If gravity isn't a force, then why does it "need" a boson?

https://old.reddit.com/r/Physics/comments/1bii6h4/if_gravity_isnt_a_force_then_why_does_it_need_a/

by thunderbong • 2 years ago

160 176 2 years ago

old.reddit.com

It's too bad the top-rated answer on Reddit says "GR does not say Gravity is not a force (or if you do say it’s not a force, then none of the other forces are forces either)" rather than explaining what people mean when they say gravity is not a force (basically, it affects the geometry of spacetime in such a way that an unaccelerated particle can still move along a path that's not a straight line in the traditional sense) and why nonetheless we can treat gravity approximately (that is, perturbatively) as if it were a force, and why this perturbative description when quantized predicts a spin-2 boson, the graviton. Oh well.

JumpCrisscross• 2 years ago

Help me with the last bit, where the perturbations when quantised predict spin-2 bosons.

CamperBob22• 2 years ago

Honestly, the flagged/dead answer from ChatGPT makes a lot of sense to me. Can someone explain where it goes wrong, without resorting to the usual snide remarks about parrots and other whistling-past-the-graveyard rhetoric?

Is it just making up BS regarding the attributes imparted by various spin values, or is that a reasonable explanation of why gravitons are presumed to be spin-2 particles?

krsrhe• 2 years ago

I recognize your name and believe you know what you are talking about, so please, please tell us the why! You or someone you know probable has a blog post or article you can link, maybe?

morcus• 2 years ago

Wow I never look at usernames but maybe I should start. I believe I read his `Gauge Fields, Knots, and Gravity` back in college (or tried to, anyways. it was a touch above my level at the time)

sandworm101• 2 years ago

I had a prof explain it using basic analogy. Forces are things that can move other things. An electrical field can move a metal ball. So too can a metal bat. put a g-meter on the ball and it will register acceleration as force is applied to the ball. Gravity is different. Put a g-meter on a falling metal ball and it will detect zero acceleration. No force is acting upon a ball. Put that metal ball in an eccentric orbit around the earth and it will speed up and slow down during each orbit, but the g-meter will register zero acceleration. The ball falls but is no not accelerated. So gravity is not a force because it doesn't move things. Gravity is something different.

gwd2• 2 years ago

But the acceleration meter won't measure any force because gravity is acting on every part of it uniformly. If you had an acceleration meter entirely made out of the same magnetic substance, and you brought a magnet near it, would the acceleration meter register anything, or would it read zero acceleration, since all parts of it were being acted on uniformly (and thus didn't "notice" any acceleration)?

ajross• 2 years ago

This is a clever analogy, but it's actually a little specious.

The reason the "g-meter" (e.g. a weight on a scale) doesn't move in the gravity case is that the weight is affected by the same field. The weight is a weight and feels gravity just like you and everything else in your environment does.

But by construction, you're imagining that the scale you have holds a different electrical charge than the object to which it's attached. Which is "normal" according to our everyday experience, but just an artifact of the way charges work on large objects (they distribute themselves on the "outside" of a conductive environment and everything inside tends to have a neutral distribution).

But that's just arbitrary. You could equally demand (in your gedankenexperiment, though doing this in practice would be very difficult) that your electrical charge be distributed just like the mass is, in which case the force measured would be zero too.

hammock2• 2 years ago

put a g-meter on the ball and it will register acceleration as force is applied to the ball. Gravity is different. Put a g-meter on a falling metal ball and it will detect zero acceleration

Put an electrical field generator on a falling metal ball and it will detect changing electric field though...

nyrikki3• 2 years ago

Gravity in GR is a 'fictitious force' or an apparent force, meaning it is is an artifact of your chosen frame of reference.

A feather falling the same speed as an elephant in the Earth reference frame is an example.

It is the same as other apperant forces like centrifugal force.

