Do Objects in Motion Create Gravitational Waves?

[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

 

[ISamson]

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

Yes.

 

[phinds]

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

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

 

[ZapperZ]

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

Maybe he/she is severely overweight.

Zz.

 

[phinds]

Maybe he/she is severely overweight.

Zz.

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

 

[Vanadium 50]

does any object with mass that moves create a gravity wave?

Yes.

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

 

[ISamson]

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

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

 

[Ross B]

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

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?

 

[Nugatory]

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

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.

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?

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.

 

[Ross B]

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.

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.

 

[Ibix]

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.

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.

 

[Orodruin]

Pet peeve: It is important to separate the concept of gravity waves from that of gravitational waves. They are not the same thing.

 

[m4r35n357]

Pet peeve: It is important to separate the concept of gravity waves from that of gravitational waves. They are not the same thing.

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

 

[Orodruin]

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

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

 

[m4r35n357]

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

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

 

[Ross B]

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

 

[Orodruin]

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

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.

 

[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?

 

[Ibix]

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

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".

 

[Nugatory]

Pet peeve: It is important to separate the concept of gravity waves from that of gravitational waves. They are not the same thing.

It's been fixed in the thread title.

 

[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?

 

[Ibix]

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?

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.

 

[Nugatory]

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

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

 

[Imager]

Popular science is great for learning about science and creating interest in it, but it is rather useless for learning actual science.

This should be on T-Shirts!

 

[timmdeeg]

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.

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.

 

[Ibix]

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.

That animation works by making a choice about what "space" means, slicing spacetime into a sequence of spatial slices, and then presenting that sequence of slices in an animation. That's fine. But it's presenting a sequence of distinct, slightly different things. In trying to understand GR it's a mistake to see it as one thing changing, because your initial choice of slicing spacetime into space was an arbitrary one.

 

[timmdeeg]

But it's presenting a sequence of distinct, slightly different things. In trying to understand GR it's a mistake to see it as one thing changing, because your initial choice of slicing spacetime into space was an arbitrary one.

Saying "one thing" do you mean the ring? To my understanding the source of the GW causes geodesic deviation which the "distortion" of the ring shows. Can you please elaborate a little more "because your initial choice of slicing spacetime into space was an arbitrary one." It seems I'm missing something important here.

 

[Ibix]

Can you please elaborate a little more "because your initial choice of slicing spacetime into space was an arbitrary one."

It's the block universe again. Take each frame (in the filmography, not physics sense) of the animation and print it. Stack up the printouts. Now dissolve the paper and leave the ink. This is the block universe and the wiggly columns of ink are the worldtubes of the dots.

To make the animation you need to put the paper back in. The paper is your choice of what "space" means. And in GR, as in SR, there's no reason to prefer horizontal or sloped pieces of paper. It's an arbitrary choice. Furthermore, the pieces of paper aren't expanding or contracting. Each one has a larger or smaller scale compared to its neighbour, but nothing is changing - unless you mistake the sequence of similar things for a single thing changing.

 

[Paul Colby]

So there's no sense of "elastic deformation of a fabric".

What about curvature? It's not zero in the neighborhood of a mass. I think I "get" that space-time doesn't "evolve" because time is included from the start and I also get vacuum is "nothing" part. I spent a lifetime viewing electromagnetic fields as dynamical why is this now verboten for curvature?

 

[PeterDonis]

I spent a lifetime viewing electromagnetic fields as dynamical why is this now verboten for curvature?

EM fields aren't dynamical either if you view them from the spacetime viewpoint. From the spacetime viewpoint, an EM field is an assignment of a 4-vector (the potential) to every event in spacetime, which has to satisfy certain equations (Maxwell's Equations in their spacetime form). Such an assignment isn't "dynamical" any more than spacetime itself is: it's a 4-dimensional object that just is, it doesn't "change".

If, OTOH, you want to view EM fields as dynamical, you have to pick some splitting of spacetime into "space" and "time", and then view the EM field as varying over "space" and evolving in "time". You can do the same thing with spacetime curvature: for example, you can view the spacetime curvature of a gravitational wave as a stretching/squeezing of "space" that changes with "time". The drawback of doing this is that there is no unique splitting of spacetime into "space" and "time", so any such choice of splitting will end up putting things into your model that are artifacts of that particular choice. (The same is true of the EM field: for example, which part of the field you call "electric" and which part you call "magnetic" is an artifact of your particular choice of how to split spacetime up into space and time.) But if you're willing to accept that drawback, the procedure itself is perfectly legitimate.

 

[timmdeeg]

To make the animation you need to put the paper back in. The paper is your choice of what "space" means. And in GR, as in SR, there's no reason to prefer horizontal or sloped pieces of paper. It's an arbitrary choice. Furthermore, the pieces of paper aren't expanding or contracting. Each one has a larger or smaller scale compared to its neighbour, but nothing is changing - unless you mistake the sequence of similar things for a single thing changing.

Yes. I just saw that also Wikipedia talks about distortion: " As a gravitational wave passes through the particles along a line perpendicular to the plane of the particles (i.e. following the observer's line of vision into the screen), the particles will follow the distortion in spacetime ". This misleads to believe that space is something physical which can be stretched ect. In deed, as in the case of the expanding universe one should think of changing distances instead.
I think the phrase "distortion of space" is ok as long as it isn't understood wrongly. The same is true talking about "curved spacetime".

 

[Paul Colby]

But if you're willing to accept that drawback, the procedure itself is perfectly legitimate.

Agreed, however, this from a space-time view nothing is dynamical seems like a vacuous viewpoint. Come rain or shine one still must solve a system of hyperbolic equations to understand the physics of an isolated or cosmological system. Other than dispelling layperson language misuse, this nothing is dynamical verbiage seems to offer little in the way of understanding, IMO.

 

[PeterDonis]

this from a space-time view nothing is dynamical seems like a vacuous viewpoint. Come rain or shine one still must solve a system of hyperbolic equations to understand the physics of an isolated or cosmological system

This is a matter of terminology, not physics. If you want to insist that any time you are solving hyperbolic differential equations, you are modeling a "dynamical" system, go ahead. As long as the precise meaning of your terminology is known, there's no problem in understanding your meaning, and "dynamical means hyperbolic differential equations" is precise. But not all physicists use that terminology, so when you see them saying that "spacetime is not dynamical", it's important to know that they mean something different by the word "dynamical" than you do--they certainly aren't claiming that the differential equations in question are not hyperbolic.

 

[Paul Colby]

it's important to know that they mean something different by the word "dynamical" than you do

Then it's certainly fair to ask what is this new meaning of the word "dynamical" of which space-time is not. I only ask because the way I was raised, space-time is governed by hyperbolic field equations as are all other fields. The subject was referred to as classical dynamics, electrodynamics, geometrodynamics ect. I must have missed a memo or something.

 

[PeterDonis]

it's certainly fair to ask what is this new meaning of the word "dynamical" of which space-time is not

I don't know that any of the scientists who use the word in the way I describe have provided a precise definition; but I think it is something along the lines of "dynamical" implying "changing with time", where "time" is some external parameter rather than an internal property of the model. Spacetime, as a model, does not have "time" as an external parameter, and does not describe a succession of "states" as a function of "time". It just describes a single 4-dimensional geometry.

One could certainly argue about which meaning is more appropriate for the word "dynamical", but as I said, that's a matter of terminology, not physics. The physics is, as you say, that the 4-dimensional spacetime geometry is obtained by solving a set of hyperbolic differential equations.