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Your favorite definition of physics

  1. Oct 6, 2011 #1
    What is your favorite definition? And what is your favorite definition of science? I.e. which are the differences between physics and other sciences as chemistry or biology?
  2. jcsd
  3. Oct 6, 2011 #2
    Some moderator moved this from the general physics forum to this PF Lounge forum.

    Does this mean that moderators think that introductory chapters defining physics and its relation to science would be eliminated from general physics textbooks? Thanks.
  4. Oct 6, 2011 #3
    Only moderators can tell what they think. Maybe they feel your question is not technical, very general and not even restricted to physics. General physics textbooks sometimes have such an introductory chapter, which is really general discussion. In fact, I recall a wonderful quote from Witten's IAS lecture on Symmetry Breaking :
  5. Oct 6, 2011 #4
    The best definition of science in one minute I know of
    Last edited by a moderator: Sep 25, 2014
  6. Oct 6, 2011 #5


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    Physics is the subset of mathematics that is easy. o:)

  7. Oct 6, 2011 #6
    Physics is the subset of Math that matters. o:)

    Not joke...
  8. Oct 6, 2011 #7


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    :approve:...he's such a badass...
    Last edited by a moderator: Sep 25, 2014
  9. Oct 6, 2011 #8

    George Jones

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    Physics is the orthogonal complement of ...
  10. Oct 6, 2011 #9
    Physics is the subset of mathematics that, as Feynman said, is charged with the duty of having to prove herself against observed reality.
  11. Oct 6, 2011 #10


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    Similar to the one that humanino posted.

    Physics = branch of science concerned with the study/knowledge/understanding of the nature/behavior/relationships/interactions and properties of matter and energy, and more generally Nature (includes the universe).

    Science = a : knowledge or a system of knowledge covering general truths or the operation of general laws (or theories) especially as obtained and tested through scientific method b : such knowledge or such a system of knowledge concerned with the physical world and its phenomena (Defn 3 from Merriam-Webster online)

    The systematized approach of study applies to chemistry, biology, and various disciplines of engineering or applied physics.

    Mathematics and physics are complementary.
  12. Oct 7, 2011 #11
    Feynman said the contrary.
  13. Oct 7, 2011 #12
    Feynman emphasized that physics is not a subset of math, and that the comparison against reality is outside mathematics. He notes the difference between a ray light (optics) and a line (geometry).
  14. Oct 7, 2011 #13
    The first definition collides with the usual definition of chemistry as the branch of science concerned with the study... and properties of matter and its transformations. The first definition seems more the definition of natural science (of which physics is a subset).

    About the second definition. I think that not everything in science is about "general truths", or "general laws". E.g. Newton's laws are not general laws valid everywhere.
    Last edited: Oct 7, 2011
  15. Oct 7, 2011 #14
    My favorite definition and the broadest is that physics is the study of motion. However, for those who prefer techno-babble:

  16. Oct 7, 2011 #15


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    Engineering is applied physics and physics is applied mathematics.
  17. Oct 7, 2011 #16


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    Chemistry is a subset of physics - on the atomic/molecular level. Of course, there is chemistry, physical chemistry, chemical physics, chemical engineering, . . . .

    And there is condensed matter physics and materials science.
    Last edited: Oct 7, 2011
  18. Oct 7, 2011 #17
    Physics is not a synonym for kinematics (subset of mechanics).
  19. Oct 7, 2011 #18
    As noted above, Feynman disagreed.
  20. Oct 7, 2011 #19
    Instead of being broad, that's only classical kinematics. The problem with this definition is that motion : "the process of changing place", implies both position and velocity to be defined at every instant along the trajectory, thus excluding quantum mechanics for instance.

    I can easily find more branches of physics which are not "the study of motion". The most obvious one is "statics", the study of distributions of stresses in solids and pressures in motionless fluids. Of course, you could always claim that static bodies are really made of zillions of atoms at finite temperature bound together. But that would be missing a very important aspect of physical sciences by opposition with mathematics : in physical science we want to purposely discard all the details which are of no relevance to the question we are asking. We want a model which captures all the complications which matter, but not more than that.

    By the same token, classical optics does not have anything which "moves". The lenses, prisms, and other mirrors are fixed and we compute the path of rays which are, for all practical purposes, instantaneous in our human sized device. Again, if you claim that photons have a finite velocity, you miss the point. For the definition of physics, whomever would be studying the laws of prisms, lenses and mirrors by assuming instantaneous propagation of the rays would still be doing physics.
  21. Oct 7, 2011 #20
    Already Mario Bunge in his classic Is Chemistry a branch of physics? got the conclusion that
    Physical chemistry is a discipline of chemistry. Chemical engineering is a discipline of engineering and chemical physics a discipline of physics. I find no reason to confound all them.

    Condensed matter physics is a discipline of physics, whereas materials science is a part of science: materials science joins the condensed matter physics, with the chemistry of materials and other disciplines.
    Last edited: Oct 7, 2011
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