Sabine Hossenfelder chimed in [0] on this discussion a couple days ago. I generally find her to be trustworthy on topics related to physics:
> The easiest way to see that gravity is not a force is to note that a force causes acceleration, but gravity does not.
> Acceleration is measurable with a device called an accelerometer. Acceleration is not relative (like velocity), it's absolute.
> If you are standing on the surface of Earth, an accelerometer will show that you are accelerated in the upward direction. That's because a force is acting on you from below, it's the solidity of Earth's crust (or whatever you are standing on), going back to a combination of electromagnetic forces and the Pauli principle.
> If you take away that support from Earth, eg by jumping off a plane, you are not accelerated. You are freely falling. Since you are not accelerated, there is no force acting on you. You experience gravity but no force, hence gravity is not a force.
> We can assign a pseudo-force to gravity by defining it as acceleration relative to the surface of Earth. This is how Newtonian gravity works. One can derive it from general relativity as an approximation.
> Physicists frequently do refer to gravity as a force anyway -- even I do -- because that's linguistically simpler. But it's like we say "internet" rather than "world wide web" even though we know that the two aren't the same, just because "internet" is simpler.
> So I usually don't pick on this. But strictly speaking, gravity is indeed not a force. If you have doubts about it, buy an accelerometer and do your own research...
If you built an accelerometer using only components that feel the electric force, would you be able to detect the acceleration caused by the electrical force?
I.e. part of the reason gravity is fundamentally different is that it's universal: everything obeys it, including the trajectories of radiation.
This makes it functionally a different thing than every other force, since for all other forces you can build a tool that would measure acceleration because there is always something available to you that is not affected by that other force
I must be really confused about physics and reality, because that argument feels like an April Fools joke.
Stick a rocket engine to the side of the mountain, and light it up. An accelerometer glued to the mountain will show a 0. So will one glued to the rocket. Does that mean thrust is not a force now?
They will both show acceleration. The one on the mountain will show an upward acceleration from the mountain pushing against the device against the pull of gravity. The rocket would too, but presumably greater if it’s accelerating faster than gravity is pulling. The key is that the gravity acceleration vector never shows up in any context. That’s because it’s not a force and the perceived acceleration is an effect of relativity, caused by the gradient of time and space. It’s entirely counter intuitive to our understanding of force and acceleration that you can observe increasing relative motion without a force based acceleration, but there it is
In the scenario you’ve set up
m >>>>>> F, because the Earth is fucking enormous. So a will be extremely small but nonzero. A precise enough accelerometer would show you this.
Also your mountain accelerometer would show ~9.8m/s^2 plus the extremely small contribution of the rocket thrust, not 0.
So… yes, you might be really confused about physics.
The accelerometer glued to the mountain will show -9.8m/s^2, not zero. The one glued to the rocket will show the same. The force of the rocket thrust is the opposite of the force of the glue holding it to the mountain, so the sum results in zero additional acceleration.
A theoretically perfect accelerometer will show something non zero as the rocket engine is in fact shifting the entire Earth by a very very very very very very tiny amount.
Stick two identical rocket engines together in deep space so their thrust cancels out. Is thrust not a force when rockets' guidance accelerometers both show 0?
But 0 is what you expect to see in that case: you have two forces acting on the body, of equal magnitude and in opposite directions, so the net force is 0, so acceleration is 0.
But if you jump out of a plane, you will see ~0. If gravity were a force, what would be the equal and opposite force acting on you?
Both engines are exerting force in opposite directions, which cancels out, as you said. There’s no acceleration happening either from gravity or this rocket arrangement, so the accelerometer reads zero. Why is this a problem? Who is saying thrust is not a force?
> The easiest way to see that gravity is not a force is to note that a force causes acceleration, but gravity does not.
Maybe I'm misunderstanding as it's been a while since physics class but I'm pretty sure gravity does cause acceleration. One of the few numbers I still remember memorizing in college 9.8m/s2 -- the acceleration by gravity on Earth.
That's the acceleration you feel on the surface of the earth. But that's not an inertial reference frame. In an inertial reference frame gravity doesn't cause acceleration (kind of by definition).
The argument is that gravity doesn't cause acceleration, resisting gravity does. Kind of how spinning an object doesn't cause a centrifugal force, the real force is whatever forces it to stay on a circular path instead of continuing straight
Objects don't fall down because of gravity, objects move along in straight lines because of inertia. It just happens that, close to large masses, "straight lines" get bent toward the center of mass, and this is what we call gravity.
But the difference is important: when you are falling down, you are in freefall, you don't experience any acceleration in spacetime. It is the surface of the earth that is experiencing acceleration towards you.
Suppose you were in a free falling box in a strong gravitational field. Is there an experiment that you can do within that box which would measure the acceleration? I think no. Not directly (perhaps you might get clever and do something with tidal forces). And if you can't measure it, why do you assume it's there?
By contrast, if you strap a rocket to the box, it's quite easy to measure the acceleration from within the box.
I think this is a distinction between push/pull? You are interpreting the reading as it being pulled down. You could also interpret it as you having to push it up.
