As Chipper Q dug up from that Living Review article, loop quantum gravity can reproduce the area-entropy formula for black holes. If you've got that, then you've got Hawking radiation right too. But it's not the triumph it sounds like.

First, it seems that any sensible microscopic theory comes up with those. In hindsight that makes sense, since Hawking et al got those results back in the 1970s before LQG or string theory were really around. All they needed were very general arguments.

Second, although every theorist I know of believes the area-entropy and radiation results - they have been derived several different ways, and it seems you would need to break some well-tested theories to not get them - they themselves have not been directly tested by experiment or observation. We have a strange situation right now where a lot of theorists have been working hard on various approaches to quantum gravity for ages and have all sorts of mathematically nice results that are very suggestive, but nothing that can be empirically tested.

Yet. Some of these people have been working lately on how various quantum gravity scenarios might affect the background of gravitational waves in the present universe. The bad news is that it's almost certainly too weak to observe with LIGO. It would be there, but probably obscured by a background from more recent events like supernovae throughout the observable universe. But it might be observable at very long wavelengths with a pulsar timing array.

As Chipper Q dug up from that Living Review article, loop quantum gravity can reproduce the area-entropy formula for black holes. If you've got that, then you've got Hawking radiation right too. But it's not the triumph it sounds like.

First, it seems that any sensible microscopic theory comes up with those. In hindsight that makes sense, since Hawking et al got those results back in the 1970s before LQG or string theory were really around. All they needed were very general arguments.

Second, although every theorist I know of believes the area-entropy and radiation results - they have been derived several different ways, and it seems you would need to break some well-tested theories to not get them - they themselves have not been directly tested by experiment or observation. We have a strange situation right now where a lot of theorists have been working hard on various approaches to quantum gravity for ages and have all sorts of mathematically nice results that are very suggestive, but nothing that can be empirically tested.

Yet. Some of these people have been working lately on how various quantum gravity scenarios might affect the background of gravitational waves in the present universe. The bad news is that it's almost certainly too weak to observe with LIGO. It would be there, but probably obscured by a background from more recent events like supernovae throughout the observable universe. But it might be observable at very long wavelengths with a pulsar timing array.

Hope this helps,
Ben

Believe it or not, I already knew all of that. (It's hard to be around physics departments these days without absorbing quite a bit about such issues.) However, what I was referring to regarding Hawking radiation (and this is born out by the article itself if you look deep enough into it) is that the last I knew LQG could get the right functional form for Hawking radiation, but couldn't get the right (well, accepted anyway) constant factor.

[...] the last I knew LQG could get the right functional form for Hawking radiation, but couldn't get the right (well, accepted anyway) constant factor.

I wouldn't say that. I admit I haven't paid attention to quantum gravity for a while, so I did a little checking up just now and here's what it looks like to me:

If you use the microscopic apparatus of LQG to compute the entropy of a (large) black hole, you get a constant times the semi-classical result that was derived in the 1970s. You fix that constant by demanding that it match the semi-classical result. (It turns out that that value of the constant makes other nice things happen too.)

To me that doesn't look like "a lot of trouble." It looks like what happens with other theories such as general relativity: Einstein worked out that this thing constructed from the curvature should be a constant times the stress-energy, and the constant is found by demanding that the theory reduce to Newtonian gravity in the right limit.

By the way, the review that Chipper Q linked to is almost a decade old. Here is a more up-to-date one, which mentions the horizon business starting around page 80. Even this one is almost two years old, and they have been working tremendously fast on applying the results to cosmology.

Still no results of the form "look for a bump like so in the gravitational wave spectrum."

## Hmm, where was I in this

)

Hmm, where was I in this thread?

Solomon,

As Chipper Q dug up from that Living Review article, loop quantum gravity can reproduce the area-entropy formula for black holes. If you've got that, then you've got Hawking radiation right too. But it's not the triumph it sounds like.

First, it seems that any sensible microscopic theory comes up with those. In hindsight that makes sense, since Hawking et al got those results back in the 1970s before LQG or string theory were really around. All they needed were very general arguments.

Second, although every theorist I know of believes the area-entropy and radiation results - they have been derived several different ways, and it seems you would need to break some well-tested theories to not get them - they themselves have not been directly tested by experiment or observation. We have a strange situation right now where a lot of theorists have been working hard on various approaches to quantum gravity for ages and have all sorts of mathematically nice results that are very suggestive, but nothing that can be empirically tested.

Yet. Some of these people have been working lately on how various quantum gravity scenarios might affect the background of gravitational waves in the present universe. The bad news is that it's almost certainly too weak to observe with LIGO. It would be there, but probably obscured by a background from more recent events like supernovae throughout the observable universe. But it might be observable at very long wavelengths with a pulsar timing array.

Hope this helps,

Ben

## RE: Hmm, where was I in

)

Believe it or not, I already knew all of that. (It's hard to be around physics departments these days without absorbing quite a bit about such issues.) However, what I was referring to regarding Hawking radiation (and this is born out by the article itself if you look deep enough into it) is that the last I knew LQG could get the right functional form for Hawking radiation, but couldn't get the right (well, accepted anyway) constant factor.

## Solomon wrote: RE:

)

Solomon wrote:

I wouldn't say that. I admit I haven't paid attention to quantum gravity for a while, so I did a little checking up just now and here's what it looks like to me:

If you use the microscopic apparatus of LQG to compute the entropy of a (large) black hole, you get a constant times the semi-classical result that was derived in the 1970s. You fix that constant by demanding that it match the semi-classical result. (It turns out that that value of the constant makes other nice things happen too.)

To me that doesn't look like "a lot of trouble." It looks like what happens with other theories such as general relativity: Einstein worked out that this thing constructed from the curvature should be a constant times the stress-energy, and the constant is found by demanding that the theory reduce to Newtonian gravity in the right limit.

By the way, the review that Chipper Q linked to is almost a decade old. Here is a more up-to-date one, which mentions the horizon business starting around page 80. Even this one is almost two years old, and they have been working tremendously fast on applying the results to cosmology.

Still no results of the form "look for a bump like so in the gravitational wave spectrum."

But we can hope-

Ben