Why gravity is not like the other forces | Hacker News Reader

Why gravity is not like the other forces

quantamagazine.org

134 pointskjhughes124 comments

I think this article is missing the biggest thing that makes gravity different: it arises from and affects everything with energy, which includes everything that we know exists.

See this graph of the known standard model interactions: https://upload.wikimedia.org/wikipedia/commons/4/4c/Elementa...

Gravity isn’t shown because every type of particle in that graph interacts with every other type of particle via gravity (i.e., the stress-energy tensor).

A lot of people may not realize that a perfectly mirrored box containing photons would have “weight”. Similarly, a spinning top weighs more than the same top that isn’t spinning, a compressed spring weighs more than the same spring in an uncompressed state, and the earth+moon system weighs less as a unit than if you pulled them apart and added together their individual weights.

In fact, most of the “weight” of an atom isn’t due to rest mass at all but rather the kinetic energy of the motion of its constituent particles.

For electron/positron collisions, it isn’t so much “annihilation” as it is a change of form. After the collision, total momentum, angular momentum, and energy all remain the same. Even though there are no longer two particles that have the property called “rest mass”, the effect on the gravitational field remains exactly the same the moment before and after the collision.

I think that gravity isn't shown simply because it isn't part of the standard model. IANAP, but standard model fields can only interact with other quantum fields and we do not yet have a quantum field version of GR.

This is one of those things that laymen often misunderstand, actually. (I don’t blame you, it’s often not well explained.) General relativity is easy to make into a QFT. There’s an action and you can easily derive amplitudes from it. And it works well, as long as you don’t ask questions of what happens at arbitrarily high energies. That’s what “nonrenormalizable” actually means: it only answers questions up to some energy scale, after which it stops being predictive. But believe it or not, physics has been here before, so we’re not totally clueless what to do! Fermi’s theory of beta decay [0] had the exact same problem. And the moral there was that there was something new that had been missed in Fermi’s theory that was only important at high enough energy. The theory was correctly predicting its own limitations. In fact we figured out what was missing. Fermi didn’t know about the W boson, and at the energy where W boson exchange becomes important, that’s where the old theory breaks down!

But Fermi’s theory works well up to that scale. It’s like how Einstein’s relativity didn’t mean that everything in Newtonian physics was totally wrong, but just that you need it if you want to think about really fast moving objects. Newton’s theory is still an outstandingly good approximation.

So if history is any guide, we don’t need to change the low energy theory of GR (at least not much), that works just fine. We just have to find out what’s the high energy phenomenon that we’re missing. That’s stuff that comes into play with black holes and very high energy collisions. Anyway we have some guesses here, too. In string theory, that’s the stringyness of the strings, their 2D nature, that fixes the problems automatically. (This was not by design, it was completely unexpected! You start playing with dimensions and it just falls out.) In LQG it’s about the fuzziness or discretization of space (which sounds appealing, although it’s really tough in practice to make this work).

[1] https://en.m.wikipedia.org/wiki/Effective_field_theory

I didn't realize it was kinetic energy, I thought most of the weight of an atom was due to the strong nuclear force

According to this article [0], you're right:

> This property of the strong nuclear force is known as asymptotic freedom, and the particles that mediate this force are known as gluons. Somehow, the energy binding the proton together, the other 99.8% of the proton's mass, comes from these gluons.

[0] https://www.forbes.com/sites/startswithabang/2016/08/03/wher...

Yep, that’s correct. I misspoke (should have said “not due to rest mass”). You can think of this by analogy with the Coulomb Hamiltonian—it is the sum of a kinetic term and an electromagnetic potential term. With the nucleus, it’s similar except the potential term corresponds to the strong force and is even much higher than the kinetic term as you say.

How come the charts that show the mass distribution of the universe don't include energy at all? This one [0] says that "visible matter" is 5%, but "energy" isn't even present. From my light research the only answer I found was "because it's too small to include", but this seems to disagree with your comment. What am I missing?

[0] https://svs.gsfc.nasa.gov/12307

It is very small and almost perfectly homogeneous, so there is basically no gravitational attraction.

amazing to think about, thank you

> I think this article is missing the biggest thing that makes gravity different: it arises from and affects everything with energy, which includes everything that we know exists.

To be fair, one of the physicists (Daniel Harlow) does say

> gravity is the only force that is felt by all kinds of matter

Does gravity affect an electric or magnetic field?

Here's a fun thought experiment / apparent paradox.

In high school physics we learn that a 1kg mass accelerates as quickly as a 2kg mass when only subjected to the force of gravity. When I used to teach physics, the intuitive explanation I gave for this hinged on a thought experiment. Suppose that you have three 1kg masses falling side by side after being dropped from the same height. Clearly they are all going to fall at the same rate since they're equivalent. Now imagine redoing the experiment but this time taking two of the masses and placing them closer together. Does anything change? Clearly not, they're still all equivalent and ought to fall at the same rate. Now imagine doing this until those two masses are right next to each other, touching. Does anything change? Well no, all three should still fall at the same rate. But now, why not glue those two masses together and call it a 2kg mass? Once you do that you've shown that a 1kg mass and a 2kg mass fall at the same rate.