The example you give also doesn't make sense. The acceleration would not go to 0 when you drop something. The acceleration continues for the entire fall. It may be offset by friction if it hits terminal velocity. But, absent that, acceleration would continue the entire fall.
If the reading of an accelerometer does change in freefall, then what it is measuring isn't the acceleration, necessarily, but the difference in acceleration between components in the device. Which makes sense. Is like watching a balloon in a car when you start moving. (With the trick of whether the windows are open or not, of course.)
This shows that your accelerometer is broken. (or, more likely, that the vector is mislabeled)
Try this thought experiment. Assume your phone is really accelerating downwards when at rest. Then let it enter a free fall i.e. drop it. Now it's accelerating down even more so the downwards acceleration should show an increase. Actually, it goes to zero. Relative to a free-fall, being stationary is an acceleration upwards.
To mimic the effect of gravity on the surface of the earth you could stand on a platform that is accelerating. As long as that platform is accelerating you will experience something similar to gravity on the surface of the earth.
The relative direction of the acceleration would be "up". This is what she means.
Your iPhone lies to you. The acceleration is indeed upwards, as you’ll see if you look at raw accelerometer data… or the feeling from your feet, which counts.
> > Acceleration is measurable with a device called an accelerometer. Acceleration is not relative (like velocity), it's absolute.
The reason that you can't measure gravitation acceleration in free-fall in a uniform gravitational field[] with an accelerometer is that all parts of the accelerometer are being accelerated together.
[] The "uniform gravitational field" part comes from Einstein's equivalence principle, and it's pretty obvious that if the field were sufficiently non-uniform then one could expect strain gauges to indicate that the free-falling object _is_ being accelerated. Imagine you have a 10km long space ship near a massive body like Earth, or the Sun, then the gravitational field will not be uniform across that space ship, and strain gauges will a) let you know, b) will even be able to tell you the strength of the field and the heading to the massive object.
All these "gravity is not a force" arguments based on the equivalence principle are simply nonsense. The better argument is that gravity is both not a force because it only curves spacetime, and also yes equivalent to a force (especially when you do a Mercator-style projection of curved spacetime onto flat spacetime).
Love the Hossenfelder reference (though she really needs to move to Bluesky...). That said, I'd love if someone could explain some physics to this noob:
If you jump out of a plane, at t=0 you'll not be moving (relative to the earth), at t=1 you'll be moving a bit (relative to the earth), and at t=2 you'll be moving even faster. How is that not acceleration? A quick wikipedia rabbit hole from "Accelerometer" --> "Inertial Reference Frame" --> "Fictitious Force" led me to this:
A pseudo force does not arise from any physical interaction between two objects, such as electromagnetism or contact forces. It is just a consequence of the acceleration a of the physical object the non-inertial reference frame is connected to, i.e. the vehicle in this case. From the viewpoint of the respective accelerating frame, an acceleration of the inert object appears to be present, apparently requiring a "force" for this to have happened.
They use the example of a passenger in an accelerating car, which makes sense. And there's some discussion of how the earth is rotating and thus has angular (?) acceleration, which also makes sense. I think this is what Hossenfelder is referencing by "the earth is accelerating you upwards". But the rotation discussion seems completely unrelated to gravity, no? An asteroid that isn't spinning would still exert gravitational (pseudo-)force, as all massive objects do of any size. Surely a person standing on a non-spinning asteroid in deep space isn't being accelerated upwards?
At the end of the day, I guess I'm missing why exactly "free fall" is the same thing as "no forces acting upon you". To my cynical arrogant brain, this reads like the physicists are harping on an unnecessary terminological thing, namely that a "force" is defined as "a physical interaction between two objects". In other words, its quantum bias, in the original sense of quantum; why can't objects interact with the continuous field of spacetime?
I'm commenting on your quote because her explanation especially "we have a machine with acceleration in the name, thus that's what acceleration is" set off a million alarm bells in my head, philosophically speaking!
That's just because gravity is also accelerating the accelerometer, and the accelerometer can only measure acceleration relative to the body of the instrument.
I've heard this argument: Because gravity is the only force that acts on all objects, including the measuring object, this makes it not a force. This is not actually an explanation, it's more of a definition, and not a useful one.
Accelerometers don't measure acceleration, they measure the difference between the force on a mass on springs and the body of your phone. If you drilled a tiny hole and pulled the mass up, the sensor would think the phone was being pulled down.
Acceleration due to gravity has an important property of acceleration in general: radiation. Spiraling black holes emit gravitational waves, in the same way that an accelerating electric charge emits light. There are free falling reference frames where uniform gravitational fields can go away, but the gravitation of a massive body isn't uniform and can't be eliminated by changing the coordinates.
The relationship between gravity and fictitious forces is an important stepping stone, but it does not have all the properties of a fictious force, only some of them.
Second, we should note that even Einstein himself cautioned against believing spacetime was actually curved. His writings inform us he didn't believe it. I don't want to appeal to authority, that's just to say smart people, including the main developer of general relativity, didn't believe it. But he didn't believe in the non-local nature of quantum theory either, which we have now, since Einstein's death, proven to be true.
