
A theoretical physics FAQ - haxiomic
http://www.mat.univie.ac.at/~neum/physfaq/physics-faq.html
======
justifier
This is a fun collection but recently I have been wanting to see how well
different theoretical models express experimental data

If I want to calculate the efficacy of relativity where can I find
observational data on mercury's orbit?

If I want to calculate the efficacy of quantised photons where can I find
observational data on blackbody radiation?

If I want to calculate the speed of light where can I find observational data
on io?

If I want to calculate the efficacy of quantum mechanical models where can I
find observational data on qm phenomena?

Are there resources of others showing from experiment to theory with
calculations of real data defending the theory's math

Or even just access to these and similar datasets?

~~~
mnl
I'm sorry, but usually people can't waste their time working in easy to
understand datasets with a wiki for amateurs (for free), unless it's for some
kind of teaching (that they're being paid for). If you're really willing to
you can start studying some of the Physics you're talking about (honestly, any
physicist could tell you still haven't), it's the easiest way to get to know
which really old papers you have to check. Maybe you can do some lab work in
an educational environment and learn what to do with raw data after you
understand the experimental setup, what's being measured and how it's being
measured: that's the key to know what you are doing. Modern big experiments,
for instance in HEP, expect from you a significant effort and having to deal
with particular frameworks, that means a few months full-time at least and you
need people around that can answer your questions. I don't mean to sound
harsh, but honestly your questions are too broad (it doesn't work like that),
and imprecise... for instance, you don't "calculate the efficacy of [a
theory]" in Physics, and data don't "defend the theory's math". Not at all.

~~~
justifier
I freely admit writing the op was difficult and I rewrote the questions many
times editing out terminology.. if I'm still coming up short perhaps you could
help

> you don't "calculate the efficacy of [a theory]"

How would you describe in a single sentence the growing precision of our
calculation of the speed of light constant over the last 300 years?

> data don't "defend the theory's math"

How would you describe in a single sentence the way Einstein used Mercury's
retrograde motion to assure himself of the advocacy of his theoretical
mathematical model for gravitation?

~~~
mnl
>How would you describe in a single sentence the growing precision of our
calculation of the speed of light constant over the last 300 years?

I don't know, first I'd talk about how it was discovered that light behaves
like c really is a constant (that's not intuitive nor trivial). You could
guess from Fizeau (1851) and Michelson-Morley (1887) experiments, but it
wasn't really until the 1905 Einstein's paper that it was understood that you
don't need aether, the speed of light can (and should) be a universal constant
and it all makes sense.

>How would you describe in a single sentence the way Einstein used Mercury's
retrograde motion to assure himself of the advocacy of his theoretical
mathematical model for gravitation?

He shows (see here:
[http://www.gsjournal.net/old/eeuro/vankov.pdf](http://www.gsjournal.net/old/eeuro/vankov.pdf))
that his theory (after reasonable approximations) predicts a precession of the
perihelion of Mercury that agrees with the experimental result, then he stops.
That's what you do in Physics: look, my nice theory predicts this observable
in agreement with the experimental data. That's a very good sign, but the data
could be explained by other theory as well. Data can "prove" that a theory is
wrong (if your theory has falsifiability, as it should), but they can never
show that your theory is all that there is, nor your maths... There are other
theories of gravitation, for instance Logunov's, that are interesting and
agree with data (more or less) but have a few problems with maths.

BTW, I don't think he ever really doubted his GR theory, it comes from a very
simple, yet profound idea that once realised had to be right. You have to do
all the technical work afterwards, that was difficult, but a few
mathematicians could have done/did it (Hilbert). In any case the idea is to
check your predictions with the measurements from more and more precise
experiments to find out if you're wrong, not the other way around (that...
well, I've met that funny epistemology a few times in social sciences.)

~~~
justifier
Why are you relying on strawmen?

> Data can "prove" that a theory is wrong (if your theory has falsifiability,
> as it should), but they can never show that your theory is all that there
> is, nor your maths.

Where you got '"prove"' and 'all that there is' I am unsure

My wording was:

op> how well different theoretical models express experimental data

Which mirrors your own reiteration of that point

> the idea is to check your predictions with the measurements from more and
> more precise experiments

Which is literally what I was asking for: measurements from .. precise
experiments

> predicts a precession of the perihelion of Mercury that agrees with the
> experimental result

Right, where's the experimental data on the orbit of Mercury?

