Using Averages To Bend the Uncertainty Principle 112
summerbreeze writes "Researchers at the University of Toronto have conducted a two-slit experiment, published in Science, that uses 'weak measurement' on photons to push back the boundaries of what can be known about them, given the Heisenberg Uncertainty Principle. Jason Palmer does a great job reporting this experiment to us mere mortals in a BBC article: 'The team allowed the photons to pass through a thin sliver of the mineral calcite which gave each photon a tiny nudge in its path, with the amount of deviation dependent on which slit it passed through. By averaging over a great many photons passing through the apparatus, and only measuring the light patterns on a camera, the team was able to infer what paths the photons had taken. While they were able to easily observe the interference pattern indicative of the wave nature of light, they were able also to see from which slits the photons had come, a sure sign of their particle nature."
Another one!?!?! (Score:4, Interesting)
Yeah Canada, again!
Canada certainly does punch above it's weight in many areas...
But this is a really interesting experiment! It really does turn the classic double slit experiment on it's ear!
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I don't get it (Score:1)
Re:I don't get it (Score:4, Informative)
In the classic experiment, if you try to find out which slit the photons are going through, they stop behaving as waves.
In this experiment, they can know which slit the photons when through, but still get the light to behave as a wave.
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I thought the photons went through both slits.
(and also took a detour around the Horsehead Nebula along the way...)
Re:I don't get it (Score:5, Insightful)
That's the idea of quantum physics : particles or waves don't move on any specific path, they move on all possible paths between 2 points. But once anything interacts with them the "potential history" function collapses, and they have taken one specific path, which had only one specific set of events taken place.
So photons only go through both slits in the function that describes their movement, not in reality. It's just that the only way to describe their behavior is to assume they go through both slits, because we can't measure these things without disturbing them.
Why not ? Well imagine you have to determine if it's the national holiday in India (they have a big elephant parade). But you don't actually have any tools smaller than elephants to measure this. So every hour or so you catapult an elephant into the main street of New Delhi, and you see if the elephant hits the detector you've set up at the other end of that street. Obviously any "detected" elephant will not be unaffected, and won't ever get to the place where the parade elephants normally end up, and your interference pattern will be gone. Now s/elephants/photons/ and you have the problem of quantum physics (and yes this is a simplification).
Now what these scientists did is they place an "elephant guide" (say a slide) in front of one of the two slits, which does not really affect the elephants, but it does alter their path a little bit, and this is reflected in the position the elephant hits the plate behind the detector. Now they know (not for certain, but better than 50%) which slit the elephant went through, yet they have managed to avoid totally destroying the normal path the elephants take, so the elephants from both slits are still in a position to interact.
A (very) nice video about this : http://www.youtube.com/watch?v=DfPeprQ7oGc [youtube.com]
Best Analogy Ever (Score:3)
> Why not ? Well imagine you have to determine if it's the national holiday in India (they have a big elephant parade). But you don't actually have any tools smaller than elephants to measure this. So every hour or so you catapult an elephant into the main street of New Delhi, and you see if the elephant hits the detector you've set up at the other end of that street. Obviously any "detected" elephant will not be unaffected, and won't ever get to the place where the parade elephants normally end up, and
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Fuck me. Someone with mod points boost this, it's about the clearest explanation of quantum physics I've seen - including four years of lectures. The amount of people who babble about "observation" apparently seriously believing that "observation" is important is startling. Your first two sentences summarise everything clearly and neatly and without any extra bullshit.
Then you carried on to use elephants, which is only even more laudable.
I once wrote a similar proposal for running a multi-slit experiment us
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I thought that was obvious from the start.
Re:I don't get it (Score:5, Informative)
Sorry to burst some bubbles, but I believe this analogy is not correct :( In fact it is not really possible to analogize quantum mechanics with anything classical, which is what people are getting at when they say that nobody really understands it.
