hawkings radiation

Reading the above article on hawkings radiation, I got the distinct inpression that what the guy was saying was that a black hole even though it is a black hole it still is made of either completely matter or antimatter and that by adding antimatter you could “evaporate” it. Am I understanding this correctly.

Lets say that in the future we are able to produce antimatter by the mega tons and if we fed it to a black hole could we create a naked singularity? If so could we create end the universe? What about if you fed it only anti protons or antineutrons or positons (perhaps misassuming that matter in a black hole still exists in these states) would we create like a unbalanced black hole? like a black hole that is made completely of electrons?

The whole point of calling the core of a black hole a “singularity” is that our current physics cannot explain what is happening there. When Chronos says that speculation about a “naked singularity” bringing about the end of the universe is a job for quantum gravity, he means that until such a comprehensive theory of physics exists nothing meaningful can be said about what will happen. All you can do is speculate wildly, with no real justification behind it.

But I’m sure that in a few minutes there will be any numbers of Dopers flocking here, ready and eager to do just that. :slight_smile:

The passage to which you’re referring to was added by Ed Zotti, based on his reading of an encyclopedia article. The answer is currently being revised (or will be, as soon as Ed reads his e-mail). The best source you can get on the subject is Hawking’s book A Brief History of Time, where he’s able to go into much more detail than I had room for.

But to answer the question, black holes don’t care what they’re made of. The matter-antimatter distinction is meaningless for a black hole. All that a black hole has is mass, angular momentum, electric charge, and (if it exists) possibly magnetic charge. So if you feed a black hole a bunch of electrons, you’ll get a negatively charged black hole (but as you put them in, it would eventually become more difficult or even impossible to add more electrons). If you then added protons, the charge would be neutralized again. The result would be identical if you instead added the same amount of positrons (antielectrons) and antiprotons.

And the part about a naked singulariy causing Big Problems is pretty well accepted; the part that calls for quantum gravity is deciding whether there would be a naked singularity in the first place.

Not antimatter, which despite it’s somewhat misleading name has mass just like ordinary matter.[sub]1[/sub]

What you might be thinking of is that theoretically you could unmake a black hole if you could feed enough negative mass or energy into it. Which is one reason some physicists are pretty certain that negative energy can’t be produced except as temporary and limited quantum fluctuations.

[sub]1. As far as we know anyway. Physicists are trying hard to obtain enough antimatter to be able to directly measure it’s response to gravity, to make sure of this.[/sub]

Antimatter’s indirect response to gravity has been thoroughly measured, though, and it behaves exactly like matter. And unmaking a black hole is the least of one’s worries concerning negative matter. You could also use negative matter to construct a time machine or FTL drive. Which, of course, raises its own questions.

Can a black hole’s charge or magnetic field extend outside the Swartzchild radius? After all, it’s just an electromagnetic field and photons are made of the same stuff.

<< You could also use negative matter to construct a time machine or FTL drive. >>

If God had meant us to invent time machines, He wouldn’t have given us clocks.

No, since the escape velocity of a black hole is greater than c.

Chronos: (and every one else too) I’m not sure i quite understood your explanation about black holes. You said that in a black hole the matter=antimatter distinction did occur in a black hole, right? The way you explained it didn’t make sense to me because you were talking about electrons and protons. But electrons are not the antiparticles to protons.

So what would happen if you injected, if you will, a lot of say antiprotons in a black hole that mostly consist of normal matter. Would the matter and the antimatter coexist? Is there a line that when crossed the matter antimatter distinction no longer occurs or matters? like say the event horizon? And lets say matter and antimatter met just before that line and the two particles anihilated each other, would the energy still be sucked in?

Anyway getting back to your explanation, what Q.E.D said above changed my perspective a bit about what you said concerning protons and electrons which have opposite charges. He (or she) said that the electric field could not escape the black hole. But if this is true how would the lets say necgative charge of a black hole, no matter how intense prevent more negatively charged electrons from entering?

If the electric field cannot escape, any electrical repulsion would not occur until said electron was completely absorbed by the black hole, way past the event horizon. So no matter the charge inside the black hole, the electron would not be able to escape unless the repulsion inside would sent the elctron flying out of the black hole at +c. But +c is not possible at least in normal space. would it be possible inside a black hole?

I really apreciate all your input. I guess a good thing to do intead of bugging you guys, is buying Hawkings book as you suggested. But I still would apreciate a responce, thanks.

I meant not, does not occur. How come one can’t edit his or her own posts?

The electromagnetic field of a black hole can, in fact, extend beyond the event horizon. Real photons can’t escape, but virtual photons can. However, the only information that the virtual photons can carry is how much charge there is inside. If the possibility that electric fields can escape disturbs you, consider that the gravitational field most definitely is able to escape, and gravity and electromagnetism aren’t really all that different.

And black holes really don’t care about the matter-antimatter distinction. If you make a black hole out of, say, three solar masses worth of hydrogen, and then add three more solar masses of antihydrogen, you’ll get a six solar mass black hole. You’d get the exact same thing if you started with six solar masses of hydrogen, or with six solar masses of antihydrogen.

