RE: Minds, Machines, and the Multiverse

From: Billy Brown (
Date: Sat May 06 2000 - 21:50:41 MDT
> For a robot to make a measurement in a reversible and non-dissipative
> manner is possible. For it to reverse that measurement is also possible.
> However for the reversal to work I believe it is necessary that the
> outside world must be in the same state that it was in at the end of
> the measurement. This was one of the points Robin made in his paper.
> If the outside world is reversed when the robot is, as is possible if
> the robot is interacting with a VR, then this condition will be met.
> But if the outside world is the real world, then in general this condition
> cannot be met. Things will have changed in the interim and when you do
> the reversal, the state of the measured system will not be the same as
> it was when the initial measurement was completed.
> Therefore measurements can be reversed in the virtual world but not in the
> real world, and you thus have additional opportunities to save energy when
> interacting with a virtual world over interacting with a physical one.

A VR can never be the environment of anything in a thermodynamic sense. It
can't even be the environment of a purely virtual device. All the bits in
your robot are represented by real bits in a physical computer. So are all
the bits representing the current state of the VR, and its past and future
states. When you try to reverse the virtual operations of your simulated
robot, it is this mass of physical bits that must be manipulated. In order
to return the VR world to a previous state, you have to return some pattern
of bits in the physical machine back to a previous state. At that point you
run into exactly the same constraints you would have if you weren't using a

This is just a special case of a much more general (and obvious) rule: no
form of software can ever perform a computation that could not, in
principle, be done by the hardware it runs on.

Billy Brown

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