Why is light affected by gravity if it has no mass?
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It’s not affected by gravity it’s affected by space-time, the light from its POV is still traveling straight but as an observers POV we see the light bend around objects with massive mass which distort space time
A good example is if you were on a boat and you happened to go over a whirlpool, you would have never stopped steering straight yet your facing a new direction now because the whirlpool distorted the ocean around you so to the people sitting on the boat they never stopped going straight but to everyone watching they got turned in a different direction
The whirlpool is the space time distortion of large mass objects
The ocean is just space
And the boat is light in this instance
Everyone says mass curves space time but no one says why. It’s because mass displaces the space time by occupying that area where space time was. Because of this space time is constantly trying to “reclaim” the spot that was displaced by mass. This is what causes gravity. Gravity is just space time trying to reclaim the area displaced by mass.
Because of this, light is bent too because light can only operate with space time.
Some good answers in here, but I’ll add a super-short one:
Light always follows the shortest path in space-time. Massive objects change where the shortest paths near them go.
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.
Gravity doesn’t bend light. Light will ALWAYS travel in a straight line.
The thing that happens is that the presence of mass bends the space-time itself. While it may appear that light bends in this situation, it does not. It moves straight in a curved medium which gives the illusion of light bending.
A way to look at it is imagine you and a friend are 1000 miles appart on the equator and you both start walking exactly north so you are walking in parallel lines. Despite your parallel paths, your paths will intersect at the pole. This isn’t because you bent your path because you did walk in a straight line the entire time. The problem is that the surface you were waking on itself was bent.
In bent mediums, straight lines appear curved.
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