Could a laser beam stay narrow over light-year distances?

[RunDMC]

Hi to all, another debunking request, if I may.

I've heard it said that the shorter the wave used to create a maser/laser the more it spreads out (from high school: 'blue is bent, and brief'). Is that true?

What I'm trying to work out is: could you make a really, really straight laser that would get no bigger in diameter than, say 15 metres over a distance of 10-25 light years? I don't have the maths/physics to work out if a laser/maser would be the size of a house or a planet by the time it traveled such long distances.

[I'm trying to settle a bet about whether those scorched patterns which appear on the side of our planet from time to time could be 'etched' on using a laser/maser from a VERY long way away. Extra brownie points if you can name the scorched patterns I'm thinking of.]

Thanks!

 

[Ivan Seeking]

Hi RunDMC. This and your previous question are appropriate for the regular engineering or physics forums. The Skepticism and Debunking is for fringe subjects - evidence and observations.

 

[RunDMC]

Patterns etched on the side of the planet? Hydro electric systems with no dam? Seems fairly fringe to me -- feel free to send me back to skepticism as this thread progresses!

Although I suppose I am asking for real physics knowledge and advice on maser/lasers and (in the other question) angular momentum. Thanks!

 

[pervect]

Hi to all, another debunking request, if I may.
I've heard it said that the shorter the wave used to create a maser/laser the more it spreads out (from high school: 'blue is bent, and brief'). Is that true?
What I'm trying to work out is: could you make a really, really straight laser that would get no bigger in diameter than, say 15 metres over a distance of 10-25 light years? I don't have the maths/physics to work out if a laser/maser would be the size of a house or a planet by the time it traveled such long distances.
[I'm trying to settle a bet about whether those scorched patterns which appear on the side of our planet from time to time could be 'etched' on using a laser/maser from a VERY long way away. Extra brownie points if you can name the scorched patterns I'm thinking of.]
Thanks!

You have it backwards - shorter wavelengths will diffract less.

For detailed calculations, you might want to look at

http://www.coseti.org/radobs14.htm

For a fully-illuminated 10 meter diameter telescope system, operating at a
wavelength of 656 nm at a range of 10 light years (632,420 A.U.), for which
THETA3 = 0.0138":

D = 0.0381 A.U.

To get to 15 meters, you'd have to increase the telescope size proportionately, or decresae the wavelength.

.0381 Astronomical Units = 5 699 683 800 meters

So you need a *lot* of improvement to get 15 meters at 10 light years

This is using the "FWHM" (full width half maximum) beamwidth, other definitions of beamwidth may give slightly different figures.

 

[RunDMC]

Thanks for the link

Thanks for the link. The paper seems to suggest you could focus a narrow beam for (theoretically) 100s of L.Y. and maintain or 'focus' a narrow beam.

Darn, my bet had been that a beam would spread far too wide to etch a pattern with features less than 15m in detail from 10-25 LY (based on some pop science I recalled of how big a laser pointer beam would be on the moon).

 

[pervect]

Thanks for the link. The paper seems to suggest you could focus a narrow beam for (theoretically) 100s of L.Y. and maintain or 'focus' a narrow beam.

Darn, my bet had been that a beam would spread far too wide to etch a pattern with features less than 15m in detail from 10-25 LY (based on some pop science I recalled of how big a laser pointer beam would be on the moon).

And so it will be, unless you have a VERY large lens. Even Robert Forward's proposal of using a Jupiter sized (200,000 km) fresnel focusing lens will give you a beam width that's much larger than 15 m at 10 lightyears at optical frequencies.

 

[Mk]

Extra brownie points if you can name the scorched patterns I'm thinking of.]
Thanks!

The Nazca lines in Peru!

 

[Reshma]

Or the Crop circles..lol!

 

[Ivan Seeking]

As for that part, that's not fringe, that's nuts!

This was a essentially a physics question but I did cringe bit when I moved it.