So you want to learn physics (2021)
susanrigetti.com
susanrigetti.com
Hydraulics are everywhere. Ever used a sink? Flushed a toilet? Contemplated an air filter? Felt both sides of a small fan? Wondered how, exactly, a utility pump causes water to go in the inlet and out the outlet, and tried to read the manufacturer’s spec? Contemplated that the ripples when you throw a rock in an actual pond really don’t resemble the average “look I made water in WebGL” animation very much?
And more fancily, and very much in “Physics”, cosmological models usually model the universe as being full of a spatially varying continuous fluid. Stars are plasma or weirder things, and those are fancy fluids.
Yet, for some reason, the basics are missing from “Physics”. You can sometimes find them in mechanical engineering departments, and Feynman covers it a bit in his lectures.
Obviously there needs to be a transition, but at some point you go from physics to engineering. I suppose it depends what specialty in physics you go into. Nobody can specialise in everything.
Engineering: practical implementation
That’s how it goes in my brain. It’s physics until we can build it reliably, then it’s engineering.
(I didn't introduce it - it was already being used).
The benefit of taking such a class or reading such a textbook is that these things have been studied extensively, we have good models for them, and it is useful to know because people are still doing fundamental research on it to this day or working on phenomena that are closely related.
1. The boundaries between disciplines are where they are in part by historical accident, and in part because that's what the people working in them find useful - there is no actual fact of the matter.
2. We don't actually know the underlying microprocesses of anything. Effective theories are all we have, and there's no fundamental difference between an effective theory for the vacuum (if it is a vacuum) and one for, say, the bulk of a semiconductor.
This is true of all categories, so not a helpful comment. I suspect someone has a good enough definition.
I think the more relevant aspect is that to reach the frontier in a wide array of fields you a solid grounding quantum physics (and several other "new" -- i.e. within the last century or so -- topics) that have displaced more "old-fashioned" topics like continuum mechanics.
if you can understand the PDEs of GR and QFT, you can apply it to this too
But I probably like abstractions like this more than most people.
Oddly, undergraduate physics also seems to be missing another, arguably even more fundamental, tensor: the moment of inertia. You can get quite far (in three dimensions, and only in three dimensions) by thinking of rotation as a vector. (Or a quaternion if that floats your boat.) But you can’t get very far by pretending that the moment of inertia is a scalar, and you get very confused very quickly if you treat it as three scalars in the magical coordinate system in which you can write it like that.
Classical ( non relativistic ) field theories by now are undergrad engineering topics, but there are only a few quantum engineers.
Most of the non quantum topics in modern undergrad physics curricula, is needed to make sense of quantum[ thermodynamics, filed theory, optics, something ]
> Solving problems is the only way to understand physics. There's no way around it.
This generalizes well to other fields. I don't want to discourage anybody from trying to educate themselves in a difficult field (be it physics or something else), but that's a very common and immediately visible problem with autodidacts. If you haven't worked through enough hard problems you lack the intuition that ties together theory.
Now I place the concrete over everything else; theory is nice when it can illuminate why the practice works. Otherwise, it’s just words.
The most frustrating is when I have friends who have derived their entire understanding of a subject I know as a practitioner (typically something tech/programming related) from watching YouTube videos/listening to podcasts.
Because they’ve heard hours and hours from experts, they have a feeling of deep understanding. But talking to them about this topic is extremely frustrating because their knowledge clearly has never had to be applied to the real world, and is grounded in nothingness, so they misunderstand lots, but they feel like they know what they’re talking about as much as you do.
> Now I place the concrete over everything else; theory is nice when it can illuminate why the practice works. Otherwise, it’s just words.
That mirrors the trajectory of all humankind, doesn’t it? From lofty Platonic ideals to nitty gritty empiricism and experimentation.
You reference Plato, but you could easily phrase it in the other direction. "From Roman engineering to Wittgenstein blithering on about meaning."
I would also deemphasize the more mathy parts of calculus - do you really need a deep dive into continuity or the fundamental theorem of calculus? Eventually, yes. But it's just like programming: you're not going to need to understand language theory or ADTs or category theory or lambda calculus to write your first program. Or your second. And, IMHO, you should only reach for this understanding when you realize you need it. Otherwise, it won't integrate well into your toolkit.
