Cause of Gravity? - Gravitons or Curvature of SpaceTime?

[Preserve]

So I'm confused on what is the actual cause of gravity because I heard that gravity is caused by the curvature of space time from the mass of objects. I also hear that gravity is cause by a massless particle called a graviton. Which one is it or do they both play a role in the cause of gravity.

[davidivad]

is it a particle or a wave...

in theory, both should be correct. it is just a matter of whether you need to use a classical approach or not.

gravitons are assumed to exist although they have not been detected yet (it would take a big machine).

consider what is going on in the double slit experiment with photons and you will understand better.

for einstein it is a curve and for the quantum world it is a graviton.

which one suits your calculations better?

actually they need to work out the math a bit better for the graviton because the current math does odd things.

these things take time.

[StringJunky]

I think the graviton is part of the quantum description of gravity which is what scientists want eventually so they can unify gravity with the other three forces (Weak, Strong and Electromagnetic) together mathematically but it's not playing ball so far so GR rules the big stuff and the other three describe the small stuff.

 

For a potential quantum gravity theory (QG) to supersede the current one (General Relativity) it must describe accurately the same as GR where GR is applicable as well as it's description of the other force domains where QG is applicable.

 

In a nutshell, Quantum Gravity is a theory-in-waiting once they've ironed the problems out.

Edited March 20, 2014 by StringJunky

[timo]

The effect of gravity is attributed to curvature in space-time. The graviton is a wave that alters space-time (not quite sure if it only bends and stretches or actually curves). The smallest possible of such waves, in a sense. Often, this means a wave that alters an otherwise-flat space-time. There is, to my knowledge, no known case of gravitational interaction between sub-atomic particles (*). On the known scale, where gravity is observed regularly, gravitons do not play a role. In fact, not even "large" gravitational waves have been detected directly, up to date (**).

 

On the sub-atomic level, we do not really know how/if gravity works. But you will readily get a lot of ideas about it if you ask around. Many of them will involve statements claiming gravitons are the ultimate source that accounts for all curvature. The reason is that this is a direct extrapolation from a simplified perturbative treatment of e.g. electromagnetism. And extrapolating from what has worked somewhat well in other cases is not exactly the worst kind of guess one can make.

 

 

(*) Please note: I do not mean to question the mainstream working hypothesis that there is a tiny interaction. I am just saying that to my knowledge no experimental observation for this exists.

(**) Haven't had the time to read the latest hype about gravitational waves having been detected. But my blind guess is that they detected patterns that can only be described if there are gravitational waves. Not that their sensors actually reacted to a gravitational wave.

[MigL]

If we consider gravity a field, then quantum field theory dictates there exists a boson excitation that 'carries' this field. We call this boson a graviton and it arises in any quantum interpretation of gravity.

 

The general relativistic classical interpretation does not invoke quantum field theory and therefore has no use for a graviton. Gravity is solely based on space-time curvature and could even be considered fictitious, i.e. two parallel lines on a positively curved surface will tend to come together as if attracted to each other by a force ( however all fictitious forces that I know of are frame dependant, and there is no alternate frame for space-time curvature ).

[K Sky]

What remains funny about this subject lays in its weight over the smartest of the smartest. Any answer on any level resembles the responses above. The intelligible facts cannot be raised. In fact, the very first letter of the gravity's equation has not been written. Y/t=x squared and such. Without certainty, I would say humanity measuring the effects of gravity moves minds. However, according to Back to the Future, we should have flying cars by now. Better than even money says we can send folks to mars, but they are stuck there after that. Why? Of course you know, Cape Canaveral and all. Our hardest methodology for breaking gravity’s hold stands no greater than jet propulsion. Muscles work fine also, but alas I cannot jump to the moon, at least without a hook line and sinker.

 

It feels like what reasoned people must have felt ages ago when they asked how the world was created. Some jerkoff would answer, God created the world. Well, what holds us down, and for the curvaturist's arguments sake, what pushes us down? Gravity. What the hell does that mean? Explain yourself! Oh, we cannot even begin to explain “gravitons” or “gravity waves.” What do you mean? The very first letter of the very first equation has not been written. It is simply amazing.

 

Surely for physicists something more complex than gravity’s equation exists. I mean seriously, nothing can go faster than light? Snicker. It might be a little early to proclaim this as fact. Although with respect, the words are usually spoken as, according to Einstein’s theory…

 

A mass of what I want bridges around chasing gravity. What holds us down? I mean you and me. I mean you, me, and society. What the $&*$ is holding us down? Why do life forms cohort like gravity? Where goes the spin man?

 

To gravity’s hold, and humankind’s most pressing search.

[MigL]

What exactly have you added to this discussion other than voicing your frustration at not understanding that nothing 'holds us down' ?

In effect the surface of the Earth holds us UP !!

[Unity+]

What exactly have you added to this discussion other than voicing your frustration at not understanding that nothing 'holds us down' ?

