Quantum mechanics for big things?

[AdrianHudson]

From my understanding QM deals with small things in the universe I use the term "small" loosely when I refer to small I'm talking about sub atomic particles. Anyways back on to the question here it goes.

Why can't QM be applied to bigger objects? I know that we have General relativity for planetary masses and galaxies to describe their behavior. Could the two be interchangeable, say we could use general relativity for things that are small and QM for stuff that is big?

 

[jarekd]

What about Schroedinger's cat? And if you need macroscopic objects with quantum behavior, check Couder's walking droplets - thread: https://www.physicsforums.com/showthread.php?t=550729

 

[DrClaude]

From my understanding QM deals with small things in the universe I use the term "small" loosely when I refer to small I'm talking about sub atomic particles. Anyways back on to the question here it goes.

Why can't QM be applied to bigger objects? I know that we have General relativity for planetary masses and galaxies to describe their behavior. Could the two be interchangeable, say we could use general relativity for things that are small and QM for stuff that is big?

QM applies to all objects. It is just that, beyond a certain size, the effects tend to be so small that they are not noticeable. But there are observable effects in "big" objects, such as big molecules. Also, you can't explain things like neutron stars without QM.

 

[ZapperZ]

Size isn't a factor here. We have seen large molecules, the size of fullerines, and a large conglomerate of particles made up of up to 10^11 electrons exhibiting quantum effects. It will get larger.

The issue here is the ability to maintain coherence so that these quantum effects can be clearly evident on our scale. Maintaining coherence gets progressively more difficult with size and with time! So it just isn't size here. I may be able to get something the size of an elephant be in a coherent state, but it is of no use and not easy to observe if it does that only in the first 10^-15 second before environmental decoherence sets in.

Zz.

 

[ModusPwnd]

Quantum mechanics was initially invented to describe black-body radiation curves, a macroscopic phenomenon.

 

[haael]

There's a wiki article for this, try searching.

One thing I find especially interesting are quantum vortices. When we make swirls in a superfluid, the vortices don't act randomly, but in arrange in cool geometrical shapes. I also like how the wavefunctions (probability density) smoothly become particle/mass density.

This convinces me that quantum phenomena may have some more familiar macroscopic interpretations. If we try to split the Schrodinger's cat's wavefunction, then we don't neccessarily get a zombie cat (half-dead, half-living). Instead it might be: very cold cat, or a cat rotating over its axis, or a cat a sound wave travels through or something else.

 

[DrClaude]

If we try to split the Schrodinger's cat's wavefunction, then we don't neccessarily get a zombie cat (half-dead, half-living). Instead it might be: very cold cat, or a cat rotating over its axis, or a cat a sound wave travels through or something else.

This is non-sensical.

 

[Vanadium 50]

180,000 gallons of liquid helium shows quantum effects. Big enough?

 

[bhobba]

The issue here is the ability to maintain coherence so that these quantum effects can be clearly evident on our scale. Maintaining coherence gets progressively more difficult with size and with time! So it just isn't size here. I may be able to get something the size of an elephant be in a coherent state, but it is of no use and not easy to observe if it does that only in the first 10^-15 second before environmental decoherence sets in.

Exactly.

Getting macro sized objects to display quantum effects aren't easy - but its not impossible. And when you do some very strange things emerge:
http://physicsworld.com/cws/article/news/2010/mar/18/quantum-effect-spotted-in-a-visible-object

Thanks
Bill

 

[meBigGuy]

This paper discusses larger things and why their quantum states are difficult to maintain

http://arxiv.org/pdf/quant-ph/0506199v3.pdf

 

[Maui]

180,000 gallons of liquid helium shows quantum effects. Big enough?

What is the quantum mechanical explanation for the liquid helium behavior and why would that be a qm effect?

 

[DrClaude]

What is the quantum mechanical explanation for the liquid helium behavior and why would that be a qm effect?

Maybe you can start with Wikipedia: Superfluid helium-4.

 

[Maui]

Maybe you can start with Wikipedia: Superfluid helium-4.

There seems to be a contradiction. It could be me(wouldn't be the 1st time anyway) or it could be that there are wrong claims in papers and textbooks on quantum theory(or possibly with the qm explanation on superfluidity which according to what I've read is still an ongoing process).

One of the 1st things one learns from high-quality books is that a wavefunction can never be observed, even in principle. And it seems that most quantum mechanical explanations on superfluidity center around the idea that at temperatures close to absolute zero, the internal random motion of atoms stops and they start behaving as a giant wavefunction, which in turn is routinely directly observed in experiments since the 1930's and filmed in videos.

