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Physicists Discover a Way Around Heisenberg's Uncertainty Principle

samzenpus posted about a year ago | from the known-unknowns dept.

Science 153

Hugh Pickens writes writes "Science Daily Headlines reports that researchers have applied a recently developed technique to directly measure the polarization states of light overcoming some important challenges of Heisenberg's famous Uncertainty Principle and demonstrating that it is possible to measure key related variables, known as 'conjugate' variables, of a quantum particle or state directly. Such direct measurements of the wave-function had long seemed impossible because of a key tenet of the uncertainty principle — the idea that certain properties of a quantum system could be known only poorly if certain other related properties were known with precision. 'The reason it wasn't thought possible to measure two conjugate variables directly was because measuring one would destroy the wave-function before the other one could be measured,' says co-author Jonathan Leach. The direct measurement technique employs a 'trick' to measure the first property in such a way that the system is not disturbed significantly and information about the second property can still be obtained. This careful measurement relies on the 'weak measurement' of the first property followed by a 'strong measurement' of the second property. First described 25 years ago, weak measurement requires that the coupling between the system and what is used to measure it be, as its name suggests, 'weak,' which means that the system is barely disturbed in the measurement process. The downside of this type of measurement is that a single measurement only provides a small amount of information, and to get an accurate readout, the process has to be repeated multiple times and the average taken. Researchers passed polarized light through two crystals of differing thicknesses: the first, a very thin crystal that 'weakly' measures the horizontal and vertical polarization state; the second, a much thicker crystal that 'strongly' measures the diagonal and anti-diagonal polarization state. As the first measurement was performed weakly, the system is not significantly disturbed, and therefore, information gained from the second measurement was still valid. This process is repeated several times to build up accurate statistics. Putting all of this together gives a full, direct characterization of the polarization states of the light."

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153 comments

Br Ba (-1)

stoolpigeon (454276) | about a year ago | (#43068783)

Yeah Science! [imgur.com]

Re:Br Ba (1)

noh8rz10 (2716597) | about a year ago | (#43069057)

teal deer: you're trying to measure 2 properties. heisenberg says you can measure one or the other. tfa says you can measure a little of one and a lot of one. it's not teleportation, but it's kinda cool.

Re:Br Ba (0)

Anonymous Coward | about a year ago | (#43069379)

Quantum teleporation is not the way. We should curve spacetime to make Portal-style wormholes. Need energy equivalent to about 71% the mass of Jupiter for a 1 meter portal.

Citation: http://www.fas.org/sgp/eprint/teleport.pdf

Re:Br Ba (4, Informative)

pjt33 (739471) | about a year ago | (#43069801)

No, Heisenberg bounds the product of the errors in the measurements of the two by means of a Schwartz inequality: i.e. if you measure one very precisely, you will get a big error in your measurement of the other one.

Re:Br Ba (1)

Anonymous Coward | about a year ago | (#43069191)

Oh c'mon. This isn't Reddit. /. may be a shit-hole at times, but it isn't a Reddit-level shit-hole. Please leave the image macros at home.

Re:Br Ba (1, Interesting)

stoolpigeon (454276) | about a year ago | (#43069381)

I got it from Reddit - so good call.

But relax - most people will never see it. It's not my fault that I think of Breaking Bad whenever I hear Heisenberg now. :)

Schrodinger would be happy (5, Funny)

Anonymous Coward | about a year ago | (#43068795)

So, is the damned cat dead or alive?

Re:Schrodinger would be happy (5, Funny)

Anonymous Coward | about a year ago | (#43068823)

Yes.

Re:Schrodinger would be happy. (5, Funny)

sidragon.net (1238654) | about a year ago | (#43068853)

And no.

Re:Schrodinger would be happy. (3, Funny)

popo (107611) | about a year ago | (#43069803)

Turns out it's a standard parlor trick. The cat has a twin sibling.

The rest is all mirrors ... and ball bearings.

Re:Schrodinger would be happy. (4, Funny)

Anonymous Coward | about a year ago | (#43071241)

...and after that it's turtles all the way down.

Re:Schrodinger would be happy (5, Interesting)

elysiuan (762931) | about a year ago | (#43068865)

My favorite part of this thought experiment is that Schrödinger constructed it to point out the ridiculousness of quantum theory and how it couldn't possibly be correct if it allowed for such a thing. Reality sure is strange, maybe the strangest thing is that we can understand it at all.

