Observerspace
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.
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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.
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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".
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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).
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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).
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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.
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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 don’t 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 "naïve" 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 naïve 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.
References
- 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)
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