If you are unable to answer the question that's fine but as is I fail to see
what you are trying to accomplish with your ire and misdirection

~~~
mnl
I'm sorry if I came across that way, didn't mean to. But you have to realise
that it's tiresome all this new internet phenomena of laymen questioning the
work ethics of actual professionals because they think they could possibly do
better, if only etc. I don't usually comment on HN about Physics because...
well, never mind, it really was a bad idea this time.

If you're interested in the data you can browse the references, go to the
libraries (as physicists have to do even at college, because it's kind of
hard), find out first what exactly you are asking... even mail someone. And
you can also measure things by yourself, this is a free universe and everybody
can study its properties (Mercury is out there and maybe you can afford a nice
telescope). What is unreasonable, very unreasonable, self-entitled in my
opinion, is expecting that experimental physicists provide some kind of
repository of (internal-notes redux) data for casual readers of pop sci. That
would cost their time (or some overworked PhD student's) and project money,
the bad news is no one is willing to do that nor paying for it. It's not that
there's some kind of conspiracy, it's that people are actually busy working.

~~~
justifier
> laymen questioning the work ethics of actual professionals because they
> think they could possibly do better, if only etc.

> What is unreasonable, very unreasonable, self-entitled in my opinion, is
> expecting that experimental physicists provide some kind of repository

> It's not that there's some kind of conspiracy

You seem to be arguing with someone else

> I don't usually comment on HN about Physics because... well, never mind, it
> really was a bad idea this time.

If you are knowledgeable on the subject I would encourage you to comment, but
keep the comment on the topic being discussed

~~~
mnl
Well, I'll stay on the post topic. It's unlikely that you can get the lab
notebooks from most of 20th century Physics, this is what you're really
looking for. It's not that important, because once the results are published
by a few different groups that don't trust each other and will scrutinize
their peers' work, all relevant data have been measured again and again. If
theories like Special Relativity were really wrong there would be consequences
like having to close all particle accelerators because they wouldn't work.
Many key experiments are routinely done as part of the university courses, and
many results are also routinely used in other fields, engineering included.
E.g. linearisations of Planck's distribution are done in Remote Sensing for
estimating surface temperatures, they work great btw. We also understand light
bulbs, pyrometers, infrared thermometers... -those work straight with Planck's
Law, how else could they find the temperature-. There are GR corrections coded
somewhere for your GPS to work, etc.

If by seeing "how well different theoretical models express experimental data"
you're planning to apply idk assorted non-linear regression models from
equations you're not familiar with to see which one has the minimum chi-square
or something like that, well I guess that's what hobbies are for. Differences
tend to appear at the limiting cases, there you need more Physics than
Levenberg–Marquardt fun, so it's better to start there.

I have to warn you that your wording matches the usual stuff you get from
Physics cranks (the bane of the trade), hence the knee-jerk reaction, sorry
about that. I'll try harder next time.

~~~
justifier
it's for educational purposes

to express concepts like these to anyone interested using real world data to
show why these concepts are important

Like why Newtonian physics is insufficient to explain Mercury's orbit

Or how infinitely divisible energy leads to the 'ultraviolet catastrophe'

Or how one can approximate the speed of light just using orbital data of io
and earth (romer's initial method)

I can appreciate that my wording may have been ambiguous

Brevity can sometimes be the enemy of clarity but questions can help avoid
assumptions

~~~
mnl
Thank you, I get it now. You're absolutely right and that could be really
interesting. The problem is you don't usually have large sets of data in
elementary uni labs, because of schedules really, but that would be the best.

I'm not an Astronomer (nor Astrophysicist, I know about GR coming from a
Theoretical Physics bkg), so I suspect you could get more recent data, but a
classic reference from the GR book of Weinberg is a paper from G. M. Clemence:
[http://adsabs.harvard.edu/abs/1947RvMP...19..361C](http://adsabs.harvard.edu/abs/1947RvMP...19..361C)
(it can be obtained if you look around, there's a similar free-to-read paper
from the same Clemence:
[https://www.jstor.org/stable/3143142?seq=1#page_scan_tab_con...](https://www.jstor.org/stable/3143142?seq=1#page_scan_tab_contents))
that has as reference this:
[http://adsabs.harvard.edu/full/1943AJ.....50..126C](http://adsabs.harvard.edu/full/1943AJ.....50..126C)
(full text), that quotes the Astronomical Papers of the American Ephemeris,
Volume 11 (it says II, but it's actually 11), part I. Google is your friend
(they've scanned those!) and you can find the data at
[https://catalog.hathitrust.org/Record/000068403](https://catalog.hathitrust.org/Record/000068403).
This is how a little of preliminary research works, you have to do it yourself
all the time... Luckily now you don't have to go hunting for a paper copy
without being sure about the contents all the time (although it still
happens).

The classical mechanics calculation might be tricky, to say the least, as
well.

For the UV catastrophe I've always liked the Jeans cubic cavity and the Gamow
observation that opening an oven doesn't kill you. And of course, the fact
that integrating Planck's law gets you Stefan-Boltzmann's, very neat result.

I should think about all this a bit more in order to give you better feedback.

------
aportnoy
Don’t limit yourself to this FAQ.

1\. Head over to physics.stackexchange.com.

2\. Sort all answers by votes.

3\. Start reading (read the corresponding questions first).

4\. Emerge three days later slightly enlighted with a stubble on your face.

5\. ???

6\. PROFIT!!!