We don’t know that. It has been posited that the force of gravity is transmitted by particles called gravitons in the same manner that the electromagnetic force is transmitted by photons, but to date no empirical evidence exists to demostrate their existence.

Also, virtual photons are emitted in photon-antiphoton pairs, which annihilate before being able to be detected. Which is why Hawking theorized that very tiny micro black holes would be able to evaporate away.

<< How come one can’t edit his or her own posts? <>>
Because we’ve had problems in the past when we permitted this, with people making some absurd statement, waiting for responses, then editing their post to say something different. It’s more of a problem with the other forums, but we decided not to allow this feature.

If you’ve made a real bonehead typo and would like a Moderator or Administrator to fix it, you can email and ask pretty-please and we’ll do our best to comply.

Thanks ck dexter, I’ll keep that in mind for later. Although, I asked the webmaster if I could change my user name and I got no response.

QED: I just read an article this weekend of how they had proven that gravity travels at the speed of light. They measured how Jupiter affected the light from a quasar. Should we not (more specifically wouldn’t you) consider this empirical evidence?

My guess why it physicists could not measure the speed of gravity fields as easily as we can measure say an electromagnetic field is because you cannot control a gravity field. We cannot just turn on or off a gravity field as we might be able to do with an electromagnetic field.

To whoever may read this, can any of you recommend or know of any online physics books or articles? I figure my knowledge of physics is pretty basic and i figure if i find one, I wouldn’t have to bug this board with every detail. Those that are free I would be more intrested in, although I wouln’t mind paying some sort of fee.

I want to thank Chronos for his excellent report. I find the subject of black holes fascinating, although there are a few things I still don’t quite get. This question is about one of them.

I’m trying to understand how a black hole would lose mass through Hawking’s radiation. So you have a black hole. Virtual particle/anti-particle pairs appear randomly throughout vacuum, including near the event horizon of this black hole. One set appears too close, and one particle goes into the event horizon, and the other goes away. [Say, for the sake of argument, the one that goes in is a positron - an anti-electron.] If I understand what Chronos said in his post above:

… then it doesn’t matter that it was a positron; the mass of the black hole is now [Old mass of black hole] + [mass of positron], which is greater (not less) than before. Even if it interacts with an electron within the black hole (assuming that interaction even makes sense in a black hole), then the released energy (which still has an equivalent mass) isn’t going to leave the event horizon, so the mass increase is still the same.

How can any particle appearing there lead to a reduction in mass (no matter how small) of the black hole? Obviously, I’m missing something basic, can someone tell me what?

Well, back when I was in grad school, and we had to make our water by hand, I could explain this. Let me see if I can now.

You can think of matter as a form of energy, and energy as a form of matter. The relationship is specified by Einstein’s famous E= m c^2 (that is "m c-squared). So when energy is radiated out from the black hole, the black hole has lost the amount of mass specified in that equation.

I have a question about the charge of a black hole. For some reason, I was pondering this very question a few days ago, and how convenient for someone else to ask it. First, to me the “existence” of virtual photons is somewhat nebulous. The arise in what are called “perturbation expansions” in quantum field theory. Basically, you start with an exact solution, and consider small deviations, or perturbations, from that. So they definitely exist as mathematical entities, but I really don’t know what it means to exist.

Consider, though, that every electron is emanating an electric (and gravitational) field as it crosses the event horizon. Any photon carrying the electrostatic field, that is leaving the electron, as it crosses the event horizon, is infinitely red shifted. But, a static field has no time dependence anyway, so what does the red shift mean? Is the electric field carried by photons that were emanated when the electron crossed the event horizon really the carriers of the electric field? Or are virtual photons crossing the event horizon and propagating to infinity (and beyond! - sorry Toy Story humor)???

A virtual particle pair zaps into existence, and one of the particles falls into the black hole while the other flies away. That is Hawking radiation. While the black hole does absorb one particle, the net effect is that a new particle flew away from the black hole, and its mass has to come from somewhere.

I’m still with NE Texan on this one, brunthilda - both the virtual particles have mass, correct? Now, if one flies away and the other gets eaten. The one which flew away gives the effect of the black hole radiating, and apparently losing mass (I might not be right on that one), but the one which disappeared across the event horizon has added mass to the BH.

Actually, thinking further about it I wonder if the problem here is that I’m imagining the virtual particles to be different (or behave differently) from how they really are.

I’ve got a much more basic question about the column. I get that it answered the basic question - if a black hole loses mass, does it un-become a black hole? The answer was a clear no - it remains a black hole.

I didn’t see anything that explained why that is, though. Gravity is directly proportional to the amount of mass - if mass decreases, won’t the gravitational pull be decreased until the escape velocity is less than the speed of light? In other words, why won’t light start escaping?

SlowMindThinking: you are thinking that the virtual photon is created inside the black hole and radiates away, but it’s not. It’s created just before the event horizon by space.

  • bruntilda: You are thinking in terms of conservation of mass + energy. How ever virtual photos are supposed to be a violation of the principle of conservation of mass and energy. They really are created out of nothing. So the creation of the virtual photon does not(or should not as far as my thinking goes) take away from the black holes mass. If anything it should add to it, since one particle falls into the bh.