speed of gravity

[PolymerTim]

Sorry to comment on a side-argument of the original poster, but it seems there is a lot of discussion about the "atomic clocks in a plane" experiment without anyone bothering to go look it up. My favorite part about being in a university is the electronic library so I took 5 minutes to read the paper and even in its short 4 pages, most of these concerns are addressed. I like this experiment since it avoids the whole issue of "light playing tricks" since the clocks start and end at the same point for comparison. To argue with this one, I think you would have to show that the trip somehow affected the vibrational frequency of the Cs without affecting time. I'm not really sure how you can separate those two, but it may not matter. If the trip affects all matter the way it does Cs vibrations then maybe that is the same thing as slowing down time.

Here's my quick summary from the paper for those who can't get it themselves.

From "Science Vol. 177 pg 166--170 (1972) (Experiment)." (My paraphrasing)

It is known that no two "real" cesium beam clocks keep precisely the same time (differing from each other from up to 1 microsec per day) and even are not completely constant since shot noise in the beam tubes cause short term fluctuations in rate. Intervals between these fluctuations can range from days to months and are unpredictable, but random. Realizing this complication, the researchers did not put one cesium beam clock on the commercial jet-liner, they put 4. In this way, any deviations in one clock from the others could be attributed to noise on that clock and factored out.

The difference in time between each of the 4 clocks and the reference ground clock was monitored for ~240 hours before being placed on a commercial jetliner and flown eastward around the earth for ~65 hours (41 in flight). Once back at the original location, the clocks were monitored for another 150 hours before being put on another jetliner flying westward for ~80 hours (49 in flight). Once at home again, the clocks were monitored for another ~100 hours. A plot of the time differences for each clock and their average is presented in the paper and shows that there is in fact a significant spread between them as expected. In this plot, time difference from ground clock (in ns) is plotted vs time (hours) covering the whole experiment. The slope of each curve for 25 hours immediately prior to flight for each clock and the mean is extrapolated linearly to the time point after the trip and compared to the first actual data point after the trip and the results reported are thus:

Clock serial # | dt(ns) east | dt(ns) west
120 | -57 | 277
361 | -74 | 284
408 | -55 | 266
447 | -51 | 266
Mean| -59 +/- 10 | 273 +/- 7
Pred| -40 +/- 23 | 275 +/- 21

Note that the predicted values shown at the bottom of the table are based off of expectations based on SR and GR. GR was used to add a kinetic term due to the ground clock and flying clocks being at different altitudes. Their final equation combines these effects into an integral over delta time for the entire known flight path, which was carefully recorded. As you can see, the agreement between the theory and the results is very convincing.

Error in the experiment was considered carefully. In addition to random errors, accounted for by multiple clocks, systematic errors were tested as well. Effects of temperature, pressure, magnetic fields, and impulse acceleration were tested on ground clocks and proven to be non-systematic if present at all.

 

[CatchPhrase]

Einstein beat you to it.

 

[CycloWizard]

Originally posted by: bwanaaa
@cyclowizard

thank you for clarifying that. since the physics community believes gravity is constant throughout the universe, it follows that that the derivative of gravity is Zero. Of course there is that Israeli physicist who talks about variable gravity because he does not want to believe in dark matter. Personally, I'm with him-it's a lot easier to believe in gravity fluctuattions that invent something new that accounts for 75% of the calculated mass variance of the universe. But I think that's how science works-propose a theory that can be the subject of the null hypothesis - and shoot it down. The tearing down of ideas I think satisfies some animal need in us.

The physics community is pretty sure that gravity depends on both time and location for all of the reasons stated above. Therefore, the rest of your post invalid since your initial premise is simply begging the question. Either that, or you need to be concise in your statement that "gravity is constant throughout the universe," since "gravity" in this sense ciould mean a variety of things (gravitational constant, actual gravitational force, acceleration due to gravity).

 

[Hulk]

Don't even twist up your brain too much thinking about gravity. It is one of the biggest mysteries in science.

