DEAR ,
THOUGHT EXPERT. A beam of 1 second long is sent from the sun to the earth with a powerful laser, so a beam of +/- 300,000 km long! Yes? What “sees” (or measures) one now if one were in space and one looks (with special equipment) at right angles to the axis sun-earth? Do one then see that beam of 300,000 km in length, towards earth, for 8 minutes + seconds? slide or not?????
Answer
So imagine an equilateral triangle, SAW, with the sun Z , the earth A and the alien observer W at the vertices.
Let’s assume that there is a perfect vacuum in space. The supposed laser in Z emits a beam towards Earth A for one second. Such a laser wave train has a lateral scatter that is very small, reaching A after eight minutes but not beaming sideways; W therefore sees nothing because it is too far away from the path of the wave train.
But interplanetary space is not a perfect vacuum, it contains free electrons, atoms and dust particles (a few per cm³). These will scatter the laser light, as you can see dust particles in a sunbeam on Earth. Supposing W had the very sensitive equipment to detect this stray light, she would indeed see a glowing streak apparently traveling at the speed of light in a vacuum; she perceives this from W as a 3/4 degree dash. Assuming, of course, that the laser beam is more powerful than the sunlight, so that you can still distinguish the phenomenon against the background of the scattered sunlight.
Answered by
prof. French Cerulus
physics, especially classical theoretical mechanics, electromagnetism, quantum mechanics, history of physics .
Old Market 13 3000 Leuven
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