This usually convinces people, but there's actually a flaw in the argument that gets to the heart of why gravity is so different from the other forces.

To see the flaw, replace the above masses by three electrons falling next to each other in an electric field. Everything goes through in exactly the same way. You end up gluing together two of the electrons and these two electrons will accelerate at the same rate as the single electron. But if you're not careful you'd conclude that all electric charges fall at the same rate in an electric field, something we know is false.

Where's the flaw? Well, all of matter is built from some particles, and as long as you restrict yourself to particles that have the same "charge/mass ratio", the argument above works. It is true that one electron accelerates the same as 100 electrons tied together but that's just because e/m is the same for all those constituents.

So, the thing that's glossed over in my high school explanation for why 1kg and 2kg accelerate at the same rate is that the constituent particles all have the same "gravitational charge / inertial mass" ratio. Because this ratio is the same for all particles, we may as well absorbed that ratio into the gravitational constant and just use "m" in place of both of them. It's this "universal coupling" that's really responsible for the equivalence principle and what sets gravity apart from the other forces.

1kg mass and a 2kg mass do not fall at the same rate. The Gravitational force is (G*m1*m2)/r^2. You are observing that m1 (the earth) is much much greater than m2 (the 1 or 2 kg masses), and you are simplifying to (G*m1)/d^2 because of the precision of the measuring device. Also, d is the same for both masses.

what no they do fall at the same rate. acceleration is F/m so the mass of the object cancels out

The mass of the earth dictates the acceleration of the individual masses towards the earth. However the acceleration of the earth itself towards the masses are dependent on how much mass is falling towards the earth. When more mass is falling to the earth, the earth accelerates towards the masses faster. So the thought experiment is flawed because with only one 1 kg weight falling towards earth, the gap between the weight closes slower than when there are three 1 kg weights spaced 1 m apart and dropped simultaneously.

If you define fall as the size of the gap. You could also take it as acceleration towards the barycenter, which would be the same. These are indistinguishable for everyday objects so could argue that the word “fall” could be interpreted either way.

Sorry, but "Mr Galileo was correct" : https://moon.nasa.gov/resources/331/the-apollo-15-hammer-fea...

Nitpicky, but shouldn't that be "Mr. Galilei"?

I was just quoting what the astronaut said

They do fall at the same rate, even with Newtonian gravity. For,

    F = m a
    F_gravity = GMm/r^2

so that

    ma = GMm/r^2.

Now cancel m from both sides and get

    a = GM/r^2

If you plug in G=6.674e-11 m^3 kg^-1 s^-2, M = M_Earth = 5.972e+24 kg and r = R_earth = 6.378e+6 m you get

    a = 9.79... m/s^2

which ought to be familiar.

The force is on both objects at the same time. The force in F = ma is a function of the mass of both and their distance. If the mass is different in the two scenarios, then the force is different. On earth with small weights, they seem the same because of the precision of the measurement.

This is why you _weigh_ less on the moon.

Is what you're getting at the fact that the distance between the earth and the other object changes from two effects (the first being the ball falling towards the Earth and the second being the Earth falling towards the ball)? That's right, of course. But that distance's second derivative is not the acceleration a in F=ma. Indeed, in both Galilean and Einsteinian relativity acceleration is detectable locally without a needed reference to another object.

Yes - I was making a mistake. I was trying to describe the effect of both masses. When one is much smaller than the other, then the movement is mostly in one direction. When they are closer in mass or even equal, they move toward each other. For example, if you have a 1 liter water bottle filled with a material that gives it the same mass as the earth, then the two bodies will move toward each other, and the water bottle will seem to move toward the earth much faster that the 1 filled with water (1kg). If it is filled with a material, that gives it much grater mass than the earth, the earth will move toward it.

>You end up gluing together two of the electrons and these two electrons will accelerate at the same rate as the single electron.

Not really. You have swept under the rug the fact that it's really hard to glue electrons together. And if you were to actually find a way to do it, you would have to add so much potential energy to the system that it's inertial mass would increase dramatically. In fact, two electrons that were actually "together" (whatever that might actually mean for a quantum particle that obeys the Pauli exclusion principle) would have a mass orders of magnitude higher than two electrons separately.

You probably want to talk about ions in an electric field, which you can "glue together", but then it becomes rather obvious that they don't all accelerate at the same rate.

The equivalence principle is not the only thing that distinguishes gravity from other forces. There is also the fact that there is only one gravitational "charge" and it's mutually attractive. (Gravity is also many orders of magnitude weaker than all other forces.)

As an experimentalist who worked in the field for a long time, my answer to the title is: The Equivalence Principle.

Because gravitational charge and inertial mass are seemingly inseparable from one another, gravity is special. It is the concept that makes General Relativity possible and the empirical fact that makes gravitational experiments difficult and unlike any other.

I've always naively assumed gravity to be a facet of momentum/movement through time. The math between acceleration, gravity, and time dilation are all too cute to be coincidental in my book.

I suspect that gravity manifests as a force but does not stem from the same underlying drivers as other forces.

Yeah gravity seem to have a time dimension to it

Gravity itself does not have a time dimension - if you had a highly changed particle acting on another one, the time dilation would be identical to that caused by gravity acting with the same force.

i.e. it's force and acceleration that cause it, not gravity.