In the experiment in TFA, they never found out which slit any particular photon went through. They have only collected some data about the average behaviour of the total set of photons. TFA suggests the scientists gathered a statistic somewhat like "X photons went through slit 1 and Y photons went through slit 2". Even here, I do not believe this is correct as I have worded it. I haven't read the paper at the article is based on, however if we follow the explanation of the first paragraph of your post, we will have an interference pattern that looks a bit different to the 50-50 one, where the possible paths between the two points have a greater 'density' of going through one of the particular slits. I would imagine that as you gradually change this ratio from 50-50 through to 0-100 the pattern would morph until it ended up being a one-slit diffusion pattern.
The rest of your post makes the same mistake as early efforts to explain the 'uncertainly principle', which was initially thought to be something like: "The particles have exact positions and momenta, but any attempt to measure them must disturb the system'. It was fairly quickly found that this was wrong, and the particles actually do not have well-defined positions and momenta (this is implicit in Schrodinger's equation and other such equations, the 'uncertainty principle' just describes a fact of the mathematical description of what a wavefunction is).
Certainly, photons behave according to the function that describes their movement. However, what is 'reality' is an open question (this is known as the interpretation of quantum mechanics). Some interpretations say that the photon travels through one slit but we cannot know which; some say that the function describing their movement *is* reality, and some say that 'reality' only consists of the photon's emission and its detection; not the stuff in between.
Re:I don't get it (Score:5, Insightful)
That's much better than the original explanation. To boil it down even further, quanta are waves when they are going somewhere (propagating) and particles when they get there (interacting). Each photon does actually go through both slits, which isn't a problem because it's a wave. When it hits the screen, it interacts in an all-or nothing, localized fashion, which gives the appearance of a particle.
The interesting thing about this experiment is that it further demonstrates that there is a continuum between particle and wave, interaction and propagation, but that this can only be shown as a statistical effect using many observations.
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I think it rearranged the wavefront a bit, which could be seen as a "partial collapse". There may have also been some times when certain photons were localized to one slit, but that would have been experimental noise rather than what they were trying to detect.
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You are correct, the OP wasn't. The wave goes through both slits, but the measurement of an individual photon must be discrete and thus particle-like.
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The 'wave' part of the particle is not so much 'the particle itself taking the form of a wave', it's the probability amplitude that if you measure it, it will be in this or this particular place.
the interference pattern only appears when you shoot many many many particles, so, in a way, you could think about the particles interfering with themselves in the past and the future which is even weirder, but that
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You're wrong. There is a pattern even in the single-shot experiment.
There CANNOT be a pattern if you shoot just one particle. If you shoot many particles, one at a time, there will be a pattern.
You're gonna have to go look for your source on that mistake. And to learn not to tell people they're wrong when you're talking obvious shit like that. One-dot pattern... you sound like an idiot.
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https://secure.wikimedia.org/wikipedia/en/wiki/Wheeler's_delayed_choice_experiment [wikimedia.org]
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So photons only go through both slits in the function that describes their movement, not in reality. It's just that the only way to describe their behavior is to assume they go through both slits, because we can't measure these things without disturbing them.
This doesn't explain the single-electron version of the double slit experiment, in which an interference pattern emerges, demonstrating that the electron wave function interfered with itself and thus must have passed through, wave-like, both slits at the same time. It's only when you try to observe which slit the electron goes through that it dutifully fulfills our expectations and goes through both. The experiment discussed above apprarently reveals a way to do some level of observation without completely
bad analogy (Score:2)
After getting up from laughing so hard, I will say this: "Well imagine you have to determine if it's the national holiday in India (they have a big elephant parade). But you don't actually have any tools smaller than elephants to measure this. So every hour or so you catapult an elephant into the main street of New Delhi, and you see if the elephant hits the detector you've set up at the other end of that street." What in reality you will actually get is: a: Dead elephants. B: crushed people, cars, wholes i
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I suspect it's mischaracterized. (Score:2)
I thought the photons went through both slits.
Ditto.
And (as I read the summary - having not read and understood the paper) it looks like they modified the amplitude, phase, and/or polarization of the wave function/photon path through one of the slits, and measured the resulting changes of the diffraction pattern.
If I've characterized the experiment correctly it does not, IMHO, constitute getting any additional measurement on "which slit each photon passed through".