Force at a distance with nothing detected between the objects! It might as well be magic. No gravity waves ever detected. No gravity particles ever detected. It cannot be manipulated or created artificially as with electric force or magnetism.

It is a fundamental property of mass and that is all we know other than how to predict it based on masses and distance between them using the universal law of gravitation.

It is a strange thing. A very strange thing.

Think about this.

Inertial mass and gravitational mass are the same for an object but if you think about it there is no reason this should be so.

A 1Kg mass has a weight of about 10N (rounded number).
A 1Kg mass when acted on by 10N force shows an acceleration equal to the that of gravity!

Why, why, why should this force of gravity be the same as the inertial force for the same mass.

There is something elegant and simple in there to be figured out but as yet no one has cracked the code.

In one case the mass is resisting a force to mitigate an otherwise infinate acceleration.

In another this mysterious force (gravity) between the two objects in the same acceleration field is showing the same numerical mass.

It's nuts. You know when people get abducted by aliens that have obviously mastered gravity they should just ask one question.

"Tell me about how gravity works please?"

 

[Gannon]

Originally posted by: taltamir
a metaphor explaining the laws of physics should not be taken literally or used to explain other phenomenon

The laws of physics are nothing more then a mathematical description of geometry and relationships.

Mathematics is a metaphorical language used to describe shapes and geometry, our entire understanding is in fact a mixed metaphor. What we do when we test 'theories' are attempts to create mapping between ideas and refining ideas as knowledge expands and tools (mental prosthetics) get better.

Whether his metaphor is valid, and the kind of metaphor it is, is the issue.

In the end science is the child of minds, not vice versa, science is meaningless without minds to interpret the results. All technologies and measurment devices are merely mind extensions.

Einstein said:

When forced to summarize the general theory of relativity in one sentence: Time and space and gravitation have no separate existence from matter.

Physical objects are not in space, but these objects are spatially extended. In this way the concept 'empty space' loses its meaning. Since the theory of general relatively implies the representation of physical reality by a continuous field, the concept of particles or material points cannot play a fundamental part, and can only appear as a limited region in space where the field strength / energy density are particularly high. (Albert Einstein, 1950)

Einstein again:

A human being is part of the whole called by us universe, a part limited in time and space. We experience ourselves, our thoughts and feelings as something separate from the rest. A kind of optical delusion of consciousness. This delusion is a kind of prison for us, restricting us to our personal desires and to affection for a few persons nearest to us. Our task must be to free ourselves from the prison by widening our circle of compassion to embrace all living creatures and the whole of nature in its beauty? The true value of a human being is determined primarily by the measure and the sense in which they have obtained liberation from the self. ? We shall require a substantially new manner of thinking if humanity is to survive. (Albert Einstein, 1954)

If we take him at face value, it would appear relativity shows there is no such thing as subjectivity, only levels of objective correctness and incorrectness. If reality is a continuous field, we are not disconnected from everything else and our individuality and our ideas of mind and thoughts, being seperate (inside vs outside) from the universe is in fact an illusion of the senses.

 

[Gannon]

Originally posted by: Hulk
Force at a distance with nothing detected between the objects!

Here lies the fallacy, according to relativity, the whole of reality is a continuous field and the myth is that reality is made of seperate "objects" is most likely just that, a myth. This would explain 'strange' quantum mechanical effects nicely as well, where 'matter' loses its 'identity'.

Einstein:

When forced to summarize the general theory of relativity in one sentence: Time and space and gravitation have no separate existence from matter.

Physical objects are not in space, but these objects are spatially extended. In this way the concept 'empty space' loses its meaning. Since the theory of general relatively implies the representation of physical reality by a continuous field, the concept of particles or material points cannot play a fundamental part, and can only appear as a limited region in space where the field strength / energy density are particularly high. (Albert Einstein, 1950)

 

[bsobel]

Originally posted by: ForumMaster
does gravity even have a speed? if we accept Einstein's theory of relativity, space is like a blanket and gravity is like the depression that a ball makes on a cloth. it's there. it's doesn't have a "speed". it is instantaneous cause it's there.

