[jeffreyH (OP)]

There are not enough gravitational wave detectors on line yet to determine if gravity works at the speed of light. What would it mean if it did? Gravity slows light, so if they are the same speed, it must also slow itself. It would only be a game changer if they are shown to operate at different speeds.

What do you think?

[Kryptid]

Gravity slows light

I was not aware of this. Are you sure?

[geordief]

Is it not gravitational waves that travel at the speed of light ? The gravitational field doesn't move does it?

Also if gravitational waves travel at the same (maximum?)speed as light does it follow that they both have to travel at the same speed in all conditions?

The  maximum speed of light is just the maximum speed of massless partcles.

[Bill S]

The  maximum speed of light is just the maximum speed of massless partcles.

That's an interesting thought, as it must assume the reality of gravitons.  Otherwise we would have to include waves, and not all waves travel at "c".

[jeffreyH (OP)]

Gravity slows light

I was not aware of this. Are you sure?

It has to. Otherwise light would escape the event horizon of a black hole.

[jeffreyH (OP)]

Is it not gravitational waves that travel at the speed of light ? The gravitational field doesn't move does it?

Also if gravitational waves travel at the same (maximum?)speed as light does it follow that they both have to travel at the same speed in all conditions?

The  maximum speed of light is just the maximum speed of massless partcles.

If the sun disappeared suddenly the gravitational field should persist for 8 minutes. This does not involve gravitational waves.

[Colin2B]

Is it not gravitational waves that travel at the speed of light ? The gravitational field doesn't move does it?

You are right, wave motion is not the same as motion of the field. The field changes and it is the change that propagates. With a sound wave or water wave the air/water doesn't move forward only the change in the field. (Ok, to be pedantic the water does move in a circle, but doesn't progress forward).
So when the sun disappears it would take 8mins for the change to propagate to earth.

[Bill S]

It has to. Otherwise light would escape the event horizon of a black hole.

Isn't light trapped by increasing curvature, rather than decreasing speed?

[geordief]

It has to. Otherwise light would escape the event horizon of a black hole.

Isn't light trapped by increasing curvature, rather than decreasing speed?

I imagine it could be said that the speed of the light attempting to escape from a Black Hole  would appear to be zero from outside the Black Hole but  would still be c for an observer within the  Black Hole.

Out of interest I have heard that light from the centre of the Sun takes hundreds(?) of years to  make its way to the  surface(not sure quite why that is so , probably the medium rather than any spacetime effect.

[geordief]

If the sun disappeared suddenly the gravitational field should persist for 8 minutes. This does not involve gravitational waves.

If the Sun disappeared suddenly  (as a thought experiment) could that not be seen (ie modeled)  as an event that produced gravitational waves?

Even if the Sun gradually looses mass (as it does all the time ) does this not also produce (imperceptible) gravitational waves?

(musing) Can there be such a thing as negative gravitational waves,can gravitational waves interfere with themselves ?

[geordief]

The  maximum speed of light is just the maximum speed of massless partcles.

That's an interesting thought, as it must assume the reality of gravitons.  Otherwise we would have to include waves, and not all waves travel at "c".

I am "speculating" beyond my education but why should gravitons not have a wave characteristic like everything else it seems?

I  wonder what is the relationship between the gravitational wave and the hypothesized graviton.....

[puppypower]

There are not enough gravitational wave detectors on line yet to determine if gravity works at the speed of light. What would it mean if it did? Gravity slows light, so if they are the same speed, it must also slow itself. It would only be a game changer if they are shown to operate at different speeds.

What do you think?

Gravity does not slow light, but only alters the wavelength of light. A red shift or blue shift does not impact the speed of light but only the wavelength.

If gravity did this to itself, the gravitational wavelength would blue shift as we get closer to a gravity source. This would make the gravitational field stronger; compounding affect.

This affect is connected to gravity as a function of mass density. The same amount of mass can exert higher gravity when it is denser because gravitational waves are more blue shifted.

[jeffreyH (OP)]

It has to. Otherwise light would escape the event horizon of a black hole.

Isn't light trapped by increasing curvature, rather than decreasing speed?

There is that word again, curvature. That is simply a way of removing the forces from the mathematics. The question is why is freefall free from the effects of acceleration and appears inertial?

Think of an evenly distributed force that is independent of mass and therefore not particulate in nature. Involving those pesky virtual non particles.

[geordief]

This affect is connected to gravity as a function of mass density. The same amount of mass can exert higher gravity when it is denser because gravitational waves are more blue shifted.

I thought mass did not cause gravitational waves but that it was changes in mass-energy distribution that caused them (a la Black Holes mergers)

[Colin2B]

Gravity does not slow light, but only alters the wavelength of light. A red shift or blue shift does not impact the speed of light but only the wavelength.

In the presence of gravity light speed becomes relative because the clock rate at various gravitational potentials varies relative to that at the location of the observer.

[Bill S]

In the presence of gravity light speed becomes relative because the clock rate at various gravitational potentials varies relative to that at the location of the observer.

Am I misinterpreting this, or are you saying that two observers, in different locations, will measure  different speeds for light, because their clocks tick at different rates, relative to each other?

[Bill S]

The original question, “Does gravity self-interact?” links to a point made in:

http://www.einstein-online.info/spotlights/gravity_of_gravity

“ One reason why the physics of general relativity is much more difficult than that of Newton's theory of gravity or the theory of electrodynamics is a property called non-linearity. In short, gravity can beget further gravity - where gravitational systems are concerned, the whole is not the sum of its parts.”

This may be sloppy wording, but as it stands it raises a serious question.  If “gravity can beget further gravity”, presumably that is because mass and energy beget gravity. Gravity contains energy, so this energy creates more gravity, ad infinitum.

If this were the case, why would we not have runaway gravity?  Why is everything not a black hole?

I think I’ve sorted out an answer that, more or less, satisfies me, but I would be interested to know other people’s thoughts.

[jeffreyH (OP)]

If gravity begets gravity it has implications for strong gravitational fields. The effects would be more noticeable near to neutron stars and black holes. I have not looked into this yet.