> If you present the hard problem first, the student may flail around and realize: I need something to help with this!
As I've grabbed my horse and lance and rushed at this windmill from assorted directions, I quickly run into my limitations that prevent the problem being solved. I run into vocabulary problems with the math, the fact that I simply don't have the math to approach the problem (which appears to be some vector calculus -- I think. "No, you idiot, it's XYZ instead", but I don't know enough to know that it's not vector calculus, if, indeed, it isn't). I try to apply basic kinematics to the problem, but I don't know if that's enough. And, finally, it could be all of those things plus, oh, some optimization issues and, also, would you like to be introduced to the several different techniques for computing numeric integration and the differential equation solvers?
"Eeep!"
To quote the film "Addams Family Values":
Wednesday: Pugsley, the baby weighs 10 pounds, the cannonball weighs 20 pounds. Which will hit the stone walkway first?
Pugsley: I'm still on fractions.
The videos that it was essentially transcribed from are great tho.
If it exists, you could probably replace most of the first book with just a really good explanation of the Lagrangian, with lots of examples, I think.
Yes.
> If you present the hard problem first, the student may flail around and realize: I need something to help with this! Now that they know they need it, and why, you can give them the tool that fits the bill.
No.
If an instructor deliberately gives a student a problem that they know the student _cannot_ solve, then it rightfully destroys trust.
I never taught at the university level, but with middle and high school math students I taught them to how (re)discover the solutions, rather than teaching them the solutions directly.
As a practical matter, many of my college classes went too quickly to do anything _but_ teach the solution -- or tell us to learn it between classes and bring questions back.
This does not destroy trust, but gives the student an important lesson: we only have the techniques to solve, say, 0.0000001% of the problems. So you have to learn brutally hard for the next many years (or rather decades) to have the minimal qualifications to be able to invent whole new techniques that no person has ever come up in history before to increase this ratio from, say,to 0.000000100000000001% (even this would trigger a whole new aera in the history of science).
> As a practical matter, many of my college classes went too quickly to do anything _but_ teach the solution
I dropped that class. I always felt it was a mistake not taking calculus in High School. I had a very good relationship with the math teacher there, and we could have done it, to some level, casually between classes. I just didn't take him up on it.
I've never learned calculus.
Issue is that often times people don't know that a certain tool exists, so they re-invent a 100x worse version and just hack something.
There is no substitute for solving problems.
Some anecdotal evidence for this:
https://www.dwarkeshpatel.com/p/dario-amodei#details
> Dario Amodei: We have generally found that if we hire someone who is a Physics PhD or something, that they can learn ML and contribute just very quickly in most cases.
In math/physics, it often won't. Solving lots of problems serves two particular purposes: To really solidify the concepts in your mind so you won't forget, and ensuring you learn the techniques and not just the knowledge.
For the former, you may find yourself in the position where you find yourself way over your head, and won't know where to start. You usually will not have a single gap, but many. You'll find yourself realizing you'll need to look up material from several textbooks to regain the knowledge you've lost. Once you begin that process, you'll pick up one of your old textbooks and while the physics knowledge may be absorbed, you'll realize you've forgotten much of the math needed to solve such problems. In the unlikely event you'll retain enough to follow the textbook, it is very unlikely you'll know the techniques well enough to solve the real world problem.
And your colleagues will. You'll be alone, and you'll drop out of that group. With physics/math, there often are hard boundaries in these groups. Those who meet the bar are in. Those who don't drop out, because it really sucks being the only person in the group who is struggling with what everyone else considers as basic.
SW engineering has a much more gradual change in skills amongst people, and usually the problems most people work on are fairly learnable in a short amount of time.
Math and its applications are a contact sport. You don’t truly appreciate it until you try to use it yourself.
I'm going to ruminate on this.
But you will spend the rest of your life arguing with people who insist that there must be a ("quick and dirty") substitute and you're responsible for finding it.
Be it microservices, coronavirus or taxation.
I emphasize this story to my kids because knowing isn’t important because everyone eventually figure it out. It’s the ones who can do the problems and get good marks that succeed in the end.