Third, the claim that only gravity can be described using geometry is false, which the author himself notes later in this article. The stress-energy-momentum tensor simply makes gravity universal, unlike the other forces. I don't see any reason why that universality confers something special to gravity with regards to interpreting it as geometry. Just because we can model gravity as geometry, doesn't make gravity a result of geometry, and the author notes that modeling gravity that way makes it so we can't unify the forces.
Finally, as the old saying goes, if you think gravity isn't a force, drop a brick on your toe! :)
I'll also point out that singularities are generally considered to be a sign of issues with a model. GR has singularities. Maybe that should tell us something.
> we should note that even Einstein himself cautioned against believing spacetime was actually curved. [...] I don't want to appeal to authority
Another example that comes to mind is Max Planck believing that light being absorbed in discrete packets of energy was only a neat mathematical hack he came up with. It took Albert Einstein to say "but what if light is discrete packets". And then as you say Einstein having major reservations against the field of quantum theory that he himself spawned.
> if you think gravity isn't a force, drop a brick on your toe
By that logic the centrifugal force has to be a force. If you don't believe it, just drive a vehicle around a curve, or whirl a rock on a string.
> But he didn't believe in the non-local nature of quantum theory either, which we have now, since Einstein's death, proven to be true.
We have not proven quantum theory to be non-local. We’ve only proven that it can’t be both local and contain particles, as opposed to particles being emergent from the wavefunction’s interactions.
MWI chooses the latter, and is therefore a local theory.
(Alternately, collapse. But collapse theories are largely nonsensical.)
> First off, the author doesn't appear to be a crank.
Is this the first most people are hearing about this? In the 90's, I had a physics textbook for the layperson (i.e. non-STEM fields). It had a fantastic chapter on general relativity, and it also went with "not a force but a spacetime curvature".
Popular scientists have been saying it for decades and always use the flexible rubber sheet as the learning aid. Serious physicists tended to stay out of the fray, preferring to just "shut up and compute", as they do for quantum mechanics.
It's a relatively recent development that physicists have entered the fray and say maybe curved spacetime isn't a model, maybe it's reality. I urge caution to all as we have no evidence that the model of curved spacetime is indeed reality. I think the caution is warranted considering that GR contains singularities, which is a sign the theory has issues.
GR is a theory of the bulk. It’s like fluid mechanics, which treats fluids as infinitely divisible and doesn’t acknowledge the existence of molecules: There’s no way it’s accurate at small enough scales that molecules matter, which is what would be happening inside a black hole.
At larger scales, however, fluid mechanics is extremely reliable.
I am sympathetic to the author's thesis. I favor the idea that gravity is a different thing from the other fundamental forces, and possibly an emergent phenomenon rather than a fundamental thing in its own right.
But, I don't buy the argument made here:
> To call gravity a force, is to privilege flat space as somehow being special.
Flat space is special, and we didn't make it special.
This is taking an important aspect of known physics---that there exist various symmetries and all elements of the corresponding group are equal players (there is no privileged reference frame, positive charge and negative charge are indistinguishable save for their oppositeness, etc.)---and attempting to apply this principle to spacetime curvature. But the zero curvature state is a unique one that is differentiatable from the others. It's the only one where a circle is perfect, having radius 2 * pi * r. And pi is a fundamental invariant of geometry, curved or otherwise. The mathematics privileges flat space. Further, experiments can be constructed to detected whether we are in flat space or not [1]. That wouldn't be possible if the whole concept of flat were only relative to an arbitrary frame.
> Is this purely a semantic difference? You could argue that it doesn't really matter whether we describe gravity as a force or through geometry, and we should conflate these two concepts. But I think this distinction is important to make because it has predictive power. If you believe that gravity is manifest through spacetime bending, then you will never find two different test particles that follow different geodesics.
Don't we get the same conclusion if we believe gravity is a force _and_ equivalence principle is true?
I like the model of an insulated tunnel bored through the Earth from the north to the south pole and filled with a vacuum (technologically implausible, yes). If we drop a steel ball into the tunnel at the north pole, what forces does it experience?
From Newton's perspective, F = ma and the ball accelerates towards the center of the Earth. The value of g diminishes to zero at the center, the ball is at its maximum velocity, and then enters the negative acceleration regime until it just reaches the surface of the Earth at the south pole. This will continue indefinitely in harmonic motion. It's not a perpetual motion machine because machines do work and we're not doing any work on the ball; it's similar to an orbiting sphere. (ChatGPT-o1 claims the period is 84.4 minutes, assuming uniform density)
The general relativity perspective seems to be the ball is just rolling up and down a bowl of spacetime, without any friction or drag, which isn't all that satisfying a picture, since it implies a restorative force being involved to keep the ball from escaping the bowl. It helps to consider the state of the ball right before it is kicked into the tunnel - it is being prevented from following its natural geodesic trajectory by the electromagnetic forces of the rocks of the Earth's crust upon which it is being held up - that's the only relevant force in this picture.
“If you accept that we live in spacetime, and it can be curved, then I think you should accept that gravity cannot be a force.”
