I’ve posted a lot over the years on interpretations of quantum mechanics. My writing has tended to focus on comparing the big three: Copenhagen, pilot-wave, and many-worlds. But there are a lot of others. One that has been gaining converts among physicists and others is Carlo Rovelli’s relational quantum mechanics (RQM) interpretation. This is an interpretation that comes up enough in conversation that I’ve always wanted to learn more. So when Rovelli’s book on it was announced, I decided I needed to read it. But Helgoland: Making Sense of the Quantum Revolution took a while to be available in the US, at least in Kindle format. My preorder finally came through last week, so I spent the last few days going through it.
Rovelli is clear at the beginning of the book that this is a partisan work, and he’s not kidding, although this type of partisanship is common in books on quantum physics. This book is about his particular interpretation. He does discuss many of the other major interpretations: many-worlds, pilot-wave, QBism, and physical collapse theories, but he makes clear that his coverage is cursory, and mentions multiple times that the reader can skip these if they want. (I read them anyway, just to see how he’d treat them.)
In another move that I’m starting to see as too common in these types of books, Rovelli’s partisanship includes his description of historical scientists. He sees his interpretation as fitting squarely within the tradition started by Werner Heisenberg, and his descriptions of Heisenberg seem pretty reverent. His view of Erwin Schrödinger, on the other hand, seems hostile, both intellectually and personally. As many authors have done, he describes Schrödinger’s polyamorous lifestyle, but goes a bit further by implying that Schrödinger had pedophilic tendencies. In contrast, in his biographical remarks about Heisenberg, he downplays Heisenberg’s collaboration with the Nazis. (He does mention another scientist who likely didn’t receive a Nobel prize because of Nazi affiliations. Heisenberg had the good luck to receive his Nobel before the Nazis came to power.)
Anyway, Rovelli’s sees Heisenberg’s chief contribution as focusing on observables and then building a theory of the relations between those observables. In his view, Schrödinger’s focus on real waves was a distraction, and the Copenhagen team were right to interpret his wavefunction as a probabilistic mathematical mechanism, a move Schrödinger himself was never happy with. (Although he did grudgingly come to admit the practical benefits.)
The main role of an interpretation is to explain what happens during the measurement process. Quantum objects move like waves, until they’re measured, then they behave like particles. In the classic Copenhagen interpretation, this is usually referred to as the wavefunction collapse. In the strong version of Copenhagen, involving a physical collapse, this was seen as problematic by Albert Einstein, because it involves an instantaneous collapse across all of time and space, leading to nonlocal “spooky action at a distance”, an issue made particularly vivid by quantum entanglement. Weaker versions of Copenhagen only have an epistemic collapse, resembling QBism, and so don’t consider themselves to have this issue.
The big question with Copenhagen is, when does the collapse occur? Niels Bohr’s answer was interaction with macroscopic systems, such as lab equipment, implying that there were different rules for microscopic and macroscopic phenomena. However, no one has managed to find any threshold where a collapse happens. Over the decades, scientists have managed to observe quantum effects in ever larger collections of quantum particles, molecules, and even tiny macroscopic objects. It looks increasingly unlikely that there is any such threshold.
This doesn’t represent an issue for non-collapse interpretations such as pilot-wave or many-worlds, but it does for most collapse interpretations. RQM is a collapse interpretation, but its innovation is to make the collapse a relative event. In RQM, what causes the collapse is an interaction with another physical system. However, the collapse only happens relative to the system interacted with, not with any other system. In other words, a quantum particle can be in superposition relative to one physical system while being collapsed relative to another.
So, if two quantum particles interact, they collapse relative to each other. But to the rest of the world, they remain in a superposition, and the interaction has left them entangled in some fashion. It also pertains to a quantum computing circuit. For each particle in the circuit, once it receives interactions, the circuit has collapsed. However, for the outside world, until there are interactions with the environment, the circuit remains in a superposition of all its possible states.
Making the collapse relative solves the question of when it occurs. It occurs on any interaction, but only relative to the particles involved in that interaction. Similar to many-worlds, this interpretation sees the entire universe as being quantum in nature. But also like many-worlds, it has radical implications. Physical reality exists in the relations, and only in the relations. This by itself isn’t too radical. It’s compatible with the ontic version of structural realism that we recently discussed. But it also implies that properties of physical systems don’t exist for another system at all until the interaction.
