# Light Years **URL:** \ **Category:** Cecil's Columns/Staff Reports\ **Created:** [September 18, 2015, 9:22am UTC](https://boards.straightdope.com/t/light-years/731653 "2015-09-18T09:22:09Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![RassleBear](https://avatars.discourse-cdn.com/v4/letter/r/3e96dc/32.png) [@RassleBear](https://boards.straightdope.com/u/RassleBear)\ **Post date:** [September 18, 2015, 9:22am UTC](https://boards.straightdope.com/t/light-years/731653/1 "2015-09-18T09:22:09Z")
In the question as to whether a planet 100 light-years away could view life on earth 100 years ago if they had a suitably strong telescope, you omitted the most obvious answer, unless i missed it. A light-year is not a measure of TIME but of DISTANCE: the distance light travels in one year. ---
**Author:** ![Kamino\_Neko](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kamino_neko/32/34_2.png) [@Kamino\_Neko](https://boards.straightdope.com/u/Kamino_Neko)\ **Post date:** [September 18, 2015, 9:44am UTC](https://boards.straightdope.com/t/light-years/731653/2 "2015-09-18T09:44:55Z")
😕 The question’s not on the homepage yet, so I can’t look at it to be sure, but that sounds less like an answer to the question, than it does the core of the question. The idea of someone 100 light years away seeing stuff happening on Earth 100 years ago is only a question that can be entertained because any light that would reach that planet from Earth would have left Earth 100 years prior, because that’s how long it would take it to travel 100 light years. ---
**Author:** ![cochrane](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cochrane/32/10441_2.png) [@cochrane](https://boards.straightdope.com/u/cochrane)\ **Post date:** [September 18, 2015, 9:55am UTC](https://boards.straightdope.com/t/light-years/731653/3 "2015-09-18T09:55:21Z")
> [@RassleBear](#): > > In the question as to whether a planet 100 light-years away could view life on earth 100 years ago if they had a suitably strong telescope, you omitted the most obvious answer, unless i missed it. A light-year is not a measure of TIME but of DISTANCE: the distance light travels in one year. Light leaving the earth would reach a point 100 light years distant in 100 years. That was the basis of that question. Also, it’s helpful to link to the column you’re posting about since it doesn’t create a link when you click “Comment on this answer.” I think this was it: > **[Would aliens looking at us from 100 light-years away see earth as it was a...](https://www.straightdope.com/21344346/would-aliens-looking-at-us-from-100-light-years-away-see-earth-as-it-was-a-century-ago)** ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [September 18, 2015, 12:03pm UTC](https://boards.straightdope.com/t/light-years/731653/4 "2015-09-18T12:03:05Z")
A light year could be a definition of time also, as in how ling light would take to reach that fixed distance. So it relates the dimensions of space and time together. ---
**Author:** ![Hail\_Ants](https://avatars.discourse-cdn.com/v4/letter/h/dc4da7/32.png) [@Hail\_Ants](https://boards.straightdope.com/u/Hail_Ants)\ **Post date:** [September 18, 2015, 12:11pm UTC](https://boards.straightdope.com/t/light-years/731653/5 "2015-09-18T12:11:01Z")
I think Cecil’s answer is accurate as usual, but I think he missed the core issue. Namely, ok fine, you couldn’t see fine detail like what clothes humans wore etc.you could only see a large-scale ‘blob’ of an image. But yes, whatever image you saw would indeed be of the object at the time in the past equal to the number of light years away it was. If humans had wiped themselves out in Star Trek’s fictional nuclear World War III and turned Earth from a blue dot into a nuclear winter atmosphere-ravaged grey dot, Trelane would see it change to that only after the number of light years away it was had passed. When we see stars go supernovae this happened their number of light years distance in the past. ---
**Author:** ![Czarcasm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/czarcasm/32/4050_2.png) [@Czarcasm](https://boards.straightdope.com/u/Czarcasm)\ **Post date:** [September 18, 2015, 12:12pm UTC](https://boards.straightdope.com/t/light-years/731653/6 "2015-09-18T12:12:29Z")
> [@kanicbird](#): > > A light year could be a definition of time also, as in how ling light would take to reach that fixed distance. So it relates the dimensions of space and time together. I’ve only heard it used as a unit of distance-how far light travels in a year. ---
