Why does gravity propagate at the speed of light?

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CycloWizard

Lifer
Sep 10, 2001
12,348
1
81
Originally posted by: f95toli
Yes, but the problem is that there is no such thing as a solid rod.
When you push one end of the stick you are only transmitting "information" to the atoms AT THAT END, in order for that information to reach the OTHER end of the rod these atoms have to interact with their neighbors etc.
Hitting sticks is therefore a very slow way of communicating; essentially the effect only travels at the speed of sound in the material.
Just to phrase it another way: in a perfectly elastic rod, the information transfer would be simultaneous (i.e. faster than the speed of light). However, there is no such thing as a perfectly elastic material. Even structural materials like steel exhibit viscoelasticity (that is, their response to stress is time-dependent). For most applications, we can consider the response instantaneous, but in reality there is always some dynamic component involved, even if it is only inertial in nature.
 

mbass

Junior Member
May 2, 2007
2
0
0
Sorry. This is LONG but I would really like to hear some thoughts on this idea I had. This would be better with graphics, but I am no artist - so maybe it really wouldn't.


For some time I have been unable to completely accept certain basic theories of Physics pertaining to gravity and other principle interactions. Recently I saw a presentation of The Elegant Universe in which a visual aid was used in explaining the way in which objects of substantial gravity warp space-time (the image was that of sun and an orbiting planet causing a trampoline-like deformation in an otherwise two-dimensional grid) and how other objects of gravity are then caught in that deformation.

When grasping behavior characteristics of Physics, I generally try to form graphical models in my own mind that are as accurate as possible in how a situation scales up to more complex/all-inclusive perspectives. So, realizing the limitations of that model, I tried to view a similar two-dimensional ?sun? object that is located within the same two-dimensional grid. However instead of having the grid warp to represent gravity?s effect, I imagined individual dots (or pixels - infinitely small points) organized around my sun, with the density of those dots at any given region representing the strength of the effect of gravity in that region.

With my sun and the simulated effects of its gravitational field established in my model, I then included the additional complexity of an orbiting earth, also including it?s own gradient of gravity dots.

Viewing this model, it quickly becomes apparent that there is the greatest concentration of these gravity dots along the straight line connecting the sun and earth. So I noted the possibility that these gravity particles could potentially form a ?chain? or ?glue,? attracting each other more strongly as they become more densely packed. This pulling force would then extend from the sun to the earth delivering the resulting force we perceive as gravity.

Now, I have never been a fan of ?pulling? forces, and that discomfort led me toward considering the possibility that the exact opposite might be happening. That is, what happens to our model if we take every point where we just said there was a gravity dot, and we take every point in our model where there currently is NOT a gravity dot, and we reverse things? Essentially we end up with a scattering of (what has now changed to ?anti-?) gravity dots across the model (approximately uniform in density) except in locations near our two gravity objects. The closer we get to our sun and earth, the fewer anti-gravity dots we find, and along that same line - directly between the sun and earth - we find gravity dots to be least dense of all.

To me, this model suddenly looks intriguingly natural. If we assume these anti-gravity dots to possess some characteristics of repulsion (where they not only repel each other, but they repel ALL objects of gravity also), then it would establish a system where this endless field of gravity dots exists and exerts a constant and surrounding force on any object exhibiting gravity. Likewise any object of gravity will push back against the field in all directions, and equilibrium will be reached in the form of a density gradient of gravity dots - increasing in density as the distance from the object increases.

(Here I will introduce the term ?grass? to replace ?gravity dot? ? grass because when this idea occurred to me, grass was everywhere I looked except in the shaded areas under trees where is was less dense. There is an impulse to use a term like graviton or some other currently proposed particle, but use of whatever particle I chose would almost certainly be wrong, and so I am going with a meaningless term that can be changed later. Surely someone else can come up with something better to call my gravity dots.)

Just as before, adding a second body of gravity to the model adds a new wrinkle. If we consider the line connecting the two objects, it is apparent that each object will feel a weaker grass-push from that region than each of the two objects feels from their other surrounding areas. The resulting sum of forces acting on each of them will drive them toward each other along that line of least density.


