5 ms·
I love these napkin calculations. The relativistic effect shifts the frequency by 22.47% for 0.2c. Given a 445nm laser (the powerful blue ones), that's 545nm
by leblancfg 11y ago
I love these napkin calculations.
The relativistic effect shifts the frequency by 22.47% for 0.2c.
Given a 445nm laser (the powerful blue ones), that's 545nm when redshifted (green). It's a bit of a jump, but that wavelength is still smack in the "solar atmospheric absorbtion spectrum window" -- you won't lose much of the energy in the atmosphere.
Reading this page diagonally [http://www.edmundoptics.com/resources/application-notes/optics/metallic-mirror-coatings/ http://www.edmundoptics.com/resources/application-notes/opti...], I don't think that the idea of getting a >99.5% reflectance value for that bandwidth is that far off.
- yongjik 11y agoUmm... 0.5% of 100GW is 500MW, or about the power of a nuclear reactor. The spaceship won't survive a millisecond. We need way better than 99.5%.
- nerfhammer 11y agoArticle says it needs to be better than 99.999%
- dharma1 11y agoiphone innards - done 99.999% reflective mirror - done (iphone gorilla glass ;)
- jessriedel 11y agoYep. "Use a material with better than 99.999% reflectivity. Usually, highly reflective surfaces are dielectric mirrors, which are composed of ‘sandwiches’ of material, with each layer reflecting back a modest fraction of the total. Each layer needs to be at least a quarter of a wavelength thick. The weight can be reduced by using a monolayer with high reflectivity at the correct wavelength. Based on recent research, this could be achieved by a ‘hole-pocked’ layer, highly reflective for very specific angles where reflectivity caustics arise. (These caustics occur for wavelengths of light that are actually longer than the sheet thickness.)" http://breakthroughinitiatives.org/index.php?controller=Forum&action=viewforum&id=5&page=1 http://breakthroughinitiatives.org/index.php?controller=Foru...
- stcredzero 11y agoIIRC, Robert Forward was writing about sails using dielectric mirrors with holes in them back in the 80's.
- jessriedel 11y agoYea, david-given links to one of Forward's old paper elsewhere in this thread. http://path-2.narod.ru/design/base_e/starwisp.pdf http://path-2.narod.ru/design/base_e/starwisp.pdf
- maxander 11y agoSo it sounds like we'd have a layered sequence of dialectric mirrors (themselves many layers each) each reflecting a different wavelength. The first layer lasts until the laser redshifts out of that layer's efficient reflection range, then vaporizes revealing the next layer tuned to the redshifted laser. That layer lasts until the laser redshifts even further, then vaporizes to reveal the next, and so on. Sacrificial layers may also help with heat management, I suppose.
- vilhelm_s 11y agoSome types of lasers (e.g. free electron lasers) can be tuned to arbitrary wavelengths, so you could maybe adjust the frequency of the laser as the craft accelerates.
- leblancfg 11y agoWell, that sure is a good point. Upon further reading, looks like you can achieve >99.9998% reflectance for a 10% wavelength band (in this case, in the infrared). [http://www.crystallinemirrors.com/wp-content/uploads/20130721_nphoton-2.pdf http://www.crystallinemirrors.com/wp-content/uploads/2013072...] Still, you're right in saying that it's probably still quite a ways off.
- skykooler 11y agoKeep in mind that a single spaceship wouldn't take up the whole beam - for one thing, there would be a bunch of them, and for another, we simply can't focus a laser on something the size of an iPhone from even as far away as the moon. That said, it is still a massive amount of power that needs to be reflected.
- leblancfg 11y agoI thought this also, but "[...] to keep the beam tightly focused on one probe at a time would require [...]" in the original article points in the opposite direction.
- ars 11y agoYou could have a sequence of lasers each tuned to a slightly different color calculated to reverse the relativistic shift. You'd probably want to do that anyway to avoid burning up your laser. Also it's easier to make a pulse at that power level vs continuous.