[HN Gopher] If gravity isn't a force, then why does it "need" a ...
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If gravity isn't a force, then why does it "need" a boson?
Author : thunderbong
Score : 114 points
Date : 2024-03-20 02:03 UTC (20 hours ago)
(HTM) web link (old.reddit.com)
(TXT) w3m dump (old.reddit.com)
| johncarlosbaez wrote:
| 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 wrote:
| Help me with the last bit, where the perturbations when
| quantised predict spin-2 bosons.
| CamperBob2 wrote:
| 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 wrote:
| 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 wrote:
| 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 wrote:
| 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.
| gwd wrote:
| 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)?
| gwd wrote:
| (Replying to myself)
|
| I guess one answer to this is that particles which are
| (supposedly?) massless, like the photon, are affected by
| the space-time warping effects of gravity. A parallel
| construction wouldn't be true of magnetism or an electric
| field. Furthermore, when we detect gravity waves, they come
| at the same time as corresponding gamma ray bursts; since
| the gamma rays are affected by the space-time warping
| effects of gravity, this means that _the gravity waves
| themselves_ are affected by the space-time warping effects
| of gravity.
|
| So gravity probably is something else. But who knows!
| pdonis wrote:
| _> the acceleration meter won 't measure any force because
| gravity is acting on every part of it uniformly_
|
| No. The acceleration meter won't measure anything because
| _there is nothing to measure_. An object in free fall is in
| free fall; there is no "gravity" acting on it at all. It's
| just as if the object were floating out in deep space, far
| from all gravitating bodies. That's the point of the
| equivalence principle.
|
| _> 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_
|
| Yes. Electromagnetism, weak, and strong interactions all
| make the acceleration meter register nonzero, even if the
| act "equally" on all parts of an object.
| hammock wrote:
| In other words, there is no gravity field, in the way
| there is an EM field that propagates these forces. Or,
| the "gravity field" is the fabric of space-time itself
| pdonis wrote:
| Yes.
| BoiledCabbage wrote:
| > The acceleration meter won't measure anything because
| there is nothing to measure.
|
| Put this way, isn't it almost begging the question? In GR
| the definition of acceleration is movement in contrast
| with the movement of gravity. If course gravity will
| never meet this criteria - all movement due to gravity
| will be aligned with movement due to gravity.
|
| If instead we had a universe where instead of all matter
| having a gravitational effect, it was that all matter had
| a magnetic effect the we'd see no acceleration due to the
| magnetic effect and gravity would "produce a field" and
| cause acceleration in your above examples.
|
| You can't use a gravitational biased tool to proclaim
| gravity is a neutral actor and everything else is a
| field.
|
| It seems like more accurately, everything is
| "gravitationally charged", so instead we say it warps
| spacetime, but really is no different.
| pdonis wrote:
| _> Put this way, isn 't it almost begging the question?
| In GR the definition of acceleration is movement in
| contrast with the movement of gravity. _
|
| No, it isn't. You have it backwards. The _definition_ of
| acceleration in GR is proper acceleration, i.e., what an
| accelerometer reads. The "movement" property is then a
| _consequence_ of this plus picking an appropriate frame
| of reference.
|
| _> If instead we had a universe where instead of all
| matter having a gravitational effect, it was that all
| matter had a magnetic effect the we 'd see no
| acceleration due to the magnetic effect_
|
| Yes, you would, because unlike gravity, magnetism does
| not obey the equivalence principle, so differently
| charged objects in the same magnetic field with the same
| initial conditions can have different motions. With
| gravity, _all_ objects in the same field with the same
| initial conditions have the same motion, regardless of
| their mass. That is _why_ it is possible to model gravity
| using spacetime curvature, and that property is unique to
| gravity.
| Ma8ee wrote:
| > Yes. Electromagnetism, weak, and strong interactions
| all make the acceleration meter register nonzero, even if
| the act "equally" on all parts of an object.
|
| I'm genuinely curious how that acceleration meter would
| work. There won't be any internal forces as a consequence
| of the external field and no relative motion.
| pdonis wrote:
| _> I'm genuinely curious how that acceleration meter
| would work._
|
| Look up how the one in your phone works. It reads nonzero
| when you are standing on Earth because of electromagnetic
| repulsion between your atoms and the atoms in the floor.
