On 5/26/2025 2:51 PM, Alan Grayson wrote:
On Monday, May 26, 2025 at 5:57:36 AM UTC-6 John Clark wrote:
On Sun, May 25, 2025 at 3:33 PM Alan Grayson <[email protected]>
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.
*Since the EP depends on measurement accuracy, it's mischaracterized
as some absolute principle. That's pretty obvious regardless of
contrary opinions, including Einstein's. AG
*
It was just an inspiring idea that Einstein had. It didn't need to have
three digit accuracy.
Brent
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 predictionpub.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 testednature.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
statenature.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 splitterhal.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 domainar5iv.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 theorypub.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
abovepub.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>
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