next-best-transport! down with ethernet!

Vadim Antonov avg at
Fri Dec 30 19:40:35 UTC 2011

On Fri, 2011-12-30 at 14:00 +0100, Vitkovsky, Adam wrote:
> Well hopefully we won't need to worry about the speed of light anymore

Nope. The laws of physics as currently understood prohibit sending information
faster than the speed of light. (The reality of FTL neutrino thingie is still
too early to tell).

> Basically when 2 photons or electrons are emitted form the same source -they 
>are somehow bound/entangled together -that means if we change the spin
>on one photon to "up" the other photon will have it's spin changed to
>"down" immediately - and it doesn't matter whether the photons are next
>to each other or light years away -this happens instantly (no energy is
>transferred yet the information is passed) -this was already tested
>between two cities

That's not what happens: the entangled particles are in superposition
state (i.e. they are carrying both |0> and |1> simultaneously).  When
the measurement on one of them is made, their common wavefunction
collapses, leaving them in random specific state.  I.e. if you measured
one |0> the other will be |1>, or vice versa.  Changing quantum state of
an entangled particle to a known state will simply break entanglement
(the story is more complicated, but I don't want to get into arcana).
Because of that the quantum entanglement *cannot be used to transmit
information* between receiving points, so this non-local action at a
distance doesn't break the relativistic prohibition on FTL information

However, this effect is still useful because it is a way to generate
random encryption keys, which will "just happen" to be the same at both
ends, hence the quantum cryptography.  Anybody trying to snoop on the
entangled photons in transit will cause premature wavefunction collapse
which can be statistically detected (in practice sources of entanglement
and phase detectors are not perfect, so quantum cryptography is not

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