What is the scientific definition of a force?

Discussion in 'Physics & Math' started by Magical Realist, Mar 22, 2022.

  1. Magical Realist Valued Senior Member

    How can a force physically exist? How can there be a pushing or a pulling in empty space without something there doing the pushing and pulling? IOW, how can there exist a pure action at a distance? And what is the medium of a force's transmission in empty space?
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  3. Michael 345 New year. PRESENT is 72 years oldl Valued Senior Member

    Force does not physically exist
    There can't
    Ditto as above
    Not required

    Point 1 - Force is determined by the measuring the attraction between two bodies or how much is expended when stuff collides

    Point 2 - Should be obvious given Point 1 explanation

    Point 3 - Ditto Point 2

    Point 4 - Gravity travels in waves and as per light does not require a medium

    Gezzzzz I hope I'm correct at 4:30am and no coffee

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  5. DaveC426913 Valued Senior Member

    There are only four fundamental forces that need modeling.

    In quantum mechanics, three of them are modeled as (virtual) particles. called bosons (W and Z, as two examples) it is the exchange of these particles that gives rise to the three forces.

    The fourth - gravity - is hypothesized to occur in QM by the exchange of gravitons, but that still awaits experimental validation. Right now, gravity is explained by GR - it is not a force at all, but simply the curvature of spacetime.

    Key words to explore:
    QM, Bosons, Fermi Diagrams, Standard Model, force carriers

    The Standard Model, in Fermi Diagrams:

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  7. Michael 345 New year. PRESENT is 72 years oldl Valued Senior Member

    OK sure

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    Is any of my explanation so bad (have mercy it was 4:30am and I was without coffee) catastrophically incorrect?

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  8. DaveC426913 Valued Senior Member

    Uh, well. Since you asked....
    What "physically exists" means is a pointless semantic argument.

    The proof is in the pudding. If we can measure it, predict it, use it, bend it to our will - to make our machines go, then it exists in any way that is meaningful.

    The force is transmitted in the form of virtual particle exchange. But no medium, no.

    Force is no more "determined by measurement" than my mass is "determined by a weigh scale".

    Force (and mass) are properties of the masses involved. Which we can measure. But they're sure there even when we don't measure them.

    Gravity does not travel in waves.
    Gravity has a value at every point in spacetime. It is always there and never travels.
    Changes in gravitational strength travel at c.
    Oscillating changes will propagate as waves.
  9. exchemist Valued Senior Member

    I don't disagree with this but I'm never really sure it explains much. It just seems to me a different model, using potentials associated with these virtual particles, rather than Newtonian forces as such. There's a rather complicated article about it all here: https://en.wikipedia.org/wiki/Static_forces_and_virtual-particle_exchange

    One useful point it does make is that you don't actually need any of this virtual particle stuff in non-relativistic QM.

    (And it's always worth bearing in mind that virtual particles are not really particles - dat why dem call virtual

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    . They are transient disturbances in the relevant quantum field that have some of the attributes of the corresponding particle.)

    P.S. But I've used the "p" word, so I'd best get out of here before Write4U shows up to deluge us in shite about potentials...enfolding and unfolding.....ZZzzzzzz.......

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  10. Michael 345 New year. PRESENT is 72 years oldl Valued Senior Member

    Not so. To physically exist means to have mass

    If gravity had mass, totally different animal

    My breakfast calls

    Thanks for the other stuff. Perhaps will digest after breakfast digest

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  11. Magical Realist Valued Senior Member

    So light doesn't have mass. Does that mean it doesn't physically exist?
  12. Michael 345 New year. PRESENT is 72 years oldl Valued Senior Member


    Anything with mass cannot travel at Lightspeed

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  13. DaveC426913 Valued Senior Member

    If your definition of "things that don't exist" includes things we can measure, capture and use, then what use is such a definition?

    That's a rhetorical question.

