# Thread: The Speed of Gravity

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## The Speed of Gravity

Let's say our Sun compresses itself into a black hole density... how long would it take for the earth to feel? What about Uranus???

I mean, my doubt is does gravity have a "speed"? I mean, if a gravitation field changes, how fast does it spread into the universe???

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It's still the speed of light in a vacuum, c. I remember something about a graviton being transferred between the two masses, and these particles move at the speed of light, but I'm not sure..

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I think if the Sun compressed itself to black-hole density, we wouldn't feel anything. The Sun's mass would remain the same, as would the distance from here to the center of the Sun.

My very rudimentary understanding of the subject is as follows: according to General Relativity, gravity is the result of curvature in spacetime caused by nearby masses, so it does not actually take any time for the Sun to attract you. It's not a force being transmitted through space from the Sun to you; it's an effect of the spacetime geometry right where you are. But changes in the position and magnitude of masses cause changes in the local spacetime curvature. These changes propagate outward at the speed of light.

So if the Sun suddenly vanished, Earth would stop feeling its "pull" some eight minutes later.

4. Well from my understanding of Gravity it's still a large unknown, but there are Two major theroies about it.

In Quantum terms when they decribe Gravity it's via a thoretical carrier called a Graviton which does obey 'c' speed. All attempts to detect said particle have failed thus far. (The Gravity Wave Detector is the latest in trying to prove this form of gravity)

However if Gravity is truely what it seems to be (warping of space via Mass/Energy, or both) then the effect of these warps changing can propagate at faster then 'c'. Abercombies warp drive is based on this priciple, and there is some evidence that a rare galaxy or two may be moving at superluminal speeds (that is thier Space Time is moving faster then C, which is the major reason some have started calling Galaxies, "Island Universes")

But to answer your Question if the sun collapsed into a Black Hole, Earth would still fell the same amount of gravity from it. Gravity in a black hole doesn't somehow multiply. What happens is the Matter that warps space becomes so dense, that light cannot escape the effects of the warp.

All gravitational bodies have this event horizon 'mathmatically', but the event horizon is 'inside' the gravitational body and is typically smaller in size then an electron, it's when it's outside of the gravitational body that you have what is known as a black hole.

I subscribe to the second understanding that Gravity is simply a warping of space/time and not some particle. If this is the one that is proven it will mean two things. No Grand Unification Theroy (as there is no Quantum of Gravity) and two, that gravitational effects propogate faster then 'c'. This is why the second one is not as popular, because it technically violates GR understanding and we humans just love to quantify things.

5. Originally Posted by dgavin
[snip]However if Gravity is truely what it seems to be (warping of space via Mass/Energy, or both) then the effect of these warps changing can propagate at faster then 'c'. Abercombies warp drive is based on this priciple, and there is some evidence that a rare galaxy or two may be moving at superluminal speeds (that is thier Space Time is moving faster then C, which is the major reason some have started calling Galaxies, "Island Universes")
Woah. #-o This just hurts to read.
I will only take a crack at one of these.

Gravity, as we know it in either theory (QM or GR) acts at the speed of light.

Island universes was a term that had been used prior to 1927...
And no galaxy is 'super-luminous,' it only appears as such due to the expansion of space...

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Originally Posted by TravisM
And no galaxy is 'super-luminous,' it only appears as such due to the expansion of space...
Psst, it's "super-luminal".

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Originally Posted by dgavin
Abercombies warp drive is based on this priciple
I think that you mean Alcubierre. I suspect that Abercombie (and Fitchs) warp drive would also have a corresponding weft drive as well.

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Yeah, if it collapsed into a black hole, gravity would only increase when you enter the previous radius of the sun.

9. Originally Posted by Sam5
Originally Posted by TravisM
And no galaxy is 'super-luminous,' it only appears as such due to the expansion of space...
Psst, it's "super-luminal".
Psst, I know... :roll:

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I think gravity is a push from external long wavelength electromagnetic pushing masses together. Thus the graviton is a quantization of the shadow cast by the mass. So gravity travels at the speed of dark.

See the chapter I wrote in the book 'Pushing Gravity' edited by Matt Edwards available on Amazon and elsewhere.

