# Hawking radiation **URL:** \ **Category:** Factual Questions\ **Created:** [March 15, 2010, 5:30pm UTC](https://boards.straightdope.com/t/hawking-radiation/532560 "2010-03-15T17:30:46Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [March 16, 2010, 3:26am UTC](https://boards.straightdope.com/t/hawking-radiation/532560/21 "2010-03-16T03:26:19Z")
When a virtual pair is created near the EH of a black hole the hole’s extreme gravitational gradient can pull them apart with such force, that they gain enough energy to both repay their energy debt to the universe and transform them into real particles. If one of the now real particles is captured by the hole, and the other escapes, then the hole has expended enough energy to create two real particles, but only got the energy of one them back. Thus it loses mass. ---
**Author:** ![Cheshire\_Human](https://avatars.discourse-cdn.com/v4/letter/c/5f8ce5/32.png) [@Cheshire\_Human](https://boards.straightdope.com/u/Cheshire_Human)\ **Post date:** [March 16, 2010, 3:57am UTC](https://boards.straightdope.com/t/hawking-radiation/532560/22 "2010-03-16T03:57:20Z")
> [@Chronos](#): > > … we’re nowhere near having telescopes capable of observing Hawking radiation directly… Very true, but you would be amazed at what astronomers these days can infer from the effects that something we can’t see has on something we can. That’s how they are finding out the atmospheric composition of some of the exoplanets they’ve found recently. When the next generation scopes come on line, if they CAN see the event horizon of one of the black holes we know about, who knows. They might see something that implies that it must exist. That’s only speculation, but some astronomers have actually suggested it as a possibility, though unlikely. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [March 16, 2010, 5:32am UTC](https://boards.straightdope.com/t/hawking-radiation/532560/23 "2010-03-16T05:32:56Z")
Eh, for a stellar-mass hole or larger, Hawking radiation is significantly cooler than the cosmic microwave background. Unless we discover a small primordial black hole, I don’t really see any way it could be detected even indirectly. ---
**Author:** ![OtakuLoki](https://avatars.discourse-cdn.com/v4/letter/o/ac8455/32.png) [@OtakuLoki](https://boards.straightdope.com/u/OtakuLoki)\ **Post date:** [March 16, 2010, 5:58am UTC](https://boards.straightdope.com/t/hawking-radiation/532560/24 "2010-03-16T05:58:24Z")
I freely admit that my knowledge of the math behind the physics, and thus my understanding of the physics, is sorely limited. I thought it was possible for a pair produced during such an event to both be consumed by the black hole, while still emitting photons due to Bremsstrahlung radiation, where a charged particle undergoes acceleration and releases a photon? And if that is happening, the black hole would be losing mass in supplying the energy for that photon, even though the black hole then consumed both members of the particle pair produced. Is this a correct understanding, a plausible one, or have I completely missed the point about Bremsstrahlung radiation? ---
**Author:** ![Cheshire\_Human](https://avatars.discourse-cdn.com/v4/letter/c/5f8ce5/32.png) [@Cheshire\_Human](https://boards.straightdope.com/u/Cheshire_Human)\ **Post date:** [March 16, 2010, 6:53am UTC](https://boards.straightdope.com/t/hawking-radiation/532560/25 "2010-03-16T06:53:30Z")
> [@Chronos](#): > > Eh, for a stellar-mass hole or larger, Hawking radiation is significantly cooler than the cosmic microwave background. Unless we discover a small primordial black hole, I don’t really see any way it could be detected even indirectly. You’re not an astronomer or an astrophysicist. I have come to expect great things (dare I say miracles) of their ingenuity in finding indirect methods for things that can’t be detected directly:D ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [March 16, 2010, 7:22am UTC](https://boards.straightdope.com/t/hawking-radiation/532560/26 "2010-03-16T07:22:51Z")
Actually, my bachelor’s degree was in astronomy and astrophysics. Trust me when I say that astronomers can’t measure temperatures of objects to a precision of better than one part in a million, which is what would be needed for any sort of indirect observation of Hawking radiation. And for any method I can actually think of, it’d be more like one part in a billion that would be needed. ---
**Author:** ![Mosier](https://avatars.discourse-cdn.com/v4/letter/m/7ea924/32.png) [@Mosier](https://boards.straightdope.com/u/Mosier)\ **Post date:** [March 16, 2010, 7:47am UTC](https://boards.straightdope.com/t/hawking-radiation/532560/27 "2010-03-16T07:47:25Z")
So gravity is energy? Or gravity _has_ energy? Why can’t we just think of Hawking radiation as "the gravity is so goddamned strong that some of it squeezes into matter outside the event horizon, and escapes? ---
