# Gravity waves or gravity delay?

• I
Or both?

The cycling between 'stretch' and 'squeeze' is caused by the orientation changing of the black holes with respect to us. Correct?

This is understood as waves (or pulses) moving through space, Correct? So they take time to get to us.

So, consider the orientation which yields us being stretched. Imagine that orientation replaced with a hypothetical single black hole, which stretches us about the same as that orientation did.

In this single hole case, it would be a continuous stretch, because there is no orientation changing happening.

So we are simply stretched. Now, if we consider a range of time of the whole time that we are stretched, is it right to say that what stretched us (during that range) moved through space to us?

FactChecker

Ibix
2020 Award
The cycling between 'stretch' and 'squeeze' is caused by the orientation changing of the black holes with respect to us. Correct?
Depends what you mean by "orientation". It's where the black holes are in their orbits.
This is understood as waves (or pulses) moving through space, Correct?
Kind of. That's acceptable as a weak field approximation, but its important to keep in mind that gravitational waves are structures in spacetime.
So, consider the orientation which yields us being stretched. Imagine that orientation replaced with a hypothetical single black hole, which stretches us about the same as that orientation did.
The stretch-and-squish effect is perpendicular to the travel direction of the waves - so effectively in the plane perpendicular to the direction to the black hole. On the other hand, static tidal forces produce lengthening towards the black hole and compression in all directions perpendicular to that. So this other black hole is somewhere else completely.
So we are simply stretched. Now, if we consider a range of time of the whole time that we are stretched, is it right to say that what stretched us (during that range) moved through space to us?
Are you asking about the propagation speed of a static gravitational field? I don't think the question is answerable. If one changes the field then the speed at which the change propagates can be measured, at least in principle. If it's not changing, what would you measure to determine whether or not anything is propagating?

In fact, it is much harder to even think about this question for gravity than, for example, electromagnetism. In EM one can start with zero net charge and then separate it into a dipole and see how long it takes for the dipole field to appear at different distances. But there's no way to do an analogous experiment in gravity because there's no negative mass to produce no net source of gravity.

FactChecker and Dale
If it's not changing, what would you measure to determine whether or not anything is propagating?

Right, there is no measurement because there is no changing. Which was key to my curiosity: The fact that there is no changing shouldn't be what determines whether the propagation occurs. Right? I understand that there is 'no topic' if there is no changing (nothing to measure).

But IF it is thought that something propagated to make the earth respond in some specific way, and IF what was propagated was a result of where the two black holes were in their orbit, then:

(imagining they stayed for a length of time to where they were at that point in their orbit) the thing that was propagated would be propagated for a longer time. That longer time would correspond to earth responding for a longer time.

Is that right?
If right, then:

We can consider a chunk of the time that the earth was responding to that propagated thing (not the whole time). That chunk of time (presumably) corresponds to a chunk of what was propagated (not all of it).

But it appears that something breaks down here. I mean, how can we say that chunk propagated? There is nothing 'surrounding' it to make it 'arrive' then 'depart' (nothing to measure). I think that was your point? Well, if it was, I agree.

But my question is:
Does the fact that we can't measure it make it be a fact that that it didn't propagate? If so, then in what sense did the 'wave' really propagate?

It seems that it's more about delay (time), than about propagation. But (perhaps) the way it works is, the delay depends on the separation amount, so it can be thought of as a propagation.

PeterDonis
Mentor
2020 Award
The fact that there is no changing shouldn't be what determines whether the propagation occurs. Right?

Wrong. "Propagation" means "effects of a change in the source propagating to distant observers". If there is no change in the source, there is nothing to propagate.

A static gravitational field is not "propagated", by gravitational waves or anything else. You simply can't model it that way; it doesn't work.

Ibix