h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

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An evaporating black hole does not go out with a flash, it blows up. This is due to the shrinking ofthe black hole. At some point, it passes the chandrasakar limit, and becomes unstable. almost immediately, it goes boom as it stops being a black hole and tries to be a sun occupying the same area as a black hole, which it can't do.

I disagree. As the hole gets smaller and smaller, more and more energy is output from it in the form of Hawking radiation until eventually the black hole is gone. This is normally described in all the books I have seen as "evaporating in a flash".

I think that we must haveread different books. how old was yours?

Hmmm,

Well. I've discovered refs to both on the web (believe me, I'd post the URLs here if I could; search for "hawking radiation" in Google). It seems that the black evaporates rather quickly when it occurs, but it's under debate whether it emits the energy in a flash of radiation (mostly gamma rays) or something more subtle occurs. The rate of energy loss is proportional to the inverse of the black hole's mass squared, so it can speed up quickly as the black hole gets smaller.

Unfortunately, it not really something we can check experimentally, since most astronomical black holes have a lifespan 10^16 times the age of the universe. The only black holes which would evaporate are those less than with 10^11 Kg of mass (would pull at 1 G from a distance of about a meter; also would be about 10^12 Kelvin). We'd apparently notice such an evaporation only if it happened within our solar system, which is not bloody likely.

Of course, I think String Theory also has some ideas concerning evaporating black holes and Hawking Radiation. Fascinating stuff, but I can't post more URLs here (Google: "string theory hawking radiation"): "Black hole physics is another area where string theory has had an impact. Black holes posed a paradox for physics. Information in an initial quantum state can be lost, thus violating principles of quantum mechanics, in the process of formation and subsequent evaporation of a black hole through Hawking radiation. It was shown that a string model of a black hole emits radiation in a fully consistent way, and that this radiation has the thermodynamic properties of Hawking radiation. This provides evidence that black holes in string theory will not suffer from the information paradox. Current work is focused on examining how the information may actually manage to leak out of the black hole through emitted radiation. "

Yours,
Jake

Hiya Jake, Gnomon and xyroth

Perhaps you could satisfy my curiosity on a point. I know nothing very much about Hawking radiation or black holes but your debate interests me.

Xyroth says an evaporating black hole does not go out with a flash, it blows up. Then Gnomon disagrees, saying the black hole evaporates in a flash. Xyroth counters that you must have read different books. Jake's reading suggests the matter's not settled one way or the other.

What I'm wondering is why, based on the reading of a book (possibly more than one), you might feel you can state categorically that a thing occurs in a very particular way, although it has never actually been observed. Is the written word so authoritative? It looks a little bit as though you're having a religious debate.

Sal

No, were not having a religious debate. What happens is that you have two people both interested in astronomy, both reading lots of books. these books are written at different times by different people.
What then happens is that they meet here (or anywhere for that matter) and suddenly find that they have conflicting information.
There are a number of ways to resolve this.

1, one of the books is a lot older than the other. in this case the more recent is likely to be true due to the rate of change in this very active science.

2, One book is written by someone like martin rees or stephen hawking, the other is written by someone relatively unknown (for example brian may from queen). In this case, hawking or rees are more likely to be right. (although brian may has been on the http://www.bbc.co.uk/skyatnight once or twice).

3, The science is changing almost daily, or is otherwise up in the air. in this case, you do your best, and finaly end up revisiting the topic months later.

I hope that this has clarified things a bit.

Thanks xyroth. I enjoy reading logical debate especially on subjects I know very little about. You can learn a lot that way. It's possible to come away from it with a more balanced view than listening to one person just explaining their interpretation of the "facts".

In subjects like this one that you're discussing here though, people often seem to treat theories as though they're proven facts and discuss them as though they know them to be rock-solid facts. It reminded me of a discussion on gm that took place here last year, where enough is unknown to give plenty of room for debate and, as seems to happen under those conditions, people were presenting what, in reality, were only their opinions, as facts. When that happens, the logic is left behind. I've heard and read a few religious debates that are conducted like that but then, with religion there's no chance of ever finding anything solid so it's bound to be open to argument of the form: "it says in the book that God said this", "yes, but he meant that and anyway, in chapter 2 it says something else entirely", "ah, but if you look in this other version of the bible your point is contradicted, so God definitely had so and so in mind".

