Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What is the difference in theory and law

  1. Dec 27, 2013 #1
    I googled this question first but there seemed to be a few different answers and I'm not sure which is correct. So take for example the theory of general relativity and the first law of thermodynamics, both are true, but why is one a theory and one a law?

    Why was Einsteins theory called theory of general relativity and not law of general relativity? Also what about Heinsenberg's uncertainty principle, is this still a theory or law or just a mathematical statement?

    Can you tell me when someone discovers something about the universe, what determines if it's classed as a theory or a law.

  2. jcsd
  3. Dec 27, 2013 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    [1] "Law of Nature". Oxford English Dictionary (3rd ed.). Oxford University Press. September 2005.

    [1] http://www.nap.edu/openbook.php?record_id=6024&page=2
    [2] AAAS Evolution Resources


    A theory seems more general than a law and broader in scope.
  4. Dec 27, 2013 #3
    A theory offers an explanation behind some phenomenon. If the theory is working, we can manipulate that phenomenon. Germ theory of disease allows us to combat disease based on our understanding.

    A law is a statement, but offers no explanation. Newton's laws of motion, for example, are statements.
  5. Dec 27, 2013 #4


    User Avatar

    Staff: Mentor

    This doesn't answer your *specific* questions, but it does explain the differences between hypothesis, theory and law.


    For those too lazy to click on the link.

    Since this is a science forum, the scientific definition would apply




    Last edited: Dec 28, 2013
  6. Dec 27, 2013 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Theories come and go; one hopes that laws are more permanent.
  7. Dec 28, 2013 #6


    User Avatar
    Homework Helper

    This is a little misleading. Newton's "laws" of motion (at least the second law, depending on how it's formulated) and "universal law" of gravitation have both been superseded by Einstein's "theories" (special and general relativity, respectively).

    Newton's "laws" continue to be excellent approximations that hold to a very high degree of accuracy in commonplace experience. But Einstein's "theories" are considered (currently, anyway) to be exact descriptions of the same phenomena.

    To the lay person, a "law" seems more definite and immutable than a "theory". But this is not necessarily true in Science.
  8. Dec 28, 2013 #7

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    What is the difference in theory and law?

    If you can tweet it, it's a law. In many cases, another name for "law" is "equation." If you need to write a book about it, or a volume of books, it's a theory. Laws are short and pithy, and oftentimes don't mean much in and of themselves. A scientific theory is needed to explain how and when to use those scientific laws.

    Some examples:
    • F=ma: Law. Newton's Principia: Theory.
    • V=IR: Law (an empirical law at that, one that fails when your resistors emit smoke). Electromagnetism: Theory.
    • j*=εσT4: Law. Quantum mechanics and thermodynamics: Theory.
  9. Dec 28, 2013 #8


    User Avatar
    Science Advisor
    Gold Member

    Its just semantics. 'Laws' of nature are merely theories that are particularly well supported.
  10. Dec 28, 2013 #9
    Hrrmm.. you make it sound like theories "become" laws, which is not true, they are different things.
  11. Dec 28, 2013 #10


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    Laws can remain practically unexplained, empirically-derived statistical regularities, such as Coulomb's law of friction.

    The deeper explanatory principles behind friction cannot as yet be seen to be satisfactorily developed in such a way that Coulomb's law of friction falls deductively out as THE principal result in every case, even though one knows, of course, that Coulomb's law of friction must be regarded as the aggregate macroscopic effect of microscopic electromagnetic interactions.

    But, even without this theory behind it, Coulomb's law is perfectly observable in its action.
    Newton's laws of motion in their totality, in contrast to the specific case like Coulomb's law, is of such a daring degree of generality that it must be regarded as a theory (and, a failed one as that)
  12. Dec 28, 2013 #11


    User Avatar
    Gold Member

    Theories consist of laws (and other things that put the laws in context)
  13. Dec 28, 2013 #12

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    Newtonian theory is not a failed theory. It remains a valid scientific theory, but now instead of being viewed as universally true theory it is a theory with a limited, non-universal domain. Physicists still teach and engineers still use Newtonian mechanics because it is extremely accurate in the realm in which it is applicable.

    Failed scientific theories include concepts such as the caloric theory of heat. Even then, the term failed is a bit strong. A better term is superseded. After all, caloric theory had some remarkable successes (e.g., the Carnot cycle), and it was the dominant theory of heat for almost 100 years. Nobody teaches or uses caloric theory any more because it was superseded by thermodynamics, which has much greater utility and a much wider range of validity.

    A scientific theory rather than a scientific law is the pinnacle of science. They represent a body of well-tested scientific knowledge and explain how to use that knowledge. F=ma doesn't mean much in and of itself, nor does σxσp≥ħ/2. Newton's laws doesn't mean much until backed up with the larger Newtonian mechanics (a scientific theory), just as Heisenberg's uncertainty principle doesn't mean much until backed up by the larger quantum theory.

    So why aren't the Heisenberg uncertainty principle, the Pauli exclusion principle,the Dirac equation, the Schrödinger equation called "laws"? These concepts are named after the key developers of the second wave of quantum mechanics. They had just seen many of the supposedly universal laws of physics cast down to the status of equations with a limited domain of applicability, and now they were working to overthrow the old quantum mechanics that had just overthrown classical physics. Perhaps they thought calling their new concepts "laws" would have been a bit too arrogant. More importantly, the developers of that second wave of quantum mechanics were strongly influenced by the concepts of anti-realism and instrumentalism. Unlike earlier generations of physicists, Bohr, Heisenberg, Pauli, et al. very specifically were not trying to elucidate God's laws. They rejected that concept. The people behind the Copenhagen interpretation were merely trying to come up with a more concise and more accurate description of what could be observed physically.
  14. Dec 28, 2013 #13
    Newton's theory was the theory of Universal Gravitation. He proposed the three laws as axioms upon which his long train of reasoning toward the existence of Universal Gravitation could be built. They have the same function the two postulates of SR have. They're basically stipulations upon which something else will be built. You agree to them or you don't. I'm not sure how the three laws, or the two postulates, could be reworked as a theory in and of themselves.

    So, I think whether or not something is called a "law" depends on how well it functions as an axiom or postulate.


    The actual chapter of Principia where Newton lays out the three laws is entitled "Axioms, or Laws of Motion." He put the word "axioms" first. All three words, "axiom," "law," "postulate," can probably be taken as synonyms.
  15. Dec 29, 2013 #14
    Laws are empirical observations which can be taken as "facts" about nature. Conservation of energy and momentum, the laws of thermodynamics, Newton's Laws, Coulomb's Law and the Law of Gravitation etc...

    They are all observed facts about nature (up to some degree of approximation) which theories, such as Thermodynamics and Classical Mechanics can be built up from. You can't really derive these laws from the theories without making some other kind of assumptions, which then just replace them as a new set of "laws".
  16. Dec 29, 2013 #15


    User Avatar
    Gold Member

    Axioms are generally more fundamental. They are types of laws, but not all laws are axioms. Mostly just because law is such a diverse term that it's used more loosely, but axioms serve more as the core laws of a theory.
  17. Dec 29, 2013 #16
    Yeah. Despite the fact I just got done suggesting they could be taken as synonyms I don't actually believe there are any true synonyms. When it comes down to a specific context there is usually one choice that is le mot juste.

  18. Dec 29, 2013 #17


    User Avatar
    Gold Member

    I like how the pronunciation of that sounds like "most used".
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook