Tuesday, June 2, 2020

Consciousness, Quantum Mechanics, and Pseudoscience

The study of consciousness is not currently “fashionable” in the physics community, and the notion that there might be any relationship between consciousness and quantum mechanics and/or relativity truly infuriates some physicists.  For instance, the hypothesis that consciousness causes collapse (“CCC”) of the quantum mechanical wave function is now considered fringy by many; a physicist who seriously considers it (or even mentions it without a deprecatory scowl) risks professional expulsion and even branding as a quack.

In 2011, two researchers took an unprovoked stab at the CCC hypothesis in this paper.  There is a fascinating experiment called the “delayed choice quantum eraser,” in which information appears to be erased from the universe after a quantum interference experiment has been performed.  The details don’t matter.  The point is that the researchers interpret the quantum eraser experiment as providing an empirical falsification of the CCC hypothesis.  They don’t hide their disdain for the suggestion that QM and consciousness may have a relationship.

The problem is: their paper is pseudoscientific shit.  They first make a massive logical mistake that, despite the authors’ contempt for philosophy, would have been avoided had they taken a philosophy class in logic.  They follow up that mistake with an even bigger blunder in their understanding of the foundations of quantum mechanics.  Essentially, they assert that the failure of a wave function to collapse always results in a visible interference pattern, which is just patently false.  They clearly fail to falsify the CCC hypothesis.  (For the record, I think the CCC hypothesis is likely false, but I am reasonably certain that it has not yet been falsified.)

Sure, there’s lots of pseudoscience out there, so why am I picking on this particular paper?  Because it was published in Annalen der Physik, the same journal in which Einstein published his groundbreaking papers on special relativity and the photoelectric effect (among others), and because it’s been cited by more than two dozen publications so far (often to attack the CCC hypothesis), only one of which actually refutes it.

What’s even more irritating is that the paper’s glaring errors could easily have been caught by a competent journal referee who had read the paper skeptically.  If the paper’s conclusion had been in support of the CCC hypothesis, you can bet that it would have been meticulously and critically analyzed before publication, assuming it was considered for publication at all.  But when referees already agree with a paper’s conclusion, they may be less interested in the logical steps taken to arrive at that conclusion.  A paper that comes to the correct conclusion via incorrect reasoning is still incorrect.  A scientist that rejects correct reasoning because it results in an unfashionable or unpopular conclusion is not a scientist.

Here is a preprint of my rebuttal to their paper.  Since it is intended to be a scholarly article, I am much nicer there than I’ve been here.

Monday, May 25, 2020

Speaking the Wrong Language

In my last post, I pointed out a fundamental problem in a particular paper – although the same problem appears in lots of papers: specifically, that there is no way to test whether an object is in a quantum superposition.  I feel like this is a point that many physicists and philosophers of physics overlook, so to be sure, I went ahead and posted the question on a few online physics forums, such as this one.  Here’s basically the response I got:
Every state that is an eigenstate of a first observable is obviously in a superposition of eigenstates of some second observable that does not commute with the first.  Therefore: of course you can test whether an object is in a quantum superposition.  Also, you are an idiot.
OK, so they didn’t actually say that last part, but it was clearly implied.  If you don’t speak the language of quantum mechanics, let me rephrase.  Quantum mechanics tells us that there are certain features (“observables”) of a system that cannot be measured/known/observed at the same time, thus the order of measurement matters.  For example, position and momentum are two such observables, so measuring the position and then the momentum will inevitably give different results from measuring the momentum and then the position – that is, the position and momentum operators do not commute.  And because they don’t commute, an object in a particular position (that is, “in an eigenstate of the position operator”) does not have a particular momentum, which is to say that it is in a superposition of all possible momenta.  In other words, the above response basically boils down to this: quantum mechanically, every state is a superposition.

Fine.  The problem is that this response has nothing to do with the question I was asking.  I ended up having to edit my question to ask whether any single test could distinguish between a “pure” quantum superposition versus a mixed state (which is a probabilistic mixture), and even then the responses weren't all that useful.

This is why I think the big fundamental problems in physics will probably not be solved by insiders.  They speak a very limited language that, by its nature, limits a speaker’s ability to discover and understand the flaws in the system it describes.  My original question, I thought, was relatively clear: is it actually possible, as Mari et al. suggest, to receive information by measuring (in a single test) whether an object is in a macroscopic quantum superposition?  But when the knee-jerk response of several intelligent quantum physicists is to discuss the noncommutability of quantum observables and come to the irrelevant (and, frankly, condescending) point that all states are superpositions and therefore of course we can test whether an object is in superposition – well, it makes me wonder whether they actually understand, at a fundamental level, what a quantum superposition is.  I feel like there’s a huge disconnect between the language and mathematics of physics, and the actual observable world that physics tries to describe. 