Inconsistencies Explained

If my claim then is that there exists a vast expanse of this grass, then we should be able to observe other properties that this explanation should exhibit. For starters, we would expect energy to be able to propagate through this field in the form of waves (in the same way energy can propagate through any medium ? just as sound will propagate differently along varying densities of matter). It so happens that electromagnetic radiation (hereafter referred to as light) behaves with many of the characteristics of a wave. Perhaps this grass is the medium over which light propagates. I personally have never been comfortable with the explanation that light sometimes is a wave and sometimes is a particle, so how can we explore this further?

We know light travels at a set speed (a property of waves) in a vacuum. When light is forced to travel through air or water, however, it slows down. It will also exhibit bending characteristics as it travels through areas of matter where the density is changing.

If we shrink down to the scale of individual atoms and use billiard balls to approximate the locations of each atom in a body of water (I don?t know enough about string theory to make any approximation it might predict), we can assume a field of gravity exists around each individual atom similar to the previous model of gravity around the sun. This field acts just like the sun and earth did previously in that the field of grass is present in the region around each atom, but as you get closer to each atom it becomes less and less dense.

So considering that grass is actually present (though in lesser densities due to gravities repulsion) in and among all this matter, it becomes clear that its density will be significantly less when averaged across a larger volume of many water molecules. That lesser density explains why light would propagate more slowly when traveling through air or water or glass.

Can we then also use this explanation to account for extreme situations where matter outright prevents the propagation of light ? when traveling through a sheet of lead for example? Recognizing that each atom?s gravity repulsion will be significantly increased due to each individual atom?s larger mass, we might entertain the possibility that light (at least, the visible spectrum) requires some minimum grass density in order to be propagated at all. That is, once a wave of energy (propagating at the wavelength/frequency properties of visible light) tries to move across a specific region with a grass-density that is less than the minimum density over which it can propagate, the wave falls apart (the energy goes somewhere for certain, but from the viewpoint of this model, it appears to cancel itself out). As well, due to the gradient nature of the grass-density decrease around each atom, some parts of the wave on either side of any given atom will be redirected around the atom ? no longer in since with the larger wave and contributing to deconstructive interference. Each atom acts as a pit for parts of a light wave to be sucked into.

Another issue this neatly addresses is the recent discovery that black holes spew superheated matter out their z-axis. This totally betrays our longstanding concept that black holes exhibit so much gravity that mass-less photons cannot even escape. Using this new explanation however, there is no problem. Black holes can still possess so much matter that their gravity field repels grass to such an extent that nowhere within some event horizon are light waves capable of propagating due to the region?s grass-density being below light?s minimum requirement for propagation. Superheated matter could then escape such a scenario without defying our understanding that nothing of mass can travel faster than light.



As I mentioned, graphics would make much of this clearer and easier to read, but the concept as a whole makes so much more sense to me than certain complexities college science classes expect students to accept without question. The universe has proven time and time again to be very simple-natured...once we see past our own complicated explanations.
 

Super Nade

Member
Oct 5, 2005
149
0
0
Mate, I didn't read through your entire post (still reading), but let me tell you that interaction of light with matter has nothing to do with gravity. L-M interaction falls under the perview of quantum theory. If you need more info about this, shoot with specific questions, because that is what my research is about.
 

greatfool66

Member
Mar 6, 2006
83
0
0
nice first post...

Not completely sure I understood it all but reminded me of an alternative theory of gravity my physics teacher told us about once.

He said imagine gravity as lines of force pushing equally from every direction in the universe.

Mass interferes with these forces so when you're standing on the Earth the forces pushing you away from the Earth have had to travel all the way through the earth and are thus weaker than the forces pushing you down towards the Earth only going through the atmosphere.

Anyone know if this theory has a name?
 

silverpig

Lifer
Jul 29, 2001
27,709
11
81
Originally posted by: greatfool66
nice first post...

Not completely sure I understood it all but reminded me of an alternative theory of gravity my physics teacher told us about once.

He said imagine gravity as lines of force pushing equally from every direction in the universe.