Lee Smolin is working on an idea where the building blocks are events where momentum is exchanged.Space, and hence position, is an emergent phenomena.

Here's a colloquium talk[1] and here's a recent paper[2].

Not exactly what you said but perhaps interesting.

As mentioned it's work in progress so remains to be seen if the idea pans out.

[1]: https://pirsa.org/21030005

[2]: https://arxiv.org/abs/2303.15546

I love how this word “emergent” has become popular by people that want to pretend they have an answer but it’s really just the “…” thing. There is no such thing as “emergent”. The motion of an automobile isn’t an “emergent” thing you can claim because you don’t know how engines and transmissions work.

Everything we can’t explain recently has people, who I assume don’t know how to say I don’t know, proclaiming “it’s emergent”. As if that’s an answer to anything. Much less a claim to knowledge.

Might as well claim “god made it happen”.

> The motion of an automobile isn’t an “emergent” thing you can claim because you don’t know how engines and transmissions work.

That's the opposite of what emergent means, isn't it? Because of force on the transmission, you can say that the motion of the vehicle is emergent rather than needing its own framework or explanation.

Seems like pretending to know what the word emergent means in physics is an emergent phenomenon on HN.

It's not wishy washy, it has a precise meaning that can be both derived and, typically, refuted.

Emergent just means that it's not the basic building block; it's made up of other, more fundamental things. So for example thermodynamics is emergent (even though it is fully studied and relatively well understood) because it arises from interactions of particles.

If you say that spacetime is emergent that is a big deal: it means that, while it can be studied on its own, it's actually composed of other, more fundamental things. But you still need to show that

a) this other fundamental thing actually exists, and

b) we can show mathematically how the spacetime works, in terms of those other fundamental thing (just like you can show how thermodynamics works in terms of statistical mechanics)

And that's where your "because you don't know how it works" thing breaks apart: one can't claim to have solved this problem without tacking b).

> it's made up of other, more fundamental things

Then explain those things and prove they exist. Otherwise this just sounds like bullshit.

There’s a difference between observing thermodynamics (and not trying to answer why but rather how we observe it - big difference!) and writing equations we can measure and just saying “it’s emergent” to “…” things we are unable to answer “I don’t know to”.

That seems to be exactly what they are trying to do:

> Lee Smolin is working on an idea where the building blocks are events where momentum is exchanged.Space, and hence position, is an emergent phenomena.

It’s perfectly OK to speculate that perhaps gravity, say, is not fundamental, but instead emerges from interactions or whatever at a lower layer that we haven’t yet identified.

What word would you use?

> Otherwise this just sounds like bullshit?

Yet it is very compelling bullshit that stands up to intense scientific scrutiny.[1]

[1] https://einstein.stanford.edu/content/relativity/a11332.html

In this case there's something more concrete, in the form of equations[1], behind the usage of the word.

[1]: https://arxiv.org/abs/1307.6167 (section 2.2)

Your [1] <https://arxiv.org/abs/1307.6167> is a really interesting choice for examining the use of "emergence" in the broad among quantum gravity researchers.

I think looking down to the start of §6 of your [1] is even more fruitful than §2.2:

"... novel form of dynamics that can be applied to a unique system -- the universe as a whole [--] that is neither quantum mechanics nor general relativity[,] from which quantum physics and space-time emerge as approximate descriptions of systems that may be regarded to a sufficient degree of approximation as isolated subsystems."

Or, more succintly and focusing on the gravitational content, "the Einstein Field Equations may be derived for some part(s) of a universe described by our theory", with the additional constraint, "but our theory cannot be derived from General Relativity".

Compare with Newton's gravitation and its ability to be derived in the weak field, low speed limit of General Relativity (and that physically interesting generally curved spacetimes have large regions that are effectively flat).

Generalizing, a theory of gravity is emergent when its fundamental equation(s) can be derived from a different dynamical theory, and the derivation is a one-way street.

However, "emergent" gets used in a different sense only a few paragraphs later in [1]: "Time reversal invariance is amongst the symmetries that must emerge from the coarse graining that neglects the unique identity of each event. From our perspective fundamental physics is time asymmetric and the apparent time symmetry of the laws of nature is approximate and emergent."

In this case they are not strictly speaking deriving time symmetry from their dynamical theory, but rather they are claiming that averaging the behaviour of their theory across some region(s) matches observation.

There’s probably some places where it’s used where it shouldn’t be, but there is such a thing as “emergent”.

I think it helps to see an easy example first.

Take a great body of water (h2o) on earth, such as the ocean. That water has waves and those waves have amplitudes, as well as other properties such as crests.

A molecule (or even a few) of water (h2o) doesn’t really have these properties. It doesn’t have crests. You could argue it has amplitudes. Crests are an emergent property of large masses of water molecules within a specific system. There’s no crest to a water molecule.

Similarly, for demonstration purposes we could say there’s no waves/crests when gravity is removed.

So when I hear emergent, I imagine properties that show up under certain conditions within a system, that were not present in individual components.

There’s a whole lot of “…” between a water molecule and a wave. There is nothing emergent about it. “Emergent” is a bullshit answer.