This is highlighted when considering RQM’s claim to local dynamics. Consider a couple of entangled particles, one held by Alice and one by Bob. Even if Alice and Bob are separated by light years, when they measure their particles at the same time, the particles collapse into compatible states. With an absolute collapse, this is a problem, because it implies faster than light communication.
But with a relative collapse, the most relevant collapse doesn’t happen until a comparison event, when the results of the measurements have been transmitted (at light speed or slower) to some party, say Charles, who does the comparison. Relative to Charles, the particles haven’t collapsed until he receives the results, even though relative to Alice and Bob their respective particles have collapsed. When Charles does receive the results, that’s when the collapse happens for him. Now we have a completely local interaction. But this only works because under RQM, the reality of the measurement outcomes don’t exist for Charles before he receives them.
So, while many-worlds implies a surplus ontology many find far too extravagant, RQM posits a radically sparse ontology that almost seems like separate interacting solipsistic realities, although centered on physical systems rather than just minds. In the latter parts of the book, Rovelli explores philosophy ranging from Marxist thought to eastern Buddhist thinking that resonates with this view.
Rovelli also veers into a discussion of consciousness. He dismisses ideas about the mind having anything to do with the collapse, or that mental processes are quantum, at least any more so than any other physical process, as well as a host of other quantum mystical notions. But in his view, seeing reality as being composed of interacting viewpoints, as RQM does, helps to close the gap between physics and the mind, eliminating a necessity to reconcile an objective view (which doesn’t exist) with subjective perspectival views. The idea is that both sides of the divide are now perspectival. It’s an interesting idea, but I suspect few troubled by the hard problem of consciousness will be convinced.
This is an interesting interpretation, but in my view it has a couple of drawbacks. One is that, as noted above, Rovelli takes a mostly anti-real stance toward the wavefunction. He notes that we never see a quantum wave, only the interference from it. That’s true but we also never see a quantum particle, only the effects it leaves in measuring equipment. And something causes the observed interference effects. The idea that the wavefunction can predict those effects with the accuracy it does, without modeling reality in some manner, seems implausible. But if we let that realism in, then RQM seems in danger of becoming many-worlds with blinders on. (It’s worth noting that an early name for many-worlds was “the relative state formulation”.)
I also see keeping the collapse postulate as a drawback. RQM does defang one of its worst implications, the instantaneous change in reality that concerned Einstein. But it also leaves in a level of indeterminism. Many will see this as a plus, preferring a physics where everything isn’t determined, and might argue that it’s a matter of taste. But I’m in the camp that sees determinism as something that works well everywhere else in science, and worked well overall for centuries before quantum physics. To me, it doesn’t seem like we should dispense with it lightly, particularly while there are options. (This doesn’t mean that quantum physics would ever be operationally deterministic.)
Finally, it shouldn’t be underestimated just how radical the sparse ontology proposed here is. The interpretation takes general relativity as an inspiration. But in the case of general and special relativity, the conclusions are a necessity driven by observation and mathematics. RQM requires a specific type of collapse postulate, a major assumption, albeit one many will consider justified given the alternatives.
But this is quantum physics. We won’t get by unscathed. Interpretations juggle things like determinism, locality, realism, the arrow of time, a single reality versus multiple realities, and now a sparse versus full reality. Every interpretation requires throwing one or more aspects of common sense reality under the bus.
What do you think of relational quantum mechanics? Do you feel like the sparse ontology is worth it?
SelfAwarePatterns 
His interpretation is almost understandable, which makes me very suspicious.
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It does require some of what Scott Aaronson calls “mad philosophical skillz”, but far less than old school Copenhagen.
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“RQM posits a radically sparse ontology that almost seems like separate interacting solipsistic realities, although centered on physical systems rather than just minds.”
It’s good to read that you were able to garner this fundamental concept from Rovelli’s book Mike, because it’s the underlying prime of motion and form at every level of interactions within systems not exclusive to just minds.