Re:Schrodinger would be happy (5, Funny)

hedwards (940851) | about a year ago | (#43068901)

That's more a matter of the way the brain selectively ignores and forgets things which would lead to inconsistency. Which until relatively recently wasn't that big of a deal, there were a small enough set of observers that things could easily be kept in sync, and without extensive records, there wasn't anything to contradict the agreement of the folks talking.

These days though, that's changed and it's going to be interesting to see what the effects are.

Re:Schrodinger would be happy (0)

Anonymous Coward | about a year ago | (#43069679)

I like how this post got modded funny, because clearly what one remembers is precisely what happened.

Re:Schrodinger would be happy (4, Insightful)

K. S. Kyosuke (729550) | about a year ago | (#43069919)

That's more a matter of the way the brain selectively ignores and forgets things which would lead to inconsistency.

Or perhaps it's simply due to the fact that our brains evolved only to cope with severely limited range of environments. We can't imagine complicated local geodetics because we didn't evolve near a black hole. We can't imagine the weird effects of special relativity because we haven't evolved at relativistic speeds. We can't grok the fractal-like nature of subatomic world and physics because we aren't molecule-sized in order to notice it. Perhaps those "inconsistencies" are no more inconsistent than, say, the hydrostatic "paradox" is paradoxical. (In fact, the very existence of the word "paradox" seems to suggest that we just get all too often confused by perfectly normal things that are simply outside the realm of our daily experience.)

Re:Schrodinger would be happy (1)

hedwards (940851) | about a year ago | (#43072145)

More likely it's just a case of use it or lose it. We don't generally start studying physics formally until high school, so we grow up thinking about the world in a way that isn't strictly speaking real. We then have to unlearn what we know so that we can understand physics if we wish to be physicists. And when you get to the quantum and relativistic areas, it's so unlike what we've conditioned ourselves to see, that it can be a hard leap to make.

I like how my previous post got modded funny, when that's pretty well established. Even our first language can be forgotten if it's left unused for long enough, the brain doesn't retain knowledge that it doesn't need because that has never led to better mating opportunities historically.

Re:Schrodinger would be happy (2)

wonkey_monkey (2592601) | about a year ago | (#43069971)

Which until relatively recently wasn't that big of a deal

Yes it was.

Re:Schrodinger would be happy (1)

hedwards (940851) | about a year ago | (#43072117)

On precisely what basis are you saying that? The brain itself accounts for about 20% of the caloric needs of a person, so having a lot of neurons hanging around that aren't needed was never desirable, now we can more readily feed ourselves, so it's not as big of an issue. But really, up until relatively recently lack of sufficient food was a much bigger concern than the ins and outs of reality.

Re:Schrodinger would be happy (0)

Anonymous Coward | about a year ago | (#43069621)

My favorite part of this thought experiment is that Schrödinger constructed it to point out the ridiculousness of quantum theory and how it couldn't possibly be correct if it allowed for such a thing.

Not all of quantum theory, just the Copenhagen interpretation.
The problem with the Copenhagen interpretation is that it is more of a cop-out to avoid dealing with a specific problem then an actual theory for how quantum mechanics works.

This is a very instructional video for the Schrödingers cat experiment [youtube.com]

Re:Schrodinger would be happy (2)

blue trane (110704) | about a year ago | (#43070153)

I didn't realize Schroedinger was so mystical. From http://en.wikiquote.org/wiki/Erwin_Schrödinger [wikiquote.org]:

Nirvana is a state of pure blissful knowledge... It has nothing to do with the individual. The ego or its separation is an illusion. Indeed in a certain sense two "I"'s are identical namely when one disregards all special contents — their Karma. The goal of man is to preserve his Karma and to develop it further... when man dies his Karma lives and creates for itself another carrier.
          - Writings of July 1918, quoted in A Life of Erwin Schrödinger (1994) by Walter Moore ISBN 0521437679

No self is of itself alone. It has a long chain of intellectual ancestors. The "I" is chained to ancestry by many factors ... This is not mere allegory, but an eternal memory.
          - Writings of July 1918, quoted in A Life of Erwin Schrödinger (1994) by Walter Moore

My personal favorite: "God knows I am no friend of probability theory, I have hated it from the first moment when our dear friend Max Born gave it birth."