11. Originally Posted by TravisM
Originally Posted by Sam5
Originally Posted by TravisM
And no galaxy is 'super-luminous,' it only appears as such due to the expansion of space...
Psst, it's "super-luminal".
Psst, I know... :roll:
I thought it was "super-liminal." Uh oh.

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Ok, so basically i can say that "gravity travels at the speed of light".

Like in the example given above, "if the Sun suddenly vanished, the Earth would stop feeling its pull some eight minutes later".

But nothing actually "vanishes" or "appears" as if by teleportation. How can anyone actually know??? Or people just assume gravity is as fast as light because light is the maximum anyway?

Well, maybe gravity can be measured by just moving an object, like as the moon revolves around the earth we could measure the "lag" between the motion and its gravitational effect on earth (tides, etc.)????? Is that how they measure the "speed" of the space-time geometry deformation caused by mass that we call gravity?????

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The question is often posed "when would we feel it if the Sun suddenly vanished?" But, as you say, this simply doesn't happen in general relativity, so general relativity isn't going to provide a coherent answer to the question.

What kinds of gravitational disturbances can happen? Changes in the shape of an object, for one: the simplest kind would be a change in the shape of the sun (due to some enormous internal eruption, say) that caused it to become more ellipsoidal, or to oscillate between a flattened and a football-shaped ellipsoid. General relativity says that the effect of that would propagate to Earth at the speed of light.

So it's fair to say that gravitational effects propagate at lightspeed in GR. Now, that does not mean that the Earth feels itself pulled toward the "retarded position" of the Sun where it was eight minutes ago. (Indeed, in a relativistic world it's hard to say what "the place where the sun was eight minutes ago" would even mean: according to what reference frame?) In a sense, the Sun's gravitational field is freely falling right along with it, and as a consequence the pull on the Earth is much closer to pointing to where the Sun is now. But if the Sun, say, suddenly blew apart into two pieces that went shooting off in opposite directions, the Earth would keep trucking along in its former path for eight minutes.

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## Ah, yes...

Yes, even though the Sun is speeding along its galactic orbit and gravitational changes due to a sudden change in the Sun's position would take 8 minutes to affect Earth...still the vector of attraction at Earth points to the position of the Sun right now and not its position 8 minutes ago.

As I recall, mathematical physics requires that this be so if the conservation laws are correct.

Commonly spacetime warpage is pictured as a dimple in a rubber sheet, in the case of the Sun and Earth, a Sun-dimple with the Earth rolling around the inside of the dimple (and the Moon rolling around the Earth's dimple).

One way to visualize the vector of attraction is to think that, if the Sun is considered in motion, then the gravity-dimple it makes is slightly lopsided -- the farther you get from the center of the dimple the more the "downhill" vector points slightly in advance of the Sun's position at Time T.

And it turns out that the adjustment in the "downhill" direction is such that at 8 light-minutes from the Sun the "downhill vector" points at the spot where the Sun will be in Time T + 8 minutes.

So in this view, yes, the Earth responds to the Sun's gravity because the Sun's gravity-dimple is already in place where the Earth is.

But the gravity-dimple only adjusts to smooth, regular motion -- like ORBITAL motions! So if you suddenly jerked the Sun 20,000 kilometers to the Galactic North, the effect -- travelling as gravity waves, ripples in spacetime -- would not reach Earth for 8 minutes.

And now, consider what real-world mechanism would give the Sun a non-orbital, non-smooth jerk...and that's why we don't see gravitational anomalies except in very special conditions like closely-orbiting neutron stars!

[addendum!] This is a visualization only, not a rigorous explanation. There are some good papers on the Web if one wants to really know what's going on. Have pity -- I only mean to give a layman's gloss on the subject.

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This stuff is boggling my brain! Ok, the vector of attraction points at the sun right now, so technically the "information on gravity" is instant. But what if the Sun takes an unusuall unpredicted path? Will the earth feel the crazy movement also instantly? Or maybe someone already answered this question?

Maybe we can transmit information instantly through gravity field in that case. Boy I'm in for a nobel.

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Originally Posted by mopc
This stuff is boggling my brain! Ok, the vector of attraction points at the sun right now, so technically the "information on gravity" is instant. But what if the Sun takes an unusuall unpredicted path? Will the earth feel the crazy movement also instantly? Or maybe someone already answered this question?
The transmission of the information isn't instant-- it's just that there is a compensatory effect that makes up for the delay, as long as the sun follows along a freely-falling path and doesn't do anything else. As I said above, if the sun were to suddenly change shape, or split in two, or start jetting out matter and rocket off in some crazy direction, we wouldn't feel that until eight minutes later.

