I’m being careful to define the expressions P[X|Ei] as the amount paid for a betting slip on X in an evidential state Ei. This is NOT the same as the agent’s credence in hypothesis X. I agree with you that credences don’t update in UDT (that’s sort of the point). However, I’m arguing that betting payments must change (“update” if you like) between the two evidential states, or else the agent will get Dutch booked.
You describe your strategy as having an infinite gain or loss in each case, so you don’t know whether it is correct (indeed you don’t know which strategy is correct for the same reason). However, earlier up in the thread I already explained that this problem will arise if an agent’s utility depends on bets won or lost by other agents. If instead, each agent has a private utility function (and there is no addition/subtraction for other agents’ bets; only for her own) then this “adding infinities” problem doesn’t arise. Under your proposed strategy (same betting payments in E0 and E1), each individual agent gets Dutch-booked and makes a guaranteed loss of 1⁄4 utile so it can’t be the optimal strategy.
What is optimal then? In the private utility case (utility is a function only of the agent’s own bets), the optimal strategy looks to be to commit to SSA betting odds (which in the simplified example means an evens bet in the state E0, and a Doomsday betting shift in the state E1).
If the agent’s utility function is an average over all bets actually made in a world (average utilitarianism) then provided we take a sensible way of defining the average, such as take the mean (betting gain—betting loss) over N Hubble volumes, then take the limit as N goes to infinity, the optimal strategy is again SSA betting odds.
If the agent’s utility function is a sum over all bets made in a world, then it is not well-defined, for the reasons you discuss: we can’t decide how to bet without a properly-defined utility function. One approach to making it well-defined may be to use non-standard arithmetic (or surreals), but I haven’t worked that through. Another approach is to sum bets only within N Hubble volumes of the agent (assume the agent doesn’t really care about far far away bets), and then only later take the limit as N tend to infinity. This leads to SIA betting odds.
Until recently, I thought that SIA odds meant betting heavily on H3.2 in the state E0, and then reverting to an evens bet in the state E1 (so it counters the Doomsday argument). However, the more recent analysis of SIA indicates that there is still a Doomsday shift because of “great filter” arguments (a variant of Fermi’s paradox), so the betting odds in state E1 should still be weighted towards H3.1.
Basically it doesn’t look good, since every combination of utility function or SSA with or without SIA is now creating a Doomsday shift. The only remaining let out I’ve been considering is a specially-constructed reference class (as used in SSA), but it looks like that won’t work either: in Armstrong’s analysis, we don’t get to define the reference class arbitrarily, since it consists of all linked decisions. (In the UDT case, all decisions that are made by any agents anywhere applying UDT).
I’m being careful to define the expressions P[X|Ei] as the amount paid for a betting slip on X in an evidential state Ei. This is NOT the same as the agent’s credence in hypothesis X. I agree with you that credences don’t update in UDT (that’s sort of the point). However, I’m arguing that betting payments must change (“update” if you like) between the two evidential states, or else the agent will get Dutch booked.
You describe your strategy as having an infinite gain or loss in each case, so you don’t know whether it is correct (indeed you don’t know which strategy is correct for the same reason). However, earlier up in the thread I already explained that this problem will arise if an agent’s utility depends on bets won or lost by other agents. If instead, each agent has a private utility function (and there is no addition/subtraction for other agents’ bets; only for her own) then this “adding infinities” problem doesn’t arise. Under your proposed strategy (same betting payments in E0 and E1), each individual agent gets Dutch-booked and makes a guaranteed loss of 1⁄4 utile so it can’t be the optimal strategy.
What is optimal then? In the private utility case (utility is a function only of the agent’s own bets), the optimal strategy looks to be to commit to SSA betting odds (which in the simplified example means an evens bet in the state E0, and a Doomsday betting shift in the state E1).
If the agent’s utility function is an average over all bets actually made in a world (average utilitarianism) then provided we take a sensible way of defining the average, such as take the mean (betting gain—betting loss) over N Hubble volumes, then take the limit as N goes to infinity, the optimal strategy is again SSA betting odds.
If the agent’s utility function is a sum over all bets made in a world, then it is not well-defined, for the reasons you discuss: we can’t decide how to bet without a properly-defined utility function. One approach to making it well-defined may be to use non-standard arithmetic (or surreals), but I haven’t worked that through. Another approach is to sum bets only within N Hubble volumes of the agent (assume the agent doesn’t really care about far far away bets), and then only later take the limit as N tend to infinity. This leads to SIA betting odds.
Until recently, I thought that SIA odds meant betting heavily on H3.2 in the state E0, and then reverting to an evens bet in the state E1 (so it counters the Doomsday argument). However, the more recent analysis of SIA indicates that there is still a Doomsday shift because of “great filter” arguments (a variant of Fermi’s paradox), so the betting odds in state E1 should still be weighted towards H3.1.
Basically it doesn’t look good, since every combination of utility function or SSA with or without SIA is now creating a Doomsday shift. The only remaining let out I’ve been considering is a specially-constructed reference class (as used in SSA), but it looks like that won’t work either: in Armstrong’s analysis, we don’t get to define the reference class arbitrarily, since it consists of all linked decisions. (In the UDT case, all decisions that are made by any agents anywhere applying UDT).