I am on the opposite time of thinking — where spacetime itself emerges from particle interaction events. Pairwise distance (i.e. metric) is just yet another interaction parameter in the world graph.
"is to say that when no force acts on a test particle in curved space, it should move along a geodesic"
Every single author who calls gravity not a force, just hand waves right past this: why should the particle move at all?
Sure if the particle is moving it will follow a curved path thinking the path is straight.
But if the particle just sits there, okay the path is curved, but it just sits there.
I've heard explanations having to do with the fact that particles move through time, that doesn't really answer the question because it can continue moving through time while just sitting there.
"in the absence of being pushed or pulled, test particles in a curved spacetime will free fall"
Really? Why exactly will they free fall? Why can't they just stay exactly where they are?
I've also never heard anyone explain what the issue is with calling gravity a force. One person said it can't be a force because it makes light move, and light is massless.
However gravity does not act on mass it acts on energy, and mass is just a form of energy. Since photons have energy obviously they gravitate.
I think the missing piece in your puzzle is adding the "time" to space-time. A particle moving through time is "falling" in the time dimension. The curvature of the 4d space makes this movement through time also move through space due to the warping effect of a gravity well.
> Really? Why exactly will they free fall? Why can't they just stay exactly where they are?
Absent any forces, they can only stay on a geodesic. It’s like the arrow that springs from the bow. No hesitation, no doubts. The path is clear. There’s only ever one geodesic, one path through spacetime that takes no energy to follow.
This is true even if you ignore general relativity and just focus on classical mechanics, as formulated by Lagrange. In Lagrangian mechanics, you must compute a number called the “action” for every path through space that your system could possibly take. Then your system will take the path that minimizes the action. If your system is a particle in space, then it will stay still if that has the minimum action, or it will follow some path through space if that has the minimum action. It can’t ever do both. Only one of them will minimize the action.
> one path through spacetime that takes no energy to follow.
Again, this if fine for an orbit for example, it does not explain how a particle will accelerate, it also does not explain momentum, because when that particle accelerate it exchanges momentum with another particle - it does not give momentum to the "geodesic".
And explain please what information is not captured by saying "Gravity is a force that acts on energy". What nuance is lost by saying that?
It seems like the point is that the particle is actually not accelerating when it follows the one path through spacetime. It only accelerates when it's stopped by something (e.g. the surface of the earth).
There is also the asymmetry of time dilation. Someone moving in a rocket ship will be younger than the person staying behind. Isn't their relative acceleration the same though? Why is the rocket man privileged in staying young? This kinda works if you imagine a grid of sand particles being acted upon by either gravity or traversed by the rocket. The rocket traverses many more grids than the person standing on Earth. The person floating in naked space will traverse the fewest grids.
The kicker is that there is no place in space with zero grid traversal. Accounting for all grid traversals becomes a nightmare.
Not arguing for or against this explanation, but it's hard enough (impossible for me) to visualize a curved 3d space, once you try to think of what curved 4d space might be like any intuitive sense of what should happen goes out the window.
Classic diagrams of curved spacetime portray space as a 2d sheet with divots in it. I wonder what a similar visualization of curved 1d time + 1d space would look like. Long valleys for gravitational wells and everything moving along the sheet in the direction of positive time?
You did the same as everyone else: You hard waved right past "end up moving". Why do I end up moving instead of just sitting there?
And if I "end up moving" that means I exchange momentum, if it's just bent space that magically makes me move, what did I exchange momentum with, if not via a force toward another particle?
Gravity is a force that acts on energy. If you disagree please give me a counter example.
You do just sit there. In the GR-realism view, the only thing particles _ever_ do is self-propagate forwards in time.
The problem is, this means the particle’s definition of “time”. Spacetime is 4D, and this can be any timelike direction. That’s what causes it to appear to move, from the perspective of someone whose time vector is pointed differently.
Then time can be curved, which reorients said vector as the particle propagates through it. That’s gravity.
Sabine Hossenfelder chimed in [0] on this discussion a couple days ago. I generally find her to be trustworthy on topics related to physics:
> The easiest way to see that gravity is not a force is to note that a force causes acceleration, but gravity does not.
> Acceleration is measurable with a device called an accelerometer. Acceleration is not relative (like velocity), it's absolute.
> If you are standing on the surface of Earth, an accelerometer will show that you are accelerated in the upward direction. That's because a force is acting on you from below, it's the solidity of Earth's crust (or whatever you are standing on), going back to a combination of electromagnetic forces and the Pauli principle.
> If you take away that support from Earth, eg by jumping off a plane, you are not accelerated. You are freely falling. Since you are not accelerated, there is no force acting on you. You experience gravity but no force, hence gravity is not a force.
> We can assign a pseudo-force to gravity by defining it as acceleration relative to the surface of Earth. This is how Newtonian gravity works. One can derive it from general relativity as an approximation.
> Physicists frequently do refer to gravity as a force anyway -- even I do -- because that's linguistically simpler. But it's like we say "internet" rather than "world wide web" even though we know that the two aren't the same, just because "internet" is simpler.
