B Black Holes, Gravitational waves and Gravitons

Raffaele

I wonder why electromagnetic waves don't escape from a black hole while gravitational waves (obviously) do.
What is the difference between the two kind of waves?
And between gravitons and photons?

Related Special and General Relativity News on Phys.org

jbriggs444

Homework Helper
I wonder why electromagnetic waves don't escape from a black hole while gravitational waves (obviously) do.
Gravitational waves do not escape from the interior of a black hole. They emerge from the region of space-time outside the event horizon(s).

Torbert

Gravity waves are a warpage of spacetime behaving similar to a compression wave, caused by the relative motion of two masses and propagating at lightspeed. Gravity begins at the surface of the BH event horizon, the motion of the horizon sends a wave of warped spacetime beginning at that point. Electromagnetic waves come from moving charges, and those generated outside the ergosphere of the event horizon can escape, just like gravity waves, but any electromagnetic waves beginning inside the event horizon cannot be detected, nor can any gravity waves caused by the singularity itself.
An observable 'naked singularity' has been theorized for the early pre-bang universe but nothing conclusive has come of it.

Ibix

Gravity waves are a warpage of spacetime behaving similar to a compression wave, caused by the relative motion of two masses and propagating at lightspeed.
Nitpick: gravity waves are a kind of surface wave on water. You are talking about gravitational waves.

Gravitational waves are not compression waves, which are longitudinal mechanical waves, but are transverse tensor waves which change the notion of distance in two direction perpendicular to their direction of travel. And they are generated by stress-energy distributions with a changing quadropole moment, not just "two masses in relative motion".
Gravity begins at the surface of the BH event horizon, the motion of the horizon sends a wave of warped spacetime beginning at that point.
Gravitational waves can never be traced back to events inside a black hole, true, but this does not mean that "gravity begins at the event horizon". We are able to describe the interior of black holes using general relativity - our current best theory of gravity. And "motion of the horizon" is a rather difficult thing to adequately describe since the horizon in this case is a null surface in a non-stationary spacetime. That's why "changing quadropole moment" is used as the source, not relative motion.
Electromagnetic waves come from moving charges
Accelerating charges, in fact.
those generated outside the ergosphere of the event horizon can escape,
The ergosphere and the event horizon are two different things. Electromagnetic waves generated outside the event horizon can escape.

Last edited:

Torbert

Ouch, simplifying and not looking up the facts does not go far here. Compression waves would require aether, I actually know that but didn't go far in making it obvious.
The innermost stable orbit is outside the event horizon but material inside will relentlessly spiral inward until the event horizon is reached.
I thought this was the same as the ergo sphere where a quick dip with an unstable orbit would transfer energy outside. But if energy could be collected then the in fall is not inevitable.
I will do better reviewing before posting in the future.

PeterDonis

Mentor
Gravity begins at the surface of the BH event horizon
This is wrong. A correct statement would be that gravitational waves observable far away are emitted from outside the horizon. But "gravity" is not limited to regions from which gravitational waves are emitted that are observable far away.

An observable 'naked singularity' has been theorized for the early pre-bang universe
What are you talking about here? Can you give a reference?

The innermost stable orbit is outside the event horizon but material inside will relentlessly spiral inward until the event horizon is reached.
I thought this was the same as the ergo sphere where a quick dip with an unstable orbit would transfer energy outside. But if energy could be collected then the in fall is not inevitable.
None of this has anything to do with emission of gravitational waves.

"Black Holes, Gravitational waves and Gravitons"

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving