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Action and reaction

  1. Mar 31, 2010 #1


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    Newton's third law of motion says that for every action there is an equal and opposite reaction.

    So how does that strong physical principle impact on philosophical questions like the creation of the universe (either by a prime mover like a god, or as some kind of QM fluctuation like the big bang). Where is the back-reaction there?

    In what sense did god's creation of the universe set up an equal and opposite action on him? Or for the big bang, where is its back-reaction?

    I ask the question mainly to highlight the standard confusion in people's thinking over causality - that there is a pusher and a pushee, so to speak. A cause and then an effect.

    Newton wrote the first two laws of motion in this spirit. He defined a local "prime mover", a force vector, first by the principle of inertia (so setting a zero baseline), then defining an acceleration (so the vector of action). But to complete the book-keeping of this scheme, he had to bring in the reaction - a mirror-image vector of action pushing right back.

    This is very weird and unrealistic. Until you step back and say Newton was just finding an economical way to deal up with the issue of global context, global constraints, in a mechanical description of the world.

    When you push to open a door, the equal and opposite action is not actually a mirror-image force vector that pops up on the part of the knob in contact with your hand. It is the whole atomic structure of the door, the frame to which it is attached, ultimately the earth to which it is anchored, that is the back-reaction. So it is in fact an asymmetry between a localised constructive push, and a global context of constraints, that explains why "a motion" occurs.

    Newton's genius was to make things much simpler than they really were (as he then did with gravity and "action at a distance").

    The success of mechanics (triumphing over the scholastic aristotelan physics in the popular self-congratulatory mythology of science) has now become embedded as a psychological given in people's causal thinking.

    But my point is, even Newton's laws included the full story of local~global causality. It just hid it. And even then, anyone who asks mechanical questions about the creation of the universe - such as what was the first push, the first cause - still ought to complete the Newtonian analysis by looking seriously for the equal and opposite reaction that should be in there as part of the mechanics.

    Interestingly, I would argue that action~reaction must also be a part of any level of mechanics - so it should be present in quantum mechanics and relativistic mechanics.

    And this would be what the observer collapse issue is all about in QM. The context that pushes back to collapse the wavefunction. Or even more concretely these days, the transactional or absorber interpretations of QM which actually treat the back-reaction as a mirror-image retrocausal action. (Again, like Newton, a promising simplification, and again like Newton, an over-simplification from the philosophical point of view).

    What about relativity? Perhaps others can help out here. But my initial thought is that the way spacetime is related in relativity is exactly the kind of causal accounting trick I am talking about. Push the speed and you get a mirror image distortion in the time and mass measurements. A more complex model, but still the same basic trick of reducing a "real" local~global scale asymmetry to an "unreal" local~local action~reaction symmetry.

    To sum up, philosophical questions are usually posed as problems of mechanical causation (from freewill to creation debates). Yet the foundational model of mechanical causation - Newton's laws of motion - includes a third law that gets generally neglected.

    1) What does this law really mean (it conceals the local~global scale story I say)?

    2) And how must it affect answers to those standard big philosophical questions?

    3) Or why would people be correct in disregarding it as they seem to?
  2. jcsd
  3. Mar 31, 2010 #2


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    I just spent several minutes trying to https://www.physicsforums.com/showpost.php?p=2646691&postcount=53" not to apply physics laws indiscriminately because it leads to junk science!

    Ah well... More power to ya...
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  4. Mar 31, 2010 #3


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    But you would agree that if you are going to apply physical laws in a philosophical setting, then you have to do so comprehensively?

    My argument here is that to base positions on Newton's first and second law, and not the complete causal package, is showing a lack of discrimination that so often leads to junk results.
  5. Apr 2, 2010 #4
    Newton's law's do not apply at the moment of creation.
    The physics involved AT THAT TIME were very different from now.
    Specifically how is a matter of debate. That it was different is not.

