Our understanding of the universe, based largely on Einstein’s theory of general relativity, relies on four fundamental forces: Gravity, electromagnetic, and the strong and weak nuclear forces. These forces stop planets, molecules and even atoms from tearing themselves apart and are the building blocks for the physical rules of the cosmos.
But in recent decades, whispers of a fifth force of nature have surfaced, and if it exists, it could overthrow everything we know about how the universe works. Now, researchers have discovered the ultimate way to test whether this fifth force is real, and it involves tracking the stars at the centre of our galaxy.
The four forces of nature hold the standard model of physics together, which is what we use to describe the behaviour of particles and matter. The smaller forces of the four are strong and weak nuclear forces. While the strong nuclear force holds atomic nuclei together, the weak nuclear force allows the radioactive decay of certain atoms.
At the large-scale end of the four forces sit gravity and electromagnetic forces. The electromagnetic force binds molecules together, while gravity stops entire planets and galaxies from being torn apart. But the problem with these four fundamental forces is that, since Einstein’s crowning theory surfaced, several holes have been poked in the way we understand gravity.
Gravity is the last of the four forces that humans are yet to figure out how to create and control. The force doesn’t explain everything that it supposedly controls – studies of our universe have shown that there is more gravity in our universe than can be produced by all of its visible matter. Up until now scientists have explained this gap with a poorly-understood entity called dark matter – though researchers are yet to prove it exists.
Based on a lack of evidence for dark matter, some physicists have controversially suggested that gravity should be removed from the list of fundamental forces. But others propose that a fifth force of nature might exist that fills the gaps in our understanding. One group of researchers has proposed the ultimate test for the theory of a fifth force and it involves tracking stars at the centre of our galaxy.
They have already begun taking measurements, and will follow the stars over the next two decades to reach a definitive conclusion on gravity and the ‘fifth force’.
“Einstein’s theory describes [gravity] beautifully well, but there’s lots of evidence showing the theory has holes. The mere existence of supermassive black holes tells us that our current theories of how the universe works are inadequate to explain what a black hole is. At the centre of our galaxy, where its supermassive black hole lies, the influence of gravity is so strong that signs of a fifth force will be easier to detect.” said team member Professor Andrea Ghez, director of the University of California, Los Angeles, Galactic Centre Group.
The team used some of the most detailed images of the centre of the Milky Way ever taken – captured by the Keck Observatory in Hawaii. They will track the orbits of stars close to the galaxy’s supermassive black hole to measure the direct influence of gravity on the movement of the stellar objects. If a fifth force is influencing the way the stars move, the team will notice as their direction will shift away from predictions made using the standard model of physics.
“This is really exciting. Our work on studying stars at the centre of our galaxy is opening up a new method of looking at how gravity works. By watching the stars move over 20 years using very precise measurements taken from Keck Observatory data, you can see and put constraints on how gravity works. If gravitation is driven by something other than Einstein’s theory of General Relativity, you’ll see small variations in the orbital paths of the stars.” said Professor Ghez.
This is the first time the fifth force theory has been tested in a strong gravitational field. Historically, measurements of our solar system’s gravity created by our sun have been used to try and detect the fifth force, but that has proven difficult because its gravitational field is relatively weak.
“It’s exciting that we can do this because we can ask a very fundamental question – how does gravity work?” said Professor Ghez.
Professor Ghez and her UCLA team are now looking forward to summer of 2018. That is when a star called S0-2 will be at its closest distance to our galaxy’s supermassive black hole, and will be pulled in at maximum gravitational strength. This will be a point where any deviations from Einstein’s theory are expected to be the greatest, giving the strongest test yet of whether the fifth force of nature exists.