A new measurement of quantum spacetime has found nothing going on


In the smallest measured units of space and time in the Universe, not much is happening. In a new search for quantum fluctuations of space-time on Planck scales, physicists have discovered that everything is fluid.

This means that, for now at least, we still can’t find a way to solve general relativity with quantum mechanics.

It is one of the most irritating problems in our understanding of the Universe.

General relativity is the theory of gravitation that describes gravitational interactions in the physical Universe on a large scale. It can be used to make predictions about the Universe; general relativity predicted gravitational waves, for example, and some behaviors of black holes.

Spacetime under relativity follows what we call the locality principle, that is, objects are only directly influenced by their immediate surroundings in space and time.

In the quantum realm, atomic and subatomic scales, general relativity breaks down and quantum mechanics takes over. Nothing in the quantum realm happens at a specific place or time until it is measured, and the parts of a quantum system separated by space or time can still interact with each other, a phenomenon known as nonlocality.

Somehow, despite their differences, general relativity and quantum mechanics exist and interact. But so far, resolving the differences between the two has proven extremely difficult.

This is where the Holometer at Fermilab comes in, a project spearheaded by astronomer and physicist Craig Hogan of the University of Chicago. This is an instrument designed to detect quantum fluctuations of space-time in the smallest possible units: a Planck length, 10-33 centimeters and Planck time, the time it takes for light to travel a Planck length.

It consists of two identical 40-meter (131-foot) interferometers intersecting in a beam splitter. A laser is fired at the splitter and sent down with two arms towards two mirrors, to be reflected back to the beam splitter to recombine. Any fluctuation on the Planck scale will mean that the returning beam is different from the beam that was emitted.

A few years ago, the Holometer did a null detection of quantum fluctuations back and forth in space-time. This suggested that spacetime itself, as we can currently measure it, is not quantized; that is, it could be divided into discrete, indivisible units or quanta.

Because the arms of the interferometer were straight, it could not detect other types of fluctuating motion, as if the fluctuations were rotational. And this could matter a lot.

“In general relativity, rotating matter drags space-time along with it. In the presence of a rotating mass, the local non-rotating frame, measured by a gyroscope, rotates relative to the distant Universe, measured by distant stars”, Hogan wrote. on the Fermilab website.

“It could well be that quantum spacetime has a Planck scale uncertainty of the local frame, which would lead to random rotational fluctuations or spins that we would not have detected in our first experiment, and too small to detect in any normal gyroscope.”

So the team redesigned the instrument. They added additional mirrors so that they could detect any rotating quantum motion. The result was an incredibly sensitive gyroscope that can detect Planck-scale rotational turns that change direction a million times per second.

In five observing runs between April 2017 and August 2019, the team collected 1,098 hours of dual interferometer time series data. In all that time, there was not a single movement. As far as we know, spacetime is still a continuum.

But that does not mean that the Holometer, as some scientists have suggested, is a waste of time. There is no other instrument like this in the world. Your results, null or not, will shape future efforts to prove the intersection of relativity and quantum mechanics at Planck scales.

“We may never understand how quantum spacetime works without some measure to guide the theory,” Hogan said. “The Holometer program is exploratory. Our experiment started with only approximate theories to guide its design, and we still don’t have a single way to interpret our null results, as there is no rigorous theory of what we are looking for.

“Are the nerves a little smaller than we think they might be, or do they have symmetry that creates a pattern in space that we haven’t measured? The new technology will allow for future experiments better than ours and possibly give us some clues about how space and time emerge from a deeper quantum system “.

The research has been published in arXiv.

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