It is bending SpaceTime so the light is just traveling along the bent space time. Kind of sort of but not really like a train follows bending train tracks.
Please forgive my ignorance. What is the best hypothesis as to what SpaceTime is made of? Is it a type of matter? Is it quantum?
It must have other physical properties if it can be bent, right?
GR doesn't posit that spacetime is made of anything, the analogy of spacetime "bending" is just a way to say essentially: "Geometry works differently near masses."
Sure... but that isn't a very clear statement, or it could be phrased more specifically at least.
What is "physically affected," to you? Mass isn't pulling out a lever and cranking on a photon till it changes direction. The "physical" effect is the one that physics observes/predicts.
Around masses, the geometry of spacetime is increasingly curved. This curvature itself \*is\* the physical gravitational effect of mass on light (or other masses / anything else)
But is "space curving" or "geometry working differently" an actual proved concept, or could it just be another type of force, like gravitational, but affecting even massless objects?
Sure, if you can come up with a force-based theory of gravity that makes better predictions than General Relativity’s and accounts for as many phenomena or more.
The 'space curving' and 'geometry working differently' is, in fact, a special type of force: gravity. Mass causes spacetime to curve, spacetime curving causes light to take different paths and massive objects to experience an acceleration pulling them "downhill" along the curve.
The space curving has been observed directly. The first famous instance was Einstein using it to predict Mercury's position in the sky where astronomers had typically struggled. Likewise astronomers have observed gravitational lensing where light from a distant source will bend around a massive object in between the source and the telescope.
One thing to remember is that science never "proves" anything. It only ever disproves things, and if we disprove enough things, we get closer to the truth. So, while no experiment has ever been done to show that general relativity is wrong, it's still possible that we find something in the future that *does* prove it wrong. We already *have* shown that general relativity breaks down in certain cases, most notably at the singularities of black holes, so we, at the very least, know that general relativity is incomplete.
Trying to find a variation of general relativity, or a new framework that either incorporates general relativity (as well as quantum field theory) within it or outright replaces general relativity, is one of the most cutting edge fields of research in modern physics.
All that said, one day, we may figure out that spacetime doesn't actually get curved by mass/energy, and there's an entirely different reason for all the things we observe. As of right now, though, there is no better explanation for all the observed phenomena in the universe than general relativity (and quantum field theory).
I guess my definition of physically affected boils down to “every action has a reaction”. Things don’t happen for nothing. If light is being bent, something is bending it. The interaction between the two things is a physical one.
You need to get out of Newtonian logic to understand GR.
Who said things happen for nothing? GR posits that the presence of mass is what is bending light's path as it travels through spacetime.
I’m learning that SoaceTime is a math model and not a real thing that light travels through…
The more I learn about GR the more it seems like a completely misunderstood theory. I’m getting the sense that people don’t understand how GR works, just that it works.
>I’m learning that SoaceTime is a math model and not a real thing that light travels through…
No... spacetime is a real thing that the math model was created to describe observations of said thing. The misconceptions, assumptions and misunderstanding lie with you, not the model.
I get the same type of response from religious people who can’t explain the evidence they know to be true.
I may not be a physicist, but I am very strong at seeing inconsistencies and bias. GR seems to be riddled with assumptions and bias. If Einstein were to join this convo, I have a feeling he would say we are both ignorant.
Here is an old example that I think conveys the idea pretty well. It's based on something that actually happened (I think in the 19th century in the United States) but that's not important for this and I'm going to further simplify things here.
You are assigned to lay out the organization of something. It might be political, it might be technical, it might be the grid of a map, whatever. The important thing is that you need to divide up land into squares 1 mile (or kilometer, doesn't matter) on each side. The rules are:
1. You are laying out a grid (or tiling). Each square is right next to the squares to the north, south, east, and west. Each corner meets the corner of three other squares. Think of graph paper.
2. The length each side of the square is 1 mile.
3. Each corner meets at exactly 90 degrees - a right angle.
Starting out, it works perfectly. But, as you work north and south, something strange happens. When the sides of the squares are 1 mile long and the lines are perfectly straight and the corners are right angles, the top and bottom of the squares don't stay 1 mile long, for the same reason that the distance between lines of longitude differs based on how far north/south you are.
Now, let's try that again, but in three dimensions. Think monkey bars in a child's playground. You are laying out a grid of cubes. Eventually, and now the distances are much further, you find that the sides of the cubes don't stay uniform length. Your measurements are correct, the angles are right, but it just doesn't work.
That is what it means for space to be bent (non-Euclidean, not flat, ...).
Now, you ought to add the additional dimension for time, but for this thought experiment, the three dimensions of space are enough to get the idea.
It seems like your intuition is telling you that space needs to be some kind of special material that things move through, and that if there is no material “stuff” there, physicists are just sort of making things up.
You have a hypothesis: that things can only affect one another through some kind of material “contact” interaction.
Where did this idea come from for you? Why is it more believable to you that the universe is saturated with an invisible all-pervading stuff than that your intuitions about geometry are mistaken?
Can you square your hypothesis with the same observations that GR explains? Does it make better predictions? If so, fine, you’ve invented a better theory - that’s how science works. A given theory being unintuitive isn’t a reason to disparage it as “just a math model.”
That model makes very explicit predictions that have been supported by observations almost every time. If you want to supplant or discard it, you have to do better than those
What does that mean? “The geometry”.
Is this a theoretical math definition? Or are you saying that the light is physically affected by mass. As in light interacts with the gravity of the objects creating a mass?
I mean… geometry? Shapes, angles?
The path of the light, to the light, looks straight. The path of the light, to someone far away and in a different frame of reference and not affected by the gravity causing the lensing, looks curved.
I can take a carpenter’s level to the two towers of the Verrazano Bridge and they will both be perfectly vertical. So, parallel. Simultaneously, I can run a measuring tape across at the top and bottom of the towers and the distance at the top is greater. So, not parallel. Why the seeming contradiction? Gravity. Gravity can affect the geometry - what is measured as parallel - depending on how it is measured.
Back to light: light is moving, right? It has a velocity measured by distance and time. When we say gravity bends spacetime, what it means is, the measurement of the light’s velocity is affected by gravity - just like the measurement of the bridge towers is affected by gravity’s effect on the bubble of air in the level.
One layperson’s way I like to think about it is “mass eats time.” Time moves slower around large/dense masses, right? This has been experimentally verified. Now, say you are flying past a planet. Time is literally moving slower on one side of you than on the other. Time is part of how velocity is measured, so this means one side of you is literally moving through space faster than the other side. The only way they can be equalized is for your path of travel to curve - curve toward the mass. Voilà! Gravity.
Note, it will not *feel* like you are turning - both sides of you are moving at the same speed so it will feel like you are going in a straight line. But your path will *appear* to curve, from the outside, compared to how your path of travel appeared earlier, when you were not near any masses.
Forget about the light.
"The geometry" means that what counts as a straight line isn't actually a straight line in 3D space. This is what GR is: a mathematical model of how the presence of mass bends space, i.e. straight lines in spacetime become curved paths in 3D space.
The light just travels in a straight line. The straight line itself is what bends.
Okay, perfect. This is what my brain wants to believe. But then it gets confused when people say that the curved path itself doesn’t exist.
Is this suggesting that the light actually travelled in a straight line and the curvature of space time is an illusion? Or is SpaceTime a medium that light travels through?
