Category Archives: Physics

Dropping the Minus Sign

Posted on by

Or, in this case, the “not”

Why aluminum should replace cesium as the standard of time

The technique involved is neat: for some atoms you can find wavelengths where the AC Stark shift is the same for the two levels in the clock transition, so the atom is unperturbed by the presence of the trapping light. So you trap them in an optical lattice, with confinement like a far off-resonance dipole force trap (FORT). This means you can continue to confine the atoms while it is in the superposition where it is oscillating between the two clock states.

The big advantage of this method is that you can trap millions of atoms easily in an optical lattice and that should make such a clock much more robust than a fountain, while achieving at least the same kind of accuracy.

Actually, that’s not the big advantage. Fountains trap millions of atoms (even billions, depending on your collection technique). The advantages are that you’d keep that many atoms (fountains lose signal from the original collection because the cloud spreads out, so the number you toss is an order of magnitude bigger than the number that return), you can interrogate the atoms for a longer period of time (an advantage shared by ion trap clocks/frequency standards) and avoiding cold-collision frequency shifts (atoms in close proximity tend to interact strongly, as they can interact for a relatively long time, and this changes the state of the atom, introducing an error in the signal)

However, “at least the same kind of accuracy” isn’t enough. I’ve noted before that international standards are a political issue. Cesium beam standards are commercially available. Furthermore, dozens of labs have or are building fountains, at some investment of time and money to gain the expertise in doing so (because atomic fountains are not, nor are they likely to become, a commercially available item). The countries doing this will likely be reluctant to switch to a standard that requires even more money and acquired expertise in a new technique for marginal gain in accuracy and precision. Especially in light of how many new options for secondary standards have emerged in just the last decade — an even better candidate may emerge as technology advances.

But Don't Put Us in a Home

Posted on by

Adopt-a-Physicist

Fall 2008 Schedule

Teachers Registration: Now – Sept. 8 (or until full)
Physicist Registration: Sept. 9 – Sept. 15 (or until full)
Teachers adopt physicists: Sept. 16 – Sept. 23
Discussion forums open: Sept. 29 – Oct. 17

This effort is led by Sigma Pi Sigma, the physics honor society, and aided by American Physical Society and the American Association of Physics Teachers. Technical support is through the ComPADRE Digital Library.

I did this two years ago, and a colleague did it last year; it sounded like the format had evolved to be a little more interactive. When I participated, I was given a list of questions, the students chose a subset to ask, and it was done through email via the teacher. Now it looks like there is a discussion forum, with more freeform interaction.

Meme, Too!

Posted on by

Jennifer has started the great pop-sci book project, a natural evolution (and yet intelligently designed progression) of The Big Read

The rules are familiar

1. Highlight those you’ve read in full
2. Asterisk those you intend to read
3. Add any additional popular science books you think belong on the list
4. Link back to me (leave links or suggested additions in the comments, if you prefer) so I can keep track of everyone’s additions. Then we can compile it all into one giant “Top 100” popular science books list, with room for honorable mentions. (I, for one, have some quirky choices in the list below.) Voila! We’ll have awesome resource for general readers interested in delving into the fascinating world of science!

I don’t read tremendous amount of pop-sci, and not much in physics since A Brief History of Time, as I’ve gone to grad school since then and really don’t need much prose on how weird quantum mechanics and relativity are. (I had to put my foot down on getting pop-sci books as gifts after getting a pop-up book of cosmology; I felt a bit like John Cleese in a Monty Python sketch

Do you like your rattle? Do you like your rattle?
Ah, yes, the rattle.
Ooh, he’s talkin’ already
Of course I can talk, I’m the Minister for Overseas Development

But I digress.)
Continue reading →

Olym-pics

Posted on by

Beijing 2008 – It’s a wrap at The Big Picture (39 photos)

If you look at the high jump of Blanka Vlasic, you can see how it’s possible to do a high jump with your center-of-mass (focus on the physics) never rising above the bar, though I think in this jump it is higher, since she’s clearing the bar by a bit. But I would guess that her COM is below her torso.

