what is the weight of light? as it can't come out of black hole, it must be having some weight

We know that light has energy, just like a ball that is hurled at a wall pushes the wall a little bit, light photons that hit any surface push the surface a bit and then bounce off. When light encounters an object, it pushes that object just a little bit.Light is very special. It has properties of both waves and particles, for example, it will bend around corners to a small degree due to diffraction as a wave. On the other hand, if you lower the intensity of a source enough (make it dim enough) you can actually count the arrival of photons which compose the light. These photons behave like particles. Here is the tricky answer to your question-- light doesn't have mass (at least it must be very, very small) but it does have signifcant momentum. So, for example, a beam of light reflecting off a mirror pushes on the mirror, but a very small amount. One way to measure the momentum is with a mirror, placed on a very sensitive balance in a vacuum jar to avoid air currents. Then a strong beam of light will push on the mirror, and one can find the momentum by relating the pressure with the intensity. This is the idea behind a "solar sail" -- a very thin mirror, thousands of meters wide which can be used to move space-craft using the light pressure of the sun. On the other hand, light does have energy -- and Einstein showed that energy and mass were related-- so in that sense, light does have mass -- but it still doesn't have "Rest Mass". i.e. a photon always travels at the speed of light -- if you try to slow one down, it merely loses energy (but doesn't go slower) until you have nothing left. Light traveling up from the earth into space, actually shifts to lower energies due to the effect of earth's gravity -- but the effect is very small. You can't measure it for visible light, but you can for gamma rays which are much higher energy light particles. Sunlight, generated near the core of the sun, can take 100,000 years to escape the surface. The pressure deep in the sun is so high, that the light hits matter and is scattered in random directions. It gets scattered so much that although it is still going the speed of light, thousands of years elapse before it gets to the surface and can esacpe. This means that the sun has over 50,000 years of generated light trapped within. This light creates pressure which makes the sun larger than it would be otherwise. In fact, for larger stars, the light pressure is larger than any other source of pressure in the star. For super big stars (200x the mass of the sun) the light pressure defeats the gravity holding the star together -- and the star falls apart. So, light has no rest mass, but it does have momentum and energy-- it can be measured by sensitive balances or by the diameter of stars. read less

Light does have weight, meaning that it is a source of gravitational fields. A box of light weighs more than an empty box. The reason is that ALL forms of energy have weight. However, under ordinary earthly conditions, the weight of light is insignificant compared to other weights. Weight (gravitational effects) comes from energy and momentum in General Relativity. Light has energy and momentum and therefore participates in gravitational effects. Not only does it move through the curved space-time produced by its neighbors, but it also contributes a little to curving spacetime. So your main question is answered, but now we get to the troublesome word "mass". Does light have mass? That depends on your definition of mass. If you mean the inertial mass m, as in momentum p=mv, where v is the velocity, the answer is yes. That's the same m which is given by E/c2. Since light has energy and momentum, it obviously has m in this sense of the term. If you mean rest mass, also called invariant mass, the answer is sort of no. That's the m in E2=p2c2+m2c4. Since for a single photon E=pc, that m is zero. As soon as you have multiple photons going different directions, their p's partially or completely cancel even though their E's just add up. In case the p's exactly cancel, you end up with E=mc2, i.e. for that batch of light the invariant mass and the inertial mass are the same and not zero. The inertial mass of that batch of light is just the sum of the inertial masses of the parts, but the invariant mass is completely different from the sum of the invariant masses of the parts. Now the gravitational effect of that light can't depend on whether we call it a bunch of separate photons, with total rest mass zero, or call it a big bag of light, with rest mass E/c2. It's easier then to just think of E as the source of gravity. (p is too, but that's a more complicated term.) read less