Even Newton himself said 'Hypotheses non fingo,' or 'I feign no hypotheses. '

It was a conjecture.

trhway3• 2 years ago

So too can a metal bat. put a g-meter on the ball and it will register acceleration as force is applied to the ball. Gravity is different. Put a g-meter on a falling metal ball and it will detect zero acceleration.

That really depends on construction of your g-meter. If instead of mass (ie. gravitational charge) you use electric charge in your accelerometer then that electric charge on the falling ball - ie. moving with acceleration - will generate EM wave thus providing clear detection of acceleration.

Wrt. the "boson" - gravity effects propagate with finite speed, i.e. wave, and the neutron in gravitational potential experiment shows that the gravitational potential/energy is quantized, and thus we have wave and quantized nature -> boson (wave packet/quant mediating interaction of a charge with the field).

moi23882• 2 years ago

Yeah this never made sense to me. Yes, it detects no acceleration. Great. But we know it IS accelerating; if you fall your speed will increase, this can only occur if you are accelerating?!

gnramires2• 2 years ago

I've recently learned about Kaluza–Klein theories (just curious wiki browsing), do they unify electromagnetism and gravity in the sense that electromagnetism also "works by changing the geometry of spacetime" ? How does this relate to QFT?

elashri3• 2 years ago

do they unify electromagnetism and gravity in the sense that electromagnetism also "works by changing the geometry of spacetime" ?

Yes, somehow actually. KK theories introduce an extra spatial dimension beyond our usual (3+1) which is postulated to be "compactified". This just means that it's curled up on itself at such a tiny scale that we don't directly perceive it. The way this extra dimension is curled and shaped affects the geometry of the overall 5-D spacetime. How it is connected to EM is that now these geometrical variations in the 5-D spacetime, when viewed from our 4-D perspective, manifest as the EM force and its associated field.

So you can say like in GR which have gravity arises as consequence of geometry, it is in KK that EM is consequence of geometry. However, the geometry and details are different.

How does this relate to QFT?

Not much in the sense that they can provide useful information to each other. QFT describes forces in terms of interactions mediated by particles (e.g., photons for EM). KK, while primarily geometrical, give hints that perhaps these force-carrying particles can be associated with specific vibrational modes of the extra dimension. Of course KK theory only include EM and gravity. So we know for sure that we need to go beyond KK. This was the actual motivation for people to think about string theory to expand the original KK work.

lubesGordi• 2 years ago

What does it mean to 'treat gravity approximately (that is, perturbatively)'? That sounds like something we do to model, which only approximates reality. That model sounds like it shouldn't be used to predict anything else? Or at least whatever is predicted shouldn't be expected to exist in 'reality'.

canjobear• 2 years ago

It’s models all the way down.

nyrikki3• 2 years ago

John von Neumann: "... the sciences do not try to explain, they hardly even try to interpret, they mainly make models. By a model is meant a mathematical construct which, with the addition of certain verbal interpretations, describes observed phenomena. The justification of such a mathematical construct is solely and precisely that it is expected to work—that is, correctly to describe phenomena from a reasonably wide area."

Both GR and QFT are insanely accurate models, but they are just models.

The N-body problem is undecidable, and Gödel, Turing, Church and other s proved that is the best we can do.

https://philsci-archive.pitt.edu/13175/1/parker2003.pdf

Western reductionism or Laplacian determinism is a good framework for practical, computable models. QFT actually is actually one of the counterexamples to Western reductionism.

But models are reductive and scientific models are just models. Don't confuse the map for the territory.

All models are wrong, some are useful; is another way of saying the same thing.

cthalupa4• 2 years ago

There isn't a model out there that doesn't break down at some point.

Looking at gravity, we can compare what General Relativity and Quantum Mechanics say about the center of a black hole. Remember, both GR and QM have both been able to accurately model the way the world works at every scale we have been able to measure and test them. But they are incompatible with each other in certain points, such as the singularity in the black hole. GR says the center of the black hole is an infinitely dense point. QM says this can't be true because everything is made up of waves in a field, which requires things be spread out over some amount of an area. These can't both be true, yet GR and QM have both stood up to every single test and observation we can throw at them. Every prediction they make that we can verify has been verified and lines up with the theories. And this is not the only place they disagree, of course, but it is one example.

