On Fri, Dec 19, 2014 at 2:13 PM, Jim Bromer <[email protected]> wrote:
> On Thu, Dec 18, 2014 at 9:52 PM, Matt Mahoney via AGI <[email protected]>
> wrote:
>> On Wed, Dec 17, 2014 at 1:27 PM, Abram Demski via AGI <[email protected]>
>> wrote:
>>> My current understanding of time is "the direction of computation".
>>
>> That is actually quite precise. The entropy of a computer can only
>> decrease. In a state transition diagram, states can merge but not
>> fork. Operations like writing a bit of memory cannot be reversed
>> because the previous bit was erased.
>
> That reasoning is confused. The reference to a state transition
> diagram is not directly related to the discussion up until that point,
Suppose a finite state machine has 2 possible states, A and B. The
program is {A -> B, B -> B}. The initial state can either be A or B.
The next state will be B. The entropy of the computer (the number of
bits you need to describe the state) has gone from 1 to 0.
If the computer is in state B, you do not know the previous state. You
cannot run the program backward.
The same argument applies to Turing machines. Writing a symbol to the
tape is not reversible in general because the previous symbol is
erased.
I realize there are some programs that are reversible, for example {A
-> B, B -> A}. In this case, entropy remains constant.
> and I don't believe anyone was talking about a computer program that
> could not respond to input.
A computation taking input is equivalent to a computation whose input
is part of its initial state.
--
-- Matt Mahoney, [email protected]
-------------------------------------------
AGI
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