Mass interferes with these forces so when you're standing on the Earth the forces pushing you away from the Earth have had to travel all the way through the earth and are thus weaker than the forces pushing you down towards the Earth only going through the atmosphere.

Anyone know if this theory has a name?

This has already been considered and was brought up in a thread here a little while ago. Basically two objects block corpuscles from each other, and the result is an attractive force. It doesn't work though.

Also, it doesn't explain space-time bends.
 

GPett

Member
Apr 14, 2007
121
0
0
I think gravity and light are more related than people would like to think.

There are particles traveling through space all the time. Some we can percieve some we cannot.

The particles have some sort of charge or state that carries some information.

In addition, the trajectory of these particles can be influenced by outside sources. (ie. the path of light can be bent or difracted, and the path of an object can be altered by gravitational forces.

It is the same physics that affect both gravity and light. They are just observed on a differnt scale.

It is beyond me to define gravity and light but I do not think there are different mechanisms in the world we live in. It is the same mechanism but we percieve the functinoality differently due to our limmeted ways of perciving our world.

If I had to try to define them.. gravity is just a byproduct of how mass and density affect the trajectory of other objects with significant mass and density. How light travels also is influcenced by other light or the absence of light.

The most glaring (or the antithisis of glaring) example of how light and gravity are tied together is the existance of black holes. It is possible for a mass to be so dense and have such a gravitational force that light particles cannot escape.

I do not think waves or wavelenght theory is correct. Waves are merely particles moving with a certain reverberation, or state that the particles are in. If particles are traveling they too have thier own density and mas that can attract or repell other particles. Wich would explain the wavelength phenominon and the multitude of odd diffraction properties of light.

Ok I have babbled enough... did any of it make sense?

Edit: We have had models and can detect magnetic fields. We have had that technology for a long time. It is just an example of what we should do with gravitational fields. If we could map and create models of gravitational fields, observe fluctuations, and corrolate that data iwth other cosmic, solar, or environmental changes that is the true way to find the next step towards understanding the true nature of gravity.
 

CycloWizard

Lifer
Sep 10, 2001
12,348
1
81
Originally posted by: GPett
Edit: We have had models and can detect magnetic fields. We have had that technology for a long time. It is just an example of what we should do with gravitational fields. If we could map and create models of gravitational fields, observe fluctuations, and corrolate that data iwth other cosmic, solar, or environmental changes that is the true way to find the next step towards understanding the true nature of gravity.
We can map local gravity fields point by point. Oil companies developed gravitometers that can measure the gravitational acceleration at a point to something like 12 significant figures because a correlation between this value and probability of the existence of oil exist for geological reasons. I'm not familiar with how EM fields are mapped, and I'm not sure how gravitometers actually work, but it would be interesting to see if the two methods could be combined.
 

PhatoseAlpha

Platinum Member
Apr 10, 2005
2,131
21
81
Originally posted by: GPett
I do not think waves or wavelenght theory is correct. Waves are merely particles moving with a certain reverberation, or state that the particles are in. If particles are traveling they too have thier own density and mas that can attract or repell other particles. Wich would explain the wavelength phenominon and the multitude of odd diffraction properties of light.

I would expect the good old dual slit experiment to torpedo this line of thought. Electrons, fired one at a time through two slits in a vacuum still exhibit characteristic dispersal patterns of waves.
 

RESmonkey

Diamond Member
May 6, 2007
4,818
2
0
That rod idea is wrong, sorry dude

I'm starting to like this website even more, there are intelligent students here!

Since 'gravitons' or however you spell them, have not yet been discovered, let's assume they exist. Massless. Why? There HAS to be something do it. What describes the Photoelectric effect? C'mon, that experiment should make use think that photons are intertwined (spelling?) with more then just 3 dimensions. They've got momentum, etc. There may be parts to every single particle that we can not detect in a 3 dimensional world like ours. But of course, there are ten hypothesized (spelling?) dimensions, eh?

