# Sliced Bread Didn't Make the List

The 50 Greatest Breakthroughs Since the Wheel

The Atlantic recently assembled a panel of 12 scientists, entrepreneurs, engineers, historians of technology, and others to assess the innovations that have done the most to shape the nature of modern life. The main rule for this exercise was that the innovations should have come after widespread use of the wheel began, perhaps 6,000 years ago. That ruled out fire, which our forebears began to employ several hundred thousand years earlier. We asked each panelist to make 25 selections and to rank them, despite the impossibility of fairly comparing, say, the atomic bomb and the plow.

# Time Travel

A nit: if “strength of gravity” means the value of g, then it’s incorrect. The amount of dilation is due to the depth in your gravitational well (the gravitational potential), which is important if you compare two planets with each other. Since the force varies as 1/r^2 and the potential as 1/r, it’s possible to contrive a planet whose mass and size are such that gravity (g) is weaker, but you are “deeper in the well” and your clock runs slower (or the opposite). If you are talking about a single planet then the distinction doesn’t matter, but the details do. You don’t want to misapply the model because of a vague description such as this.

At the end he tells us that 24k miles will slow you be about 5 nanoseconds, but you may already have known that.

# Two For the Price of One

A while back I put up a post explaining that you can add photons together under the right circumstance. The other day at work someone pointed me to an interesting doubling paper: High-efficiency frequency doubling of continuous-wave laser light (arXiv link)

Second harmonic generation (SHG, or doubling) is a nonlinear process, meaning it has a nonlinear dependence on the strength of the electric field. In other words, it becomes more efficient as you go to higher intensity. Naively, if you need two photons interacting at the same time, the odds of having two right where you need them is higher if you have more photons in your volume.

The higher intensity here is achieved with a resonant cavity, comprised of a mirror and the doubling crystal. The left-hand side of the cavity is the mirror which is mounted on a piezoelectric transducer (PZT) so the cavity length can be tuned to give you a standing wave inside the cavity, made up of an integral number of wavelengths of the light.

The crystal is periodically poled potassium titanyl phosphate crystal (PPKTP), and the right-hand surface is curved and has a dielectric coating to make it highly reflective at both the 1550 nm pump wavelength and the 775 nm output wavelength. Clever! This way you don’t have to worry about additional losses from reflection off of the crystal surface; the other surface is antireflection coated, but that’s never going to give you 100% transmittance.

Since the mirror is reflective at 1550 nm, these photons will bounce multiple times before leaving, so the power can build up. This is shown schematically with the dotted line in a loop, but the actual light profile will be a bowtie shape, so the intensity is even higher inside the crystal. However, the mirror is antireflection coated for 775 nm, so once the light is doubled it leaves. The input beamsplitter to the cavity is dichroic, so it only reflects at 1550 nm and the 775 nm light is free to pass through it.

With this they got 95% conversion efficiency with an input of 1.1 Watts, and think they can bump it to 98% with 1.3 Watts.

# Splish Splash

Humble drops of water levitate their way to stardom

First the researchers increase the strength of the field, which flattens the floating drops into discs. They then turn the drops into stars by tuning the field to the resonant frequency of the drops – or exact multiples of that frequency. Using a particular multiple produces a star with the corresponding number of spikes.

# Robots at Work and Play

Robots at Work and Play

Advancements in robotics are continually taking place in the fields of space exploration, health care, public safety, entertainment, defense, and more. These machines — some fully autonomous, some requiring human input — extend our grasp, enhance our capabilities, and travel as our surrogates to places too dangerous or difficult for us to go. Gathered here are recent images of robotic technology at the beginning of the 21st century, including robotic insurgents, NASA’s Juno spacecraft on its way to Jupiter, and a machine inside an archaeological dig in Mexico.

# Not That Kind of Atomic Wristwatch

No, this is not one of the pretenders that link up to NIST’s atomic time via a radio signal.

\$12,000 watch has its own built-in atomic clock

I link to this article because it actually mentions USNO, but there’s the original, which mentions it’s made (or will be made) in Switzerland, meaning this is probably not just a Symmetricom CSAC that’s been marked up with a counter and a display attached, but it’s undoubtedly the same technology.

This watch actually points to a problem in timekeeping, that there are two elements one must worry about: telling the time, and disseminating the time. Having a great clock is not particularly useful if you can’t transfer the information to anyone, so there is a dual, usually parallel effort to improve clocks and to improve time transfer. Time transfer can’t lag too far behind timekeeping or else there’s no point in pushing the boundaries.

Here we have the time transfer problem in reverse. If the input is the stem and you have to look at a display (or listen to a voice) to get the time, it is going to be limited to the feature of not gaining or losing a whole second over some long interval. Which goes out the window because you have to reset it when you change the batteries. The watch really doesn’t require or exploit its precision, so why? It’s really nothing more than an expensive trophy, while some pretty incredible technology is basically wasted. And an analog display? I’d want a digital one that showed the time to better than a second.

However, this does point out the ridiculousness of an episode of Person of Interest from last season, where a very rich guy™ supposedly had a watch that kept time to the nanosecond. 1 second in 1000 years is roughly a part in 10^10, so that’s not even a microsecond per day.