Cause of Gravity? - Gravitons or Curvature of SpaceTime?

[jajrussel]

Doesn't zero mean null?

 

I think I understand what they are saying when they say gravity is the curvature of space, but wouldn't the curvature of space lend itself more to defining direction rather than force?

 

Isn't direction an effect of force?

Edited June 19, 2014 by jajrussel

[imatfaal]

Doesn't zero mean null?

 

I think I understand what they are saying when they say gravity is the curvature of space, but wouldn't the curvature of space lend itself more to defining direction rather than force?

 

Isn't direction an effect of force?

 

this is beyond me - but I think that to understand it you need to also have the notion that we are objects continually travelling at a constant rate through 4d spacetime. We will take the easiest route - the straightest line - the geodesic ... When spacetime is distorted by the presence of mass-energy we need to follow the shortest route on a curved "surface" (obviously not 2d but you get the idea) - this changed route instead of the flat "surface" of euclidean space is experienced as a force in our 3d reality; it is manifest as gravity.

[jajrussel]

the table exerts an equal force upon the cup as the downward force so the net force is zero.

I finally see your point.

 

When I apply this to an object in freefall I see an imbalance of force that in effect describes its direction of movement. The net force its not equal to zero. Yet, it is commonly said that an object in freefall has no force acting upon it. What applies to the cup on the table should apply to an object in freefall.

 

Which seems to imply that gravity is a force, and that the geodesic is an effect of that force. Am I wrong?

[Bird11dog]

If it did not have a force acting on it there would be no acceleration during free fall.

[imatfaal]

I finally see your point.

 

When I apply this to an object in freefall I see an imbalance of force that in effect describes its direction of movement. The net force its not equal to zero. Yet, it is commonly said that an object in freefall has no force acting upon it. What applies to the cup on the table should apply to an object in freefall.

 

Which seems to imply that gravity is a force, and that the geodesic is an effect of that force. Am I wrong?

 

 

"Yet, it is commonly said that an object in freefall has no force acting upon it." Freefall is when the only force is gravity. There is no force opposing gravity so you accelerate. However if you are in orbit you are going fast enough that you keep falling and keep "missing the earth"; but as everything (ie the space ship etc) is accelerating in the same direction and with the same magnitude you do not recognize the acceleration and appear to float motionless.

Gravity is a force in the amazingly accurate Newtonian Mechanics, it is curvature of space time caused by mass energy in the amazingly accurate General Relativity, and it is predicted to be the result of interaction through a gauge boson called the Graviton which will allow it to fit in with the other parts of quantum field theory (can you guess how accurate the current bits of quantum electro- and quantum chromo-dynamics are - yep amazingly). In the fullness of time Gravity will be one aspect of a unified theory that covers all interaction - but that is a while off.

 

And to forestall a possible follow up - there is no answer to "but what is it really". All we have are models that allow us to put in parameters and get out predictions; these range from the instinctive sort (seeing a ball in flight and kicking it mercilessly into the Croatian Goal) to the highly technical (fancy solving the einstein field equations)

[Mordred]

good answer, couldn't have said it better myself

 

The answer of why the body remains stationary when in free fall is rather complex in terms of GR, here is a quote from Lecture notes in General relativity by Mathius Blau. The article itself would do a far better job of it than I possibly ever could. I'm lousy at explaining GR related questions, so I would make too many mistakes at it. page 76 (the article itself is an excellent resource, I refer to it all the time)

 

http://www.blau.itp.unibe.ch/newlecturesGR.pdf "Lecture Notes on General Relativity" Matthias Blau

 

"How does this viewpoint of general relativity that there is no such thing as gravitational force square with the well known "fact" that there is a gravitational force field at the surface of the Earth of 980 cm s-Z? Recall that in the standard Newtonian viewpoint this gravitational force on an object placed on the Earth's surface is balanced by the force the surface exerts, leaving the body in equilibrium,i.e., "at rest." In the viewpoint of general relativity, the only force acting on the body is the force of the surface of the Earth. On account of this force, the body accelerates(i.e., deviates from geodesic motion) at the rate of 980 cm s-2. Nevertheless, it remains in a stationary state, because in the curved spacetime geometry in the vicinity of the Earth, the orbits of time translation symmetry differ from the geodesics of the metric."

 

However as mentioned in the previous post there is no answer "to what is it really"

Edited June 19, 2014 by Mordred

[jajrussel]

So, it was my understanding of the term freefall that was causing my confusion. Thank you for telling me.

 

Is it common for scientist to say that no force is acting on an object when there is only one force acting on it?

