# Quantum behaviour of particles

What is the difference between classical particle and quantum particle?

I was thinking that we can distinguish between classical mechanics and quantum mechanics by changing our length scale. But I don't understood in what way or on what parameter we differentiate between classical particle and quantum particle.

you take one proton or neutron and say that it quantum particle, but when you make a solid out out of proton and neutron you say we have a classical body.

just a matter of size, I think?

just only size determines whether something will be a classical or quantum particle? or some inherent deep meaning?

There are no Question discussing Quantum and classical particle and can pose doubt to a beginner.

• I think you need to clarify the question. Right now it sounds like "Apples and oranges are different. But is there any concrete difference?" No offense. – Vadim Jul 27 '20 at 3:00
• Classical and quantum are not even the same theory. There is classical-to-quantum correspondence and some intricacies related to measurement - as was pointed in my answer. – Vadim Jul 27 '20 at 3:25
• This is mostly about photons, but it may help. How can a red light photon be different from a blue light photon? – mmesser314 Jul 27 '20 at 4:19
• You have some scales (in the old quantum theory), depending on your problem, that defines the 'sizes' in which quantum effects matter. One is deBroglie wavelength and another is the thermal wavelength, though both are related. – user2820579 Aug 31 '20 at 1:48

The quantum and classical particles are governed by different principles and described by different equations. However, in a certain limit a quantum particle behaves like a classical one, which is expressed by the [Eherenfest theorem][1].

The relationship between quantum and classical is somewhat complex, as discussed, for example, in [this thread][2].

Update
In response to the updated question, let me first clarify the terminology.

• Particle may mean different things, depending on the context. It can mean a point like object, i.e. an object whose internal structure is not essential for the problem that we are solving. Thus cars and stars are often treated as particles in mechanics. Particle may also mean an elementary particle - a special class of physical entities, such as electrons, protons, neutrons, photons, etc.
• When speaking of classical and quantum one also has to distinguish between the actual physical world and mathematical formalisms that we use to model it. All the physical objects in this world are quantum and relativistic, but in many cases simplified classical description is quite appropriate for practical purposes.
• Finally, it is necessary to mention that classical has special role in the sense that it is the limit most familiar to us from our conventional everyday experience, this is where we have most "intuition". [1]: https://en.wikipedia.org/wiki/Ehrenfest_theorem [2]: Is quantum mechanics applicable to only small things?
• yes, that's only I have written in question. one says that they are governed by different principles therefore they are different, but question is why they are governed by different principles? the answer written is itself quoted in question , I was expecting some deep answer. – crabNebula Jul 27 '20 at 2:46
• try to think "they are governed by different principles therefore they are different or they are different therefore governed by different principles" ? the question is with respect to second argument – crabNebula Jul 27 '20 at 2:51
• @Kunalkumar the models used to describe ( and predict) mathematically the data at macroscopic distances "are governed by different principles therefore they are different". The modedl use for microscopic values (quantum mechanics) can be shown to aggree at the limits with the macroscopic model. – anna v Jul 27 '20 at 7:47

You seem to have the idea that quantum mechanics is simply given a different name because it is talking about smaller scales, but the truth is that the entire world is quantum mechanical including the macroscopic world. Quantum mechanics is a qualitatively different update to classical mechanics. We just didn't notice that we needed a better description of the world until we looked at small scales, since this is where quantum effects are most noticable.

• i have edited the question, so please review it – crabNebula Aug 28 '20 at 17:40
• @Kunalkumar There is no such thing as a "classical particle". Everything in the world is quantum mechanical. These particles begin to act classically when there are many of them interacting, so it's more about the number of particles than about the length scale. This process is known as quantum decoherence. – Jeff Bass Aug 29 '20 at 20:38
• @JeffBass things may be explained with QM but it’s still incomplete. You can’t just dismiss classical when many are searching for classical description to complete QN. – Bill Alsept Aug 30 '20 at 1:42
• @Bill Alsept would you please expand that point of View – crabNebula Aug 30 '20 at 10:49
• @Kunalkumar contrary to answer above there are many classical particles like photons, quarks, electrons, protons and neutrons. It is the quantum particle that can’t be described or found. Many, including Einstein Believe the underlying principles that describe these particles is incomplete. A classical particle has a local reality and always takes the path of least action. A quantum particle is a mathematical description that somehow takes every path. I’m not sure why your question was closed because this same question has been asked by some of the greatest minds to ever live. – Bill Alsept Aug 30 '20 at 17:52