Then again, in the process of teaching I always found myself teaching people to work problems, which required me to be able to work the problems myself. In a way, it's kind of impressive you managed to avoid doing that.
The gap between knowers and doers will only get larger as math explainers improve their content.
Nowadays it's a lot easier when there are so many free materials from top school online. And stack exchange and reddit is available almost 24-7 if one ever has a question.
Many textbooks still employ the deplorable practice of not presenting the answer to all exercises at the end, unfortunately.
The reason is that you think you understand what you read, but as Richard Feynman said:
> The first principle is that you must not fool yourself, and you are the easiest person to fool.
You think you understand 90% of what you read, but in reality it's probably only 20-30%. By doing the exercises, at the very least you'll know that you don't know that much. And if you then reread the materials a few pages before, you'll realize that you have skimmed (or worse, skipped) some parts because you mistakenly thought you already understood it.
Another tips from my personal experience: When you're reading a textbook, keep asking in your mind questions with the types of "what if" and "how about," which are sometimes not yet explained in the section you're reading. Also, keep associating what you've recently learned with what you've already known (days ago, years ago).
Be curious and validate that you really understand what you think you understand.
I agree that there is a division between who loves that book (like the author) and the majority of the graduate students who had nightmares (and sometimes still gets). I like this goodreads review of the book [1]
> A soul crushing technical manual written by a sadist that has served as the right of passage for physics PhDs since the dawn of time. Every single one of my professors studied this book, and every single one of them hates it with a passion. While I've no intention of becoming a professor, I still wonder, will my colleagues also inflict this torture on their students? Will the cycle be perpetuated ad infinitum? How many more aspiring physicists will we leave battered and bruised at the gates of insanity before switching to a textbook that seeks to make electrodynamics clear and intuitive rather than a mind-numbing trip through the seventh circle of hell?
[1] https://www.goodreads.com/review/show/1266180525
* personal note: If this book is really the bible of classical mechanics, then I'm atheist.
> Now, a few years after writing that review, I must return to say that as much as I hate this book, it's probably the best textbook that I have. I constantly return to it to reteach myself basic concepts or math. The problem with the text is that in order for it to be useful, you pretty much have to already understand the material. It's a dense, technical manual that, when paired with an easier to understand text such as Griffiths, grants tremendous power. Don't get me wrong, if there is a hell, I personally hope John David Jackson is burning in it right now, but I also have to tip my hat to him
Classical Electrodynamics by Jackson (essential). This is the bible of classical electrodynamics, and everyone who works through either loves it or hates it (I loved it).
If you're smart enough that advanced college physics comes as easily as learning to talk, I guess this would be true. The author of this guide is such a huge outlier in every respect of life. I have seen and read many smart people and she's easily in Witten or Tao territory of just being otherworldly smart. I don't think she ever encountered anything being hard. Jackson for her is like a walk in park, which is otherwise regarded as a formidably hard text.
While generally little known and appreciated among modern theorists and mathematical physicists, physics is actually an empirical science. In other words, every single section of that reading list is based directly or indirectly on a diverse and sophisticated set of devices and measurement configurations (aka experiments). Also, most progress in our understanding the physical universe follows simply from inventing ever better probes and opening new observation windows.
A computer analogy of the theoretical/empirical physics relation might be fun: You can spend your whole life writing application software and never even know what digital devices you are actually using. That's totally legit. But if you want to write a new computer language (= a new theory) you most likely will have to dig into memory architectures and caches and all that stuff. If you want to dramatically increase the speed of computation (= a new observation window) you have to design a new chip. And if you want to go really deep and invent new computing paradigms, well then you need to learn quantum mechanics :-)
In fairness, she does have a final sentence about that weird place called laboratory (= a place of labor).
> And, finally, a note on learning in a laboratory vs. learning from textbooks. Physics is both an experimental and theoretical science, and while research happens in laboratories and on blackboards and computers, the majority of any physics education does not take place in a laboratory but in lecture classes that teach from textbooks and assign homework problems that are found in textbooks.
My recommendation for a comprehensive intro into theoretical physics is The Road to Reality by Roger Penrose. Alas there is no such profound review of all experimental physics.
