Classical physics?

[Mystery111]

I suppose this is a question on terminology. I see we have a page dedicated to relativity, but we seem to have a page dedicated to classical physics. Relativity is part of physics, it is a physical theory in itself. But it is also a classical theory, in that it does not take into account the Uncertainty Principle.

 

So what is this subforum dedicated to mostly... Newtonian Mechanics perhaps?

[Klaynos]

I suppose this is a question on terminology. I see we have a page dedicated to relativity, but we seem to have a page dedicated to classical physics. Relativity is part of physics, it is a physical theory in itself. But it is also a classical theory, in that it does not take into account the Uncertainty Principle.

 

So what is this subforum dedicated to mostly... Newtonian Mechanics perhaps?

 

 

Classical physics normally refers to not special/general relativity or quantum mechanics.

 

So, newtonian mechanics would fall in here, but so would a lot of other things.

[timo]

"Classical" does not necessarily equate with "non-quantum-mechanical". It is just as often used as "non-relativistic". In this case it seems to mean both.

Keep in mind that this is just a choice for creating a forum structure. You could also translate it with "school physics" in this case. Or, if you are as cynical as me, with "the part of physics were people know what they are talking about" or "quantitative physics" .

I think a much more dubious forum name is "Modern and Theoretical Physics", but from previous debates I know that I am pretty alone with that impression.

Edited August 30, 2011 by timo

[ajb]

Classical physics normally refers to not special/general relativity or quantum mechanics.

 

There are some conventions here.

 

Generally I would say that classical physics is any physics in which [math]\hbar[/math] plays no role. That is anything that is not quantum in nature. So here I would include special and general relativity as well as many other things.

 

Non-classical would be any physics in which [math]\hbar[/math] does play a role. This may be true quantum phenomena or things a little more subtle like semi-classical or quasi-classical.

 

Sometimes by classical people also include theories that do not obey "Einsteinian relativity"- so they exclude special and general relativity.

[Mystery111]

Then perhaps a more fitting name for the subforum here would be Newtonian Mechanics, to destinguish it from the classical nature itself - which is inherent in describing theories like relativity, which are clearly not quantum.

[Klaynos]

But then what about Legrangian mechanics? Or thermal physics...

[ajb]

Also as everything is related I expect it will be impossible to create subforums that completely satisfy everyone's points of view and philosophy. There will always be some arbitrariness to the names and debate as to what posts belong where.

 

Classical mechanics would also include classical field theory, fluid dynamics, geometric mechanics (Lagrangians and Hamiltonians etc.), kinematics of particles and rigid bodies and so on...

 

Not just Newtonian mechanics.

[Mystery111]

Well, Langrangian dynamics is almost undisputably classical in origin. The Langrangian however, also plays it's part in relativity - the relativistic Langrangian derived from [math]-mc^2\frac{d \tau}{dt}= -mc^2 \sqrt{1 - \frac{v^2t}{c^2}}[/math] would almost certainly fit within the regime of the relativity subforum. If it is generally simply the Langrangian [math]L = T - V[/math] in of course, it's most simplest form without any relativistic or field density relationship, then it is strictly a mathematical statement, which should be kept in the mathematical subforum. Of course, as just noted, there is the field density langrangian found in field theories, [math]S = \int Ldt[/math] then this should be presented in a physics subforum.

 

They are all classical, but catagorizing your type of Langrangian in easy, depending on the nature of the langrangian you are positing. You also have a Newtonian Langrangian, so no guesses where that should lie

 

Thermal physics, is of course classical as well. But it is harder to catagorize. If I be so bold, I would allow it to sit in the catagory of the physical sciences.

 

But I don't want to be a nazi. Obviously some conversations made in physics, will require a discussion of relativity, so it should be acceptable to present a relativistic langrangian in a physics subforum, if your arguement requires it. Obviously this means a little leniency, not too much moderation on behalf of certain posts.

 

p.s. sorry I haven't wrote out the relativistic langrangian in it's full form, i'm in a bit of a rush -- not because I am lazy.

Edited August 30, 2011 by Mystery111

[timo]

I think people were trying to tell you that not all Classical Physics is well-described with "Newtonian Physics", and that this forum is supposed to contain more than just NP. You seem to be arguing against the statement that anything involving a Lagrangian must be "Classical Physics". While that is true no one claimed that in the first place. To me it is a bit unclear what your point is.

