speed of gravity

[CycloWizard]

Originally posted by: Foxery
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)

If you google "Mercury perihelion," you can find quite a bit of information to back this up. Einstein himself computed the precession of Mercury's perihelion as perhaps the first experimental validation of part of general relativity. Newton's formulation failed to predict it correctly due to the assumption that gravity propagates instantaneously.

edit: For those of you having a hard time accepting that even the curvature of spacetime cannot occur faster than the speed of light, it might also be worth addressing the blanket analogy. If I try to make a "dip" in a blanket for my ball to roll into, the blanket's deformation also proceeds over a finite amount of time, with the maximum velocity fixed at the speed of sound in the material of the blanket. It simply takes a finite amount of time for the forces to propagate through the blanket material to cause a deformation at a remote location. For example, what happens if you pick up a bedsheet at one end and shake it? Depends on how fast you shake it. If you shake it an infinitesimal amount infinitely fast, it will still take the length of the bedsheet divided by the speed of sound in the sheet for the far end to move.

 

[3NF]

I think gravity is some manifestation of electromagnetism that we haven't figured out yet. Obviously I can't prove it, but that's what I think

 

[hellokeith]

Originally posted by: Foxery
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)

All very good points. Indeed, once you start critically thinking about gravity, some problems arise with the "light speed or bust" mindset.

Originally posted by: CycloWizard
If you google "Mercury perihelion," you can find quite a bit of information to back this up. Einstein himself computed the precession of Mercury's perihelion as perhaps the first experimental validation of part of general relativity. Newton's formulation failed to predict it correctly due to the assumption that gravity propagates instantaneously.

Interestingly enough, two independent researchers recently came to the same calculated conclusion that there is no such thing as planetary precession, but in fact that our entire solar system orbits/shares an orbit with the star Sirius. There are alot of problems with our current universal time system, hence why scientists continually have to add/subtract seconds each year. So the point here being that some doubt is being cast on using planetary precession as support for the current assumptions regarding gravity.

Originally posted by: CycloWizard
edit: For those of you having a hard time accepting that even the curvature of spacetime cannot occur faster than the speed of light, it might also be worth addressing the blanket analogy. If I try to make a "dip" in a blanket for my ball to roll into, the blanket's deformation also proceeds over a finite amount of time, with the maximum velocity fixed at the speed of sound in the material of the blanket. It simply takes a finite amount of time for the forces to propagate through the blanket material to cause a deformation at a remote location. For example, what happens if you pick up a bedsheet at one end and shake it? Depends on how fast you shake it. If you shake it an infinitesimal amount infinitely fast, it will still take the length of the bedsheet divided by the speed of sound in the sheet for the far end to move.

I have seen some argue that in fact gravity is non-existent. That the bending/curving of space-time by massive objects provides satisfactory explanations for all the movements of celestial bodies, and that the gravity force equation is better described as "strength of space-time curvature".

Not sure I personally buy that, but I haven't critically thought about it in depth. The real question regarding space-time curvature is if it is powerful enough in of itself to "pull" massive objects into the curve, or does there need to be an attractive force as well.

 

[silverpig]

Originally posted by: hellokeith

Not sure I personally buy that, but I haven't critically thought about it in depth. The real question regarding space-time curvature is if it is powerful enough in of itself to "pull" massive objects into the curve, or does there need to be an attractive force as well.

No no, that stuff is all nicely taken care of in GR.

 

[PowerEngineer]

<groan>

It isn't my intention to insult anyone here, but let me point out that the reality out there isn't limited by what we can visualize, what "sits well" with us, or what we think. Our thinking is not a challenge to reality -- reality is a challenge to our thinking!

The internet may be a wonderful blessing in some ways, but finding reliable science on it can be a bit like panning for gold. There's a lot of unfounded claims of truth and beauty out there. I'm amazed at how many websites claim to have proofs that Einstien's theory of general relativity is wrong. (Doesn't the undeniable existance of nuclear weapons count for anything?) There certainly are a lot of people claiming that the additional precession caused by relativistic affects on Mercury doesn't exist. Frankly, I'll bet that there are many more than just two "independent researchers" making that claim.

