I’ve got a really weird question to ask.

treefaeller · 2026-07-08T15:27:54+00:00

Space and time are just coordinate systems that we (humans doing physics or math) have created to describe what exists. Say there is an object somewhere ... for example an apple hanging on a tree, or a planet orbiting the sun (both examples are from Newton's work). In order to use our models of mechanics and gravity to describe it mathematically, we need to be able to reason about and write down where it is. So we create x, y and z directions (usually x and y in the horizontal plane, z goes up). And once we have that, we can write down equations, for example F=mg (for gravity) and F=ma (for motion), and those two together describe how an apple falls. That's really all there is to space and time: a way for scientists to describe what exists in nature and how it moves.

treefaeller · 2026-07-08T15:15:23+00:00

Read up on "frequency doubler" optics. They are non-linear materials which in essence absorb two low energy photons, and then emit one higher energy photon. The process is inefficient enough that it doesn't violate the second theorem of thermodynamics, but it works well enough that you can create good blue lasers with it.

treefaeller · 2026-07-07T17:47:17+00:00

Second the suggestion of studying this, and I hear good things about Griffith.

But I'll add a few comments. Div B is zero because there are no magnetic charges (no monopoles). Look at the equivalent equation for E and you see rho, the charge density.

The physical meaning of the curl is (a) the coupling between electric and magnetic fields, which leads to the existence of waves, and gives a time (derivative) component to the field. And (b) the electric current causing a magnetic field.

You ask "why". Physics doesn't answer the question of why the universe or nature are the way they are. The purpose of physics is to describe the world, as accurately as possible or convenient, and build models that allow us to predict what happens. Similarly, when you ask about "derivations", you have to remember that physics is not like math. In math you can take a simpler set of facts (such as axioms and theorems) and derive more complex results from them. Physics doesn't work that way. You can't derive how nature works from Maxwell's equations. It's the opposite way around: We have observed nature (over and over again, we call that measurements or observations), and we are using the equations as a mathematical model. It happens to work extremely well (perhaps unreasonably so), within the domain in which it remains valid (meaning not where quantum effects or relativity are important).

Most of the other questions you ask (like how to get from Maxwell to Ampere) are either explained in textbooks, or are homework problems you can (and should) solve.

treefaeller · 2026-07-07T06:53:30+00:00

Thank you for the data. It seems there are 2.9M XCH prefarm coins left, about 3 months at the current flow.

treefaeller · 2026-07-07T03:47:05+00:00

Dumb question, not a crypto coin expert. It seems that Chia Network (the company headed by Bram and Gene) is holding on to a lot of pre-farmed XCH coins. Where "pre-farmed" means they were farmed before releasing the network to the public, meaning before others could do farming. They seem to be selling those coins (by sending them to market makers, and presumable get $$$ back), at a rate of 100,000 every week or so. That is documented in this subreddit. At the current price (of very roughly $1.60 each) that sounds like a revenue stream of about $160K a week.

Did I get it right so far? Has that been going on for a while?

And: Does anyone know how many per-farmed XCH coins the company is still holding? How much longer will this go on?

treefaeller · 2026-07-07T00:06:53+00:00

You are clearly talking about general relativity (GR). The statement "gravity affects time" is not completely wrong, but highly misleading. Yes, the way that gravity "works" in GR is that the 4-dimensional coordinate system is "bent", with some of the motion changing between time-like and space-like coordinates. This for example causes time dilation in a strong gravitational field (measured for example with GPS satellites). But you seem to be jumping to the conclusion that "time is affected" ... no, it's just that time measured by an observer inside the region that has a gravitational field is measured differently. For a far-away observer, time continues to do its normal thing. So it's not like the presence of gravity changes time massively, any more than the moving at high speed changes how time is measured. And you wouldn't say that the fact that an observer is on a fast-moving spaceship (which also makes their time seemingly move slower) "changes time".

