Selasa, 13 Desember 2011

Lubang hitam

Dari Wikipedia bahasa Indonesia, ensiklopedia bebas

Perubahan tertunda ditampilkan di halaman iniBelum Diperiksa

Relativitas umum

Accretion disk.jpg

Topik-topik terkait

sunting

http://upload.wikimedia.org/wikipedia/commons/thumb/c/cd/Black_Hole_Milkyway.jpg/329px-Black_Hole_Milkyway.jpg

http://bits.wikimedia.org/skins-1.18/common/images/magnify-clip.png

Lukisan rekaan dari lubang hitam di depan galaksi Bima Sakti yang bermassa 10x massa matahari kita, dilihat dari jarak 600 km.

Lubang hitam adalah sebuah pemusatan massa yang cukup besar sehingga menghasilkan gaya gravitasi yang sangat besar. Gaya gravitasi yang sangat besar ini mencegah apa pun lolos darinya kecuali melalui perilaku terowongan kuantum. Medan gravitasi begitu kuat sehingga kecepatan lepas di dekatnya mendekati kecepatan cahaya. Tak ada sesuatu, termasuk radiasi elektromagnetik yang dapat lolos dari gravitasinya, bahkan cahaya hanya dapat masuk tetapi tidak dapat keluar atau melewatinya, dari sini diperoleh kata "hitam". Istilah "lubang hitam" telah tersebar luas, meskipun ia tidak menunjuk ke sebuah lubang dalam arti biasa, tetapi merupakan sebuah wilayah di angkasa di mana semua tidak dapat kembali. Secara teoritis, lubang hitam dapat memliki ukuran apa pun, dari mikroskopik sampai ke ukuran alam raya yang dapat diamati.

[sunting] Sejarah

Teori adanya lubang hitam pertama kali diajukan pada abad ke-18 oleh John Michell and Pierre-Simon Laplace, selanjutnya dikembangkan oleh astronom Jerman bernama Karl Schwarzschild, pada tahun 1916, dengan berdasar pada teori relativitas umum dari Albert Einstein, dan semakin dipopulerkan oleh Stephen William Hawking. Pada saat ini banyak astronom yang percaya bahwa hampir semua galaksi dialam semesta ini mengelilingi lubang hitam pada pusat galaksi.

Adalah John Archibald Wheeler pada tahun 1967 yang memberikan nama "Lubang Hitam" sehingga menjadi populer di dunia bahkan juga menjadi topik favorit para penulis fiksi ilmiah. Kita tidak dapat melihat lubang hitam akan tetapi kita bisa mendeteksi materi yang tertarik / tersedot ke arahnya. Dengan cara inilah, para astronom mempelajari dan mengidentifikasikan banyak lubang hitam di angkasa lewat observasi yang sangat hati-hati sehingga diperkirakan di angkasa dihiasi oleh jutaan lubang hitam.

[sunting] Asal-mula lubang hitam

Lubang Hitam tercipta ketika suatu obyek tidak dapat bertahan dari kekuatan tekanan gaya gravitasinya sendiri. Banyak obyek (termasuk matahari dan bumi) tidak akan pernah menjadi lubang hitam. Tekanan gravitasi pada matahari dan bumi tidak mencukupi untuk melampaui kekuatan atom dan nuklir dalam dirinya yang sifatnya melawan tekanan gravitasi. Tetapi sebaliknya untuk obyek yang bermassa sangat besar, tekanan gravitasi-lah yang menang.

