Black Holes and Gravity....

[Slinkey]

Some other thoughts that disturb me about Black Holes.

 

BHs have intense gravitational fields. They are objects borne of the classical theory of GR. According to GR gravity propagates at c. It cannot move any faster than this according to the rules of GR. Thus if the sun was to suddenly disappear we wouldn't know about it for over 8 minutes - no signal can reach us at super-luminal speeds to warn us of our impending doom.

 

We have experiments currently trying to detect gravity waves. The idea is to detect gravity waves as they pass us and warp spacetime. I haven't checked the research lately but last time I looked we had not detected any gravity waves.

 

According to QM for every wave there is an associated particle. In the case of gravity it is the graviton. No one has detected one of these but it should exist if gravity waves exist and is therefore detectable in principle.

 

The problem for me is when GR and QM meet at the EH of a BH.

 

According to GR, the gravitational field at the EH is so intense the escape velocity is equal to c. As gravity can only travel at c, whether as wave or particle, how can gravity permeate into the surrounding space of a BH?

 

Shouldn't a BH also be gravitationally black?

[Klaynos]

The below is my speculations....

 

The force that is stopping stuff escaping BHs is gravity, mediated by gravitons, assuming that gravitons act like other mediating particles like photons, they will only have a very small interaction, so they would not feel the same force of gravity as normal matter...

[Mr Skeptic]

That's a really clever question. I think the answer is that you are mixing GR gravity with quantum gravity. So far, we haven't figured out how to get these two to play nice.

[thedarkshade]

I think the answer is that you are mixing GR gravity with quantum gravity.

Wouldn't every living physicist like to see that done:rolleyes: !

[D H]

Being a frequently asked question, the answer is in the physics FAQs:

 

http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/black_gravity.html

[Slinkey]

Thanks DH for the link however this is more disturbing for me....

 

... from the link DM gave So how do the virtual photons get out of the event horizon? Well, for one thing, they can come from the charged matter prior to collapse, just like classical effects. In addition, however, virtual particles aren't confined to the interiors of light cones: they can go faster than light! Consequently the event horizon, which is really just a surface that moves at the speed of light, presents no barrier.

 

Hmmm...hmmmm...hmmmmmmmmm

 

I have to read up on virtual particles again (I like Feynman but it gets so damn complicated! ), but at first glance it looks like the mechanism for Hawking Radiation (HR) is very similar to the mechanism for transmitting the gravitational field. At the EH we cannot have an empty field. Even for gravity. Thus there is a probability that one of a pair of virtual particles gets out into space... but this would mean the BH loses mass simply by gravitating, surely? In this case it would be the bigger it is the quicker it loses mass rather than vice versa with HR.

 

Or am I barking up the wrong tree here entirely?

[SkepticLance]

Slinkey

 

You are assuming a graviton has mass.

[D H]

At the EH we cannot have an empty field. Even for gravity. Thus there is a probability that one of a pair of virtual particles gets out into space... but this would mean the BH loses mass simply by gravitating, surely? I

No. To do that the BH would have to emit real gravitons as opposed to virtual ones. Virtual photons are the carriers of force for a static electric or magnetic field, but real photons are needed to transmit a time-varying electromagnetic field. Similarly, virtual gravitons (if they exist) are the carriers of force for a static gravitational field but real gravitons are required to carry a gravity wave. Real gravitons, like real photons, cannot escape from a black hole.

 

=====================

 

Slinkey, you are assuming a graviton has mass.

Black holes are called "black holes" because the escape velocity exceeds the speed of light -- not even real photons can escape a black hole. Virtual particles can however escape.

[Slinkey]

The below is my speculations....

 

The force that is stopping stuff escaping BHs is gravity, mediated by gravitons, assuming that gravitons act like other mediating particles like photons, they will only have a very small interaction, so they would not feel the same force of gravity as normal matter...

 

Photons feel gravity. That was one of the experiments that vindicated GR in the early days. They looked at the apparent position of stars next to the suns surface during a total eclipse. GR said their apparent position would be deflected by the gravitational field of the sun.... they were.

 

Slinkey

 

You are assuming a graviton has mass.

 

No. A graviton only need carry energy away from the BH for it to lose mass, but see DH's reply.

 

........Similarly, virtual gravitons (if they exist) are the carriers of force for a static gravitational field but real gravitons are required to carry a gravity wave. Real gravitons, like real photons, cannot escape from a black hole.

