Generalised Uncertainty Relations from Finite-Accuracy Measurements

In this short note we show how the Generalised Uncertainty Principle (GUP) and the Extended Uncertainty Principle (EUP), two of the most common generalised uncertainty relations proposed in the quantum gravity literature, can be derived within the context of canonical quantum theory, without the need for modified commutation relations. A GUP-type relation naturally emerges when the standard position operator is replaced by an appropriate Positive Operator Valued Measure (POVM), representing a finite-accuracy measurement that localises the quantum wave packet to within a spatial region $\sigma_g>0$. This length scale is the standard deviation of the envelope function, $g$, that defines the POVM elements. Similarly, an EUP-type relation emerges when the standard momentum operator is replaced by a POVM that localises the wave packet to within a region $\tilde{\sigma}_g>0$ in momentum space. The usual GUP and EUP are recovered by setting $\sigma_g \simeq \sqrt{\hbar G/c^3}$, the Planck length, and $\tilde{\sigma}_g \simeq \hbar\sqrt{\Lambda/3}$, where $\Lambda$ is the cosmological constant. Crucially, the canonical Hamiltonian and commutation relations, and, hence, the canonical Schr{\"o}dinger and Heisenberg equations, remain unchanged. This demonstrates that GUP and EUP phenomenology can be obtained without modified commutators, which are known to lead to various pathologies, including violation of the equivalence principle, violation of Lorentz invariance in the relativistic limit, the reference frame-dependence of the `minimum' length, and the so-called soccer ball problem for multi-particle states.


Introduction
In canonical quantum mechanics the Heisenberg uncertainty principle (HUP) implies a fundamental trade-off between the precisions of position and momentum measurements. 4 It can be introduced heuristically, via the famous Heisenberg microscope thought experiment, giving [1,2] ∆x i ∆p j 2 δ i j , (1.1) or derived rigorously from the canonical quantum formalism, yielding [3,4] ∆ ψ x i ∆ ψ p j ≥ 2 δ i j . (1. 2) The inequality in Eq. (1.2) is exact and, unlike the heuristic uncertainties ∆x i and ∆p j in Eq.
(1.1), ∆ ψ x i and ∆ ψ p j represent well-defined standard deviations of the probability distributions |ψ(x)| 2 and |ψ (p)| 2 , respectively, where the momentum space representation of the particle wave function is given by the -scaled Fourier transform of its position space representation: We emphasise the scale-dependence of the canonical quantum Fourier transform, which is often neglected in standard treatments, by introducing the subscript . The relation (1.2) is obtained by combining the Schrödinger-Robertson relation for arbitrary Hermitian operators,Ô 1 and O 2 [5,6], with the canonical position-momentum commutator, (1. 5) In recent years, thought experiments in quantum gravity research have suggested the existence of generalised uncertainty relations (GURs). By reconsidering Heisenberg's 1927 gedanken experiment, and accounting for the gravitational interaction between the massive particle and the probing photon, we obtain the generalised uncertainty principle (GUP), where α 0 is a numerical constant of order unity [7][8][9]. By minimising the right-hand side with respect to ∆p j , the GUP implies the existence of a minimum position uncertainty of the order of the Planck length, l Pl = G/c 3 10 −33 cm. Reconsidering Heisenberg's arguments in the presence of a constant dark energy density ρ Λ = Λc 2 /(8πG) 10 −30 g . cm −3 [10,11], or, equivalently, an asymptotically de Sitter background with minimum scalar curvature of the order of the cosmological constant, Λ 10 −56 cm −2 [12,13], gives the extended uncertainty principle (EUP), where η 0 is of order one [14][15][16]. The EUP implies the existence of a minimum momentum uncertainty of the order of the de Sitter momentum, m dS c = Λ/3 10 −56 g . cm s −1 . This is physically reasonable since it is the minimum momentum that a canonical quantum particle can possess, when its wave function is localised within the asymptotic de Sitter horizon, which is comparable to the present day radius of the Universe r U (t 0 ) l dS = 3/Λ 10 28 cm.
