Beyond the limits of analogue experiments

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Analogue experiments have attracted interest for their potential to shed light on inaccessible domains. In 1981, Unruh found a striking mathematical analogy between the propagation of light waves near a black hole and the propagation of sound in fluids. In fact, a number of distinct such 'analogue' systems can be found, from hydrodynamical systems to Bose-Einstein condensates. The remarkable discovery of an analogy between black holes and 'dumb holes' in fluids ('swallowing' sound) or Bose-Einstein condensates has spawned a rich literature exploring the emerging field of 'analogue gravity'. Moreover, it has led to an active experimental field of studying such analogue systems in labs, culminating in the observation of analogue Hawking radiation in Bose-Einstein condensates. Analogue gravity thus naturally leads to the question of whether such analogue models can confirm the existence of (gravitational) Hawking radiation in astrophysical black holes. Can we learn anything about black holes from these analogue models? More generally, can analogue models confirm hypotheses regarding inaccessible target systems? While Dardarshti et al. have argued that analogue gravity can indeed confirm gravitational Hawking radiation, Crowther et al. have criticized their argument as circular: in order to ascertain that the analogue model and black holes in fact fall into the same universality class, one must assume that black holes are adequately described by the modelling framework from which Hawking radiation is derived---but this is precisely what was to be confirmed. Analogue experiments whose target systems are inaccessible in some epistemically relevant sense generally suffer from this weakness: they must assume the physical adequacy of the modelling framework in order to underwrite the analogy to the accessible lab system when it is often precisely this adequacy which is at stake. Recently, Evans and Thébault (and to some degree Field) have equated concerns about this circularity with general inductive scepticism. Extrapolating from lab experiments on fluids to inaccessible black holes, is, according to them, no different in principle from inferring from today's experimental results to those of tomorrow. In support of this claim, they enlist the example of stellar nucleosynthesis, i.e., of reactions transforming hydrogen to helium in the interior of main sequence stars such as our sun. These reactions are unmanipulable and (photonically) inaccessible to us; nevertheless, astrophysical observations and terrestrial experiments in nuclear physics largely confirm the theory of stellar nucleosynthesis. This argument raises the subtle and rich question of under what circumstances we can think of two systems as being of the same or of a different 'type', i.e., under what conditions can experiments on one system be considered confirmatory of a theory on another system. This talk will show that this question is relevantly different from Hume's general inductive scepticism, particularly if it concerns inferences to inaccessible target systems. When we make inferences from experimental results of analogue systems to inaccessible target systems, we require the substantial assumption that our experimental and target systems are of the same type. It is for this assumption that we need independent support.
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University of Geneva

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