I thought I'd spotted a bit of that sort of "logic" blossoming here. Was I mistaken?

Hello,

No, it's not a religious debate. Xyroth pretty much sums it up rather well. He does a good job for a vile infidel.

Yours,
Jake

Hiya Jake. Long time no chat. You've changed your nickname, haven't you? I think we had a near-simulpost there.

Are you able to explain this phenomenon of debaters presenting the latest (or indeed any) theory as an absolutely solid fact? I noticed you didn't do that, by the way. I'm speaking generally because there seems to be a lot of it about.

Sal

Hey Sal,

Yes. It's still me. I decided to change the name to something a bit punchier...

I don't think it's often a case of debaters consciously presenting theory as solid fact, but it's more a way that our minds work. We build models and revise them when we get better information. But we usually don't have the luxury of waiting until all the facts are known. And a lot of public policy is guided as much by ideology and irrational notions as it is with real logic. For example, the debate of GM food involves some serious concerns as well as some overblown fears. I think Europe might be skittish, but we're too overconfident and underinformed over here (we've had several scandals recently where business was supposed to regulate itself and didn't). Another example is food irradiation. Most of the scientific community is for irradiation to stop food poisoning, but we don't know the long-term effects and people have a real fear of radiation.

But I'm rambling. Back to a point. In this case, it's just what science does all the time. Almost everything in science is built on theory, but that doesn't stop progress from moving forward. For instance, chemistry has seen the mathematics of the atom go through several complete revisions in its lifetime, but there are still some principles that can be argued on the old theory, even if it's not correct anymore. It gets a bit dicier when we reach the limits of knowledge (we'll never be able to verify what it's like inside a black hole or how the big bang really started), but we can try to find a theory that's at least consistent with what we do know (and hopefully relatively simple to boot).

Yours,
Jake

Thank you Jake. If I understand you correctly then, saying that "this is the case" is a sort of short-hand for "based on our current knowledge and way of thinking about this matter, it would appear that such and such is very likely the case". Is that it? It saves time pussy-footing about the way I normally do, trying to make it clear that I claim to know nothing with absolute certainty, but just believe a thing to be the case based on my experience. That makes sense because there's often a lot to say and my way would be very long-winded if the uncertainty of the "facts" had to be stated and restated over and over again.

I wonder if it might be better to state the "work in progress" nature of the argument at the outset, just to make it clear, especially in an environment where lots of readers are not familiar with the conventions of scientific debate.

On the irradiated food: the best argument I've heard about that, is it can be used to sterilize food that's just going or has just gone off. People could be buying old prawns for instance, thinking they're fresh. That might turn out to be unhealthy.

Sal

Hello,

Yes. It seems to me that, when you get down to it, it's near impossible to prove anything with certainty. So a lot of our arguments based on the truth are based on theories, cultural assumptions, historical knowledge, or even what certain other people are saying. I've noticed all the time in recent news stories where periodicals will validate their claims by citing each other.

I'm not saying you should claim unknown or unprovable things with absolute certainty, but just that you can't get anywhere if you require the truth to proceed. So we basically muddle forth and build up a more correct view of the world as we proceed. But those are just my quick thoughts while I'm waiting for the debugger to start up. I haven't thought things through.

Yours,
Jake

Your quick thoughts make very satisfying answers Jake.

That leaves us with the other case that no one has mentioned.
Where we are both absolutely sure what we think we are talking about but when it comes down to it we have entirely different ideas of what the words mean. These can become redefined during the discussion making things even trickier.
The nobel prize winning physicist richard feynman once sat through a philosophy course, and during the last lesson, he asked about a term that everyone was using, but no one had defined. so he asked if a brick had that property, to give him an idea of how to apply the term. Everyone had a different idea of how it applied to a brick, even though they had been acting as if they all agreed what it meant.
Some things arejust tricky to talk about.

Good point xyroth. I'm sure there's more of that sort of misunderstanding than we realize. I'd guess that a lot of people would be surprised at how fuzzy their (not sure what to call them) "internal, taken-for-granted definitions" of some words are. It's not until we're called upon to explain exactly what we mean by a particular term that we realize how vague and, possibly at odds with other people's understanding, our own definition is.

"internal, taken-for-granted definitions" do indeed have a name, they are called unstated assumptions, and if there is a flaw in your reasoning, this is where it usually hides.