All the fundamental forces are thought to be carried by a particle, but a new mathematical model suggests gravity may be a striking exception. Building on a decades-old idea, researchers have revealed a path towards understanding this possibility in detail and testing it experimentally.
at the Lawrence Berkeley National Laboratory (LBNL) in California says that gravity stands out from the other fundamental forces. One reason for this is that our current best theory of gravity, Albert Einstein鈥檚 theory of general relativity, contains exotic objects like black holes where gravity behaves wildly differently than it does anywhere else we know of.
Black holes also exhibit strange properties when it comes to their thermodynamics, the laws that govern their temperature, energy and entropy. Other researchers had previously identified this connection as a possible hint at gravity鈥檚 true nature, and the team behind the new work was guided by the same intuition.
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Karydas and his colleagues have constructed a gravitational model that reproduces how we experience gravity in our world, but ditches gravitons, the supposed particle carriers of gravity, altogether.
鈥淭he rules are, you have to make the [theoretical] model capable of reproducing everything you鈥檝e observed so far. For example, you still need the Earth to orbit the sun stably,鈥� says team member , also at LBNL.
He says their model accomplishes this 鈥� reproducing the gravity that we 鈥渒now and love鈥� 鈥� by considering a situation analogous to a pair of massive pistons with a gas between them. Here, the laws of thermodynamics dictate that gas particles should move to maximise their collective entropy, assuming a very disordered configuration, which eventually makes the pistons move too. Effectively, the pistons are subject to what is known as an entropic force arising from the gas.
The researchers derived equations for gravity as an entropic force, with two massive objects playing the role of pistons and the gas being a collection of quantum bits, or the smallest objects that can carry quantum information. It is commonly thought that two objects gravitationally attract each other by exchanging gravitons, but in this model gravitational attraction emerges from the space around them being filled with quantum objects.
Karydas says this is the first time the idea, which was originally put forward in 1995, has been developed in enough detail to make predictions for specific physical scenarios, including several laboratory experiments that are currently being built.
He and his team calculated what researchers may measure in these experiments, which will probe whether gravity can link two objects through quantum entanglement, as well as other experiments examining random fluctuations called noise that should arise due to how the force of gravity is distributed across space. They completed these calculations for several different versions of their model so that it can be either fully ruled out or further refined based on future experimental findings.
鈥淭he kind of question we were driven by was: 鈥楽o what? What does [the theoretical model] change in what we can observe?鈥�,鈥� says team member at the University of Maryland.
at Tulane University in Louisiana says the connection to realistic experiments is often difficult to achieve for novel models of gravity, so the new work presents a significant step forward in the debate about gravity鈥檚 true nature. 鈥淥ver the last decade, we have [had] tremendous progress in not only quantum, but also gravitational measurements, so I am very optimistic that we will soon have more inputs from experimentalists,鈥� he says.
Carney says he and his colleagues also must work on many more mathematical tests for their model, such as teasing out how it may reproduce all the features of general relativity.
arXiv