    Hopes this helps.
  6. Apr 3, 2010 #5


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    Yeah, unsupported, unreferenced, argument is always absolutely conclusive round these parts.
  7. Apr 3, 2010 #6


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    Newton's laws are emergent properties of the "true" fundamental laws (note, I'm not saying that I have any special access to the "true" fundamental laws of the universe, but I do think that we approach them as our time spent researching reality goes toward infinite).

    Inertia itself is an interesting concept that we still don't quite understand the nature of. Mach's principle is an interesting explanation about what gives rise to inertia. But consider the whole universe compressed so tightly that other fundamental forces outweigh gravitational effects. Would the property of inertia still have the same meaning?

    Anyway, the point is that Newton's laws makes assumptions and all of reality doesn't satisfy those assumptions.
  8. Apr 3, 2010 #7


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    Mach claimed inertia was due to motion with respect to the stars. I find that explanation unsatisfactory - how would they know? I perceive inertia is mainly a local effect. Inertia, at large scales, is probably as different as GR is from SR.
  9. Apr 3, 2010 #8


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    Thanks for a response with content.

    I would agree that classical mechanics would be emergent (as I take a developmental or process view of ontology generally, so what exists is always emergent).

    So I would also agree that the "laws of physics" would be different during the big bang and beforehand. But in a specific way. They would have been vague, where as now they are reasonably crisp.

    But all this was not the point I was making.

    My point was that people keep insisting on using mechanical thinking to try to answer fundamental questions. Yet they don't actually use all of that thinking. Somehow the principle of action~reaction is deemed irrelevant (at least I have never seen it mentioned). I would expect some argument would be needed to justify this move (a better argument than, well, everything was different way back when).

    The subsidiary point was then that Newton was smuggling in what systems thinking holds so important - the causal interaction between events and contexts.

    So first point is that if you claim to be taking a mechanical approach, then why doesn't the third law have consequences for you?

    And second point is that a systems thinker would take an enlarged view of action~reaction that actually makes a lot of sense in answering these kinds of questions.

    And regardless of its validity, I thought just for fun, people might want to try applying the third law to questions about universe creation and see where it leads. Where does it leave the unmoved mover required by so many thinkers?

    A quick google, for example, reveals that this question is being asked occasionally (but no sensible answers)...
    Existing is of course not necesssarily an "action" - god's presence would be inertial, no? :tongue:

    That which is not constrained is by definition free to happen. So a body has inertia to the extent it is not in interaction with its context, with its world. This is of course the Machian approach. And precisely the kind of wider view taken in systems logic (Mach was a systems guy).

    So imagine the universe compressed to a point as you suggest. In your view, there would now be no context, just all "local event" and so no greater scale source of constraints.

    My approach actually wouldn't allow me to imagine things that way - a singularity. Instead I would view the big bang moment as a state of vagueness - a state of symmetry where both events and contexts (atoms and void) are merged (and so can later jointly, synergistically, e-merge).

    But again, that is a different story. The question here is really meant just to get people as far as the first step of questioning the logic they want to bring to creation stories.

    If you are being confidently mechanical - talking in terms of efficient causality - then where does the third law fit into your scheme? If you haven't thought about the issue, why is this? If you want to reject its legitimacy, what is your actual rationale?
  10. Apr 3, 2010 #9
    Well, what is an 'action', Newton actually said that if an object applies a force to another object, it must receive the negation of the force back. Action thus means 'force', a physical magnitude which has an additive inverse. So what Newton more or less said is that the net force of the universe must always be null.

    Your argument here takes the word 'action' out of context to no longer only apply to force. Creatio ex nihilo is not a 'force', it is eh.. no idea what. But Newton never claimed that if particles magically appear, some particles must also disappear, in fact, I doubt Newton knew what particles were or believed in zero point creation or whatever.
  11. Apr 3, 2010 #10


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    Gimee a break. His response is no more unsupported and suppostional than your initial post. You want a rigorous answer, you'll have to start with a rigorous question.

