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Physicists Claim First Observation of a Quantum Cheshire Cat

Soulskill posted about 9 months ago | from the it-is-both-smiling-and-not-smiling dept.

Science 148

KentuckyFC writes "Last year, a group of theoretical physicists suggested a bizarre experiment based on a quantum phenomenon known as weak measurement. Unlike ordinary measurements that always change the state of a quantum object, a weak measurement extracts such a small amount of information that it leaves the quantum state intact. For example, a weak measurement can detect the presence of a photon by the deflection it causes when it bounces off a mirror. However, this does not change the photon's quantum state. The new idea was to make two weak measurements on a quantum system that is in a superposition of states, the goal being to separate the location of this quantum system from its properties, like a Cheshire cat. Now a group of experimentalists say they've observed a quantum Cheshire cat for the first time in an experiment involving neutrons. They passed a beam of neutrons through a magnetic field to align their spins and then sent them through an interferometer in which the neutrons pass down both arms of the experiment at the same time. They then used weak measurements to locate the neutrons in one arm while measuring their magnetic properties in the other. Voila! A quantum Cheshire cat."

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"MEOW" (-1)

Anonymous Coward | about 9 months ago | (#45921769)

But I don't see any cat...

Re:"MEOW" (3, Insightful)

phrostie (121428) | about 9 months ago | (#45922039)

and the photon that leaves the cat isn't really the same photon that reflects off the mirror.

Re:"MEOW" (2)

x2A (858210) | about 9 months ago | (#45923973)

The splitter is functionally equivalent to taking a measurement, you may not be extracting the information, but it is an interaction, and it's the interaction part of "taking a measurement" that changes it, not the information extraction/conversion into another form.

We know the splitter changes it, because its velocity changes... it leaves along a different vector.

Re:"MEOW" (0)

Anonymous Coward | about 9 months ago | (#45925089)

good point, thanks

I need to know... (3, Funny)

3seas (184403) | about 9 months ago | (#45921775)

Did they kill the cat, by looking?

Schrodinger called (3, Funny)

s.petry (762400) | about 9 months ago | (#45922293)

wrong cat buddy

Re:I need to know... (5, Funny)

Em Adespoton (792954) | about 9 months ago | (#45922435)

Did they kill the cat, by looking?

No... you did, by being curious.

Re:I need to know... (0)

Anonymous Coward | about 9 months ago | (#45925047)

He and other 6 curious readers of slashdot.

Re:I need to know... (1)

Tablizer (95088) | about 9 months ago | (#45922491)

Did they kill the cat, by looking?

Yes, except for that damned smile.
   

Weak tea (1)

goombah99 (560566) | about 9 months ago | (#45924385)

from the summary:

For example, a weak measurement can detect the presence of a photon by the deflection it causes when it bounces off a mirror. However, this does not change the photon's quantum state.

Cough.... say what? If the photon produces a measurable deflection of the mirror then it transferred energy to the mirror. Therefore the QM state of the photon was changed. This sounds like a bunch of rubbish.

Neat! (-1)

Anonymous Coward | about 9 months ago | (#45921789)

Ve

Quantum bullshit (-1, Flamebait)

Anonymous Coward | about 9 months ago | (#45921825)

I know what my cat felt like when I tried to explain the Schrodinger's cat thing to her... nothing but horror.

The truth behind all this is that when God wrote his simulator, he didn't think his sims would ever look at things small enough to see the individual bits. He's going to smite us all for this when his mom ungrounds him from the CosmicAC. There: everything solved. No more need for science.

Chesire Cat (3, Funny)

BorgDrone (64343) | about 9 months ago | (#45921921)

Is it just me or does that sound a lot like a Heisenberg Compensator ?

Beam me up!

Re:Chesire Cat (0)

Anonymous Coward | about 9 months ago | (#45921975)

It certainly sounds similar.

The problem is the superposition state as well as the orderedness of the system.

In the case of the body, this isn't really the case.
You'd need to figure out some way to freeze time for a moment in a small space as well as make everything stable.
So, essentially you'd need to explode the body at lightspeed and at nospeed at the same time. I think.

Re:Chesire Cat (2)

Lord Apathy (584315) | about 9 months ago | (#45922087)

So, essentially you'd need to explode the body at lightspeed and at nospeed at the same time. I think.

Sounds painful, I think I'll walk.

