[HN Gopher] Gravity is a double copy of other forces
___________________________________________________________________
Gravity is a double copy of other forces
Author : cjg
Score : 288 points
Date : 2021-05-09 08:06 UTC (14 hours ago)
(HTM) web link (www.quantamagazine.org)
(TXT) w3m dump (www.quantamagazine.org)
| wyager wrote:
| > Most theorists assume that gravity actually pushes us around
| through particles
|
| Is this true, and if so, why? The interpretation of gravity as
| something that warps spacetime very elegantly yields its
| "gravitational force" via its effect on the action integral, and
| is easily understood using a path integral style framework. It
| seems like a particle-based framework would necessarily be a lot
| more complicated, although maybe it's necessary for some reason I
| don't know.
| zachf wrote:
| So you're right that (low-energy) gravitational physics is
| understood because you know how to write down the action. The
| question is whether the sum over paths in the path integral
| should also include a sum over metrics. That sum over metrics
| is equivalent to saying that gravitons exist (in the same way
| that the sum over electromagnetic potentials is equivalent to
| saying that photons exist).
|
| Now if you want to include gravitational effects and do it in a
| consistent way, you have to sum over metrics, meaning you have
| to have gravitons. That's because trying to treat gravity like
| it's classical but treating everything else like it's quantum
| mechanical is inconsistent. For example, classical gravity
| could tell you which slit an individual electron passed through
| in a double slit experiment, if you measured the gravitational
| field accurately enough--you could say definitively that it
| came through one slit or the other by measuring which way the
| gravitational field that it generated is pointing. This would
| destroy the interference pattern and you wouldn't be able to
| conduct the double-slit experiment at all.
| wizzwizz4 wrote:
| Everything else (that we know about) in the universe is
| quantised. (I don't know whether experiments have shown gravity
| to be quantised yet.) If something's quantised, that means
| there's a smallest possible unit of it (a "particle").
| ssivark wrote:
| Classically that's true, but if we want consistency with
| quantum mechanics, the only starting point we know is to
| quantize the force carrying fields (gravitons, just like photos
| and gluons), but then it turns out we can't get far from the
| starting point.
|
| Why do we want to quantize gravity? Imagine you are doing a
| scattering experiment. To predict the full results consistently
| (in principle), you would need to include the gravitational
| forces between the scattering particles. This would be
| particularly important in early universe cosmology where you
| might have lots of heavy stuff zipping around at high speed!
| The classical GR picture is not too helpful in these
| situations.
| evanb wrote:
| >It seems like a particle-based framework would necessarily be
| a lot more complicated.
|
| That's true for the other forces as well! And yet after very
| careful study we know the electromagnetic field IS quantized,
| particles of light are photons, and to get them to do the right
| thing (exhibit interference, for example) you need a quantum
| mechanical framework.
|
| So why not for gravity too?
| Google234 wrote:
| If this is related to string theory then the claims of any
| usefulness should be taken with a grain of salt. I remember all
| the ADS/CTF people claiming that they could provide huge insights
| into condense matter which later turned out to be very
| underwhelming.
| Cancan82 wrote:
| Sorry - I laughed at your first sentence and spit coffee on my
| keyboard; just thought you should know!
| 123pie123 wrote:
| sorry for this daft question
|
| I imagine a very very slow moving rock in space - going at 1 m/s
| (relative to earth) in a straight line, the earths mass causes
| spacetime to bend making the rock head towards earth, but then
| the rock starts to accelerate
|
| the bit I don't understand is why does the rock accelerate
| towards mass? Why does the bending of spacetime make it not carry
| on at 1 m/s towards earth
|
| and since this rock has increased its speed due to accelation,
| where has this extra energy come from?
| ItsMonkk wrote:
| I really have no business answering this, but in the spirit of
| Cunningham's law I'm going to try to give an answer how I think
| of it. Would like some feedback and some guidance. This is
| purely a intuitive answer and not at all an academic one.
|
| As we know from relativity and time dilation, as our GPS
| satellites need to be resynchronized as they move fast and at a
| lesser mass level. The center of the Earth is very heavy and is
| therefore going through time slower than the rock or surface
| objects. Gravity doesn't really exist. Gravity is merely the
| side-effect of heavy-mass objects going through time slower
| than lighter objects. This is what is meant when it is said
| that spacetime is curved.
|
| So back to your question, where does the extra energy come from
| to accelerate the rock towards the Earth? An easy way to think
| of it, expanding on E=mc^2, Et=mc^2, where t is time. As the
| ball is now falling further and further into the Earth's
| gravity well, it is now going through time slower and slower.
| With t going slower, E must increase to balance the equation,
| which speeds it up.
| 123pie123 wrote:
| I've always thought that at slow speeds - ie less than 1/10
| speed of light, then time can be ignored
| cercatrova wrote:
| > sorry for this daft question
|
| No need to feel dumb about asking questions, that's how we all
| learn :)
|
| With regards to your question, looks like someone answered this
| in another comment chain:
| https://news.ycombinator.com/item?id=27096279
| 123pie123 wrote:
| thanks, so the rock that was heading towards earth at 1 m/s
| will technically not accelerate - with in its own reference
| point, but the rock will accelerate to the earth (from the
| earths reference point)
|
| slight change of question, if the rock was heading directly
| in a straight line towards earth at 1 m/s, as the rock nears
| the earth it will accelerate (from the earths perpective)
| where has the extra energy come from? before it was heading
| at 1 m/s, but as it hits earth it will be travelling much
| faster
| lukeplato wrote:
| Anybody here know if this double copy technique is related to the
| cobordism property found in Donaldson's Theory [1]. From wiki:
|
| > Donaldson was able to show that in specific circumstances (when
| the intersection form is definite) the moduli space of ASD
| instantons on a smooth, compact, oriented, simply-connected four-
| manifold X gives a cobordism between a copy of the manifold
| itself, and a disjoint union of copies of the complex projective
| plane CP^2.