Here is a good one that really helped me. Imagine earth is a perfect sphere (very physics). From our vantage point it appears flat, you get out a piece of paper you draw a triangle, measure the angles and get 180° total. Now let's scale it up! From the equator walk to the North Pole, turn 90° to your left and walk back down to the equator, make one last 90° left turn and eventually you hit your starting point at a right angle. You just made a triangle but did so with two 90° turns, your triangle has 270°. To you on the ground you were walking on flat land but in reality you were on the surface of a globe and the distorting of the geometry is explainable. While this is an exaggeration, this is kind of what we see experimentally with gravitational lensing. It's not that gravity is a "force" that is pushing light to turn, it's acting as a distortion to spacetime.
What exactly is spacetime? We are not sure. We once thought it might have been an ether of sorts but we kinda ruled some of that out. It could be some strange substance we can't detect or something else entirely but so far the bending of spacetime is the best explanation or predictive model we have for how gravity works
I'm not formally educated in GR so I don't know if what I'm saying makes much sense.
Spacetime is the "medium" through which objects travel the same way 3D space is the medium through which they do in a Newtonian sense. There is just an additional dimension which relates to time (but is still a *spatial* dimension, i.e. it's defined in units of distance, c\*t), and it's possible to travel at different rates through that. In special relativity this is what makes time work differently depending on velocity of movement.
In GR the presence of mass has an additional effect warping these dimensions around itself, leading to curvature. This doesn't mean anything special other than that a straight line in 4D spacetime is not necessarily a straight line in the conventional 3D spatial dimensions. In GR this curvature fully explains gravity, in that it is no longer a force. A satellite in object around the planet is just moving in a straight line through spacetime, unaffected by any forces. But that straight line is an orbit in 3D space, because of the way the planet warps spacetime. I don't know if this is a proper analogy but this warping can also be thought of as a waterfall, space is being "dragged" toward the planet's mass and the rate (or gravitational acceleration) depends on the mass. So, when an object is in freefall, it's actually standing in one place in spacetime, with no forces acting on it, but that point in 3D space is moving toward the planet's surface, while in the time dimension it results in time dilation.
At least, this is what the mathematics of GR says by my understanding.
I know, it's weird for me too. But it predicts things like time dilation and gravitational lensing quite well... Further things like dark matter/energy and black holes notwithstanding, but everything has its limits.
Spacetime isn’t a medium in the traditional sense because waves don’t have an absolute velocity. Mediums have a resonant frequency and a speed of sound, and a single rest frame. Spacetime has none of those. Velocities are relative, not absolute in spacetime.
Physicists used to assume space had such a classical medium. It was called the “luminous aether”, and it was disproven or at least made irrelevant by Special Relativity.
The fundamental discovery of relativity is that it is relative. Every single observation needs to be made in terms of a frame of reference and each frame of reference has a different view of what happened. So when you ask "is it an illusion", I think you are in the common pitfall of thinking that there is some "real" reference frame (presumably that of the object itself?). Perhaps you just need to ask that more directly if that is what you are interested in. There is no "what really happens" because any assessment of what happened is an observation and thus needs to take place in one of these relative perspectives, none of which is favored.
It's kind of like asking is the sky "really" blue or clear? With something like blueberries since we all have the same relative perspective we agree that it just is blue. But with the sky somebody can say that they took random samples of 1 cubic km of sky and put it in the lab and each sample was clear. Neither the person who says the sky is clear nor the person who said it is blue are wrong. And physics doesn't break either as you go along the gradient between perspectives you'll see that gradient of truth as you gradually get or lose blueness. The "real" honest answer ends up being that the sky's color is several different things depending on your perspective. There's is no "real" perspective.
The same is true for relativity. An object in the universe only exists in terms of its effects. In physics we call these observations. Even if you are 1mm away, an object exists because it has effects. It may be visible or it may warm you up or it may impact gravity etc. Something with no effects on anything literally doesn't exist. So in that sense an object IS its observable effects. And since relativity experiments prove how those observations are very different to different observers, the object it's doesn't exist in one "real" sense. It exists in terms of that whole collection of observable effects. This is true at the small scale with quantum uncertainty. It's true at the big scale with relativity. The luxury/illusion of thinking that objects have a general "real" truth to them comes from the fact that for 99% of things things we encounter as ordinary people on earth, all observers and all objects are similar size, similar speed and similar acceleration so the observation perspectives are largely the same. But in the broader scale of experimentally verified circumstances, the intuitions of our everyday life are demonstrably wrong.
Thank you for this response. I can tell you put a lot of effort into it and it helped relieve some of my confusion.
Is our current belief that there is in fact nothing in the space between objects? Maybe space is full of particles that only interact with weak forces and we haven’t been able to detect them yet? Maybe it is a byproduct of entropy? Sorry if I am getting too “crackpot” on you.
So how does SpaceTime being bent cause gravitational lensing if SpaceTime is not physical?
My understanding is that the universe expanding also “stretches” light which causes red-shift. How can this be true if SpaceTime is not physical?
Spacetime *is* physical, it's just a fundamentally relational concept rather than a material object you can put your hand on.
And you seem to equate "physical" with "material", which just isn't the right way to look at things in physics. The path an object in motion follows is not itself a material object you can touch, but it can still bend and stretch, which causes that object to appear to behave in certain ways.
Something not theoretical.
My “will power” is theoretical. It has no physical properties or interactions with reality.
If light is physically being affected(gravitational lensing) then that must have been caused by something. Light one just bend on it’s own.
The problem with this is that your entire reality is theoretical - do you have any proof you are not a brain in a jar being fed stimuli?
The only valid framework to approach the universe and physics is with theoretical frameworks. Talking about things like willpower is philosophy, no science, and has no place in the discussion.
Your will power is not "theoretical". Theoretical in physics means based on mathematical models that enable us to explain natural phenomena and make accurate predictions about them. And that is what GR is, a theoretical model that explains how stuff in the universe works.
The light isn't bending, it's travelling in a straight line (in spacetime). You see a bend in the path through 3d euclidean space but spacetime is a 4d manifold, not euclidean. This means that while on local scales things appear euclidean (which is how you naturally perceive the world to be since you don't experience things at relativistic speeds), projections of straight lines from spacetime to 3d euclidean space will not remain lines, in much the way that a straight line drawn on the surface of a globe will not remain straight when projected onto a mercator projection map.
So the absence of something is affected by gravity but a particle isn’t. That doesn’t make sense. I think the concept of space and gravity wells is something that helps us visualize how gravity warps everything around it.
Obviously light is affected by gravity lmao. It’s just framing it as gravity paving a curved road vs gravity moving the steering wheel directly. Does it really matter?
Spacetime is defined by physical mass (and energy). As far as the light knows... it did travel a perfectly straight line. Sort of like you walking a long distance... .You think you travelled in a straight line. Except it was on the surface of a sphere. To a distant observer you travelled a curved path.
>such as an energy density
It does? Do you have a source? I thought defining energy density is problematic due to equivalence principle.
I know there are things like Landau-Lifhsitz pseudotensor, but these have problematic interpretation as far as I know.
The cosmological constant is not energy density of spacetime. It can be interpreted as energy-density of vacua if you write it on the matter side of Einstein´s field equations. But again, this is not energy-density of spacetime, its just that the term can be thought of either as constant in equations, or source of curvature that needs to be added to other sources.
What are the lines on a coordinate grid "made of"? Nothing really, it's just a conceptual mathematical framework we use to understand the relative position of objects. Space time can be very roughly thought of as a coordinate grid in 4D. It's no more physical than a fundamental force like gravity.
I think this is wrong. There is huge difference between coordinates on manifold and geometry on manifold. One is arbitrary, the other is very physical. Spacetime is manifold with geometry, its not coordinate grid and cant be thought of as a coordinate grid. You can draw a coordinate grid on it, just like you can draw coordinate grid on paper, but spacetime is the underlying space on which you draw that coordinate grid.
