most QM paradoxes, and this is the basis for his gravitationally-based QMSL theory discussed above. Like its competitors, Penrose\u2019s proposal has challenges of its own, not the least of which are the difficulties that have been encountered in producing a rigorous formulation $$\\hat M$$ along these lines. But of everything I\u2019ve seen so far, I find it to be particularly promising!<\/p>\n<p>But then again, maybe the deepest secrets of the universe are beyond us. Isaac Newton once said,<\/p>\n<blockquote><p>\u201cI do not know what I may appear to the world, but to myself I seem to have been only like a boy playing on the seashore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me.\u201d<\/p><\/blockquote>\n<p>As scientists, we press on, collecting our shiny pebbles and shells on the shore of the great ocean with humility and reverence as he did. But it would be the height of hubris for us to presume that there\u2019s no limit to how much of it we can wrap our minds around before we have any idea what\u2019s beyond the horizon. As J. B. S. Haldane once said,<\/p>\n<blockquote><p>&#8220;My own suspicion is that the Universe is not only queerer than we suppose, but queerer than we <em>can<\/em> suppose.&#8221; (Haldane, 1928)<\/p><\/blockquote>\n<p>Who knows? Perhaps he was right. God has chosen to reveal many of His thoughts to us. In His infinite grace, I imagine He\u2019ll open our eyes to many more. But He certainly isn\u2019t under any obligation to reveal them all, nor do we have any reason to presume that we could handle it if He did. But of course, only time will tell.<\/p>\n<p>One final thing\u2026 Astute readers may have noticed one big elephant in the room that I\u2019ve danced around, but not really addressed yet\u2026 <em>relativity<\/em>. Position, momentum, energy, and time have been a big part of our discussion today\u2026 and they\u2019re all inertial frame dependent, and our formal treatment of $$\\hat U$$ and $$\\hat M$$ must account for that. There are versions of the Schr\u00f6dinger equation that do this\u2014most notably the Dirac and Klein Gordon equations. Both however are <em>semi-classical<\/em> equations\u2014that is, they dress up the traditional Schr\u00f6dinger equation in a relativistic evening gown and matching handbag, but without an invitation to the relativity ball. For a ticket to the ball, we need to take QM to the next level\u2026 <em>quantum field theory<\/em>.<\/p>\n<p>But these are topics for another day, and I\u2019ve rambled enough already\u2026 so once again, stay tuned!\u00a0<strong><\/strong><\/p>\n<p>&nbsp;<\/p>\n<p><strong><span style=\"text-decoration: underline\">References<\/span><\/strong><\/p>\n<p>Haldane, J. B. S. (1928). Possible worlds: And other papers. Harper &amp; Bros.; 1st edition (1928). Available online at <a href=\"https:\/\/www.amazon.com\/s\/ref=nb_sb_noss?url=search-alias%3Daps&amp;field-keywords=Possible+worlds%3A+And+other+papers\">www.amazon.com\/s\/ref=nb_sb_noss?url=search-alias%3Daps&amp;field-keywords=Possible+worlds%3A+And+other+papers<\/a>. Accessed May 17, 2017.<\/p>\n<p>Marshall, W., Simon, C., Penrose, R., &amp; Bouwmeester, D. (2003). Towards quantum superpositions of a mirror. Physical Review Letters, <strong>91 <\/strong>(13). Available online at <a href=\"https:\/\/journals.aps.org\/prl\/abstract\/10.1103\/PhysRevLett.91.130401\">journals.aps.org\/prl\/abstract\/10.1103\/PhysRevLett.91.130401<\/a>. Accessed June 9, 2017.<\/p>\n<p>Pepper, B., Ghobadi, R., Jeffrey, E., Simon, C., &amp; Bouwmeester, D. (2012). Optomechanical superpositions via nested interferometry. Physical review letters, <strong>109<\/strong> (2). Available online at <a href=\"https:\/\/journals.aps.org\/prl\/abstract\/10.1103\/PhysRevLett.109.023601\">journals.aps.org\/prl\/abstract\/10.1103\/PhysRevLett.109.023601<\/a>. Accessed June 9, 2017.<\/p>\n<p>Penrose, R. (2016).\u00a0Fashion, faith, and fantasy in the new physics of the universe. <em>Princeton University Press,<\/em> Sept. 13, 2016. ISBN:\u00a00691178534; ASIN: B01AMPQTRU. Available online at <a href=\"https:\/\/www.amazon.com\/Fashion-Faith-Fantasy-Physics-Universe-ebook\/dp\/B01AMPQTRU\/ref=sr_1_1?ie=UTF8&amp;qid=1495054176&amp;sr=8-1&amp;keywords=penrose\">www.amazon.com\/Fashion-Faith-Fantasy-Physics-Universe-ebook\/dp\/B01AMPQTRU\/ref=sr_1_1?ie=UTF8&amp;qid=1495054176&amp;sr=8-1&amp;keywords=penrose<\/a>. Accessed May 16, 2017.<\/p>\n<p>Weaver, M. J., Pepper, B., Luna, F., Buters, F. M., Eerkens, H. J., Welker, G., &#8230; &amp; Bouwmeester, D. (2016). Nested trampoline resonators for optomechanics. Applied Physics Letters, <strong>108 <\/strong>(3). Available online at <a href=\"http:\/\/aip.scitation.org\/doi\/abs\/10.1063\/1.4939828\">aip.scitation.org\/doi\/abs\/10.1063\/1.4939828<\/a>. Accessed June 9, 2017.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>In the first installment of this series, we immersed ourselves in the quantum realm that lies beneath our everyday experience and discovered a universe that bears little resemblance to it. Instead of the solid, unambiguously well-behaved objects we\u2019re familiar with, &hellip; <a href=\"https:\/\/www.wall.org\/~aron\/blog\/interpreting-the-quantum-world-ii-what-does-it-mean\/\">Continue reading <span class=\"meta-nav\">&rarr;<\/span><\/a><\/p>\n","protected":false},"author":4,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[17,3],"tags":[],"class_list":["post-6411","post","type-post","status-publish","format-standard","hentry","category-metaphysics","category-physics"],"_links":{"self":[{"href":"https:\/\/www.wall.org\/~aron\/blog\/wp-json\/wp\/v2\/posts\/6411","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.wall.org\/~aron\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.wall.org\/~aron\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.wall.org\/~aron\/blog\/wp-json\/wp\/v2\/users\/4"}],"replies":[{"embeddable":true,"href":"https:\/\/www.wall.org\/~aron\/blog\/wp-json\/wp\/v2\/comments?post=6411"}],"version-history":[{"count":35,"href":"https:\/\/www.wall.org\/~aron\/blog\/wp-json\/wp\/v2\/posts\/6411\/revisions"}],"predecessor-version":[{"id":6450,"href":"https:\/\/www.wall.org\/~aron\/blog\/wp-json\/wp\/v2\/posts\/6411\/revisions\/6450"}],"wp:attachment":[{"href":"https:\/\/www.wall.org\/~aron\/blog\/wp-json\/wp\/v2\/media?parent=6411"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.wall.org\/~aron\/blog\/wp-json\/wp\/v2\/categories?post=6411"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.wall.org\/~aron\/blog\/wp-json\/wp\/v2\/tags?post=6411"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}