You seem to have a different definition of "emergent" to the rest of the world. Just because there _is_ an explanation for waves doesn't stop them from being an emergent behaviour.

I think 'emergent property' just means it's something that depends on the particular configuration of a set of elements that vanishes if you just look at the elements in isolation. E.g. words are an emergent property of the elements of an alphabet.

It is kind of hand-wavy and you often find the phrase 'holistic' in the same paragraph as 'emergent', which reflects that it's a kind of pushback against entirely reductive approaches, but it really just means that if you want to reconstruct a complex entity (like an automobile) you can't just count up the parts, at each stage of reduction you need to save this set of information describing the relationships between elements if you ever want to reconstruct the overall system from the most fundamental components.

It's essentially just an abstraction, I think. The justification is that you can wrap up complexity into hierarchies of little black boxes that have predictable behavior. E.g.

https://guava.physics.uiuc.edu/~nigel/courses/569/Essays_Spr...

>"The idea of emergence in physics is pervasive especially in condensed matter and cold atom physics. For example, superconductivity and superfluidity are both described by a condensate, which is an emergent state of matter with finite fraction of particles in the ground state and long range order."

The suggestion here (further developed in above paper) is that spacetime is also a kind of 'effective description' based on some other underlying microscopic organized structure, which is a theory that's likely hard to test.

> just means it's something that depends on the particular configuration of a set of elements that vanishes if you just look at the elements in isolation

Sort of like how all the parts of an engine, even when connected, do nothing if fuel isn’t being pumped through. We can not proclaim we have knowledge of the engine by simply understanding many of its parts but failing to grasp how fuel moves through it. And instead say when these parts are together motion “emerges” and believe we are profound.

Linear and highly predictable systems like automobile engines are not really comparable to chaotic systems like the weather or the universe: they demand separate modes of analysis. Emergent behavior is one of these lenses of analysis that is useful for chaotic systems, so it is natural that trying to make comparisons to an engine don’t really work with it. For a simple physical system that is chaotic, i.e. does display emergent behavior, good examples I’ve heard are things like triple pendulums, or the currents of water in a river.

I mean, couldn't you absolutely say you know more about an engine than before by invoking emergence? A skilled mechanic ignorant of physics could measure fuel flow and heat produced under different conditions, and try to figure out how the pieces fit without ever understanding the Carnot cycle - it's a useful way to describe what we know, even when we know it's not a complete picture.

We haven't stopped studying consciousness after having concluded that it's a phenomenon that emerges from brains and their neurochemical environment by studying the pieces.

I feel like your critique is fundamentally misguided.

Here's an article[1] trying to define the term a bit more rigorously.

Here we discuss a sharp definition of the narrow concept of emergence in physics; we give a mathematically precise meaning to the notion of ‘qualitative difference.’ We propose the following:

An emergent behavior of a physical system is a qualitative property that can only occur in the limit that the number of microscopic constituents tends to infinity.

Which is essentially the opposite of what you mention (vanishes when looking at the microscopic constituents).

[1]: https://www.nature.com/articles/npjquantmats201624

Interesting article. It mentions phase changes here:

> "Critical phenomena represent another distinct form of emergent properties."

A single water molecule won't undergo observable sharp phase changes from ice to water to steam as the temperature is increased, so the emergent property vanishes when looking at the (isolated) microscopic constituent.

In the case of spacetime, I think emergent just means that spacetime exists as a result of the interaction of energy, and there would be no spacetime without its interaction. It's more a claim about whether spacetime is a thing-in-itself.

You don’t think it’s important to identify “position” as an “emergent phenomenon”? That idea in itself is novel as fuck. Nevermind how someone could actually begin to explain it.

Here’s an example from your daily life. Wetness. Wetness is an emergent property of a liquid. A single molecule is neither wet nor dry, but when you have many such molecules, they collectively behave in ways that exhibit new properties such as phase transitions.

Emergent properties are definitely a thing. The reason you’re hearing more about them is because we’re paying more attention to them in lots of domains, one of the most recent and date I say common around these here parts, being the study of “intelligence”.

As others have pointed out, ocean waves are an emergent property of a body of a sufficiently large number of water molecules in a place with gravity and a dynamic atmosphere. I'd like to add—what is an ocean wave? It does exert measurable forces, it has a measurable weight and speed and height, but when it moves it will be made up of a 100% distinct set of material particles than just a moment ago. It can arise and subside without discernible trace although of course both matter and energy will be preserved.

When you think about it a wave is not so different from anything else that exists. Yesterday's rock is tomorrows top-soil, before it becomes part of a plant, after which some of its former constituents will seep back into the ground while others take to the skies. In the future, all of these components might meet again to form a new rock.

All of these things are emergent and evanescent. Their properties can hardly be predicted from the sum of their parts. When you think of it, there are many words for concepts—'liberty', 'inflation', 'boredom', 'exaggeration'—that are essential to the human experience and can even be partly made measurable, but that do not have any manner of physical reality.

Their properties can hardly be predicted from the sum of their parts.

That’s not true. We know when water boils, we know tension laws, we know a ton to predict and define the interactions.