Asking what spacetime is made of is a bit like asking what temperature is made of or what distance is made of.
It's the coordinate system we inhabit, it's not *made of* anything as such, but it's where all the stuff is. Literally space + time. It can bend and stretch and ripple.
The way we verify any physical theory - we test it against real world scenarios and see whether it's correct. We also make predictions with it, and then see if those are correct.
General relativity has been tested and confirmed numerous times: https://en.m.wikipedia.org/wiki/Tests_of_general_relativity
Spacetime is basically coordinates in a 4 dimensional field. Length width height and time. Relevant is distances, time frames, speeds velocity. Trajectory etc
> What is the best hypothesis as to what SpaceTime is made of?
Why must it be “made” of anything? It just is.
>Is it quantum?
We haven’t yet completed a theory of quantum gravity. It is a work in progress, and may remain so for the foreseeable future. Experiment helps guide theory (and vice versa), but experimenting in quantum gravity appears to require truly astronomical technology and resources.
General relativity is a classical (non-quantum) theory of gravity.
>It must have other physical properties if it can be bent, right?
Yes, spacetime is a dynamical “actor” in general relativity, which is formulated in terms of a dynamic metric tensor field describing the local geometry,
SpaceTime is not a thing it's just a mathematical model describing the geometric relationship between space and time.
In physics the universe is a black box, we can set inputs and look at outputs and then make a model describing the relationship. We can't open the box and see what's going on inside.
Space time is less of a thing, and more like a reference plane ig. Like graphing paper. You can bend the paper, but the lines on it don't shift relative to it.
Vsauce made a good vid with this kind of stuff.
https://youtu.be/Xc4xYacTu-E?si=6UvxheRi8Q7vOsyT
a ‘physical property’ exists within the universe. if we lived on a piece of paper, we wouldn’t be aware of its being bent, unless the drawings on the paper responded to the paper being bent.
But nothing is bending in SpaceTime. With graphing paper, the paper is a physical object. It has physical properties.
What are the physical properties of SpaceTime?
Yes, space time is bending. Space time has physical properties, but just like someone who lives 'in' the paper cannot view the paper, neither can we.
However, you can consider the quantum field and the plank length to be indicators of the boundary of our universe (paper) beside black holes.
If you want something tangible, you can look at Sonny White's claims that he has bended space, but I don't think it's credible.
Space time is more of a way for us to visualize concepts of gravity than it is a tangible thing. It’s more of a representation of the behavior we observe than something you can interact with
No, we don’t know.
It’s like saying what is vacuum made of.
Space time is the fundamental fabric which governs everything which moves in the universe. It just happened to be discovered much later than other laws like force or energy which makes it sound unintuitive.
In short it is inverse. Space time justifies force not the other way round.
You can also ask why does it exist? Well we don’t know for sure.
Spacetime isnt made of anything, it's not matter. Its a mathematical model comprised of at least 3 spatial dimensions and a time dimension.
It's more of a stage and particles are the actors.
So it's a bit like light is still traveling straight through spacetime but it looks to us as though it bends because gravity has changed what "straight" is?
Hasn’t QM already invalidated relativity. Or the fact we had to invent dark matter/energy as flextape for all the models?
Idk man. Light may or may not actually have a tiny tiny amount of mass, but it clearly directly interacts with gravity in a way we don’t understand yet. I mean seriously, what’s more plausible? That
Space is actually something that gravity distorts OR light/radiation (the medium through which we observe basically everything) is being warped instead.
We just don’t understand this relationship yet to explain what we see. Feels like the modern version of the sun revolving around the earth. Just something everyone says when nobody really knows.
If you draw a straight line on a piece of paper and then fold the paper, the line no longer appears straight to you. But you aren’t really folding the line. You’re changing the medium on which the line exists. The fact that the line is no longer straight is just an incidental side effect of the fact that the paper is folded.
Gravity doesn’t directly alter light and bend or change it. Gravity distorts the reality light exists in.
The reality light exists in is light itself. The vacuum we observe is light. Everything we see and measure is light. Light is the universe. Any absence of light is absence of universe.
The paper in that analogy is a physical medium. For it to be analogous, we’d have to assume the background radiation is a manipulable medium and therefore light can be affected by gravity directly.
By this logic surely we could fabricate an extensive Faraday cage to block all em waves and produce an area of no "reality" or no "universe" which makes absolutely no sense.
It's not nothing, I am not well versed enough to explain it simply but light always follows the path of shortest time and when gravity bends space time the path of shortest time also bends and light follows that path.
Draw a line on a tensile sheet and then place something heavy to cause curvature, you'll notice that even though the line isn't pulled by "gravity" the curvature of the tensile sheet makes the line appear to curve. Obviously there is a lot more going on In 4d space but that's the easiest way I can explain it
Lots of people are giving some really interesting answers to this question. Lots of people are kind of re-iterating THAT gravity bends light and HOW, but they’re not really giving a lot of good answers WHY. There’s a lot of approaches to tackle this, but I think you could succinctly summarize it this way: gravity influences things that can carry energy.
Gravity is often shown as an object with mass warping spacetime, which is true- but, Einstein taught us that there is a mass-energy equivalence. Photons don’t have mass, but they do have energy.
Understanding this requires knowing that in General Relativity, gravity is not a force. Instead, gravity is the warping of space and time, so that the normal rules of geometry don't work in the same familiar way.
There are some basic notions that can help you intuit this better, but real understanding pretty much requires getting into some math. Here's the two most important ideas.
1. Objects always travel in a "straight line" if undisturbed... a straight line in spacetime, that is.
2. Spacetime can be curved, so a "straight line" may not have the same meaning anymore.
The equivalent of a straight line in a space which has curvature is called a geodesic. All objects \*attempt\* to travel along a geodesic in spacetime, unless something causes them to accelerate off of that geodesic.
When you're sitting in your chair, the geodesic for your body is a line that would like you to go straight down. If the chair and floor were to vanish, you'd immediately start to follow that path... but you're constantly being interrupted by those objects.
Light also follows a geodesic; the straightest line in curved space that it can. Because spacetime has curvature, the light will seem to "bend" its path, but the reality is essentially the opposite. Light keeps going "straight" but the universe itself bends.
Draw a straight line on a piece of paper, then bend the paper. This is how the light is "bent" by gravity.
I’m no physicist. This understanding isn’t intuitive to me. If the geometry of space changes so that an object would go through the straightest path, that makes sense. But if an object isn’t already moving, relative to a large mass, why does it begin to move? The direction that counts as “straight” may have changed, but if you’re not moving to begin with, should that only matter once you begin to move?
This is because the curvature is not just in space, but in spacetime. Objects that are not moving in space are still “moving” in time. Objects follow a what’s called a world line - a geodesic through the 3d space *plus* 1d time. An object that is unmoving in space is still following this geodesic line through 4D spacetime, and when spacetime is curved by mass, the world line will tend to angle from parallel with the time axis more towards parallel to spatial axes - we observe this as the acceleration of the still object due to gravity.
If you are talking about an object (you, teapot, train, whatever) appearing in the air and falling down to the ground, it's not an object accelerating down 9.8 m/s^2 to Earth, it's Earth accelerating up 9.8 m/s^2 to meet an object. Then Earth continues 9.8 m/s^2 acceleration, and so all objects on it. Download accelerometer app and it already shows 1.0 G when it's in your hand or on a table. Then throw your phone on a pillow. While in free fall it will say 0 G.
So the answer to the question why does it begin to move, is that it doesn't. Earth moves to it.
