# how fast would an object need to go to turn invisible? **URL:** \ **Category:** In My Humble Opinion\ **Created:** [September 29, 2010, 11:29am UTC](https://boards.straightdope.com/t/how-fast-would-an-object-need-to-go-to-turn-invisible/555457 "2010-09-29T11:29:36Z")\ **Posts on this page:** 14\ **Page:** 2
**Author:** ![Theophane](https://avatars.discourse-cdn.com/v4/letter/t/77aa72/32.png) [@Theophane](https://boards.straightdope.com/u/Theophane)\ **Post date:** [January 15, 2012, 4:16pm UTC](https://boards.straightdope.com/t/how-fast-would-an-object-need-to-go-to-turn-invisible/555457/21 "2012-01-15T16:16:55Z")
What would be the optical equivalent of a sonic boom? Is there such a thing? Looking at you, science guys. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [January 15, 2012, 4:44pm UTC](https://boards.straightdope.com/t/how-fast-would-an-object-need-to-go-to-turn-invisible/555457/22 "2012-01-15T16:44:21Z")
> [@Theophane](#): > > What would be the optical equivalent of a sonic boom? Is there such a thing? Looking at you, science guys. Something exceeding the speed of light. Perhaps a pretty big boom. ---
**Author:** ![Nametag](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nametag/32/406_2.png) [@Nametag](https://boards.straightdope.com/u/Nametag)\ **Post date:** [January 15, 2012, 9:04pm UTC](https://boards.straightdope.com/t/how-fast-would-an-object-need-to-go-to-turn-invisible/555457/23 "2012-01-15T21:04:50Z")
A sonic boom is the result of the nature of sound waves, compressing and rarefying their medium, and the pile-up that comes when you hit the limit on the medium’s capacity to propagate the wave. Light has no medium, and there is no exceeding the speed of light. The closest thing light has to that effect is red or blue-shifting – a hypothetical object traveling at _c_ would emit light that is shifted according to your angle of view. In your frame of reference, the object is traveling at the same speed as any photons it emits, so you’d see a more-or-less instantaneous blip of light containing the entire spectrum, just as (well, just after) the object passes you. ---
**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:** [January 15, 2012, 9:22pm UTC](https://boards.straightdope.com/t/how-fast-would-an-object-need-to-go-to-turn-invisible/555457/24 "2012-01-15T21:22:57Z")
Y’all are approaching the problem wrong: in order for the object to be rendered invisible, you merely need light traveling from it to be [redshifted](http://en.wikipedia.org/wiki/Redshift) to above about 750 nm. So get the object to travel away from you at relativistic speeds and voila: invisible. ---
**Author:** ![si\_blakely](https://avatars.discourse-cdn.com/v4/letter/s/d9b06d/32.png) [@si\_blakely](https://boards.straightdope.com/u/si_blakely)\ **Post date:** [January 16, 2012, 12:29pm UTC](https://boards.straightdope.com/t/how-fast-would-an-object-need-to-go-to-turn-invisible/555457/25 "2012-01-16T12:29:38Z")
> [@Theophane](#): > > What would be the optical equivalent of a sonic boom? Is there such a thing? Looking at you, science guys. [Cherenkov Radiation](http://en.wikipedia.org/wiki/Cherenkov_radiation), but it only occurs when the speed of light in the traversed dielectric media is slower than the charged particle - c is not exceeded. Si ---
**Author:** ![Rhythmdvl](https://avatars.discourse-cdn.com/v4/letter/r/85f322/32.png) [@Rhythmdvl](https://boards.straightdope.com/u/Rhythmdvl)\ **Post date:** [January 16, 2012, 12:37pm UTC](https://boards.straightdope.com/t/how-fast-would-an-object-need-to-go-to-turn-invisible/555457/26 "2012-01-16T12:37:33Z")
> [@TriPolar](#): > > Something exceeding the speed of light. Perhaps a pretty big boom. [MtM]Where’s the kaboom? There’s supposed to be a light-shattering kaboom![/MtM] (Why Mary Tyler Moore is saying that, I don’t know. Is the Earth blocking her view or something?) ---
**Author:** ![Boyo\_Jim](https://avatars.discourse-cdn.com/v4/letter/b/87869e/32.png) [@Boyo\_Jim](https://boards.straightdope.com/u/Boyo_Jim)\ **Post date:** [January 16, 2012, 2:23pm UTC](https://boards.straightdope.com/t/how-fast-would-an-object-need-to-go-to-turn-invisible/555457/27 "2012-01-16T14:23:08Z")
How about artillery shells? [According to Wiki](http://en.wikipedia.org/wiki/M114_155_mm_howitzer), the muzzle velocity of this 155mm howitzer is 1854 ft./second. I have read claims, though, that from certain angles and under certain conditions they can be seen. ---
**Author:** ![Boyo\_Jim](https://avatars.discourse-cdn.com/v4/letter/b/87869e/32.png) [@Boyo\_Jim](https://boards.straightdope.com/u/Boyo_Jim)\ **Post date:** [January 16, 2012, 2:36pm UTC](https://boards.straightdope.com/t/how-fast-would-an-object-need-to-go-to-turn-invisible/555457/28 "2012-01-16T14:36:13Z")
[Here is a video of Minuteman re-entry vehicles striking Kwajalein at 60,000 mph](http://www.youtube.com/watch?v=ChhYOO1s-nY). Unfortunately, the video quality and camera positioning is poor. And it’s impossible to say for sure whether the video was slo-mo, though the debris cloud afterward suggest that it isn’t. So what counts as invisible? You can’t see the warheads, but you can see the superheated gases surrounding them. If somehow these gases weren’t there, could you still see the warhead? I dunno. ---