We know how to add carbon to iron, we know how make glass withstand huge loads. We know material limits because we rely on those material limits to build our world.

We choose to ignore those limits, the outcomes, the effects, climate change, earth is round, smoking is bad for you, etc.

If we wanted to know that chemical reaction takes place in your body that makes you feel boredom, I am sure we could, we figured out cocaine and pain killers, there is just no profit in making a boredom pill.

The world is not as romantic as poets make it out to be. Sun goes up sun goes down never a miscommunication. Complex, sure.

Even if you know a lot many things, you cannot predict for sure how the system will behave on another level. See this classic paper by Anderson titled "More is different" on this topic which considers hard science and its inability to predict emergent phenomena

https://cse-robotics.engr.tamu.edu/dshell/cs689/papers/ander...

"Emergent property" is a very fitting, useful and appropriate description for complex behavior that results from simple rules.

An example would be the centrifugal force, which "emerges" from basic Newtonian inertia.

Or the skin effect in AC electrics which (like pretty much ALL of circuit theory) is just a consequence of a few electrodynamic laws (Maxwell etc.).

But just knowing the basic laws does NOT mean that we can grasp all of the emergent behavior that necessarily follows-- physics => chemistry => biology would be the same thing, otherwise :P

In examples from the physical world, I think one could conceivably claim that there are some underlying basic rule(s) that we haven't discovered yet, which would explain the "emergent" behavior.

However if you take a look at cellular automata, which are artificial and thus completely defined, it's much clearer that complex "emergent" behavior really can result from nothing but very simple rules.

and that is what happens when one thinks they've developed an understanding of a term solely from reading how the term is used

you should probably read over https://en.wikipedia.org/wiki/Emergence to convince yourself that emergence as a distinct concept is a useful abstraction to have

System availability is an emergent property - I can build five nines of availability out of components with just two nines. Retries, algorithmic handling of failure, and the like.

That is the basic idea that gives rise to the theory of general relativity — that gravity isn't really a force, it's just the observable effect of being in curved spacetime. (Take a look at the "equivalence principle" and its role in the development of GR as a theory.)

> gravity isn't really a force

Yeah, this fact made me cringe at explanation #2: the guy keeps saying things like "gravity is the only force that is felt by all kinds of matter". Stuff doesn't "feel" gravity; it's not a force, it's just the shape of spacetime. Also, gravity doesn't just affect matter; photons are not matter, but are affected by the curvature of spacetime (i.e. gravity).

/me not a physicist; but I'm not a baby either, and I don't appreciate babytalk oversimplifications.

Gravity is an effect of the curve of space and relativity coming together in what we now call spacetime. Right? Einstein told us so. But... Maybe not?

There's promising math behind the idea that there are multiple additional dimensions, but where are they? How can we perceive them? What do they have to do with gravity?

The unifying theory of dimensional geometry and interaction: https://youtu.be/fvqXshyuvOg

But gravity isn't really a force. In the same way that inertia is not really a force. It's just an artifact of the coordinate system being curved.

By that definition electrical attraction is also not a force, because a charged particle will curve in an electrical field.

Maybe indeed electromagnetic charges curves space time as well, is there a theory for that?

According to general relativity, in absence of forces, an object follow a geodesic in space-time. Which we approximate to an uniform motion in straight line for high school physics. But masses "curve" the space time, so there are no straight line.

If there was no intramolecular forces to keep the apple on the tree, it would follow a geodesic that would make it fall towards earth. The same way that if the seat was not applying a force to my back, I would be pushed backwards in an accelerating car.

Kaluza-Klein theory is a geometric unification of general relativity and electromagnetism. A description of electricity as a curvature of a fifth dimension.

https://en.m.wikipedia.org /wiki/Kaluza%E2%80%93Klein_theory

If I recall correctly, the description of charged matter is somewhat problematic.

> Maybe indeed electromagnetic charges curves space time as well, is there a theory for that?

There is https://en.wikipedia.org/wiki/Kaluza%E2%80%93Klein_theory

Electromagnetic field has some energy. Energy and mass are equivalent (e=mc2). So electromagnetic field curves space time.

A key difference is that we have identified a force-carrying particle for the electromagnetic force (photon), whereas we have not been able to prove a force-carrying particle for the effect we observe as gravity.

Except that the radius of curvature in that case depends on the charge and mass of the particle, but all particles follow the same trajectory in a gravitational field. The trajectory is a geodesic, a non-Euclidean generalization of a straight line. All objects travel on straight lines unless acted upon by forces: inertia.

> but all particles follow the same trajectory in a gravitational field

That's not true. The "all masses fall at the same speed" thing is only true when one mass is much larger than the other. (Well, it's never actually true, it's more "is only a useful approximation".)

So heavier masses follow different trajectories in a gravitational field, just like more charged particles do.

Er what? This is outright false.

You forget that both particles attract each other.

A heavier particle falling toward the sun will cause the sun to move which will in turn influence the path of the particle.

But this is not what anybody means when they talk about the equivalence principle.

Sure, but I wasn't talking about the equivalence principle.

We were comparing the motion of a charged particle and a mass particle.

The equivalence principal was never mentioned.

