On Sun, May 25, 2025 at 3:33 PM Alan Grayson <agrayson2...@gmail.com> wrote:
*> I'm disagreeing with anyone, including you, who thinks the EP is an > absolute, when in fact it's a relative, an approximation.* The Equivalence Principle, which is the foundation of General Relativity, states that at sufficiently small scales there is no way to tell the difference between a gravitational field and a simple acceleration. And it is not an approximation. But is it always correct? That is not certain because General Relativity does not take Quantum Mechanics into account, nevertheless so far at least the Equivalence Principle has easily passed every experimental test put to it. For fun I asked GPT to do "Deep Research" on the following questions: "Have there been any proposed experimental tests to see if Einstein's Equivalence Principle remains true even when quantum mechanics is taken into account? In other words tests to determine if gravity can change the quantum entanglement of two particles, and if not are any such tests planned?" *SPOILER ALERT*: GPT's ultimate conclusion was "So far, all completed tests have found *no anomalous effect* – entangled states remain intact in different gravitational or accelerated frames, consistent with EEP and standard quantum theory" GPT put those words in boldface type, not me. Below is GPT's full response: ======== Tests of the Equivalence Principle with Quantum Entanglement Einstein’s equivalence principle (EEP) – that all objects fall identically in a gravitational field – is classically tested to extreme precision. Extending EEP to quantum systems, especially entangled ones, poses conceptual challenges (the EEP is classically formulated and its quantum generalization “requires careful considerations”ar5iv.org <https://ar5iv.org/pdf/2203.11628#:~:text=The%20EEP%20is%20however%20formulated,the%20notion%20of%20test%20particles>). Recent proposals and experiments therefore ask: *Can gravity alter quantum entanglement?* Would any change signal a breakdown of EEP? Below we summarize key efforts, completed or proposed, that probe this question using entangled photons or atoms in varying gravitational or inertial conditions. Space- and Satellite-Based Experiments - *Micius satellite test (Xu et al. 2019)* – An experiment by Pan, Ren, Ralph *et al.* (China/U.S.) sent one photon of a time-energy–entangled pair to orbit aboard the Micius satellite while keeping the other photon on the groundpub.math.leidenuniv.nl <https://pub.math.leidenuniv.nl/~gillrd/Peking/Files/Xu-2019-Satellite-testing-of-a-gravitationa.pdf#:~:text=containing%20closed%20timelike%20curves%20and,the%20prediction%20of%20event%20formalism>. This setup exposed the two photons to different gravitational potentials. The goal was to test predictions of a speculative “event formalism” model that entanglement would *probabilistically decohere* when particles traverse different gravity gradients. The result (published in *Science*) was that no abnormal decoherence was seen: the entanglement correlations matched standard quantum theory, *not* the model’s decohering prediction pub.math.leidenuniv.nl <https://pub.math.leidenuniv.nl/~gillrd/Peking/Files/Xu-2019-Satellite-testing-of-a-gravitationa.pdf#:~:text=containing%20closed%20timelike%20curves%20and,the%20prediction%20of%20event%20formalism>. In other words, despite the large gravitational potential difference, the quantum entanglement survived intact. *(Team: Jian-Wei Pan et al., USTC and collaborators; status: completed 2017/19.)* - *Space-QUEST mission concept (Joshi et al. 2017)* – A European “Quantum Entanglement Space Test” proposal (ESA/Topical Team) led by Ursin, Ralph, Cacciapuoti *et al.*. This feasibility study outlined sending entangled photon pairs between ground and the International Space Station (ISS) ar5iv.labs.arxiv.org <https://ar5iv.labs.arxiv.org/html/1703.08036v1#:~:text=Models%20of%20quantum%20systems%20on,theoretical%20predictions%20of%20Ralph%20and>. One photon would be sent upward while its partner remained on Earth, testing theories (e.g. by Ralph and co-workers) that such a setup could *induce decoherence of entanglement* if each photon samples a different gravitational field gradientar5iv.labs.arxiv.org <https://ar5iv.labs.arxiv.org/html/1703.08036v1#:~:text=Models%20of%20quantum%20systems%20on,theoretical%20predictions%20of%20Ralph%20and>. The paper presents a