Since Einstein published his theories of general relativity and special relativity in Annalen der Physik in 1905 and 1916, physics has struggled to further understand the structure of space and time. Since then, the two theories: quantum loop theory and string theory, both attempt to understand the universe; quantum loop theory is focused on understanding the structure of space-time, while string theory deals with combining gravity and quantum physics (1).
One of the main challenges facing string theory and quantum loop theory is the existence of theoretical particles called gravitons. First proposed in 1934 by Soviet physicists Dmitrii Blokhintsev and F. Gal'perin, gravitons carry the gravitational force of matter in the universe (2). Don Lincoln, an American physicist, explains the rationale of the graviton: since all forces of the universe (except gravity as far as we know) are transmitted by force-carrying particles - photons convey electromagnetism, the strong nuclear force is transmitted by gluons, and the weak nuclear force is imparted by the movement of the W and Z bosons - gravity should work similarly (3). Both theories affirm the existence of gravitons, but scientists have not been able to find any proof of their existence - central to both theories.
So far, scientists know that if gravitons do exist, they must travel at the speed of light and be massless; otherwise, their mass would disrupt the inverse square law that describes how gravity weakens with distance (3). However, although we know that much about gravitons, scientists have been unable to detect their existence. Because the force of gravity and gravitons are so weak (the electromagnetic force is 1039 times larger between a proton and an electron in a hydrogen atom), detecting them has posed a significant technical challenge for physicists as they would need a universe-sized detector to detect one graviton. Luckily, one universe-sized detector exists: our universe. Lawrence Krauss, a cosmologist at ASU, and Frank Wilczek, a physicist with MIT and ASU, proposed an ingenious idea: measuring slight changes in the cosmic background radiation of the universe caused by gravitons could prove their existence (4). However, some proponents of string theory have even speculated that gravitons could be from other spatial dimensions (5). While these researchers haven’t found any gravitons yet, their research provides many new ideas for physicists to detect them.
Even with these research ideas, gravitons may never be detected. However, if gravitons were detected, it would be one of the most significant scientific discoveries that would help cement our theories of quantum gravity into a theory of everything.
References:
1) Rovelli, Carlo, et al. Reality Is Not What It Seems: the Journey to Quantum Gravity. Riverhead Books, 2018.
2) Kumar, Manjit. Quantum: Einstein, Bohr, and the Great Debate About the Nature of Reality. W. W. Norton & Company, 2008.
3) Lincoln, Don. “What Are Gravitons?” PBS, Public Broadcasting Service, 14 May 2014, www.pbs.org/wgbh/nova/article/what-are-gravitons/.
4) Robbins, Lisa. “Researchers Propose a New Way to Detect the Elusive Graviton.” ASU Now: Access, Excellence, Impact, ASU News, 5 Mar. 2014, asunow.asu.edu/content/researchers-propose-new-way-detect-elusive-graviton.
5) Inglis-Arkell, Esther. “What Are Gravitons and Why Can't We See Them?” io9, io9, 16 Dec. 2015, io9.gizmodo.com/what-are-gravitons-and-why-cant-we-see-them-1643904640.