Two snapshots taken in high speed flight

The high speed flight that we show here is a trip through a gate, modelled after the Brandenburg gate in Berlin and brightly colored for purpose of illustration (Figure 1 (top), Figure 2): The front is yellow, the archways have dark red roofs and red and orange walls, the back side is blue. The ground is covered with quadratic grey tiles.


v=0	Figure 2: The gate seen obliquely from behind, picture taken with a camera at rest. The back side is the only part of the gate that is blue.

v=0.9 c	v=0.99 c

Figure 1: Two photographs of the gate. Top: The camera is at rest. Bottom: The camera approaches the gate at 90% of the speed of light c. Both photographs are taken at the same distance to the gate (see sketch). The camera is looking towards the gate.	Figure 3: Snapshot taken during the sprint through the gate. The camera is looking in the direction of motion. At the instant of making the exposure, the camera is already beyond the gate (see sketch).

Figure 1 illustrates the sight of the gate during the appoach. For comparison, two pictures are shown, one taken by a camera at rest (top), the other by a camera moving at 90% of the speed of light towards the gate (middle). Both pictures are taken at the same distance from the gate (bottom) and with identical cameras. The Doppler effect has not been taken into account; the colours are used only for clarity. On the picture taken by the moving camera, the gate appears to be much smaller and both gate and tiles are distorted. At first sight this difference is surprising: The moving camera receives the "same light" as the camera at rest, being at the same place. The resulting picture, though, is not the same. We will come back to this issue in the next section.

Figure 3 shows the sight of the gate towards the end of the short trip from one side to the other. The camera is moving at 99% of the speed of light and is looking in the direction of motion. The distorted tiles behind the gate are clearly visible, the gate itself is only partly in the field of view of the camera und appears strongly distorted. The colours indicate that what we see of the gate are the insides of the archways and the back side. But how is it possible for light from the back side to enter the camera? A first clue is provided by the sketch in Figure 3. It shows the position of the camera at the instant that the picture is taken: The camera is already beyond the gate. It is clear that light from the back side reaches this place. That fact that it enters the camera (that is oriented in the direction of motion) means that somehow a moving camera "can look backwards".