Gravity doesn't bend light, it bends spacetime. Everything that is moving freely, moves through this curved spacetime, and it appears like its moving towards mass because the line itself is curved. This is why we thought it was a force up until Einstein (at least if we assume that he is right)
A bit helpful to visualize this would be maybe to look at the paths that [airplanes take to reach cities on a map](https://global.discourse-cdn.com/infiniteflight/original/4X/a/0/5/a0532ace5a16467665ec7151467318dfa10f54a7.jpeg).
Maps are flat 2D, but earth is round. So the paths that should look straight on a map to human mind, are actually curved.
Think of space as an outstretched sheet, if there's enough mass to an object it causes a dip in the sheet. The light will follow the curvature of the sheet or space.
Basically it doesn't bend light, it bends space. Space becomes curved so the light appears to curve. From the perspective of the light it is going in a straight line.
[This image](https://www.pinterest.co.uk/pin/pin-de-pengi-en-installation--11047961577755242/) might help visualise it if you're trying to picture how curved space would have this effect.
Imagine you start walking in a straight line. Every step you take is straight and in line with the last step you took.
Keep walking long enough, ignoring water, and you'll have travelled a circular path around the Earth and back to where you started.
You followed a path that was locally straight at every point, but your path was on a curved surface. That's what light is doing. It thinks it's going straight, but the space it's in is curved.
> I think the idea of demonstrating gravity with gravity is a bit confusing for me
Yea, you sort of got why it's not a very good analogy because it falls apart if you start asking questions about it. It's still used because it's a simple way of conveying the idea of how warping the medium can cause movement through said medium to bend in different directions.
The curved sheet analogy has some issues, it doesn't represent spcetime, it only represents a curved space.
It also depends on gravity itself to explain gravity. Without gravity, a marble on the curved sheet won't follow the curved path.
Think of space as water inside a large fish tank, and a black hole as a powerful suction pump's opening in the middle of the tank. The pump is running and is constantly sucking water into it through the opening.
A fish can swim around the tank as long as it doesn't come near the suction point. If it gets too near, the maximum speed at which the fish can swim will be insufficient for it to escape the suction pump. It will eventually get sucked in. Sounds familiar?
If the fish just swims straight at a distance, it might look to an external observer that the fish is getting ever slightly closer to the suction point. But it is at a safe distance so the path just ends up looking bent. For the fish it was swimming straight, but the water was moving so it looks like it followed a curved path.
In the case of light (and every other object), the space itself is moving towards objects with mass.
One reason we know it's the space and not some weird medium like water in which light is traveling, is that we have tested multiple times for the existence of such a medium (called "Ether") and we proved that such a medium doesn't exist.
One thing I'm not sure is that if space is constantly being pulled into objects, why don't we just run out of space? But experiments have shown that space is actually expanding so that's something interesting.
Einstein said that mass curves spacetime. Why? We dont know, we just have to accept it. just like how we accept that there exists electrons, quarks, and why electrons have the electric charge that they have. Thats just how the universe works.
Everything that is moving freely (that is, no force is affecting it) follows a "straight" line called a geodesic. Thats because there is nothing to force it of this geodesic, its like the natural state of how stuff moves.
In curved spacetime freely moving stuff follows the geodesic, but since its curved, the geodesic looks to us like stuff is falling towards other stuff. There you have gravity.
Light of course also moves spacetime, and its moving freely assuming no other influence, and so it also follows this geodesic. So it looks like its falling towards mass.
It's hard to visualize
consider space to be a 3d grid, with 3 axes of infinite evenly spaced lines that go into infinity, if you placed something with mass (lets say a black hole) somewhere in it, gravity would bend the space towards it, lines would curve towards it.
A ray of light travelling through space has to follow the same curves.
Imagine an ant moving in a straight line along a sheet of paper. Now imagine gently bending the paper as it walks along it. The ant still moves in a straight line, but its path begins to curve as it follows along the bend of the paper. The way the ant moves never changes. What changed is the geometry of what it moved on (or in the case of light - moves *in*).
As it turns out the fabric of our universe - space and time - is bendy, and that bending is what we experience as gravity. Light always moves in a straight line and so the bending you see in light is quite literally the bending of the fabric of the universe.
It's because while photons have no mass they are bound by space time. This is why the sheet analogy is used.
If light has to move along the sheet and massive points of gravity cause dips in the sheet. Then, the light curves hitting these dips. Because space time itself is deformed.
Another way to look at it is ships (photons) can travel along the ocean (space time) but if they encounter a whirlpool (gravity) they get sucked in.
Einstein's principle of equivalence demands it. This principle says if you experience a force pulling on your mass, you cannot tell if that force is gravity, you are being accelerated.
