>Gravitationally, they look like any other object of similar mass.
A nitpick, and a minor one at that: Gravitationally, items of the same mass give gravitational fields that are similar at a given distance only to a first order or approximation. Gravitational fields have various details to them, though. For example, recall that spacecraft orbiting planets or moons demonstrate dynamic behavior that let us deduce details of the overall and local structure of those planets or moons.
The hardest homework problem I ever succeeded in doing involved comparing different level analyses of the gravitational field of an almost but not quite uniformly dense, almost but not quite spherical, body.
I bet swapping a same-mass black hole with the Earth would make the Moon behave noticeably differently within a human lifetime. Whether doing that with the Sun would make Earth behave noticeably differently, I don’t want to guess.
The only way I can think of that the force of a black hole doesn’t behave like other things we have witnessed, would be that black holes should impose way impressive tidal forces on objects that get too close to them. You could get your feet half as far from a black hole as your head, which you can’t do with a planet. While it lasted, the pull along the length of your body would be spectacular. But that’s just because a black hole is compact enough to let you position yourself this way. And, we never witness such extreme things, because we don’t do this painful experiment.
There are, though, ways to use gravitational fields to distinguish between the natures of some of these animals.
Whack-a-Mole suggested that from the point of view of an object falling into a black hole’s singularity, it takes an infinite amount of time to reach that singularity. (Incidentally, I recall having read this somewhere as well. If I had to bet, I would bet I read it in the book Stephen Hawking’s Universe long long ago.)
MikeS said this is not true–that “the singularity is always in the future” just means the singularity is inescapable.
But Stranger on a Train also addressed Whack-A-Mole’s comment, and seemed to be agreeing that it is true–that “the singularity is always in the future” in the sense that the meeting with it is never in the present.
If you fall into the singularity, you – or rather, whatever tidal forces have turned your constituent particles into – do encounter the singularity after a finite amount of time; that the singularity is always in your future only means that there exists no path that does not meet it beyond the event horizon.
The often-repeated ‘it takes forever to fall into a black hole’ only is true from the point of view of external observers – to those, due to gravitational time dilatation, an object approaching the event horizon seems to asymptotically slow down to a standstill just before actually encountering it (of course, its light will also be redshifted beyond visibility, so you couldn’t look at a black hole and see all the stuff that ever fell in just kinda hang around at the event horizon). However, the object falling in knows nothing of this – in fact, it can’t even tell when it crosses the horizon.
What Half Man Half Wit said is absolutely correct, however a couple of other points.
Even though the total energy of the infalling particle is negative the energy of the particle itself is still positive.
And another way to look at is that the hole via its tidal gravity has expended enough energy to create two real particles (from the virtual particle pair) but only gets the energy of one particle back. Effectively radiating a particle.
Has anyone ever figured out the minimum IQ you need to have to be able to understand stuff like general relativity and particle physics? I like to think I’m a pretty smart guy and I have a law degree from a good school, but this stuff goes way over my head. Oh sure, I can parrot back the stuff I’ve read like “massive objects warp spacetime” but I don’t really understand, for example, how space and time can be the same thing. It’s fascinating stuff as long as you are content with only understanding every few sentences you read.
I guarantee you most physicists couldn’t get through a court opinion without specialized help. I’ve had the misfortune to read hundreds of medical journal articles and they are often incomprehensible. Interest, aptitude, and prolonged study are the key in almost every field.
I grok most concepts in advanced physics without an advanced degree. I can’t explain them the way the physicists do but I’ve read about each advance over the past 40 years and I’ve read all the good popular books explaining the concepts by going back to the beginning and leading me through the logic. I’m not coming at it suddenly from the outside and trying to let it sink in by deep end immersion.
Law is no different. How many threads have there been in which Gfactor quotes yards of law jargon and people have to ask for a simple explanation. Anybody else’s jargon is impenetrable until you can find the time to hack a pathway through it with the aid of a good translator. That doesn’t make you less smart.
The only group that really is smarter than everybody else is writers.
Exapno I don’t doubt you grok the concepts, but that’s the easy part. Until you grok the Mathematics you don’t really understand why you don’t understand it.
So.
