Daily Archives: January 27, 2009

How Atomic Clocks Don't Work

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I was listening to a podcast recently that delved into timekeeping and atomic clocks, and was surprised that they got a couple of details wrong. I haven’t done a post explaining how atomic clocks work, because that’s something easily found on the intertubes, and so I’m not particularly motivated to recreate Wikipedia or HowStuffWorks.

But someone was wrong on the internet, and the basis of that “wrongness” has some physics behind it. The claim was made in explaining clocks that when electrons absorb energy they jump up a level, and then radiate it when they jump back down. And while that’s true, it’s not the basis for a Cesium or Rubidium clock. The thing is that you don’t want the atom to radiate on its own if you are going to make a clock out of it. Transitions between atomic states are not infinitely narrow, i.e. there is an uncertainty in the energy of the emitted photon. This is known as the linewidth of the transition, and for a good clock you want a really narrow transition so that you know what the frequency is. While there are several factors that can increase that linewidth, the fundamental width is due to the Heisenberg Uncertainty relation between energy (or frequency) and time.

The uncertainty of the frequency and the lifetime of the transition are inversely related, and \(Delta omega Delta t = 1 \) (that should be greater than or equals, but latex is choking on that for some reason)

In order to get a narrow transition, you want a long-lived state. So you don’t want something that radiates readily on its own, and atomic clocks don’t. Cesium and Rubidium devices are passive: you shine radiation on them, and then read out whether or not your radiation was on resonance by looking at which state the atoms are in. Active masers do radiate, but as the acronym tells us, the radiation is stimulated, rather than being spontaneous. (Left on its own, the lifetime of the Hydrogen atom state is about 10 million years) The search for long-lived states becomes even more important for optical clocks, since the larger energy differences tend to lead to shorter lifetimes. What is generally done is to search for so-called forbidden transitions, in which the strong coupling of electric dipole transitions aren’t present, and you are left with other types of transitions or ones that must couple through other states and end up taking much longer.

Happy Blogoversary to Me

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How do you measure yourself against other bloggers?
By height.

Wow. I’ve been doing this for a whole year.

It’s not surprising that I started at this time of year, since the football season is mostly over and there’s a huge gap in the day wanting to be filled, and the weather isn’t consistently good enough to expect to be outside much. I had kicked around the idea of blogging for a little while, once I realized there were blogs out there that weren’t political rants or diaries, neither of which hold must interest for me.

And, of course, it’s a work in progress. My idea of what the blog should be about has morphed over time as I discovered how much harder it is to write (hopefully good) posts than I naively thought it would be. It doesn’t help that my general niche in physics (atomic) was already very ably occupied by Uncertain Principles, so even if I were capable of discussing atomic physics as well as Chad, there still wouldn’t be much point in covering the same ground. The more popular aspects of physics would run me up against Jennifer at Cocktail Party Physics (now with several other contributors) and later Twisted Physics. The discussion of cornerstones of physics is better left to those teaching it, as Built on Facts and Dot Physics do so well. I’m not going to blog about particle physics, condensed matter, or any of the other disciplines within physics. Other physics blogs that survey the physics news use pretty much the same sources that I do, so there’s no new ground there — you have to comment on it sometime to add value. (And I’ve only mentioned a small slice of the blogs out there. No slight intended by that or the fact that I probably need to update my blogroll)

I’m also a little bit hobbled by working where I do — the government, especially the military, have rules. If it’s classified, you can’t talk about it. If it’s not classified, you still can’t talk about it, because it might still be sensitive; you have to wait until it’s cleared for official release. (Anything I’ve posted specifically having to do with my technical work has been published somewhere). I’ve been doing quite a bit of administrative work in the last few years, and even when I can spend time on the atomic clock project, there’s the tunnel vision that sets in — focusing only on the project and things immediately related to it. I try to keep my head in the game by discussing physics online, particularly at this blog’s host and occasionally elsewhere, but refuting crackpots gets old and repetitive. I don’t work in academia, so there isn’t that experience to draw upon; the teaching experience I have has some limited relevance, but not a lot.

So I’ve tried to carve out another niche. Blogging gives a different perspective, and forces me to go out and see what’s going on in the world of physics. I think I’ve read more papers because of blogging than for work this past year. I also like technology and weird things, so I’m going to find those links anyway, but at least now I feel like it’s not a waste of time to do so, since I share. I like learning about those little do-it-yourself science projects, and I’ve added a DIY science category)

Milestones:

I went over 1000 posts sometime in mid-December. Not as impressive as it sounds, though, because my links tend to each be in a separate post. Almost 50,000 visitors. That number is a little uncertain because the stats counter wasn’t in place when I first started.

What I’ve observed:

Writing posts or email when you’re sick is a bad idea. There’s too much “The hell? Why did I say that? I’m an idiot!” involved. At least post drafts can be deleted, and you can start over.
Writing well is hard. When you’re explaining things to a person you get feedback to let you know if you’re being effective.
I often think of something to add after I’ve posted that would have made it just that much better. Call it blogger’s remorse.

Ok, then. Back to the physics, the geekdom, and the nonsense.