And that's really, from my understanding, the more fundamental answer to the question asked in the reddit post. It's not that unifying the two requires a gauge boson like the graviton, though that it is one possible outcome when quantizing gravity, but that we have two very useful and very tested models of how things work that are incompatible with each other in certain ways. Maybe gravitons exist, though it currently seems impossible for us to reach the point where we can detect them - Dyson calculated that using an Earth sized detector we'd be able to detect about one graviton from the sun per billion years, if they exist - and maybe they don't.

As for predicted things not expecting to exist in reality, this is really just par for the course for models. It's not like tensors are some real physical thing either, for example.

vinnyvichy• 2 years ago

Unaccelerated particle= inertial frame.

Approximately: consider gravity as small deviations from inertiality, so it's approximately linear, like the other forces we know and love.

fuzzfactor• 2 years ago

Q. What's the difference between a boson and a bison?

A. When you incessantly hunt for every boson you can find anywhere, you end up with more of them.

pdonis31• 2 years ago

I think that more generally the answer is not responsive to the actual question, which is: why do GR and QM need to be unified? To be fair, the question itself wrongly conflates "gravity needs a gauge boson" with the question about GR and QM being unified. Not all theories of quantum gravity involve a "gauge boson" for gravity along the lines of the other Standard Model interactions.

Nor does the basic rationale for why we think gravity needs to be quantized involve a "gauge boson". It involves simple reasoning about how QM works. Say we have an experiment which puts an object with non-negligible stress-energy into a superposition of being in two different positions (for example, we make its position depend on the outcome of a spin measurement on a qubit). QM would say that spacetime would then need to also be in a superposition of two different geometries. But GR, as a classical theory, has no way to handle that. We would need a quantum theory of gravity, i.e., a quantum theory that can handle superpositions of different spacetime geometries.

trhway3• 2 years ago

Say we have an experiment which puts an object with non-negligible stress-energy into a superposition of being in two different positions (for example, we make its position depend on the outcome of a spin measurement on a qubit). QM would say that spacetime would then need to also be in a superposition of two different geometries.

The energy to move the object into a given position is an additional element here unaccounted for in your model. 2 different positions to move object into - 2 different energies (more specifically 2 different changes to the starting, before the experiment, stress energy distribution of the Universe). When corresponding moving energies (ie. their GR effects) are accounted for in those 2 cases it may as well be that those 2 cases are indistinguishable from the GR point of view, ie. those 2 supposedly different spacetime geometries happen to be the same. The superposition of 2 indistinguishable cases - it doesn't really matter is it superposition or not.

bena• 2 years ago

I would imagine the desire to unify GR and QM is because if we expect the universe to operate on a set of rules, they should by unified. And we just need to find out how.

But the forces and factors that work on the smallest particle should be the same forces that work on the largest of galaxies. If they are not, that's a completely different mystery and means our entire view of the universe is missing something substantial.

rhdunn• 2 years ago

My layperson understanding is that GR specifies that energy/mass-momemtum influences space-time curvature which then produces gravity (as particles travelling along straight lines in the curved space). That would imply that fermions are therefore the force carriers for gravity, not a hypothetical new boson.

me_me_me• 2 years ago

it would be so much easier to visualize it if we could imagine 4th dimension.

layer8• 2 years ago

You can say similar things about the field of the other forces too, though. The path of a charged particle in the EM field could be described as that particle experiencing a different space-time geometry, arising from the EM and gravitational field combined, and thus EM could also be seen as a geometry and not a force. In fact, the impulse of photons, which are vibrations in the EM field, does affect the curvature of space-time, similar to how particles with mass do.

pdonis31• 2 years ago

The path of a charged particle in the EM field could be described as that particle experiencing a different space-time geometry

No, it can't, because the EM interaction does not obey the equivalence principle, as gravity does. The geometric interpretation of gravity relies on the equivalence principle.

