user: text: Write a concise one-sentence TLDR summary of the scientific paper content below.

Title: Relational Neural Expectation Maximization: Unsupervised Discovery of Objects and their Interactions

Paper content:
Common-sense physical reasoning is an essential ingredient for any intelligent agent operating in the real-world.
For example, it can be used to simulate the environment, or to infer the state of parts of the world that are currently unobserved.
In order to match real-world conditions this causal knowledge must be learned without access to supervised data.
To address this problem we present a novel method that learns to discover objects and model their physical interactions from raw visual images in a purely unsupervised fashion.
It incorporates prior knowledge about the compositional nature of human perception to factor interactions between object-pairs and learn efficiently.
On videos of bouncing balls we show the superior modelling capabilities of our method compared to other unsupervised neural approaches that do not incorporate such prior knowledge.
We demonstrate its ability to handle occlusion and show that it can extrapolate learned knowledge to scenes with different numbers of objects.
Humans rely on common-sense physical reasoning to solve many everyday physics-related tasks BID32 .
For example, it enables them to foresee the consequences of their actions (simulation), or to infer the state of parts of the world that are currently unobserved.
This causal understanding is an essential ingredient for any intelligent agent that is to operate within the world.Common-sense physical reasoning is facilitated by the discovery and representation of objects (a core domain of human cognition BID45 ) that serve as primitives of a compositional system.
They allow humans to decompose a complex visual scene into distinct parts, describe relations between them and reason about their dynamics as well as the consequences of their interactions BID4 BID32 BID48 .The
most successful machine learning approaches to common-sense physical reasoning incorporate such prior knowledge in their design. They
maintain explicit object representations, which allow for general physical dynamics to be learned between object pairs in a compositional manner BID3 BID8 BID49 . However
, in these approaches learning is supervised, as it relies on object-representations from external sources (e.g. a physics simulator) that are typically unavailable in real-world scenarios.Neural approaches that learn to directly model motion or physical interactions in pixel space offer an alternative solution BID46 BID47 ). However
, while unsupervised, these methods suffer from a lack compositionality at the representational level of objects. This prevents
such end-to-end neural approaches from efficiently learning functions that operate on multiple entities and generalize in a human-like way (c.f. BID4 ; BID32 ; BID41 , but see BID39 ).In this work we
propose Relational N-EM (R-NEM), a novel approach to common-sense physical reasoning that learns physical interactions between objects from raw visual images in a purely unsupervised fashion. At its core is
Neural Expectation Maximization (N-EM; , a method that allows for the discovery of compositional object-representations, yet is unable to model interactions between objects. Therefore, we
endow N-EM with a relational mechanism inspired by previous work BID3 BID8 BID41 , enabling it to factor interactions between object-pairs, learn efficiently, and generalize to visual scenes with a varying number of objects without re-training.
We have argued that the ability to discover and describe a scene in terms of objects provides an essential ingredient for common-sense physical reasoning.
This is supported by converging evidence from cognitive science and developmental psychology that intuitive physics and reasoning capabilities are built upon the ability to perceive objects and their interactions BID43 BID48 .
The fact that young infants already exhibit this ability, may even suggest an innate bias towards compositionality BID32 BID37 BID45 .
Inspired by these observations we have proposed R-NEM, a method that incorporates inductive biases about the existence of objects and interactions, implemented by its clustering objective and interaction function respectively.
The specific nature of the objects, and their dynamics and interactions can then be learned efficiently purely from visual observations.In our experiments we find that R-NEM indeed captures the (physical) dynamics of various environments more accurately than other methods, and that it exhibits improved generalization to environments with different numbers of objects.
It can be used as an approximate simulator of the environment, and to predict movement and collisions of objects, even when they are completely occluded.
This demonstrates a notion of object permanence and aligns with evidence that young infants seem to infer that occluded objects move in connected paths and continue to maintain objectspecific properties BID44 .
Moreover, young infants also appear to expect that objects only interact when they come into contact BID44 , which is analogous to the behaviour of R-NEM to only attend to other objects when a collision is imminent.
In summary, we believe that our method presents an important step towards learning a more human-like model of the world in a completely unsupervised fashion.Current limitations of our approach revolve around grouping and prediction.
What aspects of a scene humans group together typically varies as a function of the task in mind.
One may perceive a stack of chairs as a whole if the goal is to move them to another room, or as individual chairs if the goal is to count the number of chairs in the stack.
In order to facilitate this dynamic grouping one would need to incorporate top-down feedback from an agent into the grouping procedure to deviate from the built-in inductive biases.
Another limitation of our approach is the need to incentivize R-NEM to produce useful groupings by injecting noise, or reducing capacity.
The former may prevent very small regularities in the input from being detected.
Finally the interaction in the E-step among the groups makes it difficult to increase the number of components above ten without causing harmful training instabilities.
Due to the multitude of interactions and objectives in R-NEM (and RNN-EM) we find that they are sometimes challenging to train.In terms of prediction we have implicitly assumed that objects in the environment behave according to rules that can be inferred.
This poses a challenge when objects deform in a manner that is difficult to predict (as is the case for objects in Space Invaders due to downsampling).
However in practice we find that (once pixels have been grouped together) the masking of the input helps each component in quickly adapting its representation to any unforeseen behaviour across consecutive time steps.
Perhaps a more severe limitation of R-NEM (and of RNN-EM in general) is that the second loss term of the outer training objective hinders in modelling more complex varying backgrounds, as the background group would have to predict the "pixel prior" for every other group.We argue that the ability to engage in common-sense physical reasoning benefits any intelligent agent that needs to operate in a physical environment, which provides exciting future research opportunities.
In future work we intend to investigate how top-down feedback from an agent could be incorporated in R-NEM to facilitate dynamic groupings, but also how the compositional representations produced by R-NEM can benefit a reinforcement learner, for example to learn a modular policy that easily generalizes to novel combinations of known objects.
Other interactions between a controller C and a model of the world M (implemented by R-NEM) as posed in BID42 constitute further research directions.

assistant: text: We introduce a novel approach to common-sense physical reasoning that learns to discover objects and model their physical interactions from raw visual images in a purely unsupervised fashion