Why is light affected by gravity if it has no mass?
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Gravity was famously “discovered” by Newton around 1700, who believed in absolute space and absolute time, and in a universe that was static and un-changing. He thought of things as set in stone because of his belief in an absolute God who created everything as we see it. This is one pit-fall of human scientists, they have preconceived beliefs that prevent them from accepting the truth. Einstein himself was unable to accept probability in the atomic realm of quantum theory (“God does not play dice”), even though his own discoveries helped bring about quantum theory.
200 years after Newton, Einstein realized that time, space, and mass are not fixed, but at extreme speeds approaching the cosmic speed limit, the speed of light, weird things occur. For instance, if you wanted to travel at light speed, your mass would increase towards the infinite, and you’d need infinite energy to attain that speed, thus only a massless photon can actually travel at light speed.
In fact, the “constant” for Einstein was the speed of light, not time or space. He realized time would slow down for those traveling very quickly, for example in the Twin’s Paradox where one twin will age slightly slower if they’ve been traveling slightly faster (see astronaut Mark Kelly and his twin on Earth).
Regarding gravity itself, Einstein’s special and general relativity from 1905-1915 showed that space and time is not a graph paper with X and Y grids that never changes. It seems that way to humans, it seems that way based on Newton, but to beams of light, space and time warp a lot around large masses like the sun. This was proven during a solar eclipse, where the light from stars near the sun was bent by the sun, proven in 1918 and making Einstein an overnight genius sensation. It was also proven by the known movements of Mercury, whose motion was slightly warped by the sun, which was not predicted by Newton’s original equation.
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