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I am having a discussion with friends about macroscopic VS microscopic in terms of physical theories, and there are parts I would like to understand better from you guys, who know those things well.

So, he started saying that it's conceptually wrong to use Quantum Mechanics to describe macroscopic phenomena, and the example lies in "touch". At microscopic level, tough doesn't exist for atoms never touch each other. What we feel like touch is repulsion energy. This is what I knew from high school physics.

But he states that this is not acceptable, because we don't know what is touch at macroscopic level, hence it makes no sense to explain it via microscopic level. He states that QM is valid only at atomic scales, like GR is valid at big scales ("no one would use GR to calculate the fall of a rock from 30 meters high, to say).

So basically he doesn't agree to explain touch with QM and microscopic theories, also questioning that "if we don't touch, how do you explain intermolecular bounds?"

Hence he claim QM has no effect at the macroscopic level and thence it makes no sense as I said to use QM at the macro level to explain touch.

Can you please tell me more about this? I don't understand how we can not use QM to explain macroscopic questions.

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    $\begingroup$ I put this comment here, but it is my "reaction" to your comment on F. Zwarts answer: It is entirely unclear what your friend means by saying "QM has no role on the macroscopic level". Indeed, everything is quantum by nature. The macroscopic physics can be understood as emerging from QM in the same way Newton's theory of gravitation emerges from General Relativity. There aren't "two distinct types of theories describing the world" (this is the way I understand your friend's position). The more fundamental, the more conceptually accurate it is. $\endgroup$ Commented May 13 at 10:46
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    $\begingroup$ I don't really think there's much physics to be explained here. It's just that his entire argument is fallacious. "Because we don't know what is touch at macroscopic level" is precisely the reason why we should look for an explanation at the microscopic level. Does he also think it's wrong to explain diseases with germ theory? $\endgroup$
    – Sturrum
    Commented May 13 at 10:58
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    $\begingroup$ This question (v2) seems very broad. Related: physics.stackexchange.com/q/65397/2451 , physics.stackexchange.com/q/70541/2451 , physics.stackexchange.com/q/22618/2451 and links therein. $\endgroup$
    – Qmechanic
    Commented May 13 at 15:39

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It has been mathematically proven that GR goes to Newton's gravity theory in the limit for small masses and small velocities. Also it has been mathematically proven that in the limit for even smaller distances in our daily environment, Newton's gravity theory results in a constant acceleration. So, the fall of a rock can be calculated in GR, but it is unnecessarily complex and the results are practically the same as when assuming a constant acceleration. Similarly, QM in the limit of macroscopic objects, results in classic theory. So, I don't understand the problem. You can use QM at the macroscopic level, but it is unnecessarily complex and has the same practical results as classical mechanics. Further, what do your friends mean with 'touch'. Why would it be relevant?

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  • $\begingroup$ Touch is meant as "to touch things". The fact that at the microscopic level we can explain touch with atoms and repulsion energy, but then what is touch at the macroscopic level? The "problem" lies in the fact that he says QM has no role in the macroscopic level, and we cannot use it to explain "the touch" in this way $\endgroup$
    – Heidegger
    Commented May 13 at 10:26
  • $\begingroup$ The answer is likely correct, but, please, keep in mind that many peopole still think that the macroscopic behaviour cannot be explained in terms of quantum mechanics. Only a few peopole, used to very strange quantum phenomena, or quantum-computer experts, understand that the "wave collapse" is just a simplification of a complex quantum phenomenon, which we do not want to model fully (although we could) So, the question makes sense and the answer is correct, but it is not trivial for everybody! $\endgroup$ Commented May 13 at 11:04
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People like to say that quantum theory can't be used to explain behaviour of macroscopic systems but they can be used to explain the behaviour of atoms. This raises the problem that since you're made of atoms and quantum theory explains the behaviour of atoms it is inconsistent to say that it can't be used to explain features of macroscopic systems.

The reason many people try to limit the applicability of quantum theory is that its predictions for macroscopic object sound incorrect at first sight. In quantum theory the behaviour of microscopic objects like electrons is explained in terms of interference. If you look at the probability of finding an electron in some region it depends on what happens along all the paths the electron could go down on the way to that point. So there are multiple versions of the electron going along all of the possible paths. But when it comes to the location of the keyboard on which I'm typing this that doesn't look like it depends on what's happening on all of the paths it could take to its current location. I don't have to worry about whether the keyboard will diffract if I walk through a doorway with it. Many people see this and say that quantum theory doesn't apply to macroscopic systems without thinking any further.

