Hopefully not too stupid question about gravitational waves

[fropome]

I've been trying to get my head around relativity etc and struggling :-(
However there's one thing that I can't find much information about at an interested-layman level - gravitational waves.
They're described as having a frequency etc (being waves) and traveling at light speed. But if they have a frequency then presumably they can have varying frequencies. What would this mean for a gravity wave? Would it have a different amount of energy? Exert a different amount of pull? Could red shift alter the effect of gravity? What about amplitude - is that fixed? Could gravitational waves interfere with each other?
Or am I taking the whole 'wave' thing too literally?

Apologies for adding to all the noob questions here, but it's got me stumped!

Thanks!

 

[mgb_phys]

Gravitational waves can have any frequency (in theory) just like any other wave. The actual frequency depend son their source - for something like two orbital bodies (eg merging stars) the frequency is twice the orbit.
So given the possible range of orbital times for stars there is only a range of generated gravity waves - the detectors are built for this range.
The amplitude of the wave depends on the mass of the objects colliding - more impressive events (like merging massive black holes) give bigger amplitudes and are more likely tobe detected.

 

[fropome]

I'm with you so far, almost - thanks for the help!

In a merging-star system like you mention, where would the waves originate? From the centre of gravity or some from each star? Is any piece of 'falling' matter radiating gravitational waves?

So would gravity be effected by interference or a version of red shift? Are there discrete packets of it, similar to photons?

 

[Chalnoth]

It's the frequency as it would be for any type of wave: the number of wave crests that pass per unit time. The frequency of a gravitational wave is determined by whatever process produces it.

Gravity waves would be effected by red shift in the same way that photons are, and yes, we believe they do exist in discrete packets we call gravitons (which would be massless, spin-2 particles: photons are massless spin-1 particles). But we don't yet know for sure what the quantum theory of gravity is.

 

[fropome]

So... if gravity is effected by red shift, does that mean that it's effect is lessened at large distances? Do we have to take that into account in any of our models, or are the distances so great that the effect is swamped by other effects?

Are gravitons thought to be affected by gravity as photons are? I assume not... but why?


Sorry - I've got far too many questions; can anyone recommend a book? I'm a maths graduate so I don't mind equations but I haven't done physics since A-level.

Many thanks for all the answers so far!

 

[fleem]

So... if gravity is effected by red shift, does that mean that it's effect is lessened at large distances? Do we have to take that into account in any of our models, or are the distances so great that the effect is swamped by other effects?

Are gravitons thought to be affected by gravity as photons are? I assume not... but why?

Current theory is that they act just like light does in all these classical things, including being affected by gravity. (Personally I'm not so sure current theory is completely right in some of this, but we won't go there).

 

[fropome]

So gravity is effected by gravity - the top of my head just fell off.

edit:
Hang on. If gravitons are effected by gravity in a similar way to photons, then how could they get out of a black hole? But if they can't get out of a black hole then a black hole wouldn't have any gravity, and then the gravitons could get out no problem. My head hurts.

 

[fleem]

So gravity is effected by gravity - the top of my head just fell off.

edit:
Hang on. If gravitons are effected by gravity in a similar way to photons, then how could they get out of a black hole? But if they can't get out of a black hole then a black hole wouldn't have any gravity, and then the gravitons could get out no problem. My head hurts.

Yikes, now there's an irritating question. I'm glad I showed criticism of current theory in my last post! I suspect the gravitational energy of a black hole exists above the black hole, so maybe it would be explained somehow along those lines. In any case I have seen papers talking about gravitational lensing of gravity waves. I'll watch this thread to see what the answer is.

 

[Janus]

So... if gravity is effected by red shift, does that mean that it's effect is lessened at large distances? Do we have to take that into account in any of our models, or are the distances so great that the effect is swamped by other effects?

Are gravitons thought to be affected by gravity as photons are? I assume not... but why?


Sorry - I've got far too many questions; can anyone recommend a book? I'm a maths graduate so I don't mind equations but I haven't done physics since A-level.

Many thanks for all the answers so far!

What's important here is not to confuse "gravity waves" with "gravity the force". Gravity waves do not carry the gravitational force, they carry information about changes in the gravitational field. In other words, if you were to erect a barrier between the Earth and Sun that blocked gravity waves produced by the Sun from reaching us, the gravitational attraction we feel towards the Sun would still be left.

The same is true for black holes. Gravity waves cannot escape the event horizon, but the gravitational field of the black hole remains.

