# Relativistic Aberration

The shape, position or orientation of an object seen by a person moving near the speed of light can be quite unusual, and even different to what is predicted by the non-optical aspects of Einstein's theory of special relativity, such as length contraction. This is due to the purely visual effects of angular compression and distortion.

## Angular compression

As we travel faster, light appears to come at us from different angles. To understand why this occurs, imagine driving in a car on a rainy day. Although to a person standing on the road the rain falls straight down to the ground, from inside the car it looks as though the rain is falling towards us at an angle. In a similar way, as we travel close to the speed of light, the angles of light coming towards us from all sides appear to contract, such that more light approaches us from in front:

Among other things, this has the effect of allowing a moving observer to see behind them. Watch what happens to an arrow originally behind the green dot as it accelerates. Aberration allows objects which normally make an obtuse angle (greater than 90 degrees) with the direction of motion to be wrapped forward into the field of view.

## Distortion

Distortion is a visual effect that arises because the speed of light is not infinite, so light takes some small amount of time to travel a certain distance. Therefore, light from distant objects must leave earlier in time than light from closer objects in order for that light to enter an observer's eye at the same time. In everyday life, the fact that we see distant objects earlier in time than nearer objects has no noticeable effect, since the difference in time is so small. However, we experience this effect when we look at the night sky: since some stars are thousands of light-years away, we are seeing them as they were thousands of years ago.

When things move at relativistic speeds, however, this effect is noticeable on a more local scale. This is because in the short time between seeing one part of an object and another, its enormous speed allows it to move a significant distance. This has the effect of making objects appear distorted; possible bent, curved or twisted. For example, long rods may appear as parabolic arcs bending away from the viewer, and cubes can present curved edges or non-parallel surfaces. Surprisingly, distortion cancels the effect of length contraction in the case of a sphere, which will always look circular to a moving observer.

Look at this animation. It shows a pulse of light, which we imagine we can see, travelling to a camera from the far corner of a box moving at close to light speed.

The light pulse has come from the back corner of the box, which we would expect to be obscured by the box itself. But the box is moving fast enough that it gets out of the way of the light pulse. This is possible because the light is moving directly towards the camera, while most of the box's movement is perpendicular to the light's path.