black hole

Stream: maths

[Kenny Lau]

can we prove that the picture we saw yesterday is that of a black hole?

[Kevin Buzzard]

I'm not even sure we can prove that anyone saw anything

[Kenny Lau]

maybe we can prove that 1/u+1/v=1/f

[Keeley Hoek]

(I could just google this, but) is gravitational lensing even the same as the optical kind?

[Mario Carneiro]

no

[Mario Carneiro]

optical lensing happens because the speed of light is slower in a medium, and gravitational lensing happens because space isn't flat

[Scott Morrison]

Media aren’t “optically flat”?

[Mario Carneiro]

I see the resemblance, but I would still argue that they are really different. As another analogy, with gravitational lensing the light goes straight the whole time, and comes out going a different direction somehow. In the material case you can locally "see" the distortion at a medium boundary or change in refraction index

[Mario Carneiro]

Of course I don't need to explain to a room of mathematicians the weird things that "straight lines" can do on curved manifolds, even when there is no curvature (e.g. flat torus)