 

[stevebd1]

So gravity is effected by gravity - the top of my head just fell off.

edit:
Hang on. If gravitons are effected by gravity in a similar way to photons, then how could they get out of a black hole? But if they can't get out of a black hole then a black hole wouldn't have any gravity, and then the gravitons could get out no problem. My head hurts.

Below is an extract from Physics FAQ-

How does the gravity get out of the black hole?

Purely in terms of general relativity, there is no problem here. The gravity doesn't have to get out of the black hole. General relativity is a local theory, which means that the field at a certain point in spacetime is determined entirely by things going on at places that can communicate with it at speeds less than or equal to c. If a star collapses into a black hole, the gravitational field outside the black hole may be calculated entirely from the properties of the star and its external gravitational field before it becomes a black hole. Just as the light registering late stages in my fall takes longer and longer to get out to you at a large distance, the gravitational consequences of events late in the star's collapse take longer and longer to ripple out to the world at large. In this sense the black hole is a kind of "frozen star": the gravitational field is a fossil field. The same is true of the electromagnetic field that a black hole may possess...

Source-
http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/black_gravity.html

 

[Chalnoth]

So... if gravity is effected by red shift, does that mean that it's effect is lessened at large distances? Do we have to take that into account in any of our models, or are the distances so great that the effect is swamped by other effects?

No, it doesn't quite work that way. It's the traveling gravitational waves that are affected by redshift, which might be seen as ripples in the underlying gravitational field. The overall shape of the gravitational field is what determines the strength of gravity, not the wavelength of ripples running across it.

 

[fropome]

Thanks for all this - it is starting to make more sense. So far my understanding is as follows:
The force of gravity isn't transmitted in waves, but fluctuations in that force can be seen as waves.
As a result, black holes capture gravitons, but this doesn't effect whether they have gravity.

Am I ok so far?

If there is a lag between an object moving and the gravitational field at an arbitrary distance changing, then would the gravitational field of a moving body appear different to the gravitational field of a static body?

If there is a minimum wave size (a single graviton) then does this mean that there is a minimum difference between gravitational field strengths?
If so, then could a movement be of the kind which produces gravitons, but involve masses / orbits so small that they can't produce a graviton?

 

[fleem]

With one slight correction. Gravity waves contain no rest mass (so they can travel at c), but they do contain energy (otherwise they wouldn't exist) so when they are captured (by a black hole or just turn into heat when they shake up some matter), then they add to the mass of that black hole or ordinary matter that captured them, increasing the gravitational and inertial mass of that black hole or ordinary matter. But that increase in gravity of the capturing mass is not because it was gravity waves in particular that were captured, its just because energy was added to that object, making it more massive.

 

[Rymer]

Actually, neither the photon nor the postulated graviton are themselves effected by 'gravity'. Such effects are due to the difference between emission source and observer -- velocities, time dilation, etc.

In General Relativity, both photon and graviton follow geodesic paths through a 'warped space-time'. There are other ways of explaining this but this is the current mainstream view. There are problems with the 'warped space-time' model -- not so much with the results the math produces but more with the confusion it causes in interpretation.

 

[fropome]

"In General Relativity, both photon and graviton follow geodesic paths through a 'warped space-time'."
Is that because of their lack of mass, their sort-of-infinite speed (I know that's not quite right but I tend to think of it that way), or are the two properties so inextricably linked that it's both?

 

[fleem]

Actually, neither the photon nor the postulated graviton are themselves effected by 'gravity'. Such effects are due to the difference between emission source and observer -- velocities, time dilation, etc.

In General Relativity, both photon and graviton follow geodesic paths through a 'warped space-time'. There are other ways of explaining this but this is the current mainstream view. There are problems with the 'warped space-time' model -- not so much with the results the math produces but more with the confusion it causes in interpretation.

Just to expound, here, I think you put quotes around 'gravity' to make clear that there are a layman's definition and a scientific definition of the word 'gravity'. I'm sure you're saying here that in current theory, the scientist's definition is that 'gravity' is a synonym of 'the warps in space-time', so using that definition, gravitons & photons are affected by gravity.

 

[fleem]

"In General Relativity, both photon and graviton follow geodesic paths through a 'warped space-time'."
Is that because of their lack of mass, their sort-of-infinite speed (I know that's not quite right but I tend to think of it that way), or are the two properties so inextricably linked that it's both?

Everything follows those geodesics. Baseballs, too. Both the light and baseball follow a straight line as seen by a local observer, because that local observer follows that warp, also. A remote observer can see that warp, though.

