Your favorite definition of physics

[juanrga]

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?

 

[juanrga]

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.

 

[humanino]

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 :

This section is explanatory and was written by the preparers of these notes in order to create a false sense of security i.e. an unsubstantiated feeling that we understand what we are talking about in the rest of the lecture. Unfortunately, this is not the case, at least if "understand" means what it usually means among mathematicians.

 

[humanino]

The best definition of science in one minute I know of

 

[micromass]

Physics is the subset of mathematics that is easy.



Joke...

 

[pergradus]

Physics is the subset of Math that matters.

Not joke...

 

[lisab]

The best definition of science in one minute I know of

...he's such a badass...

 

[George Jones]

Physics is the orthogonal complement of ...

 

[BadBrain]

Physics is the subset of mathematics that, as Feynman said, is charged with the duty of having to prove herself against observed reality.

 

[Astronuc]

Similar to the one that humanino posted.
https://www.youtube.com/watch?v=Ffr69ZovHKc

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.

 

[juanrga]

Physics is the subset of Math that matters.

Not joke...

Feynman said the contrary.

 

[juanrga]

Physics is the subset of mathematics that, as Feynman said, is charged with the duty of having to prove herself against observed reality.

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).

 

[juanrga]

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.

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.

 

[wuliheron]

My favorite definition and the broadest is that physics is the study of motion. However, for those who prefer techno-babble:

Physics
noun
the science that deals with matter, energy, motion, and force.

 

[FlexGunship]

Engineering is applied physics and physics is applied mathematics.

 

[Astronuc]

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.

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.

 

[juanrga]

My favorite definition and the broadest is that physics is the study of motion.

Physics is not a synonym for kinematics (subset of mechanics).

 

[juanrga]

Engineering is applied physics and physics is applied mathematics.

As noted above, Feynman disagreed.

 

[humanino]

My favorite definition and the broadest is that physics is the study of motion.

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.

 

[juanrga]

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.

Already Mario Bunge in his classic Is Chemistry a branch of physics? got the conclusion that

chemistry, and in particular quantum chemistry, is not a part of physics

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.

 

[FlexGunship]

As noted above, Feynman disagreed.

Meh, I don't like there this thread is going. You asked a question and then just contradict answers. Feynman was a great man, but if you already have a definition in mind, then let us know and then the thread can be closed.

 

[humanino]

Already Mario Bunge in his classic "Is Chemistry a branch of physics?" got the conclusion that "chemistry, and in particular quantum chemistry, is not a part of physics".

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.

You appeal to authority by saying "Mario said so". Fine, but the point of a scientific discussion is not to appeal to authority "as Feynman says" in the first video

I believe the point Astronuc was raising is rather mundane and mostly everybody agrees upon it : the laws of chemistry are derived from the laws of quantum physics. If ever chemists would be to exhibit a system which does not obey quantum physics, physicists would want to study this system and become knowledgeable about it. Until then, physicists do not know much about chemistry. The fact that the laws of chemistry are derived from the laws of quantum physics should not be a matter of debate. Now, if Mario or you feel that this fact does not qualify chemistry as a branch of physics, fine, you can ask Astronuc for clarification as to what he meant.

 

[juanrga]

Meh, I don't like there this thread is going. You asked a question and then just contradict answers. Feynman was a great man, but if you already have a definition in mind, then let us know and then the thread can be closed.

Therefore, I have the freedom to ask, but not to reply the answers, nor to cite Feynman or Bunge works. Maybe when I was said above that physics is the same than kinematics, I would remain silent... Is that?

 

[juanrga]

You appeal to authority by saying "Mario said so". Fine, but the point of a scientific discussion is not to appeal to authority "as Feynman says" in the first video

I believe the point Astronuc was raising is rather mundane and mostly everybody agrees upon it : the laws of chemistry are derived from the laws of quantum physics. If ever chemists would be to exhibit a system which does not obey quantum physics, physicists would want to study this system and become knowledgeable about it. Until then, physicists do not know much about chemistry. The fact that the laws of chemistry are derived from the laws of quantum physics should not be a matter of debate. Now, if Mario or you feel that this fact does not qualify chemistry as a branch of physics, fine, you can ask Astronuc for clarification as to what he meant.

You were the first to name Feynman here, but he is well-known for emphasizing the differences between physics and maths. Sorry if he disagreed with so many posters who appealed to him!

I have not appealed to authority but I have cited a paper authored by Bunge. I find no argument in your reply neither in Astronuc, but only a repetition of clichés and personal beliefs.