To state this another way: if I put two objects with different masses at a given point in spacetime and give them both the same initial velocity in the same direction, their paths through spacetime under gravity will be the same. But if I put two objects with different charges at a given point in the same electromagnetic field and give them both the same initial velocity in the same direction, their paths through spacetime will not be the same. And this remains true even if I add "extra dimensions" to "spacetime" along the lines of Kaluza-Klein theory, to represent the EM field.

codexb• 2 years ago

I feel like it's easier to explain that gravity is a force between matter and space(time) and not necessarily between matter and matter, like we presume the other forces are.

Biganon• 2 years ago

Woaw, a top comment by John Carlos Baez himself!

machina_ex_deus• 2 years ago

Non gravity forces are quite linear and simple, the coupling with fermions is only through covariant derivative.

Gravity is extremely non linear, when you look at the expression for Ricci tensor, it is much more complicated than other forces.

pizzafeelsright• 2 years ago

I like to think of gravity as displacement of the "ether ". I suppose negative mass would have a type of repelling effect on mass which I figure is energy of various negative mass.

Radio waves are relatively low in negative mass along with photons.

slowmovintarget5• 2 years ago

I didn't think there was such a thing as negative mass. That is, you can hypothetically have gravity that is repulsive, but you cannot have negative mass because mass is a property of matter conveyed by the Higgs field. All matter has some positive value for mass; the measure of the matter's resistance to acceleration.

Photons (a.k.a energy, including radio waves) do not interact with the Higgs field and so have zero mass.

There is no negative Higgs field, and no negative mass.

There's also no ether either as originally constructed (it was ruled out by experiment) but you could be speaking metaphorically.

scotty79• 2 years ago

So is it really the energy of coupling between particles and Higgs field (along with other kinds of energy and momentum) that reshapes the spacetime?

mnw21cam2• 2 years ago

Photons have zero rest mass. But they're never at rest, they are always travelling at the speed of light. They have mass equivalent to their energy, which is related to their wavelength.

fooker• 2 years ago

Physicists have been down that path along time ago.

https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_exper...

elashri3• 2 years ago

I like to think of gravity as displacement of the "ether ".

You are only behind physics community by ~140 years [1]. We know for sure that there is no "ether" so its displacement is not what causes gravity. It was a trial to explain how light would move if it is wave, which we didn't know about EM waves then.

[1] https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_exper...

int_19h2• 2 years ago

From MM experiment, we know for sure that there's no ether with the properties that were ascribed to it. However, you can easily come up with a theory of ether that completely matches those observations - it's just not as useful as a model because the concept itself becomes kinda redundant at that point. However, this is just as much an issue of terminology. Here's Einstein on the subject:

"More careful reflection teaches us however, that the special theory of relativity does not compel us to deny ether. We may assume the existence of an ether; only we must give up ascribing a definite state of motion to it, i.e. we must by abstraction take from it the last mechanical characteristic which Lorentz had still left it. We shall see later that this point of view, the conceivability of which I shall at once endeavor to make more intelligible by a somewhat halting comparison, is justified by the results of the general theory of relativity ... according to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists an ether. According to the general theory of relativity space without ether is unthinkable."

Zambyte3• 2 years ago

I have no background in physics beyond my highschool class, but how I understand gravity is this: matter has inertia, the universe is expanding "up" (into the future, away from the Big Bang) and out (stretching the surface), the inertia of the matter causes a depression in the spacetime, and the normal force of that curved spacetime causes objects to "attract" each other. This also seems to explain the relationship between gravity and the rate at which time passes (time dilation).

Is this understanding wrong by generally accepted science?

pdonis31• 2 years ago

Is this understanding wrong by generally accepted science?

