Blog Post 12, Free Form: A Brief History of Gravitational Lensing

So, this week we worked with gravitational lensing and its subtype: microlensing.  Let's take a step back and examine the history of this phenomenon.

https://upload.wikimedia.org/wikipedia/commons/1/11/A_Horseshoe_Einstein_Ring_from_Hubble.JPG

In 1784 and 1801 respectively, Henry Cavendish and Johann Georg von Soldner (both really epic names in my opinion) used Newtonian mechanics just like we did in WS 4.1, Problem 1 to determine a photon's deflection due to the gravity of a massive object.  Of course, at the time, the photon had yet to be discovered or accepted, rather light was still considered to be a simple wave, so the conclusion was that "starlight" was bent by massive objects.  Later in 1915, Einstein's General Relativity explained this phenomenon as light following straight paths in curved spacetime.  Just like we did in WS 4.1, general relativity added an additional factor of two into the bending calculation to yield a final bending angle of:
\[\alpha = \frac{4GM_L}{bc^2}\]
Where \(G\) is Newton's gravity constant, \(M_L\) is the mass of the lensing object, \(c\) is the speed of light in a vacuum, and \(b\) is the impact parameter, or the distance through which a photon is mostly affected by the lens' gravity.

In a rather famous experiment, this figure was experimentally tested during a solar eclipse.  The approach was clever.  At the time, the sun was the easiest object to use as a lens.  It was massive, and quite close by.  The primary problem was the the sun is rather... bright.  So to view the effect on the light of background stars from the sun's gravity,the sun had to be booked out.  Conveniently, the moon takes care of that during solar eclipses.  (Just for the record, tonight (September 27, 2015) a superman lunar eclipse will occur.  This should be very cool, as the next one of these will not occur until 2033).  The solar eclipse occurred in 1919, and successfully confirmed the lensing equation.

Later in 1937, Fritz Zwicky decided to think outside the box, and realized that entire galaxies could be used for lensing.  Zwicky's idea was not tested until 1979 when eh first observable lens was discovered.  Discovered by Dennis Walsh, Bob Carswell, and Ray Weymann, it was named SBS 0957+561.  If this is a bit too boring, it also had the nickname "Twin QSO" (QSO = Quasi-Stellar Object) due to the fact that it looked like two QSO's next to one another.  This was actually just a consequence of the lensing effect, there is only one object, the other is just a lensed image.

Since then, the number of known lenses has exploded with our better telescope and computer technology and CCD cameras.  We currently use lenses to look into deep space to observe galaxies that would normally be far too far away to normally observe.  Lenses are also important in the search for MACHO's (MAssive Compact Halo Objects) since most of these objects should serve as excellent lenses due to their high mass and lack of self-illumination.  With that, let's look at the future brightly with a smile.

https://upload.wikimedia.org/wikipedia/commons/b/b9/HST-Smiling-GalaxyClusterSDSS-J1038%2B4849-20150210.jpg

Sources:
https://en.wikipedia.org/wiki/Gravitational_lens
https://en.wikipedia.org/wiki/Photon
http://www.planetary.org/explore/space-topics/exoplanets/microlensing.html