English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

We show, through an explicit calculation of the relevant Green's functions,that the transverse-traceless (TT) portion of the gravitational perturbationsof Minkowski spacetime and of spatially flat cosmologies with a constantequation-of-state $w$ receive contributions from their isolated mattersource(s) outside the past null cone of the observer. This implies the TTgravitational wave (GW) cannot be a standalone observable -- despite widespread(apparent) claims in the gravitational wave literature to the contrary. About aMinkowski background, all 4 of the gauge-invariant variables -- the twoscalars, one vector and tensor -- play crucial roles to ensure the spatialtidal forces encoded within the gauge-invariant linearized Riemann tensor arecausal. These gravitational tidal forces do not depend solely on the TTgraviton but rather on the causal portion of its acceleration. However, in thefar zone radiative limit, the flat spacetime `TT' graviton Green's functiondoes reduce to the causal `tt' ones, which are the ones commonly used tocompute gravitational waveforms. Similar remarks apply to the spin-1 photon;for instance, the electric field does not depend solely on the photon, but isthe causal part of its velocity. As is known within the quantum theory ofphotons and linearized gravitons, there are obstacles to the construction ofsimultaneously gauge-invariant and Lorentz-covariant descriptions of thesemassless spin-1 and spin-2 states. Our results transparently demonstrate thatthe quantum operators associated with the helicity-1 photon and helicity-2linear graviton both violate micro-causality: namely, they do not commuteoutside the light cone in flat and cosmological spacetimes.

The Transverse-Traceless Spin-2 Gravitational Wave Cannot Be A Standalone Observable Because It Is Acausal