Re:Chesire Cat (1)

dunng808 (448849) | about 9 months ago | (#45923247)

Ah, yes. Grad school, Saturday night, Old Milwaukee beer and Papa Del's Pizza. Lot's of both. Ice on the sidewalk. Exploding bodies going nowhere fast.

Re:Chesire Cat (1)

PopeRatzo (965947) | about 9 months ago | (#45923557)

Is it just me or does that sound a lot like a Heisenberg Compensator ?

Yeah, either that or some other Big Bang episode.

Re:Chesire Cat (0)

Anonymous Coward | about 9 months ago | (#45924883)

It was in ST:THG. The episode where Moriarty wanted to beam his wife and himself off the holodeck.

Why a Cheshire Cat? (3, Insightful)

medv4380 (1604309) | about 9 months ago | (#45921951)

Why the acid trip Alice in wonderland analogy? Does it convey additional information about what they're doing, or is it just obfuscating what they're doing. I vote obfuscation, but it might just over my head right now. Stupid, grinning cat with no head.

Re:Why a Cheshire Cat? (1)

Tenebrarum (887979) | about 9 months ago | (#45922049)

I guess that it being a cat is the most obvious reference possible. As for it being the Cheshire Cat, I suppose that it's because said cat can willfully be either comprised as a regular cat, an abnormal cat (composed of say, just a head), or nothing at all.

Re:Why a Cheshire Cat? (3, Informative)

Em Adespoton (792954) | about 9 months ago | (#45922453)

Its because the CC's attributes can be in one place while its body is somewhere else... after all, it can be between the state of abnormal and nothing: the last thing to fade is the smile (not the teeth and lips, but the smile) and it can interact without in fact being there.

Re:Why a Cheshire Cat? (1)

pellik (193063) | about 9 months ago | (#45924037)

Wait, so the cheeks are last?

Re:Why a Cheshire Cat? (1)

zacherynuk (2782105) | about 9 months ago | (#45922053)

I think it's an analogy to try and make people understand the principles of information and metadata when related to a physical object - indeed when that data cannot be 'read' directly the use of an acid trip may well be the best way of describing it!.

For the majority of people, I would imagine, the concept of weak measurements would be a little boring, if not confusing. A bit of (spatially separated magnetic) spin is probably required to get noticed.

Re:Why a Cheshire Cat? (1)

Anonymous Coward | about 9 months ago | (#45922069)

I vote banning medium.com links.

Re:Why a Cheshire Cat? (3, Informative)

retroworks (652802) | about 9 months ago | (#45922193)

Per the article:

"The paradox arises when the team carried out two weak measurements. The first found the presence of neutrons in one arm while the second noted their magnetic properties in the other arm. “The neutrons behave as if particle and magnetic property are spatially separated while travelling through the interferometer,” they say. In other words, they observed a quantum Cheshire cat."

Per the peer review: "Twas brillig, and the slithy toves. Did gyre and gimble in the wabe: All mimsy were the borogoves, And the mome raths outgrabe."

the slithy toves. Did gyre and gimble in the wabe (0)

Anonymous Coward | about 9 months ago | (#45924659)

re: "Twas brillig, and the slithy toves. Did gyre and gimble in the wabe: All mimsy were the borogoves, And the mome raths outgrabe."

Awesome lesbian erotic poetry dude!

Re:Why a Cheshire Cat? (1)

Zalbik (308903) | about 9 months ago | (#45924693)

“The neutrons behave as if particle and magnetic property are spatially separated while travelling through the interferometer,”

Is it just me, or does anyone else find that completely freaky? Ok, I kinda get how quantum effects don't really occur in a "location", but at a superimposed potential of different locations....but having different properties measured at different locations just freaks me out...

Re:Why a Cheshire Cat? (0)

Anonymous Coward | about 9 months ago | (#45922433)

I wondered the same thing. Might as well say that it was "like an angry horse" or "like a lump of putty I found in my armpit one midsummer morning".

Re:Why a Cheshire Cat? (5, Informative)

kaoshin (110328) | about 9 months ago | (#45922463)

Quantum Cheshire Cat [scientificamerican.com]

Re:Why a Cheshire Cat? (1)

Anonymous Coward | about 9 months ago | (#45922555)

Why the acid trip Alice in wonderland analogy?

Because an acid trip is the only thing in this world that's as weird as quantum physics.

Re:Why a Cheshire Cat? (1)

sjames (1099) | about 9 months ago | (#45922943)

Probably because since Schrodinger, cats have been associated with quantum physics. In this case, the seemingly non-local Cheshire cat is more relevant.

