Monthly Archives: January 2008

A Ghost of Physics Past

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In which I am haunted by Bernoulli

I’m helping to get a new building finished and operational; this has been ongoing for some time. It’s for very sensitive equipment (atomic clocks of various flavors), so there are some stringent environmental controls, and it’s taken a little while to get the spaces to meet spec. The big problem has been the dynamic response — in steady-state, everything looks very good, easily meeting the 0.1 C tolerance, but it’s clear that the design didn’t quite anticipate transient responses. Because of the need for a robust system, there are a lot of redundancies, such as two take-off ducts for each room. All of the fine-tuning for heating and flow adjustments happen in the take-off duct, so there are two ducts per room, with one active and the other acting as a back-up. One room has been a particular thorn: it and its twin have the highest flow and largest heat load, and for some heretofore unknown reason, the two rooms would not behave in an identical fashion to identical changes. Whenever a duct change occurred, the one troublesome room’s temperature would fluctuate wildly, and there were smaller effects in other rooms.

Today the proverbial light bulb went on. The engineer who has been wrestling the building into shape discovered a design condition that probably falls under the category of the left hand not knowing what the right hand was doing. There is a static pressure sensor in the main supply duct, placed at some arbitrary point. There are also all of the take-off ducts going to the individual rooms, and the placement of the ducts and sensor was conflicting, and this is where Bernoulii steps in, moaning creepily, chains a-clanking and laughing (eerily, of course) because it turns out this has been a problem all along. You see, the pressure sensor was placed between two duct take-offs for the same room.

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A Quantum Complaint

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It’s not a big deal.

By that, I meant that “quantum” does not mean “big.” (This is a peeve of mine. Not so sure it’s a pet peeve. Perhaps a feral peeve?) I ran across a blog entry about the use of quantum that wasn’t wrong as it usually is. (But the blog is by a physicist, so there you go.) Now the entry is a little awkward in that, at the end, it seems to imply that quantum means “small,” and it doesn’t mean that either. But quantum things are usually small, which is why the use of the phrase “quantum leap” is doubly irritating to a pedantic, anal meticulous physicist. Years ago I tried convincing Phil Plait of this, back when the Bad Astronomy website was an only child (no forum, no blog, just posts that addressed misconceptions), but he politely disagreed, though he thought my day job was cool. But he was wrong (about the former, not the latter).

The “opposite” of quantum is continuum, because quantum means discrete. A quantum jump can be the smallest possible transition there is. If I were to offer you a quantum pile of money (and for a limited time, for only five dollars!) it could be whatever number of pennies constituted a “pile” because the penny is the quantum of currency (in the US, at least). Money is discrete, not continuous. Now, it doesn’t have to be small, either. That’s just it — size isn’t inherently part of the definition. “Quantum leap” doesn’t really tell you anything. You need to know what the quantum unit is; if the leap is actually big it should involve many quanta. If it only involves one, then it means it’s the smallest leap possible, and you shouldn’t be impressed.

The comment “Perhaps, just perhaps, we’ll finally have an example of quantum meaning small!” is probably better stated as “Perhaps, just perhaps, we’ll finally have an example of quantum being small!” since most quantum things in physicsland are small.

The Relativistic Van

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Who cares about gas mileage? This sucker warps time!

When relativity is discussed in popular literature it’s often couched in terms of affecting objects moving at a significant fraction of the speed of light, and that’s a true statement: kinematic time dilation cannot generally be ignored in that situation. But the implication that the opposite is true — that you can ignore these effects under other circumstances — doesn’t hold. At least, it doesn’t hold if you have some expensive toys at your disposal.

Let’s say you were going to drive across the US and back, and you had the aforementioned expensive toys. Maybe you wanted to calibrate clocks and check on the reliability of a satellite time-transfer system, and you have a mobile system that would do time transfer at the source and at the target site, allowing you to check on that calibration. Or something like that.