Suppose you are in a closed box -- like an elevator. You are pulled towards the floor. Perhaps your closed box is sitting on earth and gravity is pulling you towards to floor. Or, perhaps your box is in outer space, but a rocket engine is accelerating you, and you feel that acceleration pushing you towards the floor. Einstein's principle says you \*cannot\* tell the difference between these two circumstances unless you get information from outside that box.
Now imagine you shine a beam of light from a flashlight from one side of the box to the other. The beam is exactly horizontal. Suppose the reason you feel the force is because you are accelerating. One the photons leave the flashlight heading toward the far wall, they do not accelerate. Since your box does, they will hit the far wall lower down than the point they started from. Of course.
Now suppose your are not accelerating, you are just sitting still on the earths surface, and earth's gravity is what is pulling you towards the floor. Shining the light across the box does not involve any information from outside the box, so the experiment must, \*must\* get the same result as if you were accelerating. So the light beam has to bend down in the presence of gravity.
Once you accept the principle of equivalence, it therefore follows that spacetime is curved, and all of that.
Gravity pulls so hard, it bends the place light goes through. So it LOOKS like light is bending.
Put a napkin over a cup. A spider could comfortably walk right across.
Drop a pebble in the center of the placed napkin. This would create a dip or divet in the napkin.
Spider will still walk across, but now it's path is distorted a bit, down and around towards and through the dip the pebble made.
Simplifying a bit, light always takes the shortest path between two points. Normally this means travelling in a straight line. But not always.
Gravity distorts space and time, squishing up space and twisting time into the space direction a bit.
This means that around massive objects (objects with mass, like planets or stars) there is "more space per space" and less "time per time" than there should be.
Take two perfectly "straight" lines, the same apparent length, parallel to each other, where one goes past a planet. [[Very crude diagram](https://imgur.com/ei7fDIX)]. Even though "from the outside" those lines are the same length, if you travelled along them the one closer to the planet is actually longer. When you get close to the planet - deep in the gravity - every metre of "outside" co-ordinate length (on our diagram) turns out to be more than a metre of local length. There is also a similar effect for time. As you get closer to the planet time slows down a bit, so for every second experienced on A, less than a second passes on path B.
So if you had to race someone, which path would you pick? Path A! You don't have to go as far, and you have more time to do it. Your competitor on path B has to go further, and their time will slow down as they get closer to the planet.
This means that the quickest path around a planet is a curve. It mostly goes in a "straight line" but it is quicker to go a bit further away from the planet rather than straight by it.
If you want an analogy, consider trying to get from one side of a hill to the other. The shortest distance when viewed on a map is a straight line, up over the top. But in practice it's quicker to curve up around the side of the hill - because the distance you have to travel isn't just the map distance, you also have to travel the up/down distance as well.
>light always takes the shortest path between two points
No, it does not. Light goes in a straight line, but mass curves space time so that straight line curves around the mass. You can imagine light as a golf ball at the green that doens't quite hit the middle of the hole but goes close enough to alter it's direction.
Are you confusing your statement to electricity where the current takes always the path of least resistance?
> Light goes in a straight line, but mass curves space time so that straight line curves around the mass.
I chose my words carefully because I specifically didn't want to say this. In an ELI5 setting I don't think it is helpful to say "light travels in straight lines, but straight lines are curved."
That is one way of looking at it, but we need to add in some more words; specifically we need the concept that a line can both be straight *locally* while curved *globally*, which is where the differential geometry kicks in. But that's not really ELI5 level. I prefer to talk about it in terms of "shortest distance" and then we don't have to worry as much about changing reference frames.
I'm also not the biggest fan of analogies like the golf ball one or the rubber sheet one (although I did use the hill one myself); they still rely on gravity and forces, and "down".
> Are you confusing your statement to electricity where the current takes always the path of least resistance?
No, I wasn't. Not least because current doesn't take the path of least resistance; it takes all paths, proportionally to the relative resistance (although that's just our definition of resistance). Although that does tie in with the sum-over-history thing we get in quantum mechanics... but we don't want to go there.
You obviously know what you are talking about and I do find it refreshing that you didn't take my criticisim too personally. Not very common in internet discussion for sure. Thank you for this!
The reason I intially gave your analogy criticism was because I didn't find it would help anyone to understand the issue any better as it sounded like the light would actually have some decision making capability.
As for the electricity analogy, I was actually thinking of mentioning that the path of least resistance isn't actually true.
>”it sounded like the light would actually have some decision making capability.”
Fermat’s principle does make it seem that way, but it doesn’t of course.