Re:Why a Cheshire Cat? (1)

femtobyte (710429) | about 9 months ago | (#45922975)

Calling this a "Cheshire Cat state" is giving it a name --- which is not the same as the thing, or what the thing is (cf. H. Dumpty, 1872). If you want a full technical description of what this state is, that will be readily found in the papers describing it (with no obfuscation intended); however, it might be a bit lengthy to use as a name. On the other hand, a little innocuous wordplay creates a unique and memorable identifier, from which a reader could look up the specifics --- i.e. a good name. Note also that "cat states [wikipedia.org] " are already common jargon in the field.

Re:Why a Cheshire Cat? (1)

NoNonAlphaCharsHere (2201864) | about 9 months ago | (#45923017)

Once upon a time, scientists explained everything with "cat" analogies. One day a "science journalist" misread one as being a "car" analogy, and it's been downhill ever since. Scientists make discoveries and announcements, and the next day we get articles on newscientist.com about how cars like to drink milk and chase mice.

Re:Why a Cheshire Cat? (0)

Anonymous Coward | about 9 months ago | (#45923367)

Because they forgot the name "Schroedinger" and went with the only other cat they could remember.

Re:Why a Cheshire Cat? (1)

chihowa (366380) | about 9 months ago | (#45923513)

Alice's Adventures in Wonderland was published in 1865 (73 years before LSD was synthesized) by man who has no known connections to drug use. The Cheshire cat [wikipedia.org] itself predates even Lewis Carroll.

The only "acid trip" aspect of any of this is a Disney movie from 1951 (which is admittedly very trippy).

Re:Why a Cheshire Cat? (1)

nickol (208154) | about 9 months ago | (#45924603)

This is a decent analogy. A smile without a cat and an acid trip without any acid (invented almost a century later).

Curioser (1)

guygo (894298) | about 9 months ago | (#45921959)

and Curioser

The original poster didn't read even the abstract (4, Informative)

Nightlight3 (248096) | about 9 months ago | (#45921973)

"a weak measurement extracts such a small amount of information that it leaves the quantum state intact."

That's not correct description -- the quantum state is changed, albeit less than with projective measurement. The paper itself [arxiv.org] calls it in the abstract "minimal disturbing" measurement, not the "non-disturbing" measurement.

Re:The original poster didn't read even the abstra (0)

Anonymous Coward | about 9 months ago | (#45922059)

It was a somewhat amusing reference to Shroedinger's cat, get over it.

Re:The original poster didn't read even the abstra (0)

Anonymous Coward | about 9 months ago | (#45922091)

sorry, the statement I was posting to disappeared.

Re:The original poster didn't read even the abstra (1)

ArsenneLupin (766289) | about 9 months ago | (#45924895)

sorry, the statement I was posting to disappeared.

You were probably trying to reply to the thread just before this one: "Why a Cheshire Cat?"

But that question should be understood "Why a Cheshire Cat?" rather than "Why a Cheshire Cat?"

Re:The original poster didn't read even the abstra (1)

thrillseeker (518224) | about 9 months ago | (#45922107)

Sounds like it was both observed and not observed ...

Re:The original poster didn't read even the abstra (0)

Anonymous Coward | about 9 months ago | (#45923263)

That's correct. Their observation is not accurate.

Re:The original poster didn't read even the abstra (0)

Anonymous Coward | about 9 months ago | (#45923515)

BINGO!

Trying to make reason. (0)

Anonymous Coward | about 9 months ago | (#45921977)

So this is abit hard to wrap my mind around.

Lets say you have two big tanks. In one tank you pour water down into. In the other you place a person.
What they have done is basically moving the state of the water into the empty tank so that the person would feel the water pressure and wetness eventhough the water itself isnt present.

Am i correct in this?

No (2)

grimJester (890090) | about 9 months ago | (#45922309)

It's a lot simpler. They had one neutron in two places and measured different properties of it in the different places.

The new thing is that it's a bit more "real" that it's the same particle in two places than has been done before. I'd guess it's theoretically impossible to measure the same thing in two places, but I really don't know that much about quantum mechanics.

This upsets Deepak Chopra. (1)

Anonymous Coward | about 9 months ago | (#45921993)

We must stop looking into things like this. Quantum physics is fairy magic. Let's leave it at that!

Re:This upsets Deepak Chopra. (0)

Anonymous Coward | about 9 months ago | (#45922127)

IANAP but to me at least, looking into this stuff makes it seem more like fairy magic not less. Maybe a ToE would change that but for now quantum mechanics, even when explained well, remains very bizarre.

