Date: September 3, 2009

Title: Gravitational Lensing

Podcaster: SkyLK

Organization: SkyLK http://skylk.com

Description: A gravitational lens is created when the light from a distant source is bent due to the existence of a massive object between the observer and source. The object could be a galaxy, black hole, or a cluster of galaxies, and this phenomenon is known as Gravitational Lensing. Light rays composed of photons are attracted by the gravity of the mass of the massive object, resulting the linear path of light be curved, thus changing the velocity as well. Hence, the apparent image of the source can be distorted and magnified. This phenomenon was first mentioned in 1924 by the St. Petersburg physicist Orest Khvolson, and was later quantified by Albert Einstein in 1936.

Bio: Skylk.com is an astronomy information portal from Sri Lanka. Since the official launch in January 2009, skylk has being offering podcasts, articles and other interactive content in native Sri Lankan language (sinhala).

Today’s sponsor: This episode of 365 Days of Astronomy is sponsored by the American Astronomical Society, the major organization for professional astronomers in North America, whose members remind everyone that One Sky Connects Us All. Find out more or join the AAS at aas.org.

Transcript:

Naratna: Hello everyone. You are tuned into 365 Days of Astronomy podcast on 3^rd of September, 2009. Today, we are gonna talk about Gravitational Lensing. My name is Naratna and I have here with me Deshaprea and Madusha. This is actually the first podcast to IYA from Sri Lanka and we are really proud about it. Okay. To begin with, Madusha, when you talk about gravitation lensing, is it something physical or is it just a theory?

Madusha: What really happens in a gravitational lens is somewhat similar to what happens in a normal lens. Usual type of lenses. The light rays passing through it get bended up as we already know. The same thing happens our there in space due to huge gravitational force. Light rays from a very distant object get warped up in the vicinity of a huge mass and this process caused the image of the distant object to get distorted. This is simply the explanation of gravitational lens effect.

Well, you might wonder how come light rays get bent up because of gravity? Well, to answer this question, we have to go back to the year of 1915 which great physicist; Albert Einstein published the General Theory of Relativity, something that revolutionized the modern day physics. In this profound theory, he put forward a sub-theory, which explains in the vicinity of a huge mass, the space-time get warped up or bended. And as a result of this, light rays can also get bended. So theoretically, it’s possible.

What about seeing this kind of effect physically? Actually this was conformed during a total solar eclipse back in the year of 1990 as an experiment done by one of the great physicists, Arthur Stanley Eddington. They could observe bending light effect of light rays coming from distant stars. So now know for sure that what Albert Einstein has suggested is something really exist.

Naratna: When it comes to Gravitational Lensing, we all know that from his Einstein’s name, he’s pretty much connected with Gravitational Lensing. Is it Einstein who came up with the idea of Gravitational Lensing?

Madusha: Well, actually as we found Einstein, he’s the first who came up with the idea of there could be something called Gravitational Lensing kind of thing in space but this same idea was put forward as a publication by Orest Khvolson back in the year of 1924 and in 1936, Einstein himself came up with another publication saying that there could be Gravitational Lensing kind of effects out there in space and this was one of the popular publication and the name if this publication is Lens Like Action of a Star by Deviation of Light In the Gravitational Field. And in 1937, Fritz Zwicky first considered the case of where a galaxy could be acting as a source of the gravitational lens.

Many people have speculations on this gravitational lens kind of idea but it was not until 1979 that this effect was confirmed by observation of the object called green quasars.

Naratna: When we talk about Gravitational Lensing, we know that there are different types of Gravitational Lensing. We have strong Gravitational Lensing, weak Gravitational Lensing, and Gravitational Microlensing. Deshaprea, can you briefly explain what’s strong and what’s weak and what this microgravitational lensing means?

Deshaprea: First, we go into strong Gravitational Lensing. Strong Gravitational Lensing is an effect, which is capable of forming multiple images, arcs, or even Einstein Rings of the distant sources. This requires that the project lens mass density be greater than the critical density, which is generally indicated by Sigma. This leads to the idea that the single point is of the background will be transformed into creating multiple images whereas the widespread background sources will convert into arcs or rings. We use the Odd Number Theorem to determine the production of multiple images.