In effect the surface of the Earth holds us UP !!

What is this supposed to mean?

[MigL]

Just what it says.

 

And if you have trouble understanding that, read up on GR theory, as that will also help alleviate some of your frustrations.

 

Don't mean to be a knob, and maybe I misunderstood your post, but GR tells us that the 'cause' of gravity is space-time curvature.

Unfortunately it doesn't tell us why mass-energy causes space-time to curve. Or is this just an explanation gained by moving the goal-posts in your mind ?

Edited March 22, 2014 by MigL

[timo]

I'm not quite convinced that reading up on GR really helps understanding your statement.

[Unity+]

Just what it says.

 

And if you have trouble understanding that, read up on GR theory, as that will also help alleviate some of your frustrations.

 

Don't mean to be a knob, and maybe I misunderstood your post, but GR tells us that the 'cause' of gravity is space-time curvature.

Unfortunately it doesn't tell us why mass-energy causes space-time to curve. Or is this just an explanation gained by moving the goal-posts in your mind ?

Your post doesn't, from what I can tell, explain anything dealing with General Relativity. All it says is nothing holds us down and the Earth holds us up, which sounds crackish like non-sense to me, though in a sense the Earth does hold us up because gravity is acting upon us as the Earth holds us up.

 

EDIT: Had to clarify a few things.

Edited March 22, 2014 by Unity+

[timo]

Your post doesn't, from what I can tell, explain anything dealing with General Relativity. All it says is nothing holds us down and the Earth holds us up, which sounds crackish to me.

Saying that a cup of coffee does not fall down because the table "pushes it up" is quite a common statement, I think. I'm pretty sure at least one of my teachers at school explained it like that (in the context of "the reason why gravitational force does not cause this to fall is because there is another force canceling it). The other half of the statement ("nothing holds us down") is debatable. Sounds more like semantics to me, and I doubt any insight is gained from supporting, discussing or denying the statement (although my school teachers might have had a different view on this ). But calling it "crackish" seems a bit inappropriate to me, especially when considering that it was in reply to a really strange comment.

Edited March 22, 2014 by timo

[MigL]

What I mean is that a free falling object, i.e. in a gravitational field, such as Timo's cup, has no forces acting on it according to GR. The ground, or a table, stops this free fall by exerting a force on the cup. That's not semantics.

 

So what part of my statement would not be clearer with an understanding of GR Timo ? Please elaborate.

[Unity+]

Saying that a cup of coffee does not fall down because the table "pushes it up" is quite a common statement, I think. I'm pretty sure at least one of my teachers at school explained it like that (in the context of "the reason why gravitational force does not cause this to fall is because there is another force canceling it). The other half of the statement ("nothing holds us down") is debatable. Sounds more like semantics to me, and I doubt any insight is gained from supporting, discussing or denying the statement (although my school teachers might have had a different view on this ). But calling it "crackish" seems a bit inappropriate to me, especially when considering that it was in reply to a really strange comment.

I don't see how it is semantics at all.

 

I don't see how calling it "crackish" is inappropriate.

 

 

 

What exactly have you added to this discussion other than voicing your frustration at not understanding that nothing 'holds us down' ?

In effect the surface of the Earth holds us UP !!

Stating that nothing holds us down is false, though the collision with Earth prevents us from "falling" and, therefore, holds us up. Also, what was the comment that it was replying to?

What I mean is that a free falling object, i.e. in a gravitational field, such as Timo's cup, has no forces acting on it according to GR. The ground, or a table, stops this free fall by exerting a force on the cup. That's not semantics.

I think there is a misunderstanding here about the idea of General Relativity and the definition of a force.

 

 

 

strength or power exerted upon an object

Considering that definition, there are two different types of forces, which are the field and contact forces. Gravity is a field force. Therefore, there are forces acting upon the cup.

Edited March 22, 2014 by Unity+

[MigL]

Hey I'm still here !!

 

But seriously, it is not crackish but accepted physics and a little understanding of GR will make it clear.

 

But you are right, I wrongly attributed Kevin D's post #6 to you. I apologise profusely.

[Unity+]

Hey I'm still here !!

 

But seriously, it is not crackish but accepted physics and a little understanding of GR will make it clear.

 

But you are right, I wrongly attributed Kevin D's post #6 to you. I apologise profusely.

What is accepted physics? That there are no gravity acting upon us that holds us to Earth? I don't see where you are getting this conclusion because gravity is a force by accepted physics. I do have an understanding of General Relativity.

[K Sky]

What exactly have you added to this discussion other than voicing your frustration at not understanding that nothing 'holds us down' ?

In effect the surface of the Earth holds us UP !!

