# RE: Real vs. Simulated Ecologies [was: Moravec article available]

From: hal@finney.org
Date: Wed May 03 2000 - 14:41:10 MDT

Billy writes:
> The thing you have to remember about a VR is that it isn't an alternate
> universe. Any transformation that happens in the virtual world has to be
> implemented as an actual transformation on real particles in a physical
> system. It doesn't have to be the *same* transformation, but it has to be
> *some* transformation of equal or greater information content. No matter how
> cleverly you design the VR, it will always have this constraint due to the
> simple fact that the virtual world has no independent existence.

So you are in the VR, and you have built a computer which holds a 1
or a 0, and you want to erase it. Obviously there is no need for the
VR to simulate anything becoming hotter. It can simulate any laws of
physics it wants to, and it need not be constrained by the usual laws
of thermodynamics.

However in the physical world if the VR erasure is mirrored by a physical
erasure, that must dissipate heat. The amount dissipated need not have
any particular bearing on the amount that the VR person would calculate,
though.

For one thing, the amount of heat dissipated per erasure depends on the
temperature. The simulated temperature in the VR doesn't have anything
to do with the temperature of the physical system. The VR may appear
to be erasing a bit in a room temperature computer while the physical
computer running the VR is cryogenically cooled.

It could also be possible to reduce the cost of erasure or even avoid
it altogether, by the trick of running the simulation forward while
preserving all the extra bits, then running it backwards to erase the
calculation and the extra bits. You can do this every so often and
just snapshot the state that is needed to proceed with the calculation.
The result is that many erasures wouldn't get counted, only those which
are necessary to go from one snapshot to the next.

Hal

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