
'Zeno effect' verified: Atoms won't move while you watch - jonbaer
http://www.news.cornell.edu/stories/2015/10/zeno-effect-verified-atoms-wont-move-while-you-watch
======
fauigerzigerk
Sometimes I feel that people writing about physics are having great fun with
deliberately confusing people by making stuff sound magical and illogical.

This particular effect has nothing to do with whether or not anyone is looking
at an object. It's the methods of making it visible that cause the effect.

~~~
DyslexicAtheist
could swear I've had some debugging / system testing sessions where the effect
of me looking into it changed its state :-)

~~~
rybosome
We call those "heisenbugs" \- one of the most terrifying and maddening things
you can encounter prior to a launch. I'm almost positive that I remember
reading a sort from a game dev who ran into one of these right before a
release, where printing some variable caused it to have the correct value; in
this case, it was left in the game.

~~~
EwanG
Well, I know that I ran into a situation (and may once have posted about it)
in a C program where I was doing printf to debug a program, and it ran fine.
Whenever I removed them, the program would crash. Finally discovered that the
printf debugging was forcing garbage collection which took care of a bad
pointer somewhere else in the program. Honestly can't remember if I ever found
the pointer or just left the printfs in the code...

~~~
nitrogen
Thank goodness we have Valgrind these days. Though I don't know how well it
would handle a C app with a garbage collector.

Calling printf can (will) change what's beyond the stack of the calling
function, so if the pointer in question pointed outside the active stack,
printf may have put a safer value there by coincidence.

------
Strilanc
If I understand correctly, what's going on is... Suppose you have a system
rotating between state |0> and state |1>, like f(t) = cos(t) |0> \+ sin(t)
|1>. If you measure at time t, you get |0> with probability cos(t)^2. After
measuring, the system starts rotating again, but reset to |0> (assuming you
measured that). If you measure n times per rotation period, the chance of
measuring the state |0> every time (i.e. of effectively keeping the system in
the |0> state) is (cos(τ/n)^2)^n per period. That converges to 100% as n gets
large.

So, by measuring more and more frequently, you can effectively stop a system
like that from transitioning between states.

It's an interesting example of measurement unavoidably affecting quantum
systems, where the problem clearly isn't due to kicking or perturbing the
state.

~~~
tomp
So, imagine you have a ball and a slope

    
    
           o
        __/\__
    

(ball should be on top in the middle).

The ball will slowly start rolling down one or the other side of the slope
(because gravity).

You have a robot hand that keeps checking if the ball is still on top by
grabbing it and then releasing it right in the middle. There is some leeway
when the hand grabs the ball but none when it releases it.

    
    
        __|__          _|_
        | o |    =>    |o|
    

So effectively, if you do this fast enough, you keep "resetting" the ball on
top of the slope.

Is this a good analogy?

~~~
Strilanc
Not really. A minor problem is that it fails to capture the fact that this
process will also hold the qubit stable in the |1> state. More importantly, it
fails to capture the limiting behavior of the success rate; it should move
smoothly towards 100% as the measurement frequency increases but the ball-and-
slope would instead jump discontinuously from not working to working as the
hand went from being too-slow to fast-enough.

A better example might be... a series of polarizing filters? If you have a
series of polarizers but skip all the way from vertical to horizontal with
nothing in between, no light gets through. If you put a diagonal polarizer
between the horizontal and vertical, some light gets through. If you have a
big long series of very gradual steps from vertical polarizer to horizontal
polarizer, almost all the light gets through. More frequent measurements
causing the zeno effect is like to adding more gradations of polarizer
direction causing the all-light-gets-through effect.

~~~
dognotdog
Er, I am fairly sure that polarizer example doesn't work, and still no light
gets through, as polarizers don't rotate the light, but simply cut off
components that are orthogonal to the polarization direction. So stacking
slightly rotated filters through to 90 degress would still leave you with no
light.

EDIT: On second thought, it might just be you wording making it seem weird.
The analogy works if you don't look at the whole stack, but the after adding
each polarizer. Still, the same effect is had as simply gradually rotating the
2nd polarizer.

~~~
Strilanc
You're one of the lucky ten thousand today. [1]

Following a vertical polarizer with a horizontal polarizer will block all
light. But putting a diagonal polarizer in between will result in some of the
light getting through. You can find videos and explanations of this effect on
youtube [2] [3].

1: [https://xkcd.com/1053/](https://xkcd.com/1053/)

2:
[https://www.youtube.com/watch?v=gm2LCsM_S5o](https://www.youtube.com/watch?v=gm2LCsM_S5o)

3:
[https://www.youtube.com/watch?v=KM2TkM0hzW8](https://www.youtube.com/watch?v=KM2TkM0hzW8)

~~~
dognotdog
Hah, nice! Looking at it now, of course it makes sense. I guess I just short-
circuited by thinking of the polarizer as blocking light only, and not that
the reduction in intensity is the result of the projection of the previous
wave direction onto the new wave plane!

EDIT: not that this makes the quantum effects any clearer ;)

------
wmt
*while illuminated by a bright laser. By dimming the laser the quantum behaviour returned, and you watching the atoms or not had no effect on the phenomenon.

Despite the misleading title, I imagine affecting the quantum behaviour of
atoms with lasers has all kinds of nice use cases!

