Does gravity move at the speed of light?

Hi Guest, you wrote: "If you can detect gravity waves, then maybe its propagation speed could be actually determined. However, for now we have theories."

You are right, but there are some indirect measurements that indicate that gravitational waves move at c. One is the binary pulsar measurements of the 1990s, where the decay of their orbits, caused by gravitational wave radiation, followed theory by ±1%. Because the speed of gravitational waves are part of general relativity (used to make the prediction), it indicates that theory probably has the 'speed of gravity' right.

A second case is the 2002 measurement by scientists Fomalont and Kopeikin, who found that the gravitational force of Jupiter and light travel at the same speed (±20%, the experimental uncertainty). There is however still some debate on the interpretation of the results, which may be settled soon.^[1]

Jorrie

[1] Physicist Defends Einstein's Theory And 'Speed Of Gravity' Measurement.

[2] I have a short article on the 'Speed of Gravity' on my web page.

'If gravity did the same thing, then the gravitational force from the Sun would be pointing at where the Earth was 9 minutes ago'

This statement seems to suggest that gravity is directional, the picture I have in my head is of a field extending at all directions from the mass, that is just there.

A better way of stating the question might be to ask, if the sun suddenly materialised from nothing, (or raw firmament, if you prefer) would the gravity 'field' appear at the same instant, or would it take 9 minutes to 'appear'?

hi PlbMak.

"... if the sun suddenly materialised from nothing, (or raw firmament, if you prefer) would the gravity 'field' appear at the same instant, or would it take 9 minutes to 'appear'?"

BTW it's closer to 8 minutes (500 seconds). According to Einstein, all changes in gravitational fields caused by non-inertial acceleration (or "suddenly materialised" matter/energy) propagate at c. Once the situation stabilizes, the gravitational field is obviously "just there", as you said.

Jorrie

So Mr Guest, if light takes 9 min to reach Earth, at the speed of light,but if gravity took 9 min would be where Earth was 9 min ago- ignores the fact that the light from the Sun shines in all directions- SO MUST GRAVITY- whatever gravity is- so that means that that as the speed of Earths motion in orbit is way less than the speed of light, it would make very little difference to light, & indeed to gravity speed, if Earth was 9 min ahead of where a straight beam, whether light, or gravity would have landed because in the situation of above Omni radiation not straight as in a torch beam. Really, the only thing that makes sense is that gravity is negative energy- whereas the output from Sol etc is positive energy- there must be a balancing act, as the nuclear reactions occur, from more active to colder (or older) masses- this is what we see in our solar system, &, by extrapolation, in the whole of the observable universe.

<gravity is negative energy>

There is an impression lurking around that Stephen Hawking has suggested that the sum of all the gravity and all the matter-energy might be equal to zero. Any developments on this, Jorrie?

Hi PWSlack, you wrote: "There is an impression lurking around that Stephen Hawking has suggested that the sum of all the gravity and all the matter-energy might be equal to zero."

One must be careful with the wording: gravity is not energy. Gravity causes potential energy (GMm/r) that can be viewed as negative energy, but only if you choose your zero point that way. In a gravity environment, free-fall kinetic energy (½mv²) and potential energy are converted into one another; if one goes up, the other one goes down. So it's better to say kinetic energy + potential energy is constant.

In cosmology, it is usual to take potential energy + kinetic energy to equal zero, so one of them must be negative. It cannot be kinetic energy unless mass is negative, so it must then be potential energy. Depending on how the zero-point energy is defined, the cosmos may be "the ultimate free lunch". (Can't remember who coined that phrase; maybe Hawking).

Jorrie

<Bows respectfully.>

Hi Neil, further to what I wrote to PWSlack above, there is a way in which gravity can be viewed as energy, but then it is positive energy. It works like this:

The static gravity in Einstein's general relativity is slightly stronger than the gravity in Newton's theory. This can be viewed as if the gravitational field itself gravitates, meaning, it adds 'positive energy' to the normal Newtonian mass-energy of the object. Since E=mc², one can say that the gravitational field has energy, or more loosely stated, 'gravity is energy'.

I do however not support this notion that some physicists hold. I prefer to think that the extra gravitational 'pull' comes from the effects of curved spacetime and the gravitational time dilation associated with it.

Jorrie

I don't think so. If you look at the overall dimensions of the calculation it would seem that the 500 second propigation rate versus the overall stability of the system would be so small as to be negligable. It would be an interesting system to force an impulse into and watch the results, wouldn't it? I wonder if there is a phenomina in the observable universe that would be an analog of a large mass suddenly appearing or disappearing and if the effects on nearby objects could be observed.

"I wonder if there is a phenomina in the observable universe that would be an analog of a large mass suddenly appearing or disappearing and if the effects on nearby objects could be observed."

