More: Seven things that don鈥檛 make sense about gravity
You jump up, and gravity brings you back down to Earth. You reach the brow of a hill and gravity accelerates you down the other side. All neat and tidy then: gravity behaves in the way Newton thought of it, as a force that affects and changes the motion of something else.
That, at least, was how it seemed until Einstein came along. His general theory of relativity tells us that gravity is not quite that simple.
Advertisement
General relativity provides a framework under which the laws of physics look the same for everyone at every moment, regardless of how they are moving. Einstein achieved this by making gravity a property of the universe, rather than of individual bodies.
General relativity describes gravity as geometry. The fabric of the universe 鈥� the four dimensions of space and time 鈥� is full of lumps and bumps created by the presence of mass and energy. This warping is unavoidable; whenever anything 鈥� be it you, me, a piece of space dust or a photon of light 鈥� tries to travel through the universe in a straight line, it actually follows a trajectory that is curved by any mass and energy in the vicinity. The result of this curvature is what we think of as gravity. To look at it a slightly different way, gravity is not what one body does directly to another, but what a body鈥檚 mass does to the surrounding universe.
Nevertheless, treating gravity as if it were a straightforward force, as Newton did, has served us well. It allowed us to send rockets to the moon and chart the orbits of the planets with astonishing accuracy. 鈥淭he Newtonian description works to high precision,鈥� says Bernard Carr of Queen Mary, University of London.
Einstein鈥檚 description has stood up equally well to scrutiny in situations when high speeds and accelerations are involved. Yet useful as they are, neither the Newtonian model nor relativity is a fundamental explanation for gravity. We still don鈥檛 know how the fundamental, quantum properties of mass, energy and space-time combine to create the phenomenon.
We do have an idea, though. At the most basic level, the other three fundamental forces of nature 鈥� the electromagnetic force, the weak nuclear force and the strong nuclear force 鈥� work through the action of particles. The electromagnetic force, for example, is delivered by photons. This picture is known as quantum field theory. In the same way, there ought to be particles that deliver the gravitational force.
There are still two problems with this, though. First, we have yet to find any proof of the existence of these hypothetical particles, which have been dubbed 鈥済ravitons鈥�. Secondly, when quantum field theory is applied to gravity, it is prone to give nonsensical answers to straightforward questions. 鈥淭hese are fundamental obstructions that need to be overcome,鈥� says Bruce Bassett of the University of Cape Town, South Africa.