|
| [1]
| https://en.wikipedia.org/wiki/Yang%E2%80%93Mills_equations#D...
| BlueTemplar wrote:
| Very exciting ! Reminds me of the time when I learned how
| imaginary numbers were related to rotations...
| andi999 wrote:
| Be prepared to get excited again when reading about
| quarternions.
| nimish wrote:
| Clifford or geometric algebra generalizes this to spinors in
| any dimension and signature, so you also get the dirac
| equation for free in a sense
| BlueTemplar wrote:
| Yeah, quaternions didn't seem _that_ exceptional, "just"
| the generalization to rotations in 3D, but Geometric
| Algebra _did_ get me excited because I finally managed to
| understand what was the whole deal with those weird
| "pseudo-vectors"...
|
| https://www.av8n.com/physics/clifford-intro.htm
| DigiDigiorno wrote:
| "If +1, -1, and [?]-1 had been called direct, inverse and
| lateral units, instead of positive, negative, and imaginary (or
| impossible) units, such an obscurity would have been out of the
| question." - Gauss (translated), a couple hundred years ago.
|
| Of all the things to rename, this may be at the top of my list.
| I think some kids get lost in the abstraction specifically
| because the name "imaginary" isnt helpful in understanding the
| underlying concept.
| gumby wrote:
| > Of all the things to rename, this may be at the top of my
| list.
|
| I think more confounding is that the electron carries a
| negative charge.
|
| This mistake goes back to Franklin but we can't blame him;
| from the data he had he had a 50/50 chance of getting it
| right.
| k__ wrote:
| I don't know much about physics, but I always wondered if gravity
| is like a shadow.
|
| Appearently shadows can move faster than light, breaking with the
| rest of physic knowledge, but when you look into the details,
| they adhere the laws of physics no problem.
|
| Maybe, gravity is like that, not really a physical force, but the
| shadow of physical forces.
| formerly_proven wrote:
| "Projections" can move faster than light in general (e.g. the
| position of an electron beam can change faster than light).
| detritus wrote:
| I just happened to watch this, which says pretty much the
| same
|
| https://youtu.be/Bq9xR5PUs6s?t=388
|
| "Is Infinity Real?" Sabine Hossenfelder, about 6:25 in.
|
| Perhaps this is what OP was referring to?
| oefnak wrote:
| A shadow is not a thing that moves. What you see as a shadow is
| just the lack of light.
| alexnewman wrote:
| That is his point.
| k__ wrote:
| Yes, that's what I meant.
|
| Maybe, gravity is like that in some more complex (or
| abstract?) way.
| hellbannedguy wrote:
| I get what you are saying completely. I have thought the
| same, but never mentioned it.
| lupire wrote:
| Static Gravity (instantaneously, when nothing is moving) is
| the curvy shape of the Universe, created by massive
| objects. In that sense gravity not really there, it's just
| a description of the shape. But when the universe changes
| shape (due to particles moving or being created or
| destroyed), those changes happen in gravitational waves
| that travel no faster than the speed of light.
| tux3 wrote:
| Note that gravity is very much bound by the speed of light! If
| you made the Sun vanish, the Earth would only stop orbitting
| around it a few minutes later.
|
| https://en.m.wikipedia.org/wiki/Speed_of_gravity
| Y_Y wrote:
| I'd like to add that you can't make the Sun vanish. Aside
| from the moral and legal objections, physics won't let you do
| anything that would just "turn off the gravity". You'll have
| to be content with moving it away bit by bit in some
| continuous flow of mass-energy, again limited by the speed of
| light.
| blueprint wrote:
| Look... we filed notice. It was on display in the bottom of
| a locked filing cabinet stuck in a disused lavatory with a
| sign on the door saying "Beware of the Leopard".
| k__ wrote:
| Yes, I didn't mean to imply that the link between a shadow
| and gravity is the speed of light.
|
| I wanted to say they are both emergent phenomenons... or
| something in that direction.
| amelius wrote:
| Yes, this breaks Newton's third law that every force should
| have an equal and opposite reaction force, and thus the sum
| of all forces are zero at all times.
| WJW wrote:
| Newtons laws are not accurate at the quantum and
| relativistic scales anymore though.
| PeterWhittaker wrote:
| Yes, this. Of all the things I learned in SR, the example
| of two particles of the same charge approaching one
| another near c and the force vectors not lining up with
| the line between them was the most mind blowing.
| Instantaneously, conservation laws were violated, they
| held only over a "long enough" consideration of the
| interaction.
|
| Newtonian mechanics is a useful approximation at certain
| scales. At other scales, they are problematic.
| codesnik wrote:
| although nothing really vanishes anywhere, no? I wonder, if
| sun or anything else would fully annihilate, would
| resulting gamma radiation still produce a gravitational
| pull just because of the energy contained in it?
| ben_w wrote:
| Correct, but this is a different hypothetical.
| Iolaum wrote:
| It doesn't. Conservation of momentum, which is an
| equivalent of Newton's third law, exists in General
| Relativity. The results of how a system would evolve under
| GR and the special case like the one above just happen to
| look strange on a superficial look. If one makes the effort
| to calculate all the constituents of momentum in such a
| system, from GR equations, they d verify their sum is
| conserved.
| blueprint wrote:
| What if that's just the speed of the ability of the change in
| gravity to have an effect, rather than the gravity change's
| actual speed of propagation
| withjive wrote:
| Sorry, "apparently" shadows do not move faster than light.
| Since they are... well... defined by light...
|
| I am curious though, as to where you got such a strong notion
| that shadows can move faster than light...
| wyager wrote:
| He's referring to the fact that if you sweep a laser pointer
| across the moon, the dot can easily move faster than light.
| This is not proscribed by physics, nor are similar phenomena
| like the phase velocity of a wave exceeding the speed of
| light. None of these things allow you to transmit information
| faster than light.