Re:This upsets Deepak Chopra. (1)

Tablizer (95088) | about 9 months ago | (#45922537)

must stop looking into things like this. Quantum physics is fairy magic. Let's leave it at that!

Indeed. One day our experiments may end up winning Earth the Galactic Darwin Award.

Is Mars still taking one-way colonizers? Then LHC can make all the mini black holes it wants.

Re:This upsets Deepak Chopra. (1)

ravenlord_hun (2715033) | about 9 months ago | (#45922607)

If for some reason they do end up with a black hole at LHC, then Mars itself will get a one-way trip to Earth...

Re:This upsets Deepak Chopra. (1)

Tablizer (95088) | about 9 months ago | (#45922723)

No, its gravity doesn't change.

Re:This upsets Deepak Chopra. (1)

Windwraith (932426) | about 9 months ago | (#45922821)

Nah, it just took a space marine to fix that the first time it happened. Something about demons and the moons of mars and certain doom. All the footage is strangely low-resolution, though.

Re:This upsets Deepak Chopra. (1)

Jeremi (14640) | about 9 months ago | (#45924449)

If for some reason they do end up with a black hole at LHC, then Mars itself will get a one-way trip to Earth...

My understanding is that if someone did somehow create a black hole that gobbled up the Earth, the resulting object would be in the same location as Earth and would have the same mass as Earth; it would just be much, much smaller.

So not only would Mars not be effected, even Earth's moon would not notice any difference.

Re:This upsets Deepak Chopra. (0)

Anonymous Coward | about 9 months ago | (#45924675)

The moon would notice a difference, but not a large one. Essentially, the Earth occupies a patch on the moon's sky, usually measured in terms of angular diameter (approx 2 degrees) or in terms of solid angles (1.2 millisteradians). That makes it only approximately point-like rather than precisely point like; the diameter of an Earth-mass black hole is no more than 9 mm (compared to Earth's 12 700 km). The important thing is not that the whole planet's apparent disc would shrink, but rather that its gravitational features -- which are analogous to visible continents and oceans -- would.

In Newtonian gravitation, one would be able to model the earth and moon as pointlike masses per the Shell Theorem; indeed this is handy even in General Relativity by *assuming* that the conditions of Birkhoff's theorem are met. However, those conditions include perfect spherical symmetry, which is not true to the ability of even current measurement technology (we have great gravity maps of both bodies and the Grail work in particular shows that moon is quite lopsided) and are only well-behaved when there is a large central mass and a (very small, very low-mass-energy) test particle, rather than two similarly large masses. Collapsing either or both bodies the respective Swarzchild radius would make that body gravitationally *more* spherical.

Although the effect will not be enormous, over long timescales the smoothing-by-collapse of the gravitational image of the Earth would eventually lead to *less* deformation from sphericalE, as in particular it would affect the seismology of the moon (the Love and Rayleigh-Lamb wave equations for the moon would differ somewhat, which would manifest in distribution of the regolith, responses to meteor strikes, and so on). That is, the Earth is somewhat lumpy as a gravitational image at its present angular size from the point of view of the moon compared to a black hole of similar mass.

Additionally, the compaction of the earth would of course make a dramatic change to the moon's view of what we call lunar eclipses, which might make an even bigger difference over very long timescales.

Re:This upsets Deepak Chopra. (2)

maxwell demon (590494) | about 9 months ago | (#45925015)

If quantum suicide is right, then if the LHC is producing black holes, you'd better stay on Earth: On earth, you'd always be in the branch where no black holes are produced, while on Mars you'd see Earth disappear in a black hole and know that the very moment something goes wrong on Mars, you've got no hope to get any help from Earth.

Well, thinking more about it, maybe your best bet is to make your decision based on a quantum random experiment.

Wrong approach (1)

Anonymous Coward | about 9 months ago | (#45922587)

Everything looks like fairy magic until you figure it out. If it works, it's worth researching it until you reach a dead end or come up with a better solution.

That said, I'm drawing the line if I have to wear a sparkly dress and dance under the moonlight.

Re:Wrong approach (1)

cavreader (1903280) | about 9 months ago | (#45923387)

I am just happy that there are people capable of working on this type of research.

FTL Communications (1)

Lord Apathy (584315) | about 9 months ago | (#45922019)

I'm going to toss this out there but I expect the answer to be "no."