**Author:** ![yyyguyff](https://avatars.discourse-cdn.com/v4/letter/y/c5a1d2/32.png) [@yyyguyff](https://boards.straightdope.com/u/yyyguyff)\ **Post date:** [September 18, 2015, 12:31pm UTC](https://boards.straightdope.com/t/light-years/731653/7 "2015-09-18T12:31:14Z")
I agree. I think Cecil missed the core issue: Would the distant planet be looking at our “past”? ---
**Author:** ![bijk](https://avatars.discourse-cdn.com/v4/letter/b/f14d63/32.png) [@bijk](https://boards.straightdope.com/u/bijk)\ **Post date:** [September 18, 2015, 12:32pm UTC](https://boards.straightdope.com/t/light-years/731653/8 "2015-09-18T12:32:03Z")
the spirit of question was not how aliens will see but what they will see if they can. they might send a drone and beam up. after all we did see the lunar landing. mighty intelligent you might be but at times you side step the issue and bombard us with unnecessary info. ---
**Author:** ![CaymanQT](https://avatars.discourse-cdn.com/v4/letter/c/94ad74/32.png) [@CaymanQT](https://boards.straightdope.com/u/CaymanQT)\ **Post date:** [September 18, 2015, 12:45pm UTC](https://boards.straightdope.com/t/light-years/731653/9 "2015-09-18T12:45:23Z")
WASTE time on Science Fiction? C’mon, Cecil SciFi is the meaning of life! ---
**Author:** ![RaffArundel](https://avatars.discourse-cdn.com/v4/letter/r/87869e/32.png) [@RaffArundel](https://boards.straightdope.com/u/RaffArundel)\ **Post date:** [September 18, 2015, 4:09pm UTC](https://boards.straightdope.com/t/light-years/731653/10 "2015-09-18T16:09:35Z")
> [@yyyguyff](#): > > I agree. I think Cecil missed the core issue: Would the distant planet be looking at our “past”? I think he addressed a much more interesting aspect of the question. That discussion would be “yep, something 100 ly away means the light would take 100 years to get there” is pretty boring - even with expanding universe caveats and fun facts like our Sun is 8 minutes in the past when we stare at it, don’t-try-it-at-home. YMMV, but I like it when the article goes in a more interesting direction and still answers the question posed. “What would they see?” is a very specific question and it was an interesting answer because it addressed something people don’t commonly know or think about. ---
**Author:** ![cochrane](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cochrane/32/10441_2.png) [@cochrane](https://boards.straightdope.com/u/cochrane)\ **Post date:** [September 18, 2015, 5:41pm UTC](https://boards.straightdope.com/t/light-years/731653/11 "2015-09-18T17:41:28Z")
But a parsec is still not a measure of time. I don’t care what Han Solo said. ---
**Author:** ![Karen\_Lingel](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karen_lingel/32/18649_2.png) [@Karen\_Lingel](https://boards.straightdope.com/u/Karen_Lingel)\ **Post date:** [September 18, 2015, 7:58pm UTC](https://boards.straightdope.com/t/light-years/731653/12 "2015-09-18T19:58:02Z")
I giggled at the thought of someone parking a car on the moon and leaving their headlights on! Hee hee! ---
**Author:** ![Senegoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senegoid/32/6606_2.png) [@Senegoid](https://boards.straightdope.com/u/Senegoid)\ **Post date:** [September 18, 2015, 8:44pm UTC](https://boards.straightdope.com/t/light-years/731653/13 "2015-09-18T20:44:16Z")
> [@Czarcasm](#): > > I’ve only heard it used as a unit of distance-how far light travels in a year. I’ve seen it _defined_ that it can be used both ways (i.e., a light-year can mean the distance light boogies in a year, or simply the time it takes light to go that distance (i.e., a year, duh!), but I don’t recall that I’ve ever actually seen the phrase _used_ as a unit of time. Only distance. So I guess the descriptivists got a bit over-descriptive with that one. ---
**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:** [September 18, 2015, 9:08pm UTC](https://boards.straightdope.com/t/light-years/731653/14 "2015-09-18T21:08:32Z")
Yeah, nobody would ever call that unit of time a “light-year”, because it already has a perfectly good name, a “year”. ---
