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Comment by sliken | Hacker News Reader

Black holes are of course much smaller than anything else of the same mass. So the gravitational gradient is much higher, which causes some detectable side effects.

One possibility is gravitational lensing. The abstract also mentions detecting "annihilation signals from the dark matter microhalo around the PBH". Not that I understand that completely, and I don't believe (corrections welcome) that dark matter annihilation by black holes has been confirmed experimentally.

I believe high energy particles are released by black holes as they consume matter, so that might be another way to detect a small black hole instead of a similar mass rock.

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> One possibility is gravitational lensing.

From a distance there is no difference in lensing between a black hole and an regular object of the same mass.

From close there would be a difference, but if you could resolve something that small you would be able to directly image the object.

I disagree. An earth like object isn't going to bend light much, the maximum of 1G acceleration is just not that noticeable.

An earth mass blackhole is going to have 10, 1000s, or even millions of Gs depending on how close the light gets.

Seems like you'd get stars periodically blinking from places they shouldn't be, as light trajectory should have missed earth, but gets bent by the black hole. So while the black hole itself would be invisible at that distance, and occultations would be invisible, seeing stars in the wrong place would still be visible.

Yes, it will happen, but the radius of the effect is simply too small to see it.

You are only bending a minute faction of the light from the star traveling toward you. It's not enough light to see, and it's too small to resolve (unless you could resolve the black hole in the first place).

Think about it: The redirected light only has the "size" of the gravitation field in question. For an earth sized mass thats 9mm. Even if you did 9km (which wouldn't bend much) you couldn't see it, never mind 9mm.

Would evaporation give out any signal? Anti-particles could be a signal, no?

Hawking radiation is a signal, one so strong it's on the order of a large nuclear bomb when a black hole finally winks out of existence.

However if it's big enough to be messing with orbits around neptune it's likely pretty large. If it's large then the hawkings radiation isn't going to be visible.

So if it's moon mass it's going to be 0.1mm and at 2.7 kelvin or so. Earth mass is 9mm, but even colder. Generally for the orbital changes they are seeing in a wide variety of objects it seems like the mass is even larger still.

Even seeing pluto is hard, even a jupyter mass black hole is only 3 meters or so. So no I don't think we could detect it via hawking radiation.

We might however see high energy particles resulting from the somewhat messy feeding that black holes are known for. If we sent a probe it could plausibly get close enough to directly observe hawkings radiation. Pretty amazing thought, maybe we will get that lucky.

>one so strong it's on the order of a large nuclear bomb

Surely a "large nuclear bomb" isn't very strong when you're talking about things on black hole scale?

Sure, but that's when it's near zero size/mass.

Ah, looks like I was "a bit" low. Wiki claims 5×10^6 megatons of TNT when it finally evaporates. Apparently about the energy the shoemaker level struck Jupiter.

A black hole of this mass is colder than the cosmic background.

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