Black hole ignorance.

[GoBlue]

I'm a PhD immunologist by training and profession so science isn't greek to me. I understand the principle of black holes. But I have never been able to comprehend how a black holes presence can be detected by it's gravitational effects.

At the risk of asking a stupid but basic question: how does gravity escape from a black hole?

[WoodrowFan]

1. black holes

I'm a historian, not a physicist, but I didn't think gravity was something that could "escape". I thought it was a force that a black hole created.

At any rate, until a real astronomer comes along here are a couple links that may help..

Black holes can be detected through a technique called gravity lensing. Gravity lensing occurs when a massive object, in this case a black hole, passes between a star and the Earth. The black hole acts as a lens when its gravity bends the star's light rays and focuses them on the Earth. From an observer's point of view on the Earth, the star would appear to brighten. Einstein's general relativity theory suggests that light should follow the path of bent time and space, which in this scenario, is bent by the black hole's gravity (Miller).

http://www.rdrop.com/users/green/school/detect.htm

More at

http://csep10.phys.utk.edu/astr162/lect/blackhole/blackhole.html


or do they just beg the question??

[pnutchuck]

2. I can't even begin to comprehend that

I don't think that's a stupid question at all. I don't have any PhDs but it was my understanding that black holes contain so much gravity that nothing can escape them. So, passing stars, and debris get pulled in by it's gravity and that's what the scientists see. But here's a website at UC Berkeley http://cosmology.berkeley.edu/Education/BHfaq.html

[Spinzonner]

3. Non sequiter

Gravity is a force, not a form of matter or energy which could be construed as 'escaping' from a Black Hole.

Basically, nothing escapes from a Black Hole because the force of gravity within the event horizon is too strong for the kinetic energy to overcome it. At the most extreme - something moving at the speed of light, i.e. photon - cannot even escape even though they move at the maximum speed possible, namely that of light.

When you refer to 'gravitational effects' you are referring to the effect it has on the mass and energy (really the same thing) outside of the black hole in that it has the effect of warping space-time as all gravitation does and in this case causing a singularity.

There are measureable and observable effects on the space-time and matter around the black hole that will reveal its presence.

BTW, the principle that nothing can escape from a Black Hole is not entirely true theoretically. Quantum-Mechanical phenomena can result in particle-antiparticle pairs being formed at the event horizon with one being consumed by the black hole while the other escapes.

http://superstringtheory.com/blackh/blackh3.html

[sam sarrha]

4. the extreme force gravity acts like a "Lens" and beds light and images

[Spinzonner]

5. That sounds sexy

Bed-ding light :-)

And images, of course, are manifestations of light, not matter or eb=nergy themselves.

In any case, I implied that by mentioning the effect a black hole has on the space-time and sourounding matter/energy.

[pnutchuck]

6. Did you read the article in Science Digest?

It was about a year ago, and it was about a group of scientists trying to disprove the big bang theory. Their model suggests that our universe is essentially a plane and that it rubbed against another plane that caused what scientists consider the big bang. However, their theory is that there is no exact creation point of time, that these planes exist simultaneously and are created over and over again. I can't seem to find the article here, and my searches have just led me to creationist websites.

[Spinzonner]

7. There is no shortage of Cosmological Theories

about the ultimate nature of reality (or realities), whatever that is and not enough definitive evidence to choose among them at this point.

And of course, we might have to break it to find the answer ...

[rman]

18. in trying to disprove the big-bang theory they provide a mechanism

that could have caused the big bang?
interesting way to disprove something.

[GoBlue]

23. Wow and thanks

I had always assumed that all forces required the presence of energy/matter.

[Sialia]

8. Detecting black holes

The gravitational effects of black holes are evident only very close to the horizon (the "edge" of the hole), which is why for the most part only supermassive black holes at the centers of galaxies are detected via gravitational effects. These effects would include things like a very pronounced spike in the speed of orbits around the object the closer in one goes, which can be detected by Doppler effects in the light the orbiting substance emits (so the orbiting bodiess or gas have to be emitting light for this to work--generally that is the case, however).