The time dilation in question gives a fractional frequency shift that goes with the square of the speed, as compared to the square of speed of light. That’s normally very small, and has to be under this approximation (c is big, v/c is small, (v/c) squared is reeaaally small), so you can usually ignore it, right? Not everyone can. The famous Hafele-Keating experiment that used airplanes and around–the–world travel was able to measure kinematic dilations. A trip across the US is ~2700 miles, and at 600 mph you’d get a frequency shift of 4 parts in 10^13 and a dilation of about 13 nanoseconds on your round-trip due to traveling at that speed. (one thing to note is that I’m using a different coordinate system than is used in the H-K writeup, in case you want to play along at home. The answer will be the same, but the east vs west contributions are accounted for differently, and I’m not showing that detail)

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The Geometry of a Cow

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There’s an old physics joke about the dairy (Motto: “Smell Our Dairy Air!”) that hires cosultant after consultant to help optimize their milk production but to no avail. Each and every time, the end result is that nothing has improved. Finally they hire a physicist, who comes in and takes all sorts of data, and then retires to his office, madly doing calculations. After waiting a while, the dairy contacts him, inquiring about his recommendations. He come in to do a presentation, with all sorts of papers and slides for the overhead (or powerpoint on his laptop if you want to update the joke). He puts up the first slide, and starts in with, “First, we assume a spherical cow…”

It’s funny if you know physics, or more specifically, physicists, who tend to idealize all sorts of things in their models. (frictionless surfaces, elephants whose mass a may be ignored, etc.)

Anyway, hop in the wayback machine to a few years ago, when several of us were having a discussion about problems on our comprehensive exams in grad school. My boss tells one about a cow on a tether attached to a point on the rim of a cylindrical silo of a given height and radius. You were supposed to calculate the grazing area available to the cow. I, being in a smartass mood (sarcasm is my ground state), ask, “Did you assume a spherical cow?” The didn’t-miss-a-beat response was, “No, I was able to use the point-cow approximation for this problem.” And I thought that was pretty funny, and something that works in a single-panel cartoon.

point_cow.jpg

Just don't show it

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Censors wagging the dog

OK, for the Nth time in recent memory, there was a movie on cable, with which they just shouldn’t have bothered. If you’re going to show Blazing Saddles but are going to blip out every instance of words like “nigger” and “faggot,” just don’t bother. It’s a satire about bigotry. It loses a whole lot in translation when you try and clean up the bigoted speech.

Similarly, if you are going to show The Jerk, either leave “blowjob” in it or cut the whole damn scene. It’s pointless otherwise.

No Sweat

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Or, the myth of “working up a good sweat”

On occasion my “private” workouts get interrupted by someone with the audacity to want to use the exercise room at the same time I do. That’s no biggie. But what bugs me is when the offendor will turn on the heat, or turn off the AC or close the window. I prefer to not be sweating just by walking in the room. It shortens my workout, and here’s why.

Your body is about a 100-watt heat source. That is, you you are shedding heat at level of about 100 watts, give or take, just by sitting around. There’s an overview here. Most of that is radiation, and takes into account that we would normally be shedding more heat if not clothed. Long sleeves and pants insulate more, and reduce the heat loss, or compensate for larger heat loss when it’s cooler. Imagine that.

Radiation heat transfer is covered by the Stefan-Boltzmann law, which tells us that heat transfer depends on the difference of the fourth power of temperature of our body vs. the ambient room. Your body is a heat engine, and once you start exercising that engine needs to reject more heat. So you hop on the treadmill or bike or stairmaster and start your workout. The little display tells you that the output is about 100 Watts, but your body isn’t 100% efficient — no engine can be. You’re actually burning carbs at about 400 Watts, and that extra 300 Watts needs to leave your body, so your temperature goes up. If radiation were to take the whole load, your body would need to rise about 30 degrees C, and obviously it can’t do that. So, you start sweating.

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