    His challenge deserves to be addressed; your question is not pursuable until its premises are accepted. So, on what basis do you make your initial claim - that Newton's third law should apply to the creation do the universe?
    Last edited: Apr 3, 2010
  12. Apr 3, 2010 #11
    Since Newton's thinking developed based on patterns of language, thought, and culture that preceded him, the more relevant question might be which theological/creationist ideas was Newton's thought derived from? Virtue/vice, good/evil, God/devil, woman/man, creation/destruction, etc. are all dichotomous concepts like action/reaction. The question of how dichotomous thought and culture evolved in the first place would be very interesting, but I don't know how such an archaeology of knowledge would work.
  13. Apr 3, 2010 #12


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    This is a matter of philosophy so requires some flexibility of thought. I repeat, you need to pay attention to how Newton completed his mechanical description of reality.

    Yes, he identified a symmetry - and so the first law of conservation. And indeed, modern cosmology says that once there was an initial "push" - a quantum fluctuation that yielded perhaps a teaspoon of matter - then so long as there was a magically exact balance between spreading matter and its clumping gravity fields, then the expansion/cooling of the universe could continue forever. The action (or if you prefer "action") and reaction was in this case the action of the spreading mass (ie: expanding spacetime) and the reaction of the clumping gravity field (ie: shrinking spacetime).

    So the law of conservation - a symmetry - applies within the expanding universe. (OK, dark energy not being discussed here.) Yet that still leaves the initial push of a teaspoon of matter (or region of condensing inflaton field, or whatever substance you posit to get the whole thing started).

    Getting back to Newton, the deep thing he was doing was identifying the need for symmetry principles when it came to causality. To have a distinct action, or event, or any "local thing", you must also have crisply everything else that it is not. The global surround. Every event must have a context, every figure must have a ground, thesis requires anti-thesis, action requires reaction.

    But as I say, a fundamental difference here between his mechanical scheme and a systems thinking approach is that he reduced this symmetry to one where action and reaction were portrayed as having the same scale. Each force vector or action was represented of being the same size.

    A systems approach (or more correctly, a scale hierarchy approach) would make the other assumption. It would presume that when symmetry breaks, you get not mirror images, but instead asymmetry - a breaking that includes also physical scale. Which is why local~global is the fundamental concept in systems thinking (the whole is greater than its parts, etc).

    So packed into the third law - so often considered trivial and overlooked in reasoning about reality - is in fact a critical metaphysical difference in modelling.

    You can see in cosmology that people do indeed take action~reaction seriously. The need to account for a quantum blip of inflaton field, for example, has led some to posit the simultaneous creation of an anti-verse, a symmetrical partner to ours in the spirit of virtual pair particle creation (itself, like also the cooper pairs of superconductors, a kluge to make life simple when accounting for "actions" at QM level).

    On the other hand, other inflation models, like Linde's eternal spawning inflation, go for an asymmetric story (with thus fractal statistics, as powerlaws represent a system with an axis of scale symmetry). In Linde's model, the "action" would be the local cooling of spots in the field which could then grow away to become little universes like ours, while the "reaction" would be the rest of the field which symmetrically (scale-symmetrically!) grows away exponentially.

    Anyway, the challenge here again was to thinkers who like to talk about gods, prime movers and creation out of nothing, confidently using logical principles. Or more often, failing to apply them fully. I could equally have asked how do they stack up the first law of conservation against their reasoning? But the first law is framed as a global systems statement (teleological cause), whereas action~reaction is a local scale statement that goes more directly to the notion of effective cause. Which is how people are thinking when they say things like a local god created a local universe as a localised act.

    Newton's mechanical approach to causes had a huge impact on Western thought as we know. It justified people to think only in terms of linear effective causality - cause and effect. So to remove the blinkers, it is a useful exercise to go back and see the little accounting trick that Newton actually played. Then in philosophy, it becomes possible to see when it is the right trick to continue playing, and when actually you need to switch to a more complex systems, or Aristotelean, approach to causality.
  14. Apr 3, 2010 #13


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    You clearly either haven't read what I wrote, or just failed to understand it.

    In philosophy, do you have to "believe" premises to be true to explore their consequences. No, proof by contradiction is a standard approach.