Tuesday, May 19, 2020

It is Impossible to Measure a Quantum Superposition

In a previous post, I discussed how and to what extent gravity might prevent the existence of macroscopic quantum superpositions.  There has been surprisingly little discussion of this possibility and there is still debate on whether gravity is quantized and whether gravitational fields are, themselves, capable of existing in quantum superpositions.

Today I came across a paper, "Experiments testing macroscopic quantum superpositions must be slow," by Mari et al., which proposes and analyzes a thought experiment involving a first mass mA placed in a position superposition in Alice’s lab, the mass mA producing a gravitational field that potentially affects a test mass mB in Bob’s lab (separated from Alice’s lab by a distance R), depending on whether or not Bob turns on a detector.  The article concludes that special relativity puts lower limits on the amount of time necessary to determine whether an object is in a superposition of two macroscopically distinct locations.

The paper seems to have several important problems, none of which have been pointed out in papers that cite it, notably this paper.  For example, its calculation of the entanglement time TB assumes that correlation of the location of test mass mB with the gravitational field of mass mA occurs when the change in position δx of the test mass mB exceeds its quantum uncertainty Δx, which seems like a reasonable argument – except that they failed to include the increase in quantum uncertainty due to dispersion.  (This is particularly problematic where they let Δx be the Planck length!)  Another problem is their proposed experiment in Section IV: Alice is supposed to apply a spin-dependent force on the mass mA which results in different quantum states, depending on whether or not Bob turned on the detector, but both quantum states correlate to mass mA located at L (instead of R).  The problem is that by the time she has applied the force, Bob’s test mass mB has presumably already correlated to the gravitational field produced by Alice’s mass mA located at L or R, but how could that happen before Alice applied the force that caused the mass mA to be located at L?

But the biggest problem with the paper is not in their determination of the time necessary to determine whether an object is in a superposition of two macroscopically distinct locations.  No – the bigger problem is that, as far as I understand, there is no way to determine whether an object is in a superposition at all! 

Wait, what?  Obviously quantum superpositions exist.

Yes, but a superposition is determined by doing an interference experiment on a bunch of “identically prepared” objects (or particles or masses or whatever).  The idea is that if we see an interference pattern emerge (e.g., the existence of light and dark fringes), then we can infer that the individual objects were in coherent superpositions.  However, detection of a single object never produces a pattern, so we can’t infer whether or not it was in a superposition.  Further, the outcome of every interference experiment on a superposition state, if analyzed one detection at a time, will be consistent with that object not having been in superposition.  A single trial can confirm that an object was not in a superposition (such as if we detect a blip in a dark fringe area), but no single trial can confirm that the object was in a superposition.  Moreover, even if a pattern does slowly emerge after many trials, every pattern produced by a finite number of trials – and remember that infinity does not exist in the physical world – is always a possible random outcome of measuring objects that are not in a superposition.  We can never confirm the existence of a superposition, but lots and lots of trials can certainly increase our confidence.

In other words, if I’m right, then every measurement that Alice makes (in the Mari paper) will be consistent with Bob's having turned the detector on (and decohered the field) -- thus, no information is sent!  No violation of special relativity!  No problem!

Look, I could be wrong.  I’ve been studying the foundations of quantum mechanics independently for a couple of years now, and very, very few references point out that there’s no way to determine if any particular object is in a quantum superposition, which is also why it’s taken me so long to figure it out.  So either I’m wrong about this, or there’s some major industry-wide crazy-making going on in the physics community that leads to all kinds of wacky conclusions and paradoxes... no wonder quantum mechanics is so confusing!

Is there a way to test whether a particular object is in a coherent superposition?  If so, how?  If not, then why do so few discussions of quantum superpositions mention this?

Update to this post here

Why Special Relativity Prevents Copying Conscious States

I was honored to be asked by Kenneth Augustyn to present to the 3rd Workshop on Biological Mentality on Jan. 6, 2020.  The talk was entitled, “Why Mind Uploading, Brain Copying, and Conscious Computers Are Impossible.”  While the talk addressed work in my earlier papers, it offers a clearer argument explaining why Special Relativity prevents the existence of physical copies of conscious states -- specifically, why two instances of physical copies of conscious states located at different points in spacetime, whether spacelike or timelike, would require either superluminal or backward causation.  I also show that because conscious states cannot be copied or repeated, consciousness cannot be algorithmic and cannot be created by a digital computer.  I mention some possible explanatory hypotheses, several of which are related to quantum mechanics, such as Quantum No-Cloning.  Finally, I touch on my related work of whether conscious states are history dependent.