> So I usually don't pick on this. But strictly speaking, gravity is indeed not a force. If you have doubts about it, buy an accelerometer and do your own research...
[0] https://x.com/skdh/status/1850120005070799153
If you built an accelerometer using only components that feel the electric force, would you be able to detect the acceleration caused by the electrical force?
I.e. part of the reason gravity is fundamentally different is that it's universal: everything obeys it, including the trajectories of radiation.
This makes it functionally a different thing than every other force, since for all other forces you can build a tool that would measure acceleration because there is always something available to you that is not affected by that other force
I must be really confused about physics and reality, because that argument feels like an April Fools joke.
Stick a rocket engine to the side of the mountain, and light it up. An accelerometer glued to the mountain will show a 0. So will one glued to the rocket. Does that mean thrust is not a force now?
They will both show acceleration. The one on the mountain will show an upward acceleration from the mountain pushing against the device against the pull of gravity. The rocket would too, but presumably greater if it’s accelerating faster than gravity is pulling. The key is that the gravity acceleration vector never shows up in any context. That’s because it’s not a force and the perceived acceleration is an effect of relativity, caused by the gradient of time and space. It’s entirely counter intuitive to our understanding of force and acceleration that you can observe increasing relative motion without a force based acceleration, but there it is
F = ma, a = F/m.
In the scenario you’ve set up m >>>>>> F, because the Earth is fucking enormous. So a will be extremely small but nonzero. A precise enough accelerometer would show you this.
Also your mountain accelerometer would show ~9.8m/s^2 plus the extremely small contribution of the rocket thrust, not 0.
So… yes, you might be really confused about physics.
The accelerometer glued to the mountain will show -9.8m/s^2, not zero. The one glued to the rocket will show the same. The force of the rocket thrust is the opposite of the force of the glue holding it to the mountain, so the sum results in zero additional acceleration.
> An accelerometer glued to the mountain will show a 0.
Incorrect. An accelerometer glued to the mountain will show -9.8m/s^2 because it's being accelerated by said mountain.
No?
A theoretically perfect accelerometer will show something non zero as the rocket engine is in fact shifting the entire Earth by a very very very very very very tiny amount.
Stick two identical rocket engines together in deep space so their thrust cancels out. Is thrust not a force when rockets' guidance accelerometers both show 0?
But 0 is what you expect to see in that case: you have two forces acting on the body, of equal magnitude and in opposite directions, so the net force is 0, so acceleration is 0.
But if you jump out of a plane, you will see ~0. If gravity were a force, what would be the equal and opposite force acting on you?
Both engines are exerting force in opposite directions, which cancels out, as you said. There’s no acceleration happening either from gravity or this rocket arrangement, so the accelerometer reads zero. Why is this a problem? Who is saying thrust is not a force?
If you are accelerating, the accelerometers will not "show 0". I have no idea where you are getting this notion from; it's completely wrong.
> If you are standing on the surface of Earth, an accelerometer will show that you are accelerated in the upward direction.
How long until this upward a gives me a little upward v? What's the dt here?
> The easiest way to see that gravity is not a force is to note that a force causes acceleration, but gravity does not.
Maybe I'm misunderstanding as it's been a while since physics class but I'm pretty sure gravity does cause acceleration. One of the few numbers I still remember memorizing in college 9.8m/s2 -- the acceleration by gravity on Earth.
[delayed]
That's the acceleration you feel on the surface of the earth. But that's not an inertial reference frame. In an inertial reference frame gravity doesn't cause acceleration (kind of by definition).
The argument is that gravity doesn't cause acceleration, resisting gravity does. Kind of how spinning an object doesn't cause a centrifugal force, the real force is whatever forces it to stay on a circular path instead of continuing straight
That’s relative to the surface of the earth
> The easiest way to see that gravity is not a force is to note that a force causes acceleration, but gravity does not.
When did objects stop falling down from gravity? Or is this some hairsplitting about how gravity takes effect by pulling instead of pushing?
Objects don't fall down because of gravity, objects move along in straight lines because of inertia. It just happens that, close to large masses, "straight lines" get bent toward the center of mass, and this is what we call gravity.
But the difference is important: when you are falling down, you are in freefall, you don't experience any acceleration in spacetime. It is the surface of the earth that is experiencing acceleration towards you.
When Einstein literally discovered General Relativity.
Gravity is the curvature of Space Time. From that perspective, objects under gravity are traveling in a straight line.
Fun Veritasium video on the topic: https://www.youtube.com/watch?v=p1W0dpunOaM
Suppose you were in a free falling box in a strong gravitational field. Is there an experiment that you can do within that box which would measure the acceleration? I think no. Not directly (perhaps you might get clever and do something with tidal forces). And if you can't measure it, why do you assume it's there?
By contrast, if you strap a rocket to the box, it's quite easy to measure the acceleration from within the box.
Some annoying student would mention that the same force is acting on the accelerometer as well so it cannot act as an independent observer.
I don't understand her comment.
This is really easy to verify.
You drop an iPhone, the gravity "acceleration" really goes to 0 - so far so good.
But at rest (i.e. holding the iPhone in your hand), the acceleration is pointing downwards, not upward as she claims.