~~~
rubidium
Affecting the quantum behavior of atoms with lasers is pretty much the entire
field of ultracold quantum gases :) A handful of Nobel prizes have been
awarded for it.

------
buzzdenver
Layman question: what constitutes observation in the world of quantum physics
? The article suggests that stronger light means less quantum effects, but
isn't observation a binary thing, so something is either watched or not.

~~~
kawa
Observation means: Interaction with a 'classical system'. Now because of
entanglement this also works if we look at chains of interactions: Interaction
with another single atom for example is no observation, but if this other atom
interacts with a 'classical system' at some point earlier or later we have
again an 'observation'.

Now 'classical systems' are systems made of so many elements that we can only
make statistical statements about it because we can't ever determine the state
of each of it's constituents.

I suspect for some time now that this is also the source for the "quantum
randomness". In principle, everything is deterministic, but to observe
something we always have interactions with a "big classical system" (in the
end it's always our brain). And because we don't know the exact state of those
systems but only statistical averages, quantum mechanics looks random for us,
even if it's perfectly deterministic in itself. But that's my personal view of
the matter.

~~~
JabavuAdams
The first couple of paragraphs are a great summary. Prof. Binney has some
interesting observations on the non-physicality of quantum measurement (the
formalism).

One of the postulates of QM is that after a measurement the system is in a
well-defined state (the one we measured). But this is aphysical.

It's an artifact of the deliberate choice to formally model measurement in a
way that simultaneously recognizes that all measurements disturb the measured
system, while also wanting to abstract away the particular hardware used.

So, I'm starting to see wavefunction collapse and the whole Copenhagen
interpretation as artifacts of the the formalism, not any kind of physical
truth.

------
fsiefken
Would there be a difference in result when only the computer looked at the
individual positions, logged the measurements, determined them and noted the
level of quantum behaviour to test this hypothesis? I read this thread on the
physics stackexchange
[http://physics.stackexchange.com/questions/110488/consciousn...](http://physics.stackexchange.com/questions/110488/consciousness-
causes-collapse-interpretation-or-free-will-excluded-by-quantum)

..and it is said most professional physicists don't believe in this
'consciousness (human or otherwise) causes collapse' Neumann-Wigner
interpretation (and that of John Wheeler and Henri Stapps) and most amateurs
do, but has it been tested in this way also by ruling out far out psi effects
like retropsychokinesis by deleting or encrypting the data in such a way it
can only be verified by a 'team' of independent artificial scientists
(networked statistical software)?

~~~
ars
"Looking" means "interacting". It does not mean a human is looking, it means
the particle is doing something with another particle.

------
pmontra
Many measurements == little change? It sounds vaguely familiar of overloaded
computing systems. Maybe we'll run timing attacks on whatever the quantum
world is.

~~~
bitJericho
I hope we can hack the universe at some point. I think it would be very
beneficial for humanity if we could break out of our universe.

~~~
Phemist
Or even achieving privilege escalation would be good enough for most.

~~~
ars
What if you caused a crash and core dump instead?

------
bandrami
"emergent classicality" is my new favorite phrase.

------
_ZeD_
_don 't blink_

------
PhasmaFelis
I really wish more science writers would explain what "observation" actually
means. This article is better than most, but generally one leaves quantum-
effect discussions with the impression that atoms can literally tell when a
human brain is perceiving them, presumably through magic science brain waves.

~~~
javert
When writing in English, unless the writer specifies otherwise, the word
"observation" means what the dictionary says it does.

The writer of this article is saying that the particles behave differently
after being observed.

Perhaps that is a lie.

My point is, if it's a lie, let's call it a lie, not bad writing.

~~~
criddell
I guess it depends on the presumed audience. As a layman, observation is a
passive event. After reading through some of the comments, I can see that
physicists have a different meaning for that word.

------
laotzu
Is this not basically saying the same thing that is shown in the double slit
experiment?

Short video on the double slit experiment for the layman:

[https://www.youtube.com/watch?v=DfPeprQ7oGc](https://www.youtube.com/watch?v=DfPeprQ7oGc)

------
ankurdhama
Natural languages have evolved around the human scale of things and humans
actions on those things. When you try to describe nature using these concepts
you will always get weird and unintuitive explanations.