This 36-minute video is probably my clearest video explanation so far as to why mind uploading and conscious computers are inconsistent with Special Relativity.

Friday, May 8, 2020

Into the Lion's Den

Two years ago, I sold my businesses and “retired” so that I could focus full-time on learning about, addressing, and attempting to solve some of the fundamental questions in physics and philosophy of mind... things like the physical nature of consciousness, whether we have free will, the measurement problem in quantum mechanics, etc.  What gave me the audacity to think I might be able to tackle these problems where so many have failed before?  Well, first, tackling a problem only requires desire.  I find these big-picture questions fascinating and looked forward to learning, analyzing, and at least trying.  But I did think I had a reasonable shot at actually solving some of these mysteries.  Why?

While I don’t (yet) have a degree in physics or philosophy, I do have an undergraduate and master’s degree in nuclear engineering as well as a law degree (which is certainly applicable to philosophical reasoning), and have taken lots of physics and philosophy classes along the way.  As an example, I’ve taken graduate-level quantum mechanics, or a course closely related or heavily dependent on QM, at UF, MIT, Princeton, and ECU, and even a fascinating course called Philosophy of Quantum Mechanics.  In other words, I’m no expert – and I plan to continue graduate studies in physics – but I certainly have more than a superficial understanding of physics.

It takes more than education to solve problems; it also takes creativity and a willingness to say or try things that others won’t.  As the sole inventor of 17 U.S. patents on a wide variety of inventions, from rocket engines to software to pumps to consumer products, I’ve always felt confident in my ability to solve problems creatively.  As for independence – let’s just say I’ve always been a maverick.  As an example, while in law school I realized that a loophole in American patent law allowed for the patenting of fictional storylines, so I published an article to that effect.  Over the next couple years, at least six law review articles were published specifically to argue that I was full of shit: great evidence that I was actually on to something!  (Since then the courts closed the loophole.)  I’m not trying to list my CV – just to explain my state of mind when I started this process.  I had plenty of free time, an independent spirit, a history of creativity in solving problems, and a strong and relevant educational foundation.  This gave me confidence that I was in a better position than most to actually solve an important riddle.  I also figured, perhaps naively, that the field of physics was one place where novel approaches, critical thinking, and objective analysis would be rewarded.

I jumped right in.  After extraordinary amounts of research and independent thought, I soon realized that special relativity would cause problems for copying or repeating conscious states.  I wrote my first paper on the topic; the most recent iteration is here.  Not long after that, I realized that QM would also, independently of relativity, cause problems for copying or repeating conscious states, and wrote my second paper; the most recent iteration is here.  In July, 2018, I sent my first paper to the British Journal for the Philosophy of Science; it was summarily rejected without comments or review.  Fuck them.  Over the next year and a half, I submitted it to four more journals, and despite getting close to publication with one, the paper was ultimately rejected by all.  Over the same period, I submitted my second paper to three journals and, again, despite getting close to publication with one, the paper was ultimately rejected.  What had gone wrong?  Was I in over my head? 

Regarding the first paper, the same objection kept coming up over and over: that copying the physical state of a person does not necessarily copy that person’s identity.  Without getting too technical, my argument was that whether or not a person’s identity depends on their underlying physical state, special relativity implied the same conclusion.  But no matter how I replied, the conversation always felt like this:
Them: “How do you know that copying a person’s physical state would copy their identity?” 
Me: “I don’t.  But if it does, then copying that state violates special relativity.  If it doesn’t, then there is nothing to copy.  Either way, we can’t copy a person’s identity.”
Them: “But wait.  First you need to show that copying a person’s physical state would copy their identity.”
Me: “No, I don’t.  Consider statements A and B.  If AàB, and also ¬AàB, then B is true, and we don’t need to figure out if A is true.”
Them: “Hold on.  How can you be so sure that statement A is true?...”
Me: [Banging head against wall]

It’s literally crazymaking.  No one seemed to have a problem with the physics or the implications of special relativity.  Instead, their problem almost always boiled down to the concept of identity and its relationship to physical reality.  I suspect that what’s happening is that people find a conclusion they’re uncomfortable with – such as “mind uploading is impossible” or “consciousness is not algorithmic” – and then work backward to find something they can argue with... and that something always happens to be some variation on “How do you know that statement A is true?”  I don’t know if it’s a case of intentional gaslighting or unintentional cognitive dissonance, but either way it took me a long time to finally rebuild my confidence, realize I’m not crazy, completely rewrite the paper to address the identity issue head-on, and submit it to a new journal.

