B Gravitational waves

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1. Oct 28, 2017

Ross B

does any object with mass that moves create a gravity wave? So if I wave my hand it is in fact creating gravity waves, just very small ones

2. Oct 28, 2017

ISamson

Yes.

3. Oct 28, 2017

phinds

I think "very small" VASTLY overstates the strength of that particular gravity wave

4. Oct 28, 2017

ZapperZ

Staff Emeritus
Maybe he/she is severely overweight.

Zz.

5. Oct 28, 2017

phinds

Yeah, but that would mean he/she would have to shake his/her belly rather that a hand wave.

6. Oct 28, 2017

Staff Emeritus
Huh? Where did you get that? An object that is moving at constant velocity does not create a gravitational wave.

7. Oct 28, 2017

ISamson

Aren't we talking about a hand moving in space. Is that constant speed?

8. Oct 28, 2017

Ross B

why does an object moving at constant velocity not not create a gravitational wave? like the wake from a boat

even if my hand was stationary the molecules/atoms within my hand are accelerating and decelerating so they would all be , individually, creating, gravity waves

if space is elastic, as a body moves thru space it deforms space and in its wake space must snap back to its original position, that deformation of space, assuming it is not a linear deformation, would cause space itself to accelerate/decelerate/strech/compact, all other factors remaining the same?

Last edited by a moderator: Oct 29, 2017
9. Oct 29, 2017

Staff: Mentor

Consider that when an object is moving at a constant velocity (relative to what?) then there is an inertial frame in which it is not moving at all.... and all frames must agree about the presence or absence of gravitational waves, so if you can demonstrate that they aren't present using one frame, then you know they aren't present using any frame.

The boat is moving through the water, displacing it and creating waves in it. But that analogy doesn't work for gravitational waves because space is not a substance that you displace as you move through it.
You may have been misled by pop-sci presentations that speak of spacetime as a "fabric", a thing that stretches and deforms and could have properties such as elasticity. It's not.

Last edited: Oct 29, 2017
10. Oct 29, 2017

Ross B

is "displace" just another way of saying "deform" which is another way of saying "changes some property of"? It is uncontroversial that a massive object changes the properties of (displaces) space around it as it move thru space

"You may have been misled by pop-sci presentations that speak of spacetime as a "fabric", a thing that stretches and deforms and could have properties such as elasticity. It's not."

space "deforms" around a massive objects, and therefore any object with mass. Implicit in that statement is space "un-deforms" after the object has passed by.

11. Oct 29, 2017

Ibix

You have to remember that GR models spacetime, not space. Space is what you get when you slice 4d spacetime up into 3d sheets. So "space when you are near a massive body" and "space after the massive body has passed" are two completely separate parts of spacetime - nothing is actually deforming. Any sensible definition of "slicing up spacetime" will give you a sequence of spaces that change geometry smoothly, but the change is the change of slice you are calling "space, now", not from any deformation.

Some spacetimes include gravitational waves (not gravity waves - those are a kind of surface wave in fluids). Some do not. The spacetime around a body moving in a straight line can't because a distant inertial observer could see the body as stationary.

12. Oct 29, 2017

Orodruin

Staff Emeritus
13. Oct 29, 2017

m4r35n357

Please everyone, it should not take until the 12th post to correct this.

14. Oct 29, 2017

Orodruin

Staff Emeritus
To be fair, Ibix mentioned it in post #11 while I was typing and linking.

15. Oct 29, 2017

m4r35n357

A couple of posters mentioned it in passing I know, but until your post nobody corrected it.

16. Oct 29, 2017

Ross B

so around a massive object, as the space component of spacetime does not deform, I assume only time deforms?

17. Oct 29, 2017

Orodruin

Staff Emeritus
There is no such thing as "the space component". The separation into space-like slices is quite arbitrary and can be done in many different ways.

The pro-tip is that you will not be able to understand what is actually going on from extrapolating popularised descriptions or even more accurate descriptions that use English language. The reason is that language is not precise enough. The terms used do have a precise mathematical meaning, but that can be misrepresented with words having several similar meanings in English. The mental picture you are painting will likely not be accurate. Bottom line is: never use what you read in popular science to extrapolate arguments. Popular science is great for learning about science and creating interest in it, but it is rather useless for learning actual science.

18. Oct 29, 2017

Ross B

Im using Serway edition 4....am I wasting my time

Surely all I have to know is that a massive body moving thru space
the space ahead of the body has a number of parameters (one of those parameters is gravity?)
the massive body changes some of those parameters within its immediate surroundings
once the massive body has passed by, space returns to empty space

is that fundamentally flawed?

19. Oct 29, 2017

Ibix

No. Nothing deforms. Spacetime is the shape it is. It doesn't change.

If our poor limited human intellects insist on slicing 4d spacetime into a sequence of 3d volumes, and calling each volume "space" and giving each one a sequence number we call "time", then each subsequent volume may be a different "shape". You could regard each one as deformed compared to its neighbour. But nothing is actually changing shape - you are simply looking at a sequence of slightly differently shaped slices of spacetime and mistaking that for one slice that changes shape. So there's no sense of "elastic deformation of a fabric".

20. Oct 29, 2017

Staff: Mentor

It's been fixed in the thread title.

21. Oct 29, 2017

Ross B

so if a massive object has absolutely no impact, of any kind what so ever, on any parameter of the space time around it - how does any other object know, at a distance, it is there?

22. Oct 29, 2017

Ibix

Of course it has an effect. Those successive slices of spacetime are (or can be, anyway) different "shapes" for a reason. It's just that describing that effect as "distorting space" is hopelessly inadequate for getting any actual physics done.

For example, in the Schwarzschild spacetime there's an obvious way of slicing spacetime so that each slice ("all of space at this time") is identical. There is gravity there. But what's being distorted? Everything is the same, always has been and always will be. Sure, the geometry near the mass is different near the mass and far from the mass. But that's not because one of them changed shape to look like the other.

23. Oct 29, 2017

Staff: Mentor

Serway is a respectable enough textbook, but its few pages on general relativity are completely superficial: no differential geometry, no tensors, no coordinate transforms, no Einstein Field Equations. This may be because of their stated goal of introducing no math beyond first year calculus.

Carroll (https://www.preposterousuniverse.com/grnotes/) would be a good place to start in on GR; you can try the "no-nonsense overview" first if you don't want to take on the whole thing. The chapter on gravitational waves is at https://preposterousuniverse.com/wp-content/uploads/grnotes-six.pdf

24. Oct 31, 2017

Imager

This should be on T-Shirts!

25. Oct 31, 2017

timmdeeg

I think "distorting space" makes sense if interpreted as periodically stretching and squashing of space, as illustrated here by means of the ring of freely falling particles.