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The communications system

by ConradDJ
Tags: communications
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ConradDJ
#1
May21-11, 01:08 PM
PF Gold
P: 302
In his 1996 paper on Relational Quantum Mechanics, Carlo Rovelli proposed that “a complete description of the world is exhausted by the relevant information that systems have about each other.” I’m interested in exploring here what such a description of the world looks like.

I’m distinguishing here between the reality of physical things in themselves – particles, fields, spacetime, etc. – and the information about them that gets physically communicated to other things. Rovelli claimed that QM does not describe the nature of reality at all, only the nature of things as “observed” by other things. And he suggested that at a basic level, it’s not even meaningful to talk about the reality of things “in themselves” – since whether or not such a reality actually exists, all physics can ever find out about concerns the web of interaction that communicates empirical information.

Of course this doesn’t mean we should stop talking about reality. Clearly it’s vastly more efficient to say what something really is than to say how it appears from many different viewpoints, in many different contexts. And we know that for most theoretical as well as practical purposes, this kind of objective, “object-oriented” description works extremely well. But we also know that QM, our best fundamental theory, does in fact represent “the state of a system” by a mathematical function that encapsulates all the possible ways it could be observed in all available contexts.

Here I don’t want to argue about how to interpret QM. But however we might interpret it, I think Rovelli is certainly right: what physics actually deals with, in practice, is some kind of system of physical interactions that communicate information. That is, what we call reality is a highly consistent “picture” of the world that gets communicated through this system of physical appearances. This “picture” is the only reality we can know, empirically. Whether it corresponds to an absolute reality existing “in itself” is not a question for science.

On the other hand, we do know that this communications system exists, if we know anything at all. So my question is, how can we best describe it? What do our best theories tell us, not about physical reality, but about how information gets communicated?

I’ll offer for discussion, below, a few different ways to approach this question. But for the sake of argument, let’s accept a couple of Rovelli’s assumptions – that any physical system counts as an “observer” insofar as it exchanges information with other systems, and that any interaction is a “measurement” if it occurs in a context in which information can be extracted from it.

That means, we’re not trying to add anything to physics that has to do with what’s special, say, about human consciousness. This is not about how we experience the world – I want to look at the interactive structure of the world itself, that can be “experienced” by a person or a measuring instrument or an atom.
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ConradDJ
#2
May21-11, 01:40 PM
PF Gold
P: 302
Let’s begin with a theory that’s much less problematic than QM, but that still says something about the basic structure of the world – i.e. Special Relativity. Here’s a standard spacetime diagram –

http://physicsforums.com/attachment....1&d=1306002190


If we ask what part of this diagram represents the physical communications system, the answer is obviously the double light-cone. All information physically available to an observer at a particular point in spacetime comes either through interaction on the light-cone – which is mainly electromagnetic – or through interaction mediated by massive particles, which is highly localized.

So this immediately gives us the large-scale structure of the communications system. It’s a 4-dimensional web of light-speed interactions, connecting the local “present moments” of different observers. This is simply the familiar structure of Minkowski spacetime.

I think there’s already a lot to comment on here. I’ll make a couple of suggestions, and probably come back to this again later on.

My first comment is that the topology of the physical communications system is very different from the topology of the R^4 manifold that we usually ascribe to physical reality.

That is, the picture of the world that gets conveyed through the communications system – the “information content” of that system, that we call reality – is what’s often described as a 4-dimensional “block universe”. This reality is vast both in space and in time. It has a long history going back more than 13 billion years, and it will no doubt go on long into the future. We’ve made measurements in recent years that give us a lot of information about the state of things nearly all the way back to the beginning. So we have a very good historical picture of reality laid out in 4-D space and time.

This “block universe” picture corresponds to the above diagram, which is a 2-D rendering of a 3-D image of this 4-D “block”, with the vertical space-axis labeled as “Time”.

However, this is not a picture of Minkowski spacetime. In the geometry of the diagram, the distance between any two points in space and time is (s2 + t2)-2. In the physical spacetime of the communications system, the “absolute” distance between two events is (s2 – t2)-2. I.e. there’s a “null” spacetime interval between any two points on a lightcone.

This makes a big difference. For example, the diagram’s geometry includes a lot of space outside the observer’s light-cone. But in the web of communicated information there are no spacelike surfaces or intervals. Any observer’s 3-dimensional “space” is on the light-cone, where information comes in from the past or goes out into the future.

Now in the case of both geometries, this 3-dimensional space is vast. An observer can get information from distant galaxies, and we can also send messages out over great distances. But the time-dimension in the observer’s world is extremely “thin”. A person or a measuring device can only operate in this moment “now”, dealing with information that’s immediately available in this “real time” physical situation. So the physical “here and now” of any observer is indeed a very thin 3-dimensional “hypersurface” – but it’s the surface of the past light-cone, not the one labeled “hypersurface of the present” in this diagram.

Another basic difference between this topology of communications and R^4 is that long-distance connections are all one-way connections in time. I can be physically present and interact “in the moment” only with people and things that are here, in my locality. Things that are distant from me in space are equally distant in time. Actually, since the speed of light is so fast relative to the speeds at which I operate, anything on Earth is close enough to be effectively “in my locality”. So we all effectively share the same “now”. But I don’t share a physical “present moment” with someone on Mars – we could communicate, back and forth, but only with a time delay of several minutes. This “delay” isn’t accidental, it’s built into the geometry of spacetime itself.

Okay, what does this mean? For purposes of this thread, it probably makes sense to gather more evidence before jumping to conclusions. So I’d like to ask what else physics has to tell us about the physical communications system, besides its large-scale spacetime structure.

But I think it’s very interesting that what the equations of Relativity describe corresponds more closely to the structure of the physical communications system itself, than to the structure of the communicated “information content” that we call reality. In other words, Relativity seems not so different from Quantum Mechanics in this respect. They both seem to be telling us about the world of interaction we observe, rather than about the reality we imagine is out there, "behind" the appearances.
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