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Is action and reaction instantanious?

  1. Aug 22, 2008 #1
    I know very little about physics, so I hope this is a pretty easy question.

    Action is always acting and reaction is always equal. Is this action and reaction instantanious, and if not, why not?

    I read around quite a bit but this alludes me.

    I hope my query is worthy of consideration. I can't quantify for myself if its a good question or not.
  2. jcsd
  3. Aug 22, 2008 #2


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    Staff: Mentor

    Yes, they are instantaneous.
  4. Aug 22, 2008 #3
    So all action and reaction in the entire universe is within an instant.

    And action is reaction.

    Which is not even action, since it is zero in an instant.

    Is this logically flawed?

    And are we within the bounds of physics?
  5. Aug 22, 2008 #4

    Doc Al

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    The terms "action" and "reaction" are somewhat problematic (and outdated) as they imply that first comes the "action", followed by the "reaction" (as the terms are used in non-physics, everyday contexts). A better term is "3rd law pair". For example, consider a contact force between two bodies: Body A and Body B exert forces on each other. The two "3rd law pairs" are: The force that A exerts on B and the force that B exerts on A. There is no implication of time order--they are two parts of a single interaction.

    (Of course the 3rd law gets a bit more complicated when fields are involved.)
  6. Aug 22, 2008 #5


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    If you are talking about Newton's third law, then it is instantaneous. But this doesn't describe our universe well at such speeds (I'm talking about the speed of light in vacuum). At most it could have the speed of light. That means that if the sun suddenly disappear, the Earth would suffer changes in its orbit about 8 minutes later and not instantaneously as Newton could have thought. I'll study this law more in details very soon, but I guess I'm right saying that the gravitation is an action-reaction law. If I made an error, just tell me, thanks.
  7. Aug 22, 2008 #6


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    The 3rd pair law action and reaction are simultaneous. However there is a delay between the time a force is initially applied to a system and the time when the system stabilizes, due to deformation of the objects involved (compression or tension at the point of application of forces).
  8. Aug 23, 2008 #7
    Thank you everyone.

    My thoughts are:

    The first action is simultaneous with the last, and all action in between (universally speaking), so all action is instantaneous and singular in nature. (like in 3rd pair law).

    Action is not of time (and perception of seemingly endless action and reaction is remarkable.)

    So action itself is not of matter.

    The action apon "matter" made frequency movement in a photon (massless) and light speed established energy form transfer rates equal to time and the "rate of perception".

    The quantum thoeries seem heavily dependent on the observer.

    So time itself is within the first (and only) action which is zero in that instant.

    So perception is more a prerequisite for time/space/matter and not so much a consequence of the movement of "matter". (However, the action itself is singular.)

    Do my logical steps still hold water and are they in keeping with quantum theories?

    I deeply appreciate your indulging me as I am not well versed in physics. (oh, you noticed).

    And I apologise in advance if this post is inappropriate.
  9. Aug 23, 2008 #8


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    Honestly, I can't make heads or tails of what you are trying to say there. It just looks like gibberish to me.
  10. Aug 23, 2008 #9


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    Where did "singular" suddenly come from?

    No, no one has said that. Action (and reaction) certainly can occur over time. The only thing that has been said is that the "action" and "reaction" occur at the same time.

    That's gibberish. "action" and "reaction" do not necessarily have anything to do with photons and I have no idea what you mean by "rate of perception".

    I cannot say anything about your "logical steps" since I see none here.

    Last edited by a moderator: Aug 24, 2008
  11. Aug 23, 2008 #10
    yes, I have been told is is off.

    I tried to edit the post and couldn't so I hope to re-direct soon.
  12. Aug 23, 2008 #11
    I must have lost the edited post so I'll try again.

    I see that action and reaction are a single action by the 3rd pair law.

    If the first action reacts instaneously and so on, all action is instantanous.

    All action is momentary.

    The primary particals (strings, loops, photons etc) are massless.

    So an infinitely small amount of energy or action is required for movement as matter is infinitely massless.

    The entirety of time is the duration between the first and last actions which is instantaneous. (By a chain of action and reaction begun by the first action.)

    Perception of time is the observation of movement, but in fact no time elapses between the first and last actions as they are a chain reaction of instant 3rd pairs.

    I'm suggesting that time/space if of perception, and 3rd pair instant action is the reality.

    I hope this is clear. If it is just gibberish I'll refrain, I think there's logic in it and would appreciate if the flaws could be pointed out.

    Thanks everyone for indulging my rather naive notions and I hope my post is appropriate.
  13. Aug 24, 2008 #12


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    What "3rd pair law" are you talking about. No, no one has said that "action and reaction are a single action". They are separate "actions" that occur at the same time.

