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Atomic clocks that are accurate enough to measure gravity



  Atomic Clock

An older version of an ultra-stable atomic clock made from Ytterbium lattices on the NIST. Ytterbium atoms are generated in an oven (large metal cylinder on the left) and placed in a vacuum chamber in the center of the photo where they are manipulated with lasers and examined. The laser light is transported by five fibers to the clock (eg the yellow fiber in the lower middle of the photo). (Credit: Burrus / NIST)

Time just seems like money when things get scarce. But for physicists, time is always a big deal. The theory of relativity tells us that the flow of time depends on the circumstances in which you measure it: clocks tick faster on mountains than on ground level, and the faster you go, the slower your clocks run. The time depends on the room.

Thanks to the technological advances in atomic clocks – the most accurate timepieces we have developed – we can now turn things around and pinpoint physical parameters by examining the time course. Our understanding of space depends on time.

The new watches described today in the journal Nature will also promise what better atomic clocks usually promise: improved timing, communication and navigation technologies. In addition to their insights into the physical space around them, the devices could also help find gravitational waves, test relativity predictions, and search for dark matter. All this, only from high-precision watches.

Atomic Clocking In

This may seem rather complicated (and it does), so let's start with the basics. The authors of the paper suggest it helpful: "The passage of time is tracked by counting vibrations of a frequency reference, such as the revolutions of the earth or oscillations. By referring to atomic transitions, the frequency (and therefore the time) can be measured more accurately than any other physical quantity. "So, count how many times certain atoms switch between energy levels, and you get the most accurate ticks and ticks.

The first name in atomic clocks is the National Institute of Standards and Technology (NIST), and that's where today's research comes from. The latest watches are based on 1,000 atoms of ytterbium cooled to near absolute zero and encased in 1-D lattices (ie columns) of laser beams. With all the measurements of atomic clock power – minimizing errors in atomic frequencies, ensuring that the ticks are stable and the results are reproducible – the NIST researchers simply produced incredibly accurate clocks. Their error bars are in the order of 10 -18 or one billionth of a billionth of a billionth.

The Shape of Things

In fact, these atomic clocks are so accurate that they are actually sensitive to the effects of gravity. Normally, the relativistic quirks of gravity that affect the flow of time are too short to notice – but no more. As the authors say, "If these watches were compared over a long baseline or used for remote comparisons with other clocks around the world, the measurement would be limited by the gravitational knowledge on the earth's surface." That is, they are so precise. The only thing that could change their ticks and toes would be gravity itself. Clocks higher up from Earth's mass would tick faster than the clocks thanks to the theory of relativity.

This is an important business for the science of geodesy, measuring the shape and influence of Earth's gravity. Our current picture of the exact surface of the planet depends on satellites and computer modeling, which offer a fairly good resolution of a few centimeters. But these atomic clocks would reduce this resolution to one centimeter. Armed with two of these watches, researchers were able to compare the sea level on two different continents, the exact height of a mountain, or other altitude measurements (and thus the gravity measurement) they would like to perform.

And because the atomic clocks are so sensitive to gravity that they can serve as a kind of detector for all related activities. Gravitational waves as they pass through us and this planet would appear in the displays of these clocks. Extremely subtle experimental tests of Einstein's theories are now possible. The technology could help discover tiny amounts of dark matter, the invisible material that only interacts with gravity and makes up the bulk of the universe's matter.

To be clear, these are just possibilities. The authors have just built these atomic clocks and shown how accurate they are. Now that they know that the technology works so well, a new future of discoveries in physics could be just ahead. It is time.


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