I think this is a distinction between push/pull? You are interpreting the reading as it being pulled down. You could also interpret it as you having to push it up.
The example you give also doesn't make sense. The acceleration would not go to 0 when you drop something. The acceleration continues for the entire fall. It may be offset by friction if it hits terminal velocity. But, absent that, acceleration would continue the entire fall.
If the reading of an accelerometer does change in freefall, then what it is measuring isn't the acceleration, necessarily, but the difference in acceleration between components in the device. Which makes sense. Is like watching a balloon in a car when you start moving. (With the trick of whether the windows are open or not, of course.)
This shows that your accelerometer is broken. (or, more likely, that the vector is mislabeled)
Try this thought experiment. Assume your phone is really accelerating downwards when at rest. Then let it enter a free fall i.e. drop it. Now it's accelerating down even more so the downwards acceleration should show an increase. Actually, it goes to zero. Relative to a free-fall, being stationary is an acceleration upwards.
To mimic the effect of gravity on the surface of the earth you could stand on a platform that is accelerating. As long as that platform is accelerating you will experience something similar to gravity on the surface of the earth.
The relative direction of the acceleration would be "up". This is what she means.
https://old.reddit.com/r/AskElectronics/comments/2ufunr/why_...
I think there's some sign inversion happening there. But don't quote me, all the details are fuzzy to me.
Your iPhone lies to you. The acceleration is indeed upwards, as you’ll see if you look at raw accelerometer data… or the feeling from your feet, which counts.
An iPhone held “at rest” cannot be accelerating downward, by definition.
Force is not acceleration.
That is extremely easy to understand but somehow you and all of these "eminent scientists" fail to grasp it.
> > Acceleration is measurable with a device called an accelerometer. Acceleration is not relative (like velocity), it's absolute.
The reason that you can't measure gravitation acceleration in free-fall in a uniform gravitational field[] with an accelerometer is that all parts of the accelerometer are being accelerated together.
[] The "uniform gravitational field" part comes from Einstein's equivalence principle, and it's pretty obvious that if the field were sufficiently non-uniform then one could expect strain gauges to indicate that the free-falling object _is_ being accelerated. Imagine you have a 10km long space ship near a massive body like Earth, or the Sun, then the gravitational field will not be uniform across that space ship, and strain gauges will a) let you know, b) will even be able to tell you the strength of the field and the heading to the massive object.
All these "gravity is not a force" arguments based on the equivalence principle are simply nonsense. The better argument is that gravity is both not a force because it only curves spacetime, and also yes equivalent to a force (especially when you do a Mercator-style projection of curved spacetime onto flat spacetime).
Love the Hossenfelder reference (though she really needs to move to Bluesky...). That said, I'd love if someone could explain some physics to this noob:
If you jump out of a plane, at t=0 you'll not be moving (relative to the earth), at t=1 you'll be moving a bit (relative to the earth), and at t=2 you'll be moving even faster. How is that not acceleration? A quick wikipedia rabbit hole from "Accelerometer" --> "Inertial Reference Frame" --> "Fictitious Force" led me to this:
They use the example of a passenger in an accelerating car, which makes sense. And there's some discussion of how the earth is rotating and thus has angular (?) acceleration, which also makes sense. I think this is what Hossenfelder is referencing by "the earth is accelerating you upwards". But the rotation discussion seems completely unrelated to gravity, no? An asteroid that isn't spinning would still exert gravitational (pseudo-)force, as all massive objects do of any size. Surely a person standing on a non-spinning asteroid in deep space isn't being accelerated upwards?At the end of the day, I guess I'm missing why exactly "free fall" is the same thing as "no forces acting upon you". To my cynical arrogant brain, this reads like the physicists are harping on an unnecessary terminological thing, namely that a "force" is defined as "a physical interaction between two objects". In other words, its quantum bias, in the original sense of quantum; why can't objects interact with the continuous field of spacetime?
I'm commenting on your quote because her explanation especially "we have a machine with acceleration in the name, thus that's what acceleration is" set off a million alarm bells in my head, philosophically speaking!
That's just because gravity is also accelerating the accelerometer, and the accelerometer can only measure acceleration relative to the body of the instrument.
I've heard this argument: Because gravity is the only force that acts on all objects, including the measuring object, this makes it not a force. This is not actually an explanation, it's more of a definition, and not a useful one.
Accelerometers don't measure acceleration, they measure the difference between the force on a mass on springs and the body of your phone. If you drilled a tiny hole and pulled the mass up, the sensor would think the phone was being pulled down.
Acceleration due to gravity has an important property of acceleration in general: radiation. Spiraling black holes emit gravitational waves, in the same way that an accelerating electric charge emits light. There are free falling reference frames where uniform gravitational fields can go away, but the gravitation of a massive body isn't uniform and can't be eliminated by changing the coordinates.
The relationship between gravity and fictitious forces is an important stepping stone, but it does not have all the properties of a fictious force, only some of them.
First off, the author doesn't appear to be a crank.