| cb321 wrote:
| Another fun example besides projections like laser pointers
| or the spot of a searchlight on the clouds is the junction
| between blades of scissors. Almost all the examples one
| sees of this relate to synchronization defined entities not
| being physical objects. In my experience explaining this,
| the larger scale of the searchlight on the clouds seems to
| help people "get it".
| lupire wrote:
| To really kill the illusion, imagine a long line of
| people, doing "The Wave". The wave can move arbitrarily
| fast as the delay between people decreases, and even
| infine speed! (everyone does their part at the same time,
| assuming they have synced clocks and aren't dependent on
| their neighbor for the trigger, which is the key feature
| of the faster-than-light illusion.)
| lenkite wrote:
| https://en.wikipedia.org/wiki/Faster-than-
| light#Light_spots_...
|
| http://thescienceexplorer.com/universe/4-ways-travel-
| faster-...
|
| https://www.discovery.com/science/darkness-is-faster-than-
| th...
| stared wrote:
| "Light thinks it travels faster than anything but it is
| wrong. No matter how fast light travels, it finds the
| darkness has always got there first, and is waiting for it."
| - Terry Pratchett
| deadite wrote:
| >>>/r/iam14andthisisdeep
| k__ wrote:
| https://physics.stackexchange.com/questions/259732/speed-
| of-...
| withjive wrote:
| Yes that is a great of example of reading something on the
| internet and believing it completely?
|
| You just linked me to a Stackoverflow post about a theory,
| which turns out to just be completely wrong, while managing
| to include a few valid facts.
|
| Was a fun exercise, but please don't get your science this
| way.
| drooby wrote:
| Hey dude, sorry our tiny brains are struggling with the
| science. Do you mind sharing a link to the actual
| science?
| formerly_proven wrote:
| Of course a shadow can move faster than light. Because a
| shadow is not actually an object moving at all.
| knome wrote:
| The moon is a little more than 0.1 light seconds in
| diameter. Any wag that took more than a tenth of a second
| to traverse the moon's surface would be moving slower
| than the speed of light across it. There's no way the
| ponderous wag displayed in the video was moving so quick.
| At best, it was moving half the speed of light across it.
|
| What would really be interesting of the things mentioned
| in the video linked from the stackoverflow question,
| would be how a perceiver would see the closing scissors
| occur.
|
| If you were 12 light years from the handle end of the
| scissors, and only 2 light years from the tips, if the
| closing motion took a year to occur, you would see the
| point move backwards from the tip to the handles, since
| the light would take longer to reach you to see the
| handles close than the tips.
| d0mine wrote:
| Claiming that something is wrong without an alternative
| theory is not how science is done too
| fastball wrote:
| Not really true. You can definitely falsify someone
| else's hypothesis without positing one of your own.
|
| This is more of how it should be done in
| business/politics - not very helpful to say "this is the
| wrong way to do it" but not propose a better solution. In
| science, however, falsify things is always useful.
| d0mine wrote:
| It is a common misconception. You can't falsify anything
| without your own theory. Some people just can't (at the
| conscious level) recognize their assumptions (if you
| don't see air, it doesn't mean there is none).
| fastball wrote:
| I don't think this is correct.
|
| Hypotheses make predictions. Experimentation can test
| those predictions without providing an alternative
| explanation for the phenomena you are testing.
|
| For example, if I came up with a theory of gravity that
| implied everything should fall towards the earth at the
| same speed, all you need to do to falsify my theory is
| show that this is not the case. You do not need to know
| that wind resistance is the confounding variable to know
| that the theory which suggests everything should fall at
| the same speed is wrong.
| d0mine wrote:
| Again, you can't make an experiment without an underlying
| theory.
|
| I understand some concepts are so ingrained, that they
| are perceived as theories but as reality itself
| (Newtonian physics before the end of XIX century) but
| physics is not constrained by our intuition (what we are
| familiar with).
| fastball wrote:
| Could you provide an example? Because I did so, and you
| repeating exactly what you said earlier doesn't really
| invalidate mine. As far as I can tell, my example is
| valid and invalidates your theory that you need a theory
| to falsify another theory.
| mortehu wrote:
| I think they're talking about the edge of a shadow moving
| perpendicular to the light, as opposed to a shadow moving
| away from the light.
| alfiedotwtf wrote:
| I've always wondered if gravity could be seen as like a flux,
| and measured by bounds vs time.
|
| So if any physicist here could shoot this down, I'd be most
| happy:
|
| Let's say I ran between opposite ends of a basketball court.
| Back and forth, back and forth... what would happen if I
| started increasing my speed, slowly all the way to the speed of
| light?
|
| So my thinking is as you become significantly faster, you
| become heavier, but to yourself, you don't see any change - so
| it's not me who's getting heavier as I start to run faster and
| faster to the speed of light, but the enclosed box of me
| between opposite ends of the court - i.e gravity is the flux of
| a moving object bound between a distance measured over some
| time period? i.e some sort of integral of moving matters
| between a distance, with respect to time.
|
| Sorry for my incorrect science... I was just curious how
| relevant my thinking is
| behindsight wrote:
| > but to yourself, you don't see any change
|
| This hints to me that you might be thinking of the concept of
| invariant vs relativistic mass[0]
|
| > i.e some sort of integral of moving matters between a
| distance, with respect to time.
|
| It might be easier if you intuit it as total energy
| (encapsulates the relativisic aspects) - but that description
| you wrote seems to be another way to rephrase the curvature
| of space-time analogy; you could say that what you are
| describing is retreading how "steep" the bend is, yes?
|
| 0: https://en.wikipedia.org/wiki/Mass_in_special_relativity
| alfiedotwtf wrote:
| Yes? To be honest, I don't know... despite 2 years of
| university physics, we sadly never got to the fun physics -
| relativity and particle physics
| IX-103 wrote:
| I don't think you can think of it as becoming heavier in the
| traditional sense, as the higher speed does not increase the
| force of gravity.
|
| There are several (roughly equivalent) ways to describe what
| happens as you approach the speed of light.
