# Gravitational Lens - multiple images vs delays in image arrival time **URL:** \ **Category:** Factual Questions\ **Created:** [March 26, 2010, 12:16am UTC](https://boards.straightdope.com/t/gravitational-lens-multiple-images-vs-delays-in-image-arrival-time/533817 "2010-03-26T00:16:35Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [March 26, 2010, 12:16am UTC](https://boards.straightdope.com/t/gravitational-lens-multiple-images-vs-delays-in-image-arrival-time/533817/1 "2010-03-26T00:16:35Z")
Although I haven’t a clue about the underlying mathematics, and almost as little of the relevant physics, it hasn’t stopped me from being awed by the phenomenon of gravitational lensing. (For anyone who’s interested, [here is a pic](http://antwrp.gsfc.nasa.gov/apod/ap090823.html) and [here’s another one](http://antwrp.gsfc.nasa.gov/apod/ap080210.html) showing nice examples. Okay, [here’s one more](http://antwrp.gsfc.nasa.gov/apod/ap100207.html) glorious one.) As those images show, an intervening mass between our eye and a more distant object can cause us to observe multiple images of the distant object (or even a ring). My question is whether gravitational lensing has also been detected by showing a time lag of a distant object’s light curve’? By ‘light curve’ I mean some time varying flux of radiation (which need not be periodic). Implicit in my question, of course, is the assumption that there will be different path lengths for the lensed radiation from the distant object due to inhomogeneities in the distribution of mass in the intervening, lensing body(ies). So, do we ever see absolutely identical ‘light curves’ (radiating from _one_ point in space) but which are separated in time by some interval? ---
**Author:** ![minor7flat5](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/minor7flat5/32/258_2.png) [@minor7flat5](https://boards.straightdope.com/u/minor7flat5)\ **Post date:** [March 26, 2010, 2:25am UTC](https://boards.straightdope.com/t/gravitational-lens-multiple-images-vs-delays-in-image-arrival-time/533817/2 "2010-03-26T02:25:18Z")
Your mention of the difference in the light flux reminded me of how NASA did certain kinds of distance measurements: They would intentionally introduce random noise in the broadcast signal so that when they got the bounced-back signal they could try to line up the random noise with part of the original signal. This was more effective than simply sending an individual pulse. Anyway, back on topic. I know I have read a nicer description of this lensing-causes-time-delays business somewhere (Perhaps Isaac Asimov?), but [Wikipedia will have to do](http://en.wikipedia.org/wiki/Gravitational_lens#Geometry_of_the_universe)… [quote=Wikipedia] There will be a time delay (around days or weeks) between multiple images of the same source because of [ul] [li]the delay due to the difference in optical path length between the two rays.[/li][li]the general relativistic Shapiro effect, which describes light rays as taking longer to traverse a region of stronger gravitation, (see: gravity well, gravitational time dilation). Because the two rays travel through different parts of the potential well created by the deflector, the clocks carrying the source’s signal will differ by a small amount.[/ul][/li][/quote] Found [another reference](http://www.starstryder.com/2007/09/19/lensing-lenses-einsteins-cross/): > [@Another reference](#): > > Each different image provides us a snap shot of the quasar from slightly different angles. While **many quasars with split up images have significant time delays between one image and another** , the images in this situation have negligible time delay. (bolding mine) ---
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [March 26, 2010, 2:27am UTC](https://boards.straightdope.com/t/gravitational-lens-multiple-images-vs-delays-in-image-arrival-time/533817/3 "2010-03-26T02:27:04Z")
I just realized that my wording of the question I ended the OP with, i.e. > [@](#): > > do we ever see absolutely identical ‘light curves’ (radiating from one point in space) but which are separated in time by some interval could be interpreted to make you think it could be answered in the affirmative by something like [the “light echo” of Supernova 1987A](http://www.aao.gov.au/images/captions/aat066.html). I hope it’s obvious that, despite that wording, I’m talking about a very different concept. ETA: Thanks **minor**. I swear I checked in Wiki before posting. Must be getting tired. ---
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [March 26, 2010, 2:39am UTC](https://boards.straightdope.com/t/gravitational-lens-multiple-images-vs-delays-in-image-arrival-time/533817/4 "2010-03-26T02:39:56Z")
It would be wonderful if the background galaxy in a lens system developed a supernova. Then, only some of the multiple images would contain the supernova. Pretty neat. (Hey, then we could signal astronomers in the non-supernova galaxies to get ready for it. ;)) ---
**Author:** ![minor7flat5](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/minor7flat5/32/258_2.png) [@minor7flat5](https://boards.straightdope.com/u/minor7flat5)\ **Post date:** [March 26, 2010, 2:44am UTC](https://boards.straightdope.com/t/gravitational-lens-multiple-images-vs-delays-in-image-arrival-time/533817/5 "2010-03-26T02:44:19Z")
Awesome, a perfect example with a solid data point: [The Twin Quasar (Twin QSO or Double Quasar or Old Faithful)](http://en.wikipedia.org/wiki/Twin_Quasar#Quasar) > [@](#): > > The Twin Quasar’s two images are separated by 6". Both images have an apparent magnitude of 17, with the A component having 16.7 and the B component having 16.5 . There is a 417 ± 3 day time lag between the two images.[1] There you go. An example of a quasar with a 417 day delay between the images. How cool is that? ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [March 27, 2010, 1:00pm UTC](https://boards.straightdope.com/t/gravitational-lens-multiple-images-vs-delays-in-image-arrival-time/533817/6 "2010-03-27T13:00:56Z")
> [@minor7flat5](#): > > There you go. An example of a quasar with a 417 day delay between the images. How cool is that? One more point for Albert!