# Is there a way to capture a laser beam... **URL:** \ **Category:** Factual Questions\ **Created:** [February 22, 2008, 2:25am UTC](https://boards.straightdope.com/t/is-there-a-way-to-capture-a-laser-beam/438662 "2008-02-22T02:25:15Z")\ **Posts on this page:** 17\ **Page:** 1
**Author:** ![solkoe](https://avatars.discourse-cdn.com/v4/letter/s/919ad9/32.png) [@solkoe](https://boards.straightdope.com/u/solkoe)\ **Post date:** [February 22, 2008, 2:25am UTC](https://boards.straightdope.com/t/is-there-a-way-to-capture-a-laser-beam/438662/1 "2008-02-22T02:25:15Z")
using a set of perfectly aligned mirrors so that the beam itself is visible as it bounces endlessly between them? Another laser question… How much would a laser beam diverge on its journey to the moon? ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [February 22, 2008, 2:39am UTC](https://boards.straightdope.com/t/is-there-a-way-to-capture-a-laser-beam/438662/2 "2008-02-22T02:39:54Z")
[QUOTE=solkoe] … as it bounces endlessly between them? [/QUOTE] “Endlessly” implies perfect reflectivity (no absorption or scattering) - that’s going to be challenging. > [@](#): > > How much would a laser beam diverge on its journey to the moon? This [NASA website](http://sunearth.gsfc.nasa.gov/eclipse/SEhelp/ApolloLaser.html) says 4 miles: > [@](#): > > “Lunar ranging involves sending a laser beam through an optical telescope,” Dickey said. “The beam enters the telescope where the eye piece would be, and the transmitted beam is expanded to become the diameter of the main mirror, then bounced off the surface toward the reflector on the Moon.” > > The reflectors are too small to be seen from Earth, so even when the beam is precisely aligned in the telescope, actually hitting a lunar retroreflector array is technically challenging. At the Moon’s surface the beam is roughly four miles wide. Scientists liken the task of aiming the beam to using a rifle to hit a moving dime two miles away. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [February 22, 2008, 2:40am UTC](https://boards.straightdope.com/t/is-there-a-way-to-capture-a-laser-beam/438662/3 "2008-02-22T02:40:36Z")
The answer to the first question is no. Mainly, because no mirror is 100% reflective; the very best we have approach 98-99% reflectivity at a specific wavelength. After about 100 reflections, only approximately 1% of the light remains and this happens within a small fraction of a second. The answer to the second depends on the beam divergence. If your laser has a divergence of one mrad (milliradian), the beam will have spread to about 380,000 meters–about 1/10 the diameter of the Moon, if I’ve done my math correctly. ---
**Author:** ![solkoe](https://avatars.discourse-cdn.com/v4/letter/s/919ad9/32.png) [@solkoe](https://boards.straightdope.com/u/solkoe)\ **Post date:** [February 22, 2008, 2:41am UTC](https://boards.straightdope.com/t/is-there-a-way-to-capture-a-laser-beam/438662/4 "2008-02-22T02:41:27Z")
[QUOTE=Xema] “Endlessly” implies perfect reflectivity (no absorption or scattering) - that’s going to be challenging. This [NASA website](http://sunearth.gsfc.nasa.gov/eclipse/SEhelp/ApolloLaser.html) says 4 miles: [/QUOTE] But haven’t they used this technique to find an accurate distance to the moon? That must be one powerful laser! ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [February 22, 2008, 2:42am UTC](https://boards.straightdope.com/t/is-there-a-way-to-capture-a-laser-beam/438662/5 "2008-02-22T02:42:28Z")
[QUOTE=solkoe] But haven’t they used this technique to find an accurate distance to the moon? [/QUOTE] Yes - that’s what the link discusses. ---
**Author:** ![WarmNPrickly](https://avatars.discourse-cdn.com/v4/letter/w/d78d45/32.png) [@WarmNPrickly](https://boards.straightdope.com/u/WarmNPrickly)\ **Post date:** [February 22, 2008, 2:54am UTC](https://boards.straightdope.com/t/is-there-a-way-to-capture-a-laser-beam/438662/6 "2008-02-22T02:54:38Z")
My cat has been trying this for years unsuccesfully. ---
**Author:** ![Jragon](https://avatars.discourse-cdn.com/v4/letter/j/e19b73/32.png) [@Jragon](https://boards.straightdope.com/u/Jragon)\ **Post date:** [February 22, 2008, 3:07am UTC](https://boards.straightdope.com/t/is-there-a-way-to-capture-a-laser-beam/438662/7 "2008-02-22T03:07:02Z")
[QUOTE=Christopher] My cat has been trying this for years unsuccesfully. [/QUOTE] _Gasp_ You, sir, owe Kevin T. Amiss and H. Abbot a hefty sum of money for using their [patented](http://www.google.com/patents?id=OfwkAAAAEBAJ) method of exercising a cat! You should feel ashamed for this! 😛 ---
**Author:** ![WarmNPrickly](https://avatars.discourse-cdn.com/v4/letter/w/d78d45/32.png) [@WarmNPrickly](https://boards.straightdope.com/u/WarmNPrickly)\ **Post date:** [February 22, 2008, 3:21am UTC](https://boards.straightdope.com/t/is-there-a-way-to-capture-a-laser-beam/438662/8 "2008-02-22T03:21:17Z")
Exercise? It takes her weeks to get all those mirrors calibrated! 😉 ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [February 22, 2008, 3:28am UTC](https://boards.straightdope.com/t/is-there-a-way-to-capture-a-laser-beam/438662/9 "2008-02-22T03:28:02Z")