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RQM rocks…. It is the most pragmatic approach to quantum physics, one that eliminates the mystery and magic all of the other models imply. In addition, RQM compliments what we already know about classical physics plus, it corresponds concisely with my ontological model of Reality/Appearance Metaphysics (RAM). The sparse ontology thesis irrevocably reduces to an imperative; and that imperative is pansentientism.
RQM is not for everybody because there are those who are enthralled by the mystery of magic and things that cannot be explained. RQM dismantles that mystery with precision and at the end of the day I would expect RQM to be the prevailing model creating a paradigm shift of how we see the world and ourselves.
Party on
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I thought of you Lee when I read the passage about that aspect. I can see how it resonates with your philosophy.
I do think RQM retains some of its own mysteries, as I noted in the post, but far less than old school Copenhagen.
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All interpretations are just interpretations; however, Rovelli’s has seemed to me the one that has to make the fewest crazy leaps of faith.
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It all depends on one’s definition of “crazy”. 🙂
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Consider the couple of entangled particles, one held by Alice and one by Bob. If they are quite a distance apart and the entanglement is relative, how does the communication between the entangled particles take place? Are they gravitational waves or emr waves and does one particle “beam” the signal only at the other particle or does it send out a spherical signal? If a direct beam, how does one particle “know” where the other is? If a spherical wave, the power of such a wave would diminish a great deal over distance, so where does that power come from?
Is great puzzlement!
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Supposedly under RQM, the signal isn’t required. Each measurement event is local and only relative to the local physical systems. The only way a discrepancy could come up is at a comparison event afterward. But for the comparer, Charles, neither measurement has collapsed until he receives the results (using normal transmission methods). When he does, the collapse happens for him, but at that point it’s all local for him. If he subsequently transmits the results to Alice and Bob, then for each of them, the other person’s results collapse when they hear about them, interacting with the already collapsed information of their local results.
This doesn’t work from an objective third person “God’s eye” view, but RQM denies that such a view actually exists.
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I’ve read several of Rovelli’s books now, and I’ve been underwhelmed in all cases. To me Rovelli dabbles in far out fantasy. Nothing wrong with that, but I can’t take it very seriously until there are probative facts. (As we were just talking about, “Experiment is better than theory.” Also in light of what we were just talking about, all QM interpretations are at this point intuitions.)
“However, no one has managed to find any threshold where a collapse happens. Over the decades, scientists have managed to observe quantum effects in every larger collections of quantum particles, molecules, and even tiny macroscopic objects. It looks increasingly unlikely that there is any such threshold.”
I think there is a threshold, a Heisenberg Cut, but that we don’t understand its mechanism yet. All those ever larger numbers of quantum systems still require special conditions: low temps, EMF shielding, vacuum, and other ways of keeping out the world. Quantum effects seem to require isolation, which, to me, implies they don’t exist out of isolation. The world is decohered.
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Wyrd,
I think that you would have to agree that the quantum realm is beyond the reach of “probative facts” because of the measurement problem. However, if the system we know as mind is indeed a quantum system, then that system would have the capacity to bridge the gap between the classical world we experience and the quantum world we experience that is mind. That gap will be bridged by the explanatory power of the mind that is intrinsic to logical consistency. Therefore, the system of mind should not be hamstrung by the limitations of our own self-imposed intellectual constructs, a prevailing paradigm which insists that predictive power through experimentation (a posteriori) is superior to the intuitions of a priori.
It’s like Kant asked: Is there any knowledge outside of experience (a posteriori)? And the short answer is yes; because today’s a priori intuitions always become tomorrow’s a posteriori. A priori is summed up best by an intuitive insight of “I know not what, only that it is of high value”. Value always comes first in hierarchy.
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Sorry, I don’t think we’re on the same page here, Lee. I don’t think I do agree the quantum realm is beyond reach of probative facts. As for our minds being quantum systems, the jury is out. Even if they are, that doesn’t require they have any special quantum understanding power. We don’t seem to have any access to our lowest-level processes.
“…the system of mind should not be hamstrung by the limitations of our own self-imposed intellectual constructs…”
This seems contradictory to me. Why would a mind hamstring itself if it has higher capacity? What would be the point? Sorry, Lee, but I find those intellectual constructs quite helpful. They weren’t made up from the whole cloth but from successful experience. We earned those constructs.