A similar thing actually happens even in electromagnetism-- though there, it is a little different and only applies to straight-line motion at constant speed. If a charged particle is moving in a straight line at constant speed (and has been moving like that for long enough), the electric force produced by its electric field actually points straight toward its current position, even if it's far away. But if the particle makes a sudden left turn, that information only propagates out at finite speed. (And if you shake it back and forth, the fields make a radio wave...)

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## I agree!

Yes, I agree entirely, Matt! And as I recall, the equations of GR are of a higher order (mathematically) than the equations describing electromagnetism.

Because of that difference in order, the retardation, the adjustment for motion, that Matt describe for an electron in unaccelerated motion applies in gravitation to systems under smooth accelerated motion as well as unaccelerated motion. (Orbital motion is accelerated motion.)

(No, electrons do not 'orbit' an atomic nucleus the way the Moon orbits the Earth, and they are not therefore forced to radiate away all their energy in radio waves, as Matt most succinctly describes a 'shaken' electron doing. Electron orbitals are resonant quantum patterns in which the electron waveform resides. Sort of, in a manner of speaking.)

And no: you cannot transmit information superluminally using gravity. The gravitational field is either already at Earth -- the usual case, which causes the vector of attraction toward the Sun to adjust itself toward the Sun's advanced position and not the retarded one. Or else the waves caused by gravitational change propagate at the speed of light.

The advanced vector of attraction -- the fact that with respect to the Galaxy the Earth is orbiting around the position of the Sun right now -- is giving us information about the Sun 8 minutes ago: it tells us that the Sun was at position X,Y,Z and in motion at Time T - 8 minutes.

It does NOT tell us that the Sun will definitely, assuredly, and without a doubt actually arrive at the place the Earth is being attracted to. Nosir! If the Sun were to magically disappear, the Earth would continue to orbit the position of the non-existent Sun for 8 minutes. No information would be transmitted superluminally.

Now, the question of reference frames makes things look funny. If you use a reference frame in which the Sun is motionless, and all the planets move around the Sun, then there is no distortion of the gravity-dimple and the planets all move around the unaccelerated, unmoving Sun. If you take into account the Sun orbiting the Galaxy, the gravity-dimple distortion described above comes into play. And if you take into account the Galaxy's motion through the Local Cluster, then the gravity-dimple is distorted to account for that motion! Even with all those apparent motions, the vector of attraction points toward the Sun's position at time T and not at T - 8 minutes.

(Please, please note carefully that in the paragraph above I am referring to T - 8 minutes as experienced by Earth. In my previous post, I referred to T + 8 minutes as experienced by the Sun. Sorry I am so darned wishy-warshy.)

But it's not really odd at all. The laws of physics look the same no matter what the observer's state of motion, and so whether you define your observer as stationary with respect to the Galaxy or stationary with respect to the center of gravity of the Local Cluster, the calculations will always have Earth's vector of attraction pointing to the advanced position of the Sun.

18. Hate to point this out, but there is no evidence yet that gravity effects are limited to 'C'.

If one of the reasons fo the gravity wave detectory. If Gravitatioanly waves can be detected then it would be contrained by 'GR'. If they arn't...

Until we know though you can't say that gravity is limited, or not limited to light speed.

It's still an unknown.

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## Quite possibly!

It's reasonable to say that, I suppose.

GR predicts gravitational waves, and some systems have been discovered which GR predicts would produce gravitational waves. The systems are closely orbiting neutron stars. The waves predicted by GR would carry energy out of the orbital system, causing measurable degeneration of the orbits.

This has been observed, and the orbital degeneration matches the predictions of GR for the amount of energy which should be carried out of the system by gravitational waves.

So there is, I think, indirect evidence for G-waves as predicted by GR. GR has not, in the regime of large bodies and large motions, been falsified.

It's hoped that the gravity observatories will not only detect these waves but give us details about the high-gravitational regime.

But certainly gravitational waves so far remain undetected.

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Originally Posted by dgavin
Hate to point this out, but there is no evidence yet that gravity effects are limited to 'C'.