Re:Schrodinger would be happy (0)

Anonymous Coward | about a year ago | (#43071731)

No, it showed that we can't possibly know the reality of the quantum decision without influencing it. Deciding that the cat is_already_ dead or alive is an act of faith, since you can't test it before you examine it. It just seems reasonable to most folks, so they accept this "hoped for reality" as truth.
  " If you are not disturbed by quantum theory, you don't understand it", said the good Dr. Bohr.

Re:Schrodinger would be happy (0)

Anonymous Coward | about a year ago | (#43069481)

The better question is as to whether the cat falls buttered side up, or buttered side down.
Now if the cat is weakly buttered on one side, and strongly buttered on the other...
Dammit, now I'm hungry.

Re:Schrodinger would be happy (1)

mikael (484) | about a year ago | (#43071309)

The cat ends up spinning perpetually without touching the ground.

Re:Schrodinger would be happy (5, Insightful)

History's Coming To (1059484) | about a year ago | (#43069923)

They're measuring the average state of multiple cats. It's not a way around the uncertainty principle, it's a way of building up a statistical picture, which is exactly what QM does. Over-hyped article.

Re:Schrodinger would be happy (0)

Anonymous Coward | about a year ago | (#43069955)

The cat is just super, thanks.

Re:Schrodinger would be happy (3, Funny)

jellomizer (103300) | about a year ago | (#43070579)

The Cat is Dead now. Otherwise Schrodinger would be famous for finding a way to greatly extend the life of Cats.
 

Re:Schrodinger would be happy (1)

Sulphur (1548251) | about a year ago | (#43072417)

The Cat is Dead now. Otherwise Schrodinger would be famous for finding a way to greatly extend the life of Cats.

This fails to account for the nine lives of a cat.

Re:Schrodinger would be happy (1)

AliasMarlowe (1042386) | about a year ago | (#43070869)

So, is the damned cat dead or alive?

Yes. But there are two cats, and we're not sure which is alive and which is dead.

Re:Schrodinger would be happy (1)

Convector (897502) | about a year ago | (#43070993)

Turns out the cat will not stay in the box. So the experiment can never be done.

Re:Schrodinger would be happy (5, Insightful)

operagost (62405) | about a year ago | (#43071131)

Shows you have never lived with cats. Put out an empty box and it be full of cats within ten minutes.

So... Quantum cryptography is doomed ? (0)

Anonymous Coward | about a year ago | (#43068821)

Just asking.

Re:So... Quantum cryptography is doomed ? (5, Insightful)

femtobyte (710429) | about a year ago | (#43069035)

Short answer: No.

Slightly more details: this technique could only "break" quantum encryption when the sender helpfully decides to send the same message over and over again --- effectively returning to the classical limit of large numbers of quanta, hence self-defeating the "quantumness" of the encryption. Used properly, the quantum encrypted signal (a series of photons sent with pre-set polarizations) is only sent once, so the large uncertainties in single "weak" measurements assure that anyone intercepting the message still gets a garbled, uninformative result (and the end receiver does too, so they know their security was compromised).

Re:So... Quantum cryptography is doomed ? (1)

T-Bone-T (1048702) | about a year ago | (#43069217)

I seem to remember an article stating that there is always some percentage error on the receiver's end and they've managed to snoop without introducing noticeably more error. The snooping still caused errors but they still fell within the expected range of errors.

Re:So... Quantum cryptography is doomed ? (3, Interesting)

femtobyte (710429) | about a year ago | (#43069593)

Indeed, the quality of the senders/receivers equipment determines how much redundant data they have to "leak" beyond the theoretical limits --- and a sender/receiver using crude technology might be vulnerable to an attacker with far more sensitive equipment. Fortunately, once the sender/receiver's equipment gets "good enough," they can be mathematically certain that there isn't enough leaked data to sneakily reconstruct the message even if an attacker had theoretically "perfect" technology. While the "expected range of errors" with one current lab setup might have been broad enough to allow sneaky snooping, further technology development might squeeze this range down to exclude this possibility.

Nothing to see here (5, Insightful)

arse maker (1058608) | about a year ago | (#43068841)

This is old news.
It doesn't violate the uncertainty principle.