|
| One is that instead of gaining mass, you gain extra inertia,
| making changing the velocity require more force. This makes
| it appears to outside observers like your acceleration slows
| down. I don't particularly like this explanation as it
| doesn't explain why your rate of energy usage appears to go
| down (as that is more a function of your clock). I prefer the
| other explanations:
|
| Another is that your clock slows down relative to the outside
| world so you have less time to apply your force so you need
| more force to apply the same externally visible acceleration.
|
| Another is that the outside world looks shrunk in the
| direction of travel, so again you have less distance to a
| apply your force to drive the externally visible
| acceleration. This is effectively the same as previous since
| distance=time*speed.
| alfiedotwtf wrote:
| Thanks for your explanation! Damn physics is fun since it
| seems intuitive and yet bizarre at the same time
| tomrod wrote:
| So, this is a really cool concept and I look forward to the
| determination if it is true or testable.
|
| I find the "double copy of other forces" verbiage to be difficult
| to follow. Walking through a formula example would be helpful,
| and formaluae exist to communicate exactly this kind of clumpy-
| wumpy-lumpy-timey-wimey awkwardness language clods through in its
| quest to communicate mathematical structures.
| alanbernstein wrote:
| This is my complaint about most of what I read about advanced
| physics. I have enough understanding of math and basic physics
| that it seems within reach for me to follow an equation-driven
| explanation, but the papers are a bit too dense for me. And I
| suppose I'm not interested enough to invest the time to truly
| understand all of the notation.
| pishpash wrote:
| Theoretical physicists are handwavy a.f. to an extent that
| you can't tell a crank from a real one.
| dmix wrote:
| There's a ton of articles about this on every science news site
| and youtube channel.
|
| Here's an old one for example that explains what they are doing
| at Fermi wit the G-2 machine:
|
| https://www.youtube.com/watch?v=O4Ko7NW2yQo
|
| And just check the science sites.
| wrycoder wrote:
| Your link is unrelated to the topic. Do you have something
| pertinent to gravity "double cover" work?
| wrycoder wrote:
| Charlie Wood is no Natalie Wolchover. The article is
| depressingly simplistic - it's something I'd expect from
| Science Alert, not Quanta.
|
| I looked for a better entry into the subject and found this:
|
| https://arxiv.org/abs/1810.08183
|
| And a longer review article:
|
| https://arxiv.org/abs/2003.12528
| canadianfella wrote:
| >clumpy-wumpy-lumpy-timey-wimey
|
| What the heck is this?
| ssivark wrote:
| (Passing comment from a phone, so forgive the brevity.)
|
| The key predictions one typically calculates using quantum
| field theory are scattering amplitudes. Let's consider both
| gravity and one of the other forces in the "weak"/perturbative
| limit -- flat space time and force carriers as (quasi)particles
| Eg: photons, gluons, etc. Now let's compare the algebraic
| expressions for the scattering amplitude in both cases (final
| answer after pages of calculations).
|
| For gluons it turns out there is one piece due to how the
| gluons are moving when they collide (kinematic piece),
| multiplied by another piece for the charge they carry (aka
| color); surprisingly the two pieces have a somewhat similar
| form if you squint (called "color kinematics duality"). Also
| gluons are spin-1.
|
| Now, gravitons don't carry charge, but they're spin-2. If you
| look at the algebraic expression for the amplitude of
| scattering gravitons (after a much more tedious calculation) --
| lo and behold -- it looks like it just has kinematic-like piece
| multiplied twice! (and no color piece.)
|
| This is what is commonly referred to as "double copy" (of
| kinematic term) or "gravity = gauge (force) squared".
|
| This is one of the seminal papers; it's very short, and has
| very few equations (but they're written in a very abstract
| form) -- feel free to stare at them if you like:
| https://arxiv.org/abs/1004.0476
| naikrovek wrote:
| so "double copy" means "squared".
|
| no wonder no one can follow any of this. they intentionally
| obfuscate everything.
| geomark wrote:
| This probably isn't the right place for this question. But the
| right kind of people will be reading this thread so I will ask
| it.
|
| In college physics my teacher insisted that, despite gravity
| being popularly referred to as a force, it is _not_ a force.
| Weight is indeed a force, but gravity is more like a field. I
| understood it like this: Say you have a point mass in an isolated
| system. There is certainly gravity all around that mass, but
| there are no forces anywhere in the system. Not until another
| mass is introduced into the system do you have any forces. It is
| apparent from the formula for force since then you have that new
| mass times the acceleration of gravity.
|
| Or is this just being pedantic and does it even matter?
| cyphar wrote:
| Seems needlessly pedantic to me, since by that definition
| electromagnetism isn't a force either -- it's not possible to
| have a force on a charged object without there being another
| object that creates the field the first object is interacting
| with (and vice versa). In fact, Newton's third law kind of
| implies that it's impossible to have _any_ force in isolation.
| The same formula argument would apply to Coulomb 's law.
|
| Now, is it fair to point out that you can model these forces as
| fields, and that such a model is very useful? Yeah. But it's
| silly to argue that something is or is not a "force" but is
| instead a "field" -- these are mathematical constructs used in
| the models we use for physics. I mean, it's not really accurate
| to say that "forces" and "fields" exist _in the first place_
| (in the sense that the mathematical models themselves are a
| real thing that exist in the world -- they are simply models).
| And in the end it definitely doesn 't matter, neither the maths
| nor the real world cares whether you personally consider
| gravity a force or not.
|
| (I expect this is also the case for the strong and weak forces,
| but I haven't done post-grad physics.)
| derefr wrote:
| EM isn't a force. EM _flux_ is a force. The pedantry is
| important because it tells you which formulae /units are
| compatible with one-another.
|
| Weight (~gravitational force), or EM flux (~electromotive
| force), being forces, can be directly translated into impulse
| in a dynamic system--reduced to an instantaneous acceleration
| upon a mass, and thus into the fundamental units of meters,
| seconds, and grams/mols.