**Author:** ![I\_Love\_Me\_Vol.I](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/i_love_me_vol.i/32/521_2.png) [@I\_Love\_Me\_Vol.I](https://boards.straightdope.com/u/I_Love_Me_Vol.I)\ **Post date:** [March 16, 2010, 8:54am UTC](https://boards.straightdope.com/t/hawking-radiation/532560/28 "2010-03-16T08:54:35Z")
The first thing I thought when I saw the thread title was: “Radiation! Get your radiation here! Only 10 cents a rad! Radiation!! Fresh from the fission factory. Step on up and get your raaaaaaaaay-dee-a-shun while it’s hot!” Sorry about that. ---
**Author:** ![fuzzypickles](https://avatars.discourse-cdn.com/v4/letter/f/9d8465/32.png) [@fuzzypickles](https://boards.straightdope.com/u/fuzzypickles)\ **Post date:** [March 16, 2010, 1:53pm UTC](https://boards.straightdope.com/t/hawking-radiation/532560/29 "2010-03-16T13:53:35Z")
> [@Chronos](#): > > A graviton is a massless particle similar in many ways to a photon, and there’s no hope of us ever detecting individual gravitons. Pardon my intrusion (I’m hardly an expert on this subject) but aren’t gravitons still purely theoretical? ---
**Author:** ![Cheshire\_Human](https://avatars.discourse-cdn.com/v4/letter/c/5f8ce5/32.png) [@Cheshire\_Human](https://boards.straightdope.com/u/Cheshire_Human)\ **Post date:** [March 16, 2010, 2:20pm UTC](https://boards.straightdope.com/t/hawking-radiation/532560/30 "2010-03-16T14:20:35Z")
> [@Chronos](#): > > Actually, my bachelor’s degree was in astronomy and astrophysics. Ok, I will defer to your superior knowledge. I’m just an amateur, as I said. ---
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [March 16, 2010, 3:18pm UTC](https://boards.straightdope.com/t/hawking-radiation/532560/31 "2010-03-16T15:18:59Z")
> [@Cheshire\_Human](#): > > The energy of the very high gravity field near the event horizon is what forms virtual particles. If one is sucked into the black hole, the other can’t recombine, and so becomes a real particle, which takes energy away from the black hole’s gravitational field, which therefore takes away some mass from the black hole. **Cheshire Human** : This is just what **I** \* and **Balance** \*\* said earlier. - “I’ll note that the virtual particles have come into existence by “borrowing” energy from the vacuum. Normally, they give back that energy as they disappear out of existence. But, when one flies off, that’s not possible. So, the energy debt comes from the black hole instead (and energy=mass, so the black hole gets smaller)” \*\* “As Karl said above, virtual particle pairs form by “borrowing” energy from local space. (I prefer to think of it as condensation, but the “borrowing” concept works better for this.) The net effect on local gravitation is nil, since the energy was already present; it has merely changed forms. If the virtual particles form right at the event horizon of a black hole, they effectively “borrow” two particles’ worth of energy from the black hole. If tidal forces prevent the pair from annihilating, causing one particle to fall into the black hole while the other is flung away, the black hole only regains one particle worth of mass. That means it suffers a net loss of mass” ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [March 16, 2010, 4:47pm UTC](https://boards.straightdope.com/t/hawking-radiation/532560/32 "2010-03-16T16:47:55Z")
If two particles are created, then I suppose they must be pretty close together. Even if the tidal forces yank them farther apart, I still don’t see how one is sucked into the gravity well of the black hole, and yet the other one somehow goes shooting off away from the black hole. Why isn’t the other one also captured by the black hole, just not quite as fast? Also, when does particle become something that we would call “radiation,” which I usually think of as being photons. > [@I\_Love\_Me\_Vol.I](#): > > The first thing I thought when I saw the thread title was: > > “Radiation! Get your radiation here! Only 10 cents a rad! Radiation!! Fresh from the fission factory. Step on up and get your raaaaaaaaay-dee-a-shun while it’s hot!” It was funny the [first](http://boards.straightdope.com/sdmb/showpost.php?p=7802515&postcount=7) [few](http://boards.straightdope.com/sdmb/showpost.php?p=8883982&postcount=2) [times](http://boards.straightdope.com/sdmb/showpost.php?p=11252330&postcount=9) :rolleyes: ---
**Author:** ![OtakuLoki](https://avatars.discourse-cdn.com/v4/letter/o/ac8455/32.png) [@OtakuLoki](https://boards.straightdope.com/u/OtakuLoki)\ **Post date:** [March 16, 2010, 5:14pm UTC](https://boards.straightdope.com/t/hawking-radiation/532560/33 "2010-03-16T17:14:48Z")
> [@CookingWithGas](#): > > Also, when does particle become something that we would call “radiation,” which I usually think of as being photons. In my experience if it was emitted as a result of some nuclear interaction it’s going to be described as radiation, whether it’s an alpha particle, beta particle, gamma ray, or a neutron. I’m sure you have other fields that will talk about neutrinos as being radiation, as well, but rad health physics finds it hard to care about something that interacts so minutely with other matter. I don’t know of any specific energy value for when an electron (or positron) stops being a beta (beta positive) ray, and reverts to being simply an electron - but I’m sure there are several definitions out there. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [March 16, 2010, 5:23pm UTC](https://boards.straightdope.com/t/hawking-radiation/532560/34 "2010-03-16T17:23:37Z")
