How do blackholes attract light when light is massless.
In: Planetary Science
Because Newton’s theory of gravity is incomplete. Gravity isn’t a force between massive objects, but the curvature of spacetime around mass. Light follows spacetime curvature, therefore it bends around massive objects. Yes, even the sun bends light.
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?
They don’t. They bend space itself.
So light is still travelling in a straight line, the bent space just bends the paper so the straight line goes into the black hole.
Objects travel in straight lines through spacetime. Mass redefines “straight” and bends those lines towards it. Mass or no mass, an object’s path becomes curved.
An object won’t feel that curvature like you would if you were going around a bend in a car for example. As far as you’d be concerned, you’d still be going down a perfectly straight highway. No sideways forces ever get exerted on the object.
Black holes have enough mass that paths are curved right into it, creating an event horizon where there are no paths leading even light back out.
Black holes attract the spacetime light’s travelling in, not light itself. As far as the light particle itself is concerned, it’s travelling in a straight line through space, and that’s all that counts.
The same effect can be measured on Earth by GPS satellites. Their clock ticks ever so slightly faster than on the surface.
They bend space time, literally folding space. Think of it like if you place a bowling ball on a trampoline, the skin of the trampoline is a representation of space and the bowling ball is the black hole; any object you roll across the trampoline will be drawn in towards the bowling ball. https://youtu.be/dEintInq0YU
Black holes don’t attract light. What they do is they bend the fabric of space and time to such an extent that light, which always moves in straight lines, moves along the bent space either into the black hole or warped around it.
Take out a piece of paper, lay the sheet flat, and then draw a straight line. That’s how light behaves in space. Now take the paper and begin folding it over so that the light points at you. Notice how the light is still moving in a “straight line” (you’ve done nothing to the line itself) but a “straight line” within a curved geometry causes the light to move differently than when the geometry was flat. This is in essence what a black hole is doing, only instead of a sheet of paper it’s the fabric of the universe itself. That bending is what we call gravity and it’s nothing special to what you experience here on Earth, only that for a black hole there is a lot more of it.
This is why newtonian physics is insufficient to describe our universe. If gravity pulls objects with mass, then yes, light shouldn’t be affected by black holes. But under General Relativity physics would look different. The mass of the black hole is bending space and time itself and so all things traveling by its influence would be affected regardless of mass — including light.
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