    "Simultaneously", not "instantaneously". Those are very different words.

    You have been told repeatedly now that "action" and "reaction" occur simultaneously. That does not imply either "instantaneous" or "momentary".

    You must be using the wrong word here. Primary particles are not all massless nor are strings and loops particles.

    Where did you get that idea? I'm made of particles and I am definitely not massless!

    What do you mean by "first and last action"? If you mean an action and reaction, there is no "duration between the first and last actions".

    Now, what do you mean by "3rd pairs"? It is hard to understand what you mean because you keep introducing new terms ("first and last actions", "3rd pairs") without defining them.

  14. Aug 24, 2008 #13

    Doc Al

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    "Action" and "reaction" are two forces associated with a single interaction. The terms "action" and "reaction" are leading you astray--I advise dropping them.
    Again, you are taking the everyday meaning of the words "action" and "reaction" as if one happens first, followed by the other, which "reacts" to the first. Not so. They act together.

    Example: My hand hits the wall. Which happens first: My hand hitting the wall or the wall hitting my hand? Neither. They are aspects of the same interaction.

    There is no "chain" of 3rd law pairs. There can, of course, be a causal chain of interactions: My shoulder pushes my arm, my arm pushes my hand, my hand pushes the wall. But these are not third law pairs. And they are certainly not "instantaneous".
  15. Aug 25, 2008 #14
    Thanks everyone,

    It is very helpful. I'm not very eloquent having little maths "language".

    I hope I'm not imposing by furthur posting but am left with one query.

    Please indulge me even though my previous posts have eroded my credibility.

    If the first action simultaneously reacted with the second which in turn simutaneously propagated the third and so on, would the very last action be simultaneous with the very first?

    And being simultaneous, would this have an instantaneous effect, seeing it is simutaneous for the duration of first action?

    I would think the very first instant of action is simultaneous with the very last instant.

    Thanks again all. I have developed a great interest in physics now I realise that it lends credibility to "whack" theory. I like the site, but will study more so I can communicate more effectively before further posting. I look forward to reading the insightful posts of everyone here in the meantime.

    In appreciation, Throng.
  16. Aug 25, 2008 #15


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    What are these other actions and why would they necessarily be simultaneous with the first? What you are saying implies to me that you think forces propagate through an object instantaneously. They don't.

    It may be instructive to consider what happens in the real-world situation of a hammer hitting a nail. Metals are almost perfectly elastic over a certain range of forces, so in this case, the objects can be modeled as collections of tiny springs. The action and reaction that occur simultaneously at impact are a force at the impact point and compression of the spring. But the compression takes a finite time to happen - the force takes time to build, and the particles time to accelerate. So the force and acceleration of the objects propagate through the objects in a pressure wave that travels at the speed of sound.
  17. Aug 25, 2008 #16

    Doc Al

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    Nope. If the first and second "actions" are 3rd law pairs, then the second and third are not. There are only two forces in a 3rd law pair, not a chain of forces.

    You can certainly have a causal chain of forces (as I and Russ have been pointing out), but that is not a chain of "action/reaction" pairs and they do not happen simultaneously.
  18. Aug 25, 2008 #17


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    You have been told several times now that there is no "first" or "second" in the so-called "action" and "reaction". Now what do you mean by "third and so on"? What forces are you talking about?

    You seem to be thinking now of a series of forces. What does that have to do with your original question about "action" and "reaction"?

    ??What do you mean by "whack" theory? I do hope that's not some "whacky" theory you made up yourself- after telling us you know nothing about physics.
  19. Aug 26, 2008 #18

    Thanks, I am now far better informed and appreciate that.
  20. Aug 27, 2008 #19
    Though the OP is, apparently, satisfied by the response on this thread, the issue itself warrants further examination.

    Action/reaction scenarios do not have a simple, single answer, as this universe is not made of billiard balls.

    Whereas A/R(Action/Reaction) could be considered a paired event, it could be argued that action precedes reaction. In the macroscopic world, a judicial reaction to a criminal act can normally only occur after the criminal action.

    In that sense, A/R is valid, whereas R/A is not. Therefore, in that scenario action and reaction is paired but action comes first.

    Or, one starts a small fire(action) within a dry forest and the fire spreads to a thousand acres(reaction)
    It would be absurd to assume that the entire forest is burned down before the action of the initial small fire.

    But back to physics, the question is if A/R can be "flipped" to R/A and remain integrity.
  21. Aug 27, 2008 #20


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    You seem to think that there is some objective distinction between "action" and "reaction". There is not. When I push against a wall, it pushes against me. Either can be considered the "action" and either can be considered the "reaction".
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