Massa dari lubang hitam terus bertambah dengan cara menangkap semua materi didekatnya. Semua materi tidak bisa lari dari jeratan lubang hitam jika melintas terlalu dekat. Jadi obyek yang tidak bisa menjaga jarak yang aman dari lubang hitam akan terhisap. Berlainan dengan reputasi yang disandangnya saat ini yang menyatakan bahwa lubang hitam dapat menghisap apa saja disekitarnya, lubang hitam tidak dapat menghisap material yang jaraknya sangat jauh dari dirinya. dia hanya bisa menarik materi yang lewat sangat dekat dengannya. Contoh : bayangkan matahari kita menjadi lubang hitam dengan massa yang sama. Kegelapan akan menyelimuti bumi dikarenakan tidak ada pancaran cahaya dari lubang hitam, tetapi bumi akan tetap mengelilingi lubang hitam itu dengan jarak dan kecepatan yang sama dengan saat ini dan tidak terhisap masuk kedalamnya. Bahaya akan mengancam hanya jika bumi kita berjarak 10 mil dari lubang hitam, hal ini masih jauh dari kenyataan bahwa bumi berjarak 93 juta mil dari matahari. Lubang hitam juga dapat bertambah massanya dengan cara bertubrukan dengan lubang hitam yang lain sehingga menjadi satu lubang hitam yang lebih besar.

Hubblesite Special Feature

HomeJourney to a Black HoleBlack Hole Encyclopedia

Black Holes: Gravity's Relentless Pull

http://hubblesite.org/explore_astronomy/black_holes/graphics/bar_left.gif

Image illustrating: What is a black hole?

French scientist Pierre-Simon Laplace (1749-1827) was one of the first to discuss the possible existence of black holes.

Image illustrating: What is a black hole?

American physicist John Archibald Wheeler (1911- ) first introduced the term black hole and led many important studies into their properties.

What is a black hole?

A black hole is an object that is so compact (in other words, has enough mass in a small enough volume) that its gravitational force is strong enough to prevent light or anything else from escaping.


The existence of black holes was first proposed in the 18th century, based on the known laws of gravity. The more massive an object, or the smaller its size, the larger the gravitational force felt on its surface. John Michell and Pierre-Simon Laplace both independently argued that if an object were either extremely massive or extremely small, it might not be possible at all to escape its gravity. Even light could be forever captured.


The name "black hole" was introduced by John Archibald Wheeler in 1967. It stuck, and has even become a common term for any type of mysterious bottomless pit. Physicists and mathematicians have found that space and time near black holes have many unusual properties. Because of this, black holes have become a favorite topic for science fiction writers. However, black holes are not fiction. They form whenever massive but otherwise normal stars die. We cannot see black holes, but we can detect material falling into black holes and being attracted by black holes. In this way, astronomers have identified and measured the mass of many black holes in the Universe through careful observations of the sky. We now know that our Universe is quite literally filled with billions of black holes.

Next Encyclopedia Topic

Back to Encyclopedia Contents

Related Questions:

Case studies of black holes:


External Links and Further Reading:



Content OverviewCreditsContact UsGlossary

http://hubblesite.org/explore_astronomy/black_holes/graphics/bar_right.gif

http://hubblesite.org/explore_astronomy/black_holes/graphics/bar_bottom2.gif

Quit

Hubblesite Special Feature

HomeJourney to a Black HoleBlack Hole Encyclopedia

Black Holes: Gravity's Relentless Pull

http://hubblesite.org/explore_astronomy/black_holes/graphics/bar_left.gif

Image illustrating: Do Black Holes Obey the Laws of Gravity?

British scientist Sir Isaac Newton (1643-1727) formulated the laws of gravity, supposedly after pondering why an apple falls from a tree.

Image illustrating: Do Black Holes Obey the Laws of Gravity?

German-Swiss-American physicist Albert Einstein (1879-1955) expanded Newton's work by formulating the theory of general relativity.

Do Black Holes Obey the Laws of Gravity?

Black holes obey all laws of physics, including the laws of gravity. Their remarkable properties are in fact a direct consequence of gravity.


In 1687, Isaac Newton showed that all objects in the Universe attract each other through gravity. Gravity is actually one of the weakest forces known to physics. In our daily life, other forces from electricity, magnetism, or pressure often exert a stronger influence. However, gravity shapes our Universe because it makes itself felt over large distances. For example, Newton showed that his laws of gravity can explain the observed motions of the moons and planets in the Solar System.