| monster_group wrote:
| IANAP but wanted to say this - Same argument could be made for
| EM force. There is electric field around a charge but only when
| you place another charge the two charges attract or repel as
| per electromagnetic force. That way gravity and EM seem to be
| similar. But gravity does seem to be different as it can be
| thought of as not a force (in agreement with your physics
| teacher). It curves space and time and objects just move along
| the geodesics (straight lines on curved surfaces). So gravity
| gives rise to space time geometry. Once that happens, you can
| stop thinking about gravity as a force. Worth emphasizing again
| - IANAP.
| geomark wrote:
| Right. Just like the force between opposite charges. That's
| how I've always thought of it.
|
| Small nit - I think it is actually called electrostatic
| force, at least when we are talking about charges at rest.
| hexane360 wrote:
| Is there a way to describe electrostatics as objects
| following inertial trajectories through curved space?
| DangitBobby wrote:
| See, that makes sense. I've heard it said before that gravity
| is not a force, and my thoughts were just that it obviously
| is a force. And of course here we are, cutting through the
| pedantry to learn that they were in fact not communicating
| anything actually meaningful with the phrase. You could
| simply say that potential energy and kinetic energy are
| different and it would be just as meaningful as what people
| mean when they say gravity is not a force. It's misleading. A
| single particle creates a gravitational potential that exerts
| nothing on its own. Okay. I say this as someone with a BS in
| Physics.
|
| Thank you for your explanation.
| [deleted]
| _Microft wrote:
| Gravity is what we call it when objects _interact via their
| masses_. Other _fundamental interactions_ are electromagnetism,
| the weak or the strong interaction (often also called "weak
| (nuclear) force", "strong (nuclear) force").
|
| In the static case (i.e. time-independent), there is a (let's
| e.g. focus on electric / gravitational) _potential_. It 's
| spatial change ("gradient" / "derivative in space") is its
| electric / gravitational _field_ which is also the ratio of the
| _force_ on an infinitesimally small charge /mass at the given
| distance to the charge/mass (it has to be negligibly small so
| that it does not influence the field).
|
| So there are few different concepts that usually all get mixed
| up and lead to some confusion. It hope the overview helps a
| bit.
|
| If you want to read more, look up the italic terms in
| Wikipedia.
| agumonkey wrote:
| I thought it was an abstraction trick. Just like they derive
| the electric field of a charge from abstracting away the second
| charge from the classic electric force equation.
| jessermeyer wrote:
| In a Newtonian context, gravity is a force.
|
| In an Einsteinian context, forces are not even discussed.
| Enginerrrd wrote:
| They are, in the context of F=ma where m is the inertial mass
| and an object is deviating from a spacetime geodesic, which
| explains why we feel a force from gravity as we sit here.
| lupire wrote:
| Do you feel a force _from gravity_? I thought you feel the
| electromagnetic force (normal force) of your chair pushing
| you away from the Earth and stopping your free fall.
| DangitBobby wrote:
| If not for the gravitational force, the normal force
| would have nothing to exert. So I guess you feel the
| magnitude of the force from gravity via the normal force.
| tsimionescu wrote:
| Well, if you are currently at rest in relation to the
| surface of the earth, that means you are moving with speed
| c towards the future (the ct direction in Minkowsky space
| time). The shortest path from the past to the future, near
| a large mass like the Earth, is curved towards the center
| of the Earth. So, because of inertia, your trajectory in
| spacetime will be curved towards the center of the Earth.
| However, the surface you are standing on is preventing this
| motion via an electromagnetic interaction.
| im3w1l wrote:
| You can have a gravitational field with waves in it without any
| charges at all. Place a single test particle in this field and
| it be affected by the field
| climech wrote:
| Veritasium made a cool video about this some months ago:
| https://www.youtube.com/watch?v=XRr1kaXKBsU
| alberth wrote:
| This video really helped me understand why it's not a force.
| Also awesome physics channel as well.
|
| https://youtu.be/XRr1kaXKBsU
| prof-dr-ir wrote:
| Others have answered your question rather well. Let me offer a
| perspective.
|
| I believe it was Feynman who said that theoretical physicists
| have a very simple goal: they just wants to predict the future.
| Now this might be impossible in full generality, but in
| controlled circumstances (which we usually call "experiments")
| they can often do a pretty good job of predicting the outcome.
|
| As far as we know, predicting the future requires mathematics
| which is therefore the physicists' main tool. In doing so it is
| practical to give a name, like 'force' or 'geodesic motion' or
| 'particle' or 'wave', to certain mathematical concepts. But in
| my view one should not get too hung up on the precise meaning
| of any of those words, simply because it is not productive if
| the mathematics is already clear enough.
|
| In fact, I think this is the most common misconception for
| people with a passing interest in physics. (See also
| discussions elsewhere on this page...) So allow me to stress
| this: these words really mean very little without the
| equations.
|
| Of course, mine is just a physicist's perspective. I imagine
| your question would be the bread and butter for a philosopher.
| forgotpwd16 wrote:
| Mathematically all interactions can be described as fields.
| Where gravity differs from other interactions/forces is that it
| lacks, according to standard model but hypothesized to exist, a
| carrier (boson). Instead gravity is described by general
| relativity as the result of curved spacetime.
|
| Edit: This is what article is about and how _double copy_ helps
| with this. The previous is described in the article 's second
| section.
| DangitBobby wrote:
| How does it being a consequence of curved spacetime make it
| not a force? When we learn the more fundamental origins
| behind the other forces, will they also cease to be forces?
| It's not helpful, IMO.
| nimish wrote:
| It's a pseudoforce in the same way centrifugal acceleration
| is "fictitious"
|
| It's a consequence of massive objects following geodesic
| motion on a curved space; e.g. To keep an ant on a sphere
| you would need "something" to ensure the ant's velocity
| doesn't lead the ant's position "off" the sphere. That
| something is interpreted as gravity.
|
| Here's a more technical explanation:
| https://physics.stackexchange.com/questions/212167/what-
| is-a...