On the first question, even if you somehow managed to get perfectly reflective mirrors, the beam itself still wouldn’t be visible. For something to be visible, light must be entering your eye, and any light which managed to reach your eye would not be part of the bouncing beam any more. ---
**Author:** ![Joey\_P](https://avatars.discourse-cdn.com/v4/letter/j/919ad9/32.png) [@Joey\_P](https://boards.straightdope.com/u/Joey_P)\ **Post date:** [February 22, 2008, 4:05am UTC](https://boards.straightdope.com/t/is-there-a-way-to-capture-a-laser-beam/438662/10 "2008-02-22T04:05:32Z")
[QUOTE=Chronos] On the first question, even if you somehow managed to get perfectly reflective mirrors, the beam itself still wouldn’t be visible. For something to be visible, light must be entering your eye, and any light which managed to reach your eye would not be part of the bouncing beam any more. [/QUOTE] Also, whenever I’ve thought about this, I’ve always wondered how you would get the beam in to begin with. ---
**Author:** ![WarmNPrickly](https://avatars.discourse-cdn.com/v4/letter/w/d78d45/32.png) [@WarmNPrickly](https://boards.straightdope.com/u/WarmNPrickly)\ **Post date:** [February 22, 2008, 4:39am UTC](https://boards.straightdope.com/t/is-there-a-way-to-capture-a-laser-beam/438662/11 "2008-02-22T04:39:46Z")
What if you pointed the laser through a Bose-Einstien condensate and onto a mirror. Then you quickly close the other face with an opposing mirror. As I understand it, and I don’t, light slows down considerably in a Bose-Einstien condensate. That would give you enough time to close off the other end before the light escaped. ---
**Author:** ![Jragon](https://avatars.discourse-cdn.com/v4/letter/j/e19b73/32.png) [@Jragon](https://boards.straightdope.com/u/Jragon)\ **Post date:** [February 22, 2008, 5:25am UTC](https://boards.straightdope.com/t/is-there-a-way-to-capture-a-laser-beam/438662/12 "2008-02-22T05:25:32Z")
Well, getting off my figurative arse and finding something I read before: [http://science.nasa.gov/headlines/y2002/27mar\_stoplight.htm](http://science.nasa.gov/headlines/y2002/27mar_stoplight.htm) They have “captured” lasers, just not in the way the OP described (the mirrors can’t have 100% reflectivity). ---
**Author:** ![Randy\_Seltzer](https://avatars.discourse-cdn.com/v4/letter/r/ee7513/32.png) [@Randy\_Seltzer](https://boards.straightdope.com/u/Randy_Seltzer)\ **Post date:** [February 22, 2008, 6:04am UTC](https://boards.straightdope.com/t/is-there-a-way-to-capture-a-laser-beam/438662/13 "2008-02-22T06:04:31Z")
Sorry the thread title isn’t very helpful, but the answer can be found herein: > **[Mathematical proof request: a laser and mirrors in a box](https://boards.straightdope.com/sdmb/showthread.php?t=442612)** > > I’ve had this question for years, and was just made to think about it again in reading the wiki on Hamiltonian paths. Okay. You’ve got a box, and a laser, and a bunch of small mirrors. The mirrors reflect on both sides - they aren’t “one-way”... In short: no. It’s impossible to capture a laser. It is possible, however, to create (in a small space) a path that will take so long to complete that it might as well be captured. ---
**Author:** ![Bryan\_Ekers](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bryan_ekers/32/183_2.png) [@Bryan\_Ekers](https://boards.straightdope.com/u/Bryan_Ekers)\ **Post date:** [February 22, 2008, 7:10am UTC](https://boards.straightdope.com/t/is-there-a-way-to-capture-a-laser-beam/438662/14 "2008-02-22T07:10:13Z")
Sure. Put it in your pocket, save it for a rainy day. ---
**Author:** ![Harmonious\_Discord](https://avatars.discourse-cdn.com/v4/letter/h/74df32/32.png) [@Harmonious\_Discord](https://boards.straightdope.com/u/Harmonious_Discord)\ **Post date:** [February 22, 2008, 12:08pm UTC](https://boards.straightdope.com/t/is-there-a-way-to-capture-a-laser-beam/438662/15 "2008-02-22T12:08:30Z")
You wouldn’t see the beam if it was able to bounce back and forth without some loss. ---
**Author:** ![Mycroft\_H](https://avatars.discourse-cdn.com/v4/letter/m/898d66/32.png) [@Mycroft\_H](https://boards.straightdope.com/u/Mycroft_H)\ **Post date:** [February 22, 2008, 4:48pm UTC](https://boards.straightdope.com/t/is-there-a-way-to-capture-a-laser-beam/438662/16 "2008-02-22T16:48:28Z")
[QUOTE=Bryan Ekers] Put it in your pocket, save it for a rainy day. [/QUOTE] “Is that a laser beam in your pocket, or…?” ---
**Author:** ![Cluricaun](https://avatars.discourse-cdn.com/v4/letter/c/bb73d2/32.png) [@Cluricaun](https://boards.straightdope.com/u/Cluricaun)\ **Post date:** [February 22, 2008, 4:54pm UTC](https://boards.straightdope.com/t/is-there-a-way-to-capture-a-laser-beam/438662/17 "2008-02-22T16:54:40Z")
[QUOTE=Bryan Ekers] Sure. Put it in your pocket, save it for a rainy day. [/QUOTE] Or until Darth Vader shows up.