How does a particle know it should behave in such and such manner?

As a person, I can set mass is so and so, charge is so and so - then set up equation to solve its equation of motion but who computes that equation of motion for a particle in real life?

I, as a person, employ smart 'tricks' such as principle of superposition to avoid having to calculate super complicated situation (calculating electrical force by a shape where large circle is hollowed out in the off-center) but if I were to calculate this in a brute force manner, this would take long time for me to calculate. However, nature doesn't seem to face these types of problems.

Given a school of fish, the ones at the edge will sense threat and gives signal to those near them and so on but this analogy doesn't seem to make sense for physical objects generally considered in general physics problems. Am I asking the wrong type of question? Would appreciate input on this.

  • 9
    $\begingroup$ Good question, but I think it's more of a philosophy question than a physics question. $\endgroup$ – PM 2Ring Sep 17 '18 at 7:03
  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – David Z Sep 17 '18 at 20:44

I think this question makes hidden, inarticulated assumptions about reality. In physics, we make observations and then try to find models that match them. The models, though, belong only to us and exist in our heads and textbooks.

We perform the calculations required to make our predictions in our models. We cannot say whether nature makes similar calculations, and asking 'how' nature or particles perform calculations seems wrongheaded, to me. Jaynes called this the mind projection fallacy; you are projecting things that exist in your mind - calculations and models - to reality.

Responding to @knzhou, I would not, though, advocate answering every question about physics with the fact that 'physics is a bunch of models'. It is possible to construct models, answer questions about them and make calculations without projecting our calculations to reality.

The question 'how does water know to boil at 100 C?' would be better phrased 'how does our model explain the observation that water boils?'. We could answer that with @knzhou's answer: 'bubbles of steam finally have enough energy to expand against the water pressure.' This is indeed a valuable explanation.

  • 6
    $\begingroup$ I tend to agree. This is closely related to the idea that the universe is a simulation. The problem is that it's impossible in principle to distinguish between a sufficiently good simulation and "the real thing". $\endgroup$ – PM 2Ring Sep 17 '18 at 7:06
  • 2
    $\begingroup$ The assumption that a particle needs to "know how to behave" is also rather similar to the argument that "every effect must have a cause, but there must have been a first effect which cannot have a cause because there was nothing that preceded it, therefore God exists" ;) $\endgroup$ – alephzero Sep 17 '18 at 9:01
  • 1
    $\begingroup$ If I believe matter is made of atoms, and somebody asks how water "knows" to boil at 100 C, I could recline in my philosophical armchair and say "boiling and atoms are merely human constructs, so the question is meaningless". But the real answer is to understand how, at 100 C, bubbles of steam finally have enough energy to expand against the water pressure. That explanation is actually worth something. $\endgroup$ – knzhou Sep 17 '18 at 9:12
  • 9
    $\begingroup$ @knzhou but the key point there is that there is a physical process going on, no calculations are done to tell the water to boil, the water molecules don't "know" anything. Just like my body doesn't "know" to go bouncing down the street if I walk into traffic without looking, it's just something that happens due to an external influence. $\endgroup$ – Baldrickk Sep 17 '18 at 9:41
  • 5
    $\begingroup$ @knzhou There is a difference in question "Why the water boils at 100C" and "How does the particle know it has enough energy to fly away and who keeps tabs on this". BTW, given that you don't like this approach, and that I really like your answers on fundamental and quantum physics, I am really interested how you approach "Who computes that equation of motion for a particle in real life? [...] if I were to calculate this in a brute force manner, this would take long time for me to calculate. However, nature doesn't seem to face these types of problems." $\endgroup$ – luk32 Sep 17 '18 at 10:37

One way to think about it is that a particle "sniffs out" its immediate surroundings and reacts to gradient: a trend like a declining potential in one direction.

Single-celled organisms do this. Plant orient toward the sun. A rock on an incline "senses" that it's center-of-mass is slight off from the point of contact with the ground. This is all loosely speaking, of course.

In physics, differential equation capture the same idea. $F=\frac{dV}{dx}=m \frac{d^2x}{dt^2}.$ Tiny differentials give marching orders to particles and charges and spacetime.

