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I Testing Quantum Gravity paper Bose Einstein helium superfluid

  1. Oct 22, 2017 #1
    Testing Quantum Gravity
    Johan Hansson, Stephane Francois
    (Submitted on 19 Oct 2017)
    The search for a theory of quantum gravity is the most fundamental problem in all of theoretical physics, but there are as yet no experimental results at all to guide this endeavor. What seems to be needed is a pragmatic way to test if gravitation really occurs between quantum objects or not. In this article we suggest such a potential way out of this deadlock, utilizing macroscopic quantum systems; superfluid helium, gaseous Bose-Einstein condensates and "macroscopic" molecules. It turns out that true quantum gravity effects - here defined as observable gravitational interactions between truly quantum objects - could and should be seen (if they occur in nature) using existing technology. A falsification of the low-energy limit, in the accessible weak-field regime, would also falsify the full theory of quantum gravity, making it enter the realm of testable, potentially falsifiable theories, i.e. becoming real physics after almost a century of pure theorizing. If weak-field gravity between quantum objects is shown to be absent (in the regime where the approximation should apply), we know that gravity then is a strictly classical phenomenon absent at the quantum level.
    Comments: Honorable Mention Award in the 2017 Essay Competition of the Gravity Research Foundation, 9 pages
    Subjects: General Relativity and Quantum Cosmology (gr-qc); Quantum Physics (quant-ph)
    Journal reference: International Journal of Modern Physics D, Vol. 26, 1743003 (2017)
    DOI: 10.1142/S0218271817430039
    Cite as: arXiv:1710.07280 [gr-qc]


    so suppose they perform experiment using bose-einstein condensates and helium superfluid

    they find weak-field gravity between quantum objects is shown to be absent.

    what are the ramifications to string theory, LQG, standard graviton in QFT, etc if gravity then is a strictly classical phenomenon absent at the quantum level on experimental grounds.

    what theories of gravity would support gravity then is a strictly classical phenomenon absent at the quantum level?

    what about the equivalence principle, black hole entropy holography if gravity then is a strictly classical phenomenon absent at the quantum level
     
  2. jcsd
  3. Oct 22, 2017 #2

    mfb

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    That would be extremely weird. We know quantum objects are normally attracted to Earth, we have neutrons bouncing above a surface for example, showing quantized energy states. A lack of force in the opposite direction would mean our bouncing neutrons violate conservation of momentum? A conservation of momentum is also a conservation of energy in other frames, and nearly all theories conserve energy.
    In addition, if we choose short timescales, we can treat Earth as big collection of quantum objects. And we know this collection leads to a force. There is no meaningful boundary where you could say "here the force appears/disappears".

    What they propose would be a nice test (although I'm skeptical if that is as feasible as they hope), but I wouldn't expect to learn much from it beside a confirmation that quantum objects do attract each other as expected.
     
  4. Oct 24, 2017 #3

    tom.stoer

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    From the abstract: they do not discuss quantum gravity but classical gravity between quantum objects; yes, falsifying some aspects of classical gravity would falsify any theory of quantum gravity having classical gravity as its classical limit.
     
  5. Oct 24, 2017 #4
    isn't this include strings loops gravitons on flat qft?
     
  6. Oct 24, 2017 #5

    tom.stoer

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    Sorry, what exactly do you mean?
     
  7. Oct 24, 2017 #6
    The abstract proposes specific tests of gravity on quantum objects like bose-einstein condensates and superfluid helium.

    If the experimental results of these tests provide evidence falsifying some aspects of classical gravity would falsify any theory of quantum gravity having classical gravity as its classical limit,

    wouldn't that include string theory, LQG, graviton based theories, etc?
     
  8. Oct 24, 2017 #7
    what about recent proposals which theorize gravity is not a fundamental force in itself, but an emergent phenoma of quantum entanglement?

    the most famous in the press is Verlinde's

    but also worth mentioning

    https://www.newscientist.com/articl...show-how-to-unite-quantum-theory-and-gravity/
    Antoine Tilloy at the Max Planck Institute of Quantum Optics in Garching, Germany, has attempted to get at gravity by tweaking standard quantum mechanics.

    In quantum theory, the state of a particle is described by its wave function. The wave function lets you calculate, for example, the probability of finding the particle in one place or another on measurement. Before the measurement, it is unclear whether the particle exists and if so, where. Reality, it seems, is created by the act of measurement, which “collapses” the wave function.
    One solution to such paradoxes is a so-called GRW model that was developed in the late 1980s. It incorporates “flashes”, which are spontaneous random collapses of the wave function of quantum systems. The outcome is exactly as if there were measurements being made, but without explicit observers.

    Tilloy has modified this model to show how it can lead to a theory of gravity. In his model, when a flash collapses a wave function and causes a particle to be in one place, it creates a gravitational field at that instant in space-time. A massive quantum system with a large number of particles is subject to numerous flashes, and the result is a fluctuating gravitational field.

    they are planning to actually do experiments

    http://www.spacedaily.com/reports/Using_space_to_study_ultra_cold_materials_999.html
    It is said that what goes up must come down. Thank gravity for that. But sometimes gravitational effects affect matter on Earth in ways that physicists would rather do without. So early next year NASA is launching to the International Space Station a novel "Cold Atom Laboratory" that will allow physicists to conduct sustained experiments into the states of matter, under nearly weightless conditions, at ultra-cold temperatures, that would be nearly impossible to conduct on Earth.
     
  9. Oct 24, 2017 #8

    tom.stoer

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    String theory certainly yes, LQG is not clear as far as I can see (they have results like a graviton propagator, but the classical limit is by no means fully understood).
     
  10. Oct 24, 2017 #9

    they are performing experiments both current and planned, including sending ultra cold atoms into orbit.

    an experimental result "falsifying some aspects of classical gravity would falsify any theory of quantum gravity having classical gravity as its classical limit" that survives peer review and is repeated in other experiments,

    what theories of BSM gravity would be still viable?

    i.e if the following

    "In this article we suggest such a potential way out of this deadlock, utilizing macroscopic quantum systems; superfluid helium, gaseous Bose-Einstein condensates and "macroscopic" molecules. It turns out that true quantum gravity effects - here defined as observable gravitational interactions between truly quantum objects - could and should be seen (if they occur in nature) using existing technology. A falsification of the low-energy limit, in the accessible weak-field regime, would also falsify the full theory of quantum gravity, making it enter the realm of testable, potentially falsifiable theories, i.e. becoming real physics after almost a century of pure theorizing. If weak-field gravity between quantum objects is shown to be absent (in the regime where the approximation should apply), we know that gravity then is a strictly classical phenomenon absent at the quantum level."

    were established with doable experiments,

    what sort of BSM theory of gravity would still be viable? are there any current theories of BSM gravity that have been proposed that is existent with such an experimental result?

    what are the implications if experiments show "gravity then is a strictly classical phenomenon absent at the quantum level"
     
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