Regarding the second paper, the referee of the third journal brought up what I believed, at the time, was a correct and fatal objection.  But by then, I had experienced 18 continuous months of essentially nothing but rejection, criticism, or being ignored (which is sometimes worse).  Prior to that, I’d spent so much of my life feeling confident about my ability to think clearly and rationally, to solve problems creatively, to analyze arguments skeptically, and to eventually arrive at correct conclusions.  So by the time I received that final rejection, I threw the paper aside and basically forgot about it – until about two weeks ago.  Somehow the human spirit can reawaken.  I took a look at the paper with fresh eyes, fully expecting to confirm the fatal error, but found exactly the opposite.  I (and the journal referee) had been wrong about my being wrong.  In other words, the error that had been pointed out, as it turns out, was not an error.  That isn’t to say that my reasoning and conclusions in the paper are ultimately correct – there could still be other errors – but the referee had been wrong.  What I argued in my second paper is original and it just may be right.  If so, its implications are important and potentially groundbreaking.  The paper needs to be rewritten, the physics tightened, and the arguments cleaned up: a project for another day.

As for now, here’s the problem I face.  On one hand, answers to some of the deepest and most important questions plaguing humanity for millennia are finally starting to become accessible via science, particularly physics.  On the other hand, it has become, for whatever reason, out of vogue in the physics community to research or even discuss these issues, which is odd for many reasons.  First, many of the giants of physics, even in modern history, routinely debated them, including Einstein, Bohr, Wigner, and Feynman.  Second, physics has itself produced several of these hard questions (like the QM measurement problem and the inconsistency between QM and general relativity).  But because physicists rarely talk about these big-picture and foundational questions, and because there’s essentially no funding to research them, the conversations are typically left to: a) self-made or retired mavericks who don’t need funding (e.g., Roger Penrose); b) writers who profit on popular viewpoints (e.g., Sean Carroll and Deepak Chopra); c) academic philosophers who may or may not (but typically don’t) have any formal training in physics; and d) crackpots, nutjobs, and wackadoodles.  And there are a LOT of wackadoodles; category d) might dwarf the others by a factor of 100, and occasionally even includes members of the other categories.  The Internet is teeming with “amateur physicists” with their own solutions to quantum gravity, theories about “quantum consciousness” (whatever the hell that is), yada yada.

I am in category a), but I understand, if on statistics alone, why I’d be assumed to be in category d).  The thing is, maybe I am a little crazy.  But the solutions to the big problems in physics, cosmology, and philosophy of mind are not going to come from tweaking the same old shit we’ve been tweaking for the past century.  They are going to require truly revolutionary ideas, and those ideas, when first proposed, WILL seem crazy.  I want to be openminded, diligent, and creative enough to explore the crazy, revolutionary ideas that ultimately lead to the correct solutions.  Still, the hardest challenge of all will be maintaining my confidence throughout the process.  Not only will I be continually discouraged by incorrect solutions, but I suspect that my journey will be somewhat lonely.

Blogger and theoretical physicist Sabine Hossenfelder points out that stagnation in physics is in large part due to a feedback mechanism in which those who pull the strings – journal editors, those who award grant funding, members of academic tenure committees, etc. – tend to reward what is most familiar to them and popular with their peers.  This has the effect of stifling innovation.  Her solution: “Stop rewarding scientists for working on what is popular with their colleagues.”  Lee Smolin made a similar point in his article, “Why No ‘New Einstein’?”  He says that the current system of academic promotion and publication has “the unintended side effect of putting people of unusual creativity and independence at a disadvantage.”   Despite the current publish-or-perish system that incentivizes scientists to do “superficial work that ignores hard problems,” the field of physics is actually “most often advanced by those who ignore established research programs to invent their own ideas and forge their own directions.”

In other words, even though I didn’t know it when I began this process two years ago, it was a foregone conclusion that my intention to independently and creatively attack some of the hard foundational problems in science would be met with contempt, condescension, and unresponsiveness.

