Hi, I usually stick to the Spirituality/Skepticism board, but I had a question for those of you who post often over here. I'm in the middle of a confrontation with someone who is basically completely ignorant of the scientific method. Among other things, this guy is claiming that evolution and much of cosmology is not science, its just a bunch of b.s. Currently, we have been discussing the nature of black holes and the singularities they are theoretically supposed to contain. I have tried my best, and cited many sources concerning singularities, but this guy refuses to listen. So I'm turning this over to the Space and Astronomy board, to help me out over here. I'm no scientist, I'm a philosopher. So on to my question: How do we know what we know about black holes? And what specifically makes scientists believe that black holes contain a singularity at their core? I know that has to do with Einstein's theory of general relativity, but I want to know if there have been any observations made that are highly supportive of the singularity theory. Thanks for your help.
When men yield up the privilege of thinking, the last shadow of liberty quits the horizon.
Beyond your expectations, beyond your imagination: The Universe.
Posted 30 April 2008 - 01:37 PM
First of all, why even waste your time with this kind of individual?
Ok let's get started:
Since black holes are small (only a few to a few tens of kilometers in size), and light that would allow us to see them cannot escape, a black hole floating alone in space would be hard, if not impossible, to see. For instance, the photograph above shows the optical companion star to the (invisible) black hole candidate Cyg X-1.
However, if a black hole passes through a cloud of interstellar matter, or is close to another "normal" star, the black hole can matter into itself. As the matter falls or is pulled towards the black hole, it gains kinetic energy, heats up and is squeezed by tidal forces. The heating ionizes the atoms, and when the atoms reach a few million degrees Kelvin, they emit X-rays. The X-rays are sent off into space before the matter crosses the Schwarzschild radius and crashes into the singularity. Thus we can see this X-ray emission.
Binary X-ray sources are also places to find strong black hole candidates. A companion star is a perfect source of infalling material for a black hole. A also allows the calculation of the black hole candidate's mass. Once the mass is found, it can be determined if the candidate is a neutron star or a black hole, since neutron stars always have masses of about 1.5 times the mass of the sun. Another sign of the presence of a black hole is random variation of emitted X-rays. The infalling matter that emits X-rays does not fall into the black hole at a steady rate, but rather more sporadically, which causes an observable variation in X-ray intensity. Additionally, if the X-ray source is in a binary system, the X-rays will be periodically cut off as the source is eclipsed by the companion star. When looking for black hole candidates, all these things are taken into account. Many X-ray satellites have scanned the skies for X-ray sources that might be possible black hole candidates.
Cygnus X-1 is the longest known of the black hole candidates. It is a highly variable and irregular source with X-ray emission that flickers in hundredths of a second. An object cannot flicker faster than the time required for light to travel across the object. In a hundredth of a second, light travels 3000 kilometers. This is one fourth of Earth's diameter! So the region emitting the x-rays around Cygnus X-1 is rather small. Its companion star, HDE 226868 is a B0 supergiant with a surface temperature of about 31,000 K. Spectroscopic observations show that the spectral lines of HDE 226868 shift back and forth with a period of 5.6 days. From the mass-luminosity relation, the mass of this supergiant is calculated as 30 times the mass of the Sun. Cyg X-1 must have a mass of about 7 solar masses or else it would not exert enough gravitational pull to cause the wobble in the spectral lines of HDE 226868. Since 7 solar masses is too large to be a white dwarf or neutron star, it must be a black hole.
Loosely speaking, a black hole is a region of space that has so much mass concentrated in it that there is no way for a nearby object to escape its gravitational pull. Since our best theory of gravity at the moment is Einstein's general theory of relativity, we have to delve into some results of this theory to understand black holes in detail, but let's start of slow, by thinking about gravity under fairly simple circumstances.
Suppose that you are standing on the surface of a planet. You throw a rock straight up into the air. Assuming you don't throw it too hard, it will rise for a while, but eventually the acceleration due to the planet's gravity will make it start to fall down again. If you threw the rock hard enough, though, you could make it escape the planet's gravity entirely. It would keep on rising forever. The speed with which you need to throw the rock in order that it just barely escapes the planet's gravity is called the "escape velocity." As you would expect, the escape velocity depends on the mass of the planet: if the planet is extremely massive, then its gravity is very strong, and the escape velocity is high. A lighter planet would have a smaller escape velocity. The escape velocity also depends on how far you are from the planet's center: the closer you are, the higher the escape velocity. The Earth's escape velocity is 11.2 kilometers per second (about 25,000 m.p.h.), while the Moon's is only 2.4 kilometers per second (about 5300 m.p.h.).
