Re: Counterfactual Quantum Thinkers

Damien Broderick (
Tue, 12 May 1998 12:19:31 +0000

At 10:14 AM 5/11/98 -0400, Alex wrote:
>Oops, ignore my last, folks. Sorry. It's in Features, it's called Crazy
>Logic, and the web edition doesn't include it. Oh well, off to the library
>for me today.

And the ideas man in question is Richard Josza, of the University of
Plymouth, UK.

David Deutsch commented thus:

> There's a weird world where computers give
> you an answer just by being there. You don't
> even have to switch them on

Ah. Well, don't worry, this is false.

>The bottom line is that some clever fellow has worked out how to get
>answers out of a quantum preparation *without turning the machine on to
>run the calculation*.

Someone has slightly misunderstood the process, and then jumped to a
conclusion that they *thought* followed logically from the description they
had seen.

>It's based on the bomb-testing idea I'm sure you're all familiar with from
>Penrose's SHADOWS OF THE MIND. Elitzur and Vaidman showed that one time
>in 4 you can determine the state of a macro-system, *without observing it*,
>if >it is coupled to a quantum detector.

Actually they also showed the result you are describing here, namely that
you can determine that state as with reliability as close to 1 as you like
(but not actually 1), and you can reduce the probability that that will
involve measuring the system (in any one universe) to an arbitrarily low
number above zero. Again, if you reduce it to zero, the scheme won't work.

>All that's needed is for the observation
>to be *possible* via that procedure,

Which means "to occur in some universes", remember.

>and quantum interference
>can deliver you information about the real world without actually looking
>at it. Some of the time, anyway. And since that's true, a sequence of
>assays can multiply up your level of confidence to whatever degree you

The whole quantum computation that does this is the "sequence of assays".

>[...] This gadget doesn't need to be
>activated until *after* it's done the job.


>(What's more, it's reported
>that objects have now been `photographed' using this principle without
>shining any light on them, simply because light *could have been* shone on

No. It's because light did fall on them in other universes. Just like any
other interference experiment.

>Penrose is evidently rubbing his hands with glee, because this sounds like
>it might be the missing link in his theory that neural microtubules can
>sustain quantal superpositions in something like a Bose condensate. The
>brain might thus leap instantly and without frying itself to conclusions
>that even the smartest Turing Jupiter brain would be agonising over to the
>end of time...
>I dunno. Sounds like witchcraft to me. Pretty logic, though.

All that microtubule stuff, and more generally all suggestions that the
brain is a quantum computer, while not witchcraft, seem entirely
unmotivated to me. The 'interaction-free measurements', however (or
counterfactual computations as Richard Jozsa has somewhat misleadingly
called them), are a matter of ordinary, uncontroversial physics. It is
indeed pretty physics. And it only sounds like witchcraft if you try to
understand it as happening in a single universe.

-- David Deutsch

Damien Broderick