|
| _> There won't be any internal forces_
|
| Yes, there will, because the object's internal state
| (including its shape, size, and internal stresses) when
| it is accelerated is _different_ from its shape when it
| is in free fall. Why? Because the acceleration sets up
| internal forces in the object that result in a different
| equilibrium from the one it was in while it was freely
| falling.
| itishappy wrote:
| > Electromagnetism, weak, and strong interactions all
| make the acceleration meter register nonzero, even if the
| act "equally" on all parts of an object.
|
| I'm struggling to wrap my head around this assertion. If
| all parts of the object are acted upon "equally" (why is
| this in quotes?) where would this acceleration come from?
| pdonis wrote:
| _> If all parts of the object are acted upon "equally"
| (why is this in quotes?) where would this acceleration
| come from?_
|
| It is proper acceleration, not coordinate acceleration.
| An object can have nonzero proper acceleration even if
| none of its parts are in relative motion. Geometrically,
| proper acceleration corresponds to path curvature of the
| worldlines of the atoms in the object. "No relative
| motion" means all the worldlines of the object's atoms
| have the _same_ path curvature (modulo some
| technicalities that don 't really matter here). It does
| _not_ require that that path curvature be zero.
|
| Physically, a typical accelerometer works by measuring
| the internal _stresses_ that are set up in an object when
| it is accelerated. These stresses put the object into a
| different equilibrium state than it was in when it was
| freely falling: the object 's size and shape can change.
| For typical solid objects at typical Earthbound
| accelerations these changes are too small for us to see
| with our unaided senses--but sensitive instruments like
| accelerometers can detect them.
| Geee wrote:
| Yeah, if every part of human body is accelerated equally,
| there's no way to feel anything (you could see it). No
| matter how hard you get hit or thrown around, you can't
| measure it internally. Each cell can only feel their
| neighbors, and there's no internal stress / force between
| cells.
| saagarjha wrote:
| This isn't true. Try jumping off a building and you'll
| find the acceleration to be quite noticeable ;)
| stetrain wrote:
| Only because you feel the air rushing past, and the
| impact with the ground.
|
| If you do this in a capsule where the air is moving with
| you, and avoid hitting the ground, we call the sensation
| "weightlessness" or "zero g", like is experienced by
| astronauts in orbit.
|
| Gravity is absolutely acting on astronauts orbiting the
| earth, at nearly the same strength as if they were
| standing on the ground. Depending on the shape of the
| orbit their linear speed may be increasing or decreasing,
| and they are definitely experiencing directional
| acceleration as their path bends in a circle around the
| Earth. But internally there is no bodily sensation of
| acceleration. It feels the same as floating, or free fall
| without the air rushing past.
| neolefty wrote:
| The point is you won't feel it (aside from air
| resistance) on the way down.
| 8note wrote:
| The air too, you only feel because the ground and other
| air is pushing it upwards
| ajross wrote:
| 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.
| User23 wrote:
| As an aside there are real gravimeters that aren't scales
| and they are sensitive enough to detect a person walking
| around the room they are in and snow accumulating on that
| building's roof. Yes that's right they detect the
| gravitational force exerted by the snow's mass.
| pdonis wrote:
| _> Yes that's right they detect the gravitational force
| exerted by the snow's mass._
|
| No, they don't. Gravimeters of the type you describe
| measure the coordinate acceleration of a freely falling
| test object in the accelerated frame of the gravimeter.
| In other words, it's the _gravimeter_ (the part that isn
| 't the freely falling test object) that has a force
| acting on it, which makes it accelerate upward (proper
| acceleration--an accelerometer attached to the gravimeter
| reads nonzero), and a freely falling test mass therefore
| appears to accelerate downward (coordinate acceleration
| in the frame of the gravimeter), just as if the
| gravimeter were inside an accelerating rocket out in deep
| space far from all gravitating bodies.
|
| In other words, gravimeters of this type rely on the
| equivalence principle, which is the same principle that
| GR uses to justify the statement that gravity is _not_ a
| force.
| ben_w wrote:
| Rather more pertinently to the question "why is gravity
| different?", we can measure the _time dilation_ caused by
| a gravitational potential change of less than 1cm near
| the surface of the Earth:
| https://physicsworld.com/a/gravitational-time-dilation-
| measu...