Does this solve the issue with using quantum entanglement as a possible means of FTL communications? I'm under the impression that quantum entanglement can't be used for this because the act of looking at the particle would change the state. But this seems to be away around that.

So am I wrong here and why?

Re:FTL Communications (1, Funny)

wonkey_monkey (2592601) | about 9 months ago | (#45922445)

I'm going to toss this out there but I expect the answer to be "no."

[...]

So am I wrong here

N... yes. Probably. But I don't know why, sorry.

This [wikipedia.org] might help. Or it might not. It's late.

Re:FTL Communications (1)

Em Adespoton (792954) | about 9 months ago | (#45922519)

I'm going to toss this out there but I expect the answer to be "no."

Does this solve the issue with using quantum entanglement as a possible means of FTL communications? I'm under the impression that quantum entanglement can't be used for this because the act of looking at the particle would change the state. But this seems to be away around that.

So am I wrong here and why?

The issue is that the information has to be minimal enough to not be verifiable -- so you could never guarantee that it's the same neutron you're measuring. But I guess if you had enough of them, you MAY be able to send a message through the properties. But that would involve quantum entanglement on a massive scale; something we can't do yet. Plus, these quantum measurements are usually done on things that can only be measured reliably for very short periods of time... which doesn't give time to get them far enough apart in space for it to be useful.

The most insightfull part of TFA (3, Interesting)

mrwolf007 (1116997) | about 9 months ago | (#45922041)

At issue is whether the result is really paradoxical or simply an ordinary consequence of the way the experiment is set up. For example, perhaps the experiment measures the properties of different neutrons in each of these places.

Personally i dont even understand why those guys are thinking they are measuring the properties of the same neutron.

Re:The most insightfull part of TFA (3, Insightful)

wonkey_monkey (2592601) | about 9 months ago | (#45922295)

Personally i dont even understand why those guys are thinking they are measuring the properties of the same neutron.

(Most insightful part of comment highlighted.) Because they're scientists with more knowledge of physics than you or me?

I don't understand why you'd automatically assume they haven't measured the same neutron. When someone with more physics degrees than me makes a new claim about physics, I tend to default to the understanding that I'm not entirely qualified to go jabbering on the internet that they've probably just got it wrong - certainly without giving any reason beyond "I don't get it so it can't be right."

Perhaps they have got it wrong; time will tell. I think it's safe to assume that at the very least they remembered to rule out the obvious alternative explanations before publishing.

Re:The most insightfull part of TFA (1)

tftp (111690) | about 9 months ago | (#45922365)

I tend to default to the understanding that I'm not entirely qualified to go jabbering on the internet that they've probably just got it wrong

That's not what GP was saying. He was asking WHY they thought those two neutrons to be one. He did not call them idiots who cannot count, for example :-) The GP's question is valid and insightful, that's why it got moderated as such.

I'd like to hear a simple explanation myself. Unfortunately, none is provided above. Appeal to authority is not good enough.

Re:The most insightfull part of TFA (1)

wonkey_monkey (2592601) | about 9 months ago | (#45922469)

He was asking...

That's kind of my point - he wasn't asking a question. He made a statement in what reads like a disingenuous tone ("i dont even understand why...")

Re:The most insightfull part of TFA (1)

wonkey_monkey (2592601) | about 9 months ago | (#45922489)

Oh yes, and also:

The GP's question is valid and insightful, that's why it got moderated as such.

[...]

Appeal to authority is not good enough.

;)

Re:The most insightfull part of TFA (1)

mrwolf007 (1116997) | about 9 months ago | (#45922763)

Because they're scientists with more knowledge of physics than you or me?

I don't understand why you'd automatically assume they haven't measured the same neutron.

Because, quoting the article:

In this experiment, the neutrons pass through a magnetic field to ensure that the spins are aligned in the same direction.

Nice, get a couple of them, guaranteed to have the same magnetic properties. Sounds fine. Lets go on.

They then enter the interferometer where the beam is split so that the neutrons pass down both arms of the device before recombining to produce an interference pattern picked up by a pair of detectors.

Fine. Split them up like a normal double slit experiment. As stated, neutrons in both arms.
But now we have a paradox?

The paradox arises when the team carried out two weak measurements. The first found the presence of neutrons in one arm while the second noted their magnetic properties in the other arm.

