Physicists Set a Record by Measuring Time Distortion Across a Single Millimeter

Get close enough to a black hole and you'll quickly learn how the force of gravity warps the very fabric of reality.

Here on Earth, gravity's time-bending effect is nowhere near as strong. It is, however, still measurable. What's more, physicists have set a new record in describing our planet's influence on the Universe's 'fabric' – they have done so on a millimeter scale.

It's a milestone well worth paying close attention to. Zooming in so close to the gentle curve of reality's foundations could help us resolve one of the most pressing problems in all of physics.

Researchers at JILA, a joint effort of the US National Institute of Standards and Technology and the University of Colorado, used a specially designed atomic clock to measure the timing of light waves separated by 1 millimeter (about 0.04 inches), resulting in a difference equal to just 0.76 millionths of a trillionth of a percent.

The difference was the result of something called gravitational redshift – a phenomenon caused by the influence of gravity over the frequency of two identical waves as compared with one another.

As incomprehensibly small as the figure might appear, it comes as no surprise to the researchers. Einstein's general theory of relativity predicts this very result, after all.

What seem like two distinct constants of space and time is in reality a single four-dimensional sheet in which the Universe lies. Any time something with mass sinks into it, the surrounding spacetime changes shape.

The result means the length of a second close to an object – be it Earth, a black hole, or even a jellybean – won't be the same length of a second further away.

The mathematics are so precise, and so thoroughly tested, we can predict this difference for incredibly small distances even when the gravitational warping is as mild as Earth's.

They also have to be wrong. At least on some tiny level.

Quantum mechanics is another area of physics that has been thoroughly tested. One of its less intuitive implications is that as you confine a measurement of one sort, other properties become fundamentally less precise.

As dependable as the two monolithic fields of physics are, they don't exactly play well together. Time isn't as central in quantum mechanics as it is in general relativity, for one thing.

More importantly, that seamless sheet of spacetime curving ever so gracefully for general relativity would be a fuzzy mess under a quantum microscope because of the problem with less precise properties we mentioned earlier. This would create a nightmare for anybody looking for a way to mesh the two ideas together.

What we need is an indication of either theory failing, which could mean finding where our predictions falter on some itty-bitty level.

A little over a decade ago, researchers managed to measure a difference in the relative frequency of light emitted by atoms separated by a vertical distance of just over 30 centimeters (about a foot).

In this new study, using a new kind of cavity for enhancing the experiment's power, researchers managed to squeeze the atomic density down by an order of magnitude, reducing the height from centimeters to a handful of millimeters.

Into this chamber they shoved 100,000 strontium atoms, which they forced to a virtual standstill by removing as much heat as possible.

They then measured the light emitted from the top and bottom of the stack of atoms and corrected for any effects that weren't gravitational in nature.

After 92 hours of watching these tiny clocks tick, they had an average that looked more or less like the result expected if general relativity were true.

The team hasn't published the work for peer review yet, but the results are available on the pre-print server arXiv for anybody to check out.

The degree of difference between the gravitationally redshifted emissions was so small, it sets a record for how fine a difference we can detect, giving us a measure of the phenomenon nearly 100 times more precise than anything achieved in the past.

It's not exactly the theory-busting result we crave, but it is a lesson in how we can shrink technology to a scale necessary for finding kinks in two of physics' greatest ideas.

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