Ok, take the simple blanke anaology. You have a blanket on a bed. You drop the ball on the bed. From the point of impact how long does it take for the blanket to be stretched out where it needs to respond to the weight of the ball? The distortion of the fabric is not instantaneous, it happens at the speed of sound (as applied to the fabric). That wave effect is the speed we are talking about, when discussing gravity it moves at c.

Bill

 

[bsobel]

It cannot be manipulated or created artificially as with electric force or magnetism.

I certainly wouldn't say that, I'd say we've yet to demonstrate how to mainuplate or create it artificially. Saying it cannot presumes facts not in evidence as it where. In fact, there is plenty of anti-grav work occuring even at this time (Nasa has the superconducting shielding experiments, they look 'interesting' if nothing else)...

 

[bwanaaa]

Originally posted by: Hulk...
Think about this.

Inertial mass and gravitational mass are the same for an object but if you think about it there is no reason this should be so.

A 1Kg mass has a weight of about 10N (rounded number).
A 1Kg mass when acted on by 10N force shows an acceleration equal to the that of gravity!

Why, why, why should this force of gravity be the same as the inertial force for the same mass.
.....

I have thought about this for years. And I always come back to the elevator analogy used by Einstein to describe the equivalence principle. That if I may paraphrase, you cant tell the difference between these 2 settings:
1) you are in a stationary elevator on earth
2) you are in an elevator in deepspace accelerating at 1 g.

For inertial and gravitational mass to be the same value, maybe we are looking at an equivalence of sorts. they may in fact be the same thing. gravity is nothing more than the acceleration of mass. but how you might ask am i accelerating.? when i am standing still.? i would answer that the space you inhabit, in fact, is expanding. not empty space, just space where there is matter . that space is expanding. because you happen to be next to a massive body, as your space and its space expand, you push against each other and 'feel the gravity'. But this simple geometric explanation fails when you consider other details.

why do things fall?-the space of the earth and the falling thing expand until they meet. but why doesnt the space between the falling thing and the earth expand too? is it because it is much less dense? there's less stuff to expand? according to this, you should weigh as much on an earth shaped planet made of cork as you would on an earth shaped planet made of uranium. unless you invoke density again.

but thinking about this geometric explanation can be fun. it would predict that the moon's orbit through a static reference frame (where you could actually witness the earth and moon expand) would be a spiral-archimede's spiral?

this space stretching reference frame we inhabit might also explain why we can actually see the expansion of the universe when we look very far into deepspace with light.

i agree with you-for inertial mass and gravitational mass to be precisely equal is mind boggling.

 

[Nathelion]

Maybe. But maybe not. Keep in mind that gravity can arguably be described as a fictitious force. Just like the centrifugal force pushing you off a merry-go-round, gravity is a result of leaving a geodesic in space-time (In the presumably euclidean reference frame of someone watching the merry-go-round, a passenger on the latter would be accelerating (in order go round in a circle). From the perspective of an observer "seeing" space-time, us poor humans on earth would be analogously accelerating in order to "stand still" in our subjective reference frame). So really, when you think about it, you could say that gravitational force is a result of inertia, and therefore it is very intuitive that gravitational mass and inertial mass must be the same.
Of course, there are some really cool experiments going on to measure that equivalence to ever greater degrees of accuracy - I read something about putting two objects enclosed by a box in orbit around the earth, and then measuring whether their orbits diverge. I can't seem to find the link though.

 

[Gannon]

Originally posted by: Nathelion
Maybe. But maybe not. Keep in mind that gravity can arguably be described as a fictitious force.

Well according to NASA, gravity and electromagnetism are coupled. It would be hard to describe gravity as a "fictitious force" if indeed gravity and electromagnetism are different forms of the same thing.

