If a stocks price is $100, and tomorrow there will be a news event where there is a 50% chance it will be worth $50, and a 50% chance it will be worth $200, the

If a stocks price is $100, and tomorrow there will be a news event where there is a 50% chance it will be worth $50, and a 50% chance it will be worth $200, the trading value today should probably be around $125 (it's expected value or EV, $200 + $50 / 2). When the defining event actually occurs, it will either move from $125 to $50, or $125 to $200. So the news event in this example is priced in at $125, but there is still scope for big moves when certainty is realised. Obviously in real life the values are never as certain. In the above example if the stocks ever trades < $125 before the event, you would make money in the long run. EMH shouldn't be taken seriously as the market actors are compromised of irrational humans.