Yes. It's also not relevant to the discussion in this thread. The properties of gravity under discussion here have nothing whatever to do with expansion of the universe.

Zambyte3• 2 years ago

Yes.

I was hoping for some elaboration.

The properties of gravity under discussion here have nothing whatever to do with expansion of the universe.

At least part of the discussion is about gravity being (caused by? understood as?) the curvature of spacetime. Why would matter curve spacetime, if not for the accelerated expansion of spacetime + matter resisting acceleration due to inertia?

I'm also OOTL on the boson part (remember, high school physics). I'm asking to hope to understand the subject better.

riperoni• 2 years ago

Well, not really.

The expansion of the universe is not responsible for explaining gravity, it was observed and aided in determining models and constants for the theory.

Start by looking at a particles property called mass. Mass interacts with the higgs field by exchanging a particle of information called the higgs boson. The interaction strength, corresponding to mass, determines how much the space time around our particle curves. Space time curvature dictates a particles movement. Now throw another particle with mass next to the initial one, also interacting with the higgs field. This is when you get into a huge loop of interacting movements and fields.

Locally and classically we approximate it by the classical law of gravity which loosely states that two masses attract each other, depending on how close they are, i.e. GMm/r^2

The universe expanding or contracting is a consequence of mass and gravotational fields, not the other way around. And calculating the expansion and thus curvature of the universe gives you a picture of the average mass distribution.

YetAnotherNick• 2 years ago

The top reddit answer in wrong in the most irritating way. And Higgs boson has nothing to do with gravity. And Higgs boson interaction only accounts for 1% of the mass of matter.

According to relativity, energy IS mass and Higgs field gives non zero energy to Higgs boson.

snarkconjecture• 2 years ago

Neither the OP nor the top answer on Reddit mentioned the Higgs boson. You're right, but I'm not sure who you're rebutting.

m3kw9• 2 years ago

I would say gravity is a force, how the heck is this 100lb weight a 100lb weight?

pdonis31• 2 years ago

how the heck is this 100lb weight a 100lb weight?

Because the scale that reads 100 lb is pushing up on the weight. The same would be true if the scale was inside a rocket accelerating at 1 g in deep space, far from all gravitating bodies. (This is called the equivalence principle.) In other words, the 100 lb of force you are calling "weight" is not a "force of gravity"; there is no such thing. It's a non-gravitational force exerted by the scale on the weight.

lazide3• 2 years ago

The force drawing the weight and the mass of the earth 'under' it together (with the scale in the middle) however clearly exists as much as the electromagnetic force though.

Or stars, planets, etc. wouldn't (and couldn't) exist.

As to the exact details of how we account for it is another matter. Calling gravity 'measured spacetime curvature' or 'gravitational force', or coming up with pseudo particles, or whatever, is an accounting method. Either way, it's still there.

But it does clearly exist, as much as light (for example) does. All one needs to do to prove that is look out the window.

silent_cal4• 2 years ago

This is my first time hearing this. I don't study physics much but this doesn't sound right at all. You're telling me if I drop something there is no force pulling it down? That makes no sense.

Jun8• 2 years ago

The top answer given here is highly reductionist to the point of misleading. This question comes up regularly in Physics SE, if you’re interested I suggest you check out the answers to the following:

https://physics.stackexchange.com/questions/219306/if-gravit...

https://physics.stackexchange.com/questions/61899/why-do-we-...

Second one has a highly upvoted but controversial answer by Lubos Motl.

This question is similar, I think, to others like “why is there a speed limit in the universe” and “does light slow down in a material” that are I doubly muddled for science enthusiasts because (1) they are concepts that require deep domain knowledge, (2) even for some who have (1) they’ve been biased by decades of well meaning but harmful educational aides.

slingnow2• 2 years ago

The top answer given here is highly reductionist to the point of misleading

This is my expectation for every single "expert" answer on Reddit. Confidence without substance.

malfist2• 2 years ago

That's what finally broke by reddit addiction and relying on the Internet for advice.

