Scientists suggest that spacetime may be made up of individual “spacetime pixels,” rather than being smooth and continuous as it appears.
The sand dunes seen from afar appear smooth and wrinkle-free, like silk sheets spread across the desert. But a closer inspection reveals much more. As one approaches the dunes, one may notice ripples in the sand. Touch the surface and find individual grains. The same is true for digital images: by enlarging a seemingly perfect portrait enough we see the different pixels that make it up.
With the above in mind, could the universe around us be similarly pixelated?
Scientists like Rana Adhikari, a physics professor at Caltech, think so. The space we live in may not be perfectly uniform and instead be made up of incredibly small discrete units.
“A pixel in spacetime is so small that if you made things the size of a grain of sand, atoms would be as big as galaxies,” He explained.
Adhikari and scientists around the world are on the hunt for this pixelation because it is a prediction of quantum gravity, one of the deepest physical mysteries of our time.
Quantum gravity refers to a set of theories, including string theories, that seek to unify the macroscopic world of gravity — governed by general relativity — with the microscopic world of quantum physics. At the center of the mystery is the question of whether gravity and the spacetime it inhabits can be “quantified” or broken down into individual components, a hallmark of the quantum world.
“Sometimes there is a misunderstanding in scientific communication that quantum mechanics and gravity are irreconcilable,” said Cliff Cheung, professor of theoretical physics at Caltech. ‘But we know from experiments that we can do quantum mechanics on this planet, which has gravity, so they are clearly consistent. Problems arise when you ask subtle questions about black holes or try to merge theories on very short distance scales. ‘
Due to the incredibly small scales involved, some scientists have found that finding evidence for quantum gravity in the foreseeable future is an impossible task. Although researchers have come up with ideas about how they might find clues to their existence – around black holes, in the early universe, or even using LIGO (the National Science Foundation-funded observatories that detect gravitational waves) – no one has found evidence of quantum gravity in nature.
Theoretical physics professor Kathryn Zurek would like to change that. He recently formed a new multi-agency collaboration, funded by the Heising-Simons Foundation, to think about how to observe quantum gravity signatures.
The project, called QuRIOS ( Quantum gravity and Its Observational Signatures ), pairs string theorists – who are familiar with the formal tools of quantum gravity but have little practice in designing experiments – with particle theorists and model builders. who have experience with experiments but do not work with quantum gravity.
“The idea that you could look for observable features of quantum gravity is a long way from the mainstream,” Zurek noted. But we will get lost in the desert if we don’t start to focus on ways to link quantum gravity to the natural world we live in. Having observational firms to think about unites us with theorists and helps us progress on new types of questions. ‘
As part of the Zurek collaboration, he will work with Adhikari to develop a new experiment using tabletop instruments. The proposed experiment, called GQuEST ( Gravity from Quantum Entanglement of Space-Time ), will not be able to detect individual pixels of space-time, but connections between the pixels that give rise to observable signatures.
Adhikari likens searching to tuning in to old televisions.
“When I was a kid, we couldn’t have NBC, and we tried to tune in to get it. But most of the time, we would see pixelated snow. We know that some of that snow comes from the cosmic microwave background or the birth of the universe, but if you tune right to the end of that, you could find snow from solar storms and other signals. That is what we are trying to do: carefully tune in to snow or fluctuations in space-time. We will look to see if the snow fluctuates in a way that aligns with our models of quantum gravity. Our idea could be wrong, but we have to try. ‘
And it is that solving the problem of quantum gravity would be one of the greatest achievements of physics, along with the two theories that the researchers want to merge.
The non-reality of space-time
Some scientists suggest that hypothetical “gravitons” could compensate for gravity on the smallest scale. Gravitons are a component of string theory that would resonate at a particular frequency. But on an even smaller scale than that, scientists are still scratching their heads on how to unify the laws of general relativity and quantum physics.
“If I drop my coffee cup and it falls, I’d like to think its gravity,” Adhikari joked. “But in the same way that temperature is not ‘real’ but describes how a lot of molecules vibrate, spacetime might not be real.”
The same can happen with space-time.
“It may be that something that arises from the pixelation of space-time has received the name of gravity because we still do not understand what the bowels of space-time are,” he concluded.