Register to reply 
What gives BH it's gravityby danihel
Tags: gravity 
Share this thread: 
#1
May2310, 01:19 PM

P: 20

hi
first im not a physicist/mathematician, i was said that black hole is this infinitely small point of infinite density and the amount of matter that collapsed into this point determines the diameter of event horizon and the overall gravitational pull of the black hole. My question is: how can the gravity of BH vary if its caused by this one infinitely small point of infinite density? are there larger or smaller infinities or whats the catch? 


#2
May2310, 01:27 PM

P: 2,456

the size of event horizon is determined by the mass.
your question is very good. How can one point be heavier than another? 1. elementary particles are also points, but they have different masses 2. nobody believes that singularity is actually a zerovolume point. Future theory must tell us that, for example, there is something different instead (without infinities) 


#3
May2310, 03:16 PM

Sci Advisor
P: 6,077

The concept of a point with infinite density is the result of using general relativity without considering quantum theory. Inside a black hole these theories are incompatible  trying to use both at the same time leads to nonsensical solutions.



#4
May2310, 03:38 PM

Admin
P: 23,599

What gives BH it's gravity



#5
May2310, 09:12 PM

P: 1,815




#6
May2310, 11:17 PM

P: 36

Neil 


#7
May2310, 11:24 PM

Sci Advisor
PF Gold
P: 9,454

The gravity of a black hole is identical in strength to that of the mass from which it formed. Black holes arise when mass condenses down to the point an event horizon forms. What happens beyond that is a guess. My guess is collapse halts at the planck density  a miniscule, but, finite volume.



#8
May2410, 01:30 AM

P: 611

I always think that this gives a good explanation of what's going on with gravity around a black hole
How does the gravity get out of the black hole? 


#9
May2410, 09:10 AM

P: 1,815

Thank you Stevebd1, unfortunately your reference leaves a number of unanswered questions. It begins by saying that gravity doesn’t have to get out of the black hole, that the gravitational field is defined by the star’s mass before its collapsed into a black hole and that the gravitational field is a “fossil field”. Presumably the gravitational field or warped spacetime is frozen due to time dilation at the event horizon. How is it then that matter is able to pass through the EH but its gravitational and electric fields do not?
Next it states that gravitons don’t exist in GR but goes on to use virtual photons as an analogy for how gravitons would behave if they did exist. In his analogy, McIrvin refers to a static electric field. Static electric and gravitational fields are not the issue. We are interested in how changes in the singularity’s mass are propagated to the event horizon. Other references, including Leonard Susskind if I’m not mistaken, talk about the gravitational field of an infalling object adding to the frozen gravitational field at the event horizon of the black hole. This explanation avoids the problem with the effects of changes in the mass of the singularity propagating backwards to the EH but still leaves us with the problem of matter being able to pass through the EH but its gravitational field remaining frozen at the event horizon. Isn’t a much simpler explanation that the electric and gravitational fields remain at the event horizon because infalling matter also remains at the horizon? 


#10
May2410, 09:17 AM

P: 2,456




#11
May2410, 09:26 AM

P: 1,815




#12
May2410, 12:32 PM

P: 2,456

Are you aware that the position of the apparent event horzon is observerdependent, so different particles dont argree where the star is 'frozen'?



#13
May2410, 01:23 PM

P: 1,815

Yes, that's what I intended to say.



#14
May2510, 12:53 PM

P: 308

hey now  inre: "nobody believes that singularity is actually a zerovolume point"
i believe it. i try to believe at least 3 impossible things before breakfast each day  otherwise, you have to just give up on physics entirely :) 


#15
May2510, 04:04 PM

P: 10

The OP asked what gives a BH its gravity, and the answer is the same thing that gives anything the same property, energymomentum. 


#16
May2910, 02:37 AM

P: 1,555




#17
May2910, 06:41 AM

P: 2,456

In particle physics, "point particle" is synonymous with "elementary particle", which is defined as a particle without structure or, equivalently, as a particle which is not made up from component parts. According to the Standard Model of fundamental particles and forces, quarks, leptons and the (noncomposite) vector bosons are point particles in this sense. There is no experimental evidence for any of the elementary particles having spatial extent, and so they are usually considered to be point particles in the more general sense too (at least to the limited extent that the concept of a "particle" is meaningful in quantum field theory.) ??? empty space. Every point has 0 density 


#18
May2910, 07:00 AM

P: 1,555




Register to reply 
Related Discussions  
Gravity: Point where Earth's Gravity and the Moon's cancel each other out?  Advanced Physics Homework  21  
Does gravity affect gravity the way gravity affects time?  General Physics  12  
[help]how to obtain gravity eqution from gravity field Action S?  Special & General Relativity  3  
[help]how to obtain gravity eqution from gravity field Action S?  Advanced Physics Homework  2  
Gravity on mass on earth's surface vs sunearth gravity  Introductory Physics Homework  2 