Fra said:
So your financial models are not corroborated by letting every player test it in infinite trials. Because the model itself will get useless while doing it.
It's a worse problem. The trouble is that you *can't* have infinite trials even if you wanted to. For example, I have a model for how bonds work in Q1 2005. I *know* that bonds work differently in Q1 2010, so how do I even test a model?
This is different from the situation in say particle physics or biology. In particle physics, you can be reasonably sure that the laws of physics are the same today as they were yesterday, so you can repeat an experiment. Also, you can be sure that one electron is pretty much the same as another electron.
For biology, you can be reasonably sure that the human body hasn't changed radically in the last week, so that you can repeat experiments. Also, you can do statistical tests to see how much human beings change from each other, and you can be pretty sure that someone won't walk into your experiment with the ability to bend steel or who is two inches high.
For finance you can't do that. Curiously the *philosophical* problems that you run into finance are similar to the once that you run into in astrophysics. In astrophysics, you really can't repeat an experiment. You see supernova 1987A or the big bang, you can't make another supernova or big bang go off, so how do you know that your statements about 1987A or the BB are true.
Your financial models are "tested" by actually generating money.
But even that is questionable. I could generate a ton of money in 2005-2006 by assuming that the price of real estate would go up forever. If you have a lot of people sitting in around a roulette wheel each with their "lucky theory" one of them is going to end up making a ton of money.
This is something that causes a lot of paranoia. Models work until they don't, and if you aren't careful, once it's obvious that they don't work, it's too late. You *think* that your lucky rabbit's foot caused the roulette wheel to show number 23, so you bet a thousand on 15. By some stroke of magic, you win, so you bet a million on 12. You lose. You figure out you are wrong, but it's too late.
Conversely, if you lose money, that doesn't mean that you were wrong. You could have just been unlucky. If you say that there is a 95% chance that you will make money, and you lose money. Well... Hello 5%. Something that one trader mentioned is that if you are, really, really, really good, then you will be wrong about the direction of the market 40% of the time.
What I tried to say far back is that while this is gut feeling for finance feeling, this view on "theory" is quite obscure for a typical physicists.
I don't think it is.
One reason that investment banks hire physicists rather than economists to create financial models is that physicists tend to ask deeper questions than economics or finance Ph.D's. What economics and finance Ph.D.'s tend to do is to take a pre-built model of the markets and then plug numbers into spreadsheets and come up with results, and about 98% of the work of banks involve doing this sort of thing.
For example, there is the Markowitz model of portfolio allocation, and a lot of what finance and econ majors do is to type numbers in spreadsheets and come up with weights. If you want to use that model or tweak it, then you want an econ major to do that. However, if you are in a situation in which Markowitz is no where close and you have to create something else totally from scratch, that's what astrophysicists get hired for.
The fact that things in finance change in both space and time is why you need relatively large numbers of physicists. If some one comes up with a "theory of everything" about electrons, we are done. They write the paper, collect the Nobel, and no one has to do it again. That's horrible for employment. I call it the "second Einstein problem." It's great to be Albert Einstein, but once you've figured out general relativity, there is no job for a second Einstein to figure it out again.
What happens in finance is that you create a model in Q1 2005. It seems to work, but by Q2 2007, it looks shaky, and by Q4 2007, it's obvious that there are some huge new things that make it wrong. At that point you have to pay some physicist money to come up with a new/revised model, which works for another years until the markets change, and you have to pay to get it fixed again.
Lots of jobs for astrophysicists...