
#1
Sep1805, 06:10 PM

P: 4

Hi,
Here's the deal: As far as I understand gravity acts upon anything that have mass. Once a star is engulfed, no information is given out by the BH since nothing escapes its gravity. Not even light. Question 1: Why photons cannot escape from the BH since photons have no mass and they are not subjected to gravitational pull ? Question 2: How do we determine the the mass of a BH ? What indication is used to calculate its mass ? Tks 



#2
Sep1805, 07:24 PM

P: 293

1) photons are attracted by the gravitational pull according to General Relativity.
2) While there are many ways to calculate the mass, one would be to measure its attraction; ie throw a ball towards the black hole and measure its acceleration. 



#3
Sep1805, 07:28 PM

P: 29

Benjamin 



#4
Sep1805, 07:32 PM

P: 29

Photon, Gravity, Mass and Black Holes.Benjamin 



#5
Sep1905, 07:15 PM

P: 4

Thanks to all of you.
Watch out for the Warnings. Don't get in trouble because of me. Once again thanks for the explanation. Sorry about my ignorace. cicsgeek 



#6
Sep2005, 12:02 AM

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#7
Sep2005, 05:46 PM

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PF Gold
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#8
Oct105, 04:36 PM

P: 23

my astronomy teacher said that black holes have zero volume and infinite density. Also he said they all have fixed mass. i realy dont get the first two and personally dont think this is true.




#9
Oct105, 04:53 PM

PF Gold
P: 8,961

Black holes don't have fixed masses. They become more massive from accumulating matter, and less massive by 'evaporation'.




#10
Oct205, 06:12 AM

P: 2,955

From The Feynman Lectures on Physics VII, Feynam, Leighton, Sands. Turn to page 711 where Feynman is speaking in the section labeled Gravity and relativity Pete 



#11
Oct205, 06:25 AM

P: 2,955

In flat spacetime in a uniform gravitational field there is no spacetime curvature. However a beam of light will be deflected iin such a field due to the gravitational pull on the light. Pete 



#12
Oct205, 12:19 PM

P: 29

Since you consider that light is attracted (in NM) like any other massiveobjects, you can caclculate how much it should be deviated. To be more precise : if you calculate the deviation of a photon (coming for a far star) next to the sun, you find approximately alpha=0.875'', wheras the real value (given by GR) is alpha= 1.745" In my sentence i wasn't taking into account the grav. effect of the light beam on the sun because.... it is very near to 0 !. 



#13
Nov305, 01:10 PM

P: 6

your second answer is:Mass of black hole is known by its radiation or temperatre. mass of black hole is inversly proportional to temperature. lower is temp higher will mass or viceversa. 



#14
Nov305, 02:05 PM

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PF Gold
P: 3,273

Although that answer is correct for Hawking radiation you will be hard pushed to measure it! Unless the BH is a tiny primordial BH that is about to 'pop' the Hawking temperature of a BH is a fraction above absolute zero. A BH's mass is determined by the gravitational pull it makes on orbiting stars. The mass of Cygnus X1 is about 68 M _{solar} the mass of the BH at the centre of the Milky Way is about 10^{6} M_{solar}. These masses are detrmined by the velocities of the orbiting companion to Cygnus X1 a O9.7 Iab type supergiant HDE 226868, and the velocities of stars/nebulae around the galactic centre respectively. Garth 



#15
Nov405, 07:27 AM

P: 24

I am sorry if this is out of place but in curved spacetime are all things affected by gravity, even with 0 mass.




#16
Nov405, 07:43 AM

Sci Advisor
PF Gold
P: 3,273

GR reinterprets gravitational force as the effect of spacetime being curved. In free fall all objects travel along 'straight lines' called geodesics, but the geodesic itself is along a curved surface. Consider ants walking in a straight line over the dip near to the stalk of an apple. They will diverge and converge on each other and an observer not seeing the dip might conclude there are forces acting on them pushing and pulling them, whereas in fact they had been 'putting one foot in front of the other' and walking in a line that was locally straight for them. Newton's apple fell off the tree because when its stalk broke the apple and the Earth continued on straight lines through a spacetime curved by the Earth's mass with the result that they converged. BTW I have visited Newton's home in Woolsthorpe, near Grantham in Lincolnshire. He retired there from Cambridge to escape the plague in 16656 and formulated his laws of motion and gravitation, eventually published in Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy) commonly known as the Principia, 1687. Guess what? His apple tree is still there  actually the original tree was struck by lightening in the 19th century but the stump sprouted shoots and they live on today. My prized possession was a tiny apple that fell from that tree until it went rotten! How do you preserve apples for posterity? Garth 



#17
Nov405, 08:50 AM

P: 24

thanks I was confused on how light was bent by the Sun until I asked that question and you cleared it up for me, thank you



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