Multiple images from graviational lensing
We've already shown that paths of light are bent by gravity. Another
way of stating this is spacetime curvature acts like a lens,
hence the term gravitational lensing.
Under certain circumstances, gravitational lensing can fool us
into thinking we are seeing several spacetime events, when in
fact we're seing several images of a single spacetime event.
The figure below shows how a single flash of light behind a
very massive object M in two space dimensions can
be perceived as two simultaneous flashes of light coming from
opposite directions by a viewer in front of M.
Gravitational lensing means that the light cone of an event
can be distorted by matter and energy. The possibility of distorting
light cones is crucial to the possibility for nontrivial time travel.
In the case of multiple images, the light cone of the event in
question has developed a crease in it, it has overlapped with
itself. We will see more of this later, in the next section.
Multiple images are observed in our Universe today. The next
frame has a picture of a very complex multiple image system
photographed by the Hubble Space Telescope in 1995.
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