 

[Rymer]

Everything follows those geodesics. Baseballs, too. Both the light and baseball follow a straight line as seen by a local observer, because that local observer follows that warp, also. A remote observer can see that warp, though.

Only massless 'particles' follow what might be interpreted as 'straight-line' for the geometry of the 'local space-time warp'.

 

[fleem]

Only massless 'particles' follow what might be interpreted as 'straight-line' for the geometry of the 'local space-time warp'.

You're right, i should have qualified that "straight line" I said to show it was a strictly local thing. Poor wording on my part. An astronaut in a spacecraft without windows can rightly consider himself not accelerating. A local 'straight line' and 'geodesic' have different meanings. For the OP: A geodesic is simply the free-fall path of an object. Geodesics are relative. A photon (and, theory says, a gravity wave) travel on a null geodesic, which means they are right between the concepts of space-like and time-like geodesics.

 

[Chalnoth]

Actually, neither the photon nor the postulated graviton are themselves effected by 'gravity'. Such effects are due to the difference between emission source and observer -- velocities, time dilation, etc.

In General Relativity, both photon and graviton follow geodesic paths through a 'warped space-time'. There are other ways of explaining this but this is the current mainstream view. There are problems with the 'warped space-time' model -- not so much with the results the math produces but more with the confusion it causes in interpretation.

The photon and the graviton are affected by gravity just like everything else is. All other matter fallows geodesic paths through warped space-time as well. That's just how GR is formulated.

The difference in the relationship between the energy and momentum of photons and gravitons makes for some differences in how they gravitate, and in how they produce a gravitational field. But they still gravitate, and respond to gravity.

 

[Rymer]

The photon and the graviton are affected by gravity just like everything else is. All other matter fallows geodesic paths through warped space-time as well. That's just how GR is formulated.

The difference in the relationship between the energy and momentum of photons and gravitons makes for some differences in how they gravitate, and in how they produce a gravitational field. But they still gravitate, and respond to gravity.

They appear to be effected by gravity yes. That does not confirm they 'create' or 'produce' a gravitational field themselves. Gravity is a phenomenon of organized matter for sure, but MAY not be one for energy (unproven).

Chalnoth, you may be right but your statement is presented as fact -- but that has not be as yet shown.

 

[Chalnoth]

They appear to be effected by gravity yes. That does not confirm they 'create' or 'produce' a gravitational field themselves. Gravity is a phenomenon of organized matter for sure, but MAY not be one for energy (unproven).

Chalnoth, you may be right but your statement is presented as fact -- but that has not be as yet shown.

If it were not true that radiation produces gravitational fields, then our observations of the CMB would be way, way off, as in the early universe, radiation was the primary energy density.

 

[Rymer]

If it were not true that radiation produces gravitational fields, then our observations of the CMB would be way, way off, as in the early universe, radiation was the primary energy density.

Yes, they might. There is no proof that gravity -- as we know it today -- existed at and prior to the time of last scattering. An no real need for it as far as I know -- but again that doesn't mean your are wrong Chalnoth -- only that it is not proven fact. Also:

The photon and the graviton are affected by gravity just like everything else is.

Is not true. The effect on the massless photon is twice what would be expected from the Newton formulation. This means that there is something different about photons and normal matter when it comes to gravity. The entirety of the difference is still in question.

 

[Chalnoth]

Yes, they might. There is no proof that gravity -- as we know it today -- existed at and prior to the time of last scattering. An no real need for it as far as I know -- but again that doesn't mean your are wrong Chalnoth -- only that it is not proven fact.

Well, I suppose you could say there's no proof if you completely ignore how well the CMB, Baryon Acoustic Oscillation, and primordial nucleosynthesis data match with theory.

Is not true. The effect on the massless photon is twice what would be expected from the Newton formulation. This means that there is something different about photons and normal matter when it comes to gravity. The entirety of the difference is still in question.

That just says that Newton's theory is incomplete. In this case it's incomplete because it doesn't consider the gravitational effect of momentum. General Relativity perfectly predicts the deflection of photons (to within current experimental bounds), so I don't know how the difference can still be in much question.

 

[Rymer]

Well, I suppose you could say there's no proof if you completely ignore how well the CMB, Baryon Acoustic Oscillation, and primordial nucleosynthesis data match with theory.


