Gravity vs. Light

The light from a distant galaxy gets warped and distorted thanks to the immense gravity of the closer reddish-orange galaxy

We usually think light is pretty fast. Usain Bolt can set new records for how quickly man can run. There are motorcycles that can travel faster than our own nerve impulses. Aircraft has long since broken the sound barrier. And yet, we have never broken the “light barrier;” in fact, we are so pathetically far away from being able to move at the speed of light that our fastest vehicles moving 10x faster than they could, still may as well not be moving at all.

Light is fast, there is no doubt. But space is a gigantic and sometimes dangerous place, even for something as speedy and evasive as a photon. Light is not invincible, and one well-known entity that can imprison light itself is the black hole – an object of infinitesimal size but infinite density, whose gravitational pull is so strong that one would need to travel faster than light in order to escape.

An artist’s perception of a black hole. The black sphere is NOT actually the black hole. The physical black hole itself, or the singularity, is incredibly tiny, theoretically infinitesimally small. The black ball is simply the event horizon – an area of space that, were you to enter, you could never leave. It is black because the photons that are unfortunate enough to wander inside this region can never return, making it a dark, desolate place from our perspective.

It is often taught in physics that gravity comes from mass, and affects other objects that have mass. It is also often taught in chemistry that photons have zero mass. This is because the universe is weird and loves to play mind games with us. Only very recently did Albert Einstein come across a startling discovery, which he described in his famous equation, E = mc^2. In shorthand, this means that mass itself is a form of energy, and vice versa. Photons may not have mass, but they do have energy (how else would a sunny day be warmer than a cold, miserable night?).

Because of this, photons are very much affected by gravity, and will absolutely be at risk for being imprisoned by black holes. In fact, anything with gravity can and will affect light. In the first picture shown above, the blue galaxy behind the red one is warped and twisted in appearance, thanks to the immense gravity of the Red messing around with the light coming from Blue. Galaxy Blue is, of course, perfectly fine – it may not even realize Galaxy Red exists – but the photons that it emits certainly do.

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