7 ms·

He's so totally right about the hand waving approximations used in solid state physics. All fields of physics need to use some approximations, but those in the

He's so totally right about the hand waving approximations used in solid state physics. All fields of physics need to use some approximations, but those in the solid state were usually the ones with the worst reasons, just things like "it works if we do that" or even "it doesn't if we don't do this". It went on like that in three courses (two on solid state physics, one on electronics). Only after that did I happen to come across a decent book which explained some of the approximations in a way that didn't make me cringe, and some of the stuff started to make sense in retrospect. Others are still a mystery to me, and probably always will be.

schmittz 16y ago
It's often poorly explained, I completely agree, but people forget that even Planck introduced his constant just because it made the data fit. It turned out to be arguably the most fundamentally important constant (along with its reduced form) in all of physics.
- arethuza 16y ago
  Aren't all fundamental physical constants like that - they are essentially meaningless values plugged into theories to make them align with observations?
  - drhodes 16y ago
    yeahbut, some are interrelated. Like the speed of light and the permittivity of freespace (1/c^2 * mu). If you measure that constant and solve back for c, then you better get ~3 * 10^8 m/s.
  - lscharen 16y ago
    Mostly because we (humans) made up physical units (i.e. meters) while we were inventing physics. There's nothing keeping you from re-deriving physics equations without the messy constants. http://en.wikipedia.org/wiki/Natural_units http://en.wikipedia.org/wiki/Natural_units
    - neutronicus 16y ago
      Even then there are some dimensionless numbers that you can't get rid of (the ratio between the strength of the gravitational and coulomb forces being one obvious example).
      - waqf 16y ago
        Pi being another.
        arethuza 16y ago
        I wouldn't have said Pi was a physical constant - it's not like we actually measure circles to derive it empirically.
        waqf 16y ago
        True, but it certainly is a dimensionless number that you can't get rid of (and that actually shows up in physical formulae, such as http://en.wikipedia.org/wiki/Coulomb%27s_law http://en.wikipedia.org/wiki/Coulomb%27s_law). Come to think of it, so is 2. I guess if you tried to measure π by constructing circles, you'd actually be measuring the curvature tensor of space (which is an experimental observation), not π (which is just π).
        derefr 16y ago
        π being, simply, the measurement of the curvature tensor of an ideal Euclidean plane. If we lived in a universe that was non-Euclidean at macro-scale, π would be just another irrational number, and some other quantity would be exalted as "fundamental."
        gjm11 16y ago
        Pi would still be fundamental in mathematics whatever the geometry of our universe. (Examples of where it would turn up: consider the differential equation f''=-f; all its solutions are periodic with period 2pi. The series 1-1/3+1/5-1/7... has sum pi/4. The series 1+1/4+1/9+1/16+... has sum pi^2/6. exp(pi sqrt(163)) is ridiculously close to being an integer. There are deep reasons for all these things, and they wouldn't go away if the universe were very far from spatially flat.)
  - Zaak 16y ago
    That's what they look like at present, but it's possible that future theories will allow us to calculate their values from first principles.
jodoherty 16y ago
Do you remember the title or author of that book?
- mechanical_fish 16y ago
  I don't necessarily know what book he's referring to, but the book that taught me solid state was Ashcroft and Mermin. That's a book that deliberately starts out with the silliest possible model and then gradually introduces more and more sophisticated models until the book runs out of pages or your head explodes, whichever comes first. ;) An important insight, however, is that the simple models are incredibly useful. When I was a teenager I used to fret that the teachers were showing me the simple models first because they were fools, or because they thought we were fools. But it turns out that they are doing so to call attention to certain important general features without distracting you with irrelevant detail. My favorite class in all of science was Roald Hoffman's chemistry class, the highest numbered class in the Cornell chemistry department, a class which represented the point where chemistry and physics merge to become the same subject. And Hoffman deliberately used a relatively simple model of molecule-molecule interactions in a solid, the Hückel model: http://en.wikipedia.org/wiki/Hückel_method http://en.wikipedia.org/wiki/Hückel_method Moreover, Hoffman would communicate in pictures as much as in math.
  - Jach 16y ago
    Have you ever read Feynman's Lectures on Physics Volume 2 (Electricity and Magnetism)? One of the things I love about it is that it starts with Maxwell's equations, in their non-simplified form, then goes into the special cases of electrostatics, magnetostatics, and then uniting them into electrodynamics. Feynman goes over the special cases in a way that you never forget that they're in the end incorrect simplifications, there's even a table at the end of the statics section showing "These equations are false, these are true in general." I don't mind simplified incorrect models so long as they're presented that way (so I don't want to read a "all that stuff we just covered? Yeah it's wrong"-ish sentence after the fact) and the full truth is eventually revealed in an understandable way. I want a full picture of something, not a partial incorrect picture of something, it scares me that some people never stop thinking of an atom as a small planetary system.
tzs 16y ago
If anyone needs a refresher in semiconductor physics, there's a good one at this Britney Spears site: http://britneyspears.ac/ http://britneyspears.ac/ edit: how the heck does a link to an excellent introduction to semiconductor physics get voted down in a thread about an article relating to semiconductor physics?
- KirinDave 16y ago
  This is a good joke, but too subtle for most of the people who moderate on HN. My advice if you care about that sort of thing: link in further, e.g., http://britneyspears.ac/lasers.htm http://britneyspears.ac/lasers.htm
- profquail 16y ago
  You're getting voted down because most people see the domain "britneyspears.ac" and assume you're spamming, without even visiting the link. That site actually does have some decent articles related to semiconductor physics, the whole "Britney Spears" thing is just a gag to show how simple they've made it (though the humor is likely to be appreciated only by Physics geeks :)
- makmanalp 16y ago
  Sweet jesus, it's true! http://britneyspears.ac/physics/vcsels/vcsels.htm http://britneyspears.ac/physics/vcsels/vcsels.htm
- biot 16y ago
  While there does appear to be some physics articles on the site, the whole thing appears to be cloaking itself in legitimacy as a scheme to get inbound links and drive up its page rank score. Their strategy is likely to get people linking for the physics so that they can sell links for heating oil prices, concert tickets, physics help, and so on. It's more about the advertising than the physics. I'm sure there's a less spammy physics site you could have linked to?
  - JonnieCache 16y ago
    On the contrary, the page is a piece of internet history. As you can see from the domain, it was created by an academic in an early example of titlebait. http://replay.web.archive.org/20001002213854/http://www.britneyspears.ac/ http://replay.web.archive.org/20001002213854/http://www.brit... Its basically a reverse rickroll. "Britney Spears" was by far the most searched-for term on the internet for a long time.

