Eli5 If you shine a laser up in the sky for ~3 seconds, does the beam eventually fizzle out or does it keep going until it hits an object?
In: 5
Neither, but closer to the second one.
Laser light is not perfectly concentrated, and it turns out that the laws of physics [do not allow it to be](https://en.wikipedia.org/wiki/Diffraction-limited_system). It’s concentrated enough that it stays the same width over typical human ranges, but even the distance to the Moon is enough to spread it out a lot. For a typical hand laser, it would be spread out over thousands of square miles by the time it got to the Moon. (See [this XKCD](https://what-if.xkcd.com/13/) discussing essentially this exact question!)
But aside from spreading out, and occasionally being absorbed or scattered by the very few particles floating around in space (and there are a fair number), depending on the direction you shone your light, it is very possible that it a big chunk of it would travel for billions of years before encountering anything.
Neither, but closer to the second one.
Laser light is not perfectly concentrated, and it turns out that the laws of physics [do not allow it to be](https://en.wikipedia.org/wiki/Diffraction-limited_system). It’s concentrated enough that it stays the same width over typical human ranges, but even the distance to the Moon is enough to spread it out a lot. For a typical hand laser, it would be spread out over thousands of square miles by the time it got to the Moon. (See [this XKCD](https://what-if.xkcd.com/13/) discussing essentially this exact question!)
But aside from spreading out, and occasionally being absorbed or scattered by the very few particles floating around in space (and there are a fair number), depending on the direction you shone your light, it is very possible that it a big chunk of it would travel for billions of years before encountering anything.
Both, actually. It fizzles out precisely because it’s hitting stuff as the laser propagates through the atmosphere. If the beam was perfectly collimated, then yes, there’s nothing internal to the laser that would cause the beam to spread out and diffuse. But in reality, the laser gets bent by air at different temperatures, absorbed by water vapor, particles in the atmosphere, etc…
Both, actually. It fizzles out precisely because it’s hitting stuff as the laser propagates through the atmosphere. If the beam was perfectly collimated, then yes, there’s nothing internal to the laser that would cause the beam to spread out and diffuse. But in reality, the laser gets bent by air at different temperatures, absorbed by water vapor, particles in the atmosphere, etc…
Both, actually. It fizzles out precisely because it’s hitting stuff as the laser propagates through the atmosphere. If the beam was perfectly collimated, then yes, there’s nothing internal to the laser that would cause the beam to spread out and diffuse. But in reality, the laser gets bent by air at different temperatures, absorbed by water vapor, particles in the atmosphere, etc…
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