How are you at groking the math?
Actually I found that groking the math wasn’t as difficult as groking the notation, especially indicial notation. That shit almost killed me.
And the real physicists here are still so far out my league that I need a telescope to even see them.
Or as the old saw attributed to Thomas Edison goes, genius is 1% inspiration and 99% perspiration. The primary characteristic of most “geniuses” is that they’re wonks; that is, that they are able to focus in and study the intimate details of some field (or subfield) in such detail and depth that their knowledge of it is intimate and vastly exceeding that which any dilettante could hope to approach. This is not to say that it doesn’t take a modicum of intelligence to understand the mathematics behind general relativity, but once you get past the notation (which, as Ring notes, is fiendishly difficult to interpret) the math is relatively straightforward…just very, very, very tedious to work through for any significant problem. And at least GR gives answers that are more or less intuitive (once you accept the underlying premises of the theory). This places it in contrast with quantum mechanics, where you just have to accept that the math just because it happens to work, not because it makes any kind of sense.
Of course, an understanding of advanced physics based upon analogized explanations is just that; comprehension based on comparative similarity. A practical mastery of the material comes when you can concoct a scenario, develop a predictive model, and match your answers to reality.
What I don’t understand is why, if black holes have a tendency to suck everything in their surroundings into them, even light, how have they not swallowed up the universe by now?
The inverse square law, and orbits. Gravity becomes weaker with distance; so once everything nearby has been pulled in ( or thrown away ) only objects that drift nearby are significantly tugged on by the hole. And in order to actually be pulled in, an object has to hit the hole. Just as planets can orbit the Sun without being “sucked in” and immolated, objects can orbit or just fly by a black hole.
From what I recall, and from my googling while an enormous amount of energy is released, it’s almost all in the form of invisible gravity waves.
Because not everything is within some black hole’s “surroundings,” so to speak.
As long as you stay away from the event horizon of a black hole, you’re good to go. Everything that hasn’t been captured by a black hole by now, then, is all the stuff that hasn’t come within the event horizon of a black hole. It’s not suprising that there’s so much stuff left, since the volume bound by event horizons of black holes is a tiny fraction of the volume of space in the universe.
I can tell you that this is true of majoring in physics. You really do a lot of slogging through problem sets, or at least I did when I was getting my undergrad degree in physics. And then there are the labs. I would say that “being willing and able to put in a lot of time doing the work for your classes” is at least as important as raw intelligence for a physics major. It really isn’t a matter of reading something like “space and time are the same thing” and immediately and intuitively understanding what that means- it’s doing a lot of problems where you have to treat space and time the same way.
In my graduate school, there was a minimum they required on the standardized tests (the math ones). It wasnt a HARD rule to be admitted, if you could convince them some other way that you could grok the math at the required level, they would consider that.
But, having said that, years of experience had shown them that statistically speaking, if you didnt test at some fairly above average math level, you just werent going to be able to cut it in the program.
And these professors seemed to be the nice ones that WANTED more folks in the program, not some elitist snobs looking for any reason to keep folks out.
I thought neutron stars (like black holes) were fairly well confirmed observationally ? As to how they work detail wise (like black holes), thats a whole nuther issue .
In my graduate school, there was a minimum they required on the standardized tests (the math ones). It wasnt a HARD rule to be admitted, if you could convince them some other way that you could grok the math at the required level, they would consider that.
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I went to grad school in astronomy, not physics. Maybe there was some minimum score on the math GRE, but nobody ever talked about it. I remember the math section of the GRE being really easy, though- supposedly, it’s easier than the math SAT.
We did also have to take the physics GRE, which was definitely not easy. It’s not an IQ test, though. It’s a physics test, and supposedly scores vary quite a bit based on where you did your undergrad degree.
There are lots of physicists who aren’t very good at explaining things, too.
Sure. If the Black Hole has something to “feed” off of it will grow.
For instance, occasionally there is a star orbiting a black hole and the black hole is siphoning off material from the star. Since the black hole is gaining mass it is growing. Eventually the BH will completely consume the star (or the companion star, if of the right size, may form into a black hole too…eventually the two black holes would merge making one bigger one).