I am planning to begin a master’s program in physics at NYU in the fall.  NYU has some of the world’s best (or at least most academically well regarded) faculty in the fields of cosmology, the foundations of physics, the philosophy of physics, and the philosophy of mind.  But I will be entering with eyes wide open: into the lion’s den.  I certainly hope some of the faculty will be legitimately interested in answering some of the big questions – and will be responsive to and encouraging of original approaches – but I won’t expect it.  Instead, I will enter with low expectations, understanding clearly that any progress I make in answering the big questions may be despite, not because of, the physics academy.  I will hope to remain guided by a burning curiosity, a passion to learn and understand, and a confidence in my abilities to think, analyze, and create.  Please wish me luck.

Wednesday, May 6, 2020

The Effect of Gravity in Preventing Macroscopic Quantum Superpositions

In a recent post, I posited that a quantum superposition just exists if and only if the facts of the universe are consistent with the superposition; i.e., a system described at time t by state |A> + |B> just means that there is no fact about whether the system is in state |A> or |B> at time t.  In other words, had the system been measured at time t in a basis that includes elements |A> and |B>, then either outcome A or outcome B would have been measured (with probabilities according to the Born rule), but since it was not measured, then information regarding whether the system was in state |A> or |B> did not exist at time t and no future measurement/observation/fact can contradict that fact.  The production of facts (or the happening of events) over time creates new information that reduces future possibilities.

Thinking about quantum mechanics in this way has helped me immensely in understanding and solving many of the various philosophical problems in QM.  To get feedback on it, I submitted a version of the explanation to an essay contest of the Foundational Questions Institute, entitled “Interpreting Quantum Mechanics and Predictability in Terms of Facts About the Universe,” and a preprint is also available here.

However, apparently this point of view is more revolutionary than I had originally thought.  The typical way to think about or describe a quantum superposition described by state |A> + |B> is that it “is kind of in state |A> and kind of in state |B>” or that it “is in both state |A> and state |B> simultaneously” or something like that.  But these descriptions are inaccurate, sloppy, and just plain wrong. 

For example, it is typical in QM to work with expectation values, such as the expectation of position <X>, which is found by taking a weighted average of an object’s position distribution (i.e., weighted by probability, which is the square of amplitude).  The problem arises when this is treated as something real as opposed to something simply mathematically useful for making predictions.  For instance, if a particle whose position expectation is <X> is actually measured/detected at a location X0 that is somewhere far away from <X>, then do we say there’s been a violation in conservation of energy if X0 and <X> are at different potentials?  Likewise if an object having momentum expectation <P> is measured having momentum P0, but <P>2/2m ≠ P02/2m.

The problem is that there was nothing real about the particle’s location when we calculated <X>.  If we were right that the particle was in a location superposition at time t, then there is no fact, nor will there ever be, about the particle’s location at time t, so there can’t be a violation of conservation of energy by detecting the particle at X0 at a later time if there is no fact about where the particle came from.

For instance, when Roger Penrose, whom I greatly admire, tried to analyze the effect of gravity on quantum state reduction, he postulated that the difference in gravitational self-energy (EΔ) between the spacetime geometries of a quantum superposition “in which one lump [of mass] is in two spatially displaced locations” produces an instability that results in a decay into one or the other of the spacetime eigenstates.  He even goes so far as to give a decay time T ≈ ℏ/EΔ, reminiscent of the quantum uncertainty principle.  The problem, as I see it, is that he treated the “two” lumps (in the superposition) as real, so real in fact that he requires taking into account “the gravitational interaction effects between the pair of lumps.”  What pair of lumps?!  There is only one lump!

But this (mis)understanding of QM seems to permeate the field.  So far, I have been unable to find my characterization of QM in the academic literature.  It certainly may be out there, but I feel comfortable in saying that nearly all characterizations of a quantum superposition treat it as if the terms represent something real.  For instance, in the classic Schrodinger’s Cat thought experiment (which is essentially the same as the Wigner’s Friend thought experiment), we are given a quantum state of the cat |Ψ> = |alive> + |dead>, which is a linear superposition of a state in which the cat is alive and one in which it is dead.  QM tells us that the likelihood of finding the cat in one state or another depends on the square of the amplitudes, which I’ve left out for simplicity.

So here’s the classic conundrum: before we look, is the cat dead or alive?  The answer: there is no fact about it being dead or alive until evidence exists (in the form of a correlation somewhere in the universe) that it is one or the other.  Until that information exists, there simply is no fact.  The real difficulty in this thought experiment, which almost no one points out, is the extreme difficulty (and likely impossibility) of creating state |Ψ> = |alive> + |dead> in the first place.  To do so requires that there is no evidence anywhere (beyond the cat itself, of course, which we assume is thermally isolated) of the cat’s being dead or alive.  Even a single photon bouncing off the cat – and keep in mind that the universe is inundated with radiation, such as CMB – would almost certainly provide evidence correlated to its being either dead or alive.

