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Physics Travel Guide – Physics concepts explained in three levels of difficulty (physicstravelguide.com)
422 points by karlicoss on April 2, 2020 | hide | past | favorite | 36 comments

From just glancing at a few articles, this site is a treasure trove of references to highly readable explanations on a variety of fairly advanced topics. In fact, just the links to Wilczek's grand unification overview [1], and Klauber's Student Friendly QFT book [2,3] made this site worth visiting for me.

Resources that explain physics in (relatively) simple language without requiring books worth of prior knowledge seem to always be targeted at school-level physics; this one is different.

Of course, this is no substitute for a university curriculum, but as a way to plug holes in one's understanding, the site seems genuinely high quality.

[1] http://frankwilczek.com/Wilczek_Easy_Pieces/172_Unification_...

[2] https://www.quantumfieldtheory.info/

[3] ftp://srdconsulting.com/USB_BackUp/Data/Articles/QFT/StudentFriendlyQFT/

I haven't read it in full but Klauber's book is supposedly loaded with errors and also (in my view) needs re-typesetting (it's written in word)

Interesting. I am slowly building a computer science dictionary with a similar concept: three levels of difficulty https://cs.quickref.dev/

Keep doing. Good stuff. Subscribed your youtube channel as well.

That's very cool!

Almost nothing about experiments. Physics is an experimental science, based on observation of nature.

Its treatments of experiments is as if they were mere side notes to the real thing. The Tools section might include something like a ruler, clock, or basic physical tools used to observe nature with.

I strongly disagree "Physics concepts explained" has no need to add anything about experiments. The focus of the site is learning concepts.

Criticizing focused educational site for not being comprehensive enough is not valid.

I strongly disagree. Experiencing concepts first-hand through experiments improves learning and understanding, excites curiosity, and teaches practices that can be used in other disciplines.

I learned about many physics concepts with simple experiments in elementary school. They can certainly be extended and adapted for an adult audience.

What you say is true about learning physics. Not valid criticism against this site.

It's not meant to be the only source for learning physics. It is focused reference site for concepts. Having focus and focusing on just one viewpoint is a good thing.

> Physics is an experimental science, based on observation of nature.

I strongly disagree with this. In my first undergrad QM course I barely learnt anything about experiments. Experiments are quite important, but they are just side notes to the real thing.

Experiments are a way to check whether certain theories make sense, but not all theories. Many concepts have been proven mathematically, but have not yet been tested through experiments. Does that mean that theories are not part of physics? How do we know how elementary particles behave on a scale so small that we can’t observe their behaviour?

When we’re not able to acquire data, what's left for us is to construct new theories purely based on mathematics. Physics does not end where we can’t do observations, it actually starts exactly there.

Might be a good idea to understand the concept in general and then take a look at the experiment to prove it.

Huh. The first thing I opened was Aharanov-Bohm which immediately describes an experimental setup.

I found this section confusing (emphasis mine):


"This means that for all systems where the Hamiltonian does not explicitly depend on the time, we known (sic) immediately how the time-dependence of the total wave function Ψ(x,t) looks like (sic), namely: Ψ(x,t) = phi(t)ψ(x) = Ae^(−Et/ℏ)ψ(x). The only thing we then have to do is to solve the stationary Schrödinger equation Hψ(x) = Eψ(x)."

It sounds like they're saying that all systems with a time-independent Hamiltonian are stationary, which is obviously wrong. This would have confused the hell out of me while learning QM, and dissuades me from learning more on this site.

Have I misread? I can infer what they're trying to say from context and previous knowledge, but that's of little help for someone new.

>Ψ(x,t) = phi(t)ψ(x) = Ae^(−Et/ℏ)ψ(x)

This equation is missing a sum symbol, also it should be e^(-iEt/ℏ), where in the correct case the amplitude does not change. What you get is a sum of products of each occupied stationary state and a time-dependent phase. So the general solution looks like this:


Indeed. But as another commenter points out, what I think they wanted was to solve only for eigenstates here. Apparently I'm not the only one this confused.

