# Eli5: How far can a burst of light from a laser go into space

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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

It will never fade. The reason light fades on earth is because we have an atmosphere, there’s tons of little particles like nitrogen, oxygen, water, dust, etc. that photons can run into as they leave a light source, which means there’s only so far they can go before they’re bound to run into something. In space there’s next to nothing for photons to run into, so they will fly on as long as it takes to hit something. This is why we are able to see stars that are ~100,000,000,000,000 miles away, there was nothing between that star and us, and the earth was the first thing that photon of light ran into.

Light can’t fade. The reason light appears dimmer at a distance (stars for example) is that fewer photons are reaching you because they’re spreading out spherically from the point of origin.

A photon emitted continues on forever unless it hit’s something and is absorbed. It would travel straight relative to the spacetime it’s in. Since spacetime curvature varies, it might appear to follow a curved path to you, but that’s actually just a straight path in curved space.

If a photon travels past a large mass, the distortion of spacetime by that mass would change the photon’s trajectory, but that’s still the straightest line possible in that curved spacetime.

As others have said, without air or other matter to absorb/scatter the photons, they will travel forever.

That being said, the intensity of the light will fade simply because the light will spread out as it travels. Laser beams have a property called divergence that describes how quickly the beam spreads out as it travels (you can picture the beam as a very narrow cone, and the divergence is the cone angle). If you point a laser pointer at something close and then something farther away, you’ll notice that the spot is larger on the surface that is farther away. So as the beam travels through space, it will get dimmer, not because the photons are lost, but simply because they’re spread out over a much larger area.

TL;DR – Depends on how you define a laser beam

The other commenter is mostly right in that the answer is “forever”. However, there is some additional physics involved.

We often think of lasers as just perfect straight lines of light that come out of a device and impact a surface a long distance away. This is a good enough approximation on human scales. In reality though, due to the wave-like nature of light, even a perfectly focused beam will spread out due to a process called diffraction. To give you an idea of how severe this effect is, by the time the light from a regular handheld laser pointer reaches the Moon, the laser spot (which starts at only a few millimeters across) is larger than the Moon. This effect can be mitigated by starting with a wider beam, but the only way to get rid of it is to have an infinitely large beam to start with.

So while the light from a laser pointer does indeed go on forever, on any astronomical distance scale, the light would no longer look like a beam and would instead look fairly similar to any other light source. The power of the beam would be spread over an increasingly large area, so any detector attempting to pick up the signal would see it dim further and further. At far enough distances, the energy from the beam would be spread so thin that any detector would be receiving individual photons at a time, and beyond that point, those signal photons would arrive with more and more time between them. Eventually, the beam would be indistinguishable from the noise.

Like many things in life, the answer to “how far can a laser beam go?” is as much a question of “what counts as a laser beam” as it is anything else.

(I’ve intentionally disregarded redshift for this explanation, since that would require a more thorough explanation of frequency and quickly get overcomplicated)

Tag on question – if we could “entangle” light particles so they did not diverge over distance, what pratical applications could there be for this type of advancement?

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.