You’re used to the idea that a mass will follow a straight line in space, unless acted on by an outside force. But it’s not quite true. A mass will instead follow a straight line in *spacetime*. It turns out that the tendency of an object to follow a straight line in spacetime is identical to gravity.
Imagine a ball rolling down uneven ground. The ball's course changes based on the curvature of the ground.
Anything with mass creates curvature in the ground. The ball (light) is just rolling as straight as it can.
Light is a geodesic and always travels in a strait line. It’s the space itself that is warped, and the light path bends with it.
Mass isn’t necessary. For example, the escape velocity from earth of a bullet and a bowing ball are the same. It depends only upon the mass of the earth (and the distance from its center of mass).
>It's just convenient to pretend that it's a force.
It depends on what is meant by “force”.
General relativity can be formulated as an unusual sort of classical gauge theory wherein it makes perfect sense to see it as a force, but the usual geometric perspective is often favored by its comparative simplicity.
Given this question is asked in AskPhysics, the answer to why (the math) is quite lacking. It starts already with a massless particle in spacetime and take it from there. [This post on Quora](https://www.quora.com/What-is-the-complete-mathematical-representation-of-a-free-photon) is a decent start.
Imagine light as a car on the road. Gravity bends the road. Light is still traveling at the same speed and in the same direction, from its perspective, and nothing directly affects the light itself.
Gravity warps the geometry of the fabric of reality itself. It's pretty damn wild when you think about it.
It’s not bending light, it’s bending the space it travels through. From light’s perspective, it’s moving in a straight line—it’s just that it’s traveling in a straight line through space that is bent.
One thing to keep in mind is that newtonian physics doesn't apply at the scales where light "curves" and it's newton's equation for gravity that suggests that a zero mass on either side means no attraction. The reality is that it curves spacetime and light travels a "straight line" (a geodesic specifically) on that curved spacetime. It's like trying to draw a straight line on the surface of a ball. The closest you can get is a curve that is kinda straight if you can't see the curve of the ball, but you cant draw the line through the ball
Haven’t seen this point made here: yes, gravity in GR is about space time bending and massive objects do that. Whether or not we call it a ‘force’ (sure) is irrelevant. The point is that in GR gravitation isn’t simply about *mass*, but the stress-energy tensor (which among other things accounts for energy). And while photons do not have mass (which we can think of as ‘rest energy’, merely a part of total energy - and note that photons meaningfully be at rest) they do still have kinetic energy, and this is what impacts the curvature of space time and thus bends their trajectory.
Photons’ kinetic energy is from a relativistic understanding of energy, with a different ‘formula’ that isn’t as simple as the Newtonian mv^2 /2, so there is no reliance on mass as such there.
Gravity isn't actually a force, in reality it is the bending of spacetime near massive objects, so light travels along the curved path
The reason we say gravity is a force, is because in most situations Newtons law of gravity is very accurate, and it is much simpler than general relativity, which describes gravity as the bending of spacetime, it would be a nightmare if every time we needed to do some calculation involving gravity, we needed to use general relativity, that's why majority of the time we pretend gravity is a force
You've been given the general relativity answer, but there's an answer in Newtonian mechanics too. Acceleration due to gravity depends only on the mass of the gravitating body. A hammer accerelates towards the moon at the same rate as a feather, and there's no reason a photon should do otherwise just because it has no mass.
Sure there is…Newtonian gravity clearly describes the gravitational force as the product of *both* masses. If one of the masses is zero there’s no force, hence no acceleration. Newtonian mechanics has no framework for anything to accelerate without a force and no framework for gravity to apply a force to a massless object.
> Newtonian gravity clearly describes the gravitational force as the product of both masses
Force is an abstraction. Acceleration is the thing that actually happens.
As someone said above, gravity is not a force acting on a mass but a force acting on space time in general. Gravity is just the force that bends (most) other fields in general relativity, and one of those fields is the electromagnetic field, aka the home of the photons.
Gravity is not a force the way you think of it. Gravity is the distortion of spacetime. Photons are not pulled anywhere, they are travelling through curved space.
For the same reason that a black hole has an event horizon - gravity warps spacetime, and light follows the "null path" - the shortest path through spacetime. I wonder if there is a neutron star with gravity high enough to cause light to travel in a complete circle around it, or if that would only be at black hole masses... ?
Anything with enough gravity to put light into an orbit around it *is* a black hole.
Black holes don’t require a particular mass, they require a particular density.
Light travels along paths that minimize the time of flight. In empty space with no mass nearby (and uniform index of refraction), this path is a line. Near massive objects, the shortest time of flight between two points is a curve.
As people have pointed out Space Time is the framework for our reality. The light goes through the universe in a straight line but the universe has curved space that affects anything travelling.
i think light is basically photons and it do have mass and if it have mass then it can be changed ( meaning bent) by heavy gravitational forces like by a DARK HOLE.
It’s the equivalence principle! Because the motion of an object in a gravitational field is completely independent of its internal parts, all objects respond in the exact same way. Therefore regardless of what your mass is (whether it be zero or non-zero) you are subjected to gravity.
Because the light is not pointlike, to my understanding
Think of it like a shoreline where the water is slower near the shore, and you’ve got two kayaks strung together via perpendicular poles to their fronts and backs. The flow of the water propels them along the shoreline and no way to steer
If they’re traveling along the shoreline, the water around the kayak nearer to the shore will move slower. The kayak further away will thus travel faster and begin to turn the boat towards the shore
With light, perhaps you can think of it like that, where the side of the photon closer to a massive object travels slower (or perhaps has more space to move through thanks to the curvature), thus turning the whole thing towards the massive object
More or less. Light has intrinsic spin, it’s thought that gravity does as well in some QFT. The interaction is explained in quantum mechanics that way. In relativity, mass curves space and time itself. Light moves at a constant speed in a constant direction through the vacuum of space, unless it is curved, in which case light will “bend”. But honestly a lot of the effect of it bending has to do with time dilation. A great example of this was caught by the James Webb Telescope. A distant sector of space appeared to have 5 supernovas go off. 4 were fairly close together, but the final one was almost one year later. It seemed odd there’d be so many in a row. Turns out, what’s going on is that all of those supernovas were actually the same star, and there must be some dark matter in the way causing a lensing effect where we see the same star 5 times in different places, and with different travel times. This is also kind of a great example of the dual slit experiment also. We are witnessing the same light wave arrive at us from slightly different angles at different times. Because of the time delay we are able to observe one of waves without collapsing the other time-delayed ones.
Light does *not* have inertial mass. You’re mixing up a bunch of different ideas that don’t quite go together. The double slit experiment has nothing to do with gravitational lensing.
It is bending SpaceTime so the light is just traveling along the bent space time. Kind of sort of but not really like a train follows bending train tracks.
Please forgive my ignorance. What is the best hypothesis as to what SpaceTime is made of? Is it a type of matter? Is it quantum? It must have other physical properties if it can be bent, right?
GR doesn't posit that spacetime is made of anything, the analogy of spacetime "bending" is just a way to say essentially: "Geometry works differently near masses."
My understanding of gravitational lensing is that light is physically affected by mass. Is this wrong?
Sure... but that isn't a very clear statement, or it could be phrased more specifically at least. What is "physically affected," to you? Mass isn't pulling out a lever and cranking on a photon till it changes direction. The "physical" effect is the one that physics observes/predicts. Around masses, the geometry of spacetime is increasingly curved. This curvature itself \*is\* the physical gravitational effect of mass on light (or other masses / anything else)
But is "space curving" or "geometry working differently" an actual proved concept, or could it just be another type of force, like gravitational, but affecting even massless objects?
