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Although we cannot see black holes, we can detect or guess the presence of one by measuring its effects on objects around it. The following effects may be used:
* Mass estimates from objects orbiting a black hole or spiraling into the core
* Gravitational lens effects
* Emitted radiation
Many black holes have objects around them, and by looking at the behavior of the objects you can detect the presence of a black hole. You then use measurements of the movement of objects around a suspected black hole to calculate the black hole's mass.
What you look for is a star or a disk of gas that is behaving as though there were a large mass nearby. For example, if a visible star or disk of gas has a "wobbling" motion or spinning AND there is not a visible reason for this motion AND the invisible reason has an effect that appears to be caused by an object with a mass greater than three solar masses (too big to be a neutron star), then it is possible that a black hole is causing the motion. You then estimate the mass of the black hole by looking at the effect it has on the visible object.
For example, in the core of galaxy NGC 4261, there is a brown, spiral-shaped disk that is rotating. The disk is about the size of our solar system, but weighs 1.2 billion times as much as the sun. Such a huge mass for a disk might indicate that a black hole is present within the disk.
Einstein's General Theory of Relativity predicted that gravity could bend space. This was later confirmed during a solar eclipse when a star's position was measured before, during and after the eclipse. The star's position shifted because the light from the star was bent by the sun's gravity. Therefore, an object with immense gravity (like a galaxy or black hole) between the Earth and a distant object could bend the light from the distant object into a focus, much like a lens can. This effect can be seen in the image below.
In the above image, the brightening of MACHO-96-BL5 happened when a gravitational lens passed between it and the Earth. When the Hubble Space Telescope looked at the object, it saw two images of the object close together, which indicated a gravitational lens effect. The intervening object was unseen. Therefore, it was concluded that a black hole had passed between Earth and the object.
When material falls into a black hole from a companion star, it gets heated to millions of degrees Kelvin and accelerated. The superheated materials emit X-rays, which can be detected by X-ray telescopes such as the orbiting Chandra X-ray Observatory.
The star Cygnus X-1 is a strong X-ray source and is considered to be a good candidate for a black hole. As pictured above, stellar winds from the companion star, HDE 226868, blow material onto the accretion disk surrounding the black hole. As this material falls into the black hole, it emits X-rays, as seen in this image:
In addition to X-rays, black holes can also eject materials at high speeds to form jets. Many galaxies have been observed with such jets. Currently, it is thought that these galaxies have supermassive black holes (billions of solar masses) at their centers that produce the jets as well as strong radio emissions. One such example is the galaxy M87 as shown below:
t is important to remember that black holes are not cosmic vacuum cleaners -- they will not consume everything. So although we cannot see black holes, there is indirect evidence that they exist. They have been associated with time travel and worm holes and remain fascinating objects in the universe.
History: The concept of an object from which light could not escape (e.g., black hole) was originally proposed by Pierre Simon Laplace in 1795. Using Newton's Theory of Gravity, Laplace calculated that if an object were compressed into a small enough radius, then the escape velocity of that object would be faster than the speed of light.
What Is A Black Hole ?
A black hole is what remains when a massive star dies.
If you have read How Stars Work, then you know that a star is a huge, amazing fusion reactor. Because stars are so massive and made out of gas, there is an intense gravitational field that is always trying to collapse the star. The fusion reactions happening in the core are like a giant fusion bomb that is trying to explode the star. The balance between the gravitational forces and the explosive forces is what defines the size of the star.
As the star dies, the nuclear fusion reactions stop because the fuel for these reactions gets burned up. At the same time, the star's gravity pulls material inward and compresses the core. As the core compresses, it heats up and eventually creates a supernova explosion in which the material and radiation blasts out into space. What remains is the highly compressed, and extremely massive,
core. The core's gravity is so strong that even light cannot escape.
This object is now a black hole and literally disappears from view. Because the core's gravity is so strong, the core sinks through the fabric of space-time, creating a hole in space-time -- this is why the object is called a black hole.
The core becomes the central part of the black hole called the singularity. The opening of the hole is called the event horizon.
You can think of the event horizon as the mouth of the black hole. Once something passes the event horizon, it is gone for good. Once inside the event horizon, all "events" (points in space-time) stop, and nothing (even light) can escape. The radius of the event horizon is called the Schwarzschild radius, named after astronomer Karl Schwarzschild, whose work led to the theory of black holes.
