Skip to comments.Physicists scoop information from Schrodinger's cat box [Quantum Mechanics]
Posted on 01/22/2014 2:53:50 PM PST by ETL
In a paper published in the current issue of the scientific journal Nature Communications and titled "Direct measurement of a 27-dimensional orbital-angular-momentum state vector," a team of physicists led by the University of Rochester's Mehul Malik describe how they circumvented a basic principle of uncertainty that requires that some states of a quantum system must be understood poorly if other states are to be understood well.
Determining a quantum state, such as the position of an electron or the momentum of a photon, is tricky, to say the least. That's because subatomic particles behave nothing at all like billiard balls, orbiting moons, or any other kind of object with which we humans are remotely familiar.
A photon, for instance, sometimes acts like a wave, diffracting, interfering, and scattering, as all good waves ought to. Yet sometimes it will also behave like a particle, for instance by bashing into an electron or by traveling with ease through a vacuum.
According to our current understanding, things at the quantum scale can exist simultaneously in these two modes, both as localized particles, with distinct measurable states, and as spread-out probabilistic waves, with multiple contradictory states.
One consequence of this "wave-particle duality" is that it imposes a fundamental limit on how much we can know about the universe. An unobserved electron, say scientists, exists as a wave of mutually contradictory states. As the German physicist Werner Heisenberg first pointed out in 1927, taking a measurement of one state, say, the electron's position, and you irreversibly alter its momentum, and vice versa. In the parlance of quantum physicists, the "wavefunction" of a system's probabilities "collapses" into a specific state when you observe it.
If the quantum-mechanical model sounds bizarre, that's because it is.
(Excerpt) Read more at csmonitor.com ...
Direct measurement of a 27-dimensional orbital-angular-momentum state vector
ABSTRACT: The measurement of a quantum state poses a unique challenge for experimentalists. Recently, the technique of \direct measurement" was proposed for characterizing a quantum state in-situ through sequential weak and strong measurements. While this method has been used for measuring polarization states, its real potential lies in the measurement of states with a large dimensionality. Here we show the practical direct measurement of a high-dimensional state vector in the discrete basis of orbital-angular momentum. Through weak measurements of orbital-angular momentum and strong measurements of angular position, we measure the complex probability amplitudes of a pure state with a dimensionality, d=27. Further, we use our method to directly observe the relationship between rotations of a state vector and the relative phase between its orbital-angular-momentum components. Our technique has important applications in high-dimensional classical and quantum information systems, and can be extended to characterize other types of large quantum state.
(it's an 8-page pdf file)
Question: What is Quantum Entanglement?
The classic example of quantum entanglement is called the EPR paradox. In a simplified version of this case, consider a particle with quantum spin 0 that decays into two new particles, Particle A and Particle B. Particle A and Particle B head off in opposite directions. However, the original particle had a quantum spin of 0. Each of the new particles has a quantum spin of 1/2, but because they have to add up to 0, one is +1/2 and one is -1/2.
This relationship means that the two particles are entangled. When you measure the spin of Particle A, that measurement has an impact on the possible results you could get when measuring the spin of Particle B. And this isn't just an interesting theoretical prediction, but has been verified experimentally through tests of Bell's Theorem.
One important thing to remember is that in quantum physics, the original uncertainty about the particle's quantum state isn't just a lack of knowledge. A fundamental property of quantum theory is that prior to the act of measurement, the particle really doesn't have a definite state, but is in a superposition of all possible states. This is best modeled by the classic quantum physics thought experiment, Schroedinger's Cat, where a quantum mechanics approach results in an unobserved cat that is both alive and dead simultaneously.
One way of interpreting things is to consider the entire universe as one single wavefunction. In this representation, this "wavefunction of the universe" would contain a term that defines the quantum state of each and every particle. It is this approach that leaves open the door for claims that "everything is connected," which often gets manipulated (either intentionally or through honest confusion) to end up with things like the physics errors in The Secret.
Though this interpretation does mean that the quantum state of every particle in the universe affects the wavefunction of every other particle, it does so in a way that is only mathematical. There is really no sort of experiment which could ever - even in principle - discover the effect in one place showing up in another location.
Quantum Mechanics ping.
If you’d like on or off this list, please let me know.
Final exam question: Does Schrodeinger’s cat box need cleaning?