[Mordred]

there is no competition the term force isn't restricted to just the 4 forces, you can also have force exerted by inertia, or force exerted by pressure. None of the latter 2 examples need to explain the cause of the force. How you define force depends on the system your examining

 

"In physics, a force is any external effort that causes an object to undergo a certain change, either concerning its movement, direction, or geometrical construction. In other words, a force can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), i.e., to accelerate, or a flexible object to deform, or both. Force can also be described by intuitive concepts such as a push or a pull. A force has both magnitude and direction"

 

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

Edited June 19, 2014 by Mordred

[jajrussel]

Mordred, thank you for the link to the lecture notes.

there is no competition the term force isn't restricted to just the 4 forces, you can also have force exerted by inertia, or force exerted by pressure. None of the latter 2 examples need to explain the cause of the force.

Sounds reasonable, again I thank you.

[Mordred]

no problem anytime

[MigL]

Let me re-state my previous position...

 

An object in free fall is weightless, while an object like a cup sitting on a table has a weight.

We measure this weight in Newtons, a unit of force.

An object which has a weight of x Newtons feels a force while one of 0 Newtons feels none.

Ergo an object in freefall feels no force while one at rest, on a table, does.

[imatfaal]

How does anything stay in orbit then? If the ISS is not acted upon by an external force why does it not obey first law and continue off at a tangent?

 

The cup experiences two equal and opposite forces - one of which is easy to measure with a scale. The fact that the other is not so easy does not mean it is not there. Until you get into the realms of GR etc gravity is a force and freefall is the state under which the only force acting is gravity

[MigL]

A fall is an orbit and an orbit is an extended fall. Anything in orbit is weightless.

 

If your argument is that an object isn't just following a spacetime path, but the force of gravity is keeping it orbiting then what is the balancing force ? Remember that there is no centrifugal force unless you are in the frame of the orbiting object. To an external observer there is no balancing force.

 

The problem with using a scale to measure the force is are you measuring the force exerted on the scale by the cup or on the cup by the scale ?

 

Or are we misunderstanding each other ?

[imatfaal]

A fall is an orbit and an orbit is an extended fall. Anything in orbit is weightless.

 

If your argument is that an object isn't just following a spacetime path, but the force of gravity is keeping it orbiting then what is the balancing force ? Remember that there is no centrifugal force unless you are in the frame of the orbiting object. To an external observer there is no balancing force.

 

The problem with using a scale to measure the force is are you measuring the force exerted on the scale by the cup or on the cup by the scale ?

 

Or are we misunderstanding each other ?

 

We are not talking GR so no spacetime paths ;-)

 

The balancing force from Newton's third law is the fravitational attraction that the earth feels which attracts it to the planet. Third law pairs cannot act on a single object - the earth attracts the satellite and the satellite attracts the earth - they orbit around the common barycentre which is internal to the earth. Newtons first says that without an external force an object will continue at a constant velocity; the satellite does not have a constant velocity therefore it must be subject to an external force.

 

I am not arguing how you define weightlessness (I see no use in this term except to confuse) but in freefall (as exemplified by the ISS and astronauts inside) then the only force is gravity. If you use the term free fall to mean anything else it is a strange definition.

 

The scale measure the force which opposes gravity - which as you are not accelerating must be equal to the force of gravity; this is another newtons third law pair.

[Bird11dog]

Back to the OP. My argument is based on the fact that time dilation occurs no matter how short the motion(the bouncing around of atoms up and down and sideways) and the fact that the equation for momentum(p_{down = md/t > pup = md/t)} is well tested. As to why time dilation occurs I don't know. We are so locked into the standard model that I seriously doubt we will ever know. If the time however comes when we understand how energy is put together to make the proton and electron maybe we will.

[MigL]

OK Imatfaal, I'll bite.

In your force pairings, where the earth and the satellite are both attracted to the barycentre, what is the balancing force which pushes the earh and satellite outward to balance the inward gravitational pull and keep the orbit stable ?

[imatfaal]

There is no balancing force - there is a net external force on the satellite. If there were no external force it would not accelerate. If it did not accelerate it would go in a straight line.

 

As it does not go in a straight line - it accelerates inwards - it must be subject to net force.

[MigL]

OK I see what you're saying. I guess I' just used to thinking in terms of the satellite following the space-time curvature and not accelerating.

[imatfaal]

OK I see what you're saying. I guess I' just used to thinking in terms of the satellite following the space-time curvature and not accelerating.

I thought as much - I did say twice that this was not GR but Newtonian ;-)

[metacogitans]

If gravitons were responsible for gravitation then gravity would not bend light.

[Mordred]

If gravitons were responsible for gravitation then gravity would not bend light.

 

 

yes it can according to QFT (assuming the graviton exists, it is after all a hypothetical particle)

 

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

"Unlike the force carriers of the other forces, gravitation plays a special role in general relativity in defining the spacetime in which events take place"

 

in other words it has a geometric influence which is the same as the stress energy tenser (in essence would be the carrier between mass and the stress energy tenser) though that statement may be a bit poor

Edited June 29, 2014 by Mordred

[Bird11dog]

Mord, just saying that something is true does not make it true.Meta is right If it is a particle and not a field it cannot bend light. Naturally QFT claims that the graviton is both a particle and a field. You can't have it both ways just because you want the physics you were taught to hold up.

