耶鲁教授Geanakoplos：实际利率才是定价核心

John Geanakoplos, Yale professor and hedge fund owner:

• there is no just interest rate

a bond costs $0.80 today and pays $1 next year, so the nominal return is 25%. sounds great, until you realize that without knowing inflation, you still have no idea whether you actually became richer. Fisher's breakthrough was separating the number printed on money from the real question: how much consumption tomorrow can you buy by giving up consumption today?

in Geanakoplos' model, the present value of 1 apple next year falls to exactly 1/3 of an apple today. give up 1 apple now and the market gives you 3 next year, which means a real interest rate of 200%. the rate is not set by a central bank inside this model; it emerges from how impatient people are and how much wealth exists today versus tomorrow.

the same logic prices every asset. if one stock pays 1 apple next year and another pays 2, the second must cost exactly twice as much; otherwise traders can short the expensive one, buy the cheap equivalent, scale the trade from 1 share to a million and collect free future cash flow. no-arbitrage forces thousands of different assets back to the same underlying exchange rate between today and tomorrow.

this is present value stripped of finance jargon. a stock is just future dividends, a bond is just future cash, and today's price is what the market charges to move those future payoffs into the present. once you know the real interest rate, Fisher's framework says you can work backwards and price the entire stream.

the lecture is free. one example turns $0.80 into $1 and gives you 25% nominal interest; another turns 1 apple into 3 and gives you 200% real interest. most people watch the first number, but the second is the one that tells you what your money actually buys.