There is another mathematical formulation, with Lagrangians and actions, where a particle chooses the path that minimizes the action, as if the particle knows which path to take. Or Fermat's theorem where light takes the path of least time, as if the photon is intelligent enough to compare a lot of paths. This may look like particles "know how to behave." However, mathematically these theorems are equivalent to differential equations. After all, you can divide a path into many tiny segments, and then you're back to the differentials of differential equations.

So it's all a very local computation that a particle needs perform. We find similar ideas all over science. For example, in (artificial) intelligence, there's the Hebb rule: many neurons are connected in a big network, but when the network "learns" each neuron makes small adjustments to how strong its connection is to nearest neighbors only. But as a result, the entire network can learn to perform a complex computation.

Hope this makes it feel a little more clear.

  • 3
    $\begingroup$ Or, a particle chooses EVERY path simultaneously and the results combine in quantum mechanical interference with weight proportional to the expontential of the action. $\endgroup$ – M.Herzkamp Sep 17 '18 at 8:32
  • 2
    $\begingroup$ @M.Herzkamp Yes, but this is still described by a local differential equation, the Schrodinger equation. The essential point of this answer is that the particles are merely reacting to their immediate environment. Sometimes, for our personal convenience, we phrase what is going on in global terms ("the particle minimizes the total action", "the particles collectively find the ground state") but the particles can do it without any magic powers. $\endgroup$ – knzhou Sep 17 '18 at 9:10
  • $\begingroup$ Great answer, it also kind of explains how animals without any intelligence appear to "know" things when, in fact, they are simply reacting to their surroundings $\endgroup$ – Dunno Sep 17 '18 at 18:02
  • $\begingroup$ @Dunno Including humans, I presume. $\endgroup$ – JiK Sep 17 '18 at 18:45

How does a particle know how to behave?

Already the title is about metaphysics, a consciousness is attributed to the particle by the verb "know".

Physics is about modelling observational measurements with mathematical formulae which can predict future behavior. The "knowledge" is collective and comes from an accumulation of an enormous number of observations.

There are metaphysical models which attribute consciousness to particles. A particular one I read in my metaphysical era ( and was always careful to separate physics from metaphysics) is the "units of consciousness" model of Janer Roberts who was channeling Seth ( you cannot get more metaphysical than this :) ).

In this model all that exists is units of consciousness, which, like many dimensional cosmic strings exist from -infinity to + infinity, building up nature as we observe it. In that metaphysical frame, the question has an answer.

There is no answer within Physics theories and models.

  • $\begingroup$ #historyofphysics It's just a paraphrase of "How does a physicist know how to observe a particle's behavior." ;) $\endgroup$ – Beanluc Sep 17 '18 at 18:52

In physics the behavior of an object (be it elementary or a complex system) is always, by construction of the theory itself, completely constrained. In other words, we create models of Nature (I put a capital to refer to reality even beyond what physics can say about it) explicitly denying any possibility of choice or decision to their constituents. This is what allows us to predict what will happen to some system under some conditions, and this is what makes physics work at all. It's all mechanical.

So asking 'how does a particle know how to behave?' rely on a sort of category error. A physical concept such as 'particle' has precisely been created so that this sort of question does not occur. The initial similar question was something like 'how does this thing that I see in Nature happen', and the answer in physics is to make a mechanical, determinist, fully-specified representation, a model, in order to have an answer of the form: 'because the system is made that way, this thing I have seen has to happen'.

Why does this work? This is the age-old philosophical question of the nature of physical laws.

  • 1
    $\begingroup$ I don't like this, just like I don't like the top answer. You can answer every single question that has ever been asked on this site with "well, philosophically, we can't know for sure, so I'm just going to refuse to answer". That's kind of interesting the first time you hear it, not interesting when it's been said tens of thousands of times on this site and three times on this question alone. $\endgroup$ – knzhou Sep 17 '18 at 9:44
  • $\begingroup$ My point is about the nature of a theoretical object. Part of its nature, by construction, even by definition, is that it does not have to know anything. These objects are here for us to know/understand/explain things. $\endgroup$ – Stéphane Rollandin Sep 17 '18 at 9:47

The question doesn't really make a sense in terms of physics. Physics is fundamentally empirical. This means you cannot tell more than comes from your observations, which has it's limits.

