What is reinforcement learning?

Reinforcement learning is an attempt at solving an extremely general and difficult problem.

Think about how we live our lives. Every minute we are presented with some decision. It could be something minor, such as “Should I scratch that itch?” or something major, such as “Should I move to this new city?”. Our choices determine what happens next. Some outcomes are bad and they teach us not to make similar choices in the future, and some outcomes feel nice and encourage us to repeat our behaviour. Over time, if we’re smart, we learn how to achieve a reasonable level of happiness. This learning process is what we want to understand better.

Reinforcement learning is a way to model this sort of learning process. When we try to say the above with more precision, we get reinforcement learning.

We first define a set A and call it the set of actions. This set contains all possible actions we could take, including “scratch that itch”, “suffer through the itch”, “move to new city”, “stay comfortable in current city”, etc.

Next, we approximate the whole universe with one symbol: S. This denotes the set of all states the universe could possibly be in. This could include things like “raining in Mumbai”, “over the counter drug for cancer invented”, “sun explodes and destroys the entire solar system”, etc.

In the framework of reinforcement learning, we assume that the universe is governed by some process that can be partially influenced by our actions. We can think of this as a game. The universe starts in some state s_0 \in S. We get to observe the state, and are allowed to take an action a_0 \in A. Based on our action, the universe moves to the next state s_1 \in S, which we can observe, and presents us with some reward r_1, which is a real number that depends on s_0, s_1, and a_0. We pick our next action a_1 \in A, and the game continues. The goal of this game is for us to come up with a way of picking actions that maximizes the rewards in the long term. The process of figuring out the optimal way of picking actions in this setting is what we call reinforcement learning.

There are some issues that need to be carefully thought out to make this definition precise:

  1. What exactly do we mean by long-term reward? It sounds like something along the lines of \sum_i r_i. But if the game goes on forever, this sum may have no guarantee to converge and we may end up with strategies that give us an undefined amount of long-term reward. If that doesn’t happen, we always get an infinite amount of total reward and thus finding an optimal strategy gets trivial. There are two popular ways to handle this issue. One option is to assume that the game ends at some point. This is enforced by including a state e in S that denotes the end. Then we need not worry about undefined rewards any more. Another option is impose no such constraint but consider discounted reward instead of just the sum, i.e., we try to maximize \sum_i \gamma^ir_i for some constant \gamma\in (0, 1)
  2. How exactly does the universe decide the next state to be in? One option is to say that there’s a function \tau: (S, A)\to S that the universe uses to determine the next state given current state and action. This, however, makes things deterministic in the sense that for a given state and action, the next state is fully determined. We may want to relax that and add some randomness by defining the function as \tau: (S, A, S)\to [0, 1] where \tau now denotes the probability of going from a state to another when a certain action is taken. This function is usually called the transition function. One might ponder if this relaxation is really needed. If we observe a universe where if we take an action a in state s sometimes it goes to state s' and sometimes to s'' could it be because we haven’t modelled the universe with a rich enough set of states? Perhaps we could capture some extra information in the set s that would be enough to uniquely determine the next state? While this could theoretically be true, as Bell’s experiments show, the universe we live in does not lend itself to determinism by merely adding more information. Moreover, even if we could make the universe deterministic by adding more information, we are often in situations where we don’t have that information and we want to be able to model it anyway. Probabilities could still be useful as a model of uncertainty that comes from lack of information.
  3. Continuing on the previous point, is it fair to constrain the universe to only use the current state, and not the whole sequence of previous states, to decide on the next state? As long as we have no constraints on the definition of S, this constrain doesn’t change anything. We could just redefine S to contain all possible sequences of states.
  4. Turning our focus to the player, how does the player decide what action to pick next? Let’s model this in a similar way. The player uses a deterministic function \alpha: S\to A, that given a state returns an action to take. This function is called the policy. If we fix the set of states, S, set of actions A, and universe’s transition function \tau, it can be shown that there exists a policy \alpha that is the best policy, i.e., it maximizes the long term reward of the game. Even if \tau is random, there is always an optimal policy that is deterministic.
  5. We can then define learning as the task of figuring out the optimal policy. If the player knows \tau, then this is a computational question and one can measure the performance of the learning algorithm using tools from computational complexity. But for most of the problems we want to model the player doesn’t have the luxury of knowing \tau. The player just starts playing the game by taking actions and observing rewards and states, and is asked to figure out the optimal policy or something close to it after several iterations. In this setting a good way to track performance is to measure the number of iterations needed to reach a desired approximation to the optimal policy. 

Let’s step back a bit again so we don’t lose sight of the big picture and see what kinds of problems can be cast as a reinforcement learning problem. We already saw in the beginning that life is a reinforcement learning problem.

A variety of scientific fields can in fact be cast as reinforcement learning. For example, all of physics is a reinforcement learning problem as follows: let S be the set of all possible states of the universe; let A be the same as S, i.e., our action is simply a prediction of what the next state is going to be; let \tau depend only on the current state and not on the action; and finally, let the reward be high if our prediction was close to the true next state and low otherwise. Clearly, the optimal policy is one that makes accurate predictions about the universe’s future, which is exactly what the laws of physics aim to do. So if one claims that reinforcement learning is a solvable problem in general, they are also claiming that the field of physics doesn’t need to exist any more.

One can also very easily cast “making money” as a reinforcement learning problem. Of course, since life itself is reinforcement learning, it’s no surprise that making money is too. But more specifically, one can pick the set of actions to be the set of all investments one can possibly make at any given time. The optimal policy in this case would be the one that maximizes the return on investments. Thus someone claiming to have solved reinforcement learning has also solved all of finance and the hedge fund industry doesn’t need to exist any more.

In fact, the problem of reinforcement learning is so general that it’s a surprise that it’s solvable at all! And yet, some recent research has shown remarkable capabilities at being able to solve it. For example, if you haven’t been living under a rock, you have probably heard of DeepMind’s algorithm that managed to learn all of Go, chess, and shogi without having given any domain knowledge about the individual games. Merely by playing the games over and over again and observing the rewards, the algorithm figured out how to beat the world champion programs in all three games. That’s getting closer and closer to solving all of life!

In this series, we will explore the frontier of this very exciting field. We will delve deep into some of the recent work and try to develop an understanding of what makes reinforcement learning solvable in certain domains and what can be done to make it solvable in others.

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