Getting back to Penrose’s paper, in making his argument about a superposition of spacetimes, he points out that “these two space-time geometries differ significantly from each other.”  But my question is this: how could such a superposition arise in the first place?  If I am right that a superposition exists if and only if the facts of the universe are consistent with the superposition, then what would it mean if there was a “significant difference” between two (or more) eigenstates?  If we say, “There would have been a significant difference had that difference been measured but it wasn’t actually measured,” then that does not justify Penrose’s treatment of the spacetime geometries as being actually significantly different.  But to say “There is a significant difference” is wrong because: by whose standards?  By what measure?  After all, if there is a measure (in the form of evidence anywhere in the universe) by which the spacetime geometries are different, then there could not have been a superposition!

The thing is – gravity may be weak (e.g., the electromagnetic attraction between a proton and electron in a hydrogen atom is something like 1040 greater than their gravitational attraction), but it is ubiquitous in the universe and always attractive.  So my question is this: wouldn’t gravity effectively prevent any macroscopic superposition?  To use Penrose’s example, imagine a macroscopic lump of matter near Earth that we are somehow able to perfectly isolate from the universe (already a ridiculous assumption) to allow it to enter a superposition of macroscopically distinct positions.  A lump creates a gravitational field that is tiny but – as far as we know – potentially affects everything in the universe.  If the gravitational field of the lump located at position A affects even a single particle differently than the field of the lump located at B, then the lump at one of these two positions will be correlated with the rest of the universe and a quantum superposition of the lump at position A and position B cannot exist.  Note that the speed of light is irrelevant here; if the lump’s gravity takes 20 years to affect the trajectory of a particle 20 light-years away, that correlation is enough to ensure that there could not have been a superposition at the time.  (This argument may be related to the production of gravity waves, which I know little about.)

Anyway, my point is that when Penrose discusses a superposition of spacetime geometries that “differ significantly from each other,” then wouldn’t significant differences correlate to measurable differences in effects, events, and/or interactions elsewhere (i.e., outside the isolated system)?  If so, such a superposition could never exist.  Which is to say, as soon as there is a fact in the universe that differentiates the two possibilities, they are no longer both possibilities and there is no superposition. 

I haven’t done the calculation yet, but I suspect that gravity would destroy a macroscopic superposition very quickly.  Interestingly, a group of researchers showed that relativistic time dilation at different heights on the Earth’s surface was enough to decohere a macroscopic quantum superposition pretty quickly.  They showed that an isolated gram-scale object in a superposition of locations vertically separated near Earth’s surface by 1mm would decohere in around a microsecond.  This implies that even a “perfectly isolated” Schrodinger’s Cat experiment could never even get off the ground if located anywhere near a planet; however it says little about performing such an experiment in deep space with flat spacetime curvature.  But even though the word “gravitational” appeared in its title, the article was really about time dilation.  So far, I haven’t found an article that deals with how the gravitational effects of a macroscopic object in different locations would correlate to measurable differences elsewhere in the universe, and how this would prevent macroscopic quantum superpositions.  If it were the case that an isolated system described by |dead> caused some correlated event different than an isolated system described by |alive>, then the superposition |Ψ> = |alive> + |dead> could not exist. 

Of course, the question is not really whether gravitational effects are relevant to the existence of quantum superpositions.  Of course they are.  The sun could not exist in a superposition of a state in which it is located at the center of our solar system and a state in which it is located a light-year away, as the gravitational differences between such states would be heavily correlated to measurable differences in other places in the universe.  (Obviously, other differences besides gravitational differences would decohere any potential superposition long before this point.)  The question is at what scale are gravitational effects relevant to the existence of quantum superpositions.  That may place an upper limit to the size of quantum superpositions and the applicability of QM.  (This whole notion that there is no limit, in principle, to the size of objects in interference experiments is driving me crazy, but I’ll save that rant for another time.)  If the answer happens to be such as to prevent any kind of Schrodinger’s Cat or Wigner’s Friend experiment anywhere in the universe, no matter how isolated, then we can finally stop being confused by (and hearing about) these thought experiments.

Before I spend time doing these calculations or trying to reinvent the wheel, it would be great to know if it’s already been done.  Do you know of any such calculation, article, or research?