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Update: this question was due to a misunderstanding that was clarified by a more thorough reading of the source (see my answer below for details). Thank you to all who contributed.

In Helgoland, which deals with the relational interpretation of Quantum Mechanics, Carlo Rovelli makes the claim that all the properties of a physical object only exists in relation to another physical object.

It is for example commonly accepted that the property of velocity only exists in relation to the position of another object (i.e. I am standing still with respect to the Earth but am moving at great speed relative to the Sun). But Rovelli claims this is true of all properties, including fundamental properties like the electric charge of a particle.

What confuses me is that, if no property of an electron exists independently of its relations to other physical objects, why when we expect to measure an electron (like when measuring the output of a ray tube) do we measure an electron? It seems to me that there is still some fundamental fact of the matter of an unmeasured particle so that when we measure it, an electron is measured (as opposed to say a neutron, or a house)?

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    See e.g Relationalism: "The relational point of view was advocated in physics by Gottfried Wilhelm Leibniz[7] and Ernst Mach (in his Mach's principle).[7] It was rejected by Isaac Newton in his successful description of classical physics. Although Albert Einstein was impressed by Mach's principle, he did not fully incorporate it into his general theory of relativity." Commented Feb 11, 2022 at 13:22
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    And see Relations Commented Feb 11, 2022 at 13:30
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    Even if there is some fundamental fact of the matter about electron "in itself" how would it be something that we measure? That would always be in relation to other things, so we might as well restrict to relational properties only. Purely qualitative non-relational properties are sometimes called qualia and are accessible only to objects themselves, if at all. In contrast, objects as known to us are individuated not by that but by their relations to other objects.
    – Conifold
    Commented Feb 11, 2022 at 18:57

4 Answers 4

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I decided to reread Rovelli's book and found a clarification hidden in the footnotes. On page 70 in the section titled "No Interaction, No Properties", it says:

"... for there are no properties outside of interactions.56"

Reading note 56:

"The properties I am referring to are those that are variables: that is, those described by functions on the phase space, not the invariant properties such as the non-relativistic mass of a particle."

So it seems I have to apologize: a physical object can have certain nonrelational properties after all.

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Relational quantum mechanics (RQM) as developed by Rovelli is one of several interpretations of quantum mechanics.

It is a radical approach which takes for serious: The physical observables of a system attain concrete values not until two systems interact. And these values are relational, i.e. they have meaning only with respect to both systems.

As a consequence the approach generalizes in a radical way the relativity of the values of other physical quantities like location, time, speed. In particular the approach radicalizes Heisenberg’s early statement: Between two observations the electron within an atom has no orbit.

Like each other proposed interpretation of quantum mechanics, also RQM is work in progress. Quantum mechanics is considered a laboratory for testing new conceptions about the microworld. The current interest in RQM is linked with its relation to Loop Quantum Gravity, the main field of Rovelli’s activity in physics.

RQM proposes to base the ontology of the microworld not on objects but on processes, i.e. on interactions in the form of events. Since long time quantum field theory conceptualizes an electron not as a fixed small object, but as a property of the Dirac field. In a metaphoric language: An electron is the event where the Dirac field materializes. Of course the Dirac field of an electron is different from a neutron field. The latter is electrically neutral, the former carries electric charge.

For a philosophical survey of RQM see https://plato.stanford.edu/entries/qm-relational/ and the references within.

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  • "Of course the Dirac field of an electron is different from a neutron field." It is not known whether the neutrino is a Majorana fermion or a Dirac fermion. The Dirac Field is described by a pair of two-component Weyl spinors, and Majorana fermions are described by one two-component Weyl spinor.
    – CriglCragl
    Commented Feb 14, 2022 at 10:05
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Information flow is the best bridge we have between Relativity and Quantum Mechanics. In Relativity it's all about light cones and light clocks, in QM it's about when systems are isolated and when state information propagates from it.

I think I understand your unease. What you need is a physicists picture of continuous symmetries under transformation, which by Noether's theorem are conservation laws.

When a particle collision happens, whichever restframe you pick the total momentum is conserved in the system as a whole, above the very short timescales of the uncertainty principle. If an uncertain interaction like say a three-body interaction happens in a black box that will magnify the uncertainties (eg The three-body problem shows us why we can’t accurately calculate the past, where uncertainties below the Planck scale matter), even so we know the total momentum of the system will be maintained. Uncertainty, doesn't mean no knowledge. An interesting point is that angular momentum is also relative like this, so when it's given we implicitly rely on a general restframe for the universe, but the universe as a whole could be spinning by any amount, and it would be unverifiable, unmeasurable.

Consider a particle-antiparticle pair. We can think of space as a sea of low-excitation pairs. When energy is added to a virtual pair, like say a gamma ray, you get a real pair, and the total quantum numbers of the massless chargeless photon are maintained, by the pair. But once they move apart, there isn't uncertainty like there was for the virtual pair, charge will be conserved even though it 'appeared from nowhere'. Feynman Diagrams can help us picture the virtual particle sea, with particle interactions shaped by all the possible things that could happen, but with probabilities and so impacts on the system observables, scaled down with the number of vertexes or crossing points of particle paths in the diagram.

So, saying all the properties are relational isn't like a kind of 'total subjectivity', where anything could be anything. There are waves of information bouncing around, limited by the speed of light (except entangled information, ER = EPR, but this can't be used to transmit FTL signals, I think of it as like spinning a coin and cutting it in half while it's still spinning, like the particle pair you know what is conserved), and paused fuzzily in isolated systems - entropy and the second law of thermodynamics can be described as state information's tendency to spread out, see the purification principle.

We can picture the whole universe as arising from uncertainty alone, with net properties of charge and mass of zero, but a series of divisions and separations that have emergent symmetries, like even space and time themselves emerging from a fuzzy soup of uncertainty (the quantum spin network).

More on the limits of causal narratives vs symmetries & conservation laws here: Is the idea of a causal chain physical (or even scientific)?

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If an electron doesn’t interact with other particles, it will disperse over space. It's position will get more and more uncertain, while its velocity will show less and less variation. Can we say the electron still exists? It will become an extended ghost-like entity, desperately seeking for interaction over larger and larger parts of space. With increasing efficiency. Will it find a positron to happily annihilate it, in a bright flash of love? Will it encounter a tau to hate, push and run away from? Without interactions and relations, there is no meaning to existence.

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  • Several ways that's not a good picture. Conservation of charge applies. Also it's interacting with virtual particles.
    – CriglCragl
    Commented Feb 13, 2022 at 11:11
  • @CriglCragl Where do I say that doesn't hold?
    – Pathfinder
    Commented Feb 13, 2022 at 11:22
  • "Can we say the electron still exists?" There. And it doesn't spread out 'more and more', it's charge always spreads out everywhere in the universe, with propagation of changes limited by the speed of light. The uncertainty is a combination of position & momentum, so the fuzziness of atomic orbitals directly relates to the confinement around a nucleus. When a limit is placed on uncertainty about position, by an interaction, a measurement, the momentum becomes uncertain - but that is still constrained by position at time if measurement, and in proportion to certainty of position (never 100%)
    – CriglCragl
    Commented Feb 13, 2022 at 12:00
  • @CriglCragl I asked if we can say the charge still exists in a meaningful way. There is "to exist" and "to exist". I used "to exist" as meaningful existence.
    – Pathfinder
    Commented Feb 13, 2022 at 12:21

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