Redefining Time

I’ve always been on board with the idea that a second is exactly a second long. I’m not really a casual believer in the length of a second, either. One second is one second, and to hear that a team of scientists are proposing otherwise is a bit annoying and confusing.

This week at the Conference on Lasers and Electro Optics in San Jose, a group of scientists led by Dr. Stefan Droste from both the Max Planck Institute of Quantum Optics and the Federal Institute of Physical and Technical Affairs in South and North Germany will be presenting their recent findings after successfully transmitting an accurate clock signal 200 kilometers, taking a giant step towards redefining the accuracy of the second as a unit of time measurement.

Good god that was an exhausting sentence to process. That sensation you’re feeling in your temporal lobe right now is kind of how I’ve been feeling through the entire process of reading and understanding Dr. Droste’s complex answers to my relatively simple questions. Scientists always seem to do this, don’t they? Once you feel that you have a legitimately solid grasp on a primitive fact of life, they do a few experiments with quantum optics and atomic clocks, explain their work at a laser conference, then suddenly the rug has been pulled right out from under you, and you’re shopping for a new wristwatch.

What, exactly, was being tested here? How does one go about redefining time in the first place? In Dr. Droste’s words, what they checked out was how well they could “transfer an optical frequency from point A to point B.” An optical frequency is comparable to the frequency of electromagnetic waves that are visible; the “how well” refers to how stable and how accurate it can be transmitted. This is important, says Dr. Droste, because the most stable signals in the world are generated by optical clocks.

“We do not question the accuracy of time,” Dr. Droste says. “We know that it’s only accurate to a certain extent. Time, as we know it, is based on cesium atomic clocks. These kinds of clocks have accuracies to the order of 1e-15. The confidence in the time that clocks show is increased by comparing hundreds of clocks around the world with one another, and this is important since with only one single clock, you cannot know whether your clock is showing the correct time or not.”

He explained that modern celcium clocks have tiny oscillating quartz crystals built in that make for a “fairly accurate clock signal.” Conversely, atomic clocks have atoms that get excited and return a less excited, very stable frequency in the microwave region. Now the optical clock, which is the crux of this experiment, is basically the same as the atomic clock, but instead of microwaves, it generates optical frequencies that have about 50,000 to 100,000 times higher frequencies. Oh OK. Gotcha. So those are clocks.

"The accuracy is a property of a clock and can therefore not be off,” Dr. Droste continues. “The clock itself can be off and the degree of how far off it is defines its accuracy. State-of-the-art optical clocks reach uncertainties of 1e-18. This is the relative accuracy, and from this you can calculate how many nanoseconds the clock will be off after a day, or how many seconds it will be off after a million years.”

However, Dr. Droste also made it clear that optical clocks are currently in an experimental state and “cannot be transported.” The principle idea behind what they tested then, was the ability to get the stable frequencies out of the lab where the clock lives, sending them with accuracy over distance.

So Dr. Droste’s answer to “Why are you redefining time?” was essentially: “So we can move a clock.”

Previously - Red Is Not Sexy Time