Key Moments
No device could prove our theory of gravity wrong... until now
VeritasiumKey Moments
Atomic clocks can now measure time dilation with unprecedented accuracy, but the ultimate test lies in observing quantum effects where general relativity may break down.
Key Insights
The experiment aims to measure the difference in time dilation predicted by general relativity, where a clock at a higher altitude should tick faster by approximately 25 billionths of a second per day compared to one at a lower altitude.
To achieve the necessary precision, scientists are using ytterbium (Yb) atomic clocks, which resonate in the optical band with a frequency 56,000 times higher than cesium clocks, enabling accuracies of one part in 10^18.
To overcome the Doppler shift caused by atom movement, lasers are used to cool ytterbium atoms to a millionth of a degree above absolute zero.
A crucial challenge for comparing distant clocks is transmitting signals; this experiment uses a 1-meter diameter beam to transmit a signal over 80 km to Denver, requiring extreme precision for aiming.
Even with advanced clocks, the 518 trillion oscillations per second of optical clocks are too fast for current electronics, necessitating the use of optical frequency combs to measure signals by creating a 'beat note' with a reference frequency.
The experiment's initial results show Einstein's general relativity still holds, with the observed time dilation aligning with predictions to within the 13th decimal place, though analysis continues to search for new physics at the 10 part per million level.
Probing gravity with extreme time measurements
The video details a groundbreaking experiment using highly precise atomic clocks to test the predictions of Einstein's general theory of relativity, specifically concerning gravitational time dilation. The core idea is to place one atomic clock at a high altitude and another at a lower altitude to measure the difference in their ticking rates. General relativity predicts that time passes faster at higher elevations due to weaker gravitational pull. In this experiment, one clock is situated in Boulder, Colorado, while the other is placed atop Mount Blue Sky, approximately 2.7 km higher. The expected difference in time passage is about 25 billionths of a second per day, a minuscule but measurable effect. The motivation for such precise testing stems from the knowledge that general relativity is expected to break down at the quantum scale, particularly when considering superposition and the behavior of gravity in quantum mechanics, a phenomenon current theories cannot fully explain.
The quest for extreme clock accuracy
Achieving the required precision demands atomic clocks far beyond those used for standard timekeeping. The experiment utilizes ytterbium (Yb) atomic clocks, which offer significantly higher accuracy than traditional cesium clocks. While cesium clocks operate in the microwave spectrum and define a second by 9,192,631,770 oscillations, Yb clocks resonate in the optical band with a frequency 56,000 times higher, equating to 518 trillion oscillations per second. This higher frequency allows for accuracies of one part in 10^18, meaning such a clock would not gain or lose a second over the entire age of the universe. This extreme precision is necessary to detect the subtle differences predicted by general relativity and potentially uncover deviations that hint at new physics.
Taming atoms for precise timekeeping
Preparing ytterbium atoms for use in an atomic clock involves overcoming several significant challenges. Initially, the ytterbium must be heated into a gas to prevent neighboring atoms from interfering with each other's electron frequencies. However, the atoms then travel at high speeds, causing a Doppler shift that alters their measured resonant frequency. To counteract this, scientists employ a sophisticated laser cooling technique. By firing precisely tuned laser beams at the atoms, they are slowed down to just a millionth of a degree above absolute zero. Subsequently, these ultra-cold atoms are trapped in an optical lattice created by intersecting laser beams, which limits their movement and ensures a stable measurement of their natural resonant frequency. This intricate process is crucial for maintaining the clock's accuracy.
Bridging vast distances: transmitting the signal
Comparing clocks at vastly different altitudes presents a significant engineering hurdle, particularly in transmitting their ultra-precise signals. Running a fiber optic cable between the mountain observatory and the lab in Boulder is impractical due to terrain and potential damage from wildlife. Instead, the signal from the mountain clock is beamed to a rooftop receiver near Denver, about 80 km away. This involves taking a precisely focused laser beam, initially only 10 cm wide, and expanding it to about 1 meter to compensate for divergence and atmospheric turbulence. The challenge is to hit a very small target, an 8-micron core optical fiber at the receiving end, with extreme accuracy. This requires aiming a '1-meter spotlight' over 80 km, a feat of precision engineering.
The optical frequency comb: a universal ruler for light
A major obstacle in using optical atomic clocks has been the inability of current electronics to process signals oscillating at 518 trillion times per second. The solution lies in the optical frequency comb, a device that acts as a 'ruler' for light. Developed by Theodor Hänsch and John Hall, who were awarded the Nobel Prize for their work, an optical frequency comb generates a spectrum of evenly spaced frequencies. By overlapping a laser from the comb with the unknown signal from the atomic clock, a 'beat note' is produced. This beat note, representing the frequency difference between the two signals, can be measured by electronics. The known frequency of the comb tooth and the measured beat note allow scientists to precisely calculate the atomic clock's frequency, overcoming the speed limitations of conventional electronics.
Initial results: Einstein prevails, for now
The experiment successfully established a link between the two clocks and began collecting data. General relativity predicts a specific time difference between the clocks based on their altitude and the local gravitational field. The calculated difference per second is approximately 2.885 x 10^-13 seconds. While the teams are keeping their most precise data secret for further analysis, preliminary results indicate that Einstein's theory of general relativity still holds true. The observed time dilation closely matches the predictions, with the data analyzed down to the 13th decimal place. Although no evidence has emerged to contradict general relativity at this level, the scientists are continuing their analysis to search for new physics at even finer scales, aiming for an accuracy of around 10 parts per million.
Future frontiers: pushing the boundaries to space
While the current experiment provides an incredibly precise terrestrial test of general relativity, the ultimate goal is to extend these tests into space. Launching optical clocks into orbit would allow for even greater gravitational differences and the exploration of physics in extreme environments. However, transmitting signals from a moving satellite to Earth presents new challenges, particularly in tracking a target that is constantly in motion. Teams are already developing systems, including testing links on helicopters and other mobile platforms, to prove the viability of ground-to-satellite optical clock signal transfer. This future work aims to refine the tracking and laser-pointing technologies needed to establish reliable links with space-based clocks.
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Common Questions
According to Einstein's general theory of relativity, gravity slows down time. The closer an object is to a massive body, the slower time passes relative to an observer further away.
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Mentioned in this video
A university where a team is developing systems to send optical clock signals from moving satellites to the ground.
The US institute responsible for developing and maintaining standards for measurement, which operates atomic clocks used in fundamental physics experiments.
A physicist awarded the Nobel Prize in Physics in 2005 for his work on the quantum theory of optical coherence, including contributions related to optical frequency combs.
A physicist who, along with Teodor Hench and Roy Glauber, engineered the optical frequency comb into a precise ruler for measuring light, contributing to Nobel Prize-winning work.
Einstein's theory that describes gravity as the curvature of spacetime caused by mass and energy, which is being tested by measuring time dilation at different altitudes.
The change in frequency of a wave in relation to an observer who is moving relative to the wave source; it affects atomic clock measurements by altering the observed resonant frequency.
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