Quantum mechanics for big things?

  • #1

Main Question or Discussion Point

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?
 

Answers and Replies

  • #2
111
2
  • #3
DrClaude
Mentor
7,269
3,424
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.
 
  • #4
ZapperZ
Staff Emeritus
Science Advisor
Education Advisor
Insights Author
35,651
4,424
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.
 
  • #5
1,254
106
Quantum mechanics was initially invented to describe black-body radiation curves, a macroscopic phenomenon.
 
  • #6
536
35
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.
 
  • #7
DrClaude
Mentor
7,269
3,424
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.
 
  • #8
Vanadium 50
Staff Emeritus
Science Advisor
Education Advisor
2019 Award
24,319
7,157
180,000 gallons of liquid helium shows quantum effects. Big enough?
 
  • #9
9,374
2,427
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
 
  • #10
meBigGuy
Gold Member
2,323
405
  • #11
706
2
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?
 
  • #12
DrClaude
Mentor
7,269
3,424
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.
 
  • #13
706
2
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.
 
  • #14
DrClaude
Mentor
7,269
3,424
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?
 
  • #15
706
2
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.
 
  • #16
DrClaude
Mentor
7,269
3,424
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.
 
  • #17
706
2
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.
 
Last edited:
  • #18
1,948
200
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.
 
  • #19
706
2
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.
 
Last edited:
  • #20
9,374
2,427
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
 
Last edited:
  • #21
9,374
2,427
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
 
  • #22
111
2
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
orbitals.jpg
 
Last edited by a moderator:
  • #23
9,374
2,427
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
 
  • #24
111
2
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 [Broken]
 
Last edited by a moderator:
  • #25
463
7
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


.
 

Related Threads on Quantum mechanics for big things?

Replies
14
Views
4K
Replies
7
Views
740
Replies
19
Views
5K
  • Last Post
Replies
8
Views
2K
  • Last Post
Replies
2
Views
1K
  • Last Post
Replies
4
Views
3K
  • Last Post
Replies
3
Views
2K
  • Last Post
Replies
4
Views
3K
Replies
3
Views
816
Top