Combining both effects yields the extended generalised uncertainty principle (EGUP), gives rise to an EGUP-type uncertainty relation, at least when both P j ψ = 0 and X i ψ = 0 [17]. Here, we use capital letters to denote modified operators, which generate modified commutators, and lower case letters to denote their canonical quantum counterparts. However, the assumption above is problematic since, even if both P j ψ = 0 and X i ψ = 0 in a given frame of reference, a simple shift of coordinate origin or a Galilean velocity boost of the observer alters the numerical value of the associated Schrödinger-Robertson bound: This leads immediately to the reference frame-dependence of the (supposedly invariant) minimum length. In fact, the situation is even worse since even a redefinition of the positioncoordinate origin alters the value of the bound on the right-hand side. This gives rise to a coordinate-dependent 'minimum' length, which is clearly unphysical, and which strongly suggests that GUR models based on modified commutation relations are not mathematically selfconsistent [18,19] In addition, the modified position-momentum commutator (1.9) implies a modification of the canonical Heisenberg equation, which immediately gives rise to mass-dependent accelerations for quantum particles, violating the equivalence principle [20,21]. Such models also violate Lorentz invariance in the relativistic limit and suffer from the so-called soccer ball problem, so that sensible GUP-compatible multi-particle states cannot be defined [22,23]. 5 The heuristic, model-independent nature of the gedanken experiments that lead to the relations (1.6), (1.7) and (1.8), together with the pathologies displayed by modified commutator models, motivate us to consider alternative ways to generate GUP, EUP and EGUP phenomenology, without modifying the canonical Heisenberg algebra. In this paper, we consider one way in which such a scheme can be implemented from within the canonical quantum formalism. The physical basis of the model is the notion of a finite-accuracy measurement and these are represented mathematically by the construction of appropriate POVM. Roughly speaking, since errors add in quadrature for independent random variables, finite-accuracy measurements of position and momentum with detection 'sweet spots' of width σ g l Pl andσ g m dS c, respectively, give rise to the GUP and EUP, to first order in the relevant Taylor expansion. These individual relations may then be combined to give the EGUP.

GUR from finite-accuracy measurements described by POVM
In this section, we show that GUP, EUP and EGUP-type uncertainty relations can be derived in an effective model, where position and momentum measurements in canonical quantum theory are not perfectly accurate and are described by POVMs, rather than perfect projective measurements.
Let us begin by replacing the usual position-measurement operator,x, with POVM elements corresponding to the result x:Ê These elements satisfy the relationsÊ † xÊx ≥ 0 and Ê † xÊx d 3 x =Î, as required, so that Eq. (2.1) defines a standard POVM in canonical quantum mechanics [24]. From here on, we refer to g as the 'envelope function' 5 In [23] an ingenious solution to the soccer ball problem was proposed by Amelino-Camelia. In this approach, the generalised momentum operators of a given modified commutator model are defined to be the generators of 'generalised spatial translations'. The unitary transformationÛ(X) := exp[(i/ )X.P], which acts nontrivially only on theX i operators, is required to leave the modified [X i ,Pj], [X i ,X j ] and [Pi,Pj] algebras, as well as the multi-particle Hamiltonian of the model,Ĥ, invariant. This defines the 'generalised translation symmetries' of the system and, when these symmetries hold, the corresponding Noether charge for an N -particle state is represented by the operatorP Total := N I=1P I , where [P Total ,Ĥ] = 0. The usual law of linear momentum addition therefore holds for multi-particle states but a different nonlinear addition law, derived ultimately from the notion of spatial locality, holds for transfers of momentum between individual particles, due to the interactions specified byĤ [23]. Unfortunately for GUP models, in the example system considered in [23], the definition of the generalised spatial translations required to maintain the linear addition law also requires one of the position-momentum commutators to equal zero, i.e., [X i ,Pi] = 0, for some i. In this case there is no Heisenberg uncertainty principle, let alone a GUP, even though a minimum length scale l still appears in the model via the position-position commutator, e.g.