| hammock wrote:
| > 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...
| coldtea wrote:
| Well, a changing electric field is not a force tho.
| nyrikki wrote:
| 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.
| trhway wrote:
| > 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).
| ultrafilter wrote:
| Your charge-based g-meter would also measure zero
| acceleration in free fall.
|
| https://en.wikipedia.org/wiki/Paradox_of_radiation_of_charg
| e...
| gnramires wrote:
| 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?
| elashri wrote:
| > 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.
| Karellen wrote:
| > 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
|
| You've just explained "compactified" in terms of being
| "curled up", but that doesn't really help (for me, at
| least). What does it mean for a dimension to be "curled
| up"?
| elashri wrote:
| I'm sorry if I'm confusing. That's the term I use,
| usually assuming that it is obvious (like referring to
| C++ templates to a python developer without explaining
| what is that).
|
| To be honest, it is hard to visualize, as it is
| counterintuitive of what we think of space. While I know
| many people would disagree, I really like the garden hose
| analogy [1]. The idea is to simply imagine a very long
| garden hose. From a great distance, it looks like a one-
| dimensional line. Now you get closer, and realize it has
| a second dimension, which is its circumference curled
| around that seemingly 1-D line. An ant walking on the
| hose can move along its length, but also in a circle
| around it. The extra dimension in KK is like this
| circumference. It is tiny, curled up so we don't directly
| notice it, but still potentially there.
|
| [1] https://www.preposterousuniverse.com/blog/2004/06/30/
| extra-d...
| Izkata wrote:
| This kind of makes me think of a spatial version of the
| Time Cube.
| gnramires wrote:
| Interesting. I ask because I have a suspicion that Quantum
| theories seem more fundamental than General relativity,
| because treating geometry within a quantum framework seems
| very hard and "non-natural", or implausible (e.g. when
| considering superposition of spacetimes!). While if somehow
| gravity is a quantum effect (within a simpler space-time
| framework), that seems much more plausible... but Kaluza-
| Klein captured my interest in the other direction. Although
| I'm still thinking the quantum framework is appears to be
| the correct one, even though the assumptions of GR are very
| strong (so something like the equivalence principle or some
| other notable principle needs to break).
| lubesGordi wrote:
| 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 wrote:
| It's models all the way down.
| nyrikki wrote:
| 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 Godel, 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.
| cthalupa wrote:
| 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.
| pdonis wrote:
| 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.
| trhway wrote:
| >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.
| qnleigh wrote:
| There is an entire sub-field of experimental science that
| involves putting ever larger objects in superpositions of
| different locations. These experiments are no closer to
| testing quantum gravity, but they falsify whatever it is
| you're trying to say here. See
| https://www.nature.com/articles/srep13884 for a random
| example.
| pdonis wrote:
| _> The energy to move the object into a given position is
| an additional element here unaccounted for in your model._
|
| You can set the experiment up so the energy is the same in
| both cases (for example, both positions at the same height,
| just horizontally separated). If you don't, then yes, you
| have to include the effects of the different energies in
| your model.
|
| _> 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._
|
| I'm not sure how this would work if the energies were
| different, since "different" _means_ a different source for
| the spacetime geometry.
|
| But in any case, yes, for such an experiment to be relevant
| at all to the question I was discussing, the spacetime
| geometries being superposed would have to be different.
| bena wrote:
| 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.
| pdonis wrote:
| _> 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._
|
| That is one key principle that drives the effort, yes.
| However, that doesn't mean things will always work out that
| way. Freeman Dyson, for one, published at least one paper
| making arguments for why gravity didn't need to be
| quantized.
|
| _> the forces and factors that work on the smallest
| particle should be the same forces that work on the largest
| of galaxies._
|
| If you mean _fundamental_ forces, then this is true (that
| 's the definition of "fundamental"), but it also means that
| you have to adopt many levels of indirection between those
| fundamental forces and what actually happens with
| macroscopic objects. Or, to put it another way, the models
| we actually use to make predictions can have "forces and
| factors" in them that are _not_ any of the fundamental
| ones, and that 's fine, as long as we have some chain of
| reasoning that connects those models to the fundamental
| forces and factors. For example, our models of macroscopic
| objects can have dissipative forces like friction and
| viscosity in them; those aren't fundamental forces. But we
| have a chain of reasoning that connects them to fundamental
| forces (electromagnetic forces between electrons in atoms).
