From Peter Holness, Hertford, UK
I have read your fascinating article on the phenomenon of superluminal shadows in astronomical contexts. It is truly captivating to explore these cosmic curiosities. However, I would like to point out that apparent superluminal motion isn’t just confined to the depths of space(27 September, p 42).
Engineering equipment capable of demonstrating similar effects on Earth has been around for decades. For instance, with an old cathode ray oscilloscope, once considered cutting-edge, you would often see the illuminated spot on the screen travel at speeds that appeared to exceed the velocity of light, due to its extremely fast horizontal sweeping.
The key term here is “apparent”. Nothing physical is travelling faster than light, of course. The spot on the oscilloscope simply moves so quickly across the screen that it creates the illusion of superluminal motion, much like a shadow may appear to travel faster than light during certain astronomical events.
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