As for a priori versus a posteriori, my most recent post is about the value of intuition and the Yin-Yang of science-intuition. But in the end “Experiment is better than theory.” Always.
From where I sit, the problem isn’t hamstrung minds but minds that believe in stuff that has little chance of being real. Yet there are gems among that dross. Sometimes today’s a priori becomes tomorrow’s a posteriori, but from what I’ve seen that’s the exception, not the rule.
In any event, I think very few things are truly a priori. Math is one along with, per our buddy Kant, time and space. And even space might require observation. (IIRC, Kant did put time as most primal.)
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“Why would a mind hamstring itself if it has higher capacity? What would be the point?”
I agree. But like it or not, this is exactly what we as individuals do to ourselves, it’s called subjectivity.
“Sometimes today’s a priori becomes tomorrow’s a posteriori, but from what I’ve seen that’s the exception, not the rule.”
I agree. Transformational a priori intuitions are rare, and it is those exceptions to the rule that will transform our understanding of the world. And this transformation will occur one individual at a time.
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What exactly is the issue with subjectivity? (Do you just mean personal bias?)
It is the rareness of correctness of intuitions that’s a problem. It’s Sturgeon’s Law for theories — most of them are crap. The difficulty is sorting out the few good ones.
Have you ever read Idiot America by Charles Pierce? You might enjoy it. A basic thesis is that culture used to be better at picking out the rare truly useful bits from the crackpottery and ignoring the rest. We seem to have lost our capacity for grounding our thinking in basic physical reasoning. These days we embrace all sorts of craziness.
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This is only the second book of Rovelli’s that I’ve read. The first was Seven Brief Lessons on Physics, which I found too brief and too basic. I’d say this one could also have gone into more depth. In truth, this post was supplemented from stuff I remember from his SEP article. Probably some of his points only clicked because I’d already spent time trying to parse that article.
I think “far out fantasy” is too strong. Everything here follows from the postulate of a relational collapse. I do think that’s a major assumption and is the weak point. But if you buy it, the rest pretty much follows.
Definitely the world is decohered. The question is whether it’s also collapsed, and if so, what leads to that collapse.
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Seven Brief Lessons on Physics was a Rovelli book I hoped I’d enjoy, but I ended up being rather underwhelmed. I suspect we’re not the audience for that one. Before that I’d read The Order of Time (his emergent time theory) and Reality Is Not What It Seems: The Journey to Quantum Gravity. Neither did anything for me. To be honest, I see Rovelli as something of a space cadet. I stand by what I said, that he “dabbles in far out fantasy.”
“I do think that’s a major assumption and is the weak point.”
Which is just another way to say “far out fantasy.”
“The question is whether it’s also collapsed, and if so, what leads to that collapse.”
The world is decohered because wave-functions collapse. As you know, I think the field has turned “collapse” into dogma, but I don’t think it’s quite as shocking as many make it out to be. In the canonical case of photons, for instance, the photon is absorbed by the electron. It physically vanishes. Something abrupt happens there, which to me suggests abrupt isn’t the issue. We just don’t have the math to understand how to modify Schrödinger at that point. A lot of the hand-wringing is just because we don’t have an equation. Yet.
We have a central mystery in QM: Why does this electron absorb the photon and not that one. Einstein’s spooky problem came with an example: a photon released at the center of a sphere will be absorbed by one of the electrons on the sphere’s inner surface, but nothing we yet know can tell us which one.
The lesser mystery is that “collapse” changes the probabilities. It’s lesser because that may be purely an epistemic issue. If so, then of course it changes. That’s what probabilities do when new information is available.
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What gives you confidence that the math needs to be modified? Or if it does, that the missing variables will provide something like a collapse? All experimental evidence to date is compatible with the equations as they stand, reportedly to several decimal places. And adding variables, as pilot-wave does, reportedly messes up QM’s generalization into QFT.
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Keep in mind an important primary fact about QM: We know it’s incomplete. There’s some possibility it’s wrong in some fundamental way, but I agree it would make QM a surprisingly effective epicycles theory. Still, we do know pieces are missing in our description of the quantum world.