Albert Einstein refined our knowledge of gravity through his theory of general relativity. He first showed, based on the fact that light moves at a fixed speed (671 million miles per hour), that space and time must be connected. Then in 1915, he showed that massive objects distort the four-dimensional space-time continuum, and that it is this distortion that we perceive as gravity. Einstein's predictions have now been tested and verified through many different experiments. For relatively weak gravitational fields, such as those here on Earth, the predictions of Einstein's and Newton's theories are nearly identical. But for very strong gravitational fields, such as those encountered near black holes, Einstein's theory predicts many fascinating new phenomena.


Image illustrating: Do Black Holes Obey the Laws of Gravity?

The theory of general relativity continues to be tested with ever more accurate measurements, for example by NASA's Gravity Probe B satellite.

Next Encyclopedia Topic

Previous Encyclopedia Topic

Back to Encyclopedia Contents

Related Questions:

Case studies of Solar System objects:


External Links and Further Reading:



Content OverviewCreditsContact UsGlossary

http://hubblesite.org/explore_astronomy/black_holes/graphics/bar_right.gif

http://hubblesite.org/explore_astronomy/black_holes/graphics/bar_bottom2.gif

Quit

Hubblesite Special Feature

HomeJourney to a Black HoleBlack Hole Encyclopedia

Black Holes: Gravity's Relentless Pull

http://hubblesite.org/explore_astronomy/black_holes/graphics/bar_left.gif

Image illustrating: What is inside a black hole?

A wormhole can provide a short-cut connection between two distant points in the curved space-time of the Universe, as this diagram shows.

Image illustrating: What is inside a black hole?

American physicist Kip Thorne (1940- ) researched the hypothetical possibilities of time travel using wormholes.

What is inside a black hole?

We cannot glimpse what lies inside the event horizon of a black hole because light or material from there can never reach us. Even if we could send an explorer into the black hole, she could never communicate back to us.


Current theories predict that all the matter in a black hole is piled up in a single point at the center, but we do not understand how this central singularity works. To properly understand the black hole center requires a fusion of the theory of gravity with the theory that describes the behavior of matter on the smallest scales, called quantum mechanics. This unifying theory has already been given a name, quantum gravity, but how it works is still unknown. This is one of the most important unsolved problems in physics. Studies of black holes may one day provide the key to unlock this mystery.


Einstein's theory of general relativity allows unusual characteristics for black holes. For example, the central singularity might form a bridge to another Universe. This is similar to a so-called wormhole (a mysterious solution of Einstein's equations that has no event horizon). Bridges and wormholes might allow travel to other Universes or even time travel. But without observational and experimental data, this is mostly speculation. We do not know whether bridges or wormholes exist in the Universe, or could even have formed in principle. By contrast, black holes have been observed to exist and we understand how they form.

Next Encyclopedia Topic

Previous Encyclopedia Topic

Back to Encyclopedia Contents

Related Questions:

Case studies of black holes:


External Links and Further Reading:



Content OverviewCreditsContact UsGlossary

http://hubblesite.org/explore_astronomy/black_holes/graphics/bar_right.gif

http://hubblesite.org/explore_astronomy/black_holes/graphics/bar_bottom2.gif

Quit

Hubblesite Special Feature

HomeJourney to a Black HoleBlack Hole Encyclopedia

Black Holes: Gravity's Relentless Pull

http://hubblesite.org/explore_astronomy/black_holes/graphics/bar_left.gif

Image illustrating: How big is a black hole?

In a non-rotating black hole, a central point singularity is surrounded by an imaginary sphere called the event horizon. Its size is called the Schwarzschild radius.

Image illustrating: How big is a black hole?

German physicist Karl Schwarzschild (1873-1916) first discovered the solutions of the equations of general relativity that describe non-rotating black holes.

How big is a black hole?

