Is there a size limit to QM?

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Is there a maximum size an object can have above which it won't follow the laws of quantum mechanics anymore? e.g. could the double slit experiment in theory work with macroscopic objects?
 

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  • #2
Vanadium 50
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Quantum mechanics always works, but you will have a hard time building a 100 micron slit that will pass a 10 ton truck.
 
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Is there a maximum size an object can have above which it won't follow the laws of quantum mechanics anymore? e.g. could the double slit experiment in theory work with macroscopic objects?

you ask, when loses the linearity ? macroscopic object superposition, where is the limit ?

maybe at 10 14 atoms.
 
  • #4
Although I have yet to find an explanation of the double slit issue from QM, the interference pattern effect does continue for larger objects, but the effect diminishes as the objects get larger. Buckyballs have been used and they produced a very tiny but distinguishable interference pattern effect (or so they tell me).
 
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actual limit, 430 atom molecule interference.
 
  • #6
i dont think theres a limit. Only the limit on what you can observe. The bigger the object the closer the wave function gets to 1, where 1 is a single state, but as i understand no object actually reaches 1. But reaches close enough to 1 that you will only see a single state unless you observe for more time than the universe is likely to exist.
 
  • #7
Is there a maximum size an object can have above which it won't follow the laws of quantum mechanics anymore? e.g. could the double slit experiment in theory work with macroscopic objects?
As others said, QM applies at all scales; however, as objects become larger, heavier and
more complex, they tend to interact more intensely with their environment, in a way that
tends to destroy many of the typical quantum mechanical effects, like interference.

Look up 'environmentally induced decoherence' for a more in-depth explanation for this
phenomenon.
 
  • #8
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Here's an post of mine (from a previous thread) that you may find interesting:
The reason there is still disagreement as to what constitutes measurement is that it makes no experimental difference according to quantum mechanics. The way QM works under the Copenhagen interpretation is that you have to split the world into two parts, the “observer” or measurement device, and the “observed” or the particles you’re measuring.

The measurement device is assumed to behave classically. The particles in the observed system are in a superposition of states described by the wave function which keeps evolving until it interacts with the classical measurement device. The question is where to draw the line. You could consider a photon to be the observed system and an atom to be the measuring device, but you can also consider the photon-and-atom system as in a superposition of states, and take a Geiger counter to be the measurement device. So there is this von-Neumann chain, going from elementary particles to Geiger counters to human beings, and we have to decide where to cut it off.

Von Neumann proved in his famous "Bible" of QM that regardless of where you cut the chain, you would get the same experimental results. But he argued that wherever you cut the chain you have things made out of particles on each side of the cut, so there’s no principled way to place the cut in the middle. So he decided that you should place the cut between the human mind and the human body, because he believed that the mind is non-physical. Hence "consciousness causes collapse" was born. Nowadays, the most popular view is decoherence, where there is no real collapse, it's just that when you have a large number of particles in the environment interacting with the system, the wave function becomes smeared out and looks like it has collapsed. So decoherence gives us a reasonable place to cut the chain, when the number of particles involved reaches a critical number so that interference effect become negligible.
 
  • #9
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i dont think theres a limit. Only the limit on what you can observe. The bigger the object the closer the wave function gets to 1, where 1 is a single state, but as i understand no object actually reaches 1. But reaches close enough to 1 that you will only see a single state unless you observe for more time than the universe is likely to exist.
i say actual experimental limit.

there are planned experiments on bigger objects, that way they test quantum mechanics versus macro-realistic theories (if macroscopic objects obey macrorealism, or whether QM prevails).
 
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  • #11
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Of course it was - way back in the 20's, or possibly 30's. These are called correspondence theorems, and they demonstrate that the predictions of quantum mechanics approach those for classical mechanics as systems get large. Paul Ehrenfest did a lot of work on these.
 
  • #13
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Of course it was - way back in the 20's, or possibly 30's. These are called correspondence theorems, and they demonstrate that the predictions of quantum mechanics approach those for classical mechanics as systems get large. Paul Ehrenfest did a lot of work on these.
You're right about the correspondence theorems, but those don't really describe the emergence of classicality from quantum theory. In fact, that emergence is still considered a mostly unsolved problem (despite all the advances in decoherence theory and related subjects).

Ehrenfest's theorem and other correspondence theorems only show that it is sensible to assume that classical behavior can be generated by quantum systems, in one way or another. That mostly refers to classical trajectories of certain quantities. But for explaining real classical behavior you have to explain the lack of interference (decoherence, in the 70s and 80s mostly) and the the uniqueness of classical properties (i.e. the measurement problem)

While the lack of interference is understood quite well, the measurement problem is still not understood fundamentally.
 
  • #14
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Quantum behavior was observed with oscillating cantilevers of ~50µm size - big enough to be visible by eye (but not during the experiment, of course, as the required light would disturb it).
It is just an experimental limit, and the limit becomes bigger and bigger.
 
  • #15
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I can't wait until a proper test is done on the Leggett-Garg inequality. I know somewhere (might be the US), there is a plan to put a 40kg mirror into superposition.
 
  • #16
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Right stevie

this one with a kg mirror.
The LIGO Science Collaboration Experiment, New J. Phys. 11 073032 2009.



-----
Large Quantum Superpositions and Interference of Massive Nanometer-Sized Objects
O. Romero-Isart, A. C. Pflanzer, F. Blaser, R. Kaltenbaek, N. Kiesel, M. Aspelmeyer, J. I. Cirac
6 Jun 2011.

...this includes experiments in a hitherto unachieved parameter regime where collapse theories predict quantum mechanics to fail, or even more general tests of quantum theory against full classes of macrorealistic theories...
...we shall discuss the application of using this experiment to test theories beyond quantum mechanics that provide an objective collapse of the wavefunction for sufficiently large objects.....



------
http://arxiv.org/PS_cache/arxiv/pdf/1103/1103.1236v1.pdf [Broken]

...Another motivation to consider the possibility that quantum physics is only an approximation to a deeper underlying theory...

.
 
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  • #17
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Buckyballs have been used and they produced a very tiny but distinguishable interference pattern effect (or so they tell me).
If you're curious, here's an experiment from 2002 which does just that: Matter-wave interferometer for large molecules.

And here's one from 2011 using molecules made up of 430 atoms: Quantum interference of large organic molecules.
I think that's what yoda jedi was talking about.

It looks like experimentalists are in a long-term race to see who can send the biggest object through a beamsplitter and still show de Broglie interference.:biggrin:
 
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