Re:Nothing to see here (3, Interesting)

medv4380 (1604309) | about a year ago | (#43068903)

It's not exactly "old news". It's using the "old news" you're thinking about from 2011 to do something else. So it's an old dog doing a slightly new trick.

Re:Nothing to see here (0)

Anonymous Coward | about a year ago | (#43070659)

It's not exactly "old news". It's using the "old news" you're thinking about from 2011 to do something else. So it's an old dog doing a slightly new trick.

Are you calling Robert Boyd an old dog?

Re:Nothing to see here (3, Insightful)

Hoi Polloi (522990) | about a year ago | (#43069485)

Yup, they just measured a little of one and a lot of the other. Still falls under the h.u.p.

It's the fault of the stupid haedline (5, Insightful)

formfeed (703859) | about a year ago | (#43069581)

While it is news, the headline really butchers it by trying to blow the claims out of proportion:

This:

Physicists Discover a Way Around Heisenberg's Uncertainty Principle

versus this:

The downside of this type of measurement is that a single measurement only provides a small amount of information, and to get an accurate readout, the process has to be repeated multiple times and the average taken.

(my emphasis)

/. editors at their best again </sarcasm>

Re:It's the fault of the stupid haedline (4, Informative)

sonnejw0 (1114901) | about a year ago | (#43070437)

They are repeating the measurement multiple times on a stream of photons. They're not measuring the same particle repeatedly, they're not even close to overcoming the uncertainty principle.

Yeah, well (-1)

Anonymous Coward | about a year ago | (#43068887)

I don't know about this.

Does this break Quantum Key Distribution? (2)

Anonymous Coward | about a year ago | (#43068889)

I thought the premise behind QKD was that you couldn't measure the polarization of one of a pair of entangled photons on two different bases at the same time, so once you perform the measurement in either basis, you're stuck with it and can't recreate that photon to forward it to the receiver (you'll only get the right basis half of the time). If this means you can get information about the photon on both the horizontal/vertical and diagonal bases, doesn't that mean you can MITM QKD?

Re:Does this break Quantum Key Distribution? (5, Informative)

johndoe42 (179131) | about a year ago | (#43069027)

No, because the summary is (as usual) thoroughly overstated. This experiment, like any other form of quantum state tomography [wikipedia.org] lets you take a lot of identical quantum systems and characterize them. For it to work, you need a source of identical quantum states.

As a really simple example, take a polarized light source and a polarizer (e.g. a good pair of sunglasses). Rotate the polarizer and you can easily figure out which way the light is polarized. This is neither surprising nor a big deal -- there are lots of identically polarized photons, so the usual uncertainty constraints don't apply.

The whole point of QKD (the BB84 and similar protocols) is that you send exactly one photon with the relevant state. One copy = no tomography.

Re:Does this break Quantum Key Distribution? (1)

maxwell demon (590494) | about a year ago | (#43072023)

No, because the summary is (as usual) thoroughly overstated.

However the linked Science Daily article has, right in its headline, "Getting Around the Uncertainty Principle". So it's not exactly the submitters fault (unless he was a physicist with access to Nature Photonics, of course, because in the actual paper you won't find such a claim).

Re:Does this break Quantum Key Distribution? (1)

mbone (558574) | about a year ago | (#43069081)

Without looking at the original paper, who knows?

If I had to bet, right now, I would bet on Heisenberg. For now. Subject to change, of course.

Re:Does this break Quantum Key Distribution? (1)

mikael (484) | about a year ago | (#43071357)

Some time ago, New Scientist I believe, researchers had claimed that they had been able to visualize a photon travelling through space. They took a sealed container, filled it with inert gas, and fired single photons through a pair of windows. The frequency of the photon was chosen so that it would have enough energy to interact with electrons around atoms, but not enough energy to dislocate them. They could actually visualize the location of the photon through changes in the state of the atoms (temperature or electric field).

When do I get my Heisenberg Compensators? (0)

tangelogee (1486597) | about a year ago | (#43068899)

Another Star Trek gadget may come true?

Re:When do I get my Heisenberg Compensators? (1)

txoof (553270) | about a year ago | (#43068907)

We're almost half way to having a working teleporter! Woot!

Re:When do I get my Heisenberg Compensators? (0)

Anonymous Coward | about a year ago | (#43069177)

We're almost half way to having a working teleporter! Woot!

Having the set built and a Whoosh.mp3 file does not count as "almost half way".

