Does clearly visible black hole lensing happen only if we happen to be at the focal point of what it is lensing. Similar to the usual optical scheme?
Does black hole lensing also include chromatic distortion? Not focusing all the frequencies of light at the same point?
If the answer to the first question is yes. Then is it correct that we might be looking at a black hole, but see nothing? Mistaking it for empty space.
If the answer to the second question is yes. Are we looking for and seeing odd unfocused various wavelengths of light that seem to be somewhat concentrated in an area without a specific source? Like a fuzzy out of focus picture? Resulting from a black hole bending the energy from something behind it. But we are not at the focal point?
Just wondering if such an effect would happen. and allow us to propose that a black hole is doing it.
This image shows what gravitational lensing looks like. I think I can see some chromatic distortion in there.
This particular example is known as SDSS J0146-0929, and is caused by the gravity of a distant cluster of galaxies - a cluster which almost certainly includes some black holes. So you could say that we’ve already detected black hole lensing, although not exclusively so.
I thought chromatic aberration had to do with refractive dispersion? A beam of light grazing a black hole is still propagating though vacuum (unless it isn’t…)
ETA also the light is not focussed to a point like with a magnifying glass
Yes, now you come to mention it, gravitational lensing is supposed to be achromatic, so the colours in tha tNASA image must be the colours present in the original object.
There shouldn’t be chromatic aberration. All the light rays are traveling in straight lines. You only think of them as bending if you’re thinking from a perspective far away from them. That’s the point of it being the spacetime itself that is curved. The object behind the black hole is in one location that lies in multiple directions from us.
If the alignment is perfect, you get the so-called Einstein ring., where the background source forms a “ring” around the lensing object. I don’t know if this has ever been observed with a black hole, but there are several known instances of the gravity of galaxies causing Einstein rings for even-more-distance galaxies behind them.
Even if the alignment isn’t perfect, though, you still see two distorted images. You can see how this might appear in this GIF, which simulates a black hole passing in front of a galaxy.
And what’s even more common is gravitational microlensing. Even if we can’t resolve the background object as an image (the background object might be a star rather than a galaxy), the lensing effect still makes the background object get brighter and dimmer in a characteristic way.
No; as pointed out by Napier, the wavelength of a light ray doesn’t affect how it travels through space.
In fact, one of the characteristic traits of gravitational microlensing is that it happens the same way at all wavelengths. There are other processes within starts that can cause them to get brighter and dimmer over time; but usually those processes cause more brightening at some wavelengths than at others. If you see a star get brighter and dimmer by exactly the same amount at all wavelengths simultaneously, you can be pretty sure that something passed between you and the star and lensed its light.
For objects other than black holes doing the lensing, there is still something analogous to a “focal point”, in that the effect will be greatest at a certain distance. This is where initially-parallel light rays, just grazing the surface of the object, will converge. Of course, the images you get there will still be horribly distorted, but in a known way, so that you could process what you see to reconstruct the original image.
For the Sun, this point would be about 1/40 of a lightyear away. One occasionally sees pie-in-the-sky proposals to send a probe out to this distance to use the Sun’s gravitational field as a sort of super-telescope. Of course, you’d have to know exactly where you wanted to aim that telescope before you launched, because the only way to change the aim would be to move the probe ludicrous distances. Still, it’s a lot easier than sending the probe to the star system you’re interested in itself.
For black holes, light rays can loop completely around the hole, multiple times, even, resulting in light rays sent off every which way. IIRC, the strongest direction of light rays is straight back at the source.
In principle, yes, one might detect a nearby black hole as looking like a faint star with a spectrum exactly matching that of the Sun. But in practice, the hole would need to be extremely close, close enough that it’s a surprise to nobody that we’ve never seen one.
That could make a good student project: what’s the upper bound mass of an undetected black hole orbiting the earth at a given distance? Explain a bunch of detection methodologies of near-earth black holes, and estimate the maximum mass that would be undetectable.
Orbiting the Earth, we’d detect it at any size. Anything small enough that we wouldn’t notice the gravitational effects, we’d be able to see the Hawking radiation.
Out by the orbit of Pluto, say, might be trickier.