So much for a serious quantum thread...
The kitty pix are better for us anyway.
Only if you don’t mind stupid answers.
stooopid question #1, i understand how actually measuring/metering could alter an objects state, but... how can observing it(under the assumption it means with your eyes) change it's state???
or is he using the words interchangeably?
Not really. Makes it more enjoyable when you mix in a little fun. QM is wacky anyway.
I have a feeling that at some point, it simultaneously does and doesn’t.
I spent a semester (long ago) studying Schrodinger's work. I've spent a lifetime studying cats. Cats are more confusing, but they are also more fun.
If they can’t see the inherent contradiction in saying that something exists simultaneously in two contradictory states, then no power on Earth can help them.
Just curious if you are still around. I used to enjoy your post’s on “science” threads (unlike what has become of them these days)...
“how can observing it(under the assumption it means with your eyes) change it’s state??? “
“Observation” necessarily implies something (e.g. a photon of light) interacts with it, thereby changing its state.
From the info provided in Post 2:
What is Schroedinger's Cat?
Answer: Erwin Schroedinger was one of the key figures in quantum physics, even before his famous "Schroedinger's Cat" thought experiment. He had created the quantum wave function, which was now the defining equation of motion in the universe, but the problem is that it expressed all motion in the form of a series of probabilities ... something which goes in direct violation to how most scientists of the day (and possibly even today) like to believe about how physical reality operates.
Schroedinger himself was one such scientist and he came up with the concept of Schroedinger's Cat to illustrate the issues with quantum physics. Let's consider the issues, then, and see how Schroedinger sought to illustrate them through analogy.
According to the quantum physics wave function, after one hour the radioactive atom will be in a state where it is both decayed and not-decayed. Once a measurement of the atom is made, the wave function will collapse into one state, but until then, it will remain as a superposition of the two quantum states.
This is a key aspect of the Copenhagen interpretation of quantum physics - it's not just that the scientist doesn't know which state it's in, but it's rather that the physical reality is not determined until the act of measurement takes place. In some unknown way, the very act of observation is what solidifies the situation into one state or another ... until that observation takes place, the physical reality is split between all possibilities.
If the atom decays, then the Geiger counter will detect the radiation, break the vial, and kill the cat. If the atom does not decay, then the vial will be intact and the cat will be alive.
After the one-hour period, the atom is in a state where it is both decayed and not-decayed. However, given how we've constructed the situation, this means that the vial is both broken and not-broken and, ultimately, according to the Copenhagen interpretation of quantum physics the cat is both dead and alive.
The Copenhagen interpretation states that the act of measuring something causes the quantum wave function to collapse. In this analogy, really, the act of measurement takes place by the Geiger counter. There are scores of interactions along the chain of events - it is impossible to isolate the cat or the separate portions of the system so that it is truly quantum mechanical in nature.
By the time the cat itself enters the equation, the measurement has already been made ... a thousand times over, measurements have been made - by the atoms of the Geiger counter, the vial-breaking apparatus, the vial, the poison gas, and the cat itself. Even the atoms of the box are making "measurements" when you consider that if the cat falls over dead, it will come in contact with different atoms than if it paces anxiously around the box.
Whether or not the scientist opens the box is irrelevant, the cat is either alive or dead, not a superposition of the two states.
Still, in some strict views of the Copenhagen interpretation, it is actually an observation by a conscious entity which is required. This strict form of the interpretation is generally the minority view among physicists today, although there remains some intriguing argument that the collapse of the quantum wavefunctions may be linked to consciousness. (For a more thorough discussion of the role of consciousness in quantum physics, I suggest Quantum Enigma: Physics Encounters Consciousness by Bruce Rosenblum & Fred Kuttner.)
Still another interpretation is the Many Worlds Interpretation (MWI) of quantum physics, which proposes that the situation actually branches off into many worlds. In some of these worlds the cat will be dead upon opening the box, in others the cat will be alive. While fascinating to the public, and certainly to science fiction authors, the Many Worlds Interpretation is also a minority view among physicists, though there is no specific evidence for or against it.
To observe the particle with your eyes requires
bouncing a photon against the particle
that then interacts with you eye.
By bouncing a photon or other particle against it,
the state of the particle is changed.
Some interaction with the particle must occur
before you can perceive it.
