Astrofisika

Oleh: Arip Nurahman Department of Physics Faculty of Sciences and Mathematics Indonesia University of Education

Astrofisika adalah cabang astronomi yang berhubungan dengan fisika jagad raya, termasuk sifat fisik (luminositas, kepadatan, suhu, dan komposisi kimia) dari objek astronomi seperti planet, bintang, galaksi dan medium antarbintang, dan juga interaksinya. Kosmologi adalah teori astrofisika pada skala terbesar.

Dalam praktek, hampir semua riset astronomi modern mecakup sebagian dari fisika. Nama sekolah program kedoktoran ("Astrofisika" dan "astronomi") di banyak tempat seperti AS seringkali banyak menyangkut sejarah departemen tersebut daripada isi programnya.

Astrofisika pengamatan adalah bidang ilmu yang berurusan dengan pengumpulan, pengukuran, dan analisis kuantitatif dari informasi mengenai benda langit. Bidang ini juga meliputi penjelasan berdasarkan ilmu fisika mengenai instrumentasi astronomi yang digunakan, seperti teleskop, spektrometer, dan detektor yang mengubah informasi tersebut menjadi sinyal.

Informasi yang diperoleh dari pengamatan astrofisika kebanyakan berupa radiasi elektromagnetik. Perbedaan pada rentang panjang gelombang elektromagnetik yang dideteksi membuka cabang-cabang baru dalam astrofisika pengamatan, di antaranya:

* Astronomi radio
* Astronomi inframerah
* Astronomi optik
* Astronomi ultraviolet
* Astronomi sinar-X
* Astronomi sinar gamma


Astrofisika teoretis ialah bidang ilmu yang mencari penjelasan fenomena yang diamati oleh astronom dalam istilah fisika dengan pendekatan teoretis. Dengan tujuan ini, para astrofisikawan teoretis menciptakan dan mengevaluasi model-model dan teori fisika untuk membuat kembali dan memperkirakan observasi. Dalam kebanyakan kasus, mencoba memahami implikasi model fisika tak mudah dan memakan banyak waktu dan usaha.

Astrofisikawan teoretis menggunakan variasi peralatan yang luas yang termasuk model analitik (sebagai contoh, politrope untuk memperkirakan perilaku bintang) dan simulasi bilangan komputasional. Masing-masing memiliki beberapa keuntungan. Umumnya model analitik sebuah proses lebih baik untuk memberikan pengetahuan ke dalam hati apa yang sedang berlangsung. Model numerik dapat mengungkap keberadaan fenomena dan efek yang sebaliknya takkan terlihat.

Para teoris dalam astrofisika berusaha menciptaklan model teoretis dan memperhitungkan konsekuensi pengamatan model-model itu. Bantuan ini memungkinkan pengamat mencari data yang dapat menyangkal model atau bantuan dalam pemilihan antara beberapa model berganti atau yang bertentangan.

Para teoris juga mencoba menghasilkan atau memodifikasi model untuk memperhitungkan data baru. Jika ada ketakkonsekuenan, kecenderungan umum ialah untuk mencoba membuat modifikasi minimal pada model itu untuk mencocokkan data. Dalam beberapa kasus, sejumlah besar data yang tak konsisten yang melebihi waktu bisa menimbulkan tertinggalnya model itu secara keseluruhan.

Dalam komunitas astronomi, secara luas para teoris dikarikaturkan seperti tak pada tempatnya secara mekanis dan tak mujur buat usaha pengamatan. Memiliki seorang teoris di sebuat observatorium mungkin dianggap membawa sial pada observasi yang sedang berjalanan dan menyebabkan mesin rusak atau merasa langit mendung di atas.

Topik-topik yang dipelajari astrofisikawan teoretis termasuk: dinamika bintang; pembentukan galaksi; struktur zat berskala besar di alam semesta; asal sinar kosmik; relativitas umum dan kosmologi. Relativitas astrofisika berjalan sebagai alat untuk mengukur sifat struktur skala besar yang mana gravitasi memainkan peran penting dalam fenomena fisika yang diamati dan berjalan sebagai dasar untuk (astro)fisika lubang hitam dan studi gelombang gravitasi.

