# How difficult to observe Hawking radiation?

humy
Science 03 Apr '19 17:15
1. 03 Apr '19 17:159 edits
I know that it must be relatively difficult to observe Hawking radiation from a natural black hole in space because so far none has ever been detected.
But lets say there was a black hole with the mass of our Sun but it was just, say, only and exactly one light year away from Earth so to make it much easier to detect any Hawking radiation coming from it. Then, using our current technology (on or around Earth) and assuming Hawking radiation exists and is emitted at an intensity and at wavelengths exactly as predicted by the most accepted current theories on how Hawking radiation behaves, would we be able to directly detect it? -if not, can any physicist here hazard a wild guess about how long would it be until we finally have the technology to detect it?

Another question;
Which wavelengths would, in theory, Hawking radiation mainly consist of and thus which wavelengths would we have to try and detect with our detectors to have any hope of detecting it?
2. sonhouse
Fast and Curious
03 Apr '19 18:02
@humy said
I know that it must be relatively difficult to observe Hawking radiation from a natural black hole in space because so far none has ever been detected.
But lets say there was a black hole with the mass of our Sun but it was just, say, only and exactly one light year away from Earth so to make it much easier to detect any Hawking radiation coming from it. Then, using our curren ...[text shortened]... ich wavelengths would we have to try and detect with our detectors to have any hope of detecting it?
https://www.sciencedaily.com/releases/2018/09/180918154831.htm

This piece says the peak wavelength is 16 times the Schwarzchild radius, which must mean the wavelength is measured in seconds or minutes per cycle. If that is true it will be very difficult indeed to detect with our technology.
3. 03 Apr '19 19:286 edits
@sonhouse said
which must mean the wavelength is measured in seconds or minutes per cycle.
Oh damn! Yes, that certainly would indeed make it VERY difficult to detect!
It means no radio telescope dish can concentrate that signal to its center because we cannot possibly make one wide enough; at least not unless we make one something like the diamater of our solar system!
Has anyone here got any ideas of how, at least purely in theory (but without making a ridiculously large radio dish telescope), such a long wavelength radio waves could be detected? I guess we can completely rule out ever using radio telescope dishes for that.
4. sonhouse
Fast and Curious
04 Apr '19 15:281 edit
@humy said
Oh damn! Yes, that certainly would indeed make it VERY difficult to detect!
It means no radio telescope dish can concentrate that signal to its center because we cannot possibly make one wide enough; at least not unless we make one something like the diamater of our solar system!
Has anyone here got any ideas of how, at least purely in theory (but without making a ridiculously ...[text shortened]... es could be detected? I guess we can completely rule out ever using radio telescope dishes for that.
Actually🙂 as it happens, I have an idea on that front. It has to do with gravitational lensing.
I worked out a few years ago the sun can focus very low frequency radio waves.
Of course there is that pesky detail you need to go out some 200+ billion km out in space but once there you can use instruments that detect focused neutrinos for instance, but also extreme low frequency radio waves.
It turns out there is a wavelength that at low enough wavelength the sun cannot further focus those waves and that is roughly the size of twice the diameter of the sun or roughly 2 million km. That wavelength is about 6 seconds per cycle.
So perhaps that is one way. One second per cycle represents a wavelength of about 300,000 km.
One point I worked out is suppose you have this super science team with spacecraft capable of going millions of times the speed of light and such. So you have two craft, say a light day apart on each side of a star.
So you start transmitting an RF signal sweeping down from some high frequency, say one Ghz, sweeping down in frequency and you have the capability to generate extreme low frequency waves. So one ship starts transmitting a signal, a day later the other ship, being in the focal line of that star picks up that one Ghz signal and then starts to build a chart of frequencies as the wavelength goes up and up in length.
At some point the frequency transmitting will fall off at the receiver side because stars have some lower frequency they can no longer focus simply because at a sufficiently long wavelength it would be like a wave on a pond going around a rock unimpeded because the peak to peak distance of the wave is just too big for the star to focus.
The result of that is you can use that data to calculate the mass of the star pretty accurately independent of other methods.
Of course that is just a thought experiment, there would be other methods but this is a new one as far as I know, have not read anything like that before.
When I get off my ass I plan to finish my book on that subject.
5. DeepThought
04 Apr '19 15:42
@humy said
I know that it must be relatively difficult to observe Hawking radiation from a natural black hole in space because so far none has ever been detected.
But lets say there was a black hole with the mass of our Sun but it was just, say, only and exactly one light year away from Earth so to make it much easier to detect any Hawking radiation coming from it. Then, using our curren ...[text shortened]... ich wavelengths would we have to try and detect with our detectors to have any hope of detecting it?
Essentially impossible. The rate of emission is very small - unless the black hole is tiny - and radiation from infalling matter will simply swamp the signal.
6. sonhouse
Fast and Curious
04 Apr '19 19:582 edits
@deepthought said
Essentially impossible. The rate of emission is very small - unless the black hole is tiny - and radiation from infalling matter will simply swamp the signal.
Not necessarily. Due to the extreme low frequency of the emission, it should be easy to filter the very low frequencies from any other, radio, visible light, IR, UV, and the like. We are talking about seconds per cycle here, not Ghz or above.
If you designed say a loop antenna, where you have one Farad in series or parallel with 1 Henry, the design frequency center would be about 6 seconds per cycle, or a wavelength of near 2 million km. Now with a loop like that, there would be essentially zero response to gamma, visible light, UV, and the like. It is not like you have to be using some kind of visual filter on one nanohertz response band in an optical scope. Of course the actual signal level would be a problem but like I said, the sun can concentrate such waves, with some low frequency limit. A bit of a problem for space transport but it could be done eventually if someone wanted to do it badly enough.
7. DeepThought
06 Apr '19 12:19
@sonhouse said
Not necessarily. Due to the extreme low frequency of the emission, it should be easy to filter the very low frequencies from any other, radio, visible light, IR, UV, and the like. We are talking about seconds per cycle here, not Ghz or above.
If you designed say a loop antenna, where you have one Farad in series or parallel with 1 Henry, the design frequency center would be ...[text shortened]... problem for space transport but it could be done eventually if someone wanted to do it badly enough.
A solar mass black hole has a radius of the order of 10km (from memory), so assuming a wavelength of 100km we get a frequency of the order of 300Hz.

