Is Gravity Really a Fundamental Force?

[centsworth_II]

Is there a reason in current mainstream physics why gravity must exist as a separate force rather than just as an effect of mass/energy on space? Could it be that there are no gravitons, no gravity field? Could gravity just be an effect that the presence of mass/energy has on space as Einstein said?

The problem of what space is -- if it is quantized, for example -- would still remain, but not the problem of how to quantize gravity.

 

[MeteorWayne]

Huh? Gravity is what you said...the effect of mass on spacetime.

Gravitons are theoretical.

So far, except in various GUT's gravity is not quanticized.

 

[centsworth_II]

I get the impression that the mainstream feeling is that gravity must be quantized in order for a TOE (theory of everything) to work. I don't recall seeing a strong feeling out there that a TOE can work without gravity because gravity does not exist as a real particle/force. I don't know why that is.

By the way, I meant to post this in "Physics".

 

[MeteorWayne]

Seems reasonable, I'll move it there.
MW

 

[ramparts]

Well, the argument that gravity needs to be quantized goes something like this:

-On small scales, everything is quantized (that seems pretty fundamental)
-Gravity can't just stop working on small scales, so it has to be quantized on small scales

More specifically, since on a quantum level matter is described by quantized, and matter creates gravity, gravity would have to be as well.

Now, on a larger scale (and in terms just of what we know for sure), yes, gravity is different from all the other forces in that we can talk about it as an effect of spacetime, rather than an effect that happens on spacetime. But we still call it a force, because that's how it acts

 

[darkmatter4brains]

One reason gravity seems so predominant out in space and on such large scales like between planets, stars, galaxies, etc, is that, on a whole, large objects such as these are electrically neutral for the most part. So, the electromagnetic force does not come into play here. After all, when given a chance, the electromagnetic force is WAY stronger than gravity.

What if the Universe was composed of large objects that WERE NOT electrically neutral? Would the electromagnetic force then be able to act on a scale, whereby it too would curve space-time?

I wonder if anybody has formulated EM theory in these terms?

 

[csmyth3025]

Is there a reason in current mainstream physics why gravity must exist as a separate force rather than just as an effect of mass/energy on space? Could it be that there are no gravitons, no gravity field? Could gravity just be an effect that the presence of mass/energy has on space as Einstein said?

The problem of what space is -- if it is quantized, for example -- would still remain, but not the problem of how to quantize gravity.

The confusion about whether gravity is a "force" or simply a distortion of space-time is still pervasive in popular descriptions and even textbooks. A discussion of the four fundamental forces (renamed "fundamental interactions" in the article) can be found on the Wikipedia site here:

http://en.wikipedia.org/wiki/Fundamental_interaction

The article states in one paragraph:

"...Hence all objects having mass are subject to gravitational force, which works in only one direction: attraction..."

Two paragraphs later the same article makes this statement:

"...Our present-day understanding of gravity stems from Albert Einstein's General Theory of Relativity of 1915, a more accurate (especially for cosmological masses and distances) description of gravity in terms of the geometry of space-time..."

I think it's fair to say that our current understanding of gravity is that it is not a force in the sense that it's a push or pull, but rather that it's just a manifestation of the geometry of space-time where you happen to be. As ramparts points out, this idea works okay most of the time - except when you're dealing with quantum mechanics.

For example, imagine that you're standing on the equator and Earth's gravity suddenly disappears. Discounting air resistance - which would unnecessarily complicate this description - you'd find yourself traveling through the atmosphere, and then through space, in a straight line tangent to the spot on the Earth's surface where you were standing. Your direction would be the same direction as the Earth's rotation and your speed would be about 1,674 km/hr.

Now the fact is that, even while you were standing on the equator (before Earth's gravity inexplicably vanished), you were still going 1,674 km/hr. This is the speed that things stuck in one place on the surface of the Earth at the equator travel due to the rotation of the Earth on its axis. The mass of the Earth "bends" space so that what you think of as a straight line is actually a curved line (a geodesic). This curved line, for the speed you're going, curves down towards the Earth.

As we all know, you don't follow this curved line and plow down toward the innards of the Earth because you're standing on firm ground. The ground you're standing upon pushes back on you and keeps you safely where you are. You perceive this "pushing back" as your weight - which most people attribute to the "pull" (or "force") of gravity.