And that last one is going in my desktop picture rotation.

Yes, John, You Had a Question?

Posted on by

I notice that the answers are posted for the “Presidential Physics quiz” in the NY Times (original quiz wording) and, well, blech. Sorta. I’ve read decent things about the Physics for Future Presidents book, but I don’t want this to be the example for “how to quiz presidents and justify the answers.”

QUESTION 1. How does the amount of energy per gram of TNT compare with the energy per gram of a chocolate chip cookie?

My answer is (d), the cookie contains nearly 10 times as much energy.

But the answer is really around 5 or 6, depending on what numbers you’ve used, and one of the other choices is “about the same.” If you’re going to do multiple-choice, try not to bracket the right answer this way.

I do like the defense of discounting the E=mc2 answer, because it shows recognition that we’re dealing with chemical rather than nuclear reactions. But in the defense of the answer, there’s

TNT explodes all by itself, no air needed.

Well, no, not really. If you balance the reaction, you’ll find that significant oxygen is needed. Sugars have oxygen in them, too. You still need external oxygen for that reaction as well.

But all of this ignores that science isn’t just a bunch of facts. What I’d rather see from a president (or student) is some reasoning, like “these are both basically combustion reactions, so to first order, I’d expect them to release similar amounts of energy” and worry about the details later on, like what difference there is because you have some nitrogen in TNT, and how that might affect the bottom line, and maybe the reasoning that you give up some energy for the convenience of a reaction that forms more moles of gases (and those gases want to occupy 22.4L each at STP, so boom!) instead of keeping the molar amount of gas the same (swapping CO2 for O2).

QUESTION 2. Based on the answer to the previous question, suggest an energy-efficient way to destroy a car.

Continue reading →

Screw Archimedes

Posted on by

Who needs a lever, man? I’m Marty McFly.

Let’s say, for the sake of argument, that you needed to move a heavy piece of equipment you’re helping to build. And you have the constraints of not wanting to tip it, and the course you need to navigate has some spots with less than 10 cm of overhead space. (This ignores one doorway that’s actually 10 cm too low — yes, we’ve done the equivalent of having built a boat in our basement. The solution there will either be slapping Daffy Duck with a frying pan in the shape of the equipment and having him run through the door, or a sawzall.) You need a smooth ride, because it’s an expensive, somewhat delicate trinket representing a several dollars and a few person-years of labor.

I tried designing a cart, but couldn’t meet all of the constraints — anything low enough would tend to bottom out on the incline (oh, that’s right, the path isn’t level the whole way. Is that a problem? I need this soon.) One day I was stressing and kvetching about it in front of the right person, who suggested air bearings/air casters. The heretofore unconnected link between the physics and the application clicked, and I knew that was the answer. Float the sucker on air. A small industrial blower and lots of small holes.

Here’s a demo of the system with a dummy load. There are also some lead blocks there, too. (cue rimshot). 160 kg for the optical table, lead and support structure, and another 110 kg or so for me.

You need to a flashplayer enabled browser to view this YouTube video

(if you want a soundtrack, turn on a vacuum cleaner. Any “Ishmael” wisecracks about my pasty-white legs will be subject to retaliatory editing)
Continue reading →

Who's the Fairest Helium Atom of Them All?

Posted on by

Smoothest surface ever is a mirror for atoms

Metal materials reflect helium atoms much better but are harder to bend precisely into the right shape. Now materials scientists from the Autonomous University of Madrid led by Amadeo Vázquez de Parga have combined silicon and metal to make what they say is the smoothest surface ever made.

They made the near-perfect mirror by coating a thin layer of lead onto a silicon surface. This is not straightforward, because when a very thin metal layer is deposited onto a flat silicon surface it usually forms an uneven coating of differently sized bumps that perform badly as a mirror for helium atoms.