That just says that Newton's theory is incomplete. In this case it's incomplete because it doesn't consider the gravitational effect of momentum. General Relativity perfectly predicts the deflection of photons (to within current experimental bounds), so I don't know how the difference can still be in much question.

Yes, the CMB, Baryon Acoustic Oscillation, and primordial nucleosynthesis data match with theory and do appear to fit together -- but it is not clear to me what role gravity really has in doing this. None that I can see.

Also, General Relativity should match the deflection of photons -- it was designed to do so.
Don't see how that makes it 'complete' either. Problems it has include:

1) Its deterministic -- on purpose. Developed that way by the accepted philosophy of the day before quantum mechanics.

2) Has a fixed three spatial and one time dimension. Very unlikely from what we know or rather suspect today.

3) Descriptive more than predictive. You need to be able to specify the geometry and symmetry of the problem before you can calculate. Likely all models will have this limit to some degree.

4) Little or no mechanism to explain 'how gravity works'. This leaves the understanding limited by the understanding of the mathematics -- very few.

 

[Rymer]

Just to expound, here, I think you put quotes around 'gravity' to make clear that there are a layman's definition and a scientific definition of the word 'gravity'. I'm sure you're saying here that in current theory, the scientist's definition is that 'gravity' is a synonym of 'the warps in space-time', so using that definition, gravitons & photons are affected by gravity.

Well, I was actually trying to avoid a specific statement like that -- since personally I don't agree fully with the 'warped space-time' idea. Warped space-time originated with General Relativity and the concept has become associated with it. Its more of a description of the geometric interpretation of the math than any physics. It does not provide a mechanism for 'gravity'.

Note, there has be a distinction made between 'gravity' and a 'gravitational filed' more along the lines you indicate. Not sure what people thought they were accomplishing by that -- a bit of scientific elitism.

 

[fleem]

Well, I was actually trying to avoid a specific statement like that -- since personally I don't agree fully with the 'warped space-time' idea. Warped space-time originated with General Relativity and the concept has become associated with it. Its more of a description of the geometric interpretation of the math than any physics. It does not provide a mechanism for 'gravity'.

Note, there has be a distinction made between 'gravity' and a 'gravitational filed' more along the lines you indicate. Not sure what people thought they were accomplishing by that -- a bit of scientific elitism.

I tend to agree. That's also why I often add phrases like "current theory says" to my posts so I can expound on my skepticism later :) In this case, the analogy I often give is, if I develop a theory that describes the behavior of a swarm of bees, and then I want to develop a theory that describes a single bee, should I insist that my single-bee theory be founded on the same axioms I created for the swarm theory? Yet modern science presumes those strictly classical concepts like continuums, manifolds, and even non-integral values (which were created solely to describe large-scale behavior) must without question be used as the foundation of a theory that describes the behavior of single particles.

 

[Chalnoth]

Yes, the CMB, Baryon Acoustic Oscillation, and primordial nucleosynthesis data match with theory and do appear to fit together -- but it is not clear to me what role gravity really has in doing this. None that I can see.

Well, then, I suggest you take a look into how these things are computed. Hint: they depend critically upon the behavior of gravity. A really, really basic way of understanding it is that how gravity acts upon matter determines, along with the various matter components of the universe, how the universe expands with time. The above predictions also depend critically upon how the universe expands with time. Change how gravity interacts with matter too much (and changing how gravity interacts with photons would be a truly dramatic change where early universe cosmology is concerned), and the above calculations are completely altered.

The onus is upon you to show that your particular model doesn't alter the above theoretical predictions too much.

Also, General Relativity should match the deflection of photons -- it was designed to do so.

What? No. It's a prediction that stems from the theory. It wasn't "designed" to do anything of a sort: this is a necessary conclusion of the assumptions used.

1) Its deterministic -- on purpose. Developed that way by the accepted philosophy of the day before quantum mechanics.

Quantum mechanics is deterministic too.

2) Has a fixed three spatial and one time dimension. Very unlikely from what we know or rather suspect today.

It's not difficult to extrapolate GR to more than four space-time dimensions.

3) Descriptive more than predictive. You need to be able to specify the geometry and symmetry of the problem before you can calculate. Likely all models will have this limit to some degree.

What? GR makes tons of predictions, such as the specific orbits of planets (Mercury comes to mind), the deflection of light by massive bodies, as well as gravitational redshift and time dilation, to name a few.

4) Little or no mechanism to explain 'how gravity works'. This leaves the understanding limited by the understanding of the mathematics -- very few.

I don't get this claim. It has as much to explain itself as any theory does.