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In mathematical physics or theories of the physical world, an observerspace description can be a "literalist", "instrumentationalist" or "experiential" interpretation of physical phenomenology, where what appears to be happening is taken as physical reality ("what you see is what is there""logical positivism").

"Observer space" can also refer to a coordinate system designed to describe reality as it might be seen for a particular observer or class of observer, (e.g.: optical coordinates).

The approach attempts to model the universe using only physical observables, with more emphasis on describing the immediate phenomena reported by "instrumentation" and less on deriving deeper underlying causes - reality as it is seen to be, rather than how it is deduced to be.

Interpretation vs. observation

Although this sort of literalist approach can sometimes seem perverse, the way that an object is "seen" by an observer relates to the way that it is "seen" by the other bodies with which it interacts. The physics of the object's interactions is therefore (in a sense) the physics of "what it looks like", and of how the other objects with which it interacts are "seen" by it in turn. In this sense, "optical illusions" can be considered to be a legitimate part of physical reality, and visual artefacts can play a legitimate role in determining the behaviour of real physical laws, such as the laws concerning electromagnetic and gravitational fields.

  • The advantage of the observerspace approach is that it sometimes allows "visible" physics behaviour to be modeled directly without requiring a deeper underlying paradigm - this can let research programmes continue even when physics is "stalled" by shortcoming in existing interpretationalist paradigms, that cannot cope with some new aspect of physics theory.

  • The disadvantage of the observerspace approach is that sometimes invoking deeper principles allows behaviour to be described more efficiently, "surface physics" can sometimes be more complex and more difficult to understand, and does not always obviously seem to be obeying basic laws over small scales (such as conventional causality, or strict conservation laws). If a distant star moves behind a high-gravity object and is seen to split into a number of separate objects due to the gravitational lensing of its image, then although we should in theory be able to construct a description in which it really does break apart and reform in a strange way involving observer dependent reality and retrograde causality, it is simpler to say that the underlying behaviour is fixed ("the star does not split"), and to explain the apparent behaviour by the effect of gravity on light.

"Quantum" effects from "classical" underpinnings

As an example we can take the case of a building viewed by its reflection on a windblown lake. The reflection yields an image of a building whose behaviour appears to be deeply nonclassical: its surface is seen to fluctuate and ripple, and pieces of building seem to continually appear and disappear, and merge into and break away from the main structure, and at some times the entire building seems to be hovering disconnected from its foundations. Since a timelapse photograph of the rippling image shows a picture of a more conventional-looking building with sides that are straight (but a little "fuzzy"), we can construct a QM-style description of the building's image in which the physics includes strange fluctuating effects at small (time)scales, but behaves classically at larger (time) scales.

Alternatively, if we know about the physics of water and air and light, we can explain the same effect in terms of air blowing across a water surface and producing ripples, which produce a rippling reflection. However, since we cannot predict precisely how gusts of air will cause a certain pattern of ripples to be in, say, one hour's time, the "classical" explanation may not be any more accurate than the abstract quantum mechanical description, and if we do not know about the precise behaviour of air and water (or do not know whether these are the real cause of what we see), the QM description may be seen as more efficient. Where the interpretative approach scores is in its ability to deal robustly with a wider range of dynamic situations - it allows us to immediately imagine the sort of image that should result if we throw a stone at the reflected image: using a quantum description, the calculations might be theoretically possible, but might be unmanageably complex, and it might be difficult to be certain whether or not the calculations had been done correctly, how far they could be trusted, or how their results could be visualised.

Examples of observation-based approaches

  • Under special relativity ("SR"), new relationships are derived for the expected characteristics of signals passed between objects, using certain simplifying assumptions (such as flat spacetime), and by assuming that the act of observation - how things are seen to be - must conform to the principle of relativity. The special theory of relativity emphasises the relationships between observers, so in one sense this is an observerspace theory however, the emphasis on coordinate systems that are calibrated according to beliefs about the round-trip behaviour of light, (including light moving away from the observer) makes the theory's status in this regard more difficult to classify.

  • James Terrell later emphasised that although "to observe" usually means to observe impartially, or "to see" (e.g. "perfect observer"), Einstein's "observers" apply slightly different rules and are instructed to report partly-interpreted results under the special theory, an observed Lorentz contraction was not necessarily the same as a visible Lorentz contraction.