**Author:** ![mutantmoose](https://avatars.discourse-cdn.com/v4/letter/m/a87d85/32.png) [@mutantmoose](https://boards.straightdope.com/u/mutantmoose)\ **Post date:** [January 17, 2012, 10:33pm UTC](https://boards.straightdope.com/t/how-fast-would-an-object-need-to-go-to-turn-invisible/555457/29 "2012-01-17T22:33:33Z")
You sure you got that speed right? At about 5:36 the narrator says they are travelling at 16 000 kph which is 9 941 mph. Still pretty fast and only a couple of thousand mph off the 12 000 we reckoned we would need. Although I’m wondering about that now having seen the minute men - they didn’t seem to be going fast enough such that you wouldn’t see them even without the superheated gases. I was envisaging something that would whizz right past you so fast you wouldn’t even see it. And on the subject of the gases - would they collect in front of any fast moving object thus negating the invisibility? If so then that could be a problem. I wonder if those missiles are the fastest moving large-size man-made objects within the earth’s atmosphere (obviously spacecraft go faster but they don’t count). It seems like they are according to this wiki page which has a list of [Orders of magnitude of speed](http://en.wikipedia.org/wiki/Orders_of_magnitude_(speed)) and examples of things that go at each speed given. ---
**Author:** ![Boyo\_Jim](https://avatars.discourse-cdn.com/v4/letter/b/87869e/32.png) [@Boyo\_Jim](https://boards.straightdope.com/u/Boyo_Jim)\ **Post date:** [January 17, 2012, 10:58pm UTC](https://boards.straightdope.com/t/how-fast-would-an-object-need-to-go-to-turn-invisible/555457/30 "2012-01-17T22:58:15Z")
> [@mutantmoose](#): > > You sure you got that speed right? At about 5:36 the narrator says they are travelling at 16 000 kph which is 9 941 mph… You are right. In myy crappy little built in laptop speakers it sounded like ‘sixty’ and it was actually ‘sixteen’. And I must have just assumed mph because I hardly ever hear Americans talking about kph. Anyway, I should have known better – at 60,000 mph they never would have re-entered the atmosphere. ---
**Author:** ![si\_blakely](https://avatars.discourse-cdn.com/v4/letter/s/d9b06d/32.png) [@si\_blakely](https://boards.straightdope.com/u/si_blakely)\ **Post date:** [January 18, 2012, 3:54pm UTC](https://boards.straightdope.com/t/how-fast-would-an-object-need-to-go-to-turn-invisible/555457/31 "2012-01-18T15:54:18Z")
> [@mutantmoose](#): > > Still pretty fast and only a couple of thousand mph off the 12 000 we reckoned we would need. Although I’m wondering about that now having seen the minute men - they didn’t seem to be going fast enough such that you wouldn’t see them even without the superheated gases. > > I was envisaging something that would whizz right past you so fast you wouldn’t even see it. Those warheads are further than 10000 m away, so they would have to be going **much** faster. Si ---
**Author:** ![Boyo\_Jim](https://avatars.discourse-cdn.com/v4/letter/b/87869e/32.png) [@Boyo\_Jim](https://boards.straightdope.com/u/Boyo_Jim)\ **Post date:** [January 18, 2012, 4:17pm UTC](https://boards.straightdope.com/t/how-fast-would-an-object-need-to-go-to-turn-invisible/555457/32 "2012-01-18T16:17:45Z")
> [@si\_blakely](#): > > Those warheads are further than 10000 m away, so they would have to be going **much** faster. > > Si What are you referring to? The impacts on the island were filmed from a camera on the island, probably no more than a mile or two away, and the final impact looks significantly closer than that. ---
**Author:** ![Enderw24](https://avatars.discourse-cdn.com/v4/letter/e/ba9def/32.png) [@Enderw24](https://boards.straightdope.com/u/Enderw24)\ **Post date:** [January 18, 2012, 4:48pm UTC](https://boards.straightdope.com/t/how-fast-would-an-object-need-to-go-to-turn-invisible/555457/33 "2012-01-18T16:48:44Z")
> [@mutantmoose](#): > > [Really?](http://www.youtube.com/watch?v=iRGacDGUGtE&NR=1&feature=fvwp) > > Light speed is 671 million miles per hour so 1% of that would be 6.71 million miles per hour. You reckon it would have to go that fast to not be seen? Seems a little on the high side to me Possibly not. First, let’s assume your field of vision on a clear horizon at ground level is 2.5 miles in either direction. Also, let’s assume that the statistic quoted above that you probably couldn’t recognize something if it flashed for just 1/300th of a second is true. So a plane travelling 5 miles in 1/300th of a second would travel at 1,500 miles a second or 5.4 million miles an hour. That’s about .8% the speed of light. ---
**Author:** ![kenobi\_65](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kenobi_65/32/13178_2.png) [@kenobi\_65](https://boards.straightdope.com/u/kenobi_65)\ **Post date:** [January 18, 2012, 5:49pm UTC](https://boards.straightdope.com/t/how-fast-would-an-object-need-to-go-to-turn-invisible/555457/34 "2012-01-18T17:49:58Z")
> [@tdn](#): > > On the other hand, I’ve launched model rockets from 15 feet away that accelerated from 0 to over 40 in the space of 3 feet, and continued to accelerate to around 100 in the next few feet. In too many cases, the last time I saw them was when they were still on the launch pad. I’ve had those, too, including the famous “teleporting” [Estes Mosquito](http://v-serv.com/usr/kits/mosquito122.gif) (tiny rocket, weighs next to nothing). 😃 [Previous page](https://boards.straightdope.com/t/how-fast-would-an-object-need-to-go-to-turn-invisible/555457.md?page=1)