if photons have no mass, why cant they escape the gravity of a black hole?

Question

gravitational force takes into account the product of both masses. if one mass is zero then why isnt the force acting on the photon zero?

Answer 1

One did escape once. It's happily living in San Francisco with its gay lover.

Answer 2

The premise behind your question, that photons have no mass is incorrect. Photons DO have mass, they simply have zero *rest mass*. The energy/mass equivalence implication of special relativity (the famous E=m*c^2) points out that energy and mass are different forms of the same quantity. A photon has a gravitational mass of h*nu/c^2 (where nu is the frequency of the photon in free space, h is Planck's constant; a VERY small number, and c is the speed of light, a rather large number) So, a photon is subject to acceleration from gravity.

In General Relativity terminology, one would say that photons contribute to the stress-energy tensor that describes a system. (See reference).

Yes, this means photons generate their own gravity due to the energy they contain, though it is VERY small given the low energy of an individual photon and that one divides that energy by c^2 to find that mass equivalence. Then throw in the diminutive value of the gravitational constant and dang, it does not amount to much. (And yes, a non-zero rest mass object in motion (rest mass plus kinetic energy) generates more gravity than when at rest.)

In addition, as some previous answers point out, the Newtonian equation describing gravitational force is inaccurate. The additional terms of GR gravity become significantly large within about 2 schwarzschild radii of the center of mass (naturally, Newtonian gravity is quite accurate far enough away) .

Nevertheless, even a Newtonian application of gravity to the energy it takes to escape from the Schwarzshild radius of a black hole shows why a photon can't do it. The photon has to lose more energy than it contains for its mass to escape.

Bottom line, photons DO have mass, thus the premise behind the question is incorrect.

Answer 3

ANONIMOUS answer explains it very well.

my version is a bit long winded:

if Newton's theory of gravity was correct it would seems that a photon has mass and in our everyday snail like speeds Newton's theory of gravity does give us very accurate readings.

newton assumed that a object of mass exerts a force onto another object of mass but did not know why.

however Einstein theory of relavitity tells us why and more which change physics forever. an object of mass warps flat space (the bigger the mass the bigger that space is warp) this has been proven many times. effectively objects are caught in the warp space of each other. i.e. the sun warp space more than the earth and therefore the sun influence our orbit more than the earth influence the sun.
in flat space the sun would leave a dent (warp) in it (something like a bowling ball on a flat rubber sheet). so a photon would have to climb over the dent to get to us. or the escape velocity of the sun is 42.1 m/s the velocity of a photon is 299792458 m/s. at these speeds the photon is hardly affected by warp space.
now if the mass of the sun increase so would the warpness of space as would the escape velocity. that dent in the fabric of space would get deeper and curving the linear path of the photon.
now if the increasing mass of the sun turns it into a black hole the dent is so deep that space is curve back onto itself aswell as the path of the photon. so it the warpness of space around an object of mass thats affects the path of the photon rather than newtons concept of gravity.
i hopes this helps.

Answer 4

In the World of physics this is a catch -22.
If photon radiation has mass and was to radiate to the outskirts of the Universe it would orbit the Universe ,hence would not escape The containment of this phenomenal mass of the Universe itself.The Universe can be considered a black hole because its very massive.
If the Photon radiation has no mass it could never bounce off any thing. Because elastic masses react during collison producing equal and opposite forces.
The delemma remains=Is light a masseles entity or is it a mass having a finite elementary structure?
Your question is a very inovative one.Very good.
Hence we have to guess and have a lot of scientific faith to say that photon radiation has no mass at all= Zero(Zielch).
So far The Science world has strong faith and do belive that photons have zero rest mass. Nerver the less photons are not at rest they continually move so how can we measure something that continually moves. May be the science of physics is right about something like light that can exist without mass and volume can be real.
Something is considered real if when it collides with another mass it produces equal and opposite forces between the coliders.

Answer 5

That's a good question. Most students are taught the Newtonian view of gravity, which is that gravity is a force that acts on masses. That is actually a very useful view and a very good approximation; but Einstein presented a whole different view of the nature of gravity. In his view, gravity is not a "force," but a curvature of space caused by the presence of a massive object. In that view, ANYTHING that flies past a massive object (whether the flying thing has mass or not) will follow a curved path.

This novel viewpoint was proven by experiment in 1919, when astronomers actually witnessed light from a distant star curving slightly as the light passed close by our sun.

The faster something moves, the harder it is to notice the gravity-induced bend in its path. That's why we usually don't notice the slight bend in the path of a light beam. But in the vicinity of a black hole, the bend is so great as to keep the light from escaping.

Answer 6

General relativity describes a black hole as a region of empty space with a pointlike singularity at the center and an event horizon at the outer edge, the description changes when the effects of quantum mechanics are taken into account. Research on this subject indicates that, rather than holding captured matter forever, black holes slowly leak a form of thermal energy called Hawking radiation. However, the final, correct description of black holes, requiring a theory of quantum gravity, is unknown.

Answer 7

BLACK HOLES
The 20th century saw a great many new discoveries regarding celestial phenomena in the universe. One of these entities, which has only recently been encountered, is the Black Hole. These are formed when a star which has consumed all its fuel collapses in on itself, eventually turning into a black hole with infinite density and zero volume and an immensely powerful magnetic field. We are unable to see black holes even with the most powerful telescope, because their gravitational pull is so strong that light is unable to escape from them. However, such a collapsed star can be perceived by means of the effect it has on the surrounding area. In Surat al-Waqi'a, Allah draws attention to this matter in this way, by swearing upon the position of stars:

http://www.harunyahya.com/articles/miracles_universe.php

Answer 8

this is cause a black hole bends space that much that a photon simply has no more straight path to the outside anymore.

the same explanation is valid for gravitational lensing
a photon seems travelling on a straight line (in fact its a bit more complex but that does not matter in this case)
and the straight line it would travel has been bent by gravitation.

so gravitation does not influence the photon itself, it modifies the space it travels through

Answer 9

The explanation of gravity you are using is Newtonian and it works relatively accurately under normal circumstances. The first experiment that proved that Einstein's method of explaining what is happening is more accurate was conducted by English scientists just after the armistice of World War One.
Gravity is not a force at a distance. Gravity is a warping of the shape of space itself. The photons, which travel in a straight line, find that no straight line points in such a direction that it crosses the event horizon of a black hole. All lines are bent

Answer 10

Well im impressed with a lot of answers.
you need to read up on what is know as "Hawking radiation"
taken from Prof Stephen Hawking (for those budding quantum physicists out there).
This put simply prof Hawking worked out a set of really difficult equations about black holes which showed that black holes emit a radiation in the form of exotic particles...there-by also loosing mass! very clever guy!

Answer 11

It's been proven that Einstein was right when he theorized that what we call gravity is actually a curvature of space caused by the presence of mass. Everything, including light, is embedded in space and when moving must move through space. If that space is so severely distorted by mass that it actually curves back on itself then even light can't escape it. That's the case in black holes.