r@la.tis.com (Richard Schroeppel) (12/19/90)
Patrick Hayes writes:
About watching atoms decay with an STM: isn't it true that every time
you scan over an atom with the needle, you have observed it, thus collapsing
the wave function to "I'm not even thinking about decaying now" state? So
the more often you scan it, the less likely it will become that it decays?
And JoSH comments:
[This isn't as nutty as it sounds. There are in fact demonstrated
physical systems where repeated measurement can prevent quantum-
mechanical transitions. However, in the specific case of the STM,
my guess is that the measurement of the state of the nucleus is far
too indirect for it to have that kind of effect.
--JoSH]
which raises some interesting questions.
When Science News reported on the "observing it prevents quantum
transistion" phenomenon, some of us wondered about postponing or
otherwise altering radioactive decay. A local quantum wizard
informed us that the atom would have to be observed 10^13 times
per second to have a noticeable effect on radioactive decay.
But what, exactly, is an observation? QM asserts that no observation
is indirect: If A causes B, or even alters the probability of B,
then an observation of B is also an observation of A, and hence
should reinitialize the state of A. So no atom can ever decay.
Obvious nonsense. What's the flaw?
Rich Schroeppel
rcs@la.tis.com
[A very interesting question but one better suited to sci.physics
than sci.nanotech. There are a number of theories out to deal
with the problem of wave-function collapse (e.g. the speculations
in Penrose's book described here about a year ago). Unfortunately,
nothing definitive is really known.
--JoSH]landman@eng.sun.com (Howard A. Landman) (12/22/90)
Followup to sci.physics. I'll summarize back to sci.nanotech if needed. >Patrick Hayes writes: >isn't it true that every time you scan over an atom with the needle, >you have observed it, thus collapsing the wave function >And JoSH comments: >[There are in fact demonstrated physical systems where repeated measurement > can prevent quantum-mechanical transitions. Yes, but these are all systems where the thing being measured interacts with the thing that may change. The so-called "quantum Zeno effect" shouldn't occur when the thing being measured is independent (formally, when the two QM operators commute). This is the same condition that determines whether there is a Heisenberg uncertainty relationship between the two quantities. >However, in the specific case of the STM, > my guess is that the measurement of the state of the nucleus is far > too indirect for it to have that kind of effect. > --JoSH] This is really the question of whether the position of the atom commutes with its quantum state with respect to decay. I believe that the answer is yes. (Anyone in sci.physics care to confirm or deny this?) Therefore, continual measurement of the position of an atom should not have a quantum Zeno effect on its radioactive decay. -- Howard A. Landman landman@eng.sun.com -or- sun!landman