  • Under general relativity ("GR") , the older characterisation of forces due to acceleration as being is rejected, and Coriolis forces are embraced as being physically real. If an accelerated body senses an "apparent" gravitational field, then for that body the gravitational field is physically "real" and must be regarded as a genuine physical effect. The mutual interaction of masses under the theory then allows supposedly "unaccelerated" observers to also see side-effects of the field's relative distortion of spacetime, (e.g. Lense-Thirring effect). Einstein described his "breakthrough moment" with general relativity as being his realisation that a free-falling observer does not feel a gravitational field: if the falling observer cannot detect the field differential, then for that observer, the field differential effectively does not exist. Einstein later referred to this idea as "the happiest thought of my life".

  • The Membrane Paradigm chooses to treat black holes as if their apparent behaviour is physically real, even though we would normally consider some of these behaviours to be optical illusions. This approach, though counter-intuitive, manages to correctly model the basic characteristics of Hawking radiation effects (where general relativity does not).

  • Under quantum mechanics ("QM"), observerspace issues present themselves in the debate about whether the Hidden Variable Interpretation or the Copenhagen Interpretation should be regarded as more "correct". Albert Einstein argued that it was more efficient to assume that QM was a form of statistical mechanics relating to an underlying "non-spooky" physics that behaved according to conventional laws but whose properties could not be directly probed ("God does not play dice with the universe"). Niels Bohr argued that if one could use QM to describe everything that was seen, then talk of an additional "underlying" physics was redundant - the job of physics theory was to predict, not to explain (Bohr-Einstein debates).

  • In acoustics, the behaviour of an acoustic horizon shows some effects that overlap with quantum mechanics. Physics based on an acoustic metric allows a form of "hidden variable" in the form of information temporarily concealed behind acoustic horizons. Acoustic horizons described using acoustic coordinates generate apparent fluctuations and causality breakdowns similar to those that appear in a GR&QM treatment of a black hole horizon: however, in the acoustic counterpart, these effects are due to a purely classical effects that underpin the more "spooky" surface behaviour. With acoustic metrics, the underlying physics is causal and continuous, and the apparently acausal and discontinuous behaviour of the perceived physics is due to a breakdown in observational ability, and a consequent breakdown of coordinate system arguments based on direct observation, and of calculations based upon them (see, e.g.: coordinate singularity). This relationship is roughly analogous to the "rippling reflection" example given above.

  • In cosmology, the predicted behaviour of a cosmological horizon seems at least superficially similar to that of an acoustic horizon. If our observable universe is only part of a much larger universe, then events outside our directly-visible region (beyond our cosmological horizon) may be able to influence our future: if this is the case, then the information available to us within our visible region is not sufficient to predict all future events, and "observerspace" arguments are again incomplete.

Einstein and quantum mechanics

Einstein repeatedly used Ernst Mach's "logical positivist" approach as a tool for attacking and overturning older paradigms, but found that while "observer-centric" arguments were useful in a "revolutionary" approach to physics, they were not so useful in more "evolutionary" research. Although he would later publicly describe general relativity as being a theoretical implementation of Mach's ideas about mass and inertia (Princeton lectures, 1921), in private he was already expressing frustration at the "positivist" emphasis on "observables" in 1917 (Jeremy Bernstein, Einstein pp.109):

" I do not inveigh against Mach's little horse; but you know what I think about it. It cannot give birth to anything living, it can only exterminate harmful vermin."
At least some of this frustration seems to have been due to the growing popularity of the view that quantum mechanics, having used observer-centric arguments to overturn earlier classical models, was now being regarded as an "end product" rather than as a stepping stone to new and better classical descriptions. What was "seen" to happen according to QM was now increasingly being described as physical reality.

Einstein discussed his view, that observed reality was secondary to "real" physics, with Werner Heisenberg in 1926:
(Bernstein, pp.155 ):

1926. Heisenberg still had the notion that Einstein then held the kind of Machian positivistic views - the idea that all quantities that entered a physical theory must have "operational definitions" in terms of measuring instruments - which characterised the analysis leading to the special theory. He did not realise that Einstein had abandoned this position many years earlier when he was seeking his final formulation of the theory of gravitation. Hence Heisenberg was astounded when Einstein asked, "But you dont seriously believe that none but observable magnitudes must go into a physical theory?" To which Heisenberg replied, with some surprise, isn't that precisely what you have done with relativity? "
As Heisenberg recalls, Einstein replied, "Possibly I did use this kind of reasoning but it is nonsense all the same. Perhaps I could put it more diplomatically by saying that it may be heuristically useful to keep in mind what one has actually observed. But on principle, it is quite wrong to try founding a theory on observable magnitudes alone. In reality the very opposite happens. It is the theory which decides what we can observe "

Heisenberg relates this story in his book Encounters with Einstein and cites Einstein's statement "the theory determines what can be observed" as the inspiration of his famous uncertainty principle.

