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Imagine pouring a box of granola into a bowl. Are you considering hundreds of individual chunks or the motion of the group as a whole? Human perceptual limits suggest we cannot be representing the individuals, implying we simulate ensembles of objects. If true, we would need to represent group physical properties beyond individual aggregates, similar to perceiving ensemble properties like color, size, or facial expression. Here we investigate whether people do hold ensemble representations of mass, using tasks in which participants watch a video of a single marble or set of marbles falling onto an elastic cloth and judge the individual or average mass. We find first that people better judge average masses than individual masses, then find evidence that the better ensemble judgments are not just due to aggregating information from individual marbles. Together, this supports the concept of ensemble perception in intuitive physics, extending our understanding of how people represent and simulate sets of objects. 

Adults can calculate probabilities by running simulations and calculating proportions of each outcome. How does this ability develop? We developed a method that lets us bring computational modeling to bear on this question. A study of 40 adults and 31 4-year-olds indicates that unlike adults, many 4-year-olds use a single simulation to estimate probability distributions over simulated possibilities. We also implemented the 3-cups task, an established test of children’s sensitivity to possibilities, in a novel format. We replicate existing 3-cups results. Moreover, children who our model categorized as running a single simulation on our novel task show a signature of running a single simulation in the 3-cups task. This signature is not observed in children who were categorized as running multiple simulations. This validates our model and adds to the evidence that about half of 4-year-olds don’t evaluate multiple candidates for reality in parallel. 

How do people perform general-purpose physical reasoning across a variety of scenarios in everyday life? Across two stud ies with seven different physical scenarios, we asked participants to predict whether or where two objects will make contact. People achieved high accuracy and were highly consistent with each other in their predictions. We hypothesize that this robust generalization is a consequence of mental simulations of noisy physics. We designed an “intuitive physics engine” model to capture this generalizable simulation. We find that this model generalized in human-like ways to unseen stimuli and to a different query of predictions. We evaluated several state-of-the-art deep learning and scene feature models on the same task and found that they could not explain human predictions as well. This study provides evidence that human’s robust generalization in physics predictions are supported by a probabilistic simulation model, and suggests the need for structure in learned dynamics models. 

Many models of intuitive physical reasoning posit some kind of mental simulation mechanism, yet everyday environments frequently contain far more objects than people could plausibly represent with their limited cognitive capacity. What determines which objects are actually included in our representations? We asked participants to predict how a ball will bounce through a complex field of obstacles, and probed working memory for objects in the scene that were more and less likely to be relevant to the ball’s trajectory. We evaluate different accounts of relevance and find that successful object memory is best predicted by how frequently a ball’s trajectory is expected to contact that object under a probabilistic simulation model. This suggests that people construct representations for mental simulation efficiently and dynamically, on the fly, by adding objects “just in time”: only when they are expected to become relevant for the next stage of simulation.