schmittz 16y ago

It's often poorly explained, I completely agree, but people forget that even Planck introduced his constant just because it made the data fit. It turned out to be arguably the most fundamentally important constant (along with its reduced form) in all of physics.

arethuza 16y ago
Aren't all fundamental physical constants like that - they are essentially meaningless values plugged into theories to make them align with observations?
- drhodes 16y ago
  yeahbut, some are interrelated. Like the speed of light and the permittivity of freespace (1/c^2 * mu). If you measure that constant and solve back for c, then you better get ~3 * 10^8 m/s.
- lscharen 16y ago
  Mostly because we (humans) made up physical units (i.e. meters) while we were inventing physics. There's nothing keeping you from re-deriving physics equations without the messy constants. http://en.wikipedia.org/wiki/Natural_units http://en.wikipedia.org/wiki/Natural_units
  - neutronicus 16y ago
    Even then there are some dimensionless numbers that you can't get rid of (the ratio between the strength of the gravitational and coulomb forces being one obvious example).
    - waqf 16y ago
      Pi being another.
      - arethuza 16y ago
        I wouldn't have said Pi was a physical constant - it's not like we actually measure circles to derive it empirically.
        waqf 16y ago
        True, but it certainly is a dimensionless number that you can't get rid of (and that actually shows up in physical formulae, such as http://en.wikipedia.org/wiki/Coulomb%27s_law http://en.wikipedia.org/wiki/Coulomb%27s_law). Come to think of it, so is 2. I guess if you tried to measure π by constructing circles, you'd actually be measuring the curvature tensor of space (which is an experimental observation), not π (which is just π).
        derefr 16y ago
        π being, simply, the measurement of the curvature tensor of an ideal Euclidean plane. If we lived in a universe that was non-Euclidean at macro-scale, π would be just another irrational number, and some other quantity would be exalted as "fundamental."
        gjm11 16y ago
        Pi would still be fundamental in mathematics whatever the geometry of our universe. (Examples of where it would turn up: consider the differential equation f''=-f; all its solutions are periodic with period 2pi. The series 1-1/3+1/5-1/7... has sum pi/4. The series 1+1/4+1/9+1/16+... has sum pi^2/6. exp(pi sqrt(163)) is ridiculously close to being an integer. There are deep reasons for all these things, and they wouldn't go away if the universe were very far from spatially flat.)
- Zaak 16y ago
  That's what they look like at present, but it's possible that future theories will allow us to calculate their values from first principles.

