Archive for February, 2013

CSI:Time Machine

Before fingerprinting

It was essentially a criminal justice Dewey Decimal System, the first step in taking police out of the dark ages. Before Bertillion standardized measurements, police just had a jumble of descriptions and photographs with no way to organize them so they’d almost never be able to cross reference existing records when people were arrested.

Fingerprinting isn’t the only technology difference in criminal investigation, of course. Much of forensic pathology dates from after this time.

The Devil is in the Dots

Maxwell’s Demon Meets Quantum Dots

While there is an increase in the total entropy — per the second law of thermodynamics — that increase doesn’t occur in the demon-y quantum dot by itself. As one of the collaborators, Massimiliano Esposito (University of Luxembourg) told “It does, of course, respect thermodynamics…. However, if the part of the system implementing the demon is disregarded, everything looks as if the remaining pat of the system was subjected to a Maxwell demon breaking the second law while keeping the first one intact.”

Why Don't These Things Cost $50k?

Pop quiz, hotshot: Your really long optical fiber isn’t letting (much) light through, so there’s obviously a break in it somewhere. You need to fix the fiber. What do you do? What…do…you…do?

Obviously, shooting the hostage is not an option here. The fiber is probably buried underground, so it would be really helpful to know where the break is, to a resolution of at least the location of the nearest manhole, so you can go in, find the fault and splice the fiber. The solution is an optical time-domain reflectometer (OTDR). You send a pulse of light down the fiber and measure the delay of any reflection, because breaks (and other faults) tend to reflect the light, as any change in index of refraction causes a reflection. Since the speed of light transmission in a medium is simply c/n, if you can measure the return time of the pulse you can figure out how far way the fault is.

To do this in a helpful way, though, one needs to locate the fault to within a few meters, and light in a fiber will be traveling at around 200,000 km/sec, or 5 nanoseconds per meter, which means we need timing at a level at around the 10 nanosecond level. That sounds like the precision realm of commercial atomic clocks, and that sounds expensive — that kind of clock can run you several tens of thousands of dollars. But there’s an important distinction: an atomic clock gives precision long-term timing, and we don’t need that. If our optical fiber is 100 km long, a round-trip signal will take no longer than a millisecond. In other words, we don’t need a clock that will add fewer than 10 nanoseconds in a day, we just need one that won’t add more than 10 nanoseconds in a millisecond. There is almost 8 orders of magnitude difference in performance in those two systems. Put another way, we don’t want to measure the time, we want to measure a short time interval. A timing error of 10 nanoseconds in a millisecond is 10 parts-per-million, a performance that is easily reached by a cheap quartz oscillator (Here’s a cheap system that does 2 parts per million along with some extra functions we wouldn’t need). As long as the oscillator is calibrated, such a device would be just fine for this task.

Another example of this time interval application is a GPS receiver. These receivers compute your location based on the time difference between signals from multiple satellites, but since the satellites have precise clocks on them and broadcast that information, the receiver only has to measure the difference in those time tags. GPS satellites orbit at altitudes of around 20,000 km, but it’s the differences in the distances that are important to us. Overhead satellites are closest, while ones nearer the horizon are farther away, by a few thousand km. That’s a factor of ~10 greater distance than our OTDR signal (though our speed is very close to c), and we want somewhat better timing, so that puts our needs closer to 0.1 ppm, but this is also achievable, though undoubtedly a little more expensive. The great part about GPS receivers, though, is that you can actually use the timing signals to synchronize a local clock, and gain the benefit of the atomic time on the satellites, which is synchronized to the earth’s atomic time, UTC. (You might recall that such synchronization was initially — and incorrectly — blamed for timing errors in the superluminal neutrino story a little over a year ago. It’s actually quite good.)

The Mystery of Magenta, and of Light Mixing

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

Interesting and fun, but about 35 seconds in he says that you can’t combine photons together, and that’s just plain wrong.

I know that you can’t mix photons together. So you can’t take a blue photon and a green photon and mix them together to get some other photon. That just doesn’t happen.

Except that it does. You have to do this in a nonlinear medium like certain crystals, but it can be done. It’s called sum frequency generation.

Energy is conserved, so in the example given a 2.5 eV (green) photon added to a 3 eV (blue) photon will result in a 5.5 eV photon, which will be ultraviolet. The addition is not what we see with our eyes, since that’s a different process.

A special case of this is where the two photons being added are the same frequency. This is called frequency doubling, and a common (amongst geeks, at least) example of this is a green laser pointer. The source inside of this is actually an infrared laser emitting at 1064 nm, which then passes through a doubling crystal to produce light at twice the frequency, or half the wavelength: 532 nm. (and cheap laser pointers may not filter the IR from the output, which can be a danger)

Boom, Boom, Out Go the Lights

BREAKING: Huge Meteor Blazes Across Sky Over Russia; Sonic Boom Shatters Windows [UPDATED]

Apparently, at about 09:30 local time, a very big meteor burned up over Chelyabinsk, a city in Russia just east of the Ural mountains, and about 1500 kilometers east of Moscow. The fireball was incredibly bright, rivaling the Sun! There was a pretty big sonic boom from the fireball, which set off car alarms and shattered windows. I’m seeing some reports of many people injured (by shattered glass blown out by the shock wave). I’m also seeing reports that some pieces have fallen to the ground, but again as I write this those are unconfirmed.

Here's the Windup …

The Most Exciting Video of Nothing Happening: Pitch Drop Experiment in 2013

If you’re lucky, you may see what no one has seen before—no one has ever seen the drop fall.

John Mainstone, a physics professor at the University of Queensland and the experiment’s current custodian, missed the last two drops by pure bad luck. Awaiting the eighth drip from a business trip, Mainstone secured a video surveillance system to trail the elusive drop. Alas, the video feed failed precisely during the fall of the eighth pitch-drop.

The Yellow, Green or Red Flash

The Physics of Sunsets

It's a Little Like Rock Polishing

A math trick with polygons

Super Toil and Trouble

ScienceShot: Electricity Gives Bubbles Super Strength

Any kid can blow a soap bubble, but only a physicist would think to electrify one.

Pop the Question

(That’s Rhett of Dot Physics)

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