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would hawking radiation be dangerous if the source was close by, like say a few million km? what effect would it have on us. and is it possible to deflect hawking radiation?

sagar

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Hawking radiation scales with the size of the hole - but inversely. So for all known, observed sizes of black hole it is incredibly weak. Hypothetically micro black holes might give off larger and larger amounts as they evaporate - I think a few thousand tonnes mass black hole would be evaporating at a dangerous rate - however it would also only last a couple of seconds IIRC.

Basically in order to evaporate a BH needs to be smaller than about a percent of the mass of the Earth - otherwise it is cooler than the CMB and so unable to net export energy to it. I think this means that unless small black holes were formed early in the universe we are not living in an epoch where we can expect to see this evaporation process going on.

3. Originally Posted by ragasssd
would hawking radiation be dangerous if the source was close by, like say a few million km? what effect would it have on us. and is it possible to deflect hawking radiation?
In general, Hawking radiation is just thermal blackbody radiation, the same as you'd see from any hot object. As Shaula notes, for stellar mass or larger black holes (the only ones we know exist), it would be so small as to be almost impossible to measure. Smaller black holes would emit more, but it would be no different from the radiation given off by an object with a similar temperature. This is a nice calculator for Hawking radiation. You can enter any of the parameters in whatever units you prefer, and it will calculate everything else based on that.

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How does the escaped virtual particle become infrared "light" photons? Neil

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Thermal blackbody dosent mean IR. The Sun is mostly a blackbody, and it is brightest in green light IIRC.

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Apparently a black hole with a mass of 1,000 kg would evaporate in a billionth of a second. That would release about 1,700,000 times as much energy as the atomic bomb used to kill humans in Hiroshima. I wouldn't recommend looking at it, but I guess we'd probably be okay if it was a few million kilometers away. I think the energy given off would be pretty much all gamma rays which can't really be deflected, but can be stopped by several metres of dirt or concrete.

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Originally Posted by neilzero
How does the escaped virtual particle become infrared "light" photons? Neil
The virtual particle model is not an accurate description of how a black hole radiates - it is just a way to show people how a black hole could radiate. It is usually mentioned because the real maths behind it (Unruh effect) is hard going. Unruh predicts a thermal emission spectrum.

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Originally Posted by Shaula
The virtual particle model is not an accurate description of how a black hole radiates - it is just a way to show people how a black hole could radiate. It is usually mentioned because the real maths behind it (Unruh effect) is hard going. Unruh predicts a thermal emission spectrum.
What? The Bogoliubov transformations are to hard for you?

9. Originally Posted by Tensor
What? The Bogoliubov transformations are to hard for you?
I can't even pronounce it!

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Originally Posted by Tensor
What? The Bogoliubov transformations are to hard for you?
We gee, not too hard ... just .... well, you know I don't have the time and my cat is allergic to them. So I can't really have them in the house and I'd have to keep them in a shed and I don't have a shed and .... yeah, they are too hard :P

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Thanks all, and it is good to see some humor and humility. I have lots of humility because my understanding of most everything is quite shallow. Neil

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Originally Posted by Shaula
We gee, not too hard ... just .... well, you know I don't have the time and my cat is allergic to them. So I can't really have them in the house and I'd have to keep them in a shed and I don't have a shed and .... yeah, they are too hard :P
Yeah, I keep them locked up in a special room and and only let them out when a real mathematician comes over, which is never. So, I sit and stare at them and wish I was a real mathematician. Of course, I sit and stare at a lot of math and wish I was a real mathematician.

13. Originally Posted by ragasssd
would hawking radiation be dangerous if the source was close by, like say a few million km? what effect would it have on us. and is it possible to deflect hawking radiation?

sagar
If you are worried about radiation from a black hole, the x-rays radiated by the accretion disc near the event horizon are a lot nastier.

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Can we believe anything we read about black holes? Energy escapes black holes by thermal radiation, not virtual particles nor infra red photons, but black body radiation such as our sun produces? Something is wrong here? The photosphere of our sun radiates mostly visible light and infrared with a small amount of other photons, neutrinos, protons, electrons, positrons, helium nuclei and traces of other particles. Is hawking radiation ionizing on human skin? Perhaps hawking radiation is all of the above? Neil

15. Originally Posted by neilzero
It depends where you read it; e.g. if it is in the ATM forum of BAUT then probably not.

Energy escapes black holes by thermal radiation, not virtual particles nor infra red photons, but black body radiation such as our sun produces? Something is wrong here?
I'm not sure what you think is wrong. Thermal radiation is (or includes) infra-red photons - depending on the temperature. Virtual particles cannot (as far as I know) carry energy away.

Black body radiation consists of a mixture of frequencies (again, depending on the temperature of the bady). It is a description of a spectrum, rather than a "type" of radiation.

The photosphere of our sun radiates mostly visible light and infrared with a small amount of other photons, neutrinos, protons, electrons, positrons, helium nuclei and traces of other particles.
Of these, only the "visible light and infrared with a small amount of other photons" would be considered (part of the) black body radiation (I would dispute the "small amount", though). The other stuff gets belted out by other processes.

Here is a nice diagram/explanation of blackbody radiation at various temperatures: http://cseligman.com/text/sun/blackbody.htm

Is hawking radiation ionizing on human skin?
If the black hole is small enough that contains significant amounts of gamma radiation.

Perhaps hawking radiation is all of the above?
Only photons, as far as I know.

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