As a first analogy consider how an “x-squared” equation models the parabolic path of a ballistic object. But it doesn’t contain anything about that object hitting something or just exploding. That requires an additional equation. As a second, consider how GR describes black holes, but they are eternal under GR. Hawking radiation adds a new interaction with reality in addition to GR under which black holes can evaporate.
It’s not a hidden variables thing, but the need to describe an additional situation. The Schrödinger equation describes the evolution of a system of one or more particles. That system interacting with some other system requires something describing that situation.
Also, in these situations, often a particle is created or annihilated. I’ve read that the Schrödinger equation can’t describe this, and certainly I’ve never seen any example in any lecture that does. They’ve all described systems of one or more existing particles that continue to exist.
If the Schrödinger equation gives us the probability of finding a particle in a given location or of measuring its momentum, spin, or other property, what should happen when that particle ceases to exist? All the probabilities have to drop to zero, suggesting either Schrödinger equation is epistemic or that an important piece is missing.
To me it points to collapse being a thing we don’t have a handle on yet. And as Einstein’s example illustrates, this observation/collapse thing is a central unsolved mystery.
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On particles being absorbed or emitted, I have to admit I’m not sure what the Schrodinger equation’s relations to that are. I haven’t read about those limitations. (I’d be interested in learning where they’re discussed.) But it’s worth remembering that Schrodinger is just one of the mathematical frameworks used to work with quantum physics, and they all reportedly reconcile with each other.
As I understand it, physicists are able to use QFT, QED, and QCD to predict when a particle will be emitted, when it will be absorbed, and what might emerge in a collision. Those are the theories obviously tested in the LHC and similar experiments. (The recent excitement is reportedly about a possible minute deviation from those predictions.) From what I understand, the Schrodinger structures are preserved, or at least affirmed, in all those other theories, even if Schrodinger itself isn’t the best way to do the calculation.
It seems like it’s possible the math might someday have to be amended. Gravity in particular might eventually lead to it. But raw quantum theory seems famously (infamously?) stubborn in making extremely accurate predictions.
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“But it’s worth remembering that Schrodinger is just one of the mathematical frameworks used to work with quantum physics, and they all reportedly reconcile with each other.”
Indeed, and that’s part of my point. The hand-wringing over the Schrödinger equation not having collapse seems unnecessary to me. Firstly, it describes the evolution of a particle system, so maybe like a parabola describing ballistics, we wouldn’t expect it to describe collapse. Secondly, as you say, there are other formulations, including various (unproven) collapse theories (even I have one).
As you go on to say, it’s QFT that describes particle creation and annihilation. Certainly the behavior of those particles is compatible with the Schrödinger equation, but my understanding is that it does require the extension of QFT to describe how they begin and end.
“…even if Schrodinger itself isn’t the best way to do the calculation.”
The point is that the Schrödinger equation may not be capable of doing the calculation. It may not address the situation.
(One reason I started getting into QM math is to find out if what I recall reading is true or to find out how to use the Schrödinger equation to fully describe a photon that is absorbed by an electron. As I mentioned, all I’ve seen so far, by lack of example, backs up what I read. There is also my understanding that the Schrödinger equation has terms for the particles and the energies that affect them, so what happens if terms need to be introduced or removed? It’s not uncommon for terms to have factors that are non-zero only in certain situations, so it’s possible something like that is going on. I need to learn more to know.)
“But raw quantum theory seems famously (infamously?) stubborn in making extremely accurate predictions.”
Except where it fails completely, such as in Einstein’s example of the omni-directional photon released from the center of a sphere. All it gives us is a smooth distribution of probability equal at every point.
There’s a similar situation with a radioactive material. QM gives us no way to predict which atoms will decay but by experiment and theory we know they always follow half-life curves overall. How do the atoms know? How do the electrons in the sphere know that one absorbed the photon, so none of the rest of us can?
Superposition, interference, and the apparently random nature of quantum interaction, are, in my view, the three big mysteries of QM. We observe them experimentally, and have math describing them in various ways (with a big hole regarding interactions), but we don’t understand them.
So I agree it’s a great theory, very effective, but to me it’s clearly not quite ready to come out of the oven. 🙂
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People want to think there is true or correct interpretation of phenomena that by their nature is telling us that isn’t one.