The Wikipedia page on Hawking radiation has an estimate of the power output of a Solar mass black hole, assuming all the emitted radiation is electromagnetic. The figure they give there is 9E-29 Watts. You are simply not going to be able to detect that from 3,000 light years after it's propagated through a plasma.
8. sonhouse
Fast and Curious
06 Apr '19 19:081 edit
@deepthought said
A solar mass black hole has a radius of the order of 10km (from memory), so assuming a wavelength of 100km we get a frequency of the order of 300Hz.

The Wikipedia page on Hawking radiation has an estimate of the power output of a Solar mass black hole, assuming all the emitted radiation is electromagnetic. The figure they give there is 9E-29 Watts. You are simply not going to be able to detect that from 3,000 light years after it's propagated through a plasma.
Not a huge signal for sure😉 But the paper I think you found for me about gravitational lensing shows a gain of over 100 DB so at the right place, a probe, say at 1000 Au or so at a far focal line, and if you are right about the frequency of 300 Hz, we are talking of a wavelength of 1 Km or so and a half wave antenna would be 500 meters and you could in theory put up in space with no or extremely low gravity dozens of them to make an antenna with more than 30 db of gain, a Yagi in space. Then with 100 db from the sun, 130 db of gain. 2E-29 times ten trillion, You are up to a signal of 2E-16 watts. You could easily in space construct a yagi for 300 Hz of hundreds of elements and get a gain of 60 Db or more. Of course you would want to do that awfully bad to go to all that trouble to listen to the sound of Hawing. If converted to an audio signal it would come out as D4 plus 37 cents, it would be clearly heard by human ears if the signal could be boosted enough.

One question though, suppose we go to great lengths to actually suss out that signal, what have we gained scientifically speaking? Isn't Hawking radiation pretty much accepted?
One thing I see, if a black hole gets smaller, the wavelength of HR would also get smaller so you can see where more energy would be emitted at the smaller holes and therefore a lower lifespan of those black holes. So if you had a wavelength of say one micron, IR, I wonder if you could ever see that with a good space telescope tuned for IR and a BIG mirror?
9. 06 Apr '19 19:48
@sonhouse said
Isn't Hawking radiation pretty much accepted?
Actually, no. Although the Hawking radiation THEORY is accepted as a perfectly valid scientific theory, Hawking radiation EXISTENCE certainly isn't accepted by all because we have yet to have any direct and/or conclusive evidence of it.
10. sonhouse
Fast and Curious
07 Apr '19 15:56
@humy said
Actually, no. Although the Hawking radiation THEORY is accepted as a perfectly valid scientific theory, Hawking radiation EXISTENCE certainly isn't accepted by all because we have yet to have any direct and/or conclusive evidence of it.
Don't we have evidence black holes are losing energy/mass?
11. DeepThought
07 Apr '19 16:512 edits
@sonhouse said
Don't we have evidence black holes are losing energy/mass?
When LIGO detected a black hole merger a considerable amount of energy was emitted in the form of gravitational waves.