 

[DrClaude]

There seems to be a contradiction.

What is the contradiction?

(or possibly with the qm explanation on superfluidity which according to what I've read is still an ongoing process)

There are always details to clear out, but nobody doubts that superfluidity is a QM effect. Landau won the Nobel prize way back in 1962 in part for that.

One of the 1st things one learns from high-quality books

What kind of books are you talking about? Popular science, textbooks, or monographies?

is that a wavefunction can never be observed, even in principle.

Not everyone agrees with that statement: Direct measurement of the quantum wavefunction

And it seems that most quantum mechanical explanations on superfluidity center around the idea that at temperatures close to absolute zero, the internal random motion of atoms stops and they start behaving as a giant wavefunction, which in turn is routinely directly observed in experiments since the 1930's and filmed in videos.

Are you saying you have a problem with that statement?

 

[Maui]

What is the contradiction?

That it is possible to directly observe a wavefunction. This should be news to a lot of folks here.

Are you saying you have a problem with that statement?

Yes, I do. It invalidates all interpretations that posit that the wavefunction is only a mathematical tool and that includes the standard interpretation found in textbooks.

 

[DrClaude]

What is the quantum mechanical explanation for the liquid helium behavior and why would that be a qm effect?

That it is possible to directly observe a wavefunction. This should be news to a lot of folks here.

Putting aside the question of whether you can observe a wave function, my problem with your initial statement is that the explanation for superfluidity is quantum mechanical. And when you see superfluid helium flow, you are seeing a QM effect even if you are not observing a wave function.

 

[Maui]

It still doesn't feel right that you can have a very large body that displays directly observable quantum behavior in daylight, in front of cameras... Even one photon was supposedly sufficient to trigger massive decoherence in less than a thousand of a second and a rapid return to classical behavior.

 

[dauto]

The wavefunction itself cannot be measured. It is a complex function and the magnitude of that complex fuction can be measure. may be that's the source of confusion? The problem of many introductory texts is that they violate the principle that statements should be made as simple as possible but not simpler.

 

[Maui]

The wavefunction itself cannot be measured. It is a complex function and the magnitude of that complex fuction can be measure. may be that's the source of confusion? The problem of many introductory texts is that they violate the principle that statements should be made as simple as possible but not simpler.

No, every measurement on the wavefunction forces the quantum state to become one of the eigenstates of the operator corresponding to the measured observable.

What is not clear is why a large body would display quantum behavior in broad daylight in front of recording equipment without observable decoherence setting in with a rapid return to classicality(see post 17) - esp. since the fluid is in contact with macroscopic objects like the fluid container?

My primitive explanation is that(perhaps contrary to commonly adopted phrasing) liquid helium is a new entirely classical behavior due to quantum effects, but not a quantum behavior in and of itself. It's still confusing as all macroscopic behavior should be due to quantum effects.

 

[bhobba]

What is the quantum mechanical explanation for the liquid helium behavior and why would that be a qm effect?

The behavior of liquid helium is complex and difficult to explain all its features even with QM.

But some features are easy to see. For example that it flows without friction is a consequence of the fact its in its lowest energy state - if it had friction it would loose energy which is not possible. You can do an internet search for explanations of other weird aspects - but they are all based on QM.

Of course that's a superficial explanation - the correct one is much more difficult and deeper eg
http://cds.cern.ch/record/808382/files/p363.pdf
'Putting it in another way, we can say that the destruction of superflow would require a transition that takes a macroscopic number of atoms from one state to another simultaneously, and such a process has very low probability.'

Thanks
Bill

 

[bhobba]

The wavefunction itself cannot be measured.

Come again. A wavefuction is simply the expansion of a state, |u>, in eigenfunctions of position ie a representation in a certain basis of the state. An observable exists that will give 1 if its in that state 0 otherwise (ie |u><u|). So in principle you can 'measure' a wavefunction - although in practice it may not be possible - and of course you need to be able to perform the experiment many times to ensure you always get a 1.

Thanks
Bill

 

[jarekd]

Talking about superfluids, it is worth to mention behavior of macroscopic fluxons/Abrikosov vortices in superconductors, which can be observed under microscope (they are kind of similar to Couder's walking droplets), like interference ( http://prl.aps.org/abstract/PRL/v71/i14/p2311_1 ) or tunneling ( http://www.nature.com/nature/journal/v425/n6954/full/nature01826.html ).

About measurement of wavefunction, measuring single state destroys it ... however if we can repeat this state many times, we can measure for example the amplitude of wavefunction - e.g. here is measured density of electrons for s and p orbitals of carbon atom: http://blogs.nature.com/news/2009/09/electron_clouds_seeing_is_beli.html

 

[bhobba]

About measurement of wavefunction, measuring single state destroys it ...

