Are gravitational waves bent by gravity?

In summary, gravitational waves travel at the speed of light and can also bend around massive objects. With an extremely sensitive differential accelerometer, we could potentially feel distant stars in a different location than we see them due to the curved path of light in a curved universe. When a massive object passes the event horizon of a black hole, it can no longer escape, including gravity waves. As more objects fall in, the event horizon expands and objects may appear stuck at different distances due to the gravitational effect on surrounding space. The concept of time dilation near the event horizon leads to questions about when an object actually passes over the event horizon. However, the understanding of black holes and the interaction of gravity and light is still an unsolved conceptual problem.
  • #1
michelcolman
175
2
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?
 
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  • #2
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
gendou2 said:
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.
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)?
gendou2 said:
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.
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
michelcolman said:
Yes, but would it also bend around a massive object in between?
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.

michelcolman said:
... I didn't know objects approached a standstill as they approached the event horizon.
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

michelcolman said:
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.
You grasp what I have come to understand as an unsolved conceptual problem.
 
  • #5
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
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
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
any wave affected by gravitational time dilation will be bent.
 
Last edited:

1) What are gravitational waves?

Gravitational waves are ripples in the fabric of space-time caused by the acceleration of massive objects, such as colliding black holes or supernovas.

2) How are gravitational waves produced?

Gravitational waves are produced when massive objects accelerate, causing distortions in the fabric of space-time that radiate outward in all directions.

3) Are gravitational waves affected by gravity?

Yes, gravitational waves are affected by gravity. In fact, they are caused by the acceleration of massive objects, which is a result of the force of gravity.

4) Can gravitational waves be bent by gravity?

Yes, gravitational waves can be bent by gravity. This is because they are a form of energy and are subject to the laws of gravity, just like other forms of energy.

5) How do we detect gravitational waves?

Gravitational waves are detected using highly sensitive instruments called interferometers, which measure tiny changes in distance caused by the passing of a gravitational wave.

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