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If I've characterized the experiment correctly it does not, IMHO, constitute getting any additional measurement on "which slit each photon passed through".
It might, however, give interesting information about whether whatever they placed in the path on one side interacts by changing the phase etc. of the wave function continuously and universally (which would produce a modified diffraction pattern) or by interacting with some photons and not others on a statistical basis (which would produce an overlay of a
Re:I don't get it (Score:5, Funny)
The key here is surreptitiousness. The researcher must act uninterested and as if they aren't trying to measure anything in particular and especially not with any fine accuracy. It helps if they whistle and distractedly reorganize bottles on a shelf while glancing fleetingly over at the experiment letting out a bored "Meh" as they do so.
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It's why we haven't found the Higgs boson, we care about finding it too much.
Re:Higgs (Score:3)
The same attitude is needed to look at Higg's Wife's bosom without getting punched.
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The key here is surreptitiousness. The researcher must act uninterested and as if they aren't trying to measure anything in particular and especially not with any fine accuracy. It helps if they whistle and distractedly reorganize bottles on a shelf while glancing fleetingly over at the experiment letting out a bored "Meh" as they do so.
Yes, but they are planning to do so.
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Quantum mechanics is a statistical theory, valid only in the statistical limit of an infinite number of measurements and looking at the ensemble. It actually places no inherent limits on a single measurement, only on an ensemble of measurements. Hence, you have no violation of the uncertainty principle because you are tracking individual photons or a very small number of them. The Stern-Gerlach experiment back in the day observed individual particle strikes but when viewed as a large average you had the int
Re:I don't get it (Score:4, Interesting)
Quantum mechanics is a statistical theory, valid only in the statistical limit of an infinite number of measurements and looking at the ensemble. It actually places no inherent limits on a single measurement, only on an ensemble of measurements. Hence, you have no violation of the uncertainty principle because you are tracking individual photons or a very small number of them. The Stern-Gerlach experiment back in the day observed individual particle strikes but when viewed as a large average you had the interference pattern characteristic of wave phenomena, while the individual flashes on the phosphor screen indicated a particle nature.
That's absurd. The interference patterns in the two-slit experiment are still created even when the intensity is reduced to the point that there is never more than one photon traversing the slits at a time. The QM rules apply to every wavicle, not just to aggregations.
You are misinterpreting Stern-Gerlach which also shows that each particle has quantized values for angular momentum and hence meets QM predictions.
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I think you misunderstand the post you're replying to. For each photon fired in the two-slit experiment, the photon can register at ANY point on the detector -- that's a fact. It is only once we have fired many photons that we find fewer photo
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Can the first one register anywhere?
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I think you have that backwards. QM places strict limits on the information obtainable from individual measurements, but much less strict limits on measurements of ensembles. Any individual interaction can only yield so much information, many interactions can yield more information - but each interaction is separate and doesn't technically say anything about any of the other individual interactions, but rather about the process producing the interactions. The Heisenberg uncertainty principle (specifically t
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The single photon wouldn't necessarily end the superposition, it would just entangle the tree with the environment (which it already was anyway).
Obviously it's absurd to suggest that humans cause collapse, but nobody has yet suggested a convincing enough explanation of what does cause collapse (or if collapse even happens at all), so I don't think the tree is dead just yet :)
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"...the falling tree would have to avoid hitting anything at all (even a single photon hitting it would end it's superposition)"
No, , (assuming the timing of the fall of the tree has some uncertainty, which it must) the tree would just entangle whatever it interacted with into that superposition between fallen and not-fallen and as those things in turn interacted with other things there would be an outward spreading wave of entanglement that would quickly become effectively irreversible (decoherent). In one
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obviously particles because of the impressions they made on the photo-sensitive paper
No, that's got nothing to do with being a particle or not. The fun of this experiment is that it shows light to be a wave (because of the interference pattern) unless you measure photons going through the slits, in which case there is no interference pattern. Also works with electrons, btw.
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In this experiment, they can know which slit the photons when through
They infer through a study of averages.
