Gravity is one of the four fundamental interactions in nature. The other three are the electromagnetic force, the weak nuclear force, and the strong nuclear force. Gravity is the weakest of the four interactions, but it is the one that is the critical factor in creating the cosmic environment as we know it, the Universe. The existence and movement of galaxies, stars and planets and ultimately the existence of life is due to gravity, which is a perennial subject of research.
A new study published in the online pre-publication archive arXiv (https://arxiv.org/abs/2205.08767) reports that gravity can be converted into light, which is understandably if true, very impressive and opens up new scientific avenues. According to the research team that but only if space-time behaves in the right way, an international research team discovered.
Under normal circumstances, we cannot make something out of nothing. In particular, the Standard Model of particle physics, the dominant theory that explains the subatomic “zoo” of particles, usually forbids the conversion of massless particles into large particles. While particles in the Standard Model are constantly changing into each other through various reactions and processes, the photon – the massless carrier of light – cannot normally be changed into other particles. But if the conditions are just right, it is possible – for example, when a photon interacts with a heavy atom, it can spontaneously decay to become an electron and a positron, both of which are particles with mass.
Based on this assumption the research team tried to answer whether gravity itself could be converted into other particles. We usually think of gravity through the lens of general relativity, where bends and distortions in spacetime affect the motion of particles. In this picture, it would be very difficult to imagine how gravity could create particles. But we can also look at gravity through a quantum lens, imaging the gravitational force as carried by countless invisible particles called gravitons. While our picture of quantum gravity is far from complete, we know that these gravitons will behave like any other fundamental particle, including potential transformation.
To test this idea, researchers studied the conditions of the Universe immediately after its birth. When our world was very young, it was also small, hot and dense. In this young world, all forms of matter and energy have grown to unimaginable scales, far greater than even our most powerful particle accelerators can achieve.
The researchers found that in this arrangement, gravitational waves – ripples in the “fabric” of spacetime created by collisions between the most massive cosmic objects – play an important role. Normally, gravitational waves are extremely weak, capable of pushing an atom no further than the width of its core. But in the early Universe, the waves could have been much stronger, and that could have seriously affected everything else.
Early gravitational waves would move back and forth in a self-reinforcing process. Everything else in the Universe would have been caught up in the push and pull of the waves, leading to a resonance effect. Like a child moving its legs when it needs to in a swing, gravitational waves would have acted as a kind of pump, driving matter into solid masses over and over again.
Gravitational waves could also affect the electromagnetic field. Because waves are ripples in spacetime itself, they are not limited to interactions with massive objects. As the waves continue to pump, they can drive radiation into the Universe at extremely high energies, causing photons to spontaneously appear. Simply put, gravity itself creates light.
The researchers found that this process is rather inefficient. The early Universe was also expanding, so the typical patterns of gravitational waves would not have lasted long. However, the team found that if the early Universe contained enough matter to slow down the speed of light (in the same way that light travels more slowly through a medium like air or water), the waves could have stuck around quite to generate massive amounts of extra photons.
Physicists still do not fully understand the complex, tangled physics of the early Universe, which was capable of feats never seen since. This new research adds a piece to the puzzle of the evolution of the Universe, and more specifically gravity’s ability to create light. This radiation will likely continue to influence the formation of matter and the evolution of the Universe, so working out the full implications of this amazing process could lead to new revolutions in our understanding of the early Universe.