What do we know about space-time?
continuous fabric Einstein suggested. According to Einstein, space-time is like a stage that remains in place whether actors are treading its boards or not – even if there were no stars or planets dancing around, space-time would still be there. However, physicists Laurent Freidel, Robert Leigh and Djordje Minic think that this picture is holding us back. They believe spacetime doesn’t exist independently of the objects in it. Space-time is defined by the way objects interact. That would make space-time an artefact of the quantum world itself, not something to be combined with it. “It may sound kooky,” says Minic, “but it is a very precise way of approaching the problem.”
The attraction of this theory – called modular space-time – is that it might help solve another long-standing problem in theoretical physics regarding something called locality and a notorious phenomenon in quantum physics called entanglement. Physicists can set up a situation whereby they bring two particles together and link their quantum properties.
They then separate them by a large distance and find they are still linked. Change the properties of one and the other will change instantly, as if information has travelled from one to the other
faster than the speed of light. Einstein was so perturbed by this phenomenon that he called it ‘spooky action at a distance’.
Modular space-time can accommodate such behaviour by redefining what it means to be separated. If space-time emerges from the quantum world, then being closer in a quantum sense is more fundamental than being close in a physical sense. “Different observers would have different notions of locality,” says Minic, “it depends on the context.” It’s a bit like our relationships with other people. We can feel closer to a loved one far away than the stranger who lives down the street. “You can have these non-local connections as long as they are fairly small,” says Hossenfelder.
Freidel, Leigh and Minic have been working on their idea for the last eight years, and they believe they are slowly making progress. “We want to be conservative and take things step by step,” says Minic, “but it is tantalising and exciting.” It’s certainly a novel approach, and one that looks to ‘gravitationalise’ the quantum world rather than quantising gravity as in LQG. Yet as with any scientific theory, it needs to be tested. At the moment the trio are working on how to fit time into their model.
This may all sound incredibly esoteric, but it could have a more profound effect on our everyday lives. “We sit in space, we travel through time, and if something changes in our understanding of spacetime this will impact not only on our understanding of gravity, but of quantum theory in general,” says Hossenfelder. “All our present devices only work because of quantum theory. If we understand the quantum structure of space-time better, that will have an impact on future technologies – maybe not in 50 or 100 years, but maybe in 200,” she says.
The current monarch is getting long in the tooth, and a new pretender is long overdue, but which of the many options is most likely to succeed? When we decide, the resulting revolution could bear fruit not just for theoretical physics, but for all.
“Whether it describes the spacetime in which we live, we don’t really know until there is an experiment”
Jorma Louko