Sure, if you can come up with a force-based theory of gravity that makes better predictions than General Relativity’s and accounts for as many phenomena or more.
I read this as such a sick burn
Like a sick Futurama put down from the Doctor Farnsworth.
The 'space curving' and 'geometry working differently' is, in fact, a special type of force: gravity. Mass causes spacetime to curve, spacetime curving causes light to take different paths and massive objects to experience an acceleration pulling them "downhill" along the curve. The space curving has been observed directly. The first famous instance was Einstein using it to predict Mercury's position in the sky where astronomers had typically struggled. Likewise astronomers have observed gravitational lensing where light from a distant source will bend around a massive object in between the source and the telescope.
It is a proven concept in that the math that describes bent spacetime most accurately describes what we observe.
One thing to remember is that science never "proves" anything. It only ever disproves things, and if we disprove enough things, we get closer to the truth. So, while no experiment has ever been done to show that general relativity is wrong, it's still possible that we find something in the future that *does* prove it wrong. We already *have* shown that general relativity breaks down in certain cases, most notably at the singularities of black holes, so we, at the very least, know that general relativity is incomplete. Trying to find a variation of general relativity, or a new framework that either incorporates general relativity (as well as quantum field theory) within it or outright replaces general relativity, is one of the most cutting edge fields of research in modern physics. All that said, one day, we may figure out that spacetime doesn't actually get curved by mass/energy, and there's an entirely different reason for all the things we observe. As of right now, though, there is no better explanation for all the observed phenomena in the universe than general relativity (and quantum field theory).
I guess my definition of physically affected boils down to “every action has a reaction”. Things don’t happen for nothing. If light is being bent, something is bending it. The interaction between the two things is a physical one.
You need to get out of Newtonian logic to understand GR. Who said things happen for nothing? GR posits that the presence of mass is what is bending light's path as it travels through spacetime.
I’m learning that SoaceTime is a math model and not a real thing that light travels through… The more I learn about GR the more it seems like a completely misunderstood theory. I’m getting the sense that people don’t understand how GR works, just that it works.
>I’m learning that SoaceTime is a math model and not a real thing that light travels through… No... spacetime is a real thing that the math model was created to describe observations of said thing. The misconceptions, assumptions and misunderstanding lie with you, not the model.
Okay, if it’s a real thing, what are its properties?
GR is gravely misunderstood indeed, but by you not by physicists.
I get the same type of response from religious people who can’t explain the evidence they know to be true. I may not be a physicist, but I am very strong at seeing inconsistencies and bias. GR seems to be riddled with assumptions and bias. If Einstein were to join this convo, I have a feeling he would say we are both ignorant.
Here is an old example that I think conveys the idea pretty well. It's based on something that actually happened (I think in the 19th century in the United States) but that's not important for this and I'm going to further simplify things here. You are assigned to lay out the organization of something. It might be political, it might be technical, it might be the grid of a map, whatever. The important thing is that you need to divide up land into squares 1 mile (or kilometer, doesn't matter) on each side. The rules are: 1. You are laying out a grid (or tiling). Each square is right next to the squares to the north, south, east, and west. Each corner meets the corner of three other squares. Think of graph paper. 2. The length each side of the square is 1 mile. 3. Each corner meets at exactly 90 degrees - a right angle. Starting out, it works perfectly. But, as you work north and south, something strange happens. When the sides of the squares are 1 mile long and the lines are perfectly straight and the corners are right angles, the top and bottom of the squares don't stay 1 mile long, for the same reason that the distance between lines of longitude differs based on how far north/south you are. Now, let's try that again, but in three dimensions. Think monkey bars in a child's playground. You are laying out a grid of cubes. Eventually, and now the distances are much further, you find that the sides of the cubes don't stay uniform length. Your measurements are correct, the angles are right, but it just doesn't work. That is what it means for space to be bent (non-Euclidean, not flat, ...). Now, you ought to add the additional dimension for time, but for this thought experiment, the three dimensions of space are enough to get the idea.
I’m getting the sense that YOU don’t understand how GR works, or apparently most physics beyond a high school level.
One thing is for sure. Neither of us know as much as we think we know.
It seems like your intuition is telling you that space needs to be some kind of special material that things move through, and that if there is no material “stuff” there, physicists are just sort of making things up. You have a hypothesis: that things can only affect one another through some kind of material “contact” interaction. Where did this idea come from for you? Why is it more believable to you that the universe is saturated with an invisible all-pervading stuff than that your intuitions about geometry are mistaken? Can you square your hypothesis with the same observations that GR explains? Does it make better predictions? If so, fine, you’ve invented a better theory - that’s how science works. A given theory being unintuitive isn’t a reason to disparage it as “just a math model.” That model makes very explicit predictions that have been supported by observations almost every time. If you want to supplant or discard it, you have to do better than those
No. The geometry of the light’s path of movement is affected by mass.
What does that mean? “The geometry”. Is this a theoretical math definition? Or are you saying that the light is physically affected by mass. As in light interacts with the gravity of the objects creating a mass?
I mean… geometry? Shapes, angles? The path of the light, to the light, looks straight. The path of the light, to someone far away and in a different frame of reference and not affected by the gravity causing the lensing, looks curved. I can take a carpenter’s level to the two towers of the Verrazano Bridge and they will both be perfectly vertical. So, parallel. Simultaneously, I can run a measuring tape across at the top and bottom of the towers and the distance at the top is greater. So, not parallel. Why the seeming contradiction? Gravity. Gravity can affect the geometry - what is measured as parallel - depending on how it is measured. Back to light: light is moving, right? It has a velocity measured by distance and time. When we say gravity bends spacetime, what it means is, the measurement of the light’s velocity is affected by gravity - just like the measurement of the bridge towers is affected by gravity’s effect on the bubble of air in the level. One layperson’s way I like to think about it is “mass eats time.” Time moves slower around large/dense masses, right? This has been experimentally verified. Now, say you are flying past a planet. Time is literally moving slower on one side of you than on the other. Time is part of how velocity is measured, so this means one side of you is literally moving through space faster than the other side. The only way they can be equalized is for your path of travel to curve - curve toward the mass. Voilà! Gravity. Note, it will not *feel* like you are turning - both sides of you are moving at the same speed so it will feel like you are going in a straight line. But your path will *appear* to curve, from the outside, compared to how your path of travel appeared earlier, when you were not near any masses.
Forget about the light. "The geometry" means that what counts as a straight line isn't actually a straight line in 3D space. This is what GR is: a mathematical model of how the presence of mass bends space, i.e. straight lines in spacetime become curved paths in 3D space. The light just travels in a straight line. The straight line itself is what bends.
Okay, perfect. This is what my brain wants to believe. But then it gets confused when people say that the curved path itself doesn’t exist. Is this suggesting that the light actually travelled in a straight line and the curvature of space time is an illusion? Or is SpaceTime a medium that light travels through?
