The weak EMH is pretty simple: there's money to be made in arbitrage, but the act of arbitrage spreads information. So a price quickly converges to reflect available information. The harder participants try to beat the market, the harder it becomes.
What's left is the lurch of prices upon truly novel information arriving. Stocks appear to move up and down at random, because future events are not fully predictable. If they were, prices would stabilise around the "true" price and remain stuck there.
A similar argument has been used to show that time travel has not and will never be invented. Arbitrage across time would be even more profitable than arbitrage across space.
The weak EMH is pretty simple: there's money to be made in arbitrage, but the act of arbitrage spreads information. So a price quickly converges to reflect available information. The harder participants try to beat the market, the harder it becomes.
What's left is the lurch of prices upon truly novel information arriving. Stocks appear to move up and down at random, because future events are not fully predictable. If they were, prices would stabilise around the "true" price and remain stuck there.
A similar argument has been used to show that time travel has not and will never be invented. Arbitrage across time would be even more profitable than arbitrage across space.