------
flak48
I'm curious as to how the scientists confirmed that tunneling was taking place
when the atoms were not being illuminated. Does anyone know how this is
typically done?

~~~
gus_massa
I hope I understood your question correctly. It's very very very difficult to
take a photograph to catch the atom while tunneling.

The atoms in this experiment form a lattice, and the lattice acts a trap, so
the atoms can be only in some fixed spots. To go from one spot to another
spot, it has to have some minimal energy, so it's equivalent to climbing a
hill to go from one valley to another valley. The atom usually get that energy
by random movements, but in this experiment they are very cold, to avoid this
possibility, so they are trapped in one of those spots.

By quantum mechanics, they can travel from one spot to another spot without
enough energy. This is equivalent to going from one valley to another valley
without energy to climb the hill, so the tunneling nickname. The probability
is something like exp(- distance * energy_difference * constants) so it's
harder to go to a spot far away and it harder to pass a tall hill (I must be
missing a the time variable there, look at Wikipedia for the exact
expression.) And it's exponentially harder, not jut harder.

To answer your question. It's very very very difficult to take a photographs
of the atoms while tunneling. If you turn off the laser and you take a
photograph from time to time, you see that some atoms jump randomly from one
spot to another between photos while "no one" is watching :). If you turn on
the laser, the "laser" is watching, so the atoms don't move between
photographs. But don't expect to get a photograph of the atoms while
tunneling.

EDIT: after a coffee and a shower, I changed "impossible" to "very very very
difficult"

~~~
flak48
I re-read the article, and combined with your answer, it makes much more sense
now. What was originally confusing me, was how the scientists were confirming
the presence of tunneling when the laser was off.

I was mistakenly assuming that they were using some special method to infer
that tunneling was taking place.

But of course they don't need to, like you said. The tunneling can be inferred
from the change in positions of atoms in different photographs. They don't
actually need to see the actual tunneling in-progress (even if it were somehow
possible/not very difficult to do so). Makes sense.

Thanks for that answer, it's very helpful for someone like me whose last brush
with quantum physics was the physics/chemistry courses in year 1 of
engineering.

Cheers

------
signa11
what i really don't quite get (amongst a large number of other things as
well), is how you could have observed an atom without disrupting it's quantum
state...

~~~
oceanofsolaris
The Zeno effect is basically due to disruption of the quantum state. You
measure repeatedly in some basis and this basically pins the state down (in
this basis representation) even though it would not stay in this state if you
did not interact with it.

------
thesz
I guess that measurement introduces energy into the system and that is what
keeps atoms in places.

I haven't seen refutation of this in the article.

------
Confusion
I was convinced this was already known and verified long ago. I've certainly
told people this was True.

~~~
kintamanimatt
You're thinking of the double slit experiment

~~~
Confusion
No, I'm definitely thinking of the fact that observation inhibits change in a
quantum system. I'm fairly certain I was taught this was the case and was
experimentally verified. 'Long ago' here means ~20 years ago.

~~~
igravious
It says at the start of the article that it has already been experimentally
confirmed for spin, this experiment confirms with respect to motion. I don't
know when the spin one was done, maybe someone can provide more info.

What I don't know is if the different quantum properties are independent of
each other. If they are then presumably one would have to verify this Zeno
effect on each property of a quantum particle. So maybe we can now say that
the Zeno effect has been experimentally verified for two quantum properties
now. Looked at this way you were both correct and incorrect.

If spin and momentum are 'connected' in that if you experimentally verify some
meta-property about one it verifies it for the other (such as, in this case,
observing a property of a quantum particle freezes that particle) then this
has been proved True and you were correct.

Does this make sense? IANAQP.

~~~
oceanofsolaris
You can do this e.g. with photon polarization. Measuring here is very simple
since it is achieved by sending your light through a linear polarizer.

If you send light through a chiral medium, the orientation of the polarization
would normally rotate. However if you measure the polarization often enough
(by putting enough polar filters that are all oriented in the same direction
in between), this rotation will not occur. This experiment is rather simple
since you only need a standard optical bench and not any low temperature
setup.

------
sjclemmy
'In the quantum world, the folk wisdom really is true: “A watched pot never
boils.”'

~~~
pistle
And yet it does boil. The folk wisdom is wrong, so is the experiment wrong as
well?

------
jlebrech
could you freeze someone/something in time by watching all those atoms?

~~~
PhasmaFelis
You really should read the rest of the article. The imaging laser does nothing
to impede normal movement, only quantum tunneling, which only happens
frequently/reliably at near-absolute-zero temperatures anyway.

------
hauget
So I guess it's safe to hypothesize (for now) that when we're not conscious,
we are in a quantum superposition?

~~~
hauget
Why the downvotes? Has no one read about the quantum mind theory?
[https://en.m.wikipedia.org/wiki/Quantum_mind](https://en.m.wikipedia.org/wiki/Quantum_mind)

~~~
yoha
Downvotes are sometimes harsh on HN. I think the majority of HNers perceives
the Quantum Mind theory as described in this article:
[http://rationalwiki.org/wiki/Quantum_consciousness](http://rationalwiki.org/wiki/Quantum_consciousness)
.

~~~
hauget
Yeah, also some people can't take a joke...

