X-Message-Number: 21020
From: "michaelprice" <>
References: <>
Subject: Coloured photons
Date: Fri, 31 Jan 2003 09:32:52 -0000

I, like Ettinger, have I received an off-list email on the subject of
identity and the red/orange photons example:

> I know that there are what look like _very_ crisp tests for
> identity in physics, e.g. which rotational states of a diatomic
> molecule are permitted depends on whether the two atoms are
> identical or distinguishable (and, in the identical case, whether
> they are bosons or fermions).

Naturally, I have no problem agreeing with this!

> But, but...
> Ettinger actually had something when he commented about red
> photons interfering with orange photons.  The do interfere, and
> the pattern oscillates at the difference frequency.

Hmm.  Good point.  Yes, photons of different colours/frequencies
can interfere, even after allowing for detector disturbances.
Although they would still be distinguishable if we had an ideal
(i.e. non-localising) frequency detector.

Thinking about this more, though, perhaps frequency/colour is a bad
attribute to base photon identity on, since it is measure of kinetic
energy - and no one would claim that two electrons with different kinetic
energies are different kinds of particles, or that an electron changes
identity when it changes speed, so perhaps we should question why we 
would expect this for photons.  However, if we come across particles 
with, say, different rest masses or electric charges we have no problem 
classifying them as different sorts of particles.  (All photons, no matter 
what colour, have zero charge and zero rest mass.)

> P.S. All of this is _way_ away from any criteria relevant to
> cryonics.  Any real life criterion of identity of people is vastly
> less stringent than coherent quantum interference.

Amen.  But thinking about the physics does place an upper
limit on how stringent any theory of identity for people could
possibly be, even though in practice personal identity is orders
of magnitude less restrictive.  For me, personal identity is just
information and, in particular, memories, so that capturing the gross
structure of cerebral neurons (including synapses and glial cells) 
should be sufficient.  Sub-atomic details and putative physical 
continuity are irrelevant.

Michael C Price

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