| lanstin wrote:
| And massless. Light follows a geodesic in GR. Also i
| don't think intrinsic curvature is well explained by the
| ant analogy. The apparent force isn't to stay in the
| manifold but to stay on the geodesic thru space time
| (which is free fall).
| nimish wrote:
| Yep, though massless particles must follow null geodesics
| specifically.
|
| Yeah, the ant analogy isn't the best, though it does show
| off that movement on a curved surface needs "corrections"
| from the movement on a flat surface (and the degree to
| which that occurs happens to entirely characterize
| curvature itself).
|
| If you force the ant to always travel on a great circle,
| then it might be a little more precise :)
| ars wrote:
| It's neither massive nor massless object that follow
| geodesics, it's objects with energy (including the mc^2
| kind).
| nimish wrote:
| Yes, and "massive" and "massless" exhaustively covers
| both, with the special highlighting of massless particles
| which are further constrained to follow null geodesics.
|
| Massive particles don't have that limitation but they
| only travel time-like geodesics (as far as we know).
| ars wrote:
| > with the special highlighting of massless particles
| which are further constrained to follow null geodesics.
|
| Only to an approximation. Photons generate gravitational
| fields, and the planets they travel near are attracted to
| the photons (not much obviously). (See: Kugelblitz)
|
| Gravity does not treat photons specially, the rules are
| the same as for other particles.
|
| For extra credit imagine a Kugelblitz traveling by at
| light speed - it's moving so fast, it doesn't have time
| to add any mass. Yet, mass transited inside the event
| horizon of this object.
| DangitBobby wrote:
| Interesting, thank you.
| forgotpwd16 wrote:
| A force is considered an interaction mediated by a carrier.
|
| When we learn, a _force_ will be described in a way that
| adheres to the new /modified framework.
| DangitBobby wrote:
| How long can that definition of a force have existed? 60
| years? "Carriers" (carrying particles if I understand you
| correctly) only makes sense in the context of the
| standard model. It rubs me the wrong way when a field co-
| opts layperson definitions of something and redefines
| them to be something incompatible with the original
| definition, but I can understand that maybe physicists
| are not interested in helping people maintain what little
| physical intuition can be gleaned from the world around
| them.
|
| But I digress, are gravitons not a carrier of gravity?
| forgotpwd16 wrote:
| There're no gravitons in the standard model and every
| attempt to introduce them has failed. That's the problem.
| Our current theoretical framework describes all
| fundamental forces but gravity.
| tsimionescu wrote:
| Gravity is sort of special compared to the 3 forces in the
| standard model because of its relation to inertia.
|
| For all 3 SM forces, there is a distinction between the
| charge of a particle and how easy it is to move that
| particle.
|
| For gravity no such distinction has ever been measured -
| the "gravity charge" of an object seems to be the same
| thing as its tendency to oppose movement (its inertial
| mass) in any experiment ever conducted, which has led to
| Einstein's observation that gravity seems to be a dual of
| acceleration in a curved space time. No similar explanation
| is required for the other forces, as they are not
| intimately tied to inertia in the same way.
|
| Now, unfortunately general relativity's curved space time
| has never been successfully meshed with QM's Uncertainty
| principle and/or wave function. Most physicists, at least
| particle physicists, do tend to believe that it is in fact
| GR that is limited in its description, since QM (in
| particular QFT) is the most precisely confirmed
| experimental theory ever devised, while GR is extremely
| difficult to work with for making precise experimental
| predictions (Einstein's equations are non-linear, and
| almost all predictions are actually based on linear
| approximations of the equations) - so there is much more
| "room" for GR to be modified at very low scales without
| contradicting the high scale results than the other way
| around.
| cygx wrote:
| Acceleration being independent of the mass of the test
| particle being acted upon only holds true for two types of
| forces: Inertial forces, and the gravitational force.
|
| According to General Relativity, this is no coincidence:
| Free-fall spacetime trajectories ('worldlines') get
| modelled as straight lines ('autoparallels'/'geodesics'),
| indicating an absence of net forces. Apparent gravitational
| forces then arise the same way any of the other inertial
| forces do.
|
| Such an interpretation of gravity is natural insofar that
| it handily explains the measurements of accelerometers,
| which only will measure nonzero values if motion deviates
| from free-fall due to the presence of non-gravitational
| forces.
|
| But note that as is often the case, it boils down to a
| semantic argument about the definition of 'force'. The
| effects of 'fictitious' forces can certainly be as real as
| the effects of 'real' forces (in case of gravity, eg tidal
| heating, or the extreme example of spaghettification, ...);
| personally, I have no issue with calling gravity a force.
| criddell wrote:
| > Mathematically all interactions can be described as fields.
|
| Is there any other way a field can be described? What's the
| physical reality of a field?
| forgotpwd16 wrote:
| Particles as shown in most introductory particle physics
| courses. Because of particle/wave (alternatively
| particle/field) duality the question of whether particles
| or fields are the fundamental constituents remains open but
| the current consensus is the later (fields).
| criddell wrote:
| Describing a field in terms of particles seems circular
| if particles are excitations of a quantum field.
| forgotpwd16 wrote:
| Indeed. That's why it is a hard question to answer which
| is fundamental.
| grae_QED wrote:
| This sums it up [1].
|
| [1] https://www.youtube.com/embed/XRr1kaXKBsU
| jerf wrote:
| In _General Relativity_ , gravity is not a force.
|
| Despite the fact we know that GR is broken and can't be a
| description of reality, even educated physicists seem to
| persist in speaking as if it is absolutely true, even though
| they know better.
|
| See also all the talk about "what happens in a black hole",
| which should almost always be qualified with "in General
| Relativity". When we have a unified theory a lot of the crazy
| stuff, like rotating singularities leading to "other
| universes", will probably disappear.
|
| Is gravity going to be a force in the Unified Theory? Probably.
| I believe it is in our current best candidates. But they aren't
| right either yet, so I don't know.
|
| Edit: Some modders seem to be confused and are probably reading
| this as some sort of criticism of science itself or something.
