Re: Planetary rovers, SETI and other musings, was Re: update.356 (fwd)

[Tim May <tcmay@got.net>]

At 10:44 AM -0800 1/28/98, Steve Schear wrote:

>If that is the case, I still don't understand why and out-of-band signal is
>required.  If the sender collapses the wave function shortly before the
>signal reaches the intended receiver its unlikely to have changed
>polarization again.

Because the state is not known until time of measurement, at which point
the other state takes on the opposite value. (Using the language of
"collapsing the wave function," which is, BTW, not the only interpretation.)

No information can be sent because the sender cannot pick the values to send.

Instead of thinking in terms of a single bit, think in terms of a message,
to make the point even clearer.

Suppose the message to be sent is "Attack at dawn."

Suppose the bit version of this is "1 0 0 0 1 0 1 1 0 1 1 1 1 0 .....etc."

Now, how does the sender possibly send this?

He can't, unless he also sends a signal saying "Ignore the first bit, keep
the second bit, keep the third bit, ignore the fourth bit,....." In other
words, a key.

(And they can't even agree on a key "in advance," because the sender cannot
control how the polarizations will come out.)

This will have to be my last post on this subject. Please read any of the
many basic treatments of these things.

As with the endothermic vs. exothermic debate, there is much out there, and
the Cypherpunks list is a very poor place to discuss basic quantum
mechanics.

--Tim May

The Feds have shown their hand: they want a ban on domestic cryptography
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Timothy C. May              | Crypto Anarchy: encryption, digital money,
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