We see that things behave similar to each other. We conclude1, there are some rules for their behaviour 2.

Physics is basically a way of predicting future, where fundamental role play those rules. They model the reality, they don't claim they exactly depict reality.

We don't know what is the source for the rules. We don't even know whether there are some rules which are fundamental. You cannot tell whether there is a god-like being moving all the particles by hand tricking you into believing there are some rules.

The method using the rules to predict the outcomes might not be truly real, but it works good enough, even without such assumption.

We just know, they statistically work, and they let us predict big part of future we are interested in.

1 The conclusion is the final step. There is an assumption - hypothesis, then we run a bunch of tests - experiments - to see whether indeed the proposed rule we test predicts the outcome. If it does well, we establish the rule, which gets to be called a law.

2 The key point is that, we can tell which things are similar to each other, and the same things behave the same in different points in time (or rather space-time). This means that rules established at one place will work in another, if they weren't transitive they would be useless.

  • 1
    $\begingroup$ I don't like this, just like I don't like the top answer. You can answer every single question that has ever been asked on this site with "well, philosophically, we can't know for sure, so I'm just going to refuse to answer". That's kind of interesting the first time you hear it, not interesting when it's been said tens of thousands of times on this site and three times on this question alone. $\endgroup$ – knzhou Sep 17 '18 at 9:44
  • $\begingroup$ After reading the title, I thought about answering, that it doesn't know and the most general rule is that each system and particle tries to minimize it's energy + some randomness, but then body of question is who calculates this and how does the particle "know". Most questions on this site can be answered within a physical framework. This asks about the framework. It's the intrinsic property of physical systems that they follow laws, and asking why and what makes them do so doesn't make sense, because it's not physics anymore. I wasn't satisfied with top answer, and didn't see the other. $\endgroup$ – luk32 Sep 17 '18 at 10:26
  • $\begingroup$ @knzhou : It could be answered, but it is just not the domain of physics to provide the answer for that. Physics is about describing the laws - the question of existence or non-existence of some form of law-giver(s) is not one that can be addressed from science, at least not unless they provided some evidence of themselves that is empirically accessible to us, which does not appear to be the case. Nonetheless, the fact that to me it seems a legitimate question is why I can't get so easily onboard with the anti-religious crowd even though I also reject dogmatic religion as having many flaws. $\endgroup$ – The_Sympathizer Sep 17 '18 at 10:29
  • 1
    $\begingroup$ Since this is a physics site, thus that answer is given. But that doesn't mean that it might not be suitable for discussion elsewhere, on other fora. Given the very philosophical and non-empirical nature of the question, it would best be suited for a site devoted to those kind of things, I'd think. $\endgroup$ – The_Sympathizer Sep 17 '18 at 10:30

If we assume all our best descriptions of the universe are literally true (we're sure they're not, because they're not perfectly consistent yet, but bear with me)...

The particle doesn't know anything. The quantum fields are all there is in the universe, and they all behave and interact according to an extremely simple set of rules. These rules are all there is to the universe - everything else is just higher-order effects of those simple rules. A particle you can observe follows from these simple rules, as does your observation instrument and yourself.

And all the way at the level of a human being, you have thought processes that assume things behave some way because they "want" to, because our brains were shaped by natural selection to empathize with other humans - and humans do things because they will to. You're confused because to your brain, a fully sentient being like Thor throwing lightning seems to you like a simpler explanation than quantum field theory - because your brain already has circuitry for understanding other humans, and adding "can throw lightning when he wants" is an easy extrapolation of that.

Developing quantum field theory (which is probably still not how the universe actually works; there's some issues that still need development) took a very long time, in part because our intuitions just give a very bad picture of what the universe actually is - and because the fundamental moving parts are so hard to observe with any useful resolution. Natural selection didn't directly prepare us for that, because a basic understanding of kinematics is just as good at throwing spears as QFT, but vastly cheaper.