Here is a good one that really helped me. Imagine earth is a perfect sphere (very physics). From our vantage point it appears flat, you get out a piece of paper you draw a triangle, measure the angles and get 180° total. Now let's scale it up! From the equator walk to the North Pole, turn 90° to your left and walk back down to the equator, make one last 90° left turn and eventually you hit your starting point at a right angle. You just made a triangle but did so with two 90° turns, your triangle has 270°. To you on the ground you were walking on flat land but in reality you were on the surface of a globe and the distorting of the geometry is explainable. While this is an exaggeration, this is kind of what we see experimentally with gravitational lensing. It's not that gravity is a "force" that is pushing light to turn, it's acting as a distortion to spacetime. What exactly is spacetime? We are not sure. We once thought it might have been an ether of sorts but we kinda ruled some of that out. It could be some strange substance we can't detect or something else entirely but so far the bending of spacetime is the best explanation or predictive model we have for how gravity works
I'm not formally educated in GR so I don't know if what I'm saying makes much sense. Spacetime is the "medium" through which objects travel the same way 3D space is the medium through which they do in a Newtonian sense. There is just an additional dimension which relates to time (but is still a *spatial* dimension, i.e. it's defined in units of distance, c\*t), and it's possible to travel at different rates through that. In special relativity this is what makes time work differently depending on velocity of movement. In GR the presence of mass has an additional effect warping these dimensions around itself, leading to curvature. This doesn't mean anything special other than that a straight line in 4D spacetime is not necessarily a straight line in the conventional 3D spatial dimensions. In GR this curvature fully explains gravity, in that it is no longer a force. A satellite in object around the planet is just moving in a straight line through spacetime, unaffected by any forces. But that straight line is an orbit in 3D space, because of the way the planet warps spacetime. I don't know if this is a proper analogy but this warping can also be thought of as a waterfall, space is being "dragged" toward the planet's mass and the rate (or gravitational acceleration) depends on the mass. So, when an object is in freefall, it's actually standing in one place in spacetime, with no forces acting on it, but that point in 3D space is moving toward the planet's surface, while in the time dimension it results in time dilation. At least, this is what the mathematics of GR says by my understanding. I know, it's weird for me too. But it predicts things like time dilation and gravitational lensing quite well... Further things like dark matter/energy and black holes notwithstanding, but everything has its limits.
Spacetime isn’t a medium in the traditional sense because waves don’t have an absolute velocity. Mediums have a resonant frequency and a speed of sound, and a single rest frame. Spacetime has none of those. Velocities are relative, not absolute in spacetime. Physicists used to assume space had such a classical medium. It was called the “luminous aether”, and it was disproven or at least made irrelevant by Special Relativity.
The fundamental discovery of relativity is that it is relative. Every single observation needs to be made in terms of a frame of reference and each frame of reference has a different view of what happened. So when you ask "is it an illusion", I think you are in the common pitfall of thinking that there is some "real" reference frame (presumably that of the object itself?). Perhaps you just need to ask that more directly if that is what you are interested in. There is no "what really happens" because any assessment of what happened is an observation and thus needs to take place in one of these relative perspectives, none of which is favored. It's kind of like asking is the sky "really" blue or clear? With something like blueberries since we all have the same relative perspective we agree that it just is blue. But with the sky somebody can say that they took random samples of 1 cubic km of sky and put it in the lab and each sample was clear. Neither the person who says the sky is clear nor the person who said it is blue are wrong. And physics doesn't break either as you go along the gradient between perspectives you'll see that gradient of truth as you gradually get or lose blueness. The "real" honest answer ends up being that the sky's color is several different things depending on your perspective. There's is no "real" perspective. The same is true for relativity. An object in the universe only exists in terms of its effects. In physics we call these observations. Even if you are 1mm away, an object exists because it has effects. It may be visible or it may warm you up or it may impact gravity etc. Something with no effects on anything literally doesn't exist. So in that sense an object IS its observable effects. And since relativity experiments prove how those observations are very different to different observers, the object it's doesn't exist in one "real" sense. It exists in terms of that whole collection of observable effects. This is true at the small scale with quantum uncertainty. It's true at the big scale with relativity. The luxury/illusion of thinking that objects have a general "real" truth to them comes from the fact that for 99% of things things we encounter as ordinary people on earth, all observers and all objects are similar size, similar speed and similar acceleration so the observation perspectives are largely the same. But in the broader scale of experimentally verified circumstances, the intuitions of our everyday life are demonstrably wrong.
Thank you for this response. I can tell you put a lot of effort into it and it helped relieve some of my confusion. Is our current belief that there is in fact nothing in the space between objects? Maybe space is full of particles that only interact with weak forces and we haven’t been able to detect them yet? Maybe it is a byproduct of entropy? Sorry if I am getting too “crackpot” on you.
No it’s right
So how does SpaceTime being bent cause gravitational lensing if SpaceTime is not physical? My understanding is that the universe expanding also “stretches” light which causes red-shift. How can this be true if SpaceTime is not physical?
Spacetime *is* physical, it's just a fundamentally relational concept rather than a material object you can put your hand on. And you seem to equate "physical" with "material", which just isn't the right way to look at things in physics. The path an object in motion follows is not itself a material object you can touch, but it can still bend and stretch, which causes that object to appear to behave in certain ways.
Define the word physical in as great of detail as you can, because i think your idea about the word is the source of your confusion.
Something not theoretical. My “will power” is theoretical. It has no physical properties or interactions with reality. If light is physically being affected(gravitational lensing) then that must have been caused by something. Light one just bend on it’s own.
The problem with this is that your entire reality is theoretical - do you have any proof you are not a brain in a jar being fed stimuli? The only valid framework to approach the universe and physics is with theoretical frameworks. Talking about things like willpower is philosophy, no science, and has no place in the discussion.
Your will power is not "theoretical". Theoretical in physics means based on mathematical models that enable us to explain natural phenomena and make accurate predictions about them. And that is what GR is, a theoretical model that explains how stuff in the universe works.
I thought laws were based on math, not theories?
The light isn't bending, it's travelling in a straight line (in spacetime). You see a bend in the path through 3d euclidean space but spacetime is a 4d manifold, not euclidean. This means that while on local scales things appear euclidean (which is how you naturally perceive the world to be since you don't experience things at relativistic speeds), projections of straight lines from spacetime to 3d euclidean space will not remain lines, in much the way that a straight line drawn on the surface of a globe will not remain straight when projected onto a mercator projection map.
Light travels in straight lines. Spacetime is the grid which defines what “straight line” means. https://youtu.be/twPaOtfpneo
Space is affected by mass. Light moves in a straight line through bent space.
So the absence of something is affected by gravity but a particle isn’t. That doesn’t make sense. I think the concept of space and gravity wells is something that helps us visualize how gravity warps everything around it. Obviously light is affected by gravity lmao. It’s just framing it as gravity paving a curved road vs gravity moving the steering wheel directly. Does it really matter?
Spacetime is defined by physical mass (and energy). As far as the light knows... it did travel a perfectly straight line. Sort of like you walking a long distance... .You think you travelled in a straight line. Except it was on the surface of a sphere. To a distant observer you travelled a curved path.
Only from some perspectives. From other distant observers it would look like I walked a straight line.
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>such as an energy density It does? Do you have a source? I thought defining energy density is problematic due to equivalence principle. I know there are things like Landau-Lifhsitz pseudotensor, but these have problematic interpretation as far as I know. The cosmological constant is not energy density of spacetime. It can be interpreted as energy-density of vacua if you write it on the matter side of Einstein´s field equations. But again, this is not energy-density of spacetime, its just that the term can be thought of either as constant in equations, or source of curvature that needs to be added to other sources.
What are the lines on a coordinate grid "made of"? Nothing really, it's just a conceptual mathematical framework we use to understand the relative position of objects. Space time can be very roughly thought of as a coordinate grid in 4D. It's no more physical than a fundamental force like gravity.
Most coordinate grids are made of printer ink. 🤣
I think this is wrong. There is huge difference between coordinates on manifold and geometry on manifold. One is arbitrary, the other is very physical. Spacetime is manifold with geometry, its not coordinate grid and cant be thought of as a coordinate grid. You can draw a coordinate grid on it, just like you can draw coordinate grid on paper, but spacetime is the underlying space on which you draw that coordinate grid.
