Category Archives: Physics

The Evidence is Pretty Thin

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The secret molecular life of soap bubbles (1913)

Most of us would look at a soap film image and marvel at the beautiful rainbow colors; others would investigate the optics underlying them. But it took an exceptional physicist, Jean Baptiste Perrin (1870-1942), to realize that these colors concealed something more: direct evidence that matter consists of discrete atoms and molecules!

Today we take for granted that all material objects in the universe are comprised of discrete “bits” of matter, which we call atoms; however, even up until the early 20th century there were still proponents of the continuum hypothesis, in which all matter is assumed to be infinitely divisible.

This Isn't Your Grandfather's Michelson Interferometer

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Frequency comb takes a measure of distance

In its simplest form, a Michelson interferometer is used with monochromatic light. However, this limits its effectiveness because, before a measurement is made, the length of the distance to be measured must be known to within one half of the wavelength (λ/2) of the light used – typically less than 500 nm. The problem is that the distance being measured can be expressed as an integer multiple of λ/2 plus a fraction of λ/2 – but this integer multiple cannot be determined from the interference data.
Physicists have found two ways round this problem. One is to use several lasers at different colours to gain more information about the system. The other is to use a light source with a range of wavelengths and then look for phase differences in the interfering light, which can be related to distance. These techniques have their own problems, not least that thousands of different lasers would ease a measurement but installing them all in a lab would be impractical.

A frequency comb has all of its wavelengths present in a single beam, though. Really cool application of the technology.

Tug-o-War, Physics Style

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Physics demonstrations: Magdeburg hemispheres

The premise is simple: with the hemispheres pressed together, air is pumped out of the interior, creating at least a partial vacuum. This seals together the hemispheres with a remarkable force.

Guericke first demonstrated this force in 1654 for the Emperor Ferdinand III. Thirty horses, in two teams of 15, were unable to pull apart the evacuated hemispheres! He performed a smaller scale performance in 1656 in his hometown of Magdeburg, using two teams of eight horses.

The Essential Parts are Not Too Complicated, and the Principle is Easily Explained

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A film produced by the NPL Film Unit in the 1950s explaining the principles behind the first accurate atomic clock, designed by Louis Essen and built at the National Physical Laboratory in 1955.

I notice they were talking about achieving good vacuum while showing someone handling a vacuum component with their bare hands, which you wouldn’t normally do — fingerprints outgas. But it was the oven, so all of that junk gets baked off pretty quickly.

The mention a performance of a ten-thousandth of a million, or a part in 10^10. Clocks/frequency standards in use today (i.e. part of time scales, reporting their values) do ~100,000 times better, and experimental ones do even better than that, albeit for relatively short durations.

That device, which is a “physics package” (all the fun stuff) plus 6 equipment racks, now fits into about 6″ of space in a single equipment rack … and the devices are orders of magnitude better.

via BoingBoing

No Use Crying Over This, Either

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The Physics of Spilled Coffee

“I cannot say for sure if coffee spilling has been detrimental to scientific research to any significant extent,” says study author Rouslan Krechetnikov, a mechanical engineer at the University of California, Santa Barbara. “But it can certainly be disruptive for a train of thought.”

Krechetnikov and his graduate student Hans Mayer decided to investigate coffee spilling at a fluid dynamics conference last year when they watched overburdened participants trying to carry their drinks to and fro. They quickly realized that the physics wasn’t simple. Aside from the mechanics of human walking, which depends on a person’s age, health, and gender, there is the highly involved science of liquid sloshing, which depends on a complex interplay of accelerations, torques, and forces.

I have a vague recollection of this being studied years ago, looking at walking resonances.

However, physicist Andrzej Herczynski at Boston College thinks Krechetnikov and Mayer’s study didn’t go far enough. “I was personally a bit disappointed that the study is limited to cylindrical mugs … leaving out the very common curved or conical cups, such as those used for cappuccinos and lattes in Italy,” he says. “Still, the paper seems at minimum destined for the Ig Nobel Prize.”

The conical spill-resistant mugs were popular with my seagoing shipmates back in the day.

Oh, That Pesky Factor of Two

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Venus to Appear in Once-In-A-Lifetime Event

On 5 and 6 June this year, millions of people around the world will be able to see Venus pass across the face of the Sun in what will be a once-in-a-lifetime experience.

Other than the fact that Venus made a transit in 2004; I got to see it and even got to see it with a telescope used in the 1874 and/or 1882 transit expeditions. So this is (for many of us; some people have been born since 2004) a twice-in-a-lifetime experience. However, if you missed the last one, this is it — you won’t get another chance until 2117. In the US, the sunset will interrupt the transit. For much of South America or western Africa (or anywhere in Antarctica), you’ll miss out if you stay where you are. Looks like Iceland has a unique situation of having sunset interrupt the middle of the transit, but will see at again at sunrise.

More information at Transit of Venus dot org.

Are You Getting Enough Fiber in Your Timing Diet?

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I saw this paper last week and I might have gotten around to doing a blog post, but for reading Chad’s book, so he beat me to it. (OK: no, not really; I have to many other unfinished posts in the queue) Clock Synchronization Done Right: “A 920-Kilometer Optical Fiber Link for Frequency Metrology at the 19th Decimal Place”

OK, I admit, that’s a lot of zeroes. But why does that matter? Isn’t the whole point of ultra-precise atomic clocks that they all work exactly the same way? Why do you need to compare them? All atomic clocks using a given type of atom have the same basic frequency, but not all clocks are equally well made. The only way to determine the performance of a new one is to compare it to one you know works, but they’re also not very portable, so you need to be able to do the comparison remotely.

A nit: I wouldn’t necessarily characterize this as an issue of not being “equally well made”. One problem is that they are not identical — they are not the ideal clock used in thought experiments: there is noise, and it’s different for each clock. Each component in the clock can impact the clock in slightly different ways. But even if they were identical in all respects, you still have natural processes present in your oscillator, and these will give you white noise in the frequency of the clock. The integral of frequency give you the time, and the integral of white frequency noise is a random walk in phase, i.e. in the time. What does that mean? It means two clocks at identical frequencies will still undergo a random walk away from each other — they will lose synchronization. So you will still need to synchronize clocks, even if you could get them to the exact same frequency and remove all other kinds of noise. (Which you can’t).

Currently we are in a regime where clocks are better than time transfer techniques, at least over interesting distances. One can compare co-located clocks pretty well, but it doesn’t do anyone else any good if the precise time cannot be disseminated to them.