You might say to me: "If I shine a laser beam at something far away, it doesn't bend - it goes in a straight line. What's up with that?". I would have to say you're right (to just about any attainable degree of measurement). The difference is that photons make up the laser beam. They have no mass and they're traveling at almost 300,000 km/sec. The Earth's "bending" effect on the geometry of the space around it just isn't enough to noticeably deflect the photons. If you ever find yourself "standing" on a neutron star (you'd be squashed flatter than a pancake, to be sure) you could try the same experiment. Then your pancake self would notice through very squinty eyes that your laser beam does, in fact, bend down towards the surface. Your neutron star bends the geometry of the space around it a lot more than the Earth does. Even a beam of light is measurably deflected by that much bending of the space it travels through.

Chris

 

[ramparts]

One reason gravity seems so predominant out in space and on such large scales like between planets, stars, galaxies, etc, is that, on a whole, large objects such as these are electrically neutral for the most part. So, the electromagnetic force does not come into play here. After all, when given a chance, the electromagnetic force is WAY stronger than gravity.

What if the Universe was composed of large objects that WERE NOT electrically neutral? Would the electromagnetic force then be able to act on a scale, whereby it too would curve space-time?

I wonder if anybody has formulated EM theory in these terms?

Well, electromagnetic fields do curve spacetime - but that's part of gravity Gravity is defined as what happens in curved spacetime. It just so happens to boil down to what we traditionally call gravity. Electromagnetic fields curving spacetime doesn't create what we call electromagnetism (that is, it doesn't give you Maxwell's laws).

E&M is E&M. If the universe were composed of objects that were all really electrically charged, the laws of physics wouldn't be any different.

 

[csmyth3025]

One reason gravity seems so predominant out in space and on such large scales like between planets, stars, galaxies, etc, is that, on a whole, large objects such as these are electrically neutral for the most part. So, the electromagnetic force does not come into play here. After all, when given a chance, the electromagnetic force is WAY stronger than gravity.

What if the Universe was composed of large objects that WERE NOT electrically neutral? Would the electromagnetic force then be able to act on a scale, whereby it too would curve space-time?

I wonder if anybody has formulated EM theory in these terms?

Normally, big things in the universe can't accumulate a lot of charge because the electrostatic repulsion between the particles (protons or any atomic nuclei, for instance) would force everything apart.

That said, black holes can have a charge. As I understand it, the three things that define a black hole are its mass, its spin, and its charge. In theory, you could set up a particle accelerator some distance from a black hole and shoot protons at it. After several millennia of doing this you would build up a charge on the black hole because the gravity of the thing is just too much for any kind of internal electrostatic repulsion to overcome. If you keep doing this long enough you'd build up a very big charge on the black hole.

Then you could do some experiments to see what effects such a massive charged body might have.

If you were to do this with two black holes orbiting each other, I expect that the electrostatic repulsion between the two would eventually overcome their mutual gravitational attraction. They would then reach a point of equilibrium where their electrostatic repulsion balances their gravitational attraction. A sort of cosmic "Mexican stand-off".

I suspect that ramparts is right that the extra charge wouldn't change their gravitational effects (except for the added mass of the protons you shot into them).

This would make for an interesting experiment - or at least for some interesting calculations by a theoretical physicist with a lot of time on his hands.

Chris

 

[ramparts]

Indeed, the added charge would have effects that you could call gravitational (the metric tensor, which describes the gravitational field in GR, is different for a charged vs non-charged black hole), but since black holes are SO big, any charged black hole would probably attract opposite charges and neutralize out pretty quickly - unless you set up something funny like Chris's experiment

 

[csmyth3025]

Indeed, the added charge would have effects that you could call gravitational (the metric tensor, which describes the gravitational field in GR, is different for a charged vs non-charged black hole), but since black holes are SO big, any charged black hole would probably attract opposite charges and neutralize out pretty quickly - unless you set up something funny like Chris's experiment

Thanks ramparts, I take your "...something funny..." reference as a compliment. Over the years I've occasionally looked up quotes from notable people just out of curiosity. Your reference jogged a distant memory, which took me a while to look up because I couldn't remember the exact words or to whom they were attributed. My perseverance finally paid off, so here it is:

The most exciting phrase to hear in science, the one that heralds new discoveries, is not 'Eureka!' but 'That's funny...'
Isaac Asimov

Chris

 

[darkmatter4brains]

One reason gravity seems so predominant out in space and on such large scales like between planets, stars, galaxies, etc, is that, on a whole, large objects such as these are electrically neutral for the most part. So, the electromagnetic force does not come into play here. After all, when given a chance, the electromagnetic force is WAY stronger than gravity.

What if the Universe was composed of large objects that WERE NOT electrically neutral? Would the electromagnetic force then be able to act on a scale, whereby it too would curve space-time?