As stated above, there are neutrons in both arms. They find evidence of presence of neutrons in one arm (obviously, since they are in both arms) and measure magnetic properties in the other arm (obviously, since there neutrons there as well).
The result is so blatantly obvious. And there is no reason given in the article to assume both measurements apply to the same neutron. Its not like there is an explanation i dont understand, i could live with that, there is no explanation given at all.

Perhaps they have got it wrong; time will tell. I think it's safe to assume that at the very least they remembered to rule out the obvious alternative explanations before publishing.

Perhaps they have ruled out the obvious explanation. In that case the article is utter crap for not mentioning it.

Re: The most insightfull part of TFA (0)

Anonymous Coward | about 9 months ago | (#45923393)

They find evidence of presence of neutrons in one arm (obviously, since they are in both arms)

For intererence to happen the neutron must be in both arms (the wave function is not collapsed), if you strongly measure one arm and detect the neutron, the wave function collapses and the interference pattern disappears. Same for the spin.

Re:The most insightfull part of TFA (1)

Carnildo (712617) | about 9 months ago | (#45923409)

Fine. Split them up like a normal double slit experiment. As stated, neutrons in both arms.

If it's like the standard double-slit experiment [wikipedia.org] , each neutron travels through both arms of the interferometer. Under quantum mechanics, any particle behaving in a wave-like manner can do this sort of thing, even if the particle is of a type (such as a neutron) that people normally think of as being a discrete object.

This is where my understanding gets fuzzy, but I think what they've done is rig things up so that the position-like attributes of each neutron's wavefunction are detectable in one arm of the interferometer, while the magnetic-property attributes are detectable in the other arm.

Re:The most insightfull part of TFA (1)

oldhack (1037484) | about 9 months ago | (#45922847)

Perhaps they have got it wrong; time will tell. I think it's safe to assume that at the very least they remembered to rule out the obvious alternative explanations before publishing.

I highlighted what you considered "insightful". Listen to your own advice and keep your jabbering within your pay grade.

Re:The most insightfull part of TFA (0)

Anonymous Coward | about 9 months ago | (#45923177)

Do not waste your time. When someone uses an appeal to authority (with some false equivalence thrown in),

Because they're scientists with more knowledge of physics than you or me?

the rest of their thought process is not just in-cogent, but invalid. More is to be gained by ignoring such people than waddling through the muck their thoughts induce.

Re:The most insightfull part of TFA (1)

Anonymous Coward | about 9 months ago | (#45922421)

They aren't measuring the "same" neutron, they're measuring groups of spin-aligned neutrons against each other.

Re:The most insightfull part of TFA (0)

Anonymous Coward | about 9 months ago | (#45923033)

Personally i dont even understand why those guys are thinking they are measuring the properties of the same neutron.

Maybe the rate is low enough that there is at most 1 neutron going through the experiment at any time. Lots of other experiments are done on single photons and they still show all of the quantum interference effects.

Weak measurements (5, Interesting)

SoftwareArtist (1472499) | about 9 months ago | (#45922235)

Here's a more familiar example of a weak measurement. QM says you can't measure the magnetic moment of a single particle along two perpendicular axes at the same time. And yet, you can easily measure the magnetic moment of a bar magnet along two perpendicular axes at the same time. How is that possible? The bar magnet's moment is just the sum of the ones from all the particles that make it up. So by measuring the total magnetic moment, aren't you measuring the moments of all the individual particles, and hence violating the uncertainty principle?

The answer is no. When you measure the total moment of a macroscopic magnet, you only need to interact very very weakly with any individual particle, so the experiment only has a tiny effect on the state of each one. The more particles you sum over, the less information you need about each one, so the less restrictive the uncertainty principle becomes.

But the mathematical details of the explanation are curious. Weak measurements were originally proposed based on time reversible interpretations of QM, in which the future can affect the past and it's basically arbitrary which direction you call "forward in time". It was later shown that other interpretations also predicted them - of course they must, since the interpretations are mathematically equivalent. But the explanations are very different. Other interpretations explain them through an incredibly complicated series of cancellations, whereas in time reversible QM the explanation is straightforward, almost obvious. So is this evidence that time reversible QM is correct? At the moment, that question is more philosophy than science, but it's interesting to think about.

Re:Weak measurements (2)

khallow (566160) | about 9 months ago | (#45923219)

QM says you can't measure the magnetic moment of a single particle along two perpendicular axes at the same time. And yet, you can easily measure the magnetic moment of a bar magnet along two perpendicular axes at the same time.