Some reading on the universality of free fall version of the strong equivalent principle, tested in a manifestly relativistic regime:

"Even Phenomenally Dense Neutron Stars Fall like a Feather -- Einstein Gets It Right Again"

https://public.nrao.edu/news/neutron-stars-fall/

More detail, Archibald et al. 2018, "Testing the universality of free fall by tracking a pulsar in a stellar triple system" (Nature volume 559, pages 73--76 (2018)) preprint: <https://arxiv.org/abs/1807.02059>.

If universality of free fall fails, so does the single metric / purely geometrical theory of General Relativity. That would be very exciting, so it has been looked for a lot.

https://duckduckgo.com/?q=tests+of+universality+of+free+fall...

You can recover a bit from your statement by considering that solutions of the geodesic equations can be a bit messy in manifestly relativistic or multi-body systems, and the equations of motion of components can be even messier. Nobody would disagree with that (it's why there's e.g. the (gravitational) Self-Force methods and the like; see e.g. <https://arxiv.org/abs/1501.07322v3>, which (cf. your second sentence) is useful when a compact mass (e.g. a neutron star) is in a close orbit around a 50x+ more massive black hole, bottom right corner of <https://en.wikipedia.org/wiki/Post-Newtonian_expansion#/medi...>).

But WHY is the coordinate system curved?

Because of mass. But WHAT is mass?

A measure of a particle's energy as pertains to the spacetime curvature it induces

This is incorrect all the energy of a particle curves space.

Even velocity and potential energy.

Right, so those kinds of energy also induce spacetime curvature and are included in what I was saying

Wikipedia says “the property of "mass" is a manifestation of potential energy transferred to fundamental particles when they interact ("couple") with the Higgs field, which had contained that mass in the form of energy.”

Energy, not mass. Photons also curve space in this way.

Mass is just one kind of energy.

Well for anything curved there is more tension in the middle compared to the ends. Maybe it's the same thing.

If it moves the needle of a weight scale or force meter, it's a force.

I think that's a fair definition of a force.

But I'd say that the force that eventually moves the needle is not gravity, but the one that the table applies to the scale to prevent it from going down.

It's hard to better than Don Koks on this point <https://math.ucr.edu/home/baez/physics/General/Weight/whatIs...> ("What is weight?", Usenet Physics FAQ, 2014).

The contact camp agrees with you.

I'm in the contact camp: a scale or balance blocks the freely falling motion of the weighed item. Or, by equivalence, the weighing apparatus (and anything supporting it, be it the whole planet Earth or a rocket engine) imparts an acceleration (i.e., applies a force) on the weighed item, so the latter cannot be in geodesic motion.

The free-body diagram of the object has to show two balanced forces: the scale pushing on it, and gravity pulling on it.

The net force is zero.

When forces balance, they may balance to zero so the object in question doesn't move; but there is compression or tension in it. The scale measures that stress. If you remove either gravity or the restraining support (floor, table, ...) the stress goes away.

Force is defined as mass times acceleration: F = ma.

A body falling due to gravity experiences a force because it has mass and is accelerating. That's how force is defined.

There are some "real" physical force that do not depend on the reference frame and are caused by some law of nature. And there are pseudo-forces that are just a mathematical artifact of a non inertial référence frame.

Gravity is a pseudo force.

https://en.wikipedia.org/wiki/Fictitious_force

Non-intertial-reference forces can be traced to real forces if we change the reference frame. In an accelerating car, the seat really is pushing on your back. This is readily evident from the stationary reference frame. You're being accelerated and so there is a F = ma there.

Centrifugal force is pseudo, but centripetal acceleration isn't.

In what reference frame can we properly understand gravitational attraction, if the obvious one is fictional?

The seat is pushing me in the car and the earth is pushing my feet when I stand up on it.

The frame of reference is the curved 4 dimensions space-time

https://en.m.wikipedia.org/wiki/Minkowski_space

The problem is gravity also affects particles with no mass

With no rest mass.

But you can create a black hole by putting enough photons in one place.

Can I? :)

You have my permission. Just as long as it's Not In My Back Yard.

The high density accommodation of matter brought in by a black hole would spoil the character of our neighborhood.

If you're standing on scales in a spaceship and it starts accelerating, the scales will show a weight even though no force is acting directly on you (other than the contact force between you and the scales).

Does the force of the scale (presumably on the ground) accelerating up towards you with the rest of the spaceship not count?

> accelerating ... no force

F = ma

To me, this is the real answer. Gravity is not a force, it is a constraint on the shape of the universe. It is geometry.

I just had four physicists in my Uber last weekend and we got into this a bit. The one guy said the biggest problem with gravity is we’ve never figured outs its opposite or if there even is an opposite. He mentioned something about negative mass, which is maybe the handwavium equivalent of anti-matter in science fiction?

Gravity only attracts. It never pushes away.

Yeah I’m way in over my head but it’s fun to poke around in things we don’t understand.

> He mentioned something about negative mass, which is maybe the handwavium equivalent of anti-matter in science fiction?

Well, anti-matter actually exists but its mass is positive. Also, the anti-particle of the (hypothetical) graviton is the graviton itself[0], so all in all nothing fancy happening there.

> Gravity only attracts. It never pushes away.

