Understanding gravitational waves (GR)

[phinds]

While I have no doubt you are completely correct, that still makes no sense to me. Two object that are attracted to each other are moving closer and if they are closer they are more attracted. In Newtonian terms, they are accelerating towards each other and the acceleration is not constant. As we observe them, are not those two black holes moving faster as time progresses until they collide? Thanks.

Yes, the attraction changes and in Newtonian physics that means they are applying a gravitational force to each other? So what? This thread is about GR, not Newtonian physics. Things just naturally travel along geodesics (ie are in "free fall")

 

[Ibix]

In Newtonian terms,

Unfortunately, GR uses rather different terms.

As we observe them, are not those two black holes moving faster as time progresses until they collide?

You can certainly find a way to state that. The problem is that it's not coordinate independent, so it always leads to a thousand "ifs" and "excepts" and "depends on your point of views". For example, the distance between two things whose "surfaces" can't rigorously be described as places in space and don't really have centers in any meaningful sense is a surprisingly difficult thing to pin down. So we use things like quadropole moments which are actually invariants.

 

[Orodruin]

Unfortunately, GR uses rather different terms.

Or fortunately, it depends on your point of view.

 

[Ibix]

Or fortunately, it depends on your point of view.

S'all relative, man.

 

[Orodruin]

S'all relative, man.

You mean it is not person-invariant?

 

[FactChecker]

While I have no doubt you are completely correct, that still makes no sense to me. Two object that are attracted to each other are moving closer and if they are closer they are more attracted. In Newtonian terms, they are accelerating towards each other and the acceleration is not constant. As we observe them, are not those two black holes moving faster as time progresses until they collide? Thanks.

You can think that way (and many great physicists did before relativity) as long as you can accept some incredible coincidences and till some calculations are wrong. Why does the "mass" that resists acceleration EXACTLY match the mass that attracts other objects? Why does a large mass "attract" something like light that has no mass and seems to only travel in straight lines through space? How does gravitational attraction work with no intermediate entity to transmit the force? Why is the orbit of Mercury a little different from the Newtonian prediction? Relativity clarified all that beautifully. It redefined what a "straight line" means and what an acceleration away from a "straight" path means.

 

[Orodruin]

More precisely, proper acceleration is measured by an accelerometer, and nothing else.

There is such a concept as coordinate acceleration, and it can be measured if a suitable reference frame is set up, so I don't think we can just say "acceleration" without qualification is measured by an accelerometer. Particularly not in a thread where the distinction between proper and coordinate acceleration is now a topic of discussion.

I think in general "acceleration" would be taken to mean proper acceleration, but I guess you are right about it being relevant for this discussion. [Moderator's note: That discussion has now been moved to a separate thread.]

However, this thread is not about the distinction between proper and coordinate acceleration ... it is about understanding gravitational waves.

 

[PeterDonis]

this thread is not about the distinction between proper and coordinate acceleration ... it is about understanding gravitational waves.

Yes, fair point. I'll look at spinning off the "proper vs. coordinate acceleration" subthread into a separate thread.

 

[PeterDonis]

I'll look at spinning off the "proper vs. coordinate acceleration" subthread into a separate thread.

Done. The proper vs. coordinate acceleration discussion is now in this thread:

https://www.physicsforums.com/threads/proper-vs-coordinate-acceleration.974328/

 

[bob012345]

I'm going to chew on these answers for a while as well as do some reading overnight about the subject. Thanks for all the answers. I do appreciate them! If it's spun off to a new thread, I'll respond there then. Thanks again!

 

[JD_PM]

If you are trying to understand gravitational waves, I'd suggest you look for papers that model them rather than looking at data.

Do you guys have in mind any paper/book which nicely models gravitational waves?

I’ve been reading chapter 7 of Carroll’s book but I’d like to have more references.

 

[Mordred]

Ryder Lewis Introductory to General Relativity has a nice intro level into GW waves. The book will step you into all the main aspects of SR and GR as well. It's one of the textbooks I used to step into GR. Very useful in its details particularly when learning the mathematics

 

[George Jones]

Do you guys have in mind any paper/book which nicely models gravitational waves?

Ryder Lewis

Eats, Shoots & Leaves, i.e., "Ryder, Lewis".

I though about recommending this book. I love its final paragraph:
"In this chapter we have considered a few of the topics that have appeared on the agenda since Einstein's day, as a consequence of his great theory. There are, of course other questions raised by by General Relativity, perhaps the most famous of which is quantum gravity. How should a quantum theory of gravity be constructed? There is as yet no generally agreed answer to this question, but many clever people have devoted many years to thinking about it. At the end of an introduction to Einstein's theory, however, it is best not immediately to start thinking about the next challenge. Like a climber who has arrived at the top of his mountain, we should simply sit down and admire the view. Is it not absolutely remarkable that Einstein was able to create a new theory of gravity in which the geometry of space itself became a part of physics? What would Euclid have thought?"

Other, perhaps less abstract references: "A General Relativity Workbook" by Thomas Moore; "Modern General Relativity: Black Holes, Gravitational Waves, and Cosmology" by Mike Guidry.

A book devoted to the topic: "Gravitational Waves Volume 1: Theory and Experiments" by Michele Maggiore.

 

[Mordred]

I liked that passage as well lol