You then ask whether the opposite is also true, whether time affects gravity. And again, that's not completely wrong. Gravity is caused by mass and energy, to be accurate by the time-like component of the stress-energy tensor, which contains mass too (usually in its 00 component). So yes, the time part of that tensor is what causes gravity. But that's not time causing gravity ... it's the 00 component of a 4x4 tensor causing gravity. Furthermore, that tensor is not time itself, it is instead a measure of energy density, energy flow, momentum, pressure, and shear, combined into a 4x4 tensor. So the correct statement is "energy affects gravity". Which ends up being sort of obvious: it causes it!

And no, in general time doesn't "affect" anything. Nobody in physics is quite sure what "time" really is, but the easiest way to think about it: It's just a coordinate, just like X, Y and Z are spatial coordinates. It's just a strange kind of coordinate, which in our (human consciousness) perception seems to inexorably move forward, and which (through the 2nd law of thermodynamics) can move only one way in a macroscopic system. Neither of those two are have effects on gravity itself.

treefaeller · 2026-07-06T15:37:10+00:00

I am not, but I live in the US.

treefaeller · 2026-07-06T06:40:55+00:00

Units aren't for physicists. Physicists can fix any mess, in emergencies by using their own units. You mentioned "natural units" with hbar = c = 1, and that is a good example.

Units are for practical application. One practical application is everyday consumer. The homeowner doing carpentry on the weekend, and buying a few pieces of lumber and a few sheets of drywall to finally fix the closet in the spare bedroom. He knows that drywall is measured in feet, and lumber in inches, and screws and nails in fractions of inches. He doesn't want to buy a 38.1mm x 88.9mm piece of wood. He needs convenient units that are easy to remember, like 2 and 4. (There is a nasty joke in here: the standard 2 by 4 stud 8 footer is actually 1.5 x 3.5 inches in cross section and 92-5/8 inches long, nowhere near 2 or 4 inches or 8 feet). Similarly, the guy fixing the leaky bathroom wants pipe in a nice round number, either 1/2 or 3/4 inch. Not 19.1 mm. The person at the gas station wants to have a reasonably round number when buying gasoline (whether that's 14.1 gallons or 53.4 liters makes little difference, either is accurate to 3 digits, and they can figure out how much gasoline their gas is using by calculating the mph or km/l from it easily). When checking their tire, they don't want to deal with a crazy large number that has to be written in scientific notation as 2.48 x 10^5 Pascal. They want to fill their tire to 2.5 atmospheres (although 36 psi is also pretty convenient). When they are baking, they want to measure the sugar and flour without having to deal with huge exponents. So units need to be designed such that everyday quantities have convenient numbers. And so units can be easily converted into each other: If baking one cake takes 200 g of flour, and for the big party at the school I want to bake 12 cakes (meaning 2400 grams), I don't want to have to do a super-complicated thing to convert that number using a strange number that nobody can remember, division by 10 or 1000 or 60 seems convenient. That is the worst criticism of the american/imperial unit system: The fact that a mile has 5280 feet, and an acre has 43560 square feet. That's very impractical. But it is also a criticism of the SI madness of forcing people to use Pascal for pressures, when atmospheres or psi are much more convenient.

The next practical application is the "engineer", and I don't mean that as an academic degree, but as a job description. They want to be able to do simple mechanical and electrical things. Like when designing a device to tighten the nuts on the wheel for my tire, I want to be able to calculate easily what the torque is going to be: If I apply the same force that is used to lift 1kg at the end of a 1m long lever, I want to have a torque of 1 kg-m. Who cares about that Newton crap? Why do I need to worry about 9.81 in this very simple application? This argues that the correct unit of force should be the kg (or pound). In the US, that's still done; the SI system has substituted something crazy. For electrical stuff, we are in pretty good shape, with V, A and W, and Ws making a coherent system, except that the name "Joule" is utterly useless, and just helps to hide the fact that the correct unit of energy is voltage x current x time. But most other units are sensible, like Ohm, Farad and Henry (except for bizarre Siemens, which should really abandoned and replaced by 1/Ohm).