The uncertainty principle says that you can't measure two properties to a greater precision than the norm of the commutator of those two properties as operators. For a single particle, that value tends to be large relative to the size of the magnetic moment components while in a bar magnet the values of the magnetic moment are much larger, being ensembles of many particles (usually 20+ orders of magnitude larger) while the commutator doesn't increase so.

So is this evidence that time reversible QM is correct?

How can a theory be more correct than an equivalent theory? It can, as apparently is the case here, be more parsimonious, but that is a different beast from correctness.

Re:Weak measurements (2)

SoftwareArtist (1472499) | about 9 months ago | (#45924213)

How can a theory be more correct than an equivalent theory?

Yes, I simplified a bit to keep my post from getting too long. There are tons of interpretations of QM: dozens we know about, and probably lots of others that no one has thought of yet. Some are "pure interpretations", meaning they make no predictions beyond the ones made by QM itself. No experiment can ever distinguish between two pure interpretations. But a lot of them aren't pure interpretations. They still reproduce the prediction of QM to high accuracy, but in principle an experiment could distinguish between them.

If we ever learn which interpretation is correct, it will be based on evidence. But right now that evidence doesn't exist, which is why I said this is more philosophy than science. Occam's razor says we should prefer a simpler explanation over a more complicated one, but that doesn't prove the simpler explanation is actually correct. But maybe some day we'll know.

Re:Weak measurements (1)

Boronx (228853) | about 9 months ago | (#45923903)

They already found out that the universe is not left-right symmetric. Doesn't that mean it's also not time symmetric ? In addition to the rather large amount of circumstantial evidence, of course.

Re:Weak measurements (1)

SoftwareArtist (1472499) | about 9 months ago | (#45924183)

As far as we can tell, CPT invariance [wikipedia.org] is an exact symmetry of the universe. So the details are slightly more complicated, but time, charge, and parity are elements of a single symmetry.

The first thing I noted.... (0)

Anonymous Coward | about 9 months ago | (#45922237)

...about this write-up was that there was NO mention of the nationality of the scientists ("group of experimentalists"?) who had performed this feat. Slashdot almost always prefaces this sort of news story with "Scientists at MIT..." or some such.

So I guessed that this meant they were not American. And I clicked on the reference to find out that I was right. They aren't. They aren't even in the US.

So why is this story even mentioned? Isn't it the case that nothing is true unless it happens here?

Re:The first thing I noted.... (1)

Em Adespoton (792954) | about 9 months ago | (#45922889)

...about this write-up was that there was NO mention of the nationality of the scientists ("group of experimentalists"?) who had performed this feat. Slashdot almost always prefaces this sort of news story with "Scientists at MIT..." or some such.

So I guessed that this meant they were not American. And I clicked on the reference to find out that I was right. They aren't. They aren't even in the US.

So why is this story even mentioned? Isn't it the case that nothing is true unless it happens here?

happens where?

Even though Dice.com is located in the US, I think you'll find a large number of posters on slashdot aren't. Same goes for submitters.

On hearing this, a physicist facepalms (2)

Applehu Akbar (2968043) | about 9 months ago | (#45922653)

Holy crap! I forgot to feed my cat! Being a physicist, I now have no idea whether he's alive or dead.

Cheshire? (1)

PPH (736903) | about 9 months ago | (#45922777)

Or Schrodinger?

If we are going to use bad analogies, could we please stick to cars?

Cheshire Cat (0)

Anonymous Coward | about 9 months ago | (#45923021)

I was gonna say, someone has been watching Alice and Wonderland too many times. lol

why ? (1)

Anonymous Coward | about 9 months ago | (#45923121)

Why is it that 90% of the comments on this interesting subject are silly attempts to make a joke?
Is it perhaps that the smart audience of /. left this place and I am stuck with the 4chan crowd?

It used to be that these subjects generated a lot of interesting discussion. IANAP but I always learned something from them. Now I just went through 47 comments and only 4 are somewhat interesting. I feel like I am wasting my time.

Re:why ? (1)

artor3 (1344997) | about 9 months ago | (#45923349)

Because no one on this site is smart enough to really understand the topic. That's always been the case. Even those times in the past where you thought you were learning something, it was most likely from a post by someone half-remembering their undergrad physics classes.

why? Well here's a bash interpretation (1)

BringsApples (3418089) | about 9 months ago | (#45923727)

Bash physics:
echo $0
quit

Only for the analogy to be correct, the script cannot have a name, location, an OS, or user running it. Now the object is to come up with a theory (as to what the hell $0 is) that's currently unable to be disqualified. GO!

