

Quantum test pricks uncertainty - Thibaut
http://www.bbc.co.uk/news/science-environment-19489385

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Xcelerate
Nobody has cast doubt on anything in modern quantum mechanics. If you look at
the abstract, it makes this much clearer than the BBC article does:

"While there is a rigorously proven relationship about uncertainties intrinsic
to any quantum system, often referred to as “Heisenberg’s uncertainty
principle,” Heisenberg originally formulated his ideas in terms of a
relationship between the precision of a measurement and the disturbance it
must create. Although this latter relationship is not rigorously proven, it is
commonly believed (and taught) as an aspect of the broader uncertainty
principle. Here, we experimentally observe a violation of Heisenberg’s
“measurement-disturbance relationship”, using weak measurements to
characterize a quantum system before and after it interacts with a measurement
apparatus. Our experiment implements a 2010 proposal of Lund and Wiseman to
confirm a revised measurement-disturbance relationship derived by Ozawa in
2003. Its results have broad implications for the foundations of quantum
mechanics and for practical issues in quantum measurement."

In other words, Heisenberg originally thought that the inability to measure
two incompatible observables (like momentum and position) was because of
something that is now called the "observer effect":
<http://en.wikipedia.org/wiki/Observer_effect_(physics)> The observer effect
applies even to classical systems without quantum mechanics! You just can't
measure something without affecting it in some way [1].

The _modern_ version of Heisenberg's uncertainty principle says nothing about
measurement disturbing the system. It instead says that there is an _inherent_
uncertainty to the system. I've been seeking clarification on what exactly
this means for a long time and have never really gotten a satisfactory answer.
I've tried Physics Stack Exchange, my QM professor, even Hacker News and most
people either confuse HUP with the observer effect or they reply in illy-
defined terms that don't help me any.

The best that I can figure out on my own -- the _true_ HUP -- is thus: You can
prepare a system in a certain state. You then take a position measurement and
a momentum measurement at the same time. You get two real numbers. Now, you
repeat the experiment. Create a system just like you did before and take
measurements of r and p. After multiple repetitions of the same experiment
over and over, you'll get two long lists of position and momentum
measurements. Take the standard deviation of the position measurements,
multiply that value by the standard deviation of the momentum measurements,
and HUP guarantees that the resulting real number will be ≥ ħ/2 (reduced
Planck's constant over 2).

[1] Well, there's a few loopholes if anyone is curious.