[swansont]

I 've seen two uses. One is that it's not quantum; as ajb states, it does not use hbar. Another is that it's based in physics that was established prior to 1905 (even if advances have been made since then), which was the birth of relativity and quantum theory.

[Mystery111]

Timo, hi,

 

You all answer so quickly here, I took one last look before heading out, saw your reply and I needed to respond

 

My post there was specifically to Klaynos, he was asking me about the Langrangian and the Thermal physics, how they fit into a subforum without the classical subforum. Anyway, yes I agree. Not all classical physics are well-described with Newtonian physics, which is why certain classical physics may apply to certain non-classical subforums, depending on their nature. This is not always the case, but it may be the case specifically. Anything involving a Langrangian is classical, but I was asked by Klaynos to catagorize I think.

 

Anyway, my point may not be so much catagorizing and renaming the subforum. This would be a lot of work, and a lot for posters to just adapt to. In theory, though, my arguement consists that I should be able to post for instance, a relativistic thread in the classical subforum, even though there is a classical subforum present. Would I be a rebel in doing so? I don't class relativity as a subject of purely classical, physical or relativistic. It has all these descriptions anyway. So why not merge all physics, all relativistic and classical subforums together?

[Klaynos]

I was thinking (and should have been more specific) of the legrangian formulation of classical mechanics.

 

These are just subfora, the names and specifics are not that important they just often make things easier. I rarely actually pay attention to where things are posted.

 

It is not and never will be straight cut because of how intertwined physics is.

 

I think for the most part, it is up to the poster to decide on the most appropriate forum, it is very rare that something will get moved unless it is very clearly more appropriate in a different section.

[Mystery111]

I was thinking (and should have been more specific) of the legrangian formulation of classical mechanics.

 

These are just subfora, the names and specifics are not that important they just often make things easier. I rarely actually pay attention to where things are posted.

 

It is not and never will be straight cut because of how intertwined physics is.

 

I think for the most part, it is up to the poster to decide on the most appropriate forum, it is very rare that something will get moved unless it is very clearly more appropriate in a different section.

 

That is actually a very modest answer and I respect it.

 

I hope this thread is not construded as something to cause conflict. It was purely educational on the idea of what a classical theory is. I am enjoying being here already, I can see there are many educated minds here.

[eclectic`]

There are some conventions here.

 

Generally I would say that classical physics is any physics in which [math]\hbar[/math] plays no role. That is anything that is not quantum in nature. So here I would include special and general relativity as well as many other things.

 

Non-classical would be any physics in which [math]\hbar[/math] does play a role. This may be true quantum phenomena or things a little more subtle like semi-classical or quasi-classical.

 

Sometimes by classical people also include theories that do not obey "Einsteinian relativity"- so they exclude special and general relativity.

 

What is "classical physics"? That is a very good question, and I have yet to find a good answer.

 

As to the role of h or har, n my opinion, the work of Max Planck in developing his radiation equation was all based on classical physics. There was no other physics at that time as far as I know. Therefore, his radiation equation was developed using classical physics. His model of the atom was based on the atom as an "oscillator", and the concept of energy states was based on chemical reactions and thermodynamics. Your last sentence is probably the best definition, which relates to the Minkowski interpretation of "space/time". Certainly, the concept of time and space varying as a function of the velocity of an observer is not easily analyzed using classical physics.

[ajb]

As to the role of h or har, n my opinion, the work of Max Planck in developing his radiation equation was all based on classical physics. There was no other physics at that time as far as I know. Therefore, his radiation equation was developed using classical physics. His model of the atom was based on the atom as an "oscillator", and the concept of energy states was based on chemical reactions and thermodynamics.

 

Planck's deviation from classical physics is in how he assumed the energy of "electromagnetic oscillators" to be distributed. The key thing is that energy is quantised in "lumps" and this involves his famous constant. This is really the conception of quantum theory.

 

Shortly after Einstein explained the photoelectric effect using the idea of quanta of light, which was the birth of quantum theory.

 

Your last sentence is probably the best definition, which relates to the Minkowski interpretation of "space/time". Certainly, the concept of time and space varying as a function of the velocity of an observer is not easily analyzed using classical physics.

 

So this comes down to the conventions already discussed. In my opinion, as special and general relativity have nothing to do with quantum they are classical subjects. But other disagree and would say they are not. Depends on one's tastes.