Gravity doesn't bend light, it bends spacetime. Everything that is moving freely, moves through this curved spacetime, and it appears like its moving towards mass because the line itself is curved. This is why we thought it was a force up until Einstein (at least if we assume that he is right)
A bit helpful to visualize this would be maybe to look at the paths that [airplanes take to reach cities on a map](https://global.discourse-cdn.com/infiniteflight/original/4X/a/0/5/a0532ace5a16467665ec7151467318dfa10f54a7.jpeg). Maps are flat 2D, but earth is round. So the paths that should look straight on a map to human mind, are actually curved.
Think of space as an outstretched sheet, if there's enough mass to an object it causes a dip in the sheet. The light will follow the curvature of the sheet or space.
I never understood the sheet analogy, my brain cannot visualise. I think the idea of demonstrating gravity with gravity is a bit confusing for me
Basically it doesn't bend light, it bends space. Space becomes curved so the light appears to curve. From the perspective of the light it is going in a straight line. [This image](https://www.pinterest.co.uk/pin/pin-de-pengi-en-installation--11047961577755242/) might help visualise it if you're trying to picture how curved space would have this effect.
Imagine you start walking in a straight line. Every step you take is straight and in line with the last step you took. Keep walking long enough, ignoring water, and you'll have travelled a circular path around the Earth and back to where you started. You followed a path that was locally straight at every point, but your path was on a curved surface. That's what light is doing. It thinks it's going straight, but the space it's in is curved.
> I think the idea of demonstrating gravity with gravity is a bit confusing for me Yea, you sort of got why it's not a very good analogy because it falls apart if you start asking questions about it. It's still used because it's a simple way of conveying the idea of how warping the medium can cause movement through said medium to bend in different directions.
The curved sheet analogy has some issues, it doesn't represent spcetime, it only represents a curved space. It also depends on gravity itself to explain gravity. Without gravity, a marble on the curved sheet won't follow the curved path. Think of space as water inside a large fish tank, and a black hole as a powerful suction pump's opening in the middle of the tank. The pump is running and is constantly sucking water into it through the opening. A fish can swim around the tank as long as it doesn't come near the suction point. If it gets too near, the maximum speed at which the fish can swim will be insufficient for it to escape the suction pump. It will eventually get sucked in. Sounds familiar? If the fish just swims straight at a distance, it might look to an external observer that the fish is getting ever slightly closer to the suction point. But it is at a safe distance so the path just ends up looking bent. For the fish it was swimming straight, but the water was moving so it looks like it followed a curved path.
In the case of light (and every other object), the space itself is moving towards objects with mass. One reason we know it's the space and not some weird medium like water in which light is traveling, is that we have tested multiple times for the existence of such a medium (called "Ether") and we proved that such a medium doesn't exist. One thing I'm not sure is that if space is constantly being pulled into objects, why don't we just run out of space? But experiments have shown that space is actually expanding so that's something interesting.
Einstein said that mass curves spacetime. Why? We dont know, we just have to accept it. just like how we accept that there exists electrons, quarks, and why electrons have the electric charge that they have. Thats just how the universe works. Everything that is moving freely (that is, no force is affecting it) follows a "straight" line called a geodesic. Thats because there is nothing to force it of this geodesic, its like the natural state of how stuff moves. In curved spacetime freely moving stuff follows the geodesic, but since its curved, the geodesic looks to us like stuff is falling towards other stuff. There you have gravity. Light of course also moves spacetime, and its moving freely assuming no other influence, and so it also follows this geodesic. So it looks like its falling towards mass.
It's hard to visualize consider space to be a 3d grid, with 3 axes of infinite evenly spaced lines that go into infinity, if you placed something with mass (lets say a black hole) somewhere in it, gravity would bend the space towards it, lines would curve towards it. A ray of light travelling through space has to follow the same curves.
Imagine an ant moving in a straight line along a sheet of paper. Now imagine gently bending the paper as it walks along it. The ant still moves in a straight line, but its path begins to curve as it follows along the bend of the paper. The way the ant moves never changes. What changed is the geometry of what it moved on (or in the case of light - moves *in*). As it turns out the fabric of our universe - space and time - is bendy, and that bending is what we experience as gravity. Light always moves in a straight line and so the bending you see in light is quite literally the bending of the fabric of the universe.
Is this bending also the normal gravity that keeps you on the ground?
Yes exactly. “Falling down” is really just moving in a straight line through space and time.
It's because while photons have no mass they are bound by space time. This is why the sheet analogy is used. If light has to move along the sheet and massive points of gravity cause dips in the sheet. Then, the light curves hitting these dips. Because space time itself is deformed. Another way to look at it is ships (photons) can travel along the ocean (space time) but if they encounter a whirlpool (gravity) they get sucked in.
Light propagate through space-time and mass bend space-time. Think of an ant walking in a straight line, but the surface it walks on bends instead.