Observerspace as "seed theory"

Although observerspace arguments are sometimes argued to be "incomplete" (e.g. by Einstein, with respect to quantum theory) they do sometimes act as a guide to physical behaviour in regions of physics where an underlying paradigm has not yet been found, has been overlooked, or appears to be faulty - it can serve as a "no paradigm" paradigm. A deeper underlying paradigm can be easier to recognise once the correct physical behaviour has already been sketched out using observerspace arguments. Sometimes, (as with the membrane paradigm) the most valuable new insights can be obtained by constructing an observerspace model that is so obviously wrong that it's consequences have not yet been properly studied.

Gravitational time dilation

In another example from Einstein's career, the idea that gravity produces a spectral shift in light (gravitational shifts, John Michell, 1784) should cause observers at different heights in a gravitational field to see each other to appear to have different rates of timeflow - in his 1911 paper, Einstein followed this argument though to its inevitable conclusion, that at least part of the effect had to be genuine, and objects placed at different heights really must age at different rates, regardless of whether or not this effect (gravitational time dilation), had any precedents in the physics theory of the day, and whether or not it was compatible with usual definitions and assumptions being used. Einstein's "nave" approach, concentrating on what different observers should see, correctly identified a new consequence of gravitation that had been overlooked by more conventional mathematical research.

Velocity as gravity

We might choose to extend these observerspace arguments (somewhat stubbornly) to moving-body problems: a photograph of a receding object appears redshifted as if a gravitational gradient exists between it and the observer, and the associated effects expected for a gravitational field - apparent alteration in the object's dimensions, distances and angles - are also "visible" (spatial Doppler effect and simple aberration due to velocity). When photographs taken from different angles are collated, the moving object appears to be associated with a polarised gravitational field, with a redshifted rear and a blueshifted front, and seems to be showing associated asymmetrical gravitational lensing effects (conventional aberration of the object's fieldlines and visible geometry).

Taking this correspondence seriously, and treating this apparent gravitational field as "real", one would predict that a moving object should be seen to have a real velocity-dependent gravitational component pointing in its direction of motion, that should drag light along with it - this produces a "quick and dirty" argument for the existence of velocity-dependent gravitomagnetic terms, for the Lense-Thirring effect for rotating bodies (by assuming that the receding redshifted side of a rotating star exerts a stronger gravitational pull than its approaching blueshifted side), and for gravitational dragging associated with higher-order Doppler effects due to acceleration (accelerational frame-dragging under GR).

Again, the observerspace principle - that as well as the underlying physics being understood to be consistent, the visible physics must also be seen to be consistent - generates unfamiliar descriptions that have a surprising degree of initial self-consistency, and which in this case successfully produce results normally associated with the general principle of relativity.

In these sorts of observerspace exercises, the challenge is often not to find a description that is considered to be conceptually "correct", but to take a nave interpretation of visible effects to what would normally be considered unreasonable lengths in order to obtain new phenomenology (from which a new paradigm can sometimes be born). This approach to physics theory can seem reckless and haphazard (rather like Douglas Adams's description of the "trick" of flying, as throwing oneself at the ground and missing) but history has repeatedly shown it to be a valuable method of last resort when more conventional "incremental" approaches have failed.

  • Ernst Mach, The Science of Mechanics, 6th ed. (1960)
  • Werner Heisenberg, Encounters with Einstein: and other essays on people, places, and particles (1989)
  • Jeremy Bernstein, Einstein (1973)
  • James Terrell, "Invisibility of the Lorentz Contraction" Physical Review 116, 1041-1045 (1959)
  • A. Einstein, "On the influence of Gravitation on the Propagation of Light" translated and reprinted in The Principle of Relativity (1923),
  • Einstein 1921 Princeton lectures, translated and reprinted in The Meaning of Relativity (1955)

This article is a modified version of the one originally submitted by the author to Wikipedia, as "Observerspace"