Now imagine an object with such an enormous concentration of mass in such a small radius that its escape velocity was greater than the velocity of light. Then, since nothing can go faster than light, nothing can escape the object's gravitational field. Even a beam of light would be pulled back by gravity and would be unable to escape.
The idea of a mass concentration so dense that even light would be trapped goes all the way back to Laplace in the 18th century. Almost immediately after Einstein developed general relativity, Karl Schwarzschild discovered a mathematical solution to the equations of the theory that described such an object. It was only much later, with the work of such people as Oppenheimer, Volkoff, and Snyder in the 1930's, that people thought seriously about the possibility that such objects might actually exist in the Universe. (Yes, this is the same Oppenheimer who ran the Manhattan Project.) These researchers showed that when a sufficiently massive star runs out of fuel, it is unable to support itself against its own gravitational pull, and it should collapse into a black hole.
In general relativity, gravity is a manifestation of the curvature of spacetime. Massive objects distort space and time, so that the usual rules of geometry don't apply anymore. Near a black hole, this distortion of space is extremely severe and causes black holes to have some very strange properties. In particular, a black hole has something called an 'event horizon.' This is a spherical surface that marks the boundary of the black hole. You can pass in through the horizon, but you can't get back out. In fact, once you've crossed the horizon, you're doomed to move inexorably closer and closer to the 'singularity' at the center of the black hole.
You can think of the horizon as the place where the escape velocity equals the velocity of light. Outside of the horizon, the escape velocity is less than the speed of light, so if you fire your rockets hard enough, you can give yourself enough energy to get away. But if you find yourself inside the horizon, then no matter how powerful your rockets are, you can't escape.
According to general relativity, a black hole's mass is entirely compressed into a region with zero volume, which means its density and gravitational pull are infinite, and so is the curvature of space-time that it causes. These infinite values cause most physical equations, including those of general relativity, to stop working at the center of a black hole. So physicists call the zero-volume, infinitely dense region at the center of a black hole a singularity.
The singularity in a non-rotating, uncharged black hole is a point, in other words it has zero length, width, and height.
But there is an important uncertainty about this description: quantum mechanics is as well-supported by mathematics and experimental evidence as general relativity, and it does not allow objects to have zero size—so quantum mechanics says the center of a black hole is not a singularity but just a very large mass compressed into the smallest possible volume. At present we have no well-established theory that combines quantum mechanics and general relativity; and the most promising candidate, string theory, also does not allow objects to have zero size.
The future belongs to those who believe in the beauty of their dreams. Eleanor Roosevelt
Do not go where the path may lead, go instead where there is no path and leave a trail. Ralph Waldo Emerson
I aM tOrGo... I tAkE cArE oF tHe PlAcE wHiLe ThE mAsTeR iS aWaY...
Posted 01 May 2008 - 01:24 AM
I would like to add: it is technically impossible to know what happens inside a black hole without actually falling into one - but then you would not be able to send your observations to the outside world anyway. How does the quote go... "What goes on beneath the veil of the event horizon... decent people shouldn't think too much about that." Theoretically however, using general relativity, one can predict what SHOULD happen as something collapses within its own event horizon. General relativity is WAY outside of my league - special relativity anybody can understand, but the tensors and other crap involved in general I don't even want to think about. I have heard two different interpretations as to the interior of a black hole. One of my astronomy professors suggests that it could be technically possible that some unknown degeneracy pressure keeps a small nugget of matter of nonzero size from collapsing to infinity. However, I have also heard it stated that when gravity gets strong enough such that the escape velocity is greater than that of light, the warping of space and time is so extreme that space takes on a time-like quality: you cannot resist going inwards any more than you can resist going forward in time. As such, it is inevetable that anything within that volume would end up at the exact geometric center. I do not know enough about general relativity to give a better answer in this regard.
There is a great deal of honesty and integrity in saying we do not know everything. There is nothing wrong in saying that we use mathematical theories in trying to understand reality around us, but we really do not know.
And tell them they do not know all there is to know either, because if they claim they do then they are claiming they have the mind of God and to claim that is to claim that they are one of the damned, so they should carefully consider their next response.
You do not have to have an answer to every question to win a debate. Just take the high road of ethics and morality and stay there. You are then already standing at the finish line.