Cosmic expansion would like to have a word.

(Though, admittedly, expansion doesn't happen in gravitationally bound systems, so from this point of view your statement is of course correct.)

> The one guy said the biggest problem with gravity is we’ve never figured outs its opposite or if there even is an opposite.

Interesting. Do you know whether he was an active researcher in the field of relativity? I can think of a lot of "big problems" in relativity but this wouldn't be one of them. I mean, don't get me wrong, it would be nice to have something to cancel out gravity since this would inspire lots of applications that we so far deem science fiction. But we already have a pretty good idea of the limits of what's possible[1].

[0]: https://physics.stackexchange.com/questions/273918/is-there-...

[1]: https://arxiv.org/abs/1611.01808 + the rigidity case of the https://en.wikipedia.org/wiki/Positive_energy_theorem suggest that one could theoretically "shield" / cancel out gravity to some degree but not everywhere (in all directions).

Respin of a comment I posted a while back:

This is what helped me wrap ahead around the fact that gravity "is not really a force": when you are in free fall you feel 0 acceleration. You appear to be accelerating relative to the ground-- but you're actually motionless in an "inertial reference frame". (Similar to how the astronauts on the ISS don't "feel" acceleration despite accelerating rapidly relative to the earth.)

The "force" of gravity is often modeled as "gravity pulling you down" and the ground "pushing you back up". This works mathematically, but isn't quite logically consistent.

In reality, on the ground you're in a region of warped spacetime, so you feel constant upward acceleration despite not actually accelerating. (Thinking of this another way, standing on earth feels identical to being in a far away spaceship accelerating at 9.8 m/s².)

This is also why time "speeds up" near more massive objects. (Separate from "acceleration".)

We're so used to gravity this it doesn't seem weird. But when you consider the fact that free-fall is when you truly do not experience acceleration... well pondering that from many angles is what ultimately led Einstein to his model of relativity.

(This is me trying to condense what could be a 10 minute explanation into a few sentences, so apologies if it's not particularly clear.)

Veritasium had a good video on this topic: https://www.youtube.com/watch?v=XRr1kaXKBsU (Thanks to u/badocr for commenting with this link last time.)

My favorite "fun fact" about gravity is still this one[0]:

> [Gravity] shapes the arena in which it acts, unlike other fields which act in a fixed spacetime background.

From what I've seen, this fact is rather underappreciated by many of the folks doing (what they call) "quantum gravity" these days. They often try to treat gravity as a rather ordinary quantum (or string) field theory, living on a fixed background.

As Hawking says in [0], "It would be rather boring if this were the case. Gravity would be just like any other field."

[0]: Hawking: The Nature of Space and Time (1994), p. 2, https://arxiv.org/abs/hep-th/9409195

> But near the center of a black hole or in the first moments of the universe, Einstein’s equations break.

Not really. The formation and continuous description of a singularity is somewhat unique in physics, but general relativity continues to provide a description, allowing even accurate descriptions of black hole mergers. The problem with the cosmic singularity is instead a lack of observation of anything before it: maybe our universe really did begin there.

Of course, inflation, dark matter and dark energy are all outstanding questions, but they don't break general relativity, even if they represent some missing extension like general relativity represents to the precession of the perihelion of Mercury.

But actually what happens when they merge is that the event horizon are the ones which gets merged and the then the two singularities comea into contact and they merge together. The resulting singularity will be be more massive of the individual singularities. You will get a singularity where GR breaks down.

The process of black holes merge is somehow can be understood without referring at all to singularity.

Yes, due to the causal structure, but you can also refer to the singularities. The simulation of black hole mergers can be carried out both by excision method and moving puncture method. The existence of black swans does not disprove white swans, it simply proves that black swans exist.

General relativity has absolutely no problem describing singularities. The existence of a singularity is not in and of itself evidence of a breakdown. Annihilation of matter via singularity formation may be the correct description of the laws of physics. We have no evidence to the contrary and it is a prediction of our best theory of gravity.

I agree with you here. Just I was referring that Black hole mergers is not something that can be taken as an example here talking about singularities.

But yes, existence of singularities does not mean a breakdown of GR. It suggests that GR is not a complete theory of gravity and means that ww cannot make a meaningful prediction about behavior of the system at the singularity points. We can even consider that GR predicts singularities when the curvature of the spacetime becomes so strong that it becomes infinite.

You are missing my point. It is entirely possible that GR is a complete theory of gravity. The final fate of collapse could be merely a simple singularity. There is absolutely no evidence to the contrary and, if general relativity is correct, no way for the scientific community to ever know.

I have no clue about this but my layman’s idea is:

Energy is entanglement. Energy creates gravity. So gravity is created by entanglement (gravity can be considered an emergent property stemming from entanglement).

> If there were any type of particle that did not feel gravity, we could use that particle to send out a message from the inside of the black hole, so it wouldn’t actually be black.

What about gravitational waves? Could you propagate a gravitational wave from inside the black hole to the outside? Or is it stopped at the event horizon?

Gravitational waves travel trough spacetime at the speed of light. So no, not even gravitational waves can escape black hole.

"We can picture many entities called fields superimposed on top of one another and extending throughout space. "

And all affected in exactly the same way by the mysterious gravity 'force' or field.