But I find that so much time has passed that I would need to brush up parts of my high school maths first, and this kind of discourages me before even starting.
A beast it still is, I think it is contained in its own walls. I can skip any topic in Quantum Physics and others that is irrelevant.
I'm wondering if it's helpful to you too to focus on something smaller.
Of course, it might also be impenetrable if you haven't had prior exposure to most of the material.. I have zero experience trying to teach physics from it.
That said, this book does look like a great text for someone with a graduate level physics knowledge who wants to refresh their memory.
There are lots of pathways to learn & do. That is especially true of people coming to topics later in life, as I might expect is more common for HN comment thread readers. I know someone who learned to program in x86 assembly before they learned C (in fact one of the best programmers I know). If you talk to such people, I think you will find that their more varied backgrounds / ages in life when they approach things make anyone's dogmas more suspect. A great numerical relativist didn't study physics until his 30s. No idea what order he did things in, but I heard it was very non-standard.
So, I would encourage you to have more imagination of what might be possible / be less dismissive / jumping to conclusions. That is needlessly discouraging to many here were bemoaning "soooo many books/years/etc". "Ambition" might be "first learn diffgeo, then learn physics". I've been recommending that lately to a friend with extreme mathematical sophistication (a professor even) but no physics exposure, actually, but already some diffgeo exposure.
And, of course, "to learn physics" is maybe not to be a "produced physicist" any more than "to learn networks" means to "build a hardware router". It all depends. IMO, there are too many levels (& even directions/dimensions) of "having learned" to really even make "just can't", "only way" statements like you did. Elsethread, the diversity of even what "physicists" wind up having learned is shown to vary considerably (e.g. continuum mechanics). I find such absolutist statements needlessly discouraging to someone who might be hopeful to do it in "fewer steps". The way for most need not be the way for all or even the recommended way for any one person. People vary.
The Post is basically about how to get a degree or at least the equivalent and would require an effort of years.
So You Want to Learn Physics (2016) - https://news.ycombinator.com/item?id=24088985 - Aug 2020 (124 comments)
So You Want to Learn Physics (2016) - https://news.ycombinator.com/item?id=18374994 - Nov 2018 (122 comments)
So You Want to Learn Physics - https://news.ycombinator.com/item?id=12691963 - Oct 2016 (129 comments)
You’ll already be one step ahead by the time you get to the second course, which is good, because you can strongly benefit from learning vector calculus at that time. I really enjoyed the text “Div, Grad, Curl and all that”.
However, I seem to be interested in a few particular questions about particle physics as a science rather than facts about particle physics. For instance, I am interested in the instruments and methods that physicists use to verify their theoretical claims empirically. I am also interested in how theorists are able to come up with theories so early on, such that they are confirmed by evidence many years later. What are the assumptions that they were able to make? I am curious about where they derived the creativity to be able to bring in so many assumptions together and the come up with their models. Now that I write this, I realize that before theories were validated there were probably competing models.
Therefore, I am not exactly sure I want to study particle physics per se, or whether a book on the history of particle physics will do. I am ok with having a popular understanding of the subject, I mostly want to gain inspiration from following the work of famous scientists.
If you have a physics education (I have an engineering education) can you tell me if you can really get a physics degree without bumping into Bernoulli or Navier-Stokes?
At least just to see the lay of the land.
It will really be dependent on what is your physics field but you can definitely survive in physics without deep knowledge of fluid mechanics except when your study require it
PS: I am a particle physicist.
I chose to take a particle physics course as an elective in my final year - I was planning to specialize in battery and capacitor technologies and wanted to learn more.
The lectures were very different to Engineering much, much more theory focused(almost nothing on applications) it was my introduction to things like Hamiltonians, Wave Functions and Fermi-Dirac statistics. I'm glad I took the course I learnt a heap especially about semi-conductors it gave me a better appreciation and understanding of things we covered in my engineering degree like magnetism and phonons/heat transfer as well. But I will say it did feel like another world compared to Engineering - there was much less in common than I would have thought.
Also, physicists don't necessarily include fluid dynamics as a core discipline. It is almost mechanical engineering to them. I'm not surprised to see it missing.