Not so sure about that. If it's in the state you are measuring, the state doesn't change. But you would need to do it many times to be sure it always gives the same state after measurement ie you would do an experiment that gives a 1 if its in that state. But you must do it many many times to ensure it always gives 1.

Thanks
Bill

 

[jarekd]

Single measurement gives us only single observable (eigenstate of the Hamiltonian) with some probability distribution - we need many measurements to estimate this density distribution.
However, measurement does not necessarily have to destroy the state as I have written - there are also more subtle "weak measurements", which allow for example to measure average paths of photons interfering in double-slit experiment: http://materias.df.uba.ar/labo5Aa2012c2/files/2012/10/Weak-measurement.pdf

 

[audioloop]

From my understanding QM deals with small things in the universe I use the term "small" loosely when I refer to small I'm talking about sub atomic particles. Anyways back on to the question here it goes.

Why can't QM be applied to bigger objects? I know that we have General relativity for planetary masses and galaxies to describe their behavior. Could the two be interchangeable, say we could use general relativity for things that are small and QM for stuff that is big?

if you ask for quantum superposition/interference on macroscopic object


Macroscopic Quantum Coherence & Macrorealism experiments
https://www.physicsforums.com/showthread.php?t=452912

actual experimental limit
around 430 atoms.

http://www.univie.ac.at/qfp/publications3/pdffiles/ncomms1263.pdf


.

 

[jarekd]

actual experimental limit
around 430 atoms.

Fluxons/Abrikosov vortices are a few orders of magnitude larger and they have observed quantum interference for them 20 years ago: http://prl.aps.org/abstract/PRL/v71/i14/p2311_1

 

[audioloop]

Fluxons/Abrikosov vortices are a few orders of magnitude larger and they have observed quantum interference for them 20 years ago: http://prl.aps.org/abstract/PRL/v71/i14/p2311_1

your opinion, because this is the consensus in the physics comunnity.


.

 

[audioloop]

Large Quantum Superposition and Interference of Massive Nano-Objects
http://arxiv.org/PS_cache/arxiv/pdf/1103/1103.4081v1.pdf
Physical Review Letters 07/2011; 107(2)


Quantum Upsizing
http://www.fqxi.org/community/articles/display/103


.

 

[StevieTNZ]

http://www.nature.com/news/entangled-diamonds-vibrate-together-1.9532

Two diamond were entangled, in room temperature, less than two years ago.

 

[audioloop]

http://www.nature.com/news/entangled-diamonds-vibrate-together-1.9532

Two diamond were entangled, in room temperature, less than two years ago.

correlation is not entanglement.


.

 

[kith]

What is not clear is why a large body would display quantum behavior in broad daylight in front of recording equipment without observable decoherence setting in with a rapid return to classicality

Decoherence happens in a certain basis. It doesn't restrict the populations of these basis states. In particular, there's no problem with all particles occupying the same state.

These populations are given by the statistics. There are many examples of quantum effects for macroscopic systems which are due to quantum statistics: lasers, semiconductors, neutron stars, superconductivity, superfluidity, etc. Even the volume of ordinary bulk matter could be dubbed a quantum effect.

 

[jarekd]

your opinion, because this is the consensus in the physics comunnity

Not only mine, also for example of reviewers from Phys. Rev. Let. as the abstract is "We have observed quantum interference of vortices in a Josephson-junction array. When vortices cross the array along a doubly connected path, the resultant resistance oscillates periodically with an induced charge enclosed by the path. This phenomenon is a manifestation of the Aharonov-Casher effect. The period of oscillation corresponds to the single electron charge due to tunneling of quasiparticles."

I think your problem is the question of what is the de Broglie's clock - for fluxons this conjugated internal periodic dynamics has a bit different nature than for electrons or photons ... but in the http://www.univie.ac.at/qfp/publications3/pdffiles/ncomms1263.pdf, in abstract they write that de Broglie's wavelength here is lambda=h/mv~1pm, while later they have oscillations with wavelengths of hundreds of nanometers - I doubt it is the same de Broglie's clock as for photons or electrons - it is rather of some effective vibrations of the whole molecule.

