Why gravity is not like the other forces

https://www.quantamagazine.org/why-gravity-is-not-like-the-other-forces-20200615/

by kjhughes • 3 years ago

134 124 3 years ago

iaw• 3 years ago

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.

m3kw9• 3 years ago

Yeah gravity seem to have a time dimension to it

ars7• 3 years ago

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.

magicalhippo3• 3 years ago

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

nemo44x4• 3 years ago

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”.

wiml• 3 years ago

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.)

denton-scratch• 3 years ago

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.

alanfalcon• 3 years ago

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

jenadine6• 3 years ago

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.

ars7• 3 years ago

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

jenadine6• 3 years ago

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.

chao-• 3 years ago

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.

BlackFly6• 3 years ago

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.

deanCommie• 3 years ago

But WHY is the coordinate system curved?

jenadine6• 3 years ago

Because of mass. But WHAT is mass?

dghughes• 3 years ago

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

kazinator5• 3 years ago

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

jenadine6• 3 years ago

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.

charlieyu1• 3 years ago

The problem is gravity also affects particles with no mass

sebzim4500• 3 years ago

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).

BlackFly6• 3 years ago

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.

jacknews• 3 years ago

"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.

BlackFly6• 3 years ago

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.

elashri2• 3 years ago

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.

BlackFly6• 3 years ago

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.

divbzero• 3 years ago

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

taejavu• 3 years ago

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.

eviks• 3 years ago

Because it's not a force?

AITree• 3 years ago

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.

wanda• 3 years ago

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]

ISL2• 3 years ago

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.

Xcelerate2• 3 years ago

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.

gpderetta• 3 years ago

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.

zachf• 3 years ago

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

blueorange8• 3 years ago

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

vecter• 3 years ago

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...

Xcelerate2• 3 years ago

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.

CGamesPlay• 3 years ago

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

marcosdumay• 3 years ago

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

mattwad• 3 years ago

amazing to think about, thank you

codethief3• 3 years ago

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

blindriver• 3 years ago

Does gravity affect an electric or magnetic field?

sghiassy• 3 years ago

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

moffkalast3• 3 years ago

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.

hollerith• 3 years ago

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

ISL2• 3 years ago

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.

stainablesteel• 3 years ago

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

it's a push, not a pull

thank me later

pontus• 3 years ago

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.

splitwheel3• 3 years ago

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.

hllooo• 3 years ago

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

KineticLensman2• 3 years ago

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

evanb3• 3 years ago

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.

lisper• 3 years ago

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.)

dist-epoch• 3 years ago

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?

MaxikCZ• 3 years ago

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

ChicagoDave• 3 years ago

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.

codethief3• 3 years ago

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).

codethief3• 3 years ago

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

tlogan• 3 years ago

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).

OnlyMortal• 3 years ago

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

darken• 3 years ago

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.)

Why gravity is not like the other forces

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