[HN Gopher] Relational Quantum Mechanics
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Relational Quantum Mechanics
Author : hackandthink
Score : 91 points
Date : 2025-02-10 22:53 UTC (1 days ago)
(HTM) web link (plato.stanford.edu)
(TXT) w3m dump (plato.stanford.edu)
| hackandthink wrote:
| by Carlo Rovelli himself
| AdmiralNines wrote:
| I was lost by the second sentence.
| SubiculumCode wrote:
| Me too, but I wanted to :)
| greenbit wrote:
| So .. a superposition of states would really just be a point in
| time during which the state variable takes no specific value?
|
| Or, it isn't so much that Schrodinger's famous cat is both alive
| and dead at the same time, as that it's neither?
|
| The thing about the cat in the box is that, when you later open
| the box, it's some kind of _cat_ that 's revealed. You don't open
| the box to find a nice loaf of bread. I.e., seems like
| _something_ must have some kind of state, in that box, when no
| one is looking, that retains at least the possible outcomes and
| precludes others.
| uoaei wrote:
| By some interpretations, Schrodinger's equations describe
| _epistemological_ state from a given (finite) perspective. So
| it 's not so much that the cat _is_ both alive and dead, it 's
| that _the best we can say is that it 's alive or dead_. This is
| referred to as "quantum Bayesianism" aka QBism (pronounced
| "cubism").
|
| "Observer" just refers to anything that has a direct causal
| connection with the thing being "observed". No consciousness
| necessary (grr...). If something isn't being "observed" then
| for all intents and purposes it exists outside of our universe
| and any questions about it become fundamentally irrelevant.
| exhaze wrote:
| Curious - how would one frame the cat before/after box reveal
| within the conceptual framework of causal closure?
| vehemenz wrote:
| That _something_ is a relation between two systems, which
| determines or correlates with the state of the cat /box system,
| but that state is not independent of the second system
| (observer).
| markisus wrote:
| RQM is an interesting viewpoint on QM that I have not encountered
| before, even though it was apparently introduced in 1996.
|
| It says that the physical world is a collection of discrete
| events, doing away with continuous time. It also says that every
| event involves two systems, in that each event takes place in one
| system _relative_ to another system.
|
| The idea seems a little strange but the article gives a more
| intuitive example. It doesn't make sense to talk about the
| absolute velocity of a particle, only its _relative_ velocity
| with respect to a reference frame.
|
| But RQM takes it further. It brings to mind the old question
| about "If a tree falls in a forest and no one is around to hear
| it." I guess RQM would say that this question isn't even wrong.
|
| The article quickly becomes too technical for me to understand,
| so I'm still confused about some aspects. For instance, if each
| event only involves two systems, how is it that we can have a
| whole population of systems (eg scientists) eventually coming to
| a consensus about some phenomenon (eg the value of some physical
| quantity?) It seems necessary to have a unified global system
| coordinating the whole thing.
| Nevermark wrote:
| > how is it that we can have a whole population of systems (eg
| scientists) eventually coming to a consensus about some
| phenomenon
|
| Are the scientists interacting? What is the barrier you see? I
| think the two systems interact simply means that any event you
| can measure is one you have interacted with (however
| indirectly). It isn't a limitation on the number of systems
| interacting with each other.
|
| > "If a tree falls in a forest and no one is around to hear
| it." I guess RQM would say that this question isn't even wrong.
|
| I think the more concrete interpretation is being used. To
| "hear" something, you have interacted with it in some way.
|
| Not hearing something, in any way, simply means you have not
| yet interacted with it. There has been no exchange of
| information. It doesn't mean it didn't exist.
|
| The point that there is no privileged system or observer
| underlines that existence isn't tied to "observers".
|
| If both we and another civilization started sending radio
| signals into space on the same date, and on another future date
| both civilizations finally got the first signals from the
| other, neither detection caused the other civilization to come
| into existence. They both existed separately before the common
| detection. There is no privileged observer from whom to relate
| what exists or not by their observations.
|
| Yes, trees that fall in a forest without human beings cause
| vibrations. They interact with the forest around them. There is
| no observer-tron particle in humans that acts in a special way
| on events around us.
|
| But between a photon being emitted and absorbed (two bracketing
| events on its existence), we might say the photon's path is
| just those interactions, and there isn't anything to talk about
| between them. If it had no interactions with anything else
| between its two bracketing events.
|
| Like a discrete edge in a simple graph, it is a connection
| between two nodes, there are no dynamics in between the nodes.
| An edge is just a connection, not a continuous path.
| yetihehe wrote:
| > Like a discrete edge in a simple graph, it is a connection
| between two nodes, there are no dynamics in between the
| nodes. An edge is just a connection, not a continuous path.
|
| Be careful talking about graphs in physics or you might
| invoke one researcher who tried to push that analogy further
| [1], but apparently has too big ego for this.
|
| [1] https://writings.stephenwolfram.com/2020/04/finally-we-
| may-h...
| markisus wrote:
| I think I'm mostly with you except a couple things.
|
| > Are the scientists interacting?
|
| So say Alice and Bob interact, forming an event in RQM. Then
| they both separately interact with Charlie, generating two
| new events. These two new events are, in general, related
| (correlated) to each other, because they may be causally
| influenced by the original event. (For example, Alice tells
| Bob the number. They both send this number to Charlie.)
|
| > Not hearing something, in any way, simply means you have
| not yet interacted with it. There has been no exchange of
| information. It doesn't mean it didn't exist.
|
| My intuition agrees with yours here, but I think RQM is
| saying something different. The article says that in RQM,
| variables simply don't have any value unless a system acts on
| another system. Here is the relevant quote.
|
| > Variables have values when a system acts on another system.