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Well, as a philosophical realist, I’m not sure I believe that’s the case.
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I imagine QM must be hard for philosophical realist.
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🙂 It has its unsolved mysteries, but I’m actually charmed by its counter-intuitive nature. (For instance, some are quite alarmed by entanglement’s apparent end-run around the speed of light, but think it’s kinda cool how local realism isn’t true, but locality is.)
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Interesting! Based on your explanations, I now think I get what the RQM interpretation is saying. I find it less simple than Everett, assuming that collapse is supposed to be real and discrete, albeit relative. If collapse is not discrete, I’m not seeing how it’s different from Everett.
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I agree that Everett is simpler, since he just takes the raw quantum formalism and follows it to the bitter end.
I’m not sure if RQM takes a stand on whether the collapse is discrete. Making it relative seems like it provides an opening for it to not be discrete. We know decoherence isn’t discrete, but that’s separate from the collapse, if there is one.
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Ok. Well. Um. There’s nothing for it, so ….
1. I finally finished it and thought it was a great read. Very non-technical, low math. Colorful imagery: I especially appreciated his variation on the two slit experiment (where you split particles into a left and right path. If there is no obstacle, all the particles go down. If you put your hand in one path, half the particles go up, half down.) Lots of background info I didn’t know. Etc.
2. While reading I compare against my understanding of metaphysics, and Rovelli’s tracks with mine almost perfectly, so RQM is my current best understanding of QM.
3. [Here we go …]. I do not see RQM as a collapse theory at all, and I was surprised that you do. There is no collapse, only interaction, described by the wave function. What does happen is that the interaction changes something such that where there would have been interference, there no longer will be. But that change happens only in reference to the thing that interacted. If you get Alice’s and Bob’s things together without interacting with them, they will interfere. But if you take out Alice’s thing and look at it/measure it/interact with it, you change it such that it will no longer interfere, but that only applies when YOU (or anything you’ve interacted with, etc.) mess with Bob’s. Same if you put your hand in the left path, you change the particles in the right path such that they no longer self-interfere. BTW, this only makes sense when you get rid of the idea that they are point particles. They’re not, and never were, and don’t become point particles. Whatever they are, they interact with one thing at a time, which is what a point particle would do, but that does not make them a point particle.
*
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I figured you’d like that book.
1. Many physicists use an interferometer setup to demonstrate what Rovelli is talking about in his modified double-slit. The intro article in the Arc Technica series I shared a while back has an example.
https://arstechnica.com/science/2021/01/the-curious-observers-guide-to-quantum-mechanics/
It’s all about wave mechanics.
3. As I understand it, RQM is not a physical collapse theory. Rovelli’s rejection of wave function realism seems to rule that out. It seems like more of an epistemic collapse theory.
The suppression of interference effects, in and of itself, is actually explained by decoherence, which Rovelli acknowledges in endnote 39 on page 209. But decoherence doesn’t explain the fate of the unobserved outcomes. With only decoherence and nothing else, we end up with Everett many-worlds. Something else is needed for only one of those outcomes to be reality. Of course, you can say there was ever only one outcome that we just became aware of (which is what an epistemic collapse amounts to), but then where did the interference effects come from?
On a particle being some kind of consistent entity that manages to have both wave-like and particle-like interactions, I don’t know. We can speculate that something like that exists, but coming up with a concrete proposal for it is another matter.
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” With only decoherence and nothing else, we end up with Everett many-worlds.”.
Actually I think Rovelli is often regarded as close to Everett, isn’t he? Aren’t the unobserved outcomes still out there ready to be measured from a different relative observer?
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It might depend on who you ask. I know the Wikipedia interpretations comparison table lists RQM as agnostic on the other outcomes. And the first physicist I heard describe it took that stance. But Rovelli himself seems pretty strongly anti-Everettian. His anti-real stance on the wavefunction seems to rule it out.
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BTW, have you read this paper, which is surprisingly readable in many sections (not so much in others) even with my limited understanding of the math.
https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.205.7153&rep=rep1&type=pdf
He has several comments on Everett’s view which also has morphed into several different varieties. Towards the end he writes:
“There is a way of having (perspectival) branching keeping all systems on the same footing: the way followed in
this paper, namely to assume that all values assignments
are completely relational, not just relational with respect
to apparatus or Minds. Notice, however, that from this
perspective Everett’s wave function is a very misleading
notion, not only because it represents the perspective of
a non-existent observer, but because it even fails to contain any relevant information about the values observed
by each single observer! There is no description of the
universe in-toto, only a quantum-interrelated net of partial descriptions”.