Now you're doing (something just like) physics! And since this is the case, not only are there no people on this site able to "understand the topic", but neither are the folks on any other site. In my opinion, physicists are trying to count to zero, in the most intelligent way possible.

Re:why ? (1)

BringsApples (3418089) | about 9 months ago | (#45924163)

It may be because of the same reason that you're posting as AC. Also, this article is a bit... odd. It's not announcing any sort of breakthrough, just theory. And who isn't tired of theories, especially on Slashdot.

There's no free lunch (1)

Anonymous Coward | about 9 months ago | (#45923243)

The quantum world doesn't work that way. A photon passing through glass will exit the exact same point every time, based on the superposition of all interactions that it had the probability of transversing. You can't cheat. Your measurements are not accurate.

physicist wackjob phony frauds (-1)

Anonymous Coward | about 9 months ago | (#45923487)

first you set up a religion to establish the smallest definable theoretical component.
then you proceed to violate your own dogma.
fucking physicists need to spend more time in the physical world. That is where the word comes from.
If you were honest, instead of theoretical physics, you would call it - TotalMadeUpBullshit_ThatPaysGood if you Get Some Government Welfare.
you guys can get rid of the 2 biggest FRAUDS ever committed on humanity, Zero and infinity!
Even your own math screamS at you -BULLSHIT-! yOU FUCKING WaKoooS

Confusing summary (1)

Anonymous Coward | about 9 months ago | (#45923571)

I am a fucking physicist and I have absolutely no idea what is happening in the experiment related to the Cheshire Cat. If this is some sort of sci-fi/fantasy lingo, it is not on the Wikipedia disambiguation page for Cheshire Cat, and it really bothers me when I see physics articles delivered to the general population that aren't even sensible to an expert in the field.

If Schrodinger had lab assistants (1)

Anonymous Coward | about 9 months ago | (#45923607)

If Schrodinger had lab assistants I can imagine this dialogue:

"A box arrived for you today, Dr.Schrodinger. I took the liberty of opening it for you. Why did you order a dead cat?"

Well ... (0)

TrollstonButterbeans (2914995) | about 9 months ago | (#45923643)

They haven't even ever quantum entangled something as large as a neutron.

They have quantum entangled photons. The amount of energy in a neutron compared to the run of the mill photon is off the scale.

I'm drawing a blank as to what the hell any of this article is supposed to mean, quite frankly ...

Entanglement? (1)

manu0601 (2221348) | about 9 months ago | (#45923673)

What about entanglement? Does that means I can not entangle two particles, send one far away, change its state, and weak-read the state change on the first article so that entanglement is not lost and I can do it over and over?

Re:Entanglement? (1)

manu0601 (2221348) | about 9 months ago | (#45923693)

Oops, sorry: I see other posted the same question in the meantime. Too bad one cannot cancel its own post. Perhaps I can still mod me down redundant :-)

synopsis (1)

die standing (2626663) | about 9 months ago | (#45923913)

(theoretical physicists whiz kids waving hands really close in front of mr. Q's face)

"We're not touching you.... we're not touching you... we're not touching you.... haha hah haaha.. we're not touching you..."

Newsflash: Quantum Physicists Discover New Quantum State: ANNOYED - Story at 11.

weak by that it still changed (0)

Anonymous Coward | about 9 months ago | (#45923955)

small amount of information that it leaves the quantum state intact

OK, so I'm seeing a measurement technique that the final value is likely 0.123456789e-8 maybe 2 times then 0.123456788e-8 during t=0 to t=3.

Is the consistent measurement computer related (aka Chaos) or physical (the cat)? That is the question.

Was the Cat dead or alive? (0)

Anonymous Coward | about 9 months ago | (#45925187)

Was the Cat dead or alive?

the mind (0)

Anonymous Coward | about 9 months ago | (#45925431)

lots of info encoded in such a way as to form superpostions rather than having a 'sub-conscious' constantly expending energy.

Re:the mind (0)

Anonymous Coward | about 9 months ago | (#45925443)

weakly reading this info may change it slightly, like memory shifts slightly as the years progress

if the mind does work like this...ghost in the mac (0)

Anonymous Coward | about 9 months ago | (#45925489)

we will have the tech to photocopy consciousness

dreamscapes, pan's lab (0)

Anonymous Coward | about 9 months ago | (#45925457)

weakly read superpostions of previously encoded scenery

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