In the very smallest measured units of space and time in the universe, not much happens. In a new quest for quantum fluctuations of space-time on Planck scales, physicists have found that everything is running smoothly.
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 troubling 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; for example, general relativity predicted gravitational waves and some black hole behaviors.
Space-time under relativity follows what we call the principle of locality – that is, objects are only directly affected by their immediate surroundings in space and time.
In the quantum realm – atomic and subatomic scales – the general theory of relativity falls apart and quantum mechanics takes over. Nothing in the quantum realm happens in a specific place or time until it is measured, and parts of a quantum system separated by space or time can still interact with each other, a phenomenon known as non-locality.
Somehow, despite their differences, there exist the general theory of relativity and quantum mechanics, which interact. But so far, it has proven extremely 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. This is an instrument designed to detect space-time quantum fluctuations on the smallest possible units – a Planck length, 10-33 centimeter and Planck time, how long it takes light to travel 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 two arms are sent down to two mirrors to be reflected back to the beam splitter for recombining. Any fluctuation on the Planck scale means that the ray that returns is different from the ray that was emitted.
A few years ago, the Holometer made a zero detection of back-and-forth quantum jitters in space-time. This suggested that space-time itself, as we can currently measure it, is not quantified; that is, it can be broken down into discrete, indivisible units or quanta.
Because the arms of the interferometer were straight, it could not detect other types of fluctuating motion, such as if the fluctuations were rotating. And this can make a big difference.
“In general relativity, rotating matter drags space-time. 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,” Hogan wrote. on the Fermilab website.
“ It could very well be that quantum space-time has a Planck-scale uncertainty of the local frame, leading to random rotational swings or twists that we would not have detected in our first experiment, and far too small to detect in a normal gyroscope. “
That’s why the team redesigned the instrument. They added extra mirrors so that they could detect any rotating quantum movement. The result was an incredibly sensitive gyroscope capable of detecting Planck-scale rotational movements 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 dual interferometer time series data. In all that time there was no movement. As far as we know, space-time is still a continuum.
But that doesn’t mean that the Holometer, as suggested by some scientists, is a waste of time. There is no other comparable instrument in the world. The results it yields – zero or not – will shape future efforts to explore the intersection of relativity and quantum mechanics at the Planck scale.
“We may never understand how quantum space-time works without some measurement to drive the theory,” Hogan said. “The Holometer program is exploratory. Our experiment started with only rough theories to guide the design and we still don’t have a unique way of interpreting our null results as there is no rigorous theory of what we are looking for.
“Are the jitters just a bit smaller than we thought they could be, or do they have a symmetry that creates a pattern in space that we haven’t measured? New technology will enable future experiments better than ours and may give us some clues. show how space and time arise from a deeper quantum system. “
The research is published on arXiv.