# Are gravitational waves bent by gravity?

1. Dec 28, 2008

### michelcolman

Gravitational waves travel at the speed of light. So do they also bend around massive objects? If we had an extremely sensitive differential accelerometer, would we "feel" a distant star to be where we saw it? This would seem logical if you consider the curved path of light to really be a straight line in a curved universe.

Also, what happens when a massive object passes the event horizon of a black hole? Nothing can escape from behind the event horizon, including gravity waves, so if we would fly around the black hole to map its gravitational field, the object would seem to remain forever at the event horizon. That's where it sent out its last gravitational wave with an update of its position to adjust space-time curvature around the black hole. And as more objects fell in later, the event horizon would expand so that successive objects would appear stuck at different distances, at least as felt by measuring their gravitational effect on the surrounding space.

So much for a black hole with no hair... OK, it would "really" have no hair, but it would feel like it did when you're outside. Right?

2. Dec 28, 2008

### gendou2

1. Yes. If we could magically remove the sun out of space, the last of it's light would arrive at the same time as the last gravitational tug.

2. I know that time dilation shifts light from objects falling into a black hole to the red end of the spectrum the further they fall. I would expect that the gravitational waves would do the same thing: smooth over the event horizon asymptotically. It does seem to beg the question: At what time does the event horizon actually expand? As an outside observer, when does an object pass over the event horizon, if time dilation makes the object seem to approach stand-still. I don't know the answer.

3. Dec 28, 2008

### michelcolman

Yes, but would it also bend around a massive object in between? For example, suppose we would measure the gravity from a star, and then place another star in between, about halfway but slightly to the left from our point of view. Of course nothing will appear to change until the light of the new star reaches us. At that time, we will both see and feel the new star, while the old star will appear to be displaced to the right because of the curvature of light around the new star. But will the old star also "feel" more to the right if we could measure its gravity? Or in other words, will the combined gravity match the observed locations of the two stars rather than what we would ordinarily tend to consider their "real" location (even though "real" is really ill-defined, obviously)?
I have to read up on black holes, I didn't know objects approached a standstill as they approached the event horizon. Of course in the reference frame of the falling object it will keep going, but I didn't know time dilation was strong enough to bring them to a halt in an outside reference frame.

I'm just guessing now, but if objects indeed slow down to an asymptotic crawl as they approach the event horizon, and at the same time the event horizon expands because of the added mass, that would be what tips them over the horizon. It's sort of tied to how you define the black hole: as soon as you start considering the new mass to be part of the black hole, it's suddenly inside the event horizon. But with the mass considered to be a foreign mass outside the black hole, it's still outside the event horizon. Or in yet other words, it could escape from the black hole, but parts of it can't escape from the combination of black hole + new object.

Weird things, those black holes. I hope I never meet one...

4. Dec 28, 2008

### gendou2

Light and gravitational both waves travel in a strait line. Strait lines drawn in curved space result in gravitational lensing, which I think was your example. So, yes, we feel the lensed tug of gravity, just like we see the lensed shape of a distant galaxy.

I originally heard this stated in "The Black Hole Wars" by Leonard Susskind
http://en.wikipedia.org/wiki/Black_hole#Before_the_falling_object_crosses_the_event_horizon

You grasp what I have come to understand as an unsolved conceptual problem.

5. Jan 9, 2009

### nhall729

If gravity was lensed by gravity like light, how come gravity can excape a black hole to make it a black hole, but light can not?

6. Jan 9, 2009

### michelcolman

Basically, when we say gravity moves at the speed of light, this really means that changes in gravity travel at the speed of light.

The actual field of gravity (the curvature of space-time) was created while the black hole was forming, and new matter falling in keeps updating this field until it reaches the event horizon.

The universe around the black hole simply remains "bent" the way it is, there's no need for any new gravity to escape the black hole and "feed" the field.

7. Jan 10, 2009

### Steely Dan

Correct me if I'm wrong, but doesn't the superposition principle means the answer is no? In other words, if a gravitational wave is truly just a vibration of the gravitational field, should it not remain unaffected by other gravitational waves?

I suppose this is affected by whether we look at gravity classically or whether your question is incorporated into a potential quantum gravity framework.

8. Jan 11, 2009

### granpa

any wave affected by gravitational time dilation will be bent.

Last edited: Jan 11, 2009