Re:When do I get my Heisenberg Compensators? (1)

bill_mcgonigle (4333) | about a year ago | (#43069779)

Bzzt. Subspace communicator. And error correction algorithms are still required (but perhaps good enough, like modern hard drives).

Hmm... (0)

Anonymous Coward | about a year ago | (#43068905)

Heisenberg Compensator?

Editors at it again (5, Interesting)

Anonymous Coward | about a year ago | (#43068935)

certain properties of a quantum system could be known only poorly if certain other related properties were known with precision.

This careful measurement relies on the 'weak measurement' of the first property followed by a 'strong measurement' of the second property.

Weak measurements are not precise. They can become statistically significant with a large data set, but on an individual event basis, they give you effectively nothing. There's no violation of the Uncertainty Principle here.

Heisenberg (0)

Anonymous Coward | about a year ago | (#43068949)

So you do have a plan?! Yeah, Mr. White! Yeah, science!

This is not a way *around* Heisenberg (5, Informative)

Wrath0fb0b (302444) | about a year ago | (#43069039)

What they are doing is assuming that their light source is broadly uniform and averaging over the double-measurement (which is clever, no doubt). So we still haven't learned anything about a particular photon that violates the uncertainty principle, only something about the entire population. If we assume that the population is uniformly polarized (which is reasonable in this case) then we can conclude that the average reflects the properties of the individual photons. If the population was not uniform, however, then the average tells us very little about the properties of the individual photons.

And before someone too clever tries to argue that you can take a single input photon and make multiple copes and send them through this process to get results about that one photon, there is the No Clone Theorem [wikipedia.org] to here to prevent that maneuver.

So really they haven't gone around Heisenberg (which talked only about individual wave-functions) but used multiple compound measurements and an assumption about the properties of the group to infer something that Heisinberg says they can't measure directly -- which is quite clever but Herr Doctor's principle still stands quite strong.

Re:This is not a way *around* Heisenberg (5, Insightful)

ceoyoyo (59147) | about a year ago | (#43069587)

Except that the no hidden variables results suggest that the photon really doesn't have both those properties at the same time. You can measure the average, but that's all it is - it doesn't tell you anything you shouldn't know about the state of a single photon, even if they are all quantum mechanically "identical." So Heisenberg gets to be right in the strong sense, as well as the weak.

Re:This is not a way *around* Heisenberg (4, Insightful)

Dr. Spork (142693) | about a year ago | (#43070169)

Thank you, I think that's exactly right. The "no hidden variables" issue was settled in the 80s, and this does nothing to overturn those results. The summary makes it sound like they weakly measured a hidden variable and strongly measured an orthogonal variable. They didn't. Quantum mechanics, including Heisenberg's own 1926 formulation of it, predicts these measurements. So let's not pretend that any theoretical results got overturned by experiment! Quantum mechanics is the same as it ever was.

Re:This is not a way *around* Heisenberg (2)

ceoyoyo (59147) | about a year ago | (#43070887)

Which is too bad actually. Bell's theorem has an out: it holds only if the universe is local. So if someone DOES figure out a way to measure hidden variables then it implies the universe is non-local, which might mean all kinds of fun sci fi technology.

Re:This is not a way *around* Heisenberg (1)

SoftwareArtist (1472499) | about a year ago | (#43071673)

Not true: tests of Bell's inequality have only ruled out some very limited classes of hidden variable theories. There are still lots of them that are very much on the table.

In fact, the whole concept of "weak measurements" originally came from studies of the two state vector model of quantum mechanics, which is a hidden variable theory. It arises very naturally from this model (and from other time reversible interpretations of quantum mechanics, all of which are hidden variable theories). It was later shown that standard QM also predicts the same results, but only through a complicated, seemingly miraculous set of cancellations. It's often been pointed out that, although standard QM does predict weak measurements should work, it's unlikely anyone would ever have discovered that if time reversible QM hadn't made the prediction first.

Re:This is not a way *around* Heisenberg (1)

sjames (1099) | about a year ago | (#43070421)

In fact, their experiment is almost exactly the same as using a half silvered mirror to send half of the photons to one detector and half to the other.

Meanwhile, Heisenberg wasn't talking about a binary condition. The uncertainty principle actually suggests that the experiment in TFA should get these results.