I wonder if anybody has formulated EM theory in these terms?

Well, electromagnetic fields do curve spacetime - but that's part of gravity Gravity is defined as what happens in curved spacetime. It just so happens to boil down to what we traditionally call gravity. Electromagnetic fields curving spacetime doesn't create what we call electromagnetism (that is, it doesn't give you Maxwell's laws).

E&M is E&M. If the universe were composed of objects that were all really electrically charged, the laws of physics wouldn't be any different.

That's not quite what I meant but I didn't explain it very well.

Gravity can potentially be formulated without resorting to space-time curvature. It can be done solely using a QFT wrapped around the Graviton, which would be the quantum of the gravitational field. Feynman was a proponent of this idea. I don't believe he was crazy about the idea of space-time curvature as many physicists aren't, surprisingly. Different math, same predictions.

Anyhow, E&M is currently formulated as a "quantum field theory". The photon is the quantum of that field and it's exchange between particles mediates the electromagnetic interaction. (For example, moller scattering in QED - two electrons exchange a photon, thereby producing what we call Coulomb repulsion in classical theory) Well, could E&M be REFORMULATED (even partially) in a way that doesn't use that concept, but rather uses space-time curvature.

Probably a wacky idea, but you know it's probably been attempted. I'd love to know the history of that attempt, if so.

 

[ramparts]

Same math, actually - Feynman's approach to GR using field theory ended up getting the same equations as Einstein did As for the interpretation... well, that's up in the air One can interpret it as being a field theory on a flat background rather than effects of spacetime, but that gets us into some thorny philosophical questions about spacetime which are probably best left for another thread.

Anywho, if I might get technical, here's the problem I see with that idea: the fundamental field in E&M (the vector potential) is a vector field. Quantizing that gives you a spin-1 particle: the photon. GR, on the other hand, is based on the metric tensor (a fundamental object of geometry), and as a tensor, upon quantization you get a spin-2 particle, the graviton. So there's no analogy there: you can't construct a GR-like theory in which E&M can be described as effects of spacetime curvature since there is no 0,2-rank tensor analogue of the E&M vector potential. Could a more general theory (one not quite like GR) that still describes E&M as spacetime curvature be formulated? Possibly... but it's not at all obvious to me how that would work. The objects one can use to describe curvature in differential geometry are all (as far as I know) either tensors with 2, 3, 4 indices, or scalars.

I have yet to take a class on QFT, so this post is about reaching the limits of my knowledge on the subject, but I think what I just said is more or less right :lol:

 

[darkmatter4brains]

Could a more general theory (one not quite like GR) that still describes E&M as spacetime curvature be formulated? Possibly... but it's not at all obvious to me how that would work. The objects one can use to describe curvature in differential geometry are all (as far as I know) either tensors with 2, 3, 4 indices, or scalars.

That's where I was heading and it's not obvious to me either. But, the last thing I remember us doing in E&M is reducing the 4 Maxwell's Equations in their vector calculus form, down to two equations utilizing two tensors with two indices each - I think they were called the field tensor and the dual field tensor. But, we didn't go any further. Is this just convenient notation, or can more be done utilizing tensors with E&M? Beats me.

I'm not sure there's any motivation to formulate it that way either - E&M works just fine. The best place would be with small masses where the electromagnetic force totally dominates, but E&M as it stands doesn't have any problems here as far as I know. I guess that's where I was heading with the planets full of charge - motivation. But, there are those glaring problems such as the infinite energies for point charges, though.

But, they're trying to quantize gravity to get it to unify with the other forces. Maybe they could space-time curvatize the other forces and get them to fit gravity instead. :lol:

Neat stuff to think about anyhow.

 

[ramparts]

I think the only benefit of formulating E&M that way would be philosophical - it might be a point against the substantivalist position on GR.

You mean this field tensor? http://en.wikipedia.org/wiki/Electromagnetic_tensor The one that allows you to write Maxwell's equations in two equations (one, if you use the right gauge)? I'd imagine if there were a way to make that the dynamical variable in a GR-like theory it would be found, but then this tensor isn't the dynamical variable in E&M (unless I'm mistaken, the gauge potential is). Again, I don't know much of any QFT, but I'm pretty sure E&M doesn't come from quantizing that field, it comes from quantizing the vector potential.

 

[csmyth3025]

It can be done solely using a QFT wrapped around the Graviton

Shucks, folks, you lost me with the "...OFT wrapped around the graviton...". It's nice to know that people with a lot of education are still thinking about the "what ifs".

