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

Forwarded message:

> Date: Tue, 27 Jan 1998 23:44:50 -0800
> From: Steve Schear <schear@lvdi.net>
> Subject: Re: Planetary rovers, SETI and other musings, was Re: update.356
>  (fwd)

> I guess I'm over my head in such matters.  From my, admitedly, shallow
> understanding of wave function collapse, etc., I was under the apparent
> misimpression that once collapsed (e.g., by Alice entangling a 'modulation'
> photon M (of a known polarization) with one member (photon A) of an
> entangled pair, one of which was sent to Alice and the other (photon B)
> which was sent to Bob, photon B's polarization state was determined and
> could not subsequently be altered by Bob's measurement with his receiver.
> Could you recommend a good article which explain this paradox to a
> non-quantum mechanic?

The state is determined *at the time of collapse*. Once the collapse occurs
the synchronization is no longer present and subsequent events can indeed
alter the polarization of one particle without altering the other. Simply
bouncing that photon via refraction off a surface can alter the
polarization.


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At 8:28 AM -0600 1/28/98, Jim Choate wrote:
>Forwarded message:
>
>> Date: Tue, 27 Jan 1998 23:44:50 -0800
>> From: Steve Schear <schear@lvdi.net>
>> Subject: Re: Planetary rovers, SETI and other musings, was Re: update.356
>>  (fwd)
>
>> I guess I'm over my head in such matters.  From my, admitedly, shallow
>> understanding of wave function collapse, etc., I was under the apparent
>> misimpression that once collapsed (e.g., by Alice entangling a 'modulation'
>> photon M (of a known polarization) with one member (photon A) of an
>> entangled pair, one of which was sent to Alice and the other (photon B)
>> which was sent to Bob, photon B's polarization state was determined and
>> could not subsequently be altered by Bob's measurement with his receiver.
>> Could you recommend a good article which explain this paradox to a
>> non-quantum mechanic?
>
>The state is determined *at the time of collapse*. Once the collapse occurs
>the synchronization is no longer present and subsequent events can indeed
>alter the polarization of one particle without altering the other. Simply
>bouncing that photon via refraction off a surface can alter the
>polarization.

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.

--Steve

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
---------:---------:---------:---------:---------:---------:---------:----
Timothy C. May              | Crypto Anarchy: encryption, digital money,
ComSec 3DES:   408-728-0152 | anonymous networks, digital pseudonyms, zero
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