More research in string theory should help us greatly. Maybe all these 'particles' we know of and hypothesize (spelling?) of, have a more extent to them in different dimensions? That just leads to the existence/entity thought. Why is everything what it is, and why?
 

f95toli

Golden Member
Nov 21, 2002
1,547
0
0
Light, or more generally, electromagnetic radiation is very well understood; perhaps it it the best understood of all phenomena in physics.
The reason for this is that we have a VERY accurate theory for it, known as quantum electrodynamics (or QED for short) which has been around for quite some time now (it was developed by among others Feynman). Light is also "easy" to work with since it doesn't interact with the environment nearly as much as other quantum systems, meaning it has been possible to perform very accurate experiments to test QED.
Moreover, good lasers (which are themselves described by QED) have also been around for long time.
Anyway, the point I am making is that light is no "mystery", and as long as we can ignore the effects of gravity(where GR becomes important) we know "everything" about is(at least as far as we know).

One of the nice things about QED is that it shows that the so-called "particle-wave paradox" is a red herring; there is no paradox. It is true that we can sometimes think of light as having "wave-like" or "particle-like" properties but this is nothing but a convenient (but confusing) way to simplify an abstract concept.
A photon is neither a particle nor a wave and as long as the "full mathematical machinery" of QED is used there are no paradoxes; the problems only start when we try to use concepts from classical physics(waves and particles) to understand it; and the only reason why we still do that is because QED ís a theory which uses relatively complex math, meaning it is very difficult for non-experts to understand.


 

superHARD

Diamond Member
Jul 24, 2003
7,828
1
0
Originally posted by: manowar821
Originally posted by: f95toli
Originally posted by: manowar821
God I love discussions like these.

I thought of a funny way to communicate with a distant world, just make a solid rod that goes from here to there, and push it back and forth. There, faster than light communication!

Doesn't work. And you don't even need general relativity to show that; the plain old special version is enough.
Again, there is no way to transmitt information faster than light; and there are no known loopholes (not counting wormholes since they actually reduce the distance between two point)

But, the other end of the solid rod is going to move in the amount, and the same time that I move my end. Even if the rod is 200 light years long. Am I wrong?

that doesn't sound right to me...I think the rod would compress when you pushed it...the hardest rod in the world would compress a little when you bumped it...in fact it it were 200 light years away the other planet might not even feel it move at all...

And if they could feel it, the solar winds and stuff like that would make it impossible to read the bumps that came from the other planet...
 

MikeyLSU

Platinum Member
Dec 21, 2005
2,747
0
71
Originally posted by: superHARD
Originally posted by: manowar821
Originally posted by: f95toli
Originally posted by: manowar821
God I love discussions like these.

I thought of a funny way to communicate with a distant world, just make a solid rod that goes from here to there, and push it back and forth. There, faster than light communication!

Doesn't work. And you don't even need general relativity to show that; the plain old special version is enough.
Again, there is no way to transmitt information faster than light; and there are no known loopholes (not counting wormholes since they actually reduce the distance between two point)

But, the other end of the solid rod is going to move in the amount, and the same time that I move my end. Even if the rod is 200 light years long. Am I wrong?

that doesn't sound right to me...I think the rod would compress when you pushed it...the hardest rod in the world would compress a little when you bumped it...in fact it it were 200 light years away the other planet might not even feel it move at all...

And if they could feel it, the solar winds and stuff like that would make it impossible to read the bumps that came from the other planet...

so lets say you could somehow build a rod of some sort that goes to a planet 1 light year away. So, if you were to move the rod "forwards" towards teh other planet by 10 feet. It would take 1 year for the movement to happen on the other planet?
 

f95toli

Golden Member
Nov 21, 2002
1,547
0
0
No, for any real rod it would take much longer than that. The atoms/ions in a materal move roughly at the speed of sound which is much, much slower than the speed of light (the speed of sound in a material is usually a few thousand m/s).
Hence, rod-communícation is not a very practical idea.


 

silverpig

Lifer
Jul 29, 2001
27,709
11
81
Originally posted by: f95toli
No, for any real rod it would take much longer than that. The atoms/ions in a materal move roughly at the speed of sound which is much, much slower than the speed of light (the speed of sound in a material is usually a few thousand m/s).
Hence, rod-communícation is not a very practical idea.

C'mon toli, be a real physicist.

Assume an incompressible spherical rod.

