In the very smallest measured units of space and time in the universe, there is not much going on. In a new search for quantum fluctuations of space-time on Planck scales, physicists have found that everything is smooth.
This means that we – for now at least – still cannot find a way to solve general relativity with quantum mechanics.
This is one of the worst problems in our understanding of 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, for example, predicted gravitational waves and some behavior of black holes.
Space time under relativity follows what we call the principle of locality – that is, objects are only directly influenced by their immediate environment in space and time.
In the quantum realm – atomic and subatomic scales – general relativity breaks out, and quantum mechanics takes over. Nothing in the quantum range happens at a specific place or time before it is measured, and parts of a quantum system that are separated by space or time can still interact with each other, a phenomenon known as not locality.
Despite their differences, there is in a way a general relativity and quantum mechanics. But so far it has been difficult to resolve the differences between the two.
This is where the Holometer at Fermilab comes into play – a project led by astronomer and physicist Craig Hogan of the University of Chicago. It is a tool designed to detect quantum fluctuations of space-time on the smallest units – A Plank Length, 10-33 centimeters and Planck time, how long it takes to move a Planck length.
It consists of two identical 40-meter (131-foot) interferometers that intersect at a beam splitter. A laser is fired at the splitter and sent with two arms to two mirrors to reflect back to the beam splitter to recombine. Any fluctuations on Planck scale will mean that the beam returning is different from the ray emitted.
A few years ago, the Holometer did a zero detection of back-and-forth quantum disturbances in space-time. This suggests that space-time itself, as we can currently measure it, is not quantified; that is, can be broken down into discrete, indivisible units, or quantas.
Because the interferometer arms were straight, it could not detect other types of movements, such as when the swings were rotating. And that can make a lot of sense.
“In general, relativity rotates matter space-time with it. In the presence of a rotating mass, the local non-rotating frame, as measured by a gyroscope, rotates relative to the distant universe, as measured by distant stars,” wrote Hogan. on the Fermilab website.
“It may well be that the quantum space time has a Planck scale uncertainty of the local frame, which will lead to random rotational fluctuations or twists that we would not detect in our first experiment, and far too small to detect in any normal gyroscope. ‘
The team therefore redesigned the instrument. They added additional mirrors so that they could detect any rotational quantum motion. The result was an incredibly sensitive gyroscope that could detect Planck-scale rotational motions that change direction a million times per second.
In five observation runs between April 2017 and August 2019, the team collected 1,098 hours of double interferometer time series data. In all that time, there was not a single giggle. As far as we know, space-time is still a continuum.
But that does not mean that the Holometer is time, as suggested by some scientists. There is no other instrument in the world. The results it yields – zero or not – will shape future efforts to investigate the intersection of relativity and quantum mechanics on Planck scales.
“We may never understand how quantum space works without any measurement to guide the theory,” Hogan said. “The Holometer program is exploratory. Our experiment started with only rough theories to guide its design, and we still do not have a unique way of interpreting our zero results, as there is no strict theory as to what we are not looking.
“Are the jitters just a little smaller than we thought, or do they have a symmetry that creates a pattern in space that we have not measured? New technology will make future experiments better than ours and we may be giving some clues as to how space and time emerge from a deeper quantum system. ‘
The research was published on arXiv.