On a (hopefully) more non-technical plane, is there some upper limit to the charge a black hole can have? I ask this question because of some statements I read in the Wikipedia article found here:

http://en.wikipedia.org/wiki/Reissner-N ... black_hole

The particular statements that puzzle me are the following:

"...These concentric event horizons become degenerate for 2rQ = rs which corresponds to an extremal black hole. Black holes with 2rQ > rs are believed not to exist in nature because they would contain a naked singularity; their appearance would contradict Roger Penrose's cosmic censorship hypothesis which is generally believed to be true. Theories with supersymmetry usually guarantee that such "superextremal" black holes can't exist..."

As far as my ability to understand what the above quote means, it might just as well have been written in Sanskrit. I get the impression, though, that it's saying that there is an upper limit to the charge a black hole can have. If it's possible, can either of you translate this into plain english?

Chris

 

[ramparts]

What it's saying is this: there is no proof (to my knowledge) that a black hole has a maximum charge. But if the black hole's charge exceeds a certain critical value, you'll get a naked singularity, where the singularity remains but the event horizon disappears, so you can see light from objects arbitrarily close to the black hole (hence the term "naked" singularity - you can be wandering around the universe and just bump right into a singularity!). There is a hypothesis called cosmic censorship which states that you can't have these naked singularities, but it's just a hypothesis - believed to be probably true (for various reasons), but not thus far proven. Some theories (called supersymmetric theories) do prove that these naked singularities cannot exist, but supersymmetry has yet to be experimentally verified. So there is possibly a maximum charge for a black hole, but no proof exists as yet

 

[csmyth3025]

What it's saying is this: there is no proof (to my knowledge) that a black hole has a maximum charge. But if the black hole's charge exceeds a certain critical value, you'll get a naked singularity, where the singularity remains but the event horizon disappears, so you can see light from objects arbitrarily close to the black hole (hence the term "naked" singularity - you can be wandering around the universe and just bump right into a singularity!). There is a hypothesis called cosmic censorship which states that you can't have these naked singularities, but it's just a hypothesis - believed to be probably true (for various reasons), but not thus far proven. Some theories (called supersymmetric theories) do prove that these naked singularities cannot exist, but supersymmetry has yet to be experimentally verified. So there is possibly a maximum charge for a black hole, but no proof exists as yet

Thanks,

Your explanation is very helpful (and understandable). I take it that the term "2rQ" refers to twice the radius of "Q" of the black hole and "rs" refers to the Swarzschild radius. I have no idea what "Q" is except that it's related to the charge of the black hole.

I'm in no position to doubt the musings of Roger Penrose (or just about anyone else, for that matter), but I am struck by the similarity of the conviction that "a naked singularity can't exist" with the conviction that even Einstein himself held that "a singularity can't exist". Since the event horizon "hides" the singularity from our view (perhaps, as some think, from our universe), we can't be sure if a singularity does or doesn't actually exist inside it. As I understand it, the laws of physics as we understand them can't be applied within the confines of the event horizon. Correct me if I'm wrong on this.

Chris

 

[ramparts]

Yep - Q is the charge of the black hole! Q is the letter we usually use to represent charge.

The laws of physics can certainly be applied within an event horizon - it's just very near the singularity that things get fishy (i.e., on very small scales where quantum effects become important).

None of this is entirely relevant right now, though. Singularities exist in GR - they're a mathematical part of the theory. What the math actually means in real life we completely don't know yet, but we can certainly say that there is an extremely strong curvature centered on a point where quantum gravitational effects become very important.

Hope that makes sense

 

[EarthlingX]

This is way out of my league, so please excuse me, for using some weird words, but this is totally nagging me:
What is keeping all these curvatures together ?
Do they interact ? Affect one another ? :? :roll:

 

[csmyth3025]

This is way out of my league, so please excuse me, for using some weird words, but this is totally nagging me:
What is keeping all these curvatures together ?
Do they interact ? Affect one another ? :? :roll:

My uniformed understanding is that there's nothing in particular "...keeping all these curvatures together..." We can fall back on the old "bowling ball on the trampoline" analogy: if the trampoline is space and the bowling ball is a big chunk of mass, space (the surface of the trampoline) curves because there's a big chunk of mass (the bowling ball) just sitting there. Although Einstein and a lot of other very smart men have managed to describe this effect on space in mathematical terms (which theoreticians have to study for years just to understand), the general concept that matter distorts space (causes curvatures) is sufficient for most of us non-scientific types to feel we "understand" General Relativity.

Chris

 

[csmyth3025]

Yep - Q is the charge of the black hole! Q is the letter we usually use to represent charge.