 

[humanino]

I have not appealed to authority but I have cited a paper authored by Bunge.

Usually when one wants to engage in a discussion, they make a point and use the quotation for further details. You only made the quote. I do not know what are the arguments in the quote you make. I was simply trying to participate in your discussion.

I find no argument in your reply neither in Astronuc, but only a repetition of clichés and personal beliefs.

I will try to re-iterate one last time then. There are laws of quantum physics which are universal. We do not know any system which do not obey those laws.

For instance when you write an electrical circuit Ohm's law :
V = IR
this law can be derived from a more fundamental set of laws, namely Maxwell's equations and electrodynamics. We do not make explicit use of Maxwell's equations when we study electrical circuit because it would be impractical. It is much more efficient to use Ohm's laws and others directly.

This is not a belief or a cliche. It is a matter of principle. It is important that we are able to make such derivation from a more fundamental layer to the next.

So we have fundamental laws at the level of fundamental particles. The protons and neutrons which make up atomic nuclei obey laws which can in principle be derived from the particle's laws, so we say that nuclear physics is a branch of the standard model of particle physics.

The electrons which orbit around the nuclei form atoms, and the laws of atomic physics can also be derived from the standard model (namely quantum electrodynamics).

The atoms form molecules and molecules undergo chemical reactions. The laws for those phenomena can be derived from the laws of atomic physics.

So the picture which is painted is that of science as One unit, with different branches talking to one another and showing interest for what the others are doing. The chemist and the physicist can help each other to learn something new and make progress together.

When we draw this picture, we never intend to get to ridiculous conclusion such as "string theory will decide for us on the next political election". If that is what you understood Astro and I were going at, then I can understand why you do not want to agree.

I will check one last time for your answer here. If your next message is again void of any constructive argument, I will not answer anymore.

 

[juanrga]

Usually when one wants to engage in a discussion, they make a point and use the quotation for further details. You only made the quote. I do not know what are the arguments in the quote you make. I was simply trying to participate in your discussion.
I will try to re-iterate one last time then. There are laws of quantum physics which are universal. We do not know any system which do not obey those laws.

For instance when you write an electrical circuit Ohm's law :
V = IR
this law can be derived from a more fundamental set of laws, namely Maxwell's equations and electrodynamics. We do not make explicit use of Maxwell's equations when we study electrical circuit because it would be impractical. It is much more efficient to use Ohm's laws and others directly.

This is not a belief or a cliche. It is a matter of principle. It is important that we are able to make such derivation from a more fundamental layer to the next.

So we have fundamental laws at the level of fundamental particles. The protons and neutrons which make up atomic nuclei obey laws which can in principle be derived from the particle's laws, so we say that nuclear physics is a branch of the standard model of particle physics.

The electrons which orbit around the nuclei form atoms, and the laws of atomic physics can also be derived from the standard model (namely quantum electrodynamics).

The atoms form molecules and molecules undergo chemical reactions. The laws for those phenomena can be derived from the laws of atomic physics.

So the picture which is painted is that of science as One unit, with different branches talking to one another and showing interest for what the others are doing. The chemist and the physicist can help each other to learn something new and make progress together.

When we draw this picture, we never intend to get to ridiculous conclusion such as "string theory will decide for us on the next political election". If that is what you understood Astro and I were going at, then I can understand why you do not want to agree.

I will check one last time for your answer here. If your next message is again void of any constructive argument, I will not answer anymore.

This thread is mainly about the definition of physics and science, but unfortunately I have received either weak replies taken from general dictionaries or just plain nonsensical replies as physics-is-kinematics

What you are asking me now is about the topic of reductionism. Reductionism is an old philosophical program that was showed to be wrong in the late 20th with such modern stuff as emergence, autonomy, and disunity in the sciences. There are lots of papers in journals as foundations of chemistry devoted to show why chemistry is not reduced to physics, there are books, conferences...

What Bunge did in the paper cited was just to explore to what extension concepts of chemical theory could be derived from quantum physics. You must not like the conclusion, but there is little more that you can do here.

Not only chemistry has not been reduced to quantum physics but some chemists are developing new equations and theories for studying the chemical phenomena they are interested in. For instance, Prigogine (Nobel Prize for chemistry) has developed a mathematically demanding extension of quantum physics, whereas Eu has proposed a non-relativistic 'kinetic' equation for condensed phases which, as he correctly notes, is not derivable from the Schrödinger equation.

