Relational Quantum Mechanics
plato.stanford.edu
plato.stanford.edu
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.
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.
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...
> 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.
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?
> 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?
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.
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.
A simpler case of the latter is uncontroversial: If you move quickly away from a photon emitter, you will detect red-shifted photons.
Clearly, the photons didn't change, only our relation to them.
Energy, like everything else, is relative and appears to have different quantities in different situations. Strange as that seems, given conservation of energy still holds.
But energy, just like space-time, is not "Euclidean".
That’s irrelevant navel gazing: the operator which describes the change depends on the continuous path between.
> Strange as that seems, given conservation of energy still holds.
I don’t believe it does when factoring in expansion — or at least, that hasn’t been shown.
I felt Wolfram Graph vibes reading this article. They seem like such a nice way to model discrete theories. I am also wondering if perhaps General Relativity could be modeled in a similar way - in a way that could be used to simulate N-body systems, vice the instantaneous Newtonian ones that are used in cosmology, and justified, IMO, on shaky grounds related to aberration. Indeed, the article mentions explicitly that RQM is related to quantum gravity.
Leibniz was partly vindicated by Einstein, although I think Leibniz' idea of relationalism was even more fundamental (and perhaps untenable).
>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.
More: https://en.m.wikipedia.org/wiki/Mach%27s_principle
>The proposition is that the existence of absolute rotation (the distinction of local inertial frames vs. rotating reference frames) is determined by the large-scale distribution of matter, as exemplified by this anecdote:[2]
>>You are standing in a field looking at the stars. Your arms are resting freely at your side, and you see that the distant stars are not moving. Now start spinning. The stars are whirling around you and your arms are pulled away from your body. Why should your arms be pulled away when the stars are whirling? Why should they be dangling freely when the stars don't move? [a][2]
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.
https://arxiv.org/abs/quant-ph/9512022
and a more accessible version that I wrote a long time ago:
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.
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.
"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.
Layperson here. Can I read this as saying yes to locality, no to realism, and violation of causality since this only depends on who measured it, another interaction could interpret cause and effect differently?