Re:This is not a way *around* Heisenberg (1)

zAPPzAPP (1207370) | about a year ago | (#43070725)

What if you send the single photon through a lot of 'weak' detectors placed in a row, before it finally hits the 'strong' one?
Would that count, or would they cease to be 'weak'?

Not so fast. (1)

mbone (558574) | about a year ago | (#43069047)

Does anyone have a link to an actual pdf of the actual paper?

Has this interpretation of the original work been subject to peer review?

Re:Not so fast. (0, Funny)

Anonymous Coward | about a year ago | (#43069151)

No, but your Mom has

what a load of horseshit (-1)

Anonymous Coward | about a year ago | (#43069059)

Physicists are plagued with amateurs, like any other profession.

Not a violation of the uncertainty principle (5, Informative)

mpoulton (689851) | about a year ago | (#43069105)

Like many non-rigorous descriptions, the summary makes the mistake of describing the uncertainty principle as if it is a measurement problem, where the lack of precision somehow arises from inadequate measurement technology. This is not a correct statement of the uncertainty principle. The fundamental issue is that the conjugate variable values are linked on a quantum level, such that there is a certain amount of natural, inherent uncertainty in their collective values due to the statistical/wavelike nature of the quantum particle. With perfect measurement, there is still uncertainty in the pair of values for any conjugate variables because the uncertainty lies in the actual values themselves. Position and momentum are the quintessential conjugate pair. The Heisenberg uncertainty principle is sometimes framed as the idea that you cannot know the speed and position of a particle at the same time. But it's more correct to say that a particle does not HAVE an exact speed and position at the same time. This weak measurement technique is certainly useful and interesting since it allows some observations of wavefunctions without collapse, but it does not actually allow the measurement of conjugate variables more precisely than the uncertainty principle allows - because the values themselves do not exist more precisely than that.

*This description is based one one of the multiple interpretations of quantum mechanics, and probably does not accurately represent physical reality, only our human understanding of a part of reality that we have not really figured out completely yet.

Re:Not a violation of the uncertainty principle (1)

Forget4it (530598) | about a year ago | (#43069447)

Well said mpoulton

John Gribbin several decades ago made a point of not mistaking the uncertainty principle a measurement problem in his book In Search Of Schrodingers Cat>
QUOTE: from Chap 8.

These startling conclusions were published in the Zeitschrift fur Physik in 1927, but while theorists such as Dirac and Bohr, familiar with the new equations of quantum mechanics, appreciated their significance at once, many experimenters saw Heisenberg's claim as a challenge to their skills. They imagined that he was saying that their experiments weren't good enough to measure both position and momentum at the same time, and tried to conceive experiments to prove him wrong. But this was a futile aim, since that wasn't what he had said at all.
This misconception still arises today, partly because of the way the idea of uncertainty is often taught.

Re:Not a violation of the uncertainty principle (0)

Anonymous Coward | about a year ago | (#43069647)

if it is a measurement problem, where the lack of precision somehow arises from inadequate measurement technology.

I think you do have to be careful about going too far the other end too. In some sense it is a measurement problem, but not a matter of inadequate measurement technology. It is instead more of a fundamental limitation of interactions. You can kind of disregard interpretation specifics, in that it doesn't matter what properties the particle has when it is not interacting, that the properties of the interaction ultimately limit what can be done.

Re:Not a violation of the uncertainty principle (2)

ceoyoyo (59147) | about a year ago | (#43069709)

*This description is based one one of the multiple interpretations of quantum mechanics, and probably does not accurately represent physical reality, only our human understanding of a part of reality that we have not really figured out completely yet.

Bell's theorem combined with all the experiments that have been done based on it, rule out local hidden variable theories. So either (1) your description is correct and the particle doesn't have an exact speed and position at the same time, (2) a LOT of experiments have suffered from horrible systematic errors, (3) the universe is non-local, (4) the universe is superdetermined or (5) mathematics doesn't work properly.

(1) seems the most likely right now, but I'm personally rooting for (3). Instantaneous communication, teleportation, etc.

Re:Not a violation of the uncertainty principle (2)

fredprado (2569351) | about a year ago | (#43069903)

(6) Some weird hypothesis you (and nobody) have thought about.

There is never any guarantee that you have identified all alternatives, no matter how carefully you think about it.

