How do blackholes attract light when light is massless.
In: Planetary Science
Masses don’t attract masses, thats a simple modell that can’t be applied to 100% of cases. Einstein’s theory of general relativity is a more accurate modell.
In GR instead of introducing gravity as an attractive force we think of masses bending spacetime. The important bit is that without the mass spacetime has flat geometry we call that flat spacetime and with a mass we get curvature which changes the geometry which is mostly interesting because it changes what straight lines are. On surfaces in general we call straight lines geodesics, for example on a globe the equator is one, longitude lines are geodesics but random latitude lines aren’t.
If you aren’t accelerating, like you don’t fire up your thrusters in your spaceship, you fallow a geodesic. Around a mass this geometric “turns” towards the mass, or rather it focuses geodesics, there are a lot of ways to word this, again think of longitude lines converging towards the poles, this is what happens to geodesics. So really things are falling because their future is on the ground.
Light also follows geodesics but as its massless it cannot follows any random path it must always follows the so called null geodesics, these are the paths that a beam of light can take. In curved space null geodesics are also “bent” (again they are straight lines, the curvature of the surface they are straight lines in changes fundamental geometry properties like whether parallels exist). This gives away to a famous effect called gravitational lensing which was the first experiment to verify GR, looking at the images of stars around the Sun during a solar eclipse and comparing them to the actual location of these stars, and we did see lensing, that experiment has an interesting history.
Now the saying that “light will enter a black holes as its future is there” is technically incorrect as null paths have no reference frames and so we can’t talk about their past and future, still from our perspective its a sort of correct statement, its better to say that mote gravity lenses light more. And this gravitational lensing is quite useful for using it as a natural telescope, clusters of galaxies can lense the light of galaxies behind and so we can collect more light from these distant objects than we have the right to do so. Occasionally this lensing splits images into multiple copies and if the lense wasn’t perfectly perpendicular to our line of sight the images will come from different distance and so they can be shifted in time relative to each other and this allows us to observe a phenomenon like a supernova explosion multiple times. Pretty cool, huh?
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