If one of the reasons fo the gravity wave detectory. If Gravitatioanly waves can be detected then it would be contrained by 'GR'. If they arn't...

Until we know though you can't say that gravity is limited, or not limited to light speed.

It's still an unknown.
Okay, then, if this is correct my question remains ananswered? :-k

What the heck is gravity any way? In quantum, it's a frigging particle that no one has ever seen, in GR it's space-time geometry distorted by mass.

But if it's a particle ("graviton") than it can have mass, then gravity has it's own gravity field? :-s . Okay then it's a particle with no mass? That no one measures? That no one detects? Than isnt it Sagan's "invisible dragon in the garage"? An "aether"?

The GR explanation seems more plausible, even though it's not actually an explanation, just the stating of the rules of a phenomenon. No one knows exactly why gravity exists, do they?

21. Originally Posted by mopc
Originally Posted by dgavin
Hate to point this out, but there is no evidence yet that gravity effects are limited to 'C'.

If one of the reasons fo the gravity wave detectory. If Gravitatioanly waves can be detected then it would be contrained by 'GR'. If they arn't...

Until we know though you can't say that gravity is limited, or not limited to light speed.

It's still an unknown.
Okay, then, if this is correct my question remains ananswered? :-k

What the heck is gravity any way? In quantum, it's a frigging particle that no one has ever seen, in GR it's space-time geometry distorted by mass.

But if it's a particle ("graviton") than it can have mass, then gravity has it's own gravity field? :-s . Okay then it's a particle with no mass? That no one measures? That no one detects? Than isnt it Sagan's "invisible dragon in the garage"? An "aether"?

The GR explanation seems more plausible, even though it's not actually an explanation, just the stating of the rules of a phenomenon. No one knows exactly why gravity exists, do they?
No it's less likely it's an Aether then one of the other two. Although if you were to approachit from a purely research stand point, you couldn't rule that out yet either.

As one gent pointed out, Gravity seems to follow GR contraints in some cases, but not in others.

There was a resent show on Nova about a galaxy or two that were super massive and thier velosity was superluminal (they wern't talking appearant motion either) so it seems space time itself doesn't obey GR contraintis. Some have started calling this behavior 'Gravitational Detachment' as space time seems to move differently around these objects.

22. Originally Posted by mopc
Okay, then, if this is correct my question remains ananswered? :-k
Well, if you want to be pedantic. But you could also say that our best theory of gravity, general relativity, which has held up to a mountain of tests, predicts that gravitational disturbances propagate at the speed of light. And though we don't have a complete quantum theory of gravity, we know enough about the form such a theory would have to take to know that it, too, would predict that gravity propagates at the same speed as light. It's possible of course that both of these will be completely overturned, but just saying "we really don't know" implies that it's equally likely either way, and that's really not accurate.

Originally Posted by mopc
But if it's a particle ("graviton") than it can have mass, then gravity has it's own gravity field? :-s . Okay then it's a particle with no mass? That no one measures? That no one detects? Than isnt it Sagan's "invisible dragon in the garage"? An "aether"?
No, the graviton would be a massless particle, just like the photon. It would however (also like the photon) have energy associated with it, and so could produce its own gravity (it's really energy that couples with gravity, not mass), which is one of the things that makes gravity more complex than electromagnetism (photons don't have charge). As for no one measuring or detecting it, it's probably just a matter of time and more precise detectors. Since gravity is so weak, comparatively, gravitons are hard to detect. It was a long time before we were able to make detectors capable of spotting the elusive neutrino, too.

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Originally Posted by Grey
No, the graviton would be a massless particle, just like the photon. It would however (also like the photon) have energy associated with it, and so could produce its own gravity (it's really energy that couples with gravity, not mass), which is one of the things that makes gravity more complex than electromagnetism (photons don't have charge).

Let me mention something that I’ve been researching during the past year, which is the origin of the words “particle” and “photon” in association with “light”. Actually, I think the English word “particle” is not a good word to use. Several German-language writers in the 19th and early 20th Century used three different German words that basically meant “part”, “smaller part”, and “smallest part”. Often these words were translated into English as “particle.” I think this might possibly have been due to the German use of the German words for “part”, “smaller part,” and “smallest part” in reference atomic “particles” like electrons and protons. But the German words would have meant something slightly different in both cases, i.e. for light and for atomic parts. For example, a “particle” of an atom might be considered to be a “little round thing”, while a “particle” of light might be considered to be a “long little group or bundle of waves”, or perhaps “one single wave cycle that has a specific wavelength”.