Beberapa teori/model yang diterima luas dalam astrofisika termasuk Big Bang, pemompaan kosmik, benda hitam, dan teori fundamental fisika. Teori astrofisika yang memiliki beberapa pendukung namun secara luas nampaknya berbeda dengan observasi ialah kosmologi plasma. Contoh teori astrofisika yang tak diterima luas namun dianggap cukup bisa bertahan untuk karya lanjutan yang pantas ialah MOND.

Voyager Plasma Science Experiment

Voyager Plasma Science Experiment

Solar wind data measured by VOYAGER 2 up through August 30, 2007

Edited and Add By:

Arip nurahman

Department of Physics, Faculty of Sciences and Mathematics

Indonesia University of Education
&
Follower Open Course Ware at MIT-Harvard University, Cambridge. USA.

As of 7 December 2007, Voyager 2 was 7.921 billion miles from Earth (84.293 Astronomical Units from the Sun), well beyond the orbit of Pluto. Voyager 2 is leaving the solar system at 36,000 miles per hour, which is 3.2 AU per year, or 1 light year per 18,600 years.

On the same date, the light travel time from Voyager 2 to Earth was 11 hours, 48 minutes, 41 seconds. Data are returned from the spacecraft at 160 bits per second, using a transmitter with about 25 watts (!) of power.

VOYAGER 2 data up through August 30, 2007
(Daily averages through November 25, 2007)


The outer atmosphere of the Sun expands outward to form the solar wind, with average speeds of 400 km/sec (roughly one million miles per hour). The Plasma Science Experiment on Voyager 2 measures that speed every 192 seconds, and that information is returned to Earth over the Deep Space Net, analyzed, and plotted here within a few days of receipt.
VOYAGER 2 SOLAR WIND SPEED PLOTS

These plots show hourly averages of the solar wind speeds measured by Voyager 2 over the last 500 and 100 days, respectively.
Acquiring the Voyager 1 and 2 Data

Voyager plasma data are available from MIT through the links below or directly through anonymous ftp to space.mit.edu. (cd pub/plasma/vgr). Please look at the README files in each directory before using these data.

Voyager 1: hourly averages and fine resolution data are available by year for 1977-1980.
Voyager 2: hourly averages and fine resolution data are available by year for 1977-present. In addition, a file of daily averages for the entire Voyager 2 mission (de-spiked and then averaged by day) will be updated periodically.
Note that hourly average values for days 2007/240 (28 August 2007) to the present are being re-analyzed.
[go to voyager events page] Show recent events.
VOYAGER 2 SOLAR WIND DYNAMIC PRESSURE



These plots show hourly averages of the solar wind dynamic pressure observed by Voyager 2 over the entire mission (100-day averages) and over the last four years (25-day averages), respectively. These pressures are normalized to 1 AU by multiplying by the square of the spacecraft's distance.
VOYAGER DATA OVERVIEW




These plots show 50-day averages of the solar wind speed, density, and temperature over the life of the Voyager mission (from 1977 to the present), and 1-day averages over the last three years, respectively. The density shown is normalized to Earth by multiplying by the distance to Voyager in AU squared.

Light and matter united

Light and matter united

Opens the way to new computers and communication systems

By William J. Cromie

Harvard News Office

Edited and Add By:

Arip Nurahman

Department of Physics,

Faculty of Sciences and Mathematics

Indonesia University of Education

Lene Hau has already shaken scientists' beliefs about the nature of things. Albert Einstein and just about every other physicist insisted that light travels 186,000 miles a second in free space, and that it can't be speeded-up or slowed down. But in 1998, Hau, for the first time in history, slowed light to 38 miles an hour, about the speed of rush-hour traffic.



Two years later, she brought light to a complete halt in a cloud of ultracold atoms. Next, she restarted the stalled light without changing any of its characteristics, and sent it on its way. These highly successful experiments brought her a tenured professorship at Harvard University and a $500,000 MacArthur Foundation award to spend as she pleased.