detailed mission design (Space-QUEST) and notes that under standard quantum field theory no decoherence is expected, but some exotic models (the same “event operator” models) predict irrecoverable loss of entanglementar5iv.labs.arxiv.org <https://ar5iv.labs.arxiv.org/html/1703.08036v1#:~:text=Models%20of%20quantum%20systems%20on,theoretical%20predictions%20of%20Ralph%20and>. (Team: international quantum optics collaboration including Austrian Academy of Sciences, University of Vienna, ESA, etc.; status: proposed/concept study.) Laboratory/Accelerated-Frame Experiments - *Acceleration (free-fall/centrifuge) test (Fink et al. 2017)* – In Vienna, Fink, Ursin, Scheidl *et al.* placed a polarization-entangled photon source and detectors on a platform that was *dropped* 12 m (microgravity) and separately spun in a centrifuge up to 30 g. They measured an entanglement witness for accelerations spanning 0.03 g (near-free-fall) up to 30 gnature.com <https://www.nature.com/articles/ncomms15304?error=cookies_not_supported&code=0de1ebbf-a81e-4d7e-a5a1-d5e23484627f#:~:text=at%20rest%20in%20gravitational%20fields%2C,uniform%20acceleration%20on%20photonic%20entanglement>. The result (Nature Communications) was that entanglement quality showed *no significant loss* beyond noise limits at any acceleration tested nature.com <https://www.nature.com/articles/ncomms15304?error=cookies_not_supported&code=0de1ebbf-a81e-4d7e-a5a1-d5e23484627f#:~:text=at%20rest%20in%20gravitational%20fields%2C,uniform%20acceleration%20on%20photonic%20entanglement>. In effect, even extreme inertial forces (equivalent to gravitational fields by the EP) did not degrade the photon entanglement. The experiment set an upper bound on any acceleration-induced decoherence of the entangled state nature.com <https://www.nature.com/articles/ncomms15304?error=cookies_not_supported&code=0de1ebbf-a81e-4d7e-a5a1-d5e23484627f#:~:text=at%20rest%20in%20gravitational%20fields%2C,uniform%20acceleration%20on%20photonic%20entanglement>. *(Affiliation: Vienna Institute of Quantum Optics (IQOQI) and University of Vienna; status: completed 2017.)* - *Rotational (Sagnac interferometer) test (Cromb et al. 2023)* – At the University of Glasgow, Cromb, Padgett, Faccio *et al.* placed a Hong–Ou–Mandel interferometer (entangled photons) on a rotating platform (fiber Sagnac loop). They observed that as rotation rate increased, the two-photon interference pattern *flipped* from the usual HOM dip to a peakjournals.aps.org <https://journals.aps.org/prresearch/abstract/10.1103/PhysRevResearch.5.L022005#:~:text=a%20Hong,curved%20space%20on%20quantum%20entanglement>. This indicates that noninertial (rotating) motion fundamentally altered the symmetry of the entangled state. While not directly a gravity test, this table-top demonstration (Phys. Rev. Research 2023) shows that entangled photons are sensitive to rotational (relativistic) effects and “paves the way” to test entanglement in curved spacetimejournals.aps.org <https://journals.aps.org/prresearch/abstract/10.1103/PhysRevResearch.5.L022005#:~:text=a%20Hong,curved%20space%20on%20quantum%20entanglement>. *(Affiliation: University of Glasgow; status: completed 2023.)* Proposed and Future Tests - *Entangled-atom interferometer (Geiger & Trupke 2018)* – A theoretical proposal (Phys. Rev. Lett.) by Geiger (Paris Observatory) and Trupke (Vienna) suggests using *two entangled atoms of different species* in a differential free-fall interferometer. Each atom (e.g. ^85Rb and ^87Rb) forms its own atom interferometer; the two are prepared in an entangled superposition via a heralded photon detection at the input beam splitter hal.science <https://hal.science/hal-01707541/document#:~:text=We%20propose%20an%20experiment%20to,the%20detection%20of%20a%20single>. The interferometers then measure the differential acceleration of the two species. Crucially, because the atoms start entangled, this probes WEP in a regime where “the two particles involved … are not classically independent, but entangled”hal.science <https://hal.science/hal-01707541/document#:~:text=We%20propose%20an%20experiment%20to,the%20detection%20of%20a%20single>. Their proposed setup (high-finesse cavity, Raman processes) predicts sensitivity to Eötvös parameter η below 10^−7hal.science <https://hal.science/hal-01707541/document#:~:text=two%20particles%20involved%20in%20the,E%C3%B6tv%C3%B6s%20parameter%20can%20be%20achieved>. *(Institutions: LNE-SYRTE/Observatoire de Paris, University of Vienna; status: proposed in PRL 2018.)