**Author:** ![Kedikat](https://avatars.discourse-cdn.com/v4/letter/k/ed655f/32.png) [@Kedikat](https://boards.straightdope.com/u/Kedikat)\ **Post date:** [September 19, 2015, 12:43am UTC](https://boards.straightdope.com/t/light-years/731653/15 "2015-09-19T00:43:07Z")
Does this idea have any validity? A flat array of detectors that are triggered by light particles ( photons ). In front of these detectors, that are packed as tightly as possible is a filter. The filter is an array of tubes packed together. The tubes are intended to only allow a photon to reach a detector at the base of the tube, if it does not collide with the wall of the tube. Maybe the material of the tube absorbs the photon, or absorbs enough of it’s energy to not trip the detector. So only photons that come in at a straight enough angle will make it to the detector and trip it. By narrowing the tubes and or increasing their lengths, can you then change the “focus” of the detector? Not allowing off axis photons to make it through. Of course you decrease the number of photons let through to trip the detectors with less diameter and longer lengths. But could it be a concept to more closely focus on distant objects? As usual, this may be a concept already in practice that I just haven’t heard of. I was looking up how narrow carbon nanotubes can be. It made me wonder about what you might see looking through such a tube. Then I wondered about how various crystal structures might seem similar to look through. The idea of such materials being a type of focusing/filtering screen crossed my mind. It seems to me that theoretically a screen that let only photons that are traveling at a perfect straight line to a detector, through a screen one photon wide, of sufficient length tubes, that absorb or deflect off axis ones, will give 1 to 1 magnification at any distance. Sacrificing brightness, but attaining extreme clarity. May take a long time to gather a usefull number of photons though. I suspect even the narrowest carbon nanotube might be too wide for astronomy use of such a design. But length might make up for that. ---
**Author:** ![Kedikat](https://avatars.discourse-cdn.com/v4/letter/k/ed655f/32.png) [@Kedikat](https://boards.straightdope.com/u/Kedikat)\ **Post date:** [September 19, 2015, 1:02am UTC](https://boards.straightdope.com/t/light-years/731653/16 "2015-09-19T01:02:27Z")
I guess if you could curve that device I imagine, that you could also magnify more than 1 to 1. ---
**Author:** ![boffking](https://avatars.discourse-cdn.com/v4/letter/b/858c86/32.png) [@boffking](https://boards.straightdope.com/u/boffking)\ **Post date:** [September 19, 2015, 3:11am UTC](https://boards.straightdope.com/t/light-years/731653/17 "2015-09-19T03:11:47Z")
How much detail can be seen at that distance? Is it possible that from 100 lightyear away from Earth, you could watch World War 1 in what seems to be real time with a telescope? ---
**Author:** ![kaylasdad99](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kaylasdad99/32/3398_2.png) [@kaylasdad99](https://boards.straightdope.com/u/kaylasdad99)\ **Post date:** [September 19, 2015, 4:35am UTC](https://boards.straightdope.com/t/light-years/731653/18 "2015-09-19T04:35:01Z")
More importantly, could an alien civilization some 52ly away get us a decent version of the Zapruder film, and settle this “grassy knoll” brouhaha once and for all? ---
**Author:** ![Tim\_R.Mortiss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tim_r.mortiss/32/142_2.png) [@Tim\_R.Mortiss](https://boards.straightdope.com/u/Tim_R.Mortiss)\ **Post date:** [September 19, 2015, 6:26am UTC](https://boards.straightdope.com/t/light-years/731653/19 "2015-09-19T06:26:19Z")
> [@kaylasdad99](#): > > More importantly, could an alien civilization some 52ly away get us a decent version of the Zapruder film, and settle this “grassy knoll” brouhaha once and for all? Even if they could, it would take them 52 years to radio it back to us. ---
**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:** [September 19, 2015, 11:53am UTC](https://boards.straightdope.com/t/light-years/731653/20 "2015-09-19T11:53:16Z")
**Kedikat** , your idea won’t work as well as you’re expecting, because light acts as a wave, not just as a particle, and so you’ll get some diffraction at the end of your tubes. That said, it will work at least somewhat, and devices not too dissimilar from that are sometimes used for forms of radiation that are very difficult to image in conventional ways, like gamma rays. [Next page](https://boards.straightdope.com/t/light-years/731653.md?page=2)