On Wednesday 14th March we stopped in at the Star of King’s Pub to look at the universe in a completely new way with our guest speaker, Benjamin Joachimi from University College London.


Benjamin Joachimi on gravitational lensing.

Amongst other astronomical endeavors, one of Joachimi’s biggest interests is the phenomenon of gravitational lensing. This happens when a massive object in space, such as a galaxy, black hole or star, distorts our view of background galaxies and stars by deflecting the light around it. From our viewpoint here on Earth, gravitational lending can turn single points of light into arcs or multiple points as they are bent around the gravitational fields of the massive objects.

As you may have guessed from the title of the event, gravitational lensing was predicted by Albert Einstein in his theory of general relativity. He made calculations on the phenomena and wrote about them back in the early 1900s, however Einstein and other physicists engaged with gravitational lens research at the time doubted that they would even be visible from the Earth. Contrary to these early expectations, the first gravitational lens was seen in 1979, when a group of scientists discovered that a pair of distant bright stars were two images of the same star bent around a giant gravitational mass.

To explain this abstract concept, Joachimi used the example of image distortion through the base of a wine glass. You can achieve the same effect of light distortion by looking at a light source, such as a candle, through the base of the glass. It’s an easy trick you can try at home, but finish your drink first before you hold it up to a light!


Potentially one of the most exciting parts of the talk was the possibility of using gravitational lensing to discover exoplanets (planets outside of our own solar system orbiting other stars). If an intermediary star is creating a gravitational lens and distorting the view of a bright star directly behind it the effect of the lens is amplified and collectively they appear as an “Einstein disk”, which is a collective circle of light from both sources. When this intermediate star has a planet in orbit you can see an effect called microlensing as the planet creates a smaller gravitational lens to boost the light source behind even further. This appears as a temporary increase in the brightness of the light mass over a few hours or days, which is a very short timescale compared with other astronomical events. This means that we can detect exoplanets that are incredibly far away and undetectable in any other way. The downside is that once the exoplanet has been detected, it is unlikely that it can ever be detected again because these microlensing events are very rare and random, requiring perfect alignment of the stars and planets.

We would love to thank Benjamin Joachimi for his interesting talk and for en-light-ening us on gravitational lensing (hope you enjoyed that pun). We hope that all who attended enjoyed themselves.

For our next Sci-Bar on the 19th April we’ll be delving into the world of microchemistry with BobWorley. We look forward to seeing as many people there as possible!

Reporter: Ben Johnson-Newbery

Pictures: Silvia Guidotti

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