# Other What's the value of Classical Physics?

#### Qurks

I don't know where to put this but it is a question which is bothering me. From the perspective of a physicist who wants to generate new theories, what's the value in spending significant time learning classical theories?

People slave always at books like Jacksons Electrodynamics but it's not clear to me why given that it's already a complete(but wrong) theory. Isn't time better spent on new horizons like QFT?

#### Orodruin

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Good luck learning QFT without knowing any electrodynamics!

Also, electrodynamics is not ”wrong”, it works perfectly well in most situations. It has a range of validity that is limited, but EM waves do not suddenly stop working as described by Maxwell’s equation just because of this.

Another point to remember is that most people studying physics at university level are not going to do research in QFT.

#### ZapperZ

Staff Emeritus
2018 Award
I don't know where to put this but it is a question which is bothering me. From the perspective of a physicist who wants to generate new theories, what's the value in spending significant time learning classical theories?

People slave always at books like Jacksons Electrodynamics but it's not clear to me why given that it's already a complete(but wrong) theory. Isn't time better spent on new horizons like QFT?
Particle accelerators works according to classical E&M. If you get into accelerator science, the first thing they check is if you've done EM at the level of Jackson's.

Zz.

#### Geofleur

Gold Member
Perhaps it's also worth pointing out that quantization (for both particles and fields) works by replacing a classical Poisson bracket with $1 / i\hbar$ times the commutator, making quantum mechanics and classical mechanics very similar theories from an algebraic standpoint. Also, Lagrangian invariances and Noetherian conservation laws work very much the same way whether they are for classical or quantum fields; understanding the former really helps in understanding the latter.

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#### Qurks

Particle accelerators works according to classical E&M. If you get into accelerator science, the first thing they check is if you've done EM at the level of Jackson's.

Zz.
Particle accelerators work approximately according to classical E&M, it doesn't mean that developing an accelerator based upon a more advanced theory wouldn't be better.

#### Orodruin

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Particle accelerators work approximately according to classical E&M, it doesn't mean that developing an accelerator based upon a more advanced theory wouldn't be better.
Accelerators are not ”based upon a theory”. They are works of engineering and to construct them you need to apply engineering principles. One of those principles is to use an appropriate physics description and it is not appropriate to use QED calculations to construct an accelerator as using an overly an unnecessarily complicated computation will be both more time consuming and more prone to errors.

If you have ”a more advanced accelerator” based on other principles, please give us the reference.

#### ZapperZ

Staff Emeritus
2018 Award
Particle accelerators work approximately according to classical E&M, it doesn't mean that developing an accelerator based upon a more advanced theory wouldn't be better.
Then do it, and then we can talk. Otherwise, this is meaningless guess work.

Zz.

#### Dr. Courtney

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2018 Award
Working theories are tools in the tool box - in several senses:
1. They are paradigms upon which new theories are often modeled.
2. They illustrate principles that pervade other theories - for example, the importance of symmetries in classical physics.
3. They provide useful approaches needed for experimental work.
4. They are scaffolding for semi-classical approaches.
5. They are necessary to understand the history of Physics.
6. They provide valuable practice for both pencil and paper problem solving AND computational methods.
7. They illustrate key relationships between theory and experiment in ways that may be far off for some new theories.
8. They weed out students with silly ideas who really have no potential for hard work, much less new theories.

#### vela

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Particle accelerators work approximately according to classical E&M, it doesn't mean that developing an accelerator based upon a more advanced theory wouldn't be better.
This reminds me of what a mentor here called solving an inclined plane problem using general relativity.

#### mpresic3

One of the best answers to your question comes from the preface of the book "Introduction to the Physics of Fluids and Solids" by James S Trefil.
My paraphrasing (does an injustice to the preface) is that physicists have historically been generalists and many if not most future physicists will be employed in a field which highlights their knowledge of classical physics. Unfortunately, there is too little emphasis on classical physics in educating current graduate students. When I began graduate study in the late 1970's, there was a full year of classical mechanics. When I concluded graduate study, my institution and many others, pared this down to one semester of classical mechanics.

Many physicists work alongside with engineers in area where a relativistic treatment of problems will buy you very little. In GPS, on the other hand a relativistic correction is important. It depends on the field you work in and what you are called on to do.

In my own case, I started graduate study with every intention of doing a thesis in the area where the interactions are not well understood like weak interactions, or meson theory. I had two excellent professors one in Quantum Mechanics, the other in Theoretical Physics (Classical mechanics mostly then fluids and elasticity the second semester). I enjoyed the QM, but I really resonated (forgive the pun) with the presentation in theoretical physics that the second professor taught. (He used his own notes)

I chose to make theoretical physics (with mainly classical theories) my life's work. Many of my colleagues and coworkers have asked me, and expressed an opinion, you are excellent in classical physics, but you are not as good in quantum physics. I am puzzled on how they would know this, because I have never chosen or been given an assignment where QM was required. Classical physics has kept me busy and will continue to keep me busy for a long time.

I did use quantum mechanics in one case, where in an explanation during a lecture on how to calculate one variable with linear functionals, the professor demonstrated the'same formalism could calculate other variables using different kernals. Reflecting, I made the connection that the quantum mechanical wavefunction is a similar animal. Apparently, the theory of linear functionals can take you a long way whether you are looking at classical or quantum systems.

#### RPinPA

Homework Helper
Maxwell's Equations are big business. They are the basis of much of my career in simulation of electromagnetic propagation, and if anything that marketplace is getting bigger. Wireless carriers have a huge economic interest, many billions of dollars worth, in accurate modeling of signals to and from towers and their interference with other signals. The problems in our models don't lie in Maxwell being inaccurate, they lie in the difficulties in calculating what happens with a classical electromagnetic wave in the presence of buildings, trees, terrain, different ground types and weather.

Because of our limited ability to model those things, to get their effects right within even a factor of 2, the modeling community is not really that interested in using even more calculation-intensive models that differ from classical E & M in the 10th decimal point.

"What's the value of Classical Physics?"

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