This chapter discusses the fundamental characteristics and properties of stock markets and the way prices vary from one instant to the next. It first introduces the standard view about price variations and returns on the stock market, using a simple toy model to illustrate why arbitrage opportunities (the possibility to get a “free lunch”) are often washed out by the intelligent investment of informed traders, giving rise to the concept of the efficient stock market. It then considers the efficient market hypothesis in relation to random walk by analyzing Louis Bachelier's thesis that the trajectories of stock market prices are identical to random walks. It also examines how information is incorporated in prices, thus destroying potential “free lunches.” Finally, it explains the trade-off between risk and expected return.
Is a tick an elementary jump in a random walks scheme on the stock market?