NASA
Link - http://www.grc.nasa.gov/WWW/bpp/overview.html

Coupling of Fundamental Forces: Electromagnetism, gravity and spacetime are coupled phenomena. Given our technical proficiency at manipulating electromagnetism, this coupling hints that we might be able to use electromagnetism to affect gravity. In principle this is true. In practice, at least from the perspective of general relativity, it would take an enormous amount of electromagnetic energy to produce a perceptible gravitational effect ? energy levels in the regime of E=mc^2, where m represents the induced gravitational mass effect. While general relativity pertains to large-scale couplings, quantum and particle physics pertains to the couplings on the atomic scale and smaller. One example of an unresolved small-scale question is the unknown inertial and gravitational properties of antimatter. Although presumed to be equal to their normal-matter counterparts, long-duration low-gravity experiments could resolve minor differences that have not been testable in terrestrial labs. [51] Such experiments might also help resolve the lingering incompatibility between general relativity and quantum mechanics. As much as these pertain to general physics, they may also have implications for propulsion physics.

 

[Nathelion]

Originally posted by: Gannon

Originally posted by: Nathelion
Maybe. But maybe not. Keep in mind that gravity can arguably be described as a fictitious force.

Well according to NASA, gravity and electromagnetism are coupled. It would be hard to describe gravity as a "fictitious force" if indeed gravity and electromagnetism are different forms of the same thing.

NASA
Link - http://www.grc.nasa.gov/WWW/bpp/overview.html

Coupling of Fundamental Forces: Electromagnetism, gravity and spacetime are coupled phenomena. Given our technical proficiency at manipulating electromagnetism, this coupling hints that we might be able to use electromagnetism to affect gravity. In principle this is true. In practice, at least from the perspective of general relativity, it would take an enormous amount of electromagnetic energy to produce a perceptible gravitational effect ? energy levels in the regime of E=mc^2, where m represents the induced gravitational mass effect. While general relativity pertains to large-scale couplings, quantum and particle physics pertains to the couplings on the atomic scale and smaller. One example of an unresolved small-scale question is the unknown inertial and gravitational properties of antimatter. Although presumed to be equal to their normal-matter counterparts, long-duration low-gravity experiments could resolve minor differences that have not been testable in terrestrial labs. [51] Such experiments might also help resolve the lingering incompatibility between general relativity and quantum mechanics. As much as these pertain to general physics, they may also have implications for propulsion physics.

Energy -> gravitation. Regardless of whether that energy is in the form of mass or EM fields.
Yeah, it's all pretty speculative. I imagine that theory of everything we keep waiting for will clear things up

 

[Wags1974]

Im withholding judgement until the announcement that ther is/isnt a Higgs field.

 

[Nathelion]

There is something I've been wondering about for a long time - if you let, say, an electron (a charged object) orbit around a massive body (say, the earth), will it emit radiation and eventually crash because of the gravitational acceleration it is undergoing, or will it keep orbiting like an uncharged object would?

 

[thebeyonder]

I think I have a much clearer explanation.

first off, gravity will not travel "much faster than" the speed of light. it will either go the speed of light, or be instant. no middle ground beyond the speed of light.

secondly, gravity does go the speed of light. the force of gravity is transferred by vector bosons, which are particle-like in nature, as much as gamma rays: not high enough mass to become a particle, but enough energy to act particle-like. so, they can only travel as fast as light.

implication: gravity is a form of radiation.

the main difference is, light is electromagnetic radiation. radio waves, microwaves, infrared, visible, xrays, gamma rays, are all propagated from an electromagnetic source. the force carrier "particles", on the other hand, do not.

thirdly, the same mechanism is responsible for the effects of magnetic force. which then could also be considered "a form of radiation" as is gravity.

 

[Foxery]

I just noticed this thread, and I love it Some thoughts...

Gravity: Conceptualized as an effect, not as a force
The current model of "Spacetime", i.e. the fabric of the Universe as best I understand it, is that massive objects create a depression in the fabric, like standing on a tampoline. Gravity isn't an attractive "force" at all; it's the effect of sliding down a slope towards the depression the way a small child who climbs onto the trampoline will fall towards my large depression in the middle. She will also start falling immediately, because the slope towards me always exists.