The laws of physics can certainly be applied within an event horizon - it's just very near the singularity that things get fishy (i.e., on very small scales where quantum effects become important).

None of this is entirely relevant right now, though. Singularities exist in GR - they're a mathematical part of the theory. What the math actually means in real life we completely don't know yet, but we can certainly say that there is an extremely strong curvature centered on a point where quantum gravitational effects become very important.

Hope that makes sense

I encountered these statements on this Wikipedia page:

http://en.wikipedia.org/wiki/Coulomb

"...A coulomb is then equal to exactly 6.241 509 629 152 65 × 10^18 elementary charges..."

and:

"...Combined with the present definition of the ampere, this proposed definition would make the kilogram a derived unit..."

and:

"...According to Coulomb's Law, two point charges of +1 C, one meter apart, would experience a repulsive force of 9 × 10^9 N, roughly the equivalent of 900,000 metric tons of mass..."

The above statements lead me to believe that, in theory, a kilogram of protons placed one meter from another kilogram of protons would have the same repulsive force as the gravitational attraction of 900,000 metric tons of mass. Is this correct, or am I reading too much into it?

I'm wondering how the unit "Coulomb" relates to "Q" used in the "2rQ = rs" equation I mentioned several posts back. If you think the explanation is too involved for me to understand, just say so - I won't be offended.

On a point of clarification, I believe you said that if 2rQ>rs, the event horizon "disappears". I take it you mean that photons can travel from inside the event horizon to the world outside. I'm guessing that anything with mass going past the event horizon towards the singularity is still on a one-way ticket to oblivion.

Chris

 

[ramparts]

I encountered these statements on this Wikipedia page:

http://en.wikipedia.org/wiki/Coulomb

"...A coulomb is then equal to exactly 6.241 509 629 152 65 × 10^18 elementary charges..."

and:

"...Combined with the present definition of the ampere, this proposed definition would make the kilogram a derived unit..."

and:

"...According to Coulomb's Law, two point charges of +1 C, one meter apart, would experience a repulsive force of 9 × 10^9 N, roughly the equivalent of 900,000 metric tons of mass..."

The above statements lead me to believe that, in theory, a kilogram of protons placed one meter from another kilogram of protons would have the same repulsive force as the gravitational attraction of 900,000 metric tons of mass. Is this correct, or am I reading too much into it?

In theory, yes - but in reality, since protons are charged, you'll be hard-pressed to find a bunch of them together without electrons, which nullify the total charge

The one thing is that that wouldn't be a kilogram of protons - that would be enough protons to make 1 C of charge (which is something like 10^-8 kg of protons).

I'm wondering how the unit "Coulomb" relates to "Q" used in the "2rQ = rs" equation I mentioned several posts back. If you think the explanation is too involved for me to understand, just say so - I won't be offended.

Oh no no, coulombs, charge, this is all very basic and understandable Coulombs are the units for measuring charge, exactly the way that kilograms are the units of mass. "Q" is the letter we use in equations for charge, the way that "m" is the letter for mass.

On a point of clarification, I believe you said that if 2rQ>rs, the event horizon "disappears". I take it you mean that photons can travel from inside the event horizon to the world outside. I'm guessing that anything with mass going past the event horizon towards the singularity is still on a one-way ticket to oblivion.

Chris

Probably, yeah :lol: But in theory, anything can escape if it tries hard enough.

 

[EarthlingX]

I encountered these statements on this Wikipedia page:

http://en.wikipedia.org/wiki/Coulomb

"...A coulomb is then equal to exactly 6.241 509 629 152 65 × 10^18 elementary charges..."

and:

"...Combined with the present definition of the ampere, this proposed definition would make the kilogram a derived unit..."

and:

"...According to Coulomb's Law, two point charges of +1 C, one meter apart, would experience a repulsive force of 9 × 10^9 N, roughly the equivalent of 900,000 metric tons of mass..."

The above statements lead me to believe that, in theory, a kilogram of protons placed one meter from another kilogram of protons would have the same repulsive force as the gravitational attraction of 900,000 metric tons of mass. Is this correct, or am I reading too much into it?

In theory, yes - but in reality, since protons are charged, you'll be hard-pressed to find a bunch of them together without electrons, which nullify the total charge

The one thing is that that wouldn't be a kilogram of protons - that would be enough protons to make 1 C of charge (which is something like 10^-8 kg of protons).

Khm, what about H plasma confined in a magnetic field ? Would that count ? :roll:

 

[MeteorWayne]

Well, if it's Hydrogen plasma, it's made up of an equal amount of electrons and protons, so the net charge is zero