There aren't such phenomena (at least at a large scale, as they would be opposed to the energy conservation law)... But there are other phenomena: i.e. two Black Holes which are spinning around each other... just before they hit each other (forming a unique Black Hole) they emit intense gravitational waves (e.g. gravitons)... and we are trying to sense these gravitational waves by special devices called reflectometers... if we observe such gravitational waves we could 1st) confirm their existence and 2nd) measure their speed by the means of two reflectometers located in a far distance to each other... Although, I'm not so sure that the virtual gravitons (which are the carriers of the gravitational force) are exactly the same thing (or of the same nature) as the real gravitons that consist a gravitational wave...

Hi ca1ic0cat, you asked: "I wonder if there is a phenomina in the observable universe that would be an analog of a large mass suddenly appearing or disappearing and if the effects on nearby objects could be observed."

I don't think so. The closest thing to a large mass suddenly disappearing is a supernova, where most of the mass is thrown off and disperses into the surrounding space, leaving a neutron star or black hole behind, but it is not really matter 'disappearing'. A certain amount of gravitational waves are however predicted to be generated by supernovae that are slightly unsymmetrical.

The main source of strong gravitational waves are thought to be when two black holes collide and oscillate before forming one hole.^[1] There is high hope that such an event will be recorded in the near future by LIGO, the 'gravitational wave observatory'.

Jorrie

[1] I've got a chapter on gravitational waves on my website if you are interested in an 'engineering view' on the details.

Similar answers at essentially the same time by both of us, Jorrie...

I thought myself, also, the supernovae, but I realized that it isn't, actually, a mass disapperance... that's why I didn't mention it at all... I didn't know, though, that the supernovae emits, also, gravitational waves that can be detectable...

Hi, Neil. I am Mr. Guest, but I forgot my password. I am back, now. ;-)

I am sorry that I did not explain this better. I tried to simplify something that is more complex and I did not do a very good job. You have a very good point and question. Permit me a second attempt.

Basically, in classical Newtonian physics the speed of gravity must be considered as instantaneous for the calculations to work. This is because we must have conservation of angular momentum. If there is a propagation delay in the effect of gravity on two bodies that orbit each other, then the orbits would not remain stable and there would be a change in the conservation of angular momentum, which we all know can not be.

The two orbiting bodies form a couple. Let me try to give an example. Jorrie may need to come to my rescue!

Imagine two skaters that are circling each other. If, while they are circling each other they grab each other's hand, they form a couple. If they pull each other inward as they circle, the conservation of angular momentum tells us that the rate of speed that they circle each other must increase. The opposite effect will happen if they extend their arms and increase their distances between each other.

Think of the couple's linked arms as an instantaneous bond between each skater. As the tighten and relax their grips they are still considered a couple and one system. We can predict the rotational or orbital speeds of the two skaters knowing their masses and their relative center of mass distances.

Now, let's look at two orbiting bodies. Let's use Jupiter and the Sun. They are locked together, but it is by the force of gravity, not a mechanical link. Still the two bodies form a couple. We can predict the motions of each to a high degree of precision using Newtonian physics, but the force of gravity must be considered to be instantaneous in order for the laws conservation of angular momentum to apply.

If there is a transit delay for the force of gravity, then there would be an imbalance of force between the two bodies and their orbiting trajectories.

Let's say that the Sun exerts a force on Jupiter and Jupiter on the Sun. Each is acting on the other's current position now. The two forces are in direct line with each other. However, if we take the speed of gravity to be the same as that of light we get a different scenario. If the Sun attracts Jupiter at the Sun's previous previous position in time ((T-zero) the position of the Sun when it's gravity began to propagate towards Jupiter) and Jupiter attracts the Sun at Jupiter's previous position at T-zero, which is when the forces of each bodies gravitational attraction begin, then those two bodies form a couple, like the skaters. However, at T-now, the actual positions of the two bodies are at a different position in space then they were at T-zero, yet the gravitational forces felt by each body were those at T-zero and not T-now. The distance between the two bodies at T-zero and T-now are different, yet you are applying the forces exerted at T-zero for the spacial positions of T-now. The couple will begin to see a a net increase in angular momentum as they orbit. This effect will be additive over time and destabilize the orbits.

You can see this more clearly if you write a short computer program that calculates the forces of gravity on between two bodies in orbit using Newtonian physics. You can simulate the orbit easily with an iterative calculation. However, if you add a delay constant to the forces between the two bodies with each calculation, say using the previous positions of each body one iteration back instead of their current position, the orbits destabilize and the objects slowly spiral away.

We know that doesn't happen in the universe, much less the Solar System, so there can not be a delay in the calculations for gravitational classic physics. If fact, Newton did feel there was a finite speed to gravity, but he pegged it at something like one million times the speed of light.

Maybe Jorrie can explain this better. While I understand the principle I am not sure I have performed an eloquent job at explaining it.

General relativity explains the phenomena and works nicely for a speed of gravity equal to that of light.

Hi AH, nice to see you back again!

"Maybe Jorrie can explain this better. While I understand the principle I am not sure I have performed an eloquent job at explaining it.

General relativity explains the phenomena and works nicely for a speed of gravity equal to that of light."

