Albert Einstein’s general theory of relativity is one of the greatest achievements of 20th-century physics. Published in 1916, it says that what we perceive as the force of gravity in fact arises from the curvature of space and time. Einstein proposed that objects such as the sun and the Earth change this geometry. Most of the major points of his theory has been proven since, yet there are some – arguably the most exciting ones – which still need to be proved or disproved. Read more about them below the image.
Gravitational lens: a distribution of matter (such as a cluster of galaxies) between a distant light source and an observer that is capable of bending the light from the source as the light travels towards the observer. Unlike an optical lens, a gravitational lens produces a maximum deflection of light that passes closest to its center, and a minimum deflection of light that travels furthest from its center. Consequently, a gravitational lens has no single focal point, but a focal line.
Gravitational waves are disturbances in the curvature of spacetime, generated by accelerated masses, that propagate as waves outward from their source at the speed of light. Gravitational waves transport energy as gravitational radiation, a form of radiant energy similar to electromagnetic radiation.
Time dilation is a difference in the elapsed time measured by two clocks, either due to them having a velocity relative to each other, or by there being a gravitational potential difference between their locations. After compensating for varying signal delays due to the changing distance between an observer and a moving clock (i.e. Doppler effect), the observer will measure the moving clock as ticking slower than a clock that is at rest in the observer’s own reference frame. A clock that is close to a massive body (and which therefore is at lower gravitational potential) will record less elapsed time than a clock situated further from the said massive body (and which is at a higher gravitational potential).
Black holes are regions of spacetime exhibiting gravitational acceleration so strong that nothing – no particles or even electromagnetic radiation such as light – can escape from them. The theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole. The boundary of the region from which no escape is possible is called the event horizon. Although the event horizon has an enormous effect on the fate and circumstances of an object crossing it, no locally detectable features appear to be observed. In many ways, a black hole acts like an ideal black body, as it reflects no light.
White holes: Physicists describe a white hole as a black hole’s ‘time reversal’, a footage of a black hole played backwards, much as a bouncing ball is the time reversal of a falling ball. While a black hole’s event horizon is a sphere of no return, a white hole’s event horizon is a boundary of no admission – space-time’s most exclusive club where no spacecraft can ever reach the region’s edge. Objects inside a white hole can leave and interact with the outside world, but since nothing can get in, the interior is cut off cut off from the universe’s past: No outside event will ever affect the inside. Although information and evidence regarding white holes remains inconclusive, the 2006 GRB 060614 has been proposed as the first documented observance of a white hole.
A wormhole is a speculative structure linking disparate points in spacetime which is based on a special solution of the Einstein field equations. A wormhole can be visualized as a tunnel with two ends at separate points in spacetime (i.e. different locations, or different points in time, or both). Wormholes are consistent with the general theory of relativity, but whether they actually exist remains to be seen. A wormhole could connect extremely long distances such as a billion light years or more, short distances such as a few meters, different universes, or different points in time.