That first sentence seems to reflect that his relational view could be thought of as a type of branching, although perhaps not the form usually thought of by Everettians.
BTW, I think he sums up his entire view in the most succinct manner possible in the paper.
“Main observation: In quantum mechanics different observers may give different accounts of the same sequence of events.”
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How different? Causality different or order different?
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You probably would do better to read the linked paper but I understand the same sequence of events which result in two observers observing different end states.
However, before the main observation he writes this which may clarify:
Thus, we have two descriptions
of the physical sequence of events E: The description (1)
given by the observer O and the description (2) given by
the observer P. These are two distinct correct descriptions of the same sequence of events E. At time t2, in the
O description, the system S is in the state |1> and the
quantity q has value 1. According to the P description,
S is not in the state |1> and the hand of the measuring
apparatus does not indicate ‘1’.
Thus, I come to the observation on which the rest of
the paper relies.
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As you say, I would have to read the paper. It’s interesting that it’s apparently a case of |1⟩ versus not |1⟩, rather than say |1⟩ versus |0⟩.
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I took a peek at the paper and read section IIA, which seems to state his thesis. It appears to me as a version of Wigner’s Friend. Observer O, after observing system S, creates a superposition of outcomes [equation (2)].
The condition at t2 is that observer P has not observed either system S or observer O, so they “observe” a superposition of those outcomes. I say “observe” because P doesn’t actually observe anything; the situation is how perceive the situation.
Part of it is that when P does make an observation (at time t3), of S and/or O, they can only collapse a wave-function where that system and observer agree.
He does see the MWI as different. Wave-functions collapse from the point of view of the interacting systems. He’s doing a Schrödinger’s Cat thing where observer O is the cat, system S is the mechanism inside the box, and observer P is the scientist who opens the box. Or O is the Geiger counter and P is the cat. Or O is the scientist and P is someone outside.
This all depends on the view that QM describes the classical world in a meaningful way. (That’s kind of where I get off the bus.)
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Thanks. I haven’t read that paper. My main source of information, aside from the book, has been Rovelli’s SEP article (co-written with someone else). It was last revised in 2019 and probably gives a more current snapshot of his views:
https://plato.stanford.edu/entries/qm-relational/
I think the differences between Rovelli and Everett is the degree of quantum state antirealism vs realism and whether it makes any sense to reconcile the relative interactions, which as you note could be seen as equivalent to branching, into a unified view of reality. Rovelli asserts that the unified view doesn’t exist, that it’s meaningless to attempt it.
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“Rovelli asserts that the unified view doesn’t exist, that it’s meaningless to attempt it”.
Yes, I think that’s it. All views are relative. It is just applying the perspective of Einstein’s relativity to the quantum.
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I never knew Schrodinger was polyamorous. Does that mean he existed in a superposition of multiple possible relationships?
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Haha! That’s a good analogy. If you read his biography, you also learn that he went to a lot of trouble to avoid having them collapse. He had career problems because of it.
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Interesting, Mike. I appreciate you taking the time to share this as I really didn’t know what Rovelli’s stance was or what relational QM was all about. I’m still a little confused but think I have an inkling. I’ll have to check the book out at some point.
Before I do though, if I understand you, the crux is this: when any two physical systems interact they each register a definite observation of the other. But to a system that hasn’t interacted with them, those two systems or particles could still be in a superposition relative to all other systems. And a third system that interacts with either of the first two could, in principle, register a different observed state of the first two particles than the particles themselves registered when they interacted.
Is that correct?
The part that’s sort of difficult to me is the entanglement example. Bob and Alice fly off to different parts of the universe and take a peek at their entangled particles. Bob observes spin up and Alice spin down let’s say. They each see what they see. A third party, Charles, has not interacted with either of the two entangled particles, so for him the two particles are still entangled. He could observe Bob’s particle to be spin up, as Bob did, or he could observe it to be spin down. And his probability of seeing either one is given by the wave equation or the matrix formulation of QM, etc.