And if we allow for any, also effective de Brogle's clock, not only we can classify fluxons for quantum interference, but also macroscopic Couder's walking droplets in double-slit experiment: http://prl.aps.org/abstract/PRL/v97/i15/e154101

Can we go even larger? Maybe celestial bodies? :) They usually have internal periodic process: rotation, what can work as de Broglie's clock ... but for interference we need also a medium carrying waves from this periodic process, such that these waves could later affect behavior of the object which created them - maybe interference of some pulsar, using gravity waves and ... a few million years :)

However, maybe we could look for a more serious "quantum-like" properties on statistical level as there is some resemblance with Bohr's atomic model. If we would average millions of years of relative position of e.g. a planet, there are plenty of looking randomly disturbances from perfect trajectory - like caused by gravity of other planets. So to predict such time average, we should use some thermodynamical model, like taking Boltzmann distribution among all paths it could travel through - exactly like in euclidean path integral formulation of quantum mechanics, it should lead to quantum statistics of this averaged positions - discussion: https://www.physicsforums.com/showthread.php?t=710790

 

[StevieTNZ]

correlation is not entanglement.

I don't quite understand how this statement has anything to do with the diamonds entangled.

 

[Vanadium 50]

liquid helium is a new entirely classical behavior due to quantum effects

Those words make no sense in that order. Behavior cannot be entirely classical if due to quantum effects. It's like talking about the corners of a circle.

 

[.Scott]

The biggest quantum effect that I recall occurs at event horizons, for example, the event horizon of a black hole.
Viewed from the outside, an object falling into a black hole will never quite make it to the event horizon. Instead, an extreme case of time dilation will be observed. So if a watch crosses the horizon at noon, we will see the watch approach noon but never reach it.

Here's where QM takes over. Under these conditions, Heisenberg Uncertainty takes over. When we can see the watch so precisely in time, it is not possible for us to know as much about its location. As a result, the watch will blur into a holographic pattern that will soon cover the entire surface of the black hole.

You can also create an event horizon by maintaining a constant acceleration. From your non-inertial reference frame, an event horizon will follow behind you with this same QM holographic affect. That QM effect will separate you from approximately half the universe.

 

[jarekd]

Scott, sure the (looking self-contradictory) hypothesized Hawking radiation is a quantum phenomena, but like Pauli exclusion principle in white dwarfs, it is not "for big things" but regards the microscopical ones ... just in presence of "a big thing".

 

[Maui]

Those words make no sense in that order. Behavior cannot be entirely classical if due to quantum effects. It's like talking about the corners of a circle.

The monitor of my computer is an entirely classical behavior of quantum fields and 'particles'. The superfluid liquid helium seems like a new classical phenomena that can only be explained(at this time?) via quantum theory. Just like temperature is explained by the motion of atoms and molecules but is not a quantum phenomenon(it's strictly classical).

 

[.Scott]

Scott, sure the (looking self-contradictory) hypothesized Hawking radiation is a quantum phenomena, but like Pauli exclusion principle in white dwarfs, it is not "for big things" but regards the microscopical ones ... just in presence of "a big thing".

I wasn't referring to Hawking radiation. I was referring to the hologram that are created as material crosses the event horizon. It comprises the entire event horizon - although it is very thin.

 

[audioloop]

I don't quite understand how this statement has anything to do with the diamonds entangled.

one thing is quantum correlation (coherence) and other is entanglement.
the effect cited by you, has been observed time ago before this one.

read
Chen, H., et. al., 2011. Observations of anti- correlations in incoherent thermal light fields. Phys. Rev. A. 84: 033835.

and from brezinski
http://www.hindawi.com/journals/jamp/2012/469043/

"The recent paper entitled by K. C. Lee et al. (2011) establishes nonlocal macroscopic quantum correlations, which they term “entanglement”, under ambient conditions. Photon(s)-phonon entanglements are established within each interferometer arm. However, our analysis demonstrates, the phonon fields between arms become correlated as a result of single-photon wavepacket path indistinguishability, not true nonlocal entanglement"

 

[audioloop]

"Our analysis is that Lee’s explanation, in the Science paper, for the quantum correlations generated between diamonds (resulting from the pump photons) is unlikely representative of the actual situation. They postulated a nonlocal entanglement between the diamonds. While we agree that quantum correlations are established, we do not believe that the data or analysis of the experimental design supports true entanglement. The essential points will be made here but the remainder of the paper will expand on these points. First, our examination supports that these nonlocal quantum correlations occur from a combination of paths indistinguishability (for a single photon wavepacket) plus nearly identical local entanglements (Raman scatterers) in each path [13–19]. The source is coherent so building the pulses up from single photon wavepackets (a photon can only interfere with itself) is a useful approach for illustrating the physics. The correlations between diamond phonons do not fit definitions of entanglement laid out, for example, by von Neumann, EPR-B, or GHZ"

 

[StevieTNZ]

I've contacted one of the authors of the diamond paper with regards to the paper you link, to get his view on it.

