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Kecepatan cahaya gak konstan?

Dimulai oleh reborn, Februari 11, 2007, 04:54:12 AM

« sebelumnya - berikutnya »

0 Anggota dan 1 Pengunjung sedang melihat topik ini.

reborn

Research sebelumnya : [pranala luar disembunyikan, sila masuk atau daftar.]

Light and matter united
By William J. Cromie
[pranala luar disembunyikan, sila masuk atau daftar.]

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.

krigjsman

wehh panjang amat artikelnya, mesti mikir nech, cuma yang gw ingat dari kuliah relativitas umum, emang cahaya itu juga trepengaruh sama gravitasi, jadi kecepatannya gak konstan

reborn

Kalo gak salah inget juga, partikel itu kecepatan grupnya yang tidak dapat melebihi kecepatan cahaya... tapi apa hubungannya ??? hehe... ngejunk. Kecapean seharian di luar baru bisa ngepost lagi jam segini :(

Didobi

untuk lebih mengetahui lebih jelas lagi apakah kecepatan cahaya konstan apa tidak harus dibuktikan dengan benda yg memiliki kecepatan cahaya

tapi masalahnya alat itu saat ini belum ada  :-\

superstring39

Kutip dari: reborn pada Februari 16, 2007, 04:12:07 AM
Kalo gak salah inget juga, partikel itu kecepatan grupnya yang tidak dapat melebihi kecepatan cahaya...

Tapi seinget gw kecepatan grup itu yang mungkin melebihi kecepatan cahaya tapi kecepatan gelombangnya tidak bisa melebihi kecepatan cahaya, eh... itu buat grup gelombang deh  :P

Karno Giyantono

boleh juga nih tuk tambahan ilmu, thanks banget reborn...
"Orang Pintar adalah Orang yang Berusaha Membangun Rumah atau Kehidupan yang Bagus di dunia dan Istana Di Surga"

[move]caranya Belajar dan Bekerja serta Beramal dan Beribadah

HyawehHoshikawa

masukin aja ke dalam medium laen, otomatis kecepatan cahayanya pasti berkurang ;D
Rationality alone isn't enough, the world is Complex.

Karno Giyantono

Kutip dari: HyawehHoshikawa pada April 08, 2009, 10:29:13 PM
masukin aja ke dalam medium laen, otomatis kecepatan cahayanya pasti berkurang ;D

yoi bener juga.
trus anehnya jika sudah keluar dari medium yang indeks biasnya kecepatannya kembali normal lagi. kalo saya sudah tau jawabannya.

gelombang cahaya bergerak tegak lurus dengan arah rambatannya. sedangkan rambatannya bergerak dengan kecepatan 3 kali 10 pangkat delapan. nah berarti kecepatan gerak gelombang lebih cepat dari kecepatan rambatnya.

kata einstein gak ada yang lebih cepat dari kecepatan rambat cahaya, namun kecepatan gerak gelombang itu sendiri lebih cepat.
"Orang Pintar adalah Orang yang Berusaha Membangun Rumah atau Kehidupan yang Bagus di dunia dan Istana Di Surga"

[move]caranya Belajar dan Bekerja serta Beramal dan Beribadah

HyawehHoshikawa

oh, iya yah...
kecepatan cahaya kan 3x10^8 itu kalo' vektornya,
tapi panjang lintasan yang dilalui oleh cahaya dalam waktu 1 detik kan pasti lebih dari itu yah...

btw, numpang tanya dong..
misal ada 3 orang pengamat(Katakanlah pengamat A,B dan C) bergerak dengan kecepatan = kecepatan cahaya...
maka siapa yang berada di bagian paling depan?
menurut pengamat A
menurut pengamat B
menurut pengamat C
Rationality alone isn't enough, the world is Complex.

Sky

Kutip dari: Karno Giyantono pada April 09, 2009, 12:00:34 PM
gelombang cahaya bergerak tegak lurus dengan arah rambatannya. sedangkan rambatannya bergerak dengan kecepatan 3 kali 10 pangkat delapan. nah berarti kecepatan gerak gelombang lebih cepat dari kecepatan rambatnya.

Ha? maksudnya apa nih?

Kutip dari: HyawehHoshikawa pada April 10, 2009, 07:52:31 AM
oh, iya yah...
kecepatan cahaya kan 3x10^8 itu kalo' vektornya,
tapi panjang lintasan yang dilalui oleh cahaya dalam waktu 1 detik kan pasti lebih dari itu yah...

Ini juga maksudnya apa,ka?
maaf, g ngerti...

HyawehHoshikawa

KutipIni juga maksudnya apa,ka?
maaf, g ngerti...

tau bedanya panjang lintasan ama perpindahan kan?
nah itu maksut ane, lintasan cahaya itu kan bentuknya sinusoidal, (gerakannya 2 dimensi ex: ke arah X yang kecepatannya C, juga kearah Y yang kecepatannya sinusoidal.)

note:sinusoidal= membentuk grafik sinus, naek turun naek turun gitu...
Rationality alone isn't enough, the world is Complex.

Sky

Oooh gitu maksudnya....

Jadi lintasan yang dilalui cahaya itu berbeda dengan lintasan yang dilalui gelombang kan?
Bener ga?

Yang searah rambat sebesar c, tapi yang searah getar tergantung sama fungsi sinus...

Lho,Lho...Ntar dulu... kalo dijumlah secara vektor ,berarti kecepatannya lebih dari c!

Aha, baru nyambung nih...

Berarti emang kecepatan cahaya ga selalu c dong? Berarti selama ini saya salah konsep,yah?

Terus pernyataan di buku-buku tentang kecepatan cahaya selalu c itu salah y?

HyawehHoshikawa

hmm...
Mungkin intinya harusnya yang C itu jarak yang ditempuh oleh cahaya dalam waktu 1 detik...

Mungkin...
Rationality alone isn't enough, the world is Complex.

superstring39

sepertinya ada miskonsepsi disini. jangan dibayangkan bahwa cahaya adalah seperti bola yang terikat pada tali dan kemudian tali digetarkan sehingga membentuk gelombang dan bolanya naik turun itu disebut cahaya. tidak ada gerakan semacam itu pada cahaya. gambar gelombang cahaya yang kalian liaht itu bukan sebuah pergerakan mekanis seperti yang kalian bayangkan. namun itu adalah osilasi "besarnya" atau magnitude dari medan listrik dan medan magnet yang saling tegak lurus satu samalain dan sefase, maka akan terbentuk gelombang elektromagnetik jika frekuensinya sesuai dengan frekuensi cahaya tampak maka kita sebut dengan cahaya.

dark_nekron

Sebenarnya cahaya itu apa seh? Partikel apa gelombang. Aq pernah nyoba sinar matahari aq fokus kan pakek lens dan bs membakar kertas tp jk sinar ini kita pantulkan dengan cermin dan sinar pantulan ini qt fokuskan dengan lens pada jarak 1-10 cm msh bs melubangi kertas tp kenapa pas jk lbh dr itu sudah tdk bs lg membakar kertas. Denger2 klo panas radiasi nggak terpengaruh oleh lingkungan karena merambat dlm bentuk gelombang. Apakah setelah menembus cermin cahaya berubah menjadi partikel sehingga dpt d pengaruhi lingkungan?
Sory oot banget nih aq..
[move]......i'am still stupid..... and always stupid......[/move]
----ORANG GENDENG----