There's lots of people out there that mean well and have strong opinions and state things with confidence that they don't necessary know for truth.

I recall thinking that I don't critically examine what appears to be expert input from redditors in a field I know little about, but those same voices in areas I am an expert in are almost always wrong, or nuanced in some way to make them useless. I finally drew the line and realized it wasn't just my field that had wrong "experts" it was every field.

I think the final straw was when I asked a really complex question about a aberration in my telescope in the astrophotography subreddits and got very authoritative sounding responses that were far far off base, I realized it wasn't just software engineering.

I wonder how much of it is teens and young adults without a lot of experience giving advice that's the problem here. Probably not, but something to think about. That person giving you advice on how to navigate your relationship with your spouse might be a 13 year old who's never dated anyone before.

user3939382• 2 years ago

Please forgive my response being a little meta, but it bothers me that 95% of discussions where physics "experts" are clarifying something to laymen is those experts clarifying that some factor is actually well understood and makes complete sense and is firmly under the belt of the academic and research community if you just had the knowledge to understand why. Almost never through these same discussions is there acknowledgement or discussion of the unknowns, limitations, or unsolved problems. Judging from years of anecdotal exposure to these "clarifications for the laymen" one would assume absolutely nothing is unsolved in physics.

Filligree2• 2 years ago

Those can both be true at the same time. Physics has unsolved problems -- that doesn't mean that the problems that come up for laymen are unsolved.

creatonez• 2 years ago

It's balanced out by laymen getting exposed to science media that constantly hypes up how mysterious, unknown, and downright magical everything is, rather than appreciating how rich the literature has become.

cthalupa4• 2 years ago

I mean, this is how it is for any domain that is sufficiently deep. It's not like we don't have similar examples in computer science - when talking with laypeople, we provide generalizations and abstractions. There are still unsolved questions, like P=NP - but the layperson out there is unlikely to think that programming is "unsolved."

This isn't an indictment of physics and physicists, it's just an example of how advanced domain knowledge is simplified for easier consumption of non-experts across basically every field deep enough to have these sorts of complex topics.

freddealmeida2• 2 years ago

No they don't. Experimentation is all we need. Nothing in this can be determined without error. It is easier to believe it is not true. It is no different than the priests that tried to figure how many angels can stand on a head of a pin. But 1) you lack that deep domain knowledge and 2) you are biased. Your arguments are empty. First principles.

pdonis31• 2 years ago

a highly upvoted but controversial answer by Lubos Motl

Which is, of course, typical of Lubos Motl. :-)

What his answer actually boils down to is the quantum field theory answer: the claim that (a) we need a quantum theory of gravity, and (b) the known quantum field theory of a massless spin-2 field is at least a good low energy approximation to whatever the final quantum theory of gravity will turn out to be. Both of these claims are believed to be valid by many physicists, but neither one actually has any experimental evidence to support it at our current state of knowledge.

tantalor• 2 years ago

highly reductionist to the point of misleading

You say "reductionist" but don't say how. You can't just just point and say "misleading" without backing that up. Now who's being reductionist :)

GeoAtreides• 2 years ago

The top answer given here is highly reductionist to the point of misleading

Is there a reason to take your opinion as being right? I am not an expert in physics, I don't want to study for the next 20 years to be one, how can I determine if you're correct, and that the top answer is indeed "highly reductionist to the point of misleading". Maybe _you_'re wrong and the other guy is right.

Filligree2• 2 years ago

how can I determine if you're correct, and that the top answer is indeed "highly reductionist to the point of misleading".

Five years of studies, at minimum...

ck22• 2 years ago

As I understand it, Boson makes particles behave as if they had mass.

Gravity doesn't work on things without mass (or pretend mass) so Boson is critical to gravity's behavior?

