When "Spooky action at a distance" gets a little _too_ spooky...and runs into Heisenberg.

http://www.wired.com/wiredscience/20...d-uncertainty/
Unfortunately, that probably limits the idea of instantaneous
point-to-point communication from any point in the universe to any other
point in the universe impossible.
So much for FTL communications, like Starfleet uses.

Pat

On 21/11/2010 12:21 AM, Pat Flannery wrote:
http://www.wired.com/wiredscience/20...d-uncertainty/
Unfortunately, that probably limits the idea of instantaneous
point-to-point communication from any point in the universe to any other
point in the universe impossible.
So much for FTL communications, like Starfleet uses.

Pat

Though I tend to suspect articles that state things like "In another
weirdness, two particles can be bound together such that observing one
causes changes in the other, even when they’re physically far apart."

That's a fairly popular idea of what phase-entanglement is about, but QM
does not say that measuring one particle affects the other. It only says
how strongly correlated two measurements will be.

The difficulty with the idea that measuring one particle affects the
other is brought into stark relief when one considers measurement events
that are too far apart for light to travel between them. In such a
scenario, the events do not occur in a defined order, so which is the
measurement that does the affecting, and which is the measurement that
is affected?

Sylvia.

On Nov 21, 4:07*am, Sylvia Else wrote:

That's a fairly popular idea of what phase-entanglement is about, but QM
does not say that measuring one particle affects the other. It only says
how strongly correlated two measurements will be.

The difficulty with the idea that measuring one particle affects the
other is brought into stark relief when one considers measurement events
that are too far apart for light to travel between them. In such a
scenario, the events do not occur in a defined order, so which is the
measurement that does the affecting, and which is the measurement that
is affected?

But the measurements will be so strongly correlated, if the same thing
is measured for the two particles, that if measuring one did _not_
affect the other, then a logical contradiction would arise, because of
Bell's inequality. If the far particle were not affected, then it
would be bound to yield the same result for the same measurement, even
if the other particle were measured in a different way. But that would
yield the kind of behavior associated with a "hidden variables"
theory, which observation rules out.

The fact that the events don't occur in a defined order is just
another way to "prove" that this effect cannot be used for FTL
communications.

John Savard

On 22/11/2010 5:26 AM, Quadibloc wrote:
On Nov 21, 4:07 am, Sylvia wrote:

That's a fairly popular idea of what phase-entanglement is about, but QM
does not say that measuring one particle affects the other. It only says
how strongly correlated two measurements will be.

The difficulty with the idea that measuring one particle affects the
other is brought into stark relief when one considers measurement events
that are too far apart for light to travel between them. In such a
scenario, the events do not occur in a defined order, so which is the
measurement that does the affecting, and which is the measurement that
is affected?

But the measurements will be so strongly correlated, if the same thing
is measured for the two particles, that if measuring one did _not_
affect the other, then a logical contradiction would arise, because of
Bell's inequality. If the far particle were not affected, then it
would be bound to yield the same result for the same measurement, even
if the other particle were measured in a different way. But that would
yield the kind of behavior associated with a "hidden variables"
theory, which observation rules out.

QM tells us what the correlation will be. When the events are time-like
separated, one occurs before the other, and it's tempting to treate the
first as a cause, and the second as an effect. But the lack of
justification for that becomes apparent when one considers space-like
separated events. There is then no basis for characterising one as a
cause and the other as an effect, because their order is not defined. So
we have two measurements, and a predicted correlation that is born out
by experiment. Beyond that it's speculation. In particular, the notion
that one measurement affects the other involves introducing an assymetry
that doesn't exist in the theory. Something mighty strange is going on
down there, for sure, but it's not cause and effect as we understand it.

Sylvia.