Why gravity?

[Jeorbit]

Hi guys,

Do we actually know what makes gravity?

I guess I understand the difference between Newton's approach (he describes it, predicts how it will behave, but does not explain it) and Einstein's (he actually offers an explanation on why it behaves as such through the curving of spacetime by mass).

But do we know *why* mass does attract mass that way? I've seen that trampoline example on Youtube, but I do not find it totally satisfying : while it does a really nice job helping to visually understand what happens, the balls actually move like they do because of Earth's gravity and the physical proprieties of the nylon cloth, so I feel I'm back to square one!

I've read up about quarks and energy, up to where my level of understanding could take me (admitly not far, not being schooled in physics), but yet I can't grasp what is the medium of that gravity force thing.

What makes two masses react when in presence of each other. What actual physical relationship is there between the two ? Why mass would "fall" in a spacetime curvature?

Just like we can observe and mesure different kind of wavelengths and particules and their physical effects, can we do the same with what occurs between two masses mutually attracting themselves?

From what I understand Einstein's offers a real good and coherent explanation through spacetime warping, but does not tell us *why* mass react that way to the curvature.

Anyway, that was the rambling of a non-math/physics guy trying to understand

 

[Artlav]

This is mostly a philosophical question.

Science describes. The general relativity is a set of equations that describe how massive objects interact. These predict what would happen to an object near one, essentially telling us how gravity works. It's the best description we have right now.

"Why" in that context would be a recursive question - sooner or later you'd reach a theory of everything, which is just a certain equation, and the question becomes "why the universe works like that and not some other way", which is either answerable by the deity of your choice or not answerable at all if there isn't any.

We don't, however, have such a universal equation yet - general relativity and quantum mechanics don't mix too well, despite being the best descriptions in their relevant fields. Getting a correct theory of quantum gravity is an open problem in physics, and would likely answer some of the specific questions you asked.

TL;DR: We have no idea.

 

[Linguofreak]

From what I understand Einstein's offers a real good and coherent explanation through spacetime warping, but does not tell us *why* mass react that way to the curvature.

Why mass reacts that way to curvature is actually fairly simple to explain: take a simple position versus time graph drawn on a flat sheet of paper. Two objects moving at the same velocity will maintain a constant distance between them, and will be represented by parallel straight lines on the graph, and will meet the X axis at the same angle. Two objects with a relative velocity between them will be represented by lines that meet at an angle, and will make different angles with the X axis.

Now imagine drawing your velocity-time graph on a sphere. Draw two lines (great circles, actually, in this case) that meet the equator at the same angle (that is, start out with the same velocity). The thing is, there are no parallel straight lines on a sphere, all great circles eventually meet! So our two objects that start out with the same velocity will eventually run into each other at some angle, meaning that they're now moving with respect to each other. The geometry of General Relativity is somewhat different, but the idea is the same: curved spacetime means curved time which means that objects moving at a "constant velocity" under their own inertia in the presence of mass will develop velocities with respect to each other.

The real question that nobody really knows the answer to is "why does mass create curvature in spacetime". Is there in the equations that it does, but nobody knows why.

 

[Jeorbit]

Thanks all for the clarification! So much remains to be discovered, this is actually quite exciting !

The real question that nobody really knows the answer to is "why does mass create curvature in spacetime". Is there in the equations that it does, but nobody knows why.

Indeed, I guess that was the big question behind all this :thumbup:

 

[Thorsten]

Indeed, I guess that was the big question behind all this

Actually, I would argue the question is misplaced.

If you look at any elementary particles, say the muon and the up-quark - what is it that defines their identity? Why can we say they're different? What is it that makes an up quark not a muon?

We can say this because they interact with each other in different ways. The muon will interact with photons (so we can note that it is charged), so will the up-quark, but in addition the up-quark will interact with gluons, so it has color charge.

So one part of what makes the 'identity' of a particle is defined by whether is has charge or color charge - and the charge is what makes it interact a certain way.

Mass is nothing but 'gravity charge' - we usually don't call it that way, but it's nothing but the strength of the coupling to a graviton field, i.e. gravity.

So, the question should not be 'why does mass bend spacetime' - because we define mass to be the property of an elementary particle to bend spacetime, just as we define color charge as the property to interact with color fields (gluons) - that's what we mean when we assign a mass to a particle. It's one of its five possible modes to interact (besides gravity, there's the electromagnetic, the weak, the strong and the Higgs force).