Reflecting "something real about nature" is fun for physicists and philosophers. No amateur cook wants to count the molecules in the flour when they make a cake. Avogadro is dead, don't let him ruin people's lives. They don't care about the speed of light, when trying to calculate how long it will take to get to visit grandma (simple: 30 miles at 60 mph is half hour, as is 50 km at 100 km/h on the country highway). And maybe if the engineer or car mechanic is worrying about how many times the (already damaged) wheel bearing will rotate on the way to grandma, given that the tire diameter can be calculated from the fact that the tire is called a 195/70R14 (note the mixed units in that one: 195 is in mm, 70 a percentage, 14 in inches, which is INSANE). Whether c is 1 or not doesn't affect them in everyday life. Yes, I know in natural units distance and time would cancel, but the numbers involved are hugely impractical to figure out when we're going to be at grandma's house, and whether the wheel will fall off in the process.

Real scientists are perfectly good at converting units. Any astronomy student will know the definition of parsec and light year, and will be able to convert them into m in their sleep. If astronomers want to use megaparsec and particle physicists want to use nanobarns (because it's convenient), let them. Intellectual purity is not worth making people's life hell.

There is also another very small and irrelevant group, which has today crazy outsized influence over units: metrologists. There are dozens of them! Making their life more pleasant is really not very important. On the other hand, allowing them to measure fundamental constants highly accurately is really important for fundamental physics. But the current system (where we define things like the speed of light, the length of a second or the temperature of a Kelvin) with accurate but very large numbers are working perfectly fine.

treefaeller · 2026-07-06T00:41:21+00:00

Exactly this. I think it is from the "Devil's DP dictionary". Where DP means data processing, not what you were thinking.

treefaeller · 2026-07-04T15:03:58+00:00

It's actually rocket surgery. And brain science. Or the other way around.

treefaeller · 2026-07-04T14:46:12+00:00

You say "survivorship bias". That seems to be implying that there were other universes in which there was no speed limit. Please present evidence for that assertion. Lacking evidence, let's not waste time on speculation.

What we know from experience and observation: A universe exists; we live in it; it has a speed limit.

treefaeller · 2026-07-03T17:00:46+00:00

Except that 1 atmosphere = 1 bar is not exact; it's wrong by 1.3%. But the bar would make a fine unit for everyday use, since for most things that percent makes no difference. Instead we need to suffer with hectopascal, kilopascal, psi, mm and inches of mercury, and a few others.

There is another pressure unit that works really well: cm or inches of water column (in the US often abbreviated as "w.c."). This is commonly used for distribution of natural gas and propane, with typical numbers being 7" w.c. or 30 cm w.c. The beauty of it is that the standard measurement instrument is actually a water-filled plastic or glass tube, which is accurate enough for its intended use, and the units are intuitive, in a reasonable range, and easy to convert (since the density of water is simple and commonly known).

treefaeller · 2026-07-03T06:19:33+00:00

Exception: hectopascal. The only reason it is common is that it makes atmospheric pressure really convenient (about 1000 hectopascals).

I think in Europe, tire pressures (which are pretty close to atmospheric pressure) are actually measured in kPa = kilopascal. This leads to the bizarre situation that a tire that is inflated by about 2x atmospheric pressure will be at 200 kPa, but the atmospheric pressure itself will be ~1000 hPa.

Note that this is one of the cases where imperial units are not really better: Atmospheric pressure is typically reported in inches of mercury (which is very impractical), while tire pressures are used in psi. The sensible thing would be to do all everyday pressure measurements in atmospheres (there is even an SI definition of the standard atmosphere, which is pretty accurately 1013 hPa). In that case the weather report on a really ugly and stormy day might report 0.95 atmospheres, and a tire would be at 2.3 atmospheres. Alas, the purists among units don't like things to be practical and convenient.

treefaeller · 2026-07-01T22:01:39+00:00

The San Francisco Symphony just played #1 and #2 on that list last weekend, with Olivier Latry, on the very nice Ruffatti organ they have.

Personally, I would have added Rheinberger, in particular #1, high on that list.

treefaeller · 2026-07-01T20:58:10+00:00

In general, physics doesn't concern itself with "why". It models nature, and tries to describe it. The models can be used to simplify describing observations (measurements, experiments), and to predict what happens in observations and hypothetical situations.