> [@](#): > > Pardon my intrusion (I’m hardly an expert on this subject) but aren’t gravitons still purely theoretical? I would say that we’re pretty confident they exist, and we know a few of their properties, but we’re still extremely hazy on precisely how they interact (with each other and with other particles). You can consider a gravitational wave to be a stream of a great many gravitons (in much the same way that a light wave is a stream of many photons), and we’re within a few years of detecting those, but I expect that we’ll never be able to detect individual gravitons. So yes, they’re theoretical, but well-grounded enough that I feel justified in talking about them. > [@](#): > > If two particles are created, then I suppose they must be pretty close together. Even if the tidal forces yank them farther apart, I still don’t see how one is sucked into the gravity well of the black hole, and yet the other one somehow goes shooting off away from the black hole. Why isn’t the other one also captured by the black hole, just not quite as fast? Usually, it is. It’s a very rare case indeed where one of them manages to escape. But it does happen every so often. And there’s nothing unusual about calling particles “radiation”. Alpha radiation is composed of helium nuclei, and beta radiation is composed of electrons. And for that matter, most of the particles we’re talking about here are photons, anyway. ---
**Author:** ![Cheshire\_Human](https://avatars.discourse-cdn.com/v4/letter/c/5f8ce5/32.png) [@Cheshire\_Human](https://boards.straightdope.com/u/Cheshire_Human)\ **Post date:** [March 17, 2010, 3:01pm UTC](https://boards.straightdope.com/t/hawking-radiation/532560/35 "2010-03-17T15:01:22Z")
> [@KarlGauss](#): > > **Cheshire Human** : This is just what **I** \* and **Balance** \*\* said earlier. Yes, I’m aware of that, but some people were seeming not to understand your explanation, so I was trying to explain the same thing, in a different manner, in the hopes that it would aid those who failed to “get” your explanations. Things can be explained in many different ways, all technically correct, but some people “get it” one way, but not another. ---
**Author:** ![sweeteviljesus](https://avatars.discourse-cdn.com/v4/letter/s/898d66/32.png) [@sweeteviljesus](https://boards.straightdope.com/u/sweeteviljesus)\ **Post date:** [March 22, 2010, 6:24pm UTC](https://boards.straightdope.com/t/hawking-radiation/532560/36 "2010-03-22T18:24:31Z")
Inspired by another thread, I also want to ask if a particle could teleport itself back out of the black hole. I expect that the answer is no, but it doesn’t depend on the particle reaching escape velocity. Thanks, Rob ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [March 22, 2010, 9:32pm UTC](https://boards.straightdope.com/t/hawking-radiation/532560/37 "2010-03-22T21:32:31Z")
What do you mean, “teleport”? The phenomenon referred to as quantum teleportation still requires that information be transmitted, which can’t be done from inside the horizon to outside. And escape speed is irrelevant here, anyway. A black hole is sometimes described as an object where the escape speed is c, but that’s an incorrect description, and it’s only coincidence that it gives you the right value for the Schwarzschild radius (there are a couple of factors of 2 that happen to cancel out). Escape speed only applies to things that are given an initial speed and are thereafter propelled, but it’s impossible to get out of a black hole even with propulsion. ---
**Author:** ![fuzzypickles](https://avatars.discourse-cdn.com/v4/letter/f/9d8465/32.png) [@fuzzypickles](https://boards.straightdope.com/u/fuzzypickles)\ **Post date:** [March 22, 2010, 11:12pm UTC](https://boards.straightdope.com/t/hawking-radiation/532560/38 "2010-03-22T23:12:19Z")
> [@Chronos](#): > > …it’s impossible to get out of a black hole even with propulsion. I’ll bet Superman could do it. ---
**Author:** ![sevenwood](https://avatars.discourse-cdn.com/v4/letter/s/6de8d8/32.png) [@sevenwood](https://boards.straightdope.com/u/sevenwood)\ **Post date:** [March 22, 2010, 11:56pm UTC](https://boards.straightdope.com/t/hawking-radiation/532560/39 "2010-03-22T23:56:14Z")
> [@fuzzypickles](#): > > I’ll bet Superman could do it. Or Chuck Norris. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [March 23, 2010, 12:06am UTC](https://boards.straightdope.com/t/hawking-radiation/532560/40 "2010-03-23T00:06:20Z")
Typo in above: > [@](#): > > Escape speed only applies to things that are given an initial speed and are thereafter propelled, but it’s impossible to get out of a black hole even with propulsion. Should be > [@](#): > > Escape speed only applies to things that are given an initial speed and are thereafter _ **un** _propelled, but it’s impossible to get out of a black hole even with propulsion. [Previous page](https://boards.straightdope.com/t/hawking-radiation/532560.md?page=1) [Next page](https://boards.straightdope.com/t/hawking-radiation/532560.md?page=3)