The Earth orbits the Sun's position without an 8-minute delay:
Now picture the bowl shape that I create in the fabric of my Wonderful Spacetime Trampoline. (Patent pending, patent pending!) If I move, the bowl moves with me. The child will stop falling towards the center of the circle, and start falling towards my new position. I picture the Earth as a Cheerio rolling along the sides of a cereal bowl - we don't fall out, we move when the bowl does.

Illustration0

Why light "bends" towards stars/massive objects
Two parts to this.
1. Light is believed to have zero mass. An attractive force can't act on a massless object. But we do know from observation that light does move towards large stars, and is captured entirely by black holes.
2. The fabric model explains this perfectly. The light falls into the bowl-shaped depression created by a massive star; the curvature of space changes the light's path. When it comes out the other side and keeps going, its formerly straight-line path appears bent. It's like hitting a pothole in the road, if you will

Illustration1

In the case of a black hole, the depression is more like a bottomless pit that the light never climbs back out of. I read somewhere recently a clarification on this: An object caught in a black hole's gravitational pull never actually collides with it. The object orbits forever, in a manner similar to an asymptote in geometry.

Illustration2

So are objects at extreme distances attracted? Yes
The effect of gravity between far away objects exists, and is felt "instantly" - perhaps "immediately" is a better word - because the space between you is always curved, and if an object moves, the curve updates accordingly. It is, however, dwarfed by that of closer objects whose curvature is stronger. From high school physics: F(g) = 1/distance^2
The strength of gravity weakens with distance squared: next time you stand on a trampoline, compare the curve 6" from your foot to the curve out at the edges. As the surface area being stretched increases, less stretching occurs.

If the only objects in the Universe were myself and the Andromeda Galaxy, 2 million LY away, I would most certainly fall towards it. In reality, this is dwarfed by comparison by my proximity to the Earth/Sun/Milky Way. It's there, it can just be reasonably ignored. The slope between me and the Earth is VERY steep and keeps me here.


A side note on calling Spacetime "elastic"
I don't like this idea because it brings up images of a rubber band being stretched thin (implies that spacetime is "thick", and might have some limit, which is nonsensical as it is not composed of anything tangible,) or that it might remain stretched out (distorted) after an object leaves, or worse that it could break, which is not an event that I want to be anywhere near! I don't hope to explain why Spacetime works... I just trust Stephen Hawking for some things.


I write too much. Someone come tell me that I'm off my rocker.

[edit: I <3 pictures!]

 

[hellokeith]

There's some problems you haven't addressed.

1. The depression in the trampoline is not what causes the child to move towards the center. An attractive force must still exist, else the curvature would not affect your position. Put another way, a depression in a trampoline in zero G in space would not cause items on the periphery to move towards the center.
2. The earth is moving around the sun, which is moving around a point in the solar system, which is moving around the edge of a galaxy, which is moving away from other galaxies. All these bodies are in motion, so the "depression" is not already there, it has to be constantly updated. The speed at which this updates is the crux of this thread.

As was said earlier, the fact that gravity is disrespectful of black holes means that we simply don't understand its mechanisms at all, other than the current theory that gravity is a property of dense massive objects.

 

[BirdDad]

Einstein's GR and SR are patched more often than Windows.
There are those out there that would do anything to prevent him from being proven wrong.

 

[CycloWizard]

There are a couple of points I'd like to make. First, it has been repeatedly stated in this thread, most recently by Foxery (not to pick on him, but he summarized the sentiment very concisely) that "The Earth orbits the Sun's position without an 8-minute delay." The bottom line is that extensive evidence exists to indicate that there is indeed an 8-minute delay in the effect of the gravitational force on earth, at least according to Hawking. Hawking actually states that the fluctuations that this non-instantaneous information transfer causes in the path of Mercury around the sun was the first proof of general relativity and the finite propagation velocity of gravity. I'm not an astrophysicist myself, so I'll have to take his word for it.

Second, general relativity has remained essentially unchanged since Einstein put forth his final version around 1916 IIRC. All of the supposed "patching" of the theory is really simply trying to apply his theory to smaller scale things, which the theory says nothing about. For things on cosmological time and length scales, his theory predicts things perfectly, at least within our ability to measure. The reason people are confused about Einstein's theory is that it does not require a particle or any other medium for the transmission of the gravity force, while quantum mechanical theory requires a particle for the transfer of any information. This is not necessarily a flaw in relativity - it could just as easily be a flaw in quantum mechanics since no particle has been observed for this purpose.

