# I New Gravitational Wave Detection question

1. Jun 1, 2017

### thetexan

This latest observation of gravity waves has brought up a question with me..

Since gravity is a mass-caused distortion in space-time aren't these waves wave distortions of the space-time?

I know there is no such thing as ether but for this analogy and my simple mind I'll use it to illustrate my question. If I think of the universe as being filled with this stuff called space-time (4 dimensional) (I know it's not "filled"...the universe IS space-time) then it makes sense to me, in trying to understand waves, that the space-time distortions propagate just like they might through water, the difference being that instead of water the "medium" is the 4 dimensional grid we call space-time. Is that close?

And if that is true, and since the first three dimensions do not seem to distort with mass, is it then true that it is the time dimension component of the 4 dimensions that is doing the distorting?

tex

Last edited: Jun 1, 2017
2. Jun 1, 2017

### FactChecker

Yes.
Why do you say that? An orbit is a "straight" line in gravity-distorted space-time.
I wouldn't say that.

3. Jun 1, 2017

4. Jun 1, 2017

### MichaelMo

Yes.

It's close. If you go back to the rubber sheet analogy where objects "dent" the sheet, gravity waves are simply waves on the sheet of spacetime.

Hmmm. I would say that it's spacetime as a whole that's being distorted. Technically the whole sheet (including the 3 dimensions) is distorted by the wave.

5. Jun 1, 2017

### thetexan

That brings up another question for me. If an orbit is simply a straight line path that an object such as a satellite takes...and the reason the satellite is following that path is simply because it travels along an undisturbed path (which is now curved because of the distortion)...it would seem to follow that the satellite doesn't "know" any better than to follow the "straight line" path. In other words, the satellite does nothing to follow the path...it doesn't "steer" along the path...it's momentum takes it straight ahead....and it's the PATH that is what is changing.

If the path is the road along which the satellite is traveling then what does velocity have to do with it? We know that if the satellite slows down its orbit will decay and it will follow a different path...one that spirals into the planet. So if the satellite is following "the road" what difference does the velocity along the road make? The path is the path

tex

6. Jun 1, 2017

### MichaelMo

The path isn't technically changing, it just has an intrinsically curved geometric shape like the funnel in this video:

Of course there isn't the same kind of friction in space. The path isn't changing, it's simply a curved path which other objects follow.

It's just the velocity along the curved path.

I'm unclear what you mean by that question. Could you elaborate a bit?

7. Jun 1, 2017

### Drakkith

Staff Emeritus
There is no set road or path. Imagine the funnel you roll a quarter down into and watch it spiral in. If the quarter didn't lose velocity, then it would be able to keep rolling in an "orbit" without falling in. Accelerating or decelerating the quarter would change its path just like accelerating or decelerating a satellite changes its orbit.

8. Jun 1, 2017

### FactChecker

I am certainly not an expert, but it's a geodesic path in space-time, not in space. Because time is intrinsic to the geodesic path, velocity has an effect on the path. I don't think that there is any non-mathematical way to clearly describe the situation. I think that the book Relativity Visualized by Lewis Carroll Epstein is a good attempt.

9. Jun 2, 2017

### thetexan

I guess my question about velocity is this...how does the velocity of the satellite factor into the path it takes?

If a satellite enters into a circular orbit then it is established on a distorted space-time path around the planet at a certain velocity. Now, I am probably wrong but as I stated earlier I don't see the satellite doing anything to stay on that path in and of itself. I imagine it like a 1/2 inch PVC pipe around the planet representing a particular path. I imagine a marble inside the pipe going around the planet along that path. In my example the path is fixed and represents a particular distortion in space-time. The marble will follow that path regardless of its velocity...whether is travels along the pipe fast or slow it will follow the pipe. The path forces the marble to follow it because that is the path the marble took.

In my analogy the path forces the marble to follow it because the marble is physically constrained within the pipe. In reality, when a satellite slows down it will take a different path...one that spirals into the planet or if it speeds up, one that will move away from the planet. So the velocity of the satellite has some bearing on which path of distortion it takes based on its velocity.

I have always visualized the distorted "grid" of space-time much like you see on the computer generated illustrations. It seems that the space-time distortion around the planet is a fixed factor based on the mass of the planet...so it follows that all distorted paths are also fixed (acknowledging that the mass of the satellite also effect the localized distortion). All that is left to do is to decide which of the millions of paths one would like to take. Paths that are closer to the mass-causing distortion will be more pronounced while those farther away will be less so. It seems to my uneducated mind that it is simply a matter of deciding which PVC pipe I want to take around the planet...not how fast I want to go through the PVC pipe. Yet velocity factors in somehow on changing the path.

I'm just wondering how if someone can help me out.

tex

10. Jun 2, 2017

### FactChecker

Objects with different velocities are on different paths in space-time. It is a space-time distortion, not just a space distortion. Time, and therefore velocity changes the geodesic paths that unaccelerated objects follow.

Last edited: Jun 2, 2017
11. Jun 2, 2017

### thetexan

Two satellites on exactly the same trajectory following each other but with different speeds will be on different distortion paths. I get that. I'm just wondering how the velocity effects which path. Is there some relationship between the mass distortions of each object (the planet and satellite) that does this. I know that mass effects the distortion but how does velocity effect the distortion.

Another way of visualizing it would be this...if one could see in 4 dimensions you could stand afar and observe the distorted fabric of the planet (in all 4 dimensions). That observation represents the "map" that any other object will be subjected to as sit passes the planet. You would think that that "map" is fixed (assuming no change in the mass). But if I look at two satellites approaching the planet in a flyby, one fast and one slow, why do I think that they will take two paths along that "map"? I accept that they will but why (in simple terms, that is, if possible). It must be the time factor in the velocity, I'm thinking. That's why I asked earlier if it's the time dimension that is distorting.

tex

12. Jun 2, 2017

### FactChecker

Nothing about the satellite significantly changes the space-time distortion. They do not have enough mass.
I thought you asked if it was only the time distortion. It is not. The effect is on space-time as a combination -- at what time is the satellite at a particular position in space.

13. Jun 2, 2017

### MichaelMo

IMO it's confusing to look at the problem as a PVC pipe problem because the sides of the PVC hold the object in place. It's more of a geometric curvature problem, like climbing up a hill, or coming down a hill. That funnel video analogy I cited earlier is a more appropriate analogy IMO. If we gave one of the balls a push while it is orbiting around inside the funnel, the ball would tend to move higher up the side of the funnel because no side of any PVC pipe holds it in place, or holds it in exactly the same orbit. Likewise if we slow it down, it will move toward the center. I think your PVC analogy is just making it considerably harder to visualize the effect of velocity on the orbit of the object.

Last edited: Jun 2, 2017
14. Jun 2, 2017

### Drakkith

Staff Emeritus
It's important to understand that we're dealing with geometry here, not physical roads or pipes. The situation is similar to the funnel example I gave in post #12. The quarter's path is initially determined by the position and velocity of the quarter. Since the funnel is smooth, the quarter is free to change from its initial path simply by accelerating or decelerating.

Spacetime is similar. An object in free-fall (like in an orbit) is free to change its path through spacetime by accelerating or decelerating.

15. Jun 2, 2017

### FactChecker

@Drakkith 's funnel visualization comes a close as I have seen to visualizing the space-time distortion. The reference I gave in post #13 (the book Relativity Visualized by Lewis Carroll Epstein) does a lot using that approach. Beyond that, I don't think anyone can do more to help you unless you want to rely on the math.

16. Jun 3, 2017

### Drakkith

Staff Emeritus
A number of off-topic posts have been removed. I remind all members to please stay on topic.