The atom as a classical object

[Bruno81]

What is the consensus- Can the atom be considered a classical object if we managed to manipulate single atoms?

 

[Vanadium 50]

No.

 

[edguy99]

I think the field will advance. Building classical models that match the standard model as best as it can will continue and advance. Start with an IBM microscopic view of a sheet of carbon atoms and add in some chemistry type diagrams with a bohr radius overlay and some Lewis valence electrons.

The next step is to add in the rest of the electron orbitals. X-ray pulses with durations on the attosecond time scale have been reported. At the attosecond time scale the electron orbitals will come into view. One assumes that when they do, we will start to see real pictures from microscopes that match the atomic orbital (probability of finding an electron) pictures we have been seeing for years.

 

[vanhees71]

To the contrary, there's no way to understand matter in terms of its microscopic constituents in terms of a classical model. Classical physics is an approximation to quantum theory, valid for many-body systems.

 

[Bruno81]

Reading this got me thinking they can behave more classically than is imagined. No?

"Researchers have lifted a single atom from a surface and then replaced it, without using any electric current."

https://physics.aps.org/story/v11/st19

 

[Bruno81]

To the contrary, there's no way to understand matter in terms of its microscopic constituents in terms of a classical model. Classical physics is an approximation to quantum theory, valid for many-body systems.

With technology getting better and finer, would not our instruments reveal that single atoms can in fact behave very very classical-like? Almost like bricks and walls?

 

[Nugatory]

What is the consensus- Can the atom be considered a classical object if we managed to manipulate single atoms?

Reading this got me thinking they can behave more classically than is imagined. No?

That would depend on how classically you've been imagining them to act.

Any massive particle has a quantum mechanical position operator and hence a property that is more or less well approximated by the classical notion of position. In the experiment you cite, the "more or less" is more "more" than in some other experiments, but the classical picture is still an approximation. Whether it's a good enough approximation to justify the claim "the atom can be considered a classical object" depends on what you're trying to do with it.

It's worth pointing out that when the first sentence of an article reads "Like a diner spearing a morsel of food with the tine of a fork..." you should not be expecting a lot of precision in the subsequent discussion. Analogies are OK, but you have to remember that they're analogies not the real thing.

[Edit: I should add that I'm using a lot more words to say the same thing vanadium50 said above]

 

[Nugatory]

With technology getting better and finer, would not our instruments reveal that single atoms can in fact behave very very classical-like?

No, but our instruments may allow us to observe single atoms behaving exactly as quantum mechanics has always predicted.

 

[Bruno81]

That would depend on how classically you've been imagining them to act.

Any massive particle has a quantum mechanical position operator and hence a property that is more or less well approximated by the classical notion of position. In the experiment you cite, the "more or less" is more "more" than in some other experiments, but the classical picture is still an approximation. Whether it's a good enough approximation to justify the claim "the atom can be considered a classical object" depends on what you're trying to do with it.

It's worth pointing out that when the first sentence of an article reads "Like a diner spearing a morsel of food with the tine of a fork..." you should not be expecting a lot of precision in the subsequent discussion. Analogies are OK, but you have to remember that they're analogies not the real thing.

[Edit: I should add that I'm using a lot more words to say the same thing vanadium50 said above]

Thanks! Wouldn't the act of manipulation force the atom to be as classical as a shoe or a chair? I understand there are no classical objects which is even more mind boggling but I am trying to understand what can be achieved in terms of single atoms.

 

[Nugatory]

Thanks! Wouldn't the act of manipulation force the atom to be as classical as a shoe or a chair?

No. Manipulation is a technique for putting quantum systems into a state in which the expectation value of the position is fairly tightly constrained, with commensurate uncertainty about the momentum. However, a classical shoe or chair has both the position and the momentum tightly constrained.

 

[Bruno81]

But aren't you starting out with the premise that you want to prove - "Manipulation is a technique for putting quantum systems"...? And if true, how would the researches lift/push a single atom?

 

[Vanadium 50]

Bruno, you seem not to be asking us, you seem to be telling us. And what you seem to be telling us is wrong.

Atomic-sized systems act quantum mechanically. Period. Full stop. The fact that they can be placed precisely does not disprove that - QM allows that.

 

[Nugatory]

But aren't you starting out with the premise that you want to prove - "Manipulation is a technique for putting quantum systems"...?

We've been testing the quantum-mechanical predictions for the behavior of atoms for almost a century now, and these predictions have been so accurate in so many different situations that we are justified in accepting quantum mechanics as a correct theory of that behavior. Quantum mechanics says that the experiment that you've linked to is possible in principle - that's one of those predictions - so the interesting thing is that it was realized in practice twelve years ago, further gilding the lily piling on confirming that quantum mechanics accurately describes the behavior of atoms. Part of that accurate description is that atoms can be pushed around and moved, and that doesn;t mean that they are any less quantum mechanical or any more like a chair or a shoe.

It's worth noting that quantum mechanics allows for classical-sounding behavior even of things much smaller than an atom. An electron is at least seven orders of magnitude smaller and three orders of magnitude less massive than an atom. But it's still easy to make an electron behave like a classical object, by moving one through a vacuum in which electrical and magnetic fields are present.

And if true, how would the researches lift/push a single atom?

By doing what the article describes... Push the probe against the substrate to dislodge an atom and encourage it to bind to the tip of the probe, then move the probe around. The interactions needed to make that be lifting and pushing are completely quantum mechanical in nature.