# Gravitational Lens - multiple images vs delays in image arrival time

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 and here’s another one showing nice examples. Okay, here’s one more 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?

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

[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:

(bolding mine)

I just realized that my wording of the question I ended the OP with, i.e.

could be interpreted to make you think it could be answered in the affirmative by something like the “light echo” of Supernova 1987A. 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.

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. ;))

Awesome, a perfect example with a solid data point:

The Twin Quasar (Twin QSO or Double Quasar or Old Faithful)

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!