No device could prove Einstein wrong... until now

Summary

The video explores whether gravity obeys quantum mechanics and describes a precision experiment using optical atomic clocks to test a prediction of Einstein’s general theory of relativity: time passes faster at higher elevations, where gravity is slightly weaker.

Why test Einstein’s theory?

• General relativity predicts that gravity affects the rate of time. A clock about 2.7 kilometres higher should gain roughly 25 nanoseconds per day compared with one in Boulder.
• This effect has been confirmed in previous experiments. A deeper question is whether Einstein’s theory remains valid at very small scales, where quantum mechanics allows particles to exist in superpositions of different positions. General relativity does not clearly describe what happens to spacetime when the mass creating gravity is in such a superposition.
• Researchers hope that more precise tests could reveal a small correction to general relativity—or provide evidence that it continues to hold.

How the clocks work

An atomic clock uses the characteristic frequency of an atom’s transition between energy levels as its timekeeping reference. A photon can trigger the transition only when its energy—and therefore its frequency—matches the energy difference between those levels.

The international definition of one second is based on 9,192,631,770 oscillations associated with caesium-133. Caesium clocks are highly accurate, but optical clocks can use much higher frequencies. The experiment’s ytterbium clock operates at about 518 trillion oscillations per second, enabling far greater precision.

To make the ytterbium measurement reliable, scientists:

• Heat the element to create a beam of atoms.
• Cool the atoms with lasers, using photon momentum to slow them.
• Trap them in an optical lattice, a standing wave of laser light that limits their motion.
• Tune a laser to the atoms’ resonant frequency and count the laser’s oscillations as a proxy for the atomic “ticks.”

The researchers distinguish stability—how consistently a clock ticks—from accuracy—how close its measured rate is to the true frequency.

Taking the clocks out of the lab

One clock was operated in Boulder and another at an observatory on Mount Blue Sky, about 4,326 metres above sea level. This was described as the largest vertical separation yet attempted with high-precision clocks.

The clocks were compared through an optical link spanning about 80 kilometres. Light travelled through fibre at each end and through the air across the long-distance segment. The team had to aim the beam into a very small fibre target.

A separate University of Western Australia team tested a system for transferring optical-clock signals between a helicopter and a ground station. This could eventually support clock comparisons with satellites. The trial achieved brief signal locks, but not enough data for a full clock comparison.

Optical links are demanding because atmospheric turbulence can distort laser beams, and the clocks’ signals must be matched in frequency as well as aimed at each other.

Measuring optical frequencies

Electronics cannot directly count hundreds of trillions of optical oscillations per second. Scientists solve this with an optical frequency comb: a laser produces evenly spaced frequencies—like the teeth of a ruler—across a broad range. Comparing an unknown optical signal with nearby comb teeth creates a slower beat signal that electronics can measure.

The comb’s measured repetition rate and beat frequency can then be used to calculate the unknown optical frequency.

What the experiment found

General relativity predicts a clock-rate difference of about 2.885 × 10⁻¹³ seconds per second between the two locations.

The team reported no evidence that Einstein’s prediction is wrong, but said its data was still being analyzed. The aim is to test the prediction far more sensitively than previous terrestrial tests, potentially reaching roughly 10 parts per million.

Researchers and sources featured

• Theodor Hänsch, John Hall, and Roy Glauber, associated with the development of optical frequency-comb techniques. Hänsch and Hall, alongside Glauber, received the 2005 Nobel Prize in Physics.
• Hidatoshi Katori, whose group is preparing a relativity test using clock-link technology.
• Sasha Leilani, named as part of the University of Western Australia team.
• Laura and Andrew, named in connection with the NIST experiment; their surnames are not given in the subtitles.
• The National Institute of Standards and Technology (NIST), the University of Western Australia, RIKEN (rendered as “Recon” in the subtitles), and the University of Tokyo.