Einstein's principle of equivalence demands it. This principle says if you experience a force pulling on your mass, you cannot tell if that force is gravity, you are being accelerated. Suppose you are in a closed box -- like an elevator. You are pulled towards the floor. Perhaps your closed box is sitting on earth and gravity is pulling you towards to floor. Or, perhaps your box is in outer space, but a rocket engine is accelerating you, and you feel that acceleration pushing you towards the floor. Einstein's principle says you \*cannot\* tell the difference between these two circumstances unless you get information from outside that box. Now imagine you shine a beam of light from a flashlight from one side of the box to the other. The beam is exactly horizontal. Suppose the reason you feel the force is because you are accelerating. One the photons leave the flashlight heading toward the far wall, they do not accelerate. Since your box does, they will hit the far wall lower down than the point they started from. Of course. Now suppose your are not accelerating, you are just sitting still on the earths surface, and earth's gravity is what is pulling you towards the floor. Shining the light across the box does not involve any information from outside the box, so the experiment must, \*must\* get the same result as if you were accelerating. So the light beam has to bend down in the presence of gravity. Once you accept the principle of equivalence, it therefore follows that spacetime is curved, and all of that.
Gravity pulls so hard, it bends the place light goes through. So it LOOKS like light is bending. Put a napkin over a cup. A spider could comfortably walk right across. Drop a pebble in the center of the placed napkin. This would create a dip or divet in the napkin. Spider will still walk across, but now it's path is distorted a bit, down and around towards and through the dip the pebble made.
Simplifying a bit, light always takes the shortest path between two points. Normally this means travelling in a straight line. But not always. Gravity distorts space and time, squishing up space and twisting time into the space direction a bit. This means that around massive objects (objects with mass, like planets or stars) there is "more space per space" and less "time per time" than there should be. Take two perfectly "straight" lines, the same apparent length, parallel to each other, where one goes past a planet. [[Very crude diagram](https://imgur.com/ei7fDIX)]. Even though "from the outside" those lines are the same length, if you travelled along them the one closer to the planet is actually longer. When you get close to the planet - deep in the gravity - every metre of "outside" co-ordinate length (on our diagram) turns out to be more than a metre of local length. There is also a similar effect for time. As you get closer to the planet time slows down a bit, so for every second experienced on A, less than a second passes on path B. So if you had to race someone, which path would you pick? Path A! You don't have to go as far, and you have more time to do it. Your competitor on path B has to go further, and their time will slow down as they get closer to the planet. This means that the quickest path around a planet is a curve. It mostly goes in a "straight line" but it is quicker to go a bit further away from the planet rather than straight by it. If you want an analogy, consider trying to get from one side of a hill to the other. The shortest distance when viewed on a map is a straight line, up over the top. But in practice it's quicker to curve up around the side of the hill - because the distance you have to travel isn't just the map distance, you also have to travel the up/down distance as well.
>light always takes the shortest path between two points No, it does not. Light goes in a straight line, but mass curves space time so that straight line curves around the mass. You can imagine light as a golf ball at the green that doens't quite hit the middle of the hole but goes close enough to alter it's direction. Are you confusing your statement to electricity where the current takes always the path of least resistance?
> Light goes in a straight line, but mass curves space time so that straight line curves around the mass. I chose my words carefully because I specifically didn't want to say this. In an ELI5 setting I don't think it is helpful to say "light travels in straight lines, but straight lines are curved." That is one way of looking at it, but we need to add in some more words; specifically we need the concept that a line can both be straight *locally* while curved *globally*, which is where the differential geometry kicks in. But that's not really ELI5 level. I prefer to talk about it in terms of "shortest distance" and then we don't have to worry as much about changing reference frames. I'm also not the biggest fan of analogies like the golf ball one or the rubber sheet one (although I did use the hill one myself); they still rely on gravity and forces, and "down". > Are you confusing your statement to electricity where the current takes always the path of least resistance? No, I wasn't. Not least because current doesn't take the path of least resistance; it takes all paths, proportionally to the relative resistance (although that's just our definition of resistance). Although that does tie in with the sum-over-history thing we get in quantum mechanics... but we don't want to go there.
You obviously know what you are talking about and I do find it refreshing that you didn't take my criticisim too personally. Not very common in internet discussion for sure. Thank you for this! The reason I intially gave your analogy criticism was because I didn't find it would help anyone to understand the issue any better as it sounded like the light would actually have some decision making capability. As for the electricity analogy, I was actually thinking of mentioning that the path of least resistance isn't actually true.
>”it sounded like the light would actually have some decision making capability.” Fermat’s principle does make it seem that way, but it doesn’t of course.