We quantify the behaviour at large scales of the beta coalescent Π={Π(t),t≥0} with parameters a,b>0. Specifically, we study the rescaled block size spectrum of Π(t) and of its restriction Πn(t) to {1,…,n}. Our main result is a law of large numbers type of result if Π comes down from infinity. In the case of Kingman’s coalescent the derivation of this so-called hydrodynamic limit has been known since the work of Smoluchowski (Z. Phys. 17 (1916) 557–585). We extend Smoluchowski’s result to beta coalescents and show that if Π comes down from infinity both rescaled spectra n−1(c1Π(tτn),…,cnΠ(tτn)),andn−1(c1Πn(tτn),…,cnΠn(tτn)), converge to (different) deterministic limits that we compute explicitly in terms of partial Bell polynomials. Here ciπ counts the number of blocks of size i in a partition π, and (τn) is a sequence such that τn∼n−(1−a) as n→∞. Along the way we study the nontrivial limits of the rescaled block counting processes {nα#Πn(tτn),t≥0}, and {nα#Π(tτn),t≥0}, where α∈[−1,−2/(3−a)), and τn∼nα(1−a) if Π comes down from infinity.
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