| It is not. It is literally true. It is not a force in GR, but
| that doesn't make it "not a force", because GR is _known_ to be
| false. It is an exceedingly good approximation to something,
| but we know it is not the underlying truth, which is why we are
| still seeking out a Unified Theory... precisely because GR isn
| 't it. Thus, I find confident proclamations about whether or
| not gravity is or is not a force to be premature. We don't
| know. The thing it is _definitely_ not a force in is known to
| be not the truth.
|
| So to the extent you are confused about why it isn't a force...
| well, stay tuned, because this whole area is ripe for
| reconsideration in the next few decades. There are also
| theories that have attempted to rewrite all of physics as "not
| a force"s like gravity too, turning all forces into geometry.
| While these are not currently favored, IIRC these are the
| orginal physics theories that added rolled-up dimensions.
| String theory built on those.
| teachingassist wrote:
| I say that 'weight is the force due to gravity' - that weight
| is the force and gravity is the abstract concept (in college-
| level physics terms).
|
| Same for 'EM force' and 'electromagnetism'
|
| Conflating the two is a type of 'metonym' that is common in
| every-day and journalistic speech as here. If it's obvious what
| aspect you mean, then it doesn't really matter.
| cygx wrote:
| Nope, that's not what a physicist means when they say gravity
| isn't a force. What is meant is that apparent gravitational
| acceleration can be understood as a consequence of Newton's
| first instead of his second law.
|
| According to Newton's first law, a body on its own (ie with no
| net external forces acting) will continue to move uniformly in
| a straight line.
|
| Now, drag a marker uniformly and in a straight line (from your
| perspective) across a spinning disc. It will trace out a curved
| line, meaning an observer sitting on the disc will conclude
| that a force must have been present as the line would have been
| straight otherwise. We call this particular apparent force the
| Coriolis force, one of the pseudo-forces (aka fictitious forces
| or inertial forces) present in rotating reference frames. Being
| accelerated by such a pseudo-force won't register on an
| accelerometer, and the force vanishes if we analyze the motion
| from an inertial reference frame.
|
| According to General Relativity, gravity is like that, a
| pseudo-force, except that there's generally no frame that can
| make gravitational (pseudo-)forces vanish in an extended
| region.
|
| Taking a differential-geometric perspective, we note that
| there's no inherent notion of "continuing on in the same
| direction" on arbitrary manifolds. We need additional structure
| such as a covariant derivative, which gives us a notion of
| velocity change along a trajectory. Gravity hooks into that,
| with bodies in free-fall moving in 'straight lines' according
| to the Levi-Civita connection of spacetime.
| dilippkumar wrote:
| > ...Being accelerated by such a pseudo-force won't register
| on an accelerometer...
|
| Just checked the accelerometer on my phone. It reads around
| 9.8m/s/s pointing downwards.
| DarmokJalad1701 wrote:
| If you were in free-fall, (preferably in a stable orbit and
| not falling off a tall building) and checked your
| accelerometer, you would read zero. Even though you are
| still being accelerated by gravity.
| m4r35n357 wrote:
| _You_ are providing that acceleration. You need to drop it!
| cygx wrote:
| Assuming you're standing on the surface of the earth,
| that's wrong: You're being accelerated upwards, not
| downwards. Cf the introductory paragraph and "Physical
| principles" section of
| https://en.wikipedia.org/wiki/Accelerometer
| dilippkumar wrote:
| Sorry, I interpreted the - sign in the opposite
| direction.
|
| Either ways, the accelerometer is registering something.
| If gravity was purely a 'pseudo force' as GGP said, I
| should've expected a 0 reading.
|
| Am I misunderstanding the point made about the
| accelerometer?
| roywiggins wrote:
| The feeling you have of force is due to resting on
| something that's holding you up. If you take that away,
| you would be in the same gravitational field but feel
| _no_ forces on you and your accelerometer would measure
| 0m /s/s.
|
| If you jump off a bridge, during your brief flight you
| could look at your accelerometer and see zero. The
| gravitational influence on you hasn't changed at all by
| jumping off a bridge- the only change was removing the
| upward force that stopped you from falling.
| o-__-o wrote:
| Or record your screen and drop your phone from a second
| floor balcony to see what the accelerometer reads
| cygx wrote:
| You're at rest relative to Earth's surface. From the
| perspective of that particular frame, the gravitational
| (pseudo-)force gets balanced by the normal force. But the
| gravitational force is 'actually' zero, leaving the
| normal force accelerating you upwards to be measured by
| your accelerometer.
| zackees wrote:
| The gravitational force is zero, but it balances out with
| a real force which is not zero?
| [deleted]
| chronial wrote:
| You misread. Your phone is accelerating upwards at
| 9.8m/s^2. That is also what it displays. If you have an
| accelerometer that displays a history (I recommend the app
| phyphox), you will also see that acceleration while your
| phone is falling.
| wrycoder wrote:
| The phone will measure zero acceleration while it's
| falling.
| zackees wrote:
| If we replace gravity with magnetism can we make the same
| conclusion? How about the forces experience from title
| forces. The difference in gravitational pull on the moon
| (tidal forces) caused the moon to be locked in orbit with
| the earth. If gravity isn't a force, what stopped the
| moon from rotating faster than its orbit?
| whoopdedo wrote:
| Any skydivers wish to confirm this? But the wind would
| push on the phone. So any lunar skydivers wish to
| confirm?
| DarmokJalad1701 wrote:
| A better analogy is something that is in a stable orbit.
| chronial wrote:
| Apparently I forgot to type the "go away" that was
| supposed to go between the "acceleration" and "while" in
| that sentence ^^.
| jon_richards wrote:
| That's because you aren't in free fall.
| ctdonath wrote:
| If I step off a bridge, why does the distance between me
| and the very large nearby spherical mass begin decreasing
| at an, umm, accelerating rate?
| gumby wrote:
| Compared to the radius of the earth the Bridge is on the
| surface of the earth. In other words you don't travel far
| enough / long enough to observe any difference in g.