The simplest way to look at it for both Newton's and Einstein's gravity is that the gravitational field "is just there", like was pointed out in reply #3 by PlbMak. Only changes to this field caused by non-inertial movement (acceleration) of matter creates corrections to the field and those corrections propagate at the speed of light (gravitational waves).

It is similar to the acceleration of charged particles that cause electromagnetic radiation. Inertially moving particles (in free space) do not radiate. Massive bodies in normal orbits around each other are in free-fall and is moving more-or-less inertially, so they do not radiate gravitational waves. More in this article.

Jorrie

gosh it's interesting being able to listen to ideas bouncing around.

I think Hawking suggested that the orbit of the earth could be conceived of as a torus around the sun and the force of gravity as a tube from the sun to the torus down which gravitons travel. Backing up a dimension, the water flowing down the drain could be seen as a torus around the drain and the force of gravity a tube that runs from the drain to the torus down which gravitons flow. Or if you spin a weight on a string, the weight is a torus and the string can be conceived of as a tube from your hand to the torus down which centripetal force flows.

The shape of space where earth sits in one frame of time is shaped differently from every other space where the earth will sit in the future or has sat in the past. The shape of space on the side of the sun facing the current position of the earth is different from the shape of space anywhere else near the sun. This distortion would indicate the place where the quantum tube begins and ends. The space where earth will be in eight minutes is nothing like the space where it is now and won't be until the quantum tube catches up with it.

At least that's the way I visualized it when Hawking was describing it to me (in a book on tape). I wrote to him to clarify this point, but he must have had something better to do that day because he never responded.

doesnt string theory allow for the passing of neutrinos in and out of our four dimensions?

This could account for things suddenly appearing and disappearing. I beleive that there was an article last month in either American Scientific or Smithsonian about recent experiments showing support of this?? yes, no?

I don't know anything about the Universe and it's secrets, but I have a persistent notion I cant get out of my head:

That the phenomenas of gravity and inertia is due to the fact that all objects move at very high speed relative to the starting point of Big Bang. (or something)

I know there is no starting point, the universe existed everywhere and nowhere in the absolute beginning, but still..........

Inertia: If You try to move an object, it resists movement as any object will do that moves at high speed, like a high speed gyro wheel. Stop the wheel and immediately it's easy to to turn the gyro wheel in another direction.

Gravity: This is harder, but my guess is that the inertia of objects mutually affects other objects due to the inertia of these......

Don't take this message too serious, but it would be fun to know if anybody

out there have had these or similar notions.

Jonas Karud

Why would the solar system go "carooming" (sic) off into space? Isn't the massive black hole, etc at the center of the Galaxy only a small portion of the mass of the Galaxy? I would think there are like 200 billion (US Billion) solar masses in the Galaxy, not counting the black hole et al in the center of the Galaxy, and dust clouds, etc.The Solar System might wander off eventually????

On a smaller scale, if you had a lead sinker (for all the fishermen out there) on a fish line and you twirled it around from where you were standing and suddenly let go it would fly off in a tangent.

Maybe gravity would act in the same way...???

"

That the phenomenas of gravity and inertia is due to the fact that all objects move at very high speed relative to the starting point of Big Bang. (or something)

"

Aren't galaxies moving in different directions, with some of them colliding? How could that happen if the all started off in a direction from the Big Bang, presumably in an outward direction?

"Aren't galaxies moving in different directions, with some of them colliding? How could that happen if the all started off in a direction from the Big Bang, presumably in an outward direction?"

This whole frame of space where galaxies are moving in different directions and colliding is presumably travelling in high speed away from the original starting point, Big Bang.

Jonas Karud

"This whole frame of space where galaxies are moving in different directions and colliding is presumably travelling in high speed away from the original starting point, Big Bang."

The starting point (Big Bang) was everywhere at once, hence everything is traveling away from everything else. Collisions happen due to gravitational clumping that brings some areas closer together and they start traveling as a group or cluster.

Like on a highway, collisions happen where you bring to many things together, although all are moving similarly and in the same direction.

Jorrie

The underlying microscopic phenomenology of gravity is not understood. The same is true for electric fields, magnetic fields and photons. I have a complete elementary particle model to account for these phenomenon. The first step is a little elementary but creative vector analysis of electric, magnetic, photon and gravity fields. They are all made of a point-like particle that has two vectors associated with it, one for the electric field and one for the magnetic field. They are at a right angle to each other. The particle is in perpetual motion and travels at the speed of light. Those vectors may be thought of as unit vectors since their length has no meaning. What they mean about this particle is that it is orientable and in some orientations has a non-super-imposable mirror image. This model is available at my website davidmartindegner.com. I have embedded my work in a screenplay A string theorists meets the fisherman's son that is available as a free PDF download. This science is outlined in 31 pages of that screenplay.

David Martin Degner

Interesting read, but I am not finished, yet.

http://www.fourmilab.ch/gravitation/orbits/

http://science.howstuffworks.com/framed.htm?parent=question232.htm&url=http://www.fourmilab.ch/gravitation/foobar/