| rhdunn wrote:
| 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.
| hughesjj wrote:
| Well, bosons also have mass and can distort space.
| Theoretically even the (rest-)massless photon distorts
| spacetime, think it's called a kugelblitz. Also how would
| fermions be the force carriers if they don't physically
| move through space themselves to interact with far away
| fermions, ex gravitational waves? Unless you're advocating
| for a relational model of space which hey I'm all for but
| introduces other issues afaik
| pdonis wrote:
| _> 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's correct. However...
|
| _> That would imply that fermions are therefore the force
| carriers for gravity, not a hypothetical new boson._
|
| That's wrong. Energy/mass-momentum (which can, as hughesjj
| points out, be bosons or fermions or both) is the _source_
| of gravity. The source is not the same as the "force
| carrier".
|
| In GR, gravity has no "force carrier" because it is not a
| force. In the simplest quantum model that has a "force
| carrier" for gravity, the quantum field theory of a
| massless spin-2 field, the "force carrier" is the massless
| spin-2 graviton, which is not the same as any source that
| occurs in ordinary matter.
| me_me_me wrote:
| it would be so much easier to visualize it if we could imagine
| 4th dimension.
| layer8 wrote:
| 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.
| pdonis wrote:
| _> 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 wrote:
| 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.
| pizzafeelsright wrote:
| 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.
| slowmovintarget wrote:
| 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 wrote:
| 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?
| slowmovintarget wrote:
| I don't think I know the math well enough to answer the
| question definitively, but given my surface understanding
| of the Standard Model and GR... I think so (though I
| suspect "energy of coupling" is the wrong way to put it).
| andrewflnr wrote:
| Most mass of massive objects doesn't come from the higgs
| mechanism. It's mostly the relativistic mass (?) of binding
| energy in protons and neutrons. So, I'm not sure the Higgs
| field needs to be involved at all.
| slowmovintarget wrote:
| Thank you.
| mnw21cam wrote:
| 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.
| slowmovintarget wrote:
| I don't think that's the accepted view (that photons have
| mass). https://profoundphysics.com/why-do-photons-have-no-
| mass-simp...
|
| When the velocity is the speed of light, solving the
| equation of special relativity yields _m_ of zero.
|
| We have a successful theory of quantum mechanics which
| integrates with special relativity, so that definition
| holds, even if you take the same approach from an Effective
| Field Theory perspective.
| andrewflnr wrote:
| I'm not enough of a physicist to explain why this framing
| is wrong, but I took enough physics in college to know it
| absolutely is wrong.
|
| Fun fact from my "Modern Physics" homework: a system of
| _two_ photons can have mass, if they 're traveling in
| different directions. In parallel though, they're still
| massless.
| fooker wrote:
| Physicists have been down that path along time ago.
|
| https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_exper...
| elashri wrote:
| > 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_19h wrote:
| 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."
| Zambyte wrote:
| 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?
| pdonis wrote:
| _> 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.
| Zambyte wrote:
| > 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.
| pdonis wrote:
| _> I was hoping for some elaboration._
|
| If you mean elaboration on how the expansion of the
| universe works, I can give that, but as I've said, it's
| irrelevant to this discussion.
|
| _> At least part of the discussion is about gravity being
| (caused by? understood as?) the curvature of spacetime._
|
| More precisely, the curvature of spacetime _around an
| isolated gravitating body_. Which is a very _different_
| spacetime geometry from the spacetime geometry of the
| universe as a whole, which is what cosmologists use to
| account for the observations that lead us to say that the
| universe is expanding.
|
| _> Why would matter curve spacetime, if not for the
| accelerated expansion of spacetime_
|
| You have it backwards. What you are calling "the
| accelerated expansion of spacetime" is an _effect_ of
| spacetime curvature, not a cause of it. Plus, as I said
| above, that spacetime curvature is the curvature of the
| universe as a whole, which is very different from the
| curvature around an isolated gravitating body, that we are
| discussing here.
