Down and Out in Whistler’s Sundial, Or, What I Learned Not to Do on E-Bikes, Or, Is the Universe Just a Glass Marble with some Defects?
This meeting was held from June 5-9/2023 at the Sundial Hotel in Whistler, and was attended by:
Rana Adhikari (Caltech), Markus Aspelmeyer(University of Vienna), Dan Carney (Lawrence Berkeley National Laboratory), Yanbei Chen (Caltech) and Mark Kasevich (Stanford University)
Some Interesting Topics that were Discussed:
- Is spacetime like a glass, fluid, or crystal?
○ People have been talking about ‘emergent’ gravity theories since Wheeler and Dewitt in the 60’s.
○ Ted Jacobsen wrote a paper1 in the 90’s describing a kind of way of coming up with the Einstein equations of General Relativity in the same way we come up with ideas like Pressure and Temperature from the microphysics of atoms.
○ As the popular phrase, “It from Qubit”, implies, perhaps the very ideas of space and time emerge as classical idea from the more fundamental representation as many strongly entangled quantum objects2. - Should we do experiments that try to estimate parameters in some model, or should we instead choose to measure things which no one has measured before?
○ Do we follow the ‘Scientific Method’ as taught in school, or do we follow the method of serendipitous discovery?
○ Its important to know the history, and so as not to reinvent old experiments. Or propose theories that have already been ruled out. - Are there interesting anomalies to search for in the speed of near-field gravity? Perhaps, but the time delay for near field gravity, is so small that it may be impossible to measure. An example experiment: spin a levitated bar around its short axis and look at its gravitational pull on a nearby mirror. If you can measure the time delay then either the experiment is mis-calibrated or you have discovered the first signature of beyond Einsteinian gravity!
- A very clean signature of quantum gravity would be spacetime superpositions:
○ You have 2 black boxes and 1 cat! Where is the cat? It is either in box 1 or box 2. Until we open the box, the cat is in a superposition state of being in both boxes at the same time.
○ But is the space around both boxes curved, or is the curvature just around 1 box?
○ I.e. has the surrounding spacetime already “measured” the cat’s location.
○ IF so, does that mean that spacetime causes decoherence in all forms of quantum entanglement?
○ Would that mean that the squishiness of spacetime forbids some extremely highly entangled states? - It is really hard to do experiments that require measurements of decoherence. They require making some kind of oscillator which has almost zero damping. Then whatever damping is left we can ascribe to something like a ‘gravitational decoherence’. But how do you ever know its quantum gravity and not just some kind of phonon or photon scattering? There’s always that last spec of schmutz.
- Are there other people we should invite who could help us think about experiments?
- Let’s not do an experiment that requires us to average measurements for > 1 month. Systematics, systematics, systematics.
- If we follow the path of serendipitous discovery (i.e. build something very sensitive and measure something new), how do we judge what to measure?
Footnotes
- Thermodynamics of Spacetime: The Einstein Equation of State (https://arxiv.org/abs/gr-qc/9504004) ↩︎
- Van Ramsdonk: https://www.science.org/doi/10.1126/science.aay9560 ↩︎
| Test | Pros | Cons |
| Two-body experiments | ||
| Entanglement generation via gravity | Clear interpretation: rules out all non-entangling models Simple delineation between classes of models | Extremely difficult experimentally |
| Noise in gravitational interactions | Less stringent experimental requirements than entanglement | Difficult to make discovery, i.e., to know the noise is from gravity Models less clear |
| One-body experiments | ||
| Gravitational decoherence | Clear experimental path to ruling this out; lots of existing tests Can be tested by a single massive object, don’t need interactions | Difficult to make discovery, i.e., to know the decoherence is really from gravity Models unclear |
| Light propagation through spacetime (e.g., glassy/dispersive effects) | Ultra long baselines → good integration Leverages multi-messenger era Doesn’t require quantum control of a source | Uncontrolled sources, possibly hard to distinguish from other effects Models unclear |