Lots of answers here make the important game-theoretic points:
- Chess is a finite, deterministic game with complete information about the game state
- You can solve a finite game and identify a perfect strategy
- Chess is however big enough that you will not be able to solve it completely with a brute force method
However these observations miss an important practical point: it is not necessary to solve the complete game perfectly in order to create an unbeatable machine.
It is in fact quite likely that you could create an unbeatable chess machine (i.e. will never lose and will always force a win or draw) without searching even a tiny fraction of the possible state space.
The following techniques for example all massively reduce the search space required:
- Tree pruning techniques like Alpha/Beta or MTD-f already massively reduce the search space
- Provable winning position. Many endings fall in this category: You don't need to search KR vs K for example, it's a proven win. With some work it is possible to prove many more guaranteed wins.
- Almost certain wins - for "good enough" play without any foolish mistakes (say about ELO 2200+?) many chess positions are almost certain wins, for example a decent material advantage (e.g. an extra Knight) with no compensating positional advantage. If your program can force such a position and has good enough heuristics for detecting positional advantage, it can safely assume it will win or at least draw with 100% probability.
- Tree search heuristics - with good enough pattern recognition, you can quickly focus on the relevant subset of "interesting" moves. This is how human grandmasters play so it's clearly not a bad strategy..... and our pattern recognition algorithms are constantly getting better
- Risk assessment - a better conception of the "riskiness" of a position will enable much more effective searching by focusing computing power on situations where the outcome is more uncertain (this is a natural extension of Quiescence Search)
With the right combination of the above techniques, I'd be comfortable asserting that it is possible to create an "unbeatable" chess playing machine. We're probably not too far off with current technology.
Note that It's almost certainly harder to prove that this machine cannot be beaten. It would probably be something like the Reimann hypothesis - we would be pretty sure that it plays perfectly and would have empirical results showing that it never lost (including a few billion straight draws against itself), but we wouldn't actually have the ability to prove it.
Additional note regarding "perfection":
I'm careful not to describe the machine as "perfect" in the game-theoretic sense because that implies unusually strong additional conditions, such as:
- Always winning in every situation where it is possible to force a win, no matter how complex the winning combination may be. There will be situations on the boundary between win/draw where this is extremely hard to calculate perfectly.
- Exploiting all available information about potential imperfection in your opponent's play, for example inferring that your opponent might be too greedy and deliberately playing a slightly weaker line than usual on the grounds that it has a greater potential to tempt your opponent into making a mistake. Against imperfect opponents it can in fact be optimal to make a losing if you estimate that your opponent probably won't spot the forced win and it gives you a higher probability of winning yourself.
Perfection (particularly given imperfect and unknown opponents) is a much harder problem than simply being unbeatable.