 

MikeyLSU

Platinum Member
Dec 21, 2005
2,747
0
71
Originally posted by: f95toli
No, for any real rod it would take much longer than that. The atoms/ions in a materal move roughly at the speed of sound which is much, much slower than the speed of light (the speed of sound in a material is usually a few thousand m/s).
Hence, rod-communícation is not a very practical idea.

I don't understand this. if you had a steel pipe that was 1 mile long. If you move it, you are saying the other side won't see it move for about 5 seconds because it moves at the speed of sound?

<----------------------------------------------->

that is the way I'm saying we are moving it, longways.
 

silverpig

Lifer
Jul 29, 2001
27,709
11
81
Originally posted by: MikeyLSU
Originally posted by: f95toli
No, for any real rod it would take much longer than that. The atoms/ions in a materal move roughly at the speed of sound which is much, much slower than the speed of light (the speed of sound in a material is usually a few thousand m/s).
Hence, rod-communícation is not a very practical idea.

I don't understand this. if you had a steel pipe that was 1 mile long. If you move it, you are saying the other side won't see it move for about 5 seconds because it moves at the speed of sound?

<----------------------------------------------->

that is the way I'm saying we are moving it, longways.

That's what he's saying, yes.
 

bwanaaa

Senior member
Dec 26, 2002
739
1
81
I would imagine that gravity has to travel faster than lightspeed. If gravity traveled slower than light or even near lightspeed, then we would be able to detect gravitional fluctuations as wavelike distortions of light - like ripples in a pond - which we cannot. If the fluctuations are transmitted through space faster than lightspeed, then you wont see waves of distortion, but simple a contraction and expansion of the space around the gravity source-which has been observed. Now, only an astrophysicist would be able to calculate what kind of distances are necessary to actually discern a difference between these 2 patterns. And I am not one of those.
 

silverpig

Lifer
Jul 29, 2001
27,709
11
81
Originally posted by: bwanaaa
I would imagine that gravity has to travel faster than lightspeed. If gravity traveled slower than light or even near lightspeed, then we would be able to detect gravitional fluctuations as wavelike distortions of light - like ripples in a pond - which we cannot. If the fluctuations are transmitted through space faster than lightspeed, then you wont see waves of distortion, but simple a contraction and expansion of the space around the gravity source-which has been observed. Now, only an astrophysicist would be able to calculate what kind of distances are necessary to actually discern a difference between these 2 patterns. And I am not one of those.

Google "LIGO"
 

bwanaaa

Senior member
Dec 26, 2002
739
1
81
thank you, silverpig for that hint. never heard of LIGO before. Sounds like it should be a village in northern italy somewhere. Anyway, my naive review of the theory and implementation suggests that the apparatus will only be effective for static imagery. Consider the following analogy: a tool that measures the geography of land, charting the peaks and valleys. But what if I want to make a map of the geography of the waves on the ocean? I need something that takes multiple measurements a lot faster than the ocean waves are moving.

Specifically, It takes 75 trips of the light beam back and forth to accumulate enough phase difference to measure. Any event that happens faster than the speed of light will have come and gone before a single measurement is taken. Events slower than the speed of light but near it, are averaged out over the measurement time (75 trips). Only for events significantly slower than the time it takes to take a measurement will a transient in gravity be detected. The gravitational maps you have shown me are therefore not maps of waves but gravity wells.

Add to that the problem of seismic distortions. And then you have our own earth and sun with their own gravitational fields. After all, if you are trying to listen to Franz Liszt but Nirvana and the Rolling Stones are playing, you need to try a lot harder to hear him than listening to his music in a quiet setting. why dont they just build that thing in space? Dont they already have a mirror to reflect back a laser on the moon? I know a few of the Apollos dropped off those things so we could measure reflected laser light. And didnt I read somewhere that a laser can be pulsed in the atto second range (10-e18 sec) with incredible power. If a gravitational wave passes through a stream of these pulses arriving at a detector, you would see that as a transient in the rhythm. (Yes I know you'd need a clock signal to compare it against and you're basically back to doing interferometry-but this would be 'cine-interferometry' not static) And since we are always looking at the moon when we do measurements, we do not have to worry about the moon's orbit passing through our detector and creating another set of gravitational variathions that we need to filter out.