But quantum theory doesn't actually predict that the keyboard would interfere with lots of versions that are very different from one another on a macroscopic scale. The reason is that if you copy information out of a system while it is interfering that suppresses the interference: this effect is called decoherence:

https://arxiv.org/abs/quant-ph/0306072

https://arxiv.org/abs/1111.2189

A keyboard is large and changes slowly compared to a lot of the systems interacting with it like air molecules, light reflecting off the keyboard, the atoms of the desk on which it is resting and so on. So interference between the version of the keyboard I can see and one that is one millimetre to the left is negligible. When you take the trouble to work out what quantum theory implies about macroscopic objects you need only take into account versions of the objects that are very similar to the version you see so quantum theory is consistent with their observed behaviour.

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But he states that this is not acceptable, because we don't know what is touch at macroscopic level, hence it makes no sense to explain it via microscopic level. He states that QM is valid only at atomic scales, [...]

So basically he doesn't agree to explain touch with QM and microscopic theories, also questioning that "if we don't touch, how do you explain intermolecular bounds?"

The opposite is true. In the video interview series Fun To Imagine, Feynman famously and eloquently stated the state of affairs that is actually happening in physics. The sensation of touch, that your hand does not pass through a chair, is a consequence of the microscopic kind of electromagnetic forces. There is no understanding of electromagnetic forces in terms of touch; we can only possibly hope to understand touch in terms of microscopic electromagnetism.

In fact, even that is an approximation. Feynman decided to stop there, but if you actually go into the details, you will realise that the quantum mechanical, Pauli Exclusion Principle, repulsion is about as strong as the microscopic electrical repulsion when it comes to actual touching. The repulsion energy that is rising, is only understandable and explanable from microscopic quantum theory.


There is a standard quip that we tell beginning university physics students. That when something is any one of big, heavy, and/or fast-moving, we will have to consider relativity. Similarly, if something is small, we have to consider quantum theory. However, we always leave vague the delineation. Fast-moving is the easiest, because we can simply compare speeds with the speed of light in vacuum, and immediately we will have some idea of whether something is fast enough to require relativistic corrections, or to use fully relativistic considerations.

But when it comes to whether something is small, the criteria is actually that "The number of particles in a given volume is big enough". In particular, if you have a gigantic number of particles, then even if the volume is large enough to be macroscopic, it is still small enough that we have to use quantum theory.

For example, every piece of metal that you touch, every insulator, every semiconductor that makes up your computer or phone, every solid, every liquid, the whole concept of touching anything at all, can only be fully explained if you consider quantum theory. There is no way to explain the "sea of electrons" in a metal without quantum theory. There is no way to explain even the simplest covalent bonds without quantum theory. It is a degree of freedom that is hithertho unavailable in classical reasoning, that is opened up in quantum theory, that allows covalent bonds to even be a thing at all.

So your friend is just way too wrong to even begin to understand how wrong he is.

In fact, the vagueness of the definition is necessary: For example, a neutron star is big and heavy, definitely requiring the General Theory of Relativity in its description, but at the same time so dense that it can only be explained with quantum theory. Only via quantum theory can its immense gravitational pressure be balanced, not by electrons and protons holding it up, but that during its formation period, the electrons get compacted so small that it goes relativistic, and in a sudden flash, the electrons gets squished into the protons, turning them all into neutrons and stablising in the form of neutrons, as opposed to hot hydrogen gas. Stars that turn into neutron stars, are heavier than about 1.44 times the mass of our Sun. That number is called the Chandrasekhar limit. Is your friend trying to say that the Sun is not macroscopic enough for him?

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LIGO is an example where quantum mechanics matters on a macroscopic scale. LIGO measures gravitational waves. These waves stretch and squeeze distances as they pass through Earth. The changes are tiny - $1/10,000^{th}$ the diameter of a proton. Learning how to build a detector sufficiently sensitive took 20 years.

At the heart of the detector are $40$ kg mirrors that reflect beams of light back and forth. A gravitational wave makes two beams get ever so slightly out of phase. The mirrors have to be isolated from other vibrations to an insane degree. So far, it hasn't been possible to get the mirrors down to their quantum ground state. But the relevant property for measurement has been reduced to 11 levels up from ground. See How can there only be "11 phonons" in the mirrors of LIGO interferometers?

Also see this - How are LIGO mirrors cooled?

From Veritasium - The Absurdity of Detecting Gravitational Waves and How Scientists Reacted to Gravitational Wave Detection

From CalTech, how quantum mechanics is being used to improve precision - LIGO Surpasses the Quantum Limit

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