Why is light affected by gravity if it has no mass?
In: 3548
Mass (gravity) doesn’t affect the photons (light) directly. Mass (gravity) affects the space-time (the continuum of space and time) that the photons (light) travel through. This causes us to perceive that the mass affected the photos (light), but in reality they did not.
A 20lb rock is twice as heavy as a 10lb rock.
Both fall at the same rate.
So… What would a 0lb rock do?
There’s no rule that says “gravity only impacts things with mass”.
It might be a bit confusing if you think of gravity as a force, like a hand pushing down. While you can often think of it this way, gravity is more of a natural motion – a thing falling under gravity experiences no force at all yet it still moves inward. Thinking of gravity as if it’s a force can lead to confusion.
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If you want a bit of a deeper dive, gravity’s relationship with light is actually quite fascinating. While light always locally moves at light speed, because this motion depends on local time, light can ‘slow down’ in regions where time itself is slowed down.
If you remember physics class, you might recall than light (and all waves) curves into a region where it’s slow. This is how lenses work.
Since gravity slows time, light curves towards the center of the gravity.
Short answer: mass bends space-time; light follows space-time.
Longer answer: a property of mass-energy is that it bends space-time. Not only does this manifest as apparent attraction between objects with mass, but it also changes the path light follows relative to an observer’s point of view. From the light’s perspective, it propagates uniformly in all directions. But when direction itself bends, to another observer the light follows an apparently circuitous path.
EDIT 1: Wow! That blew up! Thanks for the awards everyone. I’m not a physicist, or even a science student, just an enthusiastic dilettante.
[Here’s](https://youtu.be/MTY1Kje0yLg) a cool video that helped gravity make more sense to me.
[Here’s](https://youtu.be/F5PfjsPdBzg) another awesome video that helps tie in the relationship between time and gravity.
And [another](https://www.youtube.com/watch?v=jlTVIMOix3I) video with a great visual analogy.
Just posting more [cool videos](https://www.youtube.com/c/pbsspacetime) as folks reply.
EDIT 2: Many replies which ask deeper questions about the subject exceed my capacity to offer what I feel are faithfully accurate answers. So if I don’t reply to your question, it’s because I’m not smart enough to offer what I feel is an edifying explanation.
Another great (https://youtu.be/XRr1kaXKBsU) shared in a reply below.
EDIT 3: Others have rightly pointed out that all energy (even light) bends space-time. But compared to mass (mass-energy), light’s effect on space-time curvature is minuscule (though light-only black holes are a thing! I never knew).
In general relativity, gravity affects anything with **energy**. While light doesn’t have mass, it still has energy, and so experiences gravity.
If you visualize gravity as a distortion in spacetime then I think it’s fairly intuitive that it doesn’t matter what the object is. As long as it exists, gravity affects it.
Don’t think of gravity as a force in this case, think of gravity more as a bending of space-time.
Imagine you’re an ant that can only walk in a straight line, and you’re on a sheet of paper. As you march forward, someone bends the piece of paper so it no longer lays flat. You can travel, from your perspective, exclusively straight, yet with the fold in the paper, you’re facing a different direction than if the paper hadn’t been folded. It’s the same idea with space and light, the light IS travelling straight, it’s that the space it’s travelling through isn’t.
So… If the space time axis is a piece of paper and light is a straight line drawn on it, the line (light) is forced to appear bent when you crush (gravity) the paper (space-time).
Another interesting explanation is that light behaves both as waves(energy) and particles(matter). See: double slit experiment.
Its tricky because its not gravity that’s affecting the light (photons) light travels through spacetime, and in that spacetime are also objects with crazy mass. When the light gets warped its not gravity its the shape of spacetime that the light is currently traveling on.
Check this out…imagine a bowling ball on a mattress, the mattress is spacetime, the bowling ball is, let’s say its the sun. Anything with mass will deform spacetime just like the bowling ball is deforming the mattress. Now take a marble that represents a photon (light) now roll it in a straight line, if the marble comes to close to the bowling ball the marble will start to curve because of the affect the bowling ball has on spacetime (the mattress) that same spacetime the light is traveling on…see what I’m saying? hopefully that helps..
Because light travels through space which IS affected by gravity.
Think of space like a fiber optic tube. Light goes through the space ‘tube’ and follows it. Gravity bends the tube. (Terrible analogy but I have to add filler so automod stops deleting my comment. )
In extreme environments like a black hole, space warps back in on itself so light just keeps ending up back inside the black hole.
Mass bends space.
Think of a train. Its going forward, its all it knows.
But if you curve the rail, the train will also curve, even though as far as it “knows”, its going forward.
This is really simplistic but may help.
EDIT:
Answering a question:
“If the space is bent, what remains in the area from which the space is bent?” from 14MTH30n3.
Imagine a weaving machine, with a lot of strings falling vertically parallel to one another. Like – a lot, just side by side without touching.
Now, with your finger pull some strings closer together. You’ll see others become more separated. But its still strings, separate, just curved in a different way. If you follow one of them, its still just the same string and, like the train, you won’t “feel” it change direction.
Its the same material, just… stretched. And you also are stretched – so far as you’re concerned, nothing changed.
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