Where we need to be a little more "critical" in our thinking is around their claims. For example, what exactly are Sirius and our solar system supposed to be orbiting around? If it's the galactic center, then why just Sirius? Why not the other four stars that are closer to us? And adding "leap" seconds has to do with the pesky fact that the earth refuses to rotate a speed to give us EXACTLY 365.25 days/year; it's not a sign of some problem with our understanding of time (thought there may be some).

I will grant you that by definition there must always be "some doubt" about the validity of every scientific theory. There's always the possibility that new facts will overturn what we thought we knew (the divider between faith and science). That said, these internet claims are hardly worth citing as a reason for doubt.

A couple of other comments:

It was suggested earlier that the effects of light-speed gravity change propagation would be greater for planets orbiting further from the sun. I suggest that it might be just the opposite because the angular change caused by the sun's movement becomes smaller as the distance between the sun and a planet increase (and that could well explain why it's Mercury's presession that shows the realitivistic effect).

The "bending/curving of space-time by massive bodies" is essentially what Einstien's theory of general relativity is! It says that masses are not really attracted to each other (i.e. no "force" of gravity between them), but that their masses curve space-time in a way that makes their straight line travel (through space-time) seem curved in our three space dimensions. This nicely explains why massless things like light are affected by gravity(which wouldn't be the case under Newtonian gravity).

 

[silverpig]

Originally posted by: PowerEngineer

<groan>

It isn't my intention to insult anyone here, but let me point out that the reality out there isn't limited by what we can visualize, what "sits well" with us, or what we think. Our thinking is not a challenge to reality -- reality is a challenge to our thinking!

The internet may be a wonderful blessing in some ways, but finding reliable science on it can be a bit like panning for gold. There's a lot of unfounded claims of truth and beauty out there. I'm amazed at how many websites claim to have proofs that Einstien's theory of general relativity is wrong. (Doesn't the undeniable existance of nuclear weapons count for anything?) There certainly are a lot of people claiming that the additional precession caused by relativistic affects on Mercury doesn't exist. Frankly, I'll bet that there are many more than just two "independent researchers" making that claim.

Where we need to be a little more "critical" in our thinking is around their claims. For example, what exactly are Sirius and our solar system supposed to be orbiting around? If it's the galactic center, then why just Sirius? Why not the other four stars that are closer to us? And adding "leap" seconds has to do with the pesky fact that the earth refuses to rotate a speed to give us EXACTLY 365.25 days/year; it's not a sign of some problem with our understanding of time (thought there may be some).

I will grant you that by definition there must always be "some doubt" about the validity of every scientific theory. There's always the possibility that new facts will overturn what we thought we knew (the divider between faith and science). That said, these internet claims are hardly worth citing as a reason for doubt.

A couple of other comments:

It was suggested earlier that the effects of light-speed gravity change propagation would be greater for planets orbiting further from the sun. I suggest that it might be just the opposite because the angular change caused by the sun's movement becomes smaller as the distance between the sun and a planet increase (and that could well explain why it's Mercury's presession that shows the realitivistic effect).

The "bending/curving of space-time by massive bodies" is essentially what Einstien's theory of general relativity is! It says that masses are not really attracted to each other (i.e. no "force" of gravity between them), but that their masses curve space-time in a way that makes their straight line travel (through space-time) seem curved in our three space dimensions. This nicely explains why massless things like light are affected by gravity(which wouldn't be the case under Newtonian gravity).

2 comments:

1. The sun and sirius would be orbiting about their common center of mass, but that's just a detail.
2. Light is deflected in Newtonian gravity. It's exactly 1/2 of what it really is (and what GR predicts).

 

[BirdDad]

Originally posted by: PowerEngineer

<groan>
(Doesn't the undeniable existance of nuclear weapons count for anything?)

