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From David Marjot

I read the letter questioning whether information can travel faster than light (22 November, p 35). Many years ago, when viewing the central globe of rotating mirrors in a dance hall throwing spots of light on the walls, I thought that such a spot of light on a circular target would appear to travel at a speed that depends on the speed of rotation of the beam at the centre of that target and the distance away of the target. It occurred to me that if the source of the beam of light rotated once per second at a circular target at 47,000 kilometres, then the spot of light would appear to travel across that target at the speed of light. If the target was at a greater distance or the light source rotated more quickly then the apparent speed would be greater than the speed of light. The more rapid the rotation, the shorter the distance to the target would need to be to achieve the same effect. The apparent speed of a beam of radiation from a pulsar must be enormous.

The target or targets need not form a complete circle but merely be placed on the circumference of the circle. If the information carried by the beam could be modified as it crossed one target then information to a subsequent target could flow faster than the speed of light. Perhaps this is how advanced civilisations communicate.

Valerie Jamieson writes:

• David Marjot is right that a beam of light from a rotating source can sweep a spot across a distant wall faster than the speed of light. However you couldn’t use that spot of light to send a message faster than light because the sweeping spot doesn’t carry any information. As each point on the wall receives light from the source, the information actually travels from the source to the wall and not along the path of the moving spot.

Weybridge, Surrey, UK