20 Euro Silver Einstein Ring Coin P
They may not really exist, but Einstein rings can actually be seen – when light from a galaxy or star is diverted by a massive object en route to Earth. Albert Einstein himself never actually saw one and thought it unlikely that the phenomenon bearing his name would ever be observed. The man synonymous with the term ‘genius’ was not wrong about many things, but he was wrong about this, as the first Einstein ring was discovered in 1979, some 24 years after the iconic scientist’s death, 70 years ago this year. The second coin in *The Beauty of the Universe* series, the *Einstein Ring* provides remarkable insights into the curvature of space-time and this very special galactic curiosity, making it the perfect gift for astronomy enthusiasts of all ages. According to Einstein’s general theory of relativity, gravity is only one aspect of the geometry of space-time. When a solid body bends space, light is deflected, making it seem to come from different places, such as the edge of a cluster of galaxies. In reality, the light’s source lies exactly behind it and the galaxy is one and the same, not one of several. If one imagines an ideal situation with a beautifully symmetrical and perfectly spherical cluster of galaxies, the ray of light as shown on the coin is just one of many coming from the source to be perceived by the observer’s telescope. At each point on the edge of the galaxy cluster, a different image of the galaxy behind it appears. A multiplication occurs in which a single galaxy takes on the appearance of a bulging ring. We would find this truly miraculous had Einstein not shed light on the phenomenon without ever having seen it. **Coin motif** A circular wave curves upwards on the obverse of the coin. Representing an Einstein ring, this is the pictorial multiplication of the light source on the left. Between the light source and the observer, a cluster of galaxies is simplified in the centre, which deflects the light many times due to its gravity. The German for ‘gravitational lens’ refers to its function in this context. The formula Δ∅=4GM/bcÇ, which can be used to calculate the deflection of light, completes the design. The coin’s reverse has the same basic shape as the obverse but features a colour-printed interpretation of an Einstein ring.