|
| _> matter resisting acceleration due to inertia?_
|
| This has _nothing_ to do with spacetime curvature at all.
| It would be present even if spacetime were flat.
| pdonis wrote:
| _> I 'm also OOTL on the boson part_
|
| The idea behind that is that, for all other interactions
| (electromagnetic, weak, strong), our Standard Model of
| particle physics tells us that there are "gauge bosons"
| (for electromagnetism it's the photon, for weak it's the
| W+, W-, and Z bosons, and for strong it's the eight gluons)
| that mediate the interaction. The full details of how this
| works involve quantum field theory and are way too long to
| go into here; all we need for this discussion is the basic
| idea that any interaction in quantum field theory has one
| or more "gauge bosons".
|
| So _if_ we are going to try to quantize gravity, the
| obvious way to do it--at least if you are a Standard Model
| particle physicist--is to treat it the same way and have a
| "gauge boson" for it. This has actually been done: the
| quantum field theory of a massless spin-2 field, which is
| what a "gauge boson" for gravity would look like, was
| developed in the 1960s and early 1970s. Furthermore, the
| classical limit of this quantum field theory is known to be
| General Relativity, i.e., the classical theory of gravity
| that we already have and that we already know works. So
| _if_ we want to treat gravity as if it were a Standard
| Model interaction and give it a "gauge boson", this would
| be one obvious way to do it.
|
| The reddit questioner was asking the more basic question of
| _why_ we would want to do that at all--why do we _need_ to
| quantize gravity. If we don 't need to, the fact that we
| _could_ do it using this known mathematical model (there
| are other issues that come into play with that, which one
| of the reddit responses mentions, but leave that aside) is
| irrelevant. Not all mathematical models have physical
| realizations.
| Zambyte wrote:
| Thank you for going deeper. I'm going to give this a more
| thorough look and investigate deeper when I have more
| time in a few hours.
| AnimalMuppet wrote:
| Probably a dumb question, but... why massless? If I
| understand GR correctly, the gravitational field itself
| (especially in the form of a gravitational wave) has
| energy, and therefore contributes to the mass-energy
| tensor, further bending spacetime. Would that correspond
| to a gauge particle with mass?
| pdonis wrote:
| _> why massless?_
|
| Meaning, why is the spin-2 field massless? The easiest
| short answer is, because gravity is a long range
| interaction, and only massless gauge bosons can give you
| a long range interaction. That's why electromagnetism is
| also long range (the photon is massless) but the weak
| interaction is not (W+, W-, and Z are massive).
|
| The strong interaction, unfortunately, breaks this simple
| heuristic (the gluon is massless but the strong
| interaction is short range), because it has other factors
| involved which aren't as easy to explain. But we know
| that those other factors don't come into play with the
| spin-2 field model of gravity, so we can still safely say
| that the graviton should be massless.
|
| _> If I understand GR correctly, the gravitational field
| itself (especially in the form of a gravitational wave)
| has energy_
|
| Only in a certain sense, which is...
|
| _> and therefore contributes to the mass-energy tensor,
| further bending spacetime._
|
| ...not this sense. The stress-energy tensor does not
| contain any "gravity" contribution. In the Einstein
| Field Equation, "gravity" is on the LHS, represented by
| the Einstein tensor, and "mass-energy" is on the RHS,
| represented by the stress-energy tensor. That has to be
| the case in order for local conservation laws to hold.
|
| What all that means is that there is no valid _local_
| concept of "energy stored in the gravitational field"
| (because there is no tensor quantity that corresponds to
| this). But _globally_ , you can still look at a system
| that emits gravitational waves and come up with a
| meaningful concept of "total energy" that decreases with
| time as gravitational waves are emitted. (This concept is
| called the "Bondi energy".) So we _can_ say that
| "gravitational waves carry energy" in this sense. We just
| can't _localize_ where that energy is.
| slingnow wrote:
| You lead with saying you have no background in physics,
| proceed to give an incorrect explanation, and then asked if
| it was wrong. Someone simply told you it was in fact wrong
| -- which apparently didn't meet your expectations. If you
| wanted someone to explain it to you, why not ask for an
| explanation or for critique rather than to _expect_ a total
| stranger to correct all of your understanding out of the
| goodness of their heart?