 

You are right of course based on General Relativity, and that is an equation defining gravity. You also said we do not know why mass causes space time to curve, which could be because our understanding of space-time's fabric tenures incomplete. That, along with the graviton will likely blend into an original equation, would be my point. Hopefully, with that equation defined, the work to counteract gravity can then begin. The matter does not frustrate me. My statements of our naivete with this matter, considering the giant scope of scientific knowledge we posses, were meant to be facetious. Maybe naivete words too strongly, especially since the corner may be right there. I viewed the question as ultimately theoretical. Anyway, I will remember to keep any dreamy eyed chatter off this channel.

[davidivad]

this seems to me to be mostly a problem with effective communication and part of first year learning.

how you communicate your thoughts must not abandon your audience.

i feel abandoned if i agree with one over the other. is that truly effective communication?

 

one may consider that gravity is a consequence of the curvature of spacetime.

one may also consider that gravity is the result of a field.

this is why my original response stated " is it a particle or a wave."

i might normally consider trying to maintain repore with my readers but, at this time, consider it more important to convey the feeling that you pass off to your readers.

 

my point is that scientists in today's arena need to do more than just argue the point. you must also be everything in between.

please consider your audience.

 

thank you for a proffesional response @Kevin D.

Edited March 22, 2014 by davidivad

[Unity+]

 

You are right of course based on General Relativity, and that is an equation defining gravity.

If you are referring to him stating that no force holds us down, that is incorrect. Gravity is a force that pulls us towards the Earth.

this seems to me to be mostly a problem with effective communication and part of first year learning.

how you communicate your thoughts must not abandon your audience.

i feel abandoned if i agree with one over the other. is that truly effective communication?

 

one may consider that gravity is a consequence of the curvature of spacetime.

one may also consider that gravity is the result of a field.

this is why my original response stated " is it a particle or a wave."

i might normally consider trying to maintain repore with my readers but, at this time, consider it more important to convey the feeling that you pass off to your readers.

 

my point is that scientists in today's arena need to do more than just argue the point. you must also be everything in between.

please consider your audience.

 

thank you for a proffesional response @Kevin D.

The answer to the question, unless someone can correct me, is no gravity is not a particle or a wave.

 

If I remember correctly, the graviton is only hypothetically responsible for mediating gravity as a force. Therefore, gravity itself is not the particle, but hypothetically is a result of gravitons

[davidivad]

you are so serious about the specifics that you missed the main idea.

[Unity+]

you are so serious about the specifics that you missed the main idea.

I thought the question was requesting the specifics and the main idea. Sorry if I am coming off as harsh.

[davidivad]

just trying to increase awareness of the box itself.

[I-try]

The fundamental dynamic nature and origin of gravity is certainly not known or logically indicated by mainstream scientific knowledge. When a scientist attempts to describe gravity, references are made to the graviton of QM or that GR states that gravitation is an illusion and gravity only results because matter is compelled (indicating a force is acting) to follow geodetic pathways formed by the curvature of what is referred to as space-time.

Despite the above, there are constant references to the pull of gravity or gravitation pull frequently made to provide an idea of the actions of this scientifically unknown phenomenon. In this regard, science will remain confused because all defenders of GR (a large majority of scientists) declare that reference to the origin of gravity as being in the realm of Philosophy is not science because of the impossibility of making measurements. Unfortunately for the advancement of the understanding of gravity and referring to other science forums, any reference by layman or professional to gravity or gravitation as being physical independent phenomena in their own right is too often greeted with ridiculing reference to the lack of knowledge of the person making such statements.

[Cosmobrain]

The idea of Gravitions is just a frustrated way to include gravity in the Standard Model. There is no such thing. As far as we know, Einstein's GR is the best we've got to explain gravity

[ajb]

The idea of Gravitions is just a frustrated way to include gravity in the Standard Model. There is no such thing. As far as we know, Einstein's GR is the best we've got to explain gravity

Gravitons are today theoretical things and we should discuss them as such...

 

Gravitions are really objects in effective quantum general relativity, that is the closest thing to a quantum theory of gravity we have right now. The idea is, following standard methods of quantum field theory, to linearise the full theory and look at small fluctuations about some fixed configuration understood as the "vacuum" or "background". The classical part works fine here for GR, you construct gravitational waves. Then you apply standard path integral methods to these fluctuations and you see that the resulting theory does not quite work. This approach is the perturbative approach.

 

Technically the theory you end up with is full of infinities and you can't remove these using known methods of quantum field theory - the theory is non-renormalisable.

 

However, one can treat it as an effective theory and not a fundamental one. These infinities do not appear at tree level or one-loop level, so we can calculate scattering amplitudes for quantum general relativity to low orders with no real problems. We can discuss gravitons in this context perfectly consistently, remembering we have an effective theory.

 

So, what about the full theory? There is some evidence that quantum general relativity, or something close to it is asymptotically safe. Without any technical details, this means that the theory may be well defined, but not as a perturbative theory. That is we cannot understand the theory properly in terms of gravitons. It is quite possible that the full quantum theory of gravity is not a theory of gravitons at all!