Just what did he have to do with nukes anyway,he did not help make the bomb and the Nazis where building one themselves and they certainly weren't using his research?

 

[silverpig]

Originally posted by: BirdDad

Originally posted by: PowerEngineer

<groan>
(Doesn't the undeniable existance of nuclear weapons count for anything?)

Just what did he have to do with nukes anyway,he did not help make the bomb and the Nazis where building one themselves and they certainly weren't using his research?

E = mc^2

 

[BirdDad]

nukes were going to be made wether he did anything or not

 

[silverpig]

Originally posted by: BirdDad
nukes were going to be made wether he did anything or not

How do you figure? He was the one who theorized a small amount of matter could be turned into a large amount of energy. Sure we would have gotten there eventually, but it wouldn't have happened when it did without him.

 

[Jeff7]

A possibly relevant question, and hopefully not too off-tangent: Does light produce gravity? It's my thinking that it should, in a sort of "conservation of gravity" rule. The problem would be measuring it.

Take 1kg, measure the gravitational attraction of it. It's going to be really minimal. Now convert that 1kg into pure energy. 8.9876 * 10^16 Joules. First, do that without completely obliterating the laboratory. Then measure the gravitational attraction of this energy. It's my thinking that it should still have the same gravitational attraction, the same "mass." It's like converting a solid to a gas, except that this is converting "solid" subatomic particles into a "gaseous" energy.


I wonder what physics will be like in a few hundred years. Matter is tightly bound energy. Energy is tightly bound spacetime. Who knows. So much to learn, and we've only just begun to see what's out there.

 

[firewolfsm]

Light would have gravitational force, and shows many clues pointing to that. It even has momentum, (solar sails)

 

[Jeff7]

Originally posted by: firewolfsm
Light would have gravitational force, and shows many clues pointing to that. It even has momentum, (solar sails)

Is this mass accounted for when they do calculations on a large scale - I'm of the opinion that "dark matter" isn't really some unseen substance out there, but that it's something missing from our calculations. The mass of all the EM radiation in the Universe might be part of this. I'd certainly hope that they wouldn't miss something so simple though.

 

[Nathelion]

Originally posted by: Jeff7

Originally posted by: firewolfsm
Light would have gravitational force, and shows many clues pointing to that. It even has momentum, (solar sails)

Is this mass accounted for when they do calculations on a large scale - I'm of the opinion that "dark matter" isn't really some unseen substance out there, but that it's something missing from our calculations. The mass of all the EM radiation in the Universe might be part of this. I'd certainly hope that they wouldn't miss something so simple though.

heh - funny you should say that. Thing is, that is exactly what dark matter and dark energy are - the calculations didn't fit reality, so "dark matter" and later "dark energy" was invented to cover the holes and achieve the proper results in cosmological calculations. That's what dark matter IS.
Now you ask what does the stuff look like, is it clingy goop, rogue groups of comets made of anti-matter, Godzilla taking a nap, or giant balls of cosmic ice cream? No one knows, because no objects or phenomena have been observed which account for the errors in these calculations. Since no one has been able to spot this "dark matter" with a telescope, it is very appropriately called dark. The "matter" part is just pure speculation.

 

[silverpig]

Originally posted by: Jeff7

Originally posted by: firewolfsm
Light would have gravitational force, and shows many clues pointing to that. It even has momentum, (solar sails)

Is this mass accounted for when they do calculations on a large scale - I'm of the opinion that "dark matter" isn't really some unseen substance out there, but that it's something missing from our calculations. The mass of all the EM radiation in the Universe might be part of this. I'd certainly hope that they wouldn't miss something so simple though.

No it isn't radiation. It has a different equation of state from matter. The dark matter behaves like matter and not like radiation at all. And all the radiation is nicely accounted for.

 

[Eeezee]

Originally posted by: firewolfsm
Light would have gravitational force, and shows many clues pointing to that. It even has momentum, (solar sails)

Light would have gravitational force? Under what circumstances?

