[“Quantum foundations” series]
Theoretical physicist Sean Carroll recently published a new book. I’ve read some of his books. His blog, podcast. Articles. He has a robust online media presence. And is skilled at promoting his new book (or has a skilled publicist). So, I’ve read several articles on different sites about his new book, as well as downloaded a Kindle sample of the book.
Here’re a few examples of articles promoting the new book, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime.
• PBS NewsHour: YouTube video “Sean Carroll: Universe a ‘tiny sliver’ of all there is” (published on Sep 15, 2019)
• NPR: In ‘Something Deeply Hidden,’ Sean Carroll Argues There Are Infinite Copies of You (September 13, 2019) by Adam Frank, astrophysics professor at the University of Rochester
• Wired: Sean Carroll Thinks We All Exist on Multiple Worlds (September 10, 2019) by Sophia Chen
• New York Times: Even Physicists Don’t Understand Quantum Mechanics
Worse, they don’t seem to want to understand it (September 7, 2019) by Sean Carroll, theoretical physicist at the California Institute of Technology
After almost a century of pretending that understanding quantum mechanics isn’t a crucial task for physicists, we need to take this challenge seriously.
I found NPR’s book review particularly interesting. That article recaps around a 100 year old debate about quantum mechanics and realism, a debate harkening back to its original seminal figures and regaining new life in the last few years after decades of what has been characterized as “shut up and calculate” successes.
There are a variety of reasons that this debate about quantum foundations – foundational questions related to quantum mechanics (QM) and quantum theory – has renewed vigor. Often cited are the rarified mathematics of current theories (particularly those pushing beyond the Standard Model), questions about the future of particle physics (viability of further discoveries by the LHC and investment in even greater colliders), and new perspectives by some modern well-known physicists.
The NPR review notes that no matter what your QM interpretation or perspective is (or take on the many popular tropes about what QM says), Carroll’s emphasis is that foundational questions be returned from the backwaters of legitimate research – whether you agree with his advocacy of the Everettian many-worlds interpretation of QM.
Carroll’s Everettian many-worlds perspective is included in his July 10, 2018, podcast “Episode 2: Carlo Rovelli on Quantum Mechanics, Spacetime, and Reality” – an interview with Carlo Rovelli.  Here’s an excerpt.
Carroll: But it’s not so far from mine in that part in the sense that I am an Everettian, a believer in the many worlds interpretation, at least I think that’s my favorite. I’m happy to change my mind some day down the road, but Everett himself, when he invented it in the 1950s, when he published his paper, he called it the relative state interpretation. And the point that he was trying to emphasize was that what you see is only relative to the thing seeing it. This interaction creates an impression of the measurement outcome, that is intrinsically part of the relationship between the thing that is doing the measuring and the thing that is being observed. So I think that there’s some similarity of words there.
Rovelli: Some similarities. I shouldn’t talk for you, but let me try it for one second. I think what you would add to what I say, is that you want to take a realist position where beyond that there is nevertheless a deterministic non-relational overall quantum state of the system, that we may take as the proper description of reality.
Carroll: I would like to say that, yeah …
Rovelli: Would that be so?
Carroll: Yeah, I would. I’m a realist. I can’t even imagine what it would be like to be a scientist and not be a realist about reality. [laughter] And yet I know …
Rovelli: Right, so I would like to be a realist, but less radically so. …
And so I find that if we want to be a realist about the wave function, we have … Actually, let me step back for a moment. I think, given the strangeness of quantum mechanics, any way we think it is, comes with a price. So we have options. We have a number of options, and we have been talking about two options, okay, the sort of many world interpretation, in which you have this wave function of everything, which come with a price, or the sort of strictly relational view which I described, which also come with a price. And there are others and they have their own price.
Carroll: Right, that’s right.
Rovelli: And many discussions about quantum mechanics are, “Well, which price are we ready to pay?” And the price of the relational interpretation I described is this weakening of realism. So to say, what the state of the system … Well, I don’t know, it doesn’t matter, it doesn’t have a state, it only has a state with respect to A, and it is the same with respect B, and it is the same with respect to C, and there is no solid objective, overall picture. The price of the many world interpretation, is that I have to accept …
Carroll: Many worlds.
Rovelli: In some sense all these many worlds.
Carroll: Yes, absolutely, it’s a big price, it’s a big price.
Rovelli: So it comes with an enormous ontology, a very heavy ontology, and which price do we wanna pay. In one way or the other, it contradicts our previous way of thinking of reality.
Carroll: Absolutely, and I … So I couldn’t agree more about the idea that there is a price to be paid. I could disagree a little bit about the heaviness of the ontology that is associated with many worlds, but I think that this is a good lesson, because all we need for today’s conversation is the clear fact that we have at the same time, a way to do quantum mechanics, we use quantum mechanics as physicists every day, it’s been spectacularly verified experimentally. We know how to use it, we have a recipe, we have a black box.
Rovelli: Yeah, and it works, it works fantastically well.
