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Would it be physically possible to have a beacon on the Earth's surface so powerful its beam would continue outwards past the Earth, further into space, and out into the cosmos? What would that beam look like to an observer on, say, Mars?

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  • $\begingroup$ Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. $\endgroup$
    – Community Bot
    Commented Apr 11 at 18:24
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    $\begingroup$ Our cities are perfectly visible at night from space. Here is a YouTube video filmed by an astronaut on the International Space Station over Europe. Note the countless beams of light visible. An observer on Mars with a modest amateur-level telescope could easily see, say, Paris. This is all well-known and satellite photos of Earth at night are not at all hard to find. Maybe I don't understand the question... $\endgroup$
    – AlexP
    Commented Apr 11 at 18:26
  • $\begingroup$ @AlexP I was thinking more along the lines of like could you ever have a pronounced beam of light going through space in a straight line $\endgroup$
    – iolim5678
    Commented Apr 11 at 18:56
  • $\begingroup$ "A pronounced beam of light": Vacuum doesn't glow. When you see a beam of light what you see is the light that doesn't go in a straight line, because the light that does go in a straight line does not come into your eyes... $\endgroup$
    – AlexP
    Commented Apr 11 at 19:20
  • $\begingroup$ A perhaps clearer question would have been: Given a spotlight placed on the moon, How would the beam appear to an observer not in the direct path of the spotlight.'s beam? (this change could invalidate existing answers so don't change this question) $\endgroup$ Commented Apr 11 at 19:37

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Unfortunately, Nothing

The reason we can see beams of light like spotlights on earth is because the light we are actually seeing is that being scattered by various particles in our atmosphere (diatomic molecules like nitrogen and oxygen, as well as larger particles like dust and debris), and those scattered photons hitting your eye. Given the lack of these kinds of particles outside our atmosphere, you would only be able to see the light if it were shining directly into your eyes.

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  • $\begingroup$ So you would need some kind of physical debris/small particles in order to get a visible streak through the sky... $\endgroup$
    – iolim5678
    Commented Apr 11 at 18:59
  • $\begingroup$ @iolim5678 The other side of the coin seems to be that the brighter the light, the less particles you have, the brighter and more powerful the light needs to be. $\endgroup$
    – Amocito
    Commented Apr 12 at 11:10
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You could see them.. if there were a lot of spaceships, spreading thruster molecules masses out there in the void. Basically, the equivalent of plane-trails in space- meaning, its very faint and highly dependent on traffic and the vector in which the particles drift away (opposite of spaceship, so most go down to earth).

You can not see that with your naked eye. You can see it though, with a computer aided light-sensitive camera, producing a open shutter picture composed of many pictures..

PS: There are gas planets close to suns, which produce constant spiraling trails of volatiles.. you can see a light column in those ice-spirals. https://en.wikipedia.org/wiki/Hot_Jupiter

Also light falls off with the https://en.wikipedia.org/wiki/Inverse-square_law so - good luck, having a light source bright enough to be visible on those distances.

TL,DR: Go shine a industrial laser at a https://en.wikipedia.org/wiki/Twilight_phenomenon

Will not be like: https://en.wikipedia.org/wiki/Light_pillar

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