Light has no mass. It does not have a gravitational force associated with it (just as a gluon does not have an EM force associated with it - the W and Z bosons are unique in that they have mass and thus have an insignificant gravitational force associated with them).

Photons have mo mass, but they still have energy associated with them. It is a relatively simple calculation to find the radiation pressure in QM or E&M while assuming massless radiation.

You do not need mass to have momentum - energy and mass are related, and both are related to momentum as well. You can have an object that has energy and momentum but no mass. The "rest mass" of light is zero. In calculating invariant energy, you still have momentum without mass.

 

[Eeezee]

Originally posted by: Jeff7
A possibly relevant question, and hopefully not too off-tangent: Does light produce gravity? It's my thinking that it should, in a sort of "conservation of gravity" rule. The problem would be measuring it.

Take 1kg, measure the gravitational attraction of it. It's going to be really minimal. Now convert that 1kg into pure energy. 8.9876 * 10^16 Joules. First, do that without completely obliterating the laboratory. Then measure the gravitational attraction of this energy. It's my thinking that it should still have the same gravitational attraction, the same "mass." It's like converting a solid to a gas, except that this is converting "solid" subatomic particles into a "gaseous" energy.


I wonder what physics will be like in a few hundred years. Matter is tightly bound energy. Energy is tightly bound spacetime. Who knows. So much to learn, and we've only just begun to see what's out there.

What does a particle need to do in order to "produce" gravity? It would need to be able to emit/absorb a gravity force-carrier (a theoretical graviton). Generally a particle has mass if it is able to emit such a particle. Photons are massless, and in all modern particle theories have no gravitational interaction associated with them.

Not everything is conserved. Look up CP-violation sometime. At the LHC we will be looking for other unique forms of violation physics that are currently considered impossible.

 

[silverpig]

Originally posted by: Eeezee

Originally posted by: firewolfsm
Light would have gravitational force, and shows many clues pointing to that. It even has momentum, (solar sails)

Light would have gravitational force? Under what circumstances?

Light has no mass. It does not have a gravitational force associated with it (just as a gluon does not have an EM force associated with it - the W and Z bosons are unique in that they have mass and thus have an insignificant gravitational force associated with them).

Photons have mo mass, but they still have energy associated with them. It is a relatively simple calculation to find the radiation pressure in QM or E&M while assuming massless radiation.

You do not need mass to have momentum - energy and mass are related, and both are related to momentum as well. You can have an object that has energy and momentum but no mass. The "rest mass" of light is zero. In calculating invariant energy, you still have momentum without mass.

Photons have no rest mass. They do produce gravity though.

Where does light produce gravity? Well I can give a mathy answer, or a hand-wavy answer.

Hand wavy: The Einstein equation relates curvature to mass-energy density. Thus, mass and energy both generate curvature, and thus gravity.

Mathy:

We can basically say (local spacetime curvature) = (local mass/energy density)

The stress-energy tensor T_(ab) is a measure of matter and energy density.
The Ricci curvature R_(ab) is a measure of local curvature.

I'll skip a lot of math and derivation (mostly because I don't really remember it) but you can use some identities of differential geometry, but you basically end up with Einstein's famous field equation in tensor form:

G_(ab) = 8 pi G T_(ab)

 

[Foxery]

Tensor equations are beyond me, but let's go simple:

A star emits light; star retains its mass, as light has no mass.
As an object with a fixed mass, the star generates a constant force of gravity throughout its entire lifespan.
If light also causes gravity, then either the sum of gravity in the Universe is constantly skyrocketing and will quickly cause it to collapse, (not happening!) or our concept of massless photons is broken.

 

[silverpig]

Originally posted by: Foxery
Tensor equations are beyond me, but let's go simple:

A star emits light; star retains its mass, as light has no mass.
As an object with a fixed mass, the star generates a constant force of gravity throughout its entire lifespan.
If light also causes gravity, then either the sum of gravity in the Universe is constantly skyrocketing and will quickly cause it to collapse, (not happening!) or our concept of massless photons is broken.