Re:Not a violation of the uncertainty principle (0)

Anonymous Coward | about a year ago | (#43070507)

When it comes down to something based on a simple math derivation, it is actually quite easy to produce a list of all the possibilities: you are correct, the math you did doesn't reflect the real world, or math/logic is wrong. Numbers 2 and 5 in the original list over the latter two options, and the rest are expansions of the first option: the theory is correct.

Re:Not a violation of the uncertainty principle (5, Informative)

pclminion (145572) | about a year ago | (#43069723)

For those with a signal processing background, it can be explained like this. The conjugate pair of momentum and position are related to each other by the Fourier transform -- the Fourier transform of the wavefunction in spatial coordinates yields the wavefunction in momentum coordinates. Anybody who has worked with a Fourier transform knows that if the input is band-limited, the output will not be, and vice versa. To know the position of a particle with exactness implies that its wavefunction is impulse-like in the spatial domain, which causes the momentum wavefunction to be a wave that extends infinitely throughout momentum-space. When you squeeze the bandwidth in one domain it grows in the other. Because the Fourier transform of a Gaussian is another Gaussian, a particle with Gaussian distribution in either space or momentum-space constitutes the most localizable wavefunction one could possibly achieve. The limit of the resolution is given by the Heisenberg relation, but this is a purely mathematical result, having nothing to do with measurement technique.

Re:Not a violation of the uncertainty principle (1)

ceoyoyo (59147) | about a year ago | (#43070919)

You would be surprised at how many people who work in signal processing don't really grasp that concept. Publishing a big chunk of my PhD took a lot longer than it should have because experts in signal processing were having trouble with that concept.

Re:Not a violation of the uncertainty principle (1)

Anonymous Coward | about a year ago | (#43069827)

>But it's more correct to say that a particle does not HAVE an exact speed and position at the same time.

That's debatable. In the Bohmian interpretation of QM, particles have precise positions and momenta at every instant.

Re:Not a violation of the uncertainty principle (2)

SoftwareArtist (1472499) | about a year ago | (#43071557)

Like many non-rigorous descriptions, the summary makes the mistake of describing the uncertainty principle as if it is a measurement problem, where the lack of precision somehow arises from inadequate measurement technology. This is not a correct statement of the uncertainty principle.

That's not quite right. Heisenberg's "uncertainty principle", as originally fomulated by Heisenberg, is a measurement problem. Heisenberg observed that any measurement will disturb the system being measured, such that its states before and after are different. This limits your ability to perform multiple measurements in a row. Physicists later came to identify the uncertainty with the intrinsic impossibility of having a system be in eigenstates of two non-conjugate variables at the same time. But these really are different things, and it was the former that Heisenberg originally proposed as his "uncertainty principle", not the latter.

Ah.. BS? (1)

Lawrence_Bird (67278) | about a year ago | (#43069149)

The direct measurement technique employs a "trick" to measure the first property in such a way that the system is not disturbed significantly and information about the second property can still be obtained.

So... they system is disturbed. And

The downside of this type of measurement is that a single measurement only provides a small amount of information, and to get an accurate readout, the process has to be repeated multiple times and the average taken.

This is therefore a statistical experiment and thus subject to all the normal caveats involved with statistics, including estimates of error.

Re:Ah.. BS? (2)

rraylion (1406761) | about a year ago | (#43069241)

all experiments are subject to error...

But the HUP is made for a case of a single strong measurement. This describes using multiple weak measurements which was proposed back in 1993. Good to see it is finally coming to light as a useful tool.

Re:Ah.. BS? (0)

Anonymous Coward | about a year ago | (#43069729)

Uncertainty principle still applies to weak measurements.

So... (0)

Anonymous Coward | about a year ago | (#43069199)

It's like equivalent-time sampling in oscilloscopes?

Re:So... (0)

Anonymous Coward | about a year ago | (#43070915)

Combined with an appropriate high-impedance probe, I suppose.

Time traveling messages coming soon. (1)

Requiem18th (742389) | about a year ago | (#43069385)

I'm probably misunderstanding things here but assuming just momentaneously that this allows for information to travel instantaneously, Special Relativity ask us to consider that this information is actually traveling back in time. isn't it? Then are we finally goinig to get a proper model of time travel so we can make Hollywood stop making shoddy movies like Looper?