    I would ask, moving forward, that when attempting to answer another member's sincere question for knowledge - if you feel you only have your personal opinion to offer, instead of accepted knowledge - you ensure you enclose your comments in IN MY PERSONAL OPINION, or some such.
  14. Michael 345 New year. PRESENT is 72 years oldl Valued Senior Member

    Rhetorical aside - you missed out PHYSICALLY exist

    Stuff which has physicality has a whole batch of properties which stuff which exists but does not have physicality

    Stuff without physicality likewise has properties stuff with physicality does not have. Chief among them would be the ability to travel at Lightspeed

    Are scientists being mis-quoted when they talk and write about looking for gravity waves

    Also is there not a debate along the lines of
    "if the Sun were to vanish instantly we would not know about such an occurrence until 8½ minutes later?"

    Both light and gravity waves taking that time to reach Earth

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  15. DaveC426913 Valued Senior Member

    No. Gravity waves do happen. They just don't have to be waves.
    I can look for waves in a lake caused by some disturbance, but that doesn't mean the only form lake water is that of waves.

    Saying "gravity travels at the speed of light" is analogous to saying "lake water travels at 5mph".

    Correct. Changes in the strength of gravity propagate at the speed of light.

    In this scenario (which can't physically happen) there would be a single, sharp drop in the local gravitational attraction. A trough with no crest, no wavelength, no period and no frequency.

    It doesn't have to be a wave.

    The Petitcodiac River, in Moncton Nova Scotia experiences a tidal bore twice a day. This manifests as a single crest of high water that advances up the river, driven by tides in the Bay of Fundy.
    It is a single crest, with no trough, no period, no frequency and no wavelength.

    This may seem a bit nit picky (as well as some what off-topic) but it is wise to distinguish the lake from the waves.

    Last edited: Mar 23, 2022
  16. Mr. G reality.sys Valued Senior Member

    Force is expressed in terms of "moderation", "content control", and a whole spectrum of other expressions that try to control you.

    Force is also punching back.

    Breaking a nose can be cathartic.
  17. exchemist Valued Senior Member

    This is a science thread, in the Physics and Maths section.
  18. Yazata Valued Senior Member

    I don't know.

    I'm tempted to call upon F = MA and define it in terms of mass and acceleration. Except that we would need a definition of mass, and mass is often defined as a measure of inertia, of how much a given force will accelerate an object. And that threatens to be circular.

    All the talk about bosons is pretty much incomprehensible to me. I don't understand it, though I suppose that it communicates something to physicists.

    I guess that sometimes I think of ideas like 'force' and 'energy' when they are being used in physics, as something akin to accounting gimmicks, quantities that make the equations of physics come out as desired when measurements of quantities that are more directly observable are plugged into the equations.

    And that highly abstract idea is given the name 'force', which kind of hijacks the age-old common-sensical meaning of that word which describes pulls and pushes in normal life. The use of the same word for two very different concepts might arguably generate confusion. (See below.)

    How can anything exist? I'm not certain what forces are or what kind of existence they have, so I'm certainly not in any position to explain the existence of forces.

    Action at a distance is an age-old question and there's a huge literature on it. It was a huge topic of discussion and controversy in classical physics and despite assertions that it's been solved today, I'm still not convinced. The argument often seems to be something along the lines that early modern atomistic 'billiard-ball physics' only recognized transmission of force by physical collisions. But we don't think that way any longer, so it's no longer an issue for us. Which kind of reduces it to it just happens, accept it.

    Or that it's implicit in the idea of a field, which threatens to become circular if fields are defined in terms of action at a distance. Just giving something a name doesn't supply it with a mechanism.

    I guess that Einstein tried to explain it for gravity in terms of the geometry of the space in which an object is located, but I don't know how generalizable that approach is to other actions at a distance.

    Maybe it has something to with quantum non-locality, I don't know. Just guessing there.

    Despite all the brave talk, I'm not 100% convinced that anyone really knows.
    Last edited: Apr 21, 2022
  19. James R Just this guy, you know? Staff Member

    F=ma is not circular. However, it requires that two of the three quantities be defined independently. Having done that, the third quantity is then defined by the equation.

    The definition of acceleration is relatively simple and transparent. With that in place, we then have a choice: we can either define mass and let F=ma define what force is, or we can define force and let F=ma define what mass is.