Ultimately, our understanding of the universe is shaped by our perception, which itself developed under pressures of natural selection. There's a decently consistent description of our universe in which time doesn't exist at all - there might not even be anything like causality (Barbour's "The End of Time" is a bit dated, but basically extrapolates general relativity in a way that eliminates time and causality without affecting our perception of time and causality). The reason time seems obvious and unavoidable to us is that our brains keep track of a certain kind of relative time - it's a hack produced through natural selection that (ultimately) makes us better at reproduction. But it would work just as well in a universe where a separate notion of time exists, as in a universe where time is just an illusion of a momentary configuration of particles that might or might not be in relation to other particles in different configurations.

Our models map to many possible underlying realities. In some cases, we literally can't tell the difference (yet). A typical argument is that our universe might very well be just a simulation running on a computer of some student in the actual universe; the only real difference is at the level of knowing the actual rules of the system - but we can't ever be 100% sure the rules we got are the actual rules. All we have is a heuristic that seems to have worked out very well for science - there's no such thing as a fundamentally complex thing (e.g. a fundamental property of an electron cannot be something as complicated as human thought - such complex things need "moving parts", so to speak), and the simpler something is, the more likely it is to be true.

The second in particular is wildly misused by both the lay public and plenty of scientists - people are quick to claim that Newtonian physics are simpler than relativistic physics. But what matters here isn't how simple it sounds (or is to calculate) to human beings - that's going back to the anthropomorphism we started at. Quantum field theory's description of electro-magnetism is the simplest we've developed so far - despite the fact that it's rather easy to model the flow of electricity in a simple circuit using older models, while we can't really do the same thing with QFT. But the universe seems to prefer (right? :P) a massive amount of extremely simple calculations to a quick approximation.

The universe seems to be a set of a couple of fundamental rules. Everything we see around us is what happens when you apply those rules. This includes our observations of particles and their "behavior". All the confusion is just from applying human-like thinking to things that just aren't human-like at all.


I'm not going to explain this very well, but bear with me.

Noether's Theorem says that every conservation law can be expressed as a symmetry. That is:

  • saying energy is conserved is the same as saying physical behavior is time symmetric (physical laws are the same regardless of time)
  • saying momentum is conserved is the same as saying physical behavior is translationally symmetric (physical laws are the same regardless of where the particle is in space)
  • saying angular momentum is conserved is the same as saying physical behavior is rotationally symmetric (physical laws are the same regardless of where the particle is pointed)

And so on. In other words, these fundamental laws of physics are a way of expressing symmetries.

In a hand-wavey way, the conservation laws governing particles exist because the physical universe is as simple as it can possibly be. There are no special rules we know of that say that the universe behaves differently at different times or in different places. That is really all that these laws of physics are expressing.



The best answer will involve fields. We understand fields, but know where they come from. And remember that the maths and physics are just a model.

So each particle will add up all the forces that are being applied to it (forces are vectors, and forces in opposite directions [partly] cancel out). Now it has one force vector. Its mass is a constant. So it just accelerates in the direction of force, by an amount proportional to the size of the force. After the move the forces may change, so it just does it all again. It only has to do this one simple calculation, but over and over (once every plank time).

You can also do it this “Just in time” method, however it will take longer. The trick is that, a particle, can do this simple calculation, in one plank time (with no time to spare). It has no way to look ahead.

Things are not always particle, sometimes they are waves.


Unlike some of the others answering, I'll say this question has a very clear answer. And I will try to keep it short and sweet:

TL;DR: it's entropy.

What ultimately determines the movement of a particle is the net force applied to it. The fundamental field forces (those that happen from a distance without physical contact) [that we know of] are:

  1. gravity,
  2. the nuclear strong force,
  3. the electromagnetic force, and
  4. the nuclear weak force.

And then there are the contact forces:

  1. applied force,
  2. friction force,
  3. air resistance,
  4. the spring force, and many more (see: here)

To elaborate on your specific question, there is no calculation happening; imagine 4 people pushing on you from different sides. You don't have to calculate which way you'll fall -- you just fall in the direction where the resistive force is least.


Not the answer you're looking for? Browse other questions tagged or ask your own question.