[X1,X2] = ilX1. This illustrates a general point, that it is by no means certain whether a particular modified momentum operator, corresponding to a particular modification of the canonical Heisenberg algebra, and, hence, a particular form of the GUP, is compatible with a linear addition law derived via Amelino-Camelia's procedure. Therefore, although this procedure represents a useful criterion for defining physically viable GUP models, it is clear that arbitrary deformations of the canonical Heisenberg algebra are not consistent with the existence of a linear momentum addition law and that further work is required to determine which models truly suffer from a soccer ball problem and which ones do not. Though some GUP models may be free from this pathology, a great many could still be afflicted by it. of the measure. For spherically symmetric functions the envelope is centred on the value x, and, for the sake of concreteness, we may imagine |g(x − x)| 2 as a three-dimensional Gaussian distribution with mean x and standard deviation σ g .
Finite-accuracy position measurements, conducted on an arbitrary state |ψ , then give rise to the first and second order moments Since |g(x − x)| 2 is a normalised function centred on x = x, x g = 0, and the corresponding variance is given by where σ g := σ i g e i and σ i g denotes the width of |g| 2 in each coordinate direction x i . By spherical symmetry, σ i g = σ g for all i, and we may rewrite Eq. (2.3) in terms of the individual components as where we have used the shorthand notation ∆ ψ E i ≡ ∆ ψ E x i . In like manner, finite-accuracy momentum measurements may be introduced via the opera-torsÊ where |g(p − p)| 2 dp = 1, but it is important to note that there is no intrinsic relation between the functions g andg, which may be chosen independently for a given POVM model. Nevertheless, if both |g| 2 and |g| 2 represent Gaussian distributions, which is perhaps the most natural choice for an envelope function, then g andg are related via a Fourier transform, where the new action scale β = is given by andσ g is the standard deviation of |g| 2 . However, it is equally important to note that there is nothing fundamental about the relation (2.6). Unlike the -scaled Fourier transform relating the position and momentum space representations of the quantum wave function, Eq. (1.3), the β-scaled transform relates the 'envelope functions' of the model. Finite-accuracy momentum measurements, conducted on an arbitrary state |ψ , then give rise to the first and second order moments where p n f := p n |f (p)| 2 d 3 p withf (p) =g(p) orψ (p). Since |g(p − p)| 2 is normalised and centred at p = p, p g = 0, and whereσ g :=σ gj e j andσ gj denotes the width of |g| 2 in each momentum space direction p j . Again employing spherical symmetry,σ gj =σ g for all j, so that (2.9) may be rewritten in terms of the individual components as where we have again used the shorthand To obtain a GUP-type relation from Eq. (2.4) we simply take the square root, Taylor expand the right-hand side to first order, and substitute for ∆ ψ x i from the HUP (1.2). Likewise, an EUP-type relation is obtained from (2.10) by taking the square root, Taylor expanding to first order, and substituting for ∆ ψ p j . Next, using the substitutions where we have relabelled ∆ ψ E i ≡ ∆ ψ X i and ∆ ψ E j ≡ ∆ ψ P j , for convenience. These expressions are formally analogous to the heuristic relations (1.6) and (1.7), respectively, but with ∆p j and ∆x i on the right replaced by the well-defined standard deviations ∆ ψ p j and ∆ ψ x i . This proves that GUP-and EUP-type relations can be derived rigorously, from within the canonical quantum formalism, but a remaining criticism of the formulae above is that the uncertainties on the right-hand sides of Eqs. (2.13)-(2.14) are not equivalent to the uncertainties on the left. Indeed, according to the POVM model, ∆ ψ p j and ∆ ψ x i are not operationally observable quantities. They arise only in the limits σ g → 0 andσ g → 0, respectively, in which both (2.13) and (2.14) reduce to the standard HUP (1.2). This objection can be overcome, however, by first substituting for ∆ ψ x i from (1.2) in Eq. (2.4) and then again for ∆ ψ p i from (2.10). This gives rise to an uncertainty relation between the observable standard deviations, ∆ ψ E i ≡ ∆ ψ X i and ∆ ψ E j ≡ ∆ ψ P j . It is straightforward to show that, taking the square root, Taylor expanding to first order, and neglecting the final term of order σ gσg l Pl . m dS c, this relation reduces to Therefore, the EGUP can be rigorously derived within the canonical quantum formalism. The GUP and EUP proper then arise as limits of this more fundamental relation.