What’s confusing to me is that Charles doesn’t actually measure the particles, he reads the results from Bob and Alice. And it sounds like you’re saying that when he receives and reads the results, then the particles are no longer entangled from his perspective, BUT, he could actually observe that the Bob’s and Alice’s particles are different from what Bob and Alice observed? That’s the part that is hard for me to understand. He’s not actually observing the system, he’s reading a text message.
What’s wrong with my restatement of this?
And if I’ve restated it correctly, how the heck do the communications from Bob and Alice permit Charles to observe anything different? He’s reading a statement of what they saw, no? Or is that where I’ve gone awry?
If he can’t observe anything different, and no other physical system can observe something different, than it’s hard to say they’re still entangled for Charles until he receives the measurements from Bob and Alice. And if he IS able to observe something different, then is the problem that this thought experiment is oversimplified and it’s not that he gets a text message from Bob with Bob’s observation, but that in truth he has to physically interact with the photon pair himself to make his own measurement, and THAT could be different?
Thanks in advance, Mike.
Michael
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Thanks Michael. It sounds like you’re on the right track, but the full implications of Rovelli’s view needs to set in for the picture to click.
On the entanglement part, when considering Charles’ comparison, remember that you have to evaluate that event in terms of RQM, not Copenhagen. In Copenhagen, Charles making the comparison is a macroscopic event, and therefore not quantum. The collapses happened at Alice and Bob’s individual measurements, not when Charles receives and compares the information.
However, with RQM, the entire universe, including macroscopic systems, are all quantum. (It shares this trait with the Everett many-worlds interpretation.) That means that, for Charles, prior to him receiving one of the measurements results, that measurement and everything that results from it is in a superposition of all the possible results. The whole thing only collapses, for him, when he receives the information. (This might be a little easier to accept when we recall that Rovelli rejects wavefunction realism, so we’re not necessarily talking about entire worlds that disappear when Charles gets his results.)
Now, on the question of him seeing something different than what Alice and Bob see, remember that, relative to Charles, what Alice observes from her measurement and its subsequent transmission, is all in a superposition of multiple results, until Charles gets the information, at which point the whole thing collapses, again, relative to Charles. The same thing happens between Charles and Bob.
In other words, there is never an opportunity for Alice, Bob, or Charles to compare their results in such a manner that they’ll be different (or incompatible). On the face of it, it seems like this says reality can be inconsistent. But that inconsistency only arises from an objective “view from nowhere” that RQM rejects the validity of.
If we reconcile it anyway, that leads to another interpretation, Everett. But if we accept Rovelli’s contention that there is no “God eye” view to reconcile, then we end up with a sparse ontology of interacting viewpoints, with each physical system being the center of a viewpoint, and there being no objective reality beyond that.
As I noted in the post, this is a radical view.
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“…we end up with a sparse ontology of interacting viewpoints, with each physical system being the center of a viewpoint, and there being no objective reality beyond that.”
It might be a radical view, but I think Rovelli is on the right track. This reminds me of how individuals cells that repair cuts, broken bones or bruises work. Those cells could care less about what the rest of systems within the body are doing or are even aware that those other systems exist. The cells that repair the body work 24/7 as solipsistic systems in complete isolation expressing their own unique qualitative properties until their job is done.
Rock on Rovelli…..
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Thanks, Mike. I think I have a clearer picture now. The key for me was understanding that, in my example, because Charles hasn’t previously interacted with Bob’s or Alice’s photon, (or Bob or Alice for that matter), he is just as likely to get a message from Bob that reads “spin up!” as he is “spin down!” and that whichever occurs, upon receipt of this information his picture of reality will be consistent. It is possible for Bob to have perceived the opposite result when he first observed the photon, but his reality will be consistent as well because what’s true in his world, is what he will report in his note to Charles, and so on and so forth. It does have an MWI feel to it for sure…
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That was my impression too when I first read about it. And some physicists seem to agree. But Rovelli sees RQM as very distinct from MWI. I think he leans on wavefunction antirealism to rule out the other worlds. Although he does say in the SEP article that RQM is “metaphysically neutral”.
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