Smaller black holes, e.g. stellar remnants, can be detected if they have a companion. If one can determine the orbit of the companion, one can compute the total mass of the binary, and if the companion is an ordinary star or a white dwarf, its mass can be estimated pretty accurately, so the mass of the unseen object can be computed. If it is more than about 3 solar masses then it has to be a black hole, since there are theoretical limits to the mass of a white dwarf (1.4 solar mass) and neutron stars (about 3 solar masses--that is not as well determined as the mass limit for white dwarfs). A complication for this is that effects such as the angle at which we are viewing the orbit affect the determination of the mass, which is why there are few objects that are definitely believed to be black holes.

Gravity doesn't "escape" from the hole--like all massive objects, the black hole has a gravitational field surrounding it. Note that this is the classical general-relativistic interpretation of gravity--unified theories of gravity and other forces are nascent at best. Please don't think of black holes as something apart from the universe--they are not. Outside their event horizons, they are just massive objects. The extremely strong gravitational field near the horizon does cause effects, but far from the hole its field settles down to precisely that of any object of its mass.

The overwhelming majority of gravitational lensing effects that we can see are due not to black holes but to galaxy clusters. *Any* massive object can produce a gravitational lens--it does *not* have to be a black hole. Most black holes are too small to create a lens effect that can be observed. Most usually, we see a lensed image of a distant cluster whose light passes close to the center of a dense ("rich" in astronomical parlance) cluster that lies between us and the more distant cluster. Incidentally, however, some light bending effects can be observed even in starlight passing close to the Sun--the solar eclipse of 1919 was used to test the then-new general theory of relativity.

No Hawking radiation (particles--mostly photons, actually--that are emitted from the horizon) can be observed from any black hole that might exist in the current universe. A solar-mass black hole has an effective temperature of something like 10^-7 Kelvin and Hawking radiation has a blackbody distribution, so the flux from even such a small black hole is unobservable. In an open universe (which it seems to be) time goes to infinity, however, so eventually the black hole will shrink due to the radiation, its temperature will rise (temperature is inversely related to mass for BHs) and the flux will become considerable. Eventually the black hole evaporates completely. For a solar-mass BH this process is extremely slow, however--it hasn't even really begun over the current 13.5-billion-year age of the universe. Most black holes are probably supermassive black holes at the centers of galaxies and galaxy clusters, so it will be an even slower process--but if time is infinite then it will happen.

[pmbryant]

9. Much useful information in this post. Here's some more.

I'd also mention that another way black holes are commonly "detected" is by the behavior of the matter surrounding them, as it falls into the black hole. As gas and other matter falls into a black hole, it falls deep down a graviational potential "well", releasing huge amounts of energy. In the immediate areas surrounding a black hole, this energy takes the form of X-rays.

So X-rays (of a certain spectral pattern) can also be evidence for black holes, especially when combined with the gravitational effects on nearby objects that Sialia mentions.

Basically, since we can never actually see black holes themselves, we can only infer them from their effect on their surroundings. It's taken a lot of work to build up all the evidence for black holes over the last few decades. Very non-trivial stuff, so it's no surprise that it can be hard to fully comprehend.

--Peter

[Beetwasher]

11. Thanks! Terrific Post! I've Always Wondered Whether

It's possible that eventually everything in the universe will be sucked into black holes and then the black holes will consume eachother and/or fizzle out...Is this essentially what the big crunch is, or would it be considered something different?

[phantom power]

10. According to some models, all forces must be mediated by particles

In these models, I believe that gravity is assumed to be mediated by a "graviton", although nobody has ever identified such a particle.

Now, I'm pretty sure that particles cannot escape the event horizon of a black hole (ignoring Hawking radiation).

So, is the gravity that we perceive around a black hole from gravitons that are created outside the event horizon? It seems as though they must be. Otherwise, the particle model of gravity must be a bit dodgy.

