Gravitational lensing

[DarkerSky]

Just read an ESO release about the APEX telescope discovering a 10-billion light year distant galaxy courtesy of gravitational lensing from a foreground galaxy grouping.

And it got me thinking...

Could gravitational lensing be articifically induced in a managable / practical way so that it could be applied to telescope design - say like in an orbiting space telescope - to enable it to see deeper into the universe wherever it is pointed?

In my mind the big challenges would be inducing such a lens of significant value, and having the technology to contain it / isolate it

Any thoughts....or is it pure sci-fi..?

[FraserClarke]

Sci-fi I'm afraid.

The scales on which galaxy clusters lens background objects are immense (they have trillions of solar masses of material and 'focal lengths' a good fraction of the size of the Universe). The often used moniker 'gravitational telescope' is a bit misleading really. It's not like any conventional telescope you might dream up.

In theory you could probably make a small enough 'lens' out of small blackhole, for example. However, the bending of the light by gravity does the opposite of what you want from a telescope lens -- the further from the centre of the 'lens'; the less the light is bent. Exactly the opposite of what you get in a conventional lens.

So, your gravitational lens wouldn't collect light from the whole area into a single point (as a telescope does). It would just focus light from a ring (technically known as a 'caustic') to a point. Considering lots of different 'rings' at different distances from the center of the lens, you'd end up with a long line of focused light along the 'optical axis' of the lens... rather than a single focal plane like you have in a telescope.

No sure I've explained that well??

[DarkerSky]

Now that is interesting...and a good explanation

I'm assuming that there is no stronger gravitational force than is produced by a black hole ?

In a hypothetical project then, you'd have to have the technology to initiate, sustain and stabilise a black hole to create the lens - and presumably isolate it to prevent any external effects. i.e. the earth getting swallowed up...

That's seems a ridiculously tall order in itself, let alone the optical challenges of getting the focused light into a useable format

Interesting to explore the concept though

[vhbelvadi]

the further from the centre of the 'lens'; the less the light is bent

OK, so this might be a silly question: supposing I'm very (many verys here) far from a galaxy I might not experience a g.lensing at all?

Now if that (above) is correct, we might not be able to identify black holes that are like really far away, would we? Because we would notice no g.lensing.

[FraserClarke]

By 'far' here I mean radially away from the middle; not distance along the line of sight.

Imagine we have a star in the background, and a mass (doesn't have to be a black hole) directly between us and the background star. Photons that come from the star directly towards us pass very close to the intervening mass, and they are bent the most. Photons which leave the star at a slightly different angle don't pass so close to the mass, and aren't bent as much by its gravitational field. I've attached a little diagram which I hope explains what I mean?? Hopefully you can see what I'm getting at with the bending of the rays as they go past the gravitational lens, and the optical lens... (obviously they wouldn't be sharp corners in the gravitational case; but I couldn't be bothered drawing curves properly!! ). The light is travelling left to right in the diagram.

For the second part of your question; Gravitational lensing isn't really used to detect black holes. We know about the supermassive (millions to billions of solar mass) black holes in the middle of galaxies from their dynamical effects on the surrounding stars (by measuring the motions of the stars you can work out how much mass they are orbiting around, and if you can measure close enough to the centre, at some point the only thing that can provide that much mass in a small enough volume is a black hole). There are some lensing effects you can detect in accretion disks of quasars which show you they are orbiting around a black hole in the middle.