Types of Black Holes
There are two types of black holes:
* Schwarzschild - Non-rotating black hole
* Kerr - Rotating black hole
The Schwarzschild black hole is the simplest black hole, in which the core does not rotate. This type of black hole only has a singularity and an event horizon.
The Kerr black hole, which is probably the most common form in nature, rotates because the star from which it was formed was rotating. When the rotating star collapses, the core continues to rotate, and this carried over to the black hole (conservation of angular momentum). The Kerr black hole has the following parts:
* Singularity - The collapsed core
* Event horizon - The opening of the hole
* Ergosphere - An egg-shaped region of distorted space around the event horizon (The distortion is caused by the spinning of the black hole, which "drags" the space around it.)
* Static limit - The boundary between the ergosphere and normal space
If an object passes into the ergosphere it can still be ejected from the black hole by gaining energy from the hole's rotation.
However, if an object crosses the event horizon, it will be sucked into the black hole and never escape. What happens inside the black hole is unknown; even our current theories of physics do not apply in the vicinity of a singularity.
Even though we cannot see a black hole, it does have three properties that can or could be measured:
* Electric charge
* Rate of rotation (angular momentum)
As of now, we can only measure the mass of the black hole reliably by the movement of other objects around it. If a black hole has a companion (another star or disk of material), it is possible to measure the radius of rotation or speed of orbit of the material around the unseen black hole. The mass of the black hole can be calculated using Kepler's Modified Third Law of Planetary Motion or rotational motion.
What is quantum mechanics?
Quantum mechanics is a relatively recent area of physics which studies the behaviour of the tiniest existing components of matter. Tiny they may be, but discoveries about these particles have turned our understanding of the world on its head.
One of the central characters in the film Watchmen is Dr Manhattan, a physicist turned superhero following an accident in his lab. Manhattan is able to teleport, see into the future and past, duplicate himself and much more. But what if the most superhuman of his powers was that of understanding quantum mechanics?
What makes quantum physics awkward to get to grips with is that in many ways the workings of particles at a subatomic scale contradict everyday logic. ‘We are in an area which is very difficult to imagine because it’s so different from the macroscopic world we live in,’ explains Vlatko Vedral, professor of quantum information science at Leeds University. ‘So it’s very difficult to relate everyday objects we know to the behaviour of small objects.’
For the most part, classical physics is intuitive: whether or not you are aware of Newton’s laws, when you kick a ball common sense and everyday experience allow you to make a pretty good estimate of where it will go.
But imagine for a second that your football began to behave like an electron. In that case, quantum physics would tell you that it's impossible to know where your ball will land, and, come to think of it, if you look at it from the right angle it is actually a wave of energy rather than a physical object.
From equations to reality
Things really begin to get hairy when you pause to consider the wider implications of quantum mechanics. As everything around us is made up of these tiny constituents, how does their odd behaviour impact upon reality as we know it?
‘For all that we know, every quantum object - a particle of light, a particle of matter like an atom even a small molecule - can simultaneously exist in different places at the same time,’ says Vedral. How such properties might translate into the reality we perceive with our human senses remains a mystery.
As a result, there is fiery debate amongst physicists on how to reconcile quantum mechanics with what we see around us on a day to day basis. ‘I would say most of us certainly agree up to a certain level what the predictions are, but as you scale it up then you really have a strong division in the community,’ comments Vedral.
‘We have something spectacularly successful when it comes to predictions, but somehow we find it very difficult to understand what these actually mean,’ he adds.
Philosophical musings aside, the fact remains that at a subatomic level, quantum mechanics just works, even if it sometimes seems to defy the human imagination.
Vedral takes the example of the structure of an atom: ‘For me it’s very difficult to tell you what an atom really is, because it’s now outside our visualisation - it’s difficult to draw it, to come up with a geometric image. But I can still use the correct mathematics to make predictions about what will happen if you move it, or shine a laser on it.’
This means that practical applications of quantum mechanics such as quantum computing or cryptography are likely to see the light of the day in the near future, even if gaining a complete understanding of quantum mechanics remains a distant dream.