Technical Report 2008 - 553 Time - Sensitive Computational Models with a Dynamic Time Component ∗

It is known that a parallel computer can solve problems that are impossible to be solved sequentially. That is, any general purpose sequential model of computation, such as the Turing machine or the random access machine (RAM), cannot simulate certain computations, for example solutions to real-time problems, that are carried out by a specific parallel computer. This paper extends the scope of such problems to the class of problems with uncertain time constraints. The first type of time constraints refers to uncertain time requirements on the input data, that is when and for how long are input data available. A second type of time constraints refers to uncertain deadlines for tasks. The main contribution of this paper is to exhibit computational problems in which it is very difficult to find out (read compute) what to do and when to do it. Furthermore, problems with uncertain time constraints, as described in this paper, prove once more, that it is impossible to define a universal computer, that is, a computer able to simulate all computable functions.