DJ_Juggernaut » July 4th, 2018, 10:09 pm wrote:Lincoln wrote:DJ_Juggernaut, your statement is simply incorrect. In GR, gravity is assumed to travel at the speed of light.
At the risk of agreeing to disagree with you, I will retort that the distance between most celestial objects (sun and earth for eg) is too large for it not to be instantaneous. If the earth suddenly changed its distance relative to the sun, then the new resultant gravity force would have to travel or take effect instantaneously in order for it to maintain a stable orbit. If speed of gravity travels at the speed of light, it would take 8 mins for the new resultant gravity force to take effect. By then the earth's orbit would travel in a tangent for 8 mins. And the earth is constantly changing its distance relative to the sun. So...
Note, that I agree with you that a 'gravitational wave' can travel at a finite velocity. But not the "speed of gravity". They are not the same entities. Apples and oranges. The evidence you speak of is highly questionable if you say that it addresses the "speed of gravity". They are not the same beasts, so to speak.
It is true that Jupiter is pulled towards where the sun is, not to a location in space where the sun "was" 43 minutes ago.
Regular magnetic fields have a property called
Lorentz Covariance. THis means that if an object, O, generates a magnetic field, then the field is in some sense "dragged along" with O , if O is coasting in an inertial reference frame. Changes to the field locally will propagate to distant observers at the speed of light, even while the field itself is being "dragged". The gravitational field likely does the same thing. Coasting along at constant velocity does not legally constitute a "change" to the field, hence the Jupiter issue.
The Einstein Field Equations say nothing about any particular physical system. Rather, EFEs have to be converted into a form in which they can make quantitative predictions. This conversion requires that you first commit to some configuration of massive objects ahead of time. The predictions that pop out of the mathematical machinery only apply to a particular observer -- and have no meaning to other observers in another frame. In short, you must set the initial conditions, and then tell me which reference frame you intend to predict for. A couple chalkboards later, it pops out some predictions that your selected observer will measure.
I will agree that thinking about physics in this way is repugnant and psychologically uncomfortable. We (being human) prefer that physics should tell us something objective about the world from a
God's eye Point-of-View. In situations of very strong gravity, and if General Relativity is correct, such a beast has no physical meaning.
Regarding the issue of the speed of gravity, we likely cannot refer to a speed that it "actually moved" unless we are accidentally in a happy scenario where that fudging and approximating is possible. (e.g. the earth is in the same free-fall as Lincoln's distant neutron star binary. )
Like other situations in special relativity , we note that speed is defined as distance divided by time. A person might take what I have said so far, and try to doctor a scenario in which it would be broken. One example would be placing an observer near the event horizon of a black hole. In that situation, the observer near the black hole (call him Bob) will still see changes to gravity propagating at the speed of light. While Alice , who is situated safely at a huge distant from the BH, also sees gravity propagating at c. We might wave our hands and cry "contradiction!". However, the reason why both Bob and Alice see gravity propagating at c, is because the universe
conspires to make him see that from his reference frame. It does this by perfectly slowing down his local clock. (..and because
speed = distance/time
etc etc). Bob and Alice will disagree on many physical facts about events (Bob will see stars blueshifted) but they will not disagree on the speed of gravity.