Now Mallinckrodt Professor of Physics and of Applied Physics, Hau has done it again. She and her team made a light pulse disappear from one cold cloud then retrieved it from another cloud nearby. In the process, light was converted into matter then back into light. For the first time in history, this gives science a way to control light with matter and vice versa.

It's a thing that most scientists never thought was possible. Some colleagues had asked Hau, "Why try that experiment? It can't be done."

In the experiment, a light pulse was slowed to bicycle speed by beaming it into a cold cloud of atoms. The light made a "fingerprint" of itself in the atoms before the experimenters turned it off. Then Hau and her assistants guided that fingerprint into a second clump of cold atoms. And get this - the clumps were not touching and no light passed between them.

"The two atom clouds were separated and had never seen each other before," Hau notes. They were eight-thousandths of an inch apart, a relatively huge distance on the scale of atoms.

The experimenters then nudged the second cloud of atoms with a laser beam, and the atomic imprint was revived as a light pulse. The revived light had all the characteristics present when it entered the first cloud of atomic matter, the same shape and wavelength. The restored light exited the cloud slowly then quickly sped up to its normal 186,000 miles a second.
Communicating by light

Light carries information, so think of information being manipulated in ways that have never before been possible. That information can be stored - put on a shelf, so to speak - retrieved at will, and converted back to light. The retrieved light would contain the same information as the original light, without so much as a period being lost.

Or the information could be changed. "The light waves can be sculpted," is the way Hau puts it. "Then it can be passed on. We have already observed such re-sculpted light in our lab."

A weird thing happens to the light as it enters the cold atomic cloud, called a Bose-Einstein condensate. It becomes squeezed into a space 50 million times smaller. Imagine a light beam 3,200 feet (one kilometer) long, loaded with information, that now is only a hair width in length but still encodes as much information.

From there it becomes easier to imagine new types of computers and communications systems - smaller, faster, more reliable, and tamper-proof.

Atoms at room temperature move in a random, chaotic way. But when chilled in a vacuum to about 460 degrees below zero Fahrenheit, under certain conditions millions of atoms lock together and behave as a single mass. When a laser beam enters such a condensate, the light leaves an imprint on a portion of the atoms. That imprint moves like a wave through the cloud and exits at a speed of about 700 feet per hour. This wave of matter will keep going and enter another nearby ultracold condensate. That's how light moves darkly from one cloud to another in Hau's laboratory.

This invisible wave of matter keeps going unless it's stopped in the second cloud with another laser beam, after which it can be revived as light again.

Atoms in matter waves exist in slightly different energy levels and states than atoms in the clouds they move through. These energy states match the shape and phase of the original light pulse. To make a long story short, information in this form can be made absolutely tamper proof. Personal information would be perfectly safe.

Such a light-to-matter, matter-to-light system "is a wonderful thing to wrap your brain around," Hau muses.

Details of the experiments appear as the cover story of the Feb. 8 issue of Nature. Authors of the report include graduate student Naomi Ginsberg, postdoctoral fellow Sean Garner, and Hau.
In a practical manner

You won't see a light-matter converter flashing away in a factory, business, or mall anytime soon. Despite all the intriguing possibilities, "there are no immediate practical uses," Hau admits.

However, she has no doubt that practical systems will come. And when they do, they will look completely different from anything we are familiar with today. They won't need a lot of wires and electronics. "Instead of light shining through optical fibers into boxes full of wires and semiconductor chips, intact data, messages, and images will be read directly from the light," Hau imagines.

Creating those ultracold atomic clouds in a factory, office, or recreation room will be a problem, but one she believes can be solved. "The atomic clouds we use in our lab are only a tenth of a millimeter (0.004 inch) long," she points out. "Such atom clouds can be kept in small containers, not all of the equipment has to be so cold. Most likely, a practical system designed by engineers will look totally unlike the setup we have in our lab today."

There are no "maybes" in Hau's voice. She is coolly confident that light-to-matter communication networks, codes, clocks, and guidance systems can be made part of daily life. If you doubt her, remember she is the person who stopped light, converted it to matter, carried it around, and transformed it back to light.