If we shoot a burst of light from our most powerful laser into space…how far could it travel before fading, it it doesn’t hit anything? And would it travel straight?
In: Physics
Assuming “fade” means “get less bright and eventually vanish” It doesn’t fade. Light of any sort doesn’t fade.
It SPREADS. Which looks superficially like fading but is different in all the important ways.
As Light spreads out it gives the appearance of being dimmer because fewer photons are reaching you eye. But each photon has exactly as much energy when it hits your eye as it did when it was first emitted and unless it runs into something it will keep going forever with that exact same amount of energy.
Shine a regular flashlight at Andromeda and some of those photons will probably get there in 2.53 million years.
But no one would notice because by then they’d have spread out so much they’d get mixed in with all the other photons headed that direction from the Milky Way. And they’d be spread out across an hundreds of thousands of light years in diameter.
Lasers are the same but the beam is tighter so the photons don’t spread as quickly. But they do spread, so over long enough distances you’d get the same problem.
If we’re talking the maximum range at which you could actually detect a powerful laser pointed in your direction it depends on the power and size of the laser and how good you are at making lasers with minimal beam spread.
With a decent sized Dyson swarm you could probably make a laser strong and tight enough to melt planets at fairly decent range, maybe as far as 100 light years.
The practical answer is: how far do you want the laser to be detectable? Then you can build a laser to get that far if you have enough energy and good enough engineering.
EDIT for omnidirectional radio at the strength we use for broadcasting the answer is “Maybe 3 light years if you have a REALLY big receiver and some good signal processing software”. So aliens even as close as Alpha Centauri won’t be watching I Love Lucy broadcasts.
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