The development of our physical theories

In my early days at the physics department, I wanted to get some small sense of how this vast field of knowledge was organised and had come about. David Jennings helped me sketch the main theories and how one led to the other via this email conversation.  The result was this large and rather scruffy charcoal drawing, 2m high on the studio wall.   


All physical theories, charcoal on paper, 200 x 100 cm

My comments are in italics.

Hi Geraldine,

That looks really interesting. Yep, it's a tough challenge to lay out all the interconnections between the various theories. If you want to spell out 'what influenced what' I think that roughly speaking thermodynamics and Newtonian mechanics gave rise to statistical mechanics.

Newtonian mechanics was in the 17th century, while the birth of thermodynamics roughly took place with the brilliantly titled book 'Reflections on the Motive Power of Fire', by Sadi Carnot in the early 19th century. The interest in notions of thermodynamics - work, heat, pressure temperature - grew out of the industrial revolution, which was in turn enabled by our understanding of Newtonian mechanics.

People wanted to understand thermodynamics in terms of the clockwork universe of Newton, and so eventually by the end of the 19th century Newtonian mechanics was reconciled with *most* of observed thermodynamics through the framework of statistical Mechanics. The stray odds and ends that were not explained by statistical mechanics gave rise to a little theory called quantum theory at the start of the 20th century.

Electromagnetism followed roughly a parallel path to thermodynamics, but it really has a long history, starting with natural magnets and static electricity and culminating in quantum electrodynamics 50 years ago. It's a nice one to put in a central location as it involves all phenomena in the classical world of humans, but also stretches into relativity and quantum theory.

Newtonian theory is an awkward one - Newtonian mechanics influenced statistical mechanics and electromagnetism, but Newtonian gravity really remained fairly static until Einstein in the 1920s, when general relativity arrived on the scene and revolutionized our simple notion of a gravitational 'force'. In this sense Newtonian gravity really hasn't *influenced* much at all - which is not to say it was not successful, Newtonian gravity is really useful and spectacularly good at predictions, even today.

I hope this helps somewhat - just shout back if what I said doesn't make sense! :)


Follow-up email I

Thanks for the questions - the chart is looking really good! I'll answer your questions in turn:

I am wondering about the position and title of the 'under development' cloud at the top.  I was anticipating that this is the theory that aims to unify the four forces at high energies for particles, so maybe the cloud should be positioned more to the left rather than implying it is appropriate for large bodies, please can you clarify for me?  

You are right, strictly it could be more to the left, however you might want to make the cloud bigger so that it extends across the large and small scales and then you can include 'dark matter' and 'dark energy/vacuum energy' which are two things that make up 95% of the universe that we still don't fully understand! They would definitely live in the cloud and be large scale - of course they are not specific *theories*, in the sense of the other elements.

I am vague about the order of development from QED which I know was the first and then electroweak or QCD.  My understanding is that Feynman and Co. created the QED model which then was found/developed to 'fit' for the other two forces.  

The development of these theories occurred more or less at the same time, and by many people.

Do you think I should have an additional separate box for Dirac's relativistic theory of the electron or is it contained in QED?  His work anticipated so much (antimatter, spin etc) that it feels like it deserves a separate area!

Hmmm interesting question. The Dirac equation is pretty amazing. It is contained within QED, but also in QCD, Electroweak theory. I think putting it is more specific than a broad theory - like E=mc2 -so it is debateable whether it should have its own box.

Also, something you might want to include (but certainly don't have to) is a box for "Quantum Fields on curved space-times".  This would go between the quantum field theories section and the general relativity section. It describes *quantum* fields on a *classical* curved space-time, as opposed to the cloud goal, which is to describe *quantum* fields in a *quantum* space-time! QFT on curved space-times is really the edge of what we're confident about - Hawking Radiation from black holes, vacuum fluctuations, particle creation due to gravity...etc. While with the cloud, we're not confident about much of it.

Follow-up email 2

Ah ha of course'dark matter + energy' - I had that on the very first picture I drew - I can't believe I lost 97% of the universe just like that :((  Anyway - my understanding is that there are multiple theories of what this might be.....I'm not sure if this includes the Higgs field....come to think of it - where does the Higgs field + boson sit?

The Higgs field lives in the standard model, and is widely believed to be the mechanism by which particles acquire their mass. At present it is the only major part of the Standard model that hasn't been detected experimentally.

As you rightly said, there are many, many, many models for dark matter and dark energy - some of which are within the cloud theories, but not all of them. I think you could easily justify the following for the cloud region:

(a) moving it to the left more.

(b) keeping it where it is.

(c) expanding it in both directions.

The important aspect should be that the left region is cloudy! :)

"Quantum Fields on curved space-times"  I am definitely open to including this.....but I am trying to at least have some intuition of what every item on this chart is about, so I can 'own' it :))  Maybe you can explain this to me when we meet up?......I don't even know what a 'quantum space time' is :(

Yes, of course - I can explain this in person. Briefly a quantum space-time is where events do not have a precise place or a precise time at which they occur, and geometric notions cannot be applied. Really no one knows what quantum space-time is :))

Follow up email 3

Hey there:

Can you elucidate for me the relationship between Newtonian Mechanics and Thermodynamics?

Hmm well it is not a perfectly clear cut thing. Newtonian mechanics deals with forces and dynamics, while thermodynamics is more interested in the energy side of matters. So the two overlap in certain concepts, but while Newtonian mechanics seeks to predict precise dynamics of well defined objects and forces, thermodynamics deals with big composite systems in which you are interested in a small number of their parameters. 

To do thermodynamics properly you need Newtonian mechanics, but the converse is not true! 

Should there be an arrow from Newtonian Gravitation to general relativity?  I kind of assumed that GR was fresh out of Einstein's head but maybe aspects of its form were influenced by NG.....I honestly don't know – as I'm not very familiar with GRyet.

Well, yes, I think there should be an arrow from Newtonian gravity to GR. While the two theories are quite different in character, they obviously deal with the same topic!  Also, a key aspect of GR is the equivalence principle which Einstein arrived at by studying the Newtonian theory. In the Newtonian theory inertial mass equals gravitational mass by assumption in the Universal Law of Gravitation - this is an important non-trivial fact!



David Jennings is a theoretical physicist specialising in Quantum Information.  On the relation of theoretical physics with the humanities, he has the following opinions:

"Personally, I feel that theoretical physics shares many aspects with the humanities. It certainly has a strong philosophical component - e.g. it is necessary to adopt some kind of interpretation of a theory - and involves a strong creative and imaginative component. In addition, I think that there is a strong linguistic element to what we do - we stumble around like clumsy primates building new nouns, new verbs, new adjectives and adverbs to describe the universe around us. Sometimes, finding the right word can be half the battle."