Or, we can picture just a single field, that has many dimensions or traits, which we call the other forces.

Isn’t gravity also an outlier in that it is far weaker than the other fundamental forces? The article only mentions that in passing.

Yes, and it's not too much of a stretch to imagine the author might reasonably assume readers of Quanta Magazine don't need more than a passing mention of that fact.

just wanna put this out there before someone else gets the nobel prize

it's a push, not a pull

thank me later

Because it's not a force?

It’s not a “force”. I’d argue that it’s more to do with lowering entropy.

I think the first lesson here is that the term "force" is pretty unhelpful. I think the term "interaction" is generally preferred to avoid confusion.

What I would say is that to bundle gravity in with electromagnetic, strong and weak interaction as "four fundamental interactions" – without explaining the rationale for doing so – is also pretty unhelpful.

The reason the other three fundamental interactions seem distinct from gravity is that, theoretically, we can "unify" them at a high enough energy scale. That is, at an energy scale that reflects conditions within a fraction of a second after the big bang, these three interactions can be understood as one interaction, and only as energy distributes and temperature decreases do the forces become distinct.

If I am not mistaken, we have experimentally demonstrated that the electromagnetic and weak interactions unify at high enough energy, and we have theories that make predictions towards unifying the strong interaction at higher energy scales.

Gravity doesn't slot into this picture quite so neatly at all because gravity resists definition at extremely high energies, and by extension resists any effort to understand it as a component of a unified interaction.

I think it is a mistake to dismiss unified interaction as impossible because 'gravity is not a force'. Whenever anyone says this, they make an analogy to the centrifugal force and local reference frames, and that's a fair point to highlight, but I think this indicates more of a linguistic problem in the context of gravity as a fundamental interaction.

Similarly, people are quick to dismiss the graviton because it is impossible to detect and philosophically counter-intuitive – how can the effects on matter by the curvature of spacetime be mediated by a particle?

But remember: there is no particle. The graviton is a mathematical abstraction, it's not some magic ball of gravity that you can fire from a gravity gun or surround your spaceship with. A graviton is just a... I don't want to say "position" so I'll say "probability density" of spacetime at a QM scale where uncertainty is part of the business. (Someone will correct me if that is wrong).

It makes sense that we should want to use that as way to describe gravity because it would use the same framework and methods that we use to successfully describe the rest of the universe.

Of course, there are deep seated problems with gravitons, and that's another thing that sets gravity apart, but I wouldn't say that these problems arise because the notion of the graviton is in any way absurd.

Gravity is different, and it perhaps doesn't help that at human-relevant scales, Newtonian or "classical" gravity is usually sufficient for our needs in application. It lets us believe gravity is intuitive, simple, and then when we look at galaxies and subatomic particles, and we encounter consequences of GR and QM – which can be rather surprising.

But all we have to bear in mind is that at relativistic scales, the uncertainty aspect of QM is still there, it's just incredibly improbable that any quantum weirdness will occur at the macro scale. Similarly, relativistic effects are accounted for at low energy scales, but their influence is so minute that we can remove them from our equations and leave ourselves with classical solutions that are faster to reason about.

As for why gravity is different to other interactions, I think any insightful answer to that question would probably require more ground than has yet been covered in efforts to develop quantum gravity.

At the moment, the difference is technical, it's different because our tools and methods don't account for it very well as long as we're trying to fit it into our current, well-tested models of physics, and to try and fit all of our physics into a model that makes gravity more sensible is even more daunting a task.

--— [IANAP but I am keen to learn so please please please correct me if I am wrong]

Gravity isn’t a force, it’s a curvature of space time

It's weird how mass just somehow perturbs spacetime creating a 3D whirlpool down which everything falls without using any energy in the process. That theory about mass actually only accelerating time and then the time dilation between atoms causing the force of gravity as a consequence is pretty nuts.

Makes you think there really ought to be a way to "stick" onto spacetime and stop yourself from being accelerated, much like pulling the parking brake on a hill. Maybe one just needs a time machine.

You can put the surface of a planet between you and the bottom of the whirlpool like us humans do.

Yeah but that's the lazy solution that's not really all that useful. I mean it's definitely useful to not keep falling towards the planet core, but for other less mundane uses.

If you want to "stick" to spacetime, there must be a reference frame that is somehow different from all the others. There is presently no experimental evidence for any such frame, despite a century or two of searches by very clever people.

Well maybe not literally, just in practical terms. If it's a time gradient that causes an object to accelerate, then if you can apply a reverse gradient that would just cancel out the forces. But yes, one would need to first figure out how to do that without an equivalent amount of mass on the other side, if it's even physically possible.

Here's a stupid idea that's based on my definitely flawed understanding of relativity: If having high relative velocity causes time dilation, then wouldn't spinning a disc at relativistic speeds cause that same time gradient? Maybe we just need 99.9% c rad/s fidget spinners to float in the air just like bricks don't.

Found the video on this theory: https://www.youtube.com/watch?v=UKxQTvqcpSg

your article showcases the perspectives of different quantum gravity researchers on this topic, highlighting the complexity and uncertainty surrounding the search for a truer theory of gravity.

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