# Debunk me, please! #2 (lasers/masers)

1. Oct 27, 2005

### 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 travelled 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!

2. Oct 27, 2005

### Ivan Seeking

Staff Emeritus
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.

3. Oct 27, 2005

### 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!

4. Oct 27, 2005

### pervect

Staff Emeritus
You have it backwards - shorter wavelengths will diffract less.

For detailed calculations, you might want to look at

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.

5. Oct 27, 2005

### RunDMC

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).

6. Oct 28, 2005

### pervect

Staff Emeritus
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.

7. Oct 28, 2005

### Mk

The Nazca lines in Peru!!

Last edited: Oct 28, 2005
8. Oct 28, 2005

### Reshma

Or the Crop circles..lol!

9. Oct 29, 2005

### Ivan Seeking

Staff Emeritus
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.