Title text: "röntgen" and "rem" are 20th-century physics terms that mean "no trespassing."
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This comic refers to elements of (mostly mathematical or engineering) notation commonly used in various fields of math and science. Each piece of notation is presented as "symbolizing" not what it specifically means, but a typical context in which it might be encountered. Many of the individual descriptions sounds look like verbiage that might be found on informational or warnings signs or placards, although typically with a silly edge.
- d/dx: An undergrad is working very hard
d/dx is the symbol for a single-variable derivative. This is a mathematical operation that, while difficult, is one of the most basic operations in calculus and consequently well within the reach of an undergraduate student, particularly in science. Thus, an equation with this operation would be one that would cause an undergraduate student to work very hard.
- ∂/∂x: A grad student is working very hard
The replacement of the standard "d" letters with the curly letters "∂" denotes the partial derivative, which generalizes the ordinary derivative to multi-variable calculus. Problems with partial derivatives, especially partial differential equations, can be extremely challenging—appropriate for hard graduate-level work.
- ħ: Oh wow, this is apparently a quantum thing
ħ (pronounced "h-bar") is a symbol used for (the reduced) Planck's constant, a universal, fundamental constant in quantum physics. ħ is equal to the energy of a photon divided by its frequency, and angular momentum in quantum mechanical systems is measured in quantized integer or half-integer units of ħ.
- Re: Someone needs to do a lot of tedious numerical work; hopefully it's not you
The Reynolds number (which is actually usually denoted by "Re," not "Re" as it appears in the comic) is the most important dimensionless group in fluid mechanics. Named for Osborne Reynolds, Re characterizes the relative sizes of inertial and viscous effects in a moving fluid. Large values of Re are indicative of turbulent flow, which cannot usually be solved for analytically, and so numerical modelling is necessary. Accurate numerical studies of high-Reynolds-number flows are notoriously difficult to create and program.
- (Ta4 – Tb4): You are at risk of skin burns
The Stefan-Boltzmann law says that a perfectly absorbing ("black body") source emits electromagnetic radiation with a power per unit area of σT4, where σ is a known constant and T is the absolute temperature. The quantity (Ta4 – Tb4) thus appears in any calculation of purely radiative energy transfer between two bodies, one at temperature Ta and the other at Tb. When radiative transfer is large enough to be the most important form of heat interchange, it is normally also large enough to sear the skin with thermal or ultraviolet burns.
- NA: You are probably about to make an incredibly dangerous arithmetic error
NA, or Avogadro's number, is the number of molecules in a mole of a substance—roughly the number of protons and neutrons in 1 gram of matter. This is an enormous number, approximately 6.02 × 1023. Adding up molecular weights and converting between grams and moles of several substances is a lot of arithmetic on a scale where intuition won't help you catch mistakes. Working with NA, it is easy to make errors of one or more factors of ten without noticing. If this kind of error is made in the calculation of the stoichiometrically correct amount of a reagent in a chemical reaction, it is possible to accidentally create dangerous amounts of unwanted chemical products.
- µm: Careful, that equipment is expensive
Micrometers are a very small unit of distance. Any equipment that is operating in these units will be incredibly finely calibrated and thus very expensive.
- mK: Careful, that equipment is very expensive
Millikelvin temperatures are very cold, barely above absolute zero, suggesting sensitive experiments probing quantum mechanical behavior that would likely only exist in an advanced laboratory. Any equipment that works down a mK temperatures is likely to be very expensive.
- nm: Don't shine that in your eye
Nanometers are most frequently seen in the listed wavelengths for lasers. Pointing a visible or infrared laser at someone's eye is notoriously dangerous; the tightly-focused coherent light can cause permanent damage very quickly.
- eV: Definitely don't shine that in your eye
Electron volt energies are typical of moderate-energy particle beams, produced by accelerating electrons (or protons) over macroscopic voltages. These particle beams can be even more damaging to soft tissues than optical-wavelength lasers.
- mSv: You are about to get into an Internet argument
The millisievert is a unit of radiation dose absorbed. It is actuality a very small dosage, but the joke refers to Internet trolls debating the effects of low-dose radiation sources, such as 5G wireless networks. Randall's comment may also be referring to this chart.
- mg/kg: Go wash your hands
This unit measures the dose of a drug or other chemical in milligrams per kilogram of body mass. If the appropriate dose - or worse, the lethal dose - is measured in mg/kg (parts per million), then the substance may be quite toxic.
- µg/kg: Go get in the chemical shower
A unit 1000 times smaller than mg/kg. If a dosage is measured in micrograms per kilogram (parts per billion), it is probably causing chemical rather than biological reactions.
- π or τ: Whatever answer you get will be wrong by a factor of exactly two
π is defined as the ratio of a circle's circumference to its diameter, while τ is defined as 2π (the ratio of the circumference to the radius). π has been used as the primary constant for describing the circumference and area of circles for millennia, but proponents of τ point out that τ is actually more natural in most constants, since it makes working in radians more straightforward. The joke here is that whichever constant you use, it will probably be the wrong one (off by a factor of two, one way or the other) for the formula you are trying to use.
The title text refers to two non-SI units of radiation measurement. In the mid-20th century when they were in use, the dangers of radiation weren't as well understood as today, so an area with radiation that was noteworthy back then is probably dangerous, hence the no trespassing part.
|This transcript is incomplete. Please help editing it! Thanks.|
- An undergrad is working very hard
- A grad student is working very hard
- Oh wow, this is apparently a quantum thing
- Someone needs to do a lot of tedious numerical work; hopefully it's not you
- (Ta4 - Tb4)
- You are at risk of skin burns
- You are probably about to make an incredibly dangerous arithmetic error
- Careful, that equipment is expensive
- Careful, that equipment is very expensive
- Don't shine that in your eye
- Definitely don't shine that in your eye
- You are about to get into an internet argument
- Go wash your hands
- Go get in the chemical shower
- π or τ
- Whatever answer you get will be wrong by a factor of exactly two
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