| More precisely, for a system S to have a variable A taking a
| value is to interact with a second system S'; the variable A
| characterizes the effect of a certain action on S'
|
| And about the tree falling in the forest, I take it to be
| more of a metaphysical question about existence. In the real
| world, a tree falling is an extremely complex system and will
| inevitably interact with many other systems -- the air, the
| ground, like you said. But the heart of the issue is whether
| it's _meaningful_ to talk about a tree which is somehow
| isolated from the rest of the universe.
| Nevermark wrote:
| > My intuition agrees with yours here, but I think RQM is
| saying something different. The article says that in RQM,
| variables simply don't have any value unless a system acts
| on another system. Here is the relevant quote.
|
| It is saying between the values defined at interaction
| events of particles, there isn't any more information
| there.
|
| That has nothing to do with observability. They are saying
| that events are the information, not "things" between
| events.
|
| This is very different from whether we observe events or
| not.
|
| And any question as to whether interactions that we have
| observed directly or indirectly yet really exist, or are
| meaningful or not to talk about, would put us in the role
| of privileged, special observers.
|
| Which they explicitly rule out.
|
| Special observers has always been deeply problematic. If
| two people far apart in the universe have not yet
| interacted, which one isn't somehow meaningful in any way
| that doesn't just boil down to we have yet to have any
| information about them.
|
| So we can say that we don't have information about things
| we don't have information about yet.
|
| It's a tautology.
|
| Layering any other interpretation on an already plain to
| understand tautology, that already describes the situation
| perfectly, is neither necessary, nor useful in any way.
|
| What other meaning or non-meaning could there be? What
| could that even reflect in the physics?
| zmgsabst wrote:
| > If it had no interactions with anything else between its
| two bracketing events.
|
| > Like a discrete edge in a simple graph, it is a connection
| between two nodes, there are no dynamics in between the
| nodes. An edge is just a connection, not a continuous path.
|
| How do you explain redshift from the universe expanding?
| Nevermark wrote:
| Perhaps space and "particles" interact?
|
| There must be some kind of interaction since photons are
| energy and all energy not only travels through space but
| warps space. Gravity responds to all energy, not just mass.
|
| But more likely their interpretation would be that this is
| where the importance of "relative" comes in.
|
| Relative to us the particle has been red shifted, but that
| is because of our viewpoint from our space-time position
| and conditions of reception differ from its space-time
| point and conditions of emission. Or the relative topology
| between those events. But not because of any change in the
| photon during its journey.
| zmgsabst wrote:
| > Or the relative topology between those events.
|
| But the interaction is no longer simply a connection
| between us and the origin: the path along which the
| photon traveled changed our measurement.
|
| And indeed, _different_ paths can change different
| portions of it, eg, if you have something like an
| Einstein ring where part of it passed through a dark
| matter filament. At the point where the entire path needs
| to be considered because it interacts with space while
| passing, I'm not sure what sense it can be described as
| "an edge like a graph". That seems like a classic
| trajectory, ie, a continuous path.
| jampekka wrote:
| The question about reference frames, even relative ones, is
| really interesting and has quite a long history. The current
| view of them really started only with Newton, and e.g. Leibniz
| actually had a different view, and rejected the idea of space
| "independent" of objects or time "independent" of events.
|
| Leibniz was partly vindicated by Einstein, although I think
| Leibniz' idea of relationalism was even more fundamental (and
| perhaps untenable).
|
| https://en.m.wikipedia.org/wiki/Relationalism
|
| https://plato.stanford.edu/entries/leibniz-physics/
| tsarchitect wrote:
| > how is it that we can have a whole population of systems (eg
| scientists) eventually coming to a consensus about some
| phenomenon (eg the value of some physical quantity?) It seems
| necessary to have a unified global system coordinating the
| whole thing.
|
| Not necessarily. It's not a "global system coordination" thing,
| it is coming to a consensus that "we will use this reference
| frame as our starting point", which might look like a global
| system coordinating. I guess you can say that 'science'
| initiallizes a reference frame in which we can all participate,
| compare answers, reproduce results, etc.
| anon291 wrote:
| This is interesting to me. Some of the various paradoxes are
| resolved, but now consciousness does become even more important,
| because I cannot escape from the fact that my perspective (which
| is unique to me according to RQM) still seems very real. Now the
| entirety of the universe is literally an observation that I (and
| I alone) experience. But then why do I 'feel' like it happened.
| Do other things feel like things happened to them? Am I the only
| one out here?
| meneton wrote:
| No I'm here too. But we are not totally correlated. There is
| more correlation within you, than there is between you and me.
| We are both nodes in the universe's correlation network complex
| enough to model our own correlations.
| lisper wrote:
| Gotta put in a plug for QIT, which is very similar, but I think a
| little easier to understand:
|
| https://arxiv.org/abs/quant-ph/9512022
|
| and a more accessible version that I wrote a long time ago:
|
| https://flownet.com/ron/QM.pdf
| katabasis wrote:
| I'm a fan of Carlo Rovelli's popular science books. "The Order of
| Time" in particular left a big impression on me. He is a
| philosopher as well as a scientist, and he has a gift for writing
| beautiful and accessible prose about some pretty heady topics.
|
| If you are interested in RQM but find this article somewhat dry,
| check out his short book "Helgoland".
|
| I find both RQM and Rovelli's notion of "thermal time" (i.e. the
| idea causality and entropy are emergent properties dependent on
| our perspective, as opposed to fundamental features of reality)
| to be very convincing.
| babycheetahbite wrote:
| I just finished Rovelli's "Reality Is Not What It Seems" last
| weekend and enjoyed it. It is a broad and high level summary of
| the progress of physics and the book ends with some of the more
| speculative ideas being pursued. I especially enjoyed the
| "thermal time" ideas you mention, which he touches on in this
| book.
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