 

[Jeorbit]

Mass is nothing but 'gravity charge' - we usually don't call it that way, but it's nothing but the strength of the coupling to a graviton field, i.e. gravity.

But isn't graviton an hypothetical explanation itself? Or maybe I'm just missing the point entirely But indeed now that you mention it, I've read about mass being energy.

 

[Urwumpe]

But isn't graviton an hypothetical explanation itself? Or maybe I'm just missing the point entirely But indeed now that you mention it, I've read about mass being energy.

Exactly - it is trying to explain gravity by the tools of the Standard Particle Model. Of course, with some limitations. But its hard to research this interaction.

 

[LordCroussette]

Wow, pretty hard question !

I dont know if we will be able to know the answer one day.

I might sounds really dumb, but maybe atoms works like little magnets ? When there is one, it is not effective, even if it can affect another one, but if they are combine to make an object, maybe they have "more power" so they attract more. An more massive atoms can attract more.

I am using magnets as a comparaison, an exemple. I dont say that each atom has is own magnetic field but more massive they are, more powerful they are.

Like Artlav said, it is pretty much philosophical at this point.

 

[Linguofreak]

So, the question should not be 'why does mass bend spacetime' - because we define mass to be the property of an elementary particle to bend spacetime, just as we define color charge as the property to interact with color fields (gluons) - that's what we mean when we assign a mass to a particle.

Not true. With other forces we can just define the appropriate charge to be whatever property causes a particle to interact with that force, but mass has other significance than just gravity: it is also what determines the inertia of a particle, so the question *should* be "why does mass bend spacetime?", because we don't know why the property that determines a particle's inertia also causes it to bend spacetime.

 

[Andy44]

Wow, pretty hard question !

I dont know if we will be able to know the answer one day.

I might sounds really dumb, but maybe atoms works like little magnets ? When there is one, it is not effective, even if it can affect another one, but if they are combine to make an object, maybe they have "more power" so they attract more. An more massive atoms can attract more.

I am using magnets as a comparaison, an exemple. I dont say that each atom has is own magnetic field but more massive they are, more powerful they are.

Like Artlav said, it is pretty much philosophical at this point.

But then magnets, how do they work?!

 

[LordCroussette]

But then magnets, how do they work?!

 

[Urwumpe]

But then magnets, how do they work?!

Movement of the charged electrons at non-zero temperatures resulting in magnetism? With some atoms having the right electron orbit configuration that the magnetic fields are not cancelling each other out?

 

[Thorsten]

Not true. With other forces we can just define the appropriate charge to be whatever property causes a particle to interact with that force, but mass has other significance than just gravity: it is also what determines the inertia of a particle, so the question *should* be "why does mass bend spacetime?", because we don't know why the property that determines a particle's inertia also causes it to bend spacetime.

That's the whole point of spacetime bending in GR that inertial and gravitational mass are the same phenomenon.

It's like electric and magnetic fields - you may ask the question that how is it that particles with electric charge cause both electric and magnetic fields - and the answer is that it's the same field seen from a different perspective.

Same with gravity - inertia and gravitational force are fundamentally the same phenomenon seen from different perspectives.

So perhaps the real question is - why would we see electric and magnetic fields as different phenomena? Why space and time? Why inertial and gravitational mass? Why do we not see nature as it actually is more easily?

But isn't graviton an hypothetical explanation itself?

There's a gravition as a weak-coupling expansion of gravity to leading order perturbation theory - just as there's a gluon or a photon.

If you then actually look into the nuts and bolts of how quantum field theory works, there is no such thing as a gluon any more - what things are depends on the resolution scale you look like, they all contain infinite probabilities - if you scatter of something you might call a photon at high resolution scale, you might get an interaction with a gluon instead - because the photon always contains the possibility that it temporarily fluctuated into a other stuff and a gluon.

Of course what we mean by 'photon' or 'gluon' in that context is a purely operational definition which has little to do with any reality, because to write down these concepts you typically invoke asymptotic in and out states (which are never there in reality) and an empty vacuum state (which also isn't there in reality).

So anything in quantum field theory has a very ephemeral existence, nothing is quite real in the sense you'd usually use the word, it's all idealized definitions with whole towers of alternative probabilities attached to them.

The difference with the gravition is that no one knows how to calculate it beyond leading order - but that's a deficiency in math, to leading order it is as well defined as the photon and allows to compute how two muons interact via gravity. And since we know that they do, there's nothing hypothetical about it.