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You can't measure both variables in the traditional double slit experiment--either you measure which slit the photons go through, or you detect an interference pattern, not both. It was one of the critical pieces of evidence in favor of the Copenhagen interpretation of quantum mechanics. Measuring which slit the photons traveled through collapses their wavefunction to a position eigenstate, changing their wavefunctions so that no interference pattern is created.
From the summary it sounds like they measure
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I thought the entire point of the experiment was that you still get interference with single photons, ie. they go through both slits.
Saying they measured which slit they went through doesn't make any sense if they go through both.
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Eh, hard to explain what I mean without drawing graphs of wavefunctions, but I'll try (and I may be wrong anyway, someone who's done QM past two 400 level classes four years ago would have to weigh in there).
The interference pattern isn't the result of the photons going through both slits per se (that's a really awkward, but accessible way of explaining the math, and I don't think it works very well), but a result of the wavefunctions of the photons from each slit overlapping and interfering with each other
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Re:I don't get it (Score:5, Interesting)
The fun thing is that you can do this with photons which were gravitational lensed around both sides of a galaxy and *still* collapse the wave function. Your measurement instantly changes something which happened a billion years ago (the lensing).
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The fun thing is that you can do this with photons which were gravitational lensed around both sides of a galaxy and *still* collapse the wave function. Your measurement instantly changes something which happened a billion years ago (the lensing).
Because World, wait for it ... wait for it ... we live in a simulation. The only way you can affect the past is by altering the initial values at run-time. COmon people, quantum mecanics and information theory explains god and bible and where are we gonna go. heaven is the simulation of all our brains subconcious mind. thats why we are connecting at bigger scale more and more, the invention of internet, mother of connectionhood is the natural pathway. someday all our brains will be somehow be linked toghete
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>>The fun thing is that you can do this with photons which were gravitational lensed around both sides of a galaxy and *still* collapse the wave function. Your measurement instantly changes something which happened a billion years ago (the lensing).
Even weirder? You can undo the wavefunction collapse, so the photon that you were about to measure appears instead two billion light years away.
http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser [wikipedia.org]
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I don't get it How does this differ from the classic two-slit experiment?
Well, the explanation might be a bit long, but try to bare with us.
You see, the main difference is
#include <article.h>
See? Simple as can be!
Sorry if that post got too long winded...
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How does this differ from the classic two-slit experiment?
Well you can't tell from the article, so I'm guessing based on the abstract.
From what I gather about this, think of what calcite does. It exhibits birefringence [tinyurl.com], meaning it has different indices of refraction for photons polarized vertically vs. horizontally. You can see that with the way unpolarized light in the room gets deflected by a piece of calcite if you put it on the page of a book, when you see two images of every letter.
So I think what they did here was the classic two slit experiment, with th
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Third, you only take a picture of the resulting pattern with a camera, having a long exposure time.You record where it hit, but you don't know which one hit there.
Oh? I thought they were using a video camera to measure the changes the interference pattern over time :S
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What I never understood about the uncertainty p. (Score:3)
So, as I understand it, the uncertainty principle tells us that in order to determine the position of a particle, we'd have to make a photograph of it using a sufficiently high frequency of light, otherwise we'd get a severe interference pattern. However, this high frequency of photons is coupled to high energy, thus knocking the original particle out of its path (in other words changing its momentum). So far so good.
However, assume that the particle is perfectly symmetric, e.g. a sphere. Then the interference pattern will also be symmetric. The image we'd get by making a photograph would look like a bunch of concentric circles. Where is the original particle? Well, at the center of those cirlces of course!
So this is what I don't understand. We can actually deduce the position of the particle precisely from the interference pattern. So where is all the fuzz coming from?
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> So, as I understand it...
You don't. Please read up on it.
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"I think I can safely say that nobody understands quantum mechanics."
- Richard Feynman
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Re:What I never understood about the uncertainty p (Score:5, Informative)
You're understanding of the basic assertion of the Uncertainty Principal is correct - in order to know the exact position of a particle at an exact moment, you have to measure the particle which changes it's position. Right on.
