# Can single black holes be observed by its lensing effect?

A comment on a question I asked left me confused. I asked about the possibility of observing a binary black hole by examining the lensing the binary produces. A binary has a different lensing effect as a single hole. My idea of lensing is turned upside down. I assumed it to be possible to see a hole by lensing. Which it is not, according to the comment. The hole is tiny indeed (only 3 kilometers in the case of a Sun mass hole) but does this mean the image of the stars behind doesn't change? Of course the hole must be moving wrt to the stars as seen from here. Is that not the case maybe?

• 3 km at a distance of 1000+ light-years is pretty small. ;) en.wikipedia.org/wiki/List_of_nearest_black_holes Aug 1 '21 at 17:24
• @PM2Ring Yes, thats true...I guess there is not enough light entering from whatever direction on the hole to be observable here. But what about the claim that BHs are mirrors? Are only big ones involved? Aug 1 '21 at 17:49
• Do the “ pictures” of the event horizon telescope count? Aug 1 '21 at 22:42
• @mmeent I think the image you're referring to is of the shadow of the black hole ias.edu/press-releases/2020/eht-gr-test Aug 2 '21 at 1:01
• @DaddyKropotkin The Event Horizon Telescope pictures absolutely show lensing by the BH; the arc at the "top" is light from the accretion disk behind the BH, bent as it passes near the BH. Aug 2 '21 at 10:57

The only problem is that the microlensing signature due to a star being the lens is pretty much identical to the signature of a BH being the lens. The only practical way to tell what's going on would be to get a mass measurement of the lens. If it were, say, $$\sim 10$$ solar masses and there was no bright star corresponding to the lens (an actual star that massive would be very bright, and probably easier to detect than the background star!), then you could conclude the lens was a dark object with that mass, and thus almost certainly a BH (since $$10 M_{\odot}$$ is much too large for a white dwarf or neutron star).