And moving onto the weak Gravitational Lensing, this is be different from the Gravitational Lensing which is much stronger. While the presence of any mass bends the path of light passing near it, this effect rarely produces the giant arcs and multiple images in which case, they fall into the category if associated with strong Gravitational Lensing. With a single background source, the deflection is unlikely noticeable where it is weak Gravitational Lensing. However, it’s possible that the presence of foreground mass be detected and lies in the background sources around the lensing mass.

Weak Gravitational Lensing is a statistical measurement but it provides a way to measure the masses of astronomical objects without requiring assumptions about their composition or dynamic state. So this weak Gravitational Lensing is basically a statistical idea.

Naratna: Then what about Gravitational Microlensing?

Deshaprea: This is an astronomical phenomenon that allows scientists to determine the masses of celestial objects varying from planets to stars. The specialty in this process is that this works out irrespective of the light they emit. Of course, this could also be a faint object, which literally doesn’t emit any light. However, Gravitational Microlensing enables to determine the mass of such objects. When a distant star or quasar gets sufficiently aligned with the mass of a compact foreground object, the bending of light, that would reach the gravitational field as discussed by Einstein in 1915 leads to two distorted, unresolved images resulting in an observable magnification.

The time scale of the apparent brightness depends on the mass of the foreground object as well as on the relative proper motion between the background source and the foreground lens object.

Since microlensing observations do not rely on radiation received from the lens object, this effect therefore allows astronomers to study massive objects no matter how faint they are. It is thus an ideal technique to study the galactic population of such faint or dark objects, such as brown dwarfs, red dwarfs, planets, white dwarfs, neutron stars, black holes, and massive compact halo objects. Moreover, the microlensing effect is wavelength independent allowing the scientist to study source objects that emit any kind of electromagnetic radiation.

Naratna: Okay, Deshaprea, I’ve heard about the Einstein Cross and Einstein Rings. What’s the resemblance of these when it comes to Gravitational Lensing?

Deshaprea: Well, let’s go through this Einstein Rings first. The Einstein Rings is a process of the visible image of a distant object being deformed in a such a way that there is ring like image upon the distortion. This takes place when the source, lens, and the observer are precisely aligned. It’s essential that the alignment should be precise. In fact, the first discovery of existing rings was observed in 1998. However, the initial academic reference on the Einstein rings was reported in writings of earlier mention, Orest Khvolson in 1924.

However, the likelihood of spotting an Einstein Ring increase with the mass of the lens, which results in the angular size of the ring being increased as well. So presently, there are about hundreds of known Einstein Rings.

This is about the Einstein Cross which is also more commonly called among the scientific folk, this is also referred to as Q2237+030 or QSO2337+0305, which is a Gravitational Lens Square. The Einstein Cross is apparently a quadruple image quasar resulting in formation of the cross together with the lensing galaxy at the center of the cross.

The quasar is located 8 billion light years away from us, whereas the lens is about 400 million light years away from us so they are too far. We do have in our mind that they are much far away from us. The Einstein Cross is located in the constellation of Pegasus. So if you are having some high resolution and observation, you might have focussed this but you need sophisticated analysis mechanisms to identify the cross and extract the important parameters from that.

Naratna: As I said earlier, this is the first podcast from Sri Lanka. There’s is a particular reason why we selected this Gravitational Lensing topic. Actually, there’s a Sri Lankan scientist named Professor Kavan Ratnatunga who has granted a great deal when it comes to Gravitational Lensing research.

Deshaprea: Yes. Professor Kavan Ratnatunga is a Sri Lankan scientist. He’s an astronomer. He’s credited with the discovery of a gravitational lens. So it’s really important as representing Sri Lanka a scientist has done such a great thing, such a wonderful discovery towards the research of the Gravitational Lensing. So we can say Sri Lanka has been able contribute to the field through activities like this. At this moment, we would like to recall his memories and extend some gratitude for him for his discovery so that’s about the Gravitational Lensing.

Naratna: And with that, we round up our podcast for 365 Days of Astronomy. Signing off from Sri Lanka, thank you very much.

End of podcast:

365 Days of Astronomy
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