You have misread slightly I fear. Note that he says:

>the stationary Schrödinger equation Hψ(x) = Eψ(x)."

which is not the original Schrödinger equation, but a Schrödinger equation which is stationary and which forms part of the solution of the original Schrödinger equation.

Thanks. That's the context I was referring to. Reading the whole thing again twice more, I can still confidently say it would have confused me more than helped me when I was first learning.

This is really great, would love something like this for pure maths.

A few answers from when a similar question was asked last time


Just looking around, but this looks like a gold mine ! Definitely going to be my go to site when I'm bored ! I love how each concepts are explained at high school, college, and grad school levels !

Generally very nice, but I did not find the 3 levels of difficulties. Where can I see them?

They are the tabs just below the title of every subject.

Some of those articles are just a modern Tantra - no one is seriously trying to refute these models because lots of people have a comfortable, high social status living out of it, just like modern theologians or tibetologists - they know the mantras and it's current interpretations, and the question of "How real it is" is frowned upon.

Generalisations aside (however valid and useful) most of this theoretical physics is nothing but a socially constructed models, agreed upon.

And yes, I have read a few of Bohm's books before forming an opinion.

I don’t endorse the tonality of the comment, it’s rambling and sort of bitter (I’ve been guilty of worse on “the Internet”).

With that said it’s at least possible that the commenter is alluding to a trend in physics research that is so concerning to practicing physicists and mathematicians that books called things like “The Trouble with Physics” and “Not Even Wrong” are being written and published.

I’m little better than a layman as concerns physics, but I’ve been around the block with organizational dysfunction amongst a group of super smart people, and I don’t find it at all implausible that String Theory is the “Docker-First” of physics funding.

> Some of those articles are just a modern Tantra

Would you care to provide concrete examples?

> Generalisations aside (however valid and useful) most of this theoretical physics is nothing but a socially constructed models, agreed upon.

Well, of course. Theories are formal systems that we construct to be a "looking glass" of the results of observations and experiments.

An 'esoteric tradition' cannot be overturned by direct observation available to anyone.

To an extent, there's certainly an 'esoteric priesthood' in many sciences which can be quite dogmatic about certain models. It's certainly true that valuable observations will sometimes be ignored for a time - but eventually, theory increasingly fits observation.

It's certainly not a 'nothing but' in the same sense as tantra. The predictive power of theories is essential to their fame. Einstein's early 1900s models - which seriously refuted earlier models - anticipated many later observations.

Esoteric traditions like tantra have been overturned by the experiences of one individual and the insights they transmit to others. All part of the same play.

Is that you jonathan tooker?

Before Hegel and his abstract bullshit, the main question of philosophy was "What is real", not what a mind can imagine.

Similarly, the main question of physics used to be "how everything is", not how do we think everything might be.

A map is not a territory, a model is not what is. It is that simple and infallible.

> the main question of philosophy

Well, philosophy is known to ask questions for which there are no answers. As to (theoretical) physics, I think we should look at it first and foremost as a computational tool (similar to applied mathematics) which is the way it has been since Newton, then it becomes clear that physics does indeed help understand "how everything is" - in the sense that it helps us predict the results of experiments, and I am not sure if it even makes sense to talk about a "higher level of understanding" than that. A map is definitely not a territory, but it can indeed be (and will always remain to be) both "true" and "false" to a certain degree; similarly, no one in their right mind would say that physics is, should, or can be equivalent to the reality (aspects of) which it merely reflects.

So, are you a model of Jonathan Tooker, or are we encountering the real deal here?

> Similarly, the main question of physics used to be "how everything is", not how do we think everything might be.

I assure you, philosophy of physics is much more complicated than that, and a lot of senior physicists and leaders in the community are aware of these subtleties, and constantly argue about it.

No Special or General Relativity - WTF?

OK found them hidden away under "models" - WTF again?

General relativity is (sort of) a classical field theory; it isn't crazy to put it under models, in the same way that QFT is under 'theory' but the standard model is under 'models'.

Wikis are intrinsicly hard to organize in a hierarchy. There is a search though.

That is how I found them, estranged from the other "theories" for some reason.

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