 

[Maui]

Decoherence happens in a certain basis. It doesn't restrict the populations of these basis states. In particular, there's no problem with all particles occupying the same state.

Correct me if I am wrong but they do not occupy the same state but a joint state similar to the joint state of entangled particles where even the slightest disturbance breaks the joint quantum state. Obviously in the superfluid helium somehow it does not.

These populations are given by the statistics. There are many examples of quantum effects for macroscopic systems which are due to quantum statistics: lasers, semiconductors, neutron stars, superconductivity, superfluidity, etc. Even the volume of ordinary bulk matter could be dubbed a quantum effect.

But have quantum statistics been directly observed so far or not? I don't think so. Not once. The effects you list all happen in a non measuring environment at scales impossible to observe directly and had there been a particularly accurate position or momentum measurement the devices likely wouldn't work as intended.

Besides, singling out a preferred basis means you are already observing classical (-like) behavior not quantum.

 

[Vanadium 50]

Maui,

Enough. Our mission is to provide a place for people (whether students, professional scientists, or others interested in science) to learn and discuss science as it is currently generally understood and practiced by the professional scientific community. Denying superfluidity is a quantum mechanical phenomena does not meet this standard. Indeed, this doesn't even meet the Wikipedia standard - the second sentence of http://en.wikipedia.org/wiki/Macroscopic_quantum_phenomena is "However, at low temperatures, there are phenomena that are manifestations of quantum mechanics on a macroscopic scale, the best-known being superfluidity and superconductivity."

Enough.

 

[jarekd]

Vanadium, I think the issue here is finding the boundary between classical and quantum world. I was trying to find it for many years, but if by "classical" we mean not just "classical mechanics", but "classical field theory" - I honestly couldn't find any boundary.
For example Coulder's walking droplets show how to see interference, tunneling, orbit quantization ... maximal entropy random walk shows that after repairing approximation of maximizing uncertainty, stochastic models are no longer in disagreement with quantum predictions, "squares" leading to violation of Bell inequities are natural for stochastics in 4D spacetime ... soliton particle models is a natural way to handle varying number of interacting particles ...

Where exactly is the boundary?

 

[Maui]

http://en.wikipedia.org/wiki/Macroscopic_quantum_phenomena is "However, at low temperatures, there are phenomena that are manifestations of quantum mechanics on a macroscopic scale, the best-known being superfluidity and superconductivity."

Enough.

I am in complete agreement with the wording. That concludes my participation in the thread.

 

[jarekd]

bhobba, sure everything can be seen from quantum perspective ... but is there anything what cannot be also seen from classical field theory perspective?

Like coupled pendulums - we can see them "classically" as just two moving balls, or through their normal modes, where the coordinates just rotate - we have "quantum" unitary evolution.
Going to infinite number of coupled pendulums, we can again see a crystal "classically": through dynamics of every atom ... or through e.g. phonons, collective excitations evolving in "quantum" unitary way.
Now taking infinitesimal limit, we can see it as a classical field theory ... or make its quantization, like operating on Feynman diagrams where particles have structure: are solitons ...

Why "classical" and "quantum" are not just two different perspectives on the same system?

 

[bhobba]

Why "classical" and "quantum" are not just two different perspectives on the same system?

I deleted my post because I realized you were talking about something different than an initial reading of your post indicated.

But in so far as QM can be derived from classical like principles the answer depends on what you mean by classical like.

Thanks
Bill

 

[bhobba]

Gave the paper a quick squiz. Not my cup of tea and don't agree with any of it.

But just one question out of many - why do you consider i (the square root of minus 1) paradoxical? It simply represents a rotation through 90% in the complex plane - its no more paradoxical than say -1 itself, which represents a rotation through 180%.

Its importance in QM is it allows the introduction of phase so you get path cancellation in Feynmans sum over histories. There are others as well such as complex spaces are required for Wigners theorem to apply. Its got nothing to do with paradox - its got to do with what required to model QM phenomena - like its used in many other areas of applied math.

Thanks
Bill

 

[bhobba]

No, quantum mechanics is not about an observer - it is something very objective.

That's just one of many many issues it has. I was going to mention that one but chose his views about complex numbers instead.

Thanks
Bill

 

[jarekd]

bhobba, indeed the complex numbers are from one side a natural mathematical tool to operate on any periodic processes.
Another place we use them in quantum mechanics is the Wick rotation to "imaginary time", which corresponds just to the Legendre transform: changing sign in kinetic term while getting from Hamiltonian to Lagrangian density (I have written about it here).