 

Biftheunderstudy

Senior member
Aug 15, 2006
375
1
81
Remember the theory says in no uncertain terms the gravity must propagate at the speed of light or at the very least, slower. Gravity waves are related to this, but there is a lot more subtleties to this than the EM case. One, only a gravitational quadrupole will radiate waves, also there are specific polarizations that can be worked out along with their effect on a test particle. Wiki has a fairly good entry on gravity wave--if they fixed it. A classmate of mine did a presentation on gravity waves and their detectors and found a few mistakes in the entry.

Anyway, when the calculations are done, one finds that the amplitude of a gravity wave from a very violent system like an infalling compact binary or supernova or something is on the order of 10E-24 meters!!! For comparison the average size of the nucleus is something like 10E-14 or 15--thats a billion times smaller than the size of an atomic nucleus.

Funny you should mention both space and interferometry because that is exactly what they are doing with LISA. The gravity wave detectors are Gigantic interferometers which are inter-compared to reduce noise. The bigger you interferometer arms, the bigger the amplitude you can detect. Taking this to the limit the next generation of detector is a pair of satellites put into orbit to create a VERY large arm.

The next thing you have to worry about with detection of gravity waves is frequency, the way the detectors work is that they are only sensitive to a small range of frequencies. This means that we are narrowing the possible sources of the gravity waves.

There is a large amount of indirect evidence for gravity waves too. There is a compact binary system in which both stars are neutron stars and one of them is a pulsar with us in the beam trajectory. As you may know, pulsars are an extremely stable system from the time measurement perspective. Since we get most of our information from orbital dynamics and distance measurements, this system provides us with a very accurate way of determining the properties of this binary. The kind of numbers from this system are astounding, they have produced numbers with accuracies that are absolutely unheard of in astrophysics -- parts in 10E5 if I'm not mistaken. With these properties in hand we can test almost all the predictions of GR: perihelion precession; gravitational time dilation and redshift; and importantly gravity waves. Since gravity waves carry off energy, we would expect to see the orbit degrading at rate which is calculatable. The results of these calculations match to within a very high precision. This is however an indirect test of GR
 

bwanaaa

Senior member
Dec 26, 2002
739
1
81
Originally posted by: Biftheunderstudy...

one finds that the amplitude of a gravity wave from a very violent system like an infalling compact binary or supernova or something is on the order of 10E-24 meters!!! For comparison the average size of the nucleus is something like 10E-14 or 15--thats a billion times smaller than the size of an atomic nucleus.
....

amplitude, eh?
how about wavelength?
 

Biftheunderstudy

Senior member
Aug 15, 2006
375
1
81
The frequencies in question are really low, on the hertz level. lamda=c/v yields a wavelength of ~10E8 meters. From this we could expect the wave to pass in a few seconds tops. I really encourage you to hit the entry on wiki about gravity waves, its fairly well written...now that the mistake has been fixed.
 

manowar821

Diamond Member
Mar 1, 2007
6,063
0
0
Originally posted by: MikeyLSU
Originally posted by: f95toli
No, for any real rod it would take much longer than that. The atoms/ions in a materal move roughly at the speed of sound which is much, much slower than the speed of light (the speed of sound in a material is usually a few thousand m/s).
Hence, rod-communícation is not a very practical idea.

I don't understand this. if you had a steel pipe that was 1 mile long. If you move it, you are saying the other side won't see it move for about 5 seconds because it moves at the speed of sound?

<----------------------------------------------->

that is the way I'm saying we are moving it, longways.

That's exactly what I was going to ask.

This is the type of experiment that we would do easily and cheaply, too.

I'm nearly 100% sure that it would be seen/felt by the receiving end at nearly the exact same perceivable time as I pushed it at. Now, light years away? Yeah, it's going to take some time, but I am still not convinced that compression of the material will slow it down to the point where the "message" would get there after a pulse of light I had also fired off at the same time.

This is of course barring any outside variables like solar wind, the movements of our the source and the recipient, asteroids and intergalactic terrorists.
 
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