Quantum mechanics allows us to describe a particle as a wave, and also a collection of particles, which a car happens to be. What separates a typical wave from a classical particle is that the position is not well defined for a wave.

I think that it is hard to tell exactly how big an atom is, because the electron of the atom may be anywhere, but the probability of finding the electron gets very small as you look far away from the nuclei. I think in some sense the atom is as large as we want, depending on the definition.

When a car drives past me, is the position of the car separate from my position in a quantum mechanical way, or is it correct to say that "part" of the car hit "part" of me? I understand that no matter the answer the force from the hit is so small that it is negligible in every way, but that is not the core of the question.

In addition, if I made any wrong claims, please correct me, thanks in advance.

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    $\begingroup$ That's purely a matter of definition. As you point out, your wave function and the car's wave function overlap, and in theory it "slightly hits you." But if I called into work with that explanation, my manager probably wouldn't agree with my definition of contact. $\endgroup$
    – Asher
    Mar 18, 2017 at 14:14
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    $\begingroup$ The obvious answer is: probably not. $\endgroup$ Mar 18, 2017 at 20:40
  • $\begingroup$ Whether the car hit you before or after noon depends on the observer's frame of reference. $\endgroup$ Mar 19, 2017 at 13:02
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    $\begingroup$ @DavidRicherby And, in some frames of reference, you hit the car. (Cue "in Soviet Russia" jokes) $\endgroup$ Mar 19, 2017 at 13:09
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    $\begingroup$ Schrödinger's crash $\endgroup$
    – Alpha
    Mar 21, 2017 at 18:12

1 Answer 1


Asher's comment written above is simply wrong, and the reason is rather fundamental in quantum mechanics. "The car slightly hits you" isn't how it works in quantum mechanics.

The reason is that weak effects – such as very small but nonzero values of the wave function of an electron very far from the nucleus – do not cause tiny but observable effects like they do in classical physics.

Instead, the wave function has a probabilistic interpretation. So its being nonzero very far from the nucleus means that there is a small probability that a finite effect takes place.

So in quantum mechanics, you can ask whether a car hit you – whether some reaction took place. In quantum mechanics, questions about physical systems (including this one) have to be answered by measurements, otherwise they are physically and scientifically meaningless.

And quantum mechanics predicts some probabilities for one outcome of the measurement or another. If only 0.0001% of the integrated $\int |\psi|^2$ of the electron was located at the distance $R$ equal to the distance of your body from the car, then it means that there was a 99.9999% probability that no interaction has taken place at all.

On the other hand, there is a 0.0001% probability that some interaction did take place, but it wouldn't be a small interaction. It would be a rather large one, such as the ionization of one atom – of the car or your body.

Again, in quantum mechanics, small wave functions don't mean small forces. They mean finite (not so small) jumps happening with tiny probabilities.

Again, it's very likely that you didn't get hit by a car in the morning at all.

One can give as many serious examples of that as we want. For example, when a radioactive nucleus decays, the wave function for an alpha particle gradually grows outside the nucleus. It doesn't mean that the alpha particle is "weakly felt" at all times. Instead, it means that there is some probability that the nucleus has decayed up to some moment, and some probability that it remained intact.

Also, one may consider collisions of particles, such as those at the LHC. Various quantum processes may take place; they are described by Feynman diagrams. The two colliding protons have an infinite cross section to interact through the electrostatic repulsion. So yes, there was always some interaction of this electrostatic kind. It's equivalently interpreted as the certainty that some very soft photons are being emitted. (One could say that the car-body contact above was electrostatic elastic between two electrons as well, in that case, it would always occur.)

But there are also other, short-range forces, that predict finite cross sections. In some reactions, some finite process like that – accompanied by the production of pions etc. – can take place. But in others, it strictly doesn't take place. The atoms effectively interact e.g. with van der Waals forces that decrease as a faster power law of the distance and they have a finite cross section. So when we talk about similar interactions between the whole atoms, then the interaction probably didn't occur at all – strictly zero at a probability that is almost 100%.

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    $\begingroup$ Re: "It would be a rather large [interaction], such as the ionization of one atom – of the car or your body": "Large" being a relative term, I suppose. To borrow Asher's reasoning, I doubt you could call into work with that. $\endgroup$
    – ruakh
    Mar 18, 2017 at 23:12
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    $\begingroup$ @ruakh To be fair, if one of my employees asked for the day off because "one of the atoms in their body was ionized by a freak quantum encounter with a passing vehicle", I would probably give them the day off with no questions just for trying. $\endgroup$
    – Jason C
    Mar 19, 2017 at 0:12
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    $\begingroup$ Yes, @ruakh, "large" was meant in relative terms. One ionized atom is still small on human terms - but it's an O(1) effect in comparison with the parameters describing a few atoms and their world, while the wave function at some big separation is exponentially smaller than that. ... Jason, your joke is amusing but I need to point out that such ionization may be serious, too. Ionizing radiation is a cause of cancer. I don't know the percentage of the ionized atom events that lead to cancer - it will be small but it won't be quite negligible. A DNA molecule may get broken, and... $\endgroup$ Mar 19, 2017 at 6:17
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    $\begingroup$ It's also like Schrodinger's Cat: Before you drive to work, your probability of being hit by a car is described by a wave function. Once you arrive safely, the wave function has collapsed to the "not hit" state. This analogous to opening the box that the cat is in, to see if it's alive or dead. $\endgroup$
    – Barmar
    Mar 19, 2017 at 6:27
  • $\begingroup$ I wonder if some math could show the probability of being hit by a car in any given instance? Assuming say, 1000 kg of steel, evenly distributed between 2-4 meters away? $\endgroup$
    – Nacht
    Mar 19, 2017 at 22:09

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