|
| Why not just come out and state "Someone please offer me
| their valuable time to explain physics exactly at my level
| and at my pace. Thanks!"
| int_19h wrote:
| > why would matter curve spacetime
|
| Perhaps because that's just how matter and spacetime work?
|
| I mean, you could ask the same question just about anything
| in physics. Why is there such a thing inertia at all, for
| example? You can try explaining things in terms of other
| things, but at some point you inevitably have to arrive at
| "it's just how the universe works".
| riperoni wrote:
| 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. G _M_ m/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 wrote:
| 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 wrote:
| 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 wrote:
| I would say gravity is a force, how the heck is this 100lb weight
| a 100lb weight?
| pdonis wrote:
| _> 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.
| lazide wrote:
| 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.
| pdonis wrote:
| _> The force drawing the weight and the mass of the earth
| 'under' it together_
|
| There is no such force. Think of the accelerating rocket:
| there is no "force drawing the weight and the scale
| together". There is just the rocket pushing. The same is
| true standing on the Earth: there is just the ground
| pushing.
|
| One of the reasons we need "spacetime curvature" in GR is
| to explain how the ground can be pushing things in
| _different directions_ in different parts of the Earth,
| while all those things still remain stationary relative to
| the Earth 's center and each other. But the whole point is
| that, even with all that factored in, there is still _no_
| "force drawing the weight and the Earth together". There is
| just the ground pushing up, plus spacetime geometry to
| account for the global configuration. That's it.
| lazide wrote:
| Except that clearly isn't it, as the weight moves even if
| there is no 'ground' (say at altitude) in the same
| predictable fashion, until it collides with something -
| which merely attempts to arrest it's movement. In fact,
| in orbit the forces felt are still (quite clearly) there
| too.
|
| So if anything, gravity wise 'pushing' is not a property
| of gravity at all. Merely of matter which happens to
| produce the spacetime distortions which result in
| gravity. Unless dark matter exists, which would be neat.
|
| Or the moon wouldn't continue to be up there, instead of
| 'down here'.
|
| And clouds of hydrogen atoms will eventually collect
| together in a vacuum barring other outside gravitational
| influences. Which is notably why we even have a star.
|
| And the weight itself also has measurable (albeit
| extremely tiny) such effects on everything else too.
|
| So how do you model/name/account for that force which
| causes that to occur? Since it does clearly exist.
|
| Because a rocket doesn't just 'push' either. We can
| clearly model the chemical and physics behaviors going on
| there to generate that 'push', and all involve forces
| which we can account for. None of them meaningfully
| appear to be gravity.
|
| And gravity also involve forces (spacetime distortions
| causing very real effects!). Which we can also account
| for.
|
| So seriously, what are you even talking about?
| pdonis wrote:
| _> the weight moves even if there is no 'ground' (say at
| altitude) in the same predictable fashion_
|
| "Moves" is frame-dependent. In the weight's own rest
| frame, it is the Earth that moves.
|
| What is _not_ frame-dependent is the fact that, if there
| is no ground and the weight (wrong word, as we 'll see in
| a moment) is freely falling, there is _no_ "weight"--the
| object feels _no_ force and an accelerometer attached to
| it reads zero. This was the basic insight that started
| Einstein on the road to curved spacetime and General
| Relativity: if an object falls freely, it will not feel
| its own weight.
|
| _> how do you model /name/account for that force which
| causes that to occur?_
|
| Using the word "force" here is already wrong as far as
| General Relativity is concerned: in General Relativity
| gravity is not a force.
|
| _> a rocket doesn 't just 'push' either. We can clearly
| model the chemical and physics behaviors going on there
| to generate that 'push', and all involve forces which we
| can account for. None of them meaningfully appear to be
| gravity._
|
| Yes, exactly. Now apply the _same_ principle to the Earth
| pushing up on the weight. We can clearly model all of the
| microscopic behaviors that lead to that push and the
| forces that account for them--and _none of them are
| gravity_.
|
| _> gravity also involve forces (spacetime distortions
| causing very real effects!)_
|
| The "spacetime distortions" you refer to, which do indeed
| cause real effects, are _not_ "forces". That's the whole
| point. They are spacetime geometry. Spacetime geometry is
| not a force. That is what General Relativity says.