We stress that, in this model, ∆ ψ E i ≡ ∆ ψ X i and ∆ ψ E j ≡ ∆ ψ P j represent the physically observable precisions, obtained from generalised position and momentum measurements with finite accuracies σ g > 0 andσ g > 0. By contrast, the canonical Hamiltonian is determined by the canonical (projective) position and momentum operators,x andp, viaĤ =p 2 /(2m)+V (x), where the former obey the canonical Heisenberg algebra: This leaves the canonical Heisenberg and Schrödinger equations unchanged and neatly evades the pathologies that afflict modified commutator models [18][19][20][21][22].

Discussion
We have shown that the three most common GURs studied in the quantum gravity literature, the GUP, EUP and EGUP, can be derived from within the formalism of canonical quantum mechanics. A GUP-type uncertainty relation is obtained when the standard (projective) position operator is replaced by an appropriate POVM, representing finite-accuracy measurements with error bars of width σ g > 0 in real space. In like manner, an EUP-type relation is obtained from finite-accuracy measurements with error bars of widthσ g > 0 in momentum space. These can be combined to give a relation that is formally analogous to the EGUP and the standard EGUP is recovered by setting σ g l Pl , the Planck length, andσ g m dS c, where m dS = ( /c) Λ/3 is the de Sitter mass.
This work suggests that GUP, EUP and EGUP phenomenology can be understood in a physically intuitive way, as a simple and natural outcome of finite-accuracy measurements. Such measurements are capable of generating all three GURs and the same phenomenology is obtained, at the level of the uncertainty relations, regardless of whether the limits (∆ ψ X i ) min = σ g and (∆ ψ P j ) min =σ g are fundamental, or merely effective, as an outcome of an imperfect measurement scheme.
We propose that this should give pause for thought to the GUP community. If modified commutators are not necessary for GUP phenomenology, and, after nearly 30 years of research, we are no closer to resolving the pathologies that have afflicted these models since they were first proposed in the mid-1990's, then serious attempts should be made to find alternative mathematical structures that give rise to GURs. These should be capable of generating, via rigorous derivation, the uncertainty relations predicted by model-independent gedanken experiments, but without the problems associated with modified commutation relations.
In this paper, we have proposed one such model, within the context of canonical quantum theory. Another, more radical, alternative is to consider additional quantum mechanical degrees of freedom, not present in the canonical theory, which are capable of describing quantum fluctuations of the background geometry. Such a model was proposed in a recent series of works [25][26][27][28][29] and shares many features with the model described here, including the existence of a new action scale that relates the accuracies of generalised position and momentum measurements, β := 2σ gσg 10 −61 (*). The fundamental difference between the two models is the existence of new degrees of freedom in the latter. From this, it follows that the new action scale β implies a modified de Broglie relation of the form p = k + β(k − k), where, here, p denotes the observable momentum. Heuristically, the non-canonical term β(k − k) can be interpreted as an additional momentum 'kick', transferred to the canonical wave function by a quantum fluctuation of the background. The interested reader is referred to [18,19,[25][26][27][28][29] for further details.
At first glance, this more radical alternative has nothing to do with the POVM approach described here. It requires extra degrees of freedom associated with the quantum state of the background geometry, contrary to the POVM formalism, which remains entirely within the context of canonical quantum theory. It follows from Stinespring's dilation theorem [30,31], however, that the two formalisms are equivalent if we assume the particular values, σ g l Pl and σ g m dS c, and hence the relation (*) above. The POVM picture results from tracing out the x (p ) degrees of freedom associated with quantum fluctuations of the background and the x (p ) degrees of freedom appear as a consequence of dilating the POVM. The POVM approach describes a quantum measurement of finite accuracy. The minimum resolution of the measurement may be due to technical limitations, or it can reflect the fact that the minimum length and momentum scales are fundamentally related. We postulate that in a universe with both fundamental and technological limitations to measurement accuracy, the complete description of a realistic quantum measurement should be a POVM extension of the model presented in [25,26]. We expect that this would give rise to two additional contributions to the position and momentum variances, i.e., σ 2 g + σ 2 h andσ 2 g +σ 2 h , respectively, where g is the fundamental smearing function that models the quantum indeterminacy of space-time, and h is the envelope function of a realistic detector. In the limit σ h σ g ,σ h σ g , which corresponds to all present-day measurements, the latter are expected to dominate the former.