    The original approach (back in Newton's time) essentially chose to define force in a common-sense way, as the strength of a push or a pull. In practice, a reliably reproducible force could be produced experimentally in various ways. One way would be to use a spring, whose force varies linearly with the distance it is compressed or stretched. Experimentally, then, we find that the measured acceleration of a mass pushed by a compressed spring (say) turns out to be directly proportional to the applied force.* If, for example, we draw a graph of F vs a, then we find a straight-line relationship, experimentally. We then define the gradient of that straight line to be the mass of the pushed object. Intuitively, mass is a measure of the object's inertia, or resistance to acceleration.

    The other approach - the more modern one - is to start by defining mass. Mass, roughly speaking, is currently defined as a certain measure of the amount of "stuff" in an object. Regular macroscopic objects are made of atoms, each of which has a characteristic mass. The mass of an object is, in principle, just the sum of the masses of its constituent atoms. Having defined mass and acceleration, we then define force F as the force required to give an object of mass m an acceleration a, using F=ma.

    There are more details, of course. Until around 2019 (?) the standard kilogram was defined with reference to a specific physical object kept in Paris. Now it is defined with reference to Planck's constant, which itself now has a defined value in the SI unit system. The reason for doing it this way is that Planck's constant eventually became measurable to far greater accuracy than the mass of the standard Paris kilogram cylinder.
    That's true to an extent. However, definitions like F=ma are not completely arbitrary. They are defined the way they are because they make the prediction and interpretation of experimental results easier. Suppose that instead of defining force F by F=ma, we chose instead to define force as F=ma^2. On paper, there's no problem with that (other than raising calculational and theoretical difficulties), but experimentally it vastly complicates things. Now, force (as defined) has a non-linear relationship to acceleration. Double the force on a given mass and the acceleration now is found experimentally to increase by a factor of sqrt(2). In simple cases, such things might not matter much, but a lot of other physical principles depend on the definition of force. Over all, it makes for a massively greater amount of work and complexity to start with F=ma^2 rather than the more intuitive F=ma.

    Occam's razor suggests that scientific theories should strive to use the simplest possible theory that explains phenomena. If we put F=ma up against F=ma^2, then the former definition wins the battle for simplicity hands down.

    I might also mention that, of course, the intuitive notion we have of a force as a push or a pull is more in line with common sense idea of F=ma, as opposed to F=ma^2. We expect that if we push something twice as hard (according to our perception), then something about its motion should be twice as big, not sqrt(2) times as big. Of course, it took us centuries to appreciate what the particular "something" that doubles actually was, so it's not all about "common sense".
    A physical field, fundamentally, is something that has a value at every point in space at all times (different values at different points). The modern picture of interactions is that "forces" are transmitted by excitations (a bit like waves, but also a bit like particles) in the relevant fields. This is not, technically, action at a distance, because something has to physically travel from the "source" location to the point where the "force" is felt. What travels are those carrier bosons you mentioned.
    This is where I start to get out of my depth. However, I think that string theories are attempts to generalise that approach beyond gravity. This seems to be possible, but it comes at the expense of introducing extra dimensions of space which are currently unmeasurable.

    * Perhaps using a spring is not the best example here, because the spring force varies continuously as compressed a spring pushes on an object. But you get the idea, hopefully.

    P.S. Thinking about this some more, we could use a compressed spring to push on and accelerate an object with a constant force. We just have to make sure that we move the spring with the object in such a way that the compression of the spring stays constant during the object's acceleration.
    Last edited: Apr 22, 2022
  20. Mr. G reality.sys Valued Senior Member

    You are quite correct. My bad.
  21. Mr. G reality.sys Valued Senior Member

    Word salad.

    Occam's razor is "don't multiply complexities unnecessarily."

    Verbosity is a violation.

    Be concise, if you want your acolytes to still be awake when you're done.
  22. exchemist Valued Senior Member

    Use the "Reply" function, so that we can understand what post you are referring to.
  23. arfa brane call me arf Valued Senior Member

    A force is a vector. So is a velocity or an acceleration. Which ones physically exist?

    You understand you have to do work to get a rigid body spinning about an axis of rotation, and you need to know its moment of inertia. This has units of kilogram square metres.
    Why doesn't it have just a value?

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