 

[Linguofreak]

That's the whole point of spacetime bending in GR that inertial and gravitational mass are the same phenomenon.

Inertial mass and *passive* gravitational mass (the reaction of a particle to another particle's gravity) are necessarily the same in GR. Inertial mass and *active* gravitational mass (the tendency to bend spacetime / generate a gravitational field) are not necessarily the same in GR, although when we actually observe the universe that is what we see. But a universe with no gravity whatsoever is imaginable, as is one in which there was some charge separate from inertial mass that bends spacetime.

It's like electric and magnetic fields - you may ask the question that how is it that particles with electric charge cause both electric and magnetic fields - and the answer is that it's the same field seen from a different perspective.

Not really: Inertia and response to gravity aren't just the same thing in different reference frames, they're the same thing in the same reference frame. And inertia and spacetime bending aren't the same thing in any reference frame (from what we presently know about gravity), they just happen to seem to be generated by the same property of a particle (from every observation we've made so far) for reasonswe don't understand.

So the electromagnetic analogy doesn't work with the correspondence between inertial and passive gravitational mass because the fit between the two concepts in the case of gravity is much closer than in the case of EM, and it doesn't fit the correspondence between inertial and active gravitational mass, because the foot between the two concepts is much looser for gravity: it happens to be true observationally, but nothing in our current model of how gravity works says it has to be that way. (Now, I don't know what various quantum gravitating theories say about whether active gravitational mad has to correspond to the other two, but we haven't managed to figure out how to construct a mathematically consistent theory of quantum gravity yet).

---------- Post added at 09:00 ---------- Previous post was at 08:46 ----------

But isn't graviton an hypothetical explanation itself?

It's not so much an explanation as something that's needed to make gravity consistent with quantum mechanics. We know that gravitational waves exist, and in QM every wave must be quantized, thus there must be a quantum of gravity, which we call the graviton. Unfortunately, we don't yet know how to construct a mathematically consistent theory of gravity involving gravitons, so we don't know how to make General Relativity and Quantum Mechanics consistent with each other, even though each accords very well with observation in its own field of applicability.

 

[Thorsten]

But a universe with no gravity whatsoever is imaginable, as is one in which there was some charge separate from inertial mass that bends spacetime.

Yeah, except the geometrized picture of spacetime (compelling though it is) maybe just another red herring, because there's other ways to derive the laws of GR without resorting to geometry.

So it seems likely (to me at least) that fundamentally it's 'just another' interaction.

But, as you say - lots of things are imaginable.

Anyway - I hold to my analogy, but accept your caveat that it's imaginable that it is ultimately wron (the remarkable coincidence being so remarkable that I believe there's an underlying reason for it which we just don't understand yet, rather than a chance coincidence...)

 

[Keatah]

Science seems to explain the effects and behavior and characteristics of gravity. But it doesn't tell us how it works or how to interface with it beyond using the tired terms of "mass" and "space-time".

In complex things there are often multiple ways of describing what is seen. All can be accurate because they are simply observations, not explanations of how they work.

 

[Thunder Chicken]

But then magnets, how do they work?!

I remember asking this question to a physics professor in college. He launched into the lecture of electron orbitals and fields and such. I interrupted him and said he was describing the mathematical model that predicts the force, and I then asked HOW do they work?

His response: "Oh, that. Damned if I know."

He was a great professor.

 

[Thorsten]

I then asked HOW do they work?

Actually - what would you accept as a valid answer to how something works if it's not math?

(genuinely curious - I think this is a difficult question, I have some ideas of my own, but I'd rather hear yours first).

 

[Jeorbit]

Love the discussion! You guys lost me long ago, but this thread tells quite a lot about the OF community :tiphat:

 

[dgatsoulis]

I remember asking this question to a physics professor in college. He launched into the lecture of electron orbitals and fields and such. I interrupted him and said he was describing the mathematical model that predicts the force, and I then asked HOW do they work?

His response: "Oh, that. Damned if I know."

He was a great professor.

This reminded me of Feynman's answer on magnets in a BBC interview.

[ame="https://www.youtube.com/watch?v=wMFPe-DwULM"]Feynman: Magnets FUN TO IMAGINE 4 - YouTube[/ame]

As far as I can tell, the HOW questions can be answered with math.
The WHY questions are much more difficult.