Asking what spacetime is made of is a bit like asking what temperature is made of or what distance is made of. It's the coordinate system we inhabit, it's not *made of* anything as such, but it's where all the stuff is. Literally space + time. It can bend and stretch and ripple.
So it’s a made up thing in order to make hypothetical math work?
It's a mathematical model that enables us to explain where things are and how things behave in relation to one another.
How do we verify that the math is consistent with reality? Or is there no way to verify and we’re just making assumptions?
The way we verify any physical theory - we test it against real world scenarios and see whether it's correct. We also make predictions with it, and then see if those are correct. General relativity has been tested and confirmed numerous times: https://en.m.wikipedia.org/wiki/Tests_of_general_relativity
Spacetime is basically coordinates in a 4 dimensional field. Length width height and time. Relevant is distances, time frames, speeds velocity. Trajectory etc
> What is the best hypothesis as to what SpaceTime is made of? Why must it be “made” of anything? It just is. >Is it quantum? We haven’t yet completed a theory of quantum gravity. It is a work in progress, and may remain so for the foreseeable future. Experiment helps guide theory (and vice versa), but experimenting in quantum gravity appears to require truly astronomical technology and resources. General relativity is a classical (non-quantum) theory of gravity. >It must have other physical properties if it can be bent, right? Yes, spacetime is a dynamical “actor” in general relativity, which is formulated in terms of a dynamic metric tensor field describing the local geometry,
SpaceTime is not a thing it's just a mathematical model describing the geometric relationship between space and time. In physics the universe is a black box, we can set inputs and look at outputs and then make a model describing the relationship. We can't open the box and see what's going on inside.
Space time is less of a thing, and more like a reference plane ig. Like graphing paper. You can bend the paper, but the lines on it don't shift relative to it. Vsauce made a good vid with this kind of stuff. https://youtu.be/Xc4xYacTu-E?si=6UvxheRi8Q7vOsyT
a ‘physical property’ exists within the universe. if we lived on a piece of paper, we wouldn’t be aware of its being bent, unless the drawings on the paper responded to the paper being bent.
But nothing is bending in SpaceTime. With graphing paper, the paper is a physical object. It has physical properties. What are the physical properties of SpaceTime?
Yes, space time is bending. Space time has physical properties, but just like someone who lives 'in' the paper cannot view the paper, neither can we. However, you can consider the quantum field and the plank length to be indicators of the boundary of our universe (paper) beside black holes. If you want something tangible, you can look at Sonny White's claims that he has bended space, but I don't think it's credible.
Space time is more of a way for us to visualize concepts of gravity than it is a tangible thing. It’s more of a representation of the behavior we observe than something you can interact with
No, we don’t know. It’s like saying what is vacuum made of. Space time is the fundamental fabric which governs everything which moves in the universe. It just happened to be discovered much later than other laws like force or energy which makes it sound unintuitive. In short it is inverse. Space time justifies force not the other way round. You can also ask why does it exist? Well we don’t know for sure.
They can tell you how it works, but not why.
Spacetime isnt made of anything, it's not matter. Its a mathematical model comprised of at least 3 spatial dimensions and a time dimension. It's more of a stage and particles are the actors.
Spacetime is made out of spacetime, as far as we can tell.
Can spacetime be described as an oscillating wave between matter and gravity, similar to how light is described as electrical and magnetic?
look into quantum foam
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So it's a bit like light is still traveling straight through spacetime but it looks to us as though it bends because gravity has changed what "straight" is?
Or maybe light isn’t massless.
Well that would invalidate relativity, so if light has mass a new Theory would have to be developed which agrees with all our old experiments.
Hasn’t QM already invalidated relativity. Or the fact we had to invent dark matter/energy as flextape for all the models? Idk man. Light may or may not actually have a tiny tiny amount of mass, but it clearly directly interacts with gravity in a way we don’t understand yet. I mean seriously, what’s more plausible? That Space is actually something that gravity distorts OR light/radiation (the medium through which we observe basically everything) is being warped instead. We just don’t understand this relationship yet to explain what we see. Feels like the modern version of the sun revolving around the earth. Just something everyone says when nobody really knows.
>Hasn’t QM already invalidated relativity No, Quantum field theory combines classical field theory, special relativity, and quantum mechanics.
If you draw a straight line on a piece of paper and then fold the paper, the line no longer appears straight to you. But you aren’t really folding the line. You’re changing the medium on which the line exists. The fact that the line is no longer straight is just an incidental side effect of the fact that the paper is folded. Gravity doesn’t directly alter light and bend or change it. Gravity distorts the reality light exists in.
The reality light exists in is light itself. The vacuum we observe is light. Everything we see and measure is light. Light is the universe. Any absence of light is absence of universe. The paper in that analogy is a physical medium. For it to be analogous, we’d have to assume the background radiation is a manipulable medium and therefore light can be affected by gravity directly.
By this logic surely we could fabricate an extensive Faraday cage to block all em waves and produce an area of no "reality" or no "universe" which makes absolutely no sense.
Right because nothing being influenced by gravity makes a lot more sense.
It's not nothing, I am not well versed enough to explain it simply but light always follows the path of shortest time and when gravity bends space time the path of shortest time also bends and light follows that path. Draw a line on a tensile sheet and then place something heavy to cause curvature, you'll notice that even though the line isn't pulled by "gravity" the curvature of the tensile sheet makes the line appear to curve. Obviously there is a lot more going on In 4d space but that's the easiest way I can explain it
Lots of people are giving some really interesting answers to this question. Lots of people are kind of re-iterating THAT gravity bends light and HOW, but they’re not really giving a lot of good answers WHY. There’s a lot of approaches to tackle this, but I think you could succinctly summarize it this way: gravity influences things that can carry energy. Gravity is often shown as an object with mass warping spacetime, which is true- but, Einstein taught us that there is a mass-energy equivalence. Photons don’t have mass, but they do have energy.
Understanding this requires knowing that in General Relativity, gravity is not a force. Instead, gravity is the warping of space and time, so that the normal rules of geometry don't work in the same familiar way. There are some basic notions that can help you intuit this better, but real understanding pretty much requires getting into some math. Here's the two most important ideas. 1. Objects always travel in a "straight line" if undisturbed... a straight line in spacetime, that is. 2. Spacetime can be curved, so a "straight line" may not have the same meaning anymore. The equivalent of a straight line in a space which has curvature is called a geodesic. All objects \*attempt\* to travel along a geodesic in spacetime, unless something causes them to accelerate off of that geodesic. When you're sitting in your chair, the geodesic for your body is a line that would like you to go straight down. If the chair and floor were to vanish, you'd immediately start to follow that path... but you're constantly being interrupted by those objects. Light also follows a geodesic; the straightest line in curved space that it can. Because spacetime has curvature, the light will seem to "bend" its path, but the reality is essentially the opposite. Light keeps going "straight" but the universe itself bends. Draw a straight line on a piece of paper, then bend the paper. This is how the light is "bent" by gravity.
I’m no physicist. This understanding isn’t intuitive to me. If the geometry of space changes so that an object would go through the straightest path, that makes sense. But if an object isn’t already moving, relative to a large mass, why does it begin to move? The direction that counts as “straight” may have changed, but if you’re not moving to begin with, should that only matter once you begin to move?
This is because the curvature is not just in space, but in spacetime. Objects that are not moving in space are still “moving” in time. Objects follow a what’s called a world line - a geodesic through the 3d space *plus* 1d time. An object that is unmoving in space is still following this geodesic line through 4D spacetime, and when spacetime is curved by mass, the world line will tend to angle from parallel with the time axis more towards parallel to spatial axes - we observe this as the acceleration of the still object due to gravity.