It's not just mass that produces gravity, but energy. The star emits energy, thus it's gravity slowly goes down over time.

 

[RideFree]

Originally posted by: Nathelion

Originally posted by: Jeff7

Originally posted by: firewolfsm
Light would have gravitational force, and shows many clues pointing to that. It even has momentum, (solar sails)

Is this mass accounted for when they do calculations on a large scale - I'm of the opinion that "dark matter" isn't really some unseen substance out there, but that it's something missing from our calculations. The mass of all the EM radiation in the Universe might be part of this. I'd certainly hope that they wouldn't miss something so simple though.

heh - funny you should say that. Thing is, that is exactly what dark matter and dark energy are - the calculations didn't fit reality, so "dark matter" and later "dark energy" was invented to cover the holes and achieve the proper results in cosmological calculations. That's what dark matter IS.
Now you ask what does the stuff look like, is it clingy goop, rogue groups of comets made of anti-matter, Godzilla taking a nap, or giant balls of cosmic ice cream? No one knows, because no objects or phenomena have been observed which account for the errors in these calculations. Since no one has been able to spot this "dark matter" with a telescope, it is very appropriately called dark. The "matter" part is just pure speculation.

I predict that there is no such thing as "Dark Matter" or "Dark Energy".
Physicists and Astronomers (Astrophysicists) simply put some lipstick on the pig.

 

[Foxery]

Question about our dark matter tangent -

What assumptions does our current estimate of the mass of the Universe make? For example, does it assume that "most" stars will have planets, asteroids, and comets, all of which are "dark"? There's a lot of room out there for objects that don't happen to be engaged in nuclear fusion... How intelligent is this estimate, and has it been updated now that we're seeing how common large extrasolar planets are?

 

[PowerEngineer]

Originally posted by: Foxery
Tensor equations are beyond me, but let's go simple:

A star emits light; star retains its mass, as light has no mass.
As an object with a fixed mass, the star generates a constant force of gravity throughout its entire lifespan.
If light also causes gravity, then either the sum of gravity in the Universe is constantly skyrocketing and will quickly cause it to collapse, (not happening!) or our concept of massless photons is broken.

Make everything as simple as possible, but not simpler.
-- Albert Einstein --

Stars fuse lighter elements (e.g. hydrogen) into heavier ones (e.g. helium), releasing energy (in the form of light/radiation) because some of the mass is converted into energy when the elements fuse. Therefore, the star's mass does change (by an extremely small amount) over time, as does its gravitational attraction (but the change is too small to measure).

 

[Biftheunderstudy]

Objects like planets, comets, and asteroids make up less than a percent of a stellar system so even if every star has such a system and we haven't included any of it we're still only talking a 1% unaccounted for mass. Now for some light statistics on dark matter..
The mass of the luminous matter in the milky way only accounts for about 5% of the total mass of the galaxy. The rest of the 95% is non-luminous, or Dark.
This is not just observed for the milky way, every galaxy as far as I'm aware has been shown to have a dark matter halo with the average somewhere around 75%.
Astronomers have tried many things to account for all this extra mass including non massless neutrinos, brown dwarfs, black holes and a host of other theories. From a physicists point of view, Dark Matter and Dark Energy are ugly so there are a lot of competing theories to get rid of it but its hard to dispute the evidence. The mass of galaxies does appear to be made up of mostly non-luminous matter.
If your curious do some searches on Dark Matter, extra-galactic X-ray gas, rotation curves, and galaxy cluster dynamics for starters.

 

[RideFree]

Originally posted by: Biftheunderstudy
If your curious do some searches on Dark Matter, extra-galactic X-ray gas, rotation curves, and galaxy cluster dynamics for starters.

What makes you think I haven't?
(Actually, I've been smoking something and made it all up. - Habinos SA Cohibas)

Physicist Neil Turok: Big Bang Wasn't the Beginning
He'll agree with you on the "Dark Matter" & "Dark Energy" thingy.

Edited for correction: "If your curious..." should have been, "If you're curious..."