Laser range in outer space

Admitedly, this question formed while watching bad science on DVD. In a typical Sci-Fi movies that uses fighters armed with laser cannons (Battlestar Galactica, Star Wars) we see opponents blaze away at each other. Many shots miss. I’ve always wondered, if such things were real, if a hundred years later a family could be touring in their happy space van only to be blasted by a laser bolt fired a hundred light years away.

So a more realistic question is this. Do lasers in a vacuum have a limited range? If I create a laser capable of burning a hole through a 5 inch plate of nickel and place that laser into space, is there a point at which the light will disperse enough that it will no longer be an effective laser? Or, barring physical obstructions, will I be able to scar an asteroid 500 light years away so long as I can calculate a clear shot and don’t mind dying off before seeing the result?

Two factors come into play in my opinion:
1: Dispersion due to not being emitted in a perfectly parallel manner. The laser would tend to spread out over distance, and become less “concentrated”.
2: Minute amount of dust and particles over distance woudl absorb some of the power, albeit very little.

There might be more, but that’s all I can think of off the top of my head.

All the lasers we have built are not perfect and the beam will disperse. As for the technology behind Star Wars lasers - I don’t know.

2 other factors that you will want to consider - gravity and matter. Gravity will distort a beam of light pulling harder on the closer photon then the one that is 2 inches further - this will tend to disperse the beam. Space also has trace gasses and other junk that can ‘absorb’ some of the beam.

So as time goes on the beam gets wider and loses strength.

There are a number of factors that might limit the effective range:

The beam will probably tend to spread as it travels, eventually it will be so spread out that it can do no real damage.

Even space isn’t completely empty; there is something like a few atoms (mostly hydrogen IIRC) per cubic metre of vacuum; it might seem unlikely that this would affect the beam, but consider the vast amount of space through which the beam travels in just a second; occasional encounters (absorption or deflection of the occasional photon) with those few atoms are all going to add up

There’s just the slight possibility that your beam might first encounter a larger object such as a comet, nebula, star, black hole etc.

I remember reading an article a while back regarding the some scientists firing a laser at the moon (for range-finding, I think). If I remember correclty, the diameter of the laser was around the size of a quarter when it left earth, and over 6 feet across when it hit the moon.

True, joemama, but they were firing from the Earth’s surface. The atmosphere significantly affects dispersion (as well as absorbing some of the output, of course). …

Along the same lines…How come they seem to miss so much? (Other than it making for a better movie) How do you miss with a laser at any kind of close range? Without doing the math, it seems to me that little or no lead would be required to hit with a laser beam, all you have to do is track the target and pull the trigger. I imagine that true space wars would be more like 19th century naval battles. You pull up next to each other and blast away until one or the other is dead or surrenders. ( I know, an over simplification, but still I don’t think fast agile fighters would be of any use against lasers)

Following up Rhum Runner and Sci-Fi films with laser weapons (esp. Star Wars 4).

When Syria foolishly tried to go against the Israeli Air Force over Lebanon in the 1980’s, most of the fighting consisted of:

Israeli Pilot: “Ooh, there’s a dot on the radar screen!. Computer, make that dot go away.” And it goes away.

It is infrequent (but not all that rare) for planes to even be within a mile of each other. You’d think that a civilization with laser cannons: a. doesn’t need humans to aim, b: can hit things very far out of visible range.

The close up shooting in Star Wars was based on WW I dogfights.

And, back to main OP, don’t worry about it on the scale beyond solar systems. Light-hours or less might be bad. (But that’s without reading the Tech. Specs.)

Scattering by interstellar gas/dust is negligible unless you are talking about hundreds of light-years. (Except for a few nebulae that can get in the way). The limiting factor is the resolution of the optics for the laser. It’s the same problem as a telescope - a 1-meter diameter telescope has a resolution of 0.1 arcseconds. Which means a 1-meter diameter laser cannon shoots a beam 0.1 arcseconds wide. If you aim this at the Moon from low earth orbit, it will illuminate a spot 1 mile across. Even if you use a 10-meter diameter laser cannon, the spot will be 0.1 mile across.

Arthur C. Clarke took this into account in the one and only space battle he depicted, in Earthlight. In order to make beam weapons effective, ships closed in to mere hundreds of meters of each other before firing. But I think that’s a severe under-estimate of technology. It may have been written before lasers became practical.

One of he dirty ittle secrets of laser beams is hat they aren’t perfectly collimated, an so, despite what everyone thinks, they spread out with distance. (There is no such thing in he Real World as a perfectly collimated beam. A Searchlight comes close, but read Miles V. Klein’s treatment of the spotlight in his book Optics.)

The best-collimated laser beams are in TEM00 mode, in what is called a gaussian beam. Such a beam is roughly colimted over a dstance called the Rayleigh Range. In fact, at the ends of this range the beam is actually about 1.414 times larger than it is at its narrowest point (the beam “waist”), and it expands approximately in a constant angle (the Far Field Angle) beyond this point. (In fact, the surface at which the inensity falls to a constant fraction of the on-axis ntensity is a hyperboloid). In this Far Field regime, therefre, the laser intensity drops as the sqare f the distance from th waist, just like your ordinary sorce. I just doesn’t spread equally in all directions. So your ypothetical family of the future is safe from accidental zapping by centuries-old laser beams.

And that’s without factoring in the scattering.

See Anthony Siegman’s book Lasers, or Amnon Yariv’s books on Quantum Electrnics, or any a number f laser handbooks.

Thanks for all the replies folks.