Re:Time traveling messages coming soon. (1)

wonkey_monkey (2592601) | about a year ago | (#43070037)

assuming just momentaneously that this allows for information to travel instantaneously

Why would it?

Statistics? (1)

LihTox (754597) | about a year ago | (#43069395)

"This process is repeated several times to build up accurate statistics." If I'm reading this right, this means that you need multiple identical copies of the photon, and it's impossible to duplicate the quantum state of an object without knowing its exact state. You can't repeat the process on the same photon because that strong measurement destroys the photon's original state. This would work if your source was producing a number of identical photons, but it wouldn't be very useful in, for example, breaking quantum cryptography.

I was in quantum information theory two decades ago so my knowledge is out of date, but nothing about this sounds theoretically surprising; I suspect it is the experimental technique that is the key.

So sick and tired of these sensationalis headlines (1)

quax (19371) | about a year ago | (#43069809)

No, this does not invalidate the Heisenberg principle because it is done on an ensemble and the measurement is just an average on the statistic that they gather.

This has no bearing on the fundamental validity of the uncertainty principle for a single quantum system. Never quite understood the point of these experiments [wavewatching.net]. But to advertise them in this misleading fashion is just asinine.

Way to go to confuse an already science sceptic pubic.

The New Slashdot: Headline Integrity? (1)

GodfatherofSoul (174979) | about a year ago | (#43070413)

I'm in a habit that when I see a headline like this, I just *assume* it's bullshit and move straight to the comments to find the physicists who refute the claims. Even if it's true, just by the sensationalism Slashdot headlines starting bringing us I'm going to assume false.

resolution in numerical analysis (2)

peter303 (12292) | about a year ago | (#43070715)

You can only know the qualities sampled on a discrete digital grid to certain resolution due the limits of the grid. Take a Fourier transform of quanity sampled on that grid. You can only reliable compute frequencies with wavelengths two grid points wide. Else "aliasing" allows you fit an arbitrary number of smaller wavelengths to the same sample points.

In nature the Planck unit of action discretizes the universe into the smallest quantities you can resolve.

Re:resolution in numerical analysis (2)

ceoyoyo (59147) | about a year ago | (#43071209)

Close, but your description is confuses frequency resolution and the Nyquist frequency. And a nitpick: the measurement can be anything, it doesn't have to be digital.

To use your example, the limit on the resolution of the frequency does not depend on the resolution of the sampling, it depends on the extent or field of view of the sampling. If you sample for twice as long you can resolve frequencies half as far apart. The maximum frequency you can represent (the Nyquist frequency), which is like the field of view in the frequency domain, depends on the sampling resolution. Sample twice as frequently and you can represent frequencies that are twice as high.

This is because sampling for a finite time period is like multiplying a signal by a boxcar function. The Fourier transform of a boxcar is a sinc. Multiplication in one domain is convolution in the other, so multiplying your time signal by a boxcar is convolving your frequency domain by a sinc (i.e. blurring it, or reducing the resolution).

Sampling is multiplying by a comb filter (a train of impulses). The Fourier transform of a comb is another comb with different spacing, which means that sampling your signal implies convolving the frequency spectrum by a comb function, i.e. replicating it at a particular spacing. The finer your sampling comb the wider spaced your frequency-domain replicant comb is, so the farther apart the replicants are, meaning you can look at higher frequencies without aliasing being a problem. Aliasing itself isn't reflection of the higher-than-Nyquist frequencies, it's superimposition of the replicants.

I'm not sure anyone is precisely sure what that means regarding the Planck length, Heisenberg uncertainty and conjugate pairs. Quantized momentum, for example, suggests (I think) that the wavefunction must have limited spatial extent. I suppose that implies the universe is finite. Quantized space (a concept not very friendly to general relativity as we understand it) implies that the wavefunction in momentum coordinates as limited extent: momentum is bounded.

Quantum encryption? (1)

KatchooNJ (173554) | about a year ago | (#43071293)

How does this bode for quantum encryption? Me thinks it makes it a bit less hack-proof. I guess the good news is that it sounds like Star Trek transporters are another tiny step closer to reality. ;-)

Perhaps... (0)

Anonymous Coward | about a year ago | (#43071503)

.. but he still makes incredible meth!

Sensationalist rag (0)

Anonymous Coward | about a year ago | (#43071837)

Did the people here really just run this sensationalist headline? This is closer to national enquirer than real news.

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