## Senin, 18 Juli 2011

### Fisika untuk Universitas

Fisika untuk Universitas

Ditujukan untuk meningkatkan kualitas proses dan hasil perkuliahan Fisika di tingkat Universitas

Today I'd like to talk with you about my early days at MIT and the research that I did here.

This is a long time ago.

I got my Ph.D. in the Netherlands, in nuclear physics, and I came to MIT in January 1966, almost 34 years ago.

And the idea was that I was only going to spend here one year on a postdoc position, but I liked it so much that I never left, and I don't regret it.

I joined the X-ray Astronomy group here of Professor Rossi.

Now, X-ray astronomy has to be done from above the Earth atmosphere or at least near the top of the Earth atmosphere because the X rays are absorbed by air, unlike optical astronomy and radio astronomy, which can be done from the ground.

The kind of X rays that we measure are not unlike those that your dentist is using when he takes an X-ray picture.

The energy range of these X rays is somewhere between one and 30, 40 kilo-electron volts, and if you don't know what a kilo-electron volt is, that's fine, too, but you never express the energy of an X ray in terms of joules, because the number becomes so ridiculously small.

During World War II, under Hitler's Germany, Wernher von Braun developed the V-2 rockets for destructive war purposes.

It was developed in Peenemuende.

And after the war, the Americans used these V-2 rockets for scientific purposes, and the first rocket flights to search for X rays from the Sun took place in 1948.

And X rays were found from the Sun.

That was quite a surprise.

And the power, energy per second that the Sun puts out in X rays divided by the power in optical, which is almost all the radiation of the Sun--

I'll give it the symbol of the Sun--

is approximately 10 to the minus 7, so only one ten-millionth of all the energy comes out in X rays.

So, from an energy point of view, it's very, very little.

It varies a great deal, too.

But it is really very little.

In 1962, scientists here in Cambridge, among them Professor Bruno Rossi, who was a professor at MIT, and Riccardo Giacconi and Herb Gursky--

who were working across the street at American Science and Engineering--

attempted to do an experiment to see whether they could detect X rays from objects outside our solar system.

Now, the odds were very low that they were going to succeed, and the reason is very simple.

If you take the Sun and you move it out to the nearest stars, which is typically ten to a hundred light-years, you wouldn't stand a chance to see X rays.

In fact, the sensitivity of the detectors in these days was too low by at least nine orders of magnitude, a factor of one billion.

To everyone's surprise--

to everyone's, yeah, happy surprise, I should say--

they succeeded, and they discovered an object which was later called Sco X-1.

It's in the constellation Scorpius, "X" stands for X rays, and "1" for the first X-ray source in that constellation.

The total power output of that source was about 10,000 times more than the Sun.

That doesn't make the source so special, because there are many stars in the sky that radiate way more energy than our Sun does, but what's so very special about Sco X-1, that the X-ray power over the optical power for Sco X-1 was approximately 1,000.

In other words, the X rays are the dominant source of energy and the optical is sort of, let's call it a by-product, whereas with the Sun, the optical is the main thing and the X rays is sort of a by-product.

And so the \$64 question in those days was, what can these objects be? They must be very different from the Sun, and that's what I want to discuss with you.

When I came to MIT in 1966, there were about six of these X-ray sources known in the sky.

Today there are thousands known, but there were six then.

And they were discovered from rocket flights.

These rockets would be launched, typically from White Sands, and they would spend about five minutes above the Earth atmosphere.

And during those five minutes they scanned the sky, and six sources were discovered.

I joined here the group of Professor George Clark, who is still a professor at MIT.

He was working on observations to be made from very high-flying balloons.

So we would build a telescope, and we would launch it on a balloon, and go near the top of the Earth atmosphere.

It's not as good as a rocket flight which gets completely out of the Earth atmosphere, but the flights on balloons can last way longer than five-minute rocket flights.

We could fly hours and, if we were lucky, even days, but the price we paid for that is that even though there was only very little atmosphere left above us--

about 0.3 percent of the atmosphere was left--

still that caused an effect of the absorption, so we did lose X rays that the rocket flights did not lose.

To give you a rough idea of what it took in those days--

I worked on this with graduate students and with many undergraduate students--

a telescope in those days, to build it cost typically a million dollars, and it would take us two years to build one.

The balloons that we needed to launch them were about \$100,000 in those days, and the helium that we needed to get it up was about \$80,000, and the weight of such a payload, of our telescope, was about 1,000 kilograms.

These balloons would go up to 140,000 feet and they were huge--

they were about 500 feet across.

I will show you pictures of them very shortly.

It was a risky business in that no guarantee of success.

You bought the balloons.

If they worked, so much the better.

If they didn't work, tough luck.

There was just no way that you could recover the money.

They were very thin; the balloons are made of polyethylene, and the thickness of the polyethylene was thinner than cigarette paper, so you can imagine how easy it is to damage them, and if you don't damage them during the launch, it's easy to damage them on the way up, due to the jet stream and the very cold layers of air that you have in the tropopause.

So I would like to show you now some slides, and then we will get back to talking a little bit more about X-ray astronomy.

All right, so let's see what we have first.

You see here two of my undergraduate students.

At the time they were undergraduate students.

Now they are both Ph.D.s, and some of you may think that science doesn't have much romance, but there is a lot.

They married and they have kids.

Pengembangan Perkuliahan

1. Buatlah sebuah Esai mengenai materi perkuliahan ini

2. Buatlah sebuah kelompok berjumlah 5 orang untuk menganalisis materi perkuliahan ini

3. Lakukan Penelitian Sederhana dengan kelompok tersebut

4. Hasilkan sebuah produk yang dapat digunakan oleh masyarakat

5. Kembangkan produk tersebut dengan senantiasa meningkatkan kualitasnya

1. Para Dosen MIT di Departemen Fisika

a. Prof. Walter Lewin, Ph.D.

b. Prof. Bernd Surrow, Ph.D.

2. Para Dosen Pendidikan Fisika, FPMIPA, Universitas Pendidikan Indonesia.

Terima Kasih Semoga Bermanfaat dan mohon Maaf apabila ada kesalahan.