In the smallest units measured by space and time in the Universe, not much happens. In a new search for quantum space-time fluctuations on the Planck scales, physicists have discovered that everything is smooth.
This means that – at least for now – we still cannot find a way to solve general relativity with quantum mechanics.
It is one of the most annoying problems in understanding the Universe.
General relativity is the theory of gravity that describes gravitational interactions in the large-scale physical universe. It can be used to make predictions about the Universe; general relativity predicted gravitational waves, for example, and some black hole behaviors.
Space-time under relativity follows what we call the principle of locality – that is, objects are influenced only directly by their immediate environment in space and time.
In the quantum realm – atomic and subatomic scales – general relativity decomposes and quantum mechanics takes over. Nothing in the quantum realm happens in a particular place or time until it is measured, and parts of a quantum system separate from space or time can 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 proved extremely difficult.
This is where the Fermilab Holometer comes into play – a project led by astronomer and physicist Craig Hogan of the University of Chicago. This is a tool designed to detect quantum space-time fluctuations in the smallest possible units – a Planck length, 10-33 centimeters and the Planck time, how long the light takes to travel a Planck length.
It consists of two identical 40-meter (131-foot) interferometers that intersect at a beam separator. A laser is pulled to the splitter and sent down two arms to two mirrors, to be reflected back to the splitter by the beam to recombine. Any fluctuations in the Planck scale will mean that the returning beam is different from the beam that was emitted.
A few years ago, the Holometer made a zero detection of quantum nervousness back and forth in space-time. This suggested that space-time itself, as we can measure it today, is not quantified; that is, they could be divided into discrete, indivisible, or quantum units.
Because the interferometer arms were straight, it could not detect other types of fluctuating motion, such as if the fluctuations were rotational. And that could matter a lot.
“In general relativity, rotating matter draws space-time with it. In the presence of a rotating mass, the non-rotating local frame, as measured by a gyroscope, rotates relative to the distant universe, measured by distant stars.” Hogan wrote on the Fermilab website.
“Quantum space-time may have an uncertainty at the Planck scale of the local frame, which would lead to random rotational fluctuations or twists that we would not have detected in our first experiment and far too small to be detected in any normal gyroscope. “
So the team redesigned the instrument. They added additional mirrors so they could detect any quantum rotational motion. The result was an incredibly sensitive gyroscope that can detect Planck-scale rotations that change direction one million times per second.
In five observation periods between April 2017 and August 2019, the team collected 1,098 hours of data with time series with dual interferometer. During all this time, there was not a single movement. As far as we know, space-time is still a continuum.
But this does not mean that the Holometer, as suggested by some scientists, is a waste of time. There is no other tool like this in the world. The results it returns – null or not – will shape future efforts to probe the intersection of relativity and quantum mechanics at Planck scales.
“We may never understand how quantum space-time works without a measurement to guide the theory,” Hogan said. “The Holometer program is exploratory. Our experiment started only with rough theories to guide its design and we still do not have a unique way to interpret our null results, because there is no rigorous theory of what we are looking for.
“Am I a little less nervous than I thought it might be or do they have a symmetry that creates a pattern in space that I haven’t measured? The new technology will allow future experiments to be better than ours and possibly give us some indications of how space and time arise from a deeper quantum system. “
The research was published on arXiv.