Related PBS Space Time

https://www.youtube.com/watch?v=G0Q4UAiKacw

qnleigh2• 2 years ago

Actually gravity does affect massless objects. It can bend the path of light, even though photons are massless. See 'gravitational lensing' for some cool photographic evidence :).

The Higg's Boson gives mass to many particles; this might be part of what you're thinking of.

Whether gravity is described by bosons at its deepest level is strictly speaking an open question, though many models of quantum gravity treat it this way.

ck22• 2 years ago

Gravity does not bend light or move photons.

Gravity changes the path light has to take, photons keep going where they were going in the first place.

Light doesn't escape a black hole because it is being pulled by gravity, it is just being given an infinite path to travel so it never gets there.

freddealmeida2• 2 years ago

It doesn't. Modern cosmology is so full of holes. Nothing can be determined through experiment.

cjfd• 2 years ago

To me it seems the equivalence principle is actually false. It was an inspiration to come up with general relativity but the resulting theory supports it kind of, but not really. To start out with, it only holds either looking at an infinitesimal extent of space or in a homogeneous gravitational field. As soon as the gravitational field cannot be seen as homogeneous, you are going to know that you are falling instead of floating through space. E.g., spaghettification near a black hole. Another case in point is the somewhat well-known question if a falling electron radiates. Yes, it should, but that violates the equivalence principle.

SamBam3• 2 years ago

My understanding is that the equivalence principle only applies to a specific, local region of space time, and this region could be arbitrarily small.

Tidal forces are only measurable across something with size, by measuring a difference between one point and another.

Even if your body is being spaghettified, an arbitrarily small region of your body still can't tell that it's under anything but freefall.

garyrob• 2 years ago

I was wondering exactly that same thing just yesterday!

Xcelerate• 2 years ago

Gravity is a bit different than the other forces because it is the only mechanism by which all known particles interact with all other known particles (since all particles have an association with the stress-energy tensor).

Razengan• 2 years ago

Gravity is a compression algorithm, for more efficient processing of matter in contiguous batches.

devaiops9001• 2 years ago

There is a modern explanation for gravity that is actually rather simple and strongly cohesive. But it doesn't align with modern orthodoxy, so it's not worth mentioning here.

silent_cal4• 2 years ago

Can someone please present the argument for why gravity is not a force? This makes no sense to me.

itodd• 2 years ago

ScienceClic on Youtube has amazing videos on this topic as does Veritasium. Einstein's happiest thought helps with understanding this. He thought that there would be no discernible difference between what a man felt while falling off a ladder (let's not factor in air resistance) and what a person in 0g space felt.

We are actually "accelerating" upwards at a constant rate that is equal to what we feel and many think of as gravity. This is because spacetime is curved by the mass of the earth.

Think of a grid being distorted or squeezed / pinched. The distortion pinching is "gravity". Everything in the grid still travels in a straight path relative tot he lines on the grid but since the [spacetime] grid is pinched, it looks as if it is not on a straight path.

silent_cal4• 2 years ago

Thank you

cthalupa4• 2 years ago

There's quite a lot of arguments back and forth about it through this whole thread if you really want to dive into it. The cliff notes is effectively that gravity is not a force in GR. It is considered a force in newtonian physics, and might be a force again if we ever make a Theory of Everything. Gravity is fairly akin to a force in string theory, so if they do end up being right, gravity would be a force. If the Loop Quantum Gravity folks are right, gravity will remain not a force. If it's some other theory, who knows what it'll end up being.

silent_cal4• 2 years ago

I'm not about to comb through threads on Hacker News and synthesize a theory from comments by people who may or may not know what they're talking about. But thanks.

amai• 2 years ago

Gravity might not be a force, but it is described by a field theory. And field theories can/must be quantized. So the question is really just a different way of asking, if we need to quantize gravity: https://physics.stackexchange.com/questions/6980/what-are-th...

If gravity isn't a force, then why does it "need" a boson?

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