Also stating the equation isn't the same thing as studying it.
They're both pretty firmly physics things rather than applied or engineering things, in their purest forms...
In my view, what's taught for a physics degree is more of a historical accident than a selection of the most important principles. In an extraterrestrial civilization, the boundaries between engineering, physics, and chemistry may be entirely different.
Dismissing Navier-Stokes as just a consequence of Newton's laws and thus unimportant can be extended further towards dismissing a large fraction of what's taught in physics degree programs. An undergraduate physics student may get more education on Bose-Einstein condensates (which are just a consequence of quantum mechanics :-) than they do on Navier-Stokes. The Navier-Stokes equations are a lot more important than Bose-Einstein condensates in my view.
And for that purpose of being an intermediate degree to becoming a physics PhD, Navier-Stokes isn't relevant. You don't use it in most fields that are generating physics PhDs in the 2000s and beyond.
There's only so much time to teach somebody in four years and there are significantly more important things that are also being left out (e.g. more thorough courses on group theory).
That's because physics degrees don't include much on fluid dynamics. If someone wants to get a PhD in fluid dynamics, they probably get a PhD in some variety of engineering. This goes back to what I said about the physics curriculum seeming weird to me, as it it's not about "physics" in itself. It's more a random selection of topics that exists for historical reasons.
> There's only so much time to teach somebody in four years and there are significantly more important things that are also being left out (e.g. more thorough courses on group theory).
In another comment, you said that you don't know what the Navier-Stokes equations are. Given that, I don't think you're in a good position to judge their value.
I have a couple of group theory books myself, and I don't agree with your assessment that group theory should get priority over fluid dynamics.
It was an exaggeration given that it never came up during my studies once. And I think that's a fantastic assessment of their value that I made it through most of a decade of studies without having to know a thing about fluid dynamics.
> I have a couple of group theory books myself, and I don't agree with your assessment that group theory should get priority over fluid dynamics.
...why? You're commenting on a physics line of education here. We don't use fluid dynamics and we extensively use group theory.
Fluid phenomena is ubiquitous. You live in a fluid. You probably drive a car through a fluid and may occasionally take a plane through a fluid at higher speed. You surely use plumbing. I don't see how you can claim that fluid dynamics is not valuable given that. It's a lot more relevant to most people than quantum mechanics.
On a more important note, the actual topics are completely irrelevant. What's important is learning to "think like a physicist". That's what has value even for those who don't go on to do academic research, which is most students. For any given physics topic that is relevant to real-life applications, there are engineers who actually know how to use it, something that would be ridiculous to expect from the superficial treatment a physics degree has to give any one topic.
To do fluid dynamics research in a physics department, sometimes one has to spin it in some way that people with physics degrees care about. For example, saying that it's to understand chaos theory.
Bernoulli principle was covered in my bachelor degree but Navier Stokes wasn't; true-blue fluid dynamics was either an optional course that I didn't take or a grad student course, I don't remember now.
I first encountered the Euler equation in the context of GR — absurd. In another decade or two, I suspect its rightful place early in the physics curriculum will be emphasized.
Thermodynamics/statistical mechanics was taught as a junior level class at my undergraduate alma mater. During that year, students would take electrodynamics, classical mechanics, and statistical mechanics as separate classes in some loose order, although of course simpler versions of these topics would have been introduced in first year physics.
The lack of fluid mechanics also, unfortunately, tracks with my experience.
This stuck out, pretty rigorous if all you want to satisfy your curiosity. If you want to actually apply any of the hard work you put in, you need a degree.
I think that is the most interesting part of learning anything, applying it interesting ways. Doing that within so many of the areas of study is still gated behind academics.
That killed my motivation for putting effort into most things, pretty much except computer science where we are still ok with trusting self taught people for some reason. But to do anything interesting physics, astronomy, philosophy too you need to be in school. sucks
100% same, except computers bore me to death now. I would even be willing to go back to school at this point if it wasn’t tens of thousands of dollars.
To save someone else a search: https://www.youtube.com/watch?v=8ptMTLzV4-I&list=PLJHszsWbB6...
P.S I mostly self taught physics for my own interest. Not sure if these books are under grad level.
anything new here?