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What is Physics?

  1. Nov 15, 2011 #1
    Below is an informal discussion on the meaning of Physics and reality, written by my mentor. He is teaching me Physics. What do you guys think?

    What is physics?

    Well, that's a tricky question and can be answered in various ways. One is that it's the primary science that underlies all other sciences, the rootstock so to speak with the rest of science being the scion. It speaks to us in the language of mathematics, but math isn't science. It's logic, and it tells us that above all, physics is logical, which reflects as a fact that the universe is also logical. No unicorns or witches on broomsticks. One must be aware though that maths can take one into perfectly logical places that are quite beautiful but have no reality, so before a mathematical model can be elevated to physical fact, the model has to pass the acid test of empiricism, that is, designed experiment and/or fortuitous observation. Math alone doesn't cut it, but every reality has a mathematical model waiting in the wings.

    Physics itself stands on a basis that all that exists in the physical universe is in some way in motion, and this gives us a starting point. So we might say it's the study of objects in motion and their consequences. Motion of course implies "displacement", or a change of position in "space" - what we might call an "elemental" (my idea) - at some "rate", which behooves us to recognise another elemental we call "time", but let's be careful out there.

    Space is "real", and reality itself needs definition. The best that I've seen is that cobbled together by Robert Dicke, called his Rules of Reality. These are:

    1/ To be real, an entity must be separable from another entity. (This means a one body universe is meaningless)
    2/ The entities must communicate in some way (any way will do, a force field, gravity, electric charge, "bump"....)
    3/ The entities must both agree that the other exists ("agree" here means 1 = true, 0 = false to the basic proposition, "the other exists")
    4/ Induction is a universal principle (allows the two body existence above to extend to three, then four then...ad infinitum)
  2. jcsd
  3. Nov 15, 2011 #2


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    Why is this in the math forum?

    Linguistics? Anthropology? Political Science? Do these need physics?
    This reminds me of http://xkcd.com/435/" [Broken]. You can claim that everything is just applied mathematical physics, just as you can claim mathematics is just applied theoretical linguistics, but I would just say "So what?" It's not a useful argument.
    None of the anthropology papers I read used physics. The scope is just too different.

    Debatable. Mathematics is classified as a http://en.wikipedia.org/wiki/Formal_sciences" [Broken], while physics and chem are empirical science. But then, is computer science a science? What about applied mathematics? It's not a simple black and white answer.

    No, at best it says we interpret our environment logically. The universe itself may not be logical. Or it may follow a different logic than the one we use. We do know our current theories are experimentally verifiable up to a certain error, so we continue to use them.

    Non sequitur. Logic doesn't discriminate against unicorns. Or flying pieces of wood.
    We just haven't observed unicorns or flying broomsticks so we exclude them from our theories.

    Agree. Just replace "physical fact" with "scientific theory".

    Hate the word "elemental".

    Ridiculously hard question. If you aren't a philosopher, it's probably best to leave it alone.
    Last edited by a moderator: May 5, 2017
  4. Nov 15, 2011 #3
    Would it be too simplistic to say that physics is the science of movement and forces between matter in all its forms?
  5. Nov 16, 2011 #4
    Actually, it is the part beginning with "movement" and "forces" which I agreed with the least. Those two concepts are not necessary for physics. Even our ordinary concepts of space and time may be an illusion, quantum physics could work well without anything but probability amplitudes for transitions between states.

    The best definition for physics I have seen (posted it several times before)

    Simple :
    • Guess a law (mathematical relation between observable quantities); compute the consequences of the guess.
    • Compare to experiment/measurement.
    • If it disagrees, it's wrong; if it agrees, call Oslo and ask them what they think.
    • Repeat until Oslo sends an invitation.
    Last edited by a moderator: Sep 25, 2014
  6. Nov 16, 2011 #5


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    similar to humaninos definition, I think physics is as simple as 'a process by which we model the interaction of observable quantities through bayesian inference.'
  7. Nov 16, 2011 #6
    But he's defining science. He says he describing the key to science. What is physics?
  8. Nov 16, 2011 #7
    I think your mentor is randomly pointing at various tools and methods used to achieve a goal while remaining unable to articulate what he thinks the goal is. His attempt to define it is premature: he doesn't know what he thinks it is yet. Hence: "Well, that's a tricky question and can be answered in various ways."