Here's the Wikipedia entry: https://en.wikipedia.org/wiki/Jonathan_Oppenheim
Second, we should note that even Einstein himself cautioned against believing spacetime was actually curved. His writings inform us he didn't believe it. I don't want to appeal to authority, that's just to say smart people, including the main developer of general relativity, didn't believe it. But he didn't believe in the non-local nature of quantum theory either, which we have now, since Einstein's death, proven to be true.
Third, the claim that only gravity can be described using geometry is false, which the author himself notes later in this article. The stress-energy-momentum tensor simply makes gravity universal, unlike the other forces. I don't see any reason why that universality confers something special to gravity with regards to interpreting it as geometry. Just because we can model gravity as geometry, doesn't make gravity a result of geometry, and the author notes that modeling gravity that way makes it so we can't unify the forces.
Finally, as the old saying goes, if you think gravity isn't a force, drop a brick on your toe! :)
I'll also point out that singularities are generally considered to be a sign of issues with a model. GR has singularities. Maybe that should tell us something.
> we should note that even Einstein himself cautioned against believing spacetime was actually curved. [...] I don't want to appeal to authority
Another example that comes to mind is Max Planck believing that light being absorbed in discrete packets of energy was only a neat mathematical hack he came up with. It took Albert Einstein to say "but what if light is discrete packets". And then as you say Einstein having major reservations against the field of quantum theory that he himself spawned.
> if you think gravity isn't a force, drop a brick on your toe
By that logic the centrifugal force has to be a force. If you don't believe it, just drive a vehicle around a curve, or whirl a rock on a string.
> But he didn't believe in the non-local nature of quantum theory either, which we have now, since Einstein's death, proven to be true.
We have not proven quantum theory to be non-local. We’ve only proven that it can’t be both local and contain particles, as opposed to particles being emergent from the wavefunction’s interactions.
MWI chooses the latter, and is therefore a local theory.
(Alternately, collapse. But collapse theories are largely nonsensical.)
> First off, the author doesn't appear to be a crank.
Is this the first most people are hearing about this? In the 90's, I had a physics textbook for the layperson (i.e. non-STEM fields). It had a fantastic chapter on general relativity, and it also went with "not a force but a spacetime curvature".
Popular scientists have been saying it for decades and always use the flexible rubber sheet as the learning aid. Serious physicists tended to stay out of the fray, preferring to just "shut up and compute", as they do for quantum mechanics.
It's a relatively recent development that physicists have entered the fray and say maybe curved spacetime isn't a model, maybe it's reality. I urge caution to all as we have no evidence that the model of curved spacetime is indeed reality. I think the caution is warranted considering that GR contains singularities, which is a sign the theory has issues.
GR is a theory of the bulk. It’s like fluid mechanics, which treats fluids as infinitely divisible and doesn’t acknowledge the existence of molecules: There’s no way it’s accurate at small enough scales that molecules matter, which is what would be happening inside a black hole.
At larger scales, however, fluid mechanics is extremely reliable.
What if singularities do in fact exist? What issues does that create?
They might not be observable if superextremal black holes don't exist and thus naked singularities do not exist.
I am sympathetic to the author's thesis. I favor the idea that gravity is a different thing from the other fundamental forces, and possibly an emergent phenomenon rather than a fundamental thing in its own right.
But, I don't buy the argument made here:
> To call gravity a force, is to privilege flat space as somehow being special.
Flat space is special, and we didn't make it special.
This is taking an important aspect of known physics---that there exist various symmetries and all elements of the corresponding group are equal players (there is no privileged reference frame, positive charge and negative charge are indistinguishable save for their oppositeness, etc.)---and attempting to apply this principle to spacetime curvature. But the zero curvature state is a unique one that is differentiatable from the others. It's the only one where a circle is perfect, having radius 2 * pi * r. And pi is a fundamental invariant of geometry, curved or otherwise. The mathematics privileges flat space. Further, experiments can be constructed to detected whether we are in flat space or not [1]. That wouldn't be possible if the whole concept of flat were only relative to an arbitrary frame.
[1] https://en.m.wikipedia.org/wiki/BOOMERanG_experiment
He lost me there:
> Is this purely a semantic difference? You could argue that it doesn't really matter whether we describe gravity as a force or through geometry, and we should conflate these two concepts. But I think this distinction is important to make because it has predictive power. If you believe that gravity is manifest through spacetime bending, then you will never find two different test particles that follow different geodesics.
Don't we get the same conclusion if we believe gravity is a force _and_ equivalence principle is true?
I like the model of an insulated tunnel bored through the Earth from the north to the south pole and filled with a vacuum (technologically implausible, yes). If we drop a steel ball into the tunnel at the north pole, what forces does it experience?