| babesh wrote:
| The surface is accelerating up at you relative to the
| gravitational field.
| mrow84 wrote:
| What happens if two people jump off bridges on opposite
| sides of the earth? How does that work in that
| interpretation?
| vincent-toups wrote:
| Whether it is pedantic depends on your goals and perspective,
| but if you are interested in a history and philosophical
| development of Force that culminates in the observation that
| gravitational forces are fictitious, I can heartily recommend
| "Concepts of Force" by Max Jammer.
| TchoBeer wrote:
| Isn't the same true of electromagnetism?
| throwaway316943 wrote:
| Someone needs to do a better visual explanation of these particle
| interactions. If you can describe it mathematically then surely
| you can create a computer simulation.
| 317070 wrote:
| People can and do make computer simulations of particle
| interactions? It's just that 12 dimensional PDE's are not
| exactly fast, but we can e.g. use them to compute the dipole
| moment of fundamental particles like muons.
|
| Think of it like knowing the rules, but the rules not being
| efficient to compute with.
| throwaway316943 wrote:
| 3D rendering was unbelievably slow for a long time too, the
| good thing about computers is they have storage so you can
| render 1 frame an hour if you want and just record it and
| play it back later.
| hirako2000 wrote:
| Exactly my thought while reading the prose. A picture is worth
| a thousand words, a formula a thousand pictures. Yet having a
| bit of the three kinds is often needed to easily comprehend the
| concept for those who are neither particularly visual, logical,
| or patient.
| bfvjkfmrmrm wrote:
| Quantum simulations ran on classical computers have exponential
| complexity class.
|
| So it's only doable for tiny N.
| throwaway316943 wrote:
| Even a visual demonstration with a few particles would help.
| I'm sure the statistical calculations could be visually
| represented as well.
| roywiggins wrote:
| Well, there's Feynman diagrams. The problem is that there's a
| combinatorial explosion that severely limits certain
| simulations. Apparently this double-copy procedure can be used
| to cut down on the combinatorics, which is why physicists are
| excited about it.
| stared wrote:
| Visualizing particles in an easy way is HARD.
|
| To the point, I've founded a startup precisely for that:
| https://quantumflytrap.com/
| gambitown wrote:
| What you have linked is just bog-standard entanglement
| simulations. There are a dozen free software apps to do that.
| It's basically just linear matrix algebra with complex
| numbers.
|
| What the parent meant was _real_ particle simulation, for
| example simulating the collision between two electrons or
| computing the dipole moment of a muon, from fundamental
| standard model constants. It requires numerically solving 12
| dimensional PDE 's. That's much more complicated than
| entanglement simulations.
| stared wrote:
| > There are a dozen free software apps to do that.
|
| Put your links when your mouth is. :) There is a handful of
| interactive simulations for qubits. I don't know any other
| interactive simulation for quantum many-particle systems.
|
| > It's basically just linear matrix algebra with complex
| numbers.
|
| It always has been. For Quantum Field Theory it gets much
| more complicated, and classical simulations are way to slow
| to simulate most systems. Quantum computers (or even
| quantum simulators) are likely to change the game. Anyway,
| being able to simulate does not result in being able to
| visualize in a meaningful way.
| gambitown wrote:
| > Put your links when your mouth is
|
| List of available QC simulators grouped by programming
| language: https://quantiki.org/wiki/list-qc-simulators
|
| I count more than 40 QC simulator projects. All open
| source. There is an abundance of quantum computer (QC)
| simulators, as opposed to a paucity of quantum field
| theory (QFT) simulators (most of them written in Fortran,
| yuck).
| stared wrote:
| QC simulation (qubits only, no dedicated vis) is much
| easier. You just make a vector, and multiply it by
| matrices. Here for Python interface I like QuTiP even
| more than Qiskit (though, I might be biased, as I
| contributed to the former). For interactive, IBM Quantum
| Computing Experience and Quirk
| (https://algassert.com/quirk).
|
| As "easier" I mean that 2^10 is still managable, as you
| can use dense vectors and operators. For, say, this
| quantum optics simulation you get roughly 1000 dimensions
| per particle. For 3 particles it is 10^9. Don't even
| think about dense vectors, let alot - dense operators.
| Sparse operators are not enough, as even identity is big;
| so for any real-time-ish simulation there are
| considerably more tricks and methods.
|
| Simulting QFT you need to deal with much more complexity
| (including continuous dimensions). Instead of numerically
| pretty much you need to solve some intergrals. (Side
| note: I wanted at some point to write an interactive
| editor of Feynman diagrams, turning it into formulae and
| integrating.)
| ddbb33 wrote:
| "someone"
| hungryforcodes wrote:
| "somebody"
| forgotpwd16 wrote:
| Particle interactions are visualized with Feynman diagrams
| which can be used for mathematical calculations (ref:
| arXiv:1602.04182). Do you mean something better than them?
| floatingatoll wrote:
| I'd like to learn more about this symmetry. Does anyone recommend
| a good article (or paper) about this?
|
| > _Researchers note that electromagnetism, the weak force and the
| strong force each follow directly from a specific kind of
| symmetry -- a change that doesn't change anything overall (the
| way rotating a square by 90 degrees gives us back the same
| square)._
| srl wrote:
| I'm _pretty_ sure that this is referring to gauge symmetry, so
| that's the term to search for.
|
| I dunno of a good article as an introduction. As a grad
| student, I found Terry Tao's explanation [1] rather helpful,
| but of course it has a strongly mathematical flavor.
|
| [1] https://terrytao.wordpress.com/2008/09/27/what-is-a-gauge/
| floatingatoll wrote:
| Thanks! It sounds like the next step is to look for curvature
| transformations associated with terms used for each of those
| three. (If someone knows more, always open to hear that too.)
|
| Also, a textbook is revealed on what seems to be this exact
| topic. Does this seem right?
| https://physicstoday.scitation.org/doi/pdf/10.1063/PT.3.2421
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