    So yeah, gimmee a break. If the third law seems so true in physics, where are its consequences in creation debates, let's hear what they would look like? And then, do we like the look of them? If not, then what do we infer from that?
  15. Apr 3, 2010 #14


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    (Tearing hair out quietly) well I can tell you. Having studied this now some 30 years.

    It is the basis of all metaphysics. It is the whole of greek philosophy.

    Anaximander, the first philosopher, put forward the idea of a vague potential - the apeiron - which broke via a series of dichotomies. First the aperiron separated out into the hot and the cold. Then this separated out further into the dry and the moist. These "materials" mixed to create fire, earth, air, water, and then all the thing composed of these elements.

    Then other set peice debates started up. In metaphysics 101, you will be told Heraclitus said all was flux, while Parmindes said all was stasis. The joker Zeno raised the question of whether reality is fundamentally discrete or fundamentally continuous. The atomists worked up their world view based on local atoms in a global void. Plato and Aristotle debated on whether reality was form and substance, or was one more primary, more essential.

    All philosophy is about suggesting thesis and so necessarily also discovering anti-thesis. You cannot have an action without a reaction! You cannot have a crisply defined concept without also its eqaually crisply definite negation. Aristotle called it the law of the excluded middle. It is the foundation of logic.
  16. Apr 3, 2010 #15
    Sorry, I should have put this in my post before this one, but this one reminded me of something I forgot:

    I think that the metaphysics of energy and matter vis-a-vis gravity as opposing aspects of the creation were present in the logic of Moses climbing a mountain to discover God and also the language of death as a "valley." Darkness and lowness were conceived as points of diminished energy whereas light and altitude were recognized as points of high energy or potential energy. To the extent that life was associated with energy and death with diminishing life/energy, there was action/reaction in the early biblical scripture.

    God and heaven are associated with infinite altitude and energy (unlimited creation of life), and the reaction to divinity is described in terms of "falling" into temptation, "descent" into hell, "falling" from grace, etc. The logic is that sin "pulls" people away (down) from high spiritual consciousness and life and that "the wages of sin is death," implying that if one descends enough from divinity and grace, the consequence is death and hell (below).

    Divinity and good living are analogized with climbing a mountain, i.e. a difficult act of investment in building up potential energy and greater vision (you can see further from higher places), but the more potential energy you build up, the more temptation there is to "cash in" that potential by going down instead of further up.

    I don't know if Newton's ideas about action and reaction were directly derived from these basic metaphors in the biblical theology, but I can't imagine that the two would be completely independent.
  17. Apr 3, 2010 #16


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    Christian mythology would be too narrow a focus here I feel. You are correct that (influenced by Plato and Aristotle in particular), the Christian church seems strong on dichotomies. But also woven in there are Platonic triads (the division of the body into base desires, noble feelings and pure reason - the basis of later alchemy...which did directly influence Newton). So tracing the full lineage of thought does quickly get complex.

    Heaven and hell as opposed directions does seem a natural and strong division. And it seems to fit Anaximander's original cosmology (and aristotelean physics) in that reality is created via the opposed forces of levity and gravity. The dry and the hot rises (so creating the sun and stars) while the moist and cold falls (so making the base realm of the material earth).

    However, greek, roman (and egyptian) mythology did not make exactly the same division. The gods were up on mt olympus perhaps, but human souls remained in the underworld, though if lucky, they could find their way to the Elysium islands.

    Anyway, you raise an interesting question of how exactly Newton arrived at the third law. What directly inspired him? He did invent his mechanics as a result of having a copy of Galileo and some atomists texts to read during his country escape from the plague. So he was responding to science rather than religion.

    I always presumed the third law arose as a consequence of the second (to make the imaginative leap to accounting for any action as a localised force vector, he needed to already be aware of the larger symmetry he wanted to break, and so the fact a reaction was also being created).
  18. Apr 3, 2010 #17
    Maybe more complex than I had first imagined.

    Ever wonder why the apple fell out of the tree in the story of Newton? I wonder if there's some connection with the fruit picked from the tree of knowledge by Eve under advice of the serpent, in the creation story.
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