If you have a paper where you believe that you can derive the Eu equation from quantum physics, please let me know

Your claim that the laws of chemical reactions can be derived from the laws of atomic physics is untrue. It is so false as the old 18th century claim that all of chemistry was derivable from Newtonian mechanics or the early 20th century claim that all of chemistry was derivable from classical physics (do you want the names of the Nobel Prize winners for physics who claimed so?)

But recall this thread is not about reductionism. I would like to read rigorous and complete definitions of physics from PF community.

 

[FlexGunship]

This thread is mainly about the definition of physics and science, but unfortunately I have received either weak replies taken from general dictionaries or just plain nonsensical replies as physics-is-kinematics

[etc...]

Your claim that the laws of chemical reactions can be derived from the laws of atomic physics is untrue. It is so false as the old 18th century claim that all of chemistry was derivable from Newtonian mechanics or the early 20th century claim that all of chemistry was derivable from classical physics (do you want the names of the Nobel Prize winners for physics who claimed so?)

But recall this thread is not about reductionism. I would like to read rigorous and complete definitions of physics from PF community.

Woah... this thread is in trouble... I'm out'a here!

 

[humanino]

Prigogine

I know his work rather well, and I do not think we are talking about the same issues, at all.

Eu has proposed a non-relativistic 'kinetic' equation for condensed phases which, as he correctly notes, is not derivable from the Schrödinger equation.

If you have a paper where you believe that you can derive the Eu equation from quantum physics, please let me know

Are you talking about Byung Chan Eu ? Can you please point out to a technical reference describing the equation you are referring to and how the equation was suggested ? I believe this work amounts to using a large computer simulation and fit it with a simple function. I hope I have the wrong reference.

You say yourself that reductionism is dead because of findings in the late XXth century. But Poincare stumbled onto chaos long before that, in fact at the end of the XIXth century, and it just took a long time for non-mathematician to realize what chaos and non-linear dynamics mean. A good reference would be "Structural Stability and Morphogenesis" for instance, much earlier than what you quote. Of course you will not get non-linear behavior out of a simple Shrodinger equation. Anyway, I will wait until a proper reference has been provided.

 

[Functor97]

Physics is the subset of Math that matters.

Not joke...

Mathematics matters just as much as Physics if not more...at least in the search for truth. We apply our mathematics to model reality, without differential geometry General relativity would have never got off the ground. Mathematics is the queen of the sciences! :!)

 

[micromass]

Mathematics matters just as much as Physics if not more...at least in the search for truth. We apply our mathematics to model reality, without differential geometry General relativity would have never got off the ground. Mathematics is the queen of the sciences! :!)

Incredibly well said!

 

[DoggerDan]

The first step of applied mathematics. Engineering is the second step. By comparison, most of the other sciences use relatively little math.

 

[juanrga]

Prigogine I know his work rather well, and I do not think we are talking about the same issues, at all.

Are you talking about Byung Chan Eu ? Can you please point out to a technical reference describing the equation you are referring to and how the equation was suggested ? I believe this work amounts to using a large computer simulation and fit it with a simple function. I hope I have the wrong reference.

You say yourself that reductionism is dead because of findings in the late XXth century. But Poincare stumbled onto chaos long before that, in fact at the end of the XIXth century, and it just took a long time for non-mathematician to realize what chaos and non-linear dynamics mean. A good reference would be "Structural Stability and Morphogenesis" for instance, much earlier than what you quote. Of course you will not get non-linear behavior out of a simple Shrodinger equation. Anyway, I will wait until a proper reference has been provided.

Prigogine knows that the laws of chemical reactions are not derivable from quantum electrodynamics in despite of your beliefs. What is more, Prigogine starts one of his last papers Chemical kinetics and dynamics with a critique of the well-known Dirac quote about the reduction of chemistry to quantum mechanics.

About Eu, a good reference is his monograph Nonequilibrium Statistical Mechanics: Ensemble Method. He suggested his equation after checking that the related Schrödinger-based theories could not explain all the transport phenomena of his interest. As a consequence he postulated his equation. As he correctly notes, the Schrödinger equation can be obtained as a special case from his equation, but the inverse is not true.

Nowhere I said that reductionism was dead because of work on chaos. At contrary I wrote about other three different topics.

I think that I already said that this thread is about definitions of physics, and I think that I do not need to write more about reductionism.

 

[juanrga]

The first step of applied mathematics. Engineering is the second step. By comparison, most of the other sciences use relatively little math.

What is for you the difference between physics and mathematical physics?

 

[Evo]

Enough.