“Gravitational and electromagnetic interactions are long-range interactions, meaning they act on objects no matter how far they are separated from each other.” -Francois Englert
One of the most spectacular predictions of Einstein’s General Relativity was the existence of gravitational lensing, whereby a large foreground mass could act as a lens, magnifying and distorting the background light source behind it. Although this was first observed for quasars, large galaxy clusters act as the most powerful lenses.
Which is why it was such a surprise that the brightest feature in the recently observed galaxy cluster SDSS J1531+3414 wasn’t from gravitational lensing, as originally thought, but was simply a gas bridge of star formation connecting two giant elliptical galaxies. It took redshift data for the individual components to arrive at that conclusion, showing once again that even the best experience and intuition is no substitute for good data.