Physics is confusing
Nov. 15th, 2012 11:54 am![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
drwexI know at least gsh reads my LJ once in a while and maybe other readers know more physics than I and can shed light (ha, you see what I did there) on this.
The BBC had a story today about the black hole at the center of our galaxy interacting with a pretty massive gas/dust cloud and us just about now having new observational tools to find out about it.
Confusion 1: Everyone in the story talks about this as a thing that "is going to happen" as though it were in the future. Really, though, this is something happened way the hell in the past, right? Because light just takes that long to reach Earth - my best searching turns up an answer somewhere around 25,000 light-years - the interaction of this cloud and that black hole actually happened that long ago, but really soon we'll start seeing the first photons from it. So I can kind of see why they talk about it as being in (our) future though it's a bit confusing.
Confusion 2: there was a comment about how this interaction would tell us more about how black holes "turn matter into energy". And that kind of brought me up short. The only way I know to turn matter into energy involves high-speed collisions, or nuclear decay and I didn't think matter absorption by black holes included either of those things.
The event is said to be likely to emit a lot of high-energy X-ray photons, which we plan to observe, but it's not clear to me how those photons get created. Matter in the vicinity of the hole is accelerated to nearly the speed of light (or so I read) but what causes it to emit X-rays? Is it not the incidental matter, but the black hole itself that emits X-rays as it absorbs the matter? And are the X-rays really the result of something like the protons or neutrons being ripped apart (converted to energy) by the intense gravity as they approach the event horizon or is the phrase "conversion of matter to energy" another convenience shorthand like referring to this event as being in the future is a convenience shorthand?
The BBC had a story today about the black hole at the center of our galaxy interacting with a pretty massive gas/dust cloud and us just about now having new observational tools to find out about it.
Confusion 1: Everyone in the story talks about this as a thing that "is going to happen" as though it were in the future. Really, though, this is something happened way the hell in the past, right? Because light just takes that long to reach Earth - my best searching turns up an answer somewhere around 25,000 light-years - the interaction of this cloud and that black hole actually happened that long ago, but really soon we'll start seeing the first photons from it. So I can kind of see why they talk about it as being in (our) future though it's a bit confusing.
Confusion 2: there was a comment about how this interaction would tell us more about how black holes "turn matter into energy". And that kind of brought me up short. The only way I know to turn matter into energy involves high-speed collisions, or nuclear decay and I didn't think matter absorption by black holes included either of those things.
The event is said to be likely to emit a lot of high-energy X-ray photons, which we plan to observe, but it's not clear to me how those photons get created. Matter in the vicinity of the hole is accelerated to nearly the speed of light (or so I read) but what causes it to emit X-rays? Is it not the incidental matter, but the black hole itself that emits X-rays as it absorbs the matter? And are the X-rays really the result of something like the protons or neutrons being ripped apart (converted to energy) by the intense gravity as they approach the event horizon or is the phrase "conversion of matter to energy" another convenience shorthand like referring to this event as being in the future is a convenience shorthand?
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I was a liberal arts (International Studies) major so ...
Date: 2012-11-15 05:35 pm (UTC)I'm guessing Dr Scharf is using the plain English phrase "converting matter into energy around black holes" as shorthand for all the issues that are involved in radiation escaping from an accretion disc.
"As the material falls into the black hole, it heats up to very high temperatures and about 10% of it's energy is radiated away. (The energy I mean is that given by Einstein's famous equation, E=mc^2, which gives the energy associated with a given mass, m). The rest of the material is just absorbed by the black hole, and as I mentioned above, adds to the mass of the black hole. Sometimes, the material doesn't have a chance to radiate away any energy, and essentially all of the mass goes right into the black hole."
http://www.astro.umd.edu/~miller/teaching/questions/blackholes.html
no subject
Date: 2012-11-15 06:04 pm (UTC)Ordinarily when we talk about the "dates" of far-distant events, we assume a model in which the world lines of the galaxies yield a global time coordinate, and we measure along that. But when you're talking about events in the vicinity of a black hole, that model won't do. So unless there's some standard trick for dealing with this that I don't know about, I don't think your question can have a meaningful answer.
no subject
Date: 2012-11-15 06:19 pm (UTC)They're probably just simplifying things a bit for the general audience.
On the other hand, special relativity says that there's no such thing as absolute simultaniety, and we can only judge the order of events relative to ourselves. As far as the photons involved are concerned, everything happens at the same time!
no subject
Date: 2012-11-15 08:13 pm (UTC)1) The center of the galaxy is about 8 kiloparsecs distant, which is about 25,000 light years, so light we are seing now went slurp into the black hole 25,000 years ago, give or take, which isn't all that long ago. A tiny tiny fraction of the age of the universe.
2) We are seeing X-rays and gamma rays from the center of the galaxy, so the matter falling in (probably via an accretion disk) is heated up. The details of how this works is pretty much totally unknown. We have guesses of course, but actual facts are few.
The infalling matter has angular momentum which limits how fast stuff can fall into the black hole, and it piles up in what we call an accretion disk. There the matter is heated up to millions of Kelvin, and the hot matter emits high energy photons. About 10 percent of the rest mass of matter gets turned into energy.
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Date: 2012-11-15 11:19 pm (UTC)Essentially, you can draw a coordinate map that has the event 25,000 years ago and the light just reaching us. That is the simple model and it works fine. You have to get in really close to the black hole before serious warping messes that up. Talking about it "about to happen" is shorthand for "the light is about to reach us." There is also the viewpoint that everything that we *see* happening now *is* happening now, which gives the alternative definition of "now" that was discussed. So with that non-intuitive definition, they are technically correct.
When electric charges are accelerated by anything *other* than a gravitational field, they radiate electromagnetic waves. That's just part of what charges do. Gravity is supposed to be the exception, a la the optimist falling off a building saying, "okay so far!" But lots of energy is churning around in the vicinity. The black hole emits nothing itself (well, okay, Hawking found a tiny loophole to that and got famous for it, but it is ignorably tiny.) It is the accretion disk that radiates, and it forms because particles coming in off-center have angular momentum (spin) and the tighter in they go, the faster they orbit, slowing down the fall so you get almost a traffic jam of infalling mass. Again, a lot of that is theory.