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WaxRubiks
2019-Jan-18, 08:03 PM
I was thinking abou the flat Earthers, and I wondered the question, Is a photon sphere really just a flat plain?

Strange
2019-Jan-18, 09:53 PM
Isn't the clue in the word "sphere"? (In other words, no.)

But do you mean, is the photon sphere a 2D surface, then I guess the answer is yes.

grant hutchison
2019-Jan-19, 12:57 AM
I was thinking abou the flat Earthers, and I wondered the question, Is a photon sphere really just a flat plain?No.
I'm finding it difficult to imagine why you'd think there was a question in that regard.

Grant Hutchison

Ken G
2019-Jan-19, 01:03 AM
It's kind of like a syllogism: "I was thinking about the flat Earthers, and I wondered [any kind of nonsense]."

WaxRubiks
2019-Jan-19, 02:46 AM
No.
I'm finding it difficult to imagine why you'd think there was a question in that regard.

Grant Hutchison

It's kind of like a syllogism: "I was thinking about the flat Earthers, and I wondered [any kind of nonsense]."

well take a real plain of flat space, and put a light somewhere in it. You'd expect the light to radiate out across the plane(as well as away from the plain), but the light radiating out across the flat plain would continue to do so. Isn't that just the same as a photon sphere?

And the flat-Earth stuff just made me think of it. It wasn't part of the argument.

Geo Kaplan
2019-Jan-19, 03:17 AM
well take a real plain of flat space, and put a light somewhere in it. You'd expect the light to radiate out across the plane(as well as away from the plain), but the light radiating out across the flat plain would continue to do so. Isn't that just the same as a photon sphere?

It's the same only in the sense that both describe isotropic radiation from a point. In two-space, you'd be looking at circles of constant phase. In three-space, you get a sphere. So you shouldn't be arguing that a plane is the same as a sphere. At best, you should say that a circle in 2-space corresponds to a sphere in 3-space, or something like that, so you'd be talking about "photon circles" as corresponding to "photon spheres". But there's no sense in which a plane or circle is "just the same as a ... sphere".

Ken G
2019-Jan-19, 06:02 AM
well take a real plain of flat space, and put a light somewhere in it. You'd expect the light to radiate out across the plane(as well as away from the plain), but the light radiating out across the flat plain would continue to do so. Isn't that just the same as a photon sphere?I'm not seeing any connection. You could have ended it with isn't that just the same as the Eiffel Tower, and it would have sounded the same to me.

Shaula
2019-Jan-19, 07:50 AM
well take a real plain of flat space, and put a light somewhere in it. You'd expect the light to radiate out across the plane(as well as away from the plain), but the light radiating out across the flat plain would continue to do so. Isn't that just the same as a photon sphere?
Maybe this is a good time to ask - what do you mean by photon sphere? Because I don't think you are using the term in the same way I am used to.

WaxRubiks
2019-Jan-19, 08:53 AM
Maybe this is a good time to ask - what do you mean by photon sphere? Because I don't think you are using the term in the same way I am used to.

the sphere around a non-spinning black hole where photons can hypothetically orbit.

Strange
2019-Jan-19, 09:38 AM
So the photons sphere is made up of (a class of) geodesics. These are the equivalent of straight lines in curved space. So, in a sense, the surface of the sphere could, perhaps, be thought of as a "plane" in curved space.

There does seem to be a difference though: light moving in a 2D plane is constrained to that plane, but an orbit at the photon sphere is (I think) unstable?

Shaula
2019-Jan-19, 09:49 AM
the sphere around a non-spinning black hole where photons can hypothetically orbit.
OK, so we are talking about the same thing. The disconnect for me is how "Volume of space around a dense object where photon paths can be closed" links to

well take a real plain of flat space, and put a light somewhere in it. You'd expect the light to radiate out across the plane(as well as away from the plain), but the light radiating out across the flat plain would continue to do so. Isn't that just the same as a photon sphere?
I honestly can't see any equivalence at all.