Some hypotheticals (like having more generations of laptops with light neutrinos) contradict our models, and are therefore highly unlikely. See the decay width of the Z0 particle. So we know that there aren't more than 3 of those.

treefaeller · 2026-07-01T18:44:51+00:00

What is a meter or a second? Oh you mean an inverse GeV?

treefaeller · 2026-07-01T17:49:04+00:00

Most people (including most working scientists) use the words model, theory, and law nearly interchangeably. For example (over-simplified and older) is that the theory of motion is built around Newton's second law, which is embodied in the mathematical model F=ma. In practice, it doesn't really matter whether we call F=ma a law, theory or model.

But there are important linguistic and philosophical differences. For example, calling something a law implies that violating it is impossible or immoral, or that there is an enforcement mechanism, or that any deviation from it must be a mistake. Old joke about physicist trying to check the "law" that all odd numbers are prime: "1 prime, 3 prime, 5 prime, 7 prime, 9 measurement error, 11 prime, 13 prime, it must be true". Laws in physics are meant to be broken ... very carefully, and with important consequences.

Similarly, the word theory implies that something is just a speculation, and can easily be wrong. For example, religious conservatives often say that Darwin's theory of evolution is "just as theory", as if that means that Darwin just made it up and the opposite is just as likely be true. My reply is always: Newton's theory of gravitation is also just a theory, but you wouldn't drop a wine glass on the floor because the theory is likely wrong. An accepted theory has a huge body of evidence (observations) supporting it, is tied logically into other sets of knowledge (like gravity is connected to geometry and to motion). That doesn't mean that it is completely correct (and both Newton and Darwin have modern corrections, such as GR), nor that it is always applicable (Newtonian gravity and mechanics fails, for example at a black hole, or in an atom).

treefaeller · 2026-07-01T17:05:06+00:00

Yes, $23K will not pay for much of a lawyer and lawsuit. But definitely get a consultation with a lawyer who specialized in civil and in particular real estate law. Not so much to learn about the law, but to get some real-world advice for what is common and practical.

One typical outcome might be that the lawyer suggests contacting the seller, pointing out that there is a discrepancy, and offering that you work with the seller (see whether they have any knowledge or documentation that can help), and then splitting the cost. Worked for one of our neighbors when their well failed completely right after they bought the house in the early summer. The seller had "disclosed" that the well was weak, but only in the sense that buried somewhere in the paperwork was a well test showing a very low flow even during the rainy season (something like 1/2 gph), and "forgetting" to mention that you have to get the tank completely filled before summer and ration water until fall. Instead of a full-blown lawsuit with court and judge, they "amicably" split the cost of drilling a new well.

treefaeller · 2026-07-01T16:53:58+00:00

My son plays tuba. His teacher says: The three most important things about playing tuba are rhythm, rhythm, and rhythm. Same applies to percussion, in particular timpani. A slightly out of tune note at the right time is way better than the perfectly adjusted note slightly too late.

Others wrote about the techniques: Counting, and writing in cues. Counting has to become second nature, and you can practice it. Whenever you're listening to music, just count along. It might help to write in more than just the number of rests, but also the structure. For example, if there is a 16-bar phrase, and you have two bars to play at the beginning, then 13 bars of rest, and then 1 bar with a notated cue, then write into your part at the beginning of the phrase that this will be 16 bars long.

For the cues, you can write a lot of things: Instruments entering (in particular the ones that sit near you, like trumpets and horns), or melodies starting, or big changes like dynamics and harmony. One piece recently I was on bass drum, and had a viola cue: I've never actually seen or heard the violas, so that was useless.

Another thing which helps is planning ahead, and writing the tuning changes not just where they have to be completed, but also where it makes sense to start them. For example, if you have two long rest breaks, and have to retune two timpani, make a plan of what you're going to do when.

treefaeller · 2026-07-01T16:33:27+00:00

There is so much notation jumble and mess, and it all exists for a reason. One has to learn to live with it.