Edit: Although of course this is not Hawking radiation. The rate of infall of cosmic microwave background radiation exceeds this for any solar mass or heavier black hole. So all known black holes must be accreting mass rather than shrinking.
12. DeepThought
07 Apr '19 16:57
@sonhouse said
Not a huge signal for sure😉 But the paper I think you found for me about gravitational lensing shows a gain of over 100 DB so at the right place, a probe, say at 1000 Au or so at a far focal line, and if you are right about the frequency of 300 Hz, we are talking of a wavelength of 1 Km or so and a half wave antenna would be 500 meters and you could in theory put up in spac ...[text shortened]... , IR, I wonder if you could ever see that with a good space telescope tuned for IR and a BIG mirror?
No, this won't work. First, the wavelength is of the order of 100km not 1km, but the precise frequency doesn't matter. The total power output of the black hole due to Hawking radiation is 9E-29 Watts. I took a quick look at Wikipedia to find the nearest black hole candidate and it is 3,000 lightyears away. So even if the gravitational lens could focus all the light the black hole were emitting onto your detector the total power received cannot exceed 9E-29 Watts.

Hawking radiation need not only be emitted as photons. Ultra-relativistic electrons are also possible. If it can be shown that no other natural process can produce an electron with an energy so high then detection of such a particle would be a "smoking gun". But again it is difficult to rule out other potential sources.
13. sonhouse
Fast and Curious
07 Apr '19 23:40
@deepthought said
No, this won't work. First, the wavelength is of the order of 100km not 1km, but the precise frequency doesn't matter. The total power output of the black hole due to Hawking radiation is 9E-29 Watts. I took a quick look at Wikipedia to find the nearest black hole candidate and it is 3,000 lightyears away. So even if the gravitational lens could focus all the light th ...[text shortened]... a particle would be a "smoking gun". But again it is difficult to rule out other potential sources.
What is the method by which it can make relativistic electrons and very low frequency RF? Seems like two ends of the stick. Also, suppose we could get say a couple light hours away from that black hole, would it still be possible to directly detect the Hawking radiation?
14. 08 Apr '19 08:453 edits
I have got another question;
Lets say we are in easy space-travelling distance from a black hole ( Yikes! ) ;
Would it then be possible to make a 'black hole satellite' probe go around the black hole with a highly elliptical orbit so at its closest approach to it it is only, say, a meter above its event horizon, and then, at that closest approach, it can be made to detect Hawkings radiation while it has relatively shorter wavelengths, and then, after it has recorded that detection on computer memory, the black hole satellite can transmit that information of detection to you from when it is much further away from the black hole (else the transmited satellite signal will have a wavelength stretched too long for detection) ?
15. sonhouse
Fast and Curious
08 Apr '19 17:221 edit
@humy said
I have got another question;
Lets say we are in easy space-travelling distance from a black hole ( Yikes! ) ;
Would it then be possible to make a 'black hole satellite' probe go around the black hole with a highly elliptical orbit so at its closest approach to it it is only, say, a meter above its event horizon, and then, at that closest approach, it can be made to detect Hawk ...[text shortened]... ole (else the transmited satellite signal will have a wavelength stretched too long for detection) ?
Actually, the frequency shift has already been dealt with in space probes but of course for much lower frequency shifts, there was a case where a probe lost the ability to change frequencies and we needed to send commands to it in spite of doppler shifts. So they computed the shift in frequency so that the signal was recognized by the probe and it went into a fixing mode and they were able to recover functionality. Of course if the frequency change went from 10 Ghz to 1 mhz, it might be a bit out of range for correction but within limits that sort of thing can be dealt with.
Of course there is that pesky detail of the nearest BH being 3000 light years away.....