* - *Entanglement-witness protocols (Bose et al. 2023)* – Sougato Bose and collaborators (UCL, Groningen, Glasgow) have introduced the idea of a “quantum equivalence principle” test using entanglement. They propose creating massive particles in large spatial superposition and using an entanglement entropy witness to detect any WEP violationar5iv.org <https://ar5iv.org/pdf/2203.11628#:~:text=quantum%20entities,principle%20via%20witnessing%20quantum%20entanglement>. In essence, both matter and the gravitational source are treated as quantum; any difference between inertial and gravitational masses in this fully quantum regime would reveal itself through lack of entanglement. Their 2022/23 paper provides a protocol for such experiments and argues it is “unique” for testing the weak equivalence principle in the quantum domain ar5iv.org <https://ar5iv.org/pdf/2203.11628#:~:text=quantum%20entities,principle%20via%20witnessing%20quantum%20entanglement>. *(This work is theoretical/preprint (Entropy 2023) but outlines concrete tabletop schemes.)* - *SUPREME-QG space mission (Shahriar et al. 2023–25)* – A NASA-funded concept (NIAC Phase I, 2025) led by Selim Shahriar (Northwestern U.) envisions a spaceborne WEP test using *macroscopic entangled atomic states*nasa.gov <https://www.nasa.gov/directorates/stmd/niac/niac-studies/supreme-qg-space-borne-ultra-precise-measurement-of-the-equivalence-principle-signature-of-quantum-gravity/#:~:text=entanglement%20to%20test%20the%20EP,N%20SC%20states%20are%20difficult>. The idea is two simultaneous atom interferometers with ^85Rb and ^87Rb, each using a Schrödinger-cat–like state of N≈10^6 entangled atoms generated by cavity spin-squeezingnasa.gov <https://www.nasa.gov/directorates/stmd/niac/niac-studies/supreme-qg-space-borne-ultra-precise-measurement-of-the-equivalence-principle-signature-of-quantum-gravity/#:~:text=entanglement%20to%20test%20the%20EP,N%20SC%20states%20are%20difficult>. Such a many-particle entangled state behaves as one massive superposition, boosting sensitivity by √N∼10^3. The goal is to measure differential free-fall with precision η∼10^−20nasa.gov <https://www.nasa.gov/directorates/stmd/niac/niac-studies/supreme-qg-space-borne-ultra-precise-measurement-of-the-equivalence-principle-signature-of-quantum-gravity/#:~:text=entanglement%20to%20test%20the%20EP,N%20SC%20states%20are%20difficult> – orders of magnitude beyond current tests. (The proposal outlines technology development for these giant entangled statesnasa.gov <https://www.nasa.gov/directorates/stmd/niac/niac-studies/supreme-qg-space-borne-ultra-precise-measurement-of-the-equivalence-principle-signature-of-quantum-gravity/#:~:text=entanglement%20to%20test%20the%20EP,N%20SC%20states%20are%20difficult>. Status: concept study, NIAC-awarded 2025.) Each of these efforts is explicitly aimed at the *intersection of gravity and entanglement*. So far, all completed tests have *found no anomalous effect* – entangled states remain intact in different gravitational or accelerated frames, consistent with EEP and standard quantum theory pub.math.leidenuniv.nl <https://pub.math.leidenuniv.nl/~gillrd/Peking/Files/Xu-2019-Satellite-testing-of-a-gravitationa.pdf#:~:text=containing%20closed%20timelike%20curves%20and,the%20prediction%20of%20event%20formalism> nature.com <https://www.nature.com/articles/ncomms15304?error=cookies_not_supported&code=0de1ebbf-a81e-4d7e-a5a1-d5e23484627f#:~:text=at%20rest%20in%20gravitational%20fields%2C,uniform%20acceleration%20on%20photonic%20entanglement>. Future and proposed experiments (e.g. entangled atoms in free-fall) aim to push these tests into new regimes; any observed entanglement degradation tied to gravity would signal new physics. *Sources:* Peer-reviewed articles and official reports as cited above pub.math.leidenuniv.nl <https://pub.math.leidenuniv.nl/~gillrd/Peking/Files/Xu-2019-Satellite-testing-of-a-gravitationa.pdf#:~:text=containing%20closed%20timelike%20curves%20and,the%20prediction%20of%20event%20formalism> > -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to everything-list+unsubscr...@googlegroups.com. To view this discussion visit https://groups.google.com/d/msgid/everything-list/CAJPayv2oimZ3naHQ93kgS8ZHEfjpO4Fwj6TZAtbTTnB6PFgD8g%40mail.gmail.com.