[Mordred]

Mord, just saying that something is true does not make it true.Meta is right If it is a particle and not a field it cannot bend light. Naturally QFT claims that the graviton is both a particle and a field. You can't have it both ways just because you want the physics you were taught to hold up.

 

 

Then explain electromagnetism it is both particles and it has a field of influence. The Higg's boson also is a particle with a field of influence.

a field cannot exist without particle interactions, energy does not exist on its own, energy is transferred through particle to particle interactions. A gravitational field would also have particle to particle interactions.

 

in order to have a field you must have particles interactions so in this case you must have it both ways. QED, is a working field theory, so is QCD. The problem with quantum gravity is the lack of being able to quantize the graviton. This is the underlying problem in unifying quantum gravity and relativity. Relativity does not work at all energy levels it works great in describing gravity in the macroscopic world however it does not work well in describing the microscopic interactions. This is why a complete theory is needed.

 

If you look at the stress energy tenser of the Einstein field equations (oh wow look the term field is used).

 

"the stress–energy tensor (sometimes stress–energy–momentum tensor or energy–momentum tensor) is a tensor quantity in physics that describes the density and flux of energy and momentum in spacetime"

http://en.wikipedia.org/wiki/Stress%E2%80%93energy_tensor

 

energy and momentum is transferred through particle to particle interactions.

please note the stress energy tenser of the Einstein field equations specifically refer to perfect fluid calculations. see the image to the right on that link.

spacetime fundamentally is a geometric description of gravitational influences upon matter (particle to particle interactions) in other words its a geometric description of the effective field strength of gravity at a given range. That geometry is essentially a field, hence the term gravitational field

I always find it amazing that so many people limit themselves to their favorite pet theory and never study the alternative theories as though they are always in competition with each other. The best policy is to study EVERY theory they can get their hands on. If they did they would realize that the alternate theories are not necessarily in direct competition, but are in fact merely different ways to describe the same interactions through different coordinate systems and differential geometry.

some theories are better at describing x influences other theories describe y interactions better. This does not mean one is necessarily better than the other.

 

here is a decent article on what spacetime means in terms of a perfect fluid

http://mathreview.uwaterloo.ca/archive/voli/2/olsthoorn.pdf

 

here is a paper that discusses GR and QFT compatibility it also mentions the various issues

http://arxiv.org/pdf/1209.3511v1.pdf

 

"effective field theory has shown that general relativity and quantum mechanics do in fact go together fine at ordinary scales where both are valid. GR behaves like an ordinary field theory over those scales. This is important progress. We still have work to do in order to understand gravity and the other interactions at extreme scales"

 

quoted from the above paper.

Edited June 29, 2014 by Mordred

[metacogitans]

What would a graviton be emitted from and when would it be emitted though? Would it be emitted from all mass constantly? What about a graviton emitted from a particle near the center of the sun; wouldn't the graviton be absorbed by other particles within the sun, never making it outside of the sun to produce the sun's gravitational field?

[Mordred]

What would a graviton be emitted from and when would it be emitted though? Would it be emitted from all mass constantly? What about a graviton emitted from a particle near the center of the sun; wouldn't the graviton be absorbed by other particles within the sun, never making it outside of the sun to produce the sun's gravitational field?

 

that's something we don't know yet, We can only theorize the gravitons properties and how it would decay or interact with photons or matter so there is no degree of certainty. So there is numerous speculations etc however it would behave in a similar fashion to the electromagnetic field, however with a spin 2 (possible set of interactions), one conjecture is that there is also virtual gravitons much like virtual photons, mediates the electromagnetic field. The virtual gravitons would mediate the gravitational field.

 

here is one paper that attempts to describe the graviton to photon interactions

"Graviton Physics"

http://arxiv.org/pdf/gr-qc/0607045.pdf page 10 describes how its possible though its extremely technical (it also shows the metrics for other possible spin values for the graviton) unfortunately the paper is extremely technical, and I've never found a good paper that simply describes the interactions without being highly questionable.

 

probably the easiest way to relate to this is to study the interactions of the electromagnetic field, on how photons travel through a medium, where the momentum and energy is passed from electron to electron via photon exchange. The graviton would act as the mediator exchanging energy from particle to particle. A higher density of matter would result in a higher number of exchanges with a shorter mean free path between particles. Making for a stronger field strength. The further away you get from the mass the greater the mean free path is between particle interactions(fewer interactions) and the weaker the field strength gets.

Edited June 29, 2014 by Mordred