|
| _> what are you even talking about?_
|
| I am talking about standard General Relativity, as it has
| been understood and taught in textbooks for decades now.
| What are _you_ talking about?
| MockObject wrote:
| GR agrees (recognizing the obvious caveats) with the
| classical law of F = Gm1m2/r2, where F stands for
| "force". This force is caused by spacetime curvature.
| pdonis wrote:
| No, GR says that the Newtonian law of gravity is an
| _approximation_ that makes reasonably accurate
| predictions when the spacetime curvature is small and all
| relative motions are slow compared to the speed of light.
| It does _not_ say that the Newtonian _interpretation_ of
| that equation is correct.
| Scubabear68 wrote:
| Here is where an have a problem. The ground isn't
| "pushing". The ground is resisting my weight, and may
| even be compressed as a result.
| SamBam wrote:
| If you're just standing in an elevator, how can you tell
| the difference between the floor of the elevator
| "resisting" your weight due to gravity, vs. being out in
| space with silent rockets propelling the elevator at 9.8
| m/s^2 and "pushing" you?
|
| Either way you'll feel like you and the elevator floor
| are being pushed together, and both you and the floor
| will experience some compression.
| Scubabear68 wrote:
| Very true you cannot tell the difference.
|
| That was not the question, though. The earth isn't
| pushing back.
| SamBam wrote:
| I'm saying the distinction between "resisting" and
| "pushing" isn't real.
|
| When a rocket is accelerating and you are pressed against
| the back and being driven forward, is the rocket
| "pushing" you or "resisting" you? Either one is a fine
| way of describing it. The net result, though, is that you
| are squished into the back of the rocket in a way that's
| completely indistinguishable from gravity.
|
| You may say "but the ground can never push me away from
| the ground." Sure, but the back of the rocket can't push
| you away from the back of the rocket either, and yet it
| is clearly pushing you through space. So long as the
| rocket keeps accelerating, you are being pushed forward,
| yet you won't feel it as a push, you'll feel it as an
| attraction to the back of the rocket.
| pdonis wrote:
| _> The earth isn't pushing back._
|
| Yes, it is. The force you feel as weight _is_ the ground
| pushing on you.
|
| By Newton's Third Law, the ground also feels a force of
| equal magnitude and opposite direction from you, which
| can, as you say, end up compressing the ground. But that
| isn't what _you_ feel as weight.
| Jun8 wrote:
| 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.
| slingnow wrote:
| > 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.
| malfist wrote:
| 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.
| pixl97 wrote:
| Heh, just remember this when you hear someone complaining
| about how much LLMs make up and hallucinate. Humans just
| happen to be excellent bullshit generators when it comes to
| topics they are uncertain about.
| CowIsBrown wrote:
| Every year reddit gets dumber. And yet it's still the one
| gem and generally only gem you can find on google.
|
| I miss the forum days. People knew what they were doing.
| Upvotes were supposed to save us and give us top notch
| content, instead it's now making us dumber.
| user3939382 wrote:
| 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.
| Filligree wrote:
| 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.
| bee_rider wrote:
| It seems like the mismatch between technical definitions
| and common ones and/or the nature of mathematical models vs
| our first person experience of the universe is a common
| source of confusion.
|
| I only have an EE's understanding of physics, so just a
| tiny bit of modern physics and some semiconductor stuff.
| But I'm glad to have had the big "it's just a model" reveal
| so I can worrying too much about what physics means.
| creatonez wrote:
| 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.
| cthalupa wrote:
| 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.
| freddealmeida wrote:
| 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.
| pdonis wrote:
| _> 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 wrote:
| > 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 wrote:
| > 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.
| Filligree wrote:
| > 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...
| ck2 wrote:
| 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
| qnleigh wrote:
| 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.
| freddealmeida wrote:
| It doesn't. Modern cosmology is so full of holes. Nothing can be
| determined through experiment.
| cjfd wrote:
| 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.
| SamBam wrote:
| 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 wrote:
| I was wondering exactly that same thing just yesterday!
| Xcelerate wrote:
| 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 wrote:
| Gravity is a compression algorithm, for more efficient processing
| of matter in contiguous batches.
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