After spending a semester in physics (many eons ago) measuring light as a particle, and measuring light as a wave function, I stopped asking those kinds of questions. OTOH, I do think of a situation as a probability cloud, until I learn the actual outcome of the situation, at which point I think of the cloud as having collapsed to the final outcome.
#5 with the cat in the rectangle of light is the absolute best
It is actually not a contradiction. Any object (be it an electron or you or a planet) is at once both a wave and a particle. It's just the way G-d set up the universe.
So if you test an electron for wave behavior, it will pass every wave test, perfectly. So an electron must be a wave.
But if you test an electron for particle behavior, it will pass every particle test, perfectly. So an electron must be a particle.
So what is an electron, a wave or a particle? It must be two entirely different things at once. How it shows itself to you just depends on how are are examining it!
how does seeing something with you eye effect what you see???
No Penny!!! Not Pajama Boy!
but aren't photons bouncing off of it anyway whether they go into my eye or not?
i guess my point is, i am not doing anything but looking at it,not trying to measure it - not affecting it in any way, scattered photons of light either enter my eye or they don't through nothing of my doing, if they enter my eye and i see them, how does that change it's state?
that is why i ask is he using the words measure and observe interchangeably
Affirming my list subscription. Thanks.
If rightwingcrazy doesn't mind, I'll take a stab at that.
Imagine that you are a blind person. And you are carrying a sack of baseballs. To tell if something is in front of you, you throw a baseball. If the ball bounces back to you, something is there.
Now suppose a cardboard box is in front of you. You throw a baseball at it. The baseball bounces back to you. But the box will also move! The box will no longer be where it was when the ball hit it. By merely "looking" at the box with your baseball, you have altered the state of the box.
This is what happens every time you look at something in real life. The "baseballs" are particles of light that bounce off the object, then go into your eyes.
So when you are looking at, say, a house, the light from the sun is actually moving the house, just the way your baseball moved the cardboard box. But the house is so big the effect not noticeable.
But electrons are very small. When light particles hit them, they move noticeably. The result is a blur. You cannot be certain just where they are. That's part of the Uncertainty Principle!
I’m far from an expert on the subject, but, as the piece I posted a little while ago points out, there are several interpretations of QM.
Interpretations of Schroedinger’s Cat
—how does seeing something with you eye
—effect what you see???
Your eye does not “See” the particle
The Eye “Sees” the photon that was
emitted or bounced against the particle
In this case, the Particle was
“measured” by the incident photon
Your Eye “measured” the incident photon,
not the particle itself
"Confusion about the Uncertainty Principle It's very common for the uncertainty principle to get confused with the phenomenon of the observer effect in quantum physics, such as that which manifests during the Schroedinger's cat thought experiment. These are actually two completely different issues within quantum physics, though both tax our classical thinking.
The uncertainty principle is actually a fundamental constraint on the ability to make precise statements about the behavior of a quantum system, regardless of our actual act of making the observation or not. The observer effect, on the other hand, implies that if we make a certain type of observation, the system itself will behave differently than it would without that observation in place."
not being argumentative, but like i said above, isn't sight passive?
kinda like the difference between active and passive SONAR is how i am looking at it, or is that a bad analogy
"The observer effect refers to situations in science where the act of observing a system has an impact on the system being observed. Checking pressure in a tire, for example, is done by causing a release of air which, in turn, causes a slight change in the tire pressure. This is a classic example. Scientists (especially psychologists) have to be especially careful when planning their research methods to avoid interfering with the results they intend to get.
In the realm of quantum physics, there's a more powerful form of the observer effect, as exhibited by the Schroedinger's Cat thought experiment and the quantum double slit experiment. It appears that in cases like these, the very act of making the measurement doesn't just modify the system, but actually results in fundamentally different physical behaviors within the system.
Yes. The effects are there even if a human is not observing the situation.
Suppose that a free electron is sitting on a lab table. And the overhead lights are on. Particles of light will "bounce" off the electron and cause the electron to shift around. It's like how throwing balls at a cardboard box will cause the box to shift around.
So a human does not cause the uncertainty in where the electron really is. The human just notices the uncertainty.
that is why i ask if he is using the words interchangeably when they really are not interchangeable
one is active the other is passive like active vs passive SONAR
or am i missing something?
thx for putting up with me here
Death to all misplaced apostrophes!!
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