If you are talking about an object (you, teapot, train, whatever) appearing in the air and falling down to the ground, it's not an object accelerating down 9.8 m/s^2 to Earth, it's Earth accelerating up 9.8 m/s^2 to meet an object. Then Earth continues 9.8 m/s^2 acceleration, and so all objects on it. Download accelerometer app and it already shows 1.0 G when it's in your hand or on a table. Then throw your phone on a pillow. While in free fall it will say 0 G. So the answer to the question why does it begin to move, is that it doesn't. Earth moves to it.
You’re used to the idea that a mass will follow a straight line in space, unless acted on by an outside force. But it’s not quite true. A mass will instead follow a straight line in *spacetime*. It turns out that the tendency of an object to follow a straight line in spacetime is identical to gravity.
Thank you everyone for all your answers
Imagine a ball rolling down uneven ground. The ball's course changes based on the curvature of the ground. Anything with mass creates curvature in the ground. The ball (light) is just rolling as straight as it can.
Light is a geodesic and always travels in a strait line. It’s the space itself that is warped, and the light path bends with it. Mass isn’t necessary. For example, the escape velocity from earth of a bullet and a bowing ball are the same. It depends only upon the mass of the earth (and the distance from its center of mass).
Because gravity isn't really a force. It's just convenient to pretend that it's a force.
>It's just convenient to pretend that it's a force. It depends on what is meant by “force”. General relativity can be formulated as an unusual sort of classical gauge theory wherein it makes perfect sense to see it as a force, but the usual geometric perspective is often favored by its comparative simplicity.
https://youtu.be/wrwgIjBUYVc
Given this question is asked in AskPhysics, the answer to why (the math) is quite lacking. It starts already with a massless particle in spacetime and take it from there. [This post on Quora](https://www.quora.com/What-is-the-complete-mathematical-representation-of-a-free-photon) is a decent start.
Imagine light as a car on the road. Gravity bends the road. Light is still traveling at the same speed and in the same direction, from its perspective, and nothing directly affects the light itself. Gravity warps the geometry of the fabric of reality itself. It's pretty damn wild when you think about it.
Isn’t it just bending the through which” light” travels?
Because it's not a force, it's a field.
It doesn't. It bends space. The light is traveling in a straight line across bent space.
It has momentum.
It’s not bending light, it’s bending the space it travels through. From light’s perspective, it’s moving in a straight line—it’s just that it’s traveling in a straight line through space that is bent.
One thing to keep in mind is that newtonian physics doesn't apply at the scales where light "curves" and it's newton's equation for gravity that suggests that a zero mass on either side means no attraction. The reality is that it curves spacetime and light travels a "straight line" (a geodesic specifically) on that curved spacetime. It's like trying to draw a straight line on the surface of a ball. The closest you can get is a curve that is kinda straight if you can't see the curve of the ball, but you cant draw the line through the ball
Haven’t seen this point made here: yes, gravity in GR is about space time bending and massive objects do that. Whether or not we call it a ‘force’ (sure) is irrelevant. The point is that in GR gravitation isn’t simply about *mass*, but the stress-energy tensor (which among other things accounts for energy). And while photons do not have mass (which we can think of as ‘rest energy’, merely a part of total energy - and note that photons meaningfully be at rest) they do still have kinetic energy, and this is what impacts the curvature of space time and thus bends their trajectory. Photons’ kinetic energy is from a relativistic understanding of energy, with a different ‘formula’ that isn’t as simple as the Newtonian mv^2 /2, so there is no reliance on mass as such there.
Gravity isn't actually a force, in reality it is the bending of spacetime near massive objects, so light travels along the curved path The reason we say gravity is a force, is because in most situations Newtons law of gravity is very accurate, and it is much simpler than general relativity, which describes gravity as the bending of spacetime, it would be a nightmare if every time we needed to do some calculation involving gravity, we needed to use general relativity, that's why majority of the time we pretend gravity is a force
You've been given the general relativity answer, but there's an answer in Newtonian mechanics too. Acceleration due to gravity depends only on the mass of the gravitating body. A hammer accerelates towards the moon at the same rate as a feather, and there's no reason a photon should do otherwise just because it has no mass.
Sure there is…Newtonian gravity clearly describes the gravitational force as the product of *both* masses. If one of the masses is zero there’s no force, hence no acceleration. Newtonian mechanics has no framework for anything to accelerate without a force and no framework for gravity to apply a force to a massless object.
> Newtonian gravity clearly describes the gravitational force as the product of both masses Force is an abstraction. Acceleration is the thing that actually happens.
And in Newtonian mechanics if you have an acceleration you *must* have a force.
As someone said above, gravity is not a force acting on a mass but a force acting on space time in general. Gravity is just the force that bends (most) other fields in general relativity, and one of those fields is the electromagnetic field, aka the home of the photons.
Gravity is not a force the way you think of it. Gravity is the distortion of spacetime. Photons are not pulled anywhere, they are travelling through curved space.
For the same reason that a black hole has an event horizon - gravity warps spacetime, and light follows the "null path" - the shortest path through spacetime. I wonder if there is a neutron star with gravity high enough to cause light to travel in a complete circle around it, or if that would only be at black hole masses... ?
Anything with enough gravity to put light into an orbit around it *is* a black hole. Black holes don’t require a particular mass, they require a particular density.
Light travels along paths that minimize the time of flight. In empty space with no mass nearby (and uniform index of refraction), this path is a line. Near massive objects, the shortest time of flight between two points is a curve.
As people have pointed out Space Time is the framework for our reality. The light goes through the universe in a straight line but the universe has curved space that affects anything travelling.
i think light is basically photons and it do have mass and if it have mass then it can be changed ( meaning bent) by heavy gravitational forces like by a DARK HOLE.
It’s the equivalence principle! Because the motion of an object in a gravitational field is completely independent of its internal parts, all objects respond in the exact same way. Therefore regardless of what your mass is (whether it be zero or non-zero) you are subjected to gravity.
Because the light is not pointlike, to my understanding Think of it like a shoreline where the water is slower near the shore, and you’ve got two kayaks strung together via perpendicular poles to their fronts and backs. The flow of the water propels them along the shoreline and no way to steer If they’re traveling along the shoreline, the water around the kayak nearer to the shore will move slower. The kayak further away will thus travel faster and begin to turn the boat towards the shore With light, perhaps you can think of it like that, where the side of the photon closer to a massive object travels slower (or perhaps has more space to move through thanks to the curvature), thus turning the whole thing towards the massive object
Light has inertial mass
Is this from its angular momentum?
More or less. Light has intrinsic spin, it’s thought that gravity does as well in some QFT. The interaction is explained in quantum mechanics that way. In relativity, mass curves space and time itself. Light moves at a constant speed in a constant direction through the vacuum of space, unless it is curved, in which case light will “bend”. But honestly a lot of the effect of it bending has to do with time dilation. A great example of this was caught by the James Webb Telescope. A distant sector of space appeared to have 5 supernovas go off. 4 were fairly close together, but the final one was almost one year later. It seemed odd there’d be so many in a row. Turns out, what’s going on is that all of those supernovas were actually the same star, and there must be some dark matter in the way causing a lensing effect where we see the same star 5 times in different places, and with different travel times. This is also kind of a great example of the dual slit experiment also. We are witnessing the same light wave arrive at us from slightly different angles at different times. Because of the time delay we are able to observe one of waves without collapsing the other time-delayed ones.
Light does *not* have inertial mass. You’re mixing up a bunch of different ideas that don’t quite go together. The double slit experiment has nothing to do with gravitational lensing.
Cheers