| Topic: |
Science > Physics |
| User: |
"mudpuppy" |
| Date: |
26 Jun 2005 11:17:42 AM |
| Object: |
quantum entanglement and decoherence |
If a bunch of particles start off quantumly entangled how come they do
not stay that way? What does decoherence mean exactly?
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| User: "Old Man" |
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| Title: Re: quantum entanglement and decoherence |
26 Jun 2005 04:00:28 PM |
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"mudpuppy" <lyricalforest@gmail.com> wrote in message
news:1119802662.703589.9770@g44g2000cwa.googlegroups.com...
If a bunch of particles start off quantumly entangled how come
they do not stay that way? What does decoherence mean exactly?
What does "quantumly entangled" mean ? Coherence
has to do with constant phase between wave functions.
"Decoherence" implies a random out-of-phase condition
between wave functions. It can't occur in free space.
[Old Man]
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| User: "mudpuppy" |
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| Title: Re: quantum entanglement and decoherence |
27 Jun 2005 12:25:18 PM |
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what do you mean by constant phase between wave functions?
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| User: "Old Man" |
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| Title: Re: quantum entanglement and decoherence |
27 Jun 2005 02:11:03 PM |
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"mudpuppy" <lyricalforest@gmail.com> wrote in message
news:1119893118.314367.312490@g49g2000cwa.googlegroups.com...
what do you mean by constant phase between wave functions?
Coherence.
Understanding of "phase" comes with understanding of "wave
function". Mudpuppy hasn't a clue.
"Quantum Mechanics" Landau & Lifshitz ISBN 0-08-029140-6
[Old Man]
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| User: "Guy Gordon" |
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| Title: Re: quantum entanglement and decoherence |
26 Jun 2005 05:58:05 PM |
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"mudpuppy" <lyricalforest@gmail.com> wrote:
If a bunch of particles start off quantumly entangled how come they do
not stay that way? What does decoherence mean exactly?
Short answer: They don't stay that way because they interact with their
environment.
Long answer:
'Entangled' means that our Quantum Mechanical description of two particles must
be a single state for the pair, rather than two independent states.
For example, let's say we start with a single particle in a pure QM state having
zero angular momentum. This particle decays into two, each of which *have*
angular momentum. Then because of conservation of angular momentum we know that
the sum of the two particles still has zero angular momentum. Therefore these
two are entangled, and must be described as a single state having zero angular
momentum. This state will be a linear combination of the various possible
states of the individual particles. (This is the Superposition Principle in
QM.)
In the Copenhagen Interpretation of QM, the two particles do not even *have*
individual values of angular momentum yet. But in the 'decoherence' picture we
say the two particles are entangled and their angular momentums are correlated.
It does not matter which picture you use, the mathematics of QM is the same.
'Decoherence' means that the two particles are no longer entangled, and their
attributes are no longer correlated. We can now describe them as two individual
QM states, rather than as a superposition of states.
Decoherence happens when a 'measurement' is performed on at least one of the
particles. The measurement reveals one particular result, so the superposition
of many possible states is no longer a valid description of the two particles.
A 'measurement' (which causes decoherence) is any interaction that irreversibly
selects one state out of the superposition.
QM allows us to calculate the probability of the result of the measurement. But
nobody knows how nature 'chooses' the result, or how the two particles stay
correlated as they move apart.
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| User: "Mike" |
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| Title: Re: quantum entanglement and decoherence |
26 Jun 2005 04:12:38 PM |
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See
http://en.wikipedia.org/wiki/Quantum_decoherence
http://superstringtheory.com/forum/qsboard/messages4/90.html
--Mike Jr.
mudpuppy wrote:
If a bunch of particles start off quantumly entangled how come they do
not stay that way? What does decoherence mean exactly?
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