 

But isn't the process in Hawking Radiation a pair of virtual particles that would ordinaruly meet back up but one is sucked into the BH whilst the other is "promoted" to a real particle and moves away from the EH?

 

In my example it would be a virtual graviton being promoted to a real graviton.

[Klaynos]

Photons feel gravity. That was one of the experiments that vindicated GR in the early days. They looked at the apparent position of stars next to the suns surface during a total eclipse. GR said their apparent position would be deflected by the gravitational field of the sun.... they were.

 

My point was not that photons don't feel gravity, but that photons only interact with each other very very very weakly, so graviton - graviton interactions are likely to be small... this is my own speculation based on extrapolating the photon - graviton similarities....

[Slinkey]

Sorry Klaynos, I misunderstood what you meant. That sounds possible, yes.

[Mr Skeptic]

But isn't the process in Hawking Radiation a pair of virtual particles that would ordinaruly meet back up but one is sucked into the BH whilst the other is "promoted" to a real particle and moves away from the EH?

 

In my example it would be a virtual graviton being promoted to a real graviton.

 

Wouldn't this mean that smaller black holes have more gravity than larger ones? As I understand it, the smaller ones give off more Hawking radiation than the larger ones.

 

What about quantum tunneling? I'm unfamiliar with it, but I understand it allows for particles to occasionally cross an impossible barrier.

[D H]

In my example it would be a virtual graviton being promoted to a real graviton.

It is important to remember that all of this discussion regarding Hawking radiation and gravitons is speculative.

 

Hawking radiation results from pair production of a virtual particle and its antiparticle. AFIK, gravitons, like photons, do not arise directly from pair production (they are their own antiparticles). What process promotes a virtual graviton to a real one? Moreover, Hawking radiation results in regions where the gradient if the gravitational acceleration is very high. While the gravitational acceleration is much larger near the event horizon of large black holes as compared to small black holes, the gravity gradient is much smaller with large black holes.

[Janus]

 

But isn't the process in Hawking Radiation a pair of virtual particles that would ordinaruly meet back up but one is sucked into the BH whilst the other is "promoted" to a real particle and moves away from the EH?

 

In my example it would be a virtual graviton being promoted to a real graviton.

 

You have to understand the different roles that virtual gravitons and real gravitons play.

 

Virtual gravitons are the quanta of the gravitational field while real gravitons are quanta of gravitational radiation.

 

While virtual gravitons mediate the gravitational force, gravitons transmit information about variations in the gravitational field.

 

The interaction of the gravitational force via virtual gravitons would work by the exchange of virtual particles. The virtual particle is emitted by one body passes between the distance between them and is absorbed by the other body. In the process there is an exchange of momentum. At no time does it become a "real" graviton.

 

So no, the exchange of gravitons that result in the gravitational force from a black hole is not the same as the mechanism that results in Hawking radiation.

 

 

In classical physics gravitational force is due to the field and gravitational radiation are gravity waves which travels at c.

 

Gravity waves are not responsible for the gravitational force, they carry information about variations in the gravitational field; they are "ripples" that travel through the gravitational field. They play the same role for gravity as electromagnetic raditation (radio waves) play in electromagnetism. For example, you can't draw an object towards you by shining a light on it.

 

For a black hole, thing work like this:

 

Assume you are an observer some distance from a star just before it collapses into a black hole. You are within the gravitational field of the star.

 

The star begins its collapse. Just before the black hole forms, the surface of the star is just above where the event horizon will be and you are still in the gravity field.

 

The star passes within the event horizon. What happens? The gravitational field can't just disappear. For one thing, your gravitational potential energy owuld have to disappear also, violating the conservation of energy. For another, for you to feel a change in gravitational force, the information of the change has to be transmitted to you by gravity waves emitted by the black hole. But once the surface passes below the event horizon, information can't pass back out. Gravity waves can't leave a black hole any more that lightwaves. Thus, you can never "get the Message" that the mass causing the gravitational field has passed the event horizon.

 

This is why a black hole is sometimes called a "frozen star", as far as any observer is concerned the star, including its gravitational field, is frozen in the state it was before the black hole formed.

[Slinkey]

...(detailed explanation snipped for brevity)...This is why a black hole is sometimes called a "frozen star", as far as any observer is concerned the star, including its gravitational field, is frozen in the state it was before the black hole formed.

 

That was an excellent reply and cleared up a few things for me. Thanks