"Relativity has been put to the test numerous times. It's been true for a long time. Quantum mechanics has been proven time and time again to be correct."
Gravity on the Earth Spacetime is distorted. Every spot on Earth, as far as we know, experiences time and space in a microscopic — but measurable — way. One method for detecting the distortion is to use atomic clocks. (This is exactly how GPS works.) Two technological developments in next-generation atomic clocks are now opening up new paths for ultra-small-scale gravity and relativity research.
In early 2022, the studies were published in Nature back-to-back. Both experiments use "optical lattice clocks," in which light traps ultra-cooled strontium atoms floating in a vacuum and causes them to "tick" with a red laser. The laser then counts the ticks to tell time. According to scientists, these clocks can retain time to a second's accuracy in tens or even hundreds of billions of years.
Jun Ye's lab at JILA (a cooperation between NIST and the University of Colorado, Boulder) created the world's most stable and exact optical lattice clock, capable of measuring the difference in gravity at a separation of just 0.2 millimetres (200 microns), significantly smaller than ever before.
“RELATIVITY HAS BEEN TESTED OVER AND OVER. IT’S BEEN HOLDING TRUE.”
Tobias Bothwell, the study's principal author, said, "We haven't done it amongst independent clocks yet." "However, this is the first work to demonstrate that this level of precision can be achieved."
Physicists at Shimon Kokowitz's lab at the University of Wisconsin developed a set of independent optical lattice clocks that could be activated and analysed simultaneously.
"Now we're actually constrained by the atoms rather than the laser," Kolkowitz explains. This paves the way for the development of reliable, precise, and portable clocks that could be used in gravitational wave and dark matter detectors in the future.
THE EINSTEIN CONNECTION
Even at these very small distances, which are approaching the area of quantum physics, Einstein's general and special relativity theories appear to hold valid. However, Einstein's theories and quantum theory are incompatible.
Quantum theory holds that everything is quantized, or broken down into basic, discrete units. Relativity imagines the world and forces like gravity as smooth and continuous, whereas quantum theory holds that everything is quantized, or broken down into basic, discrete units. And that's just the start of their differences. The issue for physicists is that they both appear to work.
"Relativity has been put to the test numerous times. It's been true for a long time. Quantum mechanics has been proven time and time again to be correct. However, we also understand that the two cannot be true at the same time. They have to break down at some point," argues Ye.
None of the scientists believe that their findings can be used to detect a quantized version of gravity. They might be able to see how these two types of physics interact if they can investigate the effects of gravity on quantum systems.
ATOMIC CLOCKS AND QUANTUM THEORY
"This is a strange time for physics," Kolowitz adds, "because it's actually very comparable to the time before Einstein came on the scene."
"When we look out into the universe, we see all these things that point to the fact that there's a lot we don't know about." But every measurement, test, and experiment we conduct is fully explained by the physics we are familiar with. We're just not sure how to put it all together."
Kolowitz's group is currently attempting to demonstrate, for the first time in a lab, that relativistic effects may be observed in any speeding system, not simply those influenced by gravity. This has been demonstrated in the past in an indirect manner, but it has never been verified experimentally.
“I THINK THAT MUST BE THE SPIRIT OF EINSTEIN.”
Kolkowitz says, "We're going to do the first direct test of this, the first implementation of this thought experiment that Einstein recommended 100 years ago."
"What's interesting and exciting about the future of our studies is that we're starting to gain sensitivity to gravity at lower and smaller scales," Bothwell says.
"We're at 200 microns now, and we believe we can get to 20 microns." That isn't an impossibility. That'll be very soon." Currently, adds Bothwell, this entails devising new ways to introduce more atoms into the system.
In the long run, both groups hope to use optical lattice clocks to delve deeper into previously undiscovered physics. Kolkolwitz is a member of the Laser Interferometer Space Antenna (LISA) collaboration, which plans to deploy atomic clocks as gravitational wave detectors in space. Ye's team intends to scale up their experiment to the point where quantum physics' uncertainties collide with gravity's positional certainty. The goal is to uncover a regime in which gravity influences the behaviour of quantum systems.
"I'm not suggesting our experiment will be the judge," Ye clarifies. "But you have to keep exploring, so that spirit, if nothing else, that spirit of going deeper and deeper into nature, I believe that must be Einstein's spirit."
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