    Any encyclopedia could give him a perfectly serviceable place to start:


    Personally I think "More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves," is nicely comprehensive and also to the point.
  9. Nov 16, 2011 #8
    There are no strict borders of course. Generally, I would count as physics whatever natural investigation which applies at the strictest level the methodology put forward by Feynman above, at the borders of our fundamental understanding. Meaning, what counted yesterday as "physics" may be today "electronical engineering" for instance.
  10. Nov 16, 2011 #9
    That seems like a good way to describe physics. Wikipedia is awesome!

    For a student it definitely involves a lot of ANALYSIS including listing assumptions, making sketches, doing calculations, perhaps verifying by experiment, checking logic and math, checking units, being clear in conclusions, etc.

    And the goal of UNDERSTANDING seems right. Im an engineer, and my goal in design is typically to construct something that acheives a valued result. Physics doesnt require all that, but does require understanding physical reality.

    PS. thanks to the moderators for moving this out of the math forum. I must have taken a wrong turn...
  11. Nov 16, 2011 #10
    Last edited by a moderator: Sep 25, 2014
  12. Nov 16, 2011 #11
    At the most fundamental level, physics is the study of motion. The motion of particles, of large bodies, and of energy.

    All forms of energy involve the movement of something, or the potential for it it move.
  13. Nov 16, 2011 #12
    I am not sure why this keeps coming up. It is clearly not valid. Take the first electronic experiments for instance. Although it is true that at a fundamental level, current corresponds to a transfer of single carriers, the physicist discovering electronic phenomena needs not know that to do physical science.

    How about "statics" ? One can analyze the distribution of loads, forces and stresses in mechanical structures, with the exact purpose that nothing should "move".

    Further, in the etymology of "motion" there is a "continuous change in position". It may have been agreeable at the beginning of the previous century that all phenomena are really collisions between particles and interferences of waves. But quantum physics gave up the notion of continuous trajectories (it is enough that one interpretation does for the general definition "physics as the study of motion" to fail). In addition, there are other concepts, such as entropy, which are universal and not merely about "kinetic energy transfer between gases of perfect spheres".
  14. Nov 17, 2011 #13
    Again, motion or the potential for motion. Statics come about because you have several different potentials which sum to zero - but if there were no potentials at all, then everything would be static and there'd be no point studying it.

    Like most people, you are misunderstanding quantum mechanics. QM says nothing about the trajectory of a particle, and whether it is continuous or not. Further, nothing about the concept of motion requires things to be continuous, the most simple definition of motion is that somethings position in space is changing with time. Even if an electron is teleporting all over the place, it's still moving.

    And Entropy? Where do you thing that comes from - entropy is a consequence of the way things move, if there was no motion, entropy would have no meaning.

    Everything in physics is either the study of motion directly, or some phenomena which is a result of something moving.
  15. Nov 17, 2011 #14
    I think humanino's point is that in some cases the motion is incidental to what's actually be focused on.
  16. Nov 17, 2011 #15

    D H

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    I have other words than awesome. That definition is far too vague and is arguably wrong. The general analysis of nature is a nice succinct definition of biology.

    I like the pragmatic but admittedly circular definition "physics is the body of knowledge taught and researched by professional physicists."
  17. Nov 17, 2011 #16
    No. While physics might include, for example, the mechanics of animal motion, biology won't include the motion of planets and stars.
    And that's not vague?
  18. Nov 17, 2011 #17


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    So D H defines "nature" as trees and rabbits? I think of fundamental forces and the universe.
  19. Nov 17, 2011 #18
    I am not sure what makes you think you understand it better than me. If quantum mechanics says nothing about the trajectory, then there is a consistent interpretation in which the trajectory does not exist, which means that putting motion at the center of the definition for physics altogether is inconsistent with one interpretation of physics, and therefore not valid in general.

    No, there are more general situations in which one can define the entropy of a configuration. For instance in quantum mechanics
    Last edited: Nov 17, 2011
  20. Nov 17, 2011 #19
    Having done physics in high school and in mech eng, we studied both movement and potential for movement, and the energies pertaining to each. Much of the mech eng study was about moment arms, angular momentum and the like, but also stress and strain as materials are bent, stretched, sheared, moved etc.
    I dislike that definition because having been a student of physics in high school and mechanical engineering, we were not professional physicists. In HS of course, it was generalized, whereas in mech eng it was towards our qualification. Our qualification and work as engineers required knowledge and use of physics, and of course it is also also used in other professions. Engineers and others also research the physics of new structural ideas or new materials, which requires more than a basic knowledge of physics.
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