All matter in a black hole is squeezed into a region of infinitely small volume, called the central singularity. The event horizon is an imaginary sphere that measures how close to the singularity you can safely get. Once you have passed the event horizon, it becomes impossible to escape: you will be drawn in by the black hole's gravitational pull and squashed into the singularity.


The size of the event horizon (called the Schwarzschild radius, after the German physicist who discovered it while fighting in the first World War) is proportional to the mass of the black hole. Astronomers have found black holes with event horizons ranging from 6 miles to the size of our solar system. But in principle, black holes can exist with even smaller or larger horizons. By comparison, the Schwarzschild radius of the Earth is about the size of a marble. This is how much you would have to compress the Earth to turn it into a black hole. A black hole doesn't have to be very massive, but it does need to be very compact!


Some black holes spin around an axis, and their situation is more complicated. The surrounding space is then dragged around, creating a cosmic whirlpool. The singularity is an infinitely thin ring instead of a point. The event horizon is composed of two, instead of one, imaginary spheres. And there is a region called the ergosphere, bounded by the static limit, where you are forced to rotate in the same sense as the black hole although you can still escape.


Image illustrating: How big is a black hole?

In a spinning black hole, a central ring singularity is surrounded by two event horizons, the ergosphere and the static limit.

Next Encyclopedia Topic

Previous Encyclopedia Topic

Back to Encyclopedia Contents

Related Questions:


External Links and Further Reading:



Content OverviewCreditsContact UsGlossary

http://hubblesite.org/explore_astronomy/black_holes/graphics/bar_bottom2.gif

Quit

Hubblesite Special Feature

HomeJourney to a Black HoleBlack Hole Encyclopedia

Black Holes: Gravity's Relentless Pull

Image illustrating: Can a black hole bend light rays?

Simulated view towards a black hole. The background stars are shown with realistic colors. Close to the black hole, they all blend together into a white haze.

Image illustrating: Can a black hole bend light rays?

Albert Einstein's prediction that gravity would bend starlight as it passed the Sun was verified during a total solar eclipse over Brazil in 1919.

Can a black hole bend light rays?

Imagine that you are in orbit around a black hole at a safe distance outside the event horizon. What would the sky look like? Normally you would just see the background stars steadily sliding by, due to your own orbital motion. But the gravitational force of a black hole changes things considerably.


Light rays that pass close to the black hole get caught and cannot escape. Therefore, the region around the black hole is a dark disk. Light rays that pass a little further away don't get caught but do get bent by the black hole's gravity. This makes the starfield appear distorted, as in a funhouse mirror. It also produces multiple images. You would see two duplicate images of the same star on opposite sides of the black hole, because light rays passing the black hole on either side get bent toward you. In fact, there are infinitely many images of each star, corresponding to light rays that circle the black hole several times before coming toward you.


Einstein's theory of general relativity predicts that every object bends light rays through its gravity. This is called gravitational lensing. For our Sun this effect is very weak, but it has been measured. For more massive and distant objects in the Universe much stronger lensing has been seen. However, it has not yet been possible to observe this effect near a black hole, or to directly photograph the dark disk surrounding a black hole. However, this may become possible in the foreseeable future.



The gravity of this galaxy cluster (called Abell 2218) bends the light from more distant galaxies behind it, creating thin arcs of light.

Next Encyclopedia Topic

Previous Encyclopedia Topic

Back to Encyclopedia Contents

Related Questions:


External Links and Further Reading:


Image illustrating: Can a black hole bend light rays?

The four blue dots (known as the "Einstein Cross") are images of the same distant quasar. Its light is bent by the gravity of a foreground galaxy.



Content OverviewCreditsContact UsGlossary

http://hubblesite.org/explore_astronomy/black_holes/graphics/bar_right.gif

http://hubblesite.org/explore_astronomy/black_holes/graphics/bar_bottom2.gif

Quit

Tidak ada komentar:

Posting Komentar