However, when speaking of electromagnetic phenomena, it's generally understood that we're speaking of something which can be either a particle or a wave, depending upon the property being observed. Call it a 'wavicle', if you like. It's the act of measuring the behavior that "collapses the wave function" - i.e., I can demonstrate exactly where a photon struck a sensor under a certain set of conditions, but doing so collapses the wave function. OR I can demonstrate the wavelike properties of light, but only by sacrificing any clue to the position of the photons which create that wave structure (oddly enough, collapsing the wave function once again).
Now, this is only my understanding of the condition, and I'm not really that certain I've got it right . . .
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I'm not a physicist but I'm pretty sure this is wrong. It is true that macroscopic objects are predicted to have wave functions, and some macroscopic objects have had quantum properties measured (in pretty esoteric experimental setups), but planet sized objects don't follow orbits around the sun based on their wave functions at all. I'm not even sure if you're suggesting that, but I wanted to clarify in case someone thought you were.
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That first article seems pretty theoretical (meaning they are postulating something), and they aren't making a case that the earth's quantum wave function impacts its orbit, they are arguing that the same *math* that can be used for calculating quantum wave functions can also be used *analogously* for describing orbits of captured satellites in star systems.
There's a notion in quantum physics (remember I am not a physicist) that the bigger an object is the smaller it's quantum wave "vibration" or function.
Wave (Score:2)
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Not to be pedantic (and I'm not a physicist) but you don't "sacrifice any clue to the position" you only sacrifice a precise clue. You still have a pretty darn good clue where the particle is via the quantum wave functions. It's much more likely to be near where it was emitted than far away from that spot for example - you have a statistical clue as to it's position in other words.
I'm not saying you don't know this, just wanted to clarify the language for other readers.
Re:What I never understood about the uncertainty p (Score:4, Informative)
The HUP is more fundamental than that. It doesn't just say that we can't know where a particle is because measurement disturbs it; rather it's telling you that the particle actually doesn't have a definite trajectory. In fact, it's so fundamental that it has its own mathematical formalism (commutativity of operators), upon which most of quantum mechanics is constructed.
It's important to realize that in quantum mechanics, the position of a particle is indefinite, and is specified by a diffuse/spread-out "cloud" probability, and only in special cases does this cloud collapse to a single point (which corresponds to the particle being in a definite place).
Note that it is possible (theoretically) to know the position or momentum of a particle, just not at the same time, since measuring one causes the other to become indeterminate.
Re:What I never understood about the uncertainty p (Score:4, Insightful)
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as I understand it, the uncertainty principle tells us that in order to determine the position of a particle, we'd have to make a photograph of it
Oh boy... a photograph? Of a subatomic particle?
we'd have to make a photograph of it using a sufficiently high frequency of light, otherwise we'd get a severe interference pattern.
I don't even...
thus knocking the original particle out of its path
This is the only part that made any sense.
If you're detecting a particle, you have to use another particle to do it, 'cause otherwise... how would you? So it's like finding out information about a car by blindly throwing other cars at it and measuring the collision: you're gonna affect the thing you're measuring by the act of measuring it.
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So it's like finding out information about a car by blindly throwing other cars at it and measuring the collision: you're gonna affect the thing you're measuring by the act of measuring it.
The point was that you could detect the position of the car by using much lighter objects (or objects with less energy), e.g. ping pong balls, and by deducing the position of the car from the interference pattern.
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The point was that you could detect the position of the car by using much lighter objects (or objects with less energy), e.g. ping pong balls
Ok, re-substitute "car" back to photon. Your ping-pong ball is a substitution for what, and how are you measuring that?
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And how would you do that when the particle you're trying to measure IS the size of the smallest thing you can reliably throw at your target?
When measuring the path of a photon, you only have other... photons to throw at it. though there's some that have the idea that they can reverse proton smash to get smaller resolutions, (ie, smash a particle that you reliably know how it should explode, and measure the interactions of those sub-atomic particles with the particle in question) it's a LONG stretch to get
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Richard Feynman, in The Character of Physical Law (1965)
That said, I think I can attempt to clarify some of your misunderstandings from my own understanding. In fact someone set me straight if I have any issues of my own :)
The entire notion of a point particle is essentially a classical approximation (as far as geometry goes). In fact, all the spatial information that can be known (ie not completely transparent to the rest of the universe
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tl;dr
A: why is this so non-intuitive???