From Newton's perspective, F = ma and the ball accelerates towards the center of the Earth. The value of g diminishes to zero at the center, the ball is at its maximum velocity, and then enters the negative acceleration regime until it just reaches the surface of the Earth at the south pole. This will continue indefinitely in harmonic motion. It's not a perpetual motion machine because machines do work and we're not doing any work on the ball; it's similar to an orbiting sphere. (ChatGPT-o1 claims the period is 84.4 minutes, assuming uniform density)
The general relativity perspective seems to be the ball is just rolling up and down a bowl of spacetime, without any friction or drag, which isn't all that satisfying a picture, since it implies a restorative force being involved to keep the ball from escaping the bowl. It helps to consider the state of the ball right before it is kicked into the tunnel - it is being prevented from following its natural geodesic trajectory by the electromagnetic forces of the rocks of the Earth's crust upon which it is being held up - that's the only relevant force in this picture.
“If you accept that we live in spacetime, and it can be curved, then I think you should accept that gravity cannot be a force.”
I am on the opposite time of thinking — where spacetime itself emerges from particle interaction events. Pairwise distance (i.e. metric) is just yet another interaction parameter in the world graph.
"is to say that when no force acts on a test particle in curved space, it should move along a geodesic"
Every single author who calls gravity not a force, just hand waves right past this: why should the particle move at all?
Sure if the particle is moving it will follow a curved path thinking the path is straight.
But if the particle just sits there, okay the path is curved, but it just sits there.
I've heard explanations having to do with the fact that particles move through time, that doesn't really answer the question because it can continue moving through time while just sitting there.
"in the absence of being pushed or pulled, test particles in a curved spacetime will free fall"
Really? Why exactly will they free fall? Why can't they just stay exactly where they are?
I've also never heard anyone explain what the issue is with calling gravity a force. One person said it can't be a force because it makes light move, and light is massless.
However gravity does not act on mass it acts on energy, and mass is just a form of energy. Since photons have energy obviously they gravitate.
I think the missing piece in your puzzle is adding the "time" to space-time. A particle moving through time is "falling" in the time dimension. The curvature of the 4d space makes this movement through time also move through space due to the warping effect of a gravity well.
> Really? Why exactly will they free fall? Why can't they just stay exactly where they are?
Absent any forces, they can only stay on a geodesic. It’s like the arrow that springs from the bow. No hesitation, no doubts. The path is clear. There’s only ever one geodesic, one path through spacetime that takes no energy to follow.
This is true even if you ignore general relativity and just focus on classical mechanics, as formulated by Lagrange. In Lagrangian mechanics, you must compute a number called the “action” for every path through space that your system could possibly take. Then your system will take the path that minimizes the action. If your system is a particle in space, then it will stay still if that has the minimum action, or it will follow some path through space if that has the minimum action. It can’t ever do both. Only one of them will minimize the action.
> one path through spacetime that takes no energy to follow.
Again, this if fine for an orbit for example, it does not explain how a particle will accelerate, it also does not explain momentum, because when that particle accelerate it exchanges momentum with another particle - it does not give momentum to the "geodesic".
And explain please what information is not captured by saying "Gravity is a force that acts on energy". What nuance is lost by saying that?
Hmm, if you look at another comment here: https://news.ycombinator.com/item?id=41971973
It seems like the point is that the particle is actually not accelerating when it follows the one path through spacetime. It only accelerates when it's stopped by something (e.g. the surface of the earth).
what do you mean by "just sits there"?
Sits where?
In the same position relative to the sun? Relative to the center of the Galaxy? Relative to the local galaxy supercluster?
A particle does not just sit still!
Site there relative to that other mass that wants to accelerate it toward each other.
There is also the asymmetry of time dilation. Someone moving in a rocket ship will be younger than the person staying behind. Isn't their relative acceleration the same though? Why is the rocket man privileged in staying young? This kinda works if you imagine a grid of sand particles being acted upon by either gravity or traversed by the rocket. The rocket traverses many more grids than the person standing on Earth. The person floating in naked space will traverse the fewest grids.
The kicker is that there is no place in space with zero grid traversal. Accounting for all grid traversals becomes a nightmare.
You’re moving in the time direction — and when spacetime is curved, you end up moving in the space direction as well, to follow the geodesic.
The river model of general relativity.
https://www.youtube.com/watch?v=hFlzQvAyH7g&list=PL__fY7tXwo...
Not arguing for or against this explanation, but it's hard enough (impossible for me) to visualize a curved 3d space, once you try to think of what curved 4d space might be like any intuitive sense of what should happen goes out the window.
Classic diagrams of curved spacetime portray space as a 2d sheet with divots in it. I wonder what a similar visualization of curved 1d time + 1d space would look like. Long valleys for gravitational wells and everything moving along the sheet in the direction of positive time?
> and when spacetime is curved, you end up moving
You did the same as everyone else: You hard waved right past "end up moving". Why do I end up moving instead of just sitting there?
And if I "end up moving" that means I exchange momentum, if it's just bent space that magically makes me move, what did I exchange momentum with, if not via a force toward another particle?
Gravity is a force that acts on energy. If you disagree please give me a counter example.
You do just sit there. In the GR-realism view, the only thing particles _ever_ do is self-propagate forwards in time.
The problem is, this means the particle’s definition of “time”. Spacetime is 4D, and this can be any timelike direction. That’s what causes it to appear to move, from the perspective of someone whose time vector is pointed differently.
Then time can be curved, which reorients said vector as the particle propagates through it. That’s gravity.