Carroll: And we don’t have anything close to an agreement on what it is, what the fundamentals of it really say. So that will be a wonderful thing to figure out once and for all, but maybe we don’t need to do that for the question of quantizing gravity. So just to put the puzzle, the task, in perspective, since we grew up, we human beings, as classical people, classical mechanics was invented first. It is very intuitive to us, even if pendulums and inclined planes tortured us as undergraduates, we kind of get it in a fundamental, visceral way. So we always start by thinking about systems from this classical point of view and then we quantize them, we elevate the classical description to some quantum mechanical theory. Now, presumably nature or God doesn’t work that way.
Okay, what’s my personal take on this topic? Well, I agree that quantum foundations is important. Studying philosophy (including philosophy of science) has shaped my perspective. Generally, I side more with Rovelli than Carroll on realism and QM.
Carroll is well grounded in the mathematical framework of QM, and my grasp of that is all too limited. I sometimes think that his conviction (no matter how provisional) is based on mathematics for which any explanation to “mere mortals” is problematical; so, metaphorical language misses the core of the argument.
[Carroll] And we look at and say yes, I see the worlds, there they are. I see them in my math. I don’t see them in my telescopes or microscopes.
The many-worlds interpretation focuses on the so-called measurement problem (wave function collapse), and there are many tropes which blend the terms measurement, observation, and even decision (by the PBS NewsHour) – as splitting other worlds. Carroll is careful in discussing this point, repeatedly stating that it’s an open question as to what “measurement” means (what counts as an “observation” or whether consciousness is required or not, for example). Yet, I thought that advances in understanding decoherence had diffused the problem. As Philip Ball said, “The universe is always looking.” 
And indeed the PBS NewsHours’ characterization that QM claims that “two objects can occupy the exact same space at the same time” is misleading. Their use of the words “object” and “exact” miss even a visualization of radio waves (EM photons) overlapping or interfering: positional and spatial concepts of waves – wavelength and superposition. Discussing wave functions is even more arcane.
[Carroll] But what is the wave function? Is it a complete and comprehensive representation of the world? Or do we need additional physical quantities to fully capture reality, as Albert Einstein and others suspected? Or does the wave function have no direct connection with reality at all, merely characterizing our personal ignorance about what we will eventually measure in our experiments?
So, Carroll talks about the measurment problem. And Rovelli’s Relational quantum mechanics replaces measurement with interaction as the way to remain a “realist about the wave function.” That is, a “property becomes concrete when two systems interact.”
You replace measurement with interaction. But to make the machine work, you have to assume that when two systems interact, the property of one system becomes actual, with respect to the other system, and not with respect to everything else. And that point is really shocking.
There are a lot of quantities in physics which are relational, we know that they are. The velocity of an object is not a property of the object. There’s no sense in which the velocity of an object is a property of the object … The velocity of an object is a property of an object with respect to another object.
So I can say that the Earth is set to velocity with respect to the Sun, but the Sun is not measuring, it’s not going out with meters and kilometers. So we use this language, measuring, and we interpret this language to say that some properties are relational. I want to be a realist about relational properties. And I want to think of reality in a realistic way … to our direct experience of reality [observables]. I see the photons coming in, I see the table being the table …
Personally I am not sure about the reality of the wave function – as ontic vs. epistemic.
Wiki: “If the wave function is physically real, in some sense and to some extent, then the collapse of the wave function is also seen as a real process, to the same extent.”
Online video: Perimeter Institute Lee Smolin Public Lecture “Einstein’s Unfinished Revolution” (published on Apr 18, 2019)
 Other podcasts by Sean Carroll:
- Episode 63 Solo: Finding Gravity Within Quantum Mechanics, September 9, 2019
- Episode 59 Adam Becker on the Curious History of Quantum Mechanics, August 12, 2019
- Episode 55 A Conversation with Rob Reid on Quantum Mechanics and Many Worlds, July 15, 2019
- Episode 36 David Albert on Quantum Measurement and the Problems with Many-Worlds, March 4, 2019
- Episode 31 Brian Greene on the Multiverse, Inflation, and the String Theory Landscape, January 28, 2019
 From Carroll’s Research overview page:
These days my physics research is a bit different, focusing on two big themes. The first theme is the foundations of quantum mechanics, especially connections to cosmology and emergent spacetime. I’m a proponent of the Everett (“many-worlds”) approach to quantum mechanics. It’s a great theory, but raises a number of questions to which the answers are not yet clear, including the origin of probability and how the furniture of our semi-classical world (space, time, fields) emerges from an underlying wave function in Hilbert space.
 Wiki: Quantum decoherence
However, decoherence by itself may not give a complete solution of the measurement problem, since all components of the wave function still exist in a global superposition, which is explicitly acknowledged in the many-worlds interpretation. All decoherence explains, in this view, is why these coherences are no longer available for inspection by local observers. To present a solution to the measurement problem in most interpretations of quantum mechanics, decoherence must be supplied with some nontrivial interpretational considerations (as for example Wojciech Zurek tends to do in his existential interpretation). However, according to Everett and DeWitt, the many-worlds interpretation can be derived from the formalism alone, in which case no extra interpretational layer is required.