sci.physics

martin wrote:
> "Bob Cain" wrote in message
> news:R-qdnep2XYOQyi_bnZ2dnUVZ_sejnZ2d@ ...
>> martin wrote:
>>> Hello,
>>>
>>> I'm trying to understand why entanglement is such a special feature in
>>> QM.
>>> As far as I understand it, it says that when 2 particles are 'born'
>>> together from the same source, their spins must be correlated. If
>>> initially the total spin is zero, then when 2 electrons are created, one
>>> should have spin up, and the other spin down.
>>> As long as the spins of the elctrons are not observed, the spins of each
>>> could be òr up òr down;
>> Here is where your understanding departs from the most widely accepted
>> interpretation. In that interpretation they do not have a property until
>> it is measured. If the measurement of one establishes both it and its
>> distant partner's property and that happens instantaneously then something
>> weird is going on.
>>
>> The way out of the weirdness is what has been called "hidden variables"
>> which disputes Copenhagen interpretation and says that the properties
>> really are established at the point of creation but "hidden" until
>> measured. This is intuitively satisfying. John Bell came up with a
>> hypothetical experiment and analysis which could disclose whether or not
>> there really were these hidden variables (which was Einstein's belief) or
>> whether the entangled properties did not exist until measured (Bohr and
>> company's belief.) The experiment has been performed in several
>> variations and all results to date seem to confirm that the hidden
>> variables don't exist.
>>
>> Thus, that something at a distance which is not determined becomes so when
>> something local is measured, and does so instantaneously, seems to
>> contradict the relativistic notion that no information can propagate
>> faster than light speed. Turns out that this limit is still true because,
>> since what is being measured is random to begin with, no information can
>> be transmitted this way to a distant observer, but when the distant
>> information is transmitted at light speed to the local observer he will
>> find that it correlates with what he measured. Because he can receive
>> this information in the time it takes for a one way light trip he can
>> deduce that the decoherence of the entangled pair happened instantaneously
>> across a distance which can be arbitrary.
>>
>>
>> Bob
>
> Can you perhaps give an example of entanglement-BEHAVIOUR?
> How can I notice that 2 electrons are entangled?

Usually by measuring the property (of interest) of one and then the other with a
negligable amount of time between the measurements but signifigant distance and
finding that the measurements correlate. Particle spin is often used as the
property of interest. It is possible to generate a pair of particles which must
have opposite yet unpredictable spins. Measure one particle and then the other
and you will find their spin to be opposites.

This result is only surprising, though, if you understand that the property of
interest of both particles, spin in this case, is not determined at the time of
generation but rather at the time one or the other is measured. It is not only
unknown until the measurement, it really has not been "set" until then.

If you believe that they really have the property at creation and that you are
just measuring that later then of course the result is not surprising.

Thing is, experiments have been devised and run to determine whether the
situation is the former or the latter and the former wins by observation.


Bob
--

"Things should be described as simply as possible, but no simpler."

A. Einstein