Physicists are finally ready to test an important Einstein theory


Earth’s gravity distorts Leisure. As far as we know, every point on Earth experiences time and space differently by a tiny — but measurable — amount. Atomic clocks are one of the ways to detect this distortion. (In fact, that’s how GPS works.) Now, two technological breakthroughs in next-generation atomic clocks are opening new avenues for studying gravity and relativity on a very small scale.

The studies were published back-to-back in nature Early 2022. Both experiments use so-called “optical lattice clocks” in which ultra-cooled strontium atoms floating in a vacuum are captured by light and made to “tick” with a red laser. The laser then tells the time by counting the ticks. According to scientists, these clocks keep time to the nearest second in tens or even hundreds of billions of years.

On his 143rd birthday The opposite celebrates the most famous physicist in the world – and questions the myth of his genius. Welcome to Einstein week.

Physicists from Jun Ye’s lab at JILA (a collaboration between NIST and the University of Colorado, Boulder) have produced the most stable and accurate optical lattice clock ever, accurate enough to detect the difference in gravity at a distance of just 0 .2 millimeters (200 microns) – much smaller than ever before.

“Relativity has been tested over and over again. It has come true.”

“We haven’t done it between independent clocks yet,” says Tobias Bothwell, the study’s lead author. “But this is the first piece to show that this level of precision is achievable.”

Separately, physicists in Shimon Kokowitz’s lab at the University of Wisconsin fabricated a series of independent optical lattice clocks that could be activated and measured together.

“Now we’re really limited by the atoms and not by the laser,” explains Kolkowitz. This offers a path to stable, precise, and wearable watches that could one day be developed into gravitational-wave and dark matter detectors.

The Einstein Connection

Einstein’s theories of general and special relativity still seem to apply even at these very small distances, approaching the realm of quantum mechanics. But Einstein’s theories and quantum theory do not go well together.

The theory of relativity envisions the world and forces like gravity as smooth and continuous, while the basic tenet of quantum theory is that everything is quantized, that is, broken down into basic, discrete units. And that’s just the beginning of where they disagree. The problem for physicists is that both seem to still work.

“Relativity has been tested over and over again. It has come true. Quantum mechanics has been tested time and time again and has proven itself. But we also know that both cannot be true at the same time. To some extent, they have to collapse,” Ye explains.

None of the researchers think this research can be used to discover a quantized version of gravity. However, if they can study the effects of gravity on quantum systems, they may be able to see how these two opposing types of physics interact.

Atomic Clocks and Quantum Theory

“This is a weird time for physics because it’s actually very similar to the time before Einstein came along,” says Kolowitz.

“We look out at the universe and we see all these things that suggest there are things that we don’t understand yet. But every measurement we make, every test we run, every experiment we run is well described by the physics we know. We just can’t bring everything together.”

The group around Kolowitz is currently trying to use their clocks to show in the laboratory for the first time that relativistic effects can be observed in all accelerating systems, not just in those that are influenced by gravity. This has been shown indirectly in the past, but never proved experimentally on its own.

“I think that must be the spirit of Einstein.”

“We’re going to do the first direct test of this and the first implementation of this thought experiment that Einstein proposed 100 years ago,” says Kolkowitz.

“What’s interesting and very exciting is that we’re starting to gain sensitivity to gravity on smaller and smaller scales,” Bothwell says of the future of her experiments.

“Now we’re at 200 microns, we think we can go to 20 microns. It’s not an impossible goal. That’s pretty soon.” For now, Bothwell says, that means finding ways to fit more atoms into the system.

In the long term, both groups want to use optical lattice clocks to delve deeper into unexplored areas of physics. Kolkolwitz is part of the Laser Interferometer Space Antenna (LISA) consortium, which aims to launch atomic clocks into space as gravitational wave detectors. Yes’s group wants to scale up their experiment to a scale where the uncertainties of quantum physics collide with the positional certainty of gravity. The hope is to find a regime in which gravity plays a role in the behavior of quantum systems.

“I’m not saying our experiment will be judgmental,” says Ye. “But we still have to do more research. So from that mind, if nothing else, from that mind, to probe deeper and deeper into nature, I think that must be the mind of Einstein.”

On his 143rd birthday The opposite celebrates the world’s most famous physicist – and questions the myth of his genius. Welcome to Einstein week.


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