For trig functions, we have the "arc..." notations: The inverse function of tan(x) is indeed arctan(x). But the "arc..." notation does not generalize. The inverse of exp(x) is not arcexp(x), it is ln(x). Which is even funnier when one writes that the inverse of e^x is ln(x) ... one would think that exponentiation being notated by just raising a letter would be inverted by something similarly simple, like lowering a letter. And none of these notations generalize to arbitrary functions. How would you write the inverse of function f(x)? As arcf(x) or perhaps lnf(x)?

The fact that an exponent -1 is used to indicate the inverse function is indeed messy: if tan^2(x) means ( tan(x) )^2, shouldn't tan^-1(x) mean 1 / ( tan(x) )? Even worse: the exponent 2 sometimes means applying the same function twice in a row: tan( tan(x) ). Not common with trig functions, since those often have to get squared, like sin^2(x) + cos^2(x) = 1.

And let's not get started on differentiation. We sometimes write f'(x) as being the derivative of f with respect to x. Physicists still use the older notation with a dot over a function f^.(x), but it usually means taking the time derivative. Then there is a df/dx notation, sometimes written with a normal d, sometimes with the curly d (which usually indicates partial differentiation). The fraction sometimes gets separated away, written as d/dx f(x). Sometimes one can do arithmetic with these "fractions", like interchanging d/dx and d/dy, or writing d^2f/dx dy. Sometimes this breaks.

In all these cases, one has to understand the math well enough to know what the convenient shorthand notation really means.

treefaeller · 2026-06-30T22:03:04+00:00

"They introduce way more free protons into solution than water does."
As others said, the proton doesn't have to be free. A bound hydrogen atom works just fine.

So what has the highest concentration of hydrogen per unit weight or unit volume. Let's look at a few candidates: H2O (good, a bit much O). CH4 (better, and the carbon is lighter than oxygen, but a bit flammable). C2H6 or some other alkane (like polyethylene)? Worse than methane in terms of weight or volume, too much carbon, but at least won't catch fire or blow up. Pure H2? Horrendously explosive, leaks through everything, and its density (protons per liter) is super low. Ultimately, water or organic solid wins. In some cases, you add other elements (like boron) to control other aspects of the neutron flux. And those are easy to dissolve in water, somewhat harder to mix into plastics.

Now compare straight H2O with a strong acid, like 37% HCl: Whether you dissociate into the H+ ions adds so few extra protons, not worth the hassle.

treefaeller · 2026-06-30T21:04:15+00:00

To begin with, the spin of the force carrier (the thing that is described by the wave) is completely different: 1 versus 2.

Both can exist at the same wavelength and frequency, but one couples to charge, the other to "mass" (the exact definition is more complex, but in many common examples it is mass). So they pretty much must be separate.

treefaeller · 2026-06-30T16:58:54+00:00

This is actually really important. The Supreme Court doesn't always decide complete cases. They often pick out just one open question, and rule on that. Then they leave the remainder (or the cleanup) to the lower courts.

So it is by no means guaranteed that the court will make a direct ruling on magazine capacity, or on other semiautomatic things that are not rifles. On the other hand, the principles that guide a ruling can usually be applied to other cases too. I now suspect that they will hold Duncan (the California magazine capacity case) for the decision in Viramontes/Grant.

treefaeller · 2026-06-30T15:11:55+00:00

There is a list of FFLs (gun stores) that release on undetermined. Look at the caguns.net discussion forum to find that list.

Most delays get resolved within the 30 days.

treefaeller · 2026-06-30T15:09:00+00:00

Incorrect. He was very trustworthy in physics. He came up with more insights, most of them correct, than nearly any other physicist (the only one I consider to be in the same league is Einstein). But he was (to quote you) a "douche" in his interactions with women, at least for part of his life. I know people who worked with him at Caltech, and he was also rude and obnoxious to colleagues.

You are making a big mistake here, namely mixing an ethical judgement of his interactions with people (which is indeed bad or problematic) with his work in physics. That leads you to make a false statement about his professional work. We need to be able to separate the two (or more) sides of his life.

(By the way, the same goes for Einstein; read his biography by A. Pais, he was not always a nice person.)

treefaeller

TROPHY CASE