Then again, my grasp on particle physics is most definitely dodgy. Any physicists out there to straighten me out?

[DireStrike]

12. I'm not a physicist, but...

I usually hear gravity explained as curvature of spacetime, rather than a particle mediated force.

[pmbryant]

13. Obviously, this isn't a perfectly understood area of physics right now

Einstein's Theory of General Relativity is the current best understanding we have of gravity, and it indeed explains gravity as arising from curvature of spacetime. General Relativity has been vastly successful, passing experimental test after test after test. It is bedrock fundamental physics.

But most physicists, seeing the quantum nature of every other force we know in the Universe, believe that ultimately gravity must be quantum in nature as well (i.e., mediated by particles, the hypothetical "gravitons"). Unfortunately, no one has yet been able to come up with a quantum theory of gravity that meshes with the battle-tested Theory of General Relativity. And also, no one has yet been able to detect "gravitons" experimentally. So the quantum nature of gravity remains merely a hypothesis.

But I think you'd be hard pressed to find a physicist that doesn't believe "gravitons" exist. For what that's worth.

--Peter

[DireStrike]

17. Does "quantum" have to mean particulate?

For example, are there "space" or "time" particles?

I would assume that, quantized meaning something like "having a smallest possible unit" doesn't necessarily require particles.

[pmbryant]

20. Quantized 'smallest possible units' units can act as particles

They are not particles in the conventional sense of the world, but can act as such, and are commonly called particles, for convenience.

The smallest possible unit of light, photons, can act as either particles or waves depending on the situation. Other "smallest possible units" are similar.

And since there is no quantum theory of gravity at the time, the nature of space time at quantum-scales is not understood.

--Peter

[Salviati]

21. In quantum theory though...

forces are mediated by the exchange of particles. So all the stuff out there like quarks and electrons, which are grouped together and called Fermions, 'talk' to each other by exchanging a different type of particle, called Bosons, like photons, and gluons. For example, two electrons communicate the electromagnetic force between them by exchanging photons, which are the mediator of the electromagnetic force.

Space and time may also be quantized, and physicists often do quantize it in order to do numerical simulations. In this case it is called doing the physics on a lattice. It's like having a grid of space where the bosons sit on the vertices, and the fermions can travel along the edges, and time progresses in discrete steps.

[mainegreen]

14. Never thought about that.

But its interesting. I suspect that the carrier particle for gravity can't have mass. If it did you would get a feed back effect creating infinite gravity everywhere. So if "gravitons" have no mass, then the effect of gravity in a black hole would have no effect on gravitons.

It was my understanding, however, that light does have a neglible amount of mass, so it gets caught.

Thoughts? Is this crazy or reasonable?

[pmbryant]

15. Light doesn't have mass

Light "gets caught" in black holes because of the curvature of space time, which becomes so extreme inside the event horizon of a black hole.

Gravitons are still hypothetical, but I believe you are right that they are not believed to have any mass.

--Peter

[mainegreen]

16. Ah yes.

I see your right.

I went http://www.phys.uni.torun.pl/~jkob/physnews/node35.html">Here to check it out. But then, why does gravity affect massless light particles but not other gravitons? And how does a graviton cause space curvature, frame dragging and all that rot anyhow if it is a particle? Or is is a wave/particle like photons? Not that that makes any more sense to me.

Anyone have a easy to understand explanation?

Nnng. Head hurt.

Must head to lounge for kitten thread.

[Salviati]

22. but the gravitons do self couple...

>Gravitons are still hypothetical, but I believe you are right that they are not believed to have any mass.

Though the graviton is belived to be massless, it still does interact with itself, i.e. the fact that there is a gravitational field in space creates gravitational potential energy in the field. This energy then gravitates, because energy and mass are related, E=mc^2 you know...

This self coupling is one of the big stumbling blocks in developing a 'good' theory of quantum gravity.

[rman]

19. gravity is not attracted by gravity

so "escaping" is not an issue.