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?


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

Technically, as it moves through the sky, it is hitting objects. Our atmosphere is filled with molecules like oxygen and nitrogen, as well as gaseous water (even on a clear day) which reflect, absorb and refract the beam (bounce it around to put it in a really simple but not entirely accurate way). So it does “fizzle out” as it travels. In space, where this atmosphere doesn’t exist, it could travel much further. But even the most focused beams will expand and appear more dim the farther away you are, similar to a far away star that, while emitting a lot of powerful light, appears very dim.

It will be absorbed slightly be the air, but then travel through space and spread out more or less forever. Maybe some distant alien astronomer will be wondering why that star blipped slightly green, though probably not.

Light continues until it is absorbed. Some of it will be absorbed hitting molecules in the air. Some will get absorbed hitting molecules in space (it’s not a complete vacuum). If you’re pointing it at the moon then a good portion or all of the photons will be absorbed by it. If you’re aiming at clear section of sky then a good portion will continue traveling through space essentially for ever.

Even though lasers tend to have small divergence (how much they spread out), they do spread out over time. A typical laser pointer might be 5 mW with a 2mm aperture (width of laser beam departing the pointer), and a divergence of 1 mrad (it deviates from a cylindrical beam by 1/1000 of a radian). That 2mm beam will become a 385m wide beam by the time it reaches the Moon (discounting dispersion due to the atmosphere).

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…