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I Gravity is a fictitious force

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  1. Oct 28, 2015 #1
    If gravity is a fictitious force, why some scientists try to unify it with other real forces of nature?
     
  2. jcsd
  3. Oct 28, 2015 #2
    You can view it the other way: they are trying to make the other 3 forces work with GR. Our current formulations of QCD etc. only work in flat space.
    Plus, we still don't know what is spacetime, so it's natural to try to find a theory explaining its properties. One such approach is quantization of spacetime, other approaches pretty much don't exist.
     
  4. Oct 28, 2015 #3
    In my eyes that's just a word game, using a peculiar set of definitions of "gravity", "force" and "fictitious". The contact force that I feel now, while sitting in my chair, is very real (deforming my buttocks approx. in accordance with Hooke's law) and it's definitely caused by the Earth's gravitation. And I won't participate in discussions about word games. :wink:
     
  5. Oct 28, 2015 #4

    PeterDonis

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    "Gravity" is an ambiguous term. When scientists talk about unifying "gravity" with the other "forces", they aren't talking about the "force" that makes a rock fall. What makes a rock fall is indeed not really a "force", for a variety of reasons (for example, you can't measure it with an accelerometer); but it is a manifestation of an interaction involving matter, energy, and spacetime in some way. Scientists would like to be able to understand that interaction as part of a unified picture involving all of the known interactions, because if we could do that, fundamental physics would look simpler.
     
  6. Oct 28, 2015 #5
    It is really hard to tell whether when posters assert that gravity is a fictitious force they are simply playing word games(I speak from recent experience discussing these notions around here) as suggested by #3, but I wonder if for instance #4 is drawing some kind of distinction between the term interaction and the term force, if so I'd be curious to know about it. Are gravitational tidal forces the non-ambiguous term that the OP should be referring to? Are the latter fictitiour or not?
     
  7. Oct 28, 2015 #6

    Dale

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    The tidal effects of gravity are not fictitious, so they do need to be unified with the other fundamental interactions. The "fictitious force" aspect is already unified by the fact that the other interactions can already be expressed in a manifestly covariant manner.
     
  8. Oct 28, 2015 #7

    PeterDonis

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    Yes, it is, at least in ordinary language. (In the language of, for example, particle physicists, the two terms mean the same thing; they both refer to the same terms in the mathematical equations.) A "force", as I was using the term in #4, is something you can measure with an accelerometer. An "interaction", as I was using the term in #4, is basically the thing that a particle physicist would call an interaction (or a force).

    The term "gravitational tidal forces" is ambiguous too, because we can measure the effects of tidal gravity purely by looking at freely falling objects, which are not subject to forces in the sense I used the term in post #4 (accelerometers attached to them read zero). What are often called "tidal forces" in pop science treatments are really non-gravitational forces (electromagnetic in the most common cases) exerted by one part of an object on another when the object as a whole is moving through curved spacetime.

    The reason these distinctions are important is that without them, people like the OP are led by our imprecise language to ask questions like the one in the OP, whose answers would be obvious (if the questions even occurred to them in the first place) if our language were more precise.
     
  9. Oct 28, 2015 #8

    Dale

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    The contact force is definitely not caused by the Earth's gravitation. As suggested by the name, it is caused by the contact with the chair (and Hookes law).

    It points up, not down. If you remove the chair then you remove the force without removing Earth's gravitation. If you do any number of alternative motions you can replicate the force with no gravity. And so forth.

    Saying that it is definitely caused by Earth's gravitation is simply untrue.
     
  10. Oct 28, 2015 #9

    Dale

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    +1 on this. Also, there are tidal effects involving time that I think are not reasonable to label as "forces". I refer to say "tidal effects", although I am not perfectly consistent in doing so.
     
  11. Oct 28, 2015 #10

    Dale

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    This is not simply semantics. There are physical similarities between fictitious forces and local gravitational forces.

    What is semantics is whether you choose to group the "fictitious" and "real" forces according to physical similarities or according to some other criteria. The modern convention is to group them according to their physical characteristics rather than according to a historical categorization.
     
  12. Oct 28, 2015 #11
    I can't bear the suspense, can you unveil the ultimate unambiguous term that should be used and formulate the OP's question precisely, as it should be asked?
    Also I feel for the poor OP, it's his second question in more than three years, I bet the next one will take him a decade to elaborate so let's not take it out on "people like the OP", shall we?
     
  13. Oct 28, 2015 #12

    PeterDonis

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    There isn't one. The unambiguous expression of the physics involved is in math, not ordinary language. Using the unambiguous math, the OP's question cannot even be posed.
     
  14. Oct 28, 2015 #13
    Then I think we should avoid talking about "gravity" being a "fictitious force" altogether (outside the Newtonian context), don't you think?
     
  15. Oct 28, 2015 #14

    A.T.

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    Or simply irrelevant to physics, as much of such causation musings.
     
  16. Oct 28, 2015 #15

    PeterDonis

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    My personal preference is to avoid using the word "force" at all in a relativistic context except in the sense I used it in post #4, to refer to something that can be measured by an accelerometer.
     
  17. Oct 28, 2015 #16
    The term fictitious force, as it's used in physics, is a classical physics term. Fictitious forces would be the centrifugal force you feel rounding a curve in your car, or the force you feel pushing you forwards as you apply the brakes. One doesn't normally refer to gravity as a fictitious force. In Einstein's treatment, gravitation is a geometrical phenomenon. I suppose you see it referred to as a fictitious force by some authors of popularizations, but I don't recall seeing that term in serious treatments of gravitation.

    Unification, as we see it practiced by physicists today, is a purely quantum mechanical construct, so it can't really be compared in the way you propose.

    Einstein's treatment of gravitation is a purely nonquantum theory. My understanding of these unification efforts is that they are all about creating a quantum theory of gravity. But I'm by no means an authority on the subject.
     
  18. Oct 28, 2015 #17
    Altogether, yes. Without the caveat regarding the newtonian context, because in newtonian physics gravity is a real force.

    One thing that distinguishes real forces from fictitious forces in newtonian physics is the presence of a Third Law partner. A real force is an interaction between two objects, a fictitious force isn't.
     
  19. Oct 28, 2015 #18

    A.T.

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    In a local context with clarificatin to what the term refers it seems fine to me.
     
  20. Oct 29, 2015 #19

    haushofer

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    Even that is debatable. The equivalence principle also holds for the Newton potential, i.e. it can always be gauged away by a local acceleration. Newtonian gravity can also be described as spacetime curvature, by Newton-Cartan formulation.
     
  21. Oct 29, 2015 #20

    vanhees71

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    I'd ban the expression "ficitious force" from the vocabulary of physics and rather talk about "inertial forces". The equivalence principle (weak form) tells us that at small enough space-time regions one can always introduce reference frames, where the laws of special relativity are valid (concerning all local phenomena). Particularly gravity can be "eliminated" by the choice of such a locally inertial refernce frame. A true gravitational field, however can never be completely compensated globally, which distinguishes it from the inertial forces appearing in special relativity from the point of view of non-inertial observers.
     
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