question about "Java Concurrency in Practice" example

I'm looking at a code sample from "Java Concurrency in Practice" by Brian Goetz. He says that it is possible that this code will stay in an infinite loop because "the value of 'ready' might never become visible to the reader thread". I don't understand how this can happen...

public class NoVisibility {
    private static boolean ready;
    private static int number;

    private static class ReaderThread extends Thread {
        public void run() {
            while (!ready)
                Thread.yield();
            System.out.println(number);
        }
    }

    public static void main(String[] args) {
        new ReaderThread().start();
        number = 42;
        ready = true;
    } 
}