jodoherty 16y ago

Do you remember the title or author of that book?

mechanical_fish 16y ago
I don't necessarily know what book he's referring to, but the book that taught me solid state was Ashcroft and Mermin. That's a book that deliberately starts out with the silliest possible model and then gradually introduces more and more sophisticated models until the book runs out of pages or your head explodes, whichever comes first. ;) An important insight, however, is that the simple models are incredibly useful. When I was a teenager I used to fret that the teachers were showing me the simple models first because they were fools, or because they thought we were fools. But it turns out that they are doing so to call attention to certain important general features without distracting you with irrelevant detail. My favorite class in all of science was Roald Hoffman's chemistry class, the highest numbered class in the Cornell chemistry department, a class which represented the point where chemistry and physics merge to become the same subject. And Hoffman deliberately used a relatively simple model of molecule-molecule interactions in a solid, the Hückel model: http://en.wikipedia.org/wiki/Hückel_method http://en.wikipedia.org/wiki/Hückel_method Moreover, Hoffman would communicate in pictures as much as in math.
- Jach 16y ago
  Have you ever read Feynman's Lectures on Physics Volume 2 (Electricity and Magnetism)? One of the things I love about it is that it starts with Maxwell's equations, in their non-simplified form, then goes into the special cases of electrostatics, magnetostatics, and then uniting them into electrodynamics. Feynman goes over the special cases in a way that you never forget that they're in the end incorrect simplifications, there's even a table at the end of the statics section showing "These equations are false, these are true in general." I don't mind simplified incorrect models so long as they're presented that way (so I don't want to read a "all that stuff we just covered? Yeah it's wrong"-ish sentence after the fact) and the full truth is eventually revealed in an understandable way. I want a full picture of something, not a partial incorrect picture of something, it scares me that some people never stop thinking of an atom as a small planetary system.

tzs 16y ago

If anyone needs a refresher in semiconductor physics, there's a good one at this Britney Spears site: http://britneyspears.ac/ http://britneyspears.ac/ edit: how the heck does a link to an excellent introduction to semiconductor physics get voted down in a thread about an article relating to semiconductor physics?

KirinDave 16y ago
This is a good joke, but too subtle for most of the people who moderate on HN. My advice if you care about that sort of thing: link in further, e.g., http://britneyspears.ac/lasers.htm http://britneyspears.ac/lasers.htm
profquail 16y ago
You're getting voted down because most people see the domain "britneyspears.ac" and assume you're spamming, without even visiting the link. That site actually does have some decent articles related to semiconductor physics, the whole "Britney Spears" thing is just a gag to show how simple they've made it (though the humor is likely to be appreciated only by Physics geeks :)
makmanalp 16y ago
Sweet jesus, it's true! http://britneyspears.ac/physics/vcsels/vcsels.htm http://britneyspears.ac/physics/vcsels/vcsels.htm
biot 16y ago
While there does appear to be some physics articles on the site, the whole thing appears to be cloaking itself in legitimacy as a scheme to get inbound links and drive up its page rank score. Their strategy is likely to get people linking for the physics so that they can sell links for heating oil prices, concert tickets, physics help, and so on. It's more about the advertising than the physics. I'm sure there's a less spammy physics site you could have linked to?
- JonnieCache 16y ago
  On the contrary, the page is a piece of internet history. As you can see from the domain, it was created by an academic in an early example of titlebait. http://replay.web.archive.org/20001002213854/http://www.britneyspears.ac/ http://replay.web.archive.org/20001002213854/http://www.brit... Its basically a reverse rickroll. "Britney Spears" was by far the most searched-for term on the internet for a long time.