 

[hellokeith]

Originally posted by: CycloWizard
There are a couple of points I'd like to make. First, it has been repeatedly stated in this thread, most recently by Foxery (not to pick on him, but he summarized the sentiment very concisely) that "The Earth orbits the Sun's position without an 8-minute delay." The bottom line is that extensive evidence exists to indicate that there is indeed an 8-minute delay in the effect of the gravitational force on earth, at least according to Hawking.

Just because Hawking says extensive evidence exists does not make it so. In fact, Hawking has been wrong before about gravity and radiation.

 

[CycloWizard]

Originally posted by: hellokeith
Just because Hawking says extensive evidence exists does not make it so. In fact, Hawking has been wrong before about gravity and radiation.

Yes, you're right. I'll concede that your take on general relativity is more informed than Stephen Hawkings since he lost a tongue-in-cheek bet once.

 

[silverpig]

Originally posted by: hellokeith
There's some problems you haven't addressed.

1. The depression in the trampoline is not what causes the child to move towards the center. An attractive force must still exist, else the curvature would not affect your position. Put another way, a depression in a trampoline in zero G in space would not cause items on the periphery to move towards the center.
2. The earth is moving around the sun, which is moving around a point in the solar system, which is moving around the edge of a galaxy, which is moving away from other galaxies. All these bodies are in motion, so the "depression" is not already there, it has to be constantly updated. The speed at which this updates is the crux of this thread.

As was said earlier, the fact that gravity is disrespectful of black holes means that we simply don't understand its mechanisms at all, other than the current theory that gravity is a property of dense massive objects.

1. That can be answered easily. Start two people on the equator 100 km apart and walk them both due north. They will move towards each other without an attractive force between them in spite of the fact that they are both moving parallel to each other. It's a consequence of curved space that the acceleration exists and does not require any force at all. If you were to ask Einstein and Newton to draw a free-body diagram of an object in orbit, Newton would draw an arrow pointing straight down and call it the gravitational force, Einstein would say there are no forces, and both would be right.

 

[PowerEngineer]

Originally posted by: CycloWizard

Originally posted by: hellokeith
Just because Hawking says extensive evidence exists does not make it so. In fact, Hawking has been wrong before about gravity and radiation.

Yes, you're right. I'll concede that your take on general relativity is more informed than Stephen Hawkings since he lost a tongue-in-cheek bet once.

Well, the bet itself might have been tongue-in-cheek, but the matter over which they disagreed has important implications for physics. I also have to concede that Hawkings has lately come out with a conjecture or two that seem to go beyond hard physics. Even so, it's hard to argue with Hawkings' opinion on the weight of evidence for speed of gravity.

Here's a link to an interesting one-page discussion: link

 

[firewolfsm]

On the topic of weather or not it's an active attractive force,


The way I see relativity and the curvature of spacetime is something I haven't really heard described before. I always imagine a mass pulling spacetime into its center, that's how I've always accepted the fact that non moving objects still accelerate towards a gravitational field.

 

[Foxery]

Originally posted by: CycloWizard
"The Earth orbits the Sun's position without an 8-minute delay." The bottom line is that extensive evidence exists to indicate that there is indeed an 8-minute delay in the effect of the gravitational force on earth, at least according to Hawking.

Wait, what? I'll hardly argue with something like Hawking, but my novice attempts to visualize a solar system responding to "slow" gravity makes me imagine that, at the best case, planets farther away from the sun will have increasingly eccentric (e.g. acutely eliptical) orbits, and in the worst case, will be unable to maintain any stable orbit. As far as I know, they remain pretty regular.

On an even larger scale, the idea that galaxies are attracted to each other's former positions on a million-year delay doesn't sit well with me at all. This would result in very chaotic movement. (on a comic time scale, so perhaps beyond our ability to measure)