B: Quantum mechanics! *winks knowingly*
A: ohhhhh
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But the "beam" can be so weak that there is never more than one particle in transit at a time.
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My bad (Score:2)
FULL TEXT (Score:1)
https://rapidshare.com/#!download|639tl|2460541193|Science-2011-Kocsis-1170-3.pdf|662|R~0
Experiment is about "Measurement" not UP (Score:5, Interesting)
Averaging over many measurements won't allow you to "defeat" uncertainty principle, as uncertainty principle tells you the width of the distribution (of measurements). If you wanted to get a precise measurement of the center of that distribution, yes, you can take many averages and reduce the error on that (see error of the mean [wikipedia.org]), but the width of the distribution (given by uncertainty principle), remains unchanged.
Reading the paper abstract:
It looks like the goal of experiment is to nail down (or get further in nailing down) what constitutes "measurement [wikipedia.org]". But I'm still trying to figure out how this experiment is different from the standard QND [wikipedia.org] (which doesn't claim not to collapse the wavefunction as all measurements ought to).
There was already an experiment that could do it! (Score:1)
I already read about an experiment, where they managed to find the slit a photon went through, without doing a measurement on the photon itself, preserving its wave nature.
It was really ingenious (sorry, can't remember 100% of it):
They entangled the input photon with another one, which went on a parallel course outside the experiment.
Now the two slits had one of the two polarizations. So if the photon went through one slit, it got its polarization. And so did its entangled partner.
Then the photon ended up i
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Hm. I'm not sure how that works, exactly. If two photons were entangled, measurement on one constitutes measurement on the other (this is the basis of EPR paradox [wikipedia.org], the seemingly superluminal signal-sending).
If the claim is that a measurement is made on one without disturbing the state of the other entangled photon (i.e. measuring its position, or, in the experiment you described, its polarization is supposed to collapse the polarization state of the entangled photon to that determined by condition of entang
...and they concluded? (Score:1)
What?
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This was on Ars yesterday or so, http://arstechnica.com/science/news/2011/06/an-experiment-that-just-keeps-on-giving.ars
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That they can reliably measure cats.
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No. They have finally made the Heisenberg Compensator so they can subsequently uncouple it and free Moriarty from the holodeck. Duh.
Be vewy vewy quiet... (Score:3)
...I'm hunting wavicles! Wehehehehehe!
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Cwazy Waskally Wavicles; they in two pwaces at once. It's wike it's not even a weal Wabbit, but a pwobability cwoud of wabbits. It's dead and awive at the same twime! Wwwaaaaaahhhhh!
What about the Heisenberg Compensator? (Score:1)
Easier if you ask me...
A Quick Comment (Score:1)
The goal of measurement is to find both the position and momentum of a photon so that they can plot a trajectory in order to predict the future speeds and positions. The uncertainty principle precludes exact measurement of both, but in this experiment they utilize a 'weak' type of measurement and by repeating the experiment they get averages of trajectories. This does not violate the uncertainty principle but does start to give average trajectories in contrasts to single dimensional data (ie position or mom
Here, robust statistics might be bad (Score:2)
Here, robust statistics might be bad. Normally, I would say, robust statistics is superior to the crap called "parametric statistics, based on the junk "arithmetic mean", etc.
Yet, I would guess that the few outliers of interest here would have been missed by the Buick version of statistics - the median. Hence, the AMC Pacer would win hands downs as it would steer away for any folly in its way.
Researchers see results & wave over collegues. (Score:2)
...to see the results themselves, and blow the whole experiment.
How it would sound on Star Trek... (Score:3)
By averaging over a great many photons passing through the apparatus, and only measuring the light patterns on a camera, the team was able to infer what paths the photons had taken. While they were able to easily observe the interference pattern indicative of the wave nature of light, they were able also to see from which slits the photons had come, a sure sign of their particle nature."
Just like over-inflating a balloon...
So What? (Score:2)