Planck, who was one of the first people to write about what we now call a single “photon,” used one of the German words around 1900, in reference to light, but early translations of his paper did not use the word “particle”. It used the English word “part”. The early English versions of the German terms Planck used in his “photoelectric effect” papers were: “discrete quantity” and “the energy element ε”. These terms have since been referred to in many English papers and books as “particle,” “quantum”, and “photon”, leading some people to believe they are “little round things”, rather than a very small wave or wave train of EM waves. The “photon” word didn’t even come along until someone invented it in 1926, so it was not used in Planck’s or Einstein’s early papers on this subject.

I have an English translation of Einstein’s famous 1905 “photoelectric effect” paper, and it does not contain the word “photon”, and I can’t find the term “light particle” in it. The English translation of his first main explanation reads like this: “consists of a finite number of energy quanta that are localized in points in space, move without dividing, and can be absorbed or generated only as a whole.”

I’ve learned enough from my research of English translations of old German papers to know that we really need to see Einstein’s original German text to determine what he actually meant by the term “points in space”. I think he might have meant a “point” where a wave front is located and another point behind that where the “wave end” is located, regarding a single “energy quanta” that was made up of EM waves and that had a wavelength. I’m not sure he meant that a single “energy quanta” could be described as being at a single point in space, and I’m sure he knew in 1905 that Planck was not talking about a point particle, but, essentially, he was talking about the smallest shortest bundle of electro-magnetic waves of light that an atom could emit, since both Plank and Einstein tended to use the Maxwell model of light waves at that time. In other words, a Maxwell-type short “wavelet” or the shortest “wave train” or perhaps just one full cycle of a Maxwell electric and magnetic wave traveling through space would have been the “the energy element ε” that Planck was talking about in 1901 and what Einstein called “a finite number of energy quanta” contained in a “ray” of light in his 1905 paper.

We can study Einstein’s paper and Planck’s paper and think of their view of a “quantum” as the smallest group of waves a single light ray or beam could be broken up into, and a single “photon” being the shortest emission of light waves by a single atom. The word “photon”, in my opinion, which they did not use, misrepresents their own ideas about what a light “quantum” was. This was not their fault, but it represents a modification (and I think a misinterpretation) of their original ideas by later science writers. The way a single “photon” of light can actually be an EM “wave” yet act like a “particle” is in the way the wave hits and resonates an atomic absorber. An EM wave can be a true “wave”, but it can hit an atom with a force of energy, and without being a physical “little round thing” type of “particle”.

For example, you can set a small round magnet on its edge on a table top, with its South pole aimed to the right, and you can pass another small round magnet by it, with its South pole pointing left, and you can knock over the first magnet without either of the magnets touching each other. The energy force that knocks over the magnet is strictly a “force field”, but not a “particle”, unless, perhaps, QM scientists think a magnetic field is made up of very tiny little “particles”, but, personally, I don’t have that particular belief at this time.

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Originally Posted by Sam5
The energy force that knocks over the magnet is strictly a “force field”, but not a “particle”, unless, perhaps, QM scientists think a magnetic field is made up of very tiny little “particles”, but, personally, I don’t have that particular belief at this time.
QED?

25. Originally Posted by Grey
Originally Posted by mopc
Okay, then, if this is correct my question remains ananswered? :-k
Well, if you want to be pedantic. But you could also say that our best theory of gravity, general relativity, which has held up to a mountain of tests, predicts that gravitational disturbances propagate at the speed of light. And though we don't have a complete quantum theory of gravity, we know enough about the form such a theory would have to take to know that it, too, would predict that gravity propagates at the same speed as light. It's possible of course that both of these will be completely overturned, but just saying "we really don't know" implies that it's equally likely either way, and that's really not accurate.

