Gravity Waves or Gravity Delay: Exploring Both

In summary: It's just a way of describing the curvature of spacetime in a particular region.In summary, the conversation discusses the concept of gravitational waves and their effect on stretching and squeezing of objects. It is noted that the stretching and squeezing is caused by the changing orientation of black holes. The idea of a single black hole causing a continuous stretch is considered, but it is pointed out that the effect is perpendicular to the direction of the waves. The possibility of measuring the propagation speed of a static gravitational field is also discussed, but it is concluded that there is no way to do so due to the lack of changing in the field. The question of whether the lack of measurement means that nothing is propagating is raised, but it is ultimately determined
  • #1
CHOP
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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?
 
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  • #2
CHOP said:
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.
CHOP said:
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.
CHOP said:
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.
CHOP said:
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.
 
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  • #3
Ibix said:
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.
 
  • #4
CHOP said:
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.
 
  • #5
CHOP said:
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).
Ultimately, if there's nothing to measure then it's not a matter for science. So, in terms of an answerable scientific question, if nothing is changing then nothing is propagating. And certainly by analogy to electromagnetism static fields do not require any propagation terms. They're just there.
CHOP said:
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?
Interpreting "we can't measure" to mean "there is nothing there to measure even in principle", then yes, in scientific terms it's a fact that there's nothing propagating. And it's difficult to know what other terms you could use.

In what sense does a wave propagate? You can detect a change at a distance from some source at a time that increases smoothly (probably linearly) with distance.
 

1. What are gravity waves?

Gravity waves are disturbances in the curvature of spacetime that propagate through space at the speed of light. They are produced by massive objects in motion, such as accelerating stars or merging black holes.

2. How are gravity waves different from electromagnetic waves?

Gravity waves are fundamentally different from electromagnetic waves in that they are a result of changes in the geometry of spacetime, rather than changes in electric and magnetic fields. Additionally, gravity waves are not affected by electrically charged particles, while electromagnetic waves are.

3. How are gravity waves detected?

Gravity waves are detected using highly sensitive instruments, such as the Laser Interferometer Gravitational-Wave Observatory (LIGO). These instruments measure tiny changes in the distance between two points caused by the passing of a gravity wave.

4. What is gravity delay?

Gravity delay, also known as gravitational time delay, is the phenomenon where light passing through a gravitational field is bent and takes longer to travel than it would in a vacuum. This effect was first predicted by Einstein's theory of general relativity and has been confirmed through various experiments.

5. How are gravity waves and gravity delay related?

Gravity waves and gravity delay are both consequences of Einstein's theory of general relativity. Gravity waves are produced by massive objects in motion, while gravity delay is a result of the bending of spacetime caused by the presence of mass. Both phenomena provide evidence for the existence of gravitational fields and the validity of general relativity.

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