AbstractThe symplectic quantization scheme proposed for matter scalar fields in the companion paper (Gradenigo and Livi, arXiv:2101.02125, 2021) is generalized here to the case of space–time quantum fluctuations. That is, we present a new formalism to frame the quantum gravity problem. Inspired by the stochastic quantization approach to gravity, symplectic quantization considers an explicit dependence of the metric tensor $$g_{\mu \nu }$$
g
μ
ν
on an additional time variable, named intrinsic time at variance with the coordinate time of relativity, from which it is different. The physical meaning of intrinsic time, which is truly a parameter and not a coordinate, is to label the sequence of $$g_{\mu \nu }$$
g
μ
ν
quantum fluctuations at a given point of the four-dimensional space–time continuum. For this reason symplectic quantization necessarily incorporates a new degree of freedom, the derivative $${\dot{g}}_{\mu \nu }$$
g
˙
μ
ν
of the metric field with respect to intrinsic time, corresponding to the conjugated momentum $$\pi _{\mu \nu }$$
π
μ
ν
. Our proposal is to describe the quantum fluctuations of gravity by means of a symplectic dynamics generated by a generalized action functional $${\mathcal {A}}[g_{\mu \nu },\pi _{\mu \nu }] = {\mathcal {K}}[g_{\mu \nu },\pi _{\mu \nu }] - S[g_{\mu \nu }]$$
A
[
g
μ
ν
,
π
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ν
]
=
K
[
g
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ν
,
π
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]
-
S
[
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]
, playing formally the role of a Hamilton function, where $$S[g_{\mu \nu }]$$
S
[
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ν
]
is the standard Einstein–Hilbert action while $${\mathcal {K}}[g_{\mu \nu },\pi _{\mu \nu }]$$
K
[
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ν
,
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]
is a new term including the kinetic degrees of freedom of the field. Such an action allows us to define an ensemble for the quantum fluctuations of $$g_{\mu \nu }$$
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ν
analogous to the microcanonical one in statistical mechanics, with the only difference that in the present case one has conservation of the generalized action $${\mathcal {A}}[g_{\mu \nu },\pi _{\mu \nu }]$$
A
[
g
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ν
,
π
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]
and not of energy. Since the Einstein–Hilbert action $$S[g_{\mu \nu }]$$
S
[
g
μ
ν
]
plays the role of a potential term in the new pseudo-Hamiltonian formalism, it can fluctuate along the symplectic action-preserving dynamics. These fluctuations are the quantum fluctuations of $$g_{\mu \nu }$$
g
μ
ν
. Finally, we show how the standard path-integral approach to gravity can be obtained as an approximation of the symplectic quantization approach. By doing so we explain how the integration over the conjugated momentum field $$\pi _{\mu \nu }$$
π
μ
ν
gives rise to a cosmological constant term in the path-integral approach.
Short-time quantum detection: Probing quantum fluctuations