Originally Posted by mopc
But if it's a particle ("graviton") than it can have mass, then gravity has it's own gravity field? :-s . Okay then it's a particle with no mass? That no one measures? That no one detects? Than isnt it Sagan's "invisible dragon in the garage"? An "aether"?
No, the graviton would be a massless particle, just like the photon. It would however (also like the photon) have energy associated with it, and so could produce its own gravity (it's really energy that couples with gravity, not mass), which is one of the things that makes gravity more complex than electromagnetism (photons don't have charge). As for no one measuring or detecting it, it's probably just a matter of time and more precise detectors. Since gravity is so weak, comparatively, gravitons are hard to detect. It was a long time before we were able to make detectors capable of spotting the elusive neutrino, too.
There is some more to think about this way, if Gravity is a paritcle that carries the force, does it have an anti particle (which would be probable), and does that explain the pushing of galaxies away from the empty area's of the universe? This GR consept of gravity does fit with what we know at current.

If Gravity is simply a warp of space time by the presence of something (no better way to explain it) and not a force, then it fits the explaintaion of the few gravitationaly detached faster then 'C' galaxies.

Could it be a combination where the Warping of Space by mass/energy has the effect of generating virtual or real particles that are gravitons?

Thats the main issue is that Gravity as we know it seems to exhibt both GR sub 'c' behavior, and warped space behavior.

Which might be a good indication it could have a duality nature like some other things in QM. But lets not go into that as that is ATM thinking.

Althoug looking at it this way, duality gravity would allow both models we know of to fit together.

26. The graviton, like the photon, is its own antiparticle!

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Originally Posted by Sam5
Let me mention something that I’ve been researching during the past year, which is the origin of the words “particle” and “photon” in association with “light”. Actually, I think....
As a translator of German myself, I understand this always difficult task of precise translation, since the semantic field of words in two languages rarely coincide. Imagine how much of science worldwide is misunderstood because of language mismatch!

But returning to the topic, I can thus say that gravity "travels" at the speed of light except under certain special conditions, when it can be faster, right? 8-[

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Originally Posted by mopc
As a translator of German myself, I understand this always difficult task of precise translation, since the semantic field of words in two languages rarely coincide. Imagine how much of science worldwide is misunderstood because of language mismatch!

But returning to the topic, I can thus say that gravity "travels" at the speed of light except under certain special conditions, when it can be faster, right? 8-[
Frankly, I don’t know, but from my own “classical” perspective, I would think that gravity “should” travel at some speed, and the speed of light is ok with me.

Regarding gravity “waves”, I think what that means is actually more like a “gravity tsunami”. Not a long series of continuous waves like light, but a sudden big “wave” of change, if the body in question suddenly moves or blows up. I think that is the kind of “wave” most people have been searching for. If bodies like rapidly revolving binaries continue to revolve, then they might send out a long series of changing gravity “waves” that are related to their changing positions.

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Originally Posted by Fortis
QED?

I like to think of things like gravity and magnetic fields as being “continuous”, but the QM guys like to talk about everything in terms of quanta and “particles”. So, maybe what I think of as a smooth and continuous gravity and magnetic field is actually made up of little things that are so small, I just don’t feel their roughness or their individual bumps. It’s like, even honey and molasses are supposed to be made up of individual molecules. So maybe if we keep examining the smallest of scales, even gravity and magnetic fields are made up of a lot of little individual things.

30. Originally Posted by Sam5
I like to think of things like gravity and magnetic fields as being “continuous”, but the QM guys like to talk about everything in terms of quanta and “particles”. So, maybe what I think of as a smooth and continuous gravity and magnetic field is actually made up of little things that are so small, I just don’t feel their roughness or their individual bumps. It’s like, even honey and molasses are supposed to be made up of individual molecules. So maybe if we keep examining the smallest of scales, even gravity and magnetic fields are made up of a lot of little individual things.
For the magnetic field, we know that's true. Well, "know" in the sense that our best available theory (quantum electrodynamics, as Fortis pointed out), which has passed all tests thus far thrown at it and shown amazing success, says that's the case. The electromagnetic force, and thus any effects ascribed to it, such as the forces caused by electric or magnetic fields, is the result of exchange of virtual photons. At the smallest scales of interaction, it is necessary to take into account this "graininess" of the EM interaction to make the right predictions, so it's quite likely that any successor to QED will also incorporate it. We expect the same to be true of gravity, but